ME412 –
MACHINING
2022
Assoc. Prof. Emel KURAM
Gebze Technical University
Department of Mechanical Engineering
ME412
Assoc. Prof. Emel KURAM
CONTENTS
1. MACHINING
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1.1. Material Removal Processes
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1.2. Theory of Chip Formation in Metal Machining
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1.2.1. Orthogonal Cutting Model
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1.2.2. Actual Chip Formation
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1.3. Force Relationships and Merchant Equation
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1.3.1. Forces in Metal Cutting
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1.3.2. Merchant Equation
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1.4. Power and Energy Relationships in Machining
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1.5. Machinability
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1.5.1. Machinability of Ferrous Metals
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1.5.2. Machinability of Nonferrous Metals
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1.5.3. Machinability of Miscellaneous Materials
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2. TOOL WEAR
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2.1. Tool Life
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2.1.1. Tool Wear
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2.1.2. Tool Life and Taylor Tool Life Equation
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3. SURFACE ROUGHNESS
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3.1. Surface Finish and Integrity
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3.2. Surface Texture and Roughness
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4. TEMPERATURE
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4.1. Factors Affecting Cutting Temperatures
5. CUTTING FLUIDS
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5.1. Types of Cutting Fluids
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5.1.1. Cutting Fluid Functions
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5.1.2. Chemical Formulation of Cutting Fluids
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5.2. Application of Cutting Fluids
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5.2.1. Application Methods
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5.2.2. Cutting Fluid Filtration and Dry Machining
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6. CUTTING TOOLS
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6.1. Tool Materials
Assoc. Prof. Emel KURAM
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6.1.1. High–Speed Steel and its Predecessors
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6.1.2. Cast Cobalt Alloys
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6.1.3. Cemented Carbides, Cermets and Coated Carbides
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6.1.4. Ceramics
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6.1.5. Synthetic Diamonds and Cubic Boron Nitride
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6.2. Tool Geometry
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6.2.1. Single–Point Tool Geometry
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6.2.2. Multiple–Cutting–Edge Tools
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7. DRILLING
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7.1. Cutting Conditions in Drilling
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7.2. Operations Related to Drilling
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7.3. Drill Presses
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7.4. Thrust Force and Torque
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7.5. Reaming and Reamers
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7.6. Tapping and Taps
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8. TURNING
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8.1. Cutting Conditions in Turning
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8.2. Operations Related to Turning
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8.3. Engine Lathe
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8.4. Other Lathes and Turning Machines
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8.5. Boring Machines
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8.6. Cutting Tools
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8.6.1. Tool Geometry
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8.7. Forces in Turning
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9. MILLING
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9.1. Types of Milling Operations
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9.2. Cutting Conditions in Milling
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9.3. Milling Machines
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1. MACHINING
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Assoc. Prof. Emel KURAM
ME412
Assoc. Prof. Emel KURAM
1. MACHINING
1.1. Material Removal Processes
The Material Removal Processes are a family of shaping operations in which excess material is
removed from a starting workpart so that what remains is the desired final geometry. The ‘‘family
tree’’ is shown in Figure 1.1. The most important branch of the family is Conventional Machining, in
which a sharp cutting tool is used to mechanically cut the material to achieve the desired geometry.
The three principal machining processes are turning, drilling and milling. Another group of material
removal processes is the Abrasive Processes, which mechanically remove material by the action of
hard, abrasive particles. This process group, which includes grinding. The ‘‘other abrasive processes’’
in Figure 1.1 include honing, lapping and superfinishing. Finally, there are the Nontraditional
Processes, which use various energy forms other than a sharp cutting tool or abrasive particles to
remove material. The energy forms include mechanical, electrochemical, thermal and chemical1.
Figure 1.1. Classification of material removal processes1.
1 M.
P. Groover, Fundamentals of Modern Manufacturing Materials, Processes, and Systems, John Wiley & Sons, Fourth
Edition, 2010.
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Assoc. Prof. Emel KURAM
Machining is a manufacturing process in which a sharp cutting tool is used to cut away material
to leave the desired part shape. The predominant cutting action in machining involves shear
deformation of the work material to form a chip; as the chip is removed, a new surface is exposed.
Machining is most frequently applied to shape metals. The process is illustrated in the diagram of
Figure 1.21.
Figure 1.2. (a) A cross–sectional view of the machining process. (b) Tool with negative rake angle;
compare with positive rake angle in (a)1.
Machining is important commercially and technologically for several reasons1:
✓ Variety of Work Materials : Machining can be applied to a wide variety of work materials.
Virtually all solid metals can be machined. Plastics and plastic composites can also be cut by
machining. Ceramics pose difficulties because of their high hardness and brittleness; however,
most ceramics can be successfully cut by the abrasive machining processes.
✓ Variety of Part Shapes and Geometric Features : Machining can be used to create any regular
geometries, such as flat planes, round holes and cylinders. By introducing variations in tool
shapes and tool paths, irregular geometries can be created, such as screw threads and T–slots.
By combining several machining operations in sequence, shapes of almost unlimited
complexity and variety can be produced.
✓ Dimensional Accuracy : Machining can produce dimensions to very close tolerances. Some
machining processes can achieve tolerances of ±0.025 mm (±0.001 in.), much more accurate
than most other processes.
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Assoc. Prof. Emel KURAM
✓ Good Surface Finishes : Machining is capable of creating very smooth surface finishes.
Roughness values less than 0.4 microns (16 μ-in.) can be achieved in conventional machining
operations. Some abrasive processes can achieve even better finishes.
On the other hand, certain disadvantages are associated with machining and other material
removal processes1:
 Wasteful of Material : Machining is inherently wasteful of material. The chips generated in a
machining operation are wasted material. Although these chips can usually be recycled, they
represent waste in terms of the unit operation.
 Time Consuming : A machining operation generally takes more time to shape a given part than
alternative shaping processes such as casting or forging.
Machining is generally performed after other manufacturing processes such as casting or bulk
deformation (e.g., forging, bar drawing). The other processes create the general shape of the starting
workpart and machining provides the final geometry, dimensions and finish1.
Machining is not just one process; it is a group of processes. The common feature is the use of a
cutting tool to form a chip that is removed from the workpart. To perform the operation, relative
motion is required between the tool and work. This relative motion is achieved in most machining
operations by means of a primary motion, called the Cutting Speed and a secondary motion, called
the Feed. The shape of the tool and its penetration into the work surface, combined with these
motions, produces the desired geometry of the resulting work surface1.
Cutting Conditions
Relative motion is required between the tool and work to perform a machining operation. The
primary motion is accomplished at a certain Cutting Speed (v). In addition, the tool must be moved
laterally across the work. This is a much slower motion, called the Feed (f). The remaining dimension
of the cut is the penetration of the cutting tool below the original work surface, called the Depth of
Cut (d). Collectively, speed, feed and depth of cut are called the Cutting Conditions. They form the
three dimensions of the machining process and for certain operations (e.g., most single–point tool
operations) they can be used to calculate the material removal rate for the process1:
𝑅𝑀𝑅 = 𝑣𝑓𝑑
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(1.1.)
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Assoc. Prof. Emel KURAM
RMR = Material Removal Rate (mm3/s, in3/min)
v = Cutting Speed (m/s, ft/min)
f = Feed (mm, in)
d = Depth of Cut (mm, in)
Machining operations usually divide into two categories, distinguished by purpose and cutting
conditions: roughing cuts and finishing cuts. Roughing cuts are used to remove large amounts of
material from the starting workpart as rapidly as possible, in order to produce a shape close to the
desired form, but leaving some material on the piece for a subsequent finishing operation. Finishing
cuts are used to complete the part and achieve the final dimensions, tolerances and surface finish. In
production machining jobs, one or more roughing cuts are usually performed on the work, followed
by one or two finishing cuts. Roughing operations are performed at high feeds and depths – feeds of
0.4 to 1.25 mm/rev (0.015–0.050 in./rev) and depths of 2.5 to 20 mm (0.100–0.750 in.) are typical.
Finishing operations are carried out at low feeds and depths – feeds of 0.125 to 0.4 mm (0.005–0.015
in./rev) and depths of 0.75 to 2.0 mm (0.030–0.075 in.) are typical. Cutting speeds are lower in
roughing than in finishing1.
Machine Tools
A Machine Tool is used to hold the workpart, position the tool relative to the work and provide
power for the machining process at the speed, feed and depth that have been set. By controlling the
tool, work and cutting conditions, machine tools permit parts to be made with great accuracy and
repeatability, to tolerances of 0.025 mm (0.001 in.) and better. The term Machine Tool applies to any
power–driven machine that performs a machining operation, including grinding. The term is also
applied to machines that perform metal forming and pressworking operations1.
The traditional machine tools used to perform turning, drilling and milling are lathes, drill presses
and milling machines, respectively. Conventional machine tools are usually tended by a human
operator, who loads and unloads the workparts, changes cutting tools and sets the cutting conditions.
Many modern machine tools are designed to accomplish their operations with a form of automation
called Computer Numerical Control1.
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Assoc. Prof. Emel KURAM
1.2. Theory of Chip Formation in Metal Machining
The geometry of most practical machining operations is somewhat complex. A simplified model of
machining is available that neglects many of the geometric complexities, yet describes the mechanics
of the process quite well. It is called the Orthogonal Cutting Model (Figure 1.3). Although an actual
machining process is three–dimensional, the orthogonal model has only two dimensions that play
active roles in the analysis1.
Figure 1.3. Orthogonal cutting: (a) as a three–dimensional process and (b) how it reduces to two
dimensions in the side view1.
1.2.1. Orthogonal Cutting Model
By definition, orthogonal cutting uses a wedge–shaped tool in which the cutting edge is
perpendicular to the direction of cutting speed. As the tool is forced into the material, the chip is
formed by shear deformation along a plane called the Shear Plane, which is oriented at an angle 𝜙
with the surface of the work. Only at the sharp cutting edge of the tool does failure of the material
occur, resulting in separation of the chip from the parent material. Along the shear plane, where the
bulk of the mechanical energy is consumed in machining, the material is plastically deformed1.
The tool in orthogonal cutting has only two elements of geometry: (1) rake angle and (2) clearance
angle. The rake angle determines the direction that the chip flows as it is formed from the workpart;
and the clearance angle provides a small clearance between the tool flank and the newly generated
work surface1.
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Assoc. Prof. Emel KURAM
During cutting, the cutting edge of the tool is positioned a certain distance below the original work
surface. This corresponds to the thickness of the chip prior to chip formation (to). As the chip is formed
along the shear plane, its thickness increases to tc. The ratio of to to tc is called the Chip Thickness
Ratio (or simply the Chip Ratio, r)1:
𝑟=
𝑡𝑜
𝑡𝑐
(1.2.)
Since the chip thickness after cutting is always greater than the corresponding thickness before
cutting, the chip ratio will always be less than 1.01.
In addition to to, the orthogonal cut has a width dimension (w), as shown in Figure 1.3 (a), even
though this dimension does not contribute much to the analysis in orthogonal cutting1.
The geometry of the orthogonal cutting model allows us to establish an important relationship
between the chip thickness ratio, the rake angle and the shear plane angle. Let ls be the length of the
shear plane. We can make the substitutions: 𝑡𝑜 = 𝑙𝑠 sin 𝜙 and 𝑡𝑐 = 𝑙𝑠 cos(𝜙 − 𝛼). Thus1,
𝑟=
𝑙𝑠 sin 𝜙
sin 𝜙
=
𝑙𝑠 cos(𝜙 − 𝛼) cos(𝜙 − 𝛼)
𝑟 cos 𝛼
tan 𝜙 =
1 − 𝑟 sin 𝛼
(1.3.)
The shear strain that occurs along the shear plane can be estimated by examining Figure 1.4. Part
(a) shows shear deformation approximated by a series of parallel plates sliding against one another
to form the chip. Consistent with our definition of shear strain, each plate experiences the shear strain
shown in Figure 1.4 (b). Referring to part (c), this can be expressed as1
𝛾=
𝐴𝐶 𝐴𝐷 + 𝐷𝐶
=
𝐵𝐷
𝐵𝐷
𝛾 = tan(𝜙 − 𝛼) + cot 𝜙
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(1.4.)
ME412
Assoc. Prof. Emel KURAM
Figure 1.4. Shear strain during chip formation: (a) chip formation depicted as a series of parallel
plates sliding relative to each other; (b) one of the plates isolated to illustrate the definition of shear
strain based on this parallel plate model; and (c) shear strain triangle used to derive Equation 1.41.
Problem 1.1 = In a machining operation that approximates orthogonal cutting, the cutting tool
has a rake angle = 10°. The chip thickness before the cut to = 0.50 mm and the chip thickness after the
cut tc = 1.125 in. Calculate the shear plane angle and the shear strain in the operation1.
𝑟=
tan 𝜙 =
𝑡𝑜
0.5
=
= 0.444
𝑡𝑐 1.125
0.444 cos 10
= 0.4738
1 − 0.444 sin 10
𝜙 = 25.4°
𝛾 = tan(𝜙 − 𝛼) + cot 𝜙 = tan(25.4 − 10) + cot 25.4 = 0.275 + 2.111 = 2.386
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Assoc. Prof. Emel KURAM
1.2.2. Actual Chip Formation
We should note that there are differences between the orthogonal model and an actual machining
process. First, the shear deformation process does not occur along a plane, but within a zone. If
shearing were to take place across a plane of zero thickness, it would imply that the shearing action
must occur instantaneously as it passes through the plane, rather than over some finite (although
brief) time period. For the material to behave in a realistic way, the shear deformation must occur
within a thin shear zone. This more realistic model of the shear deformation process in machining is
illustrated in Figure 1.5. Metal–cutting experiments have indicated that the thickness of the shear
zone is only a few thousandths of an inch. Since the shear zone is so thin, there is not a great loss of
accuracy in most cases by referring to it as a plane1.
Second, in addition to shear deformation that occurs in the shear zone, another shearing action
occurs in the chip after it has been formed. This additional shear is referred to as Secondary Shear
to distinguish it from primary shear. Secondary shear results from friction between the chip and the
tool as the chip slides along the rake face of the tool. Its effect increases with increased friction
between the tool and chip. The primary and secondary shear zones can be seen in Figure 1.51.
Figure 1.5. More realistic view of chip formation, showing shear zone rather than shear plane. Also
shown is the secondary shear zone resulting from tool–chip friction1.
Third, formation of the chip depends on the type of material being machined and the cutting
conditions of the operation. Four basic types of chip can be distinguished, illustrated in Figure 1.61:
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Assoc. Prof. Emel KURAM
Discontinuous Chip : When relatively brittle materials (e.g., cast irons) are machined at low
cutting speeds, the chips often form into separate segments (sometimes the segments are loosely
attached). This tends to impart an irregular texture to the machined surface. High tool–chip friction
and large feed and depth of cut promote the formation of this chip type1. Discontinuous chips usually
form under the following conditions2:
֍ Brittle workpiece materials, because they do not have the capacity to undergo the high shear
strains involved in cutting
֍ Workpiece materials that contain hard inclusions and impurities or have structures such as the
graphite flakes in gray cast iron
֍ Very low or very high cutting speeds
֍ Large depths of cut
֍ Low rake angles
֍ Lack of an effective cutting fluid
֍ Low stiffness of the toolholder or the machine tool, thus allowing vibration and chatter to occur
Continuous Chip : When ductile work materials are cut at high speeds and relatively small feeds
and depths, long continuous chips are formed1. Continuous chips usually are formed with ductile
materials that are machined at high rake angles. The deformation of the material takes place along a
narrow shear zone called the Primary Shear Zone. Continuous chips may develop a secondary shear
zone because of high friction at the tool–chip interface; this zone becomes thicker as friction
increases2. A good surface finish typically results when this chip type is formed. A sharp cutting edge
on the tool and low tool–chip friction encourage the formation of continuous chips. Long, continuous
chips (as in turning) can cause problems with regard to chip disposal and/or tangling about the tool.
To solve these problems, turning tools are often equipped with chip breakers1.
Continuous Chip with Built–Up Edge : When machining ductile materials at low–to–medium
cutting speeds, friction between tool and chip tends to cause portions of the work material to adhere
to the rake face of the tool near the cutting edge. This formation is called a Built–Up Edge (BUE). The
formation of a BUE is cyclical; it forms and grows, then becomes unstable and breaks off. Much of the
detached BUE is carried away with the chip, sometimes taking portions of the tool rake face with it,
2
S. Kalpakjian, S. R. Schmid, Manufacturing Engineering and Technology, Pearson Prentice Hall, Sixth Edition, 2010.
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ME412
Assoc. Prof. Emel KURAM
which reduces the life of the cutting tool. Portions of the detached BUE that are not carried off with
the chip become imbedded in the newly created work surface, causing the surface to become rough1.
The tendency for BUE formation can be reduced by one or more of the following means2:
֍ Increase the cutting speeds
֍ Decrease the depth of cut
֍ Increase the rake angle
֍ Use a sharp tool
֍ Use an effective cutting fluid
֍ Use a cutting tool that has lower chemical affinity for the workpiece material
The preceding chip types were first classified by Ernst in the late 1930s. Since then, the available
metals used in machining, cutting tool materials and cutting speeds have all increased and a fourth
chip type has been identified1:
Serrated Chips (Segmented, Shear–Localized) : These chips are semi–continuous in the sense
that they possess a saw–tooth appearance that is produced by a cyclical chip formation of alternating
high shear strain followed by low shear strain. This fourth type of chip is most closely associated with
certain difficult–to–machine metals such as titanium alloys, nickel–base superalloys and austenitic
stainless steels when they are machined at higher cutting speeds1. Metals with low thermal
conductivity and strength that decreases sharply with temperature (thermal softening) exhibit this
behavior, most notably titanium2. However, the phenomenon is also found with more common work
metals (e.g., steels) when they are cut at high speeds1.
Figure 1.6. Four types of chip formation in metal cutting: (a) discontinuous, (b) continuous, (c)
continuous with built–up edge, (d) serrated1.
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Assoc. Prof. Emel KURAM
1.3. Force Relationships and Merchant Equation
1.3.1. Forces in Metal Cutting
Consider the forces acting on the chip during orthogonal cutting in Figure 1.7 (a). The forces
applied against the chip by the tool can be separated into two mutually perpendicular components:
friction force and normal force to friction. The Friction Force (F) is the frictional force resisting the
flow of the chip along the rake face of the tool. The Normal Force to Friction (N) is perpendicular to
the friction force. These two components can be used to define the coefficient of friction between the
tool and the chip1:
𝜇=
𝐹
𝑁
(1.5.)
The friction force and its normal force can be added vectorially to form a resultant force (R), which
is oriented at an angle β, called the Friction Angle. The friction angle is related to the coefficient of
friction as1
𝜇 = tan 𝛽
(1.6.)
In addition to the tool forces acting on the chip, there are two force components applied by the
workpiece on the chip: shear force and normal force to shear. The Shear Force (Fs) is the force that
causes shear deformation to occur in the shear plane and the Normal Force to Shear (Fn) is
perpendicular to the shear force. Based on the shear force, we can define the shear stress that acts
along the shear plane between the work and the chip1
𝜏=
𝐹𝑠
𝐴𝑠
(1.7.)
𝑡𝑜 𝑤
sin 𝜙
(1.8.)
As = Area of the Shear Plane
𝐴𝑠 =
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Assoc. Prof. Emel KURAM
The shear stress in Equation 1.7 represents the level of stress required to perform the machining
operation. Therefore, this stress is equal to the shear strength of the work material (τ = S) under the
conditions at which cutting occurs1.
Vector addition of the two force components Fs and Fn yields the resultant force (R’). In order for
the forces acting on the chip to be in balance, this resultant R’ must be equal in magnitude, opposite
in direction and collinear with the resultant R1.
Figure 1.7. Forces in metal cutting: (a) forces acting on the chip in orthogonal cutting and (b) forces
acting on the tool that can be measured1.
None of the four force components F, N, Fs and Fn can be directly measured in a machining
operation, because the directions in which they are applied vary with different tool geometries and
cutting conditions. However, it is possible for the cutting tool to be instrumented using a force
measuring device called a Dynamometer, so that two additional force components acting against the
tool can be directly measured: cutting force and thrust force. The Cutting Force (Fc) is in the direction
of cutting, the same direction as the cutting speed (v) and the Thrust Force (Ft) is perpendicular to
the cutting force and is associated with the chip thickness before the cut (to). The cutting force and
thrust force are shown in Figure 1.7 (b) together with their resultant force (R’’). The respective
directions of these forces are known, so the force transducers in the dynamometer can be aligned
accordingly1.
Equations can be derived to relate the four force components that cannot be measured to the two
forces that can be measured. Using the force diagram in Figure 1.8, the following trigonometric
relationships can be derived1:
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Assoc. Prof. Emel KURAM
𝐹 = 𝐹𝑐 sin 𝛼 + 𝐹𝑡 cos 𝛼
(1.9.)
𝑁 = 𝐹𝑐 cos 𝛼 − 𝐹𝑡 sin 𝛼
(1.10.)
𝐹𝑠 = 𝐹𝑐 cos 𝜙 − 𝐹𝑡 sin 𝜙
(1.11.)
𝐹𝑛 = 𝐹𝑐 sin 𝜙 + 𝐹𝑡 cos 𝜙
(1.12.)
Figure 1.8. Force diagram showing geometric relationships between F, N, Fs, Fn, Fc and Ft1.
If cutting force and thrust force are known, these four equations can be used to calculate estimates
of shear force, friction force and normal force to friction. Based on these force estimates, shear stress
and coefficient of friction can be determined1.
Note that in the special case of orthogonal cutting when the rake angle α = 0, Equations 1.9 and
1.10 reduce to F = Ft and N = Fc, respectively. Thus, in this special case, friction force and its normal
force could be directly measured by the dynamometer1.
Problem 1.2 = Suppose in Problem 1.1 that cutting force and thrust force are measured during an
orthogonal cutting operation: Fc = 1559 N and Ft = 1271 N. The width of the orthogonal cutting
operation w = 3.0 mm. Based on these data, determine the shear strength of the work material1.
α = 10°
𝜙 = 25.4°
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Assoc. Prof. Emel KURAM
𝐹𝑠 = 𝐹𝑐 cos 𝜙 − 𝐹𝑡 sin 𝜙 = 1559 cos 25.4 − 1271 sin 25.4 = 863 𝑁
𝐴𝑠 =
(0.5)(3.0)
𝑡𝑜 𝑤
=
= 3.497 𝑚𝑚2
sin 𝜙
sin 25.4
𝜏=𝑆=
863
𝑁
= 247
= 247 𝑀𝑃𝑎
3.497
𝑚𝑚2
𝐹𝑠 = 𝑆𝐴𝑠
𝐹𝑐 =
𝑆𝑡𝑜 𝑤 cos(𝛽 − 𝛼)
𝐹𝑠 cos(𝛽 − 𝛼)
=
sin 𝜙 cos(𝜙 + 𝛽 − 𝛼) cos(𝜙 + 𝛽 − 𝛼)
𝐹𝑡 =
𝑆𝑡 𝑤 sin(𝛽 − 𝛼)
𝐹𝑠 sin(𝛽 − 𝛼)
=
sin 𝜙 cos(𝜙 + 𝛽 − 𝛼) cos(𝜙 + 𝛽 − 𝛼)
(1.13.)
(1.14.)
1.3.2. Merchant Equation
One of the important relationships in metal cutting was derived by Eugene Merchant. Its derivation
was based on the assumption of orthogonal cutting, but its general validity extends to three–
dimensional machining operations. Merchant started with the definition of shear stress expressed in
the form of the following relationship derived by combining Equations 1.7, 1.8 and 1.111.
𝜏=
𝐹𝑐 cos 𝜙 − 𝐹𝑡 sin 𝜙
(𝑡𝑜 𝑤/ sin 𝜙)
(1.15.)
Merchant reasoned that, out of all the possible angles emanating from the cutting edge of the tool
at which shear deformation could occur, there is one angle 𝜙 that predominates. This is the angle at
which shear stress is just equal to the shear strength of the work material and so shear deformation
occurs at this angle. For all other possible shear angles, the shear stress is less than the shear strength,
so chip formation cannot occur at these other angles. In effect, the work material will select a shear
plane angle that minimizes energy. This angle can be determined by taking the derivative of the shear
stress (S) in Equation 1.15 with respect to 𝜙 and setting the derivative to zero. Solving for 𝜙, we get
the relationship named after Merchant1:
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𝜙 = 45 +
Assoc. Prof. Emel KURAM
𝛼 𝛽
−
2 2
(1.16.)
Among the assumptions in the Merchant equation is that shear strength of the work material is a
constant, unaffected by strain rate, temperature and other factors. Because this assumption is violated
in practical machining operations, Equation 1.16 must be considered an approximate relationship
rather than an accurate mathematical equation1.
Problem 1.3 = Using the data and results from our previous problems, determine
(a) the friction angle
(b) the coefficient of friction1.
(a)
α = 10°
𝜙 = 25.4°
𝛽 = 2(45) + 10 − 2(25.4) = 49.2°
(b)
𝜇 = tan 49.2 = 1.16
Lessons Based on the Merchant Equation
The real value of the Merchant equation is that it defines the general relationship between rake
angle, tool–chip friction and shear plane angle. The shear plane angle can be increased by (1)
increasing the rake angle and (2) decreasing the friction angle (and coefficient of friction) between
the tool and the chip. Rake angle can be increased by proper tool design and friction angle can be
reduced by using a lubricant cutting fluid1.
The importance of increasing the shear plane angle can be seen in Figure 1.9. If all other factors
remain the same, a higher shear plane angle results in a smaller shear plane area. Since the shear
strength is applied across this area, the shear force required to form the chip will decrease when the
shear plane area is reduced. A greater shear plane angle results in lower cutting energy, lower power
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requirements and lower cutting temperature. These are good reasons to try to make the shear plane
angle as large as possible during machining1.
Figure 1.9. Effect of shear plane angle (𝜙): (a) higher 𝜙 with a resulting lower shear plane area; (b)
smaller 𝜙 with a corresponding larger shear plane area. Note that the rake angle is larger in (a),
which tends to increase shear angle according to the Merchant equation1.
Approximation of Turning by Orthogonal Cutting
The orthogonal model can be used to approximate turning and certain other single–point
machining operations so long as the feed in these operations is small relative to depth of cut. Thus,
most of the cutting will take place in the direction of the feed and cutting on the point of the tool will
be negligible. Figure 1.10 indicates the conversion from one cutting situation to the other1.
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Figure 1.10. Approximation of turning by the orthogonal model: (a) turning; and (b) the
corresponding orthogonal cutting1.
The interpretation of cutting conditions is different in the two cases. The chip thickness before the
cut (to) in orthogonal cutting corresponds to the feed (f) in turning and the width of cut (w) in
orthogonal cutting corresponds to the depth of cut (d) in turning. In addition, the thrust force (Ft) in
the orthogonal model corresponds to the feed force (Ff) in turning. Cutting speed and cutting force
have the same meanings in the two cases. Table 1.1 summarizes the conversions1.
Table 1.1. Conversion key: turning operation vs. orthogonal cutting1.
Turning Operation
Orthogonal Cutting Model
Feed f = Chip thickness before cut to
Depth d = Width of cut w
Cutting speed v = Cutting speed v
Cutting force Fc = Cutting force Fc
Feed force Ff = Thrust force Ft
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1.4. Power and Energy Relationships in Machining
A machining operation requires power. The cutting force in a production machining operation
might exceed 1000 N (several hundred pounds). Typical cutting speeds are several hundred m/min.
The product of cutting force and speed gives the power (energy per unit time) required to perform a
machining operation1:
(1.17.)
𝑃𝑐 = 𝐹𝑐 𝑣
Pc = Cutting Power (N-m/s, W, ft-lb/min)
Fc = Cutting Force (N, lb)
v = Cutting Speed (m/s, ft/min)
In U.S. customary units, power is traditionally expressed as horsepower by dividing ft-lb/min by
33000. Hence1,
𝐻𝑃𝑐 =
𝐹𝑐 𝑣
33000
(1.18.)
HPc = Cutting Horsepower (hp)
The gross power required to operate the machine tool is greater than the power delivered to the
cutting process because of mechanical losses in the motor and drive train in the machine. These losses
can be accounted for by the mechanical efficiency of the machine tool1:
𝑃𝑔 =
𝑃𝑐
𝐸
𝐻𝑃𝑔 =
𝐻𝑃𝑐
𝐸
(1.19.)
Pg = Gross Power of the Machine Tool Motor (W)
HPg = Gross Horsepower
E = Mechanical Efficiency of the Machine Tool (Typical values of E for machine tools are around
90%.)
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It is often useful to convert power into power per unit volume rate of metal cut. This is called the
Unit Power (Pu) or Unit Horsepower (HPu), defined1:
𝑃𝑢 =
𝑃𝑐
𝑅𝑀𝑅
𝐻𝑃𝑢 =
𝐻𝑃𝑐
𝑅𝑀𝑅
(1.20.)
RMR = Material Removal Rate (mm3/s, in3/min)
The material removal rate can be calculated as the product of vtow. This is Equation 1.1 using the
conversions from Table 1.1. Unit power is also known as the Specific Energy (U)1.
𝑈 = 𝑃𝑢 =
𝑃𝑐
𝐹𝑐 𝑣
𝐹𝑐
=
=
𝑅𝑀𝑅 𝑣𝑡𝑜 𝑤 𝑡𝑜 𝑤
(1.21.)
U = Specific Energy (N-m/mm3, J/ mm3, in-lb/in3)
Problem 1.4 = Continuing with our previous examples, let us determine cutting power and
specific energy in the machining operation if the cutting speed = 100 m/min. Summarizing the data
and results from previous examples, to = 0.50 mm, w = 3.0 mm, Fc = 1557 N1.
𝑃𝑐 = (1557)(100) = 155700 𝑁 − 𝑚/𝑚𝑖𝑛 = 155700 𝐽/𝑚𝑖𝑛 = 2595 𝐽/𝑠 = 2595 𝑊
𝑈=
155700
155700
=
= 1.038 𝑁 − 𝑚/𝑚𝑖𝑛3
3
100(10 )(3)(0.5) 150000
1.5. Machinability
The Machinability of a material is usually defined in terms of four factors2:
1. Surface finish and surface integrity of the machined part.
2. Tool life.
3. Force and power required.
4. The level of difficulty in chip control.
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Thus, good machinability indicates good surface finish and surface integrity, a long tool life and
low force and power requirements. As for chip control and as stated earlier regarding continuous
chips, long, thin, stringy and curled chips can interfere severely with the cutting operation by
becoming entangled in the cutting zone2.
Because of the complex nature of cutting operations, it is difficult to establish relationships that
quantitatively define the machinability of a particular material. In machining practice, tool life and
surface roughness generally are considered to be the most important factors in machinability.
Although not used much anymore due to their qualitative and misleading nature, approximate
Machinability Ratings (Indexes) have been available for many years for each type of material and
its condition2.
In these ratings, the standard material is AISI 1112 steel (resulfurized), with a rating of 100. This
means that, for a tool life of 60 min, this steel should be machined at a cutting speed of 100 ft/min (30
m/min). Examples of typical ratings are 3140 steel at 55; free–cutting brass at 300; 2011 wrought
aluminum at 200; pearlitic gray iron at 70; and precipitation–hardening 17–7 steel at 202.
1.5.1. Machinability of Ferrous Metals
Steels
Carbon steels have a wide range of machinability, depending on their ductility and hardness. If a
carbon steel is too ductile, chip formation can produce built–up edge, leading to poor surface finish; if
the steel is too hard, it can cause abrasive wear of the tool because of the presence of carbides in the
steel. Cold–worked carbon steels are desirable from a machinability standpoint2.
An important group of steels is Free–Machining Steels, containing sulfur and phosphorus. Sulfur
forms manganese–sulfide inclusions (second–phase particles), which act as stress raisers in the
primary shear zone. As a result, the chips produced break up easily and are small, thus improving
machinability. The size, shape, distribution and concentration of these inclusions significantly
influence machinability. Elements such as tellurium and selenium, both of which are chemically
similar to sulfur, act as inclusion modifiers in resulfurized steels2.
Phosphorus in steels has two major effects: (a) It strengthens the ferrite, causing increased
hardness and resulting in better chip formation and surface finish and (b) it increases hardness and
thus causes the formation of short chips instead of continuous stringy ones, thereby improving
machinability. Note that soft steels can be difficult to machine because of their tendency for built–up
edge formation and the resulting poor surface finish2.
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In Leaded Steels, a high percentage of lead solidifies at the tips of manganese–sulfide inclusions.
In nonresulfurized grades of steel, lead takes the form of dispersed fine particles. Lead is insoluble in
iron, copper and aluminum and their alloys. Because of its low shear strength, lead acts as a solid
lubricant and is smeared over the tool–chip interface during cutting2.
When the temperature developed is sufficiently high, such as at high cutting speeds and feeds, the
lead melts directly in front of the tool, acting as a liquid lubricant. In addition to having this effect, lead
lowers the shear stress in the primary shear zone, thus reducing cutting forces and power
consumption. Lead can be used with every grade of steel and is identified by the letter L between the
second and third numerals in steel identification (e.g., 10L45). (Note that in stainless steels, a similar
use of the letter L means “low carbon”, a condition that improves their corrosion resistance.)2.
However, because lead is a well–known toxin and a pollutant, there are serious environmental
concerns about its use in steels (estimated at 4500 tons of lead consumption every year in the
production of steels). Consequently, there is a continuing trend toward eliminating the use of lead in
steels (Lead–Free Steels). Bismuth and tin are substitutes for lead in steels, although their
performance is not as good2.
Calcium–Deoxidized Steels contain oxide flakes of calcium silicates (CaSO) that reduce the
strength of the secondary shear zone and decrease tool–chip interface friction and wear. Increases in
temperature are reduced correspondingly. Consequently, these steels produce less crater wear,
especially at high cutting speeds2.
Alloy Steels can have a wide variety of compositions and hardnesses. Consequently, their
machinability cannot be generalized, although they have higher levels of hardness and other
mechanical properties. Alloy steels at hardness levels of 45 to 65 HRC can be machined with
polycrystalline cubic–boron–nitride cutting tools, producing good surface finish, integrity and
dimensional accuracy2.
Effects of Various Elements in Steels = The presence of aluminum and silicon in steels is always
harmful, because these elements combine with oxygen to form aluminum oxide and silicates, which
are hard and abrasive. As a result, tool wear increases and machinability is reduced2.
Carbon and manganese have various effects on the machinability of steels, depending on their
composition. Plain low–carbon steels (less than 0.15% C) can produce poor surface finish by forming
a built–up edge. Cast steels are more abrasive, although their machinability is similar to that of
wrought steels. Tool and die steels are very difficult to machine and usually require annealing prior
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to machining. The machinability of most steels is improved by cold working, which hardens the
material and reduces the tendency for built–up edge formation2.
Other alloying elements (such as nickel, chromium, molybdenum and vanadium) that improve the
properties of steels generally reduce machinability. The effect of boron is negligible. Gaseous elements
such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel.
Oxygen has been shown to have a strong effect on the aspect ratio of the manganese–sulfide
inclusions: The higher the oxygen content, the lower the aspect ratio and the higher the
machinability2.
Sulfur can reduce the hot workability of steels severely because of the formation of iron sulfide
(unless sufficient manganese is present to prevent such formation). At room temperature, the
mechanical properties of resulfurized steels depend on the orientation of the deformed manganese–
sulfide inclusions (anisotropy). Rephosphorized steels are significantly less ductile and are produced
solely to improve machinability2.
Stainless Steels
Austenitic (300 series) steels generally are difficult to machine. Chatter can be a problem,
necessitating machine tools with high stiffness. Ferritic stainless steels (also 300 series) have good
machinability. Martensitic (400 series) steels are abrasive, tend to form a built–up edge and require
tool materials with high hot hardness and crater–wear resistance. Precipitation–hardening stainless
steels are strong and abrasive, thus requiring hard and abrasion–resistant tool materials2.
Cast Irons
Gray irons generally are machinable, but they can be abrasive depending on composition,
especially pearlite. Free carbides in castings reduce their machinability and cause tool chipping or
fracture. Nodular and malleable irons are machinable with hard tool materials2.
1.5.2. Machinability of Nonferrous Metals
Aluminum
Aluminum is generally very easy to machine, although the softer grades tend to form a built–up
edge, resulting in poor surface finish. Thus, high cutting speeds, high rake angles and high relief angles
are recommended2. As with magnesium, the melting points of aluminum (659 °C) and its alloys are
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low. Consequently, the temperatures generated during cutting are never high enough to be damaging
to the heat–treated structures of high speed steel tools3.
Wrought aluminum alloys with high silicon content and cast aluminum alloys are generally
abrasive; hence, they require harder tool materials. Dimensional tolerance control may be a problem
in machining aluminum, because it has a high thermal expansion coefficient and a relatively low
elastic modulus2.
Beryllium
Beryllium generally is machinable, but because the fine particles produced during machining are
toxic, it requires machining in a controlled environment2.
Cobalt–Based Alloys
Cobalt–based alloys are abrasive and highly work hardening. They require sharp, abrasion–
resistant tool materials and low feeds and speeds2.
Copper
Copper is another highly ductile metal with a face–centered cubic structure, like aluminum, but it
has a higher melting point (1083 °C). In general, copper–based alloys also have good machinability
for the same reason as aluminum alloys. Although the melting point is higher, it is not high enough for
the temperatures generated by shear in the flow–zone to have a very serious effect on the life or
performance of cutting tools. Both high speed steel and cemented carbide tools are employed. Good
tool life is obtained, the wear on the tools being in the form of flank wear or cratering or both3.
Copper in the wrought condition can be difficult to machine because of built–up edge formation,
although cast copper alloys are easy to machine. Brasses are easy to machine, especially with the
addition of lead (leaded free–machining brass). Note, however, the toxicity of lead and associated
environmental concerns. Bronzes are more difficult to machine than brass2.
Magnesium
Of the metals in common engineering use, magnesium is the easiest to machine, scoring a top
ranking by almost all the criteria. Rates of tool wear are very low because magnesium does not alloy
3
E. Trent, P. Wright, Metal Cutting, Butterworth–Heinemann, Fourth Edition, 2000.
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with steel and the metal and its alloys have a low melting point (650 °C). Temperatures at the tool–
work interface are low even at very high cutting speed and feed rate3.
The tool forces when cutting magnesium are very low compared with those when cutting other
pure metals and they remain almost constant over a very wide range of cutting speeds. The low tool
forces are associated with the low shear yield strength of magnesium, and, more important, associated
with the small area of contact on the rake face of the tool over a wide range of cutting speeds and rake
angles. This ensures that the shear plane angle is high and the chips thin – only slightly thicker than
the feed3.
Steel or carbide tools can be used and the surface finish produced is good at both low and high
cutting speeds. The chips formed are deeply segmented and easily broken into short lengths, so that
chip disposal is not difficult even when cutting at very high speed. The hexagonal structure of
magnesium is probably mainly responsible for the low ductility. This leads to the fragile segmented
chips and the short contact length on the tool rake face3.
Magnesium is very easy to machine, with good surface finish and prolonged tool life. However, care
should be exercised because of its high rate of oxidation (pyrophoric) and the danger of fire2.
Molybdenum
Molybdenum is ductile and work hardening. It can produce poor surface finish; thus, sharp tools
are essential2.
Nickel–Based Alloys and Superalloys
Nickel has a lower melting point (1452 °C) than iron (1535 °C). The metal and its alloys are, in
general, more difficult to machine than iron and steel. Nickel is a very ductile metal with a face–
centered cubic structure and unlike iron, it does not undergo transformations in its basic crystal
structure up to its melting point3.
Commercially pure nickel has poor machinability on the basis of almost all the criteria. Tool life
tends to be short and the maximum permissible rate of metal removal is low. The tools fail by rapid
flank wear plus deformation of the cutting edge, at relatively low cutting speeds. Tool forces are higher
than when cutting commercially pure iron. The contact area on the rake face is very large, with a small
shear plane angle and very thick chips3.
As with iron and other pure metals, no built–up edge is formed and the tool forces decrease
steadily as the cutting speed is raised. The contact area becomes smaller and the chip thinner.
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However, over the whole speed range, the forces are relatively high. The high temperatures generated
in the flow–zone lead to high rates of tool wear. In particular, with commercially pure nickel, there is
a characteristic adverse distribution of temperature in the tools, which is very different from that
when cutting pure iron3.
Nickel–based alloys and superalloys are work hardening, abrasive and strong at high
temperatures. Their machinability depends on their condition and improves with annealing2.
Tantalum
Tantalum is very work hardening, ductile and soft. It produces a poor surface finish and tool wear
is high2.
Titanium
Titanium and its alloys have very poor thermal conductivity (the lowest of all metals), causing a
significant temperature rise and built–up edge. They are highly reactive and can be difficult to
machine2.
Tungsten
Tungsten is brittle, strong and very abrasive; hence, its machinability is low, although it improves
greatly at elevated temperatures2.
Zirconium
Zirconium has good machinability, but it requires a coolant–type cutting fluid because of the
danger of explosion and fire2.
1.5.3. Machinability of Miscellaneous Materials
Thermoplastics
Thermoplastics generally have low thermal conductivity and a low elastic modulus and they are
thermally softening. Consequently, machining them requires sharp tools with positive rake angles (to
reduce cutting forces), large relief angles, small depths of cut and feed, relatively high speeds and
proper support of the workpiece. External cooling of the cutting zone may be necessary to keep the
chips from becoming gummy and sticking to the tools. Cooling usually can be achieved with a jet of
air, a vapor mist or water–soluble oils2.
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Thermosets
Thermosetting plastics are brittle and sensitive to thermal gradients during cutting; machining
conditions generally are similar to those of thermoplastics2.
Polymer–Matrix Composites
Polymer–matrix composites are very abrasive because of the fibers that are present; hence, they
are difficult to machine. Fiber tearing, pulling and edge delamination are significant problems and can
lead to severe reduction in the load–carrying capacity of the machined component. Machining of these
materials requires careful handling and removal of debris to avoid contact with and inhaling of the
fibers2.
Metal–Matrix and Ceramic–Matrix Composites
Metal–matrix and ceramic–matrix composites can be difficult to machine, depending on the
properties of the matrix material and the reinforcing fibers2.
Graphite
Graphite is abrasive; it requires sharp, hard and abrasion–resistant tools2.
Ceramics
Ceramics have a steadily improved machinability, particularly with the development of
machinable ceramics and nanoceramics and with the selection of appropriate processing parameters,
such as ductile–regime cutting2.
Wood
Wood is an orthotropic material with properties varying with its grain direction. Consequently, the
type of chips and the surfaces produced also vary significantly, depending on the type of wood and its
condition. Woodworking, which dates back to 3000 B.C., remains largely an art. Two basic
requirements are generally sharp tools and high cutting speeds2.
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2. TOOL WEAR
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2. TOOL WEAR
2.1. Tool Life
There are three possible modes by which a cutting tool can fail in machining1:
1. Fracture Failure = This mode of failure occurs when the cutting force at the tool point becomes
excessive, causing it to fail suddenly by brittle fracture.
2. Temperature Failure = This failure occurs when the cutting temperature is too high for the
tool material, causing the material at the tool point to soften, which leads to plastic deformation
and loss of the sharp edge.
3. Gradual Wear = Gradual wearing of the cutting edge causes loss of tool shape, reduction in
cutting efficiency, an acceleration of wearing as the tool becomes heavily worn and finally tool
failure in a manner similar to a temperature failure.
Fracture and temperature failures result in premature loss of the cutting tool. These two modes of
failure are therefore undesirable. Of the three possible tool failures, gradual wear is preferred because
it leads to the longest possible use of the tool, with the associated economic advantage of that longer
use1.
Product quality must also be considered when attempting to control the mode of tool failure. When
the tool point fails suddenly during a cut, it often causes damage to the work surface. This damage
requires either rework of the surface or possible scrapping of the part. The damage can be avoided by
selecting cutting conditions that favor gradual wearing of the tool rather than fracture or temperature
failure and by changing the tool before the final catastrophic loss of the cutting edge occurs1.
2.1.1. Tool Wear
Cutting tools are subjected to (a) high localized stresses at the tip of the tool, (b) high temperatures,
especially along the rake face, (c) sliding of the chip along the rake face and (d) sliding of the tool along
the newly cut workpiece surface. These conditions induce Tool Wear, which is a major consideration
in all machining operations. Tool wear adversely affects tool life, the quality of the machined surface
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and its dimensional accuracy and consequently, the economics of cutting operations 2. Gradual wear
occurs at two principal locations on a cutting tool: the top rake face and the flank. Accordingly, two
main types of tool wear can be distinguished: crater wear and flank wear, illustrated in Figures 2.1
and 2.2. Crater Wear, Figure 2.2 (a), consists of a cavity in the rake face of the tool that forms and
grows from the action of the chip sliding against the surface. High stresses and temperatures
characterize the tool–chip contact interface, contributing to the wearing action. The crater can be
measured either by its depth or its area1. Crater wear generally is attributed to a diffusion mechanism
– that is, the movement of atoms across the tool–chip interface. Since diffusion rate increases with
increasing temperature, crater wear increases as temperature increases2. Flank Wear, Figure 2.2 (b),
occurs on the flank or relief face, of the tool. It results from rubbing between the newly generated
work surface and the flank face adjacent to the cutting edge. Flank wear is measured by the width of
the wear band (FW). This wear band is sometimes called the Flank Wear Land1.
Figure 2.1. Diagram of worn cutting tool, showing the principal locations and types of wear that
occur1.
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Figure 2.2. (a) Crater wear and (b) flank wear on a cemented carbide tool, as seen through a
toolmaker’s microscope1.
Certain features of flank wear can be identified. First, an extreme condition of flank wear often
appears on the cutting edge at the location corresponding to the original surface of the workpart. This
is called Notch Wear. It occurs because the original work surface is harder and/or more abrasive than
the internal material, which could be caused by work hardening from cold drawing or previous
machining, sand particles in the surface from casting or other reasons. As a consequence of the harder
surface, wear is accelerated at this location. A second region of flank wear that can be identified is
Nose Radius Wear; this occurs on the nose radius leading into the end cutting edge1. Nose wear is
the rounding of a sharp tool due to mechanical and thermal effects. It dulls the tool, affects chip
formation and causes rubbing of the tool over the workpiece, raising its temperature and possibly
inducing residual stresses on the machined surface2.
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In addition to being subject to wear, tools may undergo Chipping, in which a small fragment from
the cutting edge of the tool breaks away. This phenomenon, which typically occurs in brittle tool
materials such as ceramics, is similar to chipping the tip of a pencil if it is too sharp. The chipped
fragments from the cutting tool may be very small (Microchipping or Macrochipping) or they may
be relatively large, in which case they are variously called Gross Chipping, Gross Fracture and
Catastrophic Failure. Chipping also may occur in a region of the tool where a small crack or defect
already exists. Unlike wear, which is a gradual process, chipping is a sudden loss of tool material and
a corresponding change in its shape. As can be expected, chipping has a major detrimental effect on
surface finish, surface integrity and the dimensional accuracy of the workpiece. Two main causes of
chipping are the following2:
֍ Mechanical shock (i.e., impact due to interrupted cutting, as in turning a splined shaft on a
lathe)
֍ Thermal fatigue (i.e., cyclic variations in the temperature of the tool in interrupted cutting)
Chipping can be reduced by selecting tool materials with high impact and thermal–shock
resistance. High positive rake angles can contribute to chipping because of the small included angle
of the tool tip. Also, it is possible for the crater–wear region to progress toward the tool tip, thus
weakening the tip because of reduced material volume and causing chipping2.
Tool wear and the changes in tool geometry during cutting manifest themselves in different ways,
generally classified as flank wear, crater wear, nose wear, notching, plastic deformation of the tool tip,
chipping and gross fracture (Figure 2.3)2.
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Figure 2.3. (a) Features of tool wear in a turning operation. The VB indicates average flank wear.
(b)–(e) Examples of wear in cutting tools: (b) flank wear, (c) crater wear, (d) thermal cracking and
(e) flank wear and built–up edge2.
The mechanisms that cause wear at the tool–chip and tool–work interfaces in machining can be
summarized as follows1:
Abrasion
This is a mechanical wearing action caused by hard particles in the work material gouging and
removing small portions of the tool. This abrasive action occurs in both flank wear and crater wear; it
is a significant cause of flank wear.
Adhesion
When two metals are forced into contact under high pressure and temperature, adhesion or
welding occur between them. These conditions are present between the chip and the rake face of the
tool. As the chip flows across the tool, small particles of the tool are broken away from the surface,
resulting in attrition of the surface.
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Diffusion
This is a process in which an exchange of atoms takes place across a close contact boundary
between two materials. In the case of tool wear, diffusion occurs at the tool–chip boundary, causing
the tool surface to become depleted of the atoms responsible for its hardness. As this process
continues, the tool surface becomes more susceptible to abrasion and adhesion. Diffusion is believed
to be a principal mechanism of crater wear.
Chemical Reactions
The high temperatures and clean surfaces at the tool–chip interface in machining at high speeds
can result in chemical reactions, in particular, oxidation, on the rake face of the tool. The oxidized
layer, being softer than the parent tool material, is sheared away, exposing new material to sustain
the reaction process.
Plastic Deformation
Another mechanism that contributes to tool wear is plastic deformation of the cutting edge. The
cutting forces acting on the cutting edge at high temperature cause the edge to deform plastically,
making it more vulnerable to abrasion of the tool surface. Plastic deformation contributes mainly to
flank wear.
Most of these tool–wear mechanisms are accelerated at higher cutting speeds and temperatures.
Diffusion and chemical reaction are especially sensitive to elevated temperature1.
2.1.2. Tool Life and Taylor Tool Life Equation
As cutting proceeds, the various wear mechanisms result in increasing levels of wear on the cutting
tool. The general relationship of tool wear versus cutting time is shown in Figure 2.4. Although the
relationship shown is for flank wear, a similar relationship occurs for crater wear. Three regions can
usually be identified in the typical wear growth curve. The first is the Break–in Period, in which the
sharp cutting edge wears rapidly at the beginning of its use. This first region occurs within the first
few minutes of cutting. The break–in period is followed by wear that occurs at a fairly uniform rate.
This is called the Steady–State Wear Region. In our figure, this region is pictured as a linear function
of time, although there are deviations from the straight line in actual machining. Finally, wear reaches
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a level at which the wear rate begins to accelerate. This marks the beginning of the Failure Region,
in which cutting temperatures are higher and the general efficiency of the machining process is
reduced. If allowed to continue, the tool finally fails by temperature failure1.
Figure 2.4. Tool wear as a function of cutting time. Flank wear (FW) is used here as the measure of
tool wear. Crater wear follows a similar growth curve1.
The slope of the tool wear curve in the steady–state region is affected by work material and cutting
conditions. Harder work materials cause the wear rate (slope of the tool wear curve) to increase.
Increased speed, feed and depth of cut have a similar effect, with speed being the most important of
the three. If the tool wear curves are plotted for several different cutting speeds, the results appear as
in Figure 2.4. As cutting speed is increased, wear rate increases so the same level of wear is reached
in less time1.
Tool Life is defined as the length of cutting time that the tool can be used. Operating the tool until
final catastrophic failure is one way of defining tool life. This is indicated in Figure 2.5 by the end of
each tool wear curve. However, in production, it is often a disadvantage to use the tool until this failure
occurs because of difficulties in resharpening the tool and problems with work surface quality. As an
alternative, a level of tool wear can be selected as a criterion of tool life and the tool is replaced when
wear reaches that level. A convenient tool life criterion is a certain flank wear value, such as 0.5 mm
(0.020 in.), illustrated as the horizontal line on the graph. When each of the three wear curves
intersects that line, the life of the corresponding tool is defined as ended. If the intersection points are
projected down to the time axis, the values of tool life can be identified, as we have done1.
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Figure 2.5. Effect of cutting speed on tool flank wear (FW) for three cutting speeds. Hypothetical
values of speed and tool life are shown for a tool life criterion of 0.50–mm flank wear1.
Taylor Tool Life Equation
If the tool life values for the three wear curves in Figure 2.5 are plotted on a natural log–log graph
of cutting speed versus tool life, the resulting relationship is a straight line as shown in Figure 2.61.
Figure 2.6. Natural log–log plot of cutting speed vs. tool life1.
The discovery of this relationship around 1900 is credited to F. W. Taylor. It can be expressed in
equation form and is called the Taylor Tool Life Equation1:
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𝑣𝑇 𝑛 = 𝐶
(2.1.)
v = Cutting Speed (m/min, ft/min)
T = Tool Life (min)
n, C = Parameters whose Values Depend on Feed, Depth of Cut, Work Material, Tooling (Material
in Particular) and the Tool Life Criterion Used. The value of n is relative constant for a given tool
material, whereas the value of C depends on tool material, work material and cutting conditions.
Basically, Equation 2.1 states that higher cutting speeds result in shorter tool lives. Relating the
parameters n and C to Figure 2.6, n is the slope of the plot (expressed in linear terms rather than in
the scale of the axes) and C is the intercept on the speed axis. C represents the cutting speed that
results in a 1–min tool life1.
The problem with Equation 2.1 is that the units on the right–hand side of the equation are not
consistent with the units on the left–hand side. To make the units consistent, the equation should be
expressed in the form1
𝑣𝑇 𝑛 = 𝐶(𝑇𝑟𝑒𝑓 𝑛 )
(2.2.)
Tref = Reference Value for C. Tref is simply 1 min when m/min (ft/min) and minutes are used for v
and T, respectively.
The advantage of Equation 2.2 is seen when it is desired to use the Taylor equation with units other
than m/min (ft/min) and minutes – for example, if cutting speed were expressed as m/sec and tool
life as sec. In this case, Tref would be 60 sec and C would therefore be the same speed value as in
Equation 2.1, although converted to units of m/sec. The slope n would have the same numerical value
as in Equation 2.11.
Problem 2.1 = Determine the values of C and n in the plot of Figure 2.6, using two of the three
points on the curve and solving simultaneous equations of the form of Equation 2.11.
160(5)𝑛 = 𝐶
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100(41)𝑛 = 𝐶
160(5)𝑛 = 100(41)𝑛
ln(160) + 𝑛 ln(5) = ln(100) + 𝑛 ln(41)
5.0752 + 1.6094𝑛 = 4.6052 + 3.7136𝑛
0.4700 = 2.1042𝑛
𝑛 = 0.223
𝐶 = 160(5)0.223 = 229
𝐶 = 100(41)0.223 = 229
𝑉 × 𝑇 0.223 = 229
An expanded version of Equation 2.2 can be formulated to include the effects of feed, depth of cut
and even work material hardness1:
𝑣𝑇 𝑛 𝑓 𝑚 𝑑𝑝 𝐻 𝑝 = 𝐾𝑇𝑟𝑒𝑓 𝑛 𝑓𝑟𝑒𝑓 𝑚 𝑑𝑟𝑒𝑓 𝑝 𝐻𝑟𝑒𝑓 𝑞
(2.3.)
f = Feed (mm, in)
d = Depth of Cut (mm, in)
H = Hardness
m, p, q = Exponents whose Values are Experimentally Determined for the Conditions of the
Operation
K = Constant Analogous to C in Equation 2.2
fref, dref, Href = Reference Values for Feed, Depth of Cut and Hardness
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Tool Life Criteria in Production
Although flank wear is the tool life criterion in our previous discussion of the Taylor equation, this
criterion is not very practical in a factory environment because of the difficulties and time required to
measure flank wear. Following are nine alternative tool life criteria that are more convenient to use
in a production machining operation, some of which are admittedly subjective1:
1. Complete failure of the cutting edge (fracture failure, temperature failure or wearing until
complete breakdown of the tool has occurred). This criterion has disadvantages, as discussed
earlier.
2. Visual inspection of flank wear (or crater wear) by the machine operator (without a
toolmaker’s microscope). This criterion is limited by the operator’s judgment and ability to
observe tool wear with the naked eye.
3. Fingernail test across the cutting edge by the operator to test for irregularities.
4. Changes in the sound emitting from the operation, as judged by the operator.
5. Chips become ribbony, stringy and difficult to dispose of.
6. Degradation of the surface finish on the work.
7. Increased power consumption in the operation, as measured by a wattmeter connected to the
machine tool.
8. Workpiece count. The operator is instructed to change the tool after a certain specified number
of parts have been machined.
9. Cumulative cutting time. This is similar to the previous workpiece count, except that the length
of time the tool has been cutting is monitored. This is possible on machine tools controlled by
computer; the computer is programmed to keep data on the total cutting time for each tool.
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3. SURFACE ROUGHNESS
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3. SURFACE ROUGHNESS
3.1. Surface Finish and Integrity
Surface finish influences not only the dimensional accuracy of machined parts but also their
properties and their performance in service. The term Surface Finish describes the geometric
features of a surface and Surface Integrity pertains to material properties, such as fatigue life and
corrosion resistance, that are strongly influenced by the nature of the surface produced2.
A dull tool has a large radius along its edges, just as the tip of a dull pencil or the cutting edge of a
knife. Figure 3.1 illustrates the relationship between the radius of the cutting edge and the depth of
cut in orthogonal cutting. Note that at small depths of cut, the rake angle effectively can become
negative and the tool simply may ride over the workpiece surface instead of cutting it and producing
chips. This is a phenomenon similar to trying to scrape a thin layer from the surface of a stick of butter
with a dull knife2.
Figure 3.1. Schematic illustration of a dull tool with respect to the depth of cut in orthogonal
machining (exaggerated). Note that the tool has a positive rake angle, but as the depth of cut
decreases, the rake angle effectively can become negative. The tool then simply rides over the
workpiece (without cutting) and burnishes its surface; this action raises the workpiece temperature
and causes surface residual stresses2.
If the tip radius of the tool is large in relation to the depth of cut, the tool simply will rub over the
machined surface. Rubbing will generate heat and induce residual surface stresses, which in turn may
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cause surface damage, such as tearing and cracking. Consequently, the depth of cut should be greater
than the radius on the cutting edge2.
In a turning operation, as in other cutting processes, the tool leaves a spiral profile (Feed Marks)
on the machined surface as it moves across the workpiece, as shown in Figure 3.2. We can see that the
higher the feed (f) and the smaller the tool–nose radius (R) the more prominent these marks will be.
It can be shown that the surface roughness for such a case is given by2
𝑓2
𝑅𝑡 =
8𝑅
(3.1.)
Rt = Roughness Height
If the tool vibrates or chatters during cutting, it will affect the workpiece surface finish adversely.
The reason is that a vibrating tool periodically changes the dimensions of the cut. Excessive chatter
also can cause chipping and premature failure of the more brittle cutting tools, such as ceramics and
diamond2.
Figure 3.2. Schematic illustration of feed marks on a surface being turned (exaggerated)2.
Factors influencing surface integrity are as follows2:
֍ Temperatures generated during processing and possible metallurgical transformations
֍ Surface residual stresses
֍ Severe plastic deformation and strain hardening of the machined surfaces, tearing and cracking
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In finish machining it is important to consider the surface finish to be produced, whereas in rough
machining the main purpose is to remove a large amount of material at a high rate. Surface finish is
not a primary consideration, since it will be improved during finish machining. Of course, it is
important that there be no subsurface–damage results from rough machining that cannot be removed
during finish machining2.
3.2. Surface Texture and Roughness
Regardless of the method of production, all surfaces have their own characteristics, which
collectively are referred to as Surface Texture. Although the description of surface texture as a
geometrical property is complex, the following guidelines have been established for identifying
surface texture in terms of well–defined and measurable quantities (Figure 3.3)2:
Flaws or Defects are random irregularities, such as scratches, cracks, holes, depressions, seams,
tears or inclusions.
Lay (Directionality) is the direction of the predominant surface pattern, usually visible to the
naked eye.
Roughness is defined as closely spaced, irregular deviations on a small scale; it is expressed in
terms of its height, width and distance along the surface.
Waviness is a recurrent deviation from a flat surface; it is measured and described in terms of the
space between adjacent crests of the waves (Waviness Width) and height between the crests and
valleys of the waves (Waviness Height).
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Figure 3.3. (a) Standard terminology and symbols to describe surface finish. The quantities are given
in microinches. (b) Common surface lay symbols2.
Surface Roughness is generally characterized by two methods. The Arithmetic Mean Value (Ra)
is based on the schematic illustration of a rough surface, as shown in Figure 3.4 and is defined as2
𝑅𝑎 =
𝑎+𝑏+𝑐+𝑑+⋯
𝑛
a, b, c, d, … = Absolute Values
n = Number of Readings
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The Root–Mean–Square Roughness (Rq, formerly identified as RMS) is defined as2
𝑅𝑞 = √
𝑎2 + 𝑏 2 + 𝑐 2 + 𝑑 2 + ⋯
𝑛
(3.3.)
The datum line AB in Figure 3.4 is located so that the sum of the areas above the line is equal to the
sum of the areas below the line2.
Figure 3.4. Coordinates used for surface–roughness measurement defined by Equations 3.2 and 3.32.
The Maximum Roughness Height also can be used and is defined as the height from the deepest
trough to the highest peak. It indicates how much material has to be removed in order to obtain a
smooth surface, such as by polishing2.
The units generally used for surface roughness are μm (microns) or μin. Note that 1 μm = 40 μin.
and 1 μin. = 0.025 μm2.
In general, a surface cannot be described by its Ra or Rq value alone, since these values are averages.
Two surfaces may have the same roughness value, but have actual topographies that are very
different. For example, a few deep troughs on an otherwise smooth surface will not affect the
roughness values significantly. However, this type of surface profile can be significant in terms of
friction, wear and fatigue characteristics of a manufactured product. Consequently, it is important to
analyze a surface in great detail, particularly for parts to be used in critical applications2.
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4. TEMPERATURE
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4. TEMPERATURE
There are several temperatures of importance in metal cutting. The shear plane temperature (θS)
is of importance for its influence on flow stress and since it has a major influence on temperatures on
the tool face (θT) and on the relief surface (θR). The latter two temperatures (θT and θR) are very
important to crater wear rate and rate of wear–land development, respectively. The temperature on
the tool face also plays a major role relative to the size and stability of the BUE. The ambient
temperature of the work approaching the cutting zone (θo) is also important since it directly affects
θS, θT and θR4.
When metal is cut, energy is expended in deforming the chip and in overcoming friction between
the tool and the workpiece5. Of the total energy consumed in machining, nearly all of it (~ 98%) is
converted into heat. This heat can cause temperatures to be very high at the tool–chip interface – over
600 °C (1100 °F) is not unusual. The remaining energy (~ 2%) is retained as elastic energy in the
chip1.
Cutting temperatures are important because high temperatures
 lower the strength, hardness, stiffness and wear resistance of the cutting tool; tools also may
soften and undergo plastic deformation; thus, tool shape is altered2,
 reduce tool life1,
 produce hot chips that pose safety hazards to the machine operator1 and
 can cause inaccuracies in workpart dimensions due to thermal expansion of the work material1.
It can be seen that the main sources of heat in machining are: (a) the work done in shearing in the
primary shear zone, (b) energy dissipated as friction at the tool–chip interface and (c) heat generated
as the tool rubs against the machined surface, especially for dull or worn tools2.
4.1. Factors Affecting Cutting Temperatures
The process parameter with the greatest influence on cutting temperatures is the cutting speed.
Since cutting forces generally don’t vary strongly with cutting speed, increasing the cutting speed
increases the rate at which energy is dissipated through plastic deformation and friction and thus the
4
5
M. C. Shaw, Metal Cutting Principles, Oxford University Press, Second Edition, 2005.
D. A. Stephenson, J. S. Agapiou, Metal Cutting Theory and Practice, Taylor & Francis, Third Edition, 2016.
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rate of heat generation in the cutting zone. Increasing the feed rate also increases heat generation and
cutting temperatures. For moderate ranges of these variables, the cutting speed has a greater
influence and the tool–chip interface temperature increases with the square root of the cutting speed
but the third root of the feed5:
𝑇𝑖𝑛𝑡 ∝ 𝑉 0.5 𝑎0.3
(4.1.)
Other parameters that affect the cutting force, such as the depth of cut and the rake angle, also
influence cutting temperatures; changes in these parameters that increase the cutting force normally
slightly increase cutting temperatures5.
Material properties also strongly influence cutting temperatures. Cutting temperatures are higher
for harder work materials because cutting forces and thus energy dissipation are increased. For
materials of similar hardness, cutting temperatures increase with ductility, since more ductile
materials can absorb more energy through plastic deformation5.
Thermal properties of the work material that influence cutting temperatures include the thermal
conductivity (k) and heat capacity (ρc). Temperatures generally decrease as these parameters
increase, since an increase implies that heat is more readily conducted away through the workpiece
(higher k) or that the temperature increases more slowly for a given heat input (higher ρc). Increasing
the thermal conductivity and heat capacity of the tool material also reduces temperatures, although
the effect does not appear to be as marked as for the work material5.
Another parameter that affects the thermal aspects of cutting is the ratio of the rate of material
removal per unit depth of cut to the thermal diffusivity of the work material, Vaρc/k, which is referred
to as the Thermal Number in the metal cutting literature. In the general literature on heat transfer,
it is defined as the Peclet Number (Pe) which reflects the relative importance of mass transport and
conduction. For low values of Pe, the speed of the tool with respect to the workpiece is relatively small
compared to the thermal diffusivity of the material and a comparatively large portion of the heat
generated in the deformation zone conducts into the workpiece. For large values of Pe, on the other
hand, almost all of the cutting remains in the chip and is transported out of the cutting zone. In
conventional machining with a sharp tool, Pe is generally on the order of 104 and the workpiece often
heats up appreciably during cutting. In high–speed machining, Pe is more often on the order of 105
and the workpiece normally remains much cooler5.
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Finally, peak tool–chip interface temperatures are influenced by the tool nose radius and included
angle. Increasing the nose radius reduces the peak temperature by reducing the maximum uncut chip
thickness and distributing frictional energy more evenly over the cutting edge. Reducing the included
or wedge angle (by increasing the rake or relief angles) increases the peak temperature by reducing
the area through which heat can diffuse from the cutting edge through the tool5.
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5. CUTTING FLUIDS
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5. CUTTING FLUIDS
A Cutting Fluid is any liquid or gas that is applied directly to the machining operation to improve
cutting performance. Cutting fluids address two main problems: (1) heat generation at the shear zone
and friction zone and (2) friction at the tool–chip and tool–work interfaces. In addition to removing
heat and reducing friction, cutting fluids provide additional benefits, such as washing away chips
(especially in grinding and milling), reducing the temperature of the workpart for easier handling,
reducing cutting forces and power requirements, improving dimensional stability of the workpart and
improving surface finish1.
5.1. Types of Cutting Fluids
A variety of cutting fluids are commercially available. It is appropriate to discuss them first
according to function and then to classify them according to chemical formulation1.
5.1.1. Cutting Fluid Functions
There are two general categories of cutting fluids, corresponding to the two main problems they
are designed to address: coolants and lubricants. Coolants are cutting fluids designed to reduce the
effects of heat in the machining operation. They have a limited effect on the amount of heat energy
generated in cutting; instead, they carry away the heat that is generated, thereby reducing the
temperature of tool and workpiece. This helps to prolong the life of the cutting tool. The capacity of a
cutting fluid to reduce temperatures in machining depends on its thermal properties. Specific heat
and thermal conductivity are the most important properties. Water has high specific heat and thermal
conductivity relative to other liquids, which is why water is used as the base in coolant–type cutting
fluids. These properties allow the coolant to draw heat away from the operation, thereby reducing the
temperature of the cutting tool1. However, water is not an effective lubricant; hence, it does not reduce
friction. Furthermore, it can cause oxidation (rusting) of workpieces and machine–tool components2.
Coolant–type cutting fluids seem to be most effective at relatively high cutting speeds, in which
heat generation and high temperatures are problems. They are most effective on tool materials that
are most susceptible to temperature failures, such as high–speed steels and are used frequently in
turning and milling operations, in which large amounts of heat are generated1.
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Lubricants are usually oil–based fluids (because oils possess good lubricating qualities)
formulated to reduce friction at the tool–chip and tool–work interfaces. Lubricant cutting fluids
operate by Extreme Pressure Lubrication, a special form of lubrication that involves formation of
thin solid salt layers on the hot, clean metal surfaces through chemical reaction with the lubricant.
Compounds of sulfur, chlorine and phosphorus in the lubricant cause the formation of these surface
layers, which act to separate the two metal surfaces (i.e., chip and tool). These extreme pressure films
are significantly more effective in reducing friction in metal cutting than conventional lubrication,
which is based on the presence of liquid films between the two surfaces1.
Lubricant–type cutting fluids are most effective at lower cutting speeds. They tend to lose their
effectiveness at high speeds [above about 120 m/min (400 ft/min)] because the motion of the chip at
these speeds prevents the cutting fluid from reaching the tool–chip interface. In addition, high cutting
temperatures at these speeds cause the oils to vaporize before they can lubricate. Machining
operations such as drilling and tapping usually benefit from lubricants. In these operations, built–up
edge formation is retarded and torque on the tool is reduced1.
Although the principal purpose of a lubricant is to reduce friction, it also reduces the temperature
in the operation through several mechanisms. First, the specific heat and thermal conductivity of the
lubricant help to remove heat from the operation, thereby reducing temperatures. Second, because
friction is reduced, the heat generated from friction is also reduced. Third, a lower coefficient of
friction means a lower friction angle. According to Merchant’s equation a lower friction angle causes
the shear plane angle to increase, hence reducing the amount of heat energy generated in the shear
zone1.
There is typically an overlapping effect between the two types of cutting fluids. Coolants are
formulated with ingredients that help reduce friction. And lubricants have thermal properties that,
although not as good as those of water, act to remove heat from the cutting operation. Cutting fluids
(both coolants and lubricants) manifest their effect on the Taylor tool life equation through higher C
values. Increases of 10% to 40% are typical. The slope n is not significantly affected1.
5.1.2. Chemical Formulation of Cutting Fluids
There are four categories of cutting fluids according to chemical formulation: (1) cutting oils, (2)
emulsified oils, (3) semichemical fluids and (4) chemical fluids. All of these cutting fluids provide both
coolant and lubricating functions. The cutting oils are most effective as lubricants, whereas the other
three categories are more effective as coolants because they are primarily water1.
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Cutting Oils are based on oil derived from petroleum, animal, marine or vegetable origin. Mineral
oils (petroleum based) are the principal type because of their abundance and generally desirable
lubricating characteristics. To achieve maximum lubricity, several types of oils are often combined in
the same fluid. Chemical additives are also mixed with the oils to increase lubricating qualities. These
additives contain compounds of sulfur, chlorine and phosphorus and are designed to react chemically
with the chip and tool surfaces to form solid films (extreme pressure lubrication) that help to avoid
metal–to–metal contact between the two1.
Emulsified Oils consist of oil droplets suspended in water. The fluid is made by blending oil
(usually mineral oil) in water using an emulsifying agent to promote blending and stability of the
emulsion. A typical ratio of water to oil is 30:1. Chemical additives based on sulfur, chlorine and
phosphorus are often used to promote extreme pressure lubrication. Because they contain both oil
and water, the emulsified oils combine cooling and lubricating qualities in one cutting fluid1.
Chemical Fluids are chemicals in a water solution rather than oils in emulsion. The dissolved
chemicals include compounds of sulfur, chlorine and phosphorus, plus wetting agents. The chemicals
are intended to provide some degree of lubrication to the solution. Chemical fluids provide good
coolant qualities but their lubricating qualities are less than the other cutting fluid types1.
Semichemical Fluids have small amounts of emulsified oil added to increase the lubricating
characteristics of the cutting fluid. In effect, they are a hybrid class between chemical fluids and
emulsified oils1.
5.2. Application of Cutting Fluids
5.2.1. Application Methods
The most common method is Flooding (shown in Figure 5.1, indicating good and poor flooding
practices), sometimes called Flood–Cooling because it is generally used with coolant–type cutting
fluids. In Flooding, a steady stream of fluid is directed at the tool–work or tool–chip interface of the
machining operation1.
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Figure 5.1. Schematic illustration of the proper methods of applying cutting fluids (flooding) in
various machining operations: (a) turning, (b) milling, (c) thread grinding and (d) drilling2.
A second method of delivery is Mist Application, primarily used for water–based cutting fluids. In
Mist Application the fluid is directed at the operation in the form of a high–speed mist carried by a
pressurized air stream. Mist application is generally not as effective as flooding in cooling the tool.
However, because of the high–velocity air stream, mist application may be more effective in delivering
the cutting fluid to areas that are difficult to access by conventional flooding1.
Manual Application by means of a squirt can or paint brush is sometimes used for applying
lubricants in tapping and other operations in which cutting speeds are low and friction is a problem.
It is generally not preferred by most production machine shops because of its variability in
application1.
5.2.2. Cutting Fluid Filtration and Dry Machining
Cutting fluids become contaminated over time with a variety of foreign substances, such as tramp
oil (machine oil, hydraulic fluid, etc.), garbage (cigarette butts, food, etc.), small chips, molds, fungi and
bacteria. In addition to causing odors and health hazards, contaminated cutting fluids do not perform
their lubricating function as well. Alternative ways of dealing with this problem are to: (1) replace the
cutting fluid at regular and frequent intervals (perhaps twice per month); (2) use a filtration system
to continuously or periodically clean the fluid; or (3) dry machining; that is, machine without cutting
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fluids. Because of growing concern about environmental pollution and associated legislation,
disposing old fluids has become both costly and contrary to the general public welfare1.
Filtration systems are being installed in numerous machine shops today to solve the contamination
problem. Advantages of these systems include1:
✓ prolonged cutting fluid life between changes – instead of replacing the fluid once or twice per
month, coolant lives of 1 year have been reported.
✓ reduced fluid disposal cost, since disposal is much less frequent when a filter is used.
✓ cleaner cutting fluid for better working environment and reduced health hazards
✓ lower machine tool maintenance
✓ longer tool life
The third alternative is called Dry Machining, meaning that no cutting fluid is used. Dry machining
avoids the problems of cutting fluid contamination, disposal and filtration, but can lead to problems
of its own1:
 overheating the tool
 operating at lower cutting speeds and production rates to prolong tool life
 absence of chip removal benefits in grinding and milling
Cutting–tool producers have developed certain grades of carbides and coated carbides for use in
dry machining1.
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6. CUTTING TOOLS
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6. CUTTING TOOLS
Machining operations are accomplished using cutting tools. The high forces and temperatures
during machining create a very harsh environment for the tool. If cutting force becomes too high, the
tool fractures. If cutting temperature becomes too high, the tool material softens and fails. If neither
of these conditions causes the tool to fail, continual wear of the cutting edge ultimately leads to
failure1.
Cutting tool technology has two principal aspects: tool material and tool geometry. The first is
concerned with developing materials that can withstand the forces, temperatures and wearing action
in the machining process. The second deals with optimizing the geometry of the cutting tool for the
tool material and for a given operation1.
A cutting tool has one or more sharp cutting edges and is made of a material that is harder than
the work material. The cutting edge serves to separate a chip from the parent work material.
Connected to the cutting edge are two surfaces of the tool: the rake face and the flank. The rake face,
which directs the flow of the newly formed chip, is oriented at a certain angle called the Rake Angle
(α). It is measured relative to a plane perpendicular to the work surface. The rake angle can be positive
or negative. The flank of the tool provides a clearance between the tool and the newly generated work
surface, thus protecting the surface from abrasion, which would degrade the finish. This flank surface
is oriented at an angle called the Relief Angle1.
Most cutting tools in practice have more complex geometries. There are two basic types, examples
of which are illustrated in Figure 6.1: (a) single–point tools and (b) multiple–cutting–edge tools. A
Single–Point Tool has one cutting edge and is used for operations such as turning. During machining,
the point of the tool penetrates below the original work surface of the part. The point is usually
rounded to a certain radius, called the Nose Radius. Multiple–Cutting–Edge Tools have more than
one cutting edge and usually achieve their motion relative to the workpart by rotating. Drilling and
milling use rotating multiple–cutting–edge tools. Figure 6.1 (b) shows a helical milling cutter used in
peripheral milling. Although the shape is quite different from a single–point tool, many elements of
tool geometry are similar1.
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Figure 6.1. (a) A single–point tool showing rake face, flank and tool point; and (b) a helical milling
cutter, representative of tools with multiple cutting edges1.
6.1. Tool Materials
Cutting tools must be made of materials capable of withstanding the high stresses and
temperatures generated during chip formation. Ideally, tool materials should have the following
properties5:
1. High penetration hardness at elevated temperatures to resist abrasive wear
2. High deformation resistance to prevent the edge from deforming or collapsing under the
stresses produced by chip formation
3. High fracture toughness to resist edge chipping and breakage, especially in interrupted cutting
4. Chemical inertness (low chemical affinity) with respect to the work material to resist diffusion
and chemical wear
5. High thermal conductivity to reduce cutting temperatures near the tool edge
6. High fatigue resistance, especially for tools used in interrupted cutting
7. High thermal shock resistance to prevent tool breakage in interrupted cutting
8. High stiffness to maintain accuracy
9. Adequate lubricity (low friction) with respect to the work material to prevent built–up edge,
especially when cutting soft, ductile materials.
The first three properties are required to prevent sudden, catastrophic failure of the tool.
Properties 1, 4 and 5 are required for the tool to resist the high temperatures generated during chip
formation; elevated tool temperatures can cause rapid tool wear to do thermal softening or diffusion
and chemical wear. Properties 3, 6 and 7 are required to prevent chipping of the tool, especially in
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interrupted cutting. Fracture toughness is usually characterized by the transverse rupture strength;
fatigue resistance is characterized by the Weibull modulus. As shown in Figure 6.2, properties 1, 4 and
5 generally determine the maximum cutting speed at which a tool can be used, while properties 3 and
6 determine allowable feed rates and depths of cut. It should be noted that fracture and fatigue
strength requirements vary with the cutting speed, since cutting forces usually decrease with
increasing cutting speed5.
Figure 6.2. The influence of tool material properties on optimization of cutting conditions5.
The three modes of tool failure allow us to identify three important properties required in a tool
material1:
Toughness = To avoid fracture failure, the tool material must possess high toughness. Toughness
is the capacity of a material to absorb energy without failing. It is usually characterized by a
combination of strength and ductility in the material.
Hot Hardness = Hot Hardness is the ability of a material to retain its hardness at high
temperatures. This is required because of the high temperature environment in which the tool
operates.
Wear Resistance = Hardness is the single most important property needed to resist abrasive
wear. All cutting–tool materials must be hard. However, wear resistance in metal cutting depends on
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more than just tool hardness, because of the other tool–wear mechanisms. Other characteristics
affecting wear resistance include surface finish on the tool (a smoother surface means a lower
coefficient of friction), chemistry of tool and work materials and whether a cutting fluid is used.
Cutting–tool materials achieve this combination of properties in varying degrees1. This situation
can readily be seen from Table 6.1 by noting the opposite directions of the long horizontal arrows2.
Table 6.2 shows the operating characteristics of tool materials in machining2.
Table 6.1. General characteristics of cutting–tool materials (These tool materials have a wide range
of compositions and properties; overlapping characteristics exist in many categories of tool
materials)2.
High–
Cast–
Speed
Cobalt
Steels
Alloys
Uncoated
Coated
Carbides
Carbides
Hot
Hardness
Toughness
Impact
Strength
Wear
Resistance
Chipping
Resistance
Cutting
Speed
Thermal–
Shock
Resistance
Tool
Material
Cost
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Polycrystalline
Ceramics
Cubic Boron
Nitride
Diamond
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Depth
of Light to Light to Light
Cut
Heavy
Heavy
to Light
Heavy
Assoc. Prof. Emel KURAM
to Light
Heavy
to Light to Heavy
Heavy
Very
Light
for
Single–
Crystal
Diamond
Processing Wrought, Cast and Cold
Chemical–
Cold
High–Pressure,
High–
Method
Cast,
Hot–
Pressing
Vapor
Pressing
High–
Pressure,
Hot–
Isostatic
and
Deposition
and
Temperature
High–
Isostatic
Pressing
Sintering
(CVD)
Sintering
Temperature
Pressing
Sintering
Sintering
or Sintering
Physical–
or
Hot–
Vapor
Isostatic
Deposition
Pressing
(PVD)
Sintering
Sintering
Table 6.2. General operating characteristics of cutting–tool materials2.
Tool Materials
General
Modes of Tool Wear
Characteristics
or Failure
toughness, Flank wear,
Limitations
High–Speed
High
Steels
resistance to fracture, wear
hardenability
wide
wear resistance
range
of
crater Low hot hardness, limited
and
limited
roughing and finishing
cuts,
good
for
interrupted cuts
Uncoated
High hardness over a Flank wear,
Carbides
wide
range
of wear
temperatures,
wear
resistance,
versatile,
range
because of cold welding of
chips and microchipping
toughness,
wide
crater Cannot use at low speeds
of
applications
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Coated Carbides Improved
wear Flank wear,
resistance
uncoated
Assoc. Prof. Emel KURAM
crater Cannot use at low speeds
over wear
because of cold welding of
carbides,
chips and microchipping
better frictional and
thermal properties
Ceramics
High
hardness
elevated
at Depth–of–cut
line Low
notching,
strength
and
thermomechanical
temperatures,
high microchipping, gross strength
abrasive
wear fracture
low
fatigue
resistance
Polycrystalline
Cubic
High
hot
hardness, Depth–of–cut
Boron toughness,
Nitride (cBN)
cutting– notching,
edge strength
line Low
strength
and
low
chipping, chemical stability at higher
oxidation,
temperature
graphitization
Diamond
High
hardness
toughness,
and Chipping,
oxidation, Low
abrasive graphitization
wear resistance
strength
and
low
chemical stability at higher
temperature
6.1.1. High–Speed Steel and its Predecessors
Before the development of high–speed steel, plain carbon steel and Mushet’s steel were the
principal tool materials for metal cutting. Today, these steels are rarely used in industrial machining
applications. The plain carbon steels used as cutting tools could be heat–treated to achieve relatively
high hardness (Rockwell C 60), because of their fairly high carbon content. However, because of low
alloying levels, they possess poor hot hardness, which renders them unusable in metal cutting except
at speeds too low to be practical by today’s standards. Mushet’s steel has been displaced by advances
in tool steel metallurgy1.
High–Speed Steel (HSS) is a highly alloyed tool steel capable of maintaining hardness at elevated
temperatures better than high carbon and low alloy steels. Its good hot hardness permits tools made
of HSS to be used at higher cutting speeds. Compared with the other tool materials at the time of its
development, it was truly deserving of its name “high speed”. A wide variety of high–speed steels are
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available, but they can be divided into two basic types: (1) tungsten–type, designated T–grades by the
American Iron and Steel Institute (AISI); and (2) molybdenum–type, designated M–grades by AISI1.
Tungsten–Type (T–Series) HSS contains tungsten (W) as its principal alloying ingredient.
Additional alloying elements are chromium (Cr) and vanadium (V). One of the original and best known
HSS grades is T1 or 18–4–1 high–speed steel, containing 18% W, 4% Cr and 1% V. Molybdenum (M–
Series) HSS grades contain combinations of tungsten and molybdenum (Mo), plus the same
additional alloying elements as in the T–grades. Cobalt (Co) is sometimes added to HSS to enhance
hot hardness. Of course, high–speed steel contains carbon, the element common to all steels1.
6.1.2. Cast Cobalt Alloys
Cast cobalt alloy cutting tools consist of cobalt, around 40% to 50%; chromium, about 25% to 35%;
and tungsten, usually 15% to 20%; with trace amounts of other elements. These tools are made into
the desired shape by casting in graphite molds and then grinding to final size and cutting–edge
sharpness. High hardness is achieved as cast, an advantage over HSS, which requires heat treatment
to achieve its hardness. Wear resistance of the cast cobalts is better than high–speed steel, but not as
good as cemented carbide. Toughness of cast cobalt tools is better than carbides but not as good as
HSS. Hot hardness also lies between these two materials1.
As might be expected from their properties, applications of cast cobalt tools are generally between
those of high–speed steel and cemented carbides. They are capable of heavy roughing cuts at speeds
greater than HSS and feeds greater than carbides. Work materials include both steels and nonsteels,
as well as nonmetallic materials such as plastics and graphite. Today, cast cobalt alloy tools are not
nearly as important commercially as either high–speed steel or cemented carbides. They were
introduced around 1915 as a tool material that would allow higher cutting speeds than HSS. The
carbides were subsequently developed and proved to be superior to the cast Co alloys in most cutting
situations1.
6.1.3. Cemented Carbides, Cermets and Coated Carbides
Cermets are defined as composites of Ceramic and Metallic materials. Technically speaking,
cemented carbides are included within this definition; however, cermets based on WC–Co, including
WC–TiC–TaC–Co, are known as Carbides (Cemented Carbides) in common usage. In cutting–tool
terminology, the term cermet is applied to ceramic–metal composites containing TiC, TiN and certain
other ceramics not including WC. One of the advances in cutting–tool materials involves the
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application of a very thin coating to a WC–Co substrate. These tools are called Coated Carbides. Thus,
we have three important and closely related tool materials to discuss: (1) cemented carbides, (2)
cermets and (3) coated carbides1.
Cemented Carbides
Cemented Carbides (also called Sintered Carbides) are a class of hard tool material formulated
from tungsten carbide (WC) using powder metallurgy techniques with cobalt (Co) as the binder.
There may be other carbide compounds in the mixture, such as titanium carbide (TiC) and/or
tantalum carbide (TaC), in addition to WC1.
The first cemented carbide cutting tools were made of WC–Co and could be used to machine cast
irons and nonsteel materials at cutting speeds faster than those possible with high–speed steel and
cast cobalt alloys. However, when the straight WC–Co tools were used to cut steel, crater wear
occurred rapidly, leading to early failure of the tools. A strong chemical affinity exists between steel
and the carbon in WC, resulting in accelerated wear by diffusion and chemical reaction at the tool–
chip interface for this work–tool combination. Consequently, straight WC–Co tools cannot be used
effectively to machine steel. It was subsequently discovered that additions of titanium carbide and
tantalum carbide to the WC–Co mix significantly retarded the rate of crater wear when cutting steel.
These new WC–TiC–TaC–Co tools could be used for steel machining. The result is that cemented
carbides are divided into two basic types: (1) nonsteel–cutting grades, consisting of only WC–Co; and
(2) steel–cutting grades, with combinations of TiC and TaC added to the WC–Co1.
The general properties of the two types of cemented carbides are similar: (1) high compressive
strength but low–to–moderate tensile strength; (2) high hardness (90 to 95 HRA); (3) good hot
hardness; (4) good wear resistance; (5) high thermal conductivity; (6) high modulus of elasticity – E
values up to around 600 × 103 MPa (90 × 106 lb/in2); and (7) toughness lower than high–speed steel1.
Nonsteel–Cutting Grades refer to those cemented carbides that are suitable for machining
aluminum, brass, copper, magnesium, titanium and other nonferrous metals; anomalously, gray cast
iron is included in this group of work materials. In the nonsteel–cutting grades, grain size and cobalt
content are the factors that influence properties of the cemented carbide material. The typical grain
size found in conventional cemented carbides ranges between 0.5 and 5 mm (20 and 200 m–in.). As
grain size is increased, hardness and hot hardness decrease, but transverse rupture strength (TRS)
increases. The typical cobalt content in cemented carbides used for cutting tools is 3% to 12%. As
cobalt content increases, TRS improves at the expense of hardness and wear resistance. Cemented
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carbides with low percentages of cobalt content (3% to 6%) have high hardness and low TRS, whereas
carbides with high Co (6% to 12%) have high TRS but lower hardness. Accordingly, cemented carbides
with higher cobalt are used for roughing operations and interrupted cuts (such as milling), while
carbides with lower cobalt (therefore, higher hardness and wear resistance) are used in finishing
cuts1.
Steel–Cutting Grades are used for low carbon, stainless and other alloy steels. For these carbide
grades, titanium carbide and/or tantalum carbide is substituted for some of the tungsten carbide. TiC
is the more popular additive in most applications. Typically, from 10% to 25% of the WC might be
replaced by combinations of TiC and TaC. This composition increases the crater wear resistance for
steel cutting, but tends to adversely affect flank wear resistance for nonsteel–cutting applications.
That is why two basic categories of cemented carbide are needed1.
One of the important developments in cemented carbide technology in recent years is the use of
very fine grain sizes (submicron sizes) of the various carbide ingredients (WC, TiC and TaC). Although
small grain size is usually associated with higher hardness but lower transverse rupture strength, the
decrease in TRS is reduced or reversed at the submicron particle sizes. Therefore, these ultrafine grain
carbides possess high hardness combined with good toughness1.
Cermets
Although cemented carbides are technically classified as cermet composites, the term Cermet in
cutting–tool technology is generally reserved for combinations of TiC, TiN and titanium carbonitride
(TiCN), with nickel and/or molybdenum as binders. Some of the cermet chemistries are more complex
(e.g., ceramics such as TaxNbyC and binders such as Mo2C). However, cermets exclude metallic
composites that are primarily based on WC–Co. Applications of cermets include high–speed finishing
and semifinishing of steels, stainless steels and cast irons. Higher speeds are generally allowed with
these tools compared with steel–cutting carbide grades. Lower feeds are typically used so that better
surface finish is achieved, often eliminating the need for grinding1.
Coated Carbides
The development of coated carbides around 1970 represented a significant advance in cutting–
tool technology. Coated Carbides are a cemented carbide insert coated with one or more thin layers
of wear–resistant material, such as titanium carbide, titanium nitride and/or aluminum oxide (Al2O3).
The coating is applied to the substrate by chemical vapor deposition or physical vapor deposition. The
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coating thickness is only 2.5 to 13 mm (0.0001 to 0.0005 in.). It has been found that thicker coatings
tend to be brittle, resulting in cracking, chipping and separation from the substrate1.
The first generation of coated carbides had only a single layer coating (TiC, TiN or Al2O3). More
recently, coated inserts have been developed that consist of multiple layers. The first layer applied to
the WC–Co base is usually TiN or TiCN because of good adhesion and similar coefficient of thermal
expansion. Additional layers of various combinations of TiN, TiCN, Al2O3 and TiAlN are subsequently
applied1.
Coated carbides are used to machine cast irons and steels in turning and milling operations. They
are best applied at high cutting speeds in situations in which dynamic force and thermal shock are
minimal. If these conditions become too severe, as in some interrupted cut operations, chipping of the
coating can occur, resulting in premature tool failure. In this situation, uncoated carbides formulated
for toughness are preferred. When properly applied, coated carbide tools usually permit increases in
allowable cutting speeds compared with uncoated cemented carbides1.
Use of coated carbide tools is expanding to nonferrous metal and nonmetal applications for
improved tool life and higher cutting speeds. Different coating materials are required, such as
chromium carbide (CrC), zirconium nitride (ZrN) and diamond1.
Classification of Carbides
Carbide grades are classified using the letters P, M and K (as shown in Tables 6.3 and 6.4) for a
range of applications2.
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Table 6.3. ISO classification of carbide cutting tools according to use2.
Symbol
Workpiece Material
Color Code
Designation in Increasing
Order of Wear Resistance
and Decreasing Order of
Toughness in Each Category
(in Increments of 5)
P
Ferrous metals with Blue
P01, P05–P50
long chips
M
Ferrous metals with Yellow
M10–M40
long or short chips,
nonferrous metals
K
Ferrous metals with Red
short
nonferrous
chips,
metals,
nonmetallic materials
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Table 6.4. Classification of tungsten carbides according to machining applications2.
ISO
ANSI
Standard Classification
Number
Materials
to be
Machined
Characteristics of
Machining
Operation
Type of
Carbide
Cut
Carbide
Increasing
Increasing
resistant
cutting
hardness
grades;
speed
and wear
(Grade)
K30–K40
C1
Cast
K20
C2
nonferrous
K10
K01
C3
C4
iron, Roughing
metals and purpose
nonmetallic
Light
materials
finishing
requiring
Precision
abrasion
resistance
P30–P50
C5
P20
C6
General
Steels
requiring
finishing
Roughing
General
crater
P10
P01
C7
C8
and purpose
deformation
Light
resistance
finishing
Wear–
generally
resistance
straight WC–
Co
with
Increasing
varying grain
sizes
strength and
Increasing
binder
feed rate
content
Crater–
Increasing
Increasing
resistant
cutting
hardness
grades;
speed
and wear
various
resistance
WC–Co
compositions
Increasing
Precision
with
TiC
finishing
and/or
TaC Increasing
alloys
strength and
feed rate
binder
content
6.1.4. Ceramics
Cutting tools made from ceramics were first used commercially in the United States in the mid–
1950s, although their development and use in Europe dates back to the early 1900s. Today’s ceramic
cutting tools are composed primarily of fine–grained aluminum oxide (Al2O3), pressed and sintered
at high pressures and temperatures with no binder into insert form. The aluminum oxide is usually
very pure (99% is typical), although some manufacturers add other oxides (such as zirconium oxide)
in small amounts. In producing ceramic tools, it is important to use a very fine grain size in the alumina
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powder and to maximize density of the mix through high–pressure compaction to improve the
material’s low toughness1.
Aluminum oxide cutting tools are most successful in high–speed turning of cast iron and steel.
Applications also include finish turning of hardened steels using high cutting speeds, low feeds and
depths and a rigid work setup. Many premature fracture failures of ceramic tools are because of non–
rigid machine tool setups, which subject the tools to mechanical shock. When properly applied,
ceramic cutting tools can be used to obtain very good surface finish. Ceramics are not recommended
for heavy interrupted cut operations (e.g., rough milling) because of their low toughness. In addition
to its use as inserts in conventional machining operations, Al2O3 is widely used as an abrasive in
grinding and other abrasive processes1.
Other commercially available ceramic cutting–tool materials include silicon nitride (SiN), sialon
(silicon nitride and aluminum oxide, SiN–Al2O3), aluminum oxide and titanium carbide (Al2O3–TiC),
and aluminum oxide reinforced with single crystal–whiskers of silicon carbide1.
6.1.5. Synthetic Diamonds and Cubic Boron Nitride
Diamond is the hardest material known. By some measures of hardness, diamond is three to four
times as hard as tungsten carbide or aluminum oxide. Since high hardness is one of the desirable
properties of a cutting tool, it is natural to think of diamonds for machining and grinding applications.
Synthetic diamond cutting tools are made of sintered polycrystalline diamond (SPD), which dates
from the early 1970s. Sintered Polycrystalline Diamond is fabricated by sintering fine–grained
diamond crystals under high temperatures and pressures into the desired shape. Little or no binder
is used. The crystals have a random orientation and this adds considerable toughness to the SPD tools
compared with single crystal diamonds. Tool inserts are typically made by depositing a layer of SPD
about 0.5 mm (0.020 in.) thick on the surface of a cemented carbide base. Very small inserts have also
been made of 100% SPD1.
Applications of diamond cutting tools include high–speed machining of nonferrous metals and
abrasive nonmetals such as fiberglass, graphite and wood. Machining of steel, other ferrous metals
and nickel–based alloys with SPD tools is not practical because of the chemical affinity that exists
between these metals and carbon (a diamond, after all, is carbon)1.
Next to diamond, cubic boron nitride is the hardest material known and its fabrication into cutting
tool inserts is basically the same as SPD; that is, coatings on WC–Co inserts. Cubic boron nitride
(symbolized cBN) does not react chemically with iron and nickel as SPD does; therefore, the
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Assoc. Prof. Emel KURAM
applications of cBN–coated tools are for machining steel and nickel–based alloys. Both SPD and cBN
tools are expensive, as one might expect, and the applications must justify the additional tooling cost1.
6.2. Tool Geometry
A cutting tool must possess a shape that is suited to the machining operation. One important way
to classify cutting tools is according to the machining process. Thus, we have turning tools, cutoff tools,
milling cutters, drill bits, reamers, taps and many other cutting tools that are named for the operation
in which they are used, each with its own tool geometry – in some cases quite unique1.
Cutting tools can be divided into single–point tools and multiple–cutting–edge tools. Single–point
tools are used in turning, boring, shaping and planing. Multiple–cutting–edge tools are used in drilling,
reaming, tapping, milling, broaching and sawing. Many of the principles that apply to single–point
tools also apply to the other cutting–tool types, simply because the mechanism of chip formation is
basically the same for all machining operations1.
6.2.1. Single–Point Tool Geometry
A more detailed drawing of a single–point cutting tool is illustrated in Figure 6.31.
In a single–point tool, the orientation of the rake face is defined by two angles, back rake angle (αb)
and side rake angle (αs). Together, these angles are influential in determining the direction of chip
flow across the rake face. The flank surface of the tool is defined by the end relief angle (ERA) and side
relief angle (SRA). These angles determine the amount of clearance between the tool and the freshly
cut work surface. The cutting edge of a single–point tool is divided into two sections, side cutting edge
and end cutting edge. These two sections are separated by the tool point, which has a certain radius,
called the Nose Radius. The Side Cutting Edge Angle (SCEA) determines the entry of the tool into
the work and can be used to reduce the sudden force the tool experiences as it enters a workpart.
Nose Radius (NR) determines to a large degree the texture of the surface generated in the operation.
A very pointed tool (small nose radius) results in very pronounced feed marks on the surface. End
Cutting Edge Angle (ECEA) provides a clearance between the trailing edge of the tool and the newly
generated work surface, thus reducing rubbing and friction against the surface1.
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Figure 6.3. (a) Seven elements of single–point tool geometry and (b) the tool signature convention
that defines the seven elements1.
Chip Breakers
Chip disposal is a problem that is often encountered in turning and other continuous operations.
Long, stringy chips are often generated, especially when turning ductile materials at high speeds.
These chips cause a hazard to the machine operator and the workpart finish and they interfere with
automatic operation of the turning process. Chip Breakers are frequently used with single–point
tools to force the chips to curl more tightly than they would naturally be inclined to do, thus causing
them to fracture. There are two principal forms of chip breaker design commonly used on single–
point turning tools, illustrated in Figure 6.4: (a) groove–type chip breaker designed into the cutting
tool itself and (b) obstruction–type chip breaker designed as an additional device on the rake face of
the tool. The chip breaker distance can be adjusted in the obstruction–type device for different cutting
conditions1.
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Assoc. Prof. Emel KURAM
Figure 6.4. Two methods of chip breaking in single–point tools: (a) groove–type and (b)
obstruction–type chip breakers1.
Effect of Tool Material on Tool Geometry
A positive rake angle is generally desirable because it reduces cutting forces, temperature and
power consumption. High–speed steel–cutting tools are almost always ground with positive rake
angles, typically ranging from +5° to +20°. HSS has good strength and toughness, so that the thinner
cross section of the tool created by high positive rake angles does not usually cause a problem with
tool breakage. HSS tools are predominantly made of one piece. The heat treatment of high–speed steel
can be controlled to provide a hard cutting edge while maintaining a tough inner core1.
With the development of the very hard tool materials (e.g., cemented carbides and ceramics),
changes in tool geometry were required. As a group, these materials have higher hardness and lower
toughness than HSS. Also, their shear and tensile strengths are low relative to their compressive
strengths and their properties cannot be manipulated through heat treatment like those of HSS.
Finally, cost per unit weight for these very hard materials is higher than the cost of HSS. These factors
have affected cutting–tool design for the very hard tool materials in several ways1.
First, the very hard materials must be designed with either negative rake or small positive angles.
This change tends to load the tool more in compression and less in shear, thus favoring the high
compressive strength of these harder materials. Cemented carbides, for example, are used with rake
angles typically in the range from -5° to +10°. Ceramics have rake angles between -5° and -15°. Relief
angles are made as small as possible (5° is typical) to provide as much support for the cutting edge as
possible1.
Another difference is the way in which the cutting edge of the tool is held in position. The
alternative ways of holding and presenting the cutting edge for a single–point tool are illustrated in
Figure 6.5. The geometry of a HSS tool is ground from a solid shank, as shown in part (a) of the figure.
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Assoc. Prof. Emel KURAM
The higher cost and differences in properties and processing of the harder tool materials have given
rise to the use of inserts that are either brazed or mechanically clamped to a toolholder. Part (b) shows
a brazed insert, in which a cemented carbide insert is brazed to a tool shank. The shank is made of
tool steel for strength and toughness. Part (c) illustrates one possible design for mechanically
clamping an insert in a toolholder. Mechanical clamping is used for cemented carbides, ceramics and
the other hard materials. The significant advantage of the mechanically clamped insert is that each
insert contains multiple cutting edges. When an edge wears out, the insert is unclamped, indexed
(rotated in the toolholder) to the next edge and reclamped in the toolholder. When all of the cutting
edges are worn, the insert is discarded and replaced1.
Figure 6.5. Three ways of holding and presenting the cutting edge for a single–point tool: (a) solid
tool, typical of HSS; (b) brazed insert, one way of holding a cemented carbide insert; and (c)
mechanically clamped insert, used for cemented carbides, ceramics and other very hard tool
materials1.
Inserts
Cutting–tool inserts are widely used in machining because they are economical and adaptable to
many different types of machining operations: turning, boring, threading, milling and even drilling.
They are available in a variety of shapes and sizes for the variety of cutting situations encountered in
practice. A square insert is shown in Figure 6.5 (c). Other common shapes used in turning operations
are displayed in Figure 6.6. In general, the largest point angle should be selected for strength and
economy. Round inserts possess large point angles (and large nose radii) just because of their shape.
Inserts with large point angles are inherently stronger and less likely to chip or break during cutting,
but they require more power and there is a greater likelihood of vibration. The economic advantage
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Assoc. Prof. Emel KURAM
of round inserts is that they can be indexed multiple times for more cuts per insert. Square inserts
present four cutting edges, triangular shapes have three edges, whereas rhombus shapes have only
two. Fewer edges are a cost disadvantage. If both sides of the insert can be used (e.g., in most negative
rake angle applications), then the number of cutting edges is doubled. Rhombus shapes are used
(especially with acute point angles) because of their versatility and accessibility when a variety of
operations are to be performed. These shapes can be more readily positioned in tight spaces and can
be used not only for turning but also for facing and contour turning1.
Figure 6.6. Common insert shapes: (a) round, (b) square, (c) rhombus with two 80° point angles, (d)
hexagon with three 80° point angles, (e) triangle (equilateral), (f) rhombus with two 55° point
angles, (g) rhombus with two 35° point angles. Also shown are typical features of the geometry.
Strength, power requirements and tendency for vibration increase as we move to the left; whereas
versatility and accessibility tend to be better with the geometries at the right1.
Inserts are usually not made with perfectly sharp cutting edges, because a sharp edge is weaker
and fractures more easily, especially for the very hard and brittle tool materials from which inserts
are made (cemented carbides, coated carbides, cermets, ceramics, cBN and diamond). Some kind of
shape alteration is commonly performed on the cutting edge at an almost microscopic level. The effect
of this Edge Preparation is to increase the strength of the cutting edge by providing a more gradual
transition between the clearance edge and the rake face of the tool. Three common edge preparations
are shown in Figure 6.7: (a) radius or edge rounding, also referred to as honed edge, (b) chamfer and
(c) land. For comparison, a perfectly sharp cutting edge is shown in (d). The radius in (a) is typically
only about 0.025 mm (0.001 in.) and the land in (c) is 15° or 20°. Combinations of these edge
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Assoc. Prof. Emel KURAM
preparations are often applied to a single cutting edge to maximize the strengthening effect1. In order
to further improve edge strength and prevent chipping, insert edges usually are honed, chamfered or
produced with a negative land (Figure 6.8). Most inserts are honed to a radius of about 0.025 mm
(0.001 in.)2.
Figure 6.7. Three types of edge preparation that are applied to the cutting edge of an insert:
(a) radius, (b) chamfer, (c) land and (d) perfectly sharp edge (no edge preparation)1.
Figure 6.8. Edge preparation for inserts to improve edge strength2.
6.2.2. Multiple–Cutting–Edge Tools
Most multiple–cutting–edge tools are used in machining operations in which the tool is rotated.
Primary examples are drilling and milling. On the other hand, broaching and some sawing operations
(hack sawing and band sawing) use multiple–cutting–edge tools that operate with a linear motion.
Other sawing operations (circular sawing) use rotating saw blades1.
Drills
Various cutting tools are available for hole making, but the twist drill is by far the most common.
It comes in diameters ranging from about 0.15 mm (0.006 in.) to as large as 75 mm (3.0 in.). Twist
drills are widely used in industry to produce holes rapidly and economically1.
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The standard twist drill geometry is illustrated in Figure 6.9. The body of the drill has two spiral
flutes (the spiral gives the twist drill its name). The angle of the spiral flutes is called the Helix Angle,
a typical value of which is around 30°. While drilling, the flutes act as passageways for extraction of
chips from the hole. Although it is desirable for the flute openings to be large to provide maximum
clearance for the chips, the body of the drill must be supported over its length. This support is
provided by the Web, which is the thickness of the drill between the flutes1.
The point of the twist drill has a conical shape. A typical value for the point angle is 118°. The point
can be designed in various ways, but the most common design is a chisel edge, as in Figure 6.9.
Connected to the chisel edge are two cutting edges (sometimes called Lips) that lead into the flutes.
The portion of each flute adjacent to the cutting edge acts as the rake face of the tool1.
Figure 6.9. Standard geometry of a twist drill1.
The cutting action of the twist drill is complex. The rotation and feeding of the drill bit result in
relative motion between the cutting edges and the workpiece to form the chips. The cutting speed
along each cutting edge varies as a function of the distance from the axis of rotation. Accordingly, the
efficiency of the cutting action varies, being most efficient at the outer diameter of the drill and least
efficient at the center. In fact, the relative velocity at the drill point is zero, so no cutting takes place.
Instead, the chisel edge of the drill point pushes aside the material at the center as it penetrates into
the hole; a large thrust force is required to drive the twist drill forward into the hole. Also, at the
beginning of the operation, the rotating chisel edge tends to wander on the surface of the workpart,
causing loss of positional accuracy. Various alternative drill point designs have been developed to
address this problem1.
Chip removal can be a problem in drilling. The cutting action takes place inside the hole and the
flutes must provide sufficient clearance throughout the length of the drill to allow the chips to be
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extracted from the hole. As the chip is formed it is forced through the flutes to the work surface.
Friction makes matters worse in two ways. In addition to the usual friction in metal cutting between
the chip and the rake face of the cutting edge, friction also results from rubbing between the outside
diameter of the drill bit and the newly formed hole. This increases the temperature of the drill and
work. Delivery of cutting fluid to the drill point to reduce the friction and heat is difficult because the
chips are flowing in the opposite direction. Because of chip removal and heat, a twist drill is normally
limited to a hole depth of about four times its diameter. Some twist drills are designed with internal
holes running their lengths, through which cutting fluid can be pumped to the hole near the drill point,
thus delivering the fluid directly to the cutting operation. An alternative approach with twist drills
that do not have fluid holes is to use a “pecking” procedure during the drilling operation. In this
procedure, the drill is periodically withdrawn from the hole to clear the chips before proceeding
deeper1.
Twist drills are normally made of high–speed steel. The geometry of the drill is fabricated before
heat treatment and then the outer shell of the drill (cutting edges and friction surfaces) is hardened
while retaining an inner core that is relatively tough. Grinding is used to sharpen the cutting edges
and shape the drill point1.
Although twist drills are the most common hole–making tools, other drill types are also available.
Straight–Flute Drills operate like twist drills except that the flutes for chip removal are straight along
the length of the tool rather than spiraled. The simpler design of the straight–flute drill permits
carbide tips to be used as the cutting edges, either as brazed or indexable inserts. Figure 6.10
illustrates the straight–flute indexable–insert drill. The cemented carbide inserts allow higher cutting
speeds and greater production rates than HSS twist drills. However, the inserts limit how small the
drills can be made. Thus, the diameter range of commercially available indexable–insert drills runs
from about 16 mm (0.625 in.) to about 127 mm (5 in.)1.
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Figure 6.10. Straight–flute drill that uses indexable inserts1.
A straight–flute drill designed for deep–hole drilling is the Gun Drill, shown in Figure 6.11.
Whereas the twist drill is usually limited to a depth–to–diameter ratio of 4:1 and the straight–flute
drill to about 3:1, the gun drill can cut holes up to 125 times its diameter. As shown in our figure, the
gun drill has a carbide cutting edge, a single flute for chip removal and a coolant hole running its
complete length. In the typical gun drilling operation, the work rotates around the stationary drill
(opposite of most drilling operations) and the coolant flows into the cutting process and out of the
hole along the flute, carrying the chips with it. Gun drills range in diameter from less than 2 mm (0.075
in.) to about 50 mm (2 in.)1.
Figure 6.11. Gun drill1.
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It was previously mentioned that twist drills are available with diameters up to 75 mm (3 in.).
Twist drills that large are uncommon because so much metal is required in the drill bit. An alternative
for large diameter holes is the Spade Drill, illustrated in Figure 6.12. Standard sizes range from 25 to
152 mm (1 to 6 in.). The interchangeable drill bit is held in a toolholder, which provides rigidity during
cutting. The mass of the spade drill is much less than a twist drill of the same diameter1.
Figure 6.12. Spade drill1.
Milling Cutters
Plain Milling Cutters = These are used for peripheral or slab milling. They are cylinder shaped
with several rows of teeth. The cutting edges are usually oriented at a helix angle to reduce impact on
entry into the work and these cutters are called Helical Milling Cutters. Tool geometry elements of
a plain milling cutter are shown in Figure 6.131.
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Figure 6.13. Tool geometry elements of an 18–tooth plain milling cutter1.
Form Milling Cutters = These are peripheral milling cutters in which the cutting edges have a
special profile that is to be imparted to the work. An important application is in gear making, in which
the form milling cutter is shaped to cut the slots between adjacent gear teeth, thereby leaving the
geometry of the gear teeth1.
Face Milling Cutters = These are designed with teeth that cut on both the periphery as well as the
end of the cutter. Face milling cutters can be made of HSS or they can be designed to use cemented
carbide inserts. Figure 6.14 shows a four–tooth face–milling cutter that uses inserts1.
Figure 6.14. Tool geometry elements of a four–tooth face milling cutter: (a) side view and (b) bottom
view1.
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End Milling Cutters = An end milling cutter looks like a drill bit, but close inspection indicates that
it is designed for primary cutting with its peripheral teeth rather than its end (A drill bit cuts only on
its end as it penetrates into the work.). End mills are designed with square ends, ends with radii and
ball ends. End mills can be used for face milling, profile milling and pocketing, cutting slots, engraving,
surface contouring and die sinking1.
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7. DRILLING
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7. DRILLING
Drilling is used to create a round hole. It is accomplished by a rotating tool that typically has two
cutting edges. The tool is fed in a direction parallel to its axis of rotation into the workpart to form the
round hole whose diameter is equal to the drill diameter, as in Figure 7.1. The tool is called a Drill or
Drill Bit (Figure 7.2). The most common drill bit is the twist drill. Drilling is customarily performed
on a Drill Press, although other machine tools also perform this operation1.
Figure 7.1. Drilling1.
The geometry that is ordinarily provided on a drill (Figure 7.2) represents a compromise of several
conflicting requirements, which include the following4:
 a small web to reduce thrust on the drill but a large web for greater resistance to chipping and
greater torsional rigidity
 large flutes to provide a larger space for chip transport but small flutes in the interest of
torsional rigidity
 an increase in the helix angle to more quickly remove chips but a decrease in helix angle in the
interest of greater strength of cutting edges
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Figure 7.2. Two common types of drills: (a) Chisel–edge drill. The function of the pair of margins is
to provide a bearing surface for the drill against walls of the hole as it penetrates into the workpiece.
Drills with four margins (double–margin) are available for improved guidance and accuracy. Drills
with chip–breaker features also are available. (b) Crankshaft drill. These drills have good centering
ability and because chips tend to break up easily, crankshaft drills are suitable for producing deep
holes2.
Twist Drill
The most common drill is the conventional standard–point twist drill [Figure 7.2 (a)]. The
geometry of the drill point is such that the normal rake angle and velocity of the cutting edge vary
with the distance from the center of the drill. The main features of this drill are as follows (with typical
ranges of angles given in parentheses): (a) point angle (118° to 135°), (b) lip–relief angle (7° to 15°),
(c) chisel–edge angle (125° to 135°) and (d) helix angle (15° to 30°)2.
Two spiral grooves (flutes) run the length of the drill and the chips produced are guided upward
through these grooves. The grooves also serve as passageways to enable the cutting fluid to reach the
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cutting edges. Some drills have internal longitudinal holes through which cutting fluids are forced,
thus improving lubrication and cooling as well as washing away the chips. Drills are available with a
chip–breaker feature ground along the cutting edges. This feature is important in drilling with
automated machinery, where a continuous removal of long chips without operator assistance is
essential2.
The various angles on a drill have been developed through experience and are designed to produce
accurate holes, minimize drilling forces and torque and optimize drill life. Small changes in drill
geometry can have a significant effect on a drill’s performance, particularly in the chisel–edge region,
which accounts for about 50% of the thrust force in drilling. For example, too small a lip relief angle
[Figure 7.2 (a)] increases the thrust force, generates excessive heat and increases wear. By contrast,
too large an angle can cause chipping or breaking of the cutting edge2.
Other Types of Drills
Several types of drills are shown in Figure 7.3. A Step Drill produces holes with two or more
different diameters. A Core Drill is used to make an existing hole larger. Counterboring and
Countersinking Drills produce depressions on the surface to accommodate the heads of screws and
bolts below the workpiece surface. A Center Drill is short and is used to produce a hole at the end of
a piece of stock, so that it may be mounted between centers of the headstock and the tailstock of a
lathe. A Spot Drill is used to spot (to start) a hole at the desired location on a surface2.
Figure 7.3. Various types of drills and drilling and reaming operations2.
Spade Drills [Figure 7.4 (a)] have removable tips or bits and are used to produce large–diameter
and deep holes. These drills have the advantages of higher stiffness (because of the absence of flutes
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in the body of the drill), ease of grinding the cutting edges and lower cost. A similar drill is the
Straight–Flute Drill [Figure 7.4 (b)]2.
Solid carbide and carbide–tipped drills [Figure 7.4 (c) and (d)] are available for drilling hard
materials (such as cast irons), high–temperature metals, abrasive materials (such as concrete and
brick – masonry drills) and composite materials with abrasive fiber reinforcements (such as glass and
graphite)2.
Figure 7.4. Various types of drills. (a) Spade drill; (b) straight–flute drill; (c) drill with indexable
carbide inserts; (d) drill with brazed–carbide tip2.
Gun Drilling
Developed originally for drilling gun barrels, Gun Drilling is used for drilling deep holes and
requires a special drill (Figure 7.5). The depth–to–diameter ratios of holes produced can be 300:1 or
even higher. The thrust force (the radial force that tends to push the drill sideways) is balanced by
bearing pads on the drill that slide along the inside surface of the hole. Consequently, a gun drill is
self–centering – an important feature in drilling straight, deep holes. A variation of this process is Gun
Trepanning, which uses a cutting tool similar to a gun drill, except that the tool has a central hole2.
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Figure 7.5. (a) A gun drill, showing various features. (b) Schematic illustration of the gun–drilling
operation2.
Cutting speeds in gun drilling are usually high and feeds are low. Tolerances typically are about
0.025 mm (0.001 in.). The cutting fluid is forced under high pressure through a longitudinal hole
(passage) in the body of the drill [Figure 7.5 (a)]. In addition to cooling and lubricating the workpiece,
the fluid flushes out chips that otherwise would be trapped in the deep hole being drilled and thus
interfere with the drilling operation. The tool does not have to be retracted to clear the chips, as is
usually done with twist drills2.
Trepanning
In Trepanning (from the Greek trypanon, or “boring a hole” or “auger”) the cutting tool [Figure
7.6 (a)] produces a hole by removing a disk–shaped piece (core), usually from flat plates. A hole is
thus produced without reducing all of the material that is removed to chips, as is the case in drilling.
The trepanning process can be used to make disks up to 250 mm (10 in.) in diameter from flat sheets,
plates or structural members such as I–beams. It also can be used to make circular grooves in which
O–rings are to be placed. Trepanning can be carried out on lathes, drill presses or other machine tools
using single–point or multipoint tools, as shown in [Figure 7.6 (b)]2.
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Figure 7.6. (a) Trepanning tool. (b) Trepanning with a drill–mounted single cutter2.
7.1. Cutting Conditions in Drilling
The cutting speed in a drilling operation is the surface speed at the outside diameter of the drill. It
is specified in this way for convenience, even though nearly all of the cutting is actually performed at
lower speeds closer to the axis of rotation. To set the desired cutting speed in drilling, it is necessary
to determine the rotational speed of the drill. Letting N represent the spindle rev/min1,
𝑁=
𝑣
𝜋𝐷
(7.1.)
v = Cutting Speed (mm/min, in/min)
D = Drill Diameter (mm, in)
In some drilling operations, the workpiece is rotated about a stationary tool, but the same formula
applies1.
Feed (f) in drilling is specified in mm/rev (in./rev). Recommended feeds are roughly proportional
to drill diameter; higher feeds are used with larger diameter drills. Since there are (usually) two
cutting edges at the drill point, the uncut chip thickness (chip load) taken by each cutting edge is half
the feed. Feed can be converted to feed rate using the same equation as for turning1:
𝑓𝑟 = 𝑁𝑓
fr = Feed Rate (mm/min, in/min)
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Drilled holes are either through holes or blind holes (Figure 7.7). In Through Holes, the drill exits
the opposite side of the work; in Blind Holes, it does not. The machining time required to drill a
through hole can be determined by the following formula1:
𝑇𝑚 =
𝑡+𝐴
𝑓𝑟
(7.3.)
Tm = Machining (Drilling) Time (min)
t = Work Thickness (mm, in)
A = Approach Allowance that Accounts for the Drill Point Angle, Representing the Distance the Drill
must Feed into the Work before Reaching Full Diameter (mm, in)
𝜃
𝐴 = 0.5𝐷 tan (90 − )
2
(7.4.)
θ = Drill Point Angle
In drilling a through hole, the feed motion usually proceeds slightly beyond the opposite side of
the work, thus making the actual duration of the cut greater than Tm in Equation 7.3 by a small
amount1.
In a blind–hole, Hole Depth (d) is defined as the distance from the work surface to the depth of
the full diameter [Figure 7.7 (b)]. Thus, for a blind hole, machining time is given by1
𝑇𝑚 =
𝑑+𝐴
𝑓𝑟
(7.5.)
The Material Removal Rate (MRR) in drilling is determined as the product of the drill cross–
sectional area and the feed rate1:
𝑅𝑀𝑅 =
𝜋𝐷2 𝑓𝑟
4
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This equation is valid only after the drill reaches full diameter and excludes the initial approach of
the drill into the work1.
Figure 7.7. Two hole types: (a) through hole and (b) blind hole1.
7.2. Operations Related to Drilling
Several operations are related to drilling. These are illustrated in Figure 7.8. Most of the operations
follow drilling; a hole must be made first by drilling and then the hole is modified by one of the other
operations. Centering and spot facing are exceptions to this rule. All of the operations use rotating
tools1.
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Figure 7.8. Machining operations related to drilling: (a) reaming, (b) tapping, (c) counterboring, (d)
countersinking, (e) center drilling and (f) spot facing1.
Reaming
Reaming is used to slightly enlarge a hole, to provide a better tolerance on its diameter and to
improve its surface finish. The tool is called a Reamer and it usually has straight flutes1.
Tapping
This operation is performed by a Tap and is used to provide internal screw threads on an existing
hole1.
Counterboring
Counterboring provides a stepped hole, in which a larger diameter follows a smaller diameter
partially into the hole. A counterbored hole is used to seat bolt heads into a hole so the heads do not
protrude above the surface1.
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Countersinking
This is similar to counterboring, except that the step in the hole is cone–shaped for flat head screws
and bolts1.
Centering
Also called Center Drilling, this operation drills a starting hole to accurately establish its location
for subsequent drilling. The tool is called a Center Drill1.
Spot Facing
Spot facing is similar to milling. It is used to provide a flat machined surface on the workpart in a
localized area1.
7.3. Drill Presses
The standard machine tool for drilling is the drill press. There are various types of drill press, the
most basic of which is the upright drill (Figure 7.9). The Upright Drill stands on the floor and consists
of a table for holding the workpart, a drilling head with powered spindle for the drill bit and a base
and column for support. A similar drill press, but smaller, is the Bench Drill, which is mounted on a
table or bench rather than the floor1.
Figure 7.9. Upright drill press1.
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The Radial Drill (Figure 7.10) is a large drill press designed to cut holes in large parts. It has a
radial arm along which the drilling head can be moved and clamped. The head therefore can be
positioned along the arm at locations that are a significant distance from the column to accommodate
large work. The radial arm can also be swiveled about the column to drill parts on either side of the
worktable1.
Figure 7.10. Radial drill press1.
The Gang Drill is a drill press consisting basically of two to six upright drills connected together
in an in–line arrangement. Each spindle is powered and operated independently and they share a
common worktable, so that a series of drilling and related operations can be accomplished in
sequence (e.g., centering, drilling, reaming, tapping) simply by sliding the workpart along the
worktable from one spindle to the next. A related machine is the Multiple–Spindle Drill, in which
several drill spindles are connected together to drill multiple holes simultaneously into the workpart1.
In addition, Computer Numerical Control (CNC) Drill Presses are available to control the
positioning of the holes in the workparts. These drill presses are often equipped with turrets to hold
multiple tools that can be indexed under control of the CNC program. The term CNC Turret Drill is
used for these machine tools1.
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Workholding on a drill press is accomplished by clamping the part in a vise, fixture or jig. A Vise is
a general–purpose workholding device possessing two jaws that grasp the work in position. A Fixture
is a workholding device that is usually custom–designed for the particular workpart. The fixture can
be designed to achieve higher accuracy in positioning the part relative to the machining operation,
faster production rates and greater operator convenience in use. A Jig is a workholding device that is
also specially designed for the workpart. The distinguishing feature between a jig and a fixture is that
the jig provides a means of guiding the tool during the drilling operation. A fixture does not provide
this tool guidance feature. A jig used for drilling is called a Drill Jig1.
7.4. Thrust Force and Torque
The Thrust Force in drilling acts perpendicular to the hole axis; if this force is excessive, it can
cause the drill to bend or break. An excessive thrust force also can distort the workpiece, particularly
if it does not have sufficient stiffness (for example, thin sheet–metal structures) or it can cause the
workpiece to slip into the workholding fixture2.
The thrust force depends on factors such as (a) the strength of the workpiece material, (b) feed,
(c) rotational speed, (d) drill diameter, (e) drill geometry and (f) cutting fluids. Accurate calculation
of the thrust force on the drill is difficult. Thrust forces typically range from a few newtons for small
drills to as high as 100 kN (23.5 klb) for drilling high–strength materials with large drills.
Experimental data are available as an aid in the design and use of drills and drilling equipment2.
A knowledge of the Torque in drilling is essential for estimating the power requirement; however,
because of the many factors involved, it is difficult to calculate2.
7.5. Reaming and Reamers
Reaming is an operation used to (a) make an existing hole dimensionally more accurate than can
be achieved by drilling alone and (b) improve its surface finish. The most accurate holes in workpieces
generally are produced by the following sequence of operations2:
1. Centering
2. Drilling
3. Boring
4. Reaming
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A Reamer [Figure 7.11 (a)] is a multiple–cutting–edge tool with straight or helically fluted edges
that remove very little material. For soft metals, a reamer typically removes a minimum of 0.2 mm
(0.008 in.) on the diameter of a drilled hole; for harder metals, about 0.13 mm (0.005 in.) is removed.
Attempts to remove smaller layers can be detrimental, as the reamer may be damaged or the hole
surface may become burnished. In this case, honing would be preferred. In general, reamer speeds
are one–half those of the same–size drill and three times the feed rate2.
Hand Reamers are straight or have a tapered end in the first third of their length. Various
Machine Reamers (also called Chucking Reamers, because they are mounted in a chuck and
operated by a machine) are available in two types: (1) Rose Reamers have cutting edges with wide
margins and no relief [Figure 7.11 (a)]. They remove considerable material and true up a hole for flute
reaming. (2) Fluted Reamers have small margins and relief, with a rake angle of about 5°. They
usually are used for light cuts of about 0.1 mm (0.004 in.) on the hole diameter2.
Shell Reamers (which are hollow and are mounted on an arbor) generally are used for holes larger
than 20 mm (0.75 in.). Expansion Reamers are adjustable for small variations in hole size and also
to compensate for wear of the reamer’s cutting edges. Adjustable Reamers [Figure 7.11 (b)] can be
set for specific hole diameters and therefore are versatile2.
Figure 7.11. (a) Terminology for a helical reamer. (b) Inserted–blade adjustable reamer2.
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Reamers may be held rigidly (as in a chuck) or they may float in their holding fixtures to ensure
alignment or be piloted in guide bushings placed above and below the workpiece. A further
development in reaming consists of the dreamer – a tool that combines drilling and reaming. The tip
of the tool produces a hole by drilling and the rest of the same tool performs a reaming operation. A
similar development involves drilling and tapping in one stroke, using a single tool2.
Reamers typically are made of high–speed steels (M1, M2 and M7) or solid carbides (K20, C2) or
have carbide cutting edges2.
7.6. Tapping and Taps
Internal threads in workpieces can be produced by Tapping. A Tap is a chip–producing threading
tool with multiple cutting teeth [Figure 7.12 (a)]. Taps generally are available with two, three or four
flutes. The most common production tap is the two–flute spiral–point tap. The two–flute tap forces
the chips into the hole so that the tap needs to be retracted only at the end of the cut. Three–fluted
taps are stronger because more material is available in the flute. Tap sizes range up to 100 mm (4
in.)2.
Figure 7.12. (a) Terminology for a tap. (b) Tapping of steel nuts in production2.
Tapered Taps are designed to reduce the torque required for the tapping of through holes.
Bottoming Taps are for tapping blind holes to their full depth. Collapsible Taps are used in large–
diameter holes; after tapping has been completed, the tap is collapsed mechanically and is removed
from the hole without rotation2.
Chip removal can be a significant problem during tapping because of the small clearances involved.
If chips aren’t removed properly, the excessive torque that results can break the tap. The use of a
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cutting fluid and periodic reversal and removal of the tap from the hole are effective means of chip
removal and of improving the quality of the tapped hole. For higher tapping productivity, drilling and
tapping can be combined in a single operation (Drapping) with a single tool. The tool has a drilling
section at its tip, followed by a tapping section2.
Tapping may be done by hand or with machines such as (a) drilling machines, (b) lathes, (c)
automatic screw machines and (d) vertical CNC milling machines combining the correct relative
rotation and the longitudinal feed. Special tapping machines are available with features for multiple
tapping operations. Multiple–spindle tapping heads are used extensively, particularly in the
automotive industry, where 30 to 40% of machining operations involve the tapping of holes2.
Taps usually are made of high–speed steels (M1, M2, M7 and M10)2.
Some tapping systems now have capabilities for directing the cutting fluid to the cutting zone
through the spindle and a hole in the tap, which also helps flush the chips out of the hole being tapped.
Chipless Tapping is a process of internal thread rolling using a forming tap2.
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8. TURNING
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8. TURNING
In Turning, a cutting tool with a single cutting edge is used to remove material from a rotating
workpiece to generate a cylindrical shape, as in Figure 8.1. The speed motion in turning is provided
by the rotating workpart and the feed motion is achieved by the cutting tool moving slowly in a
direction parallel to the axis of rotation of the workpiece. Turning is traditionally carried out on a
machine tool called a Lathe, which provides power to turn the part at a given rotational speed and to
feed the tool at a specified rate and depth of cut1.
Figure 8.1. Turning1.
The cutting conditions for a turning operation are depicted in Figure 8.2. Typical units used for
cutting speed are m/s (ft/min). Feed in turning is expressed in mm/rev (in./rev) and depth of cut is
expressed in mm (in.)1.
Figure 8.2. Cutting speed, feed and depth of cut for a turning operation1.
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The tool is held rigidly in a tool post and moved at a constant rate along the axis of the bar, cutting
away a layer of metal to form a cylinder or a surface of more complex profile. This is shown
diagrammatically in Figure 8.33.
Figure 8.3. Lathe turning showing a vertical cross–section at top right and a detail of the insert
geometry at bottom right3.
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8.1. Cutting Conditions in Turning
The rotational speed in turning is related to the desired cutting speed at the surface of the
cylindrical workpiece by the equation1
𝑁=
𝑣
𝜋𝐷𝑜
(8.1.)
N = Rotational Speed of the Workpiece (rev/min)
v = Cutting Speed (m/min, ft/min)
Do = Original Diameter of the Part (m, ft)
The turning operation reduces the diameter of the work from its original diameter (Do) to a final
diameter (Df), as determined by the depth of cut (d)1:
𝐷𝑓 = 𝐷𝑜 − 2𝑑
(8.2.)
The feed in turning is generally expressed in mm/rev (in./rev). This feed can be converted to a
linear travel rate in mm/min (in./min) by the formula1
𝑓𝑟 = 𝑁𝑓
(8.3.)
fr = Feed Rate or Linear Speed of the Tool along Workpiece Length (mm/min, in/min)
f = Feed (mm/rev, in/rev)
The time to machine from one end of a cylindrical workpart to the other is given by1
𝑇𝑚 =
𝐿
𝑓𝑟
Tm = Machining Time (min)
f = Length of Cylindrical Workpart (mm, in)
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A more direct computation of the machining time is provided by the following equation1:
𝑇𝑚 =
𝜋𝐷𝑜 𝐿
𝑓𝑣
(8.5.)
Do = Work Diameter (mm, in)
L = Workpart Length (mm, in)
f = Feed (mm/rev, in/rev)
v = Cutting Speed (mm/min, in/min)
*** As a practical matter, a small distance is usually added to the workpart length at the
beginning and end of the piece to allow for approach and overtravel of the tool. Thus, the
duration of the feed motion past the work will be longer than Tm1.
The Material Removal Rate (MRR) in turning is the volume of material removed per unit time,
with the units of mm3/min or in3/min 2. The volumetric rate of material removal can be most
conveniently determined by the following equation1:
𝑅𝑀𝑅 = 𝑣𝑓𝑑
(8.6.)
RMR = Material Removal Rate (mm3/min, in3/min)
f = Feed (mm, in)
8.2. Operations Related to Turning
A variety of other machining operations can be performed on a lathe in addition to turning; these
include the following, illustrated in Figure 8.41.
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Figure 8.4. Machining operations other than turning that are performed on a lathe: (a) facing, (b)
taper turning, (c) contour turning, (d) form turning, (e) chamfering, (f) cutoff, (g) threading, (h)
boring, (i) drilling and (j) knurling1.
Facing
The tool is fed radially into the rotating work on one end to create a flat surface on the end 1.
Taper Turning
Instead of feeding the tool parallel to the axis of rotation of the work, the tool is fed at an angle,
thus creating a tapered cylinder or conical shape1.
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Contour Turning
Instead of feeding the tool along a straight line parallel to the axis of rotation as in turning, the tool
follows a contour that is other than straight, thus creating a contoured form in the turned part1.
Form Turning
In this operation, sometimes called Forming, the tool has a shape that is imparted to the work by
plunging the tool radially into the work1.
Chamfering
The cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming what is
called a Chamfer1.
Cutoff
Cuttoff is the process to cut a piece from the end of a part, as is done in the production of slugs or
blanks for additional processing into discrete products2. The tool is fed radially into the rotating work
at some location along its length to cut off the end of the part. This operation is sometimes referred to
as Parting1.
Threading
Threading is the process to produce external or internal threads2. A pointed tool is fed linearly
across the outside surface of the rotating workpart in a direction parallel to the axis of rotation at a
large effective feed rate, thus creating threads in the cylinder1.
Boring
Boring is the process to enlarge a hole or cylindrical cavity made by a previous process or to
produce circular internal grooves2. A single–point tool is fed linearly, parallel to the axis of rotation,
on the inside diameter of an existing hole in the part1.
Drilling
Drilling can be performed on a lathe by feeding the drill into the rotating work along its axis.
Reaming can be performed in a similar way1.
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Knurling
This is not a machining operation because it does not involve cutting of material. Instead, it is a
metal forming operation used to produce a regular cross–hatched pattern in the work surface1.
Turning, facing, taper turning, contour turning, chamfering and boring are all performed with
single–point tools. A threading operation is accomplished using a single–point tool designed with a
geometry that shapes the thread. Certain operations require tools other than single–point. Form
turning is performed with a specially designed tool called a Form Tool. The profile shape ground into
the tool establishes the shape of the workpart. A cutoff tool is basically a form tool. Drilling is
accomplished by a drill bit. Knurling is performed by a knurling tool, consisting of two hardened
forming rolls, each mounted between centers. The forming rolls have the desired knurling pattern on
their surfaces. To perform knurling, the tool is pressed against the rotating workpart with sufficient
pressure to impress the pattern onto the work surface1.
8.3. Engine Lathe
The basic lathe used for turning and related operations is an Engine Lathe. It is a versatile machine
tool, manually operated and widely used in low and medium production. The term engine dates from
the time when these machines were driven by steam engines1.
Engine Lathe Technology
Figure 8.5 is a sketch of an engine lathe showing its principal components. The Headstock contains
the drive unit to rotate the spindle, which rotates the work. Opposite the headstock is the Tailstock,
in which a center is mounted to support the other end of the workpiece1. It is equipped with a center
that may be fixed (dead center) or it may be free to rotate with the workpiece (live center). Drills and
reamers can be mounted on the tailstock quill (a hollow cylindrical part with a tapered hole) to drill
axial holes in the workpiece2.
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Figure 8.5. Diagram of an engine lathe, indicating its principal components1.
The cutting tool is held in a Tool Post fastened to the Cross–Slide, which is assembled to the
Carriage. The carriage is designed to slide along the Ways of the lathe in order to feed the tool parallel
to the axis of rotation. The ways are like tracks along which the carriage rides, and they are made with
great precision to achieve a high degree of parallelism relative to the spindle axis. The ways are built
into the Bed of the lathe, providing a rigid frame for the machine tool1. The bed supports all major
components of the lathe. Beds have a large mass and are built rigidly, usually from gray or nodular
cast iron2.
The carriage is driven by a leadscrew that rotates at the proper speed to obtain the desired feed
rate. The cross–slide is designed to feed in a direction perpendicular to the carriage movement. Thus,
by moving the carriage, the tool can be fed parallel to the work axis to perform straight turning; or by
moving the cross–slide, the tool can be fed radially into the work to perform facing, form turning or
cutoff operations1.
The conventional engine lathe and most other machines are Horizontal Turning Machines; that
is, the spindle axis is horizontal. This is appropriate for the majority of turning jobs, in which the
length is greater than the diameter. For jobs in which the diameter is large relative to length and the
work is heavy, it is more convenient to orient the work so that it rotates about a vertical axis; these
are Vertical Turning Machines1.
The size of a lathe is designated by swing and maximum distance between centers. The Swing is
the maximum workpart diameter that can be rotated in the spindle, determined as twice the distance
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between the centerline of the spindle and the ways of the machine. The actual maximum size of a
cylindrical workpiece that can be accommodated on the lathe is smaller than the swing because the
carriage and cross–slide assembly are in the way. The Maximum Distance Between Centers
indicates the maximum length of a workpiece that can be mounted between headstock and tailstock
centers. For example, a 350 mm × 1.2 m (14 in. × 48 in.) lathe designates that the swing is 350 mm (14
in.) and the maximum distance between centers is 1.2 m (48 in.)1.
Methods of Holding the Work in a Lathe
There are four common methods used to hold workparts in turning. These workholding methods
consist of various mechanisms to grasp the work, center and support it in position along the spindle
axis and rotate it. The methods, illustrated in Figure 8.6, are (a) mounting the work between centers,
(b) chuck, (c) collet and (d) face plate1.
Figure 8.6. Four workholding methods used in lathes: (a) mounting the work between centers using
a dog, (b) three–jaw chuck, (c) collet and (d) faceplate for noncylindrical workparts1.
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Holding the work Between Centers refers to the use of two centers, one in the headstock and the
other in the tailstock, as in Figure 8.6 (a). This method is appropriate for parts with large length–to–
diameter ratios. At the headstock center, a device called a Dog is attached to the outside of the work
and is used to drive the rotation from the spindle. The tailstock center has a cone–shaped point which
is inserted into a tapered hole in the end of the work. The tailstock center is either a ‘‘live’’ center or a
‘‘dead’’ center. A Live Center rotates in a bearing in the tailstock, so that there is no relative rotation
between the work and the live center, hence, no friction between the center and the workpiece. In
contrast, a Dead Center is fixed to the tailstock, so that it does not rotate; instead, the workpiece
rotates about it. Because of friction and the heat buildup that results, this setup is normally used at
lower rotational speeds. The live center can be used at higher speeds1.
The Chuck, Figure 8.6 (b), is available in several designs, with three or four jaws to grasp the
cylindrical workpart on its outside diameter. The jaws are often designed so they can also grasp the
inside diameter of a tubular part. A Self–Centering Chuck has a mechanism to move the jaws in or
out simultaneously, thus centering the work at the spindle axis. Other chucks allow independent
operation of each jaw. Chucks can be used with or without a tailstock center. For parts with low
length–to–diameter ratios, holding the part in the chuck in a cantilever fashion is usually sufficient to
withstand the cutting forces. For long workbars, the tailstock center is needed for support1.
A Collet consists of a tubular bushing with longitudinal slits running over half its length and
equally spaced around its circumference, as in Figure 8.6 (c). The inside diameter of the collet is used
to hold cylindrical work such as barstock. Owing to the slits, one end of the collet can be squeezed to
reduce its diameter and provide a secure grasping pressure against the work. Because there is a limit
to the reduction obtainable in a collet of any given diameter, these workholding devices must be made
in various sizes to match the particular workpart size in the operation1.
A Face Plate [Figure 8.6 (d)] is a workholding device that fastens to the lathe spindle and is used
to grasp parts with irregular shapes. Because of their irregular shape, these parts cannot be held by
other workholding methods. The face plate is therefore equipped with the custom–designed clamps
for the particular geometry of the part1.
8.4. Other Lathes and Turning Machines
Other turning machines are (1) toolroom lathe, (2) speed lathe, (3) turret lathe, (4) chucking
machine, (5) automatic screw machine and (6) numerically controlled lathe1.
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The toolroom lathe and speed lathe are closely related to the engine lathe. The Toolroom Lathe
is smaller and has a wider available range of speeds and feeds. It is also built for higher accuracy,
consistent with its purpose of fabricating components for tools, fixtures and other high–precision
devices1.
The Speed Lathe is simpler in construction than the engine lathe. It has no carriage and cross–
slide assembly and therefore no leadscrew to drive the carriage. The cutting tool is held by the
operator using a rest attached to the lathe for support. The speeds are higher on a speed lathe, but the
number of speed settings is limited. Applications of this machine type include wood turning, metal
spinning and polishing operations1.
A Turret Lathe is a manually operated lathe in which the tailstock is replaced by a turret that holds
up to six cutting tools. These tools can be rapidly brought into action against the work one by one by
indexing the turret. In addition, the conventional tool post used on an engine lathe is replaced by a
four–sided turret that is capable of indexing up to four tools into position. Hence, because of the
capacity to quickly change from one cutting tool to the next, the turret lathe is used for high–
production work that requires a sequence of cuts to be made on the part1.
As the name suggests, a Chucking Machine (nicknamed Chucker) uses a chuck in its spindle to
hold the workpart. The tailstock is absent on a chucker, so parts cannot be mounted between centers.
This restricts the use of a chucking machine to short, lightweight parts. The setup and operation are
similar to a turret lathe except that the feding actions of the cutting tools are controlled automatically
rather than by a human operator. The function of the operator is to load and unload the parts1.
A Bar Machine is similar to a chucking machine except that a collet is used (instead of a chuck),
which permits long bar stock to be fed through the headstock into position. At the end of each
machining cycle, a cutoff operation separates the new part. The bar stock is then indexed forward to
present stock for the next part. Feeding the stock as well as indexing and feeding the cutting tools is
accomplished automatically. Owing to its high level of automatic operation, it is often called an
Automatic Bar Machine. One of its important applications is in the production of screws and similar
small hardware items; the name Automatic Screw Machine is frequently used for machines used in
these applications1.
Bar machines can be classified as single spindle or multiple spindle. A Single Spindle Bar Machine
has one spindle that normally allows only one cutting tool to be used at a time on the single workpart
being machined. Thus, while each tool is cutting the work, the other tools are idle. (Turret lathes and
chucking machines are also limited by this sequential, rather than simultaneous, tool operation). To
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increase cutting tool utilization and production rate, Multiple Spindle Bar Machines are available.
These machines have more than one spindle, so multiple parts are machined simultaneously by
multiple tools. For example, a six–spindle automatic bar machine works on six parts at a time, as in
Figure 8.7. At the end of each machining cycle, the spindles (including collets and workbars) are
indexed (rotated) to the next position. In our illustration, each part is cut sequentially by five sets of
cutting tools, which takes six cycles (position 1 is for advancing the bar stock to a ‘‘stop’’). With this
arrangement, a part is completed at the end of each cycle. As a result, a six–spindle automatic screw
machine has a very high production rate1.
Figure 8.7. (a) Type of part produced on a six–spindle automatic bar machine; and (b) sequence of
operations to produce the part: (1) feed stock to stop, (2) turn main diameter, (3) form second
diameter and spot face, (4) drill, (5) chamfer and (6) cutoff1.
The sequencing and actuation of the motions on screw machines and chucking machines have
traditionally been controlled by cams and other mechanical devices. The modern form of control is
CNC, in which the machine tool operations are controlled by a ‘‘program of instructions’’ consisting of
alphanumeric code. CNC provides a more sophisticated and versatile means of control than
mechanical devices. This has led to the development of machine tools capable of more complex
machining cycles and part geometries and a higher level of automated operation than conventional
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screw machines and chucking machines. The CNC lathe is an example of these machines in turning. It
is especially useful for contour turning operations and close tolerance work. Today, automatic
chuckers and bar machines are implemented by CNC1.
8.5. Boring Machines
Boring is similar to turning. It uses a single–point tool against a rotating workpart. The difference
is that boring is performed on the inside diameter of an existing hole rather than the outside diameter
of an existing cylinder. In effect, boring is an internal turning operation. Machine tools used to perform
boring operations are called Boring Machines (also Boring Mills). One might expect that boring
machines would have features in common with turning machines; indeed, as previously indicated,
lathes are sometimes used to accomplish boring1.
Boring mills can be horizontal or vertical. The designation refers to the orientation of the axis of
rotation of the machine spindle or workpart. In a Horizontal Boring operation, the setup can be
arranged in either of two ways. The first setup is one in which the work is fixtured to a rotating spindle
and the tool is attached to a cantilevered boring bar that feeds into the work, as illustrated in Figure
8.8 (a). The boring bar in this setup must be very stiff to avoid deflection and vibration during cutting.
To achieve high stiffness, boring bars are often made of cemented carbide, whose modulus of elasticity
approaches 620 × 103 MPa (90 × 106 lb/in2). Figure 8.9 shows a carbide boring bar1.
Figure 8.8. Two forms of horizontal boring: (a) boring bar is fed into a rotating workpart and (b)
work is fed past a rotating boring bar1.
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Figure 8.9. Boring bar made of cemented carbide (WC–Co) that uses indexable cemented carbide
inserts1.
The second possible setup is one in which the tool is mounted to a boring bar and the boring bar
is supported and rotated between centers. The work is fastened to a feeding mechanism that feeds it
past the tool. This setup [Figure 8.8 (b)] can be used to perform a boring operation on a conventional
engine lathe1.
A Vertical Boring Machine is used for large, heavy workparts with large diameters; usually the
workpart diameter is greater than its length. As in Figure 8.10, the part is clamped to a worktable that
rotates relative to the machine base. Worktables up to 40 ft in diameter are available. The typical
boring machine can position and feed several cutting tools simultaneously. The tools are mounted on
tool heads that can be fed horizontally and vertically relative to the worktable. One or two heads are
mounted on a horizontal cross–rail assembled to the machine tool housing above the worktable. The
cutting tools mounted above the work can be used for facing and boring. In addition to the tools on
the cross–rail, one or two additional tool heads can be mounted on the side columns of the housing to
enable turning on the outside diameter of the work1.
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Figure 8.10. A vertical boring mill1.
The tool heads used on a vertical boring machine often include turrets to accommodate several
cutting tools. This results in a loss of distinction between this machine and a Vertical Turret Lathe.
Some machine tool builders make the distinction that the vertical turret lathe is used for work
diameters up to 2.5 m (100 in.), while the vertical boring machine is used for larger diameters. Also,
vertical boring mills are often applied to one–of–a–kind jobs, while vertical turret lathes are used for
batch production1.
8.6. Cutting Tools
8.6.1. Tool Geometry
The various angles in a single–point cutting tool have important functions in machining operations.
These angles are measured in a coordinate system consisting of the three major axes of the tool shank,
as can be seen in Figure 8.112.
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Figure 8.11. Designations for a right–hand cutting tool. Right–Hand means that the tool travels from
right to left2.
Rake Angle is important in controlling both the direction of chip flow and the strength of the tool
tip. Positive rake angles improve the cutting operation by reducing forces and temperatures.
However, positive angles also result in a small included angle of the tool tip, possibly leading to
premature tool chipping and failure, depending on the toughness of the tool material2.
Side Rake Angle is more important than the Back Rake Angle, although the latter usually controls
the direction of chip flow. For machining metals and using carbide inserts, these angles typically are
in the range from -5° to 5°2.
Cutting–Edge Angle affects chip formation, tool strength and cutting forces to various degrees.
Typically, the cutting–edge angle is around 15°2.
Relief Angle controls interference and rubbing at the tool–workpiece interface. If it is too large,
the tool tip may chip off; if it is too small, flank wear may be excessive. Relief angles typically are 5°2.
Nose Radius affects surface finish and tool–tip strength. The smaller the nose radius (sharp tool),
the rougher the surface finish of the workpiece and the lower the strength of the tool. However, large
nose radii can lead to tool chatter2.
8.7. Forces in Turning
The three principal forces acting on a cutting tool are shown in Figure 8.12. These forces are
important in the design of machine tools, as well as in the deflection of tools and workpieces for
precision–machining operations. The machine tool and its components must be able to withstand
such forces without causing significant deflections, vibrations and chatter in the overall operation2.
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Figure 8.12. Forces acting on a cutting tool in turning. Fc is the cutting force, Ft is the thrust or feed
force (in the direction of feed) and Fr is the radial force that tends to push the tool away from the
workpiece being machined2.
The Cutting Force (Fc), acts downward on the tool tip and thus tends to deflect the tool downward
and the workpiece upward. The cutting force supplies the energy required for the cutting operation.
The product of the cutting force and its radius from the workpiece center determines the torque on
the spindle. The product of the torque and the spindle speed determines the power required in the
turning operation2.
The Thrust Force (Ft), acts in the longitudinal direction. It also is called the Feed Force, because
it is in the feed direction of the tool. This force tends to push the tool towards the right and away from
the chuck2.
The Radial Force (Fr), acts in the radial direction and tends to push the tool away from the
workpiece. Because of the many factors involved in the cutting process, forces Ft and Fr are difficult to
calculate directly; they usually are determined experimentally if desired2.
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9. MILLING
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9. MILLING
In Milling, a rotating tool with multiple cutting edges is fed slowly across the work material to
generate a plane or straight surface. The direction of the feed motion is perpendicular to the tool’s
axis of rotation. This orientation between the tool axis and the feed direction is one of the features
that distinguishes milling from drilling. In drilling, the cutting tool is fed in a direction parallel to its
axis of rotation. The speed motion is provided by the rotating milling cutter. The two basic forms of
milling are peripheral milling and face milling, as in Figure 9.1 (a) and (b). The cutting tool in milling
is called a Milling Cutter and the cutting edges are called Teeth. The conventional machine tool that
performs this operation is a Milling Machine1.
The geometric form created by milling is a plane surface. Other work geometries can be created
either by means of the cutter path or the cutter shape. Owing to the variety of shapes possible and its
high production rates, milling is one of the most versatile and widely used machining operations1.
Figure 9.1. Two forms of milling: (a) peripheral or plain milling and (b) face milling1.
Milling is an Interrupted Cutting operation; the teeth of the milling cutter enter and exit the work
during each revolution. This interrupted cutting action subjects the teeth to a cycle of impact force
and thermal shock on every rotation. The tool material and cutter geometry must be designed to
withstand these conditions1.
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9.1. Types of Milling Operations
Peripheral (Plain) Milling
In Peripheral Milling, the axis of the tool is parallel to the surface being machined and the
operation is performed by cutting edges on the outside periphery of the cutter. Several types of
peripheral milling are shown in Figure 9.2: (a) Slab Milling, the basic form of peripheral milling in
which the cutter width extends beyond the workpiece on both sides; (b) Slotting, also called Slot
Milling, in which the width of the cutter is less than the workpiece width, creating a slot in the work
– when the cutter is very thin, this operation can be used to mill narrow slots or cut a workpart in two,
called Saw Milling; (c) Side Milling, in which the cutter machines the side of the workpiece; (d)
Straddle Milling, the same as side milling, only cutting takes place on both sides of the work; and
Form Milling, in which the milling teeth have a special profile that determines the shape of the slot
that is cut in the work. Form milling is therefore classified as a Forming Operation1.
Figure 9.2. Peripheral milling: (a) slab milling, (b) slotting, (c) side milling, (d) straddle milling and
(e) form milling1.
In Peripheral Milling, the direction of cutter rotation distinguishes two forms of milling: up
milling and down milling, illustrated in Figure 9.3. In Up (Conventional) Milling, the direction of
motion of the cutter teeth is opposite the feed direction when the teeth cut into the work. It is milling
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“against the feed”. In Down (Climb) Milling, the direction of cutter motion is the same as the feed
direction when the teeth cut the work. It is milling “with the feed”1.
The relative geometries of these two forms of milling result in differences in their cutting actions.
In up milling, the chip formed by each cutter tooth starts out very thin and increases in thickness
during the sweep of the cutter. In down milling, each chip starts out thick and reduces in thickness
throughout the cut. The length of a chip in down milling is less than in up milling (the difference is
exaggerated in our figure). This means that the cutter is engaged in the work for less time per volume
of material cut and this tends to increase tool life in down milling1.
The cutting force direction is tangential to the periphery of the cutter for the teeth that are engaged
in the work. In up milling, this has a tendency to lift the workpart as the cutter teeth exit the material.
In down milling, this cutter force direction is downward, tending to hold the work against the milling
machine table1. However, because of the resulting impact forces when the teeth engage the workpiece,
this operation must have a rigid work–holding setup and gear backlash must be eliminated in the
table feed mechanism. Climb milling is not suitable for the machining of workpieces having surface
scale, such as hot–worked metals, forgings and castings. The scale is hard and abrasive and causes
excessive wear and damage to the cutter teeth, thus shortening tool life2.
The advantages to conventional milling are that
✓ tooth engagement is not a function of workpiece surface characteristics and
✓ contamination or scale (oxide layer) on the surface does not adversely affect tool life2.
Figure 9.3. Two forms of peripheral milling operation with a 20–teeth cutter: (a) up milling and (b)
down milling1.
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Face Milling
In Face Milling, the axis of the cutter is perpendicular to the surface being milled and machining
is performed by cutting edges on both the end and outside periphery of the cutter. As in peripheral
milling, various forms of face milling exist, several of which are shown in Figure 9.4: (a) Conventional
Face Milling, in which the diameter of the cutter is greater than the workpart width, so the cutter
overhangs the work on both sides; (b) Partial Face Milling, where the cutter overhangs the work on
only one side; (c) End Milling, in which the cutter diameter is less than the work width, so a slot is
cut into the part; (d) Profile Milling, a form of end milling in which the outside periphery of a flat part
is cut; (e) Pocket Milling, another form of end milling used to mill shallow pockets into flat parts; and
(f) Surface Contouring, in which a ball–nose cutter (rather than square–end cutter) is fed back and
forth across the work along a curvilinear path at close intervals to create a three–dimensional surface
form. The same basic cutter control is required to machine the contours of mold and die cavities, in
which case the operation is called Die Sinking1.
Figure 9.4. Face milling: (a) conventional face milling, (b) partial face milling, (c) end milling, (d)
profile milling, (e) pocket milling and (f) surface contouring1.
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Because of the relative motion between the cutter teeth and the workpiece, face milling leaves feed
marks on the machined surface (Figure 9.5). Note that the surface roughness of the workpiece
depends on the corner geometry of the insert and the feed per tooth2.
Figure 9.5. Schematic illustration of the effect of insert shape on feed marks on a face–milled surface:
(a) small corner radius, (b) corner flat on insert and (c) wiper, consisting of a small radius followed
by a large radius, resulting in smoother feed marks. (d) Feed marks due to various insert shapes2.
The terminology for a face–milling cutter, as well as the various angles, is shown in Figure 9.6. As
can be seen from the side view of the insert in Figure 9.7, the lead angle of the insert in face milling
has a direct influence on the undeformed chip thickness, as it does in turning operations. As the lead
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angle [positive, as shown in Figure 9.7 (b)] increases, the undeformed chip thickness decreases (as
does chip thickness) and the length of contact (and hence chip width) increases. The lead angle also
influences the forces in milling. It can be seen that as the lead angle decreases, there is a smaller
vertical–force component (axial force on the cutter spindle). The lead angles for most face–milling
cutters typically range from 0° to 45°. Note in Figure 9.7 that the cross–sectional area of the
undeformed chip remains constant2.
Figure 9.6. Terminology for a face–milling cutter2.
Figure 9.7. The effect of the lead angle on the undeformed chip thickness in face milling. Note that as
the lead angle increases, the chip thickness decreases, but the length of contact (i.e., chip width)
increases. The edges of the insert must be sufficiently large to accommodate the contact length
increase2.
In a typical face–milling operation, the ratio of the cutter diameter (D) to the width of cut (w)
should be no less than 3:2 2.
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9.2. Cutting Conditions in Milling
The cutting speed is determined at the outside diameter of a milling cutter. This can be converted
to spindle rotation speed using a formula that should now be familiar1:
𝑁=
𝑣
𝜋𝐷
(9.1.)
The feed (f) in milling is usually given as a feed per cutter tooth; called the Chip Load, it represents
the size of the chip formed by each cutting edge. This can be converted to feed rate by taking into
account the spindle speed and the number of teeth on the cutter as follows1:
𝑓𝑟 = 𝑁𝑛𝑡 𝑓
(9.2.)
fr = Feed Rate (mm/min, in/min)
N = Spindle Speed (rev/min)
nt = Number of Teeth on the Cutter
f = Chip Load (mm/tooth, in/tooth)
Material removal rate in milling is determined using the product of the cross–sectional area of the
cut and the feed rate. Accordingly, if a slab–milling operation is cutting a workpiece with width (w) at
a depth (d), the material removal rate is1
𝑅𝑀𝑅 = 𝑤𝑑𝑓𝑟
(9.3.)
This neglects the initial entry of the cutter before full engagement. Equation 9.3 can be applied to
end milling, side milling, face milling and other milling operations, making the proper adjustments in
the computation of cross–sectional area of cut1.
The time required to mill a workpiece of length (L) must account for the approach distance
required to fully engage the cutter. First, consider the case of slab milling (Figure 9.8). To determine
the time to perform a slab milling operation, the approach distance (A) to reach full cutter depth is
given by1
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𝐴 = √𝑑(𝐷 − 𝑑)
(9.4.)
d = Depth of Cut (mm, in)
D = Diameter of the Milling Cutter (mm, in)
The time (Tm) in which the cutter is engaged milling the workpiece is therefore1
𝑇𝑚 =
𝐿+𝐴
𝑓𝑟
(9.5.)
Figure 9.8. Slab (peripheral) milling showing entry of cutter into the workpiece1.
For face milling, let us consider the two possible cases pictured in Figure 9.9. The first case is when
the cutter is centered over a rectangular workpiece as in Figure 9.9 (a). The cutter feeds from right to
left across the workpiece. In order for the cutter to reach the full width of the work, it must travel an
approach distance given by the following1:
𝐴 = 0.5 (𝐷 − √𝐷2 − 𝑤 2 )
D = Cutter Diameter (mm, in)
w = Width of the Workpiece (mm, in)
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(9.6.)
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If D = w, then Equation 9.6 reduces to A = 0.5D. And if D < w, then a slot is cut into the work and A
= 0.5D 1.
The second case is when the cutter is offset to one side of the work, as in Figure 9.9 (b). In this case,
the approach distance is given by1
𝐴 = √𝑤(𝐷 − 𝑤)
(9.7.)
w = Width of the Cut (mm, in)
In either case, the machining time is given by1
𝑇𝑚 =
𝐿+𝐴
𝑓𝑟
(9.8.)
Figure 9.9. Face milling showing approach and overtravel distances for two cases: (a) when cutter is
centered over the workpiece and (b) when cutter is offset to one side over the work1.
It should be emphasized in all of these milling scenarios that Tm represents the time the cutter
teeth are engaged in the work, making chips. Approach and overtravel distances are usually added at
the beginning and end of each cut to allow access to the work for loading and unloading. Thus the
actual duration of the cutter feed motion is likely to be greater than Tm1.
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9.3. Milling Machines
Milling machines must provide a rotating spindle for the cutter and a table for fastening,
positioning and feeding the workpart. Various machine tool designs satisfy these requirements. To
begin with, milling machines can be classified as horizontal or vertical. A Horizontal Milling Machine
has a horizontal spindle and this design is well suited for performing peripheral milling (e.g., slab
milling, slotting, side and straddle milling) on workparts that are roughly cube shaped. A Vertical
Milling Machine has a vertical spindle and this orientation is appropriate for face milling, end milling,
surface contouring and die sinking on relatively flat workparts1.
Other than spindle orientation, milling machines can be classified into the following types: (1)
knee–and–column, (2) bed type, (3) planer type, (4) tracer mills and (5) CNC milling machines1.
The Knee–and–Column Milling Machine is the basic machine tool for milling. It derives its name
from the fact that its two main components are a Column that supports the spindle and a Knee
(roughly resembling a human knee) that supports the worktable. It is available as either a horizontal
or a vertical machine, as illustrated in Figure 9.10. In the horizontal version, an arbor usually supports
the cutter. The Arbor is basically a shaft that holds the milling cutter and is driven by the spindle. An
overarm is provided on horizontal machines to support the arbor. On vertical knee–and–column
machines, milling cutters can be mounted directly in the spindle without an arbor1.
Figure 9.10. Two basic types of knee–and–column milling machine: (a) horizontal and (b) vertical1.
One of the features of the knee–and–column milling machine that makes it so versatile is its
capability for worktable feed movement in any of the x–y–z axes. The worktable can be moved in the
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x–direction, the saddle can be moved in the y–direction and the knee can be moved vertically to
achieve the z–movement1.
Two special knee–and–column machines should be identified. One is the Universal Milling
machine [Figure 9.11 (a)] which has a table that can be swiveled in a horizontal plane (about a vertical
axis) to any specified angle. This facilitates the cutting of angular shapes and helixes on workparts.
Another special machine is the Ram Mill [Figure 9.11 (b)] in which the toolhead containing the
spindle is located on the end of a horizontal ram; the ram can be adjusted in and out over the
worktable to locate the cutter relative to the work. The toolhead can also be swiveled to achieve an
angular orientation of the cutter with respect to the work. These features provide considerable
versatility in machining a variety of work shapes1.
Figure 9.11. Special types of knee–and–column milling machine: (a) universal – overarm, arbor and
cutter omitted for clarity: and (b) ram type1.
Bed–Type Milling Machines are designed for high production. They are constructed with greater
rigidity than knee–and–column machines, thus permitting them to achieve heavier feed rates and
depths of cut needed for high material removal rates. The characteristic construction of the bed–type
milling machine is shown in Figure 9.12. The worktable is mounted directly to the bed of the machine
tool, rather than using the less rigid knee–type design. This construction limits the possible motion of
the table to longitudinal feeding of the work past the milling cutter. The cutter is mounted in a spindle
head that can be adjusted vertically along the machine column. Single spindle bed machines are called
Simplex Mills, as in Figure 9.12, and are available in either horizontal or vertical models. Duplex
Mills use two spindle heads. The heads are usually positioned horizontally on opposite sides of the
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bed to perform simultaneous operations during one feeding pass of the work. Triplex Mills add a
third spindle mounted vertically over the bed to further increase machining capability1.
Figure 9.12. Simplex bed–type milling machine horizontal spindle1.
Planer Type Mills are the largest milling machines. Their general appearance and construction
are those of a large planer; the difference is that milling is performed instead of planing. Accordingly,
one or more milling heads are substituted for the single–point cutting tools used on planers and the
motion of the work past the tool is a feed rate motion rather than a cutting speed motion. Planer mills
are built to machine very large parts. The worktable and bed of the machine are heavy and relatively
low to the ground and the milling heads are supported by a bridge structure that spans across the
table1.
A Tracer Mill, also called a Profiling Mill, is designed to reproduce an irregular part geometry
that has been created on a template. Using either manual feed by a human operator or automatic feed
by the machine tool, a tracing probe is controlled to follow the template while a milling head
duplicates the path taken by the probe to machine the desired shape. Tracer mills are of two types:
(1) x–y Tracing, in which the contour of a flat template is profile milled using two–axis control; and
(2) x–y–z Tracing, in which the probe follows a three–dimensional pattern using three–axis control.
Tracer mills have been used for creating shapes that cannot easily be generated by a simple feeding
action of the work against the milling cutter. Applications include molds and dies. In recent years,
many of these applications have been taken over by CNC milling machines1.
Computer Numerical Control Milling Machines are milling machines in which the cutter path is
controlled by alphanumerical data rather than a physical template. They are especially suited to
profile milling, pocket milling, surface contouring and die sinking operations, in which two or three
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axes of the worktable must be simultaneously controlled to achieve the required cutter path. An
operator is normally required to change cutters as well as load and unload workparts1.
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