UNIT-I
ELENTARY TREATEMENT OF METAL CUTTING
INTRODUCTION
Design of single point cutting tool is an important aspect of tool engineering. This unit deals with
the design of tool shank, design of single point cutting tool, and various forces involved during
machining of the workpiece. Strength and rigidity of tool is also taken into account while designing
single point cutting tool.
Shank: Shank is main body of a tool. It is held in a holder.
Flank: Flank is the surface or surfaces below and adjacent to cutting edge.
Heel: Heel is intersection of the flank and base of the tool.
Base: Base is the bottom part of the shank. It takes the tangential force of cutting.
Face: Face is surface of tool on which chip impinges when separated from workpiece.
Nose: It is the intersection of side cutting edge and end cutting edge.
ANGLES FORMATION:
End Cutting Edge Angle
The angle between the end cutting edge and a line perpendicular to the shank of tool is
called end cutting edge angle.
Side Cutting Edge Angle:The angle between side cutting edge and side of the tool shank is called side cutting
edge angle. It is also called as lead angle or principle cutting angle.
Side Relief Angle
The angle between the portion of the side flank immediately below the side cutting
edge and line perpendicular to the base of tool measured at right angles to the side flank is known as side
relief angle. It is the angle that prevents interference, as the tool enters the work material.
End Relief Angle
End relief angle is the angle between the portion of the end flank immediately below
the end cutting edge and the line perpendicular to the base of tool, measured at right angles to end flank. It
is the angle that allows the tool to cut without rubbing on the workpiece.
Back Rake Angle
The angle between face of the tool and a line parallel with the base of the tool, measured
in a perpendicular plane through the side cutting edge is called back rake angle. It is the angle which
measures the slope of the face of the tool from the nose toward the rear. If the slope is downward toward
the nose, it is negative back rake angle. And if the slope is downward from the nose, it is positive back
rake angle. If there is not any slope, back rake angle is zero.
Side Rake Angle
The angle between the face of the tool and a line parallel with the base of the tool, measured in a plane
perpendicular to the base and side cutting edge is called side rake angle. It is the angle that measures the
slope of the tool face from cutting edge. If the slope is towards the cutting edge, it is negative side rake
angle. If the slope is away from the cutting edge, it is positive side rake angle.
Nose Radius
Nose radius is provided to increase strength of tip of the tool. This is done by thinning the chip
where it approaches tip of tool and by enlarging the chip over a larger area of the point. It is also
provided to increase the surface finish. If the radius is more, the surface finish will be good. But
due to too large nose radius, contact between tool and workpiece increases, which in turn increase
friction. Thus, power consumption increases, along with increase in vibration and chatter occurs.
Types of Chip Produced in Metal Cutting
The type of chip produced when cutting the metal depends on the material being machined
and the cutting

the tool,

the rate of cutting,

the type or condition of the machine and,

the use or absence of a cutting lubricant.
The types of chip are described in the following order.
a) Continuous
b) Discontinuous or segmental
c) Continuous Built-up edge
a) Continuous

Continuous chips are usually formed at high rake angles and/or high cutting
speeds.

A good surface finish is generally produced.

continuous chips are not always desirable, particularly in automated machine
tools,

tend to get tangled around the tool
 operation has to be stopped to clear away the chips.
b) Discontinuous
or segmental
•
Discontinuous chips consist of segments that may be firmly or loosely
attached to each other
•
These chips occur when machining hard brittle materials such as cast
iron.
•
Brittle failure takes place along the shear plane before any tangible
plastic flow occurs
•
Discontinuous chips will form in brittle materials at low rake angles
(large depths of cut).
c) Continuous

Built-up edge
BUE consists of layers of material from the work piece that are gradually
deposited on the tool.

BUE then becomes unstable and eventually breaks up

BUE material is carried away on the tool side of the chip

the rest is deposited randomly on the work piece surface.

BUE results in poor surface finish reduced by increasing the rake angle and
therefore decreasing the depth of cut
CHIP BREAKERS
•
Long continuous chip are undesirable
•
Chip breaker is a piece of metal clamped to the rake surface of the tool which bends the
chip and breaks it
•
Chips can also be broken by changing the tool geometry, thereby controlling the chip
flow
Fig:Various chips produced in turning: a)tightly curled chip b)chip hits workpiece and
breaks c)continuous chip moving away from workpiece;and d)chip hits tool shank and
breaks off
ORTHOGONAL AND OBLIQUE CUTTING.
ORTHOGONAL CUTTING.

The cutting edge of the tool remains normal to the direction of tool feed or work feed

The direction of the chip flow velocity is normal to the cuttingedge of the tool

Here only two components of the forces are acting: cuttting force and thrust force. So the metal
cutting may be considered as two dimensional cutting.

Orthogonal cutting is a special kind of cutting where the cutting edge is normal to cutting velocity
vector. Being a two dimensional problem in view of forces in two perpendicular directions
OBLIQUE CUTTING

The cutting edge of the tool remains inclined at an acute anle to the direction of tool feed or work
feed

The direction of the chip flow velocity is at angle with the normal to cutting edge of the tool. The
angle is known as chip flow angle.

Here three components of forces are acting : cutting force , radial force and thrust force. So metal
cutting may be considered as three dimensional cutting.

The cutting edge being oblique, the shear force acts on a larger area and tool life is increased.

Oblique cutting is the most common type, normally found in several machining operations. The
cutting edge is inclined to the normal to cutting velocity vector by an angle called the inclination
angle. This type of cutting is also called three dimensional cutting due to forces raised in three
directions perpendicular to one another.
Fig: a)right hand cutting tool.Although these tools have traditionally been produced from
solids tool-steel bars,they have been largely replaced by carbide or other inserts of
various shapes and sizes,as shownin b).The vcarious angles on these tools and their
effects on machining are described
MERCHANT’S FORCE DIAGRAM:Most of the time cutting force acting on a tool is measured experimentally. But it is also important
to predict quantity of cutting force and how different cutting parameters are affecting cutting force even before
setting up the machining operation due to following reasons.
1. In order to design of mechanical structure of cutting machine which will withstand cutting force and thrust
force effectively.
2. To determine power consumption during machining process. This will help in selecting suitable motor
drive.
3. To predict tool life.
4. To increase productivity
.
Next, we can begin to consider cutting forces, chip thicknesses, etc.
First, consider the physical geometry of cutting,
Next, we assume that we are also measuring two perpendicular cutting forces that are
horizontal, and perpendicular to the figure above. This then allows us to examine specific
forces involved with the cutting. The cutting forces in the figure below (Fc and Ft) are
measured using a tool force dynamometer mounted on the lathe.
5.2.1 Force Calculations
The forces and angles involved in cutting are drawn below,
Having seen the vector based determination of the cutting forces, we can now look at
equivalent calculations
The velocities are also important, and can be calculated for later use in power calculations.
The Velocity diagram below can also be drawn to find cutting velocities.
A final note of interest to readers not completely familiar with vectors, the forces F c and Ft,
are used to find R, from that two other sets of equivalent forces are found.,
Merchant's Force Circle
Merchant’s force circle is a method for calculating the various forces involved in the cutting process.
This will first be explained with vector diagram these in turn will be followed by a few formulas.
The procedure to construct a merchant force circle diagram
1. Set up X-Y axis labeled with forces, & the origin iin the centre of the page. The scale should be
enough to include both the measured forces. The cutting forces(Fc) is drawn horizontally , & the
tangential force (Ft) is drawn vertically.
2. Draw in the resultant (R) of Fc & Ft.
3. Locate the centre of R, & draw a circle that encloses vector R. If done correctly, the
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heads and tails of all 3 vectors will lie on this circle.
Draw in the cutting tool in the upper right hand quadrant, taking care to draw the
correct rake angle () from the vertical axis.
Extend the line that is the cutting face of the tool (at the same rake angle) through
the circle. This now gives the friction vector (F).
A line can now be drawn from the head of the friction vector, to the head of the
resultant vector (R). This gives the normal vector (N). Also add a friction angle ()
between vectors R and N. As a side note recall that any vector can be broken down
into components. Therefore, mathematically, R = Fc + Ft = F + N.
We next use the chip thickness, compared to the cut depth to find the shear force.
To do this, the chip is drawn on before and after cut. Before drawing, select some
magnification factor (e.g., 200 times) to multiply both values by. Draw a feed
thickness line (t1) parallel to the horizontal axis. Next draw a chip thickness line
parallel to the tool cutting face.
Draw a vector from the origin (tool point) towards the intersection of the two chip
lines, stopping at the circle. The result will be a shear force vector (Fs). Also measure
the shear force angle between Fs and Fc.
Finally add the shear force normal (Fn) from the head of Fs to the head of R.
Use a scale and protractor to measure off all distances (forces) and angles.
The resulting diagram is pictured below,
CUTTING SPEED:
–
It has the greatest influence on tool life.
–
As the cutting speed increases the temperature also rises.
–
The heat is most concentrated on tool than the work & the hardness of the too,
matrix changes so the relative increase in the hardness of the work accelerates the
abrasive action
–
Speed is the rate of rotation of the spindle where the tool is held. It is measured in
revolutions per minute (RPMs).
The relation of cutting speed to tool life is expresses by
VTn = C
Where V= cutting speed in m per min
T= tool life in min
N=exponent which depends on tool & the work piece.
The value of exponent n is about 0.1 for HSS tool,
0.20 to 0.25 for carbide tools,
0. 4 to 0.55 for ceramic tools.
C= constant which is numerically equals to cutting speed that gives a tool life of one
min.
FEED & DEPTH OF CUT:
The tool life is influenced by the feed rate also with a fine feed the area of chip passing over
the tool face is greater than that of a course feed for a given volume of swarf removal but chip
will greater.
The effect of feed & depth of cut on tool life
V= 257/T 0.19*s 0.36*t 0.08
Where
s= feed in mm per min, t= depth of cut in mm
MACHINABILITY:
Mach inability is a system property that indicates how easy a material can be machined at low
cost. It is customary to speak of mach inability as a material property

Chemical composition of work piece material.

Micro structure

Mechanical properties

Physical properties.

Cutting conditions.
MACHINABILITY INDEX:

The rated mach inability of two or more metals being compared may vary for different
processes of cutting such as heavy turning, light turning, forming, milling, drilling,etc

It an average rating stated in comparison with reference materials. This measure can be
misleading.

The mach inability of different metals to be machined may be compared by using the
mach inability index of each Defined as
MI%= cutting speed of metal investigated for 20 min. tool- life / cutting speed of standard steel
for 20 min. tool-life.
Tool life:
Service time in minutes or seconds to total failure by chipping or cracking of the tool at
certain cutting speed, or the volume of material removed before total failure.
TOOL MATERIAL:

Carbon steels

Medium alloy steels

HSS

Stellites

Cemented carbides

Ceramics

Diamonds

Abrasives