Fundamentals of Metal
cutting and Machining
Processes
Lecture 6-7
1
Contents
A. THEORY OF METAL MACHINING
B. MACHINING OPERATIONS AND
MACHINING TOOLS
C. CUTTING TOOL TECHNOLOGY
Material Removal Processes
A family of shaping operations, the common
feature of which is removal of material from a
starting workpart so the remaining part has the
desired geometry
 Machining – material removal by a sharp
cutting tool, e.g., turning, milling, drilling
 Abrasive processes – material removal by
hard, abrasive particles, e.g., grinding
 Nontraditional processes - various energy
forms other than sharp cutting tool to remove
material
Machining
Cutting action involves shear deformation of work material to form a chip
 As chip is removed, new surface is exposed
(a) A cross-sectional view of the machining
process, (b) tool with negative rake angle;
compare with positive rake angle in (a).
Why Machining is Important



Variety of work materials can be machined
 Most frequently used to cut metals
Variety of part shapes and special geometric
features possible, such as:
 Screw threads
 Accurate round holes
 Very straight edges and surfaces
Good dimensional accuracy and surface finish
Disadvantages with Machining
 Wasteful of material
 Chips generated in machining are wasted
material, at least in the unit operation
 Time consuming
 A machining operation generally takes more
time to shape a given part than alternative
shaping processes, such as casting, powder
metallurgy, or forming
Machining in Manufacturing Sequence
 Generally performed after other manufacturing
processes, such as casting, forging, and bar
drawing
 Other processes create the general shape
of the starting workpart
 Machining provides the final shape,
dimensions, finish, and special geometric
details that other processes cannot create
Speed and Feed
 Speed is rotational motion of spindle which
allows the tools to produce cut into blank
OR the relative movement between tool
and w/p, which produces a cut
 Feed is linear motion of tool which spreads
cut on the blank
OR the relative movement between tool
and w/p, which spreads the cut
Machining Operations
 Most important machining operations:
 Turning
 Milling
 Drilling
 Other machining operations:
 Shaping and planing
 Broaching
 Sawing
Turning
Single point cutting tool removes material from a
rotating workpiece to form a cylindrical shape
Three most common machining processes: (a) turning,
Drilling
Used to create a round hole, usually by means of
a rotating tool (drill bit) with two cutting edges
Milling
Rotating multiple-cutting-edge tool is moved
across work to cut a plane or straight surface
 Two forms: peripheral milling and face
milling
(c) peripheral milling, and (d) face milling.
Cutting Tool Classification
1. Single-Point Tools
 One dominant cutting edge
 Point is usually rounded to form a nose
radius

Turning uses single point tools
2. Multiple Cutting Edge Tools
 More than one cutting edge
 Motion relative to work achieved by rotating

Drilling and milling use rotating multiple cutting
edge tools
Cutting Tools
(a) A single-point tool showing rake face, flank, and tool point; and (b)
a helical milling cutter, representative of tools with multiple cutting
edges.
Cutting Conditions (parameters) in Machining
 Three dimensions of a machining process:
 Cutting speed v – primary motion
 Feed f – secondary motion
 Depth of cut d – penetration of tool into
work piece
 For certain operations, material removal
rate can be computed as
RMR = v f d
where v = cutting speed; f = feed; d =
depth of cut
Cutting Conditions for Turning
Speed, feed, and depth of cut in turning.
Roughing vs. Finishing
In production, several roughing cuts are usually
taken on the part, followed by one or two
finishing cuts
 Roughing - removes large amounts of material
from starting workpart
 Creates shape close to desired geometry,
but leaves some material for finish cutting
 High feeds and depths, low speeds
 Finishing - completes part geometry
 Final dimensions, tolerances, and finish
 Low feeds and depths, high cutting speeds
Machine Tools
A power-driven machine that performs a
machining operation, including grinding
 Functions in machining:
 Holds workpart
 Positions tool relative to work
 Provides power at speed, feed, and depth
that have been set
 The term is also applied to machines that
perform metal forming operations
Chip Thickness Ratio
to
r 
tc
where r = chip thickness ratio; to =
thickness of the chip prior to chip
formation; and tc = chip thickness after
separation
 Chip thickness after cut is always greater than
before, so chip ratio always less than 1.0
Chip Formation
More realistic view of chip formation, showing shear zone rather
than shear plane. Also shown is the secondary shear zone resulting
from tool-chip friction.
Four Basic Types of Chip in Machining
1.
2.
3.
4.
Discontinuous chip
Continuous chip
Continuous chip with Built-up Edge (BUE)
Serrated chip
Type of chip depends on material type and cutting
conditions
Discontinuous Chip
 Brittle work materials
 Low cutting speeds
 Large feed and depth
of cut
 High tool-chip friction
Continuous Chip
 Ductile work materials
 High cutting speeds
 Small feeds and
depths
 Sharp cutting edge
 Low tool-chip friction
Continuous with BUE
 Ductile materials
 Low-to-medium cutting
speeds
 Tool-chip friction
causes portions of chip
to adhere to rake face
 BUE forms, then
breaks off, cyclically
Serrated Chip
 Semicontinuous saw-tooth
appearance
 Cyclical chip forms
with alternating high
shear strain then low
shear strain
 Associated with
difficult-to-machine
metals at high cutting
speeds
Orthogonal Cutting
-
Cutting tool is considered as a wedge
The cutting edge is perpendicular to
cutting speed
Shear plane angle can be calculated using this relation:
r: chip thickness ratio
= to/tc
Orthogonal Cutting- Shear Strain
Example 21.1
Φ
α= 10 deg
1. Shear plane angle: Φ
;
;
2. Shear strain:
Cutting Forces
F: Friction force b/w chip and rake face
N: Normal to friction force F
Fs: Shear force applied by w/p on chip
Fn: Normal to shear force Fs
These force can not be measured directly.
These need to be calculated using force
diagram
Fc: Cutting force acting in direction of
cutting speed
Ft: thurst force acting perpendicular to
Fc. Ft increases with increase in chip
thickness b4 cut
* Fc & Ft both increase as shear
strength of material increases
These force can be measured using
dynamometer
Approximation of Turning by Orthogonal Cutting
Power and Energy Relationships
 A machining operation requires power
 The power to perform machining can be
computed from:
Pc = Fc v
where Pc = cutting power; Fc = cutting force;
and v = cutting speed
Cutting Temperature
 Approximately 98% of the energy in machining is
converted into heat
 This can cause temperatures to be very high at the
tool-chip interface
 The remaining energy (about 2%) is retained as elastic
energy in the chip
 Tool-Chip thermocouple is used for measuring
temperatures in machining
- One wire is linked to tool
- 2nd wire is linked to chip
- Voltage difference is measured and then converted into
current and temp using appropriate relations
Cutting Temperatures are Important
High cutting temperatures
1. Reduce tool life
2. Produce hot chips that pose safety hazards to
the machine operator
3. Can cause inaccuracies in part dimensions
due to thermal expansion of work material
B - MACHINING OPERATIONS AND
MACHINE TOOLS
1.
2.
3.
4.
5.
6.
Turning and Related Operations
Drilling and Related Operations
Milling
Machining Centers and Turning Centers
Other Machining Operations
High Speed Machining
Machining
A material removal process in which a sharp
cutting tool is used to mechanically cut away
material so that the desired part geometry
remains
 Most common application: to shape metal parts
 Most versatile of all manufacturing processes
in its capability to produce a diversity of part
geometries and geometric features with high
precision and accuracy
 Casting can also produce a variety of
shapes, but it lacks the precision and
accuracy of machining
Classification of Machined Parts


Rotational - cylindrical or disk-like shape
Nonrotational (also called prismatic) block-like or plate-like
Machined parts are classified as: (a) rotational, or (b) nonrotational,
shown here by block and flat parts.
Machining Operations and Part Geometry
Each machining operation produces a part
geometry due to two factors:
1. Relative motions between tool and workpart
• Generating – part geometry determined
by feed trajectory of cutting tool
2. Shape of the cutting tool
• Forming – part geometry is created by
the shape of the cutting tool
Generating Shape
Generating shape: (a) straight turning, (b) taper turning, (c) contour
turning, (d) plain milling, (e) profile milling.
Forming to Create Shape
Forming to create shape: (a) form turning, (b) drilling, and (c)
broaching.
Forming and Generating
Combination of forming and generating to create shape: (a) thread
cutting on a lathe, and (b) slot milling.
Turning
A cutting operation in which single point cutting tool removes
material from a rotating work-piece to generate a cylinder
 Performed on a machine tool called a lathe
 Variations of turning performed on a lathe:
 Facing
 Contour turning
 Chamfering
 Threading
A Turning Operation
Close-up view of a
turning operation on
steel using a titanium
nitride coated carbide
cutting insert
Cutting Conditions in Turning
Rotational speed N (rev/min):
Cutting speed at cylinder surface v (m/min)
Final diameter of part:
Feed (mm/rev): f
Feed rate (mm/min): fr
Time to machine:
L: Length of cut/part
Alternatively,
Material Removal rate:
v (m/min); f (m/rev); d (m).
Neglect rotational xtic; v (m3/min)
Operations Related to Turning: Facing
Tool is fed
radially inward
- An operation of
reducing
length/thickness of
stock
Operations Related to Turning: Taper Turning
 Instead of feeding tool
parallel to axis of
rotation, tool is fed at
an angle thus creating
tapered rotational
shape
Operations Related to Turning: Contour Turning
 Instead of feeding tool
parallel to axis of
rotation, tool follows a
contour that is other
than straight, thus
creating a contoured
shape
Operations Related to Turning: Form Turning
 The tool has a certain shape that is
imparted on the w/p by feeding the tooling
radially
Operations Related to Turning: Chamfering
 Cutting edge cuts an angle on the corner
of the cylinder, forming a "chamfer"
 How is the tool motion?
Operations Related to Turning: Cut Off
 Tool is fed radially into rotating work at
some location to cut off end of part
Operations Related to Turning: Threading
 Pointed form tool is fed linearly across surface
of rotating workpart parallel to axis of rotation
at a large feed rate, thus creating threads
Operations Related to Turning: Drilling & Reaming
 Drilling is an operation of making a hole. The
drill (multi-point cutting tool) is fed parallel to
axis of rotation.
 Reaming is an operation of making a drilled
hole accurate and clean.
Operations Related to Turning: Boring
 A single point tool is fed linearly, parallel to the
axis of rotation, on the inside diameter of an
existing hole in the part.
 The purpose of boring is to enlarge the size of
an existing hole
Operations Related to Turning: Knurling
 This is an operation in which regular cross
hatched pattern is imparted on the w/p. This
pattern facilitates holding of a part
 Knurling is not a machining operation, as no
cutting takes place. Instead it is metal forming
operation done in lathe m/c
Engine Lathe
Called engine lathe?
Dates from time
when these
machines were
driven by steam
engines
Types of Lathe:
Horizontal lathe: Used when length
of part is larger than its dia
Vertical Lathe: Used if part dia is
larger than its length and part is
heavy
Lathe Specification:
1.
2.
3.
Center to center distance
Swing dia (2* distance from spindle center to guide-ways)
Weight holding capacity of spindle
Methods of Holding the Work in a Lathe




Holding the work between centers
Chuck
Collet
Face plate
Holding the Work Between Centers
- Work is held b/w
head-stock and tail
stock centers
- Tail-stock center can
be live or dead center
- Live center is held in a
bearing so rotates
- Dead center is fixed on
tailstock shaft, does
not rotate: Result is
friction.
- Used for holding parts
having a large length
to diameter ratio
(a) mounting the work between centers using a "dog”
Holding the Work in a Chuck
- Used when length to dia ratio of w/p is low.
- Can be used with and without support of tail-stock center
- Can hold w/p from outside as well as from inside
- Two types: 3 jaws/ 4 jaws
- 3 jaws is self centering chuck
- For 4 jaws, w/p centering along
the spindle axes is carried manually.
Also, these can handle irregular
stocks
(b) three-jaw chuck
Holding the Work in a Collet
- Collet consists of tubular bushing with longitudinal slits running
over half of its length; and equally spaced around its circumference
- Due to slits, one end of collet can be squeezed to reduce diameter
and provide a secure grasping pressure against the work
Holding the Work in a Face Plate
- Use to clamp irregular w/p (non
cylinders)
- The face plate is fastened to the
lathe spindle
- The face plate has several
slots/holes inside with special
clamps so that irregular shape can be
clamped
(d) face plate for non-cylindrical workparts
Types of Lathe & Turning Machines
1. Turret Lathe
Turret: tool post that can hold many tools
Tailstock replaced by “turret” that holds up to
six tools
-Tools rapidly brought into action by indexing
the turret
-Conventional tool post is replaced by
four-sided turret to index four tools
-Used for high production work that requires a
sequence of cuts on the part
-It is operated manually
Types Lathe & Turning Machines
2. Tool room lathe: small in size used for making precise tools.
3. Speed Lathe: No carriage and cross slide assembly. Tool post is fixed
with lathe bed. This provides high speed. Used for wood turning and
spinning
4- Chucking Machine:
- Uses a chuck to hold a w/p
- Don’t uses tail stock to hold work. So it can handle light weight and
low length w/p
- Operates similar to turret (means have lathe except the feeding of
tools is done automatically
Types Lathe & Turning Machines
5. Bar Machine:
- Similar to chucking m/c, except that a collect (instead of chuck) is
used , which permits long bar stock to be fed through head stock into
position.
- At the end of each machining operation a cut-off operation separates
the new part. The bar stock is then fed forwarded for machining of
next part.
- Two types: Single spindle & Multispindle
Fig.
a. Type of part produced on a 6 spindle m/c
b. Sequence of operations to produce the part: 6
operations are done simultaneously
Types Lathe & Turning Machines
5. CNC Lathe Machine:
- In conventional machines, the machines motions are controlled
through cam (a m/c element)
- In CNC machines, the motion is controlled through a program of
instructions. These instructions are given to servo-motors to further
control the m/c motions
Boring
 Difference between boring and turning:
 Boring is an operation of enlarging inside diameter
of an existing hole
(inside operation)
 Turning is an operation of reducing outside
diameter of a cylinder (outside operation)
 In effect, boring is internal turning operation
 Types of Boring machines
 Horizontal: The rotational axis of w/p is horizontal
 Vertical - The rotational axis of w/p is vertical
Horizontal Boring Mill
Fig. Horizontal boring m/c
- Used when Length of part is larger than its diameter; and
the weight is low
.
Vertical Boring Mill
A boring bar made of
cemented carbide
A vertical lathe or boring m/c
- Used when Length of part is smaller than its Diameter; and
the part is heavy
Drilling
 Creates a round hole in
a workpart
 Compare to boring
which can only enlarge
an existing hole
 Cutting tool called a drill
or drill bit
 Machine tool: drill press
Through Holes vs. Blind Holes
Through-holes - drill exits opposite side of work
Blind-holes – does not exit work opposite side
Two hole types: (a) through-hole, and (b) blind hole.
Cutting Conditions in Drilling
-Cutting speed (v) : mm/min
-Feed (f): mm/rev (f~ drill dia)
-Since there are 02 cutting edges, uncut
chip thickness taken by each cutting edge is
half the feed.
-Feed rate (fr) in mm/min: f×N
- Time to machine a through hole:
-
-Time to machine a blind hole:
- Material removal rate:
=A× fr
Operations Related To Drilling: Reaming
 Used to slightly
enlarge a hole,
provide better
tolerance on
diameter, and
improve surface
finish
Operations Related To Drilling: Tapping
 Used to provide
internal screw
threads on an
existing hole
 Tool called a tap
Operations Related To Drilling: Counter-boring
 Provides a stepped
hole, in which a
larger diameter
follows smaller
diameter partially
into the hole
Operations Related To Drilling: Counter-Sinking
 Similar to counterboring except the
step in a hole is
cone-shaped for the
flat head screws
and bolts
Drill Press
 Upright drill press
stands on the floor
 Bench drill similar
but smaller and
mounted on a
table or bench
Drill Machines
Radial Drill: Large drill press
designed for large parts
- The arm can move radially
- Gang drill machine:
Consists of 2-6 upright
drills. Each spindle is
controlled and operated
separately
- Multiple drill machine:
several spindles are
connected together.
Operated simultaneously
to make multiple hole into
a w/p
Work Holding Devices
- Fixture: is work holding device designed for
clamping a specific shape
- Jig: is work holding device designed for clamping
work as well as for guiding the tool
- Vise: A general purpose work holding device
possessing 02 jaws that grasp the work in
position
Milling
Machining operation in which work is fed past a rotating tool with
multiple cutting edges
Diff b/w Drilling & Milling?
Axis of tool rotation is perpendicular to
feed
Creates a planar surface
Other geometries possible either by
cutter path or shape
Other factors and terms:
Cutting tool called a milling cutter,
cutting edges called "teeth"
Machine tool called a milling
machine
Interrupted cutting operation:
The cutter teeth enter and
exit w/p in each revolution.
This interrupted cutting
imposes sudden loads and
thermal shocks. So teeth design should be robust
Two Forms of Milling
Two forms of milling: (a) peripheral milling, and (b) face milling.
Peripheral Milling vs. Face Milling
 Peripheral milling
 Cutter axis is parallel to surface being machined
 Cutting is performed by cutting edges on outside periphery of
cutter
 Face milling
 Cutter axis perpendicular to surface being milled
 Cutting edges on both the end and outside periphery of the cutter
are used in cutting
Types of Peripheral Milling: Slab Milling
 Basic form of peripheral milling in which the
cutter width extends beyond the work-piece
on both sides
 Width of cutter larger than width of w/p
Types of Peripheral Milling: Slotting
 Width of cutter is less than width of w/p,
creating a slot in the work
 If width of cutter is too
small, the tool will
become a saw and the
operations will be called
sawing
Types of Peripheral Milling: Side Milling
Cutter machines the side of a w/p
Types of Peripheral Milling: Straddle Milling
Cutter simultaneously machines the 02 sides of a w/p
Two Forms of Peripheral Milling
Down- Milling
Up- Milling
-
Cutter rotation and feed are in
opposite direction
Chip length is large
Chip thinner at start than its end
Tool engages in material for long
time
Tool life is smaller
Cutting force is along tangent of
teeth, so force tries to lift the part
-
Cutter rotation and feed are in same
direction
Chip length is small
Chip thicker at start than its end
Tool engages in material for short time
Tool life is longer
Cutting force presses the part. Result is
low vibration and better surface finish
Types of Face Milling: Conventional Face Milling
The cutter overhangs
both side of w/p
slab milling
High speed face
milling using
indexable inserts
Types of Face Milling: Partial Face Milling
The cutter overhangs
one side of w/p
Any difference b/w partial face milling
& side milling?
face milling
side milling
End Milling
 Cutter diameter is
less than work
width, so a slot is
cut into part
 Also diameter of
tool is smaller than
its height
Difference b/w face & end milling?
- In face milling, cutter dia is larger than
its height but in end milling cutter dia
smaller than its height
slotting
End milling: Profile Milling
Form of end milling
in which the
outside periphery
of a flat part is cut
End milling: Pocket Milling
 Another form of
end milling used
to mill shallow
pockets into flat
parts
End milling: Surface Contouring
 Ball-nose cutter fed
back and forth
across work along a
curvilinear path at
close intervals to
create a three
dimensional surface
form
Cutting Conditions in Milling
Rotational speed:
Feed (f): Feed/tooth in mm/tooth/rev
nt: no of teeth
Feed rate:
RMR: (Area of cut × fr)
If w is width of cut; d is depth of cut:
Cut time:
Horizontal Milling Machine
Spindle axis is parallel
to the work surface
Suitable for peripheral
milling
horizontal knee-and-column milling machine.
Vertical Milling Machine
Spindle axis is
perpendicular to
the work surface
Suitable for face milling
vertical knee-and-column milling machine
Machining Centers
Highly automated machine tool can perform multiple
machining operations under CNC control in one
setup with minimal human attention
 Typical operations are milling and drilling
 Three, four, or five axes
 Other features:
 Automatic tool-changing
 Automatic work-part positioning
Types:
Horizontal
Vertical
Universal
Universal machining center; highly automated, capable of multiple
machining operations under computer control in one setup with
minimal human attention
Turning Centers
CNC 4-axis turning center; capable of turning and related
operations, contour turning, and automatic tool indexing, all
under computer control.
Mill-Turn Centers
Highly automated machine tool that can
perform turning, milling, and drilling
operations
 General configuration of a turning center
 Can position a cylindrical work-part at a
specified angle so a rotating cutting tool
(e.g., milling cutter) can machine features
into outside surface of part
 Conventional turning center cannot
hold work-part at a defined angular
position and does not include rotating
tool spindles
Operation of Mill-Turn Center
Operation of a mill-turn center: (a) example part with turned, milled, and drilled
surfaces; and (b) sequence of operations on a mill-turn center: (1) turn
second diameter, (2) mill flat with part in programmed angular position, (3)
drill hole with part in same programmed position, and (4) cutoff.
Shaping and Planing
 Similar operations
 Both use a single point cutting tool moved
linearly relative to the workpart
(a) Shaping, and (b) planing.
Shaping and Planing
 A straight, flat surface is created in both
operations
 Interrupted cutting
 Subjects tool to impact loading when
entering work
 Low cutting speeds due to start-and-stop
motion
 Typical tooling: single point high speed steel
tools
Shaper
Components of a shaper.
Planer
Open side planer.
Broaching
 Moves a multiple tooth cutting tool linearly
relative to work in direction of tool axis
Broaching
Advantages:
 Good surface finish
 Close tolerances
 Variety of work shapes possible
 High material removal rate
Cutting tool called a broach
 Owing to complicated and often
custom-shaped geometry, tooling is expensive
Internal Broaching
 Performed on internal surface of a hole
 A starting hole must be present in the part to
insert broach at beginning of stroke
Work shapes that can be cut by internal broaching; cross-hatching
indicates the surfaces broached.
C - CUTTING TOOL TECHNOLOGY
1.
2.
3.
4.
Tool Life
Tool Materials
Tool Geometry
Cutting Fluids
Cutting Tool Technology
Two principal aspects:
1. Tool material
2. Tool geometry
Three Modes of Tool Failure
1. Fracture failure
 Cutting force becomes excessive at the tool
point, leading to brittle fracture
2. Temperature failure
 Cutting temperature is too high for the tool
material causing softening of tool point. This
leads to plastic deformation and loss of sharp
edge.
3. Gradual wear
 Gradual wearing of the cutting edge causes
loss of tool shape, reduction in cutting
efficiency. Finally tool fails in a manner similar
to temp failure
Preferred Mode: Gradual Wear
 Fracture and temperature failures are
premature failures (how can u avoid these
failures to occur?)
 Gradual wear is preferred because it leads to
the longest possible use of the tool
 Gradual wear occurs at two locations on a tool:
 Crater wear – occurs on top rake face
 Flank wear – occurs on flank (side of tool)
Tool Wear
Crater wear occurs because
of tool chip flow on top
rake face. High friction,
temp and stresses at the
face/chip interface are
responsible. Measured as
area or depth of dip
Flank wear results from
rubbing of flank (& or relief)
face to the newly generated
surface. Measured by width
of wear band called wear
land.
Notch wear occurs
because of tool rubbing
against original work
surface, which is harder
than machined one
Diagram of worn cutting tool, showing the principal locations and types of wear that occur.
Mechanisms of Tool Wear:
Abrasion: This is a mechanical wearing action due to hard
particles in w/p. These hard particles cause gouging and
remove small portions of the tool. It occurs in both crater and
flank wear.
Adhesion: When 02 metals are forced into contact under high
pressure & temp, adhesion or welding occurs b/w them. This
mechanism occurs in crater wear. The chip material welds on
rake face and later this welded mass is removed due to
subsequent chip flow, hence producing dips into the rake face.
Diffusion: This is a process in which an exchange of atoms
take place across a close contact boundary (like chip-rake face)
. At high temp, the atoms responsible for tool hardness diffuse
from tool into chip, thus softening top surface of tool. Later this
promotes both abrasion and adhesion at rake face. Diffusion
causes crate wear.
Chemical Reactions: At high speeds, due to high temp at the
chip-rake interface, oxidation layer form. This layer is sheared
down and a new layer is formed. This process continues and
causes crater wear.
Plastic Deformation: At high temp, the plastic deformation of
tool nose and cutting edge takes place. This further promotes
abrasion. This is major reason for flank wear.
Crater wear
Flank wear
Tool Wear vs. Time
-The tool
performance is
dictated by uniform
wear rate (or slop of
steady state region).
-The slop of steady
state region changes
with change in
cutting conditions.
- Speed is the major
influential parameter
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 curve.
Effect of Cutting Speed on Wear
Effect of cutting speed on tool flank wear (FW) for three cutting speeds
Tool Life
 Length of cutting time that the tool can be used.
- Time till tool fracture?
- If so, tool needs to re-sharp again and again. This is not so easy in
production. Also, re-sharpening will affect surface finish
- Better to define a
a level of tool wear ( say 0.5)
- Tool life against each
curve is shown in Fig.
Tool Life vs. Cutting Speed
Natural log-log plot of cutting speed vs tool life.
Taylor Tool Life Equation
Relationship is credited to F. W. Taylor
n
vT  C
where v = cutting speed; T
= tool life; n is the slope of
the plot; C is the intercept
on the speed axis at one
minute tool life
C
n
n and C are parameters
that depend on feed,
depth of cut, work
material, tooling material,
and the tool life criterion
used
Tool Life Criteria in Production
Practically, it is not always easy to measure flank wear (0.5mm) and
time to know TOOL LIFE. Therefore, in shops any of these criterion can
be used for changing a tool:
1. Complete failure of cutting edge
2. Visual inspection of flank wear (or crater wear) by the machine
operator
3. Fingernail test across cutting edge
4. Changes in sound emitted from operation
5. Chips become ribbon-like, stringy, and difficult to dispose off
6. Degradation of surface finish
7. Increased power
8. Work-piece count: Dispose off tool after certain no of pieces
9. Cumulative cutting time
Tool Materials
 Tool failure modes identify the important
properties that a tool material should possess:
 Toughness - to avoid fracture failure
 Hot hardness - ability to retain hardness at
high temperatures
 Wear resistance - hardness is the most
important property to resist abrasion
Hot Hardness
Typical hot hardness relationships for selected tool materials. Plain
carbon steel shows a rapid loss of hardness as temperature
increases. High speed steel is substantially better, while cemented
carbides and ceramics are significantly harder at elevated
temperatures.
Typical Values of n and C
Tool material
High speed steel:
Non-steel work
Steel work
Cemented carbide
Non-steel work
Steel work
Ceramic
Steel work
vT n  C
n
C (m/min)
C (ft/min)
0.125
0.125
120
70
350
200
0.25
0.25
900
500
2700
1500
0.6
3000
10,000
High Speed Steel (HSS)
Highly alloyed tool steel capable of maintaining
hardness at elevated temperatures better than
high carbon and low alloy steels
 One of the most important cutting tool
materials
 Especially suited to applications involving
complicated tool geometries, such as drills,
taps, milling cutters, and broaches
 Two basic types (AISI)
1. Tungsten-type, designated T- grades
2. Molybdenum-type, designated M-grades
High Speed Steel Composition
 Typical alloying ingredients:
 Tungsten and/or Molybdenum
 Chromium and Vanadium
 Carbon, of course
 Cobalt in some grades
 Typical composition (Grade T1):
 18% W, 4% Cr, 1% V, and 0.9% C
Cemented Carbides
Class of hard tool material based on tungsten
carbide (WC) using powder metallurgy
techniques with cobalt (Co) as the binder
 Two basic types:
1. Non-steel cutting grades - only WC-Co
2. Steel cutting grades - TiC and TaC added
to WC-Co
Cemented Carbides – General Properties
 High compressive strength but
low-to-moderate tensile strength
 High hardness (90 to 95 HRc)
 Good hot hardness
 Good wear resistance
 High thermal conductivity
 High elastic modulus - 600 x 103 MPa
 Toughness lower than high speed steel
Non-steel Cutting Carbide Grades
 Used for nonferrous metals and gray cast iron
 Properties determined by grain size and cobalt
content
 As grain size increases, hardness and hot
hardness decrease, but toughness
increases
 As cobalt content increases, toughness
improves at the expense of hardness and
wear resistance
Steel Cutting Carbide Grades
 Used for low carbon, stainless, and other
alloy steels
 TiC and/or TaC are substituted for some of
the WC
 Composition increases crater-wear
resistance for steel cutting
 But adversely affects flank wear
resistance for non-steel cutting
applications
Cermets
Ceramic-metal composite
Cemented carbide is a kind of cermet
Combinations of TiC, TiN, and titanium carbonitride
(TiCN), with nickel and/or molybdenum as binders.
 Some chemistries are more complex
 Applications: high speed finishing and semifinishing of
steels, stainless steels, and cast irons
 Higher speeds and lower feeds than steel-cutting
carbide grades
 Better finish achieved, often eliminating need for
grinding
Coated Carbides
Cemented carbide insert coated with one or
more thin layers of wear resistant materials,
such as TiC, TiN, and/or Al2O3
 Coating applied by chemical vapor
deposition or physical vapor deposition
 Coating thickness = 2.5 - 13 m (0.0001 to
0.0005 in)
 Applications: cast irons and steels in turning
and milling operations
 Best applied at high speeds where dynamic
force and thermal shock are minimal
Coated Carbide Tool
Photomicrograph
of cross section of
multiple coatings
on cemented
carbide tool
Co/Ni
WC/TiC
Ceramics
Primarily fine-grained Al2O3, pressed and sintered
at high pressures and temperatures into insert
form with no binder
 Applications: high speed turning of cast iron
and steel
 Not recommended for heavy interrupted cuts
(e.g. rough milling) due to low toughness
 Al2O3 also widely used as an abrasive in
grinding
Synthetic Diamonds
Sintered polycrystalline diamond (SPD) fabricated by sintering very fine-grained
diamond crystals under high temperatures
and pressures into desired shape with little
or no binder
 Usually applied as coating (0.5 mm thick)
on WC-Co insert
 Applications: high speed machining of
nonferrous metals and abrasive nonmetals
such as fiberglass, graphite, and wood
 Not for steel cutting, Why??
Cubic Boron Nitride
 Next to diamond, cubic boron nitride (cBN) is
hardest material known
 Fabrication into cutting tool inserts same as
SPD: coatings on WC-Co inserts
 Applications: machining steel and nickel-based
alloys
 SPD and cBN tools are expensive
Tool Geometry
Two categories:
 Single point tools
 Used for turning, boring, shaping, and
planing
 Multiple cutting edge tools
 Used for drilling, reaming, tapping,
milling, broaching, and sawing
Tool Geometry- Single Point Cutting Tool
Chip breaker
Tool Geometry: Multi-Point Cutting Tool
Plain/ Peripheral Milling Cutter
Tool Geometry: Multi-Point Cutting Tool
Twist dill
 The "business end" of a twist drill has two cutting
edges The included angle of the point on a
conventional twist drill is 118°
 Margins are the outside tip of the flutes and are
always ground to the drill diameter
Twist Drills
 An essential feature of drilling is the variation in
cutting speed along the cutting edge. The speed is
maximum at the periphery, which generates the
cylindrical surface, and approaches zero near the
center-line of the drill where the cutting edge is
blended to a chisel shape.
 Drills are slender, highly stressed tools, the flutes of
which have to be carefully designed to permit chip
flow while maintaining adequate strength.
Twist Drill Operation - Problems
 Chip removal
 Flutes must provide sufficient clearance to
allow chips to be extracted from bottom of
hole during the cutting operation
 Friction makes matters worse
 Rubbing between outside diameter of drill
bit and newly formed hole
 Delivery of cutting fluid to drill point to
reduce friction and heat is difficult because
chips are flowing in opposite direction
Cutting Fluids
Any liquid or gas applied directly to machining operation to improve
cutting performance
 Two main problems addressed by cutting fluids:
1. Heat generation at shear and friction zones
2. Friction at tool-chip and tool-work interfaces
 Other functions and benefits:
 Wash away chips (e.g., grinding and milling)
 Reduce temperature of workpart for easier handling
 Improve dimensional stability of workpart
Classification of Cutting Fluids by
Functions
Cutting fluids can be classified according to
function:
 Coolants - designed to reduce effects of heat
in machining
 Lubricants - designed to reduce tool-chip and
tool-work friction
Coolants
 Water is used as base in coolant-type cutting
fluids
 Most effective at high cutting speeds where
heat generation and high temperatures are
problems
 Most effective on tool materials that are most
susceptible to temperature failures (e.g., HSS)
Lubricants
 Usually oil-based fluids
 Most effective at lower cutting speeds
 Also reduce temperature in the operation
Dry Machining
 No cutting fluid is used
 Avoids problems of cutting fluid contamination,
disposal, and filtration
 Problems with dry machining:
 Overheating of tool
 Operating at lower cutting speeds and
production rates to prolong tool life
 Absence of chip removal benefits of cutting
fluids in grinding and milling
Gear Cutting
 Gear cutting is the process of creating a gear. The most
common processes include hobbing, broaching, and
machining; other processes include shaping, forging,
extruding, casting, and powder metallurgy.
 Hobbing is a machining process for making gears, on a
hobbing machine,
 The teeth or splines are progressively cut into the
workpiece by a series of cuts made by a cutting tool called
a hob.
 Compared to other gear forming processes it is relatively
inexpensive but still quite accurate, thus it is used for a
broad range of parts and quantities
Hobbing
Hobbing Process
 Hobbing uses a hobbing machine with two non-parallel spindles,
one mounted with a blank workpiece and the other with the hob.
 The angle between the hob's spindle and the workpiece's spindle
varies, depending on the type of product being produced.
 If a spur gear is being produced, then the hob is angled equal to
the helix angle of the hob; if a helical gear is being produced
then the angle must be increased by the same amount as the
helix angle of the helical gear
Hobbing Process
 The two shafts are rotated at a proportional
ratio, which determines the number of teeth
on the blank; for example, if the gear ratio is
40:1 the hob rotates 40 times to each turn
of the blank
 The hob is then fed up into workpiece until
the correct tooth depth is obtained.
 Finally the hob is fed into the workpiece
parallel to the blank's axis of rotation
Hob
 The hob is the cutter used to cut the
teeth into the workpiece.
 It is cylindrical in shape with helical
cutting teeth. These teeth have grooves
that run the length of the hob, which aid
in cutting and chip removal.
 The cross-sectional shape of the hob
teeth are almost the same shape as
teeth of a rack gear that would be used
with the finished product
Assignment No. 2
 What is Powder Metallurgy? What are its capabilities?
What are the common Powder Compacting
techniques? How is Sintering performed?
 Last date of submission: 01- 06-2014
 Any two mutually copied assignments will be cancelled