__theory_of_metal_cutting.pdf

By Prof.A.Chandrashekhar
Workshop Practice
TA 102
Lec – 6 & 7 :Theory of Metal Cutting

Theory of Metal cutting
 INTRODUCTION:
The process of manufacturing a component
by removing the unwanted material using a machine is
known as Machining and when used with metals it is
referred to as metal machining or metal cutting.
The raw material which is subjected to the
process of machining is called work piece or stock.

The unwanted material removed from the raw
material to give it the required shape is known as the chip or
scrap.
The device used to cut the material is known as
cutting tool or simply tool.
“ a machine tool is a power driven machine that shapes metals to
produce component”

RAW MATERIAL FINISHED PRODUCT
Metal to be cut
Unwanted material
to be removed
Getting desired shape by metal cutting (machining process)

The metal cutting process (machining) consists of removing a
layer of metal from the work piece. The required shape and
dimensions of the part with the specified surface finish are obtained
by repetitively removing layers of the material from the work
piece.

Machining process is necessary for the following
reasons
1. Closer dimensional accuracy may be required than is
available from casting, forming, or other shaping processes.
2. Some parts are heat treated to improve hardness and
wear resistance. Since heat treated parts may undergo
distortion,they generally require additional finishing
operations, such as grinding, to obtain the desired final
dimensions and surface finish.

3. Special surface characteristics or texture which cannot be
produced by other means may be required on all or parts of the
surfaces of the product. Machining with a diamond cutting tool,
for example, makes copper mirrors with very high reflectivity.
4. Machining a part may be economical than manufacturing it by
other methods of manufacturing, especially, if the required
quantity is small.
5. Shape and/or size of a component may not be suitable for
economic production by other methods.

Machine Tools
machine tools are the kind of machines
on which the metal cutting or metal forming
processes are carried out.
The functions of machineTool are
Holding the work piece
Holding the tool
Moving the tool or the work piece or both relative to
each other and
Supply energy required to cause the metal cutting

Machine Tools
A machine is a device, which converts some form of input in
to output.
A machine tool imparts the required shape to the work piece
with the desired accuracy by removing the metal from the work
piece in the form of chips.
All machine tools that we use in manufacturing can be
broadly classified in to:
1. Metal cutting machine tools
2. Metal forming machine tools

Machine Tools
Machine tools can be classified in to different
categories based upon several criteria. some of these are
as follows:
Degree of specialization
1. General purpose machine tool like lathe, shaper, milling
machine.
2. Special purpose machine tools like gear-cutting (hobbing)
machines.
Type of surface produced
1. Cylindrical surface producing machine tools like lathe,
drilling machine.
2. Flat surface producing machine tools like milling
machine, shaper.

Machine Tools
Type of motion
1. Reciprocating type of machine tools like shaper, planer,
slotter.
2. Rotary type of machine tools like lathe, drilling machine,
grinding machine.
Degree of Automation
1. Manual control machine tools.
2. Semi – automatic machine tools
3. Automatic machine tools
Duty cycle
1. Light duty machine tools
2. Medium duty machine tools
3. Heavy duty machine tools

Machine Tools
Type of energy used
1. Conventional machine tools that use mechanical energy e.g.,
lathe, drilling machine, milling machine, shaper.
2. Non-conventional machine tools which may use chemical,
electro – discharge energy etc.,

Cutting Tools
Cutting tool is a device used to remove the
unwanted material from a given work piece.
1. What is the mechanism of metal cutting and what
properties a cutting tool material should possess?
2. What should be its geometry (size, shape and angles) for
efficient cutting?
3. How cutting tools are classified ?

Mechanism of metal cutting
Essential Properties of cutting tool are :
 Hard and tough
 Strong
 Heat resistance to maintain hardness at high temperature and
 Wear resistance to prevent fast wear of cutting edge.

TOOL MATERIAL
The performance of a cutting tool material in given
machining application is mainly determined by properties
such as wear resistance, hot hardness and toughness.
To cater to the need of industry e.g.,
higher rates of production, good surface finish,
close tolerances, etc various tool materials have
been developed

TOOL MATERIAL
 Tool steel
 HSS (High speed steel)
 Carbides
 Abrasives
 Diamond
 CBN Cubic Boron Nitride
 Ucon
 Ceramics
 Tipped tools
 Coated tools

Tool steel (TS)
Carbon steel or carbon tools steel is the kind of steels
having carbon percentage ranging from 0.8 – 1.5%.
Tools made from carbon steels can be easily hardened
and used for cutting ferrous and non-ferrous metals.
Disadvantages of carbon steel are its less hot hardness
( ability of a material to withstand high temperatures)
and poor wear resistance.They loose their hardness at
around 200 – 2500C Operate at low cutting speeds.

Effect of alloying element on steel
Alloying Element
Mn
Ni
Cr
P
S
Si
Effect of alloying element
Improves machining ability and such type of steels
are known as free coating steels
Improves strength, toughness and resistance to corrosion
Improves strength, toughness and resistance to corrosion
Causes brittleness and its presence is undesirable
Reduce hot hardness and its presence is undesirable
Makes the steel tough and harder

High Speed Steel (HSS)
 Alloying steel with tungsten, chromium, molybdenum, etc
produce alloy steel known as High speed steel (HSS).
 It can retain hardness up to 6000C
 It can operate at cutting speeds 2-3 times greater than that of
carbon steels.
 HSS can be classified in to three types based on alloying
elements and their percentage.

18-4-1 HSS
 This alloy steel contains 18% tungsten, 4% chromium and
1% vanadium.
 This type of steel provides good hot hardness and form stability.
 This type of steel is also known as tungsten based HSS.

8-4-1 HSS
 This alloy steel contains 8% Molybdenum, 4% chromium
and 1% vanadium..
 It has excellent toughness and cutting ability.
 This type of steel is also known as Molybdenum based HSS.

Cobalt based HSS
 This steel contains 2-15% cobalt to increase the hot hardness
and wear resistance.
 It can operate at very high speeds.
 This steel is also known as super high- speed steel.

Effect of alloying element on HSS
Alloying
Element
Tungsten
Molybdenum
Vanadium
Cobalt
Effect of alloying element
Increases hot hardness, wear resistance and
form stability
Increases hot hardness, maintains keenness
of cutting edge
Increases wear resistance
Increases wear resistance and hot hardness

Carbides
 The basic ingredient of most carbides is tungsten carbide
 Carbides suitable for steel machining consists of 82%
tungsten carbide, 10% titanium carbide and 8% cobalt
 Carbides have high hardness over a wide range of
temperatures and have relatively high thermal conductivity.

Abrasives
 Abrasives are mainly used for machining harder material
and/or where a superior finish is required.
 Abrasive particles are used in the manufacture of grinding
wheels.

Diamond
 It is the hardest known material and can be used at
cutting speeds about 50 times those used for HSS
 Under proper conditions, diamond can machine 10,000
– 50,000 pieces or some times even 1,00,000 pieces in a
single setup where as carbides can machine only 300 –
400 pieces before requiring a re-sharpening or
replacement.
 Diamond perform well at highest possible speeds, low
feeds and low depth of cut.

Cubic boron nitride (CBN)
 Diamond being a form of carbon is not thermally very stable and
at high temperature readily reacts with iron.
Cubic boron nitride has been developed as an alternative to
diamond, for machining ferrous materials.
 CBN is the second hardest material known and has red hardness
up to 10000C

Ucon
 Ucon is an alloy of 50% columbium, 30 % titanium, and
20% tungsten.
 It has excellent thermal shock resistance, high hardness and
toughness.
 It gives 3-5 times more tool life than conventional carbides.

Ceramics
 It is an inexpensive tool material and in many cases, they
are efficient substitutes for cemented carbides.
 Their main constituent is aluminium oxide.
 Ceramic tool can cut almost any metal and withstand
high heat, but they are very brittle and will not take
shock.
 Ceramic tools can give extremely fine finishes to the
surface of a material.
 The red hardness of ceramic tool is of the order of
12000C.

Tipped Tools
 In case of alloy steel and HSS, the tool is made as one
piece. In the case of carbides, CBN and diamond , the
tool is not made as single piece owing to increased
cost, non availability and difficulty of manufacturing. In
these cases, only the cutting edges of the tool are made
from these materials and are brazed or clamped to the
tool shank.

Tool insert
Shank
Brazing
A tipped tool with tool insert Brazed

Coated Tools
 The performance of HSS tools can be improved by giving
coating of carbides over cutting edges .
 Giving a very thin coating of 5-7μm thickness of a carbide
material can improve the wear resistance, hardness, hot
hardness etc.,
 Coating material used for HSS include titanium, carbide,
titanium nitride and so on.

Types of Tools
All the cutting tools used in metal cutting can be broadly
classified in to two categories
1. Single point cutting tool
2. Multi point cutting tool
A single point tool has only one cutting
edge.These types of tools are used in
lathes, shapers and planers.

Types of Tools
single point tool

A multi point cutting tools have more than
one cutting edge.
Example include milling cutter, drills,
reamers.
Types of Tools

Tool Geometry
The tool geometry implies the size and
shape of the tool.
Material
Sharp Edge
Work
Chisel
Force
Cutting of material with a Chisel

θ
∞
Chip
Force
Setting Angle
∞ = Rake Angle
Θ = Clearance angle
Three Basic angles of a tool

Geometry of Single –Point Tool
face
flank
Side Cutting edge
End cutting
edge
End
Flank
Nose radius

Geometry of Single –Point Tool
face
flank
a
c
f
e
b
d
a = side rake angle
b= End cutting edge angle
c = side cutting edge angle
d = Back rake angle
e = End relief angle
f = side relief angle

Single point cutting Tool
 A single point cutting tool consists of a neck which is known
as operating end, and the shank or body.
 Shank is used to hold the tool in the tool
post or tool holder.
 The tool neck has the following elements:
Face, Flank, Cutting edges and nose.

Face: it is the surface on which the chip impinges and along which
it flows as it is separated from the work.
Flanks: the flanks are two surfaces of the tool facing the work.They
are called the side or main flank and the end or auxiliary flank.
Cutting Edge: they are formed by intersections of the face and the
flanks.They are called the side or main cutting edge and end
cutting edge.

Nose: The nose is the element formed at the junction of the
side and end cutting edges. This junction or the nose has a
curve of small radius, known as nose radius.

Cutting Tool Angles
Back Rake Angle:
Commonly called as rake angle, is the angle between
the face of the cutting tool and the normal to the machined
surface at the cutting edge.
Rake angle may be +ve, zero or –ve.

Cutting Tool Angles
 Side rake angle
it is the angle between the tool face and a line parallel
to the base of the tool and is measured in a plane
perpendicular to the base and the side cutting edge. This
angle gives the slope of the face of the tool from the cutting
edge.
Relief angle:
it is the angle between the flank of the cutting tool and
the tangent to the machined surface of the cutting edge. The
relief angle enables the flank of cutting tool to clear the work
piece surface and prevent rubbing.

Cutting Tool Angles
 Side cutting edge angle:
angle formed by the side cutting
edge with normal to machined surface. it is essential
for enabling the cutting tool at the start of cut to first
contact the work back from the tool tip.
End cutting edge angle:
the angle formed by the end cutting
edge with the machined surface. It provides a
clearance for that portion of the cutting edge, which
is behind the nose radius.this reduces the length of
cutting edge in contact with the work.

TYPES OF CHIPS
1. DISCONTINUOUS OR SEGMENTAL CHIPS
2. CONTINUOUS CHIPS
3. CONTINUOUS CHIPSWITH BUILTUP EDGE

CONTINUOUS CHIPS
continuous chips are formed while machining ductile
material like mild steel.
a continuous chip comes from the cutting edge of a
cutting tool as single one piece, and it will remain as
one piece unless purposefully broken.
 it will remain as one piece unless purposefully broken. it
is done by using chip breakers.

CONTINUOUS CHIPS WITH BUILTUP EDGE
 during cutting operation, the temperature rises and as hot
chip passes over the face of the tool, alloying and welding
action take place due to high pressure, which results in
builtup edge. it forms false cutting edge. it gives rough
surface.
 the presence of builtup edge is undesirable.

DISCONTINUOUS OR SEGMENTAL CHIPS
 in discontinuous or segmental chips, the chip is produced in
the form of small pieces. these type of chips are obtained
while machining brittle material like cast iron.
 good surface finish and tool life

THERMAL ASPECTS IN MACHINING
 knowledge of temperature rise is essential because
1. the rise in temperature adversely affects the properties of the
tool material
2. increased temperature causes dimentional changes in the
workpiece and hence th accuracy of machining

3. increased temperature can even distort the
accuracy of the machine tool itself.
the heat generated depends on the rate of metal cutting, cutting
speed, specific heat and thermal conductivity of the
workpiece and tool materials.

Q = Q1+Q2+Q3+Q4
AMOUNT OF HEATTAKENAWAY BY
Q1=THE CHIPS
Q2=AMOUNT OF HEAT CONDUCTED INTOTHETOOL
Q3=AMOUNT OF HEAT CONDUCTED INTO
WORKPIECE
Q4=AMOUNT OF HEAT DISSIPATEDTOTHE
SURROUNDINGS

 th amount of heat dissipated on the surrounding is very
small.
 the chip carries much of the heat generated.
 q1:q2:q3=80:10:10

FAILURE OF TOOL
1. CATASTROPHIC FAILURE
it will occur when the cutting force acting on the tool
exceeds the critical strength of the tool material, and tool
fails without giving any indication.
2. GRADUAL OR PROGRESSIVEWEAR
it may happen because of creater formation leading to
creater wear or flank wear.

CRATER WEAR
 crater wear occurs on the rake face of the cutting tool
and it occurs at a distance from cutting edge.
 crater wear is a result of rubbing between the chip and
the rake face of the tool.
 as the crater wear progresses, the cutting edge becomes
weaker and it may lead to chipping of cutting edge .
 crater wear can be reduced by using chip breakers

FLANK WEAR
 flank wear occurs on the face of the tool. flank wear is due to
continuous contact between the newly machined surface and flank
face of the tool.

Cutting Fluids
The functions that a cutting fluid is expected to fulfill are:
1. Absorb and carry away the generated heat, thereby
lowering the cutting zone temperature.
2. Cool the work piece and tool,
3. Reduce friction between tool and work piece by acting as
a lubricant( this reduces the heat generation),
4. Wash away the chips,

Cutting Fluids
5. Carry away the built-up edges formed,
6. Give very fine surface finish to the workpiece, and
7. Prevent corrosion of the machined surface, tool and
machine.

Operating conditions in Metal Cutting
 The term operating conditions include the parameters
such as
a) Cutting speed
b) Feed and
c) Depth of cut

Cutting speed,ν
 Cutting speed is the distance traveled by the work surface in
a unit time with reference to the cutting edge of the tool.
 Usually expressed in m/min

Feed, f
 The feed is the distance advanced by the tool (assuming job
remains stationary) for each revolution of the work.
 Expressed in mm/rev

Depth of Cut, d
 It is the measured perpendicular distance between the
machined surface and the unmachined (uncut) surface or the
previously machined surface of the workpiece.
 Expressed in mm.

Material Removal Rate and Machining Time
 The total material to be removed, cutting speed, feed and
depth of cut determine the material removal rate and the
time required to machine a work piece , the machining time.

Machining Time
 The time required to make one cut or pass on the work piece
 Total machining time = machining time X no. of cuts

Tool Life
 Tool Life is defined as the time for which a tool can cut
effectively or it is the time between two successive re-
sharpenings of a cutting tool.
 The tool life is a function of cutting speed as shown in figure:

Taylor’s expression or Tool life equation
νTn = C
ν = cutting speed in m/min
T =Tool life in minutes
C = a constant which depends on the tool and work material
n = slope of the curve which depends on tool and work
material

Taking consideration the values of depth of cut and feed rate
are
Where n1 and n2 constants

Machinability
 The term machinability refers to the evaluation of the
work materials with reference to machining.
 Machinability may be defined as the ease or difficulty
with which a material can be machined under a given
set of conditions.
 If vs is the specific cutting speed to produce a tool life
T for a standard material, vt is the cutting speed to
produce a tool life T for a test material,
Machinability or Machinability index
= vt/vs X 100%

Problems
Theory of metal cutting

Problem 1
While machining cast iron using a high speed steel
tool, tool life of 50 minutes was observed when
machined with a cutting speed of 100 m/min.
determine
a) GeneralTaylor‟s tool life equation, and
b) Tool life for a cutting speed of 80 m/min.
Assume n=0.09 inTaylor's tool life expression.

Problem 2
 UsingTaylor‟s equation for tool life and taking n=0.5 and
c=300, calculate the change in tool life when the cutting
speed is reduced by 25%.

This indicates that the tool life change is
T2-T1/T1 =T2/T1 – 1 = 1.78-1 = 0.78
i.e., tool life increases by 78% when cutting speed is reduced by
25%

Problem 4.3
 A carbide- cutting tool when used for machining mild steel
work piece material at a cutting speed of 50m/min, lasted
for 100 minutes. Determine a) life of the tool when the
cutting speed is increased by 25% and b) cutting speed of the
tool to get a tool life of 180 minutes.Assume n=0.26 in the
Taylor‟s expression.

Problem 4.4
Work material Tool life in Min Cutting speed m/min
A 25
10
100
150
B 40
20
200
250
 In assessing machinability of different work piece materials, the
data obtained during machining is given below. Determine the
cutting speed for„A‟ and„B‟ to get a tool life of 50 min.
Estimate the relative machinability, considering material„A‟ as
standard material.

Problem 27
 During the machining of low carbon steel with HSS cutting
tool, it was found that for cutting speeds of 40 and 50m/s
tool life was 40 and 10 minutes respectively. Derive v-T
relationship.

Problem 28
 The tool life for a high speed steel tool is expressed by the
relation vT1/7=C1, and for tungsten carbide is expressed as
vT1/5=C2. if at a speed of 24m/min the tool life is 128
minutes, compare the tool life of the two tools at a speed of
30m/min.
 Ans:T1/T2=0.64

Problem 29
 A tool life of 80 minutes is obtained at a speed of 30m/min
and 8 minutes at 60m/min. Determine the following a)Tool
life equation b) Cutting speed for 4 minutes tool life
Ans: a)vT0.3=C b) 73.69 m/min

Problem 30
 A carbide tool while machining a mild steel work piece was
found to have a life of 1 hour and 40 min when cutting at
50m/min. Find the tool life if the tool is to operate at a
speed 30% higher than previous one.Also, calculate the
cutting speed if the tool is required to have a life of 2hours
and 45 minutes.Assume n=0.28
 Ans:T=39 minV=43.4 m/min

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