Theory of Metal Cutting
Dr.A.Suresh babu,Ph.D
Asst.Professor
College of Engineering,Guindy
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UNIT I FUNDAMENTALS OF METAL CUTTING
10
Mechanics of orthogonal and oblique cuttingMechanics of chip formation-Types of chips
produced in cutting- Cutting forces and powerTemperature in cutting-Tool life –numerical
problems-Wear and failure-surface finish and
integrity- Machine tools structures-Vibration and
chatters in machining-machining economics Cutting tools steels, cobalt alloys, coated tools Diamond tools -Cutting fluids.
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Contents
1. Common
processes,
features
of
machining
2. geometry of single point tool and tool
signature
3. concept of speed, feed and depth of cut
4. Temperature in cutting
5. Tool life
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Mechanics of Metal Cutting
Tool must be sharp (what do you mean by sharp?
Relative velocity
Interference
Tool material shall be harder than the work piece
material
Physical Phenomenon in Machining
Plastic flow
Fracture
Friction
Heat
Molecular diffusion
Chatter
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At extreme condition
• Sticking friction at tip
• Deformation at high strain
and strain rate
• Nascent surface exposed
after deformation is very
active
4
Objectives During Machining
High Material Removal Rate (MRR)
Good accuracy and Surface finish
Long tool life
Cost
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Processing Parameters in Machining
Machine Related
Depth of cut
Spindle speed
Feed rate
Cutter Related
Material
Geometry
Mounting
Workpiece Related
Material (composition, homogeneity)
Geometry (bar, block, casting etc.)
Mounting
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Others
– Cutting fluid type and application
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Effects of Processing Parameters
Cutting forces and
Torques and power
Tool temperature
Frictional
effects
Work hardening
Thermal softening
Hot spots on the
machined surface
on tool face
Built
up
edge
Formation
Chatter, noise and
Vibrations
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Deflection and
diameter variations
Tool life
Surface finish
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Tool signature
•
The numerical code that describes all the key
angles of a given cutting tool is called tool
signature
•
Convenient way to specify tool angles by use of
standardized abbreviated system is known as
tool signature or tool nomenclature. The tool
signature comprises of seven elements and is
specified in different systems .
Systems of description of tool geometry
1. Tool-in-Hand System
2. Machine Reference System - ASA system
3.
Tool Reference Systems - Orthogonal Rake
System - ORS
1. Tool-in-Hand System
• There is no quantitative information, i.e.,
value of the angles.
Tool signature
• seven element defining the tool signature
2. Machine Reference System - ASA system
• ASA ( American Standards Association)
system
ASA system
• πR = Reference plane; plane perpendicular to
the velocity vector
• πX = Machine longitudinal plane; plane
perpendicular to πR and taken in the direction
of assumed longitudinal feed
• πY = Machine Transverse plane; plane
perpendicular to both πR and πX [This plane is
taken in the direction of assumed cross feed]
• The axes Xm, Ym and Zm are in the direction of
longitudinal feed, cross feed and cutting
velocity (vector) respectively.
The main geometrical features and angles
of single point tools in ASA systems
Definition
Rake angles
• αs= side (axial rake: angle of inclination of the rake surface from the reference
plane (πR) and measured on Machine Ref. Plane, πX.
• αb= back rake: angle of inclination of the rake surface from the reference plane and
measured on Machine Transverse plane, πY.
Clearance angles:
• θs= side relief angle: angle of inclination of the principal flank from the machined
surface and measured on πX plane.
• θ e = End relief angle: same as θs but measured on πY plane.
Cutting angles:
• Cs = side cutting edge angle: angle between the principal cutting edge (its
projection on πR) and πY and measured on πR
• Ce = end cutting edge angle: angle between the end cutting edge (its projection on
πR) from πX and measured on πR
Nose radius, r (in inch)
• R = nose radius : curvature of the tool tip. It provides strengthening of the tool nose
and better surface finish.
Tool signature according to ASA system
• Tool signature : αb- αs- θe - θs- Ce- Cs-R
• Tool signature : 8 -14 - 6 – 6 - 6 – 15 -1
• This system does not indicate
behaviour of tool in actual practice.
the
• Hence actual cutting condition include the
side cutting edge (OR) principle cutting
edge.
3.Orthogonal Rake System – ORS
• Planes and axes of reference
3.Orthogonal Rake System – ORS
• πR = Refernce plane perpendicular to the cutting
velocity vector, V
• πC = cutting plane; plane perpendicular to πR and
taken along the principal cutting edge
• πO = Orthogonal plane; plane perpendicular to
both πR and πC and the axes;
• Xo = along the line of intersection of πR and πO
• Yo = along the line of intersection of πR and πC
• Zo = along the velocity vector, i.e., normal to both
Xo and Yo axes.
The main geometrical angles used to express
tool geometry in Orthogonal Rake System (ORS)
3.Orthogonal Rake System – ORS
Rake angles
• i= orthogonal rake: angle of inclination of the rake surface from Reference
plane, πR and measured on the orthogonal plane, πo
• α= inclination angle; angle between πC from the direction of assumed
longitudinal feed [πX] and measured on πC
Clearance angles
• γ = orthogonal clearance of the principal flank: angle of inclination of the
principal flank from πC and measured on πo
• γ 1 = auxiliary orthogonal clearance: angle of inclination of the auxiliary flank
from auxiliary cutting plane, πC’ and measured on auxiliary orthogonal plane,
πo’ as indicated in Fig.
Cutting angles
• λ = principal cutting edge angle: angle between πC and the direction of
assumed longitudinal feed or πX and measured on πR
• Ce = auxiliary cutting angle: angle between πC’ and πX and measured on πR
Nose radius, r (mm)
• r = radius of curvature of tool tip
Orthogonal Rake System (ORS)
i –α – γ – γ1 – Ce –λ – R
0 –10–6 – 6 – 8 –75 –1
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Effect of tool Elements
Side cutting edge angle
1.
2.
3.
4.
5.
6.
End cutting edge angle
Prevents interference as the
1.
cutting tool enters the work
material
Enable the cutting tool to contact
the work first behind the tip
2.
Affects tool life and surface finish
Can vary from 0o to 90o but the
3.
satisfactory value is 15o to 30o
Smaller angle will result in more
contsct area , thinner chip and
heat distribution
Larger angle results in chatter ,
separate the work and tool
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Prevents rubbing
(or) drag
between machined and non
cutting part of the cutting edge
8o to 15o is satisfactory
Too large angle cuts the material
that supports the conduction of
heat
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Effect of tool Elements
Back rake angle
1.
2.
Side rake angle
Rake angles are three types,
positive,
zero
(sometimes
referred to as neutral) and
negative
Increase in the rake angle
reduces
the
horsepower
consumption per unit volume of
the layer being removed
1.
2.
3.
4.
5.
6.
Provides easier flow of chip
May be ‘0’, ’-’ , ’+’
Large angle lower the cutting
force and power
Lesser angles leaves lesser
material supports the cutting
edge to conduct away the heat
Practical rake angle
a
compromise between these two.
Negative rake angle is used with
carbide tools since they are
having more chance to brittle
and less shock resistance
–
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Effect of tool Elements
Side relief angle
End relief angle
1.
2.
3.
4.
Avoid rubbing of work piece and
tool
Small angle given strength to tool
increased values –cut more
efficiently,reduced cutting forces
Too large angle –weaken the
cutting edge
1.
2.
3.
4.
5.
Avoid rubbing of work piece and
tool
5o to 15o for general turning
Small angle given strength to tool
increased values –cut more
efficiently,reduced cutting forces
Too large angle –weaken the
cutting edge
Nose radius
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1.
Tool life and surface finish
2.
Varies from 0.4 to 1.6 mm
3.
Too large lead to chatter
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Difficulties in Machining Mechanics studies
Several physical phenomenon such as plastic flow,
fracture, friction, heat, molecular diffusion and chatter are
involved. Some of them occur in extrême conditions
Friction – sticking; deformation – high strain and strain
rate; nascent surface exposed after deformation is very
active causing diffusion
The cutting zone is covered by chips and coolant.
Typical machining is oblique, i.e., forces, torques and
deflections exist in all 3 directions.
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Cutting Models
Tool
workpiece
workpiece
OBLIQUE GEOMETRY
ORTHOGONAL GEOMETRY
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Tool
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Difficulties in Machining Mechanics studies
The typical machining operations are too short and the
stock (depth and width of cut) keeps changing.
Furthermore, velocity also may change along the cutting
edge as well as over time. These changes further
compound the difficulties to observe the process carefully.
Orthogonal cutting experiments were developed to
overcome these difficulties
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Orthogonal cutting
Pure orthagonal cutting
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Pure Orthogonal Cutting
Facing of thin pipe on a lathe with the cutting edge radial to
the pipe.
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Oblique cutting
Angle of deviation
of chip
Causes
1.Restricted
cutting
effect
2.Inclination angle(i≠0)
3.Tool nose radiusing
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Characteristics of Orthogonal Cutting
A wedge shaped tool is used
Cutting edge is perpendicular to the direction of cut. In
other words, cutting edge angle
Ce = 90o
and
cutting edge inclination angle i=0
Uncut chip thickness to
is constant along the cutting
edge and with respect to time.
Cutting edge is longer than the width of the blank and it
extends on its both sides.
Cutting velocity v is constant along the cutting edge and
with respect to time
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Orthogonal Cutting - Experiments
Quick stopping devices to freeze the chip formation
Cutting wax manually slowly so as to observe it
Marking grids on the side of the work piece and study their
deformation.
Microscopic studies
Photoelastic studies (tools made of transparent material such
as persbex or resin (araldite); work piece is wax. Resulting
fringe patterns are observed under polarized glasses.
Observation using high speed cameras
Force, torque and power measurements using dynamometers.
Temp measurements
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Mechanics of chip formation
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Theories of Chip Formation
Chip
formation
studies
helps
in
understanding
mechanics of metal cutting or physics of machining
They
lead
to
equations
that
describe
the
interdependence of the process parameters such as
depth of cut, relative velocity, tool geometry etc.
These relations help us in selecting optimal process
parameters.
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Theories of Chip Formation – Theory of Tear
A crack propagates ahead of the tool tip causing tearing similar
to splitting wood [Reuleaux in 1900]
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Theories of Chip Formation – Theory of Tear
Against the traditional wisdom, the tool was observed to
wear, not at the tip, but a little distance away from it.
Therefore, this theory was subscribed by many researchers
for a long time.
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Theories of Chip Formation – Theory of Tear
Further studies attributed the wear away from the tip to the
following:
Chip velocity with respect to the tool is zero at the tip.
The tip is protected by BUE.
Temp is also high a little away from the tip due to the
frictional heat.
Subsequent studies proved the chip formation as shear and
not tear. Thus the theory of tear was rejected.
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Theories of Chip Formation – Theory of Compression
The tool compresses the material during machining.
This was based on the observation that the chip length
was shorter than the uncut chip length.
Later it was established that this shortage in length
corresponds to the increase chip thickness.
Thus this theory too was wrong
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Theories of Chip Formation – Theory of Shear
The excessive compressive stress causes shear of the chip
at an angle to the cutting direction [Mallock in 1881].
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Theories of Chip Formation – Theory of Shear
Mallock’s other contributions
Emphasis on the influence of friction at chip-tool interface
Studied the effect of cutting fluids
Studied the influence of tool sharpness
Studied chatter
His observations on the above studies still hold good
although he could not explain all of them at that time.
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Types of chips
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Types of chips produced
The type of chip produced depends
geometry, and operating conditions.
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upon workpiece material, tool
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Types of chips produced
Built-up Edge
A built-up edge (BUE),consisting of
layers of material from the
workpiece that are gradually
deposited on the tool, may form at
the tip of the tool during cutting.
As it becomes larger, the BUE
becomes unstable and eventually
breaks up. Part of the BUE material
is carried away by the tools side of
the chip,the rest is deposited
randomly on the workpiece surface.
The process of BUE formation and
destruction is repeated continuously
during cutting operation unless
measures are taken to eliminate it.
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Types of chips produced
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Mechanics of Metal cutting
The angle formed by the shear plane and the direction of the tool travel is
called the shear angle
Compressive deformation will cause it to be thicker and shorter than the
layer of workpiece material removed
The work required to deform this material usually accounts for the largest
portion of forces and power involved in a metal removal operation
The ratio of chip thickness, to the un-deformed chip thickness (effective
feed rate) is called the chip thickness ratio. The lower the chip thickness
ratio, the lower the force and heat, and the higher the efficiency of the
operation
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Shear angle
1.Tool is perfectly sharp and contacts
the chip on its face only
2.The primary deformation takes place
in a very thin zone adjacent to ab
tc
3.There is no side flow of chip
t0
Chip compression coefficient
1 tc
k   1
r to
Work
r cos 
tan  
1  r sin 
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Derive an expression for the shear angle in orthogonal cutting
In terms of rake angle and chip thickness ratio?
There fore t0 * l0 = tc * lc
The out ward flow of the
r =metal causes the
Sinchip
 to be thicker after separation from the
t = l = r
tc
lc
From the triangle abc bc = Sin 
But bc = t0
r Cos
 = ratio
1 – rorab
Sin
 ratio is the ratio of uncut chip thickness to the cut
The chip
thickness
cutting
ab = t0
------ (1)
Tan
chip thickness.
Sin 
From the triangle abd bd = Sin 90-()
Tan  = abr Cos 
When metal is cut there is no change in volume of the metal cut therefore
1 r SinAnd

But Sin 90-() cos ()
bd= tc
0
0
parent metal.
Metal
prior toCos
being
Is much
 cut
. Cos
 + longer
Sin  than
Sin the
 chip which is removed.
t0 x b0 x l0 =tc x bc x lc
& ttccare angle
chip
thickness
before
and after
cutting
ab t0 shear
Therewhere
fore
can(2)be
shown
from
the above equation
()
lCos
0 & lc are length of chip before and after cut.
From (1) and(2)
b0 &tbc are
= width
Sinofcut before
= r and after cut , b0 = bc =b
0
it is observed that b1 = b2
tc
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Cos ()
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Effect of shear angle
tc
t0
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α
φ
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Shear stress and shear strain
Dis tan ce Sheared
Shear strain, s 
Thickness of Zone
α

AC
BD
φ
A
Shear
Shear stress
stress,,ss 
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B
φ
α
Shear force
Shear stress ,  s 
Shear Area
B
D
A
90-α
C
Distance sheared
90-φ
C
Fs sinFs
(tt00bb00/ sin  )
Shear strain, s  cot   tan(    )
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Velocity relationships
Cutting speed:
Velocity of the tool relative to the workpiece
Shear velocity:
velocity at which shearing takes place(i.e)
velocity of the chip relative to the work
Chip velocity:
velocity of the chip up the face of the tool
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Velocity relationships
90-(φ- α)
VC
VC
α
Vs
Vs
(φ-α)
90- α
φ
V
V
From sine rule(or) Lame’s theorem
Vc
Vs
V


sin  cos  cos(   )
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Numerical problems
Pg 14(my ref.)
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Cutting forces and power
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Cutting forces
Cutting force determination is required for
• Estimation of cutting power consumption, which also enables selection of
the power source(s) during design of the machine tools
• Structural design of the machine –fixture – tool system
• Evaluation of role of the various machining parameters on cutting forces
Process –Speed (V), feed (f or to), depth of cut (b),
Tool —material and geometry,
environment
Cutting fluid
•
Study of behaviour and machinability characterization of the work
materials
• Condition monitoring of the cutting tools and machine tools.
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Cutting forces during turning
The single point cutting tools being used for turning, shaping, planing, slotting, boring etc. are
characterised by having only one cutting force during machining.
But that force is resolved into two or three components for ease of analysis and exploitation.
Fig. visualises how the single cutting force in turning is resolved into three components along the three
orthogonal directions; X, Y and Z.
These three components are:
c
Cutting force (Fc) acts in tangential direction. It
is also called power component as it being
acting along and being multiplied by cutting
speed (V) decides cutting power consumption.
c
t
r
Feed Force (Ft) acts in the direction of feed
(axial direction). Generally, this force is small in
magnitude but is responsible for causing
dimensional inaccuracy and vibration.
Thrust Force (Fr): acts in radial direction. This
force is least harmful and hence least
significant.
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Cutting models
Turning model
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Orthogonal model
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Other force components
• shear force(Fs): A force that attempts to cause the
internal structure of a material to slide against itself.
• Normal force(Fn): Force normal to the shear force
• Friction force(Ff) Force resist the chip along the rake
face of the tool
• Normal reaction(N) Force normal to the frictional force
Coefficient of friction,

Ff
N
• Resultant force(R) forces Ff and N added vectorially
and oriented at an angle β
  tan 
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Force Relationships
R
Ff
β
Fs
N
Fc
R
R
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Ft
Fn
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Merchants force analysis-Assumptions
•
•
•
•
•
Tool is perfectly sharp
Chip doesn’t flow either side
Depth of cut is constant
Width of the tool is greater than of work piece
Work moves relative the tool at uniform
velocity
• Continuous chips produced without any built
up edge.
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Merchant circle
β
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β
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Force Relaionships
G
Fs
A
Fc
αφ
D
Ft
α
K
Fn
E
L
αφ
B
O
φ
To find Fsf
OC=OE+BD
KL=AL-AK
∆OAE, AL=F
∆OAL,
OE=FccCos
Sin α
φ
∆ABD,BD=FttSin
∆ABK,AK=F
Cosφα
therefore
Fsf=Fc Sin
Cosα+F
φ-FttCos
Sin φα
Ff
N
J
C
To
To find
find FNn
OC=OE+BD
BC=AE-AD
∆OAL,
∆OAE, OL=F
AE=Fcc Sin
Cosφα
∆ABK,BK=F
∆ABD,AD=FttCos
Sinαφ
therefore
therefore
FFn=F
=F Sin φ +Ft Cos
αφ
f ccCos α-Ft Sin
Relation between FC, Ft, Fsf and N
Fn
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• Numerical problems?
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Power and Energy Relationships
Power=Force x Velocity
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Power and Energy Relationships
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Power and Energy Relationships
E  Es  E f
Total Specific Energy required per unit time
E
bt0V
F
bt0
Total specific Energy=Total
Energy/Unit
volume of material removal
e
 c
ef 
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Ef
bt 0V

F f Vc
bt 0V

Ff r
bt 0

Ff
bt c
es 
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Es
FV
Fs cos 
 s s 
bt 0V bt 0V bt 0 cos(   )
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Temperature in cutting
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Temperature in cutting-Effects
Work piece
• Affects the
strength,
hardness and
wear resistance
• Affects the
properties of
machined surface
and its properties
• Causing
distortion of
machine
Tool
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Work piece
• Poor
dimensional
control of work
piece
Machine tool
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Factors affect the temperature pattern
(temperature distribution) in cutting zone
Feed
Depth of cut
Cutting
speed
Specific heat
,Thermal
conductivity
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Type of
cutting fluid
Temperature
pattern
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Chip tool
Contact
length
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Heat Zones
Shear Zone
• More heat generated due to shearing action of
tool
• Heat carried by chip(90 to 95%)
• Heat conducted by work piece(5 to 10%)
Friction zone
• Heat generated due to friction and also due to
secondary deformation of the built up edge
• Heat carried by chip(90 to 95%)
• Heat conducted by work piece(5 to 10%)
w/p tool contact zone
• Heat is generated due to burnishing action of the
tool
• Minimum with fresh tool
• Temperature increases with cutting time
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Methods of Measuring Temperature
Tc 
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T  W
mC
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Analytical calculation
Assumptions
1. Orthogonal cutting of second kind
2. Effect of tool flank wear neglected
3. Entire heat supplied as reappear heat
4. Shear and friction energy distributed
uniformly over the contact region
5. Thermal properties of tool and work are
independent of working temperature
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Analytical model
T  Ts  T f
Temperature rise due to shear
Heat=mass x Temp.difference x sp.heat
 t0Vb0 (Ts  T0 )C
x
0.9 Es  t0Vb0 (Ts  T0 )C
y
Rearranging for Ts
Ts 
Vc
a  chip tool contact length
  kg m 3
b x
V m s
0.9 FsVs
 T0
t0bC V
y a
T K
C  J Kg o K
Orthogonal model
o
0.9( Fc cos   Ft sin  ) cos 
k

W
m
Ts 
 T0          (1)
t0bC cos(   )
K  m2 s
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Analytical model
Temperature rise due to friction
According to joggers solution temperature rise at the interface of sliding (insulated)with a
conducting surface at constant velocity
 
L
Vt
2K
W mK
0.754 q t
k L
where Thermal diffusivity, K 
k
C
m
2
s
For our case,
qf 
Ff  Vc
ab
       (2)
and
L
Vc aC
       (3)
4k
Only 90% heat goes to the chip tool interface and 10% to the tool
Tf 
0.754  0.9  q f  a
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2k L
       (4)
and T f 
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b
Vc
       (5)
akC
73
Analytical model
Therefore chip tool interface temperature
T  Ts  T f
0.679 F f
0.9( Fc cos   Ft sin  ) cos 
T 
 T0 
t0bC cos(   )
b
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Vc
akC
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Tool life
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Machinability
The ease with which a given material may be worked with a
cutting tool under a set of cutting conditions is defined as
The
of the tool
machinability
Chip
hardness
Ease of
chip
disposal
Cutting
forces
generated
Heat
generated
Shear
angle/chip
thickness
ratio
Dimension
stability of
the work
The quality of the Machined surface
Specific
power
consumptio
n
Tool
wear/tool
life
Quality of
the
machined
surface
The power consumption per unit volume of the material removed
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Machinability index
Relative to the machinability of the standard material
Machinability of free cutting steel is arbitrary fixed as 100%
Machinabil ityIndex 
Cutting speed of material for 20 minutes tool life
x 100
Cutting speed of free cutting steel for 20 minutes tool life
FCS-sulphur0.08-0.3,Phosporous, carbon 0.1 to 0.45
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Tool Life
The period during which a tool cuts
satisfactorily is called its tool life
Expressed in minutes
Most widely used criteria for machinability
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Measures for tool life
Machine time
Actual cutting time
Average length of cut/tool edge
Average volume of metal removed/cutting edge
Average number of identical components/cutting edge
Cutting speed for a standard value of machining time
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Tool life Relationship by F.W.Taylor
VT  C
n
V-Cutting speed in m/min
T-Tool life in minutes
n- an exponent
C-Machining constant(Depending on the cutting
condition /work material)
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Tool life -Materials
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Modified Taylor’s Tool life equation
VT d f  C
n
x
y
Where,
D and f are in mm
V in m/min
T in Minutes
X and y must be determined for each
cutting condition
Note: For Constant tool life
1. F and d increased V should be decreased
2. Depending on the exponents , a reduction in the speed result in the increase in
the volume of material removed because of increase in the depth of cut and
feed rate.
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Numerical Problems?
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Tool Wear and Failure
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Temperature in Primary and Secondary Machining
Regions
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Cubic Boron Nitride
Heat
Control all the mechanisms
of tool failure so tool life is
limited only by abrasion
wear
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Tool Failure Mechanisms
1. Abrasive wear
2. Built-up edge
• Rake surface
• Flank surface
3. Thermal/mechanical cracking/chipping
4. Cratering
5. Thermal deformation
6. Chipping
•
•
Mechanical
Thermal expansion
7. Notching
8. Fracture
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Wear
a) Flank wear and crater wear
in a cutting tool;
b) View of the flank face of a
turning tool, showing
various wear patterns.
c) View of the rake face of a
turning tool, showing
various wear patterns.
d) Types of wear on a turning
tool:
1. flank wear;
2. crater wear;
3. chipped cutting edge;
4. thermal cracking on
rake face;
5. built-up edge;
6. catastrophic failure.
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Comparison of Catastrophic and Progressive Failure
Catastrophic Failure
Caused by dynamic changes
 Intermittent cutting
 Ramping
 Sudden changes in tool load
 In-homogeneity (hard particles or
voids) in the raw material
 Micro-cracks in tool during HT
 Temp gradient due to non-uniform
coolant flow
Progressive Wear
Caused by gradual wear of the tool
due to
 Adhesion,
 Abrasion
 Diffusion.
Undesirable since
Desirable since
 Tool is lost for ever
 The tool can be reused by
 Damage the part or injure the
regrinding or indexing/
operator
 Changing the bit
 Unpredictable and hence
 Predictable and hence corrective
corrective action is not possible
action is possible
Closed loop control system used to Time bound regrinding is suggested
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prevent
approach
Comparison of Crater and Flank Wear
Crater Wear
Flank Wear
Occurs on the rake face
Highly sensitive to temperature
Occurs on the flank face
Not as much sensitive to
temp as crater wear
Undesirable wear
Used as failure criteria for
brittle tools such as WC and
Al2O3 tools
Most desirable wear
Used as failure criteria for
tough tools such as HSS
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Locations of Tool Wear
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Rapid failure
Constant Period
Break In Period
Abrasive Wear (Abrasion)
Abrasive wear occurs as a result of
the
interaction
between
the
workpiece and the cutting edge.
The width of the wear land is
determined by the amount of
contact between the cutting edge
and the workpiece.
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Heat Related Tool Failure Mechanisms
Cratering (Chemical Wear)
The chemical properties of the tool-material and the affinity of the toolmaterial to the workpiece material determine the development of the
crater wear mechanism
Hardness of the tool-material does not have much affect on the process.
The metallurgical relationship between the materials determines the
amount of crater wear.
Ex: Tungsten carbide and steel have an affinity to each other
The mechanism is very temperature-dependent, making it greatest at
high cutting speeds. Atomic interchange takes place with a two-way
transfer of ferrite from the steel into the tool. Carbon also diffuses into
the chip.
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Heat Related Tool Failure Mechanisms
Built-up Edge (Adhesion)
It occurs mainly at low machining temperatures on the chip face of
the tool. It can take place with long chipping and short-chipping
workpiece materials—steel and aluminum.
This mechanism often leads to the formation of a built-up edge
between the chip and edge.
It is common for the build-up edge to shear off and then to reform.
At certain temperature ranges, affinity between tool and workpiece
material and the load from cutting forces combine to create the
adhesion wear mechanism.
Machining work-hardening materials, such as austenitic stainless
steel, this wear mechanism can lead to rapid build-up at the depth of
cut line resulting in notching as the failure mode.
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Heat Related Tool Failure Mechanisms
Built-up Edge (Adhesion)
Increased surface speeds, proper application of coolant, and tool
coatings are effective control actions for built-up edge
Thermal Cracking (Fatigue wear)
Thermal cracking is a result of thermo mechanical actions
Temperature fluctuations plus the loading and unloading of cutting
forces lead to cracking and breaking of the cutting edge
Carbide and ceramics are relatively poor conductors of heat which
leads to fatigue wear
Thermal Deformation
As the cutting edge loses its hot hardness the forces created by the
feed rate cause the cutting edge to deform
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Mechanical Failure Mechanisms
Chipping (Mechanical)
Small chipping of tool material
Cutting force should be less than shearing force.
Chipping is larger on flank surface than on a face
Rake Surface
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Flank Surface
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Mechanical Failure Mechanisms
Insert Fracture
When the edge strength of an insert is exceeded by the forces
of the cutting process the inevitable result is the catastrophic
failure called fracture.
Excessive flank wear land development, shock loading due to
interrupted cutting, improper grade selection or improper
insert size selection are the most frequently encountered
causes of insert fracture
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Heat Related Tool Failure Mechanisms
Property
Carbon and
low to
medium
alloy steels
H
S
S
Cast
Cobalt
alloys
Cemented
carbide
Coated
carbide
Ceramics
Poly crystalline
CBN
Diamond
Hot
hardness
Toughness
Wear
resistance
Chipping
resistance
Cutting
speed
Thermal
shock
resistance
Total material
cost
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Surface finish and integrity
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Surface finish and integrity
A PART SURFACE has two important aspects that must be
defined and controlled.
The first concerns the geometric irregularities of the surface.
second concerns the metallurgical alterations of the surface and
the surface layer.
This second aspect has been termed surface integrity.
Both surface finish and surface integrity must be defined,
measured, and maintained within specified limits in the
processing of any product.
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Surface Finish
Surface finish is often the
superposition of two effects:
result
of
Ideal surface roughness:
Due to tool geometry and feed rate
Natural surface roughness:
Due to irregularities in the cutting operation
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Factors
Surface finish is determined by many factors,
which can be grouped in to
1.Geometric factors
2.Work material factors
3.Machine tool factors and vibration
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1.Geometric factors
High feeds produce more prominent marks on
surface
Most evident from rotating tool or workpiece
Turning, boring, milling, reaming, grinding
Tool geometry =SCEA, ECEA, Nose radius
Feed marks due to tool geometry and tool
advance per revolution
Roughness, R, is proportional to feed for a
given geometry
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2.Work material factors
• Build up edge cyclically forms and breaks
away, particles are deposited on the newly
generated work surface causes rough surface
• Curling chip back to the work piece produce
damage to the work surface.
• Tearing of the work surface during chip
formation when machining ductile material
• Cracks in the work surface during
discontinuous chip formation when machining
brittle material.
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3.Vibration and machine tool factors
• These factors related to the machine tool and
operation set up. They include chatter and vibration in
the machine tool, deflection fixturing and backlash in
the feed mechanism.
• These machine tool factors can be minimized or
eliminated by
–providing damping and adding stiffness
–proper cutting speeds
( cutting speeds do not cause cyclical forces whose
frequency approaches natural frequency of the machine
tool)
–reducing feeds and depth of cut
–changing the cutter design to reduce forces
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Surface finishes achieved in various
Machining operations
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Machine Tool Vibration
•Chatter refers to vibration of tool and
workpiece
Produces undesirable finish
Can also be damaging to tool life
-May introduce shock loads
•High frequency vibration can be frustrating
•Low frequency vibration may be transmitted
to other equipment
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Types
1. Forced vibrations
Usually caused by an oscillating force not directly related to the cutting process itself
Transmission from other machines
Unbalanced shafts, gears, motors
2. Self-excited vibrations
Caused by instability in the cutting process itself
Generally occurs at or near the natural frequency of vibration of the part
3. Regenerative chatter
Generally occurs when cuts overlap or when subsequent cuts are taken after an
initial one
Thus chatter marks left by previous cut may induce a forced vibration of higher
amplitude than previous cut
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Machine Tool Vibration – Reduction
• Can be minimized by
– Increasing damping
• Making natural frequency of system much less
than frequency of the exciting force
• Damping arises in:
Material of the machine tool
Frictional damping in bolted joints
Viscous damping in lubricated interfaces
Slideways, bearings, etc.
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STRUCTURE OF MACHINE TOOLS
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Required performance
1. Strength
2. Stiffness
3. Rigidity
4. Provision to ensure relative location and
alignment
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Machine body
• Basic structure of machine tool
• Bed, column or upright or combination of both
• Locating datum surfaces(guide ways)
Must possess
• Shape invariability along with strength
• Producibility
• Low material requirement
• Low cost
Depends upon
• Proper selection of material and manufacturing process
• Provision for static and dynamic rigidity
• High wear resistance of guide ways
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Stiffening devices
1.Parallel
2.Diagonal ribs
Plate
or Rib section
• Superior compared to above
Body
• Strong in vertical direction
An example of a
• Less against torsional strainmachine-tool
structure. The boxtype, one-piece
Box section
design with internal
diagonal ribs
• Best stiffness in torsion
significantly
• Withstand bending forces improves the stiffness
of the machine.
Source: Okuma
• Easy to produce
Machinery Works
Tubular Section
Ltd.
• No internal corners
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ii)Guide ways
Guide way constrain the work or tool path in a
definite way
Requirements
– Accuracy of travel
– Durability
– Rigidity
Types of guide ways
– Slide ways
– Antifriction ways
– Hydrostatic guide ways
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Slideways
Vee slide
Flat ways
Dovetail
Cylindrical
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Antifriction
•
•
•
•
•
Overcomes “slipstick”
Roller or ball
Higher cost
Accurate working surfaces
Used for precision machine tools
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Hydrostatically lubricated ways
• Pressurized oil
• Pressurized air(0.3 to 0.5
N/mm2)
• Less clearance(15 to 25
microns) is the requirement
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Material for machine body and guide ways
• Damping out vibrations
• Wear resistant
Integral unit
Material -Grey cast iron
1.Any complex shape
2.Good machinability
3. Less cost
1. Large machining allowance
2. More time
3. Production defects
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Material for machine body and guide ways
Welded structures
Material-Rolled steel
1.
2.
3.
4.
Less cost
Time saving
Lightness
Higher mechanical
properties
1. Not sensitive to shock loads
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Material for machine body and guide ways
Material: Plastic
1.
2.
3.
4.
3.
Antiscoring
Low modulus of elasticity
Anticorrosion
Low
coefficient of thermal
conductivity
Less friction
Excellent wearing
Tendency
to swell qualities
when
absorb oil’
Material: Pads of zinc or of bronze
1. Good wear resistance
2. Expensive
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iii)Spindle and spindle bearings
Requirements
Withstand forces
Vibration-proof
properties
• Rotational accuracy
•Affect accuracy
•Essential high speed and
high surface finish
operation
•Able to rotate at varying
speeds
•Reduced service life of
bearings
•Essential in relatively
long longitudinal motion
of spindles
•Nose run out (radial and
axial)
•Rigidity
•Wear resistance
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Material for spindle
Rigidity –young’s modulus
(i)Medium carbon structural steel(C-45)
• Heat treated (quenched and tempered) to 22-28 Rc
Surface at 40-50 Rc
• Induction hardening recommended for less distortion
with high hardness
ii)Low carbon case hardening steel
• Carburization, quenching and tempered to 56-62 Rc
• Nitriding, quenching and tempered is recommended
for precision machine tools
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Material for spindle
iii) Manganese steel
• Heavy machines
• Heavy loads
• Normalization or hardening , tempering to 28-35 Rc
iv) Grey or nodular cast iron
• Hollow spindles
• Large diameter
• Horizontal boring machines and other
Bearings
• Sliding contact bearing -Babbitt, Cast iron, Bronze, Nylon Delrin
• Hydrostatic bearings
• Air bearing
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Machining Economics
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Machining Economics
Graphs showing
(a)cost per piece and
(b)time per piece in machining.
Note the optimum speeds
for both cost and time.
The range between the two is
known as the high-efficiency
machining range.
Tool material
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Desirable properties of tool material
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Desirable properties of tool material
Hot hardness
– High hot hardness means higher
speeds and feed rates (higher
production rates and lower costs).
Toughness and Impact strength
– Tool does not chip or fracture
Thermal shock resistance
Wear resistance
– Tool does not have to be replaced
as often
Chemical stability and inertness
– To minimize adverse reactions
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Development of Cutting Tool Materials
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Characteristics of Tool Materials
Property
Carbon
and low
to
mediu
m alloy
steels
HSS
Cast
Cobalt
alloys
Cemente
d carbide
Coated
carbide
Ceramics
Poly crystallin
e
CBN
Diamond
Depth of
cut
Light to
medium
Light to
heavy
Light to
heavy
Light to
heavy
Light to
heavy
Light to
heavy
Light to
heavy
Very light
for single
crystal
Finish
Obtainable
Rough
Rough
Rough
Good
Good
Very good
Very good
excellent
Method of
processing
Wrought
Wrought,
cast, HIP,
sintering
Cast, HIP
and
sintering
Cold
pressing
and
sintering
CVD
Cold
pressing
and
sintering
High
pressure
and high
temp.
sintering
High
pressure
and high
temp
sintering
Fabrication
Machini
ng and
grinding
Machinin
g and
grinding
Grinding
Grinding
Grinding
Grinding
Grinding
and
polishing
Grinding
and
polishing
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Factors affecting selection of Tool Materials
Hardness and condition of the workpiece material
Operations to be performed
Amount of stock to be removed
Accuracy and finish requirements
Type, capability, and condition of the machine tool to be used
Rigidity of the tool and workpiece
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Factors affecting selection of Tool Materials
Production requirements influencing the speeds and feeds
selected
Operating conditions such as cutting forces and temperatures
Tool cost per part machined, including initial tool cost, grinding
cost, tool life, frequency of regrinding or replacement, and
labor cost—the most economical tool is not necessarily the one
providing the longest life, or the one having the lowest initial
cost
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No Tool Material
Satisfies All These
Criterion
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High Speed Steel
High alloy steel
They are either
molybdenum or tungsten
based but necessarily
contains 4% chromium
M = Molybdenum
T = Tungsten
M >40 = Super HSS materials; capable of treating to high hardness
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High Speed Steel
Advantages of HSS
 Heat treated to high hardness within the range of Rc 63–68
 M40 series of HSSs is normally capable of being hardened to Rc70, but
a maximum of Rc68 is recommended to avoid brittleness
 HSSs also possess a high level of wear resistance
 HSS tools possess an adequate degree of impact toughness and are
more capable of taking the shock loading of interrupted cuts than
carbide tools
 When HSSs are in the annealed state they can be fabricated, hot
worked, machined, ground, and the like, to produce the cutting tool
shape
 Toughness in HSSs can be increased by adjusting the chemistry to a
lower carbon level
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High Speed Steel
Limitations of HSS
 Tendency of the carbide to agglomerate in the centers of large
ingots
 Improved properties and grindability are important advantages of
powdered metal HSSs
 hardness of these materials falls off rapidly when machining
 temperatures exceed about 538–593°C
 use of lower cutting speeds than those used with carbides,
ceramics
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High Speed Steel
Applications of HSS
 Most drills, reamers, taps, thread chasers, end mills, and gear
 Cutting tools are made from HSSs
 HSS tools are usually preferred for operations performed at low
cutting speeds and on older, less
 Rigid, low-horsepower machine tools
Powder metallurgy HSS
 Uniform structure with fine carbide particles and no segregation
 Lower in cost because of reduced material, labor, and machining
costs, compared to those made from wrought materials
 Near net shape
 more design flexibility
 Applications : Milling cutters
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Cemented Tungsten Carbide
Most carbide grades are made up of tungsten carbide with a
cobalt binder
Advantages of WC
 Hardness of softest WC is higher than hardened steel
 High hot hardness
Grades of WC
Straight WC
 Co as a binder
 Best suited for material having abrasion as a primary tool wear e.g.
cast iron, non ferrous materials, non metals
Complex WC
 Comprises carbides : TiC, TaC, NbC with Co as a binder
 ferrous materials, non metals
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Cemented Tungsten Carbide
Tungsten carbide is extremely hard and offers the excellent
resistance to abrasion wear
The most significant benefit of TiC is a reduction in the
tendency of the tool to fail by cratering.
The most significant contribution of TaC is that it increases the
hot hardness of the tool, which in turn reduces thermal
deformation
Effect of Co as a binder
Co is more sensitive to heat, abrasion and welding
The more cobalt present, the softer the tool, making it more sensitive
to thermal deformation, abrasive wear and chip welding
Cobalt is stronger than carbide. Therefore, more cobalt improves the
tool strength and resistance to shock
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Cemented Tungsten Carbide
Classification system
ISO classification number ranges from 05 to 50 : e.g. P20, K35, M40;
05 is most wear resistance whereas 50 is most fracture resistance
Coated carbide tools is the most significant advance in cutting tool
materials since the development of WC tooling
Various single and multiple coatings of carbides and nitrides of
titanium, hafnium, and zirconium and coatings of oxides of
aluminum and zirconium, as well as improved substrates better
suited for coating, have been developed to increase the range of
applications for coated carbide inserts.
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C- Classification
ISO- Classification
C1 to C4 for Cast iron
C5 to C8 for Steel
P = Stainless Steel
M = Steel
K = Cast Iron
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Ceramics
Ceramics are primarily aluminum oxides
Inconsistent and unsatisfactory results during initial periods
of development
Improvements : better control of microstructure (primarily
in grain size refinement) and density, improved processing,
the use of additives, the development of composite
materials, and better grinding and edge preparation
methods. Tools made from these materials are now stronger,
more uniform, and higher in quality
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Ceramics
Types of ceramics
Plain ceramics, which are highly pure (99 percent or more)
and contain only minor amounts of secondary oxides
(produced by powder metallurgy)
Composite ceramics : are Al203-based materials containing
15–30 percent or more titanium carbide (TiC) and/or other
alloying ingredients
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Ceramics
Advantages
Increased productivity: Ceramic cutting tools are operated at
higher cutting speeds than tungsten carbide tools
Good hot hardness, low coefficient of friction, high wear
resistance, chemical inertness, and low coefficient of thermal
conductivity
Most of the heat generated during cutting is carried away in
the chips, resulting in less heat buildup in the workpiece,
insert and tool holder
Better size control by less tool wear
Machining of many hard materials
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Ceramics
Limitations
Brittle than carbides
Less mechanical and thermal shock resistance
Less interchangeability with the carbide tool holders
Applications
High speed machining of steel and cast iron requiring
continuous machining
Most suitable for machining of chemically active materials
Face milling and turning applications
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Single crystal and polycrystalline diamonds(PCD)
Best suited for precision machining with very high surface finish and to
increase productivity by reducing downtimes
Diamond is the cubic crystalline form of carbon that is produced in
various sizes under high heat and pressure. Natural, mined single-crystal
stones of the industrial type used for cutting tools are cut (sawed,
cleaved, or lapped) to produce the cutting-edge geometry required for
the application.
Advantages
Hardest material known. Indentation hardness is five times than carbide.
Extreme hardness and abrasion resistance can result retaining their
cutting edges virtually unchanged throughout most of their useful lives
Because of the diamond’s chemical inertness, low coefficient of friction,
and smoothness, chips do not adhere to its surface or form built-up
edges when nonferrous and nonmetallic materials are machined.
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Cubic Boron Nitride
Super abrasive crystal that is second in hardness and abrasion
resistance only to diamond
CBN crystals are used most commonly in super abrasive wheels for
precision grinding of steels and super alloys
Advantages
Greater heat resistance than diamond tools
High level of chemical inertness
Compacted CBN tools are suitable, unlike diamond tools, for the high
speed machining of tool and alloy steels with hardness to Rc70, steel
forgings and Ni-hard or chilled cast irons with hardness from Rc45–68,
surface-hardened parts, and nickel or cobalt-based super alloys
They have also been used successfully for machining powdered metals,
plastics, and graphite.
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Cutting fluids
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Cutting fluids
Cutting fluid is used to:
1. Reduce friction and wear
2. Cool the cutting zone
3. Reduce forces and energy consumption
4. Flush away the chips from the cutting zone
5. Protect the machined surface from environmental corrosion
•
Depending on the type of machining operation, a coolant, a
lubricant, or both are used
•
Effectiveness of cutting fluids depends on type of machining
operation, tool and workpiece materials and cutting speed
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Cutting-fluid Action
• Cutting fluid seep from the sides of the chip
through the capillary action of the interlocking
network of surface asperities in the interface
• Discontinuous cutting operations have more
straightforward mechanisms for lubricant
application, but the tools are more susceptible
to thermal shock
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Effects of Cutting Fluids on
Machining
If no coolant,
1. Friction at the tool–chip interface will increase
2. The shear angle will decrease in accordance
3. The shear strain will increase
4. The chip will become thicker
5. A built-up edge is likely to form
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Effects of Cutting Fluids on Machining
As a result:
1. The shear energy in the primary zone will increase
2. The frictional energy in the secondary zone will
increase
3. The total energy will increase
4. The temperature in the cutting zone will rise
5. Surface finish will to deteriorate and dimensional
tolerances may be difficult to maintain
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Types of Cutting Fluids
1. Oils - mineral, animal, vegetable, compounded, and
synthetic oils,
2. Emulsions - a mixture of oil and water and additives
3. Semi synthetics - chemical emulsions containing little
mineral oil
4. Synthetics - chemicals with additives
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Methods of Cutting-fluid
Application
4 basic methods:
1. Flooding
2. Mist
3. High-pressure
systems
4. Through the cutting
tool system
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Selection of a cutting fluid
1. Workpiece material and machine tools
2. Biological considerations
3. Environment
•
Machine-tool operator is in close proximity to cutting fluids,
thus health effects is a primary concern
•
Progress has been made in ensuring the safe use of cutting
fluids
•
Recycling involves treatment of the fluids with various
additives, agents, biocides, deodorizers and water treatment
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Near-dry and Dry Machining
•
Near-dry cutting is the application of a fine mist of an air–fluid
mixture containing a very small amount of cutting fluid
•
Dry machining is effective on steels, steel alloys, and cast
irons, but not for aluminum alloys
•
One of the functions of a metal-cutting fluid is to flush chips
from the cutting zone
Cryogenic Machining
•
Using nitrogen or carbon dioxide as a coolant
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The chips are more brittle and machinability is increased
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