Mechanics of metal cutting

Mechanics of Metal Cutting
Prof. S.S.Petkar

Single point Cutting tool
Nomenclature
Prof. S.S.Petkar, Amgoi

Terms and definitions

Chip Formation

Types of Chips

Cutting tool Materials
Basic Requirements
• Hardness
• Hot Hardness
• Wear resistance
• Toughness
• Low friction
• Better thermal characteristics- high thermal conductivity

Comparative properties of cutting tool materials
Material
Room Temperature
Hardness, Ra
Transverse rupture
strength x 1000
MPa
540°C
760 ° C
HSS 85 -87 77-82 Very low 3.8-4.5
Cast Cobalt 82-85 75-82 70-75 1.4-2.8
Carbides 89-94 80-87 70-82 To 2.4
Ceramics 94 90 87 0.5-0.4
Diamond 7000 Knoop 7000 Knoop 7000 Knoop -

Carbon –Tool Steel (Plain Carbon steel)
• C- 0.6 – 1.5%, small amount of manganese, silicon, tungsten, molybdenum,
chromium, vanadium.
• Limitations-
In ability to withstand high temp.
more than 200°C looses hardness and cease to cut
• Applications
At low cutting speeds (up to 0.15m/s) for wood, aluminium, brass, magnesium.
They are used for form tool making,
Used for low quantity production.

High Speed Steel
Cutting speed times more than PCS
Contains tungsten,moly.,chr.,van.
Very good hardness & abrasion resistance
Advantages:-
High hardness, hot hardness, good wear
resistance, high toughness & reasonable
cost
• Limitations:-
Hardness falls rapidly beyond 650 °C
They are limited to low cutting speeds.
0.5-0.75 m/s
HSS
T type M type

Cemented Carbides
• Largest % of cutting tool is used in
metal cutting production
• They are produced by cold
compaction of Tungsten carbide
powder in a binder such as cobalt.
• High hot hardness, high young’s
modulus
• Advantages:-
High toughness, high impact strength,
Sustain high temperature as well as
speed
Limitations:
Not suitable for low cutting speeds
Not economical to use at low speeds
Expensive in inserts shape

Cemented carbide Classification
Cemented
Carbides
Lower Designation
number
Higher designation
number
P10, M10, K10 P40, K40,P50

Ceramics (Al2O3)
Base System Density g/cm3
Hardness
Transverse rupture
strength, MPa
25 °C HRA 1000° C HV
Al2O3 3.98 93.9 710 50
Al2O3 +TiC
4.24 94.3 770 80
Si3N4 3.27 92.6 1100 100
Properties of Ceramic Material

Requirements with ceramics:-
High cutting speed,
Rigid machine with high spindle speed
Machine rigid workpiece
Adequate and uninterrupted power supply
Use negative rake angle
Large nose radius
Avoid coolants with aluminium oxides based ceramics

Diamond
• It is the hardest known (Knoop hardness
8000kg/mm2)
• Good thermal conductivity, low friction,
good wear resistance, non adherence to
most material.
Limitation:-
High cost, possibility of oxidation in air,
very high brittleness.
• Applications
• Used for relatively light cuts
• Artificial diamonds are used in industrial
application
• They are used with –ve rake angle -5°
Typical materials machined with diamond
tools are:-
Al alloys, Cu, Brass, Bronze, Carbon,
graphite, plastic composites

CBN (Cubic Boron Nitride)
• It is next in hardness only after diamond (Knoop hardness 4700
kg/mm2)
• It is less reactive with steel, hard chill cast iron, cobalt based alloys
• It is used to machined alloys
• Expensive than cemented carbide

Summary of Applications
Tool Material Work Material Remarks
Carbon Steel Low strength, soft materials, non fe alloys, plastics Low cutting speed, Low strength
material.
Low/Med Alloy
Steel
Low strength, soft materials, non fe alloys, plastics Low cutting speed, Low strength
material.
HSS All materials of low and medium strength &
hardness
Low to medium cutting speed, Low to
medium strength material.
Cemented carbides All materials of low and medium strength &
hardness
Not suitable for low speed applications
Coated Carbides CI, Alloy steels , Stainless steel, super alloys Not for titanium ,non Fe alloys,
CBN Hardness alloy steel,HSS, Ni based super alloys, pure
nickel, Hardened chill CI
High Strength, hard materials
Diamond Pure cu, Al, Al-Si alloys, cold pressed cemented
carbides, rock, cement, glass, plastics
Not fr machining low carbon steel, Co,
Ni, Ti, Zr

TEMPERATUR IN METAL CUTTING

Temperature in Metal Cutting :(Heat Generation &
Dissipation)
• Heat is required to be less during operation
• Efforts must be taken at operations to minimise loss of energy & Power
Main cause:-
1. Friction between chip and tool :-
velocity of chip, ʯ, F, N . Causes 15-25% heat
Controlling points:-
optimum cutting speed, depth of cut, rake angle, use of lubricants at friction points

2. Frictional force between the w/p & Tool:-
Causes 10 % Heat. Frictional force is expected at relieving surfaces of
tool. Hence..
Side relief angle & End relief angle are required to be larger
3. Friction between outgoing chip & Workpiece:
it will come into existence if chips remain undisposed from w/p. In
shear plane 65-75 % heat is generated
proper use of chip breakers can disconnect the chips from w/p, due
to this friction can be minimised

Heat Dissipation
• Why heat should be removed from operation??
1. Loss of accuracy at w/p
2. Loss of cutting properties of tool
3. Loss of tool life
4. Loss of efficiency
• Cooling media
1. Air in form of jet
2. Water in form of continuous flowing jet.
3. Water mixed with some oils
4. Mineral and synthetic oils

Regions of Heat
Generation

There are number of methods for measuring
the chip tool interface temperature.
• Radiation pyrometers
• Embedded thermocouples
• Temperature sensitive paints
• Temper colours
• Indirect calorimetric technique
• Tool work thermocouple

Tool Material, cutting speed
Work Piece
Material
H.S.S Cemented Carbide
Mild Steel 30-40 m/min 80-120 m/min
Grey Cast Iron 20-30 m/min 60-90 m/min
Brass 60-80 m/min 80-200 m/min
Aluminium 80-120 m/min 200-300 m/min
Cast Steel 15-125 m/min 40-80 m/min

Temperature v/s Cutting Speed
• Cutting speed depends on number
of variables.
• Higher cutting speed results high
temperature
• High temperatures are likely to
affect the w/p, tool properties

Tool Wear
• Crater Wear:
• It is on rake face & less circular
• It doesn’t always extend to tool tip
• It may end at some distance from
tip
• It increases the cutting forces,
modifies the tool geometry
• It also increases the softness at
tool tip

Flank Wear (Wear Land)
• It is on the clearance surface of
the tool
• It can be characterised by the
length of wear land (w)
• It also modifies the tool geometry
• It changes the cutting parameters
like depth of cut.
• Tools are subjected to severe
conditions
1. Metal to metal contact
2. Very high stress
3. Very high temperature
4. Very high stress gradient
5. Very high temperature
gradient
6. Virgin metal

Progressive Crater Wear
Progressive Flank Wear

Tool Life Criteria
Weal Land (mm) Tool Material Remarks
0.75 Carbides Roughing
0.25-0.38 Carbides Finishing
1.50 HSS Roughing
0.25-0.38 HSS Finishing
0.25-0.38 Oxides Roughing & Finishing

Possible tool failure criteria
Based on Tool Wear
• Fine cracks Developing at cutting
edge.
• Wear land size
• Crater depth, width
• Combination of above two
• Volume or weight of material worn
off the tool
• Total destruction of the tool
Based on consequences of Worn
Tool
• Limiting value of surface finish
• Limiting value of change in
component size
• Fixed increase in cutting force or
power required to perform a cut

Cutting Fluids
Functions
• Cool the tool and workpiece
• Reduce friction
• Protect the work against rusting
• Improve the surface finish
• Wash away the chips from the
cutting zone
• To control the total Heat
Cooling action
• Cooling the tool chip interface
helps in retaining original
properties of tool hence increase in
life
• Reduction in temperature increases
the shear flow stress of w/p hence
decrease tool life

Selection of cutting fluids
• Water based Emulsion
• Mineral Oils
• Neat oils (Mineral oil +
Additives)
• Cutting Fluid Selection
1. Workpiece material
2. Machining operation
3. Cutting tool material
4. Other factors

Cutting fluids based on Work Material
Material Cutting fluid to be used
Selection
guidelines
Gey cast Iron Soluble oils, thinner neat oils for flushing swarf and dust
Al alloys Soluble oil, straight neat oil
Mild steel, L.
C. Steel
Milky type soluble oil, neat cutting oil
High Carbon
steel
Extreme Pressure(EP) cutting oil, milky soluble oil in
some applications
Alloy steels Extreme Pressure(EP) cutting oil, milky soluble oil in
some applications
Cu Alloys Water based fluids. For tougher alloys neat blended oil
used
S.S & H.R.A. High neat oil, EP Oils Prof. S.S.Petkar, Amgoi

Cutting fluids based on Tool Material
Tool Material Cutting Fluid Requirements
High Carbon Steel Water based coolants
High speed steel
For general machining water based, heavy duty work EP neat
oils
Non Ferrous
Material
Neat cutting oils are most suitable
Carbides, Ceramics
and diamond
Water based coolants , EP based oils

Machineability
• It is ease of machining
• Depending factors:-
1. raw material
2. Volume of material being removed
3. Volume of material being removed per tool resharpening
4. The quality of the machined surface in terms of the surface texture
5. The precision at the dimensions
6. The specific power consumption (power / mm3 of material to be removed)

Due to complexity , machineability is expressed as
a Comparative index as…
Cutting speed for material for 20 min. tool life
• M.I.= ----------------------------------------------------------------------------------------------- x 100
Cutting speed for free cutting steel for 20 min. tool life
The various values of the M.I. for different materials are
Cu- 70, AL Alloys – 300 to 1000,
S.S.- 25, C-45 steel- 60, Brass -180

Tool Designation
• Under ASA
8-14-6-6-6-15-1/8
αb= 8, αs=14, θe= 6, θs= 6, Ce= 6, Cs=15, R=1/8
• Under ORS
0-10-6-6-8-90-1
i=0, α=10, ϒ=6, ϒ1=6, Ce=8, lambda=90, R=1

Theory of Lee and Shaffer
• They applied the theory of plasticity
for ideal plastic material
• They assumed that deformation
occurred on thin shear plane
• There must be a stress field within
in metal chip to transmit the cutting
force from shear plane to tool face
•𝜑=
𝜋
4
− (𝛽 − 𝛼)

Cutting power
• The cutting power or rate
of energy consumption ,is
the product of cutting
speed and cutting force.
• E= Fc x V
• Power consumed in cutting, if Fc in Kg.f
and V is in m/min
Pc=
𝑭𝒄∗𝑽
𝟔𝟎∗𝟏𝟎𝟐
kW
Power consumed in cutting, if Fc
in N and V is in m/s
Pc=
𝑭𝒄∗𝑽
𝟏𝟎𝟎𝟎
kW

PLOWING FORCE AND SIZE EFFECT.
The resultant too force in metal cutting is distributed over
the areas of the tool that contact the chip and workpiece.
No cutting tool is perfectly sharp, and in the idealized
picture as shown in Figure 2
the cutting edge is represented by a cylindrical surface
joining the tool flank and tool face
Neither the force acting on the tool edge nor the force that
may act on the tool flank contributes to removal of chip
and these forces will be referred to collectively as plowing
force. Fp
The existence of the plowing force results in certain
important effects and can explain the so called size effect.
This term refers to the increase in specific cutting energy

Mechanics of metal cutting

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