Theory of metal cutting
Dr S B Patil
Assistant Professor Mechanical Engineering
College of Engineering, Pune 411005
sbp.mech@coep.ac.in
1
You should know
1. What do you understand by conventional manufacturing
processes?
Explain
how
the
conventional
manufacturing processes can be classified.
2. What do understand by conventional and non
conventional manufacturing process of manufacturing?
3. Explain the need of non conventional manufacturing
with the help of examples.
4. State and explain the parameters influencing machining
operation.
5. With the help of neat figure explain orthogonal and
oblique cutting operation.
6. With the help of neat figure explain the mechanics of
chip formation.
2
Objectives of machining
1.
2.
3.
4.
5.
Quick metal removal (or high MRR)
High class surface finish
Economy in tool cost
Less power consumption
Economy in cost of replacement and
resharpening of tool….
3
Classification of manufacturing processes
1. Primary shaping processes
-Casting, forging, rolling, drawing, extrusion, squeezing,
shearing, piercing, forging
2. Machining processes
-Shaping, turning, milling, drilling, planning, grinding,
boring, knurling, sawing, broaching
3. Surface finishing processes
- Sand blasting, buffing, lapping, belt grinding, polishing,
honing, electroplating, metal spraying, anodizing,
phosphating, tumbling, galvanizing
4
Classification of manufacturing processes
4. Joining processes
-Welding, soldering, brazing, riveting, screwing,
adhesive joining, pressing, coupling, key and cotter
joining, nut and bolts joints
5. Processes affecting change in properties
- Annealing, normalizing, hardening, tempering, grain
refining, age hardening
5
Parameters influencing machining
1. Workpiece material- Chemical and physical properties
2. Cutting tool-Type, material, tool geometry
3. Cutting environment- Cutting fluid-type (chemical
composition and rate of flow)
4. Cutting parameters- Cutting speed, DOC, Feed
5. Machine tool- type, Rigidity
6
Basic elements of machining operation
7
Mechanics of chip formation
Rake Angle (γ)
Rake
Chip
Shear Zone
Tool
Flank
Clearance Angle (α)
8
• Chip formation is a complex phenomenon
• When tool is forced against the work piece, the crystals
are elongated
• The surface along which the chip slides is called rake
face
• The surface which is relived to clear the newly machined
surface is known as flank
• The rake face makes an angle (γ) with the normal to the
finished surface is called rake angle
• Clearance angle is the angle between the flank and new
work piece surface.
9
Types of metal cutting operations
• Orthogonal cutting (Two dimensional cutting)
The cutting edge of the cutting tool is at right angles to
the direction of relative motion between tool and work
piece.
Example: Parting
• Oblique cutting (Three dimensional cutting)
The cutting edge of the cutting tool is inclined at an angle
to the direction of relative motion between tool and work
piece.
Example: Turning
10
Orthogonal cutting
11
Orthogonal Cutting assumptions
1. The tool is perfectly sharp and there is no contact
along
the
clearance
face
2. The Cutting edge of the tool remains normal to the
direction
of
tool
feed
or
work
feed.
3. The direction of the chip flow velocity is normal to the
cutting edge of the tool.
4. The cutting edge is a straight line, extending
perpendicular to the direction of motion and
generates a plane surface as the work moves past it.
12
Orthogonal &Oblique cutting
13
Oblique cutting assumptions
1.The cutting edge of the tool always remains inclined at an
acute angle to the direction of tool feed or work feed
2. The direction of the chip flow velocity is at an angle ‘β’
with the normal to the cutting edge of the tool. The
angle is known as Chip flow Angle.
3. The cutting edge of the tool is inclined at an angle ‘i’ with
the normal to the direction of work feed or tool feed i.e.,
the velocity Vc.
4. Three mutually perpendicular components of cutting
forces act at the cutting edge of the tool.
5.The cutting edge may or may not be longer than the
width of the cut.
14
You should know
1. “The quality of the chip determines number of important
aspects” Explain
2. What do you understand by tool signature. Explain with
the help of neat figure the tool signature of single point
tool.
3. Describe the various types of the chips produced during
machining operation with the help of neat figure.
4. Describe the various situations during which a particular
type of chip will be produced.
5. State the adverse effects of continuous chips with built
up edge formation and state the precautions required to
avoid.
15
Principle angles of single point tool
16
Tool signature
17
Chip formation
• Cutting tool is required to be harder and wear
resistant than the work piece.
• Interface between the tool and the work piece is
designated as feed and depth of cut.
• Relative motion between the tool and workpiece
is termed as cutting speed or velocity to
overcome the resistance.
18
Types of chips
• Size, shape and quality of the chip determines
number of aspects
1. Segmental or discontinuous chips
2. Continuous chips
3. Continuous chips with built up edge
4. Non homogenous chips
19
Segmental or discontinuous chips
• Separate , plastically deformed segmental chips through
actual fracture
• This causes excessive friction between the chip and tool
face leading to fracture of the chip into small segments.
• Loosely adhere each other
• Produced during machining of brittle material
• Also produced during machining of ductile materials at
low speed and also with high DOC
• CI, Brass
20
Segmental or discontinuous chips…
• This causes excessive friction
between chip and tool face,
leading to fracture of chip into
small segments.
• This will result in excessive tool
wear and poor surface finish.
• Other factors responsible:
smaller rake angle and large
depth of cut
21
Continuous chips
• Continuous chips are produced due to continuous plastic
deformation of the metal ahead of the tool.
• The chips will be moving smoothly up the tool face.
• Need of chip breakers
• This type of chips will be produced while machining
ductile materials like copper, mild steel etc under
favorable cutting conditions such as high cutting speeds
and minimum friction at tool-chip interface
22
Continuous chips
• The friction at
chip-tool
interface can be minimized
by polishing tool face and
adequate use of coolant.
• Other responsible factors:
bigger rake angle, finer feed
and keen cutting edge.
23
Continuous chips with built up edge
• While machining ductile materials
when high friction exists at the
chip-tool interface results in
continuous chips with built up
edge.
• The normal reaction of the chip on
the tool face is quite high and it is
maximum at the cutting edge or
nose of the tool.
24
Continuous chips with built up edge
• This gives rise to an extensively
high temperature and compressed
metal adjacent to the tool nose
gets welded to it.
• The chip is sufficiently hot and
gets oxidized as it comes off the
tool and turns blue in colour.
• The extra metal welded to the
nose of the tool is called built up
edge.
25
Continuous chips with built up edge
• Adverse effects of built up edge formation
- Rough surface finish
- Fluctuating cutting force, causing vibration in
cutting tool.
- Chances of carrying out tool material by built up
edge and crater formation on cutting tool face
and causing tool wear.
26
Continuous chips with built up edge
• Precautions to avoid built up edge formation
- The coefficient of friction at the chip tool
interface should be minimized by means of
polishing the tool face.
- Adequate supply of coolant
- Larger rake angle
- High cutting speeds and low feeds
27
Non homogenous chips
• Produced due to non-uniform strain in the
material during chip formation
• Characterized by notches on the free side
of the chip while adjoining the tool face is
smooth
28
Types of chips
29
You should know
1. Explain the terms cutting ratio and chip compression
factor.
2. Derive an expression for cutting ratio/chip compression
factor in terms of shear plane angle and rake angle.
3. Derive an expression for shear plane angle in terms of
cutting ratio and rake angle.
4. State and explain the velocities during metal cutting.
5. Derive an expression for velocities along shear plane
and chip velocity in terms of cutting velocity, rake angle
and shear plane angle.
30
Cutting ratio or chip thickness ratio
31
Chip thickness ratio
32
Cutting ratio (r)
• Cutting ratio is the ratio of thickness of the chip
before removal to thickness after removal from
the material being cut.
Cuttingratio(r ) 
Chipthicknessbeforecutting
Chipthicknessaftercutting
t1
r
t2
…… (1)
33
Chip compression/reduction factor
• The inverse of the chip thickness ratio is known
as chip compression factor or chip reduction
coefficient (k).
k= 1/r
• Cutting ratio is always less than unity. If the ratio
(r) is large, the cutting is good. A ratio of 1:2
yields good results.
•
34
35
36
37
38
Velocity relationships
39
40
Velocities in metal cutting
i) Cutting velocity (v)
It is the velocity of the tool relative to the work piece and
directed parallel to the cutting force
ii) Chip velocity (vc)
It is the velocity of chip relative to the tool and is directed
along the face of the tool
iii) Shear velocity (vs)
It is the velocity of the chip relative to the work piece and
is directed along the shear plane
41
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43
44
You should know
1. With the help of neat sketch explain the various
component forces during orthogonal and oblique cutting
operations.
2. Draw Merchant circle diagram and label it. State the
various relationships.
3. What are the assumptions in Merchant’s theory?
4. Derive an expression for shear strain.
5. Explain work done and power requirement in metal
cutting.
45
Forces in metal cutting
• In case of oblique cutting three
component
forces
act
simultaneously on the tool point .
1. Feed or thrust force (Ft or Fx )
acting in horizontal plane
parallel to the axis of the work.
2. Radial force (Fr or Fy )
acting in horizontal plane but
along the radius of the work
piece (along the axis of the tool).
46
Forces in metal cutting
3. Cutting or tangential force (Fc or
Fz ) acting in the vertical plane
and is tangential to the work
piece.
The resultant force (R)
In case of orthogonal cutting Ft= 0
47
Merchant circle
48
49
50
51
Merchant’s Theory
• For equilibrium, the force R between tool face and chip
and force R’ between work piece and chip along the
shear plane should be equal.
• For simplicity, forces R and R’ are assumed to act at the
tool point and represented by the diameter of the circle.
• Forces can be resolved as
i) In the horizontal and vertical direction, Fz & FY
ii) Along and perpendicular to the tool face, Fs & FD
iii) Along and perpendicular to the tool face FT & FN
52
Merchant’s Theory…
• Fz is the main cutting force and is in the direction of
the tool travel. The feed force FY acts in direction
perpendicular to main cutting force Fz
• Force FS acts along shear plane and represents the
force required to shear material. Force FD acts
normal to Fs and results in compressive stress
being applied to the plane of shear
• Force FT acts along tool face and represents
frictional resistance met by chip as it slides over the
tool and FN is the force normal to FT.
53
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55
56
57
58
59
60
61
62
Work done and power required in metal cutting
Let
v= cutting speed (m/min)
Fz= cutting force (kgs)
W= total work done in cutting
W= Fz X v ……. (1)
W1 = work done in shear
Fs = Shear force
vs = velocity along shear plane
W1 = Fs X vs …… (2)
63
Work done and power required in metal cutting
W2 = work done in friction
Ft = Shear force
vc = chip velocity
W2 = Ft X vc …… (3)
W= W1 + W2
Fz X v = Fs X vs + Ft X vc
Let P= Power in metal cutting
P= Fz X v /4500 H. P.
64
65
You should know
1.
What are the desirable properties of cutting tool material?
2.
State the different types of materials used for cutting tool and
describe them.
3.
State the properties of the cemented carbide tools due to which
they are used as common industrial cutting tools.
1.
Describe the following as industrial cutting tool
a. Cemented carbide tools
b. Ceramic tools
66
Cutting tool materials
•
Cutting tool has to be harder than the work material
•
Desirable properties of cutting tool
1. It should be rigid enough to withstand the forces being applied due
to cutting- broaching
2. It should be tough (resistant to shock loads). It is quite important
when tool is used for intermittent loads.
3. It should be sufficient harder (resistant to wear, abrasion, and
indentation) than the material being cut.
67
Cutting tool materials…
4. It should be able to withstand high temperatures.
5. It should be capable of withstanding the sudden cooling effect of
coolant used during cutting.
6. The coefficient of friction between the chip and the tool should be as
low as possible in the operating range of speed and feed.
7. It should be easily formed to the required cutting shape.
68
Types of Cutting tool materials
1. Carbon tool steel
2. Alloy steel
3. High speed steel
4.
Stellite
5. Cemented carbides
6. Ceramics or Cemented oxides
7.
Diamonds
69
1. Carbon tool steel
-
Plain carbon steels having carbon % as high as 1.5%
are commonly used as tool for general class of work.
-
As the operating temperature range is just 250°- 300° C
-not suitable for industrial production, also have less
wear resistance.
-
These tools can be used as hand tools or cutting soft
materials or at low cutting speeds.
-
These tools are less costly, easily forgeable and easy
to heat treat.
70
2. Alloy steel
• Medium alloy steel are similar to carbon tool steel,
however in medium alloy steels elements like chromium,
molybdenum and tungsten are added to improve certain
properties such as hardenability, wear resistance etc
• Plain carbon steel if added with alloying elements such
as chromium, vanadium, tungsten, cobalt, nickel, silicon,
etc we will get En series
such as En-8, En-24, En-30, En-31, En-36, En-56
71
3. High Speed Steel (HSS)
• It is a special alloy which may contain the alloying
elements like tungsten, chromium, vanadium, cobalt,
molybdenum etc up to 25%
• these alloying elements increase its strength, toughness,
wear resistance, cutting ability to retain its hardness at
elevated temperature of 550º - 600º C.
• Have the capability of operating 2 to 3 times higher
cutting speeds than those of high carbon steels.
72
3. High Speed Steel (HSS)
• The most commonly used HSS is better known by its
composition of alloying elements as 18-4-2 i.e. 18%W,
4%Cr and 2% V.
• Another class of HSS contains high proportion of cobalt
(2-15%) and known as cobalt HSS. It has high wear
resistance and hot hardness.
• A highly tough variety of HSS is known as vanadium
HSS carries 2% V, 6% W, 6% Mo & 4% Cr. This HSS is
highly favoured for tools which have to bear impact
loading and perform intermittent cutting.
73
3. High Speed Steel (HSS)…
• Sometimes HSS tools are treated to improve its
performance
Super finishing- to reduce friction
Chromium electrolytic plating- to reduce friction
Oxidation- to reduce friction
Nitriding- to increase wear resistance
74
4. Stellite
• It is a non-ferrous alloy consisting of cobalt, tungsten and
chromium, other elements added in varying proportion
are tantalum, molybdenum and boron.
• It has good shock and wear resistance and retains its
hardness up to 920° C
• These are used to machine materials like hard bronzes,
and cast and malleable iron etc
75
Stellite
• Tools made of stellite are capable of operating at speeds
up to 2 times more than those of common HSS tools
• Stellite does not respond to usual heat treatment
process and also it can not be easily machined by
conventional methods only grinding can be used for its
machining.
• A stellite may contain 40-50% Co, 15-35% Cr, 12-25%
W and 1-4% C.
76
5. Cemented or sintered carbides
• Manufactured by powder metallurgy
• Consists of tungsten, tantalum, and titanium carbides
together with a binder cobalt.
• Compacted to the desired shape and sintered. During
the process the cobalt binder fused to the carbides
producing a hard and dense substance
• Cemented carbides are extremely hard (90-93 HRC) and
can be used at cutting speeds 200 to 500% greater than
those used for HSS.
77
5. Cemented carbides…
• Usual practice is to confine the size to a relatively small
shape known as “insert”. Which is clamped/brazed to a
tough steel shank or holder .
• Inserts may have 3 to 8 edges and is so designed that
each of its cutting edges can be used in turn.
• These cemented carbides posses a very high degree of
hardness and wear resistance and diamond is the only
material harder than it.
• It retains its hardness up to 1000° C
78
Properties of cemented carbide tools
1. They have high thermal conductivity, low specific heat
and low thermal expansion.
2. They have high hardness and over a wide range of
temperature (900º C).
3. Their compressive strength is more than tensile
strength.
4. They are very stiff and their Young’s modulus is about
three times than the steel
79
5. Ceramic tools
•
It mainly consists of Al2O3 which is cheaper than any of
the main constitute of cemented carbides.
•
Boron nitrides in powdered form are added and mixed
with aluminum oxide powder and sintered at 1700º C.
•
They are compacted in different shapes. Usually used
in the form of disposable tips (throw away tips)
•
These tools have capability
temperature 1200° C.
to
withstand
high
80
5. Ceramic tools
•
These tools have better wear resistance as compared
to cemented carbide tools.
•
But they are brittle and posses low resistance of
bending as a result can not be employed for rough
machining and intermittent cutting.
•
However, their application for finishing operations yields
very satisfactory results.
81
•
.
5. Ceramics…
•
•
Can be operated at 2 to 3 times higher cutting speeds
than the tungsten carbide tools and usually do not
require coolant .
Can be used to cut at the top speeds up to 1500m/min
•
Ceramics are very hard and with good compressive
strength.
•
Under similar conditions, the ceramic tool are capable
of removing (MRR) 4 times material than the tungsten
carbide tool with a consumption 20% less power than
latter.
82
5. Ceramics…
•
Three types of ceramic tools are common in use
i. Al2O3 ceramic tools- the most common in use and
used for finishing and super finishing operations
ii. Sialon (a combination of silica, aluminum, oxygen
and nitrogen)
iii. Silicon nitride (Si3N4)
83
6. Diamonds
•
The hardest material ever known
•
It is brittle and offers a low resistance to shock, but is
highly wear resistance
•
It has low coefficient of friction, and high compressive
strength.
•
It gives very good surface finish at high speeds with
good dimensional accuracy.
84
6. Diamonds
•
Diamonds are employed for only light cuts and finishing
operations on material like Bakelite, plastics, glass ,
ceramics, carbon, plastics, aluminum and brass etc
•
However, on account of their excessively high cost, its
use in industry is confined.
•
Other applications includes dressing of grinding wheels.
•
Diamond particles are used in diamond wheels and
laps.
85
You should know
1.
State and describe the causes of heat during machining.
2.
Explain with the help of neat diagram the sources of heat during
metal cutting.
3.
State and describe the various functions of cutting fluid.
4.
State the desirable properties of cutting fluid.
5.
State the various types of cutting fluids used during metal cutting
and describe them.
6.
List out the various factors which affect on the selection of cutting
fluid
86
Causes of heat in metal cutting
• Heat in metal cutting is produced due to
1. Friction
2. Plastic deformation of metal
3. Chip deformation
87
Causes of heat in metal cutting
1. Friction
- Lot of friction is always takes place between the
cutting tool and work piece and chips passing over it.
-The total amount of heat generated depends on many
factors such as cutting parameters, tool material etc.
- This heat is called as heat of friction
88
Causes of heat in metal cutting
2. Plastic deformation of metal
- As the cutting is started, the cutting tool exerts
significantly high pressure on the adjacent metal grains.
-
This causes deformation or slipping of these grains over
adjacent layers in contact causes friction between them
- This friction leading to heat generation is known as heat
of deformation.
-The total heat generated depends on many factors such
as cutting parameters, work piece material etc.
89
Causes of heat in metal cutting
3. Chip distortion
- In metal cutting, as the cutting proceeds and the chip curl
out, the inside grains of the chip metal are subjected to
compression and tension respectively.
- This causes distortion of the chip grains leading to internal
friction among them results the generation of heat.
-
This heat is also called heat of chip distortion.
- The amount of heat generated depends largely on feeds
and depth of cut
90
Sources of heat in metal cutting
Heat during metal cutting is produced at
1. Around shear plane
2. Tool-chip interface
3. Tool work piece interface
91
Sources of heat in metal cutting
1. Around
shear
plane
(Primary deformation zone)
It is the region in which
actual deformation occurs
during machining which
results in heat generation.
A part of this heat is carried
away by the chip and rest of
heat is retained by the work
piece.
92
Sources of heat in metal cutting
2. Tool-chip interface
(Secondary deformation zone)
As the chip slides upward during face of
the tool friction occurs between their
surfaces due to which heat is
generated.
A part of this heat is carried away by the
chip due to which temperature
increases and rest to tool.
The amount of heat is generated due to
friction increases with increase in
cutting speed.
93
Sources of heat in metal cutting
3. Tool work piece interface
That portion of tool flank which
rubs against the work surface
is another source of heat
generation due to friction.
The heat is also shared by tool
and work piece.
The heat increases when the
tool is not sufficiently sharp.
94
Sources of heat in metal cutting
- On an average 70% of the total
heat is carried out by chip, about
15% is transferred to the tool and
remaining 15% to the work piece.
- With an increase in the cutting
speed a higher amount of heat is
absorbed by the chip and lesser
amount of heat is transferred to tool
and work piece.
-
A large value of shear angle leads
to smaller heat generation in
primary deformation zone.
95
Cutting fluids
• The use of the metal working fluids is essential
in all metal working operations.
• In metal cutting, a lot of heat is generated proves
harmful to the tool and work piece.
• These fluids help in minimizing these adverse
effects of heat and thus helps in increasing tool
life and surface finish.
96
Functions of cutting fluids
1. It cools the cutting tool and work piece by carrying away
the excessive heat.
2. It lubricates the cutting tool and thus reduces the
coefficient of friction between the chip and tool. This
increases the tool life.
3. To prevent the adhesion of chips to the tool or work or
both
97
Functions of cutting fluids
1. To cool the tool and work piece
-
The cutting fluid employed at low temperature, as
compared to the temperature of the tool, work and chips
-
The heat generated flows from them outwards the fluid,
which absorbs and drives it away along with it.
-
The fluid is thus heated up and needs a constant
replacement by a fresh amount of cooler fluid.
-
For this reason only a steady flow of the cutting fluid in
ample quantity is always needed during machining.
98
Functions of cutting fluids
2. To provide lubrication
-
It implies the reduction of friction between the tool and
work piece and tool and chips.
-
This helps in preventing a direct metal contact amongst
the work piece, tool and the chip (film lubrication) which
reduces friction.
-
A lesser amount of heat is generated and less power is
consumed in machining.
99
Functions of cutting fluids
3. To prevent adhesion of chips to the tool or work or both
-
To prevent this, the addition of chemically active agents,
like compounds of sulfur or chlorine are made to the
cutting fluids.
-
The compounds produce soapy films between the work
and tool and chip and tool face which prevents direct
contact and hence chances of welding or adhesion.
-
This film also provides lubrication, called metal
lubrication between the mating surface.
100
Desirable properties of cutting fluids
1. It should have a high specific heat, high thermal
conductivity and high film coefficient.
2. It should posses good lubricating properties to reduce
frictional forces and to decrease the power consumption
3. It should be odorless.
4. It should be non corrosion to work and machine
101
Desirable properties of cutting fluids…
5. It should be non toxic to the operating personnel.
6. It should posses low viscosity to permit free flow .
7. It should be stable in use and storage.
8. It should permit clear view of work which is specially
desirable in precision work.
9. It should be safe particularly with regards to fire and
accident hazards.
102
Types of cutting fluids…
•
A cutting fluid mainly severs the following functions
i. Cooling
ii. Lubrication and
iii. Antiwelding
Types of cutting fluids
i. Water based cutting fluids
ii. Straight or neat oil based cutting fluids
iii. Lubricants
103
Water based cutting fluids
•
Water based cutting fluids are very common in use
•
The most common in use is soluble oil which is mixed
(1 to 5%) to form emulsion.
•
It has excellent cooling properties and good lubrication
effect.
104
Water based cutting fluids…
•
Modern soluble oil contain corrosion inhibitor and a
biocide to keep down the growth of bacteria that would
otherwise cause health hazard.
•
Soda solutions are often used for grinding operations as
it has good flushing action and cooling effect.
•
Water itself is seldom used as coolant as it causes rust
and corrosion.
105
Straight or neat oil based cutting fluids
The term straight when applied to lubricants and
coolants means diluted.
Types of straight or neat oil based cutting fluids
1. Mineral oils
2. Straight fatty oils
3. Compounded or blended oils
4. Sulphurized oils
5. Chlorinated oils
106
Straight or neat oil based cutting fluids…
1. Mineral oils
- Used for light machining operations
- machining of free cutting brass and steel
2. Straight fatty oils
- the most common is lard oil
- these oils are not stable and rapidly lose their
lubricating properties
-Neither they are satisfactory coolants as they have
high viscosity.
-Mainly used during cutting with taps and dies.
107
Straight or neat oil based cutting fluids…
3. Compounded or blended oils
-Mixture of mineral oil (75%) and fatty oils (25%)
-They are very cheaper than fatty oils
- Suitable for heavy duty operations
4. Sulphurized oils
-5% Sulphur is added to lard oil so it is called as Sulphurized oil
- Used for heavy duty operations
5. Chlorinated oils
-About 3% chlorine is added to mineral oils.
- When chlorine and sulphur (5%) are present in mineral oil, they
give good lubricating properties and are suitable for machining of
strong and tough materials
-very often used for broaching
108
Lubricants
•
Solid lubricants are employed in a finely divided state
and are kept in suspension in the liquid form by means
of a depressing agent.
•
Under certain conditions the lubricants reduces friction
on the tool face and reduces power consumption,
increases tool life and surface finish.
109
Factors affecting selection of cutting fluid
1.
2.
3.
4.
5.
6.
Cutting speed, feed and depth of cut
Cutting tool material
Work piece material
Viscosity of cutting fluid
Expected tool life
Cost of cutting fluid
110
Selection of cutting fluid
•
•
•
•
Low speed and shallow cuts require little cooling or
lubricants.
A lubricant of considerable oiliness is required while
machining of tough materials at low speeds and heavy
cuts.
Shallow cuts at high speed require good coolants
therefore emulsions of soluble and sulphur base cutting
oils are employed.
Brittle materials like cast iron are often cut without the
use of a lubricant although emulsions of soluble oil in
water are sometimes used
111
Selection of cutting fluid
Sr.
No.
Operation
Cutting conditions and other
requirements
Suggested fluids
1
Turning
Process parameters and material
being cut
Emulsion or
straight oils
2
Sawing
For cleaning saw teeth, and carry
away chips
Soluble oils
3
Tapping and
threading
Lubrication as cutting speeds are
low
Fatty oils
4
Drilling and
boring
Lubrication and cooling
Soluble oils
5
Reaming
Lubrication
Soluble oils
6
Broaching
Heavier cuts are taken
Heavy and active
type of cutting oils
112
Selection of cutting fluid
Sr.
No.
Operation
7
Planing and
shaping
8
Milling
9
Cutting conditions and other
requirements
Suggested fluids
---
No cutting oils
Cooling, lubrication and prevent tool
chatter
Sulphurized
mineral oil in
ample quantity
Thread rolling
---
Straight mineral
oil or emulsions
10
Gear cutting,
shaping and
shaving
---
Active type
mineral oils and
compounds
11
Grinding,
lapping and
honing
Lubrication
Active type
mineral oils and
compounds
113
Selection of cutting fluids
Material
Cutting fluids
Steel and wrought iron
Water soluble oils or sulphur based
mineral oils
Aluminum
Mineral oils and fatty oils or soluble
oils
Brass, copper , bronze, malleable iron
Soluble oils
Cast iron
Dry machining
114
You should know
1.
2.
State the various symptoms of unsatisfactory cutting.
What are the causes of tool failure. Explain the following in detail
a.
b.
Thermal cracking and softening
Mechanical chipping
3.
With the help of neat diagram explain the following in detail
a. Crater wear
b. Flank wear
4. Explain the following wear mechanism responsible for tool wear
a. Adhesion
b. Abrasion
c. Diffusion and
d. Chemical wear
115
Unsatisfactory cutting
A properly designed and ground cutting tool is expected to
perform satisfactory.
The tool when it is not performing satisfactory then
following adverse effects may be observed
1. Extremely poor surface finish
2. Higher consumption of power
3. Work dimensions not being produced
4. Overheating of cutting tool
5. Appearance of a burnishing band on the work piece
116
Causes of tool failure
A tool may fail during an operation or perform
unsatisfactorily due to
1. Thermal cracking and softening
2. Mechanical chipping
3. Gradual wear
117
Causes of tool failure
1. Thermal cracking and softening
- Due to heat the tool tip and the area closer to the
cutting edge becomes very hot and the tool starts
loosing its hardness after attaining some temperature.
- After the operating temperature, the tool material starts
deforming plastically at the tip and adjacent to the
cutting edge under the action of the cutting pressure
and the high temperature.
- Thus the tool loses its cutting ability and is said to have
failed due to softening.
- Factors responsible: high values of cutting parameters,
smaller nose radius and selection of wrong tool material
118
Causes of tool failure
• Thermal cracking and softening
- On the account of fluctuations in temperatures and severe
temperature gradients the
tool material is subjected to
local expansion and
Contraction.
- This results in development of thermal stresses due to
which thermal cracks are developed in the material.
- The tool failure due to this aspect is known as failure
due to thermal stresses.
119
Causes of tool failure
2. Mechanical chipping
- Mechanical chipping of the nose
and/or the cutting edge of the tool
are commonly observed.
- Chipping occurs due to high cutting
pressure, mechanical impact,
excessive wear, high vibration and
weak tip and cutting edge etc.
- Chipping is more pronounced in carbide tipped and
diamond tools due to their high brittleness.
120
Causes of tool failure
3. Gradual wear
- Loss of mass from tool is
due to wear
- Two types of wear
generally found in
cutting tools
1. Crater wear
2. Flank wear
121
Crater wear
• The principal region where
wear takes place in a cutting
tool is its face, at a small
distance ‘a’ from its cutting
edge.
• This type of wear takes place
while machining ductile
materials, where continuous
chips are produced.
122
Crater wear
• The resultant feature of this is
the crater or depression at the
tool chip interface.
• This type of wear is due to the
pressure of the hot chip
sliding up the face of the tool.
• The metal from the tool face is
supposed to be transferred to
the sliding chip by means of
diffusion process.
123
Crater wear
• The shape of the crater
formed corresponding to the
shape of the underside of the
chip.
• The principal dimensions of
the formed crater are its
breadth ‘b’ and depth ‘d’.
• A continued growth of crater
will result in the cutting edge
of the tool becoming weak
and finally lead to tool failure.
124
Crater wear
• At very high speed, and the
consequent high temperatures
(say 1000° C), the HSS tool
fail due to thermal softening of
material, while the tools made
from harder materials, like
those containing tungsten
carbide, cobalt etc will not
wear so rapidly.
• Higher feeds and lack of
cutting fluid increases the rate
of crater wear.
125
Flank wear
• Another region where an
appreciable amount of wear
occurs is the flank below the
cutting edge.
• It occurs due to abrasion between
the tool flank and the work piece
and excessive heat generated as
a result of the same.
• The abrasive action is aided by
the hard micro constituents of the
cut material.
126
Flank wear
• The entire area subjected to flank
wear is known as wear land.
.
• This type of wear mainly occurs
on the tool nose, front and side
relief faces.
• The magnitude of this wear
mainly depends on the relative
hardness of the work piece and
tool materials at the time of
cutting operation and also the
extent of strain hardening of the
chip.
127
Flank wear
• When the tool is subjected to
this type of wear, the work
piece loses its dimensional
accuracy, energy
consumption is increased
and the surface finish is
poor.
128
Flank wear
The effect of flank wear is expressed in
Terms of width (or height) of wear land
which is dependent on time.
This height is a linear measure and
expressed in mm
129
Flank wear
• The total flank wear consists of
three components, drawn
between the wear land height
(VB) and time (t).
• The first component (A) which
exists for a small duration,
represents the period during
which initial wear takes place
at a rapid rate.
130
Flank wear
• The second segment (B),
which exists for a very long
duration, represents the period
during which the wear
progress uniformly.
• The last segment (C)
represents the region in which
wear occurs at a very rapid
rate and results in tool failure.
• Thus the region is known as
period of destructive wear.
131
Effect of cutting speed on flank wear
•
Effect of cutting speed on the tool flank wear (VB) for
three cutting speeds, using a tool life criterion of 0.50
mm flank wear.
132
Wear mechanism
• The wear mechanism of
cutting tool is a complex
phenomenon.
• The common
mechanisms responsible
for wear are
1. Abrasion
2. Adhesion
3. Diffusion
4. Chemical wear
133
Wear mechanism- Abrasion
• It is a type of mechanical wear.
• Under this mechanism, hard particles on the underside
of the sliding chip, which are harder than the tool
material, plough into the relatively softer material of the
tool face and remove metal particles by mechanical
action.
• The material of the tool face is softened due to the high
temperature.
134
Wear mechanism- Abrasion
• The hard particles present on the underside of the chip
may be:
a. Fragments of hard tool material
b. Broken pieces of built up edge, which are strain
hardened.
c. Extremely hard constituents like carbides, oxides, scales
etc present in the work material.
135
Wear mechanism- Adhesion
• Due to excessive pressure a lot
of friction occurs in between the
sliding surfaces of the chip and
tool face.
• This gives rise to an extremely
high localized temperature,
causing metallic bond between
the materials of the tool faces
and the chip
• But, the surfaces which are
actually microscopically rough
produces point contacts.
136
Wear mechanism- Adhesion
• Due to extremely high
temperature at the tool-chip
interface a metallic bond in
between chip and tool material
takes place at the contact
points in the form of small spot
welds.
• When the chip slides, these
small welds are broken. But this
separation is not along the line
of contact.
137
Wear mechanism- Adhesion
• A small portion of the welded
tool contact is also carried away
by the sliding chip.
• Thus, small particles from the
tool face continue to be
separated through this
phenomenon so transferred
from the tool face to the chip
will depended upon the contact
area and relative hardness of
chip and the tool material.
138
Wear mechanism- Diffusion
• Solid state diffusion, which consists of transfer of atoms
in a metal crystal lattice, is another cause of wear.
• This transfer of atoms takes places at elevated
temperature from the area of high concentration to that
of low concentration.
• The favorable condition for diffusion is provided by the
rise in localized temperature over the actual contract
area between the chip underside and the tool faces.
139
Wear mechanism- Diffusion
• In such a condition, the tool material to the chip material
at the points of contact.
• This weakens the surface structure of the cutting tool
and may ultimately lead to tool failure.
• The amount of diffusion depends upon:
a. Temperature at the contact area between the tool faces
and the chip
b. The period of contact between the tool face and chip
c. The bonding affinity between the materials of the tool
and the chip.
140
Wear mechanism- Chemical wear
• This type of wear occurs when such a cutting fluid is
used in the process of metal cutting which is chemically
active to the material of the tool.
• This is clearly the result of chemical reaction taking
places between the cutting fluid and the tool material,
leading to a change in the chemical composition of the
surface material of tool.
141
You should know
1.
2.
3.
4.
5.
What do understand by “Machinability”? List out the various factors
on which Machinability depends on.
List out the possible Machinability evaluation criterions.
Write short note on “Machinability index”.
Write short note on
1. Economics of metal cutting
2. Relationship between cutting speed, production rate, and cost
3. High efficiency range for cutting speed
4. Optimum cutting speed
5. Efficient metal removal
What could be the possible through which metal removal rates can
be improved?
142
Machinability
•
Machinability of a material gives the idea of the ease
with which it can be machined.
•
The parameters generally affecting the machinability of
material are:
Physical properties of the material
Mechanical properties of the material
Chemical composition of the material
Micro-structure of the material
Cutting conditions
1.
2.
3.
4.
5.
143
Machinability evaluation
1.
Rate of metal removal per tool grind
2.
Tool life between successive grinds
3.
Magnitude of cutting forces
4.
Quality of surface finish
5.
Shape and size of chips
6.
Temperature during cutting
7.
Power consumed
144
Machinability index
•
The machinability for different materials are compared
in terms of their machinability index.
•
For this purpose the machinability index of free cutting
steel serves as datum and it is taken as 100.
•
The machinability of the material can be computed as
145
Economics of metal cutting
•
•
•
•
•
One of the basic objective of metal cutting is producing
at minimum possible cost.
The objective may be achieved through different
possible ways such as optimizing tool life, increasing
MRR etc.
In order to achieve maximum tool life the values of
process parameters are required to be the minimum.
Cutting speed among the process parameters has the
highest bearing on tool life.
Thus there is need to determine optimum cutting which
maximizes the tool life.
146
Optimum cutting speed
•
•
It is observed that the
tooling cost increases while
the machining cost
decreases with increase in
cutting speed.
The lowest point ‘P’ on the
curve determines the
minimum cost of
production. And the
corresponding cutting
speed gives the optimum
cutting speed.
147
Optimum cutting speed
•
•
1.
2.
3.
4.
The production cost per
piece (Km) is the minimum
Total production cost of
component comprises
several components such
as
Machining cost
Tool changing cost
Tool sharpening cost
Idle cost …
148
Relation between cutting speed, production rate
and cost
•
•
Figure shows the plots for
minimum cost and maximum
production.
It is found that the optimum
cutting speed for which the
production cost is minimum is
not same for the highest
production rate.
149
Relation between cutting speed, production rate
and cost
•
•
The area lying in between
these two values of cutting
speeds is known as high
efficiency range (Hi-E Range)
and the cutting speeds lying in
this range are either
economical or more
productive.
For efficient and economical
production cutting speed is
required to be selected from
this range.
150
Efficient metal removal
1.
2.
3.
4.
Optimum cutting speeds
- tool costs are reduced
- production rates are improved
Highest feed rate and depth of cut
- productivity is improved
Largest nose radius
- surface of the work piece is improved
- heat dissipation is better
Optimum tool geometry
151
You should know
1. What do understand by tool life? Explain
2. State and explain the various methods of expressing
tool life.
3. Explain volume of material removed as a method of
calculating tool life.
4. Sate the various factors affecting tool life and describe
them in detail.
5. Explain Taylor’s tool life equation.
152
Tool Life
•
Tool life can be defined as the time interval for which
the tool works satisfactorily between two successive
grinding (sharpening).
•
When the tool wear is increased considerably, the tool
loses its ability to cut efficiently and requires to be
reground.
•
The tool life can be effectively uses as the basis to
evaluate the performance of the tool material, assess
machinability of the work piece.
153
Methods of expressing Tool Life
•
There are three common methods of expressing tool
life.
1. As time period in minutes between two successive
grindings.
2. In terms of number of components machined between
two successive grindings. This method is used when
the tool operates continuously, as in case of automatic
machines
3. In terms of volume of material removed between two
successive grindings. This method is commonly used
when the tool is primarily used for heavy stock removal.
154
Tool Life- Volume of material removed
155
Tool Life- Volume of material removed
156
Taylor’s Tool Life equation
157
Factors Affecting Tool Life
1.
2.
3.
4.
5.
6.
7.
8.
Cutting speed
Feed and depth of cut
Tool geometry
Tool material
Work material
Nature of cutting
Rigidity of machine tool
Use of cutting fluid
158
1. Effect of cutting speed
159
2. Feed and depth of cut
160
3. Tool geometry- Rake angle
• Many tool angles influence tool life such as Rake angle
has a mixed effect.
• If it is increased in a positive direction the cutting force
and the amount of heat generated will reduce and hence
tool life increases.
• But if it is very large then the cutting edge is weakened
and also the capacity to conduct heat is reduced.
• Hence the optimum range for rake angle is -5° to +10°
• Cemented carbide and ceramic tools are generally
provided with negative rake angle.
161
3. Tool geometry- Clearance angle
• Clearance angle is provided on tool to reduce rubbing
action of tool to newly generated surface.
• Large value of clearance angles results in weakening of
tool and hence results in reduced tool life.
• Smaller value of clearance angles will lower the heat
generated during the cutting but tool may rub against the
work piece.
• Hence the optimum range for clearance angle is 5° to 8°,
but in special cases, such as in carbide tipped tools, a
higher value up to 10° is used to prevent the rubbing of
shank.
162
3. Tool geometry- End cutting edge angle
• The two cutting edge angles have their influence on tool
life.
• The front cutting edge angle also known as end cutting
edge angle effects tool wear..
• Up to a certain optimum value an increase in this angle
permits the use of higher cutting speeds without adverse
effect on tool life.
• But an increase beyond that value will result in reduction
in tool life.
• It generally varies from 5° to 8°.
163
3. Tool geometry- Side cutting edge angle
• The side cutting edge angle or the plane
approach angle has a complex effect on tool life.
• If this angle is smaller, higher speeds can be
employed.
• A larger side cutting edge angle increases tool
life.
164
3. Tool geometry
• Some other geometrical parameters affecting the tool life
are
a. Inclination angle- Tool life increases with the increase in
this angle t an optimum value.
b. Nose radius- while it increase abrasion, it also helps in
improving surface finish and tool strength and the tool
life.
165
5. Work material
• The microstructure of the work material has significant
effect on the hardness of material.
• For example presence of free graphite and ferrite in cast
iron and imparts softness to them.
• Pearlitic structure is harder than ferrite and martensitic is
the hardest.
• Similarly scale formation and presence of oxide layer on
the work surface serve as abrasives and therefore have
detrimental effect on tool life.
166
5. Work material
• Consequently, higher the hardness of the work material
greater will be the tool wear and shorter will be tool life.
• Adverse effects on tool life are also experience in pure
metal as they have tendency to stick to the tool face,
specially at higher temperatures.
• This results in more friction and hence high amount of
wear on tool and therefore a shorter tool life.
167
6. Nature of cutting
• Tool life is also affected by nature of cutting i.e.
continuous or intermittent.
• In case of intermittent cutting the tool is subjected to
repeated impact loading and may give way much earlier
than expected until it is made substantially strong and
tough.
• In continuous cutting, a similar tool will have a longer tool
life.
168
7. Rigidity of Machine tool
• Both the machine tool and work should remain rigid
during the machining operation.
• If not, vibration will take place and then the cutting tool
will be subjected to intermittent loads and hence results
in shorter tool life.
169
Tool life- Use of cutting fluids
• Cutting fluids are used in machining work for helping the
efficient performance of the tool operation.
• They assist in the operation in many ways, such as by
cooling tool and work, reducing friction, improving
surface finish, helping in breaking chips and washing
them away, etc
• These factors help in improving tool life, permitting
higher metal removal rate and improving quality of
surface finish.
170