Unit-I Theory of Metal Cutting-1.pptx

ME5402
METAL CUTTING AND MACHINE TOOLS

UNIT-I
THEORY OF METAL CUTTING

Production System - The Big Picture
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Process
Manufacturing
Process
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People, Money, Equipment, Materials and Supplies, Markets, Management
4

Material Removal Processes
It is shaping operations, the common feature of which is
removal of material from a starting work part so the
remaining part has the desired geometry.
 Machining – material removal by a sharp cutting tool,
e.g., turning, milling, drilling.
 Abrasive processes – material removal by hard,
abrasive particles, e.g., grinding.(Surface Finishing)
4

Why Machining is Important
• Variety of work materials can be machined
– Most frequently used to cut metals
• Variety of part shapes and special geometric features
possible, such as:
– Screw threads
– Accurate round holes
– Very straight edges and surfaces
• Good dimensional accuracy and surface finish
5

Disadvantages with Machining
• Wasteful of material
– Chips generated in machining are wasted material, at least
in the unit operation
• Time consuming
– A machining operation generally takes more time to shape
a given part than alternative shaping processes, such as
casting, powder metallurgy, or forming
6

Examples of Cutting Processes
8

Machining Parameters
• Cutting speed is the primary cutting motion, which
relative the velocity of the cutting tool relative to the
work pieces. unit is (m/min) or (m/s)
• Feed rate is the distance the tool travel per unit
revolution of the work piece (mm/rev) or mm/min,
mm/rev
• DOC is distance of the tool is plunged into surface. It
is difference b/w initial & final diameter

Diagram Representation of Material
Removal Operations
10

General characteristics of a metal cutting tool

Cutting action involves shear deformation of work
material to form a chip.
As chip is removed, new surface is exposed.
Figure 21.2 (a) A cross-sectional view of the machining process, (b)
tool with negative rake angle; compare with positive rake angle in (a).
Chip Formation

Mechanism of Chip formation
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The cutting tool removes the metal from the workpiece in the form
of ‘‘chips’’. As the tool advances into the workpiece, the metal in
front of the tool is compressed and when the compression limit of
the metal has been exceeded, it is separated from the workpiece
and flows plastically in the form of chip. The plastic flow of the
metal takes place in a localized region called shear plane, which
extends from the cutting edge obliquely up to the uncut surface in
front of the tool. The cutting tool causes shearing action bearing
the metal along the plane (Fig. 8.1). The region between the lines
LM and NP is called shear zone

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The metal in front of the tool rake face gets immediately
compressed first elastically and then plastically. This zone
is traditionally called the shear zone, in view of the fact the
material in the final form would be removed by shear from
the parent metal.
The actual separation of the metal starts from the cutting
tool tip as yielding or fracture, depending upon the cutting
conditions. Then the deformed metal (called chip) flows
over the tool (rake) face. If the friction between the tool
rake face and the underside of the chip (deformed
material) is considerable, then chip gets further deformed,
which is termed as secondary deformation. The chip after
sliding over the tool rake face would be lifted away from
the tool, and the resultant curvature of the chip is termed

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case strained layers of material would get displaced over
other layers along the slip-planes which coincide with the
direction of maximum shear stress. Piispanen presented
an interesting mechanism to account for the deformation
process taking place at the cutting edge. He considered
the undeformed metal as a
stack of cards which would slide over one another as the
wedge shaped tools moves under these cards as shown
in Fig. 2.4.

The form of the chips is an important index of machining
because it directly or indirectly indicates :
 Nature and behaviour of the work material under
machining condition
 Specific energy requirement (amount of energy required
to remove unit volume of work material) in machining
work
 Nature and degree of interaction at the chip-tool
interfaces.
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The form of machined chips depend mainly upon :
 Work material
 Material and geometry of the cutting tool
 Levels of cutting velocity and feed and also to some extent on
depth of cut
 Machining environment or cutting fluid that affects
temperature and friction at the chip-tool and work-tool
interfaces.
Knowledge of basic mechanisms of chip formation helps to
understand the characteristics of chips and to attain
favourable chip forms.
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TYPES OF CHIP
• The chip formation in metal cutting could be
broadly categorized into three types:
(i) Discontinuous chip
(ii) Continuous chip, and
(iii) Continuous chip with BUE
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DISCONTINOUS CHIP
• When brittle materials like cast iron are cut, the
deformed material gets fractured very easily and
thus the chip produced is in the form of
discontinuous segments as shown in Fig. 2.5
22

DISCONTINOUS CHIP
• Also they generally provide better surface finish.
However, in case of ductile materials they cause
poor surface finish and low tool life. Higher depths
of cut (large chip thick ness)
• low cutting speeds and small rake angles are
likely to produce dis continuous chips.
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CONTINOUS CHIP
• Continuous chips are normally produced when
machining steel or ductile metals at high cutting
speeds. The continuous chip, which is like a ribbon
flows (Fig. 2.7) along the rake face. Continuous
chip is possible because of the ductility of metal
(steel at high temperature generated due to
cutting) flows along the shear plane instead of
rupture.
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CONTINOUS CHIP
• Some ideal conditions that promote continuous chips in metal
cutting are: sharp cutting edge, small chip thickness (fine feed),
large rake angle, high cutting speed, ductile work materials and
less friction between chip tool interfaces through efficient
lubrication.
• This is the most desirable form of chip since the surface finish
obtained is good and cutting is smooth. It also helps in achieving
higher tool life and lower power consumption. However, because
of the large coils of chips, the chip disposal is a problem. To help
in this direction various forms of chip breakers have been
developed which are in the form of a step or groove in the tool
rake face. The chip breakers allow the chips to be broken into
small pieces so that they can be easily disposed of.
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CONTINOUS CHIP with BUE
• When the friction between tool and chip is high while machining
ductile materials, some particles of chip adhere to the tool rake
face near the tool tip. When such sizeable material piles up on
the rake face, it acts as a cutting edge in place of the actual
cutting edge as shown in Fig 2.8. This is termed as built up edge
(BUE). By virtue of work hardening, BUE is harder than the
parent work material.
• The conditions that normally induce the formation of BUE are low
cutting speed, high feed and low rake angle
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Types of cutting tools
• Single point: These tools have only one
cutting edge. e.g., turning tools, shaping,
planning and slotting tools and boring tools.
• Multipoint (more than two): These tools
have more than one cutting edges. e.g.,
milling cutters, broaching tools, hobs, gear
shaping cutters, drilling, reaming

Nomenclature of single point cutting tool
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Tool Elements and Tool Angles :
Tool elements. The definitions of various tool elements are :
(i) Shank. It is the main body of the tool at one end of which the cutting
portion is formed.
(ii) Flank. The surface (or surfaces) below and adjacent to the cutting edge
is called the flank of the tool.
(iii) Face. The surface on which the chip slides is called the face of the
tool.
(iv) Heel. It is the intersection of the flank and base of the tool.
(v) Nose. It is the point where the side cutting edge and end cutting edge
intersect.
(vi) Cutting edge. It is the edge on the face of the tool which removes the
material from the workpiece.

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33
Tool angles :
(i) Side cutting edge angle. It is angle between the side cutting edge
and the side of the tool shank.
— It is also known as ‘lead angle’.
— Its complementary angle is called ‘Approach angle’.
This angle prevents interference as the tool enters the work material.
Its satisfactory values vary from 15° to 30° for general machining.
(ii) End cutting edge angle. This is the angle between the end cutting
edge and a line normal to the tool shank. This angle provides a
clearance or relief to the trailing end of the cutting edge to prevent
rubbing or drag between the machined surface and the trailing part of
the cutting edge. Only a small angle is sufficient for the purpose.
An angle of 8° to 15° has been found satisfactory in most cases on
side cutting tools, like boring and turning tools. End cutting tools, like
cut off and necking tools often have no end cutting-edge angle.

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(iii) Side relief angle. It is the angle between the portion of the side flank
immediately below the side cutting edge and a line perpendicular to the
base of the tool, and measured at right angle to the side flank.
(iv) End relief angle. It is the angle between the portion of the end flank
immediately below the end cutting edge and a line perpendicular to the
base of the tool, and measured at right angle to the end flank.
The side and relief angles are provided so that the flank of the tool clears
the workpiece surface and there is no rubbing action between the two.
— These angles range from 5° to 15° for general turning.
— Small relief angles are necessary to give strength to the cutting edge
when machining hard and strong materials.
— Tools with increased values of relief angles penetrate and cut the
workpiece material more efficiently and this reduces the cutting forces.
— Too large relief angles weaken the cutting edge and there is less mass
to absorb and conduct the heat away from the cutting edge.

35
(v) Back rake angle. It is the angle between the face of the tool and a line parallel to
the
base of the tool and measured in a plane (perpendicular) through the side cutting
edge.
— This angle is positive, if the side cutting edge slopes downwards from the point
towards
the shank and is negative if the slope of the side cutting edge is reverse.
(vi) Side rake angle. It is the angle between the tool face and a line parallel to the
base of the
tool and measured in a plane perpendicular to the base and the side cutting edge.
— This angle gives the slope of the face of the tool from the cutting edge.
The side rake is negative if the slope is towards the cutting edge and positive if the
slope is
away from the cutting edge.
The ‘‘rake angle’’ specifies the ease with which a metal is cut.
— Higher the rake angle, better is the cutting and less are the cutting forces. There is
a maximum limit to the rake angle and this is generally of the order of 15° for high
speed steel
tools cutting mild steel (increase in rake angle reduces the strength of the tool chip as
well
as the heat dissipation).

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(vii) Clearance angle. This is the angle between the machined
surface and underside of the tool called the flank face.
The clearance angle is provided such that the tool will not rub the
machined surface thus spoiling the surface and increasing the cutting
forces.
A very large clearance angle reduces the strength of the tool lip, and
hence normally an angle of the order of 5°–6° is used.
(viii) Nose angle. It is the angle between the side cutting edge and
end cutting edge.
Nose radius is provided to remove the fragile corner of the tool ; it
increases the tool life and improves surface finish. Too large a nose
radius will induce chatter.

Tool signature
• A tool having 7, 8, 6, 7, 5, 6, 0.1 as designation
(Signature) have the following angles and nose
radius.
• Back rack angle = 7
• Side rake angle = 8
• End relief angle = 6
• Side relief angle = 7
• End cutting edge angle = 5
• Side cutting edge angle = 6
• Nose radius = 0.1 mm

ORTHOGONAL AND OBLIQUE CUTTING
39
1. Orthogonal cutting : Refer to Fig. 8.6.
When the tool is pushed into the workpiece, a layer of material is removed from
the
workpiece and it slides over the front face of the tool called rake face. When the
cutting
edge of wedge is perpendicular to the cutting velocity, the process is called
orthogonal
cutting. In this case, the material gets deformed under plane strain conditions ;
the chip slides directly up the tool face. Rarely in practice, however, is the cutting
edge at right angles to the direction of cutting (i.e., orthogonal cutting).

40
2. Oblique cutting : Refer to Fig 8.7.
In most practical metal-cutting processes, the cutting edge of the
tool is not perpendicular to the cutting velocity but set at angle with
the normal to the cutting velocity. Cutting in this case takes place
in three-dimensions (turning or milling) and represents the general
case of oblique cutting. In oblique cutting a lateral direction of chip
movement is obtained.

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FORCE OF A SINGLE PONT TOOL
42

43

44
• The forces are not changed significantly by a change in cutting
speed.
• The greater the ‘feed’, of the tool, the larger the forces.
• The greater the ‘depth’ of the cut, the larger the forces.
• Tangential force increases with chip size.
Measurement of cutting forces :
Although an indirect method of measuring cutting forces acting on the tool is with
the aid of a ‘‘wattmeter’’, yet a more exact method is with the aid of a tool
dynamometer.
The total force during metal cutting, in most metal cutting dynamometers, is
determined by measuring the deflections or strains in the elements supporting the
cutting tool. The design of the dynamometer should be such as to give strains or
displacements large enough to be measured accurately.
The commonly used tool dynamometer are :
1. Mechanical dial gauge type.
2. Strain gauge dynamometer.
A strain gauge dynamometer is more accurate than a mechanical dial gauge.
3. Pneumatic and hydraulic dynamometers.
4. Electrical dynamometers.
5. Piezoelectrical dynamometers.

Shear Zone, Shear Plane and Shear Angle
45
When cutting tool is introduced into the work material, plastic deformation
takes place in a narrow region in the vicinity of the cutting edge. This region
is called shear zone (see Fig. 8.10). The width of this zone is small and
therefore chip formation is often described as a process of successive shears
of thin layers of the work material along particular surfaces. At high speeds,
this zone can be assumed to be restricted to a plane called shear plane (see
Fig. 8.11) inclined at an angle (shear angles).

50
In an orthogonal cutting test with a tool of rake angle
10°, the following observations were made:
Chip thickness ratio = 0.3
Horizontal component of the cutting force = 1290 N
Vertical component of the cutting force = 1650 N
Breath = 6mm - thickness = 0.10mm
From the Merchant's theory, calculate the various
components of the cutting forces, all angle, shear
stress and the coefficient of friction at the chip tool
interface.

Thermal Aspect
51
During metal cutting, the energy dissipated gets converted into heat.
Consequently, high temperatures are generated in the region of the tool
cutting edge, and these temperatures have a controlling influence on the
rate of wear of the cutting tool and on the friction between the chip and
tool. When a material is deformed elastically, the energy required for the
operation is stored in the material as strain energy, and no heat is
generated. However, when the material is deformed plastically, most of
the energy used is converted into heat. In metal cutting the material is
subjected to extremely high strains and the elastic deformation forms a
very small proportion of the total deformation ; therefore it may be
assumed that all the energy is converted into heat. In fact, heat is
generated in three distinct regions (Fig) these are :

Thermal Aspect
52
(i) The shear zone : Here the energy needed to shear the chip is the source of
heat. In this region about 80–85% of the heat is generated.
(ii) The chip-tool interface region. Here the energy needed to overcome friction
is the
source of heat. Some plastic deformation also occurs in this region. About 15–
20% heat
is generated in this region.
(iii) The tool-work interface region. Here energy needed to overcome frictional
rubbing
between flank face of the tool and workpiece is the source of heat. In this region
only
1–3% of heat is generated.
Measurement of Chip-Tool Interface Temperature
Follow are the various methods of measuring chip-tool interface temperature :
1. Tool work thermocouple.
2. Embedded thermocouples.
3. Infra-red photographic technique.
4. Temperature sensitive techniques.
5. Temper colours.
6. Indirect calorimetric technique.

Thermal Aspect
53
1. Tool work thermocouple :
In this technique, the hot end of the tool and workpiece and their cold
ends act as thermocouple and e.m.f. proportional to temperature
difference is produced. The workpiece is insulated from the chuck and
tailstock centre. The end of workpiece is connected to a copper wire
which dipped in mercury cup enables further connection serving as cold
end. This point and a connection from tool provide output for connection
to a milli voltmeter.
2. Embedded thermocouples :
The thermocouples are embedded in fine holes eroded in H.S.S. tool
from bottom face up to a fixed distance from the rake face. This
arrangement enables measurement of temperature at several points
along the rake face of tool.
3. Infra-red photographic technique :
This technique of temperature measurement is based on taking
photographs of the side face of tool-chip while cutting and comparing
them with strips whose temperatures are known.

TOOL WEAR
• Wear can be defined as the loss of weight or mass that
accompanies the contact of sliding surfaces.
• The wear mechanism associated with gradual or
progressive wear include :
• (i) Abrasion wear.
• (ii) Adhesion wear.
• (iii) Diffusion wear.

Tool wear mechanism
• Adhesion wear: This form of wear takes place when two
surfaces are brought into intimate contact under normal
loads and form welded junctions, which, when subjected to
shearing loads, are subsequently destroyed.
• Abrasion: Hard particles, microscopic variations on the
bottom surface of the chips rub against the tool surface and
break away a fraction of tool with them.
• Diffusion wear: At high temperatures, atoms from tool
diffuse across to the chip; the rate of diffusion increases
exponentially with temperature; this reduces the fracture
strength of the crystals.

TOOL LIFE
• Tool life is defined as the time interval between two successive
regrinds.
• Factors affecting tool life :
• Tool life depends upon the following factors :
• (i) Tool material.
• (ii) Hardness of the material.
• (iii) Type of material being cut.
• (iv) Type of the surface on the metal (Rough or smooth).
• (v) Profile of the cutting tool.
• (vi) Type of the machining operation being performed.
• (vii) Microstructure of the material.
• (viii) Finish required on the workpiece.
• (ix) Cutting speed. As the cutting speed is reduced, the tool life
increases.
• (x) Feed and depth of cut. An increase in feed and depth of cut will
shorten tool life but not nearly as much as an increase in cutting
speed.

TOOL LIFE EQUATION
• Tool life of a cutting tool may be calculated by using the
following relation :
• VTn = C This equation is known as Taylor’s tool
equation.
• where, V = Cutting speed in m/min.,
• T = Tool life in min.,
• C = A constant (which is numerically equal to cutting
speed that gives the tool life of one min.), and
• n = Another constant (depending upon finish, workpiece
material and tool material)
• n= 0.1 for H.S.S. steel tools : 0.2 to 0.25 for carbide tools
and 0.4 to 0.55 for ceramic tools.

TOOL LIFE EQUATION
• Calculate the cutting speed for a tool to have a tool life of
160 min. The same tool had a life of 9 min when cutting at
250 m/min. Take n = 0.22 in the Taylor’s tool life equation.
• During straight turning of a 24 mm diameter steel bar at
300 r.p.m. with an H.S.S. tool, a tool life of 9 min. was
obtained. When the same bar was turned at 250 r.p.m.,
the tool life increased to 48.5 min. What will be the tool life
at a speed of 280 r.p.m. ?

CUTTING TOOL MATERIALS
• (i) Higher hardness than that of the work piece material being machined, so
that it can penetrate the work material.
• (ii) Hot hardness, which is the ability of the material to retain its hardness at
elevated temperatures, in view of the high temperatures existing in the cutting
zone. This requirement becomes more and more stringent with the increasing
emphasis on higher cutting speeds to bolster productivity.
• (iii) Wear resistance – The chip-tool and chip-work interfaces are exposed to
such severe conditions that adhesive and abrasion wear is very common.
The cutting tool material should therefore have high abrasion resistance to
improve the effective life of the tool.
• (iv) Toughness – Even though the tool is hard, it should have enough
toughness to withstand the impact loads at the beginning of the cut or to force
fluctuations due to imperfections in the work material. This requirement is
more useful for interrupted cutting, e.g. milling.
• (v) Low friction – The coefficient of friction between chip and tool should be
low which would allow for lower wear rates and better chip flow.
• (vi) Better thermal characteristics – Since a lot of heat is generated at the
cutting zone, it is necessary that the tool material should have higher thermal
conductivity to dissipate this heat in the shortest time, otherwise the tool
temperature will become too high thus reducing its life.

TYPES OF CUTTING TOOL MATERIALS
1. High-speed steels
2. Cast-cobalt alloys
3. Carbides
4. Coated tools
5. Alumina-based ceramics
6. Cubic boron nitride
7. Silicon-nitride-based ceramics
8. Diamond
9. Whisker-reinforced materials and nanomaterials.

Comparison cutting tool material

Cutting fluid
• Cutting fluid are used to carry away the heat
produced during machining. It reduce friction
between tool and work piece.
• Properties of cutting fluid
• It should have high specific heat and k
• High flash point
• Non corrosive to tool
• Low viscosity to permit free flow
• It should be transparent , economical use

Function of cutting fluid
• To reduce the Friction
• Disposal chip
• To cool the cutting tool and the workpiece.
• To lubricate the chip, tool, and workpiece.
• To help carry away the chips.
• To lubricate some of the moving parts of the
machine tool.
• To improve the surface finish.
• To prevent the formation of built-up-ridge.
• To protect the work against rusting.

Types of cutting fluid
• Water based cutting fluid
Added soft soap or mineral oil to water. 80%
water and 20% mineral oil
• Straight or heat oil based cutting fluid
Straight oil based cutting fluid means undiluted
or pure oil based fluid. oil with chemical such
as sulphur and chlorine.

Types of cutting fluid
• 1. Oils (also called straight oils), including mineral, animal,
vegetable, compounded, and synthetic oils, typically are used for
low-speed operations where temperature rise is not significant.
• 2. Emulsions (also called soluble oils), a mixture of oil and water
and additives, generally are used for high-speed operations because
the temperature rise is significant. The presence of water makes
emulsions highly effective coolants. The presence of oil reduces or
eliminates the tendency of water to cause oxidation.
• 3. Semi synthetics are chemical emulsions containing little mineral
oil, diluted in water, and with additives that reduce the size of oil
particles, making them more effective.
• 4. Synthetics are chemicals with additives, diluted in water, and
containing no oil.

Other types of cutting fluid :
• Mineral oil
• Straight fatty oil
• Mixed or compound oil
• Sulphurised oil
• Chlorinated oil
Types of flushing systems:
• Flooding System
• Mist
• High Pressure system
• Through the cutting tool system

Machinability
• Machinability is defined as the ease with which
material can be satisfactorily machined.
• The life of tool before tool failure or resharpen
• Quality of machined surface
• Power consumption per unit volume of material
removed.

Factor affect machinability
• Work piece variable
• Tool material
• Machine variable
• Cutting condition
Machinability index
It is quantitative measure of machinability
I - vi/vs
Vi – cutting speed of metal investigated for 20min tool
life
Vs –standard steel for 20min tool life

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