Principles of Metal Cutting

Chapter 1
Principles of Metal Cutting
1.1 Introduction
Metal Cutting or Machining is the process of producing a workpiece by removing
unwanted material from block of metal, in the form of chips. This process is most
important since almost all the products get their final shape and size by metal
removal, either directly or indirectly, The major drawback of the process is loss of
material in the form of chips. In this chapter, we shall have a fundamental under-
standing of the basic metal cutting process.
1.2 Fundamentals of Cutting
Cutting processes remove material from the surface of a workpiece by producing
chips. Some of the more common cutting processes are illustrated in Fig. 1.1.
In the turning operation (Fig. 1.1 a) the workpiece is rotated and a cutting tool
removes a layer of material as it moves to the left. Fig. 1.1 (b) shows a cutting-off
operation, where the cutting tool moves radially inward and separates the right
piece from the bulk of the blank. Fig. 1.1 (c) shows a slab-milling operation in which
a rotating cutting tool removes a layer of material from the surface of the workpiece
and Fig. 1.1 (d) shows a end-milling operation, in which a rotating cutter travels
along a certain depth in the workpiece and produces a cavity.
One of the most common processes is turning, illustrated in Fig. 1.2. The cutting
tool is set at a certain depth of cut (measured in mm or in) and travels to the left with
a certain velocity as the work piece rotates. The feed or feed rate is the distance the
tool travels per unit revolution of the workpiece (mm/rev or in/rev). As a result of
this action, a chip is produced which moves up the face of the tool.
In order to analyze this process in detail, a two-dimensional model of it its presented
in Fig. 1.3. In this idealized model, a cutting tool moves to the left along the workpiece
at a constant velocity, V, and depth of cut, to. A chip is produced ahead of the tool by
deforming and shearing the material continuously along the shear plane.

1.2 Metal Cutting
Fig 1.1. Examples of cutting processes
Fig 1.2. Basic principle of the turning operation
Fig 1.3. Schematic illustration of a two dimensional cutting
process also called orthogonal cutting

Principles of Metal Cutting 1.3
Table. 1.1 Factors influencing cutting process
Parameter Influence and interrelationship
Cutting speed, feed,
depth of cut, cutting
fluids Tool angles
Continuous chip
Built-up edge chip
Discontinuous chip
Temperature rise
Tool wear
Machinability
Forces, Power, temperature rise, tool life, type of chip,
surface finish.
As above; influence on chip flow direction;
resistance to tool chipping.
Good surface finish; steady cutting forces;
undesirable in automated machinery.
Poor surface finish; thin stable edge can protect
tool surfaces.
Desirable for ease of chip disposed, fluctuating
cutting forces; can affect surfaces finish and cause
vibration and chatter.
Influences tool life, particularly crater wear, and
dimensional accuracy of workpiece; may cause
thermal damage to workpiece surface.
Influences surface finish, dimensional accuracy,
temperature rise, forces and power.
Related to tool life, surface finish, forces and
power.
In comparing Figs. 1.2 and 1.3, note that the feed in turning is now equivalent to t0
and the depth of cut in turning is equivalent to the width of cut (dimension
perpendicular to the page) in the idealized model. These relationships can be visualized
by rotating Fig. 1.3 clockwise by 90°. Table 1.1 outlines the factors that influence a
cutting process.
1.2.1 Independent Variables in Cutting Process
The major independent variables in the cutting process are as follows
1) Tool material, coatings, and tool condition.
2) Tool shape, surface finish, and sharpness.
3) Workpiece material, Condition, and temperature.
4) Cutting parameters, such as speed, feed, and depth of cut.
5) Cutting fluids.
6) The characteristics of the machine tool, such as its stiffness and damping.
7) Work holding and fixturing.

1.4
1.2.2 Dependent Variables in Cutting Process
Those that are influenced by changes in the independent variables are the following.
1) Type of chip produced.
2) Force and energy dissipated in the cutting process.
3) Temperature rise in the workpiece, the chip and the tool,
4) Wear and failure of the tool.
5) Surface finish produced on the workpiece after machining.
1.3 Types of Cutting Tools
A Cutting tool may be used either for cutting a part, as with a knife, or for removing
chips, parts are produced by removing metal mostly in the form of small chips.
Chip removal in the metal-cutting process may be performed either by cutting
tools having distinct cutting edge or by abrasives used in grinding wheels, abrasive
sticks, abrasive cloth, etc. These abrasives have a very large number of hard grains
with sharp edges which remove metal from the workpiece surface in such operations
as grinding.
All cutting tools can be divider into two groups.
These are:
1) Single - Point tools.
2) Multi -Point tools.
Single-Point Cutting tools having a wedge-like action find, a wide application on
lathes, and slotting machine. Multi-point cutting tools are merely two or more single-
point tools arranged together as a unit. The milling cutter and broaching tool are
good example of this type.
The simplest form of cutting tool is the single. Point tool. The cutting process as
performed by multi point tools closely resembles machining as performed by single-
point tools. In this text the cutting action of a single-point tool is dealt with elaborately.
1.4 Single point cutting tool
A single point cutting tool consists of a sharpened cutting part called its point and the
shank, Fig. 1.4. The point of the tool is bounded by the face (along which the chips
slides as they are cut by the tool), the side flank or major flank, the end flank, or
minor flank and the base. The side cutting edge a-b, is framed by the intersection of
the face and the side flank. The end cutting edge a-c is formed by the intersection of
the face and the end flank. The chips are cut from the work piece by the side-cutting
edge. The point a where the end and side-cutting edges meet is called the | nose of the
tool. Fig. 1.4 is for a right hand tool. Below; we give the definitions of the various tool
elements and tool angels.
1.4.1 Shank
it is the main body of the tool.

Fig 1.4. A single point cutting tool
1.4.2 Flank
The surface or surfaces below and adjacent to the cutting edge is called flank of the
tool.
1.4.3 Face
The surface on which the chip slides is called the face of the tool.
1.4.4 Heel
It is the intersection of the flank and base of the tool.
1.4.5 Nose
It is the pint where the side cutting edge and end cutting edge intersect.
1.4.6 Cutting edge
It is the edge on the face of the tool
which removes the material from the
work piece. The total cutting edge
consists of side cutting edge (major
cutting edge), end cutting edge (minor
cutting edge and the nose).
A single point cutting tool may be
either right or left hand cutting tool
depending on the direction of feed.
In a right cutting tool the side
cutting edge is on the side of the
thumb when the right a hand is
placed on the tool with the palm
downward and the fingers pointed Fig. 1.5. Left and Right cutting tools

1.6
towards the tool nose (Fig. 1.5). Such a tool will a cut when fed from right to left as in a
lathe in which the tool moves from tailstock to headstock. A left-cutting tool is one in which
the side cutting edge is on the thumb side when the left hand is applied (Fig. 1.5). Such a
tool will cut when fed from left to right.
The various types of surfaces and planes in metal cutting are explained below with the
help of Fig. 1.6, in which the basic turning processes, is shown. The three types of surfaces
are:
1) The work surface, from which the material is cut.
2) The machined surface which is formed or generated after removing the chip.
3) The cutting surface, which is formed by the side cutting edge of the tool.
The references from which the tool angles are
specified are the "cutting'plane" and the 'basic
plane' or the "principal plane". The
cutting plane is the plane tangent to the cutting
surface and passing through and containing the side
cutting edge. The basic plane is the plane parallel to
the longitudinal and cross feeds, that is, this plane lies
along and normal to the longitudinal axis of the
workpiece. In a lathe tool, the basic plane
coincides with the base of the tool.
»
Fig.1.6. Principal surfaces and
. _ rr.i ' t. ry c c *. plances in metal cutting
1.5 Tool-point Reference Systems
To determine the orientation and inclination of the rake face and flank surfaces, a coordinate
system is essential, resulting in a set of planes with reference to which the orientations can
be determined. Two planes are of significance: the first one is called the tool reference plane
or principal plane, which is perpendicular to the cutting plane, which is tangential to the
cutting edge and contains the velocity vector. The cutting plane is also perpendicular to the
principal plane or the tool reference plane.
There are three different types of coordinate systems that are popular, when it comes to
tool nomenclature. They are:
1) Machine reference system.
2) Orthogonal tool reference system, and
3) Oblique or normal tool reference system.
1.6 Machine reference system
The machine reference system defines the coordinates with respect to the machine axis. For
example in a lathe, x x1
is the spindle or work piece axis, yy1
is the tool axis and zz1
is the
vertical axis. Fig. 1.7 depicts the x-y-z axes and fig. 1.8 shows the principal plane and the
cutting plane under this system, along with the reference plane formed by the coordinates.

Fig. 1.8. Reference planes in the machine reference system
Fig. 1.9 shows the orientation of rake face and flank surfaces with respect to the machine
reference system along with the tool angles in symbolic form. The following terminology is
applicate to this system of tool nomenclature:
yp = back rake angle
Yi■ = side rake angle
ccp = front or end clearance or relief angle
ctj = side clearance or relief angle
Fig. 1.7. x-y-z Coordinates of machine reference system

Fig. 1.9. Machine reference system
The machine reference system of defining angles is sometimes called the old
American system of tool nomenclature, because of its popularity in earlier times. In
its abbreviated form it is referred to as ASA (American Standards Association) sys-
tem. The shape of the tool is normally specified in a special sequence, often referred
to as tool signature, as shown below;
1.7 Orthogonal Tool Reference System
Generally referred to as ORS (Orthogonal Rake System) nomenclature, this consists
of the following reference
(a) The principal or the tool reference plane I1R, which is the same as for the
machine reference system.
(b) The cutting plane nc, which contains the cutting edge, normal to riR, and
(c) A third plane rio which is perpendicular to both riR and nc, and contains the
cutting velocity vector.
1.8
= end cutting edge angle
= side cutting edge angle
= nose radius.

Fig.1.10. Orthogonal tool reference system of planes
Fig. 1.11 shows the orientation of rake face and flank surfaces with respect to the
orthogonal tool reference system and the tool angles in symbolic form. The following
terminology is applicable to this system of tool nomenclature.
= inclination angle
= orthogonal rake
= orthogonal clearance angle
= auxiliary clearance angle
= principal cutting edge angle
= nose radius.
The shape of the tool is usually specified in a special sequence, called ORS tool
signature as given below:
The ORS features have been incorporated by the new American system as ANSI
(American National standards Institute) nomenclature.
Fig.1.10 depicts the orthogonal tool reference system of planes to define the tool angles.

Fig.1.11. Orthogonal Rake System (ORS) geometry
1.8 Normal tool reference system
Fig.1.12. Normal tool reference systems of planes
Also called the oblique tool reference system. This system consists of the following
planes:
/. 10

(a) The principal of tool reference plane I1K, which is the same as in the machine
reference and orthogonal tool reference system.
(b) Cutting plane nc, which is the same as that of the orthogonal tool reference
system, and
(c) The third plane nn, which is not perpendicular to the riR plane is known as
the normal plane because it is perpendicular to the cutting edge. This is
inclined to the normal to velocity by the inclination angle X%.
Fig. 1.12 depicts the normal tool reference system (NRS) of planes to define the
tool angles, and is generally referred to as NRS (Normal Rake System) nomenclature.
Fig.1.13 shows the orientation of face and flank surfaces with respect to the NRS
system and tool angles in symbolic form. The following terminology is applicable to
this system of tool nomenclature:
Fig. 1.13. Orthogonal Rake System (ORS) and Normal
Rake System (NRS) geometry
= inclination angle
= normal rake
= normal clearance angle
= auxiliary normal clearance angle
= principal cutting edge angle =
nose radius.

1.12
The NRS tool signature is given below:
Table: 1.2 of gives a summary and comparison of the three systems of nomenclature,
and Fig. 1.14, depicts all three systems
Table.1.2: Tool nomenclature systems
Tool reference system
Orthogonal
rake system
Normal rake
system
Machine
reference
system.
(ASA)
1. Location of cutting edges
2. Orientation of face
to A Y«A rPA
Fig. 1.14. Single-Point tool geometry
Tool signatures:

3. Orientation of principal
flank
4. Orientation of auxiliary
flank
5. Nose radius
<
r
7 ap,af
y
1.9 The Mechanics of Chip Formation
Atypical metal cutting process can be schematically represented as in Fig. 1.15. A
wedge-shaped tool is made to move relative to the workpiece.
Fig. 1.15. Schematic representation of machining process.
As the tool makes contact with the metal it exerts a pressure on it resulting in the
compression of the metal near the tool tip. This induces shear-type deformation
with in the metal and it starts moving upward along the top face of the tool. As the
tool advances, the material ahead of it is sheared continuously along a plane called
the "shear plane". This shear plane is actually a narrow zone (of the order of about
0.025mm) and extends from the cutting edge of the tool to the surface of the
workpiece.
The cutting edge of the tool is formed by two intersecting surfaces. The surface
along which the chip moves upwards is called "Rake surface" and the other surface,
Which is relieved, to avoid rubbing with the machined surface, is called "Flank".
The angle between the rake surface and the normal is known as "Rake angle" (which
may be positive or negative), and the angle between the flank and the horizontal
machined surface is known as the "relief or clearance angle". Most cutting processes
have the same basic features as in Fig. 1.15, where a single point cutting tool is used
(a milling cutter, a drill, and a broach can be regarded as several single-point tools
joined together and are known as multi-point tools).

/. 14
1.10 Methods of Machining
In the metal cutting operation, Fig. 1.15, the tool is wedge-shaped and has a straight cutting
edge. Basically, there are two methods of metal cutting, depending upon the arrangement of
the cutting edge with respect to the direction of relative work-tool motion.
1) Orthogonal cutting or two dimensional cutting.
2) Oblique cutting or three dimensional cutting.
Fig. 1.16. Methods of machining
In orthogonal cutting, Fig.1.16, the cutting edge of the tool is arranged perpendicular to the
cutting velocity vector, V, whereas in oblique cutting, it is set at some angle other than 90°
to the cutting velocity vector, which gives an "inclination angle i". The analysis of oblique
cutting being very complex, the relatively simple arrangement of orthogonal cutting is,
therefore, widely used in theoretical and experimented work.
In pure orthogonal cutting i =0°, Ce=0°, and A.=90°. This is also known as orthogonal
system of second kind. When i=0, and 0<?i<90°, it is called as orthogonal system of first
kind. A common example of pure orthogonal cutting process of the turning of a thin pipe
with a straight edged tool set normal to the longitudinal axes.
1.11 Types of Chips
Whatever the cutting conditions can be, the chips produced may belong to one of the
following three types, Fig.1.17:
1) Discontinuous chips.
2) Continuous chips.
3) Continuous chips with built up edge (BUE).

Fig.1.17. Types of chips
1.11.1 Discontinuous Chips
These types of chips are usually produced when cutting more brittle materials like
grey cast iron, bronze and hard brass. These materials lack the ductility necessary
for appreciable plastic chips formation. The material ahead of the tool edge fails in
a brittle fracture manner along the shear zone. This produces small fragments of
discontinuous chips. Since the chips break up into small segments, the friction
between the tool and the chips reduces, resulting in better surface finish. These
chips are convenient to collect, handle and dispose of, Discontinuous chips are also
produced when cutting more ductile materials under the following conditions.
1) Large chip thickness.
2) Low cutting speed.
3) Small rake angle of the tool.
4) Cutting with the use of a cutting fluid.
1.11.2 Continuous Chips
These types of chips are produced when, machining more ductile materials. Due to
large plastic deformations possible with ductile materials, longer continuous chips
are produced. This type of chip is the most desirable, since it is stable cutting,
resulting in generally good surface finish. On the other hand, these chips are difficult
to handle and dispose off. The chips coil in a helix (chip curl) and curl around the
work and the tool and may injure the operator when break loose. Also, this type of
chip remains in contact with the tool face for a longer period, resulting in more
frictional heat.
These difficulties are usually avoided by attaching to the tool face machine on
the tool face a "chip breaker", Fig. 1.18. The function of chip breaker is to reduce
the radius of curvature of the chip and thus break it..

Fig.1.18. Chip Breaker
The following cutting conditions also help in the production of continuous chips
1) Small chip thickness.
2) High cutting speed.
3) Large rake angle of the cutting tool.
4) Reducing the friction of the chip along the tool face, by: imparting high surface
finish to the tool face, use of tool material with low co-efficient of friction, and use
of a good cutting fluid.
1.11.3 Continuous Chips with Built Up Edge (BUE)
When machining ductile materials, conditions of high local temperature and extreme
pressure in the cutting zone and also high friction in the tool-chip interface, may cause the
work material to adhere or weld to the cutting edge of the tool forming the built-up edge.
Successive layers of work material are then added to the built-up edge. When this edge
becomes larger and unstable, it breaks up and part of it is carried up the face of the tool
along with the chip while the remaining is left over the surface being machined, which
contributes to the roughness of the surface, the built-up edge changes its size during the
cutting operation. It first increases,then decreases etc. this cycle is a source of vibrations and
poor surface finish.
Although, the built-up edge protects the cutting edge of the tool, it changes the geometry
of the cutting tool. Low cutting speed also contributes to the formation of the built-up edge.
Increasing the cutting speed, increasing the rake angle and using a cutting fluid contribute to
the reduction or elimination of the built-up edge.
1.12 Built-up Edge
A built-up edge (BUE), consisting of layer, of material from the workpiece that are
gradually deposited on the tool (hence the term built-up), may form at the tip of the

tool during cutting (Fig. 1.20). As it becomes larger, the BUE becomes unstable and
eventually breaks up. Part of the BUE material is carried away by the tool side of the chip,
the rest is deposited randomly on the workpiece surface, the process of BUE formation and
destruction is repeated continuously during the cutting operation, unless measures are taken
to eliminate it.
The built-up edge is commonly observed in practice. It is one of the factors that most
adversely affects surface finish in cutting, as can be seen in Figs. 1.19 and 1.21.
Fig. 1.19. Continuous chip with Fig.1.20. Continuous chip built-up edge large
primary shear zone
A built-up edge in effect changes the geometry of the cutting edge. Note for example, the
large tip radius of the BUE and the rough surface finish produced.
Fig.1.21.
(a) Hardness distribution in the cutting zone for 3115 steel. Note that some regions in the
built-up edge are as much as three times harder than the bulk metal, (b) Surface finish in
turning 5130 steel with a built-up edge, (c) Surface finish on 1018 steel in face milling.
Magnifications: 15x.

1.18 Metal Cutting & Machine Tools
Because of work hardening and deposition of successive layers of material, BUE
hardness increases significantly (Fig. 1.2la). Although BUE is usually regarded as
desirable becomes it reduces wear by protecting the rake face of the tool
As the cutting speed increases, the size of the BUE decreases: in fact it may not
form at all. The tendency for a BUE to form is also reduced by any at the following
practices:
1) Decreasing the depth of cut,
2) Increasing the rake angle,
3) Using a sharp tool, and
4) Using an effective cutting fluid.
In general, the higher the affinity (tendency to form a bond) of the tool and
workpiece materials, the greater the tendency for a BUE formation. In addition, a
cold-worked metal generally has less tendency to form BUE than one that has been
annealed.
1.13 Chip Breakers
Fig.1.22. (a) Schematic illustration of the action of a chip breaking. Note that the chip
creaker decreases the radius of curvature of the chip
(b) Chip breaker clamped on the rake face of a cutting tool.
(c) Grooves in cutting tools acting as chip breaker.

As stated earlier, long, continuous chips are undesirable becomes they are a potential
safety hazard they tend to become entangled and interfere with cutting operations.
This situation is especially trouble some in high-speed automated machinery and in
untended machining cells using computer numerically controlled machining variables
are under control, the usual procedure employed to the chip intermittently with a
chip breaker
Although the chip breaker has traditionally been a piece of metal clamped to the
rake face of the tool (Fig. 1.22 a and b) which bends the chip and breaks it, most
cutting tools and inserts are now equipped with designs (Fig. 1.22 c) for individual
inserts. Chip breakers increase the effective rake angle of the tool and, consequently,
increase the shear angle.
Fig.1.23. Various chips produced in turning
(a) Tightly curled chip
(b) chip hits workpiece and breaks
(c) continuous chip moving away from workpiece, and
(d) chip hits tool shank and breaks off.
Chips can also be broken by changing the tool geometry, there by controlling chip
flow, as in tne turning operations shown in Fig. 1.23. Experience has indicated that
the ideal chip is in shape of the letter c or the number 9 and fits within a 25mm (1
in) square block.
With soft workpiece materials such as pure aluminum or copper, chip breaking
by such means is generally not effective. Common techniques used with such mate-
rials, include machining at small increments and then pausing (so that a chip is not
generated) of reversing the feed by small increments. In interrupted cutting opera-
tions, such as milling, chip breakers are generally not necessary, since the chips
already have finite lengths because of the intermittent nature of the operation.
1.14 Determination of Shear Angle
In the simplified modal of two dimensional cutting operation the cutting tool is
completely defined by the rake angle a and clearance angle y. In addition the
following assumptions are made:
1) The tool is perfectly sharp and contacts the chip on its front or rake face.
2) The primary deformation takes place in a very thin zone adjacent to the
shear plane AB.

1.20
3) There is no side flow of the chip that is plain strain condition.
Shear angle, 0, is defined as the angle made by the shear plane, with the direction
of the tool travel. If
t = uncut or unreformed chip thickness
and tr = chip thickness after the metal is cut.
(1.1) is
called the "cutting ratio" or "chip- thickness ratio".
(1.2)
Is called the "chip reduction factor",
't' is the distance between two consecutive positions of the cutting edge, measured
in a direction normal to the cutting edge along the plane of the tool face. In a case of
turning with X = 90°. t = feed.
The shear angle can be determined in the following ways:
fal From Fie. 1.24. simnle eeometrv Rives,
Solving this equation for
By dividing numerator and denominator with Cos 0

Prin ciples of Metal Cutting 1.21
[1.4)
After the cut has been taken, the measurements of't' and V can be taken, and so the
cutting ratio can be determined. Then the shear angle existing during the cut can be
obtained with the help of Eqn.1.4.
(b) If the length of cut, 1 is known then the continuity equation gives,
Plbt = Plchtc
Where p is the density of the material, assumed incompressible and 1 is the length
ofchip.
(1.5)
Then, with the help of equation (1.4), 0 can be estimated.
Fig.1.24
(c) when the length of the cut is not directly known, then it can be estimated by
weighing aknown length of chip. Then weight of the chip,
(1.6)
Knowing p, b and t, the length of cut 7 can be determined. Then 0 can be estimated
from equarions (1.5) and (1.6)
(d) If the chip and workpiece velocities Vc and V are known, then from the continuity
equation:
(1.71
Then 0 can be determined from equ

1.22
1.15 Shear Strain
The shearing process in metal cutting can be represented by a simple model shown
inFig.1.25
Fig.1.25. Model of chip formation
The shearing process is analogous to the displacement of a stack of cards, with each
card sliding slightly over the adjacent Gard.
From this model, the shear strain that the material is undergoing can be deter-
mined. Based on its definition the shear strain will be given as, (View at tool tip)
(1.8) 1.16 Velocities
There are three types of velocities involved in any metal cutting process:
1) The velocity of the tool relative to the workpiece, V usually called the cutting
speed.
2) The velocity of the chip relative to the work, Vs> called the shear velocity
that is the velocity at which shearing takes place.
3) The velocity of the chip up the face of the tool Vc, called the chip velocity.

We have from constancy of volume,
Vt = Vc tc, width remaining constant
(1.9)
The relationships in velocities can be determined from Fig
Fig. 1.26
It can be shown that,
(1.10)
From here Vs, can be determined, and so the shear strain rate during cutting can be
estimated, which is given as
The maximum value of V is approximately 25 x 10~3
mm.
1.17 Force Relations
Here the analysis is limited two dimensional or orthogonal cutting, which is simpler
to understand as compared to the complicated three-dimensional cutting process.
When a cut is made, Fig.1.27 (a), the forces acting on the metal chip are:
1) Force Fs. Which is the resistance to shear of the metal in forming the chip. It
acts the shear plane.
2) Force Fn normal to the shear plane. This is the "backing up" force on the
chip provided by the workpiece.
3) Force N at the tool chip interfaces acting normal to the cutting face of the
tool and is provided by the tool.
4) Force F is the frictional resistance of the tool acting on the chip. It acts down
ward against the motion on the chip as it moves upwards along the tool face.
Fig 1.27 (b) is a free body diagram showing the forces acting on the chip. R is the
resultant force of F and N, and R1
that of Fs and Fn Neglecting the couples, which
(1.11)

1.24
curl the chip, considering the equilibrium of the chip, R and R1
are equal in magnitudes
opposite in direction and collinear as shown.
All these forces can be represented with the help of a circle known as the
"Merchant force circle"
Fig.1.28. Here the two force triangles have been superimposed by placing the
two equal forces R and R1
together. In the figure, B is the angle of friction. In this
diagrame for convenience, the resultant forces have been moved to the point of the
tool. Since the forces Fs and Fn are at right angle to each other their intersection lies
on a circle with diameter R1
. The forces F and N may be placed in the diagram as
shown to form the circle diagram.
Fig.1.27. Forces in metal cutting
The two orthogonal components (horizontal and vertical) Fc and Ft of the resultant
force R1
can be measured by using a dynamometer. The horizontal component is the
cutting force Fc and the vertical component is the thrust force, Ft. The power
consumed during the process is the product of Fc and V. the thrust force does not
contribute to the work done. It holds the tool against the workpiece. After Fc and F(
are known, they can be laid as shown. The rake angle a can be laid off and the forces
F and N determined. The shear angle can be determined as explained earlier. Knowing,
Fc, Ft, a and 0, all the component forces acting on the chip can be determined. It is
easily shown that
F = F,.sin a +F, cos a
CHIP
Fig.1.28. Merchant Force Circle

'" N = Fr cosa - Ft sin a The co-efficient
of friction will then be given as
(1.15)
B = tan"1
/i on
the shear plane.
The shear angle, 0 , can be determined from equation (1.4) Now shear plane area,
(1.17) The averge stresses on the shear plane
area are then
(1.18)
The shear angle has great influence on the overall cutting geometry. So, its relationship
to material and process varibles should be determined so that this angle can be predicted.
Shear angle should be such that the cutting force is a minmmum or that the maximum
shear stress occurs on the shear plane. Now the shear force can be written as
.-. Fromequation (1.18)
Assuming that p is independent of 0 . therefore, for maximum shear stress,
(1.16)

1.26
(1.19)
For practical purposes, the following approximate values for 0 have been suggested,
(1.20)
It is clear from equation (1.19), that for the same rake angle, a low frication angle, i.e. less
frication between chip and tool results in a higher shear angle, which reduces the cutting
force, which in turn results in less frication. A higher shear angle also indicates a thin chip
and less severe deformation of the chip, since the shear strain is also lower, equation (1.8).
The above results can also be obtained by increasing the rake angle.
In making the shear angle predictions (Equ.1.19). Merchant made the following
assumptions:
1) The work material behaves like an ideal plastic.
2) The theory involves the minimum energy principle.
3) As, Ts and p are assumed to be constant, independent of 0
The second assumption is not supported by evidence and also, it is an experimental fact that
(3 (friction angle) varies greatly with a, and also is not independent of 0. On reconsidering
these assumptions, Merchant included the following relation into his theory:
(1.21)
Where K is a constant and as is the normal stress on the shear plane.
At,
Then obtained the following relation:
(1.22) where
1.18 Cutting Speed
Cutting speed is expressed in terms of meters per minute and it is speed at which the metal is
removed by the cutting edge from work piece. In a lathe it is the peripheral speed of the
work, past the cutting tool. In turning, cutting speed is given by the relation.

Where
D = diameter of work in mm
N = spindle (work) speed, RPM.
The cutting speed employed in machining depends on work material, cutting tool
material and tool geometry. Cutting speeds for turning different materials with HSS
tool are presented in table. 1.3.
Table. 1.3: Cutting speed of turning
Materials Cutting speed
m/min
Rake angle Clearance
angle
Mild steel 24-30 20° 8°
High carbon steel 12-18 10° 6°
Gray cast iron 18-24 0-5° 6°
Brass 60-120 8-12° 10°
Bronze 30-90 0° 6°
Gunmetal 45-90 8-10° 8°
The average feed and depth of cut at the above cutting speeds:
Average feed = 0.2to 0.8 mm/rev Average of cut = 0.5 to 1mm fort
finishing operation = 2 to 5 mm for rough operation.
Fig.1.29

1.28
1.19 Feed
Feed is defined as the distance of the cutting tool advance per revolution. It may be
expressed as mm/rev. Feed depends on tool geometry, surface finish required on
work, rigidity of tool and coolants used.
1.20 Depth of Cut
This is the distance between the unfinished surface of the work and the bottom of
the cut, measured in a direction normal to the machined surface of the work
dx -d9
Depth of cut = — ---------------------------
v
2
Where
dj = diameter of unfinished work, and
d, = diameter of finished work.
1.21 Introduction on tool wear and tool life
During any machining process the tool is subjected to three distinct factors: forces,
temperature and sliding action due to relative motion between tool and the work
piece. Due to these factors, the cutting tool will start giving unsatisfactory performance
after some time. The unsatisfactory performance may involve: lpss of dimensional
accuracy, increased surface roughness, and increases power requirements etc. the
unsatisfactory performance results from tool wear due to its continued use. When the
tool wears out it is either replaced or reconditioned, usually by grinding. This will
result in loss of production due to machine down time in addition to the cost of
replacing or reconditioning the tool. Thus, the study of tool wear is very important
from the standpoint of performance and economics. Due to a large number of factors
over, which the tool wear, depends (hardness and type of tool material, type and
condition of work piece, dimensions, of cut, i.e, feed and depth of cut, tool geometry,
tool temperature, which in turn, is a function of cutting speed surface finish of tool
temperature and cutting fluid), the majority of studies in tool wear are based on
experimental observation, since the analytical study will be very difficult.
1.22 Tool Life
Tool life is the time involved in machining between two successive grindings of a
cutting tool. The various ways in which it can be expressed are given below:
1) Time of machining between removed;
2) Volume of material removed;
3) Number of pieces machined;
4) Equivalent cutting speed.
However the time of machining between grinds is a popular measure of tool life.

1.23 Tool Life Equations
Tool life T is the cutting time at the end of which a given tool may be termed unusable on
the basis of a selected tool failure criterion.
A tool life equation is an empirical relationship between the tool life and one or more
variables of the cutting process, e.g cutting speed V, feed f and depth of Cut d, etc. the most
famous tool life equation is due to Taylor. On the bass of his experimental work, Taylor
showed that tool life T and cutting speed V are related to each other as follows
VP = C (1.23)
Where the constant n is called the tool life exponent and the parameter c is known as the
Taylor constant. Making T = 1 in the above equation, we find that C is equal to the cutting
speed for 1 min tool life. The constants n and c depend upon the tool and work materials,
fed and depth of cut, type of coolant and tool geometry etc. equation (1.23) can be rewritten
as
LogV + nlogT= logc (1.24)
(or) log T= (%) logc - ( % ) log V
On a log-log graph the Taylor's tool life equation
represents a straight line as shown in Fig.1.30.
Equation (1.23) can be generalized to include the
effects of feed and depth of cut; one such
relationship is of the form
VT t"]
d"2
= C1
Where the exponents n, na and the constant c} depend
upon tool and work materials tool geometry, and
type of coolant, etc. (Table.1.4)
Table:1.4 Exponents n, na and n2 in tool-life equation (1.25)
S.No Tool material Work material n nx n2
1
2
3
Cemented carbide Ti-coated
tungsten carbide Cemented
oxide
Steel
Steel
Steel
0.30
0.27
0.38
0.31
0.42
0.48
0.13
0.15
0.12
Colding assumed that the tool life is directly related to the tool temperature and considered
the so-called chip equivalent q defined below as the chief factor controlling the tool
temperature.
Fig.1.30. Taylor's tool life
plot on log-log scale

1.30
(1.26)
Where f is the feed rate, d the depth of cut, rn the nose radius of the tool, and Cs
the side cutting edge angle in radians.
On the basis of the above assumptions, colding proposed the following tool life
equation.
(1.27)
Where K,.is the thermal diffusivity of the too material Tthe tool life C2 a constant,
Vthe cutting speed, q the chip equivalent given by equation (1.26) and m the tool
life exponent.
Fig.1.31. Chip equivalent q
Brewer and Rveda expressed equitation equation (1.27) in the form
(1.28)
Where (3 is a constant depending upon the tool and work material and type of
coolant, etc, te is called the equivalent (uncut) chip thickness = 1/q, and p is a
constant.
Anther form of the tool life equation is as follows:
(1.29)

where Cj.s the cutting speed (m/min) for 60 min tool life when the area of cut is
1mm2
and the slenderness ration G=d/f=5, and g,z and n are constants, Table.1.5
gives the typical values of Cv,z,g and n for different tool work combinations.
Table.1.5: Values of constants for use in Equation (1.29).
Tool material
HSS Carbides
Work
material
Cv z 8 n Cv z g n
Steels
Cast iron
(nodular)
Cast iron
(flank
graphite)
20-80
25-100
20-50
0.28
0.20
0.20
0.14
0.10
0.10
0.15
0.20
0.20
35-350
60-220
45-130
0.28
0.20
0.20
0.14
0.10
0.10
0.30
0.25
0.25
1.24 Wear mechanisms (or) Types of wear
Some of the important tool wear mechanisms of a hard metal (tool) which is in
contact with a softer but deforming metal sliding past the former at a fairly high
speed are described below.
1.24.1 Shearing at high temperature
The hard metal systems at high temperature. There fore, its shear yield stress becomes
several times smaller than what it is at room temperature though the metal slides
over it has lower yield stress, it may get so much work hardened as to be able to
exert frictional stress sufficient to cause yielding by shear of the hard tool metal
(Fig.1.32). The higher the temperature at the interface, the greater is this effect.
Fig.1.32. Wear by plastic shear

1.32
1.24.2 Diffusion wear
When a metal is in sliding contact with
another metal and the temperature at
their interface is high conditions may
become right for atoms from the harder
metal to diffuse in to the softer matrix,
there by increasing the latter's hardness
and abrasiveness, on the contrary atoms
from the softer meta may also diffuse
into the harder medium weakening the
surface layer of the latter to such an
extent that particles on it are dislodged
torn (or sheared off) and are carried
away by the flowing medium
(Fig.1.33). Diffusion is a phenomenon
strongly dependent upon temperature.
For example diffusion rate is approximately
doubled for an increment of the order of 20° C in the case of machining steel
with HSS tools.
1.24.3 Adhesive wear (Attrition wear)
Let a softer metal slide over a harder metal such that it always presents a newly-
formed (nascent) surface to the same portion of the hard metal on account of friction
high temperature and pressure, particles of the softer a material adhere to a few high
spots of the harder metal. As a result, flow of the softer metal over the surface of the
hard metal becomes irregular or less continuous. More particles join up with those
already adhering and a so-called built-up edge is formed. Sooner or later some of
these fragments which may have grown up to microscopic size are torn from the
surface of the hard metal. When this recess continues for some time it appears as if
the surface of the hard metal has been nibbled away and made uneven (Fig.1.34).
Fig.1.33. Wear by diffusion
Fig. 1.34. Adhesive wear

1.24.4 Abrasive wear
The softer metal sliding over the surface of
the harder metal may contain appreciable
concentrations of hard particles. For example,
castings may have pockets of sand in then.
In these conditions, the hard particles act
as small cutting edge like those of a grinding
wheel on the surface of a hard metal which
in due course, is worn out through abrasion.
In addition, the particles of the hard metal
its intermittently come unstuck (or get torn
out) from its surface are dragged along or
rolled over. These particles plough grooves
into the surface of the hard metal (Fig.l .35). Fig.1.35. Abrasive wear
1.24.5 Fatigue wear
When two surfaces slide in contact with each other under pressure, asperities on
one surface interlock with those of the other. Due to the frictional stress, compressive
stress is produced on one side of each interlocking asperity and tensile stress on the
other side. After a given a pair of asperities have moved over or through each other,
the above stresses are relieved. New pairs of asperities are, however, soon formed
and the stress cycle is repeated. Thus the material of the hard metal near the surface
undergoes cyclic stress. This phenomenon causes surface cracks, which ultimately
combine with one another and lead to the crumbling of the hard metal. Further, the
hard metal may also be subjected to variable thermal stress owing to temperature
changes brought about by cutting fluid, chip breakage and variable dimensions of
cut, again contributing to fatigue wear.
Fig.1.36. Stress distribution around the interlocking asperities
1.25 Kinds if tool damage (or) type of tool failure
The main manifestations of tool wear are flank wear and crater wear (Fig.1.37)

Fig.1.37. Kinds of tool wear
1.25.1 Flank wear
This wear produces wear lands on the side and end flanks of the tool on account of
the rubbing action of the machined surface. In the begining, the tool is sharp and the
wear land on the flank has zero width. However very soon, the wear land develops and
grows in size (Fig. 1.38) on account of abrasion, adhesion, shear, etc.
In Fig.1.38, we note that the wear land is not of uniform width. It is widest at
some distance away from the nose. This is due to the fact that the material cut by
this part of the cutting edge had been work hardened during the precious cut. The
frictional stress and maximum temperature at the flanks also go on increasing
becomes the predominant wear mode on the flank. After a critical wear land has
formed, further wear takes places at an accelerating rate (Fig.1.39).
1.34
Fig.1.38. Progression of flank wear

Fig.1.39. Change in flank wear land width with time
It is advisable to change the tool well before the onset of rapid wear in order to avoid
catastrophic tool failure. During the steady wear phase, flank wear is caused mainly
through abrasion, whereas during the rapid wear phase, it is caused by diffusion.
1.25.2 Crater wear
It occurs on the rake face of the tool in the form of a pit called the crater. The crater
is formed at some distance from the cutting edge. Careful measurements have shown
that the locations of maximum cratering and maximum chip-tool interfacial
temperature coincide with each other. It may therefore be assumed that cratering is
a temperature dependent phenomenon caused by diffusion adhesion, etc.
Fig.1.40. Progress of cratering

1.36
Fig. 1.40 shows how the radius of curvature Re, the depth of crater dc, the width
of crater b and the distance of the start of the crater from the tool tip a change with
time. The crater significantly reduces the strength of the tool and may lead to its
total failure.
1.26 Effect of process parameters on tool life
The different wear mechanisms are influenced by the hardness and strength of the
different constituents of the work material, hot hardness, toughness and resistance
to abrasive wear of the tool material, tool temperature and affinity between work
and tool materials. Below, we discuss the effect of the various machining parameters
on tool life in terms of the above factors.
1.26.1 Tool material
The properties of the tool material which enhance its life include the following:
(a) High hot-hardness to resist deformation, adhesion and abrasion at the
relatively high cutting temperatures occurring on the rake and flank faces of
the tool, especially at high cutting speeds.
(b) Toughness to resist sudden loads in interrupter cutting, and large steady
cutting loads when heavy cuts are taken.
(c) Wear resistance
(d) Lack of chemical affinity with the work material, and
(e) High thermal conductivity and specific heat (combined with low coefficient
of thermal expansion) to minimize thermal stress and thermal shock.
1.26.2 Work Material
The properties of the work material that tend to increase the life of the tool include
the following:
(a) Softness (or lack of hardness) to reduce cutting forces, cutting temperature
and abrasive wear,
(b) Absence of abrasive constituents such as surface scale, and sand slag inclusions,
(c) Presence of desirable additives like lead t act as boundary lubricants and
sulphur to reduce cutting forces and temperatures by acting as a stress raiser,
and
(d) Lack of work-hardening tendency that tends t reduce cutting forces and
temperature and also abrasive wear and
(e) Occurrence of favorable microstructure, e.g. in high carbon steels.

1.26.3 Speed feed and depth of cut
Higher cutting speed increases tool temperature and softens the tool material. If
there by aids abrasive, adhesive and diffusional wear the cumulative effect is an
exponential decrease in tool life as given by Taylor's tool life equation.
The larger the feed, the greater is the cutting force per unit area of chip-tool
contact on the rake face and work-tool contact on the flank face. Cutting temperatures
and therefore the different types of wear are increased. An increase in cutting force
as a result of larger feed also increass the likelihood of chipping of the cutting edge
through mechanical shock.
If the depth of cut is increased, the area of the chip-tool contact increases roughly
in equal proportion to the change in depth of cut. Consequently the rise in tool
temperature is relatively small.
1.26.4 Tool Geometry
Rake angles, cutting edge
angles, relief angles and nose
radius affect tool life by varying
degrees.
The effective (or velocity)
rake angle (ae or av) increases
when the side rake angle or
the back rake angle or both
are increased. The cutting
forces, tool temperatures and
tool wear decrease with in-
crease in the rake angle.
Consequently, tool life
improves when rake angles
are increased. However, large
rake angles make the cutting
edge sharper and reduce its mechanical strength making the tool liable to chipping.
There fore, there is an optimum rake angle associated with every tool-work pair
(Fig.1.41).
The side cutting edge angle alters the length of the engaged cutting edge with out
affecting the area of cut. As a result, the chip equivalent q is changed. When the side
cutting edge angle is increased, the chip equivalent q as well as the area of chip-tool
contact are increased without significantly changing the cutting forces. Therefore,
tool temperature and rate of wear are decreased while the tool life is increased. As
noted previously, the cutting edge angle is beneficial in another respect also. By
moving the point of impact away from the tool tip, it saves it from chipping.
Increase in nose radius also increases the value of the chip equivalent q and improves
tool life. From Fig. 1.42, we observe that if the relief angle is increased, the volume of
wear required to reach a particular width of flank wear land is also increased. Therefore
larger tool life values are obtained in the case of larger relief angles and vice versa.
Fig.1.41. Effective rake angle versus tool life

1.38
On the other hand of larger the relief angles, smaller is the more liable the tool is to
chipping or fracture.
Fig.1.42. Effect of relief angle on tool life
The tool life is optimum when the relief angles are optimum (Fig. 1.43)
Fig.1.43. Optimality of relief angle
1.26.5 Cutting Fluid
The cutting fluid cools the chip and the work piece and may even reduce to some
extent the frictional stress at the tool-work and tool-chip interfaces. Therefore the
cutting temperatures are decreased. If the tool material has low value of hot hardness
(e.g. in the case of carbon steel and HSS tools), there is appreciable increase in tool
life as a result of supply of the coolant to the cutting zone. However in the case of
carbides and oxides which have high values of hot-hardness, cutting fluid has
negligible effect on tool forces or tool life.
1.26.6 Vibration Behavior of the Machine-Tool work system
The machine tool and work constitute a vibratory system having several significant
modes of vibration with corresponding modal stiffness and damping. The larger the
individual stiffness of machine, tool and work, and larger the inherent or inbuilt

damping, the greater is the dynamic rigidity of the system and less prone it is to
large amplitude vibration or chatter. On the other hand, if the machine is not properly
designed, if the work-piece is long and thin or if the tool overhang is excessive,
chatter may occur during cutting. It is known that chatter may cause fatigue failure
or catastrophic failure of the tool due to mechanical shock.
1.26.7 Interruptions in the cut
Sometimes the job is such that cutting edge has to frequently enter and exit from the
cut as, for example, inturning a work piece having long itudinal slots. Each entrance
and exit means an impact on the cutting edge that can shorten the tool life, especially
if the tool material is hard or brittle.
1.26.8 Built-up edge
The built-up edge increases the effective rake angle of the tool and reduces the
cutting force, tool temperature and wear. However the built-up edge has a tendency
to grow in size become unstable and finally break-up. The net effect of the built-up
edge is an increase in flank wear and decreases in crater wear.
1.27 Cutting fluids or coolants
A large amount of heat is generated during the process of metal cutting. The major
part of this heat is carried away by the chips and the rest shared by the work piece
and the tool. The heat thus generated has detrimental effects upon the cutting edge
of the tool; the keenness of cutting edge is lost. To reduce the detrimental influence
of heat on the tool as well as on the work piece, the heat generated during metal
cutting should be effectively removed using proper type of cutting fluids or coolants.
The use of cutting fluids improve mach inability, tool life, surface finish and also
provides considerable reduction in machining forces and chip interference. For higher
metal removal rates the heat generated during cutting is more. Therefore cutting
fluids are used to remove heat more rapidly.
1.28 Functions of Lubricants and Coolants
The coolants and lubricants used in the machining process perform the following
functions:
1) They cool the tool and the work surface.
2) They carry away the chips.
3) They improve surface finish.
4) They protect the finished surface from corrosion.
5) They cause the chips to break into small pieces.
6) They increase the tool life.
7) They provide cushioning effect between the job surface and the tool.

1.40
8) They prevent adhesion of chips to the tool or work piece or both.
9) They improve mach inability and reduce machining forces.
1.29 Properties of cutting fluids
In order to perform the above functions to use a fluid as a cutting fluid it should possess the
following properties.
1) High heat absorption.
2) Good lubrication qualities.
3) High flash point
4) Stability
5) Odourless
6) Non-toxic to the operator.
7) Low viscosity
8) Non-corrosive and non poisonous.
9) Cause no discolouration to the work.
10) Should not produce fog and smoke during usage.
1.30 Methods of applying cutting fluids
The method of application of a cutting fluid is very important if one wishes to realize the full
benefit of its use and to conserve it or reduce the wastage. In most machine tools, the system
consists of a pump mounted on a tank containing the cutting fluid. The outlet of the pump is
connected to a nozzle through a flexible pipe. The nozzle can be adjusted to direct the stream
of the fluid at the desired point (cutting zone). This is the most common method and is
called flood application.
In a second method, the fluid may be passed through a specially designed nozzle so that
it forms very fine droplets of cutting fluid or produces a mist. The drop size may range from
5 to 25 um. This mist is directed at the cutting zone at a high velocity. This method is called
mist application.
This method of application has an advantage over the flood method because during most
formation the temperature of fluid falls due to expansion, and so it can absorb more heat.
Since it can be directed at high speeds it has a better cooling effect.
In cutting processes, particularly in machining with carbide tools, flood application may
cause cracks. In such cases, mist application can be highly useful in enhancing tool life.
A more recent method of application which promises greater benefits over the existing
methods is called "high-jet method"

Fig.1.44. The high-jet method
The method was first suggested by Piggott and colwell and later it becomes the subject
matter for many investigators. In this method a narrow jet at high velocity is directed at the
flank surface of the cutting tool as shown in Fig. 1.44.
Both straight mineral oils and water-based fluids can be used. Fig.l .45 shows the effect
of jet pressure on the interface temperature, using straight mineral oil. It shows that
minimum temperature occurs at about 14kg/cm2
. Similar trends are observed with water-
based fluids. However, the optimum occurs at a medium value of jet
pressure.
Fig.1.45. Effect of jet pressure on chip-tool interface temperature

1.42
1.31 Types of cutting Fluids
1.31.1 Straight cutting oils
These are of three type, i.e.
1) Mineral oils
2) Fatty oils
3) Combination of mineral and fatty oils. These oils have high lubricating
properties but poor heat absorption quality. The heat transfer rates achieved
by this application are the lowest among the cutting fluids. Hence, these are
only suitable for low cutting speeds.
1.31.2 Oils with Additives
The beneficial effect of mineral oils can be improved with the help of additives,
which are generally compounds of sulphur or chlorine. Addition of sulphur
compounds reduces chances of chip welding on the tool rake face.
Microbiocides may be added to prevent any organic growth. Fatty oils may be
added to improve the lubricity of these oils. These oils can be used for light to
heavy-duty cuts at low or medium cutting speeds.
1.31.3 Soluble Oils
These are also called water-based cutting fluids. These are mineral oils, fat mixtures
and emulsifiers added to water. The oil is held in the form of microscopic droplets
(collidal) in water, which assumes a white milky appearance. Because of water,
these fluids have excellent cooling effects.
The mixtures can be prepared in different ratios of cutting oil and water in order
to get the desired heat transfer and lubricity characteristics. For higher heat transfer,
light mixtures may be made. For high lubricity richer mixtures of oil with water
would be necessary. The mixtures may be from 1:5 to 1:100 of oil to water.
1.32 Selection of cutting fluids
The selection of a particular cutting fluid for a certain operation depends on many
factors and they are
1) The service expected of the fluid
2) Cutting conditions and
3) Cost of the cutting fluid.
If the fluid is to be used only for the cooling purpose the emulsions are the best.
In case the anti welding and lubricating properties are the primary requirements, the
straight mineral oils are used. However, if the performance of both is similar, the cost
will be the deciding factor.The following table shows the common applications of
various cutting fluids in different machining operations.

Table. 1.6 shows the use of cutting fluids in different machining operations.
Table 1.6: Use of cutting fluids indifferent machining operations.
S.No Operation Cutting conditions and other
requirements
Suggested fluid
1 Turning Selection of cutting oil depends
upon the cutting speed, feed
and the material being
turned.
(1) For steel-25% sulphur base oil
with 75% mineral oil.
(2) For aluminum-mineral oil with
10% soluble oil.
(3) Brass-mineral oil with 10% fatty-
oil
(4) Copper, bronze, monel metal,
malleable cast iron-soluble oil with 90
to 95% water
2 Drilling
and
boring
The requirements are of
lubrication and cooling both
so as to avoid chatter,
minimize heat generation and
cool the and work.
(1) For steel soluble oil with 75%
water.
(2) Aluminium-soluble oil with 75 to
90% water.
(3) Brass-soluble oil 75% to 90% water
or lard oil with 30 to 70% and
remaining mineral oil.
3 Sawing Cutting fluid is required for
clearing the saw teeth, carry
away the heat generated
and prevent adhesion of chips.
For steel -soluble oils.
4 Broaching Heavier cuts are taken Soluble oils, sulphurised oils.
5 Shaping Usually no cutting oil is used.
6 Milling The common requirements
include cooling, lubrication
of tool chatter.
Sulphurised mineral fatty oils or
emulsions in sample quantity.
1.33 Machinability
The 'ease' with which a given material may be worked with a cutting tool is
machinability.
Machinability depends on:
1) Chemical composition of workpioce material.
2) Micro-structure.
3) Mechanical properties
4) Physical properties
5) Cutting conditions.

1.44
In evaluating machinability the following criterion may be considered:
1) Tool life between grinds.
2) Value of cutting forces.
3) Quality of surface finish.
4) Form and size of chips.
5) Temperature of cutting.
6) Rate of cutting under a standard force
7) Rate of metal removed.
The main factor to be chosen for judging machinability depends on the type of
operation and production requirements.
Some factors that are used to predict and calculate machinbility are tensile strength,
brinell hardness and shear angle. The shear angle of a given material may be calculated,
as explained before, by comparing chip thickness before removal t with the chip
thickness after removal t.
c
i.e., interms of . In terms of shear angle this is found by use of the formula
1.33.1 Machinability Index
Good machinability implies satisfactory results in machining. But this machinability
is not a basic standard, but is relative. The rated machinability of two or more metals
being compared may very for different processes of cutting, such as heavy turning,
light turning, forming, milling, drilling, etc. Good machinability indicates many
aspect, but many times one or more objectives must be satisfied to obtain others.
The machinability of different metals to be machined may be compared by using
the machinability index of each. This is defined as follows:
Machinability index,
A free-cutting steel, which is machined relatively easily and machinability index
of which is arbitrarily fixed at 100 percent, is considered a standard steel. This steel
has carbon contents of 0.13 maximum, manganese of 0.06 to 1.10 and sulphur of
0.08 to 0.03 percent.
1.34 Cutting Tool Materials
The machining process involves shearing of the work material in a narrow region a
head of the tool. This region is subjected to severe conditions such as high temperature.

A successful cutting tool for machining must resist these severe conditions and provide
good quality finished surface
Cutting tool materials must meet the following requirements:
1) High hardness and wear resistance to retain sharp cutting edge
2) High toughness and strength to resist the breakage of cutting edge.
3) High hot hardness to retain hardness and strength at elevated temperatures.
4) Non-deformable characteristics (resistance to deformation) to prevent the distortion
of cutting tool during heat treatment and machining process.
5) Good chemical stability to prevent rusting
6) High thermal conductivity to dissipate heat from the cutting zone
7) Low-coefficient of friction to reduce heat generation during machining process
8) Easy sharpen ability during service
9) Easy availability, and
10) Low cost.
The following materials are most commonly used for cutting tools.
1) High-carbon steel,
2) High-speed steel,
3) Cast alloys,
4) Cemented carbides, and
5) Ceramics.
1.34.1 High-Carbon Steel
Plain carbon steel containing 0.6 to 1.2% c is the oldest metal cutting tool material. ;
They
are easy to manufacture, but their chief draw back is low wear resistance and hot hardness.
They are used for cutting soft material at low speed. They are particularly useful for form
tools.
1.34.2 High-Speed Steel
A complex alloy steel containing tungsten, chromium, vanadium and other elements' (cobalt,
molybdenum) is called high-speed steel.
HSS is a heat-treatable, tough material that can be retain its cutting edge up to 650°c.
These are employed in cutting tools operating at high speeds and therefore should possess
an appropriate red hardness.
High-speed steels of carrying compositions are produced for different application, they
may be grouped in three main classes.
1.34.2.1 18-4-1: It contains 18% tungsten, 4% chromium, 1% vanadium and 7% carbon.
This tool is referred to as the tungsten-type HSS. It has good wear and heat resistance. It is
called as a standard tool steel. Chromium increases the depth of

1.46
hardness and add some wear resistance: vanadium increase toughness and limits
grain growth: tungsten provides red-hardness. 18-4-1 H.S.S. is extensively used for
drills, lathe and planer tools and milling cutters.
1.34.2.2 6-6-4-2: It contains 6% molybdenum, 6% tungsten 4% chromium, 2%
vanadium and 0.8 to 1%, carbon. This tool referred as molybdenum type H.S.S. it
has good wear and impact resistance, molybdenum added to HSS provides excellent
cutting ability and toughness.
1.34.2.3 Super HSS
It is made be adding 2-15% cobalt to T-Type HSS. Cobalt performs a very special
function in improving red hardness. Super HSS had good cutting ability for heavier
cuts (rough machining) that result in higher temperatures: but it is extremely hard
and more brittle than steels of other class.
1.34.3 Stellite (Cast Alloys)
Stellites consist of cobalt (45%), chromium (35%) tungstan (18%) and carbon (2%).
The iron content instellite verylow (3-4%). It may be regarded as cutting material
with properties intermediate between those of H.S.S and the cemented carbides.
Stellite is inherently hard and requires no heat-treatment, however:it can not be
forged and must be cast to shape.
Stellite is non-magnetic, non- corrodible, and possess low coefficient of fraction
and high strength. It is used in the form of tips brazed to shank of plain carbon steel.
It is also used for gauges.
1.34.4 Cemented carbides
Cemented carbide tools are made of powder metallurgy technique by using powdery
mixture of tungsten, titanium carbide and metallic cobalt. During the production:
cobalt is used to cement (bond) the carbide grains together. Cemented carbides are
produced in the form of chips and does not required the heat treatment. The cemented
carbide tips are brazed on the steel shank. These alloys are extremely hard wear
resistant and retain their hardness at temperature up to 1000°c. These are widely
used for wire-drawing dies and other similar application.
1.34.5 Ceramics
Ceramics are latest tool material. These are prepared as tool tips from finely divided
oxide (Al2 03 and 10% of other oxides of magnesium, titanium and chromium)
powders. These powders are formed into shape by cold pressing and sintering or by
hot pressing. The hardness of ceramics is greater than cemented carbides, but it is
more brittle. The advantage of ceramics is lower cost and better wear resistance. The
ceramic tips are supplied in throw away form.

Application of these cutting tools are presented in Table 1.7
Table. 1.7 Cutting tool application.
S.No Cutting tool Application
1 Tool steel (High carbon steel) For machining soft material at low speed.
2 High speed steel General purpose
3 Cobalt high speed steel For machining hard surfaces and abrasive
material.
4 Cost alloy (Satellite) For machining hard cast iron, and steel.
5 Tungsten carbide For machining gray cast iron,
stainless steel, die cast alloys, non-ferrous
metals.
6 Tungsten carbide and cobalt For machining tool steel, alloy steel,
alloy cast iron.
7 Titanium carbide and nickel For precision machining requiring
(or molybdenum) high quality surface finish
8 For machining abrasive castings and
hard steel.
1.35 Gear Boxes
A gearbox for machine tool should posses the following requirements:
1) Provide adequate spindle speeds
2) Transmit desired power
3) Provide smooth silent operation of the transmission and accurate rotation of the
spindle with out vibrations.
4) Simple construction in respect of total number of shafts, gears, clutches, bearing and
control system components.
5) Easy of carrying out preventive maintenance and to make the adjustments in bearing
clutches etc. by taking care of easy access.
1.35.1 Methods for Changing Speeds in Gear Boxes
The speed of gearbox can be changed by engaging the various transmissions in gearboxes
by changes (slip) gears, or using friction clutches.
Change gears are used when the spindle drive is to be changed in frequently. Lot of time is
lost in changing speeds. Gear boxes with sliding purpose machine tools because of their
simplicity and reliability. Gear boxes with jaw clutches are commonly used in heavy
machine tools. Friction clutch for changing speed is used mainly in the group transmissions
of small medium size and turret lathes.

Fig. 1.47
Sliding gears type gear boxes (Fig. 1.47) can transmit heavy torque and can run at
high speeds. These gear boxes have sliding cluster gears and the gears not participating
in the transmission of power to the spindle in a given engagement are not in mesh.
1.36 Feed Gear Boxes
Feed gearbox is located in front to change the feed rate as desired for given application.
The drive of the feed mechanism may be powered by a separate electric motor or
from the head stock spindle through a gear, chain or belt transmission. Various
drives for feed gearboxes are discussed below.
1.36.1 Norton Gear Box
It is commonly found in engine lathes. The different speeds of driven shaft are;
obtained by engaging the tumbler gear with gear cone fitted on the driving shaft.
The sliding gear is keyed to the driven shaft and meshes with tumbler gear, which
is held in a bracket pivoted on the driven shaft.
1.48

Fig.1.48
Fig. 1.48 shows a better version of Norton gear in which one gear is slided on splined
input shaft to obtain different speeds by engaging desired gear on intermediate shaft.
Fig.1.49
1.36.2 Feed Gear Box of Meander Type
It is a three-shaft mechanism made up of a series of identical cluster gear and a
sliding gear with a tumbler gear. In this gearbox all the cluster gears rotate continuously
in mesh including cluster gears, which do not participate in a particular engagement.
1.36.3 Clutched System
A clutch can be used to transfer power from the driving shaft to feed screw shaft.

1.50
Fig.1.50.
A clutch can slide on splined shaft. When the clutch is connected at left the speed (iVj) of
feed screw shaft is given by
When the clutch is connected to right gear the speed [N2] of feed screw shaft is given by
1.36.4 Ruppert Drive
It is a clutched drive using two clutches and six gears 1,2,3,4,5 and 6 to provide 4 speeds to
feed screw shaft .When clutch 1 is towards left and clutch 2 is also towards left, the output
shaft will have one speed. When clutch 1 is towards right and clutch 2 is towards left the
feed screw shaft will have second speed. When the clutch I is towards right and 2 is also
towards right the feed screw shaft will have third speed and when clutch 1 is towards right
and clutch 2 is towards right the feed screw shaft will have towards speed. A typical ray
diagram for this drive is shown in Fig.1.51 and actual cross sectional diagram is shown in
Fig. 1.52.
Fig.1.51 Fig.1.52

Principles of Metal Cutting /.ix—
1.36.5 Pre-Optive Gear Box
Such gear boxes are quite commonly used in capstan and turret lathes. In a pre-optive
gear box the operator preselects the speed on a dial and then pulls a lever to obtain it.
Fig. 1.53
A clutch is operated to get the desired speed without stopping the machine. Thus
where frequent speed changes are necessary this type of gearbox is quite helpful.
The arrangement of an eight speed gear box is shown in Fig. 1.53 (a) and Fig. 1.5 3(b)
shows ray diagram for the same.

Principles of Metal Cutting

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Principles of Metal Cutting

Similar to Principles of Metal Cutting (20)

More from nmahi96

More from nmahi96 (20)

Recently uploaded

Recently uploaded (20)

Principles of Metal Cutting