Machining processes and cost calculations

By
R.ARUL
Assistant Professor
Mechanical Engineering Department
arulsivagiri.r@gmail.com
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OBJECTIVE
» Enable students to understand the use of
conventional and modern machining
processes.
» Develop knowledge on machining time
estimation and machining cost
calculations.

Mechanics of chip formation, single point
cutting tool, forces in machining, Types of
chip, cutting tools– nomenclature,
orthogonal metal cutting, thermal aspects,
cutting tool materials, tool wear, tool life,
surface finish, cutting fluids and
Machinability.

» Mechanism of metal cutting
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The metal cutting is done by a relative motion between the work
piece and the hard edge of a cutting tool.
Metal cutting could be done either by a single point cutting tool or
a multi point cutting tool.
There are two basic types of metal cutting by a single point cutting
tool. They are orthogonal and oblique metal cutting.
INTRODUCTION
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If the cutting face of the tool is at 90o to the direction of the tool
travel the cutting action is called as orthogonal cutting.
If the cutting face of the tool is inclined at less than 90o to the path
of the tool then the cutting action is called as oblique cutting
INTRODUCTION
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Orthogonal metal cutting Oblique metal cutting
Cutting edge of the tool is
perpendicular to the direction of tool
travel.
The cutting edge is inclined at an
angle less than 90o to the direction of
tool travel.
The direction of chip flow is
perpendicular to the cutting edge.
The chip flows on the tool face
making an angle.
The chip coils in a tight flat spiral The chip flows side ways in a long
curl.
For same feed and depth of cut the
force which shears the metal acts on
a smaller areas. So the life of the
tool is less.
The cutting force acts on larger area
and so tool life is more.
Produces sharp corners. Produces a chamfer at the end of the
cut
Smaller length of cutting edge is in
contact with the work.
For the same depth of cut greater
length of cutting edge is in contact
with the work.
Generally parting off in lathe,
broaching and slotting operations are
done in this method.
This method of cutting is used in
almost all machining operations.

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Elements of Metal Cutting :
Cutting speed : It is the distance traveled by work surface
related to the cutting edge of Tool
v = πdN / 1000 m / min
Feed (s) : The motion of cutting edge of tool with reference to
one revolution of work piece.
Depth of cut (t) : It is measured perpendicular to axis of work
piece and in straight turning in one pass. This can be estimated
from the relation
t = ( D - d ) / 2 mm
Undeformed chip (Fc) : The cross sectional area of chip before it
is removed from work piece. it is equal to the product of feed
and depth of cut.
Fc = s x t mm2

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Different types of chips produced during machining
process
• When the tool advances into the work piece, the metal in
front of the tool is severely stressed.
• The cutting tool produces internal shearing action in the
metal. The metal below the cutting edge yields and flows
plastically in the form of chip.
• Compression of the metal under the tool takes place. When
the ultimate stress of the metal is exceeded, separation of
metal takes place.

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Different types of chips produced during
machining process
The plastic flow takes place in a localized area
called as shear plane. The chip moves upward on the face of
the tool. There are three different types of chips. They are
1. Continuous chips
2. Discontinuous chips and
3. Chips with built up edge

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Different types of chips produced during
machining process

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Continuous chips :
• The conditions that favor the production of continuous chips is
small chip thickness, high cutting speed, sharp cutting edge,
large rake angle in cutting tool and fine feed, smooth tool face
and efficient lubricating system.
• Such chips are produced while machining ductile materials like
mild steel, copper and aluminum. Because of plastic
deformation of ductile material long and continuous chips are
produced.
• This is desirable because it produces good surface finish, low
power consumption and longer tool life.
• These chips are difficult to handle and dispose off. Further the
chips coil in a helix and curl around work and tool and may
injure the operator when it is breaking. The tool face is in
contact for a longer period resulting in more frictional
heat. However this problem could be rectified by the use of
chip breakers

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Chip breakers:
• During machining, long and continuous chip will affect
machining.
• It will spoil tool, work and machine. It will also be difficult to
remove metal and also dangerous.
• The chip should be broken into small pieces for easy removal,
safety and to prevent damage to machine and work.
• The function of chip breakers is to reduce the radius of
curvature of chips and thus break it.
• The upper side of continuous chips notches while the lower
side which slides over the face tool is smooth and shiny. The
chips have the same thickness through

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Discontinuous chips :
• These chips are produced when cutting more brittle materials
like bronze, hard brass and gray cast iron.
• Since there chips break up into small segments the friction
between chip and tool reduces resulting in better surface finish.
• These are convenient to handle and dispose off.
• Discontinuous chips are produced in ductile materials under the
conditions such as large chip thickness, low cutting speed, small
rake angle of tool etc.
• Brittle materials lack the ductility necessary for appreciable
plastic chip deformation. The amount of deformation which the
chip undergoes by deformation is limited by repeated fracturing.
• If these chips are produced from brittle materials, then the
surface finish is fair, power consumption is low and tool life is
reasonable however with ductile materials the surface finish is
poor and tool wear is excessive.

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Chips with built up edge :
• This is nothing but a small built up edge sticking to the nose of
the cutting tool. These built up edge occurs with continuous
chips.
• When machining ductile materials due to conditions of high
local temperature and extreme pressure the cutting zone and
also high friction in the tool chip interface, there are
possibilities of work material to weld to the cutting edge of
tool and thus forming built up edges.
• This weld metal is extremely hard and brittle. This welding
may affect the cutting action of tool.
• Successive layers are added to the build up edge. When this
edge becomes large and unstable it is broken and part of it is
carried up the face of the tool along with chip while remaining
is left in the surface being machined. Thus contributing to the
roughness of surface.

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Chips with built up edge :
• Thus the size of the built up edge, varies during the machining
operation. It first increases, then decrease and again
increases.
• This built up edge protects the cutting edge of tool, thus
changing the geometry of the cutting tool.
• Low cutting speeds lead to the formation of built up edge,
however with high cutting speeds associated with sintered
carbide tools, the build up edge is negligible or does not exist.
• Conditions favoring the formation of build up edge are low
cutting speed, low rake angle, high feed and large depth of
cut.
• This formation can be avoided by the use of coolants and
taking light cuts at high speeds. This leads to the formation of
crater on the surface of the tool.

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Part Description
Shank It is the body of the tool which is ungrounded.
Face It is the surface over which the chip slides.
Base It is the bottom surface of the shank.
Flank It is the surface of the tool facing the work
piece. There are two flanks namely end flank and side
flank.
Cutting edge It is the junction of the face end the flanks. There are
two cutting edges namely side cutting edge and end
cutting edge.
Nose It is the junction of side and end cutting edges.
Single point cutting tool:
Parts of a single point cutting tool:

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Angle Details
Top rake angle It is also called as back rake angle. It is the slope given to the face or
the surface of the tool. This slope is given from the nose along the
length of the tool.
Side rake angle It is the slope given to the face or top of the tool. This slope is given
from the nose along the width of the tool. The rake angles help easy
flow of chips
Relief angle These are the slopes ground downwards from the cutting
edges. These are two clearance angles namely, side clearance angle
and end clearance angle. This is given in a tool to avoid rubbing of the
job on the tool.
Cutting edge
angle
There are two cutting edge angles namely side cutting edge angle and
end cutting edge angle. Side cutting edge angle is the angle, the side
cutting edge makes with the axis of the tool. End cutting edge angle is
the angle, the end cutting edge makes with the width of the tool.
Lip angle It is also called cutting angle. It is the angle between the face and end
surface of the tool.
Nose angle It is the angle between the side cutting edge and end cutting edge.
Important angles of a single point cutting tool:

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To = Depth of cut
Tc = Chip thickness

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MACHINABILITY
 The term machinability refers to the ease with
which a metal can be cut (machined) permitting
the removal of the material with a satisfactory
finish at low cost.
 Materials with good machinability require little
power to cut, can be cut quickly, easily obtain a
good finish, and do not wear the tooling much;
such materials are said to be free machining.

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MACHINABILITY
 Machinability can be difficult to predict because
machining has so many variables.
 Two sets of factors are the condition of work
materials and the physical properties of work
materials.
 The condition of the work material includes eight
factors: microstructure, grain size, heat treatment,
chemical composition, fabrication, hardness, yield
strength, and tensile strength.

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MACHINABILITY
 Physical properties are those of the individual
material groups, such as the modulus of elasticity,
thermal conductivity, thermal expansion, and work
hardening.
 Other important factors are operating conditions,
cutting tool material and geometry, and the
machining process parameters.

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Basic properties that cutting must posses are:
1. Tool material must be at least 30 to 50% harder than the work
piece material.
2. Tool material must have high hot hardness temperature.
3. High toughness
4. High wear resistance
5. High thermal conductivity
6. Lower coefficient of friction
7. Easiness in fabrication and cheap
REQUIRED PROPERTIES OF CUTTING TOOL MATERIAL

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Required properties of cutting tool material:
Hot hardness:
This is the ability of the material to with stand very high
temperature without loosing its cutting edge. The hardness of
the tool material can be improved by adding molybdenum,
tungsten, vanadium, chromium etc which form hard
carbides. High hardness gives good wear resistance but poor
mechanical shock resistance.
Wear resistance:
The ability of the tool to withstand wear is called as wear
resistance. During the process of machining, the tool is affected
because of the abrasive action of the work piece. If the tool does
not have sufficient wear resistance then there are possibilities of
failure of cutting edge. Lack of chemical affinity between the tool
and work piece also improve wear resistance.

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Toughness:
This property posses limitation on the hardness of the tool
because of very high hardness the material becomes brittle and
weak.
Low friction:
In order to have a low tool wear and better surface finish
the co-efficient of friction between the tool and chip must be
low. The thermal conductivity must be high for quick removal of
heat from chip tool interface.
In addition to the above, it must posses the following
mentioned properties.
1. Mechanical and thermal shock resistance,
2. Ability to maintain the above properties at the high operating
temperatures.
3. Should be easy to regrind and easy to weld the tool.

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Tool life:
It is an important factor in cutting tool
performance. The tool can not cut effectively for an
unlimited period of time. It has a definite life. Tool life
is the time for which the tool will operate satisfactorily
until it becomes blunt. It is the time between two
successive grinds. Following are the factors influencing
tool life.
Cutting speed:
It has the greatest influence. When the cutting
speed increases, the cutting temperature
increases. Due to this, hardness of the tool
decreases. Hence the tool flank wear and crater wear
also occurs easily.

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Cutting speed:
It has the greatest influence. When the cutting
speed increases, the cutting temperature
increases. Due to this, hardness of the tool
decreases. Hence the tool flank wear and crater wear
also occurs easily. The relation ship between tool life
and cutting speed is given by the Taylor's formula which
states
VTn = C
V is the cutting speed in meters / minute
T is the tool life in minutes.
n depends on the tool and work.
C a constant.

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Feed and depth of cut:
The tool life depends upon the amount of material
removed by the tool per minute. For a given cutting speed if the
feed or depth of cut is increased, tool life will be reduced.
Tool geometry:
Large rake angle reduces the tool cross section. Area of the
tool which will absorb heat is reduced. So the tool will become
weak. Hence correct rake angle must be used for longer tool
life. If the cutting angle increases, more power will be required
for cutting. Clearance angle of 10o to 15o is optimal.
Other factors include the material of tool (Carbon steel,
medium alloy steel, high speed steel, molybdenum high speed
steel, cobalt high speed steel, stellites, carbides, ceramics and
diamond are the commonly used tool materials.), use of cutting
fluids and work material.

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ELEMENTS USED IN CUTTING TOOL MATERIALS
ELEMENT PROPERTIES
Tungsten
Increases hot hardness
Hard carbides formed
Abrasion resistance
Molybdenum
Increases hot hardness
Hard carbides formed
Improving resistance
Chromium
Depth hardenability during heat treat hard
carbides are formed
improving abrasion resistance
some corrosion resistance
Vanadium
combines with carbon for wear resistance
retards grain growth for better toughness
Cobalt Increases hot hardness, toughness
Carbon Hardening element forms carbides

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DIFFERENT CUTTING TOOL MATERIALS
Different cutting tool materials used for cutting
operations in practice are
1. High carbon steel
2. High speed steel
3. Non -ferrous cast alloys
4. Cemented carbides,
5. Ceramics and sintered oxides
6. Ceremets
7. Diamond
8. Cubic boron nitride
9. UCON and
10.Sialon.

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1. High Carbon Steel tools
•Its composition is C = 0.8 to 1.3%, Si = 0.1 to 0.4% and Mn = 0.1 to
0.4%.
•It is used for machining soft metals like free cutting steels and
brass and used as chisels etc.
•These tool loose hardness above 250°C.
•Hardness of tool is about Rc = 65.
•Used at cutting speed of 5m/min
2. High speed steel (H.S.S)
General use of HSS is 18-4-1.
18- Tungsten is used to increase hot hardness and stability.
4 – Chromium is used to increase strength.
1- Vanadium is used to maintain keenness of cutting edge.
In addition to these 2.5% to 10% cobalt is used to increase red hot hardness.

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3. Non – ferrous cast alloys
It is an alloy of
Cobalt – 40 to 50%,
Chromium – 27 to 32%,
Tungsten – 14 to 29%,
Carbon – 2 to 4%
It can not heat treated and are used as cast form.
It looses its hardness above 800°C
It will give better tool life than H.S.S and can be used at slightly higher cutting speeds.
They are weak in tension and like all cast materials tend to shatter when subjected to
shock load or when not properly supported.
4. Cemented carbides
Produced by powder metallurgy technique with sintering at 1000°C.
Speed can be used 6 to 8 times that of H.S.S.
Can withstand up to 1000°C.
High compressive strength is more than tensile strength.
They are very stiff and their young’s modulus is about 3 times that of the steel.
High wear resistance.

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5. Ceramics and sintered oxides
Ceramics and sintered oxides are basically made of Al2O3, These are made by powder
metallurgy technique.
Used for very high speed (500m/min).
Used for continuous cutting only.
Can withstand upto 1200°C.
Have very abrasion resistance.
6. Cermets
Cermets is the combination of ceramics and metals and produced by Powder
Metallurgy process.
When they combine ceramics will give high refractoriness and metals will give high
toughness and thermal shock resistance.
For cutting tools usual combination as Al2O3 + W + Mo + boron + Ti etc.
Usual combination 90% ceramic, 10% metals.
Increase in % of metals reduces brittleness some extent and also reduces wear
resistance.

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7. Diamond
Diamond has
Extreme hardness
Low thermal expansion.
High thermal conductivity.
Very low coefficient of friction.
Cutting tool material made of diamond can withstand speeds ranging from 1500 to
2000m/min.
On ferrous metals diamond are not suitable because of the diffusion of carbon
atoms from diamond to work-piece.
Can withstand above 1500°C.
A synthetic (man made) diamond with polycrystalline structure is recently
introduced and made by powder metallurgy process.
8. Cubic Boron Nitride (CBN)
The trade name is Borozone.
Consists of atoms of Nitrogen and Boron and produced by power metallurgy
process.
Used as a substitute for diamond during machining of steel.
Used as a grinding wheel on H.S.S tools.
Excellent surface finish is obtained.

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9. UCON
UCON is developed by union carbide in USA.
It consists of Columbium 50%, Titanium 30 % and Tungsten 20%.
This is refractory metal alloy which is cast, rolled into sheets and slit into blanks.
though its hardness is only 200 BHN, it bis hardened by diffusing nitrogen into surface
producing very hard surface with soft core. It is not used because of its higher costs.
10. Sialon (Si-Al-O-N)
Sialon is made by powder metallurgy with milled powders of Silicon, Nitrogen,
Aluminium and oxygen by sintering at 1800°C.
This is tougher than ceramics and so it can be successfully used in interrupted cuts.
Cutting speeds are 2 to 3 times compared to ceramics.
At present this is used for machining of aerospace alloys, nickel based gas turbine
blades with a cutting speed of 3 to 5 m/sec.

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DIFFERENT CUTTING TOOL MATERIALS
CUTTING SPEED
HARDNESS
CUTTING TEMPERATURE
COMPOSITION
MRR
WEAR RESISTANCE
SURFACE FINISH
TOOL LIFE

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Functions of cutting fluids:
1. To cool the tool and work piece and carry away the
heat generated from cutting zone. It is essential to
maintain a temperature of 200o C for carbon tools
and 600o C for HSS.
2. At low speeds the surface finish obtained by using
cutting fluids is better than what is obtained without
using cutting fluids.
3. To wash away the chips and keep the cutting region
free.

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Functions of cutting fluids:
4. It helps to keep the freshly machined surface bright
by giving a protective coating against atmospheric
oxygen and thus protect the finished surface from
corrosion.
5. Cutting fluids improves machinability and reduces
machining forces.
6. To prevent the expansion of work piece and
7. To cause the chips to break into small parts rather
than remain as long ribbons which are hot and sharp
and difficult to remove from work piece

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Requirements of cutting fluid:
A cutting fluid should posses the following
properties.
1. High heat absorption to remove the heat developed
immediately,
2. Good lubricating properties to have a low coefficient
of friction,
3. High flash point to avoid fire hazard,
4. Stability must be high to that it does not oxidize with
air,
5. It must not react with chemical and must be neutral,

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Requirements of cutting fluid:
1. Odorless, so that at high temperatures, it does not
give a bad smell,
2. Harmless to the skin of operators,
3. Harmless to the bearings,
4. Should not have a corrosive action on the machine
or work piece,
5. Cutting tool must be transparent so that the cutting
action could be observed,
6. Low viscosity to permit the free flow of the cutting
tool and
7. It must be economic.
Choice of a cutting fluid depends upon type of operation,
material of tool and work piece, rate of metal removal
and cost of cutting fluid.

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Types of cutting fluids:
Water based cutting fluids:
In this water is mixed with soluble oil and
soaps. Following are the important characteristic
features.
1. It is a excellent cooling medium having maximum
amount of specific heat,
2. The disadvantage in using this is that it causes rust
and corrosion

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Types of cutting fluids:
Water based cutting fluids:
1. But a mixture of water and oil provides the best
lubricating properties
2. The ratio of oil to water is different for different
machining process. The usual ratio are
Operation Ratio
Turning 1:25
Milling 1:10
Drilling 1:25
Grinding 1:50

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Oil based cutting fluids:
These are fixed oil and mineral oil. Fixed oil
has greater oiliness to become gummy and decompose
when heated.
1. To combine stability of mineral oil with lubricating
properties of fixed oils they are often mixed.
2. There are different types of oil based cutting
fluids. They are soluble oils, straight fatty cutting
oils, sulphurised and aqueous solution.

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Oil based cutting fluids:
Following are the different types of cutting
fluids based on different operating conditions.
1. Straight mineral oils for light duty and high speed
work
2. Mineral oil for light and medium duty
3. Mineral oil with extreme pressure additives, such
that they are suitable for heavy duty and
4. Mineral oil and extreme pressure additives for the
heaviest duty.

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Effect of cutting fluid on cutting speed, tool life and
chip concentration:
Cutting speed:
These are not only used to carry away the heat
generated by also because of the lubricating effect of
the fluid on the working surface of the tool. When a
cutting fluid is sued for machining touch material the
productivity may be increased from 15% to 30% more
when compared with dry operation. But using cutting
fluids, high speeds may be used.
Tool life:
By using cutting fluids effectively during
machining operations the tool life increases. Carbon
steel rods have less heat resistant have maximum
increase in tool life for HSS it is around 25%.

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Effect of cutting fluid on cutting speed, tool life and
chip concentration:
Tool life:
By using cutting fluids effectively during
machining operations the tool life increases. Carbon
steel rods have less heat resistant have maximum
increase in tool life for HSS it is around 25%.
Chip concentration:
Without the use of cutting fluid chips are
accumulated near the work tool interface and are
difficult to remove because of its high temperature. By
the use of cutting fluid the temperature of the chip is
reduced and also the chips are washed away from the
work tool interface.

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Application of cutting fluids:
The cutting fluids may be applied to the cutting
tool in the following ways.
1. By hand, using brush,
2. By means of drip tank and
3. By means of a pump.
For effective use of cutting fluid and for heavy
and continuous cutting the fluid should penetrate into
the cutting zone. The following are the famous methods
of cutting fluid application.

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1. Flood application (Hi-jet application):
• Here there is a continuous stream of cutting fluid is
directed to the cutting zone with the help of nozzle.
• The used cutting fluid drops into a tank at the
bottom. Before it is re-circulated by the pump, it
passes through many filters to remove chips and
dirt.
• In some applications the cutting fluid is supplied
through the tool itself and directed along the flank
face of the tool. Though economic it is not adopted
universally because the high pressure jet may be
dangerous to the operation.

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Mist method of application:

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• In this the cutting fluid is atomized the order of 10 - 25
mm. The mist is sprayed on cutting zone at high velocities
of about 300 mpm and more under high pressure. This
method is used in all cutting operation, but is generally
more useful with high hardness work materials. The
benefits of this process are listed below.
• Due to high velocity the heat is dispersed immediately and
maintains desired temperature gradient near tool surface.
• The surface area of coolant is greater when compared to
flood application and hence increases the cooling capacity.
• Due to expansion of the mist in the issuing nozzle, it
temperature falls down considerably.

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The basic components of the system are
1. Air pump with air storage,
2. Cutting fluid container
3. Piping and
4. Spray nozzle.

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Benefits of cutting fluids:
Cooling:
• By flowing over a tool, chip and job a cutting fluid
can remove heat and reduce temperature at he
cutting zone.
• This reduction in temperature leads in increase in
tool life and decrease in tool wear.
• The cooling effect is also important in reducing
thermal expansion and distortion of work piece.
• The cooling action also bring about good surface
finish, increase chip curl and reduces BUE formation.

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Friction reduction:
A fluid passing through the cutting zone may be
subjected to any one of the following conditions.
1. High temperature approaching melting point,
2. Clean freshly produced surface and
3. High local pressure approaching the hardness of the
metal cut.
Under these conditions the chip may be made to
react wit the fluid fro form a low shear strength solid
lubricant. This thin layer prevents the formation of the
weld between the chip and the tool and hence reduces
the co-efficient of friction between chip and tool.

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Reduce shear strength:
• When the co-efficient of friction is reduced there is
also a decrease in shear work, sue to the resulting
increase in shear angle.
• An increase in shear angle results in a decrease in
shear strain giving rise to smaller shear stress and
hence the net result is a decrease of shear energy
per unit volume when cutting with an increased
shear angle.

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Tool geometries:
There are two distinct tool geometries. The are
positive and negative rake angles. Positive is suitable for
machining soft, ductile materials (like aluminum) and
negative is for cutting hard materials, where the cutting
forces are high (Hard material, high speed and feed).

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Forces on a single point cutting tool :
Following are the three forces acting on a tool
1. Axial force
2. Tangential force and
3. Radial force

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Forces on a single point cutting tool :
In the above figure (a) is for orthogonal cutting
and figure (b) is for oblique cutting. Wattmeter is a
indirect method for measuring cutting force. More
exact method is the use of dynamometer. Of the total
heat generated during machining process, given below
is the rough heat distribution.
1. Chip carries 70 % of heat.
2. Work piece carries 15 % of heat and
3. Tool carries the remaining 15 % of heat generated

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Tool life :
It could be defined from any of the below
mentioned criteria.
1. Volume of material removed between two
successive tool grind.
2. Number of work piece machined between two
successive tool grinds.
3. time of actual cutting between 2 successive tool
grinds.
4. Tool failure occurs by chipping or breakage or wear (
Takes place by crater formation or by flank wear ) or
deformation.

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Ways of measuring tool life
 No. of pieces of work machined
 Total volume of material removed
 Total length of cut.
 Limiting value of surface finish
 Increase in cutting forces
 Dimensional accuracy
 Overheating and fuming
 Presence of chatter

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Machinability :
It could be evaluated by using
1. Tool life
2. mm3 of stock removed
3. Cutting force required.
4. Temperature of tool and chip.
5. Machinability Index ( % ) = ( Cutting speed of work
piece for 20 mm Tool life ) / ( Cutting speed of SAE
1112 steel for 20 mm min tool life ) X 100.

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TOOL FAILURE:
A tool is said to fail when it losses its usefulness
though wear, breakage, chipping and
deformation. During the machining operation high
temperatures are reached and leads to the softening of
tool point. At a high temperature localized phase
transformation occurs. This gives rise in residual stress
due to which cracks appear on tool point and it is more
prone to failure. In some cases tool point may even
melt and is frequently accompanied by sparking and
hence can be easily recognized.
Thermal cracking occurs when there is a steep
temperature gradient due to intermittent
cutting. Failure can be reduced by the proper selection
of cutting parameters.

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Modes of tool failure
1. Temperature failure
a. Plastic deformation of Cutting
Edge due to high temp
b. Cracking at the Cutting Edge due to
thermal stresses.
2. Rupture of the tool point
a. Chipping of tool edge due to
mechanical impact
b. Crumbling of Cutting Edge due to
Build Up Edge
3. Gradual wear at tool point
a. Flank wear
b. Crater wear

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Wear of cutting tools:
Flank wear ( or edge wear ):
1. This type of wear takes place when machining
materials like cast iron or when the feed is less than
0.15 mm / rev. The worn region at the flank is called
as wear land. This wear land is measured with the
help of brinell microscope.
2. The work and the tool are in contact at the cutting
edge only. Usually wear appears on the clearance
face of the tool and is mainly the result of friction
and abrasion.
3. Flank wear is a flat portion worn behind the cutting
edge, which eliminates some clearance on relief.

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Wear of cutting tools:
Flank wear ( or edge wear ):
1. Flank wear is a progressive form of detoriotion and
will result in failure in spite of best precautions.
2. There are three stages in flank wear. They are
primary, secondary and tertiary stage. In the
primary stage wear is rapid due to high stress at tool
point. In secondary stage, wear is less and linear. In
the third and final stage called as the tertiary stage
the wear increases leading to catastrophic failure.
3. Abrasion by hard particles and inclusions in
the work piece, shearing of micro welds between
tool and work material and abrasion by fragments of
build up edge plowing against the clearance face of
the tool are some of the causes of this wear.

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Crater wear ( or face wear ):
1. This is caused by the pressure of the chip as it slides
up the face of the cutting tool. Due to the pressure
of the sliding chips the cool face wears out gradually.
2. On the faces of the tool there is a direct contact of
tool with the chip. Wear takes place in the form of
cavity or crater, which as its origin above the cutting
edge.
3. The crater occurs on the rake face and does not
actually reach the cutting edge by ends near the
nose.

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1. This type of wear takes place when cutting ductile
material. This wear weakens the tool. Cutting
temperature is increased. Friction and cutting force
will also increase. When the crater becomes large
the tool will totally fail.
2. Severe abrasion between chip and tool interface and
high temperature in the tool-chip interface reaching
the softening (or melting temperature) of tool
resulting in increased rate of wear. These are the
two causes of crater wear.

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• To combat crater wear, tool manufacturers can increase the
chemical stability of the tool material, as when they added
titanium carbide (TiC) to tungsten carbide (WC) in the first
successful steel-cutting carbide tool.
• Applying a hard coating to put a hard, inert barrier between
tool and work piece at high cutting speeds will also minimize
crater wear. Tool geometry can also make a difference.
• A positive-rake tool will reduce tool pressure and decrease
contact between the chip and the insert, and the reduction in
pressure and contact can reduce crater wear.

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Nose wear:
This is similar to flank wear in certain operations
like finish turning. It takes place at the nose of the
tool. When the nose of the tool is rough, abrasion and
friction between the tool and work piece will be
high. Due to this, too much heat is generated. Also
more cutting force is required. As a result the nose of
the tool wears quickly. This is more pre-dominant than
flank wear.
Breakage:
Because of high pressure acting on cutting edge
of a tool there ay be immediate failure. Breakage is
usually attributed to mechanical shock, thermal shock,
thermal cracks and fatigue.

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Chipping:
The cutting edge may crumble due to improper
relief angle, excess clearance and insufficient support of
the tool. This could also happen if the work piece is very
hard. It is a microscopic form of breakage due to loss of
many small particles caused due to unhoned carbide
edges, excessive vibration and chatter.

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Deformation:
When a heavy load is applied close to the
cutting edge of tool the surface becomes indented
while the adjacent face shows a bulge. Because of
which crack occurs on periphery of indentation and
finally leads to failure.

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MERCHANT’S CIRCLE DIAGRAM
For establishing the relationship
between measurable and actual forces
Merchant’s circle diagram will be used.
1. Merchant circle diagram is used to analyse the
forces acting in metal cutting.
2. The analysis of three forces system, which
balance each other for cutting to occur. Each
system is a triangle of forces.

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ASSUMPTIONS MADE IN DRAWING MERCHANT’S CIRCLE
1. Shear surface is a plane extending upwards from the
cutting edge.
2. The tool is perfectly sharp and there is no contact
along the clearance force.
3. The cutting edge is a straight line extending
perpendicular to the direction of motion and
generates a plane surface as the work moves past it.
4. The chip doesn’t flow to either side, that is chip
width is constant.
5. The depth of cut remains constant.
6. Width of the too, is greater than that of the work.
7. Work moves with uniform velocity relative tool tip.
8. No built up edge is formed.

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THREE TRIANGLES OF FORCES IN MERCHANT’S CIRCLE DIAGRAM
The three triangles of forces in merchant’s circle
diagram are
1. A triangle of forces for the cutting forces,
2. A triangle of forces for the shear forces,
3. A triangle of forces for the frictional forces.

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MERCHANT’S CIRCLE DIAGRAM

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90
FORCES ACTING ON A CUTTING TOOL
Let F = Frictional force
N = Normal to frictional force
Fs = Shear force
Fsn = Normal to shear force
Fc = Cutting force or tangential
component of force
Ft =Thrust force or feed force
β = Friction angle
μ = Coefficient of friction = tanβ
Fc and Ft are along and normal
to the direction of velocity.

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