Unit _ 1

Theory of Metal Cutting

1.1 INTRODUCTION

Production or manufacturing of any object is a value addition process by which raw

material of low utility and value due to its irregular size, shape and finish is converted into a high
utility and valued product with definite size, shape and finish imparting some desired function
ability.

Machining is an essential process of semi-finishing and often finishing by which jobs of

desired shape and dimensions are produced by removing extra material from the preformed
blanks in the form of chips with the help of cutting tools moved past the work surfaces in
machine tools. The chips are separated from the workpiece by means of a cutting tool that
possesses a very high hardness compared with that of the workpiece, as well as certain
geometrical characteristics that depend upon the conditions of the cutting operation. Among all
of the manufacturing methods, metal cutting, commonly called machining; is perhaps the most
important. Forgings and castings are subjected to subsequent machining operations to acquire
the precise dimensions and surface finish required. Also, products can sometimes be
manufactured by machining stock materials like bars, plates, or structural sections.

1.2 BASIC FUNCTIONAL PRINCIPLES OF MACHINE TOOL OPERATIONS

Machine Tools produce desired geometrical surfaces on solid bodies (preformed blanks)
and for that they are basically comprised of;
• Devices for firmly holding the tool and work
• Drives for providing power and motions to the tool and work
• Kinematic system to transmit motion and power from the sources to the tool-work
• Automation and control systems
• Structural body to support and accommodate those systems with sufficient strength and
rigidity.
For material removal by machining, the work and the tool need relative movements and
those motions and required power are derived from the power source(s) and transmitted through
the kinematic system(s) comprised of a number and type of mechanisms.

1.3 INTRODUCTION TO METAL MACHINING

Three main areas of metal machining are Turning, Milling and Grinding.
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 In each topic, the basic essential concepts of the types of machines and their structures,
the cutting tools and the operations will be discussed.

Turning

 Applications : Usually to form round or taper cylindrical profiles.

 Work-pieces : Rotating, usually cylindrical shape

 Cutters : Single-point cutter, move in linear direction.

 Machines : Turning machine, or lathe

Milling

 Applications : for Flat or formed profiles.

 Work pieces : Move linearly

 Cutters : Rotating about its own axis, Multiple cutting edges.

 Machines : Milling machines (Horizontal, Vertical and universal Milling

Machines).

Grinding

 Applications : For better surface finishing, better accuracy or close tolerances.

 Work pieces : For hardened steels (materials), Very little metal removal
 Cutters : Grinding wheels
 Machines :
• Surface grinding machines (for flat or formed profiles)
• Cylindrical Grinding machines
• Universal grinding machines

1.4 BROAD CLASSIFICATION OF MACHINE TOOLS

th
Number of types of machine tools gradually increased till mid 20 century and after that
started decreasing based on Group Technology.
However, machine tools are broadly classified as follows:
 According to direction of major axis :
 Horizontal center lathe, horizontal boring machine etc.
 Vertical – vertical lathe, vertical axis milling machine etc.
 Inclined – special ( e.g. for transfer machines).
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  According to purpose of use :
 General purpose – e.g. center lathes, milling machines, drilling machines etc.
 Single purpose – e.g. facing lathe, roll turning lathe etc.
 Special purpose – for mass production.
 According to degree of automation
 Non-automatic – e.g. center lathes, drilling machines etc.
 Semi-automatic – capstan lathe, turret lathe, hobbing machine etc.
 Automatic – e.g., single spindle automatic lathe, swiss type automatic lathe, CNC
milling machine etc.
 According to size :
 Heavy duty – e.g., heavy duty lathes (e.g. ≥ 55 kW), boring mills, planning machine,
horizontal boring machine etc.
 Medium duty – e.g., lathes – 3.7 ~ 11 kW, column drilling machines, milling machines
etc.
 Small duty – e.g., table top lathes, drilling machines, milling machines.
 Micro duty – e.g., micro-drilling machine etc.
 According to precision :
 Ordinary – e.g., automatic lathes
 High precision – e.g., Swiss type automatic lathes
 According to number of spindles :
 Single spindle – center lathes, capstan lathes, milling machines etc.
 Multi-spindle – multispindle (2 to 8) lathes, gang drilling machines etc.
 According to blank type :
 Bar type (lathes)
 Chucking type (lathes)
 Housing type
 According to type of automation :
 Fixed automation – e.g., single spindle and multispindle lathes
 Flexible automation – e.g., CNC milling machine
 According to configuration :
 Stand alone type – most of the conventional machine tools.
 Machining system (more versatile) – e.g., transfer machine, machining center, FMS
etc.
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 1.5 COMPARISONS OF VARIOUS METAL MACHINING PROCESSES

Work pieces Cutters Applications Remarks

Round or
Move linearly,
Turning Rotating cylindrical Turning machine
single point
shapes
Rotating Flat or formed
Milling Linear Milling machine
Multi point profiles
Linear
Shaping Linear Flat Shaper
Single Point
Planer Move Linear Single point Flat Planer
Better finishing
Surface Rotating Better accuracy Small metal
Linear
Grinding Grinding Wheel removal
For hardened
surfaces
Better finishing
Cylindrical Rotating Better accuracy Small metal
Rotating
Grinding Grinding Wheel removal
For hardened
surfaces
Rotating and
Drilling Stationery Move linear Cylindrical holes Drilling machine
along the axis
Rotating and
Reaming Stationery Move linear Better Finishing Drilling machine
along the axis
Better
Single point
Stationery or straightness or Jig borer, Milling
Boring rotating or linear
rotating correcting the machine, Lathe
motion
ovality

1.6 OBJECTIVES OF METAL MACHINING

 Quality:
 Better precision, smaller tolerances
 Better surface finishing
 Productivity:
• Faster machining time
• Shorter delivery
 Cost :
• Lower cost, cheaper price
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 1.7 CUTTING PARAMETERS

Cutting Speed: Cutting speed is the distance traveled by the work surface in unit
time with reference to the cutting edge of the tool. The cutting speed, V is simply referred to as
speed and usually expressed in m/min.

Fig.1 Cutting speed

Feed: The feed is the distance advanced by the tool into or along the workpiece
each time the tool point passes a certain position in its travel over the surface.
In case of turning, Feed rate is defined as tool’s distance travelled during one spindle
revolution.
Feed f is usually expressed in mm/rev. Sometimes it is also expressed in mm/min and is
called feed rate.

Fig. 2. Feed and depth of cut

Depth of cut: It is the distance through which the cutting tool is plunged into the
workpiece surface. Thus it is the distance measured perpendicularly between the machined
surface and the unmachined (uncut) surface or the previously machined surface of the
workpiece. The depth of cut d is expressed in mm.
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 1.8 SELECTION OF CUTTING SPEED AND FEED

The selection of cutting speed and feed is based on the following parameters:
• Workpiece material
• Tool Material
• Tool geometry and dimensions
• Size of chip cross-section
• Types of finish desired
• Rigidity of the machine
• Types of coolant used

1.9 CHIP FORMATION

Regardless of the tool being used or the metal being cut, the chip forming process occurs
by a mechanism called plastic deformation. This deformation can be visualized as shearing.
That is when a metal is subjected to a load exceeding its elastic limit.

This action, shown in Figure is similar to the action that takes place when a deck of cards
is given a push and sliding or shearing occurs between the individual cards.

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 1.10 MECHANICS OF CHIP FORMATION

 During metal cutting, the metal is severely compressed in the area in front of the cutting
tool.
 This causes high temperature shear and plastic flow if the metal is ductile.
 When the stress in the workpiece just ahead of the cutting tool reaches a value exceeding
the ultimate strength of the metal, particles will shear to form a chip element, which
moves up along the face of the work.
 The outward or shearing movement of each successive element is arrested by work
hardening and the movement transferred to the next element.
 The process is repetitive and a continuous chip is formed.
 The plane along which the element shears, is called shear plane.

A chip has two surfaces:

1. One that is in contact with the tool face (rake face). This surface is shiny, or burnished.
2. The other from the original surface of the work piece.
This surface does not come into contact with any solid body. It has a jagged, rough
appearance, which is caused by the shearing mechanism.

1.11 TYPES OF CHIP

Every Machining operation involves the formation of chips. The nature of which differs
from operation to operation, properties of work piece material and the cutting condition. Chips
are formed due to cutting tool, which is harder and more wearer-resistant than the work piece
and the force and power to overcome the resistance of work material. The chip is formed by the
deformation of the metal lying ahead of the cutting edge by a process of shear.
1. Discontinuous or segmented chips
2. Continuous chips
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 3. Continuous Chips with a built-up edge (BUE)
4. Serrated Chips

1. DISCONTINUOUS CHIP

Discontinuous or segmented chips are produced when brittle metal such as cast iron and
hard bronze are cut or when some ductile metals are cut under poor cutting conditions.
They are formed when the amount of deformation to which chips undergo is limited by
repeated fracturing. Discontinuous chips consist of segments that may be firmly or loosely
attached to each other.

Brittle failure takes place along the shear plane before any tangible plastic flow occurs.
Discontinuous chips will form in brittle materials at low rake angles (large depths of cut).
Discontinuous Chips usually form under the following conditions:
• Brittle work piece materials
• Work piece materials that contain hard inclusions and impurities, or have structures such
as the graphite flakes in gray cast iron.
• Very low cutting speeds.
• Large depths of cut.
• High feed rate
• Low rake angles.
• Lack of an effective cutting fluid.
• Low stiffness of the machine tool.
Because of the discontinuous nature of chip formation, forces continually vary during
cutting. Hence, the stiffness or rigidity of the cutting-tool holder, the Work holding devices, and
the machine tool are important in cutting with both DC and serrated-chip formation.

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 2. CONTINUOUS CHIPS

Continuous chips are usually formed with ductile materials such as mild steel, copper etc,
at high rake angles and/or high cutting speeds. A good surface finish is generally produced.
continuous chips are not always desirable, particularly in automated machine tools, it tends to
get tangled around the tool and operation has to be stopped to clear away the chips.
Continuous chips usually form under the following conditions:
• Small chip thickness (fine feed)
• Small cutting edge
• Large rake angle
• High cutting speed
• Ductile work materials
• Less friction between chip tool interface through efficient lubrication

3. CONTINUOUS CHIPS WITH BUILT UP EDGE (BUE)

This type of chip is very similar to the continuous chip. With the difference that it
has a built up edge adjacent to tool face and also it is not so smooth. It is obtained
by machining on ductile material. Due to high local temperature, extreme pressure in
the cutting and high friction in the tool chip interference, it may cause the work
material to adhere or weld to the cutting edge of the tool on the rake surface. Successive
layers of work material are then added to form the built up edge. When this edge
becomes larger and unstable, it breaks up and it carries some part 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, and then again increases etc.
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 The tendency for a BUE to form is reduced by any of the following practices:
• Increase the cutting speeds
• Decreasing depth of cut
• Increasing the rake angle
• Using a sharp tool
• Using an effective cutting fluid
• Use a cutting tool that has lower chemical affinity for the work piece material.

 Effects of BUE formation

Formation of BUE causes several harmful effects, such as:

 It unfavorably changes the rake angle at the tool tip causing increase in cutting
forces and power consumption
 Repeated formation and dislodgement of the BUE causes fluctuation in cutting
forces and thus induces vibration which is harmful for the tool, job and the machine
tool.
 Surface finish gets deteriorated
 May reduce tool life by accelerating tool-wear at its rake surface by adhesion and
flaking

4. SERRATED CHIPS

Serrated chips: semi-continuous chips with alternating zones of high shear strain
then low shear strain. Metals with low thermal conductivity and strength that decreases sharply
with temperature, such as titanium, exhibit this behavior.

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 The Semi-continuous chips have a saw-tooth-like appearance and associated with difficult-to
machine metals at high cutting speeds.

Summary of conditions favorable for the different types of chips

Continuous chip
Parameters Continuous chip Discontinuous chips
with BUE

Ductile material such

Brittle material such
Material as Mild steel, Ductile Material
as cast iron
aluminium, copper

Cutting Speed High Low Low

Feed rate Less High high

Depth of cut Less High High

Positive / More rake Positive / More rake Less Rake angle /

Rake angle
angle angle Negative rake angle

Friction at the chip

Less More More
tool interface

Rigidity of Machine
More rigid More rigid Less rigid
tool

Lack of effective
cutting fluid /
Cutting fluid should
Cutting fluid sometimes cutting
act as lubricant to Lack of cutting fluid
fluid at high velocity
reduce the friction
may act as chip
breaker

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 1.11 CHIP BREAKERS
A continuous chip flows away from the work at high speed. If this chip is allowed to
continue, it may wrap around the tool post, the workpiece, the chuck, and perhaps around the
operator’s arm. Not only is the operator in danger of receiving a nasty laceration, but if the chip
winds around the workpiece and the machine, he must spend considerable time in removing it.
A loss of production will be encountered. Therefore it is imperative that this chip be controlled
and broken in some manner. Hence chip breakers are used to break up the long continuous chip
in small pieces.

Chip breakers may be of the following types:

Step type: A step is ground on the face of the tool along the cutting edge.
Groove type: A small grove is ground behind the cutting edge.
Clamp type: A thin carbide plate or clamp is brazed or screwed on the face of the tool

1.12 METHODS OF MACHINING

In the metal cutting operation, 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:
 Orthogonal cutting or two dimensional cutting
 Oblique cutting or three dimensioning cutting.

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 ORTHOGONAL CUTTING (2D) OBLIQUE CUTTING (3D)

Orthogonal cutting takes place when the cutting If the cutting face is inclined at an angle
face of the tool is 900 to the line of action or path less than 900 to the path of the tool, the
of the tool. cutting action is known as oblique.
The direction of the chip flow velocity is at an
The direction of the chip flow velocity is normal angle with the normal to the cutting edge of
to the cutting edge of the tool. the tool. The angle is known as chip flow
angle.
The cutting edge clears the entire width of the The cutting edge may or may not clear the
workpiece on either ends entire width of the workpiece on either ends
Here only two components of forces are acting: Here three components of forces are acting:
Cutting Force and Thrust Force. So the metal Cutting Force, Radial force and Thrust Force
cutting may be considered as a two dimensional or feed force. So the metal cutting may be
cutting. considered as a three dimensional cutting.
For a given depth of cut, the contact line For a given depth of cut, the contact line
between the tool and the workpiece will be less between the tool and the workpiece will be
and it will be equal to the depth of cut more and it will be more than the depth of cut

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 For the same feed and depth of cut, the force For the same feed and depth of cut, the force
which shears the metal acts on a smaller area which shears the metal acts on a larger area
on the cutting tool. So the tool life is less on the cutting tool. So the tool life is more
Orthogonal cutting in the machine shop is
The bulk of machining being done by oblique
confined mainly to such operations as knife
cutting.
turning, broaching and slotting

1.13 NOMENCLATURE / GEOMETRY OF SINGLE-POINT TURNING TOOL

A single point cutting tool consist Flank, face, cutting edge, nose, rack angle, clearance
angle, cutting edge angle etc. All these parts control the cutting condition, tool life and cutting
speed of tool. These parts are described as follows.

1. Shank:

The main body of the tool is known as shank. It is the backward part of tool which is hold
by tool post.

2. Face:

The top surface tool on which chips passes after cutting is known as face. It is the
horizontal surface adjacent to the cutting edges.

3. Flank:

Sometime flank is also known as cutting face. It is the vertical surface adjacent to cutting
edge. Accordingly, there are two flank viz. side flank and end flank.

4. Nose or Cutting point:

The point where both cutting edge meets known as cutting point or nose. It is at the front
of the tool.

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 5. Base:

The bottom surface of tool is known as base. It is just opposite surface of face.

Fig. 1.6 Geometry of single-point turning tool.

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 6. Back rake angle

Back rake angle is the angle between the face of the single point cutting tool and a line
drawn parallel to the base of the tool from the tool tip towards the backside of the tool and. Rake
angles may be positive, zero or negative, respectively.
If the slope face is downward from the nose, it is positive rake angle and if it is upward
away from the nose, it is negative back rake angle. Back rake angle helps in removing the chips
away from the workpiece. When the sloping is zero, it is called as zero rake angle.

The strength of the tool is a function of rake angle. A tool with positive rake angle, Fig.1.7
(a) , has got less cross-sectional area for resisting the cutting forces when compared with tools
in Fig. 1.7(b) and 1.7(c). Hence, the strength of the tool with positive rake is less as compared to
other tools. Tool shown in Fig. 1.7(b) has zero rake angle. Positive rake angle is provided for
easy removal of chip, but chip of brass is discontinuous so it will not affect tool, so no need of
positive angle

Tool with negative rake angle is illustrated In Fig. 1.7(c) and it has more cross-sectional
area for resisting the cutting forces: Hence, the strength of the tool is maximum when rake angle
is negative. Further negative rake is used for machining at higher cutting speeds

Fig. 1.7 Positive, zero and negative rake angles on a tool.

7. Side rake angle

The angle between the face and plane perpendicular to the side cutting edge is known as
side rack angle. It allows chips to flow smoothly when material cut by side cutting edge.

Relief angle

The relief angle is the angle between the flank of the cutting tool and the tangent to the
machined surface at the cutting edge. The side and the end face of flank form side and end
relief angles, respectively.

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 The relief angles enable the flank of the cutting tool to clear the workpiece surface and prevent
rubbing. These relief angles are also referred to as clearance angles.

8. End relief angle:

End relief angle is defined as the angle between the portion of the end flank immediately
below the cutting edge and a line perpendicular to the base of the tool, measured at right angles
to the flank. End relief angle allows the tool to cut without rubbing on the workpiece.

9. Side relief angle:

Side rake angle is the angle between the portion of the side flank immediately below the
side edge and a line perpendicular to the base of the tool measured at right angles to the side.
Side relief angle is the angle that prevents the interference as the tool enters the material. It is
incorporated on the tool to provide relief between its flank and the workpiece surface.

10. Side cutting edge angle

Angle formed by the side cutting edge with the normal to machined surface is known as
side cutting edge angle. It is essential for enabling the cutting tool at the start of cut to first
contact the work back from the tool tip. A large side cutting edge angle increases the force
component, which tends to force the cutting tool away from the workpiece.

11. End cutting edge angle

The angle formed by the end cutting edge with the machined surface is called end cutting
edge angle. It provides a clearance for that portion of the cutting edge, which is behind the nose
radius. This reduces the length of the cutting edge in contact with the work. Also, it is
undesirable to have a cutting edge, just contact the work surface without actually cutting. These
results in rubbing action causing more tool wear and may spoil the surface finish.

Fig. 1.8 Different cutting angles of single-point tool.

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 The different angles explained above are shown in Fig. 1.8. It is important to note that all the
tool angles are defined relative to the machined surface of workpiece. Hence, their magnitude
may be different if the tool is not properly set.

1.14 TOOL SIGNATURE

Tool signature is a numerical code that describes all the key angles of a given cutting tool
or a convenient way to specify tool angles by use of standardized abbreviated system is known
as tool signature or tool nomenclature.
Tool signature of a single point cutting tool consists of Seven elements:
1. Back rake angle (0°)
2. Side rake angle (7°)
3. End relief angle (6°)
4. Side relief angle (8°)

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 5. End cutting edge angle (15°)
6. Side cutting edge angle (16°) and
7. Nose radius (0.8 mm)
It is usual to omit the symbols for degrees and mm, simply listing the numerical value
of each component in single point cutting tool:

A typical tool signature is 0-7-6-8-15-16-0.8

1.15 RIGHT HAND AND LEFT HAND TOOLS

With respect to the direction of feed, single point tools may be classified as either left
hand or right hand tools. As shown in Fig. 1.9 they have their cutting edge on the specified side
and will cut when moved from left to right or right to left.

Fig. 1.9 Right hand and left hand tools.

A right hand tool (Fig. 1.9 (a)) is one in which the side cutting edge is on the side of the thumb
when the right hand is placed on the tool with the fingers pointed towards the tool nose. In a
lathe, a right hand tool cuts from right to left.
A left hand tool (Fig. 1.9(b)) is one in which the side cutting edge is on the thumb side when the
left hand is applied.

1.16 CUTTING TOOL MATERIALS

Cutting tool is a device, used to remove the unwanted material from given work piece. For
carrying out the machining process, cutting tool is fundamental and essential requirement. The
cutting tool material of the day and future essentially require the following properties to resist or
retard the phenomena leading to random or early tool failure:
 High mechanical strength; compressive and tensile
 Fracture toughness – high or at least adequate
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  High hardness for abrasion resistance
 High hot hardness to resist plastic deformation and reduce wear rate at elevated
temperature
 Chemical stability or inertness against work material, atmospheric gases and cutting
fluids
 Resistance to adhesion and diffusion
 Thermal conductivity – low at the surface to resist incoming of heat and high at the
core to quickly dissipate the heat entered
 High heat resistance and stiffness
 Manufacturability, availability and low cost.

1.16.1 PROPERTIES OF TOOL MATERIALS

The tool material must be harder than the work piece material. Higher
Hardness :
the hardness, easier it is for the tool to penetrate the work material.
Hot Hardness is the ability of the cutting tool to maintain its Hardness
and strength at elevated temperatures. This property is more
Hot hardness :
important when the tool is used at higher cutting speeds, for
increased productivity.
In spite of the tool being tough, it should have enough toughness to
withstand the impact loads that come in the start of the cut due to
Toughness : force fluctuations, due to imperfections in the work material.
Toughness of cutting tools is needed so that tools don’t chip or
fracture, especially during interrupted cutting operations like milling.
The tool-chip and chip-work interface are exposed to severe
Wear conditions that adhesion and abrasion wear is very common. Wear
:
Resistance resistance means the attainment of acceptable tool life before tools
need to be replaced
The coefficient of friction between the tool and chip should be low.
Low friction :
This would lower wear rates and allow better chip flow.
Since a lot of heat is generated at the cutting zone, the tool material
Thermal should have higher thermal conductivity to dissipate the heat in
:
characteristics shortest possible time; otherwise the tool temperature would become
high, reducing its life.

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 1.16.2 NEEDS AND CHRONOLOGICAL DEVELOPMENT OF CUTTING TOOL MATERIALS

With the progress of the industrial world it has been needed to continuously develop and
improve the cutting tool materials and geometry;

 To meet the growing demands for high productivity, quality and economy of machining

 To enable effective and efficient machining of the exotic materials that are coming up with
the rapid and vast progress of science and technology

 For precision and ultra-precision machining

 For micro and even nano machining demanded by the day and future.

It is already stated that the capability and overall performance of the cutting tools depend
upon,

 The cutting tool materials

 The cutting tool geometry

 Proper selection and use of those tools

 The machining conditions and the environments

Out of which the tool material plays the most vital role. The relative contribution of the cutting
tool materials on productivity, for instance, can be roughly assessed from Fig. 3.3.1

Fig. 3.3.1 Productivity raised by cutting tool materials.

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 Fig. 3.3.2 Chronological development of cutting tool materials

1.16.3 DIFFERENT ELEMENTS USED IN CUTTING TOOL MATERIALS

Different elements used in cutting tool materials and their properties are

Element Properties

Increases hot hardness

Tungsten Hard carbides formed
Abrasion resistance

Increases hot hardness

Molybdenum Hard carbides formed
Improving resistance

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 Depth hardenability
Chromium Improving abrasion resistance
some corrosion resistance

combines with carbon for wear resistance

Vanadium
retards grain growth for better toughness

Cobalt Increases hot hardness, toughness

Carbon Hardening element forms carbides

1.16.4 CUTTING TOOL MATERIALS

1. Carbon and Medium alloy steels : These are the oldest of the tool materials dating back
hundreds of years. In simple terms it is a high carbon steel (steel which contains about
0.9 to 1.3% carbon). Inexpensive, easily shaped, sharpened. No sufficient hardness and
wear resistance. Limited to low cutting speed operation

2. High Speed Steel (1900): The major difference between high speed tool steel and plain
high carbon steel is the addition of alloying elements (manganese, chromium, tungsten,
vanadium, molybdenum, cobalt, and niobium) to harden and strengthen the steel and
make it more resistant to heat (hot hardness). They are of two types: Tungsten HSS
(denoted by T), Molybdenum HSS (denoted by M).
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.
Rest iron
 H.S.S is used for drills, milling cutters, single point cutting tools, dies, reamers etc.
 It loses hardness above 600°C.
 Sometimes tungsten is completely replaced by Molybdenum.
 Molybdenum based H.S.S is cheaper than Tungsten based H.S.S and also slightly
greater toughness but less water resistance.

3. Non – ferrous cast alloys

It is an alloy of
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 Cobalt – 40 to 50%,
Chromium – 27 to 32%,
Tungsten – 14 to 29%,
Carbon – 2 to 4%

 Also known as cast cobalt alloys or stellites

 It cannot be heat treated and are used as cast form.
 It loses 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 or Sintered Carbides (1926-30):

 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
 They are very stiff and their young’s modulus is about 3 times that of the steel.
 High wear resistance.
 High modulus of elasticity.
 Low coefficient of thermal expansion.
 High thermal conductivity, low specific heat, low thermal expansion.
According to ISO the various grades of carbide tool materials grouped as
1. For cutting CI and non ferrous metals are designated as K10 to K50
2. For cutting steel are designated as P10 to P50
3. For general purpose application are designated as M10 to M50.

The advantages of carbide tools are

 They have high productivity capacity.

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  They produce surface finish of high quality.
 They can machine hardened steel.
 Their use leads to reduction in machining costs.
Carbide tool are available as brazed tip tools (carbide tip is brazed to steel tool) and inserts
(inserts are of various shapes- triangular, square diamond and round).

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.
 Used for machining CI and plastics.
 Has less tendency to weld metals during machining.
 Generally used ceramic is sintered carbides.
 Another ceramic tool material is silicon nitride which is mainly used for CI.

6. Cermets
 Cermets are 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.
 TiC, nickel, TiN, and other carbides are used as binders.
 Usual combination 90% ceramic, 10% metals.
 Increase in % of metals reduces brittleness some extent and also reduces wear
resistance.

7. Diamond
 Diamond has
1. Extreme hardness
2. Low thermal expansion.
3. High thermal conductivity.
4. Very low coefficient of friction.
 Cutting tool material made of diamond can withstand speeds ranging from 1500 to
2000m/min.
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  Because diamonds are pure carbon, they have an affinity for the carbon of ferrous
metals. Therefore, they can only be used on non-ferrous metals.
 Can withstand above 1500°C.
 A synthetic (man made) diamond with polycrystalline structure is recently introduced
and made by powder metallurgy process.
 Feeds should be very light and high speeds Rigidity in the machine tool and the setup is
very critical because of the extreme hardness and brittleness of diamond.

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.

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 is 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.

1.17 MECHANICS OF METAL CUTTING

1.17.1 Chip thickness ratio / Cutting ratio

Chip Thickness ratio is defined as the ratio of the undeformed thickness of the chip to
the chip thickness after cutting.
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  The outward flow of the metal causes the chip to be thicker after the separation from the
parent metal. That is the chip produced is thicker than the depth of cut.

The chip separated from work by the action of cutting tool ,under goes severe plastic
deformation. This means the chip cannot return to original dimension ,hence there shall always
be strain associated with it. so separated chip will have larger dimension than uncut chip
thickness. Hence the chip thickness ratio will always be lesser than one

t
Thus, Cutting ratio, r =
tc
Where
t = undeformed chip thickness (i.e. before cutting) and
tc = mean thickness of chip ( i.e., after cutting )
Coefficient of chip contraction or Chip reduction Coefficient
Coefficient of chip contraction or Chip reduction Coefficient is invese of Chip Thickness
ratio. It is a quantitative measurement of plastic deformation occurred during the cutting process.

1 tc
Chip reduction coefficient k = 
r t
1.17.1.1 Methods to determine cutting ratio
1. The cutting ratio "r" can be obtained by direct measurement of "t" & "tc". However
since underside of chip is rough the correct value of "tc" is difficult to obtain and hence tc
can be calculated by measuring length of chip (Lc) and weight of the piece of chip "W c".

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 Weight of the chip = Wc = Volume of the chip x density
= Wc = L c x b c x tc x ρ

Wc
tc 
Lc x bc x ρ
Where, bc = width of the chip
Lc = length of the chip
ρ = Density of material assumed to be unchanged during chip
formation.
2. Alternatively, the length of chip (1c) & length of work (l) can be determined. The
length of work can be determined by using a work piece with slot, which will break the
chip for each revolution of work piece. The length of chip can be measured by string.

When metal is cut, there is no change in volume of metal cut. Hence volume of chip
before cutting is equal to volume of chip after cutting i.e.

L x d x t  Lc x bc x t c

t L
r c [for orthogonal cutting bc = d]
tc L
L = Length of cut
Lc = Length of the chip
d = depth of cut
bc = width of the chip

3. Cutting ratio can also be determined by finding chip velocity (Vc) and cutting speed
(V). The chip velocity (Vc) can be accurately determined by determining length of chip
with a string for a particular cutting time measured with the help of a stopwatch.
From the continuity equation, we know that volume of metal flowing per unit time before
cutting is equal to volume of metal flowing per unit time after cutting.
V.d.t. = Vc .bc.tc

t V
r c [for orthogonal cutting bc = d]
tc V
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 1.17.2 Shear Plane Angle
The shear angle is the angle made by shear plane with the direction of tool travel. In fig
1.7a it is the angle made by the line AC with direction of tool travel. The value of this angle
depends on cutting conditions, tool geometry, tool material & work material. If the shear angle is
small, the plane of shear is larger, the chip is thicker and therefore higher fore is required to
remove the chip.
From the figure,
t  ACsin φ
t c  AC cos (φ  α)
t sin φ
r 
t c cos( φ  α)
sin φ
r 
cos φ cos α  sin φ sin α From
r cos φ cos α  r sin φ sin α  sin φ
r cos φ cos α  sin φ (1  r sin α) .....(1)
 eqn 1 by cos φ
r cos α  tan φ (1  r sin α)

r cos α
tan φ 
(1  r sin α)
1.17.3 SHEAR STRAIN IN CHIP

The total shear strain for chip formation can be calculated by assuming that the chip formation
process is in pure shear, in the direction of Vs as shown in Figure 8(b).

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 The shear strain ε is defined as the ratio of the tangential displacement of an element of the
material to its height and given as :
Tangential Displaceme nt
Shear Strain   
Length
S AB AD  DB AD DB
     
Y CD CD CD CD
  tan (   )  cot 
sin (   ) cos  sin  sin (   )  cos  cos(   )
   
cos(   ) sin  sin  cos (   )
cos (     )
 
sin  cos (   )

cos 
 
sin  cos (   )
1.17.4 SHEAR STRAIN RATE IN CHIP

 S V S
Shear Strain rate =     s   Vs
dt Y dt Y dt


V Cos
 
Cos (   ) Y
Mean thickness of primary shear zone (ΔY) can be taken as = 25 microns
1.17.5 VELOCITY RELATION IN METAL CUTTING
In orthogonal machining, cutting velocity (VC) , chip flow velocity (Vf) and shear velocity
(VS) are interrelated. These three velocity vectors together form a triangle, which is
called velocity triangle in machining. A typical velocity triangle is depicted below. Here length
of the sides of the triangle indicates the magnitude of corresponding velocity. The triangle is
based on the assumptions of orthogonal machining (chip is flowing in orthogonal direction) and

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 single shear plane (indicates shearing is occurring in a concentrated 2-D shear plane rather than
through a region).

Since all three angles of the triangle is known, so Sine Rule can be applied. Law of sine
states that, for an arbitrary triangle, the ratio between the length of a particular cord and the sine
of its opposite angle is a constant. Hence, for this particular velocity triangle, the following
equation can be written. This equation is very important in mechanics of machining as it
provides an interrelation between cutting velocity, chip velocity and shear velocity.

V V Vs
 c 
sin(90  (φ  α)) sinφ sin(90  α)

V V V
 c  s
cos(φ  α) sinφ cosα

V sin   sin 
Vc    r 
cos(   )  cos( -  ) 

Vc V  r

V cos 
Vs  .........(1)
cos(   )
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 Multiply and divide the above expression by sinФ

V Sin  cos 
Vs 
Sin  cos(   )

Vs  V  Sin 
1.18 FORCE MEASUREMENT IN METAL CUTTING
The force acting on a cutting tool during the process of metal cutting are the fundamental
importance in the design of cutting tools. The determination of cutting forces necessary for
deformation the work material at the shear zone is essential for several important requirements:
 to estimate the power requirements of a machine tool
 to estimate the straining actions that must be resisted by the machine tool components,
bearings, jigs and fixtures
 to evaluate the role of various parameters in cutting forces
 to evaluate the performance of any new work material, tool material, environment,
techniques etc. with respect to machinability (cutting forces)
Cutting forces
The force system in the general case of conventional turning process is shown in the following
Figure.

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 The largest magnitude is the vertical force Fc which in turning is larger than feed force FF,
and FF is larger than radial force Fr. For orthogonal cutting system Fr is made zero by placing
the face of cutting tool at 90 degree to the line of action of the tool

1.18.1 MERCHANTS FORCE RELATION

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

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 7. Work moves with uniform velocity relative tool tip.
8. No built up edge is formed.
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.

Fs = Shear Force, which acts along the shear plane, is the resistance to shear of the
metal in forming the chip.
Fn = Force acting normal to the shear plane, is the backing up force on the chip
provided by the workpiece.

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 F = Frictional resistance of the tool acting against the motion of the chip as it moves
upward along the tool.
N = Normal to the chip force, is provided by the tool.

Relationship of various forces acting on the chip with the horizontal and vertical cutting
force from Merchant circle diagram

Frictional Plane / Tool Plane Forces

D
α
C
(90-α)
F  OA  CB  CG  GB  ED  GB
α
E α Fc O  F  FC sin   Ft cos 
(β - α)
(90-α) G N  AB  OD  CD  OD  GE
 N  FC cos   Ft sin 
Ft R
α
The coefficient of friction
α F
F
  tan 
β
N
B N Where   Friction angle
A
Shear Force System
B
α
FS  OA  OB  AB  OB  CD
A
Fs
(90-∅)
Fc  FS  FC cos   Ft sin 
C ∅ O
∅
(90-∅)
(β - α)
FN  AE  AD  DE  BC  DE
Fn D
Ft R
α
 FN  FC sin   Ft cos 
∅
FN  FS tan(     )
E

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 F  FC sin   Ft cos 
N  FC cos   Ft sin 
FS  FC cos   Ft sin 
FN  FC sin   Ft cos 
FN  FS tan(     )
1.19 POWER REQUIRED IN METAL CUTTING

Power or energy consumed per unit time is the product of the cutting force and cutting velocity.
Hence power at the cutting spindle / cutting power = Pc = Fc * V N-m/s or watts

The cutting power is dissipated in the shear zone and on the rake surface

The Power consumed/ work done per sec in shear: = Ps = Fs* Vs N-m/s or watts

The Power consumed/ work done per sec in friction = Pf = F * Vc N-m/s or watts

Total Power required = Pc = Ps + Pf

Pc = Fs* Vs + F * Vc

Pc
Motor power required = Pm = ; where η m  Efficiency of the motor
ηm

1.19 SPECIFIC CUTTING ENERGY

Specific Energy, Uc is defined as the total energy required to remove a per unit
volume of material removed.
Energy Energy per unit time
UC  
Volume Removed Volume Removed per unit time
Cutting Power (Pc )
UC 
Material Removal Rate (MRR)

FC V FC
Uc   N  m/mm 3 or Joule/mm 3
bt V bt

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 If uf and us be specific energy for friction and specific energy for shearing , then

F Vc F V Fr Fv
Uc  U f  Us   s s   s s
bt V bt V bt bt V

1.20 STRESS AND STRAIN ACTING ON THE CHIP

Fs
Mean shear stress τ s 
As
AS = Area of the shear Plane
= L*b
t
Length of the shear plane L =
sin 

bt
As 
sin φ
Fs (FC cosφ  Ft sinφ) sin φ
Mean shear stress τ s  
As bt

FN (FC sinφ  Ft cosφ )sin φ

Mean shear stress σ s  
As bt
We know, work done in shearing unit volume of the material = Shear stress * Shear strain

Fs  Vs
 τs  ε
btV

F V Fs  Vs
 s s 
; Fs
s b t V bt V
As
Fs  Vs
 Vs 1
Fs ;  x
bt V V sin 
bt
sin φ
cos  Vs cos 
  [Note -  ]
sin  cos (   ) V cos(   )
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 1.21 TOOL WEAR
Cutting tools are subjected to extremely severe cutting conditions such as:
 Metal to metal contact with chip and work
 Very high stress
 Very high temperature
 Very high temperature gradients
 Very high stress gradients
Because of all the above-mentioned factors, the tool-chip interface exhibits a loss in tool
material which is known as tool wear. As tool wear progresses, cutting forces increase and
vibrations increase. Tool tip softens and flows plastically and gets blunt edge which will result in
further progressing of plastic deformation from the tool tip to the interior.
Cutting tools generally fail by :
1. Mechanical breakage due to excessive forces and shocks. Such kind of tool failure is
random and catastrophic in nature and hence is extremely detrimental.
2. Quick dulling by plastic deformation due to intensive stresses and temperature. This
type of failure also occurs rapidly and is quite detrimental and unwanted.
3. Gradual wear of the cutting tool at its flanks and rake surface.
The first two modes of tool failure are very harmful not only for the tool but also for the job
and the machine tool. Hence these kinds of tool failure need to be prevented by using suitable
tool materials and geometry depending upon the work material and cutting condition.

But failure by gradual wear, which is inevitable, cannot be prevented but can be slowed
down only to enhance the service life of the tool.

1.21.1 Types of Wear

o Flank Wear

o Crater Wear

o Notch Wear

o Nose wear

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 a. Flank Wear

Flank wear occurs on the clearance / relief face of the tool

and is mainly caused by the rubbing of the newly machined
workpiece surface on the contact area of the tool edge. This type of
wear occurs on all tools while cutting any type of work material.
Flank wear begins along the lead cutting edge and generally
moves downward, away from the cutting edge. The edge wear is also commonly known as the
wear land. During the initial and steady wear phase (stage I & II), the root cause is due to
abrasion, whereas during stage III, it is by diffusion. Flank Wear generally occurs when the
speed of cutting is very high. It causes many losses but one of them is increased roughness of
surface of the final product. Also when the cutting speed is increased, the wear curve shifts
towards left side, thereby decreasing the tool life

Figure : Flank Wear at different cutting speed

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 The main reasons for occurrence of flank wear and increase in rate of flank wear
are listed below:
i) One of the main reason for flank wear is increase in cutting speed that causes flank
wear to grow rapidly.
ii) Another reason for flank wear is high value of feed and depth of cut.
iii) Abrasion by hard panicles of the workpiece.
iv) Shearing of micro welds between tool and work material.

Effects and Losses due to Flank Wear:

1. Flank wear increases the total cutting force required to cut.

2. It affects component’s dimensional accuracy.
3. It also increases the final product surface roughness.
4. Sometimes flank wear also changes the shape of the components produced.
Flank Wear can be prevented by following ways:
1. Flank wear can be prevented by reducing the cutting speed.
2. Reducing the feed and depth of cut.
3. By using good quality of carbide in the cutting tool.

b. Crater Wear

Typically, crater wear occurs on the rake face of

the tool. It is essentially the erosion of an area parallel to
the cutting edge. This erosion process takes place as the
chip being cut, rubs the top face of the tool. Under very
high-speed cutting conditions and when machining tough
materials, crater wear can be the factor which determines
the life of the tool. Crater wear is caused mainly by
diffusion and adhesion.

Crater formation increases the effective rake angle of the tool and thus may reduce
cutting forces. However, excessive crater wear weakens the cutting edge and can lead to
deformation or fracture of the tool, and should be avoided because it shortens tool life and
makes resharpening the tool difficult. Severe crater wear usually results from temperature-
activated diffusion or chemical wear mechanisms. Crater wear can be minimized by increasing

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 the chemical stability of the tool material or by decreasing the tool's chemical solubility in the
chip;

c. Notch Wear

Tools used in rough turning often develop notch

wear on the tool face, especially at the point of contact
between the tool and the unmachined part surface or
free edge of the chip. Depth of cut notching usually
results from abrasion and is especially common when
cutting parts with a hard surface layer or scale, or work
hardening materials which produce an abrasive chip
(e. g. stainless steels and nickel-based superalloys). Severe notch wear makes resharpening
the tool difficult and can lead to tool fracture, especially with ceramic tools. Notch wear can be
reduced by increasing the lead angle, which increases the area of contact between the tool and
part surface, by varying the depth of cut in multipass operations, and by increasing the hot
hardness and deformation resistance of the tool material.

d. Nose Wear

Nose radius wear occurs on the nose radius of the tool, on the
trailing edge near the end of the relief face. It resembles a
combined form of flank and notch wear, and results primarily
from abrasion and corrosion or oxidation Severe nose radius
wear degrades the machined surface finish.

1.21.2 WEAR MECHANISMS

Five basic wear mechanisms are categorized as: Abrasion, adhesion, diffusion, oxidation and
chemical corrosion
a. Abrasion:
Abrasive wear occurs when hard particles abrade and remove material from the tool. The
abrasive particles may be contained in the chip, as with adhering sand in sandcast parts,
carbide inclusions in steel, or free silicon particles in aluminum-silicon alloys. They may also
result from the chip form or from a chemical reaction between the chips and cutting fluid, as with
powder metal steels (which form a powdery chip) or cast irons alloyed with chromium. Abrasion
occurs primarily on the flank surface of the tool. Abrasive wear by hard particles entrained in
the cutting fluid is sometimes called erosive wear. Abrasive wear is usually the primary
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 cause of flank wear, notch wear, and nose radius wear, and as such is the often, the form of
wear which controls tool life, especially at low to medium cutting speeds.

b. Adhesion:

Adhesive or attritional wear occurs when small particles of the tool adhere or weld to the
chip due to friction and are removed from the tool surface. It occurs primarily on the rake face of
the tool and contributes to the formation of a wear crater. Adhesive wear rates are usually low,
so that this form of wear is not usually practically significant. However, significant adhesive wear
may accompany built-up edge (BUE) formation, since the BUE is also caused by adhesion, and
can result in chipping of the tool.

c. Diffusion:

When a metal is in sliding contact with another metal and the temperature at their
interface is high, conditions may become right for the alloying atoms from the harder metal to
diffuse into the softer matrix; thereby increasing the latter’s hardness and abrasiveness. On the
other hand atoms from the softer metals may also diffuse into harder metal, thus weakening the
surface layer of the latter to such an extent that particles on it are dislodged and are carried
away by flowing chip material. Because of high temperatures and pressures in diffusion wear,
micro transfer on an atomic scale takes place. The rate of diffusion increases exponentially with
increases in temperature. .

d. Oxidation:

Oxidation occurs when constituents of the tool (especially the binder) react with
atmospheric oxygen. It most often occurs near the free surface of the part, where the hot portion
of the tool in and around the tool-chip contact region is exposed to the atmosphere. Oxidation
often results in severe depth-of-cut notch formation and can be recognized by the fact that the
tool material is typically discolored in the region near the notch. Oxidation of wear debris or
particles of the work material may also result in the production of hard oxide particles which
increase abrasive wear. Oxidation does not occur with aluminum oxide-based ceramic
tools.
e. CHEMICAL WEAR OR CORROSION
Chemical wear or corrosion, caused by chemical reactions between constituents of the
tool and the workpiece or cutting fluid, produces both flank and crater wear, with flank wear
dominating as the cutting speed is increased. Chemical wear scars are smooth compared to
wear scars produced by other mechanisms and may appear to be deliberately ground into the

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 tool. This type of wear is commonly observed when machining highly reactive materials such as
titanium alloys.

Chemical wear may also result from reactions with additives (e.g., free sulfur or
chlorinated EP additives) in the cutting fluid. (EP additives, in fact, are used to reduce adhesive
wear by producing controlled chemical wear. The surface layer of the tool is changed to the
reaction product, which is typically soft and wears rapidly by abrasion. Changing the tool
material (or coating) or the additives in the cutting fluid will often reduce this type of wear.

1.22 TOOL LIFE

Tool life is a most important factor in the evaluation of machinability, it is the period of
time in which the tool cuts effectively and efficiently. Tool life is defined as the time period
between two successive regrinding of tool and two successive replacement of tool. A cutting tool
should have lone tool life. The cost of grinding and replacement is very high, so the short tool
life will be uneconomical. Now a day’s tool material improvement increases the tool life.

When a tool no longer performs the desired function then it is said that tool reaches end
of useful life. The following sign indicates that the tool life is over.

 Poor surface finish, and dimensional error and presence of chatter marks on the
workpiece
 Overheating of workpiece – tool interface due to friction’
 Spoiled cutting edges
 A sudden increase in power consumption
The methods of expressing the tool life are
 Time unit - it is the most commonly used tool life unit
 Number of Workpieces machined by a tool.
 Total length of cut.
 Volume of material removed y tool during its total life span
Volume of metal removed per minute = π . D .t . f . N mm3 / min
Total volume of metal removed for a given time = π . D. t . f . N . T mm3
Total vol. of metal removed for tool failure = V x 1000 x t x f x T mm3
 DN
[note V = ]
1000
where,

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 D = workpiece dia in mm
t = Depth of cut in mm
f = Feed rate in mm/rev
N = no. of revolutions of workpiece per minute.
T = Time of operation in min
V = Cutting speed in m/min
The tool life is most commonly expressed in minute, expected life of some tool material is
given below
Cast tool steel = 120 min
High speed steel tool = 60 to 120 min
Cemented carbide tool = 420 to 480 min

1.22.1 FACTORS AFFECTING TOOL LIFE

The tool life will be affected by various factors, which are mentioned below
 Machining variables – Feed, cutting speed and depth of cut
 Tool material and its properties
 Properties of workpiece material
 Tool geometry – Profile of the cutting tool
 Machining conditions like temperature, rigidity of the machine tool, nature of cutting
a. Cutting speed:
Cutting speed has the greatest influence on tool life. As the cutting speed increases the
temperature also rises. The heat is more concentrated on the tool than on the work and the
hardness of the tool matrix changes, so the relative increase in the hardness of the work
accelerates the abrasive action. The criterion of the wear is dependent on the cutting speed
because the predominant wear may flank or crater if cutting speed is increased

A common method of forecasting tool wear is to use Taylor’s equation; his study on tool
life was done in 1907. Taylor thought that there is an optimum cutting speed for best
productivity. This is reasoned from the fact that at low cutting speeds, tools have higher life but
productivity is low, and at higher speeds the reverse is true. This inspired him to check up the
relationship of tool life and cutting speed. Based on the experimental work he proposed the
formula for tool life.
Taylor’s Empirical Equation: VT n =C
Where,
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 T = tool lifetime; usually in minutes
V = cutting velocity, m/min
C = constant; the cutting velocity for 1 minute of elapsed time before
reaching the wear limit of the tool
n = constant which is considered a characteristic of the tool material,
called tool life index.
Note: at T = 1 minute, C becomes equal to the cutting speed
Each combination of workpiece, tool material and cutting condition has its own n and C
values, both of which are determined experimentally. The Value of “C” and “n’ for different tool
materials are listed in the table.

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 The tool life obviously decreases with the
increase in cutting velocity keeping other conditions
unaltered as indicated in Fig. If the tool lives, T , T , T ,
1 2 3

T etc are plotted against the corresponding cutting

4

velocities, V , V , V , V etc as shown in Fig. a smooth

1 2 3 4

curve like a rectangular hyperbola is found to appear.

When F. W. Taylor plotted the same figure taking both V
and T in log-scale, a more distinct linear relationship
appeared as schematically shown in Fig.

Figure shows the typical variation of tool life with speed for HSS, WC, and ceramic tools,
keeping the other conditions the same. It is clear that the tool life for a given speed is normally
much higher with WC than that with HSS. A ceramic tool performs better at a high cutting speed.

b. FEED AND DEPTH OF CUT

Tool life is a direct function of temperature. At higher feed, the cutting force per unit area
of chip tool contact on rake face & work tool contact on flank face is increased there by
increasing the temperature and hence wear rate.
Similarly, at higher depth of cut, the area of chip tool contact is increased roughly in
proportion to change in depth of cut (such is not the case with feed change where the chip tool
contact area changes by larger proportion than change in depth of cut), increasing the
temperature & consequently the wear rate.

The cumulative effect of speed, feed & depth of cut can be seen from the modified
Taylor’s tool life equation. Increase in any one of the above reduces the tool life, but cutting
speed has more impact on tool life followed by feed & depth of cut.
Modified Taylor’s Equation:

f = Feed mm/rev; d = depth of cut in mm

p,q are constants < 1
q < p indicates that tool life is more sensitive to the uncut slip chip thickness than to the
width of cut.

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 c. TOOL GEOMETRY
Rake angles and relief angles & nose radius affect the tool life by varying degree.

Increasing the rake angle decreases the cutting force and heat produced at the tool tip. However
increasing the rake angle to a large value reduces the tool material available at the tool tip for
conducting heat generated, thus increasing the tool tip temperature. This would decrease tool
life, thus again an optimum value has to be selected.

Large relief angle increases volume of wear required to reach a particular width of flank
wear land as seen from fig and also reduces the tendency of rubbing between flank & work
piece surface, there by increasing
the tool life. However, on the other
hand, larger the relief angle, smaller
is the mechanical strength of cutting
edge & more liable the tool is to
chipping fracture. Thus there is
maximum tool life for optimum relief
angle as seen for fig.

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 d. Work material:

The properties of the work material that tend to increase the tool life are as follows,
a) softness (or lack of hardness) to reduce cutting forces, cutting temperature & abrasive
wear,
b) absence of abrasive component such as slag inclusions, surface scale & sand,
c) presence of desirable additives like lead to act as boundary lubricants and sulphur to
reduce cutting forces & temperatures by acting as stress raiser, and
d) lack of work hardening tendency that tend to reduce cutting forces and temperatures and
also abrasive wear and
e) Occurrence of favorable microstructure, e.g. presence of spheroidized pearlite instead of
lamellar pearlite in high carbon steel improves tool life.
Similarly in cast irons, a structure that contains large amount of free graphite & ferrite
leads to greater tool life than one, which contains free iron carbide.

e. TOOL MATERIAL

Tool material which can withstand maximum cutting temperature without losing its
mechanical properties and geometry will ensure maximum tool life. Hence higher the
mechanical properties (mostly hardness and toughness) in the tool materials, longer will be the
tool life.

f. CUTTING FLUID

The cutting fluid cools the tool & work piece, acts as lubricant and reduces friction at chip
tool interface. Therefore the cutting temperatures are decreased & the use of cutting fluid in the
tool materials with low value of hot hardness (e.g.) shows appreciable increase in tool life.
However in carbides & oxides, which have high value of hot hardness, the cutting fluid has
negligible effect on tool forces or tool life.

g. NATURE OF CUTTING:

It has also great influence on tool life; e.g. in the case of continuous cutting the tool life is
much better than in intermittent cutting. The intermittent cutting gives regular impacts on the tool
leading to its failure much earlier.

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 h. VIBRATION / RIGIDITY BEHAVIOR OF MACHINE TOOL WORK SYSTEM

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 tool due to mechanical shock

1.23 MACHINABILITY: -

Machinability is the property of material to be machined, which governs the case or the
difficulty with which it can be machined under a given set of conditions. In spite of the efforts
made by the number of investigators, so far, there has been no exact quantitative definition of
Machinability. It is due to large number factors involved & their complexity in metal cutting
process viz. forces & power, tool life, surface finish etc. These are dependent upon number of
variable such as work material, cutting conditions, M/C tool rigidity tool geometry. Due to this, it
is impossible to combine these factors so as to give a suitable definition for Machinability. It is of
a considerable economic importance for production engineer to know in advance the
Machinability of work material so that he can its processing in an efficient manner.

1.25 TYPES OF TOOLS

All the cutting tools used in metal cutting can be broadly classified into two categories viz. single
point tools and multipoint tools. Fig.1.4 shows examples of single point and multipoint tools.

Fig. 1.4 Two types of cutting tools.

a. Single point tools
A single point tool has only one cutting edge. An example of a single point tool is shown in Fig.
1.4(a). These types of tools are used in lathes, shapers and planers.
b. Multipoint tools
As the name implies, multipoint tools have more than one cutting edge. Fig. 1.4(b) shows a
multipoint tool. The tool has cutting teeth on its periphery. During the process of machining, each

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 tooth does the cutting action independently on the workpiece. The tool shown In Fig. 1.4(b) is
called a milling cutter and is used in milling operation. Examples of other multipoint tools include
drills and reamers.

1.26 TEMPERATURE DISTRIBUTION IN METAL CUTTING

During machining heat is generated at the cutting point from three sources, as indicated
in Fig. 2.7.1. Those sources and causes of development of cutting temperature are:

 Primary shear zone (1) where the major part of the energy is converted into heat

 Secondary deformation zone (2) at the chip – tool interface where further heat is
generated due to rubbing and / or shear

 At the work tool interface (3) due to rubbing between the tool and the finished surfaces.

Figure : Apportionment of heat amongst chip, tool and work

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 The heat generated is shared by the chip, cutting tool and the blank. The apportionment
of sharing that heat depends upon the configuration, size and thermal conductivity of the tool –
work material and the cutting condition. Fig. 2.7.2 visualises that maximum amount of heat is
carried away by the flowing chip. From 10 to 20% of the total heat goes into the tool and some
heat is absorbed in the blank. With the increase in cutting velocity, the chip shares
For example, in a typical study of machining mild steel at 30 m/min at about 750 deg of
cutting temperature at tool-chip interface, the distribution of total energy developed at the shear
zone is as follows
 Energy at chip – 60 percent
 Energy to workpiece – 30 percent
 Energy to tool - 10 percent

1.26.1 EFFECTS OF THE HIGH CUTTING TEMPERATURE ON TOOL AND WORK

High cutting temperatures are detrimental to both the tool and the job. The major portion of
the heat is taken away by the chips. But it does not matter because chips are thrown out. So
attempts should be made such that the chips take away more and more amount of heat
leaving small amount of heat to harm the tool and the job. The possible detrimental effects of
the high cutting temperature on cutting are:
On tool
 Rapid tool wear , which reduces tool life
 Cutting edges plastically deform and tool may loose its hot hardness
 Thermal flaking and fracturing of cutting edges may take place due to thermal
 shock
 Built up edge formation

On work
 Dimension inaccuracy of work duet to thermal distortion and expansion and
 contraction during and after machining
 Surface damage by oxidation, rapid corrosion, burning etc.
 Tensile residual stresses and microcracks at the surface and sub surfaces.

1.26.2 Tool work thermocouple technique for measuring Temperature

Fig. 2.7.3 shows the principle of this method. In a thermocouple two dissimilar but
electrically conductive metals are connected at two junctions. Whenever one of the junctions is
heated, the difference in temperature at the hot and cold junctions produce a proportional
current which is detected and measured by a milli-voltmeter. In machining like turning, the tool
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 and the job constitute the two dissimilar metals and the cutting zone functions as the hot
junction. Then the average cutting temperature is evaluated from the mV after thorough
calibration for establishing the exact relation between mV and the cutting temperature.

Tool-work thermocouple technique of measuring cutting temperature.

1.26.3 Control of cutting temperature

It is already seen that high cutting temperature is mostly detrimental in several respects.
Therefore, it is necessary to control or reduce the cutting temperature as far as possible.
Cutting temperature can be controlled in varying extent by the following general methods:

 Proper selection of material and geometry of the cutting tool(s)

 Optimum selection of cutting speed and feed rate without sacrificing MRR
 Proper selection and application of cutting fluid
 Application of special technique, if required and feasible.

1.27 CUTTING FLUIDS

Cutting fluids, sometimes referred to as lubricants or coolants are liquids and gases applied to
the tool and work piece to assist the cutting operation.

1.27.1 Functions of cutting fluids:

•To cool the tool.

•To cool the work piece.
•To lubricate and reduce friction
•To improve surface finish.
•To protect the finished surface from corrosion.
•To wash the chips away from the tool.

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 1.27.2 Properties of cutting fluids:
• High heat absorption capability.
• Good lubricating quality.
• High flash point so as to eliminate the hazard of fire.
• Stability so as not to get oxidized in presence of air.
• Neutral so as not to react chemically.
• Odourless so as not to produce bad smell even when heated.
• Harmless to the skin of the operators.
• Non corrosive to the work or the machine.
• Transparency so that the cutting action of tool may be observed by the operators.
• Low viscosity to permit free flow of the liquid.
• Low priced to minimize production cost.

1.27.3 TYPES OF CUTTING FLUIDS

4 general types:
 Oils - mineral, animal, vegetable, compounded, and synthetic oils,
 Emulsions - a mixture of oil and water and additives
 Semi synthetics - chemical emulsions containing little mineral oil
 Synthetics - chemicals with additives

1. Straight oils

These oils are non-emulsifiable and very useful in machining operations where they
function in undiluted form. Their composition is a base mineral or even petroleum oil. Often they
contain polar lubricants like vegetable oils, fats and esters.

They may also contain extreme pressure additives including sulphur, chlorine, and
phosphorus. To achieve the best lubrication use straight oils however they may have poor
cooling characteristics.

2. Synthetic fluids

They do not contain mineral oil base or petroleum. Instead, they’re formulated from the
alkaline organic and inorganic compounds alongside additives to prevent corrosion. They
function well in their diluted form. Of all the varieties of cutting fluids, synthetic fluids offer the
best cooling performance.

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 3. Soluble oils

Soluble Oils usually form an emulsion after mixing them with water. The resulting
concentrate contains emulsions and a base mineral oil to produce a stable emulsion. They
function well in their diluted form and offer a great lubrication in addition to heat transfer
performance. They are the least expensive and are the most widely used fluids in the industry.

4. Semi-synthetic fluids

These fluids are basically a combination of the soluble oils and synthetic fluids. Besides,
the heat transfer performance and cost of the semi-synthetic fluids falls between those of the
soluble and synthetic fluids.

1.28 TOOL DYNAMOMETERS

Dynamometers are devices used to measure cutting forces in machining operation. The cutting
force cannot be detected or quantified directly but their effect can be sensed using Transducer.
For example, a force which can neither be seen nor be gripped but can be detected and also
quantified respectively by its effect and the amount of those effects (on some material) like
elastic deflection, deformation, pressure, strain etc. These effects, called signals, often need
proper conditioning for easy, accurate and reliable detection and measurement. In other words,
Measurement involves three stages
 Conversion into another suitable variable (deflection, expansion etc)
 Amplification, filtration and stabilization
 Reading or recording

Measurement of cutting force(s) is based on three basic principles:

1. Measurement of elastic deflection of a body subjected to the cutting force
2. Measurement of elastic deformation, i.e. strain induced by the force
3. Measurement of pressure developed in a medium by the force.

1.28.1 Measuring deflection caused by the cutting force(s)

Under the action of the cutting force, say FC in turning, the tool or tool holder elastically deflects
as indicated in Fig.1 Such tool deflection, δ is proportional to the magnitude of the cutting force,
FC, simply as, Treating the tool as a cantilever beam, we can write the deflection of the tool as

Since for a given cutting tool and its holder, E and I are fixed, we can write, δ α Fc
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 Figure :- Deflection of cutting tool

Fig. 10.3 Electrical transducers working based on deflection measurment

(a) linear pot (b) circular pot (c) capacitive pick up (d) LVDT type

The deflection, δ, can be measured

1. mechanically by dial gauge (mechanical transducer)
2. electrically by using several transducers like; ⎯ potentiometer; linear or circular ⎯

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 3. capacitive pickup ⎯ inductive pickup
4. LVDT ( Linear Variable Differential Tranformer)

1.28.2 Measuring cutting force by monitoring elastic strain caused by the force.
Increasing deflection, δ enhances sensitivity of the dynamometer but may affect
machining accuracy where large value of δ is restricted, the cutting forces are suitably measured
by using the change in strain caused by the force. Fig. 10.5 shows the principle of force
measurement by measuring strain, ε, which would be proportional with the magnitude of the
force, F (say P ) as,
Z

where,
M = bending moment
Z = sectional modulus (I/y) of the tool section
I = plane moment of inertia of the plane section
y = distance of the straining surface from the neutral plane of the beam (tool)

Measuring cutting forces by strain gauges

Force measurement by strain gauge based transducer.

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 The change in resistance of the gauges connected in a wheatstone bridge produces voltage
output ΔV, through a strain measuring bridge (SMB) as indicated in Fig. 10.6. Out of the four
gauges, R , R , R and R , two are put in tension and two in compression as shown in Fig. 10.6.
1 2 3 4

The output voltage, ΔV, depends displacement of the strain gauge.,

1.28.3 Measuring cutting forces by pressure caused by the force

This type of transducer functions in two ways :
1. the force creates hydraulic or pneumatic pressure (through a diaphragm or piston) which
is monitored directly by pressure gauge as indicated in Figure 6.
2. the force causes pressure on a piezoelectric crystal and produces an emf proportional to

the force or pressure as indicated in Figure 7

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