Gives basics on THEORY OF METAL CUTTING, chips types, wear, tool material.

1. By
V. THULASIKANTH
Assistant Professor
Mechanical Engineering Department
vtkvsk@gmail.com
THEORY OF METAL CUTTING
1

2. Orthogonal and oblique cutting– Classification of cutting tools: single,
multipoint – Tool signature for single point cutting tool – Mechanics of
orthogonal cutting – Shear angle and its significance – Chip formation–
Cutting tool materials– Tool wear and tool life – Machinability – Cutting
Fluids– Simple problems.
Syllabus
2
TEXT BOOKS
1. Sharma, P.C., A textbook of Production Technology – Vol I and II, S. Chand &
Company Ltd., New Delhi, 1996.
2. Rao, P.N., Manufacturing Technology, Vol I & II, Tata McGraw Hill Publishing
Co., New Delhi, 1998.

3. Metal Cutting
3
Metal cutting or Machining operation is to produce a desired shape, size and
finish of a component by removing excess material in the form of chips.
So, primary objective of metal cutting is to produce chips which are thrown away.
Chips may constitute more than 50% of initial work piece.
Machining processes are performed on metal cutting machines, more commonly termed
as machine tools using various types of cutting tools
Metal cutting process in general should be carried out at high speeds and feeds
with least cutting effort at minimum cost.
Factors affecting metal cutting
1. Properties of Work material
2. Properties & geometry of cutting tool
3. Interaction between tool and work

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External
Internal
threadingforminggroovingfacingturning

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Mechanics of Metal Cutting
A cutting tool exerts compressive force on the workpiece which stresses the work
material beyond the yield point and therefore metal deform plastically and shears off.
Plastic flow takes place in a localized
region called the shear plane.
Sheared material begins to flow along
the cutting tool face in the form of chips.
Flowing chips cause tool wear.
Applied compressive force which leads
to formation of chips is called cutting
force.
Heat produced during shearing action raises the temperature of the workpeice,
cutting tool and chips.
Temperature rise in cutting tool softens and causes loss of keeness in cutting
edge.
Cutting force, heat and abrasive wear are important features in metal cutting.

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Types of Metal Cutting Process
Orthogonal cutting is also known as two dimensional metal cutting in which the cutting
edge is normal to the work piece. (angle = 90deg)
Oblique cutting is also known as three dimensional cutting in which the cutting action
is inclined with the job by a certain angle called the inclination angle. (angle ≠ 90deg)

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Types of Cutting Tools
Cutting tools performs the main machining operation.
It is a body having teeth or cutting edges on it.
They comprise of single point cutting tool or multipoint cutting tools.
Single point cutting tool : This type of tool has a effective cutting edge and
removes excess material from the workpeice along the cutting edge.
These tools may be left-handed or right-handed.
Again single point cutting tools classified as solid type and the tipped tool.
Brazed tools are generally known as tool bits and are used in tool holders.
The tipped type of tool is made from a good shank steel on which is mounted a tip
of cutting tool material.
Tip may be made of high speed steel or cemented carbide.
Different types of carbide tips are generally used on tipped tool.

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Geometry comprises mainly of nose, rake face of the tool, flank, heel and shank etc.
The nose is shaped as conical with different angles.

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Types of Chips
Chips are separated from the workpiece to impart the required size and shape.
The chips that are formed during metal cutting operations can be classified into four
types:
1. Continuous chips
2. Continuous chips with built-up edge
3. Discontinuous or segmental chips.
4. Non homogenous chips
1. Continuous chips
Chip is produced when there is low friction between the chip and tool face
This chip has the shape of long string or curls into a tight roll
Chip is produced when ductile materials such as Al, Cu, M.S, and wrought Iron are
machined.
Formation of very lengthy chip is hazardous to the machining process and the
machine operators.

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It may wrap up on the cutting tool, work piece and interrupt in the cutting operation.
It becomes necessary to deform or break long continuous chips into small pieces.
It is done by using chip breakers and this can be an integral part of the tool design
or a separate device.
2. Continuous chips with built-up edge
When high friction exists between chip and tool, the chip material welds itself to the
tool face.
Welded material increases friction further which in turn leads to the building up a
layer upon layer of chip material.
Build up edge grows and breaks down when it becomes unstable.
Chips with build up edge result in higher power consumption, poor surface finish and
large tool wear

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3. Discontinuous or segmental chips
Chip is produced in the form of small pieces.
These types of chips are obtained while
machining brittle material like cast iron, brass
and bronze at very low speeds and high feeds.
For brittle materials it is associated with fair
surface finish, lower power consumption and
reasonable tool life.
For ductile materials it is associated with poor surface finish excessive tool wear.
4. Non-homogeneous chips
It will be in the form of notches and formed due to non-uniform strain in materal
during chip formation.
Non homogenous chips are developed during machining highly hard alloys like
titanium.

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Chip Control and Chip Breakers
During machining high tensile strength materials chips has to be properly
controlled.
Carbide tip tools will be used for high speeds which leads to high temperature and
produce continuous chips with blue color.
If the above mentioned chips are not broken means it will adversely effect the
machining in following ways,
•Spoiling cutting edge
•Raising temperature
•Poor surface finish
•Hazardous to machine operator
Two ways are employed to overcome all the above drawbacks.
First one is Proper selection of cutting conditions and second one is
chip breakers are used to break the chips.

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Proper selection of cutting conditions
Since the cutting speed influences to the great extend the productivity of
machining and surface finish, working at low speeds may not be desirable.
 If the cutting speed is to be kept high, changing the feed and depth of cut is a
reasonable solution for chip control.
Chip breaker
There are two types of chip breakers
1.ΠExternal type, an inclined obstruction clamped to the tool face
2. Integral type, a groove ground into the tool face or bulges formed onto the tool
face
clamped

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NOMENCLATURE Of SINGLE POINT TOOL

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Feed
Back rake angle (αb)
It is the angle between the face of the tool and a line parallel with base of the tool
measured in a perpendicular plane through the side cutting edge.
This angle helps in removing the chips away from the work piece.

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Side rake angle (αs)
It is the angle by which the face of tool is inclined side ways.
This angle of tool determines the thickness of the tool behind the cutting edge.
It is provided on tool to provide clearance between work piece and tool so as to
prevent the rubbing of work- piece with end flank of tool.
End relief angle
It 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.
It is the angle that allows the tool to cut without rubbing on the work- piece.
Side relief angle
It is the angle that prevents the interference as the tool enters the material.
It 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.

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End cutting edge angle
It is the angle between the end cutting edge and a line perpendicular to the shank
of the tool.
It provides clearance between tool cutting edge and work piece.
Side cutting edge angle
It is the angle between straight cutting edge on the side of tool and the side of the
shank.
It is also known as lead angle.
It is responsible for turning the chip away from the finished surface.

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Tool Signature
Convenient way to specify tool angles by use of a standardized abbreviated
system is known as tool signature or tool nomenclature.
The seven elements that comprise the signature of a single point cutting tool can
be stated in the following order:
Tool signature 0-7-6-8-15-16-
0.8
1. Back rake angle (0°)
2. Side rake angle (7°)
3. End relief angle (6°)
4. Side relief angle (8°)
5. End cutting edge angle (15°)
6. Side cutting edge angle (16°)
7. Nose radius (0.8 mm)

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Properties of cutting tool materials
1. Red hardness or Hot Hardness: It is the ability of a material to retain its hardness
at high temperature
2. Wear resistance: It enables the cutting tool to retain its shape and cutting efficiency
3. Toughness: It relates to the ability of a material to resist shock or impact loads
associated with interrupted cuts
Classification tool materials
1. Carbon-Tool Steels:
 0.6-1.5% carbon + little amount of Mn, Si, Cr, V to increase hardness.
 Low carbon varieties possess good toughness & shock resistance.
 High carbon varieties possess good abrasion resistance
2. High Speed Steels (HSS):
 High carbon+ little amount Tungsten, Molybdenum, Cr, V & cobalt to increase
hardness, toughness and wear résistance.
 High operating temperatures upto 600o
C.

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 Two types of HSS i.e, is T-type and M-Type
 Vanadium increases abrasion resistance but higher percentage will decreases
grindability.
 Chromium increases hardenability
 Cobalt is added to HSS to increase red hardness.
3. Cast Cobalt Base Alloys:
 It is a combination of W, Cr, carbon and Cobalt which form an alloy with red
hardness, wear resistance and toughness. It is prepare by casting.
 Used for machining Cast iron, alloy steels, non-ferrous metals and super alloys
4. Cemented Carbides:
 These are carbides of W, Titanium and tantalum with small amount of cobalt
produced by means of powder metallurgy route.
 Two types i.e, Straight Tungsten Carbide Cobalt Grade and Alloyed Tungsten
Carbide Grade

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Straight Tungsten Carbide Cobalt Grade : Cast iron, non ferrous alloys, plastics,
wood, glass etc.
Alloyed Tungsten Carbide Grade: All grades of steel at 3 to 4 times more speeds
than HSS
5. Ceramic Tools:
 Aluminium Oxide, Silicon Carbide, Boron Carbide, Titanium Carbide, Titanium
Boride
 High speed, longer tool life, superior surface finish, No coolant is required.
6. Diamond Tools:
 More abrasion resistance
 Used for turning grinding wheels
 Used to produce mirror surface finish.
 Diamond abrassive belts are used to produce TV screens
 Poly crystalline diamond inserts are brazed into cutting edges of circular saws for
cutting construction materials like concrete, refractories, stone etc.

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Tool Life
Properly designed and ground cutting tool is expected to perform the metal cutting
operation in an effective an smooth manner
If a tool is not giving satisfactory performance it is an indicative of tool failure.
Following are the adverse effects observed during operation;
During operation cutting tool may fail due to following;
Extremely poor surface finish on the workpiece
Higher consumption of power
Work dimensions are not produced as specified
Overheating of cutting tool
Appearance of burnishing band on the work surface
Thermal cracking and softening
Mechanical Chipping
Gradual wear

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1.Thermal Cracking and Softening
During cutting operation lot of heat will be generated due to this cutting tool tip
and area closer to cutting edge will become hot.
Cutting tool material will be harder up to certain limit (temperature & pressure), if
it crosses the limit it starts deforming plastically at tip and adjacent to the cutting
edge under the action of cutting pressure and high temperature.
Tool looses its cutting ability and it is said to have failed due to softening.
Main factors responsible for this condition are;
High cutting speed
High feed rate
More depth of cut
Small nose radius
Choice of wrong tool material
Tool life is defined as the time interval for which tool works satisfactorily
between two successive grinding or re-sharpening of the tool.

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Working temperatures for common tool materials are;
Carbon tool steels 200o
C - 250o
C
High speed steel 560o
C - 600o
C
Cemented Carbides 800o
C - 1000o
C
Tool material is subjected to local expansion and contraction due to severe
temperature gradient.
Gives rise to thermal stresses further leads to thermal cracks.

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2. Mechanical Chipping
Mechanical chipping of nose an cutting edge of the tool are commonly observed
causes for tool failure.
Reasons for failure are High cutting pressure, Mechanical impact, Excessive
wear, too high vibrations and chatter, weak tip an cutting edge, etc.
This type of failure is pronounced in carbide tipped and diamond tools due to
high brittleness of tool material.
Chipped Tip

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3. Gradual wear
When a tool is in use for some time it is found to have lost some weight or
mass implying that it has lost some material from it due to wear.
Wear locations:
Crater wear location
Flank wear location
Crater wear
Due to pressure of the hot chip
sliding up the face of the tool, crater
or a depression is formed on the face
of tool. (Ductile materials)
By diffusion shape of crater formed
corresponds to the shape of
Crater wear

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Flank wear
Occurs between tool and workpiece
interface
Due to abrasion between tool flank
and workpiece
Hard microconstituents of cut
material and broken parts of BUE.
Common in Brittle material.
The entire area subjected to flank
wear is known as WEAR LAND (VB),
occurs on tool nose, front and side
relief faces

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Mechanism of wear
Adhesion wear: Fragments of the work-piece get welded to the tool
surface at high temperatures; eventually, they break off, tearing
small parts of the tool with them.
• Abrasion: Hard particles, microscopic variations on the bottom
surface of the chips rub against the tool surface and break away a
fraction of tool with them.
• Diffusion wear: At high temperatures, atoms from tool diffuse
across to the chip; the rate of diffusion increases exponentially with
temperature; this reduces the fracture strength of the crystals.
•Chemical wear: Reaction of cutting fluid to material of tool

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Heat Generated During Machining
Heat finds its way into one of three places
Workpiece, tool and chips
Heat Dissipation
Ideally most heat taken off in chips
Indicated by change in chip color as heat causes chips to
oxidize
Cutting fluids assist taking away heat
Can dissipate at least 50% of heat created during machining

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Cutting Fluids—Types and Applications
Cutting Fluids
Essential in metal-cutting operations to reduce heat and friction
Centuries ago, water used on grindstones
100 years ago, tallow used (did not cool)
Lard oils came later but turned rancid
Early 20th
century saw soap added to water
Soluble oils came in 1936
Chemical cutting fluids introduced in 1944

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Cutting fluid is a type of coolant and lubricant designed specifically for
metalworking and machining processes.
There are various kinds of cutting fluids, which include oils, oil-water emulsions,
pastes, gels, aerosols (mists), and air or other gases.
 They may be made from petroleum distillates, animal fats, plant oils, water and
air, or other raw ingredients.
Depending on context and on which type of cutting fluid is being considered, it
may be referred to as cutting fluid, cutting oil, cutting compound, coolant, or
lubricant.
What is Cutting Fluid ?

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Economic Advantages to Using Cutting Fluids
Reduction of tool costs
Reduce tool wear, tools last longer
Increased speed of production
Reduce heat and friction so higher cutting speeds
Reduction of labor costs
Tools last longer and require less regrinding, less downtime, reducing
cost per part
Reduction of power costs
Friction reduced so less power required by machining

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Characteristics of a Good Cutting Fluid
Good cooling capacity
Good lubricating qualities
Resistance to rancidity
Relatively low viscosity
Stability (long life)
Rust resistance
Nontoxic
Transparent
Nonflammable

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Types of Cutting Fluids
Most commonly used cutting fluids
Either aqueous based solutions or cutting oils
Fall into three categories
Cutting oils
Emulsifiable oils
Chemical (synthetic) cutting fluids

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Refrigerated Air System
Another way to cool chip-tool interface
Effective, inexpensive and readily available
Used where dry machining is necessary
Uses compressed air that enters vortex generation
chamber
Cooled 100ºF below incoming air
Air directed to interface and blow chips away

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