Metal cutting theory
Requirements of good machining Practice
• Quick metal removal.
• Good surface finish.
• Economy in tool cost.
• Less power consumption.
• Economy in the cost of replacement and
sharpening tools.
• Minimum idle time of machining tools.
Basic elements of machining
a) Work piece b) Tool and c) Chip
keeping
• the work piece
stationary and moving
the tool
Or
• by keeping the tool
stationary and moving
the work
or
• by moving both tool
and work piece
Good characteristics of Cutting Tool Materials:
1. Ability to retain its hardness at elevated temperatures called
hot hardness.
2. Ability to resist shock, called toughness.
3. High resistance to wear to ensure longer tool life.
4. Low co-efficient of friction at the chip –tool interface, so that
the surface finish good and wear in minimum.
5. Should be cheap.
6. Should be able to be fabricated and shaped easily.
7. If it is to be used in the form of brazed tips, its other physical
properties like tensile strength, thermal conductivity, co
efficient of thermal expansion and modulus of elasticity etc.
should be as close to the shank material as possible to avoid
Cracking.
Cutting Tool Materials
1. Tool steels having a carbon percentage as high as 1.5% are in common
use as tool materials for general class of work. They can’t be used at high
speeds. The required hardness is lost by them at temperature 200- 250C. They
are used mainly for hand tools as they are less costly, easily forgeable and easy
to heat treat.
2. High carbon medium alloy steels - small amount of
Tungsten, Chromium, Molybdenum, Vanadium etc. able to operate
temperatures of 350C. (5% alloying elements)
3. High Speed Steel: It is a special alloy-steel containing the alloying
elements like Tungsten, Chromium, Vanadium, Cobalt and Molybdenum up to
25%. These alloying elements increase its strength, toughness, wear resistance,
cutting ability and retains it’s hardness at elevated temperature of 550 -600 C
the high speed steel tools are capable of operating at 2 to 3 times higher cutting
speeds than high carbon steel Tool steels .
The most commonly used high speed steel has composition alloying
elements as 18-4- 1 i.e. 18%W, 4%Cr, and 1%V
4. Cemented Carbides:
These Carbides are formed by the mixture of Tungsten, Titanium with Carbon. The
carbides in the powder form are mixed with Cobalt which acts as binder.
The mixture with powder metallurgy process, sintered at high pressures of
1500kg/sq cm to 4000kg/sq cm and temperatures of above 1500C.
It is shaped in to desired forms of tips.
These Carbide tips are then brazed or fastened mechanically to the shank made of
medium Carbon steel. These cemented carbides possess a very high degree of
hardness and wear resistance.
They are able to retain this hardness at temperature up to 1000C
with the result, the tools tipped with cemented carbide tips are capable of
operating at speeds 5 to 6 times higher than those of high speeds
5. Stellite: It is a non ferrous alloy mainly of Cobalt, Tungsten and
Chromium. Other elements added in varying proportions are Tantalum,
Molybdenum and Boron. It has good shock and wear resistance and retains its
hardness at a red heat up to 920C. It is used for machining materials like hard
bronzes, cast and malleable Iron etc.
Tools made of Stellite are capable of operating at speed up to 2 times more
than those of common high speed steel tools. Only grinding can be used for
machining it effectively.
A satellite may contain 40-50% Co, 15-35%Cr, 12-25%W and 1-4%Carbon.
6. Ceramics:
• It mainly consists of Aluminum oxide which is comparatively much cheaper than any
of the chief constituents of cemented Carbides.
• Boron nitrides in powdered form are added and mixed with Aluminum oxide
powder and sintered together at a temperature of 1700C. They are then compacted
in to different tip shapes.
• Tools made of ceramic material are capable of withstanding high temperatures,
without loosing their hardness up to 1200C.
• They are much more wear resistant than cemented carbide tools. They are more
brittle and low resistance to bending.
• They can’t be used for rough machining work and mainly used for finishing
operations. They are capable of removing 4 times more material than Tungsten
carbide tools and2-3 times high cutting speeds under similar conditions.
• No coolant is needed while machining with ceramic tools.
7. Diamond:
• It is the hardest material known and used as cutting tool material. It is brittle and
low resistance to shock but it is highly wear resistant.
• Diamonds are used for only light cuts on materials like Bakelite, Carbon, Plastics,
Aluminum and Brass etc.
• Because of low co efficient of friction they produce a high grade of surface finish.
• Because of high cost only limited use in tool industry
Geometry of Single Point Tools:
1. Rake angle:
• It is the angle formed between face of the tool and plane parallel to its base. If this
inclination is towards shank, It is known as back rake or top rake. When it is measured
towards side of the tool, it is called side rake.
• These rake angles guide the chips away from the cutting edge, thereby reducing the
chip pressure on the face and increasing the keenness (sharpness) of the tool, so
that less power is required for cutting.
• An increased rake angle will reduce the strength of cutting edge. Therefore tools used
for cutting hard materials are given small rake angles, whereas those used for soft
metals contain large rake angles.
• The above rake angles are called positive rake angles. When no rake is provided on
the tool, it is said to have zero rake angle. When the face of the tool is so ground that
it slopes upwards from the point, it is said to contain a negative rake. (5 to 10 ).
Such rake is usually provided on carbide tipped tools when they are used for
machining extra- hard surfaces, hardened steel parts and for taking intermittent cuts.
2. Lip angle: The angle between the
face and flank of the tool is known as Lip
angle. It is also called angle of keenness of
the tool. Strength of the cutting edge or
point of the tool is directly affected by this
angle. Larger the lip angle, stronger will
be cutting edge and vice versa. This angle
varies inversely as the rake angle. It is only
for this reason that when harder metals are
to be machined a stronger tool is required,
the rake angle is reduced and consequently
the lip angle is increased.
3. Clearance angle:
It is the angle formed by the front or side surface of the tool which are adjacent and
below the cutting edge when the tool is held in a horizontal position. It is the angle
between one of these surfaces and a plane normal to the base of the tool. When the
front surface is considered it is called front clearance and when the surface below cutting
edge is considered, the angle formed is known as side clearance angle. The purpose of
providing front clearance is to allow the tool to cut freely without rubbing against the
surface of the job. The side clearance is to direct the cutting thrust to the metal area
adjacent to the cutting edge.
The type and geometry of chip formed are greatly affected by the
• metal of work piece,
• geometry of cutting tool and
• method of cutting.
Relief angle: It is the angle
formed between flank of the
tool and a perpendicular drawn
from the cutting point to the
base of the tool.
Nose radius: If the cutting tip of a single point tool carries a sharp cutting point, the
cutting tip is weak. It is therefore highly stressed during the operation, may fail or
loose its cutting ability soon and produces marks on the machined surface. In order to
prevent these harmful effects the nose is provided with a radius, called Nose radius. It
enables greater strength to cutting tip, a prolonged tool life and superior surface finish
on the work piece. As the value of this radius increases, a higher cutting speed can be
used. If it is too large, it may lead to chatter. So a balance has to be maintained. Its
value normally varies from 0.4mm to 1.6mm depending upon several factors like depth
of cut, amount of feed, type of cutting and type of tool
Chip Formation:
Chips are formed due to tearing and shearing.
In the chip formation by tear, the
work piece material adjacent to the tool face is
compressed and crack runs ahead of the
cutting tool and towards body of the work-
piece. The chip is highly deformed and the
work piece material is relatively under formed.
Cutting takes place intermittently and there is
no movement of the work piece material over
the tool face.
In chip formation by shear, there is a general
movement of the chip over tool face
Types of chips:
Continuous chip:
This type of chip is produced while machining a ductile material, like mild steel
and copper at very high cutting speed and minimum friction between the chip
and
the tool face. The friction at the chip-tool inter face can be minimized by
polishing the tool face and adequate use of coolant. The basis of production of
a continuous chip is the continuous plastic deformation of the metal ahead of
the cutting tool, the chip moving smoothly up the tool face. Other factors
responsible are
• bigger rake angle,
• finer feed and
• keen cutting edge of the tool
Discontinuous chips: This type of chips is produced during machining of
brittle materials like cast-iron and bronze. These chips are produced in the form of small
segments.
In machining of such materials, as the tool advances forward, the shear-plane angle
gradually reduces until the value of compressive stress acting on the shear plane
becomes too low to prevent rupture. At this stage, any further advancement of the
tool results in the fracture of the metal ahead of it, thus producing a chip. With further
advancement of the tool, the processes of metal fracture and production of chips goes on
repeatedly producing discontinuous chips.
Such chips are also sometimes produced in machining of ductile materials, when low
cutting speeds are used and adequate lubrication is not provided. This causes excessive
friction between the chip and tool face, leading to fracture of chip in small segments. This
will also result in excessive wear on the tool and poor surface finish on the work-piece.
Other factors responsible for production of discontinuous chips are smaller rake angle on
the tool and too much depth of cut.
Continuous chip with built-up edge: It is very similar to the continuous type
and not as smooth as continuous chip. It has a built-up edge adhering on nose of the tool,
which changes the effective geometry of cutting. It is obtained by machining ductile metals
with high speed tools at ordinary cutting speeds, thus introducing high friction between
the chip and tool face. The form and size of such an edge depends largely on the cutting
speed, being absent at very low and very high cutting speeds. This type of chip results in
poor surface finish.
The chip is also sufficiently hot and gets oxidized as it comes off the tool and turns blue in
colour. The extra metal welded to tool nose or point of the tool is called built-up edge.
This metal is highly strain hardened and brittle. With the result, as the chip flows up the
tool, the built-up edge is broken and carried away with the chip while the rest of it
adheres to the surface of the work-piece, making it rough. Due to the built-up edge the
rake angle is also altered and so is the cutting force. The common factors responsible for
formation of built-up edge are low cutting speed, excessive feed, small rake angle and lack
of lubricant.
Chip-Breakers
Types
The chips produced during machining at higher speeds in machining of high tensile
strength materials, need to be effectively controlled. Carbide tipped tools are used in case
of higher speeds and due to high temperature the chip will be continuous of blue colour
and take the shape of coil. Such a chip, if not broken in to parts and
removed from the surroundings of the metal cutting area, will adversely affect the
machining in the following way.
a) Adversely affect tool life by spoiling the cutting edge, creating crater and raising the
temperature.
b) Lead to poor surface finish on the work-piece.
c) The chips get curled around the rotating work-piece and cutting tool, it may be
hazardous to the machine operator.
d) If large and continuous coil is allowed to be formed it may endanger the machine and
even the work place.
e) Very large coils offer a lot of difficulty in their removal.
The chip breakers break the produced chips in to small pieces.
The work hardening of the material of the chip makes the work of the chip breakers easy
The common methods used for chip breaking are:
i) By control of tool geometry i.e. grinding proper back rake and side rake angles according
to the speeds and feeds used.
ii) By obstruction method i.e. by inters posing a metallic obstruction in the path of the coil.
The following types of chip breakers are commonly used:
a) Groove type: It consists of a grinding groove on the face of the tool, behind the cutting
edge, leaving a small land near the tip.
b) Step type: It consists of a grinding a step on the face of the tool, adjacent to the cutting
edge.
c) Secondary rake type: It consists of providing a secondary rake on the tool, through
grinding, together with a small step.
d) Clamp type: This type of chip breaker is very common with the carbide tipped tools. The
chip breaker is a thin and small plate, which is either brazed to or held mechanically on the
tool face.
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)
Cutting Speed, Feed and Depth Of Cut:
Cutting speed of a tool can be defined as the rate at which its cutting edge
passes over the surface of the work-piece in unit time. It is normally
expressed in terms of surface speed in meters per minute.
Cutting speed affects the tool life and efficiency of machining.
So, Selection of proper cutting speed has to be made very
judiciously
If it is too high, the tool gets over heated and its cutting edge may fail,
needing regrinding. If it is too low, too much time is consumed in machining
and full cutting capacities of the tool and machine are not utilized, resulting
in lowering of productivity and increasing the production cost.
Feed of the cutting tool can be defined as the distance it travels along or in to the
work-piece for each pass of its point through a perpendicular position in unit time. In
turning operation of lathe, it is equal to the advancement of the tool corresponding to
each revolution of work. In planning it is the work, which is fed and not the tool. In
milling work, the feed is considered per tooth of the cutter.
The cutting speed and feed of a cutting tool is largely influenced by the following factors:
1. Material being machined.
2. Material of the cutting tool.
3. Geometry of the cutting tool.
4. Required degree of surface finish.
5. Rigidity of the machine tool being used
6. Type of coolant being use
Depth of cut:
It is indicative of the penetration of the cutting edge of the tool in to the work piece
material in each pass, measured perpendicular to the machined surface
i.e. it determines the thickness of metal layer removed by the cutting tool in one pass.
Example: In turning operation on a lathe it is given by
𝐷−𝑑
Depth of cut =𝑥 =
2
Where D = Original diameter of the work-piece in mm
D = Diameter obtained after turning in mm in one pass.
coolants
coolants are used in metal machining to perform the following main functions.
1. They cool the tool and the work piece.
2. They provide lubrication between the tool and work piece and tool and chips.
3. They prevent the adhesion of chips to the tool or work piece or both
The sources of heat generation during metal cutting are the following.
1 Friction: A lot of friction always takes place between the cutting tool and the work piece
and between the tool face and the chips passing over it. The total amount of heat
generated depends upon many factors viz. cutting speed, feed, tool material, depth of cut
and metal being machined. The heat so generated is known as heat of friction.
2. Plastic deformation of metal: Cutting tool exerts high pressure on the
adjacent metal grains which due to this pressure start slipping along their planes of
weakness. This causes deformation of all of them. The action of slipping of these grains
in contact with one another causes friction, leading to the generation of the heat of
deformation. The total amount of heat generated again depends upon the cutting speed,
feed, depth of cut and the metal being machined. Higher speeds, feeds, more depth of
cut, tougher materials contribute to greater heat generation.
3. Chip distortion: In machining, as the cut proceeds and the chips curl out, the
inside and the outside grain of the chip metal are subjected to compression and tension
respectively. This causes distortion of the chip grains are the chips leading to a sort of
internal friction amongst the grains and consequently generation of heat of chip
distortion. The amount of heat generated depends on feeds and depth of cut. Heavier
the feed and deeper the cut, the longer will be the area of cross-section of the chip and
more distortion amongst the grains, resulting in higher amount of heat generation
Machinability:
Machinability: Gives the idea of ease with which it can be machined.
The parameters influencing the machinability of a work piece material
are:
1. Physical Properties of material.
2. Mechanical Properties of material.
3. Chemical composition of material.
4. Micro-Structure of material
5. Cutting conditions.
Machinability of the material depends on various other variable factors such as
1. Tool Life: Longer tool life, it enables at a given cutting speed on the speed the
better is the machinability.
2. Surface finish: It is directly proportional, i.e. better surface finish the higher in
machinability.
3. Power Consumption: Lower power consumption per unit of metal removal-better
machinability.
4. Cutting Forces: Lesser amount of cutting force required for removal of higher
volume of metal under standard conditions, the higher will be the machinability.
5. Shear angle: Larger shear angle denotes better machinability.
6. Rate of metal removal under standard cutting conditions.
Tool Life:
Tool life can be defined on the time interval for each tool works satisfactorily
between into successive grindings.
These are three common ways of expressing Tool life.
1. As time period in minutes between two successive grindings.
2. In terms of no. of components machined between two successive grindings.
3. In terms of the volume of the material removed between two successive
grindings
Volume of metal removed/min = ∏ D t f N cu.mm/ min
Where D = Dia of work piece in mm
t = depth of cut in mm
f = feed rate mm/rev
N = no. of revolutions of work per min
Factors affecting Tool Life:
1. Cutting Speed.
2. Feed and Depth of cut.
3. Tool Geometry.
4. Tool Material.
5. Work Material.
6. Nature of Cutting.
7. Rigidity Machine tool and work.
8. Use of cutting fluids
1. Effect of cutting speed:
The tool life varies inversely on cutting speed i.e. higher the cutting speed the smaller
the tool life.
(V T)^n = C
V = Cutting speed m/min.
T = Tool life minutes.
n = An exponent – Its value depends on the tool material.
C = Machining Constant.
n = 0.1 to 0.15 HSS Tools
= 0.2 to 0.5 Carbide Tools
= 0.6 to 1.0 Ceramic Tool
2. Effect of feed rate and depth of cut: It will appreciably effect in reduction
in tool life.
V = Cutting Speed m/min
T = Tool Life in min
F = Feed rate mm/min
t = Depth of a cut in mm
If the tool life in considered on constant, the cutting speed
will decrease if the feed rate and depth of cut are
increased.
3. The Geometry: Geometrical parameters (Tool angles) of a cutting tool influence
its performance. The Rake angle has mixed effect. If it is increased, the amount of
heat generated are reduced and help in increasing the life of cutting tool. But if it is
very large the cutting edge is weakened and also its capacity to conduct the heat is
reduced results in reduction in mechanical strength and lowering tool life. For
effective economical tool life it is necessary to strike a balance. The optimum value
of rake angle needs to be used. This value varies from -5 to +10. The minus sign
indicates negative rake i.e. rake angle sloping up words from Tip. Tools carrying
negative rake angle provide a stronger cutting edge and hence a stronger tool.
Carbide and ceramic tools are generally provided – ve angle.
Similarly relief angle or clearance angle (5 to 8)on influence the tool performance.
These angles are provided on cutting tools to prevent the rubbing of tool flank
against the machine work surface. They thus help in lowering the amount of heat
generated and therefore increasing the tool life. But very large relief angles beyond
certain level results in weakening of tool resulting in reduction of tool life.
The two cutting edge angels also have their influence on tool performance. The
front cutting edge angle/end cutting edge angle effects the tool wear. Up to a
certain optimum value an increase in this angle permits the higher speeds
without an adverse effect on tool life. But an increase beyond certain value will
result in reduction of tool life. It generally varies from 5 to 8 . If the side cutting
edge angle is smaller the higher speeds can be used. However it has complex
effect on Tool life. A larger end cutting edge angle increases tool life
Nose radius: While it increases the abrasion, it also helps in improving
surface finish and tool strength and hence tool life.
4. Tool material: The main characteristics of good cutting tool material are its hot
hardness, wear resistance, impact resistance, abrasion resistance, heat
conductivity and strength etc. An ideal tool material is the one which will
remove the largest volume of work material at all speeds. It is not possible to get
truly ideal tool material. The tool material which can with stand max cutting
temperature without loosing its principal mechanical properties and geometry
will ensure max tool life. The higher hot hardness and toughness in tool material,
the longer the tool life.
5. Work Material: The micro-structure of work material is significant as it directly
effects the hardness of material. Higher the hardness of the work material
greater will be the tool wear and shorter will be the tool life. In machining pure
metals, because of their tendency to stick to the tool face. Specially at high
temperatures results in more friction and high amount wear on tool and
therefore shorter tool life.
6. Nature of cutting: Tool life is affected by nature of cutting i.e. whether it is continuous
or intermittent. In the intermittent cutting the tool is subjected to impact loads and may
give away much earlier than expected until it is made strong and tough. In continuous
cutting similar tool will have relatively longer life.
7. Rigidity of machine tool and work : Both the machine tool and work – piece should
remain rigid during the machining operation. If not , vibrations will take place and the
cutting tool will be subjected to intermittent cutting, instead of continuous cutting. This
will result in impact loading of tool and therefore shorter life.
8. Use of cutting fluids: Cutting fluids are used in machining work for helping the efficient
performance of the operation. They are used either in liquid or gaseous form. They assist
the operation by cooling the tool and work, reducing the friction, improving the surface
finish, helping in breaking the chips and washing them away etc. These factors help in
improving the tool life, permitting higher metal removal rate and improving the quality of
surface finish.
S.No Orthogonal Cutting Oblique Cutting
1 The cutting angle of the tool makes a right The cutting angle of the tool does not make the
angle to the direction of motion right angle to the direction of motion
2 The flow of the chip is perpendicular to the The flow of the chip is not perpendicular to the
cutting edge. cutting edge.
3
The tool has lesser cuttings life The tool has a higher cuttings life.
4 The shear forces per unit area is high, which The shear force per unit area is low, which
increases the heat per unit area. decreases heat per unit area.
5 In orthogonal cutting, the surface finish is
In oblique cutting surface finish is good.
poor.
6 Cutting edge is longer than the edge of the Cuttings may or may not be longer than the
cut edge of the cut.
7 In oblique cutting, three components of force
In orthogonal cuttings, only two components are considered, cutting force, thrust force, and
of force are considered cutting force and radial force, which cannot represent by 2D
thrust force, which can be represented by a coordinate. It used a 3D coordinate to represent
2D coordinate system. the forces acting during cutting, so it is known
as 3D cutting.
8
Two mutually perpendicular cutting Three mutually perpendicular forces are
forces act on the work piece involved.
What is Merchant’s circle diagram?
(cutting force analysis)
