2. Introduction
Kinematic system in any machine tool is comprised of chain(s) of several mechanisms to
enable transform and transmit motion(s) from the power source(s) to the cutting tool and the
workpiece for the desired machining action.
The kinematic structure varies from machine tool to machine tool requiring different type
and number of tool-work motions.
Even for the same type of machine tool, say column drilling machine, the designer may
take different kinematic structure depending upon productivity, process capability, durability,
compactness, overall cost etc targeted.
2
3. Kinematic System And Working Principle Of Lathe Machine
• Amongst the various types of lathes, centre lathes are the most versatile and commonly used.
• Fig. 2.1 schematically shows the typical kinematic system of a 12 speed centre lathe.
• For machining in machine tools the job and the cutting tool need to be moved relative to each
other.
The tool-work motions are:
Formative motions :
• cutting motion
• feed motion
Auxiliary motions :
• indexing motion
• relieving motion etc 3
4. Fig 2.1 Schematic diagram of a center lathe.
Kinematic System And Working Principle Of Lathes
4
5. Kinematic System And Working Principle Of Lathes
In lathes:
Cutting motion is attained by rotating the job
Feed motion by linear travel of the tool:
-either axially for longitudinal feed
-or radially for cross feed
It is noted, in general, from Fig.2.1
• The job gets rotation (and power) from the motor through the belt-pulley, clutch and then the speed gear box
which splits the input speed into a number (here 12) of speeds by operating the cluster gears.
• The cutting tool derives its automatic feed motion(s) from the rotation of the spindle via the
gear quadrant, feed gear box and then the appron mechanism where the rotation of the feed
rod is transmitted
- either to the pinion which being rolled along the rack provides the longitudinal feed
- or to the screw of the cross slide for cross or transverse feed. 5
6. Kinematic System And Working Principle Of Lathes
• While cutting screw threads the half nuts are engaged with the rotating leadscrew to
positively cause travel of the carriage and hence the tool parallel to the lathe bed i.e., job axis.
• The feed-rate for both turning and threading is varied as needed by operating the Norton gear
and the Meander drive systems existing in the feed gear box. The range of feeds can be
augmented by changing the gear ratio in the gear quadrant connecting the feed gear box with
the spindle
• As and when required, the tailstock is shifted along the lathe bed by operating the clamping
bolt and the tailstock is moved forward or backward or is kept locked in the desired location.
6
7. Kinematic System of Drilling Machines and Their Principle of Working
The kinematic system enables the drilling machine the following essential works:
Cutting motion:
• The cutting motion in drilling machines is attained by rotating the drill at different speeds
(r.p.m.). Like centre lathes, milling machines etc, drilling machines also need to have a
reasonably large number of spindle speeds to cover the useful ranges of work material, tool
material, drill diameter, machining and machine tool conditions.
• It is shown in Fig. 2.2 that the drill gets its rotary motion from the motor through the speed
gear box and a pair of bevel gears. For the same motor speed, the drill speed can be changed
to any of the 12 speeds by shifting the cluster gears in the speed gear box.
• The direction of rotation of the drill can be changed, if needed, by operating the clutch in the
speed reversal mechanism, RM-s shown in the figure.
7
8. Kinematic System of Drilling Machines and Their Principle of Working
Feed motion:
• In drilling machines, generally both the cutting motion and feed motion are imparted to the
drill. Like cutting velocity or speed, the feed (rate) also needs varying (within a range)
depending upon the tool-work materials and other conditions and requirements.
• Fig. 2.2 visualizes that the drill receives its feed motion from the output shaft of the speed
gear box through the feed gear box, and the clutch.
• The feed rate can be changed to any of the 6 rates by shifting the gears in the feed gear box.
And the automatic feed direction can be reversed, when required, by operating the speed
reversal mechanism, RM-s as shown.
• The slow rotation of the pinion causes the axial motion of the drill by moving the rack
provided on the quil.
8
9. Kinematic System of Drilling Machines and Their Principle of Working
• The upper position of the spindle is reduced in diameter and splined to allow its passing
through the gear without hampering transmission of its rotation.
Tool work mounting
• The taper shank drills are fitted into the taper hole of the spindle either directly or through
taper socket(s).
• Small straight shank drills are fitted through a drill chuck having taper shank.
• The workpiece is kept rigidly fixed on the bed (of the table).
• Small jobs are generally held in vice and large or odd shaped jobs are directly mounted on the
bed by clamping tools using the T-slots made in the top and side surfaces of the bed as
indicated in Fig. 2.2.
9
11. • The kinematic system comprising of a number of kinematic chains of several mechanisms enables
transmission of motions (and power) from the motor to the cutting tool for its rotation at varying
speeds and to the work-table for its slow feed motions along X, Y and Z directions.
• In some milling machines the vertical feed is given to the milling(cutter) head. The more versatile
milling machines additionally possess the provisions of rotating the work table and tilting the vertical
milling spindle about X and / or Y axes.
• Fig. 2.3 typically shows the kinematic diagram of the most common and widely used milling
machine having rotation of the single horizontal spindle or arbor and three feed motions of the
work-table in X, Y and Z directions
• The milling cutter mounted on the horizontal milling arbor, receives its rotary motion at different
speeds from the main motor through the speed gear box which with the help of cluster gears splits the
single speed into desirably large number (12, 16, 18, 24 etc) of spindle speeds..
Kinematic System of Milling Machine and Their Principle of Working
11
12. Kinematic System of Milling Machine and Their Principle of Working
• Power is transmitted to the speed gear box through Vee-belts and a safety clutch as shown in
the diagram.
• For the feed motions of the workpiece (mounted on the work-table) independently, the cutter
speed, rotation of the input shaft of the speed gear box is transmitted to the feed gear box
through reduction (of speed) by worm and worm wheels as shown.
• The cluster gears in the feed gear box enables provide a number of feed rates desirably.
• The feeds of the job can be given both manually by rotating the respective wheels by hand as
well as automatically by engaging the respective clutches.
• The directions of the longitudinal (X), cross (Y) and vertical (Z) feeds are controlled by
appropriately shifting the clutches. The system is so designed that the longitudinal feed can be
combined with the cross feed or vertical feed but cross feed and vertical feed cannot be
obtained simultaneously. 12
13. Kinematic System of Milling Machine and Their Principle of Working
• This is done for safety purpose. A telescopic shaft with universal joints at its ends is incorporated to
transmit feed motion from the fixed position of the feed gear box to the bed (and table) which moves
up and down requiring change in length and orientation of the shaft.
• The diagram also depicts that a separate small motor is provided for quick traverse of the bed and
table with the help of an over running clutch.
• During the slow working feeds the rotation is transmitted from the worm and worm wheel to the
inner shaft through three equi-spaced rollers which get jammed into the tapering passage.
• During quick unworking work-traverse, the shaft is directly rotated by that motor on-line without
stopping or slowing down the worm. Longer arbours can also be fitted, if needed, by stretching the
over-arm. The base of the milling machine is grouted on the concrete floor or foundation.
13
14. Kinematic System of Milling Machine
Fig.2.3 Kinematic
diagram of a milling
machine
14
15. Kinematic System of Shaping Machine
The usual kinematic system provided in shaping machine for transmitting power and motion from
the motor to the tool and job at desired speeds and feeds is schematically shown in Fig. 2.4.
Fig. 2.4 Kinematic diagram
of a shaping machine.
15
16. .
• The central large bull gear receives its rotation from the motor through the belt-pulley,
clutch, speed gear box and then the pinion.
• The rotation of the crank causes oscillation of the link and thereby reciprocation of the ram
and hence the tool in straight path.
• Cutting velocity which needs to be varied depending upon the tool-work materials, depends
upon:
-The stroke length, S mm
-Number of strokes per min., Ns and
-The Quick return ratio, QRR (ratio of the durations of the forward stroke and the
return stroke)
16
17. Kinematic System of Planing Machine
• The simple kinematic system of the planing machine enables transmission and
transformation of rotation of the main motor into reciprocating motion of the large work
table and the slow transverse feed motions (horizontal and vertical) of the tools.
• The reciprocation of the table, which imparts cutting motion to the job, is attained by rack-
pinion mechanism.
• The rack is fitted with the table at its bottom surface and the pinion is fitted on the output
shaft of the speed gear box which not only enables change in the number of stroke per
minute but also quick return of the table.
• The blocks holding the cutting tools are moved horizontally along the rail by screw-nut
system and the rail is again moved up and down by another screw-nut pair.
17
18. Kinematic System of Slotting Machine
• The schematic view of slotting machine is typically shown in Fig.2.5. The vertical slide
holding the cutting tool is reciprocated by a crank and connecting rod mechanism, so here
quick return effect is absent.
• The job, to be machined, is mounted directly or in a vice on the work table. Like shaping
machine, in slotting machine also the fast cutting motion is imparted to the tool and the feed
motions to the job.
• In slotting machine, in addition to the longitudinal and cross feeds, a rotary feed motion is
also provided in the work table.
• The intermittent rotation of the feed rod is derived from the driving shaft with the help of a
four bar linkage as shown in the kinematic diagram.
18
19. Kinematic System of Slotting Machine
• It is also indicated in Fig. 2.5 how the intermittent rotation of the feed rod is transmitted to
the lead screws for the two linear feeds and to the worm – worm wheel for rotating the work
table.
• The working speed, i.e., number of strokes per minute, Ns may be changed, if necessary by
changing the belt-pulley ratio or using an additional “speed gear box”, whereas, the feed
values are changed mainly by changing the amount of angular rotation of the feed rod per
stroke of the tool. This is done by adjusting the amount of angle of oscillation of the paul as
shown in Fig. 2.5. The directions of the feeds are reversed simply by rotating the tapered paul
by 180° as done in shaping machines.
19
22. Introduction
What is Machine Tool?
• Machine tool is a power operated, non-portable and valuable machine that can perform
multiple machining operations by remove excess material from a pre-formed blank with the
help of a suitable cutting tool.
• Machine tool is strictly restricted within the metal-working (or machining) field.
• So a machine having following five characteristics can be considered as a machine tool.
• It must be power driven (human operated machines are not machine tools). The form of
power at input to the machine tool can be either electrical, mechanical, hydraulic, pneumatic
or a non-conventional one.
• It must be non-portable (portability irrespective of size). Thus machine tools are always
firmly installed with the shop floor. 22
23. Introduction
What is Machine Tool?
• It must have sufficient value (value in terms of capability and performance; not on the basis
of cost).
• It can perform more than one machining or metal cutting operations.
• It utilizes a cutting tool to shear off excess materials from workpiece.
• Examples of machine tool include Lathe machine tool, Milling machine tool, Shaping
machine tool, drilling & boring machine tool, etc.
23
24. Cutting Tool
• A cutting tool is a small device having one or more wedge shaped and sharp cutting edges to
facilitate shearing during metal cutting.
• So a cutting tool basically removes (shears off) material from workpiece.
• It is rigidly mounted on the machine tool in appropriate location.
• Shape and features of the cutting tool varies widely based on the required machining
operation and intended performance.
• Cutting tool cannot provide any motion required for cutting. All intended motions are
provided by the machine tool.
• So cutting tool is mounted on a machine tool using suitable tool holding arrangements so that
it can compress a thin layer of workpiece material to gradually shear it off in the form of
chips for material removal.
• For example, lathe is a machine tool, while the single point turning tool is a cutting tool.
24
25. Cutting Tool Materials
Carbon and medium alloy Steels
High Speed Steel(HSS)
Cemented Carbides
Ceramics
Polycrystalline Diamonds
Cubic Boron Nitride (CBN)
25
26. Properties of Cutting Tool Materials
The desirable properties of tool material includes:
1.Wear Resistance
• Wear resistance should be as high as possible.
• Wear of tool is caused by abrasion, adhesion and diffusion.
• Wear resistance refers to the ability of tool material to retain its sharpness and shape for
longer duration while machining is continued.
2. Hot Hardness
• It is the measure of the ability of tool material to retain its hardness at high temperature.
• Hot hardness should be as high as possible especially at high temperature.
26
27. Properties of Cutting Tool Materials
3. Toughness
• It is the ability of material to absorb energy and deform plastically before failure and fracture.
• Tougher the material more is the ability to withstand external load, impact and intermittent
cuts.
• Hence, toughness should be as high as possible.
4. Coefficient of Thermal Expansion
• Coefficient of thermal expansion determines the influence of thermal stresses and thermal
shocks on a material.
• It should be as low as possible so that tool does not get distorted after heat treatment, and
remains easy to regrind and also easy to weld to the tool holder.
27
28. Properties of Cutting Tool Materials
• Carbide have lower coefficient of thermal expansion than high speed steel and they develop
lower thermal stress but are more sensitive to thermal shock because of their brittleness.
5. Hardness
• It is the ability of material to resist the penetration, scratching, abrasion or cutting.
• Hardness of tool material should be as high as possible.
• Generally it should be higher than workpiece.
6. Thermal Conductivity
• It should be as high as possible with a view to remove the heat quickly from chip tool
interface.
7. Chemical Stability or Inertness against work material, atmospheric gases and cutting fluids.
8. Manufacturability, Availability and Low cost 28
29. Cutting Tool Materials
a). Carbon and medium alloy Steels
• High carbon tool steel is the oldest cutting tool materials, having carbon content
ranging from 0.7 – 1.5%.
• Inexpensive, easily shaped, sharpened.
• Maximum hardness is about HRC 62 and hence has low wear resistance
• It has low hot hardness-poor properties above 200OC.
• Limited to low cutting speed operation (9 mm/min).
• Uses: Drills taps, broaches ,reamers for machining soft materials and wood
working tools
29
30. Cutting Tool Materials
b) High Speed Steel (HSS)
• The basic composition of HSS is 18% W, 4% Cr, 1% V, 0.7% C and rest Fe.
• HSS tools are suitable for machining of mild steel materials and it is used as cutting tool
material where:
The tool geometry and mechanics of chip formation are complex, such as helical twist drills,
reamers, gear shaping cutters, hobs, form tools, broaches, etc.
The tool is to be used number of times by resharpening.
• With time the effectiveness and efficiency of HSS tools and their application range were
gradually enhanced by improving its properties and surface condition through:
• Refinement of microstructure.
30
31. Cutting Tool Materials
• Addition of large amount of cobalt and Vanadium to increase hot hardness and wear
resistance respectively.
• Manufacture by powder metallurgical process.
c) Carbides (Cemented or Sintered Carbides)
i) Straight or Single Carbide
• The straight or single carbide tools or inserts were produced powder metallurgically by mixing,
compacting and sintering 90 to 95% WC powder with cobalt.
• The hot, hard and wear resistant WC grains are held by the Co binder which provides the necessary
strength and toughness.
• Such tools are suitable for machining grey cast iron, brass, bronze etc. which produce short
discontinuous chips and at cutting velocities two to three times of that possible for HSS tools.
31
32. Cutting Tool Materials
ii) Composite Carbides
• For machining steels successfully, another type called composite carbide have been
developed by adding (8 to 20%) a gamma phase to WC and Co mix.
• The gamma phase is a mix of TiC, TiN, TaC, NiC etc. which are more diffusion resistant
than WC due to their more stability and less wettability by steel.
iii) Mixed Carbides
• Titanium carbide (TiC) is not only more stable but also much harder than WC.
• So for machining ferritic steels causing intensive diffusion and adhesion wear a large quantity
(5 to 25%) of TiC is added with WC and Co to produce another grade called mixed carbide.
• But increase in TiC content reduces the toughness of the tools.
• Therefore, for finishing with light cut but high speed, the harder grades containing up to 25%
TiC are used and
• For heavy roughing work at lower speeds lesser amount (5 to 10%) of TiC is suitable. 32
33. Cutting Tool Materials
d) Plain Ceramics
• The plain ceramic tools are brittle in nature and hence had limited applications.
• Basically three types of ceramic tool bits are available in the mark:
1) Plain alumina with traces of additives are used mainly for machining cast iron and similar
materials at speeds 200 to 250 m/min.
2) Alumina with or without additives machining steels and cast iron at VC = 150 to 250
m/min.
3) Carbide ceramic (Al2O3 + 30% TiC) cold or hot pressed, black colour, quite strong and
enough tough - used for machining hard cast irons and plain and alloy steels at 150 to 200
m/min.
• The plain ceramic outperformed the existing tool materials in some application areas like
high speed machining of softer steels mainly for higher hot hardness.
33
34. Advanced Tool Materials
1) Coated Carbides
Coated carbides remarkably enhance overall machining economy through:
Reduction of cutting forces and power consumption.
Increase in tool life by 200 to 500 %.
Improved product quality.
Effective and efficient machining of wide range of work materials.
Pollution control by less or no use of cutting fluid,
Reduction of abrasion, adhesion and diffusion wear.
Reduction of friction and BUE formation.
Heat resistance and reduction of thermal cracking and plastic deformation.
34
35. Advanced Tool Materials
2) Cermets
• These sintered hard inserts are made by combining „cer‟ from ceramics like TiC, TiN or
TiCN and „met‟ from metal (binder) like Ni, Ni-Co, Fe, etc.
• The modern cermets providing much better performance are being made by TiCN which is
consistently more wear resistant, less porous and easier to make.
• Application wise, the modern TiCN based cermets with beveled or slightly rounded cutting
edges are suitable for finishing and semi-finishing of steels at higher speeds, stainless steels
but are not suitable for jerky interrupted machining and machining of aluminium and similar
materials.
35
36. Advanced Tool Materials
3) Coronite
• Coronite is made basically by combining HSS for strength and toughness and tungsten
carbides for heat and wear resistance.
Unlike solid carbide, the coronite based tool is made of three layers:
• The central HSS or spring steel core.
• A layer of tungsten carbides of thickness around 15% of the tool diameter.
• A thin (2 to 5 µm) PVD coating of TiCN.
• Such tools are not only more productive but also provide better product quality.
• The coronite tools made by hot extrusion followed by PVD-coating of TiN or TiCN
outperformed HSS tools in respect of cutting forces, tool life and surface finish.
36
37. Advanced Tool Materials
4) High Performance Ceramics (HPC)
• Ceramic tools as such are much superior to sintered carbides in respect of hot hardness,
chemical stability and resistance to heat and wear but lack in fracture toughness and strength.
Through last few years‟ remarkable improvements in strength and toughness and hence
overall performance of ceramic tools could have been possible by several means which
include:
Sinterability, microstructure, strength and toughness of Al2O3 ceramics were improved to
some extent by adding TiO2 and MgO.
Introducing nitride ceramic (Si3N4) with proper sintering technique - this material is very
tough but prone to built-up-edge formation in machining steels.
Adding carbide like TiC (5 ~ 15%) in Al2O3 powder - to impart toughness and thermal
conductivity. 37
38. Advanced Tool Materials
4.1) Nitride Based Ceramic Tools
i) Plain Nitride Ceramics Tools: Compared to plain alumina ceramics, Nitride (Si3N4) ceramic
tools possess higher bending strength, toughness and higher conductivity, consequently. exhibit
more resistance to fracturing by mechanical and thermal shocks. Hence such tool seems to be
more suitable for rough and interrupted cutting of various material excepting steels, which
cause rapid diffusion wear and BUE formation.
ii) Sialon Tools: Hot pressing and sintering of an appropriate mix of Al2O3 and Si3N4 powders
yielded an excellent composite ceramic tool called SIALON which are very hot hard, quite
tough and wear resistant. These tools can machine steel and cast irons at high speeds (250 - 300
m/min). But machining of steels by such tools at too high speeds reduces the tool life by rapid
diffusion.
38
39. Advanced Tool Materials
iii) SiC Reinforced Nitride Tools:
The toughness, strength and thermal conductivity and hence the overall performance of nitride ceramics could
be increased remarkably by adding SiC whiskers or fibers in 5 - 25 volume %. The SiC whiskers add fracture
toughness mainly through crack bridging, crack deflection and fiber pull-out. Such tools are very expensive but
extremely suitable for high production machining of various soft and hard materials even under interrupted
cutting.
4.2) Alumina Based Ceramic Tools
i) Zirconia Toughened Alumina (ZTA) Ceramic: ZTAs more widely applicable and more productive than
plain ceramics and cermets in machining steels and cast irons.
ii) Zirconia Toughened Alumina (ZTA) Ceramic: ZTA hardness has been raised further by proper
control of particle size and sintering process. Hot pressing and HIP raise the density, strength and
hot hardness of ZTA tools but the process becomes expensive and the tool performance degrades
at lower cutting speeds. 39
40. Advanced Tool Materials
iii) Alumina Ceramic Reinforced by SiC Whiskers:
The properties, performances and application range of alumina based ceramic tools have been improved
spectacularly through drastic increase in fracture toughness (2.5 times). After optimization of the
composition, processing and the tool geometry, such tools have been found too effectively and
efficiently machine wide range of materials, over wide speed range (250 - 600 m/min) even under large
chip loads. But manufacturing of whiskers need very careful handling and precise control and these
tools are costlier than zirconia toughened ceramic tools.
iv) Silver Toughened Alumina Ceramic:
Compared to zirconia and carbides, metals were found to provide more toughness in alumina ceramics.
ain compared to other metal-toughened ceramics, the silver-toughened ceramics can be manufactured by
simpler and more economical process routes like pressureless sintering and without atmosphere control.
40
42. Types of Cutting Tools
Cutting tools may be classified according to the number of major cutting edges
(points) involved as follows:
Single-point cutting:
– The cutting tool has only one major edge.
Examples: turning tools, shaping, planning and slotting tools and boring tools
Double-point cutting:
– The cutting tool has more than one major cutting edge.
– Examples: drilling
Multi-point (more than two):
Examples milling cutters, broaching tools, hobs, gear shaping cutters etc.
42
43. Types of Cutting Tools
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.
43
49. Cutting Tool Nomenclature
• Size: it is determined by the width of shank, height of shank and overall length.
• Shank: it is main body of a tool. It is held in a holder.
• Flank: it is the surface or surfaces below and adjacent to cutting edge.
• Heel: it is intersection of the flank and base of the tool.
• Base: it is the bottom part of the shank. It takes the tangential force of cutting.
• Face: it is surface of tool on which chip impinges when separated from workpiece.
• Cutting Edge: it is the edge of that face which separates chip from the workpiece.
• The total cutting edge consists of side cutting edge, the nose and end cutting edge.
• Tool Point: it is part of tool, which is shaped to produce the cutting edge and the face.
• The Nose: it is the intersection of side cutting edge and end cutting edge.
49
50. Cutting Tool Nomenclature
• Neck: it is the small cross section behind the point.
Side Cutting Edge Angle:
• The angle between side cutting edge and side of the tool shank is called side cutting edge
angle. It is also called as lead angle or principle cutting angle.
End Cutting Edge Angle:
• The angle between the end cutting edge and a line perpendicular to the shank of tool is
called end cutting edge angle.
Side Relief Angle:
• The angle between the portion of the side flank immediately below the side cutting edge and
line perpendicular to the base of tool measured at right angles to the side flank is known as
side relief angle.
• It is the angle that prevents interference, as the tool enters the work material. 50
51. Cutting Tool Nomenclature
End Relief Angle:
• End relief angle is the angle between the portion of the end flank immediately below the end
cutting edge and the line perpendicular to the base of tool, measured at right angles to end
flank.
• It is the angle that allows the tool to cut without rubbing on the workpiece.
Back Rake Angle:
• The angle between face of the tool and a line parallel with the base of the tool, measured in
a perpendicular plane through the side cutting edge is called back rake angle.
• It is the angle which measures the slope of the face of the tool from the nose toward the rear.
• If the slope is downward toward the nose, it is negative back rake angle.
• And if the slope is downward from the nose, it is positive back rake angle.
• If there is not any slope, back rake angle is zero.
51
52. Cutting Tool Nomenclature
Side Rake Angle:
• The angle between the face of the tool and a line parallel with the base of the tool, measured
in a plane perpendicular to the base and side cutting edge is called side rake angle.
• It is the angle that measures the slope of the tool face from cutting edge.
• If the slope is towards the cutting edge, it is negative side rake angle.
• If the slope is away from the cutting edge, it is positive side rake angle.
• All the tool angles are taken with reference to the cutting edge and are, therefore, normal to
the cutting edge.
• A convenient way to specify tool angle is by use of a standardized abbreviated system called
tool signature. Sometimes it is also called as tool character.
• Tool signature also describes how the tool is positioned in relation to the workpiece. 52
54. Chip Formation
Chips are formed as a result of metal cutting process/machining process.
• Machining process include:
Conventional machining process - Chip formation process:
• Use a wedge shaped cutting tool.
• Direct contact between work and tool.
Non-conventional machining process - Chip less machining process:
– The shape of the tool depends on the shape of cut.
– No direct contact between work and tool.
54
55. Chip Formation
For machining:
Holding the work piece properly.
In metal cutting operation, the work piece is securely clamped in a machine tool vice or clamps
or chuck or collets.
1. Fix the tool properly
Use a wedge shaped cutting tool.
Set the tool to a certain depth of cut.
Force to move in direction of cut.
2. Create all the necessary motions in the machining time.
• Work piece motions – Tool motions. 55
57. Chip Formation
Shear off the metal is done provided
1. When the tool is harder than the metal to be cut.
2. The tool have good strength.
To resist cutting pressures.
Strength to keen enough to sever the metal.
3. When the tool have Proper tool geometry.
4. When there is a relative work-tool movement.
57
58. Chip Formation
Shearing process
1. All metals in the solid state have a characteristic crystalline structure/grain structure.
Fine grain structure
Coarse grain structure
2. When the cutting tool advances in the work piece.
Heavy forces are exerted on the crystals in front of the tool face.
These crystals, in turn exert similar pressures on crystals ahead of them.
3. Shear point formation
As the tool continues to advance, the material at the sheared point is sheared by the cutting edge of the tool or it
may be torn loose by the action of the bending chip which is being formed.
4. Shear plane formation
•As the tool advances, maximum stress is exerted along sheared line, which is called the shear plane. This
plane is approximately perpendicular to the cutting face of the tool. There exists a shear zone on both sides of
the shear plane.
58
59. Chip Formation
5. Shear takes place
When the force of the tool exceeds the strength of the material at the shear plane, rupture
or slippage of the crystalline grain structure occurs, thus forming the metal chip.
59
60. Chip Formation
Metal cutting
• In metal cutting operation, the position of cutting edge of the cutting tool is important based
on which the cutting operation is classified as: Orthogonal cutting and Oblique cutting
1. Orthogonal Cutting
• Orthogonal cutting is also known as two dimensional metal cutting in which the cutting edge
is normal to the work piece.
• In orthogonal cutting no force exists in direction perpendicular to relative motion between
tool and work piece.
2. Oblique cutting
• Oblique cutting is the common type of three dimensional cutting process used in various
metal cutting operations in which the cutting action is inclined with the job by a certain angle
called the inclination angle. 60
63. Forces in metal cutting: (a) forces acting on the chip in orthogonal cutting, and(b) forces acting
on the tool that can be measured.
• Consider the forces acting on the chip during orthogonal cutting in Figure (a).
• The forces applied against the chip by the tool can be separated into two mutually
perpendicular components: friction force and normal force to friction.
• The friction force F is the frictional force resisting the flow of the chip along the rake face of
the tool.
• The normal force to friction N is perpendicular to the friction force. These two components
can be used to define the coefficient of friction between the tool and the chip.
63
64. Chip Formation
In metal cutting (machining) process, working motion is imparted to the work piece and
cutting tool by the mechanisms of machine tool so that the work and tool travel relative to
each other and machine the work piece material in the form of shavings known as chips.
The chip gets separated from the work piece material and moves up along the tool face.
In addition, when the metal is sheared, the crystals are elongated, the direction of
elongation being different from that of shear. The circles which represent the crystals in the
uncut metal get elongated into ellipses after leaving the shearing plane.
Chips are separated from the work piece to impart the required size and shape to the work
piece.
The type of chips edge formed is basically a function of the work material and cutting
conditions. 64
65. Chip Formation
Types of chip:
Continuous chip
Continuous chip with built up edges (BUE).
Discontinuous/Segmental chip
Non homogenous chip
1. Continuous Chips
• Formed with ductile materials machined at high cutting speeds and/or high rake angles
• Deformation takes place along a narrow shear zone called the (primary shear zone)
• Continuous chips may develop a secondary shear zone due to high friction at the tool–chip interface
• This zone becomes thicker as friction increases
• Continuous chips may also occur with wide primary shear zone with curved boundaries
• Occurs: machining soft metals at low speeds, low rake angles
65
66. Chip Formation
Note, lower boundary of deformation zone drops below machined surface ⇒ distortion in
workpiece, poor finish
Fig to show continuous chip formation
66
67. Chip Breakers
• Formation of very lengthy chip is hazardous to the machining process and the machine
operators. It may wrap up on the cutting tool, work piece and interrupt in the cutting
operation. Chip breaker can be an integral part of the tool design or a separate device.
• Long, continuous chips are undesirable since: become entangled and greatly interfere with
machining potential safety hazard
• Chip-breaker: breaks chips intermittently with cutting tools
• Traditionally are clamped to rake face: bend and break the chip
• Modern tools: built-in chip breakers Ideal chip: “C” or “9” shape
67
69. Chip Formation
2. Continuous chip with built up edges
During cutting operation, the temperature rises and as the hot chip passes over the
face of the tool, alloying and welding action may take place due to high pressure,
which results in the formation of weak bonds in microstructure and these weakened
particles might pullout.
Owing to high heat and pressure generated, these particles get welded to the cutting
tip of the tool and form a false cutting edge. This is known as built-up edge.
• Consists of layers of material from the workpiece that are deposited on the tool tip.
69
70. • As it grows larger, the BUE becomes unstable and eventually breaks apart. BUE: partly
removed by tool, partly deposited on workpiece
BUE can be reduced by:
Increase the cutting speeds
Decrease the depth of cut
Increase the rake angle
Use a sharp tool
Use an effective cutting fluid
Use cutting tool with lower chemical
affinity for workpiece materials
70
71. Chip Formation
3. Discontinuous Chips
Consist of segments that are attached firmly or loosely to each other. Discontinuous chips form under
the following conditions:
• Brittle workpiece materials like cast iron, brass and bronze.
• Materials with hard inclusions and impurities
• Very low or very high cutting speeds
• Large depths of cut
• Low rake angles
• Lack of an effective cutting fluid
• Low stiffness of the machine tool (⇒ vibration, chatter)
• Fairly good surface finish is obtained and tool life is increased with this type of chips. 71
73. Chip Curl
• Chips will develop a curvature (chip curl) as they leave the workpiece surface
Factors affecting the chip curl conditions are:
Distribution of stresses in the primary and secondary shear zones.
Thermal effects.
Work-hardening characteristics of the workpiece material
Geometry of the cutting tool
Cutting fluids. Note, as cutting depth ↓, chip radius ↓ (i.e. curlier)
73
74. Non homogenous type of chip (Serrated Chips )
• Non homogenous chips are developed during machining highly hard alloys like titanium
which suffers a marked decrease in yield strength with increase in temperature
• Also called segmented or Serrated chips
• They are semicontinuous chips with:
large zones of low shear strain and
small zones of high shear strain (shear localization)
• Example: metals with low thermal conductivity and strength that decreases sharply with
temperature, i.e. thermal softening (e.g. titanium)
• Chips have a sawtooth-like appearance. Note, do not confuse this with dimension
74
75. Four types of chip formation in metal cutting:
(a) discontinuous, (b) continuous, (c) continuous with built-up edge, (d) serrated.
75
