1. MODULE 4
OUTLINES
Mechanical cutting techniques
Tool materials
Non –conventional machining
Economics of metal cutting
2. Why Machining is Important ?
Variety of materials can be machined
Most frequently applied to metals
Variety of part shapes & special geometry features possible
Accurate round holes
Very straight edges and surfaces
Good dimensional accuracy and surface finish
3. Disadvantages of Machining
Wasteful of material
– Chips generated in machining are wasted material, at least in the
unit operation
Time consuming
– A machining operation generally takes more time to shape a
given part than alternative shaping processes, such as casting,
powder metallurgy or forming
Lot of Power (electrical energy) is required
4. Three basic categories of material removal processes are
1. Conventional machining,
2. Abrasive processes, and
3. Non-conventional processes
5. THEORY OF METAL CUTTING
The metal cutting is done by a relative motion between
the work piece & the cutting tool.
Metal cutting could be done either by a single point
cutting tool or a multi point cutting tool.
The two basic types of metal cutting by a single point
cutting tool are orthogonal and oblique metal cutting.
If the cutting face of the tool is at 90o to the direction of
the tool travel the cutting action is called as orthogonal
cutting.
6. If the cutting face of the tool is inclined at less than 90o to
the path of the tool then the cutting action is called as
oblique cutting.
Elements of Metal Cutting :
Cutting speed : It is the distance traveled by work surface related
to the cutting edge of tool
v = (πdN / 1000 ) m/min
Feed (s) : The motion of cutting edge of tool with reference to one
revolution of work piece.
Depth of cut (t) : It is measured perpendicular to axis of work piece
and in straight turning in one pass.
7. Chip Formation
Machining = Chip formation by a tool
In any traditional machining process, chips are formed by a
shearing process
When the ultimate stress of the metal is exceeded, separation of
metal takes place. The plastic flow takes place in a localized area
called as shear plane
8. Chip Formation
α = rake angle
•Cutting action involves shear deformation of work
material to form a chip.
• As chip is removed, new surface is exposed
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9. Types of Chips
Type of chips produced influences surface finish,
integrity and machining operation.
1. Continuous
2. Continuous chips with Built up edge
3. Discontinuous
10. (1) Continuous Chip
• Ductile work materials
(e.g., low carbon steel, Al,
Copper)
• Usually formed at high cutting
speed or high rake angle
• Small feeds and depths
11. Continuous chip
• Sharp cutting edge on the tool
• Low tool-chip friction
• Suitable cutting fluid
• Continuous “ribbon” of metal that flows up
the chip/tool zone.
• Usually considered the ideal
condition for efficient
cutting action.
12. This is desirable because it produces good surface finish,
low power consumption and longer tool life.
Problems with Continuous chip
• Chips are difficult to handle and dispose off.
• The chips coil in a helix and curl around work and tool and
may injure the operator when it is breaking.
• The tool face is in contact for a longer period resulting in
more frictional heat. However this problem could be
rectified by the use of chip breakers.
13. (2) Continuous with BUE
• Forms at the tip of the tool
• Plastic deformation causes
adhesion between chip and
tool face
• Tool-chip friction causes
portions of chip to adhere to
rake face
• BUE forms, then breaks off,
cyclically
14. Continuous with BUE
• Adversely affects the surface finish
• Changes the geometry of cutting tool
• This type of chip is common in softer non-ferrous
metals and low carbon steels.
15. Continuous with BUE
To avoid BUE : 1. Decrease Depth of cut,
2. Increase rake angle,
3. Reduce tool tip radius,
4.Use proper cutting fluid
16.
17. (3) Discontinuous Chip
• Brittle work materials ( like bronze,
hard brass and gray cast iron )
• Generally non metals like ceramics,
polymers, composites, form
discontinuous chips
• Low cutting speeds
• Large feed and depth of cut
18. Discontinuous Chip
• Materials that contain hard inclusions &
impurities
• Lack of proper cutting fluid
• Low stiffness of the m/c
• These are convenient to handle and
dispose off
If these chips are produced from brittle materials, then the surface
finish is fair, power consumption is low and tool life is reasonable
however with ductile materials the surface finish is poor and tool
wear is excessive.
19. Single point cutting tool:
Parts of a single point cutting tool:
Part Description
Shank It is the body of the tool which is ungrounded.
Face It is the surface over which the chip slides.
Base It is the bottom surface of the shank.
Flank It is the surface of the tool facing the work
piece. There are two flanks namely end flank and
side flank.
Cutting It is the junction of the face end the flanks. There
edge are two cutting edges namely side cutting edge
and end cutting edge.
Nose It is the junction of side and end cutting edges.
20. Angle Details
Top rake angle It is also called as back rake angle. It is the slope given to the face
or the surface of the tool.
Side rake angle It is the slope given to the face or top of the tool. This slope is
given from the nose along the width of the tool. The rake angles
help easy flow of chips
Relief angle These are the slopes ground downwards from the cutting
edges. These are two clearance angles namely, side clearance angle
and end clearance angle. This is given in a tool to avoid rubbing of
the job on the tool.
Cutting edge There are two cutting edge angles namely side cutting edge angle
angle and end cutting edge angle. Side cutting edge angle is the angle,
the side cutting edge makes with the axis of the tool. End cutting
edge angle is the angle, the end cutting edge makes with the width
of the tool.
Lip angle It is also called cutting angle. It is the angle between the face and
end surface of the tool.
Nose angle It is the angle between the side cutting edge and end cutting edge.
21.
22. CUTTING TOOL MATERIALS CHARATERISITCS
• Selection of cutting tool materials is very
important
What properties should cutting tools have ?
– Hardness at elevated temperatures (hot
hardness)
– Toughness so that impact forces on the tool
can be taken
– Wear resistance
– Chemical stability or inertness
23. Different tool materials
1. Carbon and medium alloy steel
2. High speed steel (HSS)
3. Cast cobalt alloys
4. Carbides
5. Coated tools
6. Ceramics
7. Diamond
24. The List below shows some commercial tool materials
CBN - Cubic Boron Nitride
Ceramics
HSS - High Speed Steel
PCD - Polycrystalline Diamond
WC - Tungsten Carbide
Coated WC - Tools coated with Tungsten Carbide
25. Hardness of Cutting Tool
Materials as a Function
of Temperature
Figure . The hardness of various
cutting-tool materials as a function
of temperature (hot hardness). The
wide range in each group of
materials is due to the variety of
tool compositions and treatments
available for that group.
26. Tool life:
Volume of material removed b/w two successive tool grind.
Number of work piece machined b/w two successive tool grinds.
Time of actual cutting b/w two successive tool grinds.
Following are the factors influencing tool life.
Cutting speed:
It has the greatest influence. When the cutting speed
increases, the cutting temperature increases. Due to this,
hardness of the tool decreases.
27. The relation ship between tool life and cutting speed is
given by the Taylor's formula which states
VTn = C
V is the cutting speed in meters / minute
T is the tool life in minutes.
n depends on the tool and work.
C a constant.
Feed and depth of cut:
For a given cutting speed if the feed or depth of
cut is increased, tool life will be reduced.
28. The useful tool life of a HSS tool at 18 m/min is 3 hours. Calculate
the tool life when the tool operates at 24 m/min. ( take n = 0.125 )
Solution:
VTn = C
V = 18 m/min
T = 3 x 60 = 180 min
Constant C = 18 x ( 180 ) 0.125 = 34.45
Now V = 24 m/min.
T = ( 34.45 / 24 ) 1/0.125
= 18 minutes.
29. Tool geometries:
There are two distinct tool geometries. The are positive
and negative rake angles.
Positive is suitable for machining soft, ductile materials
(like aluminum) and negative is for cutting hard materials,
where the cutting forces are high (Hard material, high
speed and feed).
30. Cutting Temperatures are Important….
High cutting temperatures …
1. Reduce tool life
2. Produce hot chips that pose safety hazards
to the machine operator
3. Can cause inaccuracies in part dimensions
due to thermal expansion of work material
31. Temperature In Cutting
Fig:Percentage of the heat generated in
Fig:Typical temperature distribution cutting going into the workpiece,tool,and
in the cutting zone. chip,as a function of cutting speed.
32. Power and Energy Relationships
• A machining operation requires power
• The power to perform machining can be
computed from:
P = Fc v
where P = cutting power;
Fc = cutting force;
v = cutting speed
33. Why Nonconventional processes?
To machine new (harder, stronger & tougher) materials
difficult or impossible to machine conventionally
For unusual & complex geometries that cannot easily
machined conventionally
To achieve stringent surface (finish & texture) requirements
not possible with conventional machining
34. Advantages of Non-conventional machining:
1) High accuracy and surface finish
2) No direct contact of tool and w/p, so there is
less/no wear
3) Tool life is more
4 ) Quieter operation
Disadvantages of non-conventional machining:
1) High cost
2) Complex set-up
3) Skilled operator required
37. Electrical discharge machining (EDM)
Based on erosion of metals by spark discharges.
EDM system consist of a tool (electrode) and work piece,
connected to a dc power supply and placed in a dielectric
fluid.
When potential difference between tool and work piece is
high, a transient spark discharges through the fluid,
removing a small amount of metal from the work piece
surface.
This process is repeated with capacitor discharge rates of 50-
500 kHz.
38. Dielectric fluid – mineral oils, kerosene, distilled and
deionized water etc.
Role of the dielectric fluid
Acts as a insulator until the potential is sufficiently high.
Acts as a flushing medium and carries away the debris.
Also acts as a cooling medium.
Electrodes – usually made of graphite.
40. Wire EDM
This process is similar to contour cutting with a band
saw.
A slow moving wire travels along a prescribed path,
cutting the work piece with discharge sparks.
Wire should have sufficient tensile strength and
fracture toughness.
Wire is made of brass, copper or tungsten. (about
0.25mm in diameter).
42. Parts with complex, precise and irregular shapes for forging, press
tools, extrusion dies, difficult internal shapes for aerospace and
medical applications can be made by EDM process. Some of the
shapes made by EDM process are shown in figure.
Applications of EDM
43. Advantages of EDM
Materials of any hardness can be machined
No burrs are left in machined surface
Thin and fragile/brittle components can be machined
without distortion
Complex internal shapes can be machined
Limitations of EDM
Suitable only for electrically conductive materials
MRR is low and the process is slow compared to
conventional machining processes
Unwanted erosion and over cutting of material can occur
Rough surface finish at high rates of material removal
45. Laser-beam machining :
Utilizes a high-energy, coherent light beam to melt and vaporize
particles on the surface of metallic and non-metallic w/p.
Lasers can be used to cut, drill, weld and mark.
LBM is particularly suitable for making accurately placed holes.
Different types of lasers are available for manufacturing operations
which are as follows:
1.CO2 Gas laser : It is a gas laser that emits light in the infrared
region. It can provide up to 25 kW in continuous-wave mode.
2. Nd:YAG: Neodymium-doped Yttrium-Aluminum-Garnet (Y3Al5O12)
laser is a solid-state laser which can deliver light through a fibre-optic
cable. It can provide up to 50 kW power in pulsed mode and 1 kW in
continuous-wave mode.
47. Advantage of laser cutting
No limit to cutting path as the laser point can move any path.
The process is stress less allowing very fragile materials to be laser
cut without any support.
Very hard and abrasive material can be cut.
Sticky materials are also can be cut by this process.
It is a cost effective and flexible process.
High accuracy parts can be machined.
No cutting lubricants required
No tool wear
Narrow heat effected zone
Limitations of laser cutting
Uneconomic on high volumes compared to stamping
Limitations on thickness due to taper
High capital cost
High maintenance cost
Assist or cover gas required
49. Applications
LBM can make very accurate holes as small as
0.005 mm in refractory metals, ceramics,
and composite material without warping
the work pieces
Used widely for drilling and cutting of
metallic and non-metallic materials.
Laser beam machining is being used
extensively in the electronic and
automotive industries.
50. Plasma Arc Machining - PAM
Plasma arc machining (PAM) is a material removal
process in which the material is removed by directing
a high velocity jet of high temperature
(11,000-30,000°C) ionized gas on the work piece.
Plasma: A mixture of free electrons, positively charged
ions and neutral atoms.
Plasma can be obtained by heating a gas to a very high
temperature so that it is partially ionized.
51.
52. Advantages Disadvantages
Cuts any metal. Large heat affected
5 to 10 times faster than zone.
oxy-fuel. Difficult to produce
150 mm thickness ability. sharp corners.
Easy to automate. Smoke and noise.
Burr often results.
PAM….
54. Electron beam machining (EBM)
• Similar to LBM except laser beam is replaced by high
velocity electrons.
• When electron beam strikes the work piece surface,
heat is produced and metal is vaporized.
• Surface finish achieved is better than LBM.
• Used for very accurate cutting of a wide variety of
metals.
55. Applications of EBM :
1. To drill gas orifices for pressure differential devices (used in
nuclear reactors, rotors and aircraft engines, etc.)
2. To produce wire drawing dies, light-ray orifices and spinnerets
to produce synthetic fibers.
3. To produce metering holes, either round or profile shaped, to
be used as flow holes on sleeve valves, rocket fuel injectors or
injection nozzles on diesel engines.
4. To scribe thin films.
5. To remove small broken taps from holes.
56. Advantages of EBM:
1. Very small holes and slots of high precision in a short time in almost
any material can be made.
2. Different shapes of holes, slots and orifices can be machined.
3. There is no mechanical contact between the tool and the w/p.
Limitations of EBM
1. High cost of equipment.
2. Limited applicability (maximum depth of cut is 4 mm).
3. Low material removal rate.
4. Non-uniformity of holes and slots like taper and the entrance of holes
and slots is cratered and bell shaped.
5. Requires skilled workmanship.
58. Electrochemical Machining
• Reverse of electroplating
• An electrolyte acts as a current carrier and high electrolyte
movement in the tool-work-piece gap washes metal ions
away from the work piece (anode) before they have a
chance to plate on to the tool (cathode).
• Tool – generally made of bronze, copper, brass or stainless
steel.
• Electrolyte – salt solutions like sodium chloride or sodium
nitrate mixed in water.
• Power – DC supply of 5-25 V.
59. Advantages of ECM
• Burr free surface.
• No thermal damage to the parts.
• Lack of tool force prevents distortion of parts.
• Capable of machining complex parts and hard materials
Limitations of ECM
• ECM is not suitable to produce sharp square corners or flat
bottoms because of the tendency for the electrolyte to erode
away sharp profiles.
• ECM can be applied to most metals but, due to the high
equipment costs, is usually used primarily for highly
specialised applications.
61. Water Jet Machining
Fig : (a) Schematic illustration of WJM
(b) A computer-controlled, WJM cutting a
granite plate.
(c) Example of various nonmetallic parts
produced by the water-jet cutting process.
62. Water jet machining (WJT)
• Water jet acts like a saw and cuts a narrow groove in the
material.
• Pressure level of the jet is about 400MPa.
Advantages
- no heat produced
- cut can be started anywhere without the need for
predrilled holes
- burr produced is minimum
- environmentally safe and friendly manufacturing
Application – used for cutting composites, plastics, fabrics,
rubber, wood products etc. Also used in food processing
industry.
64. Abrasive Jet Machining (AJM)
• In AJM a high velocity jet of dry air, nitrogen or CO2
containing abrasive particles is aimed at the work piece.
• The impact of the particles produce sufficient force to
cut small hole or slots, deburring, trimming and
removing oxides and other surface films.
