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UNIT-1
INTRODUCTION
Traditional or Conventional Machining
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Need for Unconventional Machining
• Greatly improved thermal, mechanical and chemical properties of modern
materials – Not able to machine thru conventional methods. (Why???)
• Ceramics & Composites – high cost of machining and damage caused during
machining – big hurdles to use these materials.
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• In addition to advanced materials, more complex shapes, low rigidity
structures and micro-machined components with tight tolerances and fine
surface finish are often needed.
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• To meet these demands, new processes are developed.
• Play a considerable role in aircraft, automobile, tool, die and mold making
industries.
Need for Unconventional Machining
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• Very high hardness and strength of the material. (above 400 HB.)
• The work piece is too flexible or slender to support the cutting or grinding
forces.
• The shape of the part is complex, such as internal and external profiles, or
small diameter holes.
• Surface finish or tolerance better than that obtainable conventional
process.
• Temperature rise or residual stress in the work piece is undesirable.
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Unconventional Machining Processes - Classification
Electrical
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Mechanical Based Processes
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USM – through mechanical abrasion in a medium (solid abrasive particles
suspended in the fluid)
WJM – Cutting by a jet of fluid
AWJM – Abrasives in fluid jet.
IJM – Ice particles in fluid jet.
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Abrasives or ice – Enhances cutting action.
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Thermal based Unconventional Processes
Thru – melting & vaporizing
Many secondary phenomena – surface cracking, heat affected zone and
striations.
Heat Source:
Plasma – EDM and PBM.
Photons – LBM
Electrons – EBM
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Ions – IBM
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Machining medium: different for different processes.
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Chemical & Electrochemical based Unconventional Processes
CHM – uses Chemical dissolution action in an etchant.
ECM – uses electrochemical dissolution action in an electrolytic cell.
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The requirements that lead to the development of nontraditional machining.
 Very high hardness and strength of the material.
 The work piece: too flexible or slender to support the cutting or grinding
forces.
 The shape of the part is complex, such as internal and external profiles, or
small diameter holes.
 Surface finish or tolerance better than those obtainable conventional
process.
 Temperature rise or residual stress in the work piece are undesirable.
 Conventional machining involves the direct contact of tool and work piece, whereas unconventional machining does not require the direct
contact of tool and work piece. Conventional machining has many
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disadvantages like tool wear which are not present in Non-conventional
machining.
Advantages of Non-conventional machining
1) High accuracy and surface finish
2) Less/no wear
3) Tool life is more
4 ) Quieter operation
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Disadvantages of non-conventional machining:
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1) High cost
2) Complex set-up
3) Skilled operator required
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Need for unconventional machining processes
Machining – produces finished products with high degree of accuracy
Utilizes cutting tools (harder than work piece material)
Needs a contact between the tool and work piece.
Needs a relative motion between the tool and work piece.
The need for higher productivity, accuracy and surface quality
Improve the capability of automation system and decreasing their
sophistication (decreasing the investment cost) requirements
Very hard fragile materials difficult to clamp for traditional machining
When the work piece is too flexible or slender
When the shape of the part is too complex
Internal and external profiles, or small diameter holes.
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Selection Process
• Selection Process is based of following parameters
– Physical Parameter
– Shapes to be Machined
– Process Capability
– Economic consideration
Physical Parameter
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Process
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Machines
Holes ( Micro, Small, deep,Shallow)
LBM, EBM,ECM, USM & EDM
Precision Work
USM & EDM
Horning
Etching
Grinding
Deburring
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ECM
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ECM & EDM
AJM & EDM
USM & AJM
Threading
EDM
Profile Cut
PAM
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Process Capability or Machining Characteristics
MRR
Accuracy
( mm3/s )
Surface Finish
(μm)
(μm)
Power (kW/
cm3/ min
LBM
0.10
0.4 – 6.0
25
2700
EBM
0.15 - 40
0.4 – 6.0
25
450
15 - 80
0.25
10
1.8
ECM
27
0.2 -0.8
50
7.5
PAM
2500
Rough
250
0.90
USM
14
0.2 – 0.7
7.5
9.0
AJM
0.014
0.5- 1.2
50
312.5
Process
EDM
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PROCESS ECONOMY
Process
Capital Cost
Tool & Fixtures
Power
Requirement
Efficiency
EDM
Medium
High
Low
High
CHM
Medium
Low
High
Medium
ECM
V. High
Medium
Medium
V. Low
V. Low
Low
Low
Low
USM
High
High
Low
Medium
EBM
High
Low
Low
V. High
LBM
Medium
Low
V. Low
V. High
PAM
V. Low
Low
V. Low
V. Low
Conventional
V. Low
Low
Low
V. Low
AJM
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UNIT-II
MECHANICAL ENERGY BASED PROCESSES
ABRASIVE JET MACHINING (AJM)
In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on
the work material at a high velocity. The high velocity abrasive particles remove
the material by micro-cutting action as well as brittle fracture of the work material.
In AJM, generally, the abrasive particles of around 50 μm grit size would
impinge on the work material at velocity of 200 m/s from a nozzle of I.D. of 0.5
mm with a standoff distance of around 2 mm. The kinetic energy of the abrasive
particles would be sufficient to provide material removal due to brittle fracture of
the work piece or even micro.
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It is the material removal process where the material is removed by high velocity
stream of air/gas or water and abrasive mixture .An abrasive is small, hard
particle having sharp edges and an irregular shape .High velocity jet is aimed at a
surface under controller condition .
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Components Of Abrasive Jet Machining
 Abrasive delivery system
 Control system
 Pump
 Nozzle
 Mixing tube
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 Motion system
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1.Abrasive Delivery System
Auto abrasive delivery system has the capability of storing abrasive &
delivery the abrasive to the bucket . It’s works auto programming system by
help of once measuring record & no adjustment or fine tuning system . High
sensitive sensor gives extremely reliable & repeatable .
2.Control system
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The control algorithm that computes exactly how the feed rate should vary
for a given geometry in a given material to make a precise part .The
algorithm actually determines desired variation in the feed rate & the tool
path to provide an extremely smooth feed rate .
3.pump
Crankshaft & intensifier pump are mainly use in the abrasive jet machine
.The intensifier pump was the only pump capable of reliably creating
pressures high .Crankshaft pumps are more efficient than intensifier pumps
because they do not require a power robbing hydraulic system ultra high
pressure & more stroke per minute .
Advantages
 Extremely fast setup & programming
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 No start hole required
 There is only one tool
 Low capital cost
 Less vibration
 No heat generated in work piece
 Environmentally friendly
Disadvantages
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 Low metal removal rate
 Due to stay cutting accuracy is affected
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 Abrasive powder cannot be reused
 Tapper is also a problem
WATER JET MACHINING
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Water jet cutting can reduce the costs and speed up the processes by
eliminating orreducing expensive secondary machining process. Since no
heat is applied on the materials, cut edges are clean with minimal burr.
Problems such as cracked edge defects,Crystallization, hardening, reduced
weald ability and machinability are reduced in this process.
Water jet technology uses the principle of pressurizing water to
extremely high pressures,and allowing the water to escape through a very
small opening called “orifice” or “jewel”. Water jet cutting uses the beam of
water exiting the orifice to cut soft materials. This method is not suitable for
cutting hard materials. The inlet water is typically pressurized between1300
–4000 bars. This high pressure is forced through a tiny hole in the jewel,
which is typically0.18 to 0.4 mm in diameter
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There are six main process characteristics to water jet cutting:
1. Uses a high velocity stream of Ultra High Pressure Water 30,000–90,000 psi
(210–620 MPa) which is produced by an with possible abrasive particles
suspended in the stream.
2. Is used for machining a large array of materials, including heat-sensitive,
delicate or very hard materials.
3. Produces no heat damage to workpiece surface or edges.
4. Nozzles are typically made of sinter
5. Produces a taper of less than 1 degree on most cuts, which can be reduced or
eliminated entirely by slowing down the cut process or tilting the jet.
6. Distance of nozzle from workpiece affects the size of the kerf and the
removal rate of material. Typical distance is .125 in (3.2 mm)
APPLICATIONS OF WJM
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 WJM is used on metals, paper, cloth, leather, rubber, plastics, food, and
ceramics.
 It is a versatile and cost-effective cutting process that can be used as an
alternative to traditional machining methods.
 It completely eliminates heat-affected zones, toxic fumes, recast layers,
work hardening and thermal stresses.
 It is the most flexible and effective cleaning solution available for a variety
of industrial needs.
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 In general the cut surface has a sandblast appearance.
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 Moreover, harder materials exhibit a better edge finish.
 Typical surface finishes ranges from 1.6 μm root mean square (RMS) to
very coarse depending on the application.
 Tolerances are in the range of  25 µm on thin material.
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 Both the produced surface roughness and tolerance depend on the machining
speed.
ADVANTAGES
 It has multidirectional cutting capacity.
 No heat is produced.
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 Cuts can be started at any location without the need for predrilled holes.
 Wetting of the workpiece material is minimal.
 There is no deflection to the rest of the workpiece.
 The burr produced is minimal.
 The tool does not wear and, therefore, does not need sharpening.
 The process is environmentally safe.
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 Hazardous airborne dust contamination and waste disposal problems that are
common when using other cleaning methods are eliminated.
 There is multiple head processing.
 Simple fixturing eliminates costly and complicated tooling, which reduces
turnaround time and lowers the cost.
 Grinding and polishing are eliminated, reducing secondary operation costs.
LIMITATIONS
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 Very thick parts can not be cut with water jet cutting and still hold
dimensional accuracy. If the part is too thick, the jet may dissipate some, and
cause it to cut on a diagonal, or to have a wider cut at the bottom of the part
than the top. It can also cause a rough wave pattern on the cut surface.
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It is not suitable for mass production because of high maintenance
requirements.
ULTRASONIC MACHINING
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Ultrasonic machining, or strictly speaking "Ultrasonic vibration machining",
is a subtraction manufacturing process that removes material from the surface of a
part through high frequency, low amplitude vibrations of a tool against the material
surface in the presence of fine abrasive particles. The tool travels vertically or
orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm (0.002 to
0.005 in.).
The fine abrasive grains are mixed with water to form a slurry that is distributed
across the part and the tip of the tool. Typical grain sizes of the abrasive material
range from 100 to 1000, where smaller grains (higher grain number) produce
smoother surface finishes.
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An ultrasonically vibrating mill consists of two major components, a transducer
and a sonotrode attached to an electronic control unit with a cable. An electronic
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oscillator in the control unit produces an alternating current oscillating at a high
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frequency usually between 18 and 40 kHz in the ultrasonic range. The transducer
usually consists of a cylinder made of piezoelectric
ceramic. The oscillating
voltage is applied to electrodes attached to the transducer, which converts the
electrical energy into mechanical vibrations.
The transducer then vibrates the sonotrode at low amplitudes and high
frequencies. The sonotrode is usually made of low carbon steel. A constant stream
of abrasive slurry flows between t and work piece. This flow of slurry allows
debris to flow away from the cutting area.
The slurry usually consists of water (20 to 60% by volume) and boron carbide,
aluminum oxide and silicon carbide particles.The sonotrode removes material from
the work piece by abrasion where it contacts it, so the result of machining is to cut
a perfect negative of the sonotrode's profile into the work piece. Ultrasonic
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vibration machining allows extremely complex and non-uniform shapes to be cut
into the workpiece with extremely high precision.
Ultrasonic Machining Advantages and Disadvantages
Get to know about the advantages and disadvantages of machining process in order
to make the right decision:
Advantages
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Machined all sorts of hard materials
Produces fine finished and structured results
Produces less heat
Various hole cut shapes due to vibratory motion of the tool
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
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Disadvantages
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Requires a higher degree of integrity and skills
No certified record of radiography
Unnecessary large grain sizes causes defects
Additional repairs might be required due to spurious signs and
misunderstanding of the process
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Ultrasonic machines are the future of machining which is used all over the world
for creating hard and brittle forms of materials for the industrial uses. A lot of
operations can be performed with the ultrasonic machine which can benefit the
industrialists in a variety of ways. The advanced technology creates solution which
helps in opening up the market opportunities and has made things easier.
PRINCIPLE OF ULTRASONIC MACHINING
• During one strike, the tool moves down from its most upper remote position with
a starting speed at zero, then it speeds up to finally reach the maximum speed at the
mean position.
• Then the tool slows down its speed and eventually reaches zero again at the
lowest position.
• When the grit size is close to the mean position, the tool hits the grit with its full
speed.
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• The smaller the grit size, the lesser the momentum it receives from the tool.
• Therefore, there is an effective speed zone for the tool and, correspondingly there
is an effective size range for the grits.
PIEZOELECTRIC TRANSDUCER
• Piezoelectric transducers utilize crystals like quartz whose dimensions alter when
being subjected to electrostatic fields.
• The charge is directionally proportional to the applied voltage.
• To obtain high amplitude vibrations the length of the crystal must be matched to
the frequency of the generator which produces resonant conditions.
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ABRASIVE SLURRY
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• The abrasive slurry contains fine abrasive grains. The grains are usually boron
carbide, aluminum oxide, or silicon carbide ranging in grain size from 100 for
roughing to 1000 for finishing.
• It is used to microchip or erode the work piece surface and it is also used to carry
debris away from the cutting area.
APPLICATIONS
It is mainly used for
(1) drilling
(2) grinding,
(3) Profiling
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(4) coining
(5) piercing of dies
(6) welding operations on all materials which can be treated suitably by abrasives.
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LIMITATIONS
• Under ideal conditions, penetration rates of 5 mm/min can be obtained.
• Power units are usually 500-1000 watt output.
• Specific material removal rate on brittle materials is 0.018 mm cubic/Joule.
• Normal hole tolerances are 0.007 mm and a surface finish of 0.02 to 0.7 micro
meters.
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Unit-III: ELECTRICAL ENERGY BASED PROCESSES
Part - A (2 Marks)
ELECTRICAL DISCHARGE MACHINING (EDM)
1. What are functions of dielectric fluid used in EDM?
1. It acts as an insulating medium
2. It cools the spark region and helps in keeping the tool and work piece cool.
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3. It maintains a constant resistance across the gap.
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4. It carries away the eroded metal particles.
2. Basic requirement of dielectric fluid used in EDM? (Dec2005, AU)
1. Stable Dielectric strength.
2. It should have optimum viscosity.
3. It should have high flash point.
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4. It should be chemically stable at high temperature and neutral.
5. It should not emit toxic vapours.
3. What the dielectric fluids commonly used in EDM?
1. Petroleum based hydrocarbon fluids.
2. Paraffin, white sprite, transformer oil.
3. Kerosene, mineral oil.
4. Ethylene glycol and water miscible compounds.
4. What are the prime requirements of tool material in EDM?
1. It should be electrically conductive.
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2. It should have good mach inability.
3. It should have low erosion rate.
4. It should have low electrical resistance.
5. Name some of the tool material used in EDM?
1. Copper, brass, alloys of Zinc &tin.
2. Hardened plain carbon steel
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3. Copper tungsten, silver tungsten, tungsten
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4. Copper graphite and graphite.
6. What is the process parameter efficiency the MRR?
1. Energy discharge
2. Capacitance.
3. Size of work piece.
4. Machine tool design
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7. Write the formula for finding the energy discharge in EDM?
W= (1/2) x EIT
W-discharge energy
I-Current
T-time
E-voltage
8. What is the effect of capacitance in EDM?
Increasing the capacitance causes the discharge to increase and increase both the
peak current and discharge time.
9. How do you increase the inductance of the circuit?
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A piece of iron or steel be allowed to lodge between the leads it would increase
the inductance of the circuit and reduce the M/C rate.
10. Define W/T ratio?
It is the ratio of volume of work removed to the volume of tool removed.
11. What is cycle time?
It is the sum of discharge time and waiting time.
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12. Define over cut?
It is the discharge by which the machined hole in the work piece exceeds the
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electrode size and is determined by both the initiating voltage and the discharge energy.
13. Define Rehardening?
While metal heated to a temperature above the critical and then rapidly cooled by
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the flowing dielectric fluid the metal is rehardened.
14. What is recast metal?
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Metal heated to a temperature above the melting point and which is not displaced
by the action of the spark discharge, resoldifies as recast metal.
15. Explain electrode wear?
A crater is produced in the electrode, which is likewise dependent on the electrode
material and the energy of the discharge.
16. What are types of power supply circuits used in EDM? (Dec2007, AU)
1. R-Ccircuit.
2. Rotary impulse generator.
3. Controlled pulse (vacuum tube).
4. Oscillator controlled pulse.
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5. Transister pulsed circuit.
17. What are the design factors to be considered while selecting the machine tool?
1. Number of parts to be produced.
2. Accuracy.
3. Size of work piece.
4. Size of electrode.
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5. Depth of cavity.
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18. Why the servo controlled system is needed in EDM? (May 2011, Dec2006, AU)
EDM requires that a constant arc gap be maintained between the electrode and the
work piece to obtain maximum machining efficiency. Therefore EDM tool in corporate
some form of servo control.
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19. Define wear ratio?
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Wear ratio=Work piece material removed/Loss of electrode material.
Part – B (16 Marks)
1. Explain the process of electrical discharge machining, its process parameters and
applications.(Dec2005,Dec2006, AU) (May/June 2015) (May2006, AU) (May 2011,
May2007,AU) (May/June 2016)
2. What are the basic requirements of tool materials in EDM process? Name any four
tool
materials. (6) (May 2011. AU)
3. (i) Explain the different types of control circuits used in EDM
process.(10)
(May2011,AU)
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Electro Discharge Machining (EDM) is an electro-thermal non-traditional machining process,
where electrical energy is used to generate electrical spark and material removal mainly occurs
due to thermal energy of the spark. EDM is mainly used to machine difficult-to-machine
materials and high strength temperature resistant alloys. EDM can be used to machine difficult
geometries in small batches or even on job-shop basis. Work material to be machined by EDM
has to be electrically conductive
Process
In EDM, a potential difference is applied between the tool and workpiece. Both the tool and the
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work material are to be conductors of electricity. The tool and the work material are immersed in
a dielectric medium. Generally kerosene or deionised water is used as the dielectric medium. A
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gap is maintained between the tool and the workpiece. Depending upon the applied potential
difference and the gap between the tool and workpiece, an electric field would be established.
Generally the tool is connected to the negative terminal of the generator and the workpiece is
connected to positive terminal. As the electric field is established between the tool and the job,
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the free electrons on the tool are subjected to electrostatic forces. If the work function or the
bonding energy of the electrons is less, electrons would be emitted from the tool (assuming it to
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be connected to the negative terminal). Such emission of electrons are called or termed as cold
emission. The “cold emitted” electrons are then accelerated towards the job through the dielectric
medium. As they gain velocity and energy, and start moving towards the job, there would be
collisions between the electrons and dielectric molecules. Such collision may result in ionisation
of the dielectric molecule depending upon the work function or ionisation energy of the dielectric
molecule and the energy of the electron. Thus, as the electrons get accelerated, more positive
ions and electrons would get generated due to collisions. This cyclic process would increase the
concentration of electrons and ions in the dielectric medium between the tool and the job at the
spark gap. The concentration would be so high that the matter existing in that channel could be
characterised as “plasma”. The electrical resistance of such plasma channel would be very less.
Thus all of a sudden, a large number of electrons will flow from the tool to the job and ions from
the job to the tool. This is called avalanche motion of electrons. Such movement of electrons and
ions can be visually seen as
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a spark. Thus the electrical energy is dissipated as the thermal energy of the spark. The high
speed electrons then impinge on the job and ions on the tool. The kinetic energy of the electrons
and ions on impact with the surface of the job and tool respectively would be converted into
thermal energy or heat flux. Such intense localised heat flux leads to extreme instantaneous
confined rise in temperature which would be in excess of 10,000oC.
Such localised extreme rise in temperature leads to material removal. Material removal occurs
due to instant vapourisation of the material as well as due to melting. The molten metal is not
removed completely but only partially. As the potential difference is withdrawn, the plasma
channel is no longer sustained. As the plasma channel collapse, it generates pressure or shock
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waves, which evacuates the molten material forming a crater of removed material around the site
of the spark. Thus to summarise, the material removal in EDM mainly occurs due to formation of
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shock waves as the plasma channel collapse owing to discontinuation of applied potential
difference. Generally the workpiece is made positive and the tool negative. Hence, the electrons
strike the job leading to crater formation due to high temperature and melting and material
removal. Similarly, the positive ions impinge on the tool leading to tool wear. In EDM, the
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generator is used to apply voltage pulses between the tool and the job. A constant voltage is not
applied. Only sparking is desired in EDM rather than arcing. Arcing leads to localised material
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removal at a particular point whereas sparks get distributed all over the tool surface leading to
uniformly distributed material removal under the tool.
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Characteristics of EDM
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(a) The process can be used to machine any work material if it is electrically conductive
(b) Material removal depends on mainly thermal properties of the work material rather than its
strength, hardness etc
(c) In EDM there is a physical tool and geometry of the tool is the positive impression of the hole
or geometric feature machined
(d) The tool has to be electrically conductive as well. The tool wear once again depends on the
thermal properties of the tool material
(e) Though the local temperature rise is rather high, still due to very small pulse on time, there is
not enough time for the heat to diffuse and thus almost no increase in bulk temperature takes
place. Thus the heat affected zone is limited to 2 – 4 μm of the spark crater
Dielectric
In EDM, as has been discussed earlier, material removal mainly occurs due to
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thermal evaporation and melting. As thermal processing is required to be carried out in absence
of oxygen so that the process can be controlled and oxidation avoided. Oxidation often leads to
poor surface conductivity (electrical) of the workpiece hindering further machining. Hence,
dielectric fluid should provide an oxygen free machining environment. Further it should have
enough strong dielectric resistance so that it does not breakdown electrically too easily but at the
same time ionise when electrons collide with its molecule. Moreover, during sparking it should
be thermally resistant as well.
Generally kerosene and deionised water is used as dielectric fluid in EDM. Tap water cannot be
used as it ionises too early and thus breakdown due to presence of salts as impurities occur.
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Dielectric medium is generally flushed around the spark zone. It is also applied through the tool
to achieve efficient removal of molten material.
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Electrode Material
Electrode material should be such that it would not undergo much tool wear when it is impinged
by positive ions. Thus the localis
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ed temperature rise has to be less by tailoring or properly choosing its properties or even when
temperature increases, there would be less melting. Further, the tool should be easily workable as
intricate shaped geometric features are machined in EDM.
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Thus the basic characteristics of electrode materials are:
•High electrical conductivity – electrons are cold emitted more easily and there is less bulk
electrical heating
•High thermal conductivity – for the same heat load, the local temperature rise would be less due
to faster heat conducted to the bulk of the tool and thus less tool wear
•Higher density – for the same heat load and same tool wear by weight there would be less
volume removal or tool wear and thus less dimensional loss or inaccuracy
•High melting point – high melting point leads to less tool wear due to less tool material melting
for the same heat load
•Easy manufacturability
•Cost – cheap
The followings are the different electrode materials which are used commonly in the industry:
•Graphite
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•Electrolytic oxygen free copper
•Tellurium copper
•Brass
Equipment
•Dielectric reservoir, pump and circulation system
•Power generator and control unit
•Working tank with work holding device
•X-y table accommodating the working table
•The tool holder
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•The servo system to feed the tool
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4. DESCRIBE
THE
WIRE
CUT
EDM
EQUIPMENT,
ITS
WORKING,
APPLICATIONS AND ADVANTAGES (DEC2005, AU) . (MAY 2013, AU) (MAY 2011,
MAY2007,AU) (16)
The Wire Electric Discharge Machining (WEDM) is a variation of EDM and is commonly
known as wire-cut EDM or wire cutting. In this process, a thin metallic wire is fed on-to the
workpiece, which is submerged in a tank of dielectric fluid such as de-ionized water. This
process can also cut plates as thick as 300mm and is used for making punches, tools and dies
from hard metals that are difficult to machine with other methods. The wire, which is constantly
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fed from a spool, is held between upper and lower diamond guides. The guides are usually CNCcontrolled and move in the x–y plane. On most machines, the upper guide can move
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independently in the z–u–v axis, giving it a flexibility to cut tapered and ransitioning shapes
(example: square at the bottom and circle on the top). The upper guide can control axis
movements in x–y–u–v–i–j–k–l–. This helps in programming the wire-cut EDM,for cutting very
intricate and delicate shapes.
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Wire Cut Electric Discharge Machining (WEDM)The Wire Electric Discharge Machining
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(WEDM) is a variation of EDM and is commonly known as wire-cut EDM or wire cutting. In
this process, a thin metallic wire is fed on-to the workpiece, which is submerged in a tank of
dielectric fluid such as de-ionized water. This process can also cut plates as thick as 300mm and
is used for making punches, tools and dies from hard metals that are difficult to machine with
other methods. The wire, which is constantly fed from a spool, is held between upper and lower
diamond guides. The guides are usually CNC-controlled and move in the x–y plane. On most
machines, the upper guide can move independently in the z–u–v axis, giving it a flexibility to cut
tapered and ransitioning shapes (example: square at the bottom and circle on the top).
This helps in programming the wire-cut EDM,for cutting very intricate and delicate shapes. In
the wire-cut EDM process, water is commonly used as the dielectric fluid. Filters and de-ionizing
units are used for controlling the resistivity and other electrical properties. Wires made of brass
are generally preferred. The water helps in flushing away the debris from the cutting zone. The
flushing also helps to determine the feed rates to be given for different thickness of the materials.
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between the tool-electrode gap thereby creating a path for each discharge. The area wherein
discharge takes place gets heated to very high temperatures such that the surface gets melted and
removed. The cut particles (debris) get flushed away by the continuously flowing dielectric fluid.
WEDM is a non-conventional process and is very widely used in tool steels for pattern and die
making industries. The process is also used for cutting intricate shapes in components used for
the electric erospace industries.
Applications of Wire-Cut EDM
Wire EDM is used for cutting aluminium, brass, copper, carbides, graphite, steels and titanium.
A schematic of the cutting through wire EDM is shown. The wire material varies with the
application requirements. Example: for quicker cutting action, zinc-coated brass wires are used
while for more accurate applications, molybdenum wires are used.
The process is used in the following areas:
Aerospace, Medical, Electronics and Semiconductor applications
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Tool & Die making industries.
For cutting the hard Extrusion Dies
In making Fixtures, Gauges & Cams
Cutting of Gears, Strippers, Punches and Dies
Manufacturing hard Electrodes.
Manufacturing micro-tooling for Micro-EDM, Micro-USM and such other micro-machining
applications.
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Unit-IV: CHEMICAL AND ELECTRO-CHEMICAL ENERGY BASED PROCESSES
Part-A (2 Marks)
CHEMICAL MACHINING (CHM)
1. What is the principle of Chemical Machining (CHM)?
Chemical attacks metals and etch them by removing small amounts of material from
the surface using reagents or etchants
2. What are the processing steps in CHM?
I. Preparing: pre- cleaning
II. Masking: application of chemically resistant material (if selective etching is
desired)
III. Etch: dip or spray exposure to the etchant
IV. Remove Mask: strip remaining mask and clean
V. Finish: inspection and post processing
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3. What is etching factor?
Ratio of undercut to depth of cut
4. What are the two major processes in CHM?
Two major CHM processes are:
I. Chemical milling - eroding material to produce blind details – pockets, channels etc.,
II. Chemical blanking – for producing details that penetrate the material entirely (holes,
slots, etc.)
5. What is the purpose of etchant used in CHM? Give some examples.
Purpose:
 To dissolve a metal by turning it into a metallic salt, which then goes into solution
 Many chemical are available as etchants:FeCl3, Chromic acid, FeNO3, HF, HNO3
6. What are the criteria used for selection of etchant?
 Required surface finish
 Removal rate
 Material type
 Etch depth
 Type of resist
 Cost

7. What is the purpose of maskant and how is it classified? (May/jun 2015,2016)
Maskants (chemically resistant coatings) are used to cover the surfaces which are not
to be machined – does not allow the etchant to react reach and react with work piece to
dissolve it.
8. Based on the techniques of applying maskants, classify them (May/june 2013)
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Cut and peel - Involve the use of relatively thick material which is scribed and
removed to create a selective exposure to the etchant - Neoprene, butyl or vinyl-based
material is used as maskants

Screen printing - A fine mesh silk or stainless steel screen, which has areas blocked-off to
allow selective passage of the maskant is used

Photo resist masks – materials that produce etchant resistant images by means of
photographic techniques.
9. How are maskants selected?
Selection of proper maskant for a particular application is accomplished by evaluation
of the job with respect to six factors – chemical resistance, part configuration, quantity of
parts, cost, ease of removal and required resolution
10. What are the applications for CHM and photochemical machining (PCM)? (Dec
2014/may-june 15)
CHM is used extensively to etch preformed aerospace parts to obtain maximum
strength to weight ratios:
• Integrally stiffened Titanium engine ducts
• Spray etching a rotating tube for cruise missile launch tubes
• Thinning and sizing of a delta booster tank bulkhead
• Chemical sizing of engine cowl inlet duct skins
• Undercut on clad aluminium
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11. What are the advantages of CHM and PCM?
• Metal removal is completely stress free
• Complex shapes and deeply recessed areas can be uniformly chemically milled
• Extremely thin sections can be chemically milled
• Metal hardness or brittleness is not a factor
12. What are the limitations of CHM & PCM?
• Fillet radius is approximately equal to depth of cut
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• Extremely deep cuts are usually not cost effective
• A homogenous metal structure is normally required for good results
• Suitable photographic facilities are not always available
13. Please identify the principle of ECM. How does it differ from electroplating?( May
2009/16)
Principle of ECM - electrolysis. When a D.C potential is applied across two
electrodes separated by a small gap and an electrolyte is pumped through the small gap, the
constituents of the anode work piece material goes into the solution and not plate on the
cathode tool. Electroplating is the reverse of ECM where the cathode is plated by the
depleted metal from the anode.
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14. Define Etchants
It is the chemical reagent (etchant) is used to remove the metal from the
workpiece. The metal removed by the chemical conversion of the metal into metallic salt
15. What are the various etchant used for various materials?
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Sl Material
No
Etchant
1
Aliminium
Caustic Soda
2
Steel
HCl or Nitric acid
3
Stainless steel
Iron chloride
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4
Magnesium
Nitric acid
5
Titanium
Nitric acid
16. What are the various maskant used for various materials?
Sl Material
No
maskant
1
Aluminium
Butyl rubber, Neoprene rubber
2
Magnesium
polymer
3
Nickel
Neoprene
4
Titanium
Translucent chlorinated
polymers
5
Ferrous metals
PVC, polythelene
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17. What are the various methods of masking?
 Scribed and peeled maskant

Photoresists maskants
18. What is the classification of chemical machining process?

Chemical blanking – material removed from entire area

Contour machining – material removed from selected area only
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ELECTRO CHEMICAL MACHINING (ECM)
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1. Define ECM? (Dec,2004)
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It is the controlled removal of metals by the anodic dissolution in an electrolytic
medium, where the work piece (anode) and the tool (cathode) are connected to the
electrolytic circuit, which is kept, immersed in the electrolytic medium
2. Write the Faraday’s first law of electrolysis?
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The amount of any material dissolved or deposited is proportional to the quantity
of electrolyte passed.
3. Write the Faraday’s second law of electrolysis?
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The amount of different substances dissolved or deposited by the same quantity of
electricity are proportional to their chemical equivalent weight.
4. Write Ohm’s law?
Current, I = V/R
Where, V = Voltage
R = resistance
5. What are the factors that influence oxidation in ECM?
(i) Nature of work piece. (ii) Type of electrolyte. (iii) Current density. (iv)
Temperature of the electrolyte.
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6.What are the materials used to make the tool electrode? (May/June 2013)
Copper and copper alloys, titanium, aluminum, brass, bronze, carbon, Monel and
reinforced plastics.
7.What are the main functions of electrolysis in the ECM? (Dec 2006)
i) For completing the electric circuit between the tool and the work piece and to
allow the reaction to proceed efficiently.
ii) To remove the products of machining from the cutting region.
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iii) To carry away the heat generated during the chemical reaction.
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iv) To avoid ion concentration at the work piece- tool gap.
6. What are the properties are expected from the electrolysis used in the ECM? (Dec
2007)
i) High thermal conductivity.
ii) Low viscosity and high specific heat.
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iii) Should chemically stable even at high temperature.
iv) Should be non-toxic and non-corrosive.
7. What is the electrolysis commonly used in ECM?
15 -20 % NaCl in water, sodium nitrate, potassium nitrate, sodium sulphate,
sodium chromate and potassium chloride.
8. What are the results which are in improper selection of electrolyte in ECM?
(i) Low machining rate. (ii) Over cut and stray cutting.
9. What are the methods generally used to filter the electrolyte?
(i) Running the system until it is contaminated completely and replaces it.
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(ii) Centrifugal separation.
(iii) Sedimentation.
(iv) Use of clarifiers
10. What are the characteristics of a good ECM tool?
(i) It should be a good conductor of electricity and heat.
(ii) Easily machinable.
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(iii) Resistant to chemical reaction.
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(iv) It offers resistance to the high electrolyte pressure
11. What are the problems that occur while improperly selecting the electrolyte flow?
Cavitations, stagnation and vortex formation
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12. What are the parameters that affect the MRR?
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(i) Feed rate. (ii) Voltage. (iii) Concentration of the electrolyte.
(iv) Temperature of the electrolyte. (v) Current density. (vi) Velocity of the electrolyte.
13. How the current density affect the MRR?
Current density is controlled not only by the amount of current but also by the
size of the gap between the tool and the work piece. A small gap results in high current
density, which in turn produces more material removal.
14. What are the advantages of ECM?
(i) ECM is simple, fast and versatile method.
(ii) Surface finish can be extremely good.
(iii) Fairly good tolerance can be obtained.
15. What are the limitations of ECM? (Dec 2007)
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(i) Large power consumption and the related problems.
(ii) Sharp internal corners cannot be answered.
(iii) Maintenances of higher tolerances require complicated contours
16. What are the applications of ECM? (Dec,2004)
ECM is used for sinking, profiling and contouring, multi hole drilling, trepanning,
broaching, honing, steel mill applications, surfacing, sawing, contour machining of hand
to hand machine materials.
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17. What are the various tool materials that can be used effectively with ECM?
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Generally aluminium, copper, brass, titanium, cupro-nickel and stainless steel are
used as tool materials.
18. What factors should be considered in selecting the tool materials in ECM?


Good stiffness- Rigidity of the tool construction and material is important because

Easy machinability- particularly important if complex shaped tools are requi

High corrosion resistance- to protect itself from the highly corrosive electrolyte
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solution
19. What are the different types ECM operations?

Electro Chemical Milling (ECM)

Electro Chemical Grinding (ECG)

Electro Chemical Honing (ECH)

Electro Chemical Deburing (ECD)

Electro chemical turning (ECT)

Electro chemical trepanning (ECTr) etc.,
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ELECTO CHEMICAL GRINDING (ECG)
1. Define ECG.
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ECG is the material removal process in which the material is removed by the
combination of Electro- Chemical decomposition as in ECM process and abrasive due to
grinding.
2. Which material is used to make the grinding wheel?
Metal bonded diamond (or) Aluminum oxide.
3. What are the important functions of abrasive particles used in ECG?
It acts as insulator to maintain a small gap between the wheel and work piece.
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They are electrolysis products from the working area. To cut chips if the wheel should
contact the work piece particularly in the event of power failure.
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4. What are the advantages of ECG?
i) No thermal damage to work piece.
ii) Wheel wear is negligible.
iii) No distortion of the work piece.
5. What are the disadvantages of ECG?
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High capital costs, because of the special wheel tool. Power consumption is quite
high. Electrolyte is corrosive.
6. What are the limitations of ECG?
1. The work material must be conductive.
2. Nit suitable for machining soft material.
3. Require dressing tools for preparing the wheels.
7. What is the application of ECG?
1. Precision grinding of hand metals economically.
2. Grinding Carbide cutting tools inserts.
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3. To grind end mill cutters more precisely.
8. Explain the difference between the ECG and conventional grinding?
In ECG, there is a grinding wheel similar to a conventional grinding wheel except
that the bonding material is electrically conductive.
Conventional grinding produces components with good surface finish and
dimensional tolerances but such components are also associated with burrs, comparatively
large heat affected zone (HAZ), and thermal residual stresses. These defects are not found in
electrochemically ground work pieces.
9. What are the advantages of ECG over conventional grinding?

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




Reduce cost of grinding (higher cost of equipment compensated by higher MRR,
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Reduced pressure of work against the wheel and so delicate parts can be
manufactured without distortion
10. What is ECD? Identify its applications.
ECD is a process for the removal of metal burrs by anodic dissolution, the same
principle as ECM.
Applications:
 Generally employed for far away located as well as inaccessible places where

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ECD process has been successfully applied in many different industries ranging
from consumer appliances and automotive to biomedical and aerospace products

The automotive industry, a heavy user of ECD, employs this process for
deburring and radiusing the cross-

Not only is ECD applicable to the high-volume environment of the automotive
11. How does ECD differ from ECM?
ECD is a special version of ECM used exclusively to deburr or radius workpieces. It
differs from ECM in that the electrolyte pressure, electrolyte flow, and current are all
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relatively low. Another major difference is that the cathode (tool) is usually held stationary in
ECD.
12. ECD is same as ECM except much simple to use. Justify the statement.
ECD is same as ECM except much simple to use because, there is no feed mechanism
needed for the ECD tool (the tool is held stationary). Moreover in ECD, the electrolyte
pressure, electrolyte flow, and current are all relatively low.
13. What are the functions served by the electrolyte in the ECM process?
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


Removes reaction products
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Part – B (16 Marks)
1. With the help of a simple schematic diagram, explain the working of Electro chemical
machining process. Also show the block diagram of a typical power supply for an ECM
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machine. (May 2011, AU) (Dec2005, AU) (May2010, AU) (May 2011) (May 2011, AU) (May
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2013, AU) (May 2013, AU). (May 2013, AU) (May/June 2014) (May/June 2014) (Dec2014)
(Dec2014) (May/June 2015) (May/June 2016)(May/June 2016)
ELECTRO CHEMICAL MACHINING (ECM, Electrochemical machining (ECM) is a metal removal process base d on the principle of reverse electroplating. In this p rocess, particles travel
from the anodic material (workpiece) toward the cathodic material (machining tool).
Electrochemical machining (ECM) is a metal -removal process base d on the principle of reverse
electroplating. In this p rocess, particles travel from the anodic material (workpiece) toward the
cathodic material (machining tool). A current of electrolyte fluid carries away the depleted
material before it has a chance to reach the machining tool. The cavity produced is the female
mating image of the tool shape.
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.
EQUIPMENTS:ECM machines come in both vertical and horizontal types. depending on the work requirements
these machines are built in many different sizes as well. the vertical machine is comprised of a
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base, column, table, and spindle head. the spindle head has a servo-mechanism that automatically
advances the tool and controls the gap between the cathode (tool) and the workpiece.
WORKING
An electrode and work piece (conductor) are placed in an electrolyte, and a potential voltage is
applied. on the anode (+ve) side the metal molecules ionize (lose electrons) break free of the
work piece, and travel through the electrolyte to the electrode (a cathode; has a -ve charge; a
surplus of electrons). in edm an arc was used to heat metal, here the metal dissolves chemically.
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variation in the current density will result in work taking the electrodes shape. the electrode is
fed with a constant velocity, and the electrolyte is fed through the tool. the tool is designed to
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eliminate deposition of the ionized metal on the electrode.
SUPPLY V = 8 TO 20V, I = >1000A.
· ELECTRODE GAP IS TYPICALLY 0.1 TO 0.2 MM.
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· MRR IS ABOUT 1600MM3/MIN. PER 1000A, OR 3KWHR FOR 16000 MM3 (NOT
VERY EFFICIENT, 30 TIMES MORE THAN STANDARD MACHINING
TECHNIQUES).
· MRR IS INDEPENDENT OF MATERIAL HARDNESS.
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· GOOD FOR LOW MACHINABILITY, OR COMPLICATED SHAPES.
· VERY LITTLE TOOL WEAR,
· FORCES ARE LARGE WITH THIS METHOD BECAUSE OF FLUID PUMPING FORCES.
· FARADAY'S LAWS STATE THAT,
ADVANTAGES:-
1. the components are not subject to either thermal or mechanical stress.
2. there is no tool wear during electrochemical machining.
3. non-rigid and open work pieces can be machined easily as there is no contact between the
tool and work piece.
4. complex geometrical shapes can be machined repeatedly and accurately
5. electrochemical machining is a time saving process when compared with conventional
machining
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2) Describe the process of ECG with a neat sketch (8) (May/June 2015)
Electro-chemical grinding (ECG) is a variant process of the basic ECM. It is a burr free and
stress free material removal process, wherein material removal of the electrically conductive
material takes place through mechanical (grinding) process and electro-chemical process. The
abrasive laden grinding wheel is negatively charged and the workpiece is positively charged.
They are separated by an electrolyte fluid. The fine chips of the material that is removed from the
workpiece (debris) stays in the electrolyte fluid, which is further filtered out. Electrochemical
grinding and electrochemical machining are similar processes with a difference that a wheel
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substitutes the tool used in ECM. The wheel shape is similar to the desired work shape
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Process of Grinding in ECM
The main feature of electrochemical grinding (ECG) process is the use of a
metallic grinding wheel which is embedded with insulating abrasive particles such
as diamond, set in the conducting material. Copper, brass, and nickel are the most
commonly used materials while aluminum oxide is a typical abrasive used while
grinding steels.
The commutator is an electrolytic spindle having carbon brushes and holds the
grinding wheel. It receives a negative charge from the DC power supply and the
workpiece is given a positive charge. In ECG process, the grinding wheel slightly
touches the workpiece. Electrolyte is supplied on-to the grinding wheel near the
workpiece such that the wheel carries it through the cutting process thereby
resulting in an electro-chemical action. A nozzle similar to the one which carries
coolant in a conventional grinding process is provided, which enables the flow of
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Process Characteristics
The life of grinding wheel in ECG process is very high as around 90% of the
metal is removed by electrolysis action and only 10% is due to the abrasive
action of the grinding wheel.
The ECG process is capable of producing very smooth and burr free edges
unlike those formed during the conventional grinding process (mechanical).
The heat produced in the ECG process is much less, leading to lesser distortion
of the workpiece.
The major material removal activity in ECG process occurs by the dissolving
action through the chemical process. There is very little tool and workpiece
contact and this is ideally suited for grinding of the following categories:
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Fragile work-pieces which otherwise are very difficult to grind by the
conventional process
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The parts that cannot withstand thermal damages and
The parts designed for stress and burr free applications
Applications
The applications of ECG process include the following:
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In production of tungsten carbide cutting tools.
In burr-free sharpening of hypodermic needles.
In grinding of super-alloy turbine blades.
In form grinding of aerospace honeycomb metals.
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In removal of fatigue cracks from steel structures that have been used for
underwater applications.
The ECG process can be applied to the following common methods of grinding.
1. Face Wheel Grinding.
2. Cone Wheel grinding.
3. Peripheral or Surface grinding.
4. Form Wheel or Square grinding.
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Unit V: THERMAL ENERGY BASED PROCESSES
Part –A (2Marks)
LASER BEAM MACHINING (LBM)
1. What is Laser?
It is acronym of light amplification by stimulated emission of radiation.
2. What is Maser?
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Laser can be melt diamond when focused by lens system. The energy density
being of two order 100,000 KW/cm2. This energy is due to atoms that have light energy
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level. When such an atom impinge with electromagnetic waves having resonant
frequency
3. What are the characteristics of Laser beam?
1. Material removal
2. Material shaping
3. Welding
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4. Thermo kinetic change.
4. What are the gases commonly used in LASER?
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The gases commonly used are: He, Ne, Argon, Co2 etc.
5. What are the advantages of Laser drilling?
No physical contact between work root pair hence there is no possibility if
breakage or wear of root. Precision location is ensured by focusing of the beam Large
aspect ratio can be achieved.
6. What are the characteristics of Laser used in Laser machining?
1. Can be focused to maximum intensity or to lower intensity as needed.
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2. Can be moved rapidly on the work.
3. Remote cutting over long standoff distances.
7. What are the fundamentals of photons used in Laser?
In the Laser the photons are in ground state at 0oC they are brought to the excited
state by means of absorption of energy by temperature change, collision etc.
8. What are the emission lines?
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The atoms when these they are bringing down goes to the excited state by stimulated
emission and emit photons within 10 nano seconds. They have the same wavelength as
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the excited photons.
9. What is the Maser principle?
The atoms at this state will impinge with electrons waves having resonate frequency.
This is known as maser.
10. What is population inversion?
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If the atoms in the excited state are greater than that of the ground state then it is
known as population inversion.
11. How does Laser melting works?
It melts and vaporizes the unwanted material by means of narrow pulsed laser
operating at 2 to 100pilses/sec Because of this high accuracy is not possible to micro
sized holes.
12. What is solid state Laser?
Solid state Laser is the Lasers, which consist of a hot nat, which may be
crystalline solid/ glass, doped with an active material whose atoms provide the lasing
action.
13. Write the properties of laser
Monochromatic, coherence, divergence, intensity, mode structure
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ELECTRON BEAM MACHINING (EBM)
1. Define EBM?
It is the thermo-electrical material removal process on which the material is
removed by the high velocity electron beam emitted from the tungsten filament made to
impinge on the work surface, where kinetic energy of the beam is transferred to the work
piece material, producing intense heat, which makes the material to melt or vaporize it
locally.
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2. What is the characteristic of the electron beam?
(i) High concentrated energy.
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(ii) Deep penetration into the metals.
(iii) Low distortion.
(iv) Any material either conductive or non-conductive can be processed
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3. Write the application of electron beam?
- Thin film machining.
- Surface treatment.
- Engraving metals and non-metals.
- Cutting of materials.
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4. What are the main elements of the EBM equipment?
(i) Electron Gun.
(ii) Beam focusing and deflecting units.
(iii) Work Table.
(iv) Vacuum chamber
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5. What is the function of magnetic lens used in EBM?
It converges the beam into a narrow spot into the work piece.
6. What are the two types of EBM?
(i) Thermal type. (ii) Non-thermal Type.
7. Explain the thermal type EBM?
In this type the electron beam is used to heat the material up to the point where it
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is selectively vaporized.
8. Explain Non-thermal type EBM?
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In this type, the EBM produces a chemical reaction.
9. Write the advantage of EBM?
(i)
High accuracy.
(ii)
Any type of material can be processed.
(iii)
No mechanical or thermal distortion.
(iv)
No physical or metallurgical damage results.
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10. Write the disadvantages of EBM?
(i) High cost of equipment.
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(ii) Skilled operator is required for operation.
(iii) Limited to 10mm material thickness.
11. Write any four application of EBM?
(i) Micro machining application on materials.
(ii) Drilling of apertures for electron microscope.
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(iii) Drilling of holes in ruby and diamond crystal.
12. Write the Richardson-Dushman Equation?
J=At2 exp- (EW/KT)
Where, J = Current Density
A =constant (120 Amphere/cm2deg2)
K =Boltzman Constant (1.3x10-23 J/K)
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T = Absolute temperature (Kelvin)
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W =work function (Volts)
13. Write general formula for focal length of a magnetic lens?
f/(S + D) = 25V/(NT)2
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Where, V =Electron accelerating voltage
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NT =Ampere turns in the lens winding
S =pole piece separation
D =Bore diameter
F =focal length
14. Why vacuum is needed in EBM?
1) To reduce corrosion 2) To get correct focusing
15. What is the drawback of electron beam machining?
One major diameter of electron beam welding has been the requirement of high
degree of vacuum essential or satisfactory operation of this process because of degassing.
PLASMA ARC WELDING (PAW)
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1. Define plasma
Plasma is defined as the gas, which has been heated to a sufficiently high
temperature to become ionized.
2. What are the advantages of plasma arc welding?
a. Exothermic oxidation takes place. b. DC power supply
3. What are the metals that can't be machined by plasma arc machining?
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a. Stainless steel b. Monel c. Super alloys
4. What is the basic heating phenomenon that takes place in plasma arc welding?
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The basic heating phenomenon that takes place at the work piece is a combination
of anode heating due to direct electron bombardment recombination of molecules on the
work piece.
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5. How the basic plasma does is generated.
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The basic plasma is generated by subjecting a stream of gas to the electron
bombardment of the electric arc.
6. How the initial ionization is accomplished in plasma arc machining.
A high voltage arc established between electrode and nozzle accomplishes initial
ionization.
7. Why does gas formed in plasma do in P.A.M?
This gas stabilizes the arc and prevents it from diverging.
8. How another source of heating achieved in P.A.M
It is desirable to achieve a third source of heating by injecting oxygen into work
area to take advantage of exothermic oxidation.
9. Write the principle of P.A.M
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Once the material has been raised to molten point the high velocity gas stream
blows the material away.
10. Write the circuitry details in PAM.
+ ve terminal connected to work piece and -- ve terminal connected to electrode.
11. Which type of power supply is used in P.A.M
DC power supply is used.
12. Which part is constricted by plasma?
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Nozzle duct is constricted by plasma.
Part – B (16 Marks)
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1.EXPLAIN THE PRINCIPLE OF LBM WITH NEAT SKETCH (10) (MAY/JUNE 2016)
(DEC2006, AU) (MAY 2011, AU) (DEC2014) (MAY/JUNE 2015)
Laser Beam Machining – the lasing process Lasing process describes the basic operation of
laser, i.e. generation of
coherent (both temporal and spatial) beam of light by “light
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amplification” using “stimulated emission”. In the model of atom, negatively charged lectrons
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rotate around the positively charged nucleus in some specified orbital paths.The geometry and
radii of such orbital paths depend on a variety of parameters like number of electrons, presence
of neighbouring atoms and their electron structure, presence of to magnetic field etc. Each of the
orbital electrons is associated with unique energy levels. At absolutezero temperature an atom is
considered to be at ground level, when all the electrons occupy their respective lowest potential
energy. The electrons at ground state can be excited to higher state of energy by absorbing
energy form external sources like increase in electronic vibration at elevated temperature,
through chemical reaction as well as via absorbing energy of the photon.
On reaching the higher energy level, the electron reaches an unstable energy band. And it comes
back to its ground state within a very small time by releasing a photon. This is called
spontaneous emission The spontaneously emitted photon would have the same frequency as that
of the “exciting” photon. Sometimes such change of energy state puts the electrons in a metastable energy band. Instead of coming back to its ground state immediately (within tens of ns) it
stays at the elevated energystate for micro to milliseconds. In a material, if more number of
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electrons can be somehow pumped to the higher meta-stable energy state as compared to number
of atoms at ground state, then it is called “population inversion”.
LASING MEDIUM
Many materials can be used as the heart of the laser. Depending on the
lasing medium lasers are classified as solid state and gas laser. Solid-state lasers are commonly
of the following type
•Ruby which is a chromium – alumina alloy having a wavelength
of 0.7μm
•Nd-glass lasers having a wavelength of 1.64 μm
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•Nd-YAG laser having a wavelength of 1.06 μm
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These solid-state lasers are generally used in material processing. The generally used gas lasers
are
•Helium – Neon
•Argon
•CO2.
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Lasers can be operated in continuous mode or pulsed mode. Typically CO 2 gas laser is operated
in continuous mode and Nd – YAG laser is operated in pulsed mode electrons,
Laser Construction
A typical Nd-YAG laser. Nd-YAG laser is pumped using flash tube. Flash tubes can be helical,
as shown in or they can be flat. Typically the lasing material is at the focal plane of the flash
tube. Though helical flash tubes provide better pumping, they are difficult to maintain
LASER BEAM MACHINING – APPLICATION
Laser can be used in wide range of manufacturing applications
•Material removal – drilling, cutting and tre-panning
•Welding
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•Cladding
•Alloying
LASER BEAM MACHINING – ADVANTAGES
•In laser machining there is no physical tool. Thus no machining force or wear of the tool takes
place.
•Large aspect ratio in laser drilling can be achieved along with acceptable accuracy or
dimension, form or location
•Micro-holes can be drilled in difficult – to – machine materials
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•Though laser processing is a thermal processing but heat affected zone specially in pulse laser
processing is not very significant due to shorter pulse duration.
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LASER BEAM MACHINING – LIMITATIONS
•High initial capital cost
•High maintenance cost
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•Not very efficient process
•Presence of Heat Affected Zone – specially in gas assist CO2
laser cutting
•Thermal process – not suitable for heat sensitive materials like
aluminium glass fibre laminate
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1. Explain the principle of CBM with neat sketch (10) (May/June 2016) (Dec2006, AU)
(May 2011, AU) (Dec2014) (May/June 2015)
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ELECTRON BEAM MACHINING – PROCESS
Electron beam is generated in an electron beam gun. The construction and working principle of
the electron beam gun would be discussed in the next section. Electron beam gun provides high
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velocity electrons over a very small spot size. Electron Beam Machining is required to be carried
out in vacuum. Otherwise the electrons would interact with the air molecules, thus they would
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loose their energy and cutting ability. Thus the workpiece to be machined is located under the
electron beam and is kept under vacuum. The high-energy focused electron beam is made to
impinge on the workpiece with a spot size of 10 – 100 μm. The kinetic energy of the high
velocity electrons is converted to heat energy as the electrons strike the work material. Due to
high power density instant melting and vaporisation starts and “melt – vaporisation” front
gradually progresses
Finally the molten material, if any at the top of the front, is expelled
from the cutting zone by the high vapour pressure at the lower part. Unlike in Electron Beam
Welding,the gun in EBM is used in pulsed mode. Holes can be drilled in thin sheets using a
single pulse. For thicker plates, multiple pulses would be required. Electron beam can also be
manoeuvred using the electromagnetic deflection coils for drilling holes of any shape
ELECTRON BEAM MACHINING – EQUIPMENT
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An electron beam gun, which is the heart of any electron beam machining facility. The basic
functions of any electron beam gun are to generate free electrons at the cathode, accelerate them
to a sufficiently high velocity and to focus them over a small spot size. Further, the beam needs
to be manoeuvred if required by the gun. The cathode as can be seen is generally made of
tungsten or tantalum. Such cathode filaments are heated, often inductively, to a temperature of
around 25000C. Such heating leads to thermo-ionic mission of electrons, which is further
enhanced by maintaining very low vacuum within the chamber of the electron beam gun.
Moreover, this cathode cartridge is highly negatively biased so that the thermo-ionic electrons
are strongly repelled away form the cathode. This cathode is often in the form of a cartridge so
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that it can be changed very quickly to reduce down time in case of failure Just after the cathode,
there is an annular bias grid. A high negative bias is applied to this grid so that the electrons
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generated by this cathode do not diverge and approach the next element, the annular anode, in
the form of a beam. The annular anode now attracts the electron beam and gradually gets
accelerated. As they leave the anode section, the electrons may achieve a velocity as high as half
the velocity of light. The nature of biasing just after the cathode controls the flow of electrons
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and the biased grid is used as a switch to operate the electron beam gun in pulsed mode. After the
anode, the electron beam passes through a series of magnetic lenses and apertures. The magnetic
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lenses shape the beam and try to reduce the divergence. Apertures on the other hand allow only
the convergent electrons to pass and capture the divergent low energy electrons from the fringes.
This way, the aperture and the magnetic lenses improve the quality of the electron beam. Then
the electron beam passes through the final section of the electromagnetic lens and deflection coil.
The electromagnetic lens focuses the electron beam to a desired spot. The deflection coil can
manoeuvre the electron beam, though by small amount, to improve shape of the machined holes.
Generally in between the electron beam gun and the workpiece, which is also under vacuum,
there would be a series of slotted rotating discs. Such discs allow the electron beam to pass and
machine materials but helpfully prevent metal fumes and vapour generated during machining to
reach the gun. Thus it is essential to synchronize the motion of the rotating disc and pulsing of
the electron beam gun. Electron beam guns are also provided with illumination facilityand a
telescope for alignment of the beam with the workpiece. Workpiece is mounted on a CNC table
so that holes of any shape can be machined using the CNC control and beam deflection in-built
in the gun. One of the major requirements of EBM operation of electron beam gun is
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maintenance of desired vacuum. Level of vacuum within the gun is in the order of 10-4 to 106Torr.
{1 Torr = 1mm of Hg}
Maintenance of suitable vacuum is essential so that electrons do not loose their energy and a
significant life of the cathode cartridge is obtained. Such vacuum is achieved and maintained
using a combination of rotary pump and diffusion pump. Diffusion pump is attached to the
diffusion pump port of the electron beam gun Diffusion pump is essentially an oil heater. As the
oil is heated the oil vapour rushes upward where gradually converging structure as The nozzles
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change the direction of motion of the oil vapour and the oil vapour starts moving downward at a
high velocity as jet. Such high velocity jets of oil vapour entrain any air molecule is present
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within the gun. This oil is evacuated by a rotary pump via the backing line. The oil vapour
condenses
due to presence of cooling water jacket around the diffusion pump.
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ELECTRON BEAM PROCESS – PARAMETERS
The process parameters, which directly affect the machining characteristics in Electron Beam
Machining, are:
•The accelerating voltage
•The beam current
•Pulse duration
•Energy per pulse
•Power per pulse
•Lens current
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•Spot size
•Power density
As has already been mentioned in EBM the gun is operated in pulse mode. This is achieved by
appropriately biasing the biased grid located just after the cathode. Switching pulses are given to
the bias grid so as to achieve pulse duration of as low as 50 μs to as long as 15 ms. Beam current
is directly related to the number of electrons emitted by the cathode or available in the beam.
Beam current once again can be as low as 200 μamp to 1 amp
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ELECTRON BEAM PROCESS CAPABILITY
EBM can provide holes of diameter in the range of 100 μm to 2 mm with a depth upto 15 mm,
i.e., with a l/d ratio of around 10. The hole can be tapered along the depth or barrel shaped. By
focusing the beam below the surface a reverse taper can also be obtained. Generally burr
formation does not occur in EBM. A wide range of materials such as steel, stainless steel, Ti and
Ni super-alloys, aluminium as well as plastics, ceramics, leathers can be machined successfully
using electron beam. As the mechanism of material removal is thermal in nature as for example
in electro-discharge machining, there would be thermal damages associated with EBM.
However, the heat-affected zone is rather narrow due to shorter pulse duration in EBM. Some of
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the materials like Al and Ti alloys are more readily machined compared to steel. Number of
holes drilled per second depends on the hole diameter, power density and depth of the hole as
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well as material type
ELECTRON BEAM MACHINING – ADVANTAGES AND LIMITATIONS

EBM provides very high drilling rates when small holes with

large aspect ratio are to be drilled.

It can machine almost any material irrespective of their mechanical properties.

No mechanical cutting force, work holding and fixturing cost is very less. Fragile and
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brittle materials can also be processed.
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The heat affected zone in EBM is rather less due to shorter pulses.

EBM can provide holes of any shape by combining beam deflection using el

Electromagnetic coils and the CNC table with high accuracy.

Limitations

high capital cost of the equipment and necessary regular maintenance

applicable for any equipment using vacuum system.

There is significant amount of non-productive pump down period for attaining desired
vacuum.

Though heat affected zone is rather less in EBM but recast layer

formation cannot be avoided
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PLASMA ARC MACHINING (PAM)
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Plasma arc machining is a nontraditional thermal process. Plasma is defined as a gas that has
been heated to a sufficiently high temperature to become partially ionized and therefore
electrically conductive. The term plasma, as employed in physics, means ionized particles. The
temperature of plasma may reach as high as 28000 o C. Various devices utilizing an electric arc
to heat gas to the Plasma
The plasma arc produced by modern equipment is generated by a plasma torch that is
constructed in such a manner as to provide an electric arc between an electrode and workpiece, a.
A typical plasma torch consists of an electrode holder, an electrode, a device to swirl the gas,
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and a water-cooled nozzle. The geometry of the torch nozzle is such that the hot gases are
constricted in a narrow column.
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Primary gasses, such as nitrogen, argon-hydrogen, or air, are forced through the nozzle and arc
and become heated and ionized. Secondary gases or water flow are often used to help clean the
kerf of molten metal during cutting.
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The stream of ionized particles from the nozzle can be used to perform a variety of industrial
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jobs. The plasma arc, as an industrial tool, is a most heavy employed in sheet and plate cutting
operations as an alternative to more conventional oxy-fuel torches or other cutting tools. Plasma
arc is routinely used as an integral component of some modern punching machines. Plasma arc
methods are also employed in special applications to replace conventional machining operations
such as lathe turning, milling and planing, heat treatment and metal deposition operations, and
plasma arc welding.
PRINCIPLES OF OPERATION
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In PAM, constricting an electric arc through a nozzle, as shown in generates the basic plasma
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jet. Instead of diverging into an open arc, the nozzle constricts the arc into a small cross section.
This action greatly increases the power of the arc so that both temperature and voltage are raised.
After passing through the nozzle, the arc exists in the form of a high-velocity, well-columnated
and intensely hot plasma jet.
The basic heating phenomenon that takes place at the workpiece is a combination of heating due
to energy transfer of electrons, recombination of dissociated molecules on the workpieces, and
connective heating from the high-temperature plasma that accompanies the arc. In some cases, it
is desirable to achieve a third source of heating by injecting oxygen into the work area and taking
advantage of the exothermic oxidation reaction. Once the material has been raised to the molten
point, the high-velocity gas stream effectively blows the material away.
For an optimized PAM cutting or machining operation, up to 45% of the electrical power
delivered to the torch is used to remove metal from the workpiece. Of the remaining power,
approximately 10% go into the cooling water in the plasma generator and the rest is wasted in
the hot gas and in heating the workpiece.
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The jet stream of ionized gases exits at sonic speed and tends to maintain a slightly diverging
columnar shape until deflected by solid material. This ionized jet serves as a conductor for the
arc; it provides directional stability. The ionized gas may be further shielded from dispersion and
heat loss, which result from impacting air molecules, as it exits from the nozzle by means of
another annular stream of gas that surrounds the plasma as it leaves the orifice nozzle.
The ionized plasma gas is usually inactive to protect the electrode from combustion and ensure
long life. When oxygen is added as either the plasma ionized gas or the secondary enveloping
gas, the speed of cutting steel is increased. Use of a secondary envelope of gas improves the kerf
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wall appearance on certain metals. This envelope also acts as a protective shield for the nozzle
during extensive piercing operations.
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A major improvement in mechanized plasma arc cutting occurred in recent years with the
development of so-called “water injection”. When the water injection technique is employed, the
arc is constricted by a flow of water around the arc. This injection of water has many advantages,
including:
1. A square cut can be made.
2. Arc stability is increased.
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3. Cutting speed can be increased.
4. Workpieces are heated less.
5. Less smoke and fumes are generated.
6. Nozzle life is increases.
APPLICATIONS
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Cutting: With appropriate equipment and techniques, the plasma arc can be successfully
employed to make cuts in electrically conductive metals.
illustrates the general setup for
cutting using a plasma arc torch.
hole piercing, reproducible and high-quality holes are made rapidly in a variety of materials
with the plasma arc. Holes can be pierced much faster than they
can be drilled. Plasma arc
hole piercing is performed with conventional plasma arc cutting equipment that has been
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modified to produce a short, carefully
controlled arc operating time, suitable arc current
programming during the short operating cycle, and effective slag ejection.
Almost instantly after ignition the arc rapidly penetrates through the plate forming a hole
approximately the same size as the diameter of the arc stream.
Continued plasma exposure
increases the size of the hole to 4 or 5 times the arc stream diameter. Moving the torch or
workpiece in a circular motion can
produce larger holes.
stack cutting, Plasma is effectively stack-cut stainless steel and aluminum. Plasma stack-cutting
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of thin carbon steel tends to weld the layers making them
difficult to separate after cutting.
During cutting, the layers should be clamped firmly enough to minimize gaps, but loose enough
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to permit slippage between
layers due to differential expansion. The upper layer may buckle if
clamping does not allow slippage.
Machining: The plasma arc can be used for “machining” or removing the metal from the surface
of a rotating cylinder to simulate a conventional lathe or turning
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operation. The process
As the workpiece is turned, the torch is moved parallel to the axis of the work. The torch is
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positioned so the arc will impinge tangentially on the workpiece and
remove the outer layer of
metal. Cutting can be accomplished with the workpiece rotating in either direction relative to the
torch, but best results are obtained
when the direction of rotation permits use of the shortest
arc length for cutting. The flow of molten metal being removed must be in such a direction that it
does
not tend to adhere to the hot surface that has just been machined.
Generally, the plasma-arc metal removal process has little if any advantage on easy-to-machine
metals, but it has considerable economic advantage for rough
metal removal of hard-to-
machine metals. The process is generally considered to be a roughing operation, which should be
followed by a conventional machine
finishing cut. Metal removal rates with the plasma arc
process on hard-to-machine metals are up to ten times faster than those achieved on a lathe using
a
tungsten carbide cutting tool. The principal disadvantages are the metallurgical alteration of
the characteristic of the surface profile produced.
OPERATING PARAMETERS
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Cutting: Quality of cut and metal removal rate are largely dependent upon proper attention to
operating variables. Several factors contribute to the quality and speed of cuts made by the
plasma arc process, including cutting-tip nozzle selection, power level, gas type and mixture, gas
flow rate, traverse speed, standoff distance, thickness of material, type of materials, impingement
angle, and equipment design.
Nozzle size: The highest quality plasma cut is usually obtained when maximum thermal intensity
is used. To achieve this, the smallest, or next to the smallest,
nozzle size that is capable of
operating at a power level suitable for the speed and thickness involved is used. Nozzle size for
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cutting usually ranges from 0.80 to
6.30 mm in diameter.
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Power: Direct current up to 200 kW and 50-1000 A is employed in plasma arc cutting
operations.
Gas mixture composition: A proportion of 10% hydrogen and 90% nitrogen or argon usually
gives good general-purpose results. Choice of plasma gas
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composition can have considerable
effect on the plasma flame since the ionized gas is the conductive path. The shielding gas or
secondary gas can be nitrogen,
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oxygen, air, or carbon dioxide. Water is sometimes used as a
shield.
Gas flow rate: For any particular nozzle size, an increase in plasma gas (primary) flow permits
an increase in current. This increases the power density of the
flame and permits greater speed
with less taper on the kerf walls. Primary gas flow rates usually range between 0.40-5.60 m3/hr.
Secondary gasses are
pressurized to flow at up to 11.3 m3/hr.
Standoff distance: Due to the columnar shape of the plasma jet, a wide range of tip-toworkpiece spacing is allowable. This permits machine cutting along
warped or irregular
surfaces. General consideration include:
Better quality cuts usually result from a short standoff distance since arc divergence is less and
the thermal intensity of the arc is greater.
Excessively close standoff distance can promote arcing due to the accumulation of slag drops
on the tip.
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Increased power input is necessary when the standoff distance is great.
Standoff distance can range from 6.5-76.2 mm.
Machining: In turning operations, torch variables include the electrical power delivered, the
gases used to form the plasma, the flow rate of the gases through the torch, the orifice diameter
through the nozzle duct, and any secondary gas streams. In general, there is an optimum exitorifice size for operation at a particular power level that produces a well-controlled, highvelocity plasma jet with maximized capacity for performing the material removal operation.
Thus gas flow rate, orifice size, and power level are intimately related.
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Metal removal rate: In the physical orientation of PAM operations, such variables as torch
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standoff, angle to work, depth of cut, feed into the work, and speed
of the work toward the
torch are involved. Feed and depth of cut determine the volume of metal removed. illustrates the
inter-relationship among some of
the factors involved in plasma arc turning. Since the torch
carriage speed and direction are not normally coupled to the work spindle, as they might be on
an
ordinary lathe, it is necessary to calculate the carriage speed in order to produce a turned
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piece with a predetermined pitch (pitch-feed). When a 50kW power
level on 50 mm rods is
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used, the maximum metal removal rate for satisfactory surface finish is about 114 cm3/min.
Surface finish: Surface finish can vary anywhere from helical ridges along the surface to a
depending upon the feed
into the work optimization of the process. Plasma arc machining for maximum removal rate does
produce a slight helical ridge as the
cut progresses.
Surface characteristics: One characteristics of the workpiece surface when it is not cooled
during an operation is a gradual inward taper in the direction of the cut. This is believed to be
due to accumulated heating of the workpiece as the cut progresses and should be minimized or
eliminated by appropriate cooling
methods. An oxidation scale normally forms behind the cut
on an unprotected specimen, but this can be minimized or eliminated by proper shielding.
METALLURGICAL EFFECTS
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Metallurgical effects of the PAM process are as widely varied as the materials used and there
respective metallurgical histories. In general, the depth of the heat-affected zone is
approximately 0.75 mm. For some operations, this hardened material may have to be removed;
for other applications, a hardened surface such as this may be desirable. The nature of individual
applications of the PAM process determines the metallurgical effects that can be tolerated.
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