Ucm class notes 18 19

ME6004 Unconventional Machining Processes Compiled By: Dr. B. Ramesh, Ph.D.,
Associate Professor/Mechanical, St. Joseph’s Institute of Technology, Chennai-600119
ME 6004 UMP 1
UNIT -1
INTRODUCTION TO UNCONVENTIONAL MACHINING PROCESS
Difference between Unconventional and Conventional machining processes
S.NO UNCONVENTIONAL MACHINING
PROCESS
CONVENTIONAL MACHINING
PROCESS
1 There is a physical contact between tool
and workpiece.
No physical between tool and workpiece.
2 The tool need has to be harder than the
work piece.
The tool need not be harder than the work
piece as there is no contact.
3 Mechanism of material removal is
shearing.
Mechanism of material removal is based
on the energy used.
4 The energy domain used for material
removal is mechanical.
The energy domain commonly used is
mechanical, electrical, thermal and
chemical.
5 Macroscopic chip formation Microscopic chip or no chip formation.
6 Harder material like titanium, ceramics
and HSTR materials cannot be
machined.
Harder materials can be machined.
7 It is difficult to machine complex shapes. Complex shape can be machined.
8 Since there is physical contact between
tool and work piece, tool life is low.
Since there is no physical contact, tool life
is high.
9 Heat is generated at the point of contact
of tool and work piece, hence properties
gets altered
No heat is generated since there is no
contact.
10 Surface finish is not so good Good surface finish can be obtained
11 Can be used for mass production. Cannot be used for mass production.
12 Production cost is low. Production cost is high.
13 Material removal rate is high. Material removal rate is low.
14 High precision cannot be achieved High precision can be achieved.
Factors that should be considered during the selection of an appropriate
unconventional machining process for a given job.

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Before selection the process the following aspects must be studied:
Physical parameters Properties of work material Shape to be obtained
Process capability Economic considerations

ME 6004 UMP 3

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Types of energy domain Limitation
Mechanical energy based
UCMP
 Cannot machine ductile materials
 Material removal rate is low
 The abrasive particles get embedded to the work
piece.
Electrical energy based
UCMP
 Electrically nonconductive material cannot be
machined
 Process is not suitable for producing sharp corners
 MRR is lesser than conventional machining
 Slow process if good surface finish and high
accuracy are required.
Chemical energy based
UCMP
 Cost of the equipment is high.
 Sharp corners cannot be produced
 The finished surface has poor fatigue strength
 Design and manufacturing of the tool electrode is
very difficult
Thermal energy based
UCMP
 Larger size holes cannot be drilled.
 Holes produced by this process may have low degree
of roundness.
 Tolerance achieved by this system is very low

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Process Capital cost Tooling cost
Power
consumption
cost
Material
removal rate
efficiency
Tool Wear
USM L L H M
AJM VL L L H L
ECM VH M M L VL
CHM M L H M VL
EDM M H L H H
EBM H L L VH VL
LBM L L VL VH VL
PAM VL L VL VL VL
The various unconventional machining process on the basis of type of
energy employed, material removal rate, transfer media and economical
aspects.

ME 6004 UMP 6

ME 6004 UMP 7
Unit II
MECHANICAL ENERGY BASED PROCESSES
The salient features of AJM.
 This process of removal of material by impact erosion through the action of a
concentrated, high velocity stream of grit abrasives
 It is used to cut hard and brittle materials and the process is free from vibration
 AJM uses a stream of fine grained abrasive mixed with carrier gases at high
pressure
Stream is directed by means of a suitably designed nozzle onto the work surface
to be machined.
Material removal occurs due to erosion caused by abrasive particles
AJM process variables that influences the rate of material removal

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The effect of stand – off distance on material removal rate and machining
accuracy in AJM.

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Stand Off Distance ( SOD)
Stand off is defined as the distance between face of the nozzle and the work surface
A large SOD results in the flaring up of jet which leads to poor accuracy
Small MRR at low SOD is due to reduction in nozzle pressure with decrease in distance
Small MRR at large SOD is due to reduction in jet velocity with increase in distance.
Various elements of abrasive water jet machining process.

ME 6004 UMP 10
Important process parameters of abrasive jet machining:
1. Abrasive mass flow rate
2. Nozzle tip distance
3. Gas Pressure
4. Velocity of abrasive particles
5. Mixing ratio
6. Abrasive grain size
Abrasive mass flow rate:
Mass flow rate of the abrasive particles is a major process parameter that influences the metal
removal rate in abrasive jet machining. In AJM, mass flow rate of the gas (or air) in abrasive
jet is inversely proportional to the mass flow rate of the abrasive particles.
Due to this fact, when continuously increasing the abrasive mass flow rate, Metal Removal
Rate (MRR) first increases to an optimum value (because of increase in number of abrasive
particles hitting the workpiece) and then decreases.
However, if the mixing ratio is kept constant, Metal Removal Rate (MRR) uniformly
increases with increase in abrasive mass flow rate.
Nozzle tip distance:
Nozzle Tip Distance (NTD) is the gap provided between the nozzle tip and the workpiece.
Upto a certain limit, Metal Removal Rate (MRR) increases with increase in nozzle tip
distance. After that limit, MRR remains constant to some extent and then decreases.
In addition to metal removal rate, nozzle tip distance influences the shape and diameter of
cut.
For optimal performance, a nozzle tip distance of 0.25 to 0.75 mm is provided.
Gas pressure:
Air or gas pressure has a direct impact on metal removal rate.
In abrasive jet machining, metal removal rate is directly proportional to air or gas pressure.
Velocity of abrasive particles:
Whenever the velocity of abrasive particles is increased, the speed at which the abrasive
particles hit the workpiece is increased. Because of this reason, in abrasive jet machining,
metal removal rate increases with increase in velocity of abrasive particles.

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Mixing ratio:
Mixing ratio is a ratio that determines the quality of the air-abrasive mixture in Abrasive Jet
Machining (AJM).
It is the ratio between the mass flow rate of abrasive particles and the mass flow rate of air (or
gas).
When mixing ratio is increased continuously, metal removal rate first increases to some
extent and then decreases.
Abrasive grain size:
Size of the abrasive particle determines the speed at which metal is removed.
If smooth and fine surface finish is to be obtained, abrasive particle with small grain size is
used.
If metal has to be removed rapidly, abrasive particle with large grain size is used.
Process of AJM:
Work Material- Hard and brittle materials like glass, quartz, ceramics, mica etc.
Abrasive-Aluminium oxide, Sic, Glass powder, Dolomite
Size of Abrasive-25µm
Flow rate- 2 to 20g/min
Medium-N2 or CO2 or Air
Velocity-125 to 300m/s
Operations-Drilling, cutting, deburring, cleaning etc.,
In abrasive jet machining (AJM) a focused stream of abrasive grains of Al2O3 or SiC carried
by high-pressure gas or air at a high velocity is made to impinge on the work surface through
a nozzle of 0.3- to 0.5-mm diameter. The process differs from sandblasting (SB) in that AJM
has smallerdiameter abrasives and a more finely controlled delivery system. The workpiece
material is removed by the mechanical abrasion (MA) action of the high-velocity abrasive
particles. AJM machining is best suited for machining holes in super hard materials. It is
typically used to cut, clean, peen, deburr, deflash, and etch glass, ceramics, or hard metals.
The abrasives used in AJM are
 Aluminium Oxide- General purpose abrasive and it is used in sizes of 10,25
and 50 micron
 Silicon Carbide- It is used for cutting on extremely hard materials & used in
the sizes of 25 and 50 microns.
 Glass powder- It is used for light polishing and deburring and used in the size
of diameter 0.30 to 0.60 mm.

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 Dolomite- It is used for light cleaning and etching and it is of 200 grit size.
Advantages of AJM:
 No heat generation during this process
 Very thin and brittle materials can be cut without any risk of breaking -used to
cut intricate hole shapes in hard and brittle materials.
Disadvantages of AJM:
 Material removal rate is
slow
 Nozzle wear rate is high
 Abrasive powder cannot be
reused
Applications of AJM:
1. Drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing.
2. Deburring of cross holes, slots, and threads in small precision parts that require a
burr-free finish, such as hydraulic valves, aircraft fuel systems, and medical
appliances
3. Machining intricate shapes or holes in sensitive, brittle, thin, or difficult-to-machine
materials
4. Insulation stripping and wire cleaning without affecting the conductor
5. Micro-deburring of hypodermic needles
6. Frosting glass and trimming of circuit boards, hybrid circuit resistors, capacitors,
silicon, and gallium
7. Removal of films and delicate cleaning of irregular surfaces because the abrasive
stream is able to follow contours
Principle of working of water jet machining process.
Water Jet Machining
Employs a fine jet of water with a high pressure ( 1500-4000 MN/sq.m) and high
velocity ( upto twice the speed of sound)
When fine jet bombarded on the work piece erodes the material.
High velocity flow is virtually stopped, then the KE is converted to pressure
energy ( called stagnation pressure)
Mechanism of Metal removal
Mechanism of water jet machining is erosion caused by localized compressive failure
which occurs when the local fluid pressure exceeds the strength of the material
Jet cutting equipment
Pump : to pressurize the liquid to 1500 – 4000 MN/sq m
Tubing : High pressure to transport fluid

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Ratio Outside to inside diameter is 5 to 10
Tubing made of solid stainless steel
Valves: most high pressure valves are of needle type
Nozzle : made of sintered diamond and sapphire
Exit diamter of nozzle 0.05 to 0.35mm
Advantages of WJM:
Water is cheap, non toxic Any contour can be cut Does not generate heat
Best suited for explosive environments
Operational Summary of WJM:

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Principle of USM with neat diagram:
Ultrasonic Machining
Ultrasonic – frequency above the upper frequency of human ear
There are two types of waves , namely longitudinal waves and shear waves.
Longitudinal waves are mostly used in ultrasonic applications since they can be
easily propagated.
Device used to any type of energy into ultrasonic waves is called transducer.
Introduction
Material is removed by the action of abrasive grains
Abrasive particles are driven into the work surface by the tool oscillating normal
to the work surface at high frequency
The tool is shaped as approximate mirror image of the configuration of the cavity
desired in the work .
Transducer is a device which converts one form of energy into another form of energy. Ultrasonic
transducer is adevice which converts any form of energy into ultrasonic vibration. In USM it converts

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high frequency electrical energy into mechanical vibration. Two types of transducer are used in USM
to convert the supplied energy into mechanical motion
Ultrasonic Transducer: It converts high frequency electrical signal into high frequency
linear mechanical vibration. In USM either of the two types of transducers are used, i.e.
piezoelectric transducers or magnetostrictive transducers.
Piezoelectric transducer: When an electric current is passed through the piezoelectric
crystal (quartz) it expands, when the current is removed the crystal attains its original size.
This effect is known as piezoelectric effect. These are available up to 900W power supply &
95% efficiency. This effect is used to produce the vibrations in ultrasonic machining.
Magneto-strictive transducer: When an object made of ferromagnetic materials (Nickel &
Nickel alloy sheets) is placed in the continuously changing magnetic field, a change in its
length takes place. These are available up to 2.4KW power supply & 20% - 30% efficiency.
For this transducer cooling is essential to remove the waste heat. The coefficient of
magnetostrictive elongation is equal to the ratio of change in length to the length of the
magnetostrictive coil.
Abrasive Slurry: Commonly used abrasives are Al2O3, Sic & B4C (Boron Carbide).
Vibrating Abrasives attain K.E. and strike the work piece surface with a force much higher
than their own weight. That is each down stroke of the tool accelerates numerous abrasive
particles resulting in the formation of thousands of tiny chips per second.
Some abrasives used are
o Alumium oxide ( Alumina)
o Boron Carbide
o Silicon carbide
o Diamond dust
Boron is the most expensive abrasive material but is best suited for tungsten
carbide , tool steel and precious stones
Silicon finds the maximum application
Diamond dust ensures good accuracy, surface finish and cutting rates
The size of abrasive vary from 200 and 2000 grit
Course grades are good for roughing, whereas finer grades (1200 to 2000) are
used for finishing
In actual practice the surface roughness of the machined face is governed by work
material, roughness on toll surface, vibration amplitude, fineness of abrasive grit,
efficient slurry circulation

ME 6004 UMP 16
Mechanism of material removal in USM
The process is performed by cutting tool which oscillates at high frequency, typically
20-40Hz. The shape of the tool is just mirror part of the shape to be produced in the work
piece. The high speed reciprocating motion of the tool drives the abrasive grains across a
small gap against the work piece. The tool is gradually fed with a uniform force. The impact
of the abrasive is the energy principally responsible for material removal in the form of small
wear of particles that are carried away by abrasive slurry. The tool material being tough and
ductile wears out at a much slower rate. Material removal can happen by hammering of grit,
impact of free abrasive grit and micro cavitation.

ME 6004 UMP 17
Applications of USM:
 Used for making dies in forging and extrusion processes.
 Used for performing machining operations like drilling, grinding, profiling and milling
operation.
 Enables a dentist to drill a hole of an shape on teeth without creating pain.
 Coining operation for materials like glass and ceramics.
 It is particularly useful in micro drilling holes of up to 0.1 mm.
Feed Mechanisms of USM process:
The feed mechanism must perform the following functions
Bring the tool very slowly close to the work piece
Provide adequate cutting force and sustain this during cutting
Decrease the force at the specified depth
Return the tool

ME 6004 UMP 18
In systems shown in fig (a) & (b) counter weights are used. The force is adjusted
through weights
Figure (c) shows a compact spring loaded system which is quite sensitive
For high rating machines, pneumatic or hydraulic systems are used ( Fig (d) )

ME 6004 UMP 19
UNIT-3 ELECTRICAL ENERGY BASED PROCESSES
EDM:
Elements of EDM machine:
Power supply:
Power supply of the machine senses the voltage between the two electrodes and maintains the
desired gap by sending the signal to the servo controller. Power supply should also be able to
control the parameters like current, duration of pulse, frequency of pulse, duty cycle and
electrode polarity. Power supply uses solid state rectifier to convert alternate current into
pulsed direct current. Cut off protection circuit is provided which terminates the power during
over voltage, over current and short circuit of electrode. Three different types of power
generating circuit are being used like resistance capacitance type, rotary impulse type
generator and electronic pulse generator.
Dielectric fluid:
Dielectric fluid is a liquid medium which is filled in the tank of the EDM machine setup.
Work piece and tool are submerging in the tank with a small gap between them. Transformer
oil, paraffin, kerosene, lubricating oil and deionised water are commonly used. The dielectric
fluid should be filtered before reuse so that chip contamination of the fluid will not affect
machining accuracy.
Tool and tool material:
In EDM both work piece and tool are the electrodes but usually the word electrode specifies
the tool. The shape of the tool will be same as that of the product desired. Tool material
should be selected such that it would not undergo much tool wear when it is impinged by
positive ions. The localised temperature rise has to be less tailoring or properly choosing its
properties choosing its properties so even when temperature increases, there would be less
melting. Further the tool material should be easily workable as intricate shaped geometric
feature are machined in EDM. The material used for the tool has influence on tool wear, side
clearance, rate of material removal and finish obtained.
Servo mechanism:
A servo system is used to maintain a predetermined gap between the tool and work piece.
The servo gets its input signal from the difference between a selected reference voltage and
the actual voltage across the gap. The signal is amplified and the tool, as it wears a little, is
advanced by hydraulic control. A short circuit across the gap causes the servo to reverse the
motion of the tool until the gap is maintained.

ME 6004 UMP 20
Working principle of EDM process.
• Consider the case of a discharge between two electrodes (tool cathode and work
anode) through a gaseous or liquid medium
• Suitable voltage is applied. The potential intensity of the electric field between them
build up , until a predetermined value
• Individual electrons break loose from the surface of the cathode and impelled towards
the anode under the influence of field forces.
• While moving in inter electrode space, the electrons collide with the neutral
molecules of the dielectric detaching electrons from them and causing ionization.
• Ionization becomes such that a narrow channel of continuous conductivity is formed
• This results in momentary current impulse or discharge.
• This leads to generation of extremely high temperature between 8000 C and 12000 C
causing fusion or partial vaporization of the metal and dielectric fluid at the point if
discharge.
• This results in the formation of tiny crater at the point of discharge in the work piece.
The mechanism of material removal in EDM.
Fundamentally the electro-sparking method of metal working involves an electric
erosion effect which connotes the breakdown of electrode material accompanying
any form of electric discharge, (The discharge is usually through a gas, liquid or
in some cases solids)

ME 6004 UMP 21
A necessary condition for producing a discharge is the ionization of the dielectric, that is ,
splitting up of its molecules into ions and electrons
• This leads to generation of extremely high temperature between 8000 0
C and 12000 0
C
causing fusion or partial vaporization of the metal and dielectric fluid at the point if
discharge.
• This results in the formation of tiny crater at the point of discharge in the work piece.
The cathode electrode is assumed to be the source of producing electrons which are emitted
either by field effect or by schottky affect.
The Electrons liberated from the cathode are accelerated until they gain sufficient energy to
ionize the liquid molecules and initiate an electron avalanche.
The applied field E, at which an electron avalanche can be initiated, is given as
eE =chv
Where e-charge; E-Applied field; =Mean free path of electron; c-Velocity of light,
hv-Ionization quantum for the liquid molecule.
This theory is used to magnify the breakdown strength of hydrocarbons. But it does not take
into account the ignition delay observed between the applied voltage and breakdown voltage.
Breakdown in gas is introduced by collisional ionization of the molecules. But in liquid, it is
not possible due to insufficient kinetic energy of the electrons. In order to avoid this, a pre-
breakdown electron current flows from the cathode to anode. This low current heats the
liquid to form a vapour bubble of sufficient pressure in between the electrodes. Then a spark
is produced in the vapour bubble according to high pressure gas-discharge mechanism.
Various electrode materials used in EDM Process.

ME 6004 UMP 22
Electrode feed control system in EDM
The Servo Mechanism Both wire and vertical EDM machines are equipped with a
servo control mechanism that automatically maintains a constant gap of about the thickness
of a human hair between the electrode and the workpiece. It is important for both machine
types that there is no physical contact between the electrode and the workpiece, otherwise
arcing could damage the workpiece and break the wire. The servomechanism advances the
electrode into the workpiece as the operation progresses and senses the work-wire spacing
and controls it to maintain the proper arc gap which is essential to a successful machining
operation.
Five key factors in electrode material selection
The performance factors that EDMers look for are metal removal rate, wear, surface finish,
machinability, and material cost. A large cavity with no detail would typically require a
material that would provide a high metal removal rate, good wear, and be available in a large
size at a reasonable cost. These performance factors would be found in a grade in
the Fine classification where particle size is larger and the material is economical. At the
other extreme, a very small cavity with razor sharp detail would typically require a grade in
the Angstrofineclassification where a good surface finish, very good wear resistance, and
machinability are critical. The small particle size and high strength give this grade the ability
to hold and maintain intricate detail. The material cost would be an insignificant factor.
Each application will have different performance requirements that should be considered
when selecting a grade. Job factors to be considered are the workmetal, the shape, size and
number of cavities to be cut, and the number of electrodes needed to rough and finishthe job.
The performance factors can mean the difference between success and failure, profit and loss.
These factors, together with the graphs from the Objective Comparison Testing program will
provide an accurate evaluation of a given material's performance.
The effect of process parameters on the metal removal in EDM.

ME 6004 UMP 23
Advantages of EDM:
 All electrically conductive materials can be easily machined.
 Complicated geometries can be produced easily which are otherwise difficult to
machine by other technique.
 No mechanical stress is present in the process since there is no direct physical contact.
 Material removal rate is more when compared with other UCM process.
 The process can maintain high level of dimensional accuracy.
 Material of any type irrespective of the mechanical properties can be cut.
 Holes are completed in one pass of electrode.
Disadvantages of EDM:
 Electrically nonconductive material cannot be machined.
 Process is not suitable for producing sharp corners.
 MRR is lesser than conventional machining process.
 Slow process, particularly if good surface finish and high accuracy are required.

ME 6004 UMP 24
Applications of EDM:
 The process can be used for manufacturing of tool.
 It is extremely useful for machining of exotic materials used in aerospace industries,
refractory metals and hardened steels.
 It is used for making stamping tools, wire drawing dies and extrusion dies, intricate
mould cavities.
 Drilling of micro holes, with high accuracy is possible using EDM.
The classification of various spark erosion generators.
Various electric circuits in EDM:
Relaxation Generator circuit:
The relaxation generator where the principal is based on the charging and discharging of the
capacitor that is connected to the power supply. The type of wave that is generated by these
arrangements is the saw tooth wave. In creating the spark, the capacitor is allowed to charge
and then it is brought to contact with the work piece and discharges.

ME 6004 UMP 25
R-C-L Circuit:
In the relaxation circuit metal removal rate increases as R is decreased. But R cannot be
decreased below a critical value. If R decreases below the critical value, arcing will takeplace
instead of sparking. Further, the capacitor charging time in R-C circuit is much higher than
discharging time. Therefore an inductance L is included in the charging circuit.
Rotary Pulse Generator circuit:
Solid state devices are used instead of capacitor and resistors in pulse generator. Replacing
the capacitor a solid-state devices such as the transistor are used. They are toggled between of
state and saturation state to generate rectangular pulse which swing between zero and supply
voltage. The idea is to increase the production efficiency which it have higher production
efficiency than the relaxation circuits. This is the rotary impulse generator power supply
where the voltage waveform is generated based on the DC motor principle, which it creates a
sinusoidal wave pattern that is similar to rectification.
Controlled pulse generator circuit:
The RC, RCL and rotary pulse generator Circuit are not having automatic prevention of the
current flow in case of a short circuit. To obtain such an automatic control, a vacuum tube or
a transistor is used as switching device.

ME 6004 UMP 26
Tool wear in EDM process.
Tool wear is the characteristic of the tool during machining process. As the tool does not
come into contact with the work, life of the tool is long and less wear and tear takes place.
The tool wear ratio is defined as the ratio of volume of work material removed to the volume
of electrode (tool) consumed.
Wear ratio=Volume of work material removed
Volume of electrode consumed
The wear ratio for brass electrode is 1:1, for copper is 2:1 and for copper tungsten is 8:1 for
non-metallic (graphite) wear ratio may vary from 5:1 to 50:1
WEDM:
Elements of WEDM:
Computerized numerical control:
The numerical control offers the capabilities of scaling, mirror imaging, rotating axis
exchange and assist program. The CNC control of the machine offers other feature including
technology to aid in the prevention of wire breaks, background editing, and graphic display of
program while the machine is running.
Power supply:
The power supply unit of WEDM provides energy for producing the spark between the work
piece and the wire.

ME 6004 UMP 27
Mechanical section:
The mechanical section of the WEDM includes work table, work stand and work drive
mechanism. The work table of the typical machine has high precise lead screws with
recirculating ball bearing driven by AC motor. Wire diameter range from 0.004-0.014 inches.
The most commonly used diameter is being 0.010”. In wire drive mechanism, the wire
originates from a supply spool and then passes through a tension device. Different diameter
wires require different amount of tension to keep it straight. It then comes in contact with
power feed contacts where the electric current applied. The wire then passes through a set of
precision round diamond guides and finally is then transported in to a waste bin.
Dielectric system:
Wire EDM uses deionized water as the dielectric. The dielectric system includes the water
reservoir, filtration system, deionization system and water chiller unit. During the cutting
process the chip from the material that is being eroded gradually changes the water
conductivity level. Resistivity levels of the water are set according to the cutting requirements
of the work piece material being machined.
Working principle of WEDM:
The Spark Theory on a wire EDM is basically the same as that of the vertical EDM process.
In wire EDM, the conductive materials are machined with a series of electrical discharges
(sparks) that are produced between an accurately positioned moving wire (the electrode) and
the workpiece. High frequency pulses of alternating or direct current is discharged from the
wire to the workpiece with a very small spark gap through an insulated dielectric fluid
(water). Many sparks can be observed at one time. This is because actual discharges can
occur more than one hundred thousand times per second, with discharge sparks lasting in the
range of 1/1,000,000 of a second or less. The volume of metal removed during this short
period of spark discharge depends on the desired cutting speed and the surface finish
required. The heat of each electrical spark, estimated at around 15,000° to 21,000°
Fahrenheit, erodes away a tiny bit of material that is vaporized and melted from the
workpiece. (Some of the wire material is also eroded away) These particles (chips) are
flushed away from the cut with a stream of de-ionized water through the top and bottom
flushing nozzles. The water also prevents heat build-up in the workpiece. Without this

ME 6004 UMP 28
cooling, thermal expansion of the part would affect size and positional accuracy. Keep in
mind that it is the ON and OFF time of the spark that is repeated over and over that removes
material, not just the flow of electric current.
Wire-cut EDM Process:
 Wire-cut Electrical discharge machining (wire-cut EDM) is a thermal material
removal process by melting and partial vaporization of the workpiece material. Source
of the energy used for melting and vaporization is found in the form of heat, generated
by electrical discharges and sparks between two electrodes in close proximity. The
electrodes (tool electrode and workpiece) are immersed in dielectric liquid or flowing
pressurized dielectric medium.
 Electrical discharge occurs when the dielectric is broken down by the application of
voltage pulse. Some of the released energy during discharge is transferred to the
electrodes and results in the heating of highly localized regions of the electrodes.
When the temperature of the heating region exceeds melting temperature of the
electrodes material removal starts in size of very small particles.
 Molten and vaporized particles (debris material) are washed away from the sparking
area by the continuously flushing dielectric fluid. Flowing pressure of the dielectric
fluid should be used in an appropriate value, high pressurized fluid result in vanishing
the influence of electrical sparks, and removed together with debris particles,
however, low pressure flow result in rising debris concentration in sparking area and
cause secondary discharge, arc, and short circuit.
 In wire-cut EDM process, mechanical properties of the workpiece do not affect the
machining process. Thermal properties such as melting point, boiling point, and
electrical conductivity of workpiece materials affect the machining characteristics.
The material removal rate of wire-cut EDM process is primarily determined by the
electrical conductivity and melting temperature of the workpiece material. A
workpiece with higher electrical conductivity and lower melting temperature can be
machined efficiently.

ME 6004 UMP 29
 Nozzle used as an injector to inject the dielectric in the machining area in Wire-cut
EDM.
Advantages of WEDM:
 Complex shapes that would otherwise be difficult to produce with conventional cutting
tools.
 Extremely hard material to very close tolerances.
 Very small work pieces where conventional cutting tools may damage the part from
excess cutting tool pressure.
 There is no direct contact between tool and work piece. Therefore delicate sections and
weak materials can be machined without perceivable distortion.
 A good surface finish can be obtained; a very good surface may be obtained by redundant
finishing paths.
 Very fine holes can be attained.
 Tapered holes may be produced.
Disadvantages of WEDM:
 The slow rate of material removal.
 Potential fire hazard associated with use of combustible oil based dielectrics.
 The additional time and cost used for creating electrodes for ram/sinker EDM.
 Reproducing sharp corners on the workpiece is difficult due to electrode wear.
 Specific power consumption is very high.
 Power consumption is high.
 "Overcut" is formed.
 Excessive tool wear occurs during machining.
 Electrically non-conductive materials can be machined only with specific set-up of the
process.
UNIT- 4 CHEMICAL ENERGY BASED PROCESSES
.Chemical machining process:

ME 6004 UMP 30
Chemical milling (CHM) is the controlled chemical dissolution (CD) of the
workpiece material by contact with a strong reagent. Special coatings called maskants protect
areas from which the metal is not to be removed. The process is used to produce pockets and
contours and to remove materials from parts having a high strength-to-weight ratio.
CHM consists of the following steps:
 Residual stress relieving: If the part to be machined has residual stresses from the
previous processing, these stresses first should be relieved in order to prevent warping
after chemical milling.
 Preparing: The surfaces are degreased and cleaned thoroughly to ensure both good
adhesion of the masking material and the uniform material removal.
 Masking: Masking material is applied (coating or protecting areas not to be etched).
 Etching: The exposed surfaces are machined chemically with etchants.
 Demasking: After machining, the parts should be washed thoroughly to prevent
further reactions with or exposure to any etchant residues. Then the rest of the
masking material is removed and the part is cleaned and inspected.
Advantages and disadvantages of CHM process:
Advantages:
1. Weight reduction is possible on complex contours that are difficult to
machine using conventional methods.
2. Simultaneous material removal, from all surfaces, improves productivity
and reduces wrapping.
3. No burrs are formed.
4. No stress is introduced to the workpiece, which minimizes the part
distortion and makes machining of delicate parts possible.
Disadvantages:
1. Only shallow cuts are practical: up to 12.27 mm for sheets and plates, 3.83
mm on extrusions, and 6.39 mm on forgings.
2. Handling and disposal of chemicals can be troublesome.
3. Hand masking, scribing, and stripping can be time-consuming, repetitive,
and tedious.
4. Surface imperfections are reproduced in the machined parts.
5. Metallurgical homogeneous surfaces are required for best results.
6. Deep narrow cuts are difficult to produce.
7. Fillet radii are fixed by the depth of cut.

ME 6004 UMP 31
Types of maskants used in chemical machining:
The maskant to be used is determined primarily by the chemical used to etch the material, and
the material itself. The maskant must adhere to the surface of the material, and it must also be
chemically inert enough with regards to the etchant to protect the workpiece. Most modern chemical
milling processes use maskants with an adhesion around 350 g cm−1; if the adhesion is too strong, the
scribing process may be too difficult to perform. If the adhesion is too low, the etching area may be
imprecisely defined. Most industrial chemical milling facilities use maskants based upon neoprene
elastomers or isobutylene-isoprene copolymers. Maskants to be used in photochemical machining
processes must also possess the necessary light-reactive properties.
The most commonly used maskants:
For aluminium
sodium hydroxide, Keller's reagent
For steels
hydrochloric and nitric acids, ferric chloride for stainless steels, Nital (a mixture of nitric acid and
ethanol, methanol, or methylated spirits for mild steels. 2% Nital is common etchant for plain carbon
steels.
For copper
cupric chloride, ferric chloride, ammonium persulfate, ammonia, 25-50% nitric acid, hydrochloric
acid and hydrogen peroxide
For silica
hydrofluoric acid (HF) is a very efficient etchant for silicon dioxide. It is however very dangerous if it
comes into contact with the body.
Characteristics of cut and peel maskants:
 It should be tough enough to withstand handling.
 It should be well adhering to the work piece surface.
 It should be easy for scribing.
 Material should be able to withstand the heat used during chemical machining.
Selection of maskants:
Work piece material Masking material
Aluminium and alloy Polymer, butyl rubber neoprene
From based alloy Polymer, polyvinyl chloride, polyethylene
butyl rubber.
Nickel Neoprene
Magnesium Polymer
Copper and alloy Polymer
Titanium Polymer
Silicon Polymer

ME 6004 UMP 32
Limitations of cut and peel maskants:
 It is difficult to get sharp corners.
 Difficult to chemically machine thick material
 Scribing accuracy is very limited causes less dimensional accuracy
 Chemical etchants are very dangerous fro operators
 Disposal of etchants are very expensive
 Time consuming process.
Working of ECM process.
Electrochemical machining is based on the principle of outlined in Fig., the workpiece
and tool are the anode and cathode respectively, of the electrolytic cell, and a potential
difference, usually at about 10 V, is applied across them. A suitable electrolyte, for example
aqueous sodium chloride solution is chosen so that the cathode shape remains unchanged
during electrolysis. The electrolyte is pumped at a rate 3 to 30 m/sec through the gap between
electrodes to remove the products of machining and to diminish unwanted effects, such as
those that arise with cathodic gas generation and electric heating. The rate at which metal is
then removed from the anode is approximately in inverse proportion to the distance between
the electrodes. As machining proceeds, and with the simultaneous movement of the cathode
at a typical rate, for example, 0.02 mm/sec towards the anode, the gap width along the
electrode length will gradually tend to a steady-state value. Under these conditions, a shape,
roughly complementary to that of cathode, will be produced on the anode. A typical gap
width then should be about 0.4 mm.
The process of metal removal by electro chemical dissolution was known as long
back as 1780 AD but it is only over the last couple of decades that this method has been used
to advantage. Itis also known as contactless electrochemical forming process. The noteworthy
feature of electrolysisis that electrical energy is used to produce a chemical reaction,

ME 6004 UMP 33
therefore, the machining process basedon this principle is known as Electrochemical
machining (ECM). This process works on the principleof of Faraday’s laws of electrolysis.
Michael Faraday discovered that if the two electrodes are placed in a bath containing
aconductive liquid and DC potential (5-25V) is applied across them, metal can be depleted
from the anode and plated on the cathode. This principle was in use for long time. ECM is the
reverse of theelectroplating.ECM can be thought of a controlled anodic dissolution at atomic
level of the work piece thatis electrically conductive by a shaped tool due to flow of high
current at relatively low potentialdifference through an electrolyte which is quite often water
based neutral salt solution.In ECM, Electrolyte is so chosen that there is no plating on tool
and shape of tool remainsunchanged. If the close gap (0.1 to 0.2mm) is maintained between
tool and work, the machinedsurface takes the replica of tool shape.
Equipment
The electrochemical machining system has the following modules:
• Power supply
• Electrolyte supply and cleaning system
• Tool and tool feed system
• Work piece and Work holding system.
Power supply:
During ECM, a high value of direct current ( may be as high as 40000 A) and a low value of
electric potential ( in range of 5-25 V) across IEG( Interelectrode gap) is desirable. The
highestcurrent density achieved so far is around 20,000 A/cm2. Hence , with the help of a
rectifier and atransformer, three phase AC is converted to a low voltage, high current DC.
Silicon controlledrectifier (SCRs) are used both for rectification as well as for voltage
regulation because of their rapidresponse to the changes in the process load and their
compactness. Voltage regulation of ± 1% isadequate for most of the precision ECM works.
However, lack of process control, equipment failure, operator’s error, and similar other
reasons may result in sparking between tool and work. Theelectrical circuitry detects these
events and power is cut off ( using the device like SCRs) within 10micro seconds to prevent
the severe damage to the tool and work. In case of precision works even asmall damage to an
electrode is not acceptable. It may be minimized by using a bank of SCRs placedacross the
DC input to ECM machine.
Electrolyte supply and Cleaning system
The electrolyte supply and cleaning system consisting of a pump, filter, pipings,
control valves, heating or cooling coils, pressure gauges, and a storage tank ( or reservoir).
Electrolyte supplyports may be made in the tool, work or fixture, depending upon the
requirement of the mode ofelectrolyte flow. Small inter electrode gap, usually smaller than
1mm, should be maintained forachieving High MRR and high accuracy. For this purpose,

ME 6004 UMP 34
smooth flow of electrolyte should bemaintained and any blockade of such a small gap by
particles carried by electrolyte, should beavoided. Hence, electrolyte cleanliness is
imperative. It is normally done with the help of filters madeof SS steel, Monel or any other
anticorrosive material.It should be ensured that the piping system does not introduce any
foreign material likecorroded particles, scale or pieces of broken seal material. Piping system
is therefore made of SSsteel, Glass fibre reinforced plastic (GFRP), plastic lined MS or
similar other anti corrosive material.The required minimum capacity of electrolyte tank is
500 gallons for each 10000 A of current. ECMis supposed to machine different metals and
alloys at optimum machining conditions and with varyingrequirements of accuracy, surface
texture, etc. Under such situations, a single tank system is notrecommended because of loss
of time and wastage of electrolyte during drilling cleaning, mixing orfilling of new electrolyte
in the tank. It results in higher cost and poor accuracy of electro chemicallymachined surface
and also poor control of operating conditions. More than one tank therefore, can beused and
their number would depend upon the range of electrolytes needed to meet the work load.
Tool and Tool Feed system
Use of anti-corrosive material for tools and fixtures is important because they are required for
a long period of time to operate in the corrosive environment of electrolyte. High thermal
conductivityand high thermal conductivity are main requirements. Easy machining of tool
material is equallyimportant because dimensional accuracy and surface finish of the tool
directly affect the work pieceaccuracy and surface finish. Aluminum, Brass, Bronze, copper,
carbon, stainless steel and monel are afew of the material used for this purpose. Further, those
areas on the tool where ECM action is notrequired, should be insulated. For example, lack of
insulation on the sides of die sinking tool causesunwanted machining of work and results in a
loss of accuracy of the machined work piece. Use of non– corrosive and electrically non
conducting material for making fixtures is recommended. Also, thefixtures and tools should
be rigid enough to avoid vibration or deflection under the high hydraulicforces to which they
are subjected.
Work piece and work holding system:
Only electrically conductive material can be machined by this process, The chemical
properties of anode (work) material largely govern the material removal rate (MRR). Work
holdingdevices are made of electrically non conductive materials having good thermal
stability, and lowmoisture absorption properties, For Example, graphite fibres reinforced
plastics, plastics, Perspex,etc., are the materials used for fabricating the work holding device.
Advantages of ECM
 The components are not subject to either thermal or mechanical stress.
 No tool wear during ECM process.
 Fragile parts can be machined easily as there is no stress involved.
 ECM deburring can debur difficult to access areas of parts.

ME 6004 UMP 35
 High surface finish (up to 25 µm in) can be achieved by ECM process.
 Complex geometrical shapes in high-strength materials particularly in the aerospace
industry for the mass production of turbine blades, jet-engine parts and nozzles can be
machined repeatedly and accurately.
 Deep holes can be made by this process.
Limitations of ECM
 ECM is not suitable to produce sharp square corners or flat bottoms because of the
tendency for the electrolyte to erode away sharp profiles.
 ECM can be applied to most metals but, due to the high equipment costs, is usually
used primarily for highly specialised applications.
Material removal rate, MRR, in electrochemical machining:
MRR = C .I. h (cm3
/min)
C: specific (material) removal rate (e.g., 0.2052 cm3
/amp-min for nickel);
I: current (amp);
h: current efficiency (90–100%).
The rates at which metal can electrochemically remove are in proportion to the current passed
through the electrolyte and the elapsed time for that operation. Many factors other than
current influence the rate of machining. These involve electrolyte type, rate of electrolyte
flow, and some other process conditions.
Electro chemical deburring process.
In electrochemical deburring (ECDB), the anodic part to be deburred is placed in a
fixture, which positions the cathodic electrode in close proximity to the burrs. The electrolyte
is then directed, under pressure, to the gap between the cathodicdeburring tool and the burr.
On the application of the machining current, the burr dissolves forming a controlled radius.
Since the gap between the burr and the electrode.
Mechanism of deburring. Faraday’s laws of electrolysis dictate how the metal is
removed by ECDB. The deburring speed may be as high as 400 to 500 mm/min. ECDB using
a rotating and feeding tool electrode enhances the deburring process by creating turbulent

ME 6004 UMP 36
flow in the interelectrode gap. The spindle rotation is reversed to increase the electrolyte
turbulence. Normal cycle times for deburring reported by Brown (1998) are between 30 to 45
s after which the spindle is retracted and the part is removed. In simple deburring when the
tool is placed over the workpiece, a burr height of 0.5 mm can be removed to a radius of 0.05
to 0.2 mm leaving a maximum surface roughness of 2 to 4 μm. When burrs are removed from
intersections of passages in housing, the electrolyte is directed and maintained under a
pressure of 0.3 to 0.5 MPa using a special tool. That tool has as many working areas as
practical so that several intersections are deburred at a time. Proper tool insulation guarantees
the flow of current in areas nearby the burr. The deburring tool should also have a similar
contour of the work part thus leaving a 0.1 to 0.3 mm inter electrode gap. Moreover the tool
tip should overlap the machined area by 1.5 to 2 mm in order to produce a proper radius. The
choice of the electrolyte plays an important role in the deburring process.
Electro chemical honing process:
It is a process in which it combines the high removal characteristics of Electrochemical
Dissolution (ECD) and Mechanical Abrasion (MA) of conventional Honing. It has much higher
rates than either of honing & internal cylindrical grinding. Cathodic tool is similar to the
conventional honing tool, with several rows of small holes so that electrolyte could enter directly
into electrode gap. Electrolyte provides electron through the ionization process which acts as
coolant and flushes away the chips that are formed off by mechanical abrasion and metal sludge
that results from electrochemical dissolution action. Tool is inserted inside the worked hole or a
cylinder. Mechanical abrasion takes place first by the stones/hones. Oxides formed due to
working from previous process will be removed by it and clean surface will be achieved. Now the
clean surface will be in contact with electrolyte and then Electrochemical Dissolution will remove
the desired material. Same procedure is continued till the required cut is made. To control surface
roughness Mechanical Abrasion is allowed to continue for a few seconds after the current has
been turned off. Majority of the material is removed by the ECD phase. Abrading stones remove
enough material to generate a round, straight, geometrically true cylinder. Mechanical abrasion
just removes the surface oxides that are formed on the work surface due to ECD. Removal of

ME 6004 UMP 37
oxides enhances the performance of ECD as it could directly remove the required material and
fresh surface is obtained for each ECD phase.
Electro chemical grinding process:
Electrochemical grinding (ECG) utilizes a negatively charged abrasive grinding
wheel, electrolyte solution, and a positively charged workpiece, as shown.The process is,
therefore, similar to ECM except that the cathode is a specially constructed grinding wheel
instead of a cathodic shaped tool like the contour to be machined by ECM. The insulating
abrasive material (diamond or aluminum oxide) of the grinding wheel is set in a conductive
bonding material. In ECG, the non conducting abrasive particles act as a spacer between the
wheel conductive bond and the anodic workpiece. Depending on the grain size of these

ME 6004 UMP 38
particles, a constant interelectrode gap (0.025 mm or less) through which the electrolyte is
flushed can be maintained.
The abrasives continuously remove the machining products from the working area. In
the machining system the wheel is a rotating cathodic tool with abrasive particles (60–320
grit number) on its periphery. Electrolyte flow, usually NaNO3, is provided for ECD. The
wheel rotates at a surface speed of 20 to 35 m/s, while current ratings are from 50 to 300 A.
When a gap voltage of 4 to 40 V is applied between the cathodic grinding wheel and
the anodic workpiece, a current density of about 120 to 240 A/cm2 is created. The current
density depends on the material being machined, the gap width, and the applied voltage.
Material is mainly removed by ECD, while the MA of the abrasive grits accounts for an
additional 5 to 10 percent of the total material removal.
Applications
The ECG process is particularly effective for
1. Machining parts made from difficult-to-cut materials, such as sintered carbides, creep-
resisting (Inconel, Nimonic) alloys, titanium alloys, and metallic composites.

ME 6004 UMP 39
2. Applications similar to milling, grinding, cutting off, sawing, and tool and cutter
sharpening.
3. Production of tungsten carbide cutting tools, fragile parts, and thinwalled tubes.
4. Removal of fatigue cracks from steel structures under seawater. In such an application
holes about 25 mm in diameter, in steel 12 to 25 mm thick, have been produced by ECG at
the ends of fatigue cracks to stop further development of the cracks and to enable the removal
of specimens for metallurgical inspection.
5. Producing specimens for metal fatigue and tensile tests.
6. Machining of carbides and a variety of high-strength alloys.
UNIT- 5 THERMAL ENERGY BASED PROCESSES
LBM:
Working principle and construction of LBM:
Introduction
Figure : Laser beam machining schematic
Laser-beam machining is a thermal material-removal process that utilizes a high-energy,
coherent light beam to melt and vaporize particles on the surface of metallic and non-metallic
workpieces. Lasers can be used to cut, drill, weld and mark. LBM is particularly suitable for
making accurately placed holes. A schematic of laser beam machining is shown in Figure 12.
Different types of lasers are available for manufacturing operations which are as follows:

ME 6004 UMP 40
· CO2 (pulsed or continuous wave): It is a gas laser that emits light in the infrared region. It
can provide up to 25 kW in continuous-wave mode. · Nd:YAG: Neodymium-doped Yttrium-
Aluminum-Garnet (Y3Al5O12) laser is a solid state laser which can deliver light through a
fibre-optic cable. It can provide up to 50 kW power in pulsed mode and 1 kW in continuous-
wave mode.
Advantages
1. Any solid metal w/o decomposition can be cut
2. No contact of tool with work.
3. The Beam can be projected through a transparent window
4. Large mechanical forces are not exerted to work PC
Limitations
– Localized thermal stresses, heat affected zones, recast layer and thermal distribution in thin
parts
– Difficulty of material processing depends on how close materials boiling and melting points
are
– Hole wall geometry can be irregular
– The cutting of flammable materials is usually inert gas assisted
Applications
LBM can make very accurate holes as small as 0.005 mm in refractory metals ceramics, and
composite material without warping the workpieces. This process is used widely for drilling
and cutting of metallic and non-metallic materials. Laser beam machining is being used
extensively in the electronic and automotive industries.
– Multiple holes in very thin and thick materials
– Non-standard shaped holes and slots
– Prototype parts
– Trimming, scribing and engraving of hard materials
– Small diameter lubrication holes

ME 6004 UMP 41
PAM:
Power source
The plasma arc is normally operated with a DC, drooping characteristic power source.
Because its unique operating features are derived from the special torch arrangement and
separate plasma and shielding gas flows, a plasma control console can be added on to a
conventional TIG power source. Purpose-built plasma systems are also available. The plasma
arc is not readily stabilised with sine wave AC. Arc reignition is difficult when there is a long
electrode to workpiece distance and the plasma is constricted, Moreover, excessive heating of
the electrode during the positive half-cycle causes balling of the tip which can disturb arc
stability.
Special-purpose switched DC power sources are available. By imbalancing the waveform to
reduce the duration of electrode positive polarity, the electrode is kept sufficiently cool to
maintain a pointed tip and achieve arc stability.
Arc starting
Although the arc is initiated using HF, it is first formed between the electrode and plasma
nozzle. This 'pilot' arc is held within the body of the torch until required for welding then it is
transferred to the workpiece. The pilot arc system ensures reliable arc starting and, as the
pilot arc is maintained between welds, it obviates the need for HF which may cause electrical
interference.
Electrode
The electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle is
copper. The electrode tip diameter is not as critical as for TIG and should be maintained at
around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore
diameter for the current level and plasma gas flow rate will lead to excessive nozzle erosion
or even melting. It is prudent to use the largest bore diameter for the operating current level..
Plasma and shielding gases
The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5%
hydrogen for the shielding gas. Helium can be used for plasma gas but because it is hotter this
reduces the current rating of the nozzle. Helium's lower mass can also make the keyhole
mode more difficult.
Operation of transferred and non-transferred arc mode in plasma
machining:
Non-transferred arc process:
The arc is formed between the electrode (-) and the water cooled constricting nozzle(+). Arc
plasma comes out of the nozzle as a flame. The arc is independent of the work piece and the
work piece does not form a part of the electrical circuit. Just as an arc flame (as in atomic

ME 6004 UMP 42
hydrogen welding), it can be moved from one place to another and can be better controlled.
The non transferred arc plasma possesses comparatively less energy density as compared to a
transferred arc plasma and it is employed for welding and in applications involving ceramics
or metal plating (spraying). High density metal coatings can be produced by this process. A
non-transferred arc is initiated by using a high frequency unit in the circuit.
Transferred arc process:
The arc is formed between the electrode(-) and the work piece(+). In other words, arc is
transferred from the electrode to the work piece. A transferred arc possesses high energy
density and plasma jet velocity. For this reason it is employed to cut and melt metals. Besides
carbon steels this process can cut stainless steel and nonferrous metals also where
oxyacetylene torch does not succeed. Transferred arc can also be used for welding at high arc
travel speeds. For initiating a transferred arc, a current limiting resistor is put in the circuit,
which permits a flow of about 50 amps, between the nozzle and electrode and a pilot arc is
established between the electrode and the nozzle. As the pilot arc touches the job main
current starts flowing between electrode and job, thus igniting the transferred arc. The pilot
arc initiating unit gets disconnected and pilot arc extinguishes as soon as the arc between the
electrode and the job is started. The temperature of a constricted plasma arc may be of the
order of 8000 - 250000C.
Under water plasma cutting:
Plasma:
We normally think of three states of matter-solid, liquid and gas. One description of plasma is
that it is the fourth states of matter. For the most commonly known substance, water these

ME 6004 UMP 43
states are ice, water and steam. When added heat energy, the ice will change from solid to
liquid and if more heat is added it will change into gas (steam). When substantial heat is
added to gas, it will change from gas to plasma- the fourth states of matter.
Conventional Plasma ARC Cutting
The plasma jet generated by conventional "dry" arc constriction techniques was introduced in
1957 by Union Carbide's Linde Division. In the same year, Dr. Robert Gage obtained a
patent, which for 17 years gave Union Carbide a virtual monopoly. This technique could be
used to sever any metal at relatively high cutting speeds. The thickness of a plate could range
from thin sheet metal to plates as thick as ten inches (250 mm). The cut thickness was
ultimately dependent on the current-carrying capacity of the torch and the physical properties
of the metal. A heavy duty mechanized torch with a current capacity of 1000 amps could cut
through 10-inch thick stainless steel and aluminium. However, in most industrial
applications, plate thickness seldom exceeded two inches. In this thickness range,
conventional plasma cuts were usually beveled and had a rounded top edge. Beveled cuts
were a result of an imbalance in the heat input into the cut face. A positive cut angle resulted
because the heat energy at the top of the cut dissipated as the arc progressed through the cut.
This heat imbalance was reduced by placing the torch as close as possible to the work piece
and applying the arc constriction principle. Increased arc constriction caused the temperature
profile of the electric arc to become extended and more uniform.Correspondingly, the cut
became squarer. Unfortunately, the constriction of the conventional nozzle was limited by the
tendency of increased constriction to develop two arcs in series, one arc between the
electrode and nozzle and a second arc between the nozzle and work piece. This phenomenon
was known as "double arcing" and damaged both the electrode and nozzle. Double arcing
severely limited the extent to which plasma cut quality could be improved. Since the
introduction of the plasma arc process in the mid-50, considerable research has focused on
increasing arc constriction without creating double arcing. Plasma arc cutting as performed
then is now referred to as "conventional plasma cutting." It can be cumbersome to apply if
the user is cutting a wide variety of metals and different plate thicknesses. For example, if the
conventional plasma process is used to cut stainless steel, mild steel, and aluminum, it is
necessary to use different gases and gas flows for optimum cut quality on all three metals.
Conventional plasma cutting predominated from 1957 to 1970, and often required very
expensive gas mixtures of argon and hydrogen.
Underwater Cutting
Further attempts in Europe to decrease the noise level of the plasma arc and to eliminate
smoke development as much as possible led to underwater cutting. This method for high
power plasma cutting with cutting currents above 100 amps has become so popular that
today, many high power plasma cutting systems cut under water. For underwater plasma
cutting, the work piece is immersed about 2 to 3 inches under water and plasma torch cut
while immersed in the water. The smoke and noise level as well as the arc glare are reduced
dramatically. One negative effect of this cutting method is that the work piece cannot be
observed while cutting and the cutting speed is reduced by 10-20%. Further, the operator can
no longer determine from the arc sound whether the cutting process is proceeding correctly

ME 6004 UMP 44
and whether the consumables are producing a good quality cut. Finally, when cutting in
water, some water surrounding the cut zone is disassociated into oxygen and hydrogen, and
the freed oxygen has a tendency to combine with the molten metal from the cut (especially
aluminum and other light metals) to form metal oxide, which leaves free hydrogen gas in the
water. When this hydrogen collects in a pocket under the work piece, it creates small
explosions when reignited with the plasma jet. Therefore, the water needs to be constantly
agitated while cutting such metals.
EBM:
EBM is a thermal material removal process that utilizes a focus beam of electrons having a
high velocity to perform high speed drilling & cutting.
The EBM process begins after the work Pc is placed in the work chamber and a vacuum is
achieved. To create a hole by an electron beam occurs in 4 stages. First the electron beam is
focused on the work piece to a dia that is slightly smaller than the final desired
diameter.Power is adjusted that the electron beam will generate a power density at the work
piece in excess of 108 W/Cm2
.
Because of the shape of the electrostatic field formed by the grid cup, the electrons are
simultaneously pre focused electrostatically and pass as a converging beam through the hole
in the anode without colliding with the anode. As soon as the electrons pass through the
anode, they have reached their maximum velocity for a given accelerating voltage and will
maintain this velocity (the process takes place in a vacuum environment) until they collide
with the work piece.

ME 6004 UMP 45
Before the electron collide with the work piece a variable strength electron magnetic lens is
used to refocus the beam to any desired diameter to less than 0.00254 cm at precise location
on the work 26 piece and thus obtain an extremely high power density on the work piece.
Process parameters
Beam current, pulse duration, lens current and beam deflection signal are 4 (four) most
important parameters associated with EBM. Determination of initial setting is through trial &
error testing. Once established each parameter is computer controlled to maintain the process.
Beam current is continuously adjusted from 100 µ amp to 1 µ amp EBM machine available
that can generate pulse energy is excess of 120 joules / pulse, which is very high compared to
other process. The extremely available energy of higher magnitude can be used for any deep
drilling of large dia.
 Longer the pulse duration widen will be diameter and deeper drilling.
Process capability
 Materials can be processed by E.B.M are S.S, Cobalt, Alloys, Copper,
Aluminium, Titanium, Ceramics, Leather & Plastic.
 Because of high concentration of electron beam, the HAZ seldom exceeds 0.025
mm, No burr is generated on exit side of the hole. In entrance side a small lip of
solidified material may remain around the hole diameter.
 Hole can be drilled by high aspect ratio to the tune of 15: 1. Hole dia that can be
drilled range from 0.1 to 1.4 mm in thickness upto 100mm
 The tolerance on hole is typically ±5% of the diameter or 0.03 mm.

ME 6004 UMP 46
Advantages:
 EBM is an excellent process for micro finishing.
 Electrical conductor’s materials can be machined.
 Extremely close tolerances are obtained.
Disadvantages
 High capital equipment cost
 Long production time due to the time needed to generate a vacuum
 The presence of a thin recast layer
 Need for auxiliary backing material
Applications:
 EBM is mainly used for micro machining operations on thin materials.
 Drilling of holes in pressure differential devices used in nuclear reactors, air craft
engines etc.

Ucm class notes 18 19

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Ucm class notes 18 19