Electrical Discharge Machining (Non-Traditional Machining)

1. Electrical Discharge
Machining
Module 4
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2. Principle of EDM
• Spark machining or spark erosion machining
• It is a controlled metal removal process based on the
principle of erosive effects of electrical discharges
(Sprk) taking place between two electrically
conducting materials immersed in a dielectric fluid.
• One of the conducting materials tool electrode (tool)
• Other is workpiece electrode (workpiece)
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3. Cont…
• The shape of the tool is similar to that desired w/p.
• The tool and the w/p are separated by dielectric fluid
and connected to DC power supply to create a
potential difference between the tool and the work
piece.
• When the potential difference high the dielectric
fluid in the gap is ionized under the pulsed
application of direct current. Thus enabling a spark
discharges to pass between the tool and w/p
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4. Cont…
• Metal removal from the w/p takes place due to
erosion caused by the electric spark.
• The amount of material removal is very small, and is
flushed away with the continuously flowing fluid.
• The downward movement of the tool will produce
the desired shape on the w/p.
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5. Need of EDM
• Extremely hard material like tungsten or tungsten
carbide are difficult to machine with conventional
machining process.
• Very small w/p may get damaged due to excessive
cutting tool pressure
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6. EDM Operation
• Tool shaped electrode is connected to the – ve
terminal
• Work piece is connected to + ve terminal
• Tool and w/p separated by small gap known as spark
gap filled with dialectic fluid.
• Spark gap usually ranging from 0.01 – 0,05 mm
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7. Cont…
• When potential differences is sufficiently high, a
transient spark discharges through the fluid
removing a very small amount of materials from the
workpiece.
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8. TSN JSSATEB

9. Mechanism of Metal Removal
• Initially the gap between the tool and work piece,
which consists of the dielectric fluid, is not
conductive, but under the pulsed application of DC,
the dielectric fluid in the gap is ionized, causing the
spark to discharge between the tool & the workpiece.
• The spark impinges on the elevated surface of the w/p
at a very high temperature around 10,000º C causing a
small portion of the w/p to melt / vaporize.
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10. Cont…
• The force of electric and magnetic fields caused by
the spark produce a tensile force resulting in tearing
of partials of molten and softened metal from the
work surface thereby causing metal removal to take
place
• The continuously flowing fluid flushes away the
excess material removed from the machining gap.
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11. Cont…
• Only one spark will be made between the tool and
w/p.
• The resulting spark in EDM process causes erosion to
take place on both the tool, as well as the w/p.
• Tool eroded less because the tool tip is subjected to a
compressive force by the spark, which helps reduce
tool wear.
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12. EDM Equipment
• Power supply / Power generator
• Dielectric medium
• Tool electrode
• Servo feed mechanism
• Pumps and filters
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13. Power supply / Power Generator
• Electrical energy in the form of short duration
impulses are to be supplied at the machining gap
between the tool and w/p
• Direct current pulse power generator are used to
achieve the purpose.
• Different EDM generator
- RC ( Resistance – Capacitance ) type or Relaxation generator
- Rotary impulse type generator
- Controlled pulse circuit ( Static pulse generator)
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14. RC ( Resistance – Capacitance ) type or Relaxation
generator
The relaxation or RC type of generator was the first to
be used in EDM process.
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15. Cont…
• The circuit consists of a DC Power source that
charges a capacitor C across a Resistance R, the
saw tooth wave form thus generated.
• When a potential differences is applied, current
will flow into the circuit and the capacitor get
charged to a value Vc which in turn discharges its
chare to the gap between the tool and the
workpiece and hence relaxed to zero voltage.
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16. Cont…
• A spark will be established between the tool and
w/p leading to machining.
• The dielectric fluid in the gap gets deionized.
• Once the voltage becomes too low to sustain the
spark, the charging of the capacitor would
continuous and the cycle repeats itself
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17. Cont…
• The spark repetition rate for a given supply voltage
and capacitance, cannot be increased beyond a
critical value.
• the spark repetition rate is determined by the speed
at which the spark gap is deionized and cleared the
m/c ing part .
• MRR Depends on
– Supply voltage and break down voltage
– Charging resistance
– Capacitance
– Gap
– Dielectric strength TSN JSSATEB

18. Dielectric fluid
• In EDM process, the tool and the w/p are separated
by a dielectric fluid
• Which may be deionized water, transformer oil,
paraffin oil, kerosene, lubricating oils
• De-ionized water is rarely used because, although it
results in high material removal rates and increased
cooling capacity, it also results in undesirably high
electrode wear rates.
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19. Functions
• It acts as an insulator between the electrode
and workpiece,
• As a coolant to draw away the small amount of
heat generated by the sparks,
• And as a flushing medium to remove the metal
by-products from the cutting gap.
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20. Dielectric fluid requirements
• Possess sufficiently high dielectric strength to remain
electrically non conductive until the required
breakdown voltage is reached.
• Deionize rapidly after the spark discharges has taken
place.
• Be chemically neutral so as not to attack the tool,
workpiece and m/c equipment.
•
• Posses high flash point to avoid any fire hazards
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21. Cont…
• Possess high viscosity for easy circulation and
wetting capacity
• Should not emit any toxic vapors or have unpleasant
odors
• Be cheap and easily available
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22. Flushing
• Wear debris in the spark gap must be removed
continuously for efficient machining .
• This is achieved by a technique known as
flushing.
• “ Flushing is a process of circulating the dielectric
fluid between the tool electrode and workpiece in
a proper way so as to maintain a clean
environment for efficient machining of the
workpiece”
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23. Methods of flushing
1. Injection or Pressure Flushing
2. Suction Flushing
3. Side Flushing
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24. Injection or Pressure Flushing
• The dielectric fluid is injected continuously into
the spark gap either through the hole drilled in the
workpiece or tool.
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25. Suction Flushing
• To overcome the tapering effect on components
resulting from injection flushing, suction flushing
is employed
• Dielectric fluid is sucked either through the pre
drilled tool or w/p
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26. Side Flushing
• Side flushing method is adopted when it is
impossible to drill flushing holes either in the tool or
w/p
• Using nozzle by carful adjustment of force required,
fluid is supplied to the periphery of the tool electrode.
• Care should be taken to allow the fluid parallel to the
w/p, to avoid the turbulence
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27. Electrodes.
• The EDM electrode is the tool that determines the
shape of the hole or cavity ultimately generated.
• The most important considerations pertaining to
electrode selection are the electrode material and
design.
• The requirements for good Electrode
Readily available,
Easily machinable,
Exhibit low wear,
Electrically conductive,
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28. Cont…
• Copper and brass are two commonly used
materials that meet most of these criteria,
although they exhibit a relatively high wear rate.
Copper tungsten provides much lower rates but is
a more difficult material to machine.
• Graphite and copper are by far the most versatile
electrode materials. Both are easily machined and
are available in various grades for application to
all work piece materials.
• Because of the temperature at which graphite
vaporizes is so much higher than any metal, the
wear rates of graphite electrodes are extremely
low. TSN JSSATEB

29. Cont…
• Grain size, the most important property of graphite
electrodes, governs wear rate, surface finish, and
removal rates.
• When a graphite electrode is composed of very small
graphite grains, as compared with coarse grains, the
electrode wear rate is decreased, surface finish is
improved, and removal rates are increased.
• The biggest draw back of using graphite for
electrodes is its brittleness. Generally, a metal wire
electrode is best for drilling small holes because of
the breakage risk associated with thin graphite
electrodes.
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30. Servo system
• The servosystem is commanded by signals from the
gap voltage sensor system in the power supply and
controls the infeed of the electrode or workpiece to
precisely match the rate of material removal.
• If the gap voltage sensor system determines that a
piece of electrically conductive material has bridged
the gap between the electrode and workpiece, the
servosystem will react by reversing direction until
the dielectric fluid flushes the gap clear.
• When the gap is clear, the infeed resumes and
cutting continues.
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31. TSN JSSATEB

32. Cont…
• The selection of the dielectric flushing technique
has a direct effect on the function of the servo
system.
• If the flushing technique being used is inefficient in
removing the process by-products from the cutting
gap, the servo system may have to spend most of
the time reversing to clear the cutting gap.
• This results in extremely long cycles. However if
the flushing technique being used effectively
removes the by products, the servo system will
spend almost no time retracting, resulting in much
faster cycles. TSN JSSATEB

33. EDM Process Parameters
The important parameters that effects on the accuracy
and surface finish.
1. Influence of spark frequency
2. Influence of current
3. Spark gap
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34. Influence of spark frequency
• Decreasing the current and increasing its
frequency result in improved surface finish
ie energy available for metal removal during a
given period is shared by a large number of
spark
Increasing spark frequency and holding all other
parameters constant, results in a decrease in
surface roughness.
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35. Cont…
• This is because the energy available for material
removal during a given period is shared by a
larger number of sparks, hence the corresponding
crater size is reduced.
• The frequency capability of EDM machines
ranges from a low of 180 Hz when performing
roughing cuts, to a high of several hundred
kilohertz when generating the fine finishes
required for finishing cuts.
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36. Influence of current
• MRR and Surface finish depends on the current
in the discharge spark
• Current increases each spark removes a larger
material from w/p
• Increases in current leads to poor surface finish
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37. Spark gap
• Gap between the tool and w/p ranges from (0.01 –
0.05 mm).
• Smaller the gap, higher the accuracy with a good
surface finish.
• Efficient flushing becomes difficult to achieve with
small gaps
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38. Heat Affected Zone in EDM
• In EDM Metal removal form w/p takes place due to
melting and vaporization of the work material by the
energy of electric spark
• The temperature of discharged spark is in the rage of
8000 to 12000° C
• High temperature may result in microstructure
changes leading to changes in mechanical properties
of the machined surface.
• Microscopic study identified three layers
- Recast layer - Heat Affected Zone
- Converted layer
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39. Cont…
.
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40. 1. Recast layer
• During m/c ing, if the molten material from the
w/p is not flushed out quickly, it will re solidify
and became harder due to cooling effect of
dielectric fluid
• This thin adhered layer of about 2 – 40 µm it is
know as recast layer
• It extremely hard, brittle and porous and contains
micro cracks
• This layer must be removed by secondary m/c ing
process before the work part put into use
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41. 2. Heat affected zone
• Around the recast layer is the heat affected zone
formed due to rapid heating and quenching cycles
during machining.
• This layer is of about 25 µm thick
• The heating and cooling cycle and diffused material
during machining are responsible reasons for
presence of this zone.
• Thermal residual stress, grain boundary cracks are
some of the effects related due to this zone
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42. 3. Converted zone
• Around the heat affected zone can be found the
converted zone.
• In this layer a change in grain structure from the
original structure is apparent.
• Around the converted zone lies the unaffected base
layer which remains in the same condition as it was
prior to EDM Process
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43. Advantages
• By this process, materials of any hardness can be
machined.
• No burrs are left in machined surface.
• Thin and fragile/brittle components can be
machined without distortion;
• Complex internal shapes can be machined
• Although the metal removal in this case is due to
thermal effects, there is no heat in the bulk of the
material TSN JSSATEB

44. Disadvantages
• This process can only be employed in electrically
conductive materials;
• Material removal rate is low and the process
overall is slow compared to conventional
machining processes;
• Unwanted erosion and over cutting of material
can occur;
• Rough surface finish when at high rates of
material removal.
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45. Applications
• Useful in machining of small holes, orifices, slots in
diesel fuel injection nozzles, airbrake valves and
aircraft engines etc.
• Blind cavities and narrow slots in dies, minimum
diameter hole can be produced.
• Mold making
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46. Electrical Discharge Grinding (ECG)
• A rotating wheel electrode is used
• Grinding process takes place without the use of
abrasive particles
•
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47. Cont…
• The tool electrode is in the from of a rotating wheel
made up of graphite, brass , steel etc
• The wheel will rotates about horizontal axis
• Wheel and w/p is separated by dielectric fluid
• The gap between tool and w/p is maintained by a
servo control mechanism
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48. Cont…
• When sufficient pulse DC power supply is applied,
the insulative properties of dielectric fluid are
momentarily brown down, ther by allowining a
small spark will generate b/t tool and w/p.
• The thermal energy of the spark melts a small
portion of the w/p at the point where spark makes
contact with the w/p
• The rotating motion of the wheel causes the molten
metal to ejected from the w/p
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49. Travelling wire EDM / Wire EDM
• The process makes use of a travelling wire made from
copper or brass, (Ø= 0.05 – 0.3 mm) as tool electrode.
• The wire unwinds from spool, feeds through the w/p
and is taken up on a second spool with constant wire
tension.
• The movement of wire is precisely by CNC system
• Deionized water is used as the dielectric fluid and
injected through the nozzle coaxially with the wire
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50. TSN JSSATEB

51. Cont…
• A small gap of about 0.025 -0.05 mm b/t w/p and
wire
• When sufficient voltage is applied, the dielectric
fluid ionizes and spark will generates
• Spark precisely erode a small portion of w/p
causing it to melt and vaporize
• The circulating dielectric fluid cools the
vaporized metal and forces the solidified eroded
particles from the gapTSN JSSATEB

52. Cont…
• The eroded particles coming along with the
deionized water
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53. Plasma Arc Machining (PAM)
• Plasma arc machining or Plasma arc cutting (PAC)
is a thermal m/c ing process.
• Used for cutting thick sections of electrically
conductive materials.
“ The principle of the process is based on utilizing a
high temperature plasma, which is generated by
heating gases to elevated temperature”.
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54. Cont…
• “The plasma in this state is composed of positive
ions, neutral atoms and free electrons that have
become disassociated from the main gas atoms”
• The temperature of plasma can be as high as
30,000°C .
• This heat can be utilized to melt and cut rapidly any
type of material.
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55. Need for PAM
• Thick metal ( drilling, sawing, machining,
punching)
• It is Economical for speed and quality on carbon
steel, aluminum, stainless steel
Compared to conventional m/c ing
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56. PAM Equipment
• Power supply
• Gas supply unit
• Cooling water system
• Control elements
• Plasma torch
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57. TSN JSSATEB

58. Plasma torch / plasma gun / plasma cutter
• Is a device in which a flowing gas is passed
through an electric arc, producing a high velocity
jet of high temperature ionized gas called plasma.
• The torch carries a 2% thoriated tungsten electrode
connected to the negative terminal of a DC power
supply.
Note : A vacuum-tube filament consisting of tungsten
mixed with a small quantity of thorium oxide to
give improved electron emission. Also known
as thoriated emitter
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59. Cont…
• The tungsten electrode acts as a cathode
• The nozzle is connected to the positive terminal
of the power supply through a suitable resistor
to limit the current through the nozzle to about
50 Amp
• The w/p is connected directly to the positive
terminal of the power supply
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60. TSN JSSATEB

61. Gas supply and cooling water system
• On one side of the torch is provided a passage for
the supply of gas into the chamber
• The type of gas used depends on the type of w/p
material
• A provision for circulating the water around the
torch is provided in order to cool the electrode and
the nozzle during the operation
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62. Commonly used gas mixture
Sl.No Material to be cut Gas/ Gas mixture
1 Aluminum and
Magnesium
Nitrogen, Nitrogen-
Hydrogen mixture
Argon – Hydrogen mixture
2 Stainless steel and
non ferrous metal
Nitrogen- Hydrogen
mixture
Argon – Hydrogen mixture
3 Carbon and alloy
steels, Cast Iron
Nitrogen- Hydrogen
mixture and compressed air
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63. Operation
• A strong arc is struck between the tungsten electrode
( cathode) and the nozzle ( anode) and mean while a
suitable gas is forced into the chamber.
• Gas molecules collide with the high velocity
electrons of arc , the gas get ionized and very large
amount of heat energy is evolved.
• This high velocity stream of hot ionized gas is called
plasma
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64. Cont…
• The maximum velocity of the jet is around 550
m/sec and temperature is as high as 28,000°C.
• The arc is maintained stable, so that it heats the
flowing gas and maintains it in plasma state
• The nozzle narrow down the high velocity jet of
high temperature gas into a small c/s of w/p
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65. Cont…
• The high heat of the gas is sufficient to raise the
w/p temperature above melting point.
• High velocity gas stream effectively blows the
molten metal away resulting in metal removal
from w/p.
• The torch may be water or gas cooled, with the
coolant usually flowing through the electrode and
around the nozzle to dissipate the high heat of the
plasma.
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66. Mechanism of metal removal
• The metal removal is basically due to the high
temperature produced.
• The heating of the work piece is, as a result of
anode heating, due to direct electron
bombardment plus convection heating from the
high temperature plasma that accompanies the
arc.
• The heat produced is sufficient to raise the work
piece temperature above its melting point and the
high velocity gas stream effectively blows the
molten metal away.
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67. Plasma torch design
It includes
Nozzle design
Electrode
Plasma gas
Insulation
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68. Nozzle design
• Nozzle focuses the stream of plasma to produce the
desired cut
• Nozzle are designed so that the orifice is slightly
larger than the stream of plasma being focused
• This helps the nozzle to contain and focus the vortex
of plasma without being affected by it
• The length of the nozzle bore is directly related to
the quality of cut achieved.
• Long bore generally yields a high density arc and
superior cut
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69. Electrode
• Cathode electrode is heated by the current passing
through it
• Cathode heating is a function of electrode diameter
and current passing through it
• Excessive heating may cause melting of the
electrode and additional cooling becomes
compulsory
• For large arc voltage it is preferable to increases the
angle of taper at the cathode tip
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70. Cont…
• For large arc voltage it is preferable to increases the
angle of taper at the cathode tip
• For transferred arc mode flat face cathode is good
• The taper of the cathode electrode and convergence
of the nozzle should be such that the arc
terminates at the tip of the cathode electrode
• The tip is hence designed to be the closest point to
the anode
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71. Plasma Gas
• Plasma torch operation is highly depends on the type
of gases being used
• Different gases have different specific heats, thermal
conductivities
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72. Insulation
• Some portion of the positive and negative section of
the torch must be electrically insulated due to high
frequency voltage used for ignition.
• The insulator portion must not be nearer to the arc
zone in order to avoid any damage to the insulator
by radiation or convection heat transfer
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73. Process Parameters
• Torch work piece distance ( Stand Off) and current
• Gas flow rate
• Cutting speed
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74. Torch work piece distance ( Stand Off) and
current
• With increases in thickness of w/p the current and
torch work distance need to be increase
• As stand of distance increases the arc diverges and
spreads over a wide area leading to low thermal
intensity of plasma arc.
• Close stand off distance can promote arcing
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75. Gas flow rate
• For thicker material high gas flow and high current
is required
• Gas flow need to set according to the current level
and nozzle bore diameter
• If gas flow is too low for current level / current
level is too high for the nozzle uneven arc will
generate
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76. Cutting speed
• The amount of heat transferred by the plasma can be
controlled by the surface cutting speed.
• If the speed is too high, then the upper edge of the
contact face comes closer to the plasma jet and
receives excessive heat it leads to high MRR
• Surface finish is not good at high cutting speed
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77. Safety Precautions
• Plasma flame emits UV and IR radiations, which is
harmful to human
• Safety eye glasses ,hand shields, gloves and head
resistant cloths must be worn during the operation
• It is necessary to carry out in ventilated room in
order to avoid inhaling of fumes and gases
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78. Cont…
• Gases like oxides of nitrogen pose significant hazard
to operator.
• Gas cylinder and their pressure need to be monitored
regularly
• Electric cable supply, power source supply gas leaks
must be check prior to operation
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79. Advantages
• The main advantage of PAM is speed. For
example, mild steel of 6mm thick can be cut at
3m/min
• The plasma arc can be used to cut any metal or
even to non- conducting materials like concrete
etc., since it is primarily a melting process
• Due to high speed of cutting the deformation of
sheet metals is reduced while the width of the cut
is minimum
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80. Cont…
• Smooth cuts free from contaminants are obtained
in the process
• Profile cutting of metals especially of stainless
steel and aluminium can be very easily done by
PAM
• Operating costs are less when compared to oxy-
fuel torch
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81. Disadvantages
• The main disadvantage of PAC is the high initial
cost of the equipment.
• Well-attached drops on the underside of the cut
can be a problem and there will be heat- affected
zone (HAZ).
• The depth of HAZ depends on the material and its
thickness
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82. Cont…
• Smoke and noise
• Sharp corners are difficult to produce because of
the wide diameter of the plasma stream
• Burr is often produced
• Taper on the work piece may occur
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83. Applications
• Used to cut stainless steel and aluminium alloys.,
magnesium, titanium, copper, nickel and alloys of
copper and nickel
• PAC can be used for stack cutting, plate beveling,
shape cutting
• It can also be used for underwater cutting.
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84. • The plasma jets are used for welding materials
like titanium, stainless steel etc.,
• Plasma arc is used for depositing filler metal on
surface to obtain desired properties like
corrosion resistance, wear resistance, toughness
or anti-friction properties – Plasma arc surfacing
• The plasma arc can also be used for spraying a
prepared surface of the base material with
droplets of molten metal to obtain a surface of
required thickness
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