Wire electrical discharge machining (EDM) is a non-traditional machining process that uses electricity to cut any conductive material precisely and accurately with a thin, electrically charged copper or brass wire as an electrode. During the wire EDM process, the wire carries one side of an electrical charge and the workpiece carries the other side of the charge. When the wire gets close to the part, the attraction of electrical charges creates a controlled spark, melting and vaporizing microscopic particles of material. Plasma arc cutting (PAC) uses a plasma torch to direct a high-velocity jet of hot plasma from an ionized gas to cut electrically conductive materials. PAC systems operate on either a non-transferred arc mode or transferred arc

1. 1
Electrical Discharge Machining
(EDM)

2. PROCESS PRINCIPLES
• Electrical discharge machining (EDM) is a thermal
process that uses spark discharges to erode electrically
conductive materials.
• A shaped electrode defines the area in which spark
erosion will occur, thus determining the shape of the
resulting cavity or hole in the workpiece.

3. 3

4. 4
• Electrical discharge machining is a relatively simple
manufacturing process to set up and perform.
• The electrically conductive workpiece is positioned in
the EDM machine and connected to one pole of a
pulsed power supply.
• An electrically conductive electrode, shaped to match
the dimensions of the desired cavity or hole, is
connected to the remaining pole of the power supply.

5. 5
• The electrode and workpiece are then positioned
in such a way that a small gap is maintained
between the two.
• To provide a controlled amount of electrical
resistance in the gap, an insulating (dielectric)
fluid is flooded between the electrode and work-
piece.

6. 6

7. 7
• As illustrated in the sequence of drawings in
the figure, when a pulse of DC electricity is
delivered to the electrode and the workpiece,
an intense electrical field is created at the
point where surface irregularities provide the
narrowest gap.
• As a result of this field, naturally occurring
microscopic contaminants suspended in the
dielectric fluid begin to migrate and
concentrate at the strongest point in this
field.
• Simultaneously, negatively charged
particles are emitted from the workpiece.
Together these contaminants and particles
result in the formation of a high-conductivity
bridge across the gap.

8. 8
• As the voltage between the electrode
and workpiece increases at the
beginning of the pulse, the temperature
of the material making up the conductive
bridge increases.
• A small portion of the dielectric fluid
and charged particles in conductive
bridge vaporizes and ionizes resulting in
the formation of a spark channel
between the two surfaces.

9. 9
• At approximately the midpoint of the
electrical pulse, the power supply decreases
the voltage delivered to the gap, but raises
the current.
• This has the effect of increasing both the
temperature and pressure in the spark
channel.
• The extremely high temperature of the spark
melts and vaporizes a small amount of
material from the surfaces of both the
electrode and the workpiece at the points of
spark contact. Fed by the gaseous by-
products of vaporization, a bubble rapidly
expands outward from the spark channel.

10. 10
• When the electrical pulse is terminated, the spark
and heating action are stopped instantly.
• This causes both the spark channel and,
consequently, the vapor bubble to collapse.
• Small, rapidly solidified balls of material and gas
bubbles represent the residue from the cycle.
• The dielectric fluid acts to remove these by-
products from the gap.
• This entire sequence takes place in a period of only
microseconds to milliseconds.

11. 11
EDM System

12. 12
• D.C. Power Supply
• Frequency Control
• Work pan
• Servo Controlled Feed Workpiece
• Pressure Guage
• Flow meter

13. 13
Power Supply
• It transforms the alternating current (AC)
from the main utility electrical supply into the
pulsed direct current (DC) required to
produce the spark discharges at the
machining gap.
• First, the input power is converted into
continuous DC power by conventional solid-
state rectifiers.
• A small percentage of this DC power is used
to generate a square wave signal via a digital
multi-vibrator oscillator circuit.

14. 14
Dielectric System
• The EDM dielectric system consists of
the dielectric fluid, delivery devices,
pumps, and filters
• Various fluids are able to provide the
requisite properties of high degree of
fluidity and high electrical resistance.
• The most popular fluids, in order of
popularity, are transformer oil, paraffin
oil, kerosene, lubricating oils
hydrocarbon oil, silicon-based oils, and
de-ionized water.

15. 15
• 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.
• Therefore de-ionized water is most
often used when drilling small diameter
holes while using wire as the electrode.

16. 16
Regardless of the fluid being used, three
functions are performed by the dielectric
fluid.
1. It acts as an insulator between the electrode
and workpiece,
2. As a coolant to draw away the small amount
of heat generated by the sparks,
3. And as a flushing medium to remove the
metal by-products from the cutting gap.

17. 17
High di electric sterngth to minimize DC arcing.
No toxic gases should be formed during ionization.
Odourless and easily available

18. 18
• Of the three dielectric fluid functions,
flushing is by far the most critical for
optimum process efficiency.
• Poor flushing results in stagnation of the
dielectric fluid and a buildup of tiny
machining residue particles in the gap.
• Stagnation usually results in low material
removal rates or short circuits.

19. 19
Methods for flushing the dielectric fluid through the
cutting zone.

20. 20
• Any of these flushing techniques can be
performed exactly as illustrated in Figure, or
with the work piece submerged in a tank of
dielectric fluid.
• Whenever inflammable dielectric liquids are
being used, submersion of the work piece is
recommended to reduce the chances of
accidental dielectric fluid fires.
• Because it is more cost effective to reuse the
dielectric, it is usually cleaned, recycled, and
returned to the cutting gap. Pumps and
disposable filters perform this function.

21. 21
PROCESS PARAMETERS
The selection of EDM parameters is
important in determining the accuracy
and surface finish obtained for a
particular application.

22. 22
Spark frequency, current & surface finish
• As current is increased, each individual spark
removes a larger crater of metal from the work
piece. Although the net effect is an increase
in material removal rates, when holding all
other parameters constant it also has the
effect of increasing surface roughness.
• The same effect is also observed when spark
voltage is raised.
• Electrical discharge machining equipment is
available that is able to operate between 0.5
and 400 amp and with voltages ranging from
40 to 400 V DC.

23. 23
• Increasing spark frequency and holding all
other parameters constant, results in a
decrease in surface roughness.
• 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.

24. 24
Spark gap
• The gap between the electrode and
workpiece is determined by the spark voltage
and current.
• Typical values for the gap range from 0.012
to 0.050 mm (0.0005 to 0.002 in.).
• The smaller the gap, the closer the accuracy
with a better finish and slower material
removal rate.
• As the gap decreases, efficient flushing
becomes difficult to achieve.

25. 25
• Increasing the pulse duration of the
sparks has the effect of increasing the
removal rate, increasing the surface
roughness, and decreasing the
electrode wear.
• The values of pulse duration available
with currently available EDM machines
range from a few microseconds to
several milliseconds.

26. 26

27. 27
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

28. 28
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.

29. 29
APPLICATION
• 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

30. Wire EDM

31. 31

32. 32

33. Special form of EDM is wire EDM,
wherein the electrode is a continuously
moving conductive wire made from
copper, brass, tungsten or molybdenum.
(about 0.25mm in diameter).
The process is widely used for the
manufacture of punches, dies and stripper
plates.

34. 34
Wire electrical discharge machining (EDM) is a non-traditional machining process that
uses electricity to cut any conductive material precisely and accurately with a thin,
electrically charged copper or brass wire as an electrode.During the wire EDM process,
the wire carries one side of an electrical charge and the workpiece carries the other side
of the charge. When the wire gets close to the part, the attraction of electrical charges
creates a controlled spark, melting and vaporizing microscopic particles of material. The
spark also removes a miniscule chunk of the wire, so after the wire travels through the
workpiece one time, the machine discards the used wire and automatically advances
new wire. The process takes place quickly—hundreds of thousands of sparks per
second—but the wire never touches the workpiece. The spark erosion occurs along the
entire length of the wire adjacent to the workpiece, so the result is a part with an
excellent surface finish and no burrs regardless of how large or small the cut.Wire EDM
machines use a dielectric solution of deionized water to continuously cool and flush the
machining area while EDM is taking place. In many cases the entire part is submerged
in the dielectric fluid, while high-pressure upper and lower flushing nozzles clear out
microscopic debris from the surrounding area of the wire during the cutting process. The
fluid also acts as a non-conductive barrier, preventing the formation of electrically
conductive channels in the machining area. When the wire gets close to the part, the
intensity of the electric field overcomes the barrier and dielectric breakdown occurs,
allowing current to flow between the wire and the workpiece, resulting in an electrical
spark.

35. 35
On most wire EDM machines, the path of the wire is controlled by computer numerically-
controlled (CNC) diamond guides, which can move independently of each other on
multiple axes for tapered cuts and complex shapes such as small-radius inside corners
and narrow slots. Additionally, wire sizes vary from 0.012” diameter down to 0.004” for
high-precision work. Wire EDM is capable of holding tolerances as tight as +/-
0.0001”.Wire EDM provides a solution to the problems encountered when trying to
machine materials that are normally difficult to work with, such as hardened steel,
aerospace-grade titanium, high-alloy steel, tungsten carbide, Inconel, and even certain
conductive ceramics.
One requirement of the wire EDM process is a start hole for threading the wire if the
part’s features do not allow you to cut an edge. Wire EDM can only machine through
features; however, we can quickly drill a hole in any conductive material using another
type of EDM, small hole drilling or “hole pop” EDM.

36. 36

37. PLASMA ARC MACHINING
(PAM)
37

38. 38
WORKING PRINCIPLE
• A gas molecule at room temperature consists of
two or more atoms. When such a gas is heated to a
high temperature of the order of 2000°C or so, the
molecules separate out as atoms.
• If the temperature is raised to 3000°C, the
electrons from some of the atoms dissociate and
the gas becomes ionized consisting of ions and
electrons. This state of gas is known as plasma.

39. Plasma Generation
39

40. 40

41. 41
• Thus, plasma is the glowing, ionized gas that results
from heating of a material to extremely high
temperature.
• It is composed of free electrons dissociated from the
main gas atoms.
• A gas in plasma state becomes electrically
conductive as well as responsive to magnetism.
Because of such behavior, plasma is also known as
a fourth state of matter.

42. 42
• The temperature of plasma can be of the
order of 33,000°C.
• When such a high temperature source reacts with
work material, the work material melts out and
may even vaporize, and finally is cut into pieces.
• Many materials (say, aluminium, stainless steel,
etc) have high thermal conductivity, large heat
capacity, and/or good oxidation resistance. As a
result, such materials cannot be cut by
conventional techniques like oxy-fuel cutting.
• But these materials can be easily cut by plasma
arc cutting (PAC).

43. 43
PLASMA ARC CUTTING SYSTEM
• PAC system uses DC power source. PAC systems
operate either on non-transferred arc mode or
transferred arc mode (Fig. 9.1).
• In non-transferred arc mode, the thermal
efficiency is low (65-75%) and power is
transferred between the electrode and the nozzle.
This non-transferred arc ionizes a high velocity
gas that is streaming towards the work piece.
• The work piece may be electrically conductive or
non-conductive.

44. 44
TRANSFERRED Fig. 9.1 NON- TRANSFERRED

45. 45
• Fig. 9.1 Schematic diagram for non-transferred
and transferred arcs.
• In case of a transferred arc mode, the arc is
maintained between the electrode (negative
polarity) and the electrically conductive work
piece (positive polarity).
• Note that only electrically conductive work piece
can be machined or cut by transferred arc system.
The arc heats a co-axial flowing gas and maintains
it in a plasma state.

46. 46
• The electro thermal efficiency is up to 85-90%.
PAC system can deliver up to 1000 A at about 200
V (DC).
• The flowing gas pressure may be up to 1.4 MPa
resulting in a plasma velocity of several hundred
metres/second. Higher the gas flow rate, more will
be momentum of the plasma jet.
• It will ease out removal of the molten material
from the machining zone.
• The plasma jet is constricted by the flowing gas
which acts as a cooling agent sandwiched between
the nozzle wall and the plasma jet.

47. 47
• In case of PAC, the material may be removed
either by melting, or by melting and vaporization
both.
• In either case, the material (in molten state or
vaporized state) is blown off from the machining
zone by high velocity plasma jet.

48. 48
ELEMENTS OF PLASMA ARC CUTTING
SYSTEM
The important elements of a PAC system are
• Plasma torch.
• Power supply,
• Gas supply,
• Cooling water system,
• Control console

49. 49
Plasma torch
There are many torch designs which are practically
used, for example
 air plasma
 oxygen injected
 dual gas
 water injected plasma torch.

50. Air plasma torch
 Air plasma torch uses compressed air as the gas that ionizes
and does cutting.
 The air to be used should be uncontaminated.
 The nozzle of this torch may result in prematured failure
because of double arcing i.e, arcing between the electrode
and the nozzle, and between the nozzle and the work piece.
 Zirconium or hafnium are used as electrode material
because of their higher resistance to oxidation.
50

51. 51
Cathode
Nozzle
Sealing Cap
Carrier Gas (Air)
Coolant (Water)
Standoff Height
Workpiece

52. 52
Oxygen injected
• To avoid oxidation of electrode (or to enhance the life of the
electrode), oxygen injected torch (Fig. 9.3) uses nitrogen as the
plasma gas.
• Oxygen is injected downstream of the electrode. However, it
lowers down the nozzle life. This torch gives high MRR and
poor squareness of the cut edges. It is commonly used for mild
steel plate cutting.
• The presence of oxygen in the air helps in increasing MRR in
case of oxidizable materials like steel.

53. 53
Schematic diagram of oxygen-injected torch construction.

54. 54
Dual gas system
It uses one gas (nitrogen) as the plasma gas while another gas as
the shielding gas (02, C02, argon-hydrogen, etc). Secondary or
shielding gas is chosen according to the material to be cut.
Secondary gas system helps in maintaining sharp corners on the
top side of the cut edges.

55. 55
Constructional details of dual gas plasma torch

56. 56
• To avoid double arcing, the lower part of the nozzle is made of
ceramic.
• Water constriction helps in reducing smoke, enhancing nozzle life,
reducing HAZ, and limiting formation of oxides on the cut edges of
the workpiece.
water injected torch
water (pressure =1.2 MPa) is injected (radially or swirling vertically)
to constrict the plasma. A small quantity (about 10%) of water
vaporizes. This thin layer of steam constricts the plasma and also
insulates the nozzle. Nitrogen at about 1 MPa is used as the plasma.

57. 57
• Water muffler (a device that produces a covering of water
around the plasma torch and extends down to the work
surface) helps in reducing smoke and noise.
• Water mixed with a dye also absorbs part of the ultraviolet
rays produced in PAC.
• In some cases, a water table is also used to reduce the level of
noise and extent of sparks. Water below the workpiece
quenches sparks and damps sound level.
• Underwater PAC systems are also available which effectively
reduce the noise and smoke levels.

58. 58
(-)o-
Constructional details of water-injected plasma torch

59. Mechanism of metal removal
 Heating of workpiece is as a result of anode heating, due to
the electron bombardment plus convection heating from
high temp plasma that accompanies the arc.
 Approx 45% of electrical power delivered to torch is used
to remove metal from workpiece.
 Arc heat is concentrated on a localized area of w.p and it
raised it to its melting pt.
59

60. PAM Parameters
• 1. Design of DC plasma torches:
• 2 Physical Configuration
• 3. Work Environment
60

61. Process Characteristics
• Cutting rates : 250-1700mm/min
• Accuracy on width of slot and diameter of holes is
ordinarily from
+_ 0.8mm on 6-30 mm thick plates and
+ _3.0 mm on 100-150mm thick plate.
• Depth of HAZ depends on work material, its thickness and
cutting speed.
61

62. 1. Gas for Plasma generation
 Gas that does not attack tungsten electrodes or workpiece
can be used as plasma gas.
 For cutting Carbon and alloy steels- a mixture of N and H
with compressed air.
 For cutting stainless steel, aluminium and other non-ferrous
metals- mixtures of argon-hydrogen or N-H are used.
 Typical gas flow rates are 2 to 10 m3/hr.
62

63. 2. Cutting Rates
 250-1700 mm/min. which depends on thickness of metal
being cut.
 Sometimes water is injected into the jet which helps in
confining the arc, in blasting away the scale and smoke
reduction.
 Water injected plasma can increase cutting rate by 40–50%
 A 5mm thick carbon steel plate can be cut at 6000mm/min.
63

64. • 3. Surface finish and accuracy:
 It gives better accuracy than oxy- acetylene gas cutting. Cut
edges are round, with corner radius of about 4mm. There is
also problem of taper (about 2-5 degree).
 cut width is around 2.5 to 8mm.
 Accuracy + _ 0.8mm on 5 to 30mm plates
+_ 3.0mm on 100 to 150mm thick plates.
64

65. 65
Advantages of PAM
1. Plasma Arc Cutting produces a high
quality, dust free cutting.
2. Straight as well as curved shapes can
be cut easily.
3. The corrosion resistance of the
stainless steel is not effected when it
is cut with plasma arc.
4. The plasma arc cutting is the fastest
of all cutting processes.

66. 66
Disadvantages of PAM
1. It needs more electrical equipments;
hence chances of electrical hazards
are more.
2. The plasma arc produce a high noise.
The use of ear plugs or ear muffs to
protect the operator is essential.
3. The initial cost of process equipments
are very high.

67. 67
Safety Precautions
• Machine the heat affected zone
(0.75-5 mm).
• Regulate gas pressure (approx. 1-
1.4 MPa).
• Maintain constant distance between
torch and work piece.
• High labor safety (i.e. goggles,
gloves, etc…).
• Proper training for operators.
• Protection against glare, spatter and
noise from the plasma.

68. 68
Gases Used
 Primary Gases:
Gases that are used to create the plasma arc.
Examples are nitrogen, argon, hydrogen,
hydrogen, or mixture of them
 Secondary Gases or Water:
Surrounds the electric arc to aid in confining it
and removing the molten material.

69. 69
System Components
 Torch
 Power Supply
 Arc Starting Circuit
www.twi.co.uk/j32k/servlet/ getFile/jk51.html

70. 70
System Components
 The torch is the
holder of the
consumable electrode
and nozzle.
 Responsible for
forming the arc and
maintain it in a vortex.
A. The Torch:
Groover 626

71. 71
System Components
 Constant DC current
source.
 Speed and cut
thickness are
determine by the
amount of output
current.
B. Power Supply:

72. 72
 High frequency
generator circuit that
produces a high AC
Current.
 To start the arc, the AC
current ionizes the
cutting gas, which
makes it conductive to
allow the DC current to
flow through it.
System Components
C. Arc Starting Circuit:

73. 73
Advantages - Disadvantages
• Cuts any metal.
• 5 to 10 times faster
than oxy-fuel.
• 150 mm thickness
ability.
• Easy to automate.
• Large heat affected
zone.
• Rough Surfaces
• Difficult to produce
sharp corners.
• Smoke and noise.
• Burr often results.

74. 74
Applications
• Pipe industry –
preparing pipe
edges for welding.
• industries for shape
cutting Cpam.engr.wesc.edu

75. 75
Other Plasma Uses
• Plasma Arc Welding (PAW)- plasma arc is
produced and aimed at the weld area to weld.
• Applications- Used for butt and lap joints
because of higher energy concentrations and
better arc stability.