Complete and Detailed Explanation of EDM

1. • INTRODUCTION
• PRINCIPLE
• CONSTRUCTION
• TOOL MATERIAL
• FLUID PROPERTIES
• WORKING
• EFFECTS OF EDM ON METAL SURFACE
• ADVANTAGES
• DISADVANTAGES
• APPLICATIONS

4. Electro Discharge Machining (EDM) is an electro-thermal non-traditional machining
process, where electrical energy is used to generate electrical spark and material removal
mainly occurs due to thermal energy of the spark.
EDM is mainly used to machine difficult-to-machine materials and high strength
temperature resistant alloys. EDM can be used to machine difficult geometries in small
batches or even on job-shop basis. Work material to be machined by EDM has to be
electrically conductive.
INTRODUCTION

5.  A potential difference is applied between the tool and work-piece (both are to be
conductors of electricity).
 The tool and the work material are immersed in a Dielectric medium.
 A gap is maintained between the tool and the work-piece (Spark Gap).
 Depending upon the applied potential difference and the gap between the tool and work-
piece, an electric field would be established.
 Tool is connected to the negative terminal of the generator and the work-piece is
connected to positive terminal.
 Due to electric field between the tool and the job, the free electrons on the tool are
subjected to electrostatic forces.
EDM Process
Kerosene
Deionised water

6.  IF Work function or Bonding energy of the electrons is less, then electrons will emit from
the tool (assuming it to be connected to the negative terminal).
 Emission of electrons are called or termed as “Cold emission”.
 “Cold emitted” electrons are accelerated towards the job through the dielectric medium.
 Electrons gain velocity and energy ---------------collisions occurs between the electrons and
dielectric molecules.
 Collisions result in “Ionisation” of the dielectric molecule”.
???? Ionization is the process by which an
atom or a molecule acquires a negative
or positive charge by gaining or losing
electrons to form ions, often in
conjunction with other chemical
changes.

7. Ionisation process depends on Ionisation energy of the dielectric molecule and energy
of the electron.
The ionization energy (IE) is qualitatively
defined as the amount of energy required to
remove the most loosely bound electron, the
valence electron, of an isolated gaseous atom
to form a cation.
 More positive ions and electrons would get generated due to collisions.
 This process would increase the concentration of electrons and ions in the dielectric
medium between the tool and the job at the spark gap.
 The concentration is very high that the matter existing in that channel could be
characterised as “plasma”.

8.  The electrical resistance of plasma channel will be very less.
 Electrons will flow from the tool to the job and ions from the job to the tool. This
flowing is called avalanche motion of electrons.
Movement of electrons and ions can
be visually seen as a spark.
Electrical energy is dissipated finally as
thermal energy of the spark. 10,000oC

9.  Localized extreme rise in temperature leads to material removal by vapourisation of the
material as well as due to melting.
 Potential difference is applied periodically as shown below………
Plasma channel
collapses regularly
Pressure or shock waves are generated, which
removes the molten material forming a crater
around the site of the spark.

10.  Dielectric flushes the eroded particles or Debris from the machining gap
Mechanism of material removal

11. EDM spark description

12. 1. Dielectric reservoir, pump and circulation system
2. Power generator and control unit
3. Working tank with work holding device
4. X-y table accommodating the working table
5. The tool holder
6. The servo system to feed the tool
Components of EDM

14. A dielectric (or dielectric material) is an electrical insulator that can
be polarized by an applied electric field.
What is Dielectric??

15. Dielectric fluids
Functions of the dielectric fluid :
1. Flush the eroded particles from the machining gap
2. Provide insulation between the electrode and the work-piece
3. Cool the section that was heated by the discharging effect
The main requirements of the EDM dielectric fluids
1. Adequate viscosity
2. High flash point
3. Good oxidation stability
4. Minimum odor
5. Low cost
6. Good electrical discharge efficiency
Kerosene is used with
certain additives gas
bubbles and de-odoring

16. Other dielectric Fluids used are
 Silicon fluids with a mixture of petroleum oils.
 Aqueous solutions of ethylene glycol
 Water in emulsions
 Distilled water.
Note :
 Flushing of the dielectric plays a major role in the maintenance of stable machining and
the achievement of close tolerance and high surface quality.
 Inadequate flushing can result in arcing, decreased electrode life, and increased
production time.

17. Four different methods are used for introducing dielectric fluid to the
machining gap
1. Normal flow
2. Reverse flow
3. Jet flushing
4. Immersion flushing

18.  Dielectric fluid is introduced, under pressure, through one or more passages in
the tool
and is forced to flow through the gap between the tool and the workpiece.
 Flushing holes are generally placed in areas where the cuts are deepest.
 Normal flow is sometimes undesirable because it produces a tapered opening in the
workpiece.
1. Normal flow

19. 2. Reverse flow
 This method is particularly useful in machining deep-cavity dies.
 The taper produced using the normal flow mode is reduced.
 The gap is submerged in filtered dielectric, and instead of pressure being applied at
the source a vacuum is used.

20. 3.Jet flushing
 In many instances, the desired machining can be achieved by using a spray or
jet of fluid directed against the machining gap
 Machining time is always longer with jet flushing than with the normal and
reverse flow modes.
4.Immersion flushing
 For many shallow cuts or perforations of thin sections, simple immersion of the
discharge gap is sufficient.
 Cooling and machining debris removal can be enhanced during immersion cutting
by providing relative motion between the tool and workpiece.

21.  Flushing through the tool is more preferred than side flushing.
 Many small flushing holes are better than a few large ones.
 Steady dielectric flow on the entire workpiece-electrode interface is desirable.
 Dead spots created by pressure flushing, from opposite sides of the workpiece,
should be avoided.
 A vent hole should be provided for any upwardly concave part of the tool-
electrode to prevent accumulation of explosive gases.

22. Electrode material
 It should not undergo much tool wear when it is impinged by positive ions.
 Rise of localised temperature need to be less by tailoring or properly choosing its properties
or even when temperature increases, there should be less melting.
Basic characteristics of electrode materials are:
1. High electrical conductivity – electrons are cold emitted more easily and there is less bulk
electrical heating
2. High thermal conductivity – for the same heat load, the local temperature rise would be
less due to faster heat conducted to the bulk of the tool and thus less tool wear
3. Higher density – for the same heat load and same tool wear by weight there would be less
volume removal or tool wear and thus less dimensional loss or inaccuracy
4. High melting point – high melting point leads to less tool wear due to less tool material
melting for the same heat load
5. Easy manufacturability and cheap

23. The followings are the different electrode materials which are used commonly in the
industry:
• Graphite
• Electrolytic oxygen free copper
• Tellurium copper – 99% Cu + 0.5% tellurium
• Brass

24. Heat-affected zone
 As the temperature of the discharges reaching 8000 to 12,000°C, metallurgical
changes occur in the surface layer of the work-piece.
 Some annealing of the work-piece can be expected in a zone just below the machined
surface.
 Not all the work-piece material melted by the discharge is expelled into the dielectric.
 Remaining melted material is quickly chilled, primarily by heat conduction into the bulk
of the work-piece, resulting in an exceedingly hard surface.
 The depth of the annealed layer is proportional to the amount of power used in the
machining operation. It ranges from 50 μm for finish cutting to approximately 200 μm for
high metal removal rates.

25.  Annealing effect can be reduced by choosing electrodes that produce more stable
machining.
 A finish cut removes the annealed material left by the previous high-speed roughing.
 The altered surface layer, which is produced during EDM, significantly lowers the
fatigue strength of alloys.
 The altered layer consists of a recast layer with or without microcracks, some of which
may extend into the base metal, plus metallurgical alterations such as rehardened and
tempered layers, heat-affected zones, and intergranular precipitates

26.  Processes which restores fatigue properties are:
1. Removal of the altered layers by low-stress grinding
2. Chemical machining
3. Metallurgical-type coating, reheat treatment
4. Shot peening.

27. A dielectric (or dielectric material) is an electrical insulator that can
be polarized by an applied electric field.
What is Dielectric??

28. Dielectric fluids
Functions of the dielectric fluid :
1. Flush the eroded particles from the machining gap
2. Provide insulation between the electrode and the work-piece
3. Cool the section that was heated by the discharging effect
The main requirements of the EDM dielectric fluids
1. Adequate viscosity
2. High flash point
3. Good oxidation stability
4. Minimum odor
5. Low cost
6. Have high dielectric strength ( remains electrically non-conductive until the required
Break down voltage between the electrodes attained)
7. High degree of fluidity
Kerosene is used with
certain additives---- gas
bubbles and de-
odoring

29. Other dielectric Fluids used are
 Silicon fluids with a mixture of petroleum oils.
 Aqueous solutions of ethylene glycol
 Water in emulsions
 Distilled water.
Note :
 Flushing of the dielectric plays a major role in the maintenance of stable machining and
the achievement of close tolerance and high surface quality.
 Inadequate flushing can result in arcing, decreased electrode life, and increased
production time.

30. Six different methods are used for introducing dielectric fluid to the machining
gap
1. Normal flow
2. Reverse flow
3. Jet flushing
4. Immersion flushing
5. Ultrasonic vibration of electrodes
6. Rotating electrode flushing

31.  Dielectric fluid is introduced, under pressure, through one or more passages in the tool
and is forced to flow through the gap between the tool and the workpiece.
 Flushing holes are generally placed in areas where the cuts are deepest.
 Normal flow is sometimes undesirable because it produces a tapered opening in the
workpiece.
1. Normal flow (Pressure through electrodes)

32. 2. Reverse flow (Suction through electrodes)
 This method is particularly useful in machining deep-cavity dies.
 The taper produced using the normal flow mode is reduced.
 The gap is submerged in filtered dielectric, and instead of pressure being applied at the
source a vacuum is used.

33. 3.Jet flushing
 In many instances, the desired machining can be achieved by using a spray or
jet of fluid directed against the machining gap
 Machining time is always longer with jet flushing than with the normal and
reverse flow modes.
 Used only when none of the other methods can be used due to tool or work
configuration
4.Immersion flushing
 For many shallow cuts or perforations of thin sections, simple immersion of the
discharge gap is sufficient.
 Cooling and machining debris removal can be enhanced during immersion cutting
by providing relative motion between the tool and workpiece.

34.  Flushing through the tool is more preferred than side flushing.
 Many small flushing holes are better than a few large ones.
 Steady dielectric flow on the entire workpiece-electrode interface is desirable.
 Dead spots created by pressure flushing, from opposite sides of the workpiece,
should be avoided.
 A vent hole should be provided for any upwardly concave part of the tool-
electrode to prevent accumulation of explosive gases.

35. Power Supply or Power Generators
1. R.C Circuits
2. Rotary Pulse Generators
3. Static Pulse Generators

36. 2. Rotary Pulse Generators
 To increase the metal removal rates, Motor Generators
are developed to supply the required Machining power
for EDM.
 During Operation the capacitor is charged by Diode on
half cycle .In the following half cycle, the sum of
the Voltage from the generator and charged capacitor
Is applied to the gap
Produces high MRR but produces very
rough surfaces
Why ???

37. 3.Static Pulse Generators or Controlled Pulse Circuits
In previous two Generators one element of control is lacking ……………
Is that the ability to cut off the current in case of short circuit
In both cases the electrode need to be removed mechanically to break the short circuit
And this takes long time which damages the work surface
Faster method for stopping the current in the event of short circuits resulted in the
Development of circuits with Vacuum tubes and transistors

38. These circuits increases
Material removal rate with high degree of accuracy
Provides very wide range of pulse duration frequencies
Independent “on” “off” controls
Power transistor devices acts as switching devices
vacuum
tube
The vacuum tube is a glass tube that has its gas
removed, creating a vacuum. Vacuum tubes contain
electrodes for controlling electron flow and were used in
early computers as a switch or an amplifier.

39. Vacuum tubes were also used in radios,
televisions, radar equipment, and telephone
systems during the first half of the 1900s. In the
1950s, the transistor started to replace the
vacuum tube. Today, vacuum tubes are no longer
used in electronic equipment
The simplest vacuum tube, the diode, contains only a heater, a heated electron-emitting
cathode (the filament itself acts as the cathode in some diodes), and a plate (anode). Current
can only flow in one direction through the device between the two electrodes, as electrons
emitted by the cathode travel through the tube and are collected by the anode. Adding one or
more control grids within the tube allows the current between the cathode and anode to be
controlled by the voltage on the grid or grids. Tubes with grids can be used for many
purposes, including amplification, rectification, switching, oscillation, and display.

40.  By replacing resistor R in case of R-C circuits
a series of vacuum tubes are connected in
parallel.
 The electronic control circuit turns on the
tube and the condenser gets charged. This
also enables the current flow to stop in case of
a short circuit.
 Vacuum tube circuits require high voltage and
low current supply.
 Vacuum tube pulse generator circuits are
further improved by using transistors in place
of vacuum tubes which are low voltage devices
Vacuum Tube pulse Generators

41. What is Transistor Device??
and Why they are used
Transistor pulse circuits
A transistor is a semiconductor device used to amplify or switch
electronic signals and electrical power. It is composed of
semiconductor material usually with at least three terminals for
connection to an external circuit.
 By turning a small input current into a large output current, the transistor
acts like an amplifier. But it also acts like a switch at the same time. When
there is no current to the base, little or no current flows between the
collector and the emitter. Turn on the base current and a big current flows.

43.  Present-day EDM machines adopt transistorized pulse circuits.
 They can give higher material removal rate with better surface finish and better
dimensional tolerance.
 The frequency duration and intensity of spark discharges are better controlled with
wider variations.
 Switching is done in this circuit by an oscillator at a selected predetermined
frequency without the capacitors.
 The oscillator is also controlled by the gap condition between the tool and work so
that the transistors are put off when short circuited.
 Transistor pulse circuits are switched by low power square wave generators and allow
independent on and off controls.

44. Electrode material
 It should not undergo much tool wear when it is impinged by positive ions.
 Rise of localised temperature need to be less by tailoring or properly choosing its
properties or even when temperature increases, there should be less melting.
 In EDM process the shape of the tool is transferred in the cavity cut during machining.
 The main factors that determine the suitability of a material for application as
electrode tool material in EDM are:
1. Higher metal removal rate
2. Lower tool wear
3. Higher degree of electrical efficiency

45. Basic characteristics of electrode materials are:
1. High electrical conductivity – electrons are cold emitted more easily and there is less
bulk electrical heating
2. High thermal conductivity – for the same heat load, the local temperature rise would
be less due to faster heat conducted to the bulk of the tool and thus less tool wear
3. Higher density – for the same heat load and same tool wear by weight there would
be less volume removal or tool wear and thus less dimensional loss or inaccuracy
4. High melting point – high melting point leads to less tool wear due to less tool
material melting for the same heat load
5. Easy manufacturability and cheap

46. The followings are the different electrode materials which are used commonly in the industry:
 Graphite (easily machinable, low wear rate and high conductivity) electrode with finer grain size
results in low TWR(Tool Wear Rate),better surface finish and higher MRR.(Draw back is its
brittleness)
 Copper and Brass (Highly stable and relatively low wear rate)
 Tellurium copper – 99% Cu + 0.5% tellurium
 Copper Tungsten (low wear rat but expensive and not easily machinable)
 Cast aluminum, Copper Boron and Silver Tungsten
EDM electrode tools are made by forming, casting, machining or by powder
metallurgy. Tools as small as 0.1 mm diameter have been successfully used in
EDM process.

47. Tool wear is quantified by Wear ratio (Ratio of Volume of Workpiece material
removed to the tool material removed)
In General electrode wear is more at corners/Edges than the rest of tool
Wear ratio ranges from 5:1 to 100:1
The wear ratio range from 5:1 to 100:1. The wear of the tool
compared with the wear of the work, i.e., the material removed
is referred to as wear ratio.

48. Wear ratio is given by:
Wr = 2.25 MT
– 2,3
Where, Wr = Work/tool wear ratio
MT = Work/tool melting point ratio.
 With increase in cross-
sectional area of work and
tool. But the wear ratio
increases with higher cutting
rate, in machining sintered
hard metals, vanadium and
molybdenum steels. Narrow
deep sections with sharp
corners results with higher
wear ratio. Tool wear can be
minimized by reversing the
polarity and using copper
tools.
How to reduce
Wear ratio of
work and tool ??

49. A good isotropic grain structured graphite with fine grains possessing same
current-carrying and wear characteristics in any direction make excellent
electrode tool for EDM. Copper, copper-tungsten and brass are also extensively
used as EDM tool material.

51. Servo System
 Used to maintain a predetermined gap between tool and work piece
 Gap voltage sensor in power supply which sends signal to servo system
 Servo system keeps the tool reciprocating towards the work-piece until the dielectric
fluid flushes the gap and clears it off the electrically conductive material
Requirements of Servo system are
1. Need to be very sensitive for even small movements
2. Enough power to overcome the weight of ram, electrode and flushing forces
Electrode Refeeding
Tool wear reduces length of the tool
Electrode feed Control

52. Process capability:
 The surface produced by electric discharge machining contains number of small
craters distributed over the surface due to sparks generated.
 The size of the crater and quality of the surface depends on the energy produced in
the spark.
 If the energy is high, deeper craters are produced leading to poor surface finish.
 The material removed per discharge will be normally in the range of 10– 6 to 10– 4
mm3.
 EDM process is a thermal process and is not a mechanical energy dependent process for
material removal, the material properties like
Hardness
Strength
Toughness of work material will not influence the material removal rate.

53. Melting point and latent heat of melting are two important
properties which determine the volume of material removed for
every discharge.
The frequency of discharge, the energy per discharge, the voltage and current
is usually varied to control the material removal rate. The material removal rate
and surface roughness increase with increase in current density and decrease
in frequency of spark.

54. With varying working parameters resulting with different energy density material
removal rate in EDM vary over a large range from 2 to 400 mm
3/min. Higher
material removal rate produces rough cut surface with molten recast structure with
poor surface texture and low fatigue properties. To get a good surface finish,
finishing cut with low material removal rate or a separate finishing operation is
essential to remove recast layer. As discussed earlier oscillating electrodes
technique gives good surface finish.

55. voltage and affecting a number of controls leading to reduction in pulse energy.
If the arcing continues the process is automatically stopped.
2. Adaptive control of off-time with arcless circuits in which gap deionisation is
measured: Here triggering of the pulse ahead will be only after complete gap
deionisation achieved by adaptive control of off-time. This control automatically
eliminates arcing allowing the process to work at maximum pulse duty factor
with stable machining conditions. This improves machining conditions to
produce better surface.
2. Adaptive process reference selection control: With the circuit the gap is
measured, varied automatically to a corresponding machining pulse efficiency till
the optimum gap voltage is reached. This is because EDM machining will be
most efficient at only one particular spark gap voltage condition. Also, the gain of
the servo system is modified till the hunting is eliminated. This control system
not only helps in making the machining process automatic, but also makes the
process parameter selection easy.

56. Applications
1. EDM is used for the machining of tools having complicated profiles and other
components.
2. It produces complex shapes to a high degree of accuracy in difficult-to-machine materials
such a heat-resistant alloys, superalloys and carbides.
3. The incorporation of EDM within a computerintegrated manufacturing (CIM) system
reduced the length of time that the unit operation, without stops for maintenance, is
required.
4. Micromachining of holes, slots, and dies; procedures for surface deposition; modification;
texturing; milling; and mechanical pulsing are typical applications.
5. It is applicable for a wide range of materials which include hard die tool steels to soft
copper and heavy components like forging dies to delicate workpieces such as copper parts
for fitting into vacuum tubes
6. It is also useful to cut workpieces that is too fragile to withstand the cutting force in
conventional machining.

57. Some of the items shaped by EDM for production applications and used in working
conditions are:
1. Dies, fixtures, gauges
2. Cutting tools
3. Press tools, extrusion dies
4. Die moulds for plastics
5. Diecasting dies, mould inserts
6. Remachining, repairing of worn dies for hot and cold forging
7. Making forging dies like connecting rod forging dies, etc.
8. Sintering dies
9. Calibrating tools
10. Shaping carbide tools, templates.

58. NOTE: In making a die cavity, by conventional method many operations like
milling, grinding, drilling and finishing operations are to be performed at different stages.
Whereas by EDM process the same die cavity can be finished by a fewer stages like making
the electrode tool and EDM machining, i.e., by faster cycle time. It reduces the machining
requirements. Further, the process is economical even for one piece. The economical
analysis reveals in majority of cases the cost of production of dies by EDM machining will
be 50% less than the cost of manufacturing by conventional methods.