2. Sparking
• Whenever sparking takes place between two electrical contacts a
small amount of material is removed from each of the contacts
• If one can harness and control the spark energy to employ it for
useful purpose, that is machining of metals
• Sparks of short duration and high frequency are needed for
efficient machining
• Further, it was also observed that if the discharge is submerged in
dielectric, the energy can be concentrated into a small area
3. • A relaxation circuit (RC circuit) was proposed in which electrodes
are immersed in the dielectric
• The capacitor is charged from a direct current source
4. • As soon as the potential across the
electrodes crosses the breakdown
voltage (Vb), the sparking takes
place at a point of least electrical
resistance.
• It usually occurs at the smallest
inter-electrode gap (IEG).
• After each discharge, capacitor
recharges and spark appears at the
next narrowest gap.
• Occurrence of each spark generates
heat energy
5. After each discharge, capacitor recharges and spark appears at the
next narrowest gap. Occurrence of each spark generates heat
energy which is shared in different modes by workpiece, tool,
dielectric, debris and other parts of the system
6. • The dielectric serves some important functions
1. cools down the tool and workpiece
2. cleans (or flushes away) the IEG
3. localizes the spark energy into a small cross-sectional area
• Energy content in each spark and frequency of sparking are
governed by the conditions in the IEG
7. Electric discharge machining
• EDM is a thermoelectric process - heat energy of a spark is used
to remove material
• Workpiece and tool - electrically conductive materials
• Spark produced between electrodes and its location is
determined by the narrowest gap - IEG
• Spark duration - very short (few micro-seconds)
• Frequency of sparking - high as thousands of sparks per second
8. Electric discharge machining
• Area over which a spark is effective is also very small
• Temperature under the spark is very high
• As a result, the spark energy is capable of partly melting and
partly vaporizing material from localized area on both the
electrodes
• The material is removed in the form of craters which spread over
the entire surface of the workpiece.
9. Electric discharge machining
• Cavity produced in the workpiece - replica of the tool
• To have machined cavity as replica of the tool, the tool wear
should be zero
• To minimize wear of the tool the operating parameters and
polarity should be selected carefully
• Particles eroded from the electrodes are known as debris
• Analysis of the debris has revealed that it is the mixture of
irregular shaped particles resulting from re-solidification from the
molten state as well as hollow spherical particles
10. Electric discharge machining
• Amount of material eroded from the tool surface is much smaller
than that from the workpiece surface
• A very small gap between the two electrodes is to be maintained
to have the spark to occur
• For this purpose, a tool driven by the servo system is continuously
moved towards the workpiece
11. WORKING PRINCIPLE OF EDM
• During EDM, pulsed DC of 80-100 V at approximately 5 kHz is
passed through the electrodes.
• It results in the intense electrical field at the location where
surface irregularity provides the narrowest gap.
• Negatively charged particles (electrons) break loose from the
cathode surface and move towards the anode surface under the
influence of the electric field forces.
• During this movement in the IEG, the electrons collide with the
neutral molecules of the dielectric medium
12. • In this process, electrons are also detached from these neutral
molecules of the dielectric resulting in still more ionization.
• The ionization soon becomes so intense that a very narrow
channel of continuous conductivity is established.
• In this channel, there is a continuous flow of considerable number
of electrons towards the anode and that of ions towards the
cathode.
• Their K E is converted into heat energy, hence heating of anode
due to the bombardment of electrons and heating of cathode due
to the bombardment of ions, take place.
• Thus, it ends up in a momentary current impulse resulting in a
discharge which may be an arc or a spark.
13. The spark energy raises the localized temperature of the tool and
workpiece to such a high value that it results either in melting, or
melting as well as vaporization of a small amount of material from
the surface of both electrodes at the point of spark contact.
14. • The amount of material eroded from the workpiece and the tool
will depend upon the contributions of electrons and ions,
respectively.
• The polarity normally used is normal polarity in which the tool is
-ve and workpiece is +ve, while in reverse polarity the tool is +ve
and workpiece is negative.
15. • The sparking takes place over the entire surface of the workpiece
hence the replica of the tool is produced on the workpiece.
• Usually, a component made by EDM process is machined in two
stages
– rough machining at high MRR with poor surface finish, and
– finish machining at low MRR with high surface finish.
16. RC PULSE GENERATOR – Characteristics
1. Low material removal rate (MRR) because of long idle time (or
charging time) and very short spark time.
2. Improved surface finish, achieved during finish machining, is
associated with further reduction in MRR.
3. High tool wear rate (TWR).
17. • Peak current attained in RC circuit is very high.
• This high value of peak current results in a very high temperature
which is not required and it may also result in thermal damage to
both the workpiece and the tool.
• Controlled pulse generator overcomes these problems
18. • Generators give low peak
current, short idle time, and
desired length of pulse, enabling
us to select either
• rough machining conditions
(high energy and low frequency
of sparking), or
• finish machining conditions (low
energy and high frequency).
19. • EDM machine tool has four major components: power supply,
dielectric system, tool and workpiece, and servosystem
20. Power Supply
• Power supply converts alternating current (AC) into pulsed direct
current (DC) used to produce sparks between tool and workpiece
• Solid state rectifier is used to convert AC into DC.
• Digital multi-vibrator oscillator helps to creates high power pulsed
output responsible for generating sparks between the electrodes
21. Power Supply
• Power supply should also be able to control the parameters like
voltage, current, duration and frequency of a pulse, duty cycle
(the ratio of on-time to pulse time), and electrode polarity.
• EDM power supplies are also equipped with cut-off protection
circuit.
• During EDM, one of the four states can occur, i e normal
discharge, free discharge (arc), short circuit and open circuit.
23. Dielectric System
Good dielectric fluid should possess certain properties
1. Have high dielectric strength (remain electrically non-conductive
until the required breakdown voltage between the electrodes is
attained)
2. Take minimum possible time to breakdown
3. Deionize the gap immediately after the spark has occurred
4. Serve as an effective cooling medium
5. High degree of fluidity
24. Dielectric System
• The fluids commonly used as dielectric are transformer oil, paraffin oil,
kerosene,
• lubricating oils, and deionized water. Deionized water gives high MRR
and
• functions as more effective cooling medium but also causes high
electrode wear
• rates. Further, it suffers from the drawback of causing corrosion. To
overcome
• this problem inhibitors are used but they result in increased electrical
conductivity
• to an unacceptable level.
25. Debris particles
• Concentration of the debris particles in the gap increases rapidly as
the machining progresses.
• These wear particles should be removed from the gap so that fresh
dielectric enters the IEG for spark discharges.
• For reproduction accuracy of the process debris need to be removed
• It is possible to correct such errors only if it could be feasible to predict
• quantitatively the error of shape.
• However, complex shaped electrodes with sharp edges or protruding
points further add for poor reproduction accuracy.
• Hence, flushing is decisive for process efficiency and product quality.
26. Effective flushing
• Effective flushing of dielectric removes by-products from the
gap.
• Ineffective flushing results in low MRR and poor surface finish.
• The effective flushing may increase MRR as much as by a factor of
10 or so, Poor flushing ends up with stagnation of dielectric and
build-up of machining residues which apart from low MRR also
lead to short circuits and arcs.
• A good flushing system is the one that shoots the dielectric to the
place where the sparking occurs.
• It is felt that adequate flushing in case of blind cavities is difficult.
27. Machining of blind cavities
• Rotating tool with an eccentric hole (off-centred holes) for dielectric supply
can be used.
• Jet flushing is less effective
• . In case of inflammable dielectric fluids, the workpiece should always be
immersed in the dielectric fluid to minimize any chance of accidental fire.
• As machining continues,
• the dielectric gets contaminated with more and more amount of debris.
• Presence of such debris in the IEG, results in bridging the gap.
• It yields more number of arcs. Arcs are undesirable in EDM because they
damage both tool and workpiece.
• Occurrence of such arcs can be eliminated by proper filtration of the dielectric
as well as appropriate flushing of the IEG.
28. Various methods for dielectric flushing
(a) suction through
electrode, (b) suction
through workpiece,
(c) pressure through
electrode, (d)
pressure through
workpiece, (e) jet
flushing, (f) periodic
cycling of electrode
29. Electrodes
• Both tool and workpiece are electrodes. However, the word
‘electrode’ is used only for tool.
• The material used as tool electrode should possess desirable
properties like easily machinable, low wear rate, good conductor
of electricity and heat, cheap, and readily available.
• Graphite, copper, brass, copper tungsten, cast aluminium, copper
boron, and silver tungsten are some of the materials that are
used for making the tools for EDM
30. • Graphite
– easily machinable
– low wear rate
– high conductivity
• Brass
– highly stable
– relatively low wear rate
• Copper tungsten
– low wear rate
– but expensive, and cannot be easily shaped
31. • Copper and graphite are more commonly used because they can
be easily machined.
• A graphite electrode with finer grains results in lower TWR (Tool
Wear Rate), better surface finish, and higher MRR.
• However, its brittleness is an undesirable characteristic because
of which it is prone to breakage.
