Non Traditional Machinig

1. CHAPTER (5)
ELECTRODISCHARGEMACHINING(EDM)

2. 1- Introduction
-The history of EDM dates
back to the days of world
wars.
- A simple servo controller
maintained the gap width
between the tool and the
WP, reduced arcing.
- Recently, the machining
speed has gone up by 20
times and improved the
surface finish by a factor
of 15%.
EDM has the following
advantages:
1- Cavities with thin walls
and fine features can
be produced.
2- Difficult geometry is
possible.
3- The use of EDM is not
affected by the
hardness of the work
material.
4- The process is burr-
free.

3. 2- Mechanism of material removal
- In EDM, the removal of
material is based upon
the electrodischarge
erosion (EDE) effect of
electric sparks occurs
between two electrodes
that are separated by a
dielectric liquid.
- Metal removal takes place
as a result of the
generation of extremely
high temperatures
generated by the high
intensity discharge that
melt and evaporate the
two electrodes.
- A series of voltage
pulses of magnitude
about 20 to 120 V
and frequency on the
order of 5 kHz is
applied between the
two electrodes,
which are separated
by a small gap,
typically 0.01 to 0.5
mm.
- Temperatures of
about 8000 to 12000
deg C and heat fluxes
up to 1017 W/ sq. m
are attained.

4. EDM components

5. 2-1- Typical EDM pulse current train for
controlled pulse generator

6. 2-2- Variation of voltage with time using an
RC generator

7. 2-3- Voltage and current waveforms during
EDM

8. 2-4- EDM spark description
- Owing to the evaporation
of the dielectric, the
pressure on the plasma
channel rises rapidly to
values as high as 200
atmospheres.
- Such great pressure
prevent the evaporation
of the superheated metal.
- At the end of the pulse,
the pressure drops
suddenly and the
superheated metal
evaporates explosively.
- Metal is thus removed
from the electrodes.
- Fresh dielectric fluid
rushes in, flushing the
debris away and
quenching the surface of
the WP.
- The relation between the
amount of material
removed from the anode
and cathode depends on
the respective
contribution of the
electrons and positive
ions to the total current
flow.
- Without a sufficient off
time, debris would collect
making the spark
unstable.

9. 2-4- EDM spark description

10. -The frequency of
discharges or sparks
usually varies between
500 and 500,000
sparks/sec.
- The position of the
electrode is controlled by
the servomechanism,
which maintains a
constant gap width (200
– 500 µm) between the
electrodes.
-EDM performance
measures such as
material removal rate,
tool wear, and surface
finish, for the same
energy, depends on the
shape of the current
pulses.
- Open gap voltage that
occurs when the
distance between both
electrodes is too large.

11. - Micro short circuits
occur when sudden
contact occurs
between the tool and
the WP.
- Arcs occur when the
plasma channel of
the previous pulse is
not fully deionized.
- It is believed that only
Sparks really
contribute to material
removal in a desired
mode.
3- The machining
system
-The main components
of the EDM system
include:
a- The tool feed servo-
controlled unit.
b- The power supply:
which is responsible
for supplying pulses at
a certain voltage,
current, on time, and
off time.
c- The dielectric
circulation unit: which
is flushes the
dielectric fluid to the
IEG after being filtered.

12. 3-a- EDM schematic

13. EDM machines

14. EDM machines

15. Constant gap
Workpiece holding system
Flushing mode
Dielectric system
Type
Open gap volt
Waveform
Power supply
Pulse current
EDM
Machining
chamber
Pulse on-time
3-b- EDM system components
Servomechanism

16. 3-1- EDM electrodes
1- Material: Metals with a
high melting point and
good electrical
conductivity are usually
chosen as EDM tool
materials.
a) Graphite is the most
common electrode
material.
b) Copper has good EDM
wear and good
conductivity.
c) Copper and silver
tungsten for making
deep slots
d) Copper graphite is
good for cross-
sectional electrodes.
e) Brass ensures stable
sparking conditions.

17. 2-Movements: In
addition to the servo-
controlled feed, the
tool electrode may
have an additional
rotary or orbiting
motion.
a) Electrode rotation:
helps to solve the
flushing difficulty,
increase the cutting
speed, and improve
the quality of the
hole.
b) Electrode orbiting:
produces cavities having
the shape of the electrode
and improves the flushing.
3- Polarity: Electrode polarity
depends on both WP and
electrode materials.
4- Electrode wear: The
melting point is the most
important factor in
determining the tool wear.
-Electrode wear ratios are:
end wear- side wear-
corner wear- volume wear.

18. Types of electrode wear in EDM

19. a) The wear rate:
Wt =(11*10 of power 3 ) i (Tt of power -2.38).
b) The wear ratio:
Rw = 2.25 (Tr of power -2.3)
Where: i: EDM current, A
Tt :melting point of the tool, deg. C
Tr : ratio of the WP to tool melting point
4- Electrode wear: The melting point is the most
important factor in determining the tool wear.
-Electrode wear ratios are: end wear- side wear-
corner wear- volume wear.

20. 3-2- Dielectric fluids
The main functions of the
dielectric fluid are to:
1- Flush the eroded
particles from the
machining gap.
2- Provide insulation
between the WP and the
electrode.
3- Cool the section that
was heated by the
discharge effect.
The main requirement
of the EDM dielectric
fluids are:
a) adequate viscosity.
b) high flash point.
c) good oxidation
stability.
d) minimum odor.
e) low cost.
f) good electrical
discharge efficiency.

21. The dielectric fluids:
1- Kerosene: is used for
most EDM operations
with certain additives that
prevent gas bubbles and
de- odoring.
2- Silicon fluids: and a
mixture of these fluids
with petroleum oils have
given excellent results.
3- Other dielectric fluids:
aqueous solutions of
ethylene glycol, water in
emulsions, and distilled
water.
Methods for introducing
dielectric fluids to the
machining gap are:
1- Normal flow: It is
introduced under
pressure through one or
more passages in the
tool.
The flushing holes are
placed in areas where
the cuts are deepest.

22. 2-Reverse flow: this
method is useful in
machining deep
cavity dies, where
the taper produced
using the normal
flow mode can be
reduced.
3- Jet flushing: the
desired machining
can be achieved by
using a spray or jet
of fluid directed
against the
machining gap
4- Immersion flushing: for
many shallow cuts or
perforations of thin
sections.
For proper flushing
conditions:
1- Flushing through the
tool is more preferred
than side flushing.
2- Many small flushing
holes are better than a
few large ones.
3- Steady dielectric flow on
the entire WP-electrode
interface is desirable.

23. Common dielectric flushing modes

24. 3-2- Continued
4- Dead spots created by pressure flushing,
from opposite sides of the WP, should be
avoided.
5- A vent hole should be provided for any
upwardly concave part of the tool to prevent
accumulation of explosive gases.
6- A flush box is useful if there is a hole in the
cavity.

25. EDM products

26. 4- Material removal rates
- In EDM the metal is
removed from both the
WP and the tool
electrode.
- In this regard a material
of low melting point
has a high metal
removal rate and a
rougher surface.
- Typical removal rates
range from 0.1 to 400
cu. mm / min
- The MRR or VRR was
described as:
VRR= (4*10 of power 4 )
*i*(Tw of power -1.23)
where: i: the EDM current
(A)
Tw: the melting point of the
WP material ( deg C)

27. Material removal rate,
Surface quality,
Accuracy
Pulse
characteristics
Crater volume
Tool electrode
Workpiece
thermal
properties
Material
Conductivity
Boiling point
Melting point
Dielectric properties
Wear
Movement
Parameters affecting EDM performance

28. Effect of pulse current on MRR and surface roughness

29. Effect of pulse on time on MRR and surface roughness

30. 5- Surface integrity
- The spark machined surface consists of a multitude
of overlapping craters.
- The depth of the resulting craters represents the
peak to valley surface roughness Rt.
Surface roughness increase linearly with an increase
in MRR.
- The introduction of oxygen gas into the discharge
gap provides extra power by the reaction of
oxygen, which in turn increased the melting of the
workpiece and created greater expulsive forces
that increased the metal removal rate and surface
roughness.
- When EDM is used for cusp removal, the silicon
powder has been suspended in the working fluid,
during the stage of finish EDM.

31. Heat-affected zone
- With the temperature of the discharges reaching
8000 to 12,000°C, metallurgical changes occur in
the surface layer of the workpiece.
- Additionally a thin recast layer of 1 μm at 5- μJ
powers to 25 μm at high powers is formed.
- Some annealing of the workpiece can be
expected in a zone just below the machined
surface.
- The depth of the annealed layer is proportional
to the amount of power used in the machining
operation.
- Choosing electrodes that produce more stable
machining can reduce the annealing effect.

32. EDM heat-affected zones.

33. b- ED Sawing
6- Applications
a- EDM Drilling: irregular, tapered, curved, as well as
inclined holes can be produced.

34. c- Machining of spheres

35. d- Machining of dies

36. e- Wire EDM

37. 8- Process control
- The quality of machining in EDM is
determined from fuzzy rules obtained from
experimental knowledge for recognizing
stability in machining.
- Information provided by a signal processor
is used to decide on the appropriate
electro machining conditions.
- Artificial neural networks are further new
tool in EDM control.

38. 9- Environmental impact