2. Contents
What is EDM?
Surface Integrity
Surface Roughness
Surface Finish
Surface Finish with Forced Circulation
Machining Accuracies
3. What is EDM?
Electric 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 which is difficult-to-
machine and high strength temperature resistant alloys.
EDM can be used to machine difficult geometries in small
batches.
Work material to be machined by EDM has to be
electrically conductive.
4. Surface Integrity
Surface consists of a multitude of overlapping craters that are
formed by the action of microsecond-duration spark discharges.
Crater size depends on
physical and mechanical properties of the material
composition of the machining medium
discharge energy and duration
Surface roughness increases linearly with an increase in MRR.
MRR in practice does increase with increase in working voltage,
current, pulse on time and decreases with increase in pulse off
time.
Kuneida and Furuoya (1991) claimed that the introduction of
oxygen into discharge gap provides extra power by the reaction of
oxygen.
5. Surface Roughness
EDM is achieved through
the formation of craters
due to the sparks.
Larger the crater size result
in rough surface.
The crater depth(hc) can be
approximately expressed in
terms of the energy
released per spark(E)
hc ≈K1E0.33
6. Surface Finish
As with conventional metal cutting, surface roughness
is inversely proportional to cutting power and speed.
In other words, the key to achieving better EDM finish
quality is to cut slower.
The surface unevenness, when machining steel under
normal conditions are approximately related as
Hrms≈ 1.11 Q0.384
The dependence of surface finish on the pulse energy
E and the comparison of surface finish with that
obtained by the conventional processes are indicated
in fig in the next slide.
7.
8. Surface Finish with Forced Circulation
The forced circulation of the dielectric has been found to
generally improve the surface finish.
It is clear that the forced circulation leads to a significant
improvement in the surface finish.
9. Machining Accuracies
The inaccuracies introduced
during the EDM operation
are mainly the following:
I. Taper of the hole
machined
II. Overcut due to sparks at
the side faces of the
electrodes
III. Errors due to the gradual
change in the electrode
shape and size
10. Taper
A taper results because the upper portion of the hole walls
is subjected to a more number of sparks than the bottom
portion.
The taper is found to depend on the square of the tool
diameter(in case of round tools).
It can be controlled by an appropriate alteration of the
electrical parameters.
11. Overcut
An overcut is that dimension by which the hole in the
workpiece exceeds the electrode size.
The magnitude of the overcut is dependent on the
spark length and to some extent on the crater
dimensions.
12. References
Manufacturing Science by Amitabha Ghosh and Asok
Kumar Mallik
http://nptel.ac.in/courses/112105127/pdf/LM-39.pdf
https://www.micropulsewest.com/our-blog/surface-
finish-achievable-with-edm/