‟
The work-piece thickness is the most significant and
influential parameter contributing to MRR and EWR.
However, work-piece thickness has only a minor effect on
circularity of holes formed. The diameter of the electrode tool
contributes the most to the circularity of micro holes.
Circularity of Micro Holes
Circularity of micro holes corresponds to the
diameter of electrode as determined from the
SN ratio. Micro-EDM gives good machining
performance when the circularity conforms to the
electrode diameter. The results show that (Figures
16 – 20):
● ●
Circularity increases with the change of
work-piece material from brass to stainless
steel;
● ●
Circularity shows slight decrease when the
work-piece thickness is increased;
● ●
Circularity is directly proportional to the
increase in electrode diameter;
● ●
Circularity first shows an increasing trend
followed by a decreasing trend when the
nominal capacitance is increased; and
● ●
Higher feed rate gives larger circularity.
Brass work-pieces have a specific gravity
value of 8.49 whereas stainless steel work-
pieces have a lower specific gravity of 8.0.
As the atom density in stainless steel is
lower, it will form a larger crater on the work-
piece whenever each spark discharge occurs.
Thus, the circularity of micro hole formed on
stainless steel is slightly larger. Work-piece
thickness does not have much influence on
the circularity of micro holes because all the
machining processes fabricate a through hole
in the end. Circularity of micro holes conforms
to the diameter of electrode used for machining.
Anyhow, there is still a little overcut on the holes
fabricated by EDM processing. Therefore, the
circularity is larger with the increase in electrode
diameter. Generally, higher nominal capacitance
applied constitutes to higher electrical spark
energy discharge and hence larger circularity.
However, the result states that the highest
nominal capacitance gives smallest circularity
of micro hole. This behaviour may be explained
by the spark generated in the EDM process point
to point in the range of gap width. Therefore,
higher nominal capacitance only gives higher
depth of hole in this case. Higher feed rate of
electrode gives faster repeated spark discharge
on the workpiece. The surroundings of the micro
hole are not able to form resolidification and
get machined by the following spark. Hence, it
shows a larger circularity micro hole as a result.
Conclusion
The main purpose of this study was to fabricate a
PEDM and conduct experiments in order to study
the machining parameters and the machining
performances of micro-EDM by using copper
electrodes together with brass and stainless steel
work-pieces. The conclusions for the experiment
results are summarised as:
1. Successfully developed a PEDM machine
for micro application.
2. The effect of the machining parameters of
micro-EDM including work-piece material,
work-piece thickness, electrode diameter,
nominal capacitance applied and feed rate
on the machining performances which
include MRR, EWR and circularity have
been investigated.
3. The work-piece thickness is the most
significant and influential parameter
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