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From：Precision Engineering 30 (2006) 414–420
Author：Kai Egashira ∗, Akihiro Matsugasako, Hachiro Tsuchiya, Makoto
Miyazaki
Presented by Tsung-Yen Wu
南台科技大學 精密製造實驗室
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The possibility of electrical discharge machining (EDM) with
ultralow discharge energy has been investigated. EDM using
an RC discharge circuit was performed at low open-circuit
voltages and a capacitance of approximately 30 pF.
The machining proceeded at voltages lower than 15V at a
vibration amplitude of 0.4 μm. The maximum discharge
energy per pulse is as small as approximately 3 nJ under these
conditions.
The volumetric electrode wear ratio can be 0.2% at voltages
lower than 40V, while it is normally more than 1% for EDM
using an RC discharge circuit.Workpiece surfaces processed
at voltages of 20V or lower are smooth and free of observable
discharge craters, and show no typical features of surfaces
machined by EDM.
南台科技大學 精密製造實驗室
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Introduction
Experimental
Possibility of EDM with ultralow discharge energy
Electrode wear ratio
Observation of drilled holes
Conclusions
南台科技大學 精密製造實驗室
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Since short discharge pulse duration and low discharge
energy are necessary for a small unit removal per
discharge, an RC discharge circuit, in which very short
discharge pulse duration can be realized, is generally
employed for such fabrication.
In this type of circuit, the maximum discharge energy
per pulse is (1/2)CE2, where C and E represent the
circuit’s capacitance and open-circuit voltage,
respectively. E must be lowered to reduce the energy
because C cannot be smaller than the stray capacitance
of the machine used.
南台科技大學 精密製造實驗室
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EDM of microholes in copper was possible at a very
low open-circuit voltage of 2V, although the
penetration rate of the electrode was very low.
However, with the capacitance being 10μF, the
discharge energy in this drilling was too high for
micromachining.
The processed surface was rough with large discharge
craters.
Furthermore, EDM with the machine’s stray
capacitance did not proceed at voltages lower than
approximately 30V.
南台科技大學 精密製造實驗室
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One of the possible reasons why EDM does not
proceed at open-circuit voltages lower than 30V with
the stray capacitance is that the discharge energy is so
low that material is neither melted nor vaporized.
Another possible reason is that debris and gaseous
bubbles are not removed from the discharge gap
between the electrode and workpiece because it is
narrow due to the low voltage.
There are many measures for that purpose: for
example, assistance of ultrasonic vibration, electrode
rotation, internal or external flushing nd jump flushing.
南台科技大學 精密製造實驗室
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Ultrasonic vibration and electrode rotation are good
selections for thin electrodes. EDM with ultrasonic
vibration has been performed in many studies so far;
most of the recent studies have focused on drilling
microholes.
Although the methods differ in what is vibrated –
electrode, workpiece or machining fluid – all results
showed that the material removal rate increased owing
to vibration expelling debris and bubbles from the
discharge gap. Electrode rotation also has the same
effect.
南台科技大學 精密製造實驗室
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This increase in the material removal rate suggests
that,by applying ultrasonic vibration, it would be
possible to carry out EDM with lower discharge energy
at lower voltages.
In the present study, therefore, we examined the
possibility of EDM with ultralow discharge energy,
which is realized by adopting open-circuit voltages
lower than 30V and a capacitance equal to the
machine’s stray capacitance while the workpiece is
ultrasonically vibrated and the electrode rotated.
南台科技大學 精密製造實驗室
Fig. 1. (a) Schematic
configuration of experimental
setup and (b) discharge circuit.
南台科技大學 精密製造實驗室
南台科技大學 精密製造實驗室
3.1. Minimum discharge energy at which EDM proceeds
 The possibility of EDM with ultralow discharge energy
was investigated by drilling microholes with the
assistance of ultrasonic vibration.
 Fig. 2 shows the results of drilling for each vibration
amplitude.
 Drilling was performed three times under each
condition.
南台科技大學 精密製造實驗室
南台科技大學 精密製造實驗室
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Fig. 2(a)shows that drilling without ultrasonic vibration
proceeded at an open-circuit voltage of 25V with the
electrode feed length of 6–11μm; however, progress
was negligible at a voltage of 22.5V. Fig. 2(b–e) shows
the results obtained with ultrasonic-vibration
amplitudes of 0.2, 0.4, 0.8 and 1.6 μm, respectively.
The results prove that drilling does not proceed at
voltages lower than 25V without vibration not because
the discharge energy is too low to remove material but
because debris and bubbles collecting in the discharge
gap prevent machining from proceeding.
南台科技大學 精密製造實驗室
3.2. Optimum ultrasonic-vibration amplitude
 All the approximate lines in Fig. 2(a–e) are drawn
together in Fig. 2(f), clearly showing the relationship
between the electrode feed length and the vibration
amplitude.
 The feed length increased with an increase in amplitude
from 0 μm; however, it decreased after the amplitude
was increased to beyond 0.4 μm.
 Although vibration of a larger amplitude expels more
debris and bubbles from the discharge gap, the material
removal rate is reduced.
 To examine the cause of this result, discharge current
waves were investigated.
南台科技大學 精密製造實驗室
Fig. 3 shows discharge-current
waves at amplitudes of 0, 0.4 and 1.6 μm, which were recorded
when neither short-circuiting nor arcing occurred.
As shown in Fig. 3(a), discharge occurred throughout the recording
span without vibration (at an amplitude of 0 μm). With vibration,
as indicated in Fig. 3(b and c), most discharge occurred periodically
at a period of approximately 25 μs, corresponding to that of the
vibration at a frequency of 40 kHz.
南台科技大學 精密製造實驗室
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Discharge was negligible when the gap between the
electrode and workpiece was widened by vibration and
the electrical field was decreased there. The time
during which much discharge occurred shortened with
an increase in vibration amplitude, thus lowering the
material removal rate.
This result suggests that a large vibration amplitude is
not always preferable and that an optimum amplitude
exists. Under the conditions of the present experiments,
the optimum vibration amplitude is 0.4 m for EDM
with ultralow discharge energy.
南台科技大學 精密製造實驗室
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One of the disadvantages of an RC discharge circuit is
a large electrode wear ratio.
The wear ratio is generally large and reducing it to less
than 1% is a difficult task with discharge energy of
higher than 10 nJ
Drilling deep holes is difficult with ultralow discharge
energy. The volumes of electrode wear and removed
part of the workpiece must be large enough for the
wear ratio to be determined without much error.
南台科技大學 精密製造實驗室
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Therefore, slots were fabricated instead of holes.
With ultrasonic vibration of 0.4 μm amplitude, EDM
was performed at open-circuit voltages of 40V and
lower, as well as 100V, to compare the wear ratio to
that in the case of high discharge energy.
The machining of 150 μm-long slots was carried out
until the vertical electrode feed length reached 30 μm
or the machining time reached 30 min. The average
charging current was 3 mA.
Fig. 4 shows the relationship between the volumetric
electrode wear ratio and the open-circuit voltage.
南台科技大學 精密製造實驗室
Fig. 4. Relationship between volumetric electrode wear ratio and open-circuit voltage.
南台科技大學 精密製造實驗室
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Three slots were machined under each condition,
except at a voltage of 100V. A high wear ratio of more
than 6% was obtained at a voltage of 100V, which is
characteristic of an RC discharge circuit.
At voltages lower than 40V, however, the wear ratio
was smaller than 1%. It was as small as 0.2% under
some conditions.
The results show that EDM with ultralow discharge
energy has the advantage of a low electrode wear ratio,
as well as high machining accuracy.
南台科技大學 精密製造實驗室
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Drilled holes were observed using a scanning electron
microscope(SEM).
Fig. 5. Bottom surfaces of holes drilled at open-circuit voltages of:
(a) 100V, (b) 25V and (c) 15V, with ultrasonic vibration of 0.4 μm amplitude.
南台科技大學 精密製造實驗室
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As shown in Fig. 5(a), the surface processed at a
voltage of 100V is covered with discharge craters,
which are normally observed on surfaces machined by
EDM.
Although the craters are much smaller at a voltage of
25V (Fig. 5(b)), the surface is still not smooth and
resembles a satin finished one.
The surface at a voltage of 15V (Fig. 5(c)) is,however,
smooth with concentric patterns and is free of
observable craters, unlike those processed under
normal conditions by EDM.
南台科技大學 精密製造實驗室
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One possible reason for such a smooth surface is that
material was removed without leaving craters because
of the ultralow discharge energy.
The concentric patterns reflect the profile of the
electrode-end surface.
Because the electrode was processed by WEDG in
which finish machining was carried out at an opencircuit voltage of 100V with the stray capacitance, its
surface was rough with craters, as shown in Fig. 6(a).
After machining at a voltage of 50V, the electrode end
became smooth without observable craters, as shown in
Fig. 6(b).
南台科技大學 精密製造實驗室
Fig. 6. Electrode-end surfaces after (a) fabrication and (b) use in machining at
open-circuit voltage of 50V.
南台科技大學 精密製造實驗室
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The processed surfaces shown in Fig. 7 have no craters
or distinct concentric patterns.
These results demonstrate that, in EDM with ultralow
discharge energy, processed surfaces are smooth
without craters, and high reproduction accuracy is
realized.
南台科技大學 精密製造實驗室
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Fig. 8 shows holes drilled at various vibration
amplitudes and an open-circuit voltage of 20V.
The drilling negligibly proceeded without vibration
(Fig. 8(a)). The diameters of the other holes are
approximately 18.5 μm, indicating little influence of
the vibration amplitude on the hole dimensions.
南台科技大學 精密製造實驗室
Fig. 8. Holes drilled at open-circuit voltage of 20V and ultrasonic-vibration amplitudes
of: (a) 0 μm, (b) 0.2 μm, (c) 0.4 μm, (d) 0.8 μm and (e) 1.6 μm.
南台科技大學 精密製造實驗室
1.
Drilling results showed that EDM is possible at opencircuit voltages lower than 15V with ultrasonic
vibration of 0.4 μm amplitude. The maximum
discharge energy per pulse is approximately 3 nJ
under these conditions, which is the lowest energy
reported at which EDM is possible, to the best of our
knowledge.
南台科技大學 精密製造實驗室
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While it has been reported that it is difficult to reduce
the volumetric electrode wear ratio to less than 1% in
EDM with an RC discharge circuit, a wear ratio of
0.2% can be obtained at voltages lower than 40V.
This result suggests that EDM with ultralow
discharge energy has the advantage of a low wear
ratio, as well as a high machining accuracy.
Workpiece surfaces processed at open-circuit
voltages of 20V and lower are smooth without
observable discharge craters, and do not show any
typical features of those machined by EDM. The
profile of the electrode end was transferred onto the
workpiece surface owing to the narrow discharge
gap, indicating high reproduction accuracy.
南台科技大學 精密製造實驗室
Thanks
for
Your Attention
南台科技大學 精密製造實驗室