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Micro slit machining using EDM with a
modified rotary disk electrode(RDE)
H.M. Chow , B.H. Yan , F.Y. Huang
Department of Mechanical Engineering, National Central University, Chung-Li, 32054, Taiwan, ROC
Name：Wen-Chen Huang
ID：M9710108
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Date：2009/5/19
Summary
1. Abstract……………………………….3
2. Introduction…………………………...4
3. Experimental procedure………….…5
4. Results and discussion…………..….8
5. Conclusions……………..……….…20
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Abstract
• The effects of polarity, discharge current, pulse
duration and rotational speed on the material
removal rate (MRR), the electrode wear rate (EWR),
the expansion of slit, the surface profile and the
recast layer of micro slit machining are reported
and discussed.
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Introduction
• MEDM equipment is too expensive to be able to be
used widely.
• WEDM suffers from the breakage susceptibility of
the superfine wire.
• This new application of RDE-EDM machining is
achieved by locating the rotating disk electrode
below the workpiece to improve the debris removal
rate.
• The benefits of this modified RDE-EDM also
include the obtain of an improved EDMed circuit
system that reduces the discharge current, and the
offering of a compact designation to stabilize RDE
vibration during machining.
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Experimental procedure
• The modified RDE-EDM developed in this study
consists of a modified non-micro EDM machine (a
die-sinking EDM) with a RDE.
Fig. 1. Schematic diagrams of EDM with:
(a) a conventional RDE
(b) a modified RDE. Note that the relative position of the workpiece
and the RDE is reversed in the modified RDE-EDM.
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Experimental procedure
The RDE-EDM experimental conditions
Conditions
Electrode size
Cu, D=ø42 mm, t=25, 50, 75, 100 μm
Workpiece
Ti–6Al–4V, t=0.45 mm
Polarity
Negative (－), positive (＋)
Dielectric
Kerosene
Peak current Ip (A)
0.06, 0.1, 0.5
High voltage (V)
280
Gap voltage (V)
25
Duty factor
0.55
Pulse duration τp (μs)
2, 5, 10, 20
Working time (min)
4
Revolutions of electrode (rpm)
0, 10, 20, 50, 150
Target depth
1.02mm
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Experimental procedure
Fig. 2. A detailed schematic diagram of the modified RDE-EDM
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proposed in this study.
Results and discussion
Fig. 3. The material removal depth vs. Fig. 4. The electrode wear vs. the rpm
the rpm of the RDE electrode
of the RDE electrode with the
with the discharge current as a
discharge current as a parameter.
parameter.
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Results and discussion
• (Fig. 3)The former was because the relative motion
between the electrode and the workpiece increased
the debris removal rate, whilst latter might be due
to the large centrifugal force at high rotational
speed that made it difficult for the dielectric fluid to
flow into the gap, thus decreasing the discharge
activity.
• The workpiece was located at the top of the RDE in
the present modified RDE-EDM, thus the debris
removal mechanism was increased not only by the
rotating electrode but also by the gravity of the
debris itself.
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Results and discussion
Fig. 5. The effects of electrode thickness on the material removal depth,
the expansion of the slit, and the electrode wear.
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Results and discussion
• At a discharge current Ip = 0.1A, the discharge
density was too high for an EDM process to be
stable.
• An optimized discharge density could be reached
by using a thicker electrode, the optimized
arrangement possibly allowing the use of a greater
material removal depth and therefore resulting in
less electrode wear.
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Results and discussion
Fig. 6. The material removal depth vs.
the pulse duration for both
negative discharge polarity and
positive discharge polarity.
Fig. 7. The expansion of the slit vs.
the pulse duration for both
negative discharge polarity and
positive discharge polarity.
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Results and discussion
• A higher MRR was observed with adopting the
RDE-EDM as the cathode.
• However, in handling the positive-polarity condition,
the dissociated carbons element in the dielectric
fluid tend to adhere to the anode (Ti alloy), which
may form a TiC recast layer by solid solubilization
and then diffuse gradually during sample melting
and solidification in the EDM process.
• This phenomenon may, somehow, reduce the
material removal rate. Furthermore, the melting
point of TiC (3150°C) is about twice that of Ti
(1660°C). It is more desirable to adopt a negative
polarity in a acquiring low EWR and a high MRR.
This practice is adopted in the present work.
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Results and discussion
Fig. 8. Cross-sectional SEMs of micro slits obtained by both positive and
negative discharge polarities for: (a) the outlook of the slit; (b) the
bottom of the slit, and; (c) the surface of the slit.
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Results and discussion
• (a)The depth of the slit was twice the depth with
negative polarity than it was with positive polarity.
• (b)The thermal effect area was smaller and the
recast layer was thinner with negative polarity.
• (c)More sub-crack surfaces are observed with
positive polarity, which is consistent with the lower
MRR associated with positive polarity.
• The deposit carbon reacts with Ti to form TiC which
has a high melting point above 3150°C and requires
a greater energy density to be removed with positive
polarity: this also accounts for lower MRR with
positive polarity.
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Results and discussion
Fig. 9. The material removal depth vs. Fig. 10. The expansion of the slit vs.
the pulse duration with discharge
pulse duration with discharge
current as a parameter (the
current as a parameter ( the
negative discharge polarity is
negative discharge polarity is
adopted).
adopted).
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Results and discussion
• Too-long a pulse duration (>6 μs) or too-high a
discharge current (0.5 A) result only in a lower
removal rate and worse surface conditions.
• Only a small slit expansion was obtained at the low
discharge current of 0.06 A and a pulse duration of
2–5 μs with negative polarity.
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Results and discussion
Fig. 11. A photograph of a single-slit
microstructure. The micro slit
is compared with a hair.
Fig. 12. Photograph of a multiple-slit
microstructure (with 10 slits)18
Results and discussion
• The resultant width of the slit was 42 mm, and the
depth was 1.02 mm
• The tolerance of the slit width and slit depth is ±1
μm, and ± 5 μm, respectively.
• The uniform wear in the radial direction was
reduced to 0.02 mm after the carrying out of the
machining of the 10-slit microstructure process.
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Conclusions
1. The modified RDE-EDM can improve MRR by
locating the workpiece above the RDE. EWR also
decreases uniformly around the periphery of the
disk electrode with this modified arrangement. The
position accuracy and vibrational stability of RDE
are improved over those of classical RDE-EDM to
achieve a high standard of micro slit machining.
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Conclusions
2. Although Ti is known to be a difficult material to
cut, an MRR as high as 1.5 mm3 /min-1 is
demonstrated in this study with the modified RDEEDM, using the optimum working condition at 10–
20 rpm, a discharge current of 0.1 A, and a pulse
duration of 5 μs.
3. Optimized discharge current is essential because
the temperature during discharge is extremely
sensitive to the discharge current due to the small
area of the micro slit. A greater MRR and lower
EWR can be obtained by properly optimizing the
discharge current.
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Conclusions
4. Negative polarity for the workpiece was adopted
for the present micro machining. A greater MMR
and lower EWR in the machining of the Ti alloy
process was observed under such working polarity.
5. The finished surface of the slit shows less
cracking, less recast layer and a smaller
expansion of the slit with negative polarity which
later is recommended for further work in this and
similar fields. However, the cracking, the recast
layer, and the expansion of the slit all increase as
pulse duration increase.
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Conclusions
6. The best working conditions are: Ip = 0.06 A;
τp = 2 μm, and; 20 rpm to obtain the smallest slit
width in these experiments. However the optimum
conditions may be different when applied to other
EDM processes. A preliminary calibration of each
EDM process to acquire the optimization is
therefore essential in applying this new technique.
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Thanks for your attention!
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