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Ultrasonic vibration electrical discharge machining in gas
Q.H. Zhang*, J.H. Zhang, J.X. Deng, Y. Qin, Z.W. NiuCollege of Mechanical Engineering, Shandong University, Jinan 250061, China
Abstract
A new method of ultrasonic vibration electrical discharge machining (UEDM) in gas is proposed in this paper. It is shown that electrical
discharge machining (EDM) with ultrasonic aid can be achieved well in a gas medium. The tool electrode is formed into a thin-walled pipe,
and a high pressure gas medium is supplied through it. During machining, ultrasonic vibration of the workpiece can improve the machining
process. Molten workpiece material can be ejected from the base body of the workpiece with the aid of ultrasonic vibration and be removed/
ushed out of the working gap without becoming reattached to the electrode. Selecting #45 steel and copper as the workpiece material andelectrode material, respectively, experiments have been carried out, the results showing that UEDM is a method with a high material removal
rate (MRR). The greatest advantages of this technique are lower pollution and a low electrode wear ratio.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: UEDM; Gas medium; MRR; Tool electrode wear
1. Introduction
Electrical discharge machining (EDM) is generally car-
ried out in a dielectric liquid. It is a thermal process where
material is removed by a succession of electrical discharges
occurring between an electrode and a workpiece plungedinto a dielectric uid. Every discharge ionizes a localized
plasma canal, where the temperature can become very high
(up to 1000 8C), leading to fusion and ebullition of metal of
both facing materials [1]. The use of liquid has been
regarded as indispensable for the stability and efciency
of the process, because it is known that the liquid serves as a
cooling medium in the discharge gap and ushes machining
debris out of the working gap. Thus it plays one of the most
important roles in the materials removal mechanism.
EDM is a useful machining method. It has a great advan-
tage in machining a workpiece with a special shape or of
hard-machining material, such as plastic moulds, blanking
dies, carbide materials and engineering materials [2,3]. Now
it has been applied widely in manufacturing engineering.
Despite its wide use in industry today, EDM has some
disadvantages. One of the most serious disadvantages is that
it can result in environmental pollution [4]. It is known that
EDM can produce waste dielectric liquid that is very harmful,
so steps should be taken not to let this waste into the
surrounding environment. Further, dielectric liquid is gen-
erally kerosene-based oil, so that it will decompose and
release harmful vapor (CO and CH4) during EDM, which
will do harm to the health of the operator. For environment
protection reason, the green method of EDM without pollu-
tion has recently become a subject of chief study in the world.
EDM in gas is a new machining method which was
proposed by Kunieda and Yoshida in 1997 [5]. In thismethod, EDM is achieved in gas instead of kerosene-based
oil, so that the pollution decreases. When this new method
appeared, all the world was astounded. It is regarded as one
of the most important methods with good prospects, but this
method has a great disadvantage, being of low stability and
having a low material removal rate (MRR).
To overcome the shortage of EDM in gas, a new method,
ultrasonic vibration electrical discharge machining (UEDM)
in gas, is developed in this paper.
2. Principle of UEDM in gas
A number of studies of EDM in gas have appeared in
engineering journals in recent years [5,6]. Descriptions of
the process also exist in some review papers [7,8]. Experi-
mental investigations have also been conducted on the
inuence of different parameters on the MRR in EDM in
gas. A number of attempts have also been made to predict
MRR in ultrasonic machining [911]. The machining theory
proposed here combines existing descriptions of the material
removal process with the ultrasonic machining process.
The process of UEDM in gas is schematically shown in
Fig. 1. In UEDM in gas, the gap between the tool electrode
Journal of Materials Processing Technology 129 (2002) 135138
* Corresponding author.
E-mail address: [email protected] (Q.H. Zhang).
0924-0136/02/$ see front matter # 2002 Elsevier Science B.V. All rights reserved.
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and the workpiece is small (about 0.01 mm), and the voltage
between them is higher than in EDM in liquid, so short
circuits are easy to take place. It is very important for
improving the MRR to avoid short circuits. Therefore, some
measures have been taken, such as a rotation and a planetary
motion being superimposed upon the tool electrode.
During UEDM in gas, the workpiece is vibrating with
ultrasonic frequency, which can cause the molten workpiece
material to be ejected from the base body of the workpiece
without being reattached to it again, which is advantageous
in increasing the MRR. The electrode is formed into a thin-
walled pipe, high pressure gas being supplied through an
internal hole and owing over the machining gap with a high
velocity. The gas enhances the removal of molten and
evaporated workpiece material. The gas, at high velocity,
also cools and solidies the removed material and prevents itfrom adhering onto the surface of the tool electrode. Further-
more, during the pulse interval, the high velocity gas blows
off the plasma formed by the previous discharge and
decreases the temperatures of the discharge spots on the
tool electrode and the workpiece due to heat transfer, thus
ensuring the recovery of the dielectric strength of the gap.
3. Experiments of UEDM in gas
The experiments were performed on an electrical dis-
charge small hole machine DK730 (made in China, modied
by the authors). The worktable of the machine was espe-
cially designed to accept an ultrasonic vibration unit, and the
clamp of the machine was designed to accept high pressure
gas when it is turning. In the experiments, the tool electrode
was a cylindrical pipe with outer and inner diameters of 10
and 9 mm, respectively. The high pressure gas was supplied
to the working gap through an internal hole of the tool
electrode. There was a gas drier between the compressor and
the regulator to eliminate the inuence of water vaporcontained in the compressed gas on the machining char-
acteristics. #45 steel and copper were selected as the work-
piece and tool electrode, respectively, whilst air and oxygen
gas were selected as the gaseous mediums.
The ultrasonic generator (made in China) had a maximum
power of 100 W with an adjustable frequency in the range of
1723 kHz. The measured amplitudes of vibration during
idling were 0.006 mm at 50 W and 0.012 mm at 100 W. The
frequency used in the experiments was set at 20.3 kHz
(controlled through an adjusting knob). The pressure of
the gas could be changed continuously from 10 to
500 kPa. The voltage supplied by the power supply could
be changed in steps of 20 V from 100 to 300 V.
The MRR was measured using a dial gauge with an
accuracy of 0.001 mm. The rate of depth penetration was
measured with a dial gauge and the MRR calculated by
multiplying by the cross-sectional area of the penetrated
aperture. The surface roughness was measured through a
Talysurf 40 surface measuring instrument (made in England)
with a relative accuracy of 5%.
Five sets of experiments were carried out to show the
effects of the open voltage, the pulse duration, the wall
thickness of the pipe electrode, the amplitude of ultrasonic
vibration and the gas medium on the MRR. Some observa-
tions of the roughness of the machined surface were alsomade. The experimental variables are summarized in Table 1.
4. Experimental results and discussion
4.1. The effect of open voltage on the MRR
Experimental results show that the MRR tends to increase
with the increase of the open voltage, as shown in Fig. 2. It
should be noted that the MRR is affected only slightly by the
open voltage. In fact, the action of the open voltage is only to
break down the gas medium. With the vibration of the
workpiece, it is easy for the gas medium to be broken down
in UEDM in gas.
Fig. 1. Principle of UEDM in gas.
Table 1
Summary of experimental condition
Experiment Open voltage (V) Pulse duration (ms) Wall thickness (mm) Vibration amplitude (mm) Gas medium
1 160, 200, 240, 300 600 0.3 12 Air
2 240 60, 120, 600, 1200 0.3 12 Air
3 240 600 0.3, 0.5, 0.8, 1.0, 2.0 12 Air
4 240 600 0.3 6, 12 Air
5 240 600 0.3 12 Air, oxygen gas
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The surface roughness was not found to be affected
clearly by the open voltage. The roughness was around
Ra 0:032 mm.
4.2. The effect of pulse duration on the MRR
Experimental results show that the MRR tends to increase
with the increase of the pulse duration, as shown in Fig. 3. A
long pulse duration not only results in a long time per
monopulse but also leads to large fusion of the material.
Both of these are of advantage for the material removal, so
the MRR increases with the pulse duration.
It is found that the surface roughness increases with the
increase of the pulse duration, rising from Ra 0:028 to
0.038 mm over the range of amplitude examined.
4.3. The effect of the wall thickness of the pipe electrode on
the MRR
Fig. 4 shows the effect of the wall thickness of the pipe
electrode on the MRR. An increase of the wall thickness of
the pipe electrode causes a decrease in the MRR. It should benoted that the MRR increases drastically when the wall
becomes thinner than the diameter of the discharge crater. It
is considered that in the high velocity gas ow, most of the
molten workpiece at the discharge spot is removed without
reattachment to the workpiece surface, especially when the
wall is thinner than the diameter of the discharge crater.
However, when the wall is much thicker than the diameter of
the discharge crater, the MRR is less because of reattach-
ment of the molten material to the workpiece surface.
The surface roughness is not found to be affected by the
wall thickness of the pipe electrode. It stabilizes at
Ra 0:032 over the range of wall thickness examined.
4.4. The effect of the amplitude of ultrasonic vibration
on the MRR
Experimental results show that the MRR tends to increase
with the increase of amplitude of ultrasonic vibration, as
shown in Fig. 5. It is considered that the workpiece, vibrating
with ultrasonic frequency, can have the molten workpiece
material ejected from the base body of the workpiece with-
out being reattached to the workpiece which is advantageous
in improving the MRR.
The surface roughness is not found to be affected clearly
by the amplitude of ultrasonic vibration, stabilizing at about
Ra 0:032 mm.
4.5. The effect of the gas medium on the MRR
The effect of the gas medium on the MRR was also
investigated. As shown in Fig. 6, the MRR in pure oxygen
gas was twice as large as that in air. It is considered that heat
Fig. 2. The effect of open voltage on MRR.
Fig. 3. The effect of pulse duration on MRR.
Fig. 4. The effect of wall thickness of pipe electrode on MRR.
Fig. 5. The effect of amplitude of ultrasonic vibration on MRR.
Fig. 6. The effect of gas medium on MRR.
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generation by oxidation of the molten and evaporated steel
enhance the machining efciency.
The surface roughness is found to be affected strongly by
the gas medium. For air and oxygen gas, the corresponding
values of Ra was measured to be 0.032 and 0.046 mm,
respectively.
5. Conclusions
The principle of UEDM in gas has been introduced and
the effect on MRR has been measured. It was found that
UEDM in gas is an effective machining method. The MRR
of UEDM in gas is much higher compared with that of EDM
in gas and conventional EDM in dielectric liquid. Experi-
mental results show that increases in the open voltage, pulse
duration, amplitude of ultrasonic vibration and decrease of
the wall thickness of the pipe electrode, result in an increase
of the MRR. As a medium, oxygen gas can produce a greater
MRR than air.
Acknowledgements
The work described in this paper is supported by Natural
Science Foundation of Shandong Province (subject number
Y2001F14).
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