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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: zhangqh@sdu.edu.cn (Q.H. Zhang).

0924-0136/02/$ see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 5 9 6 - 4

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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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