Elsevier

CIRP Annals

Volume 70, Issue 1, 2021, Pages 147-150
CIRP Annals

High rising speed discharge current pulse for EDM generated by inductive boosting voltage circuit

https://doi.org/10.1016/j.cirp.2021.04.014Get rights and content

Abstract

This paper introduces a newly developed EDM pulse generator whose high open voltage for discharge is generated by induced electromotive force using a power supply of only 5 V. The pulse generator can generate short duration and high discharge current pulses. The rising time of discharge current pulses to 5 A can be made shorter than 50 ns with a pulse duration of 200 ns or shorter. The results proved that such discharge current pulse shapes increase material removal volume per discharge and that the higher boosting open voltage facilitates the machining of high resistivity materials.

Introduction

In electrical discharge machining (EDM), it is well-known that removal efficiency is defined as the ratio of the volume of removed material to the volume of total material melted by a single discharge. This value is usually no more than several percent [1], which is the reason for the low material removal rate of EDM compared to conventional machining methods. Since the discharge current pulse plays a main role in the discharge process, the influence of discharge current pulse shapes on the removal volume by a single discharge should be investigated carefully. Shinohara et al. [2] compared the machining performances of different discharge pulse shapes under the same discharge energy conditions. They used a transistor generator to ignite a discharge, and a function generator for supplying various triangular current shapes after ignition. They found that low increasing speed right-angled triangular pulses produced the largest removal volume when using brass as the workpiece material, while high rising and low falling speed right-angled triangular pulses produced the largest removal for steel workpieces. However, given that they used 200 μs as the discharge duration, their study lacks findings on short pulse durations normally used in wire EDM or micro EDM.

It is known that in EDM, even under identical energy conditions, different performances can be observed between rectangular current pulses with higher peak current at shorter discharge duration and those with lower peak current at longer discharge duration [3]. Zahiruddin et al. [4] concluded that discharge current pulses with shorter duration like those in micro-EDM result in higher energy efficiency and removal efficiency than conventional macro-EDM since considerably higher power density is generated on workpieces. Therefore, investigations on the influences of different types of pulse shapes like triangular pulses with short duration are important.

In conventional EDM, with the transistor pulse generator, gap current and pulse duration can be controlled efficiently by changing the resistance and duty of the transistor, but it is hard to obtain short pulse duration. On the other hand, RC generators can be built easily and pulse durations shorter than transistor generators can be generated. However, both types of generators require a power supply that provides high voltage to cause dielectric breakdown in the EDM gap. Recently, boost converters [5] are widely used to increase DC voltage to high levels for photovoltaic systems or motor drivers. In micro EDM, Chung et al. [6] used a switching circuit comprised of inductor and MOSFET to realize bipolar pulses using water as the dielectric liquid. With their method, high voltage power supply is not required. However, the principle of discharge is the same as the RC generator since the capacitor is inserted parallel to the gap for storing energy.

This paper introduces an LC pulse generator based on the inductive voltage boosting circuit capable of generating triangular discharge current pulse shapes with high increasing speed and low decreasing speed, different from conventional generators such as RC generators. The new generator is a type of switching circuit which uses an inductor to store energy and a power supply of only 5 V. Like the RC generator, the discharge current pulse of the proposed generator is characterized by short discharge duration, but differs in shape under the same discharge energy conditions, which results in different machining characteristics.

Section snippets

Principles of proposed generator

Fig. 1(a) shows the newly developed LC pulse generator. In the circuit shown, Uin is the constant DC power source, C1 is the capacitor used to prevent leak current flowing through the gap, L is the inductor used to store energy and inductively boost the gap voltage, and S is the switch for controlling the current direction.

When the switch S is turned on, Uin supplies power to L from its positive terminal for charging the inductor as shown in Fig. 2(a). The increase in current causes back

Determinant factors of peak open voltage in non-discharge conditions

The experimental results in Fig. 3 show that when the inductance or switching frequency was increased, the peak open voltage of the gap decreased. This is due to the decrease in the induced electromotive force with the decrease in iL as a result of the large L and short ton in Eq. (1). Moreover, the Zener diode installed in the MOSFET also had a huge influence on the peak voltage. In the above experiment, the peak voltage was limited to 100 V due to the use of a 100 V Zener diode. This peak

Observation of cross section of discharge craters

To compare different discharge current pulse shapes, single discharge craters generated by three types of generators shown in Figs. 1(a), (b) and (c) were observed. Fig. 7 shows the discharge current pulses obtained from the three generators and Table 1 shows the parameters used to generate the pulse shapes. Fig. 1(c) shows the pulse generator used for generating discharge current pulse shapes with high decreasing speed as shown in Fig. 7(c). This pulse generator is a type of RC generator with

Applications of LC generator

In this study, we practically applied the LC generator to micro-EDM, and used WEDG [8] to make tungsten micro-rods using the LC generator with a discharge current of 200 mA and discharge duration of 200 ns as shown in Fig. 12.

EDM drilling characteristics were compared between the LC and RC generators using relatively high discharge current of 5 A and discharge duration of 1.6 μs. Fig. 13 shows the through holes machined in a 0.8 mm thick stainless-steel plate under the conditions shown in Table

Conclusions

The newly developed LC pulse generator can realize short duration and high rising speed discharge current pulses for EDM using a power supply of 5 V voltage, not possible with conventional generators. The results of this study showed that in single discharges, higher removal volume can be obtained from discharge current pulses with short duration and high rising speed. These results demonstrate that the LC pulse generator provides better machining efficiency compared to the RC generator under

Declaration of Competing Interest

None.

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