Research on electrical discharge machining (EDM) in Japan all began with Professor Seizaburo Ho of the University of Tokyo (Department of Electrical Engineering, Faculty of Engineering) watching a science movie made in the former Soviet Union. Together with Professor Hisao Kurafuji, also from the University of Tokyo (Department of Precision Engineering), they embarked on studies in this area by establishing the foundations of EDM and electrical machining societies in Japan. In this essay, I would like to talk about the roots of EDM as well as the International Symposium on Electromachining (ISEM), focusing on my own experiences of participating in ISEMⅡ onwards. In 1989, Japan hosted the ISEM (IX) for the first time. It proved to be a huge success and became by far the largest conference since it was started. From around 1995, mold-making trends using high speed milling have been accelerating. Cuttable shapes and large molds are gradually being replaced by high speed milling, and sinking EDM is declining. This has led to the need to find new applications for sinking EDM in areas other than mold-making. One example is its application to the electrical discharge coating of jet engine parts. There is also a need for further fundamental studies on sinking EDM as well as the development of electrochemical machining (ECM) techniques for cemented carbide besides EDM.
In this paper, we deal with the performance of electrochemical discharge machining (ECDM) with multiple electrodes. ECDM has been used to machine grooves and holes on insulated materials such as glass. However, many cracks are often observed around a machined hole because of the accumulation of discharge heat. While some holes were being machined by ECDM on glass, the discharge current was forced to disperse by switching the electrodes to decrease the number of cracks (divided power). The performance of ECDM was compared with those of multiple electrodes connected electrically in parallel (equipotential power) and of a single electrode. Then hole shapes were observed. The hole entrance was round because of the divided power. In the cases of equipotential power and a single electrode, cracks were frequently generated around the hole entrance by heat expansion. The cracks were the origins of protrusions by erosion. Eventually, long protrusions were formed. Because bubbles collapse during pulse intervals because of the divided power, the hole diameter was not enlarged.
To understand the mechanism of material removal in electrical discharge machining, the process of a single discharge must first be understood. In this study, aiming to investigate the influence of bubble behavior on removal volume of the crater, we observed bubble behavior during single discharge using a high-speed video camera. In experiment, a low-melting-temperature alloy expected to allow the generation of large craters was used to conduct detailed observations of individual single craters. A thin tungsten electrode was used, and the current was fixed at 20 A. To investigate whether change in bubble behavior is related to removal, double-pulse discharge experiments were conducted. And the influence of the expansion direction of a bubble was modified by using a jig that limited the height of the bubble. In addition, laser irradiation in pure water was conducted under conditions of limited bubble height. On these results, we conclude that the expansion and shrinkage directions of the bubble are important factors that affect the removal volume in EDM.
In a large-area electron beam (EB) irradiation method developed recently, high energy EB can be obtained without focusing the beam. Therefore, the large-area EB of 60mm in diameter with almost uniform energy distribution can be used for instant melting and evaporating of metal surface. The previous studies clarified that the surface smoothing of metal mold made of steel, cemented carbide and ceramics could be performed efficiently. Also this method can be applied to the surface finishing of biomaterials, such as stainless steel and titanium alloy. In this method, the sharp edge is possibly rounded to about half hundred microns in radius under relatively high energy density conditions, since the material removal remarkably progresses there due to the electron concentration and heat accumulation at the edge. In this study, the possibility of micro-deburring was investigated by using the preferential edge removal effect in the large-area EB irradiation. Experimental results showed that the micro-deburring was possible by large-area EB irradiation, and the burr height decreased with increasing energy density of EB and number of irradiation. The burr of about 50μm in height generated in EDM process could be removed completely. Furthermore, micro-burrs at the edge of several small holes within the wide area of 60mm in diameter could be removed simultaneously.
A laser singulation method by the superposition of a pulsed fiber laser and an SHG:YAG laser (SHG: Second Harmonic Generation) was proposed to perform the high-quality processing of semiconductor packages, which are consisted of thermosetting epoxy-resin with silica as the molding compounds and a glass epoxy board with insulator coatings for the semiconductor package. The superposition of the pulsed fiber laser and SHG:YAG laser with a high pulse repetition rate led to a straighter kerf shape with a smaller kerf width under the same pulse energy condition. A smaller kerf width was achieved by controlling the time-delay between the laser pulses compared with synchronized laser pulses. The heat affected zone from the cut surface was reduced by the superposition of the pulsed fiber laser and SHG:YAG laser. Moreover, synchronized laser pulses led to a smaller heat affected zone compared with unsynchronized pulses.
Since Electro-chemical Machining (ECM) is a contactless machining method, it is necessary to detect and control the gap-width in order to estimate the machining amount and improve the machining speed and accuracy. In this research, a tool electrode which is capable of detecting the gap-width by the suction pressure was proposed. With this method, the electrolyte is confined and circulated beneath the tool electrode by a single suction pump. It was found the electrolyte is successfully circulated by the pump. In addition, experimental results showed that the suction pressure indicates the gap-width and the pressure signal can be successfully used to control the tool electrode feed.
During the process of electrochemical machining (ECM), bubbles and sludge are generated in the inter-electrode gap, affecting the machining accuracy significantly. To solve this problem, an electrolyte flow is supplied into the gap to flush them away. Hence, this study aimed to observe the flow of electrolyte and bubbles in the gap using a transparent electrode. SiC single crystal semiconductor was used as the cathode material because it is electrically conductive and optically transparent. The gap was observed through the SiC cathode in the direction normal to the electrode surface using a high speed video camera. Distributions of the flow speed and diameter of bubbles were investigated to understand the influence of flushing flow on the machining process.
Insulating ceramic materials can be machined by electrical discharge machining (EDM) using the assisting electrode method with a sinking or wire EDM machine and electrically conductive material. In this machining process, electrically conductive carbon and carbide products adhere to the surface of the workpiece during discharge. The characteristics of the layer are affected by the electrical machining conditions and the physical characteristics of the workpiece. Typically, the machining conditions for SEDM and WEDM are very different. In this paper, to investigate the difference in the machining properties of SEDM and WEM, several experiments were carried out by varying the amount of Al2O3 added to the Si3N4 ceramics, and the size of the tool electrode. The material removal rate and the surface roughness were estimated for workpieces following EDM. Discharge waveforms were observed with a digital oscilloscope and analyzed by using the designed program. Machining properties were analyzed by considering the structure of the workpiece and the electrode diameter.