Key Engineering Materials Vols. 523-524

Abstract: Local wet etching (LWE) is a non-conventional deterministic surface figuring and finishing technique in ultra-precision optics fabrication fields. The general removal function in LWE is cylinder, so fringe of the removal function is sharp and scale of the removal function is determined by inner diameter of the nozzle head. When fabricating some specimen with high frequency figure error, ideal designed shape can’t be achieved easily. Compared with general LWE removal function, Gaussian removal function is more suitable for figuring owing to its smoother fringe and the centralization of its energy. At the same time Gaussian removal function can improve the efficiency in calculation of the dwelling time, because it’s very suitable for Fourier transform. What’s more, theoretical residual figure error can also be reduced for Gaussian removal function’s high spatial resolution. Ideal Gaussian function is difficult to obtain in LWE, so we have proposed near-Gaussian removal function by eccentric rotation of the nozzle head. Through controlling offset of the eccentric rotation, we achieve the optimal near-Gaussian removal function in LWE. Aims of the introduction of near-Gaussian removal function in LWE are to improve the fabrication efficiency and to remove the surface’s high frequency residual figure error.

Abstract: In wire electrical discharge machining (WEDM), discharge current is one of the important factors which determine the machining properties, such as machining speed or machining accuracy. Previously developed machining simulators of WEDM, however, neglected the influence of workpiece conditions on the discharge current. Hence, this study developed a simulation tool for analyzing the discharge current in WEDM, where the electromagnetic field analysis around the wire electrode was coupled with the analysis of the equivalent circuit of the entire pulse generator circuit. The simulation results showed that, under the same pulse voltage and duration, the discharge current may change depending on the electromotive force which is determined by material properties, geometry and size of workpiece and wire electrode. In this study, the discharge current was calculated with different workpiece thicknesses. In addition, the discharge current was measured and compared with the calculation results in order to show the validity of the simulation tool.

Abstract: Brittle materials, such as silicon, silicon carbide and sapphire have been conventionally sliced for wafers by a multi-wire saw method with slurry in industrial fields. Recently, the multi-wire saw method with a fixed diamond abrasive wire has been available as a commercial product at acceptable cost, and the high slicing performance is expected compared with the normal multi-wire saw method with slurry. However, there still remain some problems such as bad working environment with abrasives, cleaning cost of sliced wafers, crack generation on the sliced surface and a large kerf loss against a wafer thickness. On the other hand, the developed multi-wire EDM slicing method would accomplish the high performance slicing of silicon and silicon carbide with a narrow kerf width under a clean process environment. However, the thinner wire is challenging process with a normal round shape wire electrode. Therefore, the new wire electrode with track-shaped section was proposed in order to satisfy both the narrow kerf width and the high wire tension even in the case of thin wire electrode. In this study, the running control of wire electrode with a track-shaped section was experimentally investigated, and the possibility of proposed process was discussed.

Abstract: The authors developed Whirling Electrical Discharge Machining (Whirling EDM) [1] with the feedback circuit to control gap distance between tool electrode and workpiece. It was found that the probability of discharge was increased by feedback circuit [2]. However, the motor rotational speed control conditions were not optimized and the machining characteristics were not clarified. In this paper, the rotational behavior of tool electrode in Whirling EDM was observed by high-speed camera, and it was considered that the influence of motor rotational speed control conditions on machining characteristics and the relationship between electrical conditions and machining characteristics. As a result, the discharge probability was able to be increased by decreasing range of motor rotational speed in feedback circuit. In addition, the machining characteristics of Whirling EDM which are influenced by electrical conditions were clarified.

Abstract: This paper deals with a combination machining method of slicing with electro-chemical discharge machining (ECDM). Glass and fine ceramics are hard to machine, and cracks are generated on their surface during cutting and grinding because of their brittleness and hardness. ECDM has been studied for machining such insulating materials. The authors have proposed a deposition method of WC layer by electrical discharge machining (EDM) with a compressed WC-Co powder as an electrode. The improvement of removal rate is expected by combining wire-sawing with ECDM. A silica glass rod was cut with a saw wire with the WC layer. The removal rate of the combination machining is larger than that of individual use of ECDM or wire sawing. The groove width by wire sawing was smaller than the others. Cracks were seldom observed on the bottom and side walls of the groove. Providing the fresh working fluid also assisted to decrease the cracks. The synergy of wire-sawing with ECDM was obtained.

Abstract: This paper describes micro electrochemical machining (ECM) using the electrostatic induction feeding method. In ECM, electrolytic dissolution can be localized in the area where the gap width is narrow by using pulse durations shorter than several tens of nano-seconds. With the electrostatic induction feeding method which has been developed for micro electrical discharge machining, the current pulse of such short durations can be obtained more easily compared with the conventional pulse generators. In this study, the influences of the pulse voltage of power supply and feeding capacitance on the machining current were investigated theoretically. It was found that the current pulse duration is nearly equal to the rise time and fall time regardless of the pulse-on time of the pulse voltage. Hence, ultra-short pulses can be obtained without a need to use an expensive pulse generator. Micro-hole drilling carried out in a sodium nitrate aqueous solution with current pulse duration of 30ns showed that significantly small side gap of 2μm could be obtained.

Abstract: A new turning method to control the thrust force to be zero has already been proposed in our laboratory, and it is shown that micro shafts which diameters are less than 1 mm can be generated stably and repeatedly by applying this turning method. As an application of this turning method, a generating method of electrodes of micro electric discharge machining, EDM, for micro holes is proposed. When drilling a micro hole by EDM using an electrode with high aspect ratio, machined chip is difficult to be exhausted. Therefore, machining time has a tendency to be long and the electrode consumption becomes large. Standing in such a viewpoint, a new method to exhaust the chip smoothly by forming the micro electrode geometry is proposed in this paper. As the results of this study, it is confirmed that the machining time of micro holes can be shorted and the consumption of electrode can also be decreased experimentally.

Abstract: In the micro EDM method by electrostatic induction feeding, a pulse voltage source is coupled to the tool electrode by a capacitor in order to reduce the minimum discharge energy per pulse. Positive pulse voltage and negative pulse voltage are applied to the tool electrode alternately, relative to the workpiece. In this study, it was found that when the feeding capacitance was large, multiple discharges occurred frequently during a single pulse duration of the pulse voltage source. Since the impedance of the pulse voltage source which was produced by the authors varied depending on the polarity, the multiple discharges occurred in the polarity with the higher impedance, while they did not occur in the opposite polarity. It was also found that when multiple discharges occurred, the discharge energy was released intermittently in the multiple discharges. In this study, using multiple discharges phenomena, better surface was obtained with higher removal speed.

Abstract: It is difficult to machine permanent magnets by traditional machining such as turning, milling and grinding, because of magnetic force. However, electrical discharge machining (EDM), which is a non-contact thermal machining method, has been used for shape machining of magnetic materials. In the EDM process, non-magnetic materials such as copper and graphite are typically used for electrodes. Magnetic materials have a Curie point; therefore, their magnetic flux density reduces when they are heated to a high temperature. Because EDM is a thermal process, it has the potential to control the magnetic flux density of a machined surface. In this study, to clarify the relationship between magnetic flux density and temperature distributions in depth direction of permanent magnest by EDM, internal temperatures of magnets were investigated using a K type thermo couple during EDM. Neodymium magnets were used as work-pieces. The magnetic flux density of a machined neodymium magnet was measured. In addition, the effects of duty factor (D.F.) were also examined. The results showed that the average temperature inside of the magnet is determined by the input energy, depending on the discharge conditions. A decrease of surface magnetic flux density after EDM is affected by the magnitude of the area and the amount of decrease is due to the increase of the internal temperature of the magnet. Therefore, it isn’t determined by the magnitude of the simple input energy.

Abstract: AlN ceramic materials have high thermal conductivity and electrical insulation, prompting consideration of their use as a semiconducting material. Although AlN should be machined with a high accuracy of form and dimension to achieve products and components with requisite precision, mechanical and other machining methods such as the micro blasting technology or laser method cannot be used because of the brittleness and high thermal conductivity of AIN. Recently, we have succeeded in machining many insulating ceramics by EDM with the assisting electrode method. We have already machined many insulating ceramic materials such as Si₃N₄, ZrO₂ and Al₂O₃. However, inferior machining characteristics were obtained with AlN than with other materials. In this study, the effects of several electrical discharge conditions were examined to obtain better machining properties, such as high material removal rate and a low electrode wear ratio. It was found that machining time decreased with an increase in capacitance, while the electrode wear ratio increased. In addition, the machining hole profiles were straight along the depth direction, and the shape of holes was non-tapered