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This paper aims to design a novel pulse generator for electrical discharge machining (EDM) which can generate discharge current pulse with high discharge frequency more efficiently. Recent research has revealed that the LC pulse generator is able to generate higher rising speed discharge current pulses and high open voltage for EDM by utilizing electromotive force induced by an inductor. Conventional LC generators use inductors to store energy and switches to control the open gap voltage and discharge current flowing through the gap by varying the charging time of inductance, thereby realizing higher peak discharge power even with a low voltage DC power supply, as well as large removal volume per discharge due to high rising speed waveforms. However, the discharge probability of LC generators is lower than 20% if the discharge frequency divided by the switching frequency of the LC generator is defined as the discharge probability of the LC generator. This is because the duration of the open voltage pulse is significantly short in the order of several microseconds, although it usually takes several microseconds or longer for discharge to be ignited. This discharge delay time is an important parameter in EDM because longer delay time results in lower material removal rate, and an excessively short delay time indicates that the gap will be short circuited, resulting in decrease machining stability. Hence, this paper proposes a modified version of the LC generator in which a high impedance transistor pulse generator is used to control the discharge ignition in EDM to produce discharge frequency 2 to 3 times higher than the conventional LC pulse generator.

A micro rod machining method which can switch between electrical discharge machining (EDM) and electrochemical machining (ECM) by attaching/detaching a diode to/from a bipolar pulse generator in parallel to the working gap was newly developed using a wire electrode made of tungsten. The problem of the wire electrode wear was eliminated by the use of the wire electrochemical turning (WECT) method in which the tungsten wire electrode is continuously running. The ultra-short bipolar pulse current was generated by the electrostatic induction feeding method where a pulse voltage is coupled to the working gap through a feeding capacitance. The machining characteristics of three types of wire guide; disk-shaped WC guide, laminated wire guide and cylindrical acrylic guide, were studied. The experimental results showed that the cylindrical acrylic guide has the best machining characteristics without the influence of guide wear and with less stray current flowing through the working gap. Using the cylindrical acrylic guide, the influences of the feeding capacitance C1, and the total amplitude of the pulse voltage on the machining characteristics were studied. Finally, a stainless steel SUS 304 micro-rod with a high aspect ratio of 14 was fabricated efficiently by using the EDM and ECM modes for rough and finish machining in sequence with the same setup, pulse generator, and neutral electrolyte.

Electrolyte Jet Machining (EJM) has been performed on stainless steel surfaces with the aim of reducing bacterial retention through the generation of nanoscale surface morphology. Following initial EJM experiments aimed at investigating the influence of machining depth, machining speed and current density on the resulting surface roughness, three characteristic surfaces were produced with a current density of 10–18 A/cm2 and a machining speed of 0.8–8 mm/s to obtain an arithmetic mean height (Sa) of 0.5–0.74 μm and a density of peaks (Spd) of 0.25–1.26 μm−1. Relatively large differences between the three surfaces in terms of Spd allowed thorough investigation into the effects of surface feature size on bacterial retention to be performed. Reductions in the order of 90% compared to control samples were achieved for gram-positive Bacillus cereus and Staphylococcus aureus across the entire tested parameter range (Spd = 0.25–1.26 μm−1), while reductions in the order of 99% were achieved for gram-negative Escherichia coli and Pseudomonas aeruginosa for surfaces characterized by Spd > 1 μm−1. Not only do the results call attention to EJM as an innovative technology for producing antibacterial surfaces, they also highlight important differences in the behavior of gram-positive and gram-negative bacteria in relation to EJM-textured surfaces with nanoscale surface morphology.

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.

The novel wire electrochemical turning (WECT) method has been proposed to machine tungsten with neutral electrolyte and bipolar pulse current in the authors’ previous research. The influence of tool wear caused by the bipolar pulse current, which is needed to remove the oxide layer generated on tungsten surface when using a neutral electrolyte, was eliminated by the WECT method because the wire tool electrode was kept being wound during the machining process. In this paper, the influences of the most important process parameters, such as the kind of electrolyte, wire material, wire diameter, rotation speed of the workpiece rod, and frequency of the pulse voltage, were studied to understand the fundamental aspects of the WECT method and improve the machining efficiency of tungsten micro-rods. The experimental results showed that the NaCl electrolyte and the NaNO3 electrolyte had higher machining efficiency and better machining accuracy, respectively. The machining accuracy was improved with the SUS304 stainless steel wire used as tool electrode than the brass wire tool electrode. It was also found that the machining efficiency and accuracy were improved with decreasing the wire diameter and increasing the rotation speed of workpiece because the influence of stray current flowing through the side and end surface of machined micro-rod was reduced. The machinable length of micro-rod peaked at an optimum feed speed of the workpiece, and the optimum speed increased with increasing the frequency of pulse voltage. As a result, a minimum rod diameter of 11 μm was fabricated with a high aspect ratio of 36. Compared with the previous method using a platinum film as tool electrode, the aspect ratio was increased significantly because the influence of tool wear was eliminated.

Electrochemical jet processing (EJP) is an easy-to-implement technology for creating complex microstructures based on anodic dissolution without requiring specially designed cathodes. In this study, a new electrolyte-jet-based surface modification method based on the principle of cathodic hydrogenation, namely, electrochemical jet hydrogenation (EJH), was proposed for the first time to selectively modify the material in aspects of its brittleness. In this method, a workpiece is innovatively set as the cathode, and hydrogen evolution occurs locally on the cathode surface within the jet impinging area, leading to a localized H-treatment of the cathode material. The hydrogen-material interaction can alter the surface material property and result in localized surface modification, for example, material ductile-brittle transition by hydrogen embrittlement (HE). In this research, the proposed method was validated on niobium metal. According to the results, evident localized embrittlement was achieved, and the degree of embrittlement was precisely controlled by adjusting the electrochemical parameters mainly including current density and processing time. As a selective surface modification method, EJH can be applied as an assistive technology in hybrid machining of difficult-to-machine superalloys where localized surface modification of ductile-brittle transition is expected.

It is known that only several percent of the meltpool in the discharge crater is removed as debris in electrical discharge machining (EDM). Hence, machining rate would be improved and thickness of the damaged layer would be decreased if most of the molten material could be removed. In the previous researches, it was found that the removal volume of single pulse discharge, machining rate of continuous pulse discharges, and tool wear are significantly influenced by the rising and falling speeds of the discharge current pulse. In the previous researches however, the influences of the pulse shapes were investigated under a limited machining condition. Machining characteristics of these pulse shapes have not been compared under different machining conditions. This study shows that the most efficient pulse shape can be different if the discharge duration and discharge current are changed. The reason why the optimum pulse shape depends on the machining conditions is discussed.

In wire electrical discharge machining (WEDM), discharge craters and heat-affected zone impair the surface roughness and generate micro cracks and residual stress. Hence, this study aimed to finish the WEDMed surface by electrochemical machining (ECM) using the same setup, wire electrode and machining fluid used in WEDM. Ultrashort bipolar pulses were used to improve the finishing speed and surface roughness. To realize electrochemical finishing on a WEDM machine, first the influences of the electrolyte concentration on machining characteristics were investigated, and it was found that machining can be performed even if the electrolyte concentration of NaNO3 aqueous solution is diluted to 0.2 wt%. With lower electrolyte concentration, smaller surface roughness was obtained at smaller machining depth. Next, the material of the wire electrode was changed from platinum to brass which is commonly used for WEDM, and the wear rate of the brass wire was investigated. Although the wear rate was 20 times higher than that of platinum, wire breakage did not occur in most of the experiments conducted in the present work. Then, machining was performed with an equal current density for different thicknesses of workpiece, and influences of the workpiece thickness on the process characteristics were investigated.

The increasing demand for optimized component surfaces with enhanced tribological, optical, chemical, biomedical, and geometric complexity is a key driver in the development of electrolyte jet machining (EJM) technology. To date the applications of EJM method have been limited to planar surface processing. Realization of three-dimensional freeform surfaces with high precision by EJM is of significant importance to advance the process. In this study, the characteristics and principle of multi-axis EJM of arbitrary-shaped workpieces were investigated for the first time. The resultant machined features were experimentally demonstrated under principal factors of inter-electrode gap distance, jet orientation relative to workpiece surface, jet orientation relative to gravity, and workpiece configuration. Furthermore, a simulation model of EJM considering jet orientation and workpiece curvature was built to analyze the electrolyte jet flow and clarify the principle of the current density distribution in the non-symmetric electrolyte flow. It is demonstrated that jet orientations directly affect the profiles of current density distribution and consequently result in varied machining results. In EJM of freeform surfaces, a smaller radius of curvature brings about more intensive concentration of current density distribution and consequently a larger machining depth. The same principles are confirmed in EJM of corner shapes with different angles.

A novel method to switch the machining mode between micro-EDM and micro-ECM in the same electrolyte using the same tool electrode and pulse generator circuit was proposed in this research. This method uses the electrostatic induction feeding method where a pulse voltage is coupled to the working gap through a capacitance. The switch between the EDM and ECM modes was realized by utilizing the oxide film formed on the surface of the tungsten electrode. To switch the ECM mode to the EDM mode, the polarity of the diode attached in parallel to the working gap was changed to control the generation of the oxide film. In the ECM mode, the diode was connected so that it is forwardly biased when the polarity of the tool electrode is positive, thereby the oxide film was not formed on the tungsten electrode. In the EDM mode, since the polarity of the diode was reversed, the oxide film was formed, thereby discharge could be ignited. The switching ability was verified from the waveforms of gap current and voltage which showed that discharge occurred in the EDM mode, whereas there was no discharge waveform observed in the ECM mode. In the ECM mode, the influences of total amplitude and rise/fall time of pulse voltage were investigated. In the EDM mode, the discharge probability with different concentrations of electrolyte was researched first. Then, the influence of the feeding capacitance C1 on the EDM mode was investigated. Finally, through-holes were machined by conducting the EDM and ECM modes in this order to obtain higher material removal rate in rough machining and better surface roughness without residual stress in finish machining.

This paper aims to visualize spatio-temporal phenomena in electro physical and chemical processes in order to understand machining mechanisms and to achieve a technological breakthrough. In situ measurement methods to visualize the time evolution of stress, strain, temperatures, fluid velocities, and species density distributions, together with 2D or 3D images of the workpieces being processed are summarized. Applications of these methods to fundamental studies on electrical discharge machining, electrochemical machining, laser processing and additive manufacturing are introduced. Signal recording and processing technologies and real time monitoring enabling closed-loop feedback control are also discussed.

In the interelectrode gap of ECM, the electrolyte flow results in ununiformly distributed temperature and volume fraction of bubbles, leading to uneven distribution of the gap width. This paper aimed to realize high precision ECM using stationary electrolyte. A single current pulse was supplied after every jump flushing motion of the tool electrode. The pulse duration used was set sufficiently short not to cause the boiling of the electrolyte. Furthermore, the stamp flushing method was newly developed to squeeze out the sludge and bubbles from the gap to achieve high precision machining.

20th CIRP Conference on Electro Physical and Chemical Machining, ISEM 2020, online, 19 Jan 2021 - 21 Jan 2021; Procedia CIRP 95, 189-194 (2021). doi:10.1016/j.procir.2020.01.198 special issue: "20th CIRP Conference on Electro Physical and Chemical Machining / Edited by Konrad Wegener, Moritz Wiessner", Published by Elsevier, Amsterdam [u.a.]

The present study proposes a novel method for detecting micrometric undercuts (UCs) generated by electrochemical machining for the production of surfaces with tailored functionality. Two different algorithms for the detection of UCs based on two-dimensional topographic maps are tested. The first is a traditional approach based on definition of UCs in terms of surface orientation with respect to a reference direction. The second is an innovative alternative approach designed to reduce sensitivity to numerical effects that potentially lead to overestimation of the number of detected UCs. Electrochemical Jet Machining (ECJM) is used to texture SUS 316L specimens with the aim of producing a measurable surface with a representative number of micrometric UCs. Generated surface textures, comprising craters with diameters ranging from a few microns to tens of microns, are cross-sectioned and inspected with Scanning Electron Microscopy. The extracted profiles allow the novel method for detection of UCs to be efficiently tested and compared with the traditional approach. The number of UCs is found to decrease with increasing electrolyte jet scanning speed, while remarkable differences are revealed between the two calculation approaches at scanning speeds below 2 mm s−1.

This paper shows that the radial flow caused by the collision of the metal vapor jets generated from the surfaces of the tool electrode and workpiece serves as one of the material removal mechanisms in electrical discharge machining (EDM). Material removal per single discharge of a steel workpiece was larger using the tool electrode made of materials with lower boiling point. Observation of the discharge gap using a high-speed video camera showed that molten material on the workpiece was swept away more significantly and larger amounts of debris was removed using the tool electrode materials with lower boiling point. The molecular dynamics simulation showed that the radial velocities of the vapor atoms removed from the workpiece were higher using the tool electrode with lower boiling point because the jets from the tool electrode were more intensive. Furthermore, the discharge reaction force acting on the wire electrode in a single pulse discharge in air was measured from the wire vibration. The results showed a larger discharge reaction force using a zinc coated brass wire compared with a non-coated wire. Thus, it can be concluded that the shear force caused by the jets serves as one of the material removal mechanisms in EDM. Furthermore, one of the reasons for the higher material removal rate obtained using zinc coated wire electrode in wire EDM is the ease of boiling of the zinc coating layer on the wire surface.

A new method, wire electrochemical grinding(WECG), is proposed for machining tungsten micro rods with neutral electrolyte and bipolar current. The inevitable problem of tool wear due to the bipolar current can be eliminated by using a running wire as the tool electrode with this method. In this study, three kinds of wire guides were used for feeding the workpiece rod in the axial direction: cylindrical cemented tungsten carbide (WC) guide, cylindrical zirconium oxide (ZrO2) guide, and disk-shaped cemented WC guide, to investigate the influence of materials and configurations of the guide on the machining characteristics. The experimental results showed that the disk-shaped WC wire guide can realize higher machining accuracy because of less influence of stray current flowing through the gap formed between the machined micro-rod and wire guide. Even if the ZrO2 material has a much lower electrical conductivity than that of the cemented WC, the machining process was still influenced by the stray current between the micro-rod and wire guide. Furthermore, micro-rods were also machined by feeding the rod workpiece in the radial direction. Compared to when the workpiece is fed in the axial direction, the current was higher, which was clarified by the simulation of the current density distribution. However, the machining time was much longer than that with the workpiece fed in the axial direction because more machining steps were necessary to obtain a smooth side surface. With the workpiece fed in the axial direction method, a tungsten micro-rod of 35 μm in diameter was machined to the length of 163 μm. The results confirmed that the new method is capable of miniaturization equivalent to micro electrical discharge machining (EDM).

Recently, a multi-wire electrical discharge slicing (EDS) process has been proposed for slicing SiC ingots into wafers. A significant reduction in kerf loss is expected with this method compared with the conventional multi-wire saw method. However, this process entails a high risk of wire breakage. Therefore, in the present study, a novel electrical discharge slicing process utilizing a running ultra-thin foil tool electrode is demonstrated for the slicing of SiC ingots. Relative to multi-wire EDS, the risk of tool breakage can be reduced with this new technique by increasing the tool cross-sectional area. A 25-mm (1-inch) SiC ingot was successfully sliced with the proposed method and a kerf loss of about 100 μm was achieved by utilizing a foil tool electrode with a thickness of 50 μm. This paper summarizes the specific characteristics and results of this process, including the machining stability, machining strategy, selection of foil tool material and multiple simultaneous slicing.

Machining of high-aspect ratio slots is a common operation in industry, particularly in the mold and die and aerospace sectors. Electrical Discharge Machining (EDM) is a competitive technology for this operation, since it does not depend on material hardness and can fulfill strict geometrical requirements. However, due to debris accumulation in the narrow gap, the machining depth for stable machining is limited. The present work includes original findings about the influence of machined holes on flushing efficiency. Different configurations for this basic approach have been proposed and their performance discussed in terms of machining time and electrode wear. A remarkable and original finding is that continuous pockets or open holes are much better than the separated or closed holes. Furthermore, the paper shows that a sudden change in material removal rate occurs when the vacant spaces of the holes are sunk in the slot. A deeper insight into the EDM phenomena has been achieved through a study of discharge parameters, by analyzing discharge frequency, duty factor, discharge delay time and discharge off-time. In this study, using the proposed electrode reduced the process time by as much as 65% in the machining of 10 mm depth slots. Moreover, machining stability can be guaranteed if the flushing pockets machined on the electrode go through the whole depth of the machine.

The gap phenomena of EDM in deionized water and oil were observed using a high speed video camera. It is known that depending on situations, higher material removal rate can be obtained in water than in oil. To understand the reason, the distribution of discharge locations was observed using a transparent electrode made of SiC single crystal. It was found that the discharge locations were dispersed more widely in water than in oil. Hence, the machining was performed more stably in water than in oil, whereas the material removal volume per single pulse discharge was the same both in water and oil. Thus, the movement of bubbles and the flow field of the dielectric liquid in the working gap were observed using the same setup. Then, it was found that the radial flow of the liquid due to discharge can propagate with longer distances from the discharge spot and the oscillation of bubbles can continue for a longer period of time in water. Using oil in contrast, the radial flow could not reach a wider range than in water, and the bubble oscillation was damped due to the viscosity of oil, which is higher than water. Therefore, the temperature at the discharge spot can be lowered by the flushing flow of the liquid generated by the subsequent discharges. Thus, the cooling effect of the liquid is higher in water than in oil, thereby higher discharge current with shorter discharge interval can be used in water without causing a concentration of discharge locations. This is presumably the reason for the higher material removal rate in water than in oil.

In this study, tungsten was machined successfully with the neutral electrolyte NaNO 3 aqueous solution and bipolar current because the nonconductive oxide layer on the surface, which interrupts the electrochemical dissolution process, was removed by the NaOH generated when the polarity of the tungsten is negative. However, the tool electrode was worn due to the bipolar current. Although the tool wear ratio was decreased when the platinum sheet was used as tool electrode than that with the stainless steel plate used as tool electrode, the platinum tool was also slightly worn. Hence, to avoid the influence of the tool wear, a new method, WECG (wire electrochemical grinding), for tungsten machining with neutral electrolyte and bipolar current is proposed, and the running brass wire is used as the tool electrode. Thereby, the problem of tool wear inevitable due to the bipolar current was eliminated because the running wire tool was used in this method.

An EDM method for manufacturing plastic micro gear pumps is proposed. Micro gates and cavities for mating gears are machined by EDM in both sides of a stainless steel foil, respectively. The foil is then transferred to an injection molding machine, and fixed to a mold to form mating gears by injection molding. The mating gears formed on the foils are then re-positioned on the EDM machine, released from the foil, and placed in a second foil pre-machined by EDM for the pump housing, using an ejector pin fabricated by reshaping the tool electrode used to machine the two foils.

This paper reveals the influence of easiness of boiling of tool electrode materials on the machining speed in EDM. It is considered that during the discharge process, both the anode and cathode emit the metal vapor jets into the gap. The interaction of the jets in the gap generates a kind of shear force acting on the molten pool, which helps remove the molten material in the workpiece. Moreover, it was found that lower boiling point of tool electrode results in stronger interaction of the jets and larger shear force, thereby the material removal volume of the discharge crater will be larger. To validate the above thoughts, the effect of the interaction of the jets on the material removal process was investigated through the molecular dynamics simulation. Then, the material removal volume of the discharge crater in a series of single discharge experiments using tool electrodes with different thermo-physical properties related to the easiness of boiling was compared to verify the simulation results. Finally, the machining experiments were performed using a sinking EDM machine to verify the higher removal performance of tool electrode materials which are easy to emit vapor jets.

Electrolyte jet machining (EJM) is an innovative form of electrochemical machining (ECM) with excellent machining flexibility and accuracy. Recently, the nozzle inclination is introduced into EJM to further expand its machinability. In this work, to clearly reveal the mechanism behind this EJM process, a three-dimensional (3-D) multiphysics model of the stationary EJM process accounting for two-phase flow field and electric field was developed for the first time and experiments were also performed for validation. With the inclined nozzle, the distribution of flow velocity and thickness of electrolyte film around the jet exhibits uneven, thus resulting in non-circular hydraulic jump and non-axisymmetric anodic current density distribution, respectively. Besides, the effects of nozzle inclination angle and electrolyte ejecting flow velocity were investigated. Larger inclination angle leads to increased asymmetry of flow velocity and current density distribution, which results in more seriously non-circular hydraulic jump and asymmetric machined dimple shape. The electrolyte ejecting flow velocity affects the jet shape and current density distribution that eventually influences the machined results with the inclined nozzle, but this effect does not exist with the conventional vertical nozzle. Moreover, this work was extended to the translating EJM process to study the effect of nozzle translating direction with the inclined nozzle. It is demonstrated that through selecting translating directions, grooves with different shapes and surface finish can be machined even using a certain nozzle inclination. The mechanism for these phenomena was clarified by simulation, paving the way for design of the EJM process with the inclined nozzle.

Micro-rods were machined by electrochemical machining using the electrostatic induction feeding method, with which ultra-short current pulse duration of several tens of ns can easily be obtained. A tungsten plate and stainless steel (SUS304) rod were used as the tool electrode and workpiece, respectively. To improve the machining accuracy, the machining characteristics when the workpiece is fed in the axial and radial directions were investigated using NaCl aqueous solution as the electrolyte. When fed in the axial direction, the machinable length of the micro-rods was found to peak at the optimum feed speed because of the influence of pitting corrosion and collision between electrodes. When the workpiece was fed in the radial direction, the influence of pitting corrosion decreased, however, the micro-rod was shortened with increasing feed distance in the radial direction because of the stray current flowing through the end of the micro-rod. The simulation results of the material removal process agreed qualitatively with experimental results. Next, machining characteristics were compared between the electrolytes, NaCl and NaNO 3 aqueous solutions, by feeding the workpiece in the axial direction. It was found that the influence of pitting corrosion was eliminated with the NaNO 3 aqueous solution, and there was no machinable length limitation with suitable feed speeds. In addition, the taper angle and gap width were smaller with the NaNO 3 aqueous solution, compared with those of the NaCl aqueous solution. Stainless steel micro-rods of 100 μm in diameter with a high aspect ratio of 20 were fabricated with the NaNO 3 aqueous solution. According to the preliminary research results, the machinable minimum diameter of the micro-rods was investigated and micro-rods with an average diameter of 9 μm and length of 78 μm were machined successfully.

Recently, the production of silicon wafers 450 mm in diameter has begun. However, a precise method for warp measurement of 450 mm wafers has not yet been established. Hence, the authors have developed a four-point-support inverting method to measure the warp shapes of large diameter wafers with high accuracy. The principle of this measurement method is equivalent to that for the three-point-support inverting method developed for warp measurement of 300 mm wafers. In the four-point-support inverting method, surface shape measurement can be performed with decreased deflection due to gravity using an additional central support. In this study, the principle of the proposed measurement method was verified experimentally. It was found that the four-point-support inverting method could measure the warp of a 300 mm wafer with high accuracy, equivalent to that of the three-point-support inverting method. However, in this experiment, a steel ball was used for additional support which caused concern regarding damage to the wafer surface at the point of contact with the support. Hence, a noncontact support method using an air bearing was proposed. It was found that the noncontact support method could measure the warp with accuracy equivalent to the contact support method described previously. Moreover, this study demonstrates the superiority of the four-point-support method to the three-point-support method regarding the repeatability of the warp measurement.

During the ECM process, the metal workpiece is dissolved and turns into sludge which contaminates the electrolyte. To realize precise ECM with high cost-effectiveness, an electrolyte treatment system which can realize reuse of the electrolyte and maintain the electrolyte quality constant is significantly important and essential. Especially, in the ECM of alloys containing a certain level of chromium, it is very likely chromium dissolves to the toxic carcinogen Cr(VI). Therefore, an electrolyte filtration system is required for removing not only the sludge but also residual toxic ions in the electrolyte for health and environment conservation reasons. In this study, activated carbon and scrap iron, which are low cost and easily available materials, were newly utilized to reduce and remove toxic Cr(VI) ions. Experiments clarified that use of activated carbon has no influence on the machining ability of NaNO 3 aqueous solution serving as the electrolyte. By adjusting the pH of the electrolyte to acidic, activated carbon can remove Cr(VI) from the NaNO 3 aqueous solution electrolyte to a concentration of less than 0.1 mg/L. On the other hand, scrap iron generated from metal cutting processes can be used to reduce Cr(VI) to non-toxic Cr(III). By mixing HNO 3 into the electrolyte solution, the reduction efficiency of scrap iron on Cr(VI) improves significantly.

Electrical discharge machining by foil electrode serves as an alternative method for SiC slicing. This technology uses a highly tensioned thin foil as the tool electrode. The main advantages over wire EDM are that the foil thickness can be made smaller than the wire diameter, vibrations can be avoided by applying high tension, and higher current can be supplied since there is less risk of tool breakage. However, due to the large side surface area of the foil electrode, there is a high occurrence probability of side surface discharges and high concentration of debris, which affects kerf width accuracy and machining stability. In the aim to overcome both problems, this study proposes two foil electrode designs: a foil electrode in which holes are machined and the insulation of the side surface areas by a resin coating layer of 5 μm thickness. The influences of both foil electrodes were tested with three different slicing strategies: no strategy, applying jump motion of the tool electrode, and applying reciprocating motion. From machining experiments and comparative studies of the discharge delay time, it was found that with both foil tools, the occurrence probability of side surface discharges can be reduced. In addition, the chip pocketing effect of the holes enhance the flushing conditions, resulting in a higher cutting speed.

This paper describes tungsten micro-rod machining using a neutral electrolyte and ultra-short-pulse bipolar current supplied by the electrostatic induction feeding method. The tungsten oxide layer can be removed by NaOH generated when the polarity of tungsten electrode is negative. The machining accuracy improves with higher electrolyte concentration, and there is less tool wear when platinum tool electrode is used. A micro-rod with a diameter of 7.1 μm and aspect ratio of 14 was fabricated successfully. The machinable minimum diameter was equivalent to that obtained by EDM. Furthermore, the current waveform could be optimized to increase the material removal rate.

Micro-rods were machined by electrochemical machining using the electrostatic induction feeding method, with which ultra-short current pulse duration of several tens of ns was obtained. A tungsten plate and stainless steel (SUS304) rod were used as the tool electrode and workpiece, respectively. The rotating workpiece rod was fed axially in the direction normal to the top surface of the tool electrode, thereby the diameter of micro-rod was determined by the depth of cut in the radial direction. The influences of voltage amplitude and the thickness and surface area of the tool electrode on machining characteristics were investigated from the aspects of machinable length limitation, straightness of the micro-rod, pitting corrosion, and surface finish to determine the optimum machining conditions. The results of machining experiments and current density simulation showed that the influence of stray current on machining accuracy markedly decreased with decreasing tool electrode thickness, allowing micro-rods with higher aspect ratio and better surface finish to be fabricated. On the other hand, the influence of the top surface area of the tool electrode on machining accuracy was small due to small changes in the current density on the frontal surface of workpiece, resulting in small changes in the material removal rate (MRR).