Electrical discharge machining of submicron holes using ultrasmall-diameter electrodes

We have carried out the electrical discharge machining (EDM) of submicron holes using ultrasmall-diameter electrodes. Two types of electrode were used: tungsten electrodes fabricated by the combination of wire electrodischarge grinding and electrochemical machining, and silicon electrodes originally designed as probes for scanning probe microscopes. The diameters of the former and latter were 1 μm or less, and less than 0.15 μm, respectively. Holes were drilled using a relaxation-type pulse generator at an open-circuit voltage of less than or equal to 20 V with the machine's stray capacitance as the only capacitance. Using tungsten electrodes, holes of less than 1 μm in diameter and more than 1 μm in depth were successfully drilled. A 1.3-μm-wide slot was also fabricated by drilling many holes with a small pitch. It was possible to drill holes of approximately 0.5 μm diameter using silicon electrodes because the electrode diameter was less than those of the tungsten electrodes. These holes have the smallest reported diameter for holes drilled by EDM, indicating the possibility of submicron- and nanoscale machining by EDM.     

Machining characteristics of micro EDM in water using high frequency bipolar pulse

Micro EDM using conventional pulse generators such as the RC type or transistor type with water as the dielectric fluid suffers from poor accuracy of machined structures due to electrolytic corrosion. In this study, a new high frequency bipolar pulse generator for micro EDM in water was developed in order to prevent electrolytic corrosion. The new pulse generator produced a high frequency bipolar pulse possessing a positive pulse duration of several hundred nanoseconds with a high repetition rate provided to the machining gap. Discharge characteristics of micro EDM using the new pulse generator were investigated. Machining characteristics were also investigated according to machining conditions such as the repetition rate, positive voltage, capacitance and resistivity. Using the new pulse generator, micro holes without electrolytic corrosion were successfully fabricated in deionized water and tap water. Machining time, tool wear and clearance using the new pulse generator in deionized water decreased compared with those using the RC circuit in kerosene.     

Three-dimensional shaping by dot-matrix electrical discharge machining

This paper deals with a new prototyping method called dot-matrix electrical discharge machining (EDM) with scanning motion. The machining process by the dot-matrix method is similar to printing motion with a dot-impact printer. This method can be applied not only to EDM but also electrochemical machining and forming. A prototype of the machining unit for the dot-matrix method has six feeding devices for thin wire electrodes. The electrodes of 300 μm in diameter are arranged with the pitches of 760 μm. To obtain a smooth surface, a planetary motion in the x-y-plane is added to the feeding of the machining unit in the z-direction, the same area is machined repeatedly, or the machining unit is moved with fine feed. By compensating for the wear of the electrode during the scanning EDM, various shapes with the accuracy of micrometers order can be obtained without a formed tool electrode.     

Investigations aiming to increase electrochemical machining accuracy

Under conditions of electrochemical sinking, the standard deviation of inter-electrode gap thickness decreases with the decrease of machined area, while the machining accuracy increases. Advantage may be taken of this fact when using an electrode with a working area smaller than its machined area and when using different electrode kinematics than for sinking. For instance, the working electrode should move above the machined area at a velocity v_p. Theoretical and expeirmental research shows, that electrochemical machining (ECM) processes can show a significant increase of machining accuracy when compared to classical electrochemical sinking. This results mainly from the fact that parameters of the process can be chosen in such a way that the uneven distribution of physical properties in the machining space does not radically affect changes of inter-electrode gap thickness. Because the productivity of the discussed case is lower than for classical electrochemical sinking, it should be applied for finishing operations in which a small material excess is removed.     

激光微涂敷层的电火花后处理及其加工性能

分析、测量了不同加工条件下的材料去除率、相对电极损耗和电火花加工表面粗糙度,并研究了表面微裂纹和微硬度分布。实验结果表明,不同的材料具有类似的电火花加工性能,材料去除率随脉冲电流的增加而增加,峰值电流比脉冲宽度对表面粗糙度的影响更显著。研究结果对于选择合适参数进行电火花后处理具有重要意义。     

小孔加工电火花脉冲电源实验研究

针对传统脉冲电源普遍存在的电能利用率低、电阻发热严重、散热设备庞大等问题,提出了一种取消限流电阻的电火花脉冲电源结构。电源由PWM控制器、DC/DC变换器、加工脉冲时序发生器等组成。电源本身具有自动调节输出电压的功能,可实现空载击穿电压、脉冲宽度、脉冲间隔独立可调,取消了体积庞大的工频变压器。通过多次小孔加工实验获得了脉冲电源加工过程中电极损耗率、小孔椭圆度以及单边放电间隙等和电源稳定性相关参数的数据曲线,所得数据表明电源具有较好的加工稳定性和可靠性。     

Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid

A surface modification method by electrical discharge machining (EDM) with a green compact electrode has been studied to make thick TiC or WC layer. Titanium alloy powder or tungsten powder is supplied from the green compact electrode and adheres on a workpiece by the heat caused by discharge. To avoid the production process of the green compact electrode, a surface modification method by EDM with powder suspended in working fluid is proposed in this paper. After considering flow of working fluid in EDM process, the use of a thin electrode and a rotating disk electrode are expected to keep powder concentration high in the gap between a workpiece and an electrode and to accrete powder material on the workpiece. The accretion machining is tried under various electrical conditions. Titanium powder is suspended in working oil like kerosene. TiC layer grows a thickness of 150 μm with a hardness of 1600 Hv on carbon steel with an electrode of 1 mm in diameter. When a disk placed near a plate rotates in viscous fluid, the disk drags the fluid into the gap between the disk and the plate. Therefore, the powder concentration in the gap between a workpiece and a rotational disk electrode can be kept high. A wider area of the accretion can be obtained by using the rotational electrode with a gear shape.     

Wire electrochemical machining using reciprocated traveling wire

Wire electrochemical machining (WECM) is a cutting process in which the workpiece acts as an anode and the wire as a cathode. WECM is typically used to cut plates and exhibits a great advantage over wire electro-discharge machining, namely, the absence of a heat-affected zone around the cutting area. The enhancement of WECM accuracy is a research topic of great interest. In WECM, the homogeneity of the machined slit has a decisive influence on the machining accuracy. This is the first study in which the integration of pulse electrochemical machining (ECM) and a reciprocated traveling wire electrode was used to improve the homogeneity of this slit. The experimental results show that the combination of pulse ECM and a reciprocated traveling wire electrode could enhance the accuracy of WECM and that generally a low applied voltage, pulse duty cycle, and electrolyte concentration; an appropriate traveling wire velocity; and a high pulse frequency and feeding rate enhance the accuracy and stability of WECM. Finally, a microstructure with a slit width of 177 μm, with a standard deviation of 1.5 μm, and with an aspect ratio of 113 was fabricated on a stainless steel substrate measuring 20 mm in thickness.     

EXPERIMENTAL INVESTIGATIONS INTO THE INFLUENCE OF MAJOR OPERATING PARAMETERS DURING MICRO-ELECTRO DISCHARGE DRILLING OF CEMENTED CARBIDE

Present study investigates the influence of major operating parameters on the performance of micro-EDM drilling of cemented carbide (WC-10wt%Co) and identifies the ideal values for improved performance. The operating parameters studied were electrode polarity, gap voltage, resistance, peak current, pulse duration, pulse interval, duty ratio, electrode rotational speed and EDM speed. The performance of micro-EDM drilling process was evaluated by machining time, material removal rate (MRR), relative electrode wear ratio (RWR), spark gap, surface finish and dimensional accuracy of micro-holes. It has been found that there are two major conflicting issues in the micro-EDM of carbide. If the primary objective is to obtain better surface finish, it can be obtained by the sacrifice of high machining time, low MRR and high RWR. However, for faster micro-EDM, the surface roughness is higher and electrode wear is again much higher. It is concluded that negative electrode polarity, gap voltage of 120 V, resistance of 33 Ω, peak current of 8 A, pulse duration of 21 μs, pulse interval of 30 μs, duty cycle of 0.47, electrode rotational speed of 700 rpm and EDM speed of 10 μm/s can be considered as ideal parameters to provide improved performances during the micro-EDM of WC-Co.     

磨粒辅助EDM与ECM复合加工技术

微机电系统(Micro electromechanical systems,MEMS)的快速发展与产品微型化的发展趋势对微细结构表面(包括微孔、微槽和微棱柱/锥等)的加工质量提出了更高的要求,为了提高微细结构表面的加工质量,提出一种磨粒辅助放电加工(Electrodischarge machining,EDM)与电化学加工(Electro chemical machining,ECM)复合加工新方法,通过建立微加工模型分析了该方法的加工机理,搭建了微加工试验平台,并进行了工艺参数优化研究,采用直径500μm和75μm的钨电极在SUS 304不锈钢上分别进行了微盲孔和微通孔加工试验研究,结果表明,在所用的EDM、EDM与ECM复合加工和磨粒辅助EDM与ECM复合加工三种方法中,磨粒辅助EDM与ECM复合加工方法获得的表面粗糙度(Ra15 nm)最高,因此该方法是微细结构表面高效和高质量加工的最佳方法之一。     

Investigating and removing the re-sticky debris on tungsten carbide in electrical discharge machining

Electrical discharge machining (EDM) is an excellent method to machine tungsten carbide with high hardness and high toughness. However, debris from this material produced by EDM re-sticking on the workpiece surface remarkably affects working efficiency and dimension precision. Therefore, this study investigated the re-sticky phenomenon of tungsten carbide and how to reduce the debris re-sticking on the workpiece surface. In general, the polarity in EDM depended on the different electrical parameters of the machine input and the different materials of the tool electrode. The first item of investigation observed the re-sticky position of the debris to study the effect of different polarities during the EDM process. Next, the tool electrode was set at different conditions without rotation and with a 200 rpm rotational speed to evaluate the rotating effect in EDM. Finally, different lift distances of the electrode and different shapes of electrode with rotation were utilized to investigate the improvement for reducing debris re-sticking on the machining surface. The results showed that only negative polarity in EDM could cause the re-sticky phenomenon on tungsten carbide. On the other hand, debris would notably re-stick on any machining position when the tool electrode was not rotated in EDM. Besides, debris significantly stuck on the center of the working area with rotation of the electrode. Additionally, a larger lift distance of the tool electrode could reduce debris re-sticking on the working surface, but this process would decrease material removal rate in EDM. In the end, a special shaped design of the tool electrode resulted in the re-sticky debris completely vanishing, when the electrode width was 0.6 times the diameter of this cylindrical electrode.     

Electrical discharge machining with ultralow discharge energy

The possibility of electrical discharge machining (EDM) with ultralow discharge energy has been investigated. EDM using an RC discharge circuit was performed at low open-circuit voltages and a capacitance of approximately 30 pF. Workpieces were ultrasonically vibrated to remove debris and bubbles from the discharge gap, thus preventing short-circuiting. The machining proceeded at voltages lower than 15 V at a vibration amplitude of 0.4 μm. The maximum discharge energy per pulse is as small as approximately 3 nJ under these conditions. The volumetric electrode wear ratio can be 0.2% at voltages lower than 40 V, while it is normally more than 1% for EDM using an RC discharge circuit. Workpiece surfaces processed at voltages of 20 V or lower are smooth and free of observable discharge craters, and show no typical features of surfaces machined by EDM.     

Development of parallel spark electrical discharge machining

This paper describes the development of parallel spark EDM method. In the discharge circuit, the electrode is divided into multiple electrodes, each of which is electrically insulated and connected to the pulse generator through a diode. A capacitor is inserted parallel to each discharge gap between each electrode and workpiece (here workpiece is common for each electrode). Compared with conventional EDM in which only a singular discharge can be generated for each pulse, multiple discharges can dispersively be generated for each pulse in parallel spark EDM. Results of experiments on parallel spark EDM and conventional EDM show that not only is the machining process more stable, but the machining speed and surface roughness can also be improved with parallel spark EDM.     

超短脉冲电流微细电解加工技术研究

利用电化学腐蚀方法,在自制的电解加工机床上连续实现微细工具电极的制作和工件的加工,通过试验研究了超短脉冲的电压幅值和脉冲宽度对侧面加工间隙的影响。结果表明,减小脉冲宽度,降低加工电压,可以提高微细电解加工的精度。利用优化的加工参数,进行了微小孔加工、微细直写加工以及成形电极微细加工的实验。     

Analysis of the machining characteristics of EDM as functions of the mobilities of electrons and ions

Electrical discharge machining (EDM) is a process that can be used effectively to machine conductive metals regardless of their hardness. In the EDM process, material removal occurs because of the thermal energy of the plasma channel between the electrode and the workpiece. During EDM, the electrode as well as the workpiece is abraded by the thermal energy. Tool wear adversely affects the machining accuracy and increases tooling costs. Many previous studies have focused on mitigating the problems of tool wear by investigating various EDM parameters. In this study, the tool wear problem was investigated on the basis of the mobilities of electrons and ions in the plasma channel. The material removal volumes of both the electrode and the workpiece were compared as functions of the gap voltage. The material removal difference according to the capacitance was also investigated. The tool wear ratio was calculated under different EDM condition and an EDM conditions for reducing the tool wear ratio was suggested.     

Micro-hole Maching of Copper Using the Electro-discharge Machining Process with a Tungsten Carbide Electrode Compared with a Copper Electrode

This paper describes micro-hole machining of a copper plate using the electro-discharge machining (EDM) process. Tungsten carbide was selected as the material for the electrode and compared with a copper-electrode. A precision centreless grinding process was employed to grind the electrode down to the desired diameter. A series of experiments were performed on a traditional EDM machine to investigate the effects of electrode material polarity setting and of a rotating electrode. Results have shown that electrode wear and hole enlargement are both smaller when positive polarity machining is selected; whereas electrode wear is larger and machining speed is higher when negative polarity machining is selected. High-quality micro-hole machining in copper can be achieved by the proposed method.     

Investigation of emulsion for die sinking EDM

Machining fluid is a primary factor that affects the material removal rate, surface quality, and electrode wear of electrical discharge machining (EDM). Kerosene is the most commonly used working fluid in die sinking EDM, but it shows low ignition temperature and high volatility; if the improper operations are undertaken, it can cause conflagration. Using distilled water or pure water as the machining fluid in EDM, no fire hazard occurs, and the working environment is well; however, using distilled water or pure water as the machining fluid in EDM, the material removal rate of machining large surface is low, and the machine tool is easily eroded. Emulsion-1 and emulsion-2 used as working fluid in die sinking EDM are developed. The compositions of emulsion-1 and emulsion-2 are introduced. In comparison with kerosene, emulsion-1 and emulsion-2 used in EDM show high material removal rate, low surface roughness, high discharge gap, and good working environment. The electrode wear ratio in emulsion-1 is lower than that in kerosene. The electrode wear ratio in emulsion-2 is higher than that in kerosene. The effects of composition and concentration of emulsifier on the emulsion property and EDM performance have been investigated. The comparative tests of EDM performance with kerosene, emulsion-1, and emulsion-2 have been done.     

微细电火花加工中电极材料的蚀除机理研究

在微细电火花加工过程中 ,由于放电时间极短 ,使得其阴阳两极的电极材料蚀除过程产生较大的差异。本文应用传热学和电场的基本理论 ,分别对微细电火花加工阴阳两极的材料蚀除机理进行了理论研究 ,得出了在窄脉宽微细电火花加工中 ,尽量缩短脉宽可提高阳极材料的去除效率 ,同时又不会明显增加阴极材料损耗的结论。为微细电火花加工脉冲电源设计及加工工艺的改进提供了理论依据     

Experimental Investigation on Electrical Discharge Drilling of Ti-6Al-4V Alloy

This article describes the experimental investigation related to creation of holes in aerospace titanium alloy workpiece using static electrode machining and electrical discharge drilling (EDD) process. Special attachment for holding and rotating the tool electrode was developed and installed on electrical discharge machining (EDM) machine by replacing the original conventional tool holder provided on die sinking EDM. The effect of input parameters such as gap current, pulse on-time, duty factor and RPM of tool electrode on output parameters for average hole circularity (C_a) and average surface roughness (R_a) have been studied. It is observed that the effect of rotating electrode machining has considerable influence on the output parameters over stationary electrode machining. The micro-graphs and photographs of few selected samples were taken by SEM and metallurgical microscope, which also commensurate with the findings of the study.     

脉冲电解加工间隙测控方法的研究进展

介绍电解加工间隙测控的意义及重要性 ;总结电解加工技术中加工间隙的测控方法 ;分析国内外加工间隙测控的现状及关键问题 ,提出一种可实现在线测控加工间隙的新思路———建立六维力、电流信号与加工间隙之间的关系方程式。