A study on micro-hole machining of polycrystalline diamond by micro-electrical discharge machining

Polycrystalline diamond (PCD), with its superior wear and corrosion resistance, is an ideal material for micro-hole parts in the field of microfabrication. This study investigated the micro-hole machining performance for PCDs by micro-electrical discharge machining (micro-EDM). A series of experiments were carried out to investigate the proper machining polarity and the impacts of micro-EDM parameters on machining performance. Experimental results indicate that negative polarity machining is suitable for micro-EDM of PCDs because of the protection brought over by the adhesion sticking to the electrode. An appropriate volume of adhesion on the tool electrode can help to increase the material removal rate (MRR) and reduce the relative tool wear ratio (TWR). By contrast, an excessive volume of adhesion can lead the machining into a vicious circle in which micro-holes are drilled with overlarge diameters. An optimal set of machining conditions was chosen among the investigated ranges of nominal capacitance and electrode rotation speed. An exemplary PCD through-hole, machined under the chosen optimal machining conditions, shows satisfactory machining results.     

A study on the quality micro-hole machining of tungsten carbide by micro-EDM process using transistor and RC-type pulse generator

Tungsten carbide (WC) is an extremely hard and difficult-to-cut material used extensively in manufacturing because of its superior wear and corrosion resistance. Besides diamond-charged grinding wheels, micro-EDM is an effective method of machining this extremely hard and brittle material. Since micro-EDM is more generally an electro-thermal process, the supplied energy from a pulse generator is an important factor determining the performance of the micro-EDM process. This study investigates the influence of major operating parameters on the performance of micro-EDM of WC with focus in obtaining quality micro-holes in both transistor and RC-type generators. Experimental investigations were conducted with view of obtaining high-quality micro-holes in WC with small spark gap, better dimensional accuracy, good surface finish and circularity. In micro-EDM, the fabrication of micro-parts requires minimization of the pulse energy supplied into the gap which can be fulfilled using the RC-generator. It was observed that the RC-generator can produce better quality micro-holes in WC, with rim free of burr-like recast layer, good dimensional accuracy and fine circularity. Moreover, the smaller debris formed due to low discharge energy in RC-type micro-EDM can be easily flushed away from the machined area resulting in surface free of burr and resolidified molten metal. Therefore, RC-type micro-EDM could be more suitable for fabricating micro-structures in WC, where accuracy and surface finish are of prime importance.     

Probability of precision micro-machining of insulating Si3N4 ceramics by EDM

Electrical discharge machining (EDM) is used as a precision machining method for the electrically conductive hard materials with a soft electrode material. But recently we succeeded to machine on insulating material by EDM. The technology is named as an assisting electrode method. The EDMed surface is covered with the electrical conductive layer during discharge. The layer holds the electrical conductivity during discharge. For micro-EDM, the wear of tool electrode becomes lager ratio than the normal machining. So the micro-machining is extremely difficult to get the precision sample.

In this paper to obtain a fine and precise ceramics sample, some trials were carried out considering the EDM conditions, tool electrodes material and assisting electrode materials. Insulating Si₃N₄ ceramics were used for workpiece. The machining properties were estimated by the removal rate and tool wear ratio. To confirm the change of micro-machining process, the discharge waveforms were observed. The micro-machining of the Ø0.05 mm hole could be machined with the commercial sinking electrical discharge machine.     

Development of a reversible machining method for fabrication of microstructures by using micro-EDM

A new micromachining method for the fabrication of micro-metal structures by using micro-reversible electrical discharge machining (EDM) was investigated. The reversible machining combines the micro-EDM deposition process with the selective removal process, which provides the ability of depositing or removing metal material using the same micro-EDM machining system. From the discharge mechanism of micro-EDM, the process conditions of micro-EDM deposition were analyzed firstly. Using the brass and steel materials as a tool electrode, the micro-cylinders with 200 μm in diameter and height-to-diameter ratio of more than 5 were deposited on a high-speed steel surface. Then the machining procedure was transformed easily from deposition to selective removal process by switching the process conditions. Different removal strategies including micro-EDM drilling and micro-EDM milling were used in the machining. Micro-holes with 80 μm in diameter are drilled successfully in the radial direction of the deposited micro-steel cylinder. Also, a brass square column with 70 μm in side length and 750 μm in height, and a micro-cylinder with 135 μm in diameter and 1445 μm in height are obtained by using micro-EDM milling. Finally, the characteristics of the deposited material were analyzed. The results show that the material components of a deposited micro-cylinder are almost the same as those of the tool electrode, and the metallurgical bonding has been formed on the interface. In addition, the Vickers-hardness of 454Hv of the steel deposited material is higher when compared to the hardness of 200Hv of the raw steel electrode.     

Modeling of radial gap formed by material dissolution in simultaneous micro-EDM and micro-ECM drilling using deionized water

For enhancing the surface finish of micro-holes, micro-EDM and micro-ECM have been combined in a unique hybrid machining process by using low-resistivity deionized water as bi-characteristic fluid. The affected material layer generated by electric sparks is further dissolved from machined surface owing to the effect of electrochemical reaction. To maintain the dimensional accuracy of micro-holes, short voltage pulses are applied to localize the material dissolution zone and thus the thickness of further removed material layer is of prime importance in deciding the final dimension of micro-holes. This study presents the modeling of radial gap distance in simultaneous micro-EDM and micro-ECM drilling by predicting the thickness of material layer further dissolved by electrochemical reaction. The analytical approach incorporating the double-layer theory, the Butler–Volmer equation and the Faraday's law of electrolysis is used to simulate the radial gap distance for different pulse parameters. The simulation data is then verified with the experimental results. It is observed that the applied pulse parameters directly affect the final dimension of obtained micro-holes. The effectiveness of short voltage pulses in localizing the material dissolution zone is found to be in accordance with the double-layer charging characteristic. When the pulse duration is too short, the material dissolution is negligible and SEDCM has no effect on improving the inner surface of micro-hole.     

Development of a grinding-drilling technique for holing optical grade glass

This study presents the development of a grinding-drilling technique for an innovative bench-top drill that combines micro-EDM with grinding and drilling to fabricate micro-holes in optical grade glass. Firstly, a novel diamond-tool, made with copper-based sintered alloy, is designed and fabricated on-line into a harbor-shaped structure with a hollow shaft and negative back rake-angles. Constructed reverse co-centric micro-hole EDM-drilling and reverse w-EDM facilitate on-line machining of the diamond-tool, which can then be directly utilized to drill micro-holes in optical glass and quartz. Application of a load-cell that detects the drilling force in real-time, providing feedback for fine tuning the feed-rate of the tool is proposed. Experimental results show that excellent geometric and dimensional accuracy of micro-holes can be achieved. The estimated reasonable tool life is determined at a machining number of 30 times. The proposed grinding-drilling technique is simple, cost effective, and can significantly contribute to the precision micromachining industry.     

Orbital electrode actuation to improve efficiency of drilling micro-holes by micro-EDM

This paper presents a novel machining technique for micro-EDM that actuates the EDM electrode on an orbital trajectory that is created by a 2-axis flexural micro-EDM head with a range of ±100 μm in both x- and y-directions. The orbital motion with its adjustable radius decouples the size of the hole to be drilled from the size of the electrode, allowing a range of hole sizes to be drilled. The orbital motion of the electrode increases the hole diameter proportional to the orbital radius, thereby creating a larger gap between the work piece and the electrode, which promotes increased flushing. For holes with large depth to diameter ratios, the increased flushing reduces electrode wear, creates a better surface finish, and eliminates the exponential reduction in material removal rates typical for EDM drilling.     

微细电火花小孔加工过程仿真

微细电火花小孔加工过程中存在的电极损耗问题,严重影响了孔的加工精度.以单脉冲放电理论为基础,改进了微细电火花小孔加工过程的仿真模型,对工具与工件加工形状的变化过程进行了仿真研究.与实验结果相比,模型能较好地预测电极损耗及其对工件形状精度的影响,从而为进一步研究电极离线补偿提供了一种经济、可行的方法.     

Fabrication of deep micro-holes in reaction-bonded SiC by ultrasonic cavitation assisted micro-EDM

Ultrasonic vibration was applied to dielectric fluid by a probe-type vibrator to assist micro electrical discharge machining of deep micro-holes in ceramic materials. Changes of machined hole depth, hole geometry, surface topography, machining stability and tool material deposition under various machining conditions were investigated. Results show that ultrasonic vibration not only induces stirring effect, but also causes cloud cavitation effect which is helpful for removing debris and preventing tool material deposition on machined surface. The machining characteristics are strongly affected by the vibration amplitude, and the best machining performance is obtained when carbon nanofibers are added into the vibrated dielectric fluid. As test pieces, micro-holes having 10 μm level diameters and high aspect ratios (>20) were successfully fabricated on reaction-bonded silicon carbide in a few minutes. The hybrid EDM process combining ultrasonic cavitation and carbon nanofiber addition is demonstrated to be useful for fabricating microstructures on hard brittle ceramic materials.     

Application of micro-EDM combined with high-frequency dither grinding to micro-hole machining

This research presents a novel process using micro electro-discharge machining (micro-EDM) combined with high-frequency dither grinding (HFDG) to improve the surface roughness of micro-holes. Micro-EDM is a well-established machining option for manufacturing geometrically complex small parts (diameter under 100 μm) of hard or super-tough materials. However, micro-EDM causes the recast layer formed on the machined surface to become covered with discharge craters and micro-cracks, resulting in poor surface quality. This affects the diameter of the micro-hole machined and undermines seriously the precision of the geometric shape. The proposed method that combines micro-EDM process with HFDG is applied to machining high-nickel alloy. As observed in SEM photographs and surface roughness measurement, HFDG method can reduce surface roughness from 2.12 to 0.85 μm Rmax with micro-cracks eliminated. Our results demonstrated that micro-holes fabricated by micro-EDM at peak current 500 mA followed by HFDG at 40 V can achieve precise shape and good surface quality after 6–8 min of lapping.     

单层多道微细电火花定长补偿加工方法

定长补偿是应用分层原理进行微细电火花加工的一种方法,它能较好地解决加工中电极损耗对精度的影响问题.通过实验证明了微细电火花铣削加工中由于底面和侧面放电间隙不同,在加工过程中要区别对待;根据实验结果建立了实际加工后单道的横截面模型,在此基础上引入了体积系数、面积系数、残切系数,对原来的定长补偿方法进行了修正.根据建立的实际加工后残切模型,给出2种典型去除残切的加工工艺路线,应用定长补偿方法解决了分层三维构件加工中的单层加工问题.     

Effect of surface roughness of tool electrode materials in ECDM performance

Electrochemical discharge machining (ECDM) is an emerging non-traditional machining process that involves high-temperature melting assisted by accelerated chemical etching. In this study, the tool electrode (200 μm in diameter) is fabricated by wire electrical discharge grinding (WEDG). After the tool electrode is machined, the surface roughness of tool electrode materials (stainless steel, tungsten carbide, and tungsten) is different because of the physical properties. However, the surface roughness affects the wettability on tool electrode, and also changed the coalesce status of gas film in ECDM. Hence, this study explores the wettability and machining characteristics of different tool electrode materials and their impact on gas film formation. Their machining performance and extent of wear under gravity-feed micro-hole drilling are also examined. Experimental results show that the optimal voltage of different tool electrode can shed light on the machining performance. Moreover, wettability of tool electrode is determined by surface roughness of tool material, which in turn affects the coalesce status of gas film, machining stability and micro-hole diameter achieved. In addition, differences in tool material also results in variations in machining speed. Significant tool wear is observed after repeated gravity-feed machining of 50 micro-holes.     

A study on the machining of high-aspect ratio micro-structures using micro-EDM

Micro-electro-discharge machining (micro-EDM or μ-EDM) has been gaining popularity as a new alternative method to fabricate micro-structures. The main advantages of the micro-EDM method are its low set-up cost, high accuracy and large design freedom. Compared to etching or deposition techniques, micro-EDM has the advantage of being able to fabricate complex three-dimensional shapes with high-aspect ratio. However, there are many operating parameters that affect the micro-EDM process. The fabrication of micro-electrodes on the machine is also an important process to remove the clamping error to maintain high accuracy in the machined micro-structures.

In this paper, the machining of micro-structures is divided into two basic processes. One is the on-machine fabrication of the micro-electrodes with high-aspect ratio, and the other is the EDM of the workpiece in micrometer range. An optical sensor has been developed to measure and control the dimension of the thin electrode during the tool fabrication process. Different methods have been investigated to fabricate a thin electrode into the desired dimension without deflection. The performance of the micro-EDM process is evaluated in terms of the material removal rate (MRR), tool wear ratio (TWR), and the stability of the machining. Influences of the various operating parameters of the micro-EDM process, such as the operating voltage, gap control algorithm, and resistance and capacitance values in the R–C spark control circuit, are discussed.     

Ultrasonic and electric discharge machining to deep and small hole on titanium alloy

Being a difficult-to-cut material, titanium alloy suffers poor machinability for most cutting process, let alone the drilling of small and deep holes using traditional machining methods. Although electric discharge machining (EDM) is suitable to handle titanium alloys, it is not ideal for small and deep holes due to titanium alloys’ low heating conductivity and high tenacity. This paper introduces ultrasonic vibration into micro-EDM and analyzes the effect of ultrasonic vibration on the EDM process. A four-axis EDM machine tool which combines ultrasonic and micro-EDM has been developed. A wire electric discharge grinding (WEDG) unit which can fabricate a micro-electrode on-line, as well as a measuring unit, is set up on this equipment. With a cylindrical tool electrode, made of hard carbide, which has high stiffness, a single-side notch was made along the electrode. Ultrasonic vibration is then introduced into the micro-EDM. Experiments have been carried out and results have shown that holes with a diameter of less than Ø0.2 mm and a depth/diameter ratio of more than 15 can be drilled steadily using this equipment and technology.     

微细电火花加工的电极补偿方法研究

微细电火花加工中,电极损耗是引起加工误差的主要原因之一,因此必须进行电极的实时补偿以保证加工精度.研究了一种根据有效放电脉冲次数来进行实时补偿的方法,设计了基于可编程逻辑控制器(CPLD)的微细电火花补偿控制电路,并试验验证了此方法的可行性.     

A study on the fine-finish die-sinking micro-EDM of tungsten carbide using different electrode materials

In recent years, tungsten carbide (WC) and its composites (WC–Co) are widely used in the die and mold industries due to their unique combination of hardness, strength and wear resistance. Micro-EDM is one of the most effective methods for machining these extremely difficult-to-cut materials. However, numerous applications of WC often involve intense mechanical demands at the surface. Therefore, fine-finish micro-EDM of WC is becoming an imminent and important issue. In this study, investigations have been conducted with view of obtaining fine surface finish in the micro-EDM of WC using tungsten (W), copper tungsten (CuW) and silver tungsten (AgW) electrodes. It was found that the surface characteristics are dependent mostly on the discharge energy during machining. The fine-finish micro-EDM requires minimization of the pulse energy supplied into the gap. In addition, the surface finish was found to be influenced greatly by the electrical and thermal properties of the electrode material. The performance of the electrodes for the finishing micro-EDM was evaluated based on the achieved surface roughness and surface characteristics with respect to material removal rate (MRR) and electrode wear ratio (EWR). It was found that AgW electrode produces smoother and defect-free nanosurface with the lowest R_a and R_max among the three electrodes. Besides, a minimum amount of material migrates from the AgW electrode to the WC workpiece during the finishing micro-EDM. On the other hand, CuW electrodes achieved the highest MRR followed by AgW. In the case of electrode wear, the W electrode has the lowest wear followed by CuW and AgW. Finally, considering all the performance parameters, AgW appears to be the best choice for finish die-sinking micro-EDM of WC.     

一种用于微细电火花加工的甚高频微能脉冲电源

作为微细电火花加工的关键技术之一,微能脉冲电源性能的优劣直接影响放电加工的精度、效率、稳定性等。从减小放电脉冲能量、增大放电间隙、可持续加工的需求出发,探索了一种基于电路共振原理的甚高频(频率在30~300 MHz)微能脉冲电源,突破了现有电火花脉冲电源的工作模式,能产生脉宽极窄、频率极高的脉冲波形,具有纳米级放电蚀除特性,提高了微细电火花加工的极限蚀除能力。放电频率为65 MHz时,相对于传统的微能脉冲电源,加工的孔边缘几乎没有重铸层,极大地减轻了在加工过程中的热损伤、重铸层和热影响区等常规缺陷,改善了工件加工的表面质量,实验结果证明所设计的甚高频微能脉冲电源具有良好的放电蚀除特性。     

Transitions of micro-EDM/SEDCM/micro-ECM milling in low-resistivity deionized water

Owing to its slight conductivity, deionized water has been used as a bi-characteristic fluid to combine micro-EDM and micro-ECM milling in a unique machining process which has been named as SEDCM milling. To attain both electrical discharge and electrochemical reaction during machining, selection of machining parameters such as feedrate and layer depth has been empirically observed to be of prime importance. This paper presents an analytical model to identify the critical conditions for transitions of material removal mechanisms in this hybrid machining process. The criteria for three distinct machining modes micro-EDM/SEDCM/micro-ECM milling are determined based on the thickness of material layer that electrochemical reaction could dissolve when the electrode scans over the surface. The critical feedrate for transitions of material removal mechanisms are then predicted using double layer theory, Butler–Volmer equation and Faraday's law of electrolysis. Experimental tests were also performed to validate the proposed model. It has been established that the SEDCM milling is only attained at moderate feedrate. For high feedrate, machining mode is changed to micro-EDM milling alone when the thickness of material layer that electrochemical reaction could dissolve is smaller than the roughness of micro-EDMed surface. On the contrary, for low feedrate, material removal mechanism is converted to pure micro-ECM when the thickness of layer dissolved by electrochemical reaction is higher than the preset layer depth. In addition, it is also found that lower feedrate is required for SEDCM milling when higher layer depth is used because more material needs to be removed by the sparks in every feed.     

A study on microgrinding of brittle and difficult-to-cut glasses using on-machine fabricated poly crystalline diamond (PCD) tool

Present study aims to investigate the feasibility of microgrinding difficult-to-machine glass materials with Poly Crystalline Diamond (PCD) tool, which is fabricated on-machine using micro-electrodischarge machining (micro-EDM). A detailed experimental investigation on the mechanism of the process including the effect of micro-EDM machining conditions on tool surface and the effect of grinding parameters on microgrinding performance are presented. In this context, a comparative study on the microgrinding performance of three glass materials (BK7, Lithosil and N-SF14) using on-machine fabricated PCD tool was carried out. It was found that during tool fabrication using micro-EDM process, higher discharge energy generates rougher surface and larger craters on the tool, which can provide higher material removal rate (MRR) during grinding but results in poorer surface finish on glass surface. In addition to micro-EDM conditions of tool fabrication, the roughness of the ground glass surface depends greatly on grinding parameters such as depth of cut, feed rate and tool rotational speed. The surface roughness increases with increasing axial depth of cut and feed rate, whereas higher rotational speed was found to improve the surface finish. Among three types of glass materials, BK7 glass was found to provide better performance in terms of the achieved surface finish and cutting force analysis.