Basic study on pulse generator for micro-EDM

An RC pulse generator can easily generate a pulse on-time as short as a several dozen nanoseconds in micro-electro-discharge machining (micro-EDM), but its discharge frequency is low due to the time needed to charge the capacitor in micro-EDM, which has a strong negative effect on the pulse generator’s working efficiency. Therefore, a new transistor-type isopulse generator has been developed for micro-EDM in this research. Evaluation of the machining characteristics proved that the transistor-type isopulse generator is suitable for micro-EDM. The experimental results reveal that the transistor-type pulse train generator is unsuitable for micro-EDM due to its low removal rate: 80-ns and 30-ns pulse on-times of discharge current can be obtained by using the transistor-type isopulse generator developed in this research, and the removal rate of this generator is two or three times higher than that of the traditional RC pulse generator.     

A computer-aided design system for foot-feature-based shoe last customization

The micro-electrical discharge machining (micro-EDM) process has proved to be an appropriate nonconventional machining method for manufacturing accurate and complex three-dimensional structural micro-features which are difficult to be produced by conventional processes. However, the miniaturisation of the EDM process requests special requirements on the machining equipment. Pulse generators which can produce small input energy pulses and high precision systems are the two major requirements. In this paper, newly developed technologies regarding these aspects are explored with the aid of a commercial micro-EDM machine. By examining the pulses, innovative strategies implemented in the pulse generator are studied. Pulse measurements reveal the correlation between the discharge pulses and the machine parameters in order to provide an overview of process capability. Conclusions are applied on machining of a ceramic composite Si₃N₄-TiN and optimised machining settings for different machining conditions are achieved. Accordingly, applications of two- and three-dimensional micro-structures on different types of materials such as a stainless steel micro-compressor and a ceramic miniature gas turbine are demonstrated. By inspecting the machining geometry and surface integrity, process characteristics of micro-EDM are discussed.     

Improvement of machining characteristics of micro-EDM using transistor type isopulse generator and servo feed control

This paper describes the improvement of machining characteristics of micro electrical discharge machining (micro-EDM) using a newly developed transistor type isopulse generator and servo feed control. The RC generator is mainly applied in conventional micro-EDM even though the transistor type isopulse generator is generally more effective for obtaining higher removal rate, because the transistor type generator is unable to generate iso-duration discharge current pulses with small pulse duration (several dozen nano-seconds), which is the normal level for micro-EDM. A new transistor type isopulse generator was therefore developed using a current sensor with high frequency response. With the new transistor type isopulse generator developed, the pulse duration can be reduced to about 30 ns, which is equivalent to the pulse duration used in finishing by the conventional RC pulse generator for micro-EDM. In order to achieve stable machining and improve machining characteristics, a new servo feed control system for micro-EDM using average ignition delay time to monitor the gap distance was also developed. By integrating the transistor type isopulse generator with this new servo feed control system, we were able to obtain a removal rate of about 24 times higher than that of the conventional RC pulse generator with a constant feed rate in both semifinishing and finishing. The effectiveness of the servo feed control proved higher in finishing than in semifinishing, whereas the transistor type isopulse generator was more effective in semifinishing than in finishing.     

An experimental study on micro-EDM in low-resistivity deionized water using short voltage pulses

Deionized water has been used as dielectric fluid for micro-electrical discharge machining (micro-EDM) because it gives higher material removal rate and lower tool wear than hydrocarbon oil. Moreover, it is a relatively low-cost and eco-friendly substance. Therefore, deionized water tends to be more favorable for micro-EDM. However, it causes weak electrochemical reaction during micro-EDM due to its slight conductivity. This leads to the unanticipated additional material removal from the workpiece which affects the machining shape and quality. The study in this paper aims to suppress the electrochemical reaction in die-sinking micro-EDM using deionized water by employing short voltage pulse. Experiments were carried out to fabricate micro-holes using the developed nanosecond pulse circuit. Different pulse parameters were applied to identify the main factor affecting the electrochemical reaction rate. Machining gap was found to be thinner and workpiece surface adjacent to the rim of micro-holes were found to be free of defects caused by material dissolution when pulse duration reached a critical value. Moreover, the influence of pulse parameters on material removal rate and machined shape was also investigated. Besides, energy-dispersive X-ray spectroscopy analysis showed that the machined surface using deionized water was less affected from material migration during micro-EDM process in comparison to hydrocarbon oil.     

Towards the effective tool wear control in micro-EDM milling

The electrode wear in micro-electrical discharge milling (micro-EDM milling) is one of the main problems to be solved in order to improve machining accuracy. This paper presents an investigation on wear and material removal in micro-EDM milling for selected process parameter combinations typical of rough and finish machining of micro-features in steel. The experiments were performed on state-of-the-art micro-EDM equipment. Based on discharge counting and volume measurements, electrode wear per discharge and material removal per discharge were measured for several energy levels. The influence of the accuracy of volume measurements on the electrode wear per discharge and on the material removal per discharge are discussed, and the issues limiting the applicability of real time wear sensing in micro-EDM milling are presented.     

Increasing discharge energy of micro-EDM with electrostatic induction feeding method through resonance in circuit

This paper describes the method to increase discharge energy in micro-EDM using electrostatic induction feeding as the pulse generator. In a previous study, controlled pulse train method was introduced to increase discharge energy and enlarge unit removal per discharge by allowing multiple discharges to occur at the same spot. During observation, it was found that the oscillation of discharge current is excited at higher amplitude even if the same capacitance is used. In this paper, the influence of frequency on discharge energy was investigated. It was found that higher discharge energy can be obtained when machining was done at the resonant frequency of the circuit. Influence of capacitance and inductance on resonant frequency was studied. Probability of discharge continuity at different frequencies was examined. The results confirmed that higher discharge continuity within the pre-determined pulse train duration can be achieved when machining is conducted at resonant frequency, leading to higher material removal rate (MRR).     

Laminated fabrication of 3D queue micro-electrode and its application in micro-EDM

3D micro-electrode used in micro electrical discharge machining (micro-EDM) is difficult to be fabricated. Based on laminated object manufacturing (LOM) process, this paper superimposed multilayer 2D micro-structures together to fit out 3D micro-electrode and applied it in micro-EDM to process 3D micro-cavity mold. Firstly, 100-μm-thick Cu foils were cut by wire-electrical discharge machining (WEDM) to obtain multilayer 2D micro-structures, and then these 2D micro-structures were connected together to fit out 3D micro-electrode through vacuum pressure thermal diffusion welding. Secondly, under the effect of 80-V voltage, 0.2-MHz pulse frequency, 800-ns pulse width, and 4200-ns pulse interval, the 3D micro-electrode was applied in micro-EDM and 3D micro-cavity mold with high surface quality was obtained. Thirdly, in order to reduce the adverse impact of electrode wear on machining precision of 3D micro-cavity mold, 3D queue micro-electrode was used to process the same 3D micro-cavity mold, in which the first electrode is for rough machining and the others for fine machining. Finally, based on the above studies, two kinds of 3D queue micro-electrodes were fabricated, and the 3D micro-cavity molds with surface roughness Ra?=?0.48 μm were obtained through micro-EDM. Compared with the scanning 3D micro-EDM process, the 3D micro-cavity mold can be obtained through up and down reciprocating method of the 3D queue micro-electrode, featuring simple machining process and high efficiency.     

微细电火花铣削影响加工精度的主要因素分析

通过微电火花铣削试验研究,优化电参数、小厚度分层加工等方法分析微小孔加工直线度和圆柱度误差。结果表明:空载电压110V,峰值电流为70A,脉冲宽度为4μs,脉冲间隔为90μs,分层厚度0.15mm,放电间隙0.012mm,微小孔铣削精度最高,其加工尺寸精度±0.005mm,表面粗糙度0.427μm。     

An optimisation of micro-EDM operation for fabrication of micro-hole

Micro electro-discharge machining (micro-EDM) operation is a versatile operation to machine any hard conducting material to fabricate micro-products of different shape and size. The micro-holes are a common feature in many micro-products. It is a challenging task to improve the surface integrity and aspect ratio of holes. A number of controlling parameters affect the micro-EDM operation. In the present investigation, the optimization of micro-EDM operation has been carried out for fabrication of micro-holes using design of experiment technique using the Taguchi method. The circularity error, the recast layer and machining time has been analysed. The significant factors influencing the process have been found out. The workpiece is taken as copper and the tool is tungsten electrode.     

空气中微细电火花沉积的工艺规律研究

论述了一种新的电火花加工方法，它通过合理选择工艺条件在空气中将金属材料放电沉积在工件上。对电火花沉积加工的基本原理进行了分析，预测了实现条件，使用通用的电火花成形加工机床和常见的电极材料黄铜，在空气介质中，通过大量实验对微细电火花沉积进行了系统研究，得出各工艺参数的影响规律。在高速钢工件表面沉积出直径为0．19mm、高度为7.35mm的微小圆柱体。对沉积材料的测试表明，沉积材料致密，与基体结合紧密，成分取决于工具电极材料，同时基体硬度得到提高。     

Development of an integrated apparatus of micro-EDM and micro-CMM

Authors have developed a 4-axis micro-CMM that has miniaturized probe. By using an L-shaped probe, the micro-CMM can measure three dimensional coordinates of the high aspect ratio structures such as micro-holes. However, an installation error of the probe primarily affects the accuracy in the micro-CMM because the probe was previously machined in other micro-EDMs. In this research, we developed an integrated apparatus of micro-EDM and micro-CMM. The micro-EDM has 5-axis control with a general plate tool electrode and it can make probes of various shapes by the electric discharge scanning method. And the 4-axis micro-CMM has a mechanism that rotates the probe and it can measure with sub-micrometer accuracy. In this paper, Authors introduce a basic configuration of the integrated apparatus, machining method of the probe, and measuring method and measurement result of the micro-hole.     

UV-LIGA和微细电火花加工技术组合制作三维金属微结构的研究

研究了UV-LIGA和微细电火花加工技术组合制作三维金属微结构的工艺方法。首先通过UV-LIGA制作二维金属微结构,再对该微结构进行微细电火花加工得到三维金属微结构。该方法具有加工精度高、可实现自由曲面三维金属微结构制作的优点。制作出了局部为梯形凸台和锥形凹槽三维微结构的镍模具。分析了微细电火花加工中放电参数对表面粗糙度的影响,通过减小放电电压和电容的方法降低了表面粗糙度。     

Performance analysis of EDM electrode fabricated by localized electrochemical deposition for micro-machining of stainless steel

Electrical discharge machining (EDM) is an important and widely used process for the fabrication of complex three-dimensional structure of micro-tools, micro-components, and parts with micro-feature. It allows high precision, low setup cost, large freedom of design, and good surface quality. However, in order to produce different varieties of high-accuracy structures on machine, microelectrode fabrication is necessary so as to reduce the clamping error which is one of the biggest challenges in the field of micro-EDM. In this study, it has been shown that localized electrochemical deposition (LECD) is one of the simplest, inexpensive, and damage-free ways to fabricate complex-shaped electrodes for micro-EDM compared to other conventional electrode fabrication processes. In this study, electrode was fabricated with the help of a non-conductive mask which was placed between the anode and the cathode where the cathode was placed above the anode and mask and the system was immersed in a plating solution of acidified copper sulfate. The micro-EDM was carried out by the deposited electrode without changing or removing its orientation. The performance of LECD electrode was evaluated in this study by micro-holes fabrication on high melting point material such as stainless steel in terms of the material removal rate, tool relative wear ratio, and dimensional accuracy. Finally, the performance of the LECD electrode was also evaluated by a comparative study with a circular EDM electrode for fabrication of complex three-dimensional structure.     

微细电火花加工用晶体管脉冲电源的研究

在传统晶体管脉冲电源的基础上，研制开发了一种适用于微细加工的晶体管脉冲电源，这种脉；中电源可以满足微细加工中粗加工、精加工的不同需要，可实现最小脉宽为50ns的放电电流。通过微细孔加工实验，对所开发的晶体管脉冲电源的加工特性与传统的RC脉冲电源进行了分析比较，实验结果表明前者的加工效率约为后者的2～6倍。     

A hybrid model using supporting vector machine and multi-objective genetic algorithm for processing parameters optimization in micro-EDM

In micro-electrical discharge machining (EDM), processing parameters greatly affect processing efficiency and stability. However, the complexity of micro-EDM makes it difficult to determine optimal parameters for good processing performance. The important output objectives are processing time (PT) and electrode wear (EW). Since these parameters influence the output objectives in quite an opposite way, it is not easy to find an optimized combination of these processing parameters which make both PT and EW minimum. To solve this problem, supporting vector machine is adopted to establish a micro-EDM process model based on the orthogonal test. A new multi-objective optimization genetic algorithm (GA) based on the idea of non-dominated sorting is proposed to optimize the processing parameters. Experimental results demonstrate that the proposed multi-objective GA method is precise and effective in obtaining Pareto-optimal solutions of parameter settings. The optimized parameter combinations can greatly reduce PT while making EW relatively small. Therefore, the proposed method is suitable for parameter optimization of micro-EDM and can also enhance the efficiency and stability of the process.     

In-situ process monitoring and adaptive control for precision micro-EDM cavity milling

The linear tool wear compensation method (LCM) is commonly applied in micro-EDM 3-D milling to compensate the tool length wear in order to achieve high machining accuracy. Traditional LCMs mainly rely on empirical models and off-line wear measurements, whereas the process dynamics are not taken into account. When machining complex 3D cavities, an increasing number of tool wear compensation cycles have usually to be performed in order to maintain the targeted machining accuracy. This negatively affects the duration of the overall machining cycle. To realize efficient precision micro-EDM cavity milling, without the necessity to predefine Z-axis tool feed in the NC trajectory before machining, an in-situ process control system is developed to adaptively control the tool wear compensation factor based on the discharge pulse behavior. Experiments have shown that the change of the compensation factor can be detected and also a continuous increase of the factor (over compensation) leads to the saturation of the mean effective pulse frequency. Pulse monitoring therefore provides valuable information for understanding the process dynamics and for selecting the machining parameters towards better machining efficiency. Furthermore, the information gathered in-situ can be utilized to predict the tool wear and perform in-situ tool wear prediction. To implement this on machine-level, a combined off-line and in-line adaptive control of the tool wear compensation factor is proposed and experimentally validated by milling different 3D cavities. The off-line adaptive control is only necessary when the predicted machining depth error exceeds a certain limit. In this way, more than 80% of the off-line adaptive control cycles can be eliminated, whereby a total save of cycle time up to 18% has been reached, while still maintaining the desired dimensional and form accuracy.     

甚高频共振式脉冲源放电蚀除

针对传统微细电火花脉冲电源普遍存在的放电频率低、脉宽较大、纳米级高效蚀除能力难以日益提高的问题,设计出了一种基于电路共振原理的甚高频微能脉冲源,该脉冲源可产生放电频率55 MHz、电压峰峰值220 V的开路电压波形,电压脉宽可压缩至9.1 ns。进行了不同开路电压下的放电实验,获得了各实验条件下的放电波形。实验结果证明所设计的甚高频微能脉冲源具有良好的加工工艺性能。     

Process modeling of hybrid machining system consisted of electro discharge machining and end milling

This paper presents a novel hybrid machining process (HMP) that combines cutting action with machining using discharge pulses. Working conditions for a machine tool capable of combining micro-electro discharge machining (EDM) with milling is still an ill-defined problem relying on heuristics because there is insufficient knowledge of the discharge mechanism and the effects of machining parameters. The proposed HMP that combines micro-EDM and milling processes was applied to a steel alloy (AISI 1045) as the workpiece and end mill tungsten carbide as the tool electrode. Test results obtained from a number of experiments showed that the developed HMP yields reasonable machining time and surface roughness. Significant controlling variables for the machining response were identified and ranked using the Taguchi method. Furthermore, the response surface method was used to develop an empirical model based on the correlation between input variables and output responses.     

High-efficiency approach for fabricating MTE rotor by micro-EDM and micro-extrusion

Micro-gas turbine engine（MTE） rotor is an important indicator of its property, therefore, the manufacturing technology of the microminiature rotor has become a hot area of research at home and abroad. At present, the main manufacturing technologies of the MTE rotor are directed forming fabrication technologies. However, these technologies have a series of problems, such as complex processing technology high manufacturing cost, and low processing efficiency, and so on. This paper takes advantage of micro electric discharge machining（micro-EDM） in the field of microminiature molds manufacturing, organizes many processing technologies of micro-EDM reasonably to improve processing accuracy, presents an integrated micro-EDM technology and its process flow to fabricate MTE rotor die, and conducts a series of experiments to verify efficiency of this integrated micro-EDM. The experiments results show that the MTE rotor die has sharp outline and ensure the good consistency of MTE rotor blades. Meanwhile, the MTE rotor die is applied to micro extrusion equipment, and technologies of micro-EDM and micro forming machining are combined based on the idea of the molds manufacturing, so the MTE rotor with higher aspect ratio and better consistency of blades can be manufactured efficiently. This research presents an integrated micro-EDM technology and its process flow, which promotes the practical process of MTE effectively.     

微细电火花加工及其关键技术

综述了微细电火花加工的基本原理及最新研究进展。比较了LIGA技术与微细电火花加工的特点与应用。简要分析了微细电火花加工的关键技术：微细电极的在线制作、微进给装置、微小能量的脉冲电源、微小电极的运动轨迹规划、电极的损耗及补偿策略。展望了微细电火花加工在微三维结构加工中的应用前景。