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.     

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。     

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).     

A two-stage servo feed controller of micro-EDM based on interval type-2 fuzzy logic

In order to improve the performance of micro-electrical discharge machining (micro-EDM), the most important issue is to develop a highly stable servo control system. In order to account for the characteristics of high frequency, serious signal distortion, and high noise in micro-EDM process, type-2 fuzzy logic sets are introduced which is able to handle these uncertainties effectively. Based on interval type-2 fuzzy sets theory, a two-stage servo feed controller is designed. The first stage of the controller is for detecting the discharge state for a single sample point, and the second stage is used to obtain the servo feed speed. In addition, the discrimination and statistical methods for discharge states of sample points are proposed to obtain the proportion of each discharge state in an analytical period. Experiments demonstrate that the new controller can obviously improve the processing efficiency of micro-EDM compared with traditional controllers. Therefore, the proposed interval type-2 fuzzy logic-based two-stage servo feed controller is an effective way to enhance the efficiency and stability of micro-EDM and meanwhile to achieve good processing quality.     

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.     

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.     

基于线电极的电火花铣削加工及其脉冲能量控制

介绍了基于线电极电火花铣削加工的产生背景及其基本原理,分析了在该加工中对单个脉冲放电能量进行控制的必要性,研制出了通过控制放电持续时间来控制脉冲能量的能量控制型脉冲电源,并给出了其放电脉冲波形。     

微细电火花线切割加工关键技术

在分析微细电火花线切割加工特点的基础上,对获得高精度、高表面质量工艺指标的关键技术进行了研究。其中重点对微能量脉冲电源、恒张力走丝系统、偏开路伺服控制策略、基于压电陶瓷电动机的微驱动进给系统、智能工艺规划系统和加工工作液间隙特性等关键技术进行了研究,解决了微小复杂零件难以切割的问题,并且获得高质量微小复杂零件,最终满足了微细电火花线切割加工的需要。     

An experimental study on micro wire-EDM of polycrystalline diamond using a novel pulse generator

This paper presents a new pulse generator for cutting of polycrystalline diamond (PCD) by micro wire electrical discharge machining (micro wire-EDM). The pulse generator using anti-electrolysis circuitry and digital signal processor-based pulse control circuit was developed to suppress damages on the machined surface of PCD while achieving stable machining. A novel pulse control method was proposed to provide high-frequency pulse control signals with a period of off duty cycle for reionization of the dielectric in the spark gap so as to reduce the consecutive occurrence of short circuits. A series of experiments were carried out to investigate the effect of open voltage on machining performance in terms of material removal rate, slit width, thickness of the damaged layer on machined surface, and surface finish. An increase of open voltage increases peak current, thus producing greater discharge energy and, thereby, contributing to improvements in material removal rate, but leading to larger slit width and thickness of the damaged layer and worse surface finish. Experimental results not only demonstrate that the developed pulse generator could achieve satisfactory machining results but also have verified the applicability of this new technique in micro wire-EDM.     

电流型电火花加工脉冲电源的研究

分析了传统独立电火花加工脉冲电源和逆变式电火花加工脉冲电源的不足,论述了电源型电火花加工脉冲电源的工作原理,大量的样机和独立式脉冲电源的对比工艺试验表明,电流型脉冲电源不仅满足了电火花加工多项性能指标的要求,而且达到了高效节能的效果。     

Diameter control of microshafts in wire electrical discharge grinding

In microelectrical discharge machining (μEDM), the machining repeatability of microshafts with the same diameter is low due to the complex and stochastic nature of sparking. To improve machining repeatability, the effects of the main error sources on diameter accuracy of microshafts fabricated by wire electrical discharge grinding (WEDG) were analyzed. Strategies to minimize diameter differences between the target microshaft and the microshaft finally obtained by semifinishing and finishing were then proposed. Determining an appropriate infeed (depth of a cut) according to the desired erosion depth in the radial direction (ΔR) of microshafts was chosen as the control strategy for the semifinishing of microshafts produced by WEDG. To determine the infeed, empirical models that evaluate the relationship between the infeed and ΔR were developed through a large number of experiments. For the finishing of microshafts using WEDG, machining strategies such as zero infeed and stopping the wire electrode running, were proposed to eliminate some of the inherent errors. Finally, the empirical models developed for determining the infeed were verified by the successful fabrication of a stepped microshaft, which has small deviations from its desired diameters. By employing the developed empirical models and machining strategies, diameters of 26 out of 28 microshafts fabricated were found to lie in the range of 45?±?2 μm. The experimental results verify that the diameter control strategies are effective and can be used to improve machining repeatability.     

Parametric optimization for surface roughness, kerf and MRR in wire electrical discharge machining (WEDM) using Taguchi design of experiment

This paper reports the effect and optimization of eight control factors on material removal rate (MRR), surface roughness and kerf in wire electrical discharge machining (WEDM) process for tool steel D2. The experimentation is performed under different cutting conditions of wire feed velocity, dielectric pressure, pulse on-time, pulse off-time, open voltage, wire tension and servo voltage by varying the material thickness. Taguchi’s L18 orthogonal array is employed for experimental design. Analysis of variance (ANOVA) and signal-tonoise (S/N) ratio are used as statistical analyses to identify the significant control factors and to achieve optimum levels respectively. Additionally, linear regression and additive models are developed for surface roughness, kerf and material removal rate (MRR). Results of the confirmatory experiments are found to be in good agreement with those predicted. It has been found that pulse on-time is the most significant factor affecting the surface roughness, kerf and material removal rate.     

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.     

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.     

A comparative experimental investigation of deep-hole micro-EDM drilling capability for cemented carbide (WC-Co) against austenitic stainless steel (SUS 304)

Microelectro-discharge machining (micro-EDM) has become a widely accepted non-traditional material removal process for machining difficult-to-cut but conductive materials effectively and economically. The present study aims to investigate the feasibility of machining deep microholes in two difficult-to-cut materials: cemented carbide (WC-Co) and austenitic stainless steel (SUS 304) using the micro-EDM drilling. The effect of discharge energy and electro-thermal material properties on the performance of the two work materials during the micro-EDM drilling has also been investigated. The micro-EDM drilling performance of two materials has been assessed based on the quality and accuracy of the produced microholes, machining stability, material removal rate (MRR), and electrode wear ratio. The results show that deep-hole micro-EDM drilling is technically more feasible in WC-Co as it offers microholes with smooth and burr-free surfaces at the rim in addition to improved circularity and lower overcut than those provided by SUS 304. Moreover, WC-Co exhibits better machinability during the deep-hole micro-EDM drilling, providing relatively higher MRR and stable machining.     

Multi-response optimization of Micro-EDM process parameters on AISI304 steel using TOPSIS

The Technique for order preference by similarity to ideal solution (TOPSIS) method of optimization is used to analyze the process parameters of the micro-Electrical discharge machining (micro-EDM) of an AISI 304 steel with multi-performance characteristics. The Taguchi method of experimental design L27 is performed to obtain the optimal parameters for inputs, including feed rate, current, pulse on time, and gap voltage. Several output responses, such as the material removal rate, electrode wear rate, overcut, taper angle, and circularity at entry and exit points, are analyzed for the optimal conditions. Among all the investigated parameters, feed rate exerts a greater influence on the hole quality. ANOVA is employed to identify the contribution of each experiment. The optimal level of parameter setting is maintained at a feed rate of 4 μm/s, a current of 10 A, a pulse on time of 10 μs, and a gap voltage of 10 V. Scanning electron microscope analysis is conducted to examine the hole quality. The experimental results indicate that the optimal level of the process parameter setting over the overall performance of the micro-EDM is improved through TOPSIS.     

Development of a linear electrostrictive servo motor

This paper presents a linear electrostrictive servo motor with high resolution and large stroke for ultra-precision motion control. High thrust force is obtained by making use of an electromagnetic clamping mechanism with force magnifying structure in the motor design. An operator alterable iterative learning control algorithm is proposed for the motion control of the motor. A prototype is designed, fabricated and tested. Experimental results show that the prototype has a mechanical resolution of 0.02 μm, yaw error less than 2 μm and maximum thrust force of 30N. Applications of the motor include producing the servo feed motions required in micro electrical discharge machining (micro-EDM) system or as a motion control device for other precision machining systems.     

ELECTRIC DISCHARGE MILLING AND MECHANICAL GRINDING COMPOUND MACHINING OF SILICON CARBIDE CERAMIC

Silicon carbide (SiC) ceramics have been widely used in modern industry. However, the manufacture of SiC ceramics is not an efficient process. This paper proposes a new technology of machining SiC ceramics with electrical discharge milling and mechanical grinding compound method. The compound process employs the pulse generator used in electrical discharge machining, and uses a water-based emulsion as the machining fluid. It is able to effectively machine a large surface area on SiC ceramics with a good surface quality. In this paper, the effects of pulse duration, pulse interval, peak voltage, peak current and feed rate of the workpiece on the process performance parameters, such as material removal rate, relative electrode wear ratio and surface roughness, have been investigated. A L₂₅ orthogonal array based on Taguchi method is adopted, and the experimental data are statistically evaluated by analysis of variance and stepwise regression. The significant machining parameters, the optimal combination levels of machining parameters, and the mathematical models associated with the process performance are obtained. In addition, the workpiece surface microstructure is examined with a scanning electron microscope and an energy dispersive spectrometer.     

基于异形孔等效放电面积的微细电火花加工技术

针对喷丝板异形孔加工这一工艺难题，分析了微细电火花加工系统的关键技术，并对传统晶体管可控式RC脉冲电源电路拓扑进行了改进，设计并研制出适合微细电火花加工的新型微能脉冲电源。针对等效放电面积对微小异形孔加工效率及加工精度的影响进行了详细的理论分析与大量实验研究，为合理地选择加工规准提供了可靠的理论与实践依据。     

进给速度对高速走丝气中线切割加工的影响

研究了在大气和乳化液两种介质中，进给速度对高速走丝电火花线切割加工的影响。实验结果表明，气中加工的直线度好、放电间隙窄、加工速度(材料去除率)高。另外还发现，无论气中或液中加工时，进给速度都存在一个最佳的数值，使加工的粗糙度值呈现为最低值，同时加工速度值呈现为最高值。