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。     

Influence of machining parameters on surface roughness in finish cut of WEDM

Surface roughness is significant to the finish cut of wire electrical discharge machining (WEDM). This paper describes the influence of the machining parameters (including pulse duration, discharge current, sustained pulse time, pulse interval time, polarity effect, material and dielectric) on surface roughness in the finish cut of WEDM. Experiments proved that the surface roughness can be improved by decreasing both pulse duration and discharge current. When the pulse energy per discharge is constant, short pulses and long pulses will result in the same surface roughness but dissimilar surface morphology and different material removal rates. The removal rate when a short pulse duration is used is much higher than when the pulse duration is long. Moreover, from the single discharge experiments, we found that a long pulse duration combined with a low peak value could not produce craters on the workpiece surface any more when the pulse energy was reduced to a certain value. However, the condition of short pulse duration with high peak value still could produce clear craters on the workpiece surface. This indicates that a short pulse duration combined with a high peak value can generate better surface roughness, which cannot be achieved with long pulses. In the study, it was also found that reversed polarity machining with the appropriate pulse energy can improve the machined surface roughness somewhat better compared with normal polarity in finish machining, but some copper from the wire electrode is accreted on the machined surface.     

气体放电加工基础工艺试验研究

采用单因素法进行了基本的工艺参数(电参数、伺服参考电压等)对气体介质放电加工性能影响的试验研究。试验结果表明:气体介质的放电加工适于采用正极性加工。在试验加工的范围内,工件的蚀除速度和表面粗糙度值随脉冲宽度和峰值电流的增加而增加,随脉冲间隔的增加而减小。极间并联合适的电容能够使加工速度和加工表面粗糙度有所改善,并对此现象进行了分析。对于某一确定的加工参数,存在一个较佳的伺服参考电压值,使加工性能较为稳定。工具电极具有较高的旋转速度能够使气体放电加工性能得到提高。使用氧气介质能够实现快速电火花加工,并根据不同气体的物理性能对不同气体介质的加工性能进行了分析。工件表面显微硬度测试结果表明:空气中放电加工的工件的表面硬度比基体硬度高,比煤油中加工的工件表面硬度低。     

Scale effects and a method for similarity evaluation in micro electrical discharge machining

Electrical discharge machining(EDM) is a promising non-traditional micro machining technology that offers a vast array of applications in the manufacturing industry. However, scale effects occur when machining at the micro-scale, which can make it difficult to predict and optimize the machining performances of micro EDM. A new concept of “scale effects” in micro EDM is proposed, the scale effects can reveal the difference in machining performances between micro EDM and conventional macro EDM. Similarity theory is presented to evaluate the scale effects in micro EDM. Single factor experiments are conducted and the experimental results are analyzed by discussing the similarity difference and similarity precision. The results show that the output results of scale effects in micro EDM do not change linearly with discharge parameters. The values of similarity precision of machining time significantly increase when scaling-down the capacitance or open-circuit voltage. It is indicated that the lower the scale of the discharge parameter, the greater the deviation of non-geometrical similarity degree over geometrical similarity degree, which means that the micro EDM system with lower discharge energy experiences more scale effects. The largest similarity difference is 5.34 while the largest similarity precision can be as high as 114.03. It is suggested that the similarity precision is more effective in reflecting the scale effects and their fluctuation than similarity difference. Consequently, similarity theory is suitable for evaluating the scale effects in micro EDM. This proposed research offers engineering values for optimizing the machining parameters and improving the machining performances of micro EDM.     

动态电容补偿在微机器人胶囊无线供能系统中的应用

微机器人胶囊采用电磁耦合无线能量传输时,为获得较高传输效率及功率,需使发射端和接收端在同一频率下达到理想谐振耦合状态。由于高频下线圈的分布电容、寄生电容以及等效电容的波动等因素影响,系统容易偏离原有的理想谐振状态,严重影响系统传输性能。分析了系统频率失谐的原因,提出一种动态电容补偿方法,通过反向补偿变化的电容值,使得系统重新恢复谐振状态,从而改善了系统的传输性能。实验结果表明该动态电容补偿方法是有效的。     

Research of micro EDM/ECM method in same electrolyte with running wire tool electrode

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.     

利用声发射信号时频特征在线监测慢走丝线切割加工表面粗糙度

针对慢走丝线切割加工(WEDM-LS)时因温度高、切缝窄等因素造成的加工过程难以监测的问题,利用声发射检测技术对占空比可调脉冲的慢走丝线切割加工过程进行在线监测。首先利用小波包能谱算法将AE信号分解到8个独立的频段上:分别为W1~W8,且频率依次降低;然后提取各频段上的能量特征,研究其与加工工件表面粗糙度值之间的相关性。试验结果表明:W8频段的能量与表面粗糙度值之间具有较高的相关性,该频段的能量与脉冲放电能量均随着脉冲信号占空比的增大而增大,且加工表面粗糙度值也随之逐渐增大。最后通过回归分析得到了反应材料表面粗糙度值与W8频段能量占比关系的数学预测模型,该模型的预测结果与实际测得的表面粗糙度值误差仅为3.51%。说明该模型具有较高的预测精度,可用于加工表面粗糙度的在线监测。     

混粉电火花加工中粉末对工件表面的影响

对不同加工条件下混粉电火花加工后工件表面的硅含量进行了对比测量。实验结果表明：当峰值电流小于4A时，混粉电火花加工后的工件表面硅含量随峰值电流的增大而急剧减小，而当峰值电流大于4A时，工件表面硅含量随峰值电流的增大而缓慢增加；混粉电火花加工后的工件表面硅含量随脉宽的增大而增加；在其他加工条件相同的情况下，对于相同的单次放电脉冲能量，混粉电火花加工获得的工件表面硅含量随峰值电流变化的关系呈近似二次曲线。引入熵的概念，对产生上述结论的原因进行了分析，并解释了混粉电火花加工可以改善工件表面质量的机理。     

超声振动改善气体介质电火花加工的机理研究

提出了超声振动-气体介质电火花复合加工技术,在简述其加工原理的基础上分析了超声振动在改善气体介质电火花加工中的作用,以及相关的理论机理。实验结果表明,附加超声振动能改善放电间隙状态,减少短路、拉弧现象,提高了材料的去除率。进行了超声振动-气体介质电火花加工单脉冲实验设计。     

Electrical discharge machining in distilled water

The performances of kerosene and distilled water as the dielectric fluid in electrical discharge machining (EDM) were compared over the pulse energy range 0.72 – 288 mJ. Machining in distilled water resulted in a higher metal removal rate and a lower wear ratio than in kerosene when a high pulse energy range (72 – 288 mJ) was used. With distilled water, the machining accuracy was poor but the surface finish was better. Electron microprobe analysis revealed that the deposition of tool material (copper) on the work surface (high carbon steel) was low when machining took place in distilled water at a high pulse energy (288 mJ) and in kerosene at a low pulse energy (72 mJ). It is concluded that distilled water may be used as a dielectric fluid in EDM at a high pulse energy range.     

Investigations on influence of process variables on crater dimensions in micro-EDM of γ-titanium aluminide alloy in dry and oil dielectric media

In micro-electro-discharge machining (EDM), challenge lies in enhancing material removal rate while retaining precision in crater dimensions. Material properties of both anode and cathode and the process variables have significant control on MRR and accuracy. In the present research, experiments were conducted on γ-titanium aluminide alloy work piece using 200-μm steel electrode. The circular craters were produced both in the presence and absence of dielectric fluid using varying micro-EDM process variables, i.e., open-circuit voltage, discharge capacitance, pulse frequency, and pulse-on time. Overcut was measured from optical microscope images using Image Analyzer software. Influences of process variables and optimal conditions for minimum overcut on crater dimensions were investigated using ANOVA test, which shows that capacitance of RC circuit contributes significantly in crater formation followed by pulse frequency. Regression equations of overcut for both dielectric mediums were developed using discharge energy and spark-on time as two functions. It was also investigated that overcut was less in air medium compared to oil medium.     

An investigation on machinability of nodular cast iron by WEDM

In this study, the machinability of standard GGG40 nodular cast iron by WEDM using different parameters (machining voltage, current, wire speed, and pulse duration) was investigated. From the results, the increase in surface roughness and cutting rate clearly follows the trend indicated with increasing discharge energy as a result of an increase of current and pulse-on time, because the increased discharge energy will produce larger and deeper discharge craters. Three zones were identified in rough regimes of machining for all samples: decarburized layer, heat affected layer, and bulk metal. High machining efficiency can be obtained when the proper electrical parameters are selected, but whether high energy or the low energy is used, a coarse surface is always obtained. The variation of surface roughness and cutting rate with machining parameters is mathematically modeled by using the regression analysis method.     

Experimental and numerical investigations of material removal process in electrochemical discharge machining of glass in discharge regime

Electrochemical discharge machining (ECDM) can be applied as a non-traditional processing technology for machining non-conductive materials such as glass and ceramics, based on the phenomena of evoked electrochemical discharges around the tool electrode. The material removal mechanism of ECDM is noticeably complex and difficult to experimentally characterize. In this paper, finite element models were proposed to predict the material removal in the ECDM discharge regime. First, the single-pulse discharge on a tapered electrode was modeled. It was found that about 30.5% of the discharge energy is transferred to the workpiece. The continuous discharge on a cylindrical electrode was thereafter modeled according to this phenomenon, in which the removal of a layer of the workpiece material starts from the projected contour of the edge of the electrode end and extends inward during the ECDM processing. The effective discharge ratio for material removal was calculated to be 10.1%. The drilling depths of holes at different applied voltages were predicted by the proposed finite element method. It was found that the predicted values were consistent with the experimental results.     

Influence of parameters and optimization of EDM performance measures on MDN 300 steel using Taguchi method

Maraging steel (MDN 300) exhibits high levels of strength and hardness. Optimization of performance measures is essential for effective machining. In this paper, Taguchi method, used to determine the influence of process parameters and optimization of electrical discharge machining (EDM) performance measures on MDN 300 steel, has been discussed. The process performance criteria such as material removal rate (MRR), tool wear rate (TWR), relative wear ratio (RWR), and surface roughness (SR) were evaluated. Discharge current, pulse on time, and pulse off time have been considered the main factors affecting EDM performance. The results of the present work reveal that the optimal level of the factors for SR and TWR are same but differs from the optimum levels of the factors for MRR and RWR. Further, discharge current, pulse on time, and pulse off time have been found to play a significant role in EDM operations. Detailed analysis of structural features of machined surface was done by using scanning electron microscope (SEM) to understand the influence of parameters. SEM of electrical discharge machining surface indicates that at higher discharge current and longer pulse on duration give rougher surface with more craters, globules of debris, pockmarks or chimneys, and microcracks than that of lower discharge current and lower pulse on duration.     

汽中电火花加工技术研究

提出了一种汽中电火花加工技术,其加工介质为水蒸汽,蒸汽由蒸汽发生器获得,经管状电极喷向工件。试验结果显示：一般情况下,材料去除率随放电电流、脉冲宽度的增大而增大,随脉冲间隔的增大而减小,但一味地增大脉冲宽度和减小脉冲间隔会造成电火花加工稳定性变差,从而降低材料去除率;汽中电火花加工技术的工具电极相对损耗率相对较低,受脉冲宽度、脉冲间隔影响小,但在大的放电电流下,电极相对损耗率会有所增加,且汽中电火花加工的工件附着物较少。
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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.     

An investigation on machinability of nodular cast iron by WEDM

In this study, the machinability of standard GGG40 nodular cast iron by WEDM using different parameters (machining voltage, current, wire speed, and pulse duration) was investigated. From the results, the increase in surface roughness and cutting rate clearly follows the trend indicated with increasing discharge energy as a result of an increase of current and pulseon time, because the increased discharge energy will produce larger and deeper discharge craters. Three zones were identified in rough regimes of machining for all samples: decarburized layer, heat affected layer, and bulk metal. High machining efficiency can be obtained when the proper electrical parameters are selected, but whether high energy or the low energy is used, a coarse surface is always obtained. The variation of surface roughness and cutting rate with machining parameters is mathe-matically modeled by using the regression analysis method.     

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.     

Experimental study on energy consumption of energy-saving pulse power for WEDM

Wire electrical discharge machining (WEDM) is widely used in aerospace, mold manufacturing, automotive, and other fields. But now, traditional WEDM pulse power contains current-limiting resistor, and its energy utilization rate is low. Meanwhile, discharge energy of WEDM pulse power cannot be accurately controlled, and there is no unified understanding of distribution about discharge energy. In this paper, the pulse power improves energy utilization rate by removing current-limiting resistor and adopting single-arm pulse width modulation control method which controls peak current and modulates long short-circuit pulse width. Experiments proved that the energy-saving pulse power improves energy utilization rate by, approximately, 67.6 % than traditional pulse power. The paper analyzed a single-pulse energy waveform of the energy-saving pulse power and calculated each spark pulse energy and total energy provided by pulse power. It found that gap spark discharge energy only accounted for about 51 % of the energy provided by the pulse power, and other energy was mainly consumed in switching loss of power switch and parasitic resistance of pulse power.     

TC4钛合金电火花小孔加工多目标优化试验研究

为提升电火花加工TC4钛合金的表面加工质量和加工效率,选取紫铜圆柱电极开展TC4钛合金电火花小孔加工试验,采用正交试验法,以电极相对损耗率、表面粗糙度、工件材料去除体积为工艺指标,分析峰值电流、维持电压、放电脉宽对工艺指标的影响重要性。采用RBF(Radial basis function)神经网络对已有试验数据进行训练,建立放电参数与工艺指标之间的数学预测模型。以该预测模型为适应度函数,将遗传算法与Skyline选择算法结合进行多目标优化仿真,得到最佳工艺指标,最后开展多目标优化验证试验。结果表明：当峰值电流为14 A、维持电压39 V/42 V、放电脉宽102μs/108μs时能够取得最优的加工结果,优化值与试验值误差较小。