Trepanning of Al2O3 by electro-chemical discharge machining (ECDM) process using abrasive electrode with pulsed DC supply

Electro-chemical discharge machining (ECDM) of electrically non-conductive high-strength–high-temperature-resistant ceramics such as aluminium oxide (Al₂O₃) by trepanning method (i.e. orbital motion of tool) has shown the possibility of drilling large size holes by comparatively smaller electrodes efficiently and economically. However, at greater machined depth, the conventional electrode configurations and machining parameters show that machining performance gradually deteriorates with increase in tool depth and finally cause micro cracks on the machined surface due to thermal shocks at high voltage. To reduce this problem and to enhance the machining performance during trepanning operation of Al₂O₃, a spring fed cylindrical abrasive electrode of 1.5 mm diameter has been used under the effect of the three most influential parameters, namely, pulsed DC supply voltage, duty factor and electrolyte conductivity, each at five different levels to assess the volume of material removed, machined depth and diameteral overcut. The results obtained from this study revealed that pulsed DC has reduced the tendency of cracking at high supply voltage compared to smooth DC and the machining ability of the abrasive electrode was better than copper electrode as it would enhance the cutting ability due to the presence of abrasive grains during machining. In addition to this, trepanning provides the scope for drilling bigger holes.     

小孔径大深度长锥孔的刀具设计

介绍对承受超高压小孔径大深度长锥孔进行分段加工，保证相邻段必须是凸棱状相交的刀具设计方法。     

精密注塑模具电极拆分的要点与技巧分析

对精密注塑模具电极拆分方法进行了研究,分析了电极拆分要点,介绍了电火花成形技术在精密注塑模具制造中的具体应用,重点阐述了电极放电位置的选择、电极的组合、电极结构设计和放电间隙处理等技巧。生产验证,提高了模具质量,降低加工成本,并提高了生产效率。     

Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool

The effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool are studied experimentally with a theoretical analysis. Problems in selecting the optimum burnishing parameters and some burnishing mechanisms are discussed. With suitable parameters employed, the new no-chip finishing process developed can eliminate or reduce the cutting marks left on the workpiece surface by diamond cutting tools, with its surface roughness reduced to Ra=0.026 μm from the original 0.5 μm.     

微细电火花加工关键技术及工艺研究

文中基于新研发的一套微细电火花精密加工系统μEDM-50,介绍了研发过程中探索出的最小脉宽可以达纳秒级的微能脉冲电源以及一些针对微细电火花加工的特点形成的特殊工艺.微能脉冲电源具有主动消电离环节,可以减少脉间的残余电荷放电,提高加工表面质量;特殊工艺有利于提高系统的加工精度和效率,提高微细电极的安全性.最后,介绍了一些金属零件上典型的微小特征精密加工实验以及放电沉积实验.该系统加工特征的尺寸范围主要介于数十微米到数毫米内.     

Geometry prediction of EDM-drilled holes and tool electrode shapes of micro-EDM process using simulation

EDM is an efficient machining process for the fabrication of a micro-metal hole with various advantages resulting from its characteristics of non-contact and thermal process. However, this process has a serious problem caused by the tool wear, which significantly deteriorates the machining accuracy. In this paper, a geometric simulation model of EDM drilling process with cylindrical tool is proposed to predict the geometries of tool and drilled hole. The geometries of tool and workpiece are represented by two-dimensional matrix. For accurate prediction of their geometries, the tool motion, the sparking gap width, the spark frequency, the crater made by a single spark, and the tool wear ratio are considered as simulation parameters. To verify the simulation model the prediction results are compared with the actual experimental ones. Consequently, it is shown that the geometry prediction results match the experimental ones well within the error of 13%. Developed model can be used in offline compensation of tool wear in the fabrication of a blind hole. For the purpose of this, a compensation scheme based on the developed model is introduced, it is then demonstrated that the scheme is successfully applied to an actual micro-hole machining.     

Improvement of hole exit accuracy in electrochemical drilling by applying a potential difference between an auxiliary electrode and the anode

Electrochemical drilling (ECD) is a promising and low-cost process for yielding multiple holes simultaneously in difficult-to-machine materials. In this process, the hole exit accuracy is very sensitive to the electrode feeding depth. In practice, excessive electrode feeding is necessary to ensure that all holes are drilled through simultaneously when there is an error in thickness of the workpiece plate. This results in stray removal at the hole exit and an etched and pitted surface. In the modification of ECD described here, a potential difference is introduced via an insoluble platinum auxiliary electrode that is attached beneath a dielectric perforated plate and arranged opposite to the hole exits to diminish the damage from the stray current and thereby improve the tolerance of the exit accuracy to excessive electrode feeding. Simulation results indicate that an appropriate value of the potential difference concentrates the current at the tool tip and may reverse the current direction on the workpiece surface. Experiments verify that this approach is effective in obtaining holes with good exit accuracy in the case of excessive electrode feeding. Furthermore, it is confirmed that this method is capable of drilling multiple holes with remarkably enhanced exit accuracy and uniformity.     

平动电极的球刀加工分析

分析了平动电极与普通放电加工中电极的工作原理,得出电极的运动轨迹不同,导致电极相对工件的尺寸、形状不同,如果直接用球刀加工平动电极,会导致电极的形状不能满足平动加工的要求。针对这一问题,根据不同的电极形状分别采用了分割、改变圆角或斜角尺寸、平移等方法,用球刀加工出了合格的平动电极。为采用球刀加工平动电极提供了一条思路。     

实验用多微孔板的电火花加工

介绍了电火花微孔加工方法,着重对加工参数、工具电极的制备以及对加工中应注意的问题进行了探讨,成功地在厚80 μm的不锈钢板材上加工出128个直径55 μm的微孔.     

石墨电极在彩电模具电火花加工中的应用

通过彩电前壳模具喇叭网孔镶件电火花加工,对紫铜电极和石墨电极加工结果进行分析比较,指出石墨材料在制作大电极,特别是在对电极损耗有严格要求的电火花加工时,石墨电极比紫铜电极有许多优点。指出专用石墨作电火花加工电极的优势正逐渐被大家认识,将成为企业首选的电火花加工电极材料。     

An experimental study of discharge mechanism in electrochemical discharge machining

The basic mechanism of the electrochemical discharge machining process is not yet completely understood and is still a subject of investigations. In the present work an attempt has been made to identify the underlying mechanism through experimental observations of time-varying current in the circuit. Based on these observations the basic mechanism of temperature rise and material removal is proposed.     

Combination of capacitance and conductive working fluid to speed up the fabrication of a narrow, deep hole in electrical discharge machining using a dielectric-encased wire electrode

An attempt was made to increase the machining speed of a new electrical discharge machining system for fabricating narrow, deep holes in metal. The method employs a wire encased in a dielectric jacket as the tool electrode, in contrast with the conventional pipe electrode. The role of the dielectric jacket is to completely suppress unnecessary secondary discharges occurring between the sidewalls of the wire and the fabricated hole. In the present study, the effectiveness of the combination of conductive working fluid and a capacitor connected to the work piece and the tool electrode was examined. Although electrode wear was severe, machining speed with this combination (saline water at 150–250 μS/cm and capacitance at 8 μF) was twice as fast compared with fabricating a hole ( 0.8–0.9 mm) without a capacitor and saline water in a 20-mm thick carbon steel block. The mechanisms involved are discussed based on electrical circuit theory and electrochemical corrosion.     

Achieving high accuracy and high removal rate in micro-EDM by electrostatic induction feeding method

With conventional relaxation pulse generators used in micro-electrical discharge machining, due to the difficulty in keeping the minimum necessary discharge interval between pulse discharges, localized discharge and abnormal arc occur frequently. In contrast, with the newly developed electrostatic induction feeding method, only a single discharge occurs for each cycle of the periodic pulse voltage. As this realizes sufficient cooling of the discharge gap between pulses, thermal stress on the machined surface is less and duty factors can be increased, resulting in higher accuracy and machining speed compared to the relaxation pulse generator.     

数控加工中的对刀方法

文章介绍了数控加工中对刀的基本原理,常见的对刀方法及特点,并分别举例说明了数控车床、数控铣床对刀问题的处理.     

Improving machining performance of wire electrochemical discharge machining by adding SiC abrasive to electrolyte

The use of wire electrochemical discharge machining (WECDM) to slice hard brittle materials has recently been studied because its effectiveness is independent of the mechanical characteristics of the machined materials. Therefore, materials with high hardness, brittleness, strength and electrical insulation, which are difficult-to-cut, can be machined. In ECDM, the electrochemical reaction produces hydrogen bubbles, which accumulate around the cathode. A thin gas layer forms on the surface of the electrode and isolates the electrode from the electrolyte. When a voltage that exceeds the critical voltage is applied, continuous discharge occurs. The material near the electrode is removed by the discharge erosion and chemical etching. The use of WECDM to cut electrically insulating materials has only recently been investigated. However, the breakdown of the gas in the bubbles and the vibration of the wire in WECDM strongly affect the shape accuracy. This work aims to improve the over cut quality by adding SiC abrasive to the electrolyte. A mechanism that combines discharge, chemical etching and abrasive cutting is studied. The effects on expansion, roughness and material removal rate (MRR) are discussed. The experimental results reveal that adding abrasive reduces the slit expansion because it increases the critical voltage. The particles disrupt the bubble accumulation to form an isolating layer around the wire, increasing the critical voltage and reducing the discharge energy. The surface roughness is improved because the abrasive helps to refine the micro-cracks and melted zone that is formed by discharge heat erosion. Meanwhile, smaller grit produces lower roughness. The quality of the slit can be controlled; its expansion and roughness of the slit are 0.024 mm and 0.84 um Ra, respectively.     

聚晶立方氮化硼刀具电火花放电磨削分析

以国产不同粒度的聚晶立方氮化硼复合片试样作为研究对象,选取电极转速、开路电压、峰值电流、脉冲宽度和脉冲间隔作为主要工艺参数,以材料去除量、电极损耗作为加工效率的评价指标,以试样表面粗糙度值作为加工质量评价标准,结合扫描电镜观测、能谱分析、粗糙度仪测试,分析主要工艺参数对放电磨削加工的影响。实验结果表明:研究表明:PCBN电火花放电磨削加工的机理是高温熔化黏结剂,从而导致聚晶立方氮化硼颗粒脱落而实现磨削去除,过程中还伴随着元素间的置换和氧化;PCBN粒度对电火花放电磨削加工效率和质量的影响不如放电加工工艺参数的影响显著;材料去除量、电极损耗随着电极旋转速度的增加、开路电压的增大均有不同幅度的增加;随着峰值电流的增加,材料去除量增加,电极损耗减小,工件材料的表面粗糙度值增大。     

电沉积法制备多孔镁电极

为了制备一种骨架稳定的有机镁燃料电池多孔镁电极,通过扫描电镜、能谱分析及电化学方法研究了在1mol/L EtMgBr/THF的电解质溶液中,镁在金属镍和铜2种基体上沉积的电化学行为。结果表明：镁在2种金属基体上均可以发生沉积,而且在铜基体上还能够形成一层连续的镀层。采用在预镀铜的泡沫镍基体上电沉积镁的方法制备了一种新型骨架稳定的多孔镁电极,这种多孔镁电极的放电性能优于平板状的镁电极。     

对加工中心自动换刀装置换刀时间的探讨

文章简单介绍了ATC结构以及目前市场上ATC换刀时间的不一致局面,对现有加工中心ATC换刀时间的评定方法做了综合论述,适应ATC的模块化趋势,提出给出ATC独立换刀时间以及刀对刀时间结合切削对切削、辅助位置对位置的换刀时间评定办法.     

数控加工中工件坐标系的建立及对刀方法的探讨

通过对数控加工中工件坐标系和机床坐标系的基本概念的分析,分别介绍了利用G92及G54-G59两种不同指令建立工件坐标系的原理及对刀方法,并介绍了如何充分运用两种指令建立工件坐标系的有效途径.     

Enhanced virtual machining for sculptured surfaces by integrating machine tool error models into NC machining simulation

Sculptured surface machining is a time-consuming and costly process. It requires simultaneously controlled motion of the machine axes. However, positioning inaccuracies or errors exist in machine tools. The combination of error motions of the machine axes will result in a complicated pattern of part geometry errors. In order to quantitatively predict these part geometry errors, a new application framework ‘enhanced virtual machining’ is developed. It integrates machine tool error models into NC machining simulation. The ideal cutter path in the NC program for surface machining is discretized into sub-paths. For each interpolated cutter location, the machine geometric errors are predicted from the machine tool error model. Both the solid modeling approach and the surface modeling approach are used to translate machine geometric errors into part geometry errors for sculptured surface machining. The solid modeling approach obtains the final part geometry by subtracting the tool swept volume from the stock geometric model. The surface modeling approach approximates the actual cutter contact points by calculating the cutting tool motion and geometry. The simulation results show that the machine tool error model can be effectively integrated into sculptured surface machining to predict part geometry errors before the real cutting begins.