RESEARCH OF MICRO ELECTRO DISCHARGE MACHINING EQUIPMENT AND PROCESS TECHNIQUES

Micro electro discharge machining (micro EDM) is a feasible way to manufacture micro structures and has potential application in advanced industrial fields. For the realization of micro EDM, it is necessary to pay careful attention to its equipment design and the development of process techniques. The present status of research and development of micro EDM equipment and process techniques is overviewed. A micro electro discharge machine incorporated with an inchworm type of micro feed mechanism is introduced, and a micro electro discharge machine for drilling micro holes suitable to industrial use is also introduced. Some of the machining experiments carried out on the micro EDM prototypes are shown and the feasibility of the micro EDM technology to practical use is discussed.     

Precision machining of micro tool electrodes in micro EDM for drilling array micro holes

Micro electro discharge machining (micro EDM) is suitable for machining micro holes on metal alloy materials, and the micro holes can be machined even to several microns by use of wire electro discharge grinding (WEDG) of micro electrodes. However, considering practicability of micro holes <Φ100 μm in batch processing, the controllable accuracy of holes’ diameter, the consistency accuracy of repeated machining and the processing efficiency are required to be systematically improved. On the basis of conventional WEDG method, a tangential feed WEDG (TF-WEDG) method combined with on-line measurement using a charge coupled device (CCD) was proposed for improving on-line machining accuracy of micro electrodes. In TF-WEDG, removal resolution of micro-electrode diameter (the minimum thickness to be removed from micro electrode) is greatly improved by feeding the electrode along the tangential direction of wire-guide arc, and the resolution is further improved by employing negative polarity machining. Taking advantage of the high removal resolution, the precise diameter of micro-electrode can be achieved by the tangential feed of electrode to a certain position after diameter feedback of on-line measurement. Furthermore, a hybrid process was presented by combining the TF-WEDG method and a self-drilled holes method to improve the machining efficiency of micro electrodes. A cyclic alternating process of micro-electrode repeated machining and micro holes’ drilling was implemented for array micro holes with high consistency accuracy. Micro-EDM experiments were carried out for verifying the proposed methods and processes, and the experimental results show that the repeated machining accuracy of micro electrodes was less than 2 μm and the consistency accuracy of array micro holes was ±1.1 μm.     

微细电火花加工设备技术研究

微细电火花加工的关键设备技术涉及电极的微进给伺服机构、电极和工件的附加树对运动机构、微小能量放电电源以及加工状态检测与控制系统等。文章围绕微细电火花加工系统的设计系统，介绍基于压电致动原理以及摩擦传动的微进给机构、工具电极的线放电磨削机构和旋转主轴、以及微小能量放电电源的设计等，并指出需要进一步研究的课题。     

喷油嘴微孔电火花加工机床机电控制系统设计

喷油嘴喷孔加工质量直接影响发动机的燃料喷射和燃烧性能。喷油嘴喷孔机械钻削加工已难以适应高压力、微细喷孔喷油嘴发展的需要。微细电火花加工喷油嘴喷孔具有无宏观加工力、无毛刺、可加工微细喷孔以及可在热处理后加工等优点。文章结合自主设计研发的喷油嘴微细孔电火花加工机床,设计了机床的机电伺服控制系统,并进行了控制系统性能优化,进行了喷油嘴微细喷孔的电火花加工实验和工业应用,较好地满足了喷油嘴微细喷孔加工的工业需求。     

微细球状电极制备研究

微细球状电极在精微加工、测量领域有很大的应用空间,为制备低成本、高精度微细球状电极,提出新的制备工艺,在微细电解加工机床上运用微细电化学刻蚀(Micro electro-chemical machining,micro-ECM)—单脉冲放电加工(One pulseelectro discharge,OPEG)组合技术制...     

A parallel link end effector for scanning electrical discharge machining process

Because a parallel mechanism has a high-frequency response, multiple degrees of freedom (DOF), and high stiffness, it can be applied to an end effector for electrical discharge machining (EDM) with a scanning motion. A prototype has 3 DOF: two tilting angles around the x- and y-axes, and the movement in the z-direction. It consists of, a base plate, a stage, a constraint link, and three inchworm devices that act as links. The inchworm devices are connected with the stage and the base plate. The z-position and inclination of the stage are changed by adjusting the length of the inchworm devices. The electrode feeding is controlled by the combination of the steplike movement with the inchworm devices and continuous extension of piezos. The frequency response of the stage by the continuous extension of the piezos is up to 200 Hz. The positioning accuracy of the end effector is less than 30 μm in height and 0.04° in inclination. Some examples of EDM by the scanning motion are demonstrated.     

Electrical discharge machining of submicron holes using ultrasmall-diameter electrodes

We have carried out the electrical discharge machining (EDM) of submicron holes using ultrasmall-diameter electrodes. Two types of electrode were used: tungsten electrodes fabricated by the combination of wire electrodischarge grinding and electrochemical machining, and silicon electrodes originally designed as probes for scanning probe microscopes. The diameters of the former and latter were 1 μm or less, and less than 0.15 μm, respectively. Holes were drilled using a relaxation-type pulse generator at an open-circuit voltage of less than or equal to 20 V with the machine's stray capacitance as the only capacitance. Using tungsten electrodes, holes of less than 1 μm in diameter and more than 1 μm in depth were successfully drilled. A 1.3-μm-wide slot was also fabricated by drilling many holes with a small pitch. It was possible to drill holes of approximately 0.5 μm diameter using silicon electrodes because the electrode diameter was less than those of the tungsten electrodes. These holes have the smallest reported diameter for holes drilled by EDM, indicating the possibility of submicron- and nanoscale machining by EDM.     

Research on ECM process of micro holes with internal features

Micro holes with internal features are widely used as spray holes and cooling holes nowadays, which are usually required to be with high aspect ratio and shape accuracy, as well as good surface quality. An electrochemical machining (ECM) process is presented to machine these micro holes with diameter <200 μm. A quantitative relation between micro-hole diameter and machining parameters including voltage, duty ratio and feedrate is obtained through orthogonal experiments. According to the designed shape of internal features, change rules of machining parameters for varied diameters in different depth are obtained, and then micro holes with internal features are shaped precisely. Taking reverse tapered hole as an example, ECM experiments by varying parameters of voltage, duty ratio and feedrate (called varying voltage machining, varying duty ratio machining and varying feedrate machining, respectively) are carried out. Micro holes with inlet diameter of 178 μm and taper angle of 1.05° are shaped on a 1.0 mm-thick workpiece of 18CrNi8. The deviation of inlet is <3 μm and the taper-angle error is <0.1° in varying voltage machining. The corresponding dimensional accuracy of taper angle is improved by 51% than that of varying duty ratio machining under the same efficiency. The machining efficiency of varying voltage machining is increased by 36% compared to the efficiency in varying feedrate machining. In addition, the micro holes with complex features of funnel shape and bamboo shape are machined.     

Study of the Batch Production of Micro Parts Using the EDM Process

The micro machining of copper plates by the electrical discharge machining (EDM) process is described. Tungsten carbide was selected as the material for the electrode. Experiments were carried out on a conventional CNC-EDM machine to investigate machining of micro holes, and micro slots. The results show that micro holes, and micro slots can be successfully processed on a conventional CNC-EDM machine. To improve the productivity of micro parts using the EDM process, a batch production method of micro EDM using multi-electrodes has been studied. A new technique for preparing multi-electrodes has been developed. Results also show that the batch production of micro parts using EDM is feasible and that the batch production of micro parts using EDM process with multi-electrodes is very effective.     

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.     

微机械研究新进展

介绍了近三五年内国际上微机械研究的新进展,阐述新的微加工技术,趋向于探讨高纵横比的真三维立体结构;适用于聚合物和金属材料的制造方法;报道了压电型和电磁型微小电机的研制水平以及一些初见成效的微机械作品。认为国外正在进行多途径的研究,并为努力攻克微机械固有的特殊技术难点而在不懈奋斗之中。     

The development of an electrodischarge machine for micro-hole boring

This paper describes an electrodischarge machine^† for micro-hole boring capable of improved accuracy and performance in boring precision small diameter holes in components such as ink jet nozzles for printers, electron gun apertures for graphic displays, micro-connectors for high-speed computers, and optical components for telecommunications. Micro-energy discharging permits machining of 15–300 μm diameter micro-holes with a roundness accuracy of 0.5 μm or better and a surface roughness less than 0.1 μm. No bending stress is applied to the tool electrode, therefore high precision machining of cylindrical surfaces, machining with very thin side-walls, and machining of overlapping multiple-holes are all possible, regardless of the hardness of any electrically conductive material. Tool electrodes of any diameter are machinable using reversed-polarity electric discharge and are replaceable in the same way as conventional drills. Travel of the tool electrode is controlled automatically by microprocessor, thus eliminating the need for a skilled operator.     

Micro ECM with ultrasonic vibrations using a semi-cylindrical tool

In recent years, there has been a growing demand for micro holes. However, electrochemical machining has rarely been employed in drilling these holes because of problems with electrolyte diffusion. In this research, a semi-cylindrical tool was used as a tool electrode to increase the flow space of the electrolyte, and electrolyte diffusion was improved via the application of ultrasonic vibrations. Micro holes with a specified diameter of 76 μm were drilled on a 304 stainless steel plate of 300μm thickness. The proposed technique reduced both the machining time and the machining gap.     

Algorithms and machining experiments to reduce depth errors in servo scanning 3D micro EDM

In servo-scanning 3D micro electro discharge machining (SS-3D MEDM), the depth errors of 3D micro cavities are accumulated layer by layer due to the contour scanning process with keeping discharge gap for compensating axial electrode wear in real time. In this research, the errors’ causes were analyzed, and then a layer depth constrained algorithm (LDCA) and an S-curve accelerating algorithm (SCAA) were proposed to reduce the depth errors. By LDCA, over-cutting errors can be avoided by controlling a tool-electrode feed maximum at every scanning spot. As a supplementary algorithm for LDCA, SCAA can compensate insufficient-machining errors at start and end of scanning paths. Implementation process and control strategy of the algorithms were also described. The purpose of this research is to efficiently machine complex 3D micro-cavities with high accuracies of shape and surface. By use of computer-aided manufacturing software of Pro/Engineer to plan complex 3D scanning paths, machining experiments were carried out to verify the proposed algorithms. The experimental results show: Typical 3D micro cavities <800 μm can be automatically machined, and the machining accuracies of micro surfaces and edges are obviously improved, and the depth errors can be controlled within 2 μm, and the material removal rate reaches 2.0 × 10⁴ μm³/s with tool electrode of ∅80 μm and its rotational speed of 1000 r/min. In addition, the 3D micro cavities designed on unknown edge or hollow workpieces can be successfully formed.     

高频窄脉冲电流微细电解加工

微细电解加工是微细加工领域很有发展前景的微细加工技术之一。适合于微细电解加工的装置被研制出来, 它包括机床进给机构、线电极电火花磨削在线制作微细电极装置、短路检测模块、脉冲电源及其他一些辅助装置, 其中,高频窄脉冲电源是微细电解加工最重要的核心技术之一。根据微细电解加工的特点,设计了微细电解加工 MOSFET脉冲电源,该微能脉冲电源能很好地满足微细电解加工的要求。运用该微细电解加工装置进行加工试验, 在低的加工电压和低的钝化电解液浓度条件下,利用高速旋转的微细电极加工微小孔和像小铣刀一样进行微细电解铣削加工微结构,得到了满意的工艺效果,因而进一步说明电解加工在微细加工领域很有发展潜力。     

Electro-chemical micro drilling using ultra short pulses

Electro-chemical machining (ECM) has been rarely applied in micro machining because the electric field is not localized. In this work, ultra short pulses with tens of nanosecond duration are used to localize dissolution area. The effects of voltage, pulse duration, and pulse frequency on the localization distance were studied. High quality micro hole with 8 μm diameter was drilled on 304 stainless steel foil with 20 μm thickness. Localization distance can be manipulated by controlling the voltage and pulse duration, and various hole shapes were produced including stepped holes and taper free holes.     

磨粒辅助EDM与ECM复合加工技术

微机电系统(Micro electromechanical systems,MEMS)的快速发展与产品微型化的发展趋势对微细结构表面(包括微孔、微槽和微棱柱/锥等)的加工质量提出了更高的要求,为了提高微细结构表面的加工质量,提出一种磨粒辅助放电加工(Electrodischarge machining,EDM)与电化学加工(Electro chemical machining,ECM)复合加工新方法,通过建立微加工模型分析了该方法的加工机理,搭建了微加工试验平台,并进行了工艺参数优化研究,采用直径500μm和75μm的钨电极在SUS 304不锈钢上分别进行了微盲孔和微通孔加工试验研究,结果表明,在所用的EDM、EDM与ECM复合加工和磨粒辅助EDM与ECM复合加工三种方法中,磨粒辅助EDM与ECM复合加工方法获得的表面粗糙度(Ra15 nm)最高,因此该方法是微细结构表面高效和高质量加工的最佳方法之一。     

Punching of noncircular micro-holes and development of micro-forming

In this paper, vibration-EDM technology was introduced by the vibrating worktable designed to fabricate noncircular micro-electrodes and micro die-openings using wire electro-discharge grinding (WEDG). Micro punches and micro die-openings with different clearances were fabricated by controlling the spark gap in micro electrical discharge machining (micro-EDM). The micro-punching experiments include producing triangular and hexagonal micro-holes with a side length of 200 μm on brass strip. Punching of successive hexagonal micro-holes was also carried out successfully. The micro-forming experiments were conducted on copper and brass, producing cup-shaped protrusions with a diameter of 300 μm. All the experimental results show that the modified micro-punching machine is able to perform both micro-punching and micro-forming operations.     

Three-dimensional shaping by dot-matrix electrical discharge machining

This paper deals with a new prototyping method called dot-matrix electrical discharge machining (EDM) with scanning motion. The machining process by the dot-matrix method is similar to printing motion with a dot-impact printer. This method can be applied not only to EDM but also electrochemical machining and forming. A prototype of the machining unit for the dot-matrix method has six feeding devices for thin wire electrodes. The electrodes of 300 μm in diameter are arranged with the pitches of 760 μm. To obtain a smooth surface, a planetary motion in the x-y-plane is added to the feeding of the machining unit in the z-direction, the same area is machined repeatedly, or the machining unit is moved with fine feed. By compensating for the wear of the electrode during the scanning EDM, various shapes with the accuracy of micrometers order can be obtained without a formed tool electrode.     

微细孔超声加工关键技术

为提高微细孔超声加工的微细程度和精度,研究微细孔超声加工的关键技术,包括高精度的机床本体、超声加工单元、微细工具的制备技术、微细工具几何参数的选择等。对微细工具的动态压杆稳定性进行分析,结果表明微细工具的压杆稳定性不仅与静压力有关,还与超声振幅有关。导出判别动态压杆稳定性的Mathieu方程,并经简化给出工具临界压力和临界长度的计算判据,解决因工具长度、静压力(压应力)大小和超声振幅选择不当而导致的加工中出现工具轴弯曲、折断、破坏孔的形状精度的问题。对工件振动方式的微细孔超声加工进行的试验表明,微细工具的磨损率随着进给速度和超声振幅的增大而增加。在所研制的微细超声加工系统上可加工出直径13 μm的微细孔。