外文翻譯--基于工具的納米精加工和顯微機(jī)械加工 英文版【優(yōu)秀】

Tool-based nanonishing and micromachining M. Rahman , H.S. Lim, K.S. Neo, A. Senthil Kumar, Y.S. Wong, X.P. Li Mechanical Engineering Department, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Abstract There is a growing demand for industrial products not only with increased number of functions but also of reduced dimensions. Micromachining is the most basic technology for the production of such miniaturized parts and components. Since miniaturization of industrial products had been the trend of technological development, micromachining is expected to play increasingly important roles in todays manufacturing technology. Micromachining based on lithography has many disadvantages unlike tool-based micromachining technology such as micro-turning, grinding, EDM and ECM have many advantages in productivity, efciency, exibility and cost effectiveness. However, difculties, as the machining unit reduced, are yet to be solved to utilize the tool-based machining technology for micromachining. In this paper, recent achievements in some important areas of tool-based micromachining are introduced. Electrolytic in-process dressing (ELID) grinding and ultra precision machining using single point diamond tool are two most widely applied techniques to produce nano-surface nish on hard and brittle materials. Recently these techniques are also being applied for nano-surface generation on silicon wafers and it is hoped that this process will be able to replace the current technique, chemical mechanical polishing (CMP) process. Micro-electro-discharge machining (micro-EDM) and micro-turning technology are widely used to produce miniaturized parts and features. Usually hybrid machining is carried out to fabricate micro-components with high precision. Usually multi-purpose miniature machine tools are used to produce such components. Recent achievements on the development of such machines are also discussed in this paper. 2006 Elsevier B.V. All rights reserved. Keywords: Tool-based machining； ELID grinding； Micro-EDM； Micro-turning； Hybrid machining 1. Introduction Micromachining is gaining popularity due to the recent advancements in Micro-Electro Mechanical Systems. Many studies have been carried out to fabricate functional microstructures and components. Micromachining technology using photolithography on silicon substrate is one of the key processes to fabricate the microstructures. However, there are some limitations in this process due to its quasi-three-dimensional structure, its low aspect ratio and limitation of the working material. Deep X-ray lithography using synchrotron radiation beam (LIGA process) and focused-ion beam machining process can produce high aspect ratio three-dimensional sub-micron structures with very high form accuracy. But, these processes require special facilities, and the maximum achievable thickness is relatively small1,2. Conventional material removal processes, such as turning, milling and grinding, are also studied to fabricate microstructures by introducing a single point diamond cutter or very ne grit sized grinding wheels. These material removal processes can machine almost every material such as metals, plastics and semiconductors. There is also no limitation in machining shape, so that at surfaces, arbitral curvatures and long shafts can be machined, which are required for the moving parts and guiding structures 3,4. Planer and aspheric surfaces with nano-surface nish can also be produced by ductile mode machining either by single point turning using diamond cutter or with xed abrasive grains using new grinding methods 5,6. To obtain nano-surface nish and accuracy on brittle materials, grinding wheels with ne abrasive size are needed. Problems such as wheel loading and glazing are encountered while grinding with ne abrasives. Periodic dressing is essential to minimize the above problems and this makes the grinding process very tedious. Micro-mould cavities are also needed for mass-production of micro-components, which can be made by injection molding process. Hard-to-machine work-piece materials should be machined very precisely in three-dimensional forms in the micron range for the purpose of microinjection. For the fabrication of complex three-dimensional molds using very tough die materials, micro-electro-discharge machining (EDM) is one of the alternative machining processes that can be used successfully. Micro-EDM can machine almost every conductive material, regardless of its stiffness. Using a very thin electrode with control of the EDM contour, micro-molds can be produced successfully. Although these methods cannot reach the dimensional magnitudes of photo fabrication techniques, such magnitudes are not required in many cases. Besides these, the set up cost for the photo fabrication and etching techniques are also comparatively more expensive than micromachining using machine tools. In this paper, recent achievements in some important areas of tool-based micromachining are introduced. 2. Nano-surface generation 2.1. Nano-surface generation using electrolytic in-process dressing (ELID) grinding 2.1.1. Principle of ELID grinding It is possible to obtain mirror surface nishes on hard and brittle materials when material removal takes place through plastic deformation rather than fracture. Ductile mode machining can be realized when using super ne abrasive grinding wheels together with ELID grinding. Smoother surface and fewer grinding mark son the glass surface was observed when metal bonded diamond grinding wheel of grit size 4000 and above was used 79. Murata et al. (1985) introduced electrolytic in-process dress-ing (ELID) to grind ceramic with metal bonded diamond wheel of grit size less than #400 and they found the method to be efcient for grinding hard and brittle materials. ELID grinding was further improved by Ohmori (1990) with metal bonded grinding wheels of ner grades with grit size more than #1000 that was electrolytically dressed during grinding to realize ne surface nish. The developed ELID grinding process is a simple technique that can be used on any conventional grinding machine5,6. The basic ELID system consists of a metal bonded diamond grinding wheel, electrode, power supply and electrolyte. A schematic of an ELID system developed in NUS is shown in Fig. 1. The metal bonded grinding wheel is made into the positive pole through the application of a brush smoothly contacting the wheel shaft and the electrode is made into negative pole. In the small clearance of approximately 0.10.3 mm between the positive and negative poles, electrolysis occurs through the supply of the grinding uid and an electrical current. Fig. 2 shows the mechanism of ELID grinding of metal bonded diamond wheel. After truing (a), the grains and bonding material of the wheel surface are attened. It is necessary for the trued wheel to be electrically pre-dressed to protrude the grains on the wheel surface. When pre-dressing starts (b), the bonding material ows out from the grinding wheel and an insulating layer composed of the oxidized bonding material is formed on the wheel surface (c). This insulating layer reduces the electrical conductivity of the wheel surface and prevents excessive ow out of the bonding material from the wheel. As grinding begins (d), diamond grains wear out and the layer also become worn out (e). As a result, the electrical conductivity of the wheel surface increases and the electrolytic dressing restarts with the ow out of bonding material from grinding wheel. The protrusion of diamond grains from the grinding wheel therefore remains constant. This cycle is repeated during the grinding process to achieve stable grinding. 2.1.2. Characteristics of ELID grinding on nano-nishing optical glass (BK7) The ELID grinding system developed has been applied to the grinding of BK7 glass, a common material for the manufacture of optical components. By applying the ELID grinding technique, there was a vast improvement in the surface roughness of the ground surface as shown in the micrographs of the ground surfaces in Fig. 3. The conventional grinding process produces poorer nish because the active sharp grits per unit area of the grinding wheel decreases during grinding until the next dressing cycle. In the case of the ELID grinding technique, the active sharp grits per unit area of the wheel remain the same due to constant dressing and this leads to improved surface integrity and surface roughness. The ELID grinding technique also has the advantage of reducing the bonding strength of the wheel- working surface, hence improving grind-ability. Fig. 1. ELID grinding setup. Fig. 2. In-process dressing in ELID grinding. Fig. 3. Comparison of ground surface generated with (a) conventional grinding and (b) ELID grinding (50% current). Fig. 4. Effect of grit size on the surface generation Fig. 5. Effect of in-process dressing condition (current duty ratio) on surface generation. Fig. 4 shows the effect of grinding wheel grit size on the surface generation. The grinding mode of the ground glass surface is inuenced by the grit size of the wheel that has very little or no inuence on the machining and pre-dressing conditions. The experiments show that the in-process dressing condition affects the surface roughness of the machined surface (Fig. 5). The grit held by the oxide layer is loosely held in the bond and the process is same as the lapping process. The oxide layer holding the diamond grit is like the lapping pad and the bonding material acts like a supporting pad. When more dressing current is applied, thickness of the oxide layer increases, the abrasives are loosely bonded and the grinding process becomes almost like polishing process. From the experiments it was observed that the surface roughness is better when the current duty ratio increases. However, there is limitation in terms of machining condition (feed rate) feed rate, due to a black strip formation on the ground surface.Fig. 6 shows the effect of feed rate and current duty ratio on the black strip formation, which will affect the surface nish. In the gure, marks indicate the formation of the black strip. To attain the desired surface nish and avoiding black strip formation, it is important to select the appropriate feed rate and current duty ratio. Experiments are also conducted to investigate the inuence of the current duty ratio on the surface roughness and wheel wear, when the feed rate and depth of cut are kept constant. From Fig. 7, it is clear that the surface roughness ha improved but the wheel wear increases proportionally with the current duty ratio. The ELID grinding system that was developed and the experiments carried out has provided a practical application solution of the process. Through appropriate selection of machining and electrolytic dressing conditions, a surface nish of 0.01 m (Ra) is easily achievable on BK 7. Fig. 8 is an example of macro lens machined on a 5 mm glass rod. 2.1.3. Nano-surface nishing of silicon wafer using ELID grinding Among the polishing techniques used for semiconductor materials, CMP (Chemical mechanical Polishing) has many advantages and few serious disadvantages too. Materials which are soft and brittle like GaAs and GaP are efciently polished by the CMP technique. However, some of the disadvantages associated with this process for polishing hard and brittle materials like silicon are (i) low efciency due to low removal rates, (ii) non-uniform wafer surface due to the variation in the back pressure of wafer, and the variation in relative cutting speed across the wafer surface, and (iii) relatively high cost involved in this process. On the other hand, the ELID grinding process has some important advantages over the CMP process, and there is a potential for the ELID grinding process to replace the CMP polishing method. Some signicant advantages over CMP are (i) high efciency due to high removal rate, (ii) uniform ground surface across the wafer, and (iii) relatively low cost involved in this process. The authors have conducted experiments to compare the over- all performance involved in CMP process with that of the ELID grinding process. The ELID grinding operation was carried out on a Computer Numeric Control (CNC) machining center (Fig. 9). The optimum conditions were determined by observing the effects of various parameters related to ELID grinding and then proper conditions for better results were selected. The ELID grinding parameters for wafer machining are as follow: feed speed 100 mm/min, wheel grit 8000 (grit size 1.76mm), spindle speed 500 rpm, depth of cut 1 m, ELID power voltage 90 V, max current 10 A. In the grinding of silicon wafer, the material removal rate is remarkably high and typically 6.596 mm3/min can be achieved. The wheel wear rate is negligible. After grinding two wafers to 195 m thickness, there is no variation in the thickness of the grinding wheel. The ground surface is perfectly ductile and mirror like nish could be achieved as shown in Fig. 10. Fig. 6. Limitation in ELID grinding. Fig. 7. Effect of dressing current condition. Fig. 8. A spheric microlens by ELID grinding (5 mm diameter) Fig. 9. Experimental setup for ELID grinding of silicon wafer. Fig. 10. Mirror nished silicon surface. 2.2. Nanosurface generation by ultra precision machining 2.2.1. Ultra precision machining using single point diamond tool (SPDT) Ultra precision machining is a technique which removes materials from a few microns to sub-micron level to achieve ductile mode machining on hard-to-machine materials such as electro-less nickel plating, silicon, quartz, glass and ceramics with no subsurface defects. Such a machining process is able to achieve mirror surface nish of less than 10 nm and form error of less than 1m easily. If properly applied to a specic range of diamond turn-able materials, the process is far superior to grinding and polishing where shape control is more difcult and processing time is longer. An important factor to achieve ultra precision machining is a machine tool capable of moving in high accuracy at nanometer resolution. Necessary features for such a machine tool includes stiffness for vibrational stability, air bearing spindles with low run-outs, straight square ways and closed loop controller using nanometer resolution feedback. One such machine used in the Advanced Manufacturing Laboratory of NUS is the Toshiba ULG100C (H3) ultra precision machine. Another important factor is to employ high quality tools made of single crystal diamond be it natural or articial. The advantages of single crystal diamond cutter include high hardness and wear resistance, good thermal conductivity for heat removal during machining, and it is possible to achieve sharp cutting edge radius of 20 nm for nanometric level cutting. Other important factors to consider include cutter geometry, tool wear, coolant supply, cutting conditions, and the characteristics of the material being machined. The employment of SPDT in a turning setup is commonly referred to as diamond turning as shown in the machining setup in Fig. 11. Fig. 11. Face turning setup. Fig. 12. Flank (a) and rake (b) face wear after cutting distance of 93.6 km on EN of 5.7% P (w/w). 2.2.2. Diamond turning of electro-less nickel plated molding dies A major application of the ultra precision machining technology is for the diamond turning of electro-less nickel plated molding dies for plastic optical parts such as LCD or projection TV. However, a big challenge posed is the short tool life of diamond cutters and polishing process is required after diamond turning. This is not desired as the form error of the polished surface is inferior to that of the diamond-turned surface. Hence, it is important to maintain only the turning process for such molds and the goal of this project is improving diamond cutting tool life by optimizing material characteristics of electro-less nickel, design of the diamond cutting tools and the machining conditions. At the Advanced Manufacturing Laboratory of NUS, a project was undertaken in collaboration with PERL of Hitachi Ltd., Japan to study into this problem. The process for diamond turning of electroless nickel molding dies has been established and surface roughness of less than 6 nm (Ra) has been achieved. Major factors affecting the wear of diamond cutter have been established； namely (a) the electro-less nickel plating composition, (b) the crystal orientation of the diamond cutter, (c) the types of diamond employed (articial or natural), and (d) the rake angle of the diamond cutters. Taking these into consideration, a long cutting distance of 200 km has been achieved and still maintaining mirror surface nish quality of 0.12 mm Ry. The following subsections show the major ndings of the project undertaken. 2.2.2.1. Investigating cutter performance for different electro-less nickel plating (EN) compositions. In diamond turning of electro-less nickel plating, the composition of nickel and phosphorus in the electro-less nickel has signicant inuence on its machinability for two reasons. Firstly, elements like nickel and iron work as catalysts, which promote the diamondgraphite transformation 10, hence a greater amount nickel compared to phosphorus will lead to greater diamond cutter wear. Secondly, the phosphorus content in electro-less nickel signicantly affects its structure and hardness11. Electro-less nickel with lower phosphorus content tend to be more brittle and harder, hence making it harder to machine. Experimental cuts were made on work-pieces of 5.7% and 11.5% (w/w) phosphorus to see the cutter performance, hence establishing the preferred electro-less nickel composition as detailed by Pramanik et al. 12. The quantitative comparisons of cutting tool performance in terms of surface nish and wear with cutting distance for different phosphorus content have been presented in Figs. 1215. It is very clear that rate of wear is very small and almost constant for 11.5% (w/w) phosphorus content, hence maintaining good surface nish. On the other hand, tool wear was so high when machining electro-less nickel with 5.7% (w/w) phosphorus content that machining had to be stopped after cutting around 100 km. The severe wear is obvious when comparing the micro- graphs (Figs. 12 and 13) of the cutters that were used to machined electroless with different phosphorus content. The phosphorus in electro-less nickel not only affects its hardness and material structure, but also to serve as a lubricant to aid the machining process. Though electroless nickel with lower phosphorus content is harder and brittle, no brittle fracture was noticed during machining. So phosphorus content plays some more roles in chemical property of electro-less nickel and tool wear, from experimental results it can be assumed that phosphorus mitigates the catalytic action of nickel for promoting diamondgraphite transformation. It is clear that phosphorus protects the cutter from damage and facilitates the machinability of electro-less nickel. Fig. 13. Flank (a) and rake (b) face wear after cutting distance of 202.3 km on EN of 11.5% P (w/w) Fig. 14. Effect of cutting distance on surface roughness (Ra), for phosphorus content. Fig. 15. Effect of cutting distance on wear for different phosphorus content. 2.2.2.2. Effects of crystal orientation on cutting tool performance for very long cutting distance (200 km). It is well known that single crystal diamond exhibits anisotropism whereby its properties vary with direction and crystal orientation. So the performance testing of crystal orientation is important before commercial use of single crystal diamond tool. Two types of crystal orientation, i.e. (1 1 0) and (1 0 0) at rake face have been studied. Figs. 1619 show micrographs of the cutters after 202 km cutting distance. It seems that the form and shape of wear development at rake and ank face are same for both types of crystal orientations but the quantity of wear is very different as shown by Fig. 18 which shows the progression of ank wear. The rate of rake wear propagation for crystal orientation (1 0 0) is very high up to a cutting distance of around 80 km after that the wear rate is low and steady. In case of crystal orientation (1 1 0) the rate of rake wear is not so high and it is very much steady all over the cutting distance. In terms of surface nish (Ra) the crystal orientation (1 1 0) shows better performance as shown in Fig. 19 due to lower progression of wear. From the experimental analysis, it can be observed that diamond cutter with (1 1 0) crystal orientation on the rake face is able to perform better. Fig. 16. Flank (a) and rake (b) face wear after cutting distance of 202 km with rake face (1 1 0) diamond cutter (500). Fig. 17. Flank (a) and rake (b) face wear after cutting distance of 202.3 km with rake face (1 0 0) diamond cutter (500). Cutting conditions of ultra precision cutting tests Fig. 18. Effect of cutting distance on wear for different cutter crystal orientation. Fig. 19. Effect of cutting distance on surface roughness (Ra), for different cutter crystal orientation. 2.2.3. Cutting conditions and tool edge radius for nano-scale ductile cutting of silicon wafer Current method of processing silicon wafer by grinding results in frequent subsurface damage which has to be removed by polishing. It is proposed to increase the yield rate by establishing the ductile mode machining regime using diamond single point cutting tool on an ultra precision machine. This way damage due to brittle fracture can be minimized and reliability of parts in service can be improved. To achieve ductile mode machining of silicon, previous work of researchers 13,14 have shown the ductilebrittle machining transition for semi-conductor materials as the unde formed chip thickness was increased, and Yan et al. 15 has shown the critical depth of cut for silicon to be 58 nm. It has also been shown the tool cutting edge geometry has signicant effect on achieving ductile mode cutting 16,17. Although previously there have been research work done on the ductile mode chip formation in cutting of silicon material, there has been no detailed report on cutting conditions and tool cutting edge radius for nano-scale ductile mode cutting of silicon wafer. In the present study, cutting conditions and tool cutting edge radius for nano-scale ductile mode cutting of silicon wafers have been investigated through experimental tests. Face turning experiments of silicon wafer were carried out on an ultra precision lathe (Toshiba ULG-100C) of 1 nm positioning resolution using diamond tools with 0 rake, 7 clearance and 0.5 mm radius. Silicon (1 1 1) wafers of 100 mm in diameter, 0.5 mm thick and having a lapped nish were used as specimens. The schematic diagram of chip formation in ductile cutting of silicon wafer is shown in Fig. 20. Here, R is the cutting edge radius, the tool rake angle, ao is the depth of cut. Cutting was performed using single crystal diamond tool of six different cutting edge radii ranging from 23 to 807 nm. The diamond tool cutting edge radius was measured through the indentation method by Li et al. 18. The cutting radii were achieved using a special lapping process that was designed 17. The cutting conditions of the ultra precision face turning experiments are listed in Table 1. Dry cutting was carried out for the purpose of collecting the cutting chips. The machined work-piece surface texture and chip formations were examined using scanning electron microscope (SEM) (JEOL JSM-5500). The machined work-piece surface topography was examined using an atomic force microscope (AFM). Surface roughness of machined silicon wafers was examined using a Form tracer (Mitutoyo CS-5000). AFM photographs of the machined surfaces in the cutting of silicon wafers using diamond tools with different cutting edge radii at cutting speed of 150 m/min are shown in Fig. 21. For the diamond tool edge radius of 23, 202, 490, 623 and 717 nm, one test was conducted under the condition that the under formed chip thickness was less than the tool edge radius and the other test was conducted under the condition that the under formed chip thickness was larger than the tool edge radius. For the diamond tool edge radius of 807 nm, both the tests were conducted under the condition that the under formed chip thickness was less than the tool edge radius. This was conrmed by SEM observations which showed continuous chips similar to those formed in cutting of ductile materials, where chip formation is dominated by dislocation. On the other hand, when the under formed chip thickness was larger than the tool cutting edge radius, the machined workpiece surfaces were very rough and fractured, which showed that the cutting was conducted under brittle mode. Fig. 20. Schematic diagram of chip formation in ductile cutting of a brittle material. 3. Tool-based micromachining using hybrid process 3.1. Development of miniature machine tool form-process machining Fig. 21. AFM photographs of machined silicon wafer surfaces under different under formed chip. Fig. 22. Multi-purpose miniature machine tool. A multi-purpose miniature machine tool has been developed for high precision micromachining 19. Fig. 22 shows the structure of the desktop miniature machine tool. The machine tool has its size of 560 mm (W) 600 mm (D) 660 mm (H), and the maximum travel range are 210 mm (X) 110 mm (Y) 110 mm (Z). Each axis has optical linear scale with resolution of 0.1m, and full closed feedback control ensures accuracy of sub-micron. Machine enables changeable high speed, middle speed and low speed spindles for micro-milling, micro-turning and micro- grinding on the machine. The low speed spindle is electrically isolated from the body of the machine so that electrical machining, such as EDM and ECM, can be performed on the machine. The motion controller can execute a program downloaded from the host computer independently； thus a good EDM gap control can be achieved in a real time. The response of each axis is important because a fast movement is required for the gap control during EDM. The Z-axis shows a linear response up to 10 Hz and a resonant frequency at 100 Hz at the amplitude of 0.1024 mm, which is sufcient distance to control the spark gap in the EDM process. 3.2. High aspect ratio micro-fabrication using micro-EDM Micro-Electro-Discharge Machining ( -EDM) is a non-traditional machining technology that has been found to be one of the most efcient technologies for fabricating microcomponents. The non-contact process requires little force between electrode and work-piece and is capable of machining ductile, brittle or super hardened-materials. With appropriate parameters, it is possible for micro-EDM to achieve high precision and high quality machining. The non-contact nature of EDM makes it possible to use a very long and thin electrode for the machining. Even though a micro-milling cutter of down to 50 m in diameter is available in the market, the length of the tool is usually 35 times of its diameter and it is also not suitable to machine a very tough die material, which only can be machined using EDM. Although micro-EDM plays an important role in the eld of micromachining, it has disadvantages such as high electrode wear ratio and low material removal rate. The wear of electrode must be compensated either by changing the electrode or by preparing longer electrode from the beginning or fabricating the electrode in situ for further machining. It is not recommended to change the microelectrode during machining, because it may reduce the accuracy due to the change in setup or re-clamping of the micro-electrode. Fig. 23 shows a conceptual process to fabricate a high aspect ratio microstructures using micro-EDM. In the micro-EDM process, the tool electrode is fabricated on the machine to avoid clamping error. From an electrode thicker than the required diameter, a cylindrical electrode is fabricated by EDM process using a sacricial electrode. Different setup of the sacricial electrode can be used in this process (Fig. 23(a). When there is a dimensional change in the sacricial electrode, the diameter of the tool electrode fabricated is usually unpredictable. An on-machine measurement of the tool electrode diameter is required in this case (Fig. 23(b). An optical measurement device has been specially developed for the measurement of a thin electrode, which consists of a laser diode, optical lter and photo detectors. After measuring the diameter, a compensated machining schedule for the tool electrode fabrication is generated and machining is carried out. These processes are repeated until the required tool electrode diameter is achieved. After nishing the tool electrode fabrication, micro-EDM is performed to fabricate the high aspect ratio microstructure (Fig. 23(c). In the study of the tool electrode fabrication process, three different sacricial electrodes are tested to compare their capability and performance. Fig. 24 shows the three different types of sacricial electrodes. Fig. 24(a) shows a stationary block, which is the simplest method to machine a tool electrode. Fig. 24(b) shows a rotating electrode with 0.5 mm in thickness and 60 mm in diameter. The rotating speed of the disk electrode is about 90 rpm during tool fabrication. Fig. 24(c) shows a guided running wire as a sacricial electrode of 0.07 mm in diameter. The wire running speed is about 35 mm/s. This method is known as wire electro-discharge grinding (WEDG), and it is a typical method for micro-EDM. During the tool fabrication process, the spindle is rotating about 300 rpm and it moves up and down according to the toolelectrode contact conditions. This means that the spindle is under control to maintain the EDM spark gap. Once the tool reaches one end of its stroke movement, the tool moves toward the electrode to a given depth of cut, and the process is repeated. Fig. 25 shows some typical electrode shapes which are fabricated using micro-EDM with different types of sacricial electrodes. Fig. 25(a) shows the tool electrode machined using a stationary copper block. The surface of the fabricated tool electrode is generally smooth. However, the shape accuracy is not as good as desired, and the tool usually has some taper. The tapered shape on the tool electrode is due to the wear of the sacricial electrode. Since the tool moves upward and downward during tool fabrication, the lower (tip) portion has a greater chance to face the sacricial electrode, and consequently is subjected to more discharges therefore machining. Slight tilting of the electrode block toward the tool electrode does not help to improve the tapered shape. The electrode still has uneven diameter as shown in Fig. 25(b). The stationary sacricial block electrode is easy to install； however, the shape and dimensions are not easy to control. Fig. 25(c) shows an example of tool electrode fabrication using a rotating sacricial electrode. In this example, there is erosion on the rotating electrode by electrical discharges during machining. However, the erosion is distributed almost uniformly over the whole perimeter of the electrode. Taking into consideration of the diameter difference between the tool electrode and sacricial electrode, the dimensional change of the rotating electrode is almost negligible in the tool electrode fabrication using micro-EDM. The 0.5 mm thickness of the rotating electrode gives the same effect as stationary electrode on the surface nish, since it is wide enough to nish a smooth surface. Fig. 25(d) is a typical surface condition of the tool electrode which is machined by EDM using a running wire. This process is known as wire electro-discharge grinding (WEDG), and is being used widely for micro-EDM machining. Since a fresh wire is continuously used, the dimensional change of the sacricial electrode is theoretically zero. This fact ensures high-accuracy dimensional control in micro-EDM. However, the surface nishing efciency is not as high as that of the rotating electrode method. This is due to the fact that the diameter of the running wire is only 0.07 mm and it is not enough to nish the machined surface smoothly using the same condition of EDM gap control used for rotating electrode. To achieve better surface using this thin wire, the speed of the nishing process must be reduced. From the comparison of the three different methods, the rotating electrode method is found to be the most efcient method to fabricate a tool electrode. Even though the wear of rotating electrode is not zero, the diameter of the electrode can be controlled using on-machine measurement followed by compensation machining. After series of test are being made, various micro-shapes are fabricated using micro-EDM. Fig. 26(a) shows a micro-slot of width 200 m made on a tungsten rod. Scanning EDM process is used for the machining. Fig. 26(b) shows a triangular hole machined on a stainless still plate. To achieve the shape, a 50m tungsten electrode was machined to the triangular shape rst using WEDG process followed by a die sinking micro-EDM. Fig. 26(c) shows a microelectrode of diameter of 50 mm machined using WEDG process. Fig. 26(d) shows simulated micro-cracks machined on an aluminum work-piece using micro-scanning EDM process. Fig. 23. Process to fabricate high aspect ratio microstructures using micro-EDM. Fig. 24. Three types of sacricial electrode for on-machine tool fabrication. Fig. 25. Typical shape of on machine fabricated tool electrodes. (a) Tapered tool electrode machined using a stationary sacricial block. (b) Uneven diameter machined using a stationary sacricial block. (c) Electrode machined using rotating disk. (d) Electrode machined using running wire. Fig. 26. Micro-features made by micro-EDM. (a) Micro-slot； (b) triangular hole； (c) microelectrode； (d) micro-crack Fig. 27. Concept of high-accuracy micro-EDM. (a) EDM with thin electrode；(b) turning-EDM hybrid machining. 3.3. Machining of thin electrode using micro-turning Fig. 28. Deection of micro-shaft during machining. Fig. 29. Effect of step size on step error and deection. Micro-Electro-Discharge Machining ( -EDM) is a kind of non-traditional machining technology. This is one of the most efcient technologies for fabricating micro-components. The non-contact process requires little force between electrode and work-piece and is capable of machining ductile, brittle or super hardened materials. Under the appropriate parameters, it is possible for micro-EDM to set high precision and high quality machining. Although micro-EDM plays an important role in the eld of micromachining, it has disadvantages such as high electrode wear ratio and low material removal rate. The wear of electrode must be compensated either by changing the electrode or by preparing longer electrode from the beginning or fabricating the electrode in situ for further machining. It is not recommended to change the microelectrode during machining, because it may reduce the accuracy due to the clamping. Machining longer electrodes introduce deection due to low stiffness as illustrated in Fig. 27(a). Fig. 27(b) illustrates the concept of turning-EDM hybrid machining. In this hybrid machining process, EDM is carried out using a turned shaft. An electrode of required length is fabricated using the micro-turning process. Using this hybrid machining, clamping error can be avoided and deection of electrode can be minimized, consequently the accuracy of machining can be improved. When different diameters of electrode are preferred, turning can signicantly reduce the electrode preparation time as compared to the WEDG method. This hybrid machining technology also can be used to fabricate a cylindrical micro-component with non-rotational portion such as a key slot or at bar by the help of EDM process followed by turning. One of the problems in machining thin electrode using turning is the deection of the work-piece during machining. Fig. 28 Fig. 30. Micro-turned parts. (a) 33mm diameter electrode； (b) high aspect ratio shaft； (c) micro-motor shaft. shows the deection of end portion of the work-piece is measured using a deection measurement sensor. From the experiment, it is observed that the work-piece is not bending only in nor- mal direction (X) of the tool work-piece contact region； it also deects in the tangential (Y) direction. In fact, the work piece deects towards top surface (rake surface) of the cutting tool. To fabricate a highly accurate thin shaft, the Y direction deection also must be compensated according to the change of the work-piece diameter. Another factor that affects the deection of the work-piece is the step size, which is given to minimize the deection. Fig. 29 shows the effect of step size on the step error and deection of the work-piece. It is observed that the effect of step size on step error is more dominant than depth of cut when the step size is increased. For the thin electrode turning, reducing step size is a promise to reduce the overall error of the electrode. Some samples of micro-turned parts