- Email: [email protected]
Tool-based micro-machining
Journal of Materials Processing Technology 192–193 (2007) 204–211
Tool-based micro-machining A.B.M.A. Asad, Takeshi Masaki, M. Rahman ∗ , H.S. Lim, Y.S. Wong 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. Micro-machining 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, micro-machining is expected to play increasingly important roles in today’s manufacturing technology. Micro-machining based on lithography has many disadvantages unlike tool-based micro-machining technology such as micro-turning, microgrinding, micro-EDM and micro-ECM have many advantages in productivity, efficiency, flexibility and cost effectiveness. However, difficulties, as the machining unit reduced, are still remained to be solved to utilize the tool-based machining technology for micro-machining. In this paper, recent achievements at NUS in the area of tool-based micro-machining are introduced. Micro-electro-discharge machining (microEDM) and micro-Turning technology are developed to produce miniaturized parts and features using a multi-purpose miniature machine tool for hybrid processes which is developed at NUS. © 2007 Elsevier B.V. All rights reserved. Keywords: Tool-based machining; Micro-EDM; Micro-EDG; Micro-EDM milling; Scanning EDM; Micro-Turning; Hybrid machining
1. Introduction Micro-machining is gaining popularity due to the recent advancements in Micro-Electro Mechanical Systems (MEMS). Many studies have been carried out to fabricate functional micro-structures and components. Micro-machining 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-3-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 3-dimensional sub-micron structures with very high form accuracy. But, these processes require very expensive and special facilities, and the maximum achievable thickness is relatively small [1,2]. Combination of conventional material removal processes, such as turning and milling, are hybridized with non-conventional machining processes like EDM/EDG to fabricate microstructures with high dimensional accuracy. These hybrid material removal processes can machine micro-features on almost every material such as metals, plastics and semiconductors. There is also no limitation in machining shape, so
∗
Corresponding author. Tel.: +65 6516 2168; fax: +65 6779 1459. E-mail address: [email protected] (M. Rahman).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.038
that flat surfaces, arbitrary curvatures and long shafts can be machined, which are required for the moving parts and guiding structures [3–9]. Micro-mould cavities are also needed for mass-production of micro-components, which can be made by injection molding process. Hard-to-machine workpiece materials should be machined very precisely in 3-dimensional forms in the micron range for the purpose of microinjection. For the fabrication of complex 3-dimensional molds using very tough die materials, micro-electro-discharge machining (-EDM) is one of the alternative machining processes that can be used successfully. -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 micro-machining using machine tools. In order to achieve meaningful implementation of micro/nano-machining techniques, the research efforts in NUS seek to address three important areas; namely (a) machine tool development, (b) on-machine micro/nano-metrology for process control, and (c) process development for the micro/nanomachining techniques to achieve the necessary accuracy and quality. An integrated effort in these areas has resulted in
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
machine tools, processes and on-machine measuring technology being developed for tool-based micro/nano-machining. 2. Development of miniature machine tool for multi-process micro-machining A multi-purpose miniature machine tool has been developed and going through continuous development cycle for high precision micro-machining [10]. Fig. 1 shows the structure of the miniature machine tool. The machine tool has its size of 1.5 m (W) × 1.1 m (D) × 1.9 m (H), including the controller unit and the dielectric supply unit. The maximum travel ranges are 210 mm (X) × 110 mm (Y) × 110 mm (Z). Each axis has optical linear scale with resolution of 0.1 m, and full closed feedback control and compensation ensures high accuracy. Machine enables changeable high speed, middle speed and low speed spindles for -Milling, -Turning and -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 using high speed communication, which creates a very user friendly environment for the operator on a standard PC. The controller has been tested to execute more than 300 motion nodes per second. At the same time, since the motion controller was developed jointly by NUS and a NUS venture company for the multi-process machine tool, programs and scheduling can be accessed and developed to meet the time critical requirements which is a key development area for micro-machining processes. As for example process parameter
Fig. 1. Multi-purpose miniature machine tool.
205
sampling can be done as fast as 20 kHz rate, while still managing other job schedules. Moreover, it has a multiprocessor-based architecture with UART-based bus network communication to distribute works between multiple processors. The response of each axis is important because a fast movement is required for the gap control during EDM. The servo system was designed to respond to the reference command within 50 ms. This was found to be excellent for gap control during EDM process. Unlike conventional machining processes, non-conventional machining processes and especially in the domain of micromachining processes, process know-how and development of process control system plays a vital role. The EDM process control hardware and software development requires continuous research. As a result of the continuous effort at NUS, recent development of EDM control hardware and software has improved the machining speed and quality by manifold. For -EDM process the amount of energy released controls the machining quality and the sparking frequency along with the control of the sparking gap decides the machining time. Fig. 2(a and b) show very fine finishing edge of the machined feature. Fig. 3 shows machining time for fabricating holes less than 20 m diameter on 50 m stainless steel plate. It was found from
Fig. 2. (a) Circularity of 50 m machined hole on stainless steel plate. (b) Edges of machined workpiece on stainless steel.
206
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
Fig. 3. Time taken for drilling holes of less than 20 m diameter.
experiment that using a 70 m tungsten wire, to cut a 12 mm stainless steel plate of 50 m thickness, takes less than 220 s. 3. Tool fabrication using -EDM process Micro-Electro-Discharge Machining (-EDM) is a nontraditional machining technology that has been found to be one of the most efficient technologies for fabricating microcomponents. The non-contact process requires little or almost no force between electrode and workpiece and is capable of machining ductile, brittle or super hardened-materials. With appropriate parameters, it is possible for -EDM process 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 machining. Even though a Milling cutter of down to 50 m in diameter is available in the market, the length of the tool is usually three to five times of its diameter and it is also not suitable to machine tough die material, which only can be machined using EDM. Although EDM plays an important role in the field of micro-machining, it has disadvantages such as 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 reduces the accuracy due to the change in setup or re-clamping of the micro-electrode.
Fig. 4 shows a conceptual process to fabricate a high-aspectratio micro-structures using -EDM. In -EDM process, 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 sacrificial electrode. Different setup of the sacrificial electrode can be used in this process (Fig. 4(a)). Due to dimensional change in the sacrificial electrode, diameter of the tool electrode fabricated is usually unpredictable. An on-machine measurement of the tool electrode diameter is required in this case (Fig. 4(b)). An optical measurement device has been specially developed for the measurement of a thin electrode, which consists of a laser diode, optical filter 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 finishing the tool electrode fabrication, -EDM is performed to fabricate the high-aspect-ratio micro-structure (Fig. 4(c)). In the study of the tool electrode fabrication process, four different sacrificial electrodes are tested to compare their capability and performance. Fig. 5 shows the three different types of sacrificial electrodes. Fig. 5(a) shows a stationary block, which is the simplest method to machine a tool electrode. This process is known as ‘Block Electro Discharge Grinding’ (BEDG) process. Fig. 5(b) shows a rotating electrode and Fig. 5(c) shows a guided running wire as a sacrificial electrode of 0.07 mm in diameter which is known as ‘Wire Electro Discharge Grinding’ (WEDG). All of the above in situ electrode fabrication processes have their advantages and disadvantages. A fourth process which is a combination of processes described in Fig. 5(a and b) was also experimented to combine the advantages of both the processes. Stationary block as a sacrificial electrode requires the simplest setup and 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 a tapered shape due to the wear of the sacrificial electrode. In this method, while the tool electrode (Z axis) controlled the EDM gap with top surface of the sacrificial block, a relative translational motion between the block electrode and tool electrode was provided along the longitudinal axis of the block electrode. There was erosion on the moving block by electrical discharges
Fig. 4. Process to fabricate high-aspect-ratio micro-structures using micro-EDM.
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
207
Fig. 5. Three types of sacrificial electrode for on-machine tool fabrication.
Fig. 6. Moving BEDG process for on-machine tool fabrication.
during machining. However, the erosion was distributed almost uniformly over a larger area of the block electrode. Moreover, most of the sparks were between the gap layer of the top surface of the block electrode and the un-machined part of the tool electrode and there were almost no spark from the side of the tool. This created electrodes with very smooth surface, very good shape accuracy and is not tapered. However, there was difference in the length of the electrode produced and is always smaller than the targeted length due to the groove created on the surface of the block electrode. Taking into consideration of the machined area between the tool electrode and sacrificial electrode, the difference of the target length and the actual length is almost negligible. Fig. 6 illustrates the concept of the tool electrode fabrication by moving BEDG process.
fabricate a 15 m diameter tool electrode of 0.5 mm length, from a commercially available 0.5 mm diameter tungsten electrode using moving BEDG process, took about 3–4 h. This is well known that tungsten electrode has very little wear against stainless steel and with this electrode 100 holes could be machined on 50 micron thick SUS-304 plate. But, at NUS a new hybrid process for -Turning has been developed that can be used to machine very fine electrodes and almost removes all the disadvantages that sacrificial electrode fabrication processes have. Fig. 7 illustrates the concept of Turning-EDM hybrid machining. In this hybrid machining process, EDM is carried out using a micro-shaft. An electrode of required length is fabricated using -Turning process. Using this hybrid machining, clamping error can be avoided and deflection of electrode can be minimized, consequently the accuracy of machining can be improved. When different diameters of electrode are preferred, turning can significantly reduce the electrode preparation time as compared to the sacrificial electrode fabrication methods. This hybrid machining technology also can be used to fabricate a cylindrical microcomponent with non-rotational portion such as a key slot or flat bar by the help of EDM process followed by turning. One of the problems in machining thin electrode by turning is the deflection of the shaft during machining. Fig. 8 shows the deflection of end portion of the workpiece is measured using a deflection measurement sensor. From the experiment, it is observed that the workpiece is not bending only in normal direc-
4. EDM electrode fabrication using a hybrid of -EDM and -Turning process As has been mentioned earlier, all of the tool electrode fabrication processes explained in Section 3, using sacrificial electrode have their advantages and disadvantages. But, they have few disadvantages in common. These processes are very difficult for automation and are always error prone. Most of the cases operator is required to check the fabrication process and perform manual compensation for the error. Moreover, the electrode fabrication process takes more than few hours. As for example, to
Fig. 7. Concept of hybrid process for micro-EDM.
208
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
Fig. 8. Deflection of micro-shaft during machining.
tool nose resolves the cutting force on the shaft in to two components, namely Fx and Fy as could be seen in Fig. 11(a). Fy component of the cutting force does the actual cutting and Fx component causes deflection of the micro-shaft. Fig. 11(b) shows the actual cutting with the commercially available PCD insert, observed under a tool scope. A commercially available PCD insert can be modified to achieve a very sharp cutting edge which would reduce the Fx
Fig. 9. A 100 m diameter micro-shaft fabricated using conventional Turning.
tion (X) of the tool-workpiece contact region; it also deflects in the tangential (Y) direction. In fact, the work piece deflects towards top surface (rake surface) of the cutting tool. It is always very difficult to achieve straight shaft below 100 m diameter and in a lot of cases, the tool is either broken or starts to wobble due to excessive radial cutting force on this micro-shaft. Fig. 9 shows one such micro-shaft machined using the conventional -Turning method. Conventional PCD inserts, designed for finishing light cut, used for -Turning has 100 m tool nose radius (Fig. 10). This
Fig. 10. The tool geometry of commercially available PCD inserts for finishing light cut.
Fig. 11. (a) Resolution of cutting forces. (b) Actual cutting observed under a tool scope with commercially available PCD insert.
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
209
Fig. 14. Wear of brass electrode for boring holes on 50 and 100 m stainless steel plate. Fig. 12. Concept of sharp edge to reduce the Fx component.
component of the cutting force significantly (Fig. 12) and thus this makes it possible to achieve straight shaft with much smaller diameter. At NUS, research was conducted to make such a sharp edged tool from the commercially available PCD tool using -EDM process. Essentially, a sacrificial electrode was used to perform EDG on the tool edge to achieve a very sharp edged tool. In Fig. 13, a micro-shaft and the EDG edged tool used for machining the micro-shaft under a tool scope is observed. This has stretched the possibility of -Turning to an extreme. Two millimeter long micro-shafts with less than 20 m diameter has been machined very easily at NUS. The fabrication process for a 2.0 mm long shaft takes less than 10 min. Experiments at NUS show that though Brass has large wear, 2.0 mm long electrode can machine about 40 holes on a 50 m thick SUS plate. Fig. 14 shows a plot for the tool electrode wear for boring holes around 50 m diameter on stainless steel, using brass electrode. This process shows much improvement on the machining time compared to the in situ tool electrode fabrication processes described in Section 3, which takes few hours to fabricate as mentioned earlier. Moreover, this process does not require much operator intervention and can mostly be automated due to the less chances of error. Fig. 15 shows a 22 m shaft fabricated by
Fig. 13. The 2 mm long shaft fabricated using modified PCD tool and observed under tool scope.
Fig. 15. The 22 m electrode fabricated by -Turning process after machining 10 micro-holes on 100 m thick stainless steel plate, showing some wear.
-Turning process and after drilling about 10 holes on 100 m thick SUS-304 plate. 5. High aspect ratio micro-fabrication using -EDM process Research has been conducted to explore the possibility of -EDM for fabrication of complex micro-shapes and microfeatures for the Biomedical, MEMS and Electronics Industry. Tool electrode, as small as 4 m diameter, was fabricated by researchers at NUS lab using BEDG process of tool electrode fabrication. This electrode was used for boring controlled holes on 50 m thick stainless steel, making it probably the world’s smallest and highest aspect ratio hole. Four controlled holes of diameter 6.5, 8.5, 10.5 and 12.5 m were machined (Fig. 16). Fig. 17 shows letter ‘N’, the initial of ‘NUS’ machined by boring holes less than 10 m in diameter. Experiments have also been conducted at NUS for the fabrication of three-dimensional complex features using Scanning EDM/EDM Milling process. The recent development in the scanning algorithm made it possible to get very smooth and controlled groove while keeping the electrode shape unchanged. Fig. 18 shows a small pyramid of 50 m height, machined using -EDM Milling technique. Fig. 19 is the image of micro-slots of less than 30 m width, machined on 50 m thick stainless steel
210
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
Fig. 19. Micro-slots of less than 30 m width and 50 m depth, machined by -EDM milling process.
Fig. 16. The 6.5 m hole machined on 50 m stainless steel plate, probably the world’s smallest hole with highest aspect ratio.
Fig. 20. Micro-flower machined on 50 m SUS plate by -EDM process. Fig. 17. Letter ‘N’ of ‘NUS’ machined with holes of less than 10 m diameter.
Fig. 18. Small pyramid machined using -EDM (150 m (L) × 140 m (W) × 50 m (H)).
Fig. 21. ‘NUS’ machined by -EDM milling process.
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
plate. Fig. 20 shows a micro-flower and Fig. 21 shows ‘NUS’ written using -EDM Milling technique.
211
continuous development of the machine tools hardware and software and for the help and support for the fabrication of micro-structures and features.
6. Conclusions References In this paper, recent achievements in the area of tool-based micro-machining has been introduced. A multi-purpose miniature machine tool has been developed for high precision micro-machining in NUS. The machine is capable of performing -EDM, -ECM, -WEDG, WEDM, -Turning and -Milling. Continuous research and development are done on the machine to push the limit of the micro-machining technology. Special hybrid processes developed for micro-machining included the fabrication of highaspect-ratio micro-structure. This is possible through fabrication of a very fine electrode on-machine by sacrificial electrodes or -Turning, and then -EDM is performed to fabricate microstructures using the electrode. For -Turning process, the PCD tool is also modified by -EDM process. Some complex microfeatures and shapes of extremely small dimensions have been successfully fabricated using the techniques developed. Acknowledgements The authors would like to express their deepest gratitude to Mikrotools Pte. Ltd., a technology partner of NUS, for the
[1] H. Okuyama, H. Takada, Micromachining with SR and FEL, Nuclear Instrum. Methods Phys. Res. B 144 (1998) 58–65. [2] S. Matsui, T. Kaito, J.-I. Fujita, M. Ishida, Y. Ochiai, Three-dimensional nanostructure fabrication by focused-ion-beam chemical vapor deposition, J. Japan Soc. Precis. Eng. 67 (9) (2001) 1412–1415 (in Japanese). [3] Z. Lu, T. Yoneyama, Micro cutting in the micro lathe turning system, J. Mach. Tools Manuf. 39 (1999) 1171–1183. [4] C. Van Ossenbruggen, Micro Spark Erosion, 20, Philips Technisch Tijdschrift, 1969, pp. 200–213. [5] T. Sato, T. Mizutani, K. Kawata, Electro-discharge machine for micro-hole drilling, National Technical Reports, 31, 1985, pp. 725–733. [6] T. Masuzawa, C.L. Kuo, M. Fujino, Drilling of deep micro holes by EDM, Ann. CIRP 38 (1989) 195–198. [7] T. Masuzawa, C.L. Kuo, M. Fujino, A combined electrical machining process for micronozzle fabrication, Ann. CIRP 43 (1994) 189– 192. [8] M. Rahman, A.S. Kumar, J.R.S. Prakash, Micro milling of pure copper, J. Mater. Process. Technol. 116 (2001) 39–43. [9] H.S. Lim, A. Senthil Kumar, M. Rahman, Improvement of form accuracy in hybrid machining of microstructure, J. Electron. Mater. 31 (10) (2002) 1032–1038. [10] H.S. Lim, Y.S. Wong, M. Rahman, M.K. Edwin Lee, A study on the machining of high-aspect ratio micro-structures using micro-EDM, J. Mater. Process. Technol. 140 (2003) 318–325.
Tool-based micro-machining A.B.M.A. Asad, Takeshi Masaki, M. Rahman ∗ , H.S. Lim, Y.S. Wong 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. Micro-machining 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, micro-machining is expected to play increasingly important roles in today’s manufacturing technology. Micro-machining based on lithography has many disadvantages unlike tool-based micro-machining technology such as micro-turning, microgrinding, micro-EDM and micro-ECM have many advantages in productivity, efficiency, flexibility and cost effectiveness. However, difficulties, as the machining unit reduced, are still remained to be solved to utilize the tool-based machining technology for micro-machining. In this paper, recent achievements at NUS in the area of tool-based micro-machining are introduced. Micro-electro-discharge machining (microEDM) and micro-Turning technology are developed to produce miniaturized parts and features using a multi-purpose miniature machine tool for hybrid processes which is developed at NUS. © 2007 Elsevier B.V. All rights reserved. Keywords: Tool-based machining; Micro-EDM; Micro-EDG; Micro-EDM milling; Scanning EDM; Micro-Turning; Hybrid machining
1. Introduction Micro-machining is gaining popularity due to the recent advancements in Micro-Electro Mechanical Systems (MEMS). Many studies have been carried out to fabricate functional micro-structures and components. Micro-machining 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-3-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 3-dimensional sub-micron structures with very high form accuracy. But, these processes require very expensive and special facilities, and the maximum achievable thickness is relatively small [1,2]. Combination of conventional material removal processes, such as turning and milling, are hybridized with non-conventional machining processes like EDM/EDG to fabricate microstructures with high dimensional accuracy. These hybrid material removal processes can machine micro-features on almost every material such as metals, plastics and semiconductors. There is also no limitation in machining shape, so
∗
Corresponding author. Tel.: +65 6516 2168; fax: +65 6779 1459. E-mail address: [email protected] (M. Rahman).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.038
that flat surfaces, arbitrary curvatures and long shafts can be machined, which are required for the moving parts and guiding structures [3–9]. Micro-mould cavities are also needed for mass-production of micro-components, which can be made by injection molding process. Hard-to-machine workpiece materials should be machined very precisely in 3-dimensional forms in the micron range for the purpose of microinjection. For the fabrication of complex 3-dimensional molds using very tough die materials, micro-electro-discharge machining (-EDM) is one of the alternative machining processes that can be used successfully. -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 micro-machining using machine tools. In order to achieve meaningful implementation of micro/nano-machining techniques, the research efforts in NUS seek to address three important areas; namely (a) machine tool development, (b) on-machine micro/nano-metrology for process control, and (c) process development for the micro/nanomachining techniques to achieve the necessary accuracy and quality. An integrated effort in these areas has resulted in
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
machine tools, processes and on-machine measuring technology being developed for tool-based micro/nano-machining. 2. Development of miniature machine tool for multi-process micro-machining A multi-purpose miniature machine tool has been developed and going through continuous development cycle for high precision micro-machining [10]. Fig. 1 shows the structure of the miniature machine tool. The machine tool has its size of 1.5 m (W) × 1.1 m (D) × 1.9 m (H), including the controller unit and the dielectric supply unit. The maximum travel ranges are 210 mm (X) × 110 mm (Y) × 110 mm (Z). Each axis has optical linear scale with resolution of 0.1 m, and full closed feedback control and compensation ensures high accuracy. Machine enables changeable high speed, middle speed and low speed spindles for -Milling, -Turning and -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 using high speed communication, which creates a very user friendly environment for the operator on a standard PC. The controller has been tested to execute more than 300 motion nodes per second. At the same time, since the motion controller was developed jointly by NUS and a NUS venture company for the multi-process machine tool, programs and scheduling can be accessed and developed to meet the time critical requirements which is a key development area for micro-machining processes. As for example process parameter
Fig. 1. Multi-purpose miniature machine tool.
205
sampling can be done as fast as 20 kHz rate, while still managing other job schedules. Moreover, it has a multiprocessor-based architecture with UART-based bus network communication to distribute works between multiple processors. The response of each axis is important because a fast movement is required for the gap control during EDM. The servo system was designed to respond to the reference command within 50 ms. This was found to be excellent for gap control during EDM process. Unlike conventional machining processes, non-conventional machining processes and especially in the domain of micromachining processes, process know-how and development of process control system plays a vital role. The EDM process control hardware and software development requires continuous research. As a result of the continuous effort at NUS, recent development of EDM control hardware and software has improved the machining speed and quality by manifold. For -EDM process the amount of energy released controls the machining quality and the sparking frequency along with the control of the sparking gap decides the machining time. Fig. 2(a and b) show very fine finishing edge of the machined feature. Fig. 3 shows machining time for fabricating holes less than 20 m diameter on 50 m stainless steel plate. It was found from
Fig. 2. (a) Circularity of 50 m machined hole on stainless steel plate. (b) Edges of machined workpiece on stainless steel.
206
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
Fig. 3. Time taken for drilling holes of less than 20 m diameter.
experiment that using a 70 m tungsten wire, to cut a 12 mm stainless steel plate of 50 m thickness, takes less than 220 s. 3. Tool fabrication using -EDM process Micro-Electro-Discharge Machining (-EDM) is a nontraditional machining technology that has been found to be one of the most efficient technologies for fabricating microcomponents. The non-contact process requires little or almost no force between electrode and workpiece and is capable of machining ductile, brittle or super hardened-materials. With appropriate parameters, it is possible for -EDM process 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 machining. Even though a Milling cutter of down to 50 m in diameter is available in the market, the length of the tool is usually three to five times of its diameter and it is also not suitable to machine tough die material, which only can be machined using EDM. Although EDM plays an important role in the field of micro-machining, it has disadvantages such as 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 reduces the accuracy due to the change in setup or re-clamping of the micro-electrode.
Fig. 4 shows a conceptual process to fabricate a high-aspectratio micro-structures using -EDM. In -EDM process, 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 sacrificial electrode. Different setup of the sacrificial electrode can be used in this process (Fig. 4(a)). Due to dimensional change in the sacrificial electrode, diameter of the tool electrode fabricated is usually unpredictable. An on-machine measurement of the tool electrode diameter is required in this case (Fig. 4(b)). An optical measurement device has been specially developed for the measurement of a thin electrode, which consists of a laser diode, optical filter 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 finishing the tool electrode fabrication, -EDM is performed to fabricate the high-aspect-ratio micro-structure (Fig. 4(c)). In the study of the tool electrode fabrication process, four different sacrificial electrodes are tested to compare their capability and performance. Fig. 5 shows the three different types of sacrificial electrodes. Fig. 5(a) shows a stationary block, which is the simplest method to machine a tool electrode. This process is known as ‘Block Electro Discharge Grinding’ (BEDG) process. Fig. 5(b) shows a rotating electrode and Fig. 5(c) shows a guided running wire as a sacrificial electrode of 0.07 mm in diameter which is known as ‘Wire Electro Discharge Grinding’ (WEDG). All of the above in situ electrode fabrication processes have their advantages and disadvantages. A fourth process which is a combination of processes described in Fig. 5(a and b) was also experimented to combine the advantages of both the processes. Stationary block as a sacrificial electrode requires the simplest setup and 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 a tapered shape due to the wear of the sacrificial electrode. In this method, while the tool electrode (Z axis) controlled the EDM gap with top surface of the sacrificial block, a relative translational motion between the block electrode and tool electrode was provided along the longitudinal axis of the block electrode. There was erosion on the moving block by electrical discharges
Fig. 4. Process to fabricate high-aspect-ratio micro-structures using micro-EDM.
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
207
Fig. 5. Three types of sacrificial electrode for on-machine tool fabrication.
Fig. 6. Moving BEDG process for on-machine tool fabrication.
during machining. However, the erosion was distributed almost uniformly over a larger area of the block electrode. Moreover, most of the sparks were between the gap layer of the top surface of the block electrode and the un-machined part of the tool electrode and there were almost no spark from the side of the tool. This created electrodes with very smooth surface, very good shape accuracy and is not tapered. However, there was difference in the length of the electrode produced and is always smaller than the targeted length due to the groove created on the surface of the block electrode. Taking into consideration of the machined area between the tool electrode and sacrificial electrode, the difference of the target length and the actual length is almost negligible. Fig. 6 illustrates the concept of the tool electrode fabrication by moving BEDG process.
fabricate a 15 m diameter tool electrode of 0.5 mm length, from a commercially available 0.5 mm diameter tungsten electrode using moving BEDG process, took about 3–4 h. This is well known that tungsten electrode has very little wear against stainless steel and with this electrode 100 holes could be machined on 50 micron thick SUS-304 plate. But, at NUS a new hybrid process for -Turning has been developed that can be used to machine very fine electrodes and almost removes all the disadvantages that sacrificial electrode fabrication processes have. Fig. 7 illustrates the concept of Turning-EDM hybrid machining. In this hybrid machining process, EDM is carried out using a micro-shaft. An electrode of required length is fabricated using -Turning process. Using this hybrid machining, clamping error can be avoided and deflection of electrode can be minimized, consequently the accuracy of machining can be improved. When different diameters of electrode are preferred, turning can significantly reduce the electrode preparation time as compared to the sacrificial electrode fabrication methods. This hybrid machining technology also can be used to fabricate a cylindrical microcomponent with non-rotational portion such as a key slot or flat bar by the help of EDM process followed by turning. One of the problems in machining thin electrode by turning is the deflection of the shaft during machining. Fig. 8 shows the deflection of end portion of the workpiece is measured using a deflection measurement sensor. From the experiment, it is observed that the workpiece is not bending only in normal direc-
4. EDM electrode fabrication using a hybrid of -EDM and -Turning process As has been mentioned earlier, all of the tool electrode fabrication processes explained in Section 3, using sacrificial electrode have their advantages and disadvantages. But, they have few disadvantages in common. These processes are very difficult for automation and are always error prone. Most of the cases operator is required to check the fabrication process and perform manual compensation for the error. Moreover, the electrode fabrication process takes more than few hours. As for example, to
Fig. 7. Concept of hybrid process for micro-EDM.
208
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
Fig. 8. Deflection of micro-shaft during machining.
tool nose resolves the cutting force on the shaft in to two components, namely Fx and Fy as could be seen in Fig. 11(a). Fy component of the cutting force does the actual cutting and Fx component causes deflection of the micro-shaft. Fig. 11(b) shows the actual cutting with the commercially available PCD insert, observed under a tool scope. A commercially available PCD insert can be modified to achieve a very sharp cutting edge which would reduce the Fx
Fig. 9. A 100 m diameter micro-shaft fabricated using conventional Turning.
tion (X) of the tool-workpiece contact region; it also deflects in the tangential (Y) direction. In fact, the work piece deflects towards top surface (rake surface) of the cutting tool. It is always very difficult to achieve straight shaft below 100 m diameter and in a lot of cases, the tool is either broken or starts to wobble due to excessive radial cutting force on this micro-shaft. Fig. 9 shows one such micro-shaft machined using the conventional -Turning method. Conventional PCD inserts, designed for finishing light cut, used for -Turning has 100 m tool nose radius (Fig. 10). This
Fig. 10. The tool geometry of commercially available PCD inserts for finishing light cut.
Fig. 11. (a) Resolution of cutting forces. (b) Actual cutting observed under a tool scope with commercially available PCD insert.
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
209
Fig. 14. Wear of brass electrode for boring holes on 50 and 100 m stainless steel plate. Fig. 12. Concept of sharp edge to reduce the Fx component.
component of the cutting force significantly (Fig. 12) and thus this makes it possible to achieve straight shaft with much smaller diameter. At NUS, research was conducted to make such a sharp edged tool from the commercially available PCD tool using -EDM process. Essentially, a sacrificial electrode was used to perform EDG on the tool edge to achieve a very sharp edged tool. In Fig. 13, a micro-shaft and the EDG edged tool used for machining the micro-shaft under a tool scope is observed. This has stretched the possibility of -Turning to an extreme. Two millimeter long micro-shafts with less than 20 m diameter has been machined very easily at NUS. The fabrication process for a 2.0 mm long shaft takes less than 10 min. Experiments at NUS show that though Brass has large wear, 2.0 mm long electrode can machine about 40 holes on a 50 m thick SUS plate. Fig. 14 shows a plot for the tool electrode wear for boring holes around 50 m diameter on stainless steel, using brass electrode. This process shows much improvement on the machining time compared to the in situ tool electrode fabrication processes described in Section 3, which takes few hours to fabricate as mentioned earlier. Moreover, this process does not require much operator intervention and can mostly be automated due to the less chances of error. Fig. 15 shows a 22 m shaft fabricated by
Fig. 13. The 2 mm long shaft fabricated using modified PCD tool and observed under tool scope.
Fig. 15. The 22 m electrode fabricated by -Turning process after machining 10 micro-holes on 100 m thick stainless steel plate, showing some wear.
-Turning process and after drilling about 10 holes on 100 m thick SUS-304 plate. 5. High aspect ratio micro-fabrication using -EDM process Research has been conducted to explore the possibility of -EDM for fabrication of complex micro-shapes and microfeatures for the Biomedical, MEMS and Electronics Industry. Tool electrode, as small as 4 m diameter, was fabricated by researchers at NUS lab using BEDG process of tool electrode fabrication. This electrode was used for boring controlled holes on 50 m thick stainless steel, making it probably the world’s smallest and highest aspect ratio hole. Four controlled holes of diameter 6.5, 8.5, 10.5 and 12.5 m were machined (Fig. 16). Fig. 17 shows letter ‘N’, the initial of ‘NUS’ machined by boring holes less than 10 m in diameter. Experiments have also been conducted at NUS for the fabrication of three-dimensional complex features using Scanning EDM/EDM Milling process. The recent development in the scanning algorithm made it possible to get very smooth and controlled groove while keeping the electrode shape unchanged. Fig. 18 shows a small pyramid of 50 m height, machined using -EDM Milling technique. Fig. 19 is the image of micro-slots of less than 30 m width, machined on 50 m thick stainless steel
210
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
Fig. 19. Micro-slots of less than 30 m width and 50 m depth, machined by -EDM milling process.
Fig. 16. The 6.5 m hole machined on 50 m stainless steel plate, probably the world’s smallest hole with highest aspect ratio.
Fig. 20. Micro-flower machined on 50 m SUS plate by -EDM process. Fig. 17. Letter ‘N’ of ‘NUS’ machined with holes of less than 10 m diameter.
Fig. 18. Small pyramid machined using -EDM (150 m (L) × 140 m (W) × 50 m (H)).
Fig. 21. ‘NUS’ machined by -EDM milling process.
A.B.M.A. Asad et al. / Journal of Materials Processing Technology 192–193 (2007) 204–211
plate. Fig. 20 shows a micro-flower and Fig. 21 shows ‘NUS’ written using -EDM Milling technique.
211
continuous development of the machine tools hardware and software and for the help and support for the fabrication of micro-structures and features.
6. Conclusions References In this paper, recent achievements in the area of tool-based micro-machining has been introduced. A multi-purpose miniature machine tool has been developed for high precision micro-machining in NUS. The machine is capable of performing -EDM, -ECM, -WEDG, WEDM, -Turning and -Milling. Continuous research and development are done on the machine to push the limit of the micro-machining technology. Special hybrid processes developed for micro-machining included the fabrication of highaspect-ratio micro-structure. This is possible through fabrication of a very fine electrode on-machine by sacrificial electrodes or -Turning, and then -EDM is performed to fabricate microstructures using the electrode. For -Turning process, the PCD tool is also modified by -EDM process. Some complex microfeatures and shapes of extremely small dimensions have been successfully fabricated using the techniques developed. Acknowledgements The authors would like to express their deepest gratitude to Mikrotools Pte. Ltd., a technology partner of NUS, for the
[1] H. Okuyama, H. Takada, Micromachining with SR and FEL, Nuclear Instrum. Methods Phys. Res. B 144 (1998) 58–65. [2] S. Matsui, T. Kaito, J.-I. Fujita, M. Ishida, Y. Ochiai, Three-dimensional nanostructure fabrication by focused-ion-beam chemical vapor deposition, J. Japan Soc. Precis. Eng. 67 (9) (2001) 1412–1415 (in Japanese). [3] Z. Lu, T. Yoneyama, Micro cutting in the micro lathe turning system, J. Mach. Tools Manuf. 39 (1999) 1171–1183. [4] C. Van Ossenbruggen, Micro Spark Erosion, 20, Philips Technisch Tijdschrift, 1969, pp. 200–213. [5] T. Sato, T. Mizutani, K. Kawata, Electro-discharge machine for micro-hole drilling, National Technical Reports, 31, 1985, pp. 725–733. [6] T. Masuzawa, C.L. Kuo, M. Fujino, Drilling of deep micro holes by EDM, Ann. CIRP 38 (1989) 195–198. [7] T. Masuzawa, C.L. Kuo, M. Fujino, A combined electrical machining process for micronozzle fabrication, Ann. CIRP 43 (1994) 189– 192. [8] M. Rahman, A.S. Kumar, J.R.S. Prakash, Micro milling of pure copper, J. Mater. Process. Technol. 116 (2001) 39–43. [9] H.S. Lim, A. Senthil Kumar, M. Rahman, Improvement of form accuracy in hybrid machining of microstructure, J. Electron. Mater. 31 (10) (2002) 1032–1038. [10] H.S. Lim, Y.S. Wong, M. Rahman, M.K. Edwin Lee, A study on the machining of high-aspect ratio micro-structures using micro-EDM, J. Mater. Process. Technol. 140 (2003) 318–325.