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High Aspect Ratio and Complex Shaped Blind Micro Holes by Micro EDM
High Aspect Ratio and Complex Shaped Blind Micro Holes by Micro EDM Z.Y. Yu, K. P. Rajurkar (1) and H. Shen Department of Industrial and Management Systems Engineering University Of Nebraska-Lincoln, USA Abstract It is difficult to drill high aspect ratio through holes and complex shaped blind holes using micro EDM. The debris concentration in the narrow discharge gap causes abnormal discharges leading to excessive electrode wear and lower machining precision. In micro EDM, the electrode size is too small for internal flushing. This paper presents a new approach for effective self-flushing using planetary movement. Through micro holes with an aspect ratio of 18 have been drilled. This approach is also demonstrated by drilling blind noncircular micro holes with sharp corners and edges. The process performance characteristics are analyzed under different machining conditions. Keywords: Micro-machining, EDM, Performance
1 INTRODUCTION Micromachining technologies are used to produce micro parts to enhance product functionality in limited space and save material and energy. These techniques are being employed not only to produce Micro Electro-Mechanical Systems (MEMS) devices with the capability of sensing and actuating but also to machine micron-size features. The selection of an appropriate micromachining technique mainly depends on the size and shape of the feature, aspect ratio (in the case of a micro hole) and work material properties. Microelectronic processing technologies (such as photolithography, etching, physical vapor deposition (PVD), and chemical vapor deposition) are not suitable to machine micro features on metallic alloys. Mechanical manufacturing methods such as drilling can be used to drill holes of 70pm in diameter. Laser has the ability of drilling holes of 40pm [ I ] . With these methods, however, it is very difficult to drill a high aspect ratio or a blind noncircular micro hole with sharp corners and edges with super alloys such as stainless steel, tungsten carbide and D2 steel. Automotive, aircraft, aerospace and medical industries are increasingly in need of such micro features and holes. Micro Electrical Discharge Machining (EDM) is able to drill burr-free micro holes with high precision regardless of the hardness of workpiece material. In micro EDM, the material removal takes place due to electrical discharges generated between closely spaced electrodes in the presence of a dielectric medium. The shape of a machined feature is the mirror image of the electrode or the combination of the electrode shape and the tool path. Micro EDM has successfully been applied to machine not only micro holes of 5pm but also 3D complex micro cavities [2,3]. Drilling a through micro hole of noncircular cross-sectional shape has been realized by micro EDM [4]. Micro holes of an aspect ratio of 15 have been achieved using deionized water as dielectric liquid and vibrating workpiece [5]. However, the rapid movement of the workpiece may cause the vibration of electrode resulting in poor precision. The common problem in EDM hole drilling is the electrode wear. In micro EDM, the discharge gap usually is of several microns. It is very difficult to remove gaseous bubbles and debris from such a small gap. The debris concentration results in abnormal discharges (arcs and/or
short circuits) leading to unstable machining and excessive electrode wear [6,7]. The electrode is too small for internal flushing, and external flushing causes vibration of small and thin electrodes. In this paper, a new approach based on the principle of planetary movement of an electrode is presented. The planetary movement in micro EDM provides extra space needed for debris removal from the narrow gap in drilling high aspect ratio micro holes as well as blind noncircular micro holes. This approach is verified by drilling through micro holes with an aspect ratio of 18 and blind noncircular micro holes with sharp corners and edges. The process performance characteristics such as material removal rate and electrode wear are analyzed under different machining conditions. 2 PLANETARY MOVEMENT IN MICRO EDM The planetary movement of tool electrodes is widely used in conventional die-sinking EDM to reduce the debris concentration at the discharge gap and thus avoid unstable machining and arcs. This results in lesser wear of the bottom edges of the tool and therefore, minimizes undesirable tapering and waviness at the bottom surface of the blind hole [8-111. The tool path depends on the complexity of the feature to be machined. Such operation requires the ability of movement of all axes of the machine tool as per control instructions provided by the machine specific cornputer. In this study, a commercially available micro EDM is modified to achieve planetary movement. For drilling of circular micro holes with a high aspect ratio, a rotating electrode is fed into the workpiece located on the worktable moving along a circular path in X-Y plane. For drilling a blind noncircular micro hole, a corresponding electrode is prepared. The tool path is designed as per cross-sectional shape of the micro hole. For instance, a square blind micro hole is machined using a square electrode and a square tool path in X-Y plane. The related tool path, named as scanning, is shown in Figure 1. HIGH ASPECT RATIO MICRO HOLES 3.1 Experiments and results The deep hole machining is carried out horizontally to reduce the influence of gravity on the debris removal. 3
Figure 3: Micro hole through 2.5mm plate.
Figure 1: Scanning tool path Mineral oil and deionized water are used to compare the effect of different dielectric fluids. The equipment for deionized water shown on the left side of Figure 2 is replaced by an oil pump when mineral oil is used as dielectric liquid. The electrode is fed along the X-axis towards the workpiece placed on the worktable which is moving along a circular path in Y-2 plane. Table 1 lists the machining conditions used in drilling high aspect ratio micro holes. The eccentric radius in planetary movement is 10pm. The use of a large eccentric radius may reduce the machining efficiency and a small eccentric radius may not provide enough space for debris removal. Water reservoir I
Valve Y-2worktable
I
\
.... ....
Conductivity meter X-axi s....... Deionization Equipment
Figure 2: Experimental set-up for circular holes
I
Electrode material Electrode size Conductivity
I I
I
Figure 4: Hole entrance.
Tungsten 60pm to 80pm 0.3 to 0.4 pS/cm
Table 1: Machining conditions for deep hole drilling. 3.2 Influence on discharge gap, hole and electrode Figure 3 shows a micro hole drilled in a 2.5mm stainless steel plate using 1OOOpF capacitance and deionized water with planetary movement. The diameters at the entrance (on the right of Figure 3) and at the exit (on the left of Figure 3), as shown in Figure 4 and 5 respectively, are 145pm and 120pm. In all such experiments, difference between hole diameters at the entrance and at the exit varied from 20pm to 30pm. An experiment without planetary movement but with the same electrode diameter is also carried out to investigate the effect of planetary movement. The difference of radiuses of the hole entrances with and without planetary movement is about 7pm, i.e. smaller than the eccentric radius of planetary movement (10pm). This may be caused by the high conductivity of deionized water and secondary discharges when debris moves out. Without planetary movement, the drilled depth of the hole is limited to 1.33mm in a 2.5mm plate. It can also be seen in figure 3 that portion A of the hole (about 550pm in length) is tapered which means that the
Figure 5:Hole exit discharge gap changes during machining. When a hole is drilled to a certain depth, the debris ejected from the working area begins to attach and accumulate on the surface of the electrode. Discharges occur between the debris and the inner side of the hole. The gaseous bubbles generated by these secondary discharges push the debris further along the axis of electrode feed. Most of the debris is eventually driven out of the hole due to secondary discharges that result in a larger diameter in portion B. At the end of the operation, the debris is easily ejected through the hole exit; the occurrence of secondary discharges reduces, resulting in the taper of portion A. The tool electrode used in micro drilling is cleaned with acetone to remove debris deposits. Figures 6 and 7 show the conditions of the electrode before and after cleaning respectively. The maximum diameter of the electrode in Figures 6 is 107pm, 38pm larger than the diameter of electrode after cleaning. Debris deposits on the surface of the electrode are observed in both cases, i.e. with and without planetary movement. The diameter difference caused by secondary discharges of the electrode before and after machining is about 2pm.
Figure 6: Electrode before cleaning after hole drilling
Figure 7: Electrode after cleaning after hole drilling.
3.3 Effect on machining parameters
The effect of machining time on electrode feed, wear length of electrode and depth of hole for 1OOOpF capacitance with and without planetary movement is shown in Figure 8. The depth of hole is the difference between the electrode feed and the wear length of electrode. It can be seen (Figure 8) that the hole depth up to 2500pm (aspect ratio of 18) can be achieved with planetary movement as opposed to only 1300pm (aspect ratio of 10) without planetary movement. In drilling without planetary movement, beyond a certain depth, the debris is not effectively removed and fresh dielectric fluid is unable to enter the gap. Hence, abnormal discharges occur frequently, resulting in tool wear instead of deep drilling. The planetary movement of the electrode provides an uneven gap, which breaks the balance between the resistance caused by the viscosity of dielectric fluid and the pressure of bubbles, and allows bubbles to escape from the side of larger gap, followed by the removal of debris. This leads to less occurrence of abnormal discharges and improves the stability of machining process. This improvement of process stability is also reflected in a reduction of more than 50% of the electrode wear (up to time point A in figure 8). 3500 3000
,
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I
I
I
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I
3000
2 2500
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-0
. I -
$ 1500
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1000
500 0 0
100
Time (minutes) Figure 9: Electrode feed vs. machining time happen at sharp corners and edges of the electrode, resulting in excessive electrode wear. The electrode with rounded edges and corners generates holes of unacceptable precision. The planetary movement of the electrode can eliminate or reduce abnormal discharges and improve the machining efficiency and precision. After the electrode is dressed by Wire Electric Discharge Grinding [13], it is fed vertically (2axis) into the workpiece placed on the worktable moving in X-Y plane (Figure 10). The machining conditions are listed in Table 2. Tool holder
2500 2000
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I
.
,
I
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eceip; ; ;J
1500 1000
Y
500 0
-
300
200
0
20 40 Machining Time (minutes)
60
Electrode Feed (with planetary movement)
u controller
worktable
c
Figure 10: Experimental system for noncircular blind holes.
- - - a - - - Electrode Wear (with planetary movement) - c - Hole Depth (with planetary movement) +Electrode Feed (without planetary movement) - - - A -- - Electrode Wear (without planetary movement) - -A- - Hole Depth (without planetary movement) Figure 8: Electrode feed, wear length and hole depth vs machining time. 3.4 Influence of dielectric medium Figure 9 shows that the electrode feed rate in deionized water is much higher than that in mineral oil. In EDM, the high temperature of the plasma channel decomposes mineral oil to generate carbon. The dissociated conductive carbon increases the debris concentration. On the other hand, the conductivity of deionized water is low. Therefore, the debris concentration remains low [12]. 4 BLIND NONCIRCULAR MICRO HOLES When a circular hole is drilled, the rotation of the electrode improves the gap conditions and process stability. When a blind noncircular micro hole is machined, without rotation of the electrode, debris concentration occurs more easily. In addition, short circuits and arcs are more likely to
Total Electrode feed Electrode size
pentagonal. IOOpm, 300pm 60pm to 80pm
Table 2: Machining conditions for blind noncircular micro hole drilling. Figures 11 - 13 show the machining results with total electrode feed of 100pm and offset of tool paths by 15 to 20pm. Blind triangular, square and pentagonal micro holes with sharp corners and edges have been machined. Figure 14 illustrates the effect of scanning speed of the planetary movement on material removal rate (MMR). There is an optimum value (0.33pmkec) for the scanning speed. When the electrode feed rate is constant, high scanning speed causes abnormal discharges. The rate of machining the non-uniform bottom surface is lower than the electrode feed rate. A lower scanning speed however results in larger unevenness of the bottom of the hole,
especially at the bottom corners of the hole, resulting in frequent short circuits
5 SUMMARY In micro EDM, the discharge gap is very small, and the size of the electrode is too small to use internal and/or external flushing to remove debris. In this paper, a new approach using planetary movement of the electrode is proposed to reduce the debris concentration and improve precision. The planetary movement of electrode provides extra space for debris removal. Therefore, the material removal rate increases and the electrode wear reduces. This method has been verified by machining of micro holes with high aspect ratio and blind noncircular micro holes.
Figure 11: Triangular blind hole 6
AC KNOW LEDGM ENTS
Authors are thankful for the support from the Nebraska Research Initiative Fund (NRI) and National Science Foundation (NSF) (DMI-9908219). Dr. K. P. Rajurkar thankfully acknowledges the support from the National Science Foundation under the Intergovernmental Personnel Act program. REFERENCES
Masuzawa T., 2000, State of the Art of Micromachining, Annals of the CIRP, 49/2:473-488.
Figure 12: Square blind hole.
Yu 2.Y., Masuzawa T., Fujino M., 1998, Micro-EDM for Three-Dimensional Cavities, -Development of Uniform Wear Method-, Annals of the CIRP, 47/1: 169-172.
Figure 13: P ntago al blind hole. 4000
I1 I
: N
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\
1000 0 0
2
4
6
8
10
Scanning speed (pdsec) Figure 14: MMR vs.scanning speed Table 3 lists the machining time (MT) and electrode wear ratio ( E M ) of blind square holes for a total electrode feed of 300pm under different machining conditions. At the same electrode feed rate (0.6pm/sec), the machining time with planetary movement is less than one-third of the time without planetary movement. The electrode wear ratio also reduces sharply. These results confirm the advantage of planetary movement of the electrode in micro EDM. Feed Rate (0.6pm/sec) without scanning Feed Rate (3pm/sec) without scanning Feed Rate (0.6pmkec) With scanning
MT (minute) 37.2
EWR (%) 6.5
13.1
6.4
10.4
1.4
Rajurkar, K. P., Yu. Z., 2000, 3D Micro-EDM Using CAD/CAM, Annals of the CIRP, 49/1:127-130. Fujino M., Okamoto N., Masuzawa T., 1995, Development of Multi-Purpose Microprocessing Machine, Proceedings of ISEM XI, 613-620. Sheu D. Y., Masuzawa T., Fujino M., 1997, Machining of Deep Microholes by EDM, Proceedings of the Annual Assembly of JSEME, 105-108 (in Japanese). Koch O., Ehrfeld W. Michel F., Gruber H. P., 2001, Recent Progress in Micro-Electro Discharge Machining, --Part 1: Technology, Proceedings of ISEM, XIII, 737-745. Masuzawa T., Cui X., Taniguchi N., 1992, Improved Jet Flushing for EDM, Annals of the CIRP, 41/1:239242. Kobayashi K., 1995, The Present and Future Development of EDM and ECM, Proceedings of ISEM XI, 29-47. Kruth J. P., Bleys P. H., 2000, Machining Curvilinear Surfaces by Electro Discharge Machining, Proceedings of the 2"d International Conference on Machining and Measurements of Sculptured Surfaces, 271-294. [ l o ] Staelens F., Kruth J. P., 1989, An Overall Optimization Strategy for Planetary EDM, Proceedings of ISEM IX, 317-320. [ I l l Altan T., Lilly B. W., Kruth J. P., Konig W., Tonshoff H. K., Van Luttervelt C. A,, Khairy A. B., 1993, Advanced Techniques for Die and Mold Manufacturing, Annals of the CIRP, 42/2:707-716. [I21 Masuzawa T., Tsukamoto J., Fujino M., 1989, Drilling of Deep Microholes by EDM, Annals of the CIRP, 38/1: 195-198. [I31 MasuzawaT., Fijino, M., Kobayashi, K., 1985, Wire Electrodischarge Grinding for Micromachining, Annals of the CIRP, 34/1:431-434.
1 INTRODUCTION Micromachining technologies are used to produce micro parts to enhance product functionality in limited space and save material and energy. These techniques are being employed not only to produce Micro Electro-Mechanical Systems (MEMS) devices with the capability of sensing and actuating but also to machine micron-size features. The selection of an appropriate micromachining technique mainly depends on the size and shape of the feature, aspect ratio (in the case of a micro hole) and work material properties. Microelectronic processing technologies (such as photolithography, etching, physical vapor deposition (PVD), and chemical vapor deposition) are not suitable to machine micro features on metallic alloys. Mechanical manufacturing methods such as drilling can be used to drill holes of 70pm in diameter. Laser has the ability of drilling holes of 40pm [ I ] . With these methods, however, it is very difficult to drill a high aspect ratio or a blind noncircular micro hole with sharp corners and edges with super alloys such as stainless steel, tungsten carbide and D2 steel. Automotive, aircraft, aerospace and medical industries are increasingly in need of such micro features and holes. Micro Electrical Discharge Machining (EDM) is able to drill burr-free micro holes with high precision regardless of the hardness of workpiece material. In micro EDM, the material removal takes place due to electrical discharges generated between closely spaced electrodes in the presence of a dielectric medium. The shape of a machined feature is the mirror image of the electrode or the combination of the electrode shape and the tool path. Micro EDM has successfully been applied to machine not only micro holes of 5pm but also 3D complex micro cavities [2,3]. Drilling a through micro hole of noncircular cross-sectional shape has been realized by micro EDM [4]. Micro holes of an aspect ratio of 15 have been achieved using deionized water as dielectric liquid and vibrating workpiece [5]. However, the rapid movement of the workpiece may cause the vibration of electrode resulting in poor precision. The common problem in EDM hole drilling is the electrode wear. In micro EDM, the discharge gap usually is of several microns. It is very difficult to remove gaseous bubbles and debris from such a small gap. The debris concentration results in abnormal discharges (arcs and/or
short circuits) leading to unstable machining and excessive electrode wear [6,7]. The electrode is too small for internal flushing, and external flushing causes vibration of small and thin electrodes. In this paper, a new approach based on the principle of planetary movement of an electrode is presented. The planetary movement in micro EDM provides extra space needed for debris removal from the narrow gap in drilling high aspect ratio micro holes as well as blind noncircular micro holes. This approach is verified by drilling through micro holes with an aspect ratio of 18 and blind noncircular micro holes with sharp corners and edges. The process performance characteristics such as material removal rate and electrode wear are analyzed under different machining conditions. 2 PLANETARY MOVEMENT IN MICRO EDM The planetary movement of tool electrodes is widely used in conventional die-sinking EDM to reduce the debris concentration at the discharge gap and thus avoid unstable machining and arcs. This results in lesser wear of the bottom edges of the tool and therefore, minimizes undesirable tapering and waviness at the bottom surface of the blind hole [8-111. The tool path depends on the complexity of the feature to be machined. Such operation requires the ability of movement of all axes of the machine tool as per control instructions provided by the machine specific cornputer. In this study, a commercially available micro EDM is modified to achieve planetary movement. For drilling of circular micro holes with a high aspect ratio, a rotating electrode is fed into the workpiece located on the worktable moving along a circular path in X-Y plane. For drilling a blind noncircular micro hole, a corresponding electrode is prepared. The tool path is designed as per cross-sectional shape of the micro hole. For instance, a square blind micro hole is machined using a square electrode and a square tool path in X-Y plane. The related tool path, named as scanning, is shown in Figure 1. HIGH ASPECT RATIO MICRO HOLES 3.1 Experiments and results The deep hole machining is carried out horizontally to reduce the influence of gravity on the debris removal. 3
Figure 3: Micro hole through 2.5mm plate.
Figure 1: Scanning tool path Mineral oil and deionized water are used to compare the effect of different dielectric fluids. The equipment for deionized water shown on the left side of Figure 2 is replaced by an oil pump when mineral oil is used as dielectric liquid. The electrode is fed along the X-axis towards the workpiece placed on the worktable which is moving along a circular path in Y-2 plane. Table 1 lists the machining conditions used in drilling high aspect ratio micro holes. The eccentric radius in planetary movement is 10pm. The use of a large eccentric radius may reduce the machining efficiency and a small eccentric radius may not provide enough space for debris removal. Water reservoir I
Valve Y-2worktable
I
\
.... ....
Conductivity meter X-axi s....... Deionization Equipment
Figure 2: Experimental set-up for circular holes
I
Electrode material Electrode size Conductivity
I I
I
Figure 4: Hole entrance.
Tungsten 60pm to 80pm 0.3 to 0.4 pS/cm
Table 1: Machining conditions for deep hole drilling. 3.2 Influence on discharge gap, hole and electrode Figure 3 shows a micro hole drilled in a 2.5mm stainless steel plate using 1OOOpF capacitance and deionized water with planetary movement. The diameters at the entrance (on the right of Figure 3) and at the exit (on the left of Figure 3), as shown in Figure 4 and 5 respectively, are 145pm and 120pm. In all such experiments, difference between hole diameters at the entrance and at the exit varied from 20pm to 30pm. An experiment without planetary movement but with the same electrode diameter is also carried out to investigate the effect of planetary movement. The difference of radiuses of the hole entrances with and without planetary movement is about 7pm, i.e. smaller than the eccentric radius of planetary movement (10pm). This may be caused by the high conductivity of deionized water and secondary discharges when debris moves out. Without planetary movement, the drilled depth of the hole is limited to 1.33mm in a 2.5mm plate. It can also be seen in figure 3 that portion A of the hole (about 550pm in length) is tapered which means that the
Figure 5:Hole exit discharge gap changes during machining. When a hole is drilled to a certain depth, the debris ejected from the working area begins to attach and accumulate on the surface of the electrode. Discharges occur between the debris and the inner side of the hole. The gaseous bubbles generated by these secondary discharges push the debris further along the axis of electrode feed. Most of the debris is eventually driven out of the hole due to secondary discharges that result in a larger diameter in portion B. At the end of the operation, the debris is easily ejected through the hole exit; the occurrence of secondary discharges reduces, resulting in the taper of portion A. The tool electrode used in micro drilling is cleaned with acetone to remove debris deposits. Figures 6 and 7 show the conditions of the electrode before and after cleaning respectively. The maximum diameter of the electrode in Figures 6 is 107pm, 38pm larger than the diameter of electrode after cleaning. Debris deposits on the surface of the electrode are observed in both cases, i.e. with and without planetary movement. The diameter difference caused by secondary discharges of the electrode before and after machining is about 2pm.
Figure 6: Electrode before cleaning after hole drilling
Figure 7: Electrode after cleaning after hole drilling.
3.3 Effect on machining parameters
The effect of machining time on electrode feed, wear length of electrode and depth of hole for 1OOOpF capacitance with and without planetary movement is shown in Figure 8. The depth of hole is the difference between the electrode feed and the wear length of electrode. It can be seen (Figure 8) that the hole depth up to 2500pm (aspect ratio of 18) can be achieved with planetary movement as opposed to only 1300pm (aspect ratio of 10) without planetary movement. In drilling without planetary movement, beyond a certain depth, the debris is not effectively removed and fresh dielectric fluid is unable to enter the gap. Hence, abnormal discharges occur frequently, resulting in tool wear instead of deep drilling. The planetary movement of the electrode provides an uneven gap, which breaks the balance between the resistance caused by the viscosity of dielectric fluid and the pressure of bubbles, and allows bubbles to escape from the side of larger gap, followed by the removal of debris. This leads to less occurrence of abnormal discharges and improves the stability of machining process. This improvement of process stability is also reflected in a reduction of more than 50% of the electrode wear (up to time point A in figure 8). 3500 3000
,
I
I
I
I
C
I
3000
2 2500
:2000
-0
. I -
$ 1500
g
0
al
w
1000
500 0 0
100
Time (minutes) Figure 9: Electrode feed vs. machining time happen at sharp corners and edges of the electrode, resulting in excessive electrode wear. The electrode with rounded edges and corners generates holes of unacceptable precision. The planetary movement of the electrode can eliminate or reduce abnormal discharges and improve the machining efficiency and precision. After the electrode is dressed by Wire Electric Discharge Grinding [13], it is fed vertically (2axis) into the workpiece placed on the worktable moving in X-Y plane (Figure 10). The machining conditions are listed in Table 2. Tool holder
2500 2000
,
I
.
,
I
I*l
eceip; ; ;J
1500 1000
Y
500 0
-
300
200
0
20 40 Machining Time (minutes)
60
Electrode Feed (with planetary movement)
u controller
worktable
c
Figure 10: Experimental system for noncircular blind holes.
- - - a - - - Electrode Wear (with planetary movement) - c - Hole Depth (with planetary movement) +Electrode Feed (without planetary movement) - - - A -- - Electrode Wear (without planetary movement) - -A- - Hole Depth (without planetary movement) Figure 8: Electrode feed, wear length and hole depth vs machining time. 3.4 Influence of dielectric medium Figure 9 shows that the electrode feed rate in deionized water is much higher than that in mineral oil. In EDM, the high temperature of the plasma channel decomposes mineral oil to generate carbon. The dissociated conductive carbon increases the debris concentration. On the other hand, the conductivity of deionized water is low. Therefore, the debris concentration remains low [12]. 4 BLIND NONCIRCULAR MICRO HOLES When a circular hole is drilled, the rotation of the electrode improves the gap conditions and process stability. When a blind noncircular micro hole is machined, without rotation of the electrode, debris concentration occurs more easily. In addition, short circuits and arcs are more likely to
Total Electrode feed Electrode size
pentagonal. IOOpm, 300pm 60pm to 80pm
Table 2: Machining conditions for blind noncircular micro hole drilling. Figures 11 - 13 show the machining results with total electrode feed of 100pm and offset of tool paths by 15 to 20pm. Blind triangular, square and pentagonal micro holes with sharp corners and edges have been machined. Figure 14 illustrates the effect of scanning speed of the planetary movement on material removal rate (MMR). There is an optimum value (0.33pmkec) for the scanning speed. When the electrode feed rate is constant, high scanning speed causes abnormal discharges. The rate of machining the non-uniform bottom surface is lower than the electrode feed rate. A lower scanning speed however results in larger unevenness of the bottom of the hole,
especially at the bottom corners of the hole, resulting in frequent short circuits
5 SUMMARY In micro EDM, the discharge gap is very small, and the size of the electrode is too small to use internal and/or external flushing to remove debris. In this paper, a new approach using planetary movement of the electrode is proposed to reduce the debris concentration and improve precision. The planetary movement of electrode provides extra space for debris removal. Therefore, the material removal rate increases and the electrode wear reduces. This method has been verified by machining of micro holes with high aspect ratio and blind noncircular micro holes.
Figure 11: Triangular blind hole 6
AC KNOW LEDGM ENTS
Authors are thankful for the support from the Nebraska Research Initiative Fund (NRI) and National Science Foundation (NSF) (DMI-9908219). Dr. K. P. Rajurkar thankfully acknowledges the support from the National Science Foundation under the Intergovernmental Personnel Act program. REFERENCES
Masuzawa T., 2000, State of the Art of Micromachining, Annals of the CIRP, 49/2:473-488.
Figure 12: Square blind hole.
Yu 2.Y., Masuzawa T., Fujino M., 1998, Micro-EDM for Three-Dimensional Cavities, -Development of Uniform Wear Method-, Annals of the CIRP, 47/1: 169-172.
Figure 13: P ntago al blind hole. 4000
I1 I
: N
U
I
\
1000 0 0
2
4
6
8
10
Scanning speed (pdsec) Figure 14: MMR vs.scanning speed Table 3 lists the machining time (MT) and electrode wear ratio ( E M ) of blind square holes for a total electrode feed of 300pm under different machining conditions. At the same electrode feed rate (0.6pm/sec), the machining time with planetary movement is less than one-third of the time without planetary movement. The electrode wear ratio also reduces sharply. These results confirm the advantage of planetary movement of the electrode in micro EDM. Feed Rate (0.6pm/sec) without scanning Feed Rate (3pm/sec) without scanning Feed Rate (0.6pmkec) With scanning
MT (minute) 37.2
EWR (%) 6.5
13.1
6.4
10.4
1.4
Rajurkar, K. P., Yu. Z., 2000, 3D Micro-EDM Using CAD/CAM, Annals of the CIRP, 49/1:127-130. Fujino M., Okamoto N., Masuzawa T., 1995, Development of Multi-Purpose Microprocessing Machine, Proceedings of ISEM XI, 613-620. Sheu D. Y., Masuzawa T., Fujino M., 1997, Machining of Deep Microholes by EDM, Proceedings of the Annual Assembly of JSEME, 105-108 (in Japanese). Koch O., Ehrfeld W. Michel F., Gruber H. P., 2001, Recent Progress in Micro-Electro Discharge Machining, --Part 1: Technology, Proceedings of ISEM, XIII, 737-745. Masuzawa T., Cui X., Taniguchi N., 1992, Improved Jet Flushing for EDM, Annals of the CIRP, 41/1:239242. Kobayashi K., 1995, The Present and Future Development of EDM and ECM, Proceedings of ISEM XI, 29-47. Kruth J. P., Bleys P. H., 2000, Machining Curvilinear Surfaces by Electro Discharge Machining, Proceedings of the 2"d International Conference on Machining and Measurements of Sculptured Surfaces, 271-294. [ l o ] Staelens F., Kruth J. P., 1989, An Overall Optimization Strategy for Planetary EDM, Proceedings of ISEM IX, 317-320. [ I l l Altan T., Lilly B. W., Kruth J. P., Konig W., Tonshoff H. K., Van Luttervelt C. A,, Khairy A. B., 1993, Advanced Techniques for Die and Mold Manufacturing, Annals of the CIRP, 42/2:707-716. [I21 Masuzawa T., Tsukamoto J., Fujino M., 1989, Drilling of Deep Microholes by EDM, Annals of the CIRP, 38/1: 195-198. [I31 MasuzawaT., Fijino, M., Kobayashi, K., 1985, Wire Electrodischarge Grinding for Micromachining, Annals of the CIRP, 34/1:431-434.