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Ultrasonic Vibration Drilling of Microholes in Glass
Ultrasonic Vibration Drilling of Microholes in Glass K. Egashira', K. Mizutani' 'Department of Mechanical Engineering, School of Biology-Oriented Science and Technology, Kinki University, Uchita, Wakayama, Japan Submitted by T. Nagao ( I ) , Tosayamada, Kochi, Japan
Abstract Microholes with a diameter of 10 pm were drilled in glass by ultrasonic vibration cutting using a microtool fabricated by wire electrodischarge grinding. The workpiece was vibrated in order to realize high-precision tool rotation. Cutting was performed in the ductile regime at a depth of cut of 0.05 pm, leaving neither fractures nor cracks around the rim of the hole. The application of ultrasonic vibrations resulted in (1) a decrease in the required cutting force, (2) an extension of the tool life, (3) an increase in the permissible penetration and tool length and (4) smoother machined surfaces. Keywords: Ultrasonic vibration cutting, Microhole, Glass
1 INTRODUCTION It is well known that hard and brittle materials can be cut in the ductile regime at a small depth of cut. This phenomenon is called ductile-regime cutting and many studies of applications of the technique such as the realization of mirror surfaces by cutting have been reported [ I ] [2] [3]. However, the application of ductileregime cutting to the fabrication of concave microshapes with a high aspect ratio is impossible since single-point monocrystalline diamond tools, which are widely used in experiments on ductile-regime cutting, have large and low-aspect-ratio dimensions. Therefore, the present authors fabricated high-aspectratio concave microshapes by ductile-regime cutting using a microtool manufactured by wire electrodischarge grinding (WEDG) [4]. As a result, we succeeded in drilling microholes in silicon with a diameter of less than 10 pm [5]. Glass is another hard and brittle material of significant industrial value. Compared to silicon, the isotropic characteristics of glass impose a limitation on the shapes obtainable by chemical etching. In addition, the laser processing technique is not effective for transparent materials. Considering these facts, mechanical machining methods using a solid tool are more useful for the machining of glass than for silicon. Therefore, in this study, we selected glass as the workpiece material and investigated the drilling of microholes by cutting. The achievable machining rate of ductile-regime cutting is low due to the low penetration and the small depth of cut, which is usually less than 1 pm. A large depth of cut or high penetration increases the cutting force, leading to tool breakage or transition of the cutting mode from the ductile regime to the brittle regime. Assistance by ultrasonic vibration is a useful technique for the reduction of the cutting force. The cutting force is reduced by the application of ultrasonic vibration during the drilling of holes in duralumin, where the tool was vibrated [6]. However, there has been no attempt
to apply the technique to the ductile-regime cutting of microholes in brittle materials. Furthermore, large tool rotation run-out occurs due to the massive assembly of ultrasonic transducer and related parts required for vibration. This is undesirable for the drilling of microholes, where high-precision tool rotation is required. Installation of the vibration assembly on the workpiece is an effective technique for the elimination of this problem and which enables for the selection of a high-precision rotation tool spindle mechanism. In this paper, we investigate the ductile-regime cutting of microholes with a diameter of 10 pm in glass and the utilization of ultrasonic vibration of the workpiece. 2
EXPERIMENTAL METHOD
2.1 Microtool Cemented carbide was chosen as the tool material because its hardness, toughness and electrical conductivity properties enable it to cut glass, endure the cutting force and enable for tool fabrication by WEDG, which is an electrodischarge machining (EDM) method. WEDG is highly applicable to the fabrication of micropins because diameters as small as 3 pm can be realized. The tool diameter and length were defined by the holes to be drilled. Figure 1 shows the shape of a microtool and scanning electron microscope (SEM) micrographs of a 17 pm-diameter and 50 pm-long tool with a semi-circular cross section. The cutting edge roundness and wedge angle are approximately 0.5 pm and 70", respectively. Although the tool surface is covered with craters generated by electrodischarge and the cutting edge is not straight or as smooth as that of a tool fabricated by precise polishing, ductileregime cutting can be realized, as shown in the next chapter.
(d) R: Gap between wire electrode and tool material in WEDG process plus radius of wire electrode
Tool length
Wedge angle
Y
/
Cutting edge
Figure 1: (a) Overview (b) close-up view parallel to the cutting edge (c) close-up view normal to the cutting edge of a 17 pm-diameter microtool with a wedge angle of 70" and cutting edge radius of 0.5 pm and (d) tool shape. 2.2 Experimental arrangement for cutting In the previous study, the cutting and tool fabrication were performed on a WEDG machine [5]. It was impossible to measure the cutting force or to vibrate the workpiece, since the machine did not have the functionality for such operations. Therefore, a microultrasonic machining (MUSM) [7] setup shown in Figure 2 was employed in the present study. This setup was originally designed for MUSM by workpiece vibration; however, ultrasonic vibration cutting by workpiece vibration is possible using a microtool designed for cutting. The setup has three numerically controlled axes driven by stepping motors and leadscrew, with a step feed
of 0.05 pm. The workpiece was attached to the end face of a bolted Langevin type ultrasonic transducer with double-sided adhesive tape. It was vibrated vertically at a frequency of 40 kHz. The amplitude was 0.8 pm in all experiments. Since the tool diameter and depth of cut are very small in ductile-regime cutting using a microtool, the cutting force is also so small that accurate measurement using a dynamometer is a difficult process. Therefore, an electronic balance, which is commonly used for weighing samples in chemical experiments, was employed. It has a minimum increment of 10 mgf (approximately 0.1 mN) and a time resolution of 50 ms. The transducer holder, which holds the transducer at its nodal point of vibration, was placed on the balance. The tool spindle mechanism, which consists of a Vshaped bearing, mandrel and DC motor, was as employed in the WEDG machine for the fabrication of the microtool. Rotation run-out was less than 0.5 pm and rotation speed was 3000 rpm [8]. 2.3 Experimental procedure Figure 3 illustrates the experimental procedure. First, a 300 pm-diameter cemented carbide pin was placed in the mandrel and clamped and the mandrel placed onto the V-shaped bearing of the WEDG machine. The mandrel holding the microtool was placed onto the Vshaped bearing of the MUSM setup. High-precision rotation of the mandrel was maintained because the bearings were of the same type. The workpiece was vibrated and the rotating mandrel moved in a vertical motion. The initial contact between the tool and the workpiece was detected by the electronic balance. The critical depth of cut of brittle-ductile transition (d,) is an important parameter in ductile-regime cutting and the depth of cut has to be equal to or less than d,. Since the tool was fed at a step feed of 0.05 pm, the depth of cut was considered to be 0.05 pm. If the cutting mode is found to be the ductile regime, d, is determined to be 0.05 pm or larger. Borosilicate glass, which is widely used as optical glass, was selected as the workpiece. Cutting oil was
Figure 3: Experimental procedure
not used 3
EXPERIMENTAL RESULTS
3.1 Machining examples Figure 4 shows microholes with a diameter of 10 pm and a depth of 20 pm, which were drilled at a penetration of 0.05 p d s . These holes are the smallest holes drilled using the cutting tool. The holes in Figures 4 (a) and 4 (b) were drilled without and with vibration, respectively. Neither fractures nor cracks are observed around the rims of both holes, indicating that cutting was carried out in the ductile regime. This result suggests that drilling microholes by ductile-regime cutting is possible in glass as well as silicon. The influence of ultrasonic vibration on the appearance of the hole was not observed. Since the cutting mode was found to be in the ductile regime at a depth of cut of 0.05 pm, d, of borosilicate glass in drilling is determined to 0.05 pm or larger. 3.2 Cutting force Figure 5 shows the thrust cutting force during drilling at a penetration of 0.05 p d s and drilling depth of 20 pm using a 9.5 pm-diameter tool both with and without vibration. The cutting force was recorded once every second. In both experiments, the cutting force gradually increased until the drilling depth reached approximately 5 pm, after which it was maintained within the range 25-30 mN without vibration and at approximately 10 mN with vibration. This indicates that the use of vibrations reduced the cutting force by 6070%. Such a result enables for an increase in the penetration and tool length. 3.3 Tool life One of the problems associated with the use of microtools is the short tool life caused by tool breakage due to their small dimensions. Furthermore, cemented carbide is inferior to steel with regard to fracture toughness, thereby further reducing tool life. Therefore, the extension of tool life is of prime
importance if significant increases in productivity are to be realized. A decrease in the cutting force will result in the extension of the tool life. Figure 6 shows the relationship between tool life and penetration when holes are consecutively drilled using a microtool with a diameter of 9 pm and a length of 25 pm at a drilling depth of 20 pm. Here, tool life is defined as the total machining time to tool breakage. Wthout vibration, no hole was completely drilled at a penetration of 0.1 p d s . Tool life at a penetration of less than 0.1 pm/s was less than 60 min. Conversely, drilling was possible at a penetration of 0.2 p d s by the use of vibrations and tool life was increased to 160 min at a penetration of 0.025 p d s . This result shows that both penetration and tool life can be increased by applying vibrations to the workpiece. Figure 7 shows microholes drilled in the experiment of Figure 6 with vibration at a penetration of 0.125 p d s , at which 40
Without vibration
35 h
30 v
25 2 P 20 a .E 15 $ 10 a,
With vibration
Y
5 5 10 15 Drilling depth (prn)
0
20
Figure 5: Thrust cutting force (tool diameter = 9.5 pm, penetration = 0.05 pm/s)
E.-- 120 .!5 100a,
0 0
I-
80-
0 0
6040
-
0
20 0
0
I
0
-
v
0.1 0.2 Penetration (pm/s)
Figure 6: Relationship between tool life and penetration (tool diameter = 9 pm, tool length = 25 pm, drilling depth = 20 pm).
Figure 4: Microholes with a diameter of 10 pm and depth of 20 pm drilled (a) without vibration and (b) with vibration (penetration = 0.05 pm/s).
Figure 7: Microholes consecutively drilled with vibration using a single tool (tool diameter = 9 pm, penetration = 0.125 pm/s, drilling depth = 20 pm).
drilling was impossible without vibration. The influence of high penetration on the appearance of the hole was not observed and all holes were drilled without fractures or cracks around the rim. The relationship between tool life and tool length is illustrated in Figure 8. Tool life was measured at a penetration of 0.05 pm/s and drilling depth 5 pm smaller than tool length for tools with a diameter of 9 pm. The tool life of tools shorter than 35 pm was approximately doubled by the application of vibrations. Furthermore, the application of vibrations enabled for drilling using tools of lengths up to 40 pm. Note that tools longer than 30 pm broke soon after contact with the workpiece when vibrations were not used. 3.4 Machined surface To investigate the influence of ultrasonic vibration on the machined surface, holes were drilled using the same tool both with and without vibrations and the surfaces observed by SEM. The first hole was drilled with vibrations and the second one without, at a penetration of 0.05 pm/s using a tool with a diameter of
120 100 h
.L 80
E
v
0
With vibration
1
22 pm. Figure 9 shows the SEM micrographs of the bottom surfaces of the holes. Figure 9 (a) shows the surface without vibration. Concentric trenches that mirror the profile of the cutting edge of the tool are clearly observed. There are approximately three trenches per 1 pm in the radial direction. However, the surface with vibration, which is shown in Figure 9 (b), is much smoother than that without vibration. Since the hole in Figure 9 (b) was drilled before the one in Figure 9 (a), the effects of tool wear can be eliminated. One possible reason for the absence of the trenches may be that cutting was performed intermittently. 4 CONCLUSION Microholes with a diameter of 10 pm were drilled in glass by ultrasonic vibration cutting and the workpiece was vibrated to realize high-precision tool rotation. Cutting was performed in the ductile regime at a depth of cut of 0.05 pm. Neither fractures nor cracks were observed around the rim of the hole whether the workpiece was vibrated or not. The measurement of cutting force by an electronic balance showed that cutting force was reduced by 6070% at a vibration amplitude of 0.8 pm compared to that without vibration. Reducing the cutting force resulted in an increase in the tool life, penetration and tool length, thereby improving machining rate and machinable depth. The observation of machined surfaces by SEM showed that the surface with vibration was smooth, while concentric trenches that mirror the profile of the cutting edge clearly remained on the surface of the hole drilled without vibration. 5
2
O 0 20
l 25
,
;
/
.
I
30 35 40 45 Tool length (pm)
50
Figure 8: Relationship between tool life and tool length (tool diameter = 9 pm, penetration = 0.05 pm/s, drilling depth = 5 pm smaller than tool length).
IOpm Figure 9: Bottom surfaces of holes (a) without vibration and (b) with vibration (penetration = 0.05 pm/s).
REFERENCES Mizutani, K., Tanaka, Y., 1987, Lengths of Crack Extension and Upper Limits of Depth of Cut Preventing Crack Extension in Orthogonal Cutting of Ceramics -Their Estimation Using the Stress Field of the Expansion of a Cylindrical Tube-, Journal of the Japan Society of Precision Engineering, 53/11:1758-1764. Blake, P., Scattergood, R., 1990, Ductile Regime Machining of Germanium and Silicon, Journal of the American Ceramic Society, 73/4:949-957.
Nakasuji, T., Kodera, S., Hara, S., Matsunaga, H., Ikawa, N., Shimada, S., 1990, Diamond Turning of Brittle Materials for Optical Components, Annals of the CIRP, 39/1:89-92. Masuzawa, T., Fujino, M., Kobayashi, K., Suzuki, T., 1985, W r e Electrodischarge Grinding for Micromachining, Annals of the CIRP, 34/1:431434. Egashira, K., Mizutani, K., Microdrilling of Monocrystalline Silicon Using a Cutting Tool, Precision Engineering. Accepted for publication, in press. Onikura, H., Ohnishi, O., Feng, J., Kanda, T., Morita, T., 1996, Effects of Ultrasonic Vibration on Machining Accuracy in Microdrilling, Journal of the Japan Society of Precision Engineering, 62/5:676680. Egashira, K., Masuzawa, T., 1999, Microultrasonic Machining by the Application of Workpiece Vibration, Annals of the CIRP, 48/1:131-134. Sato, T., Mizutani, T., Kawata, K., 1985, ElectroDischarge Machine for Micro-Hole Boring, National Technical Report, 31/5:105-113.
Abstract Microholes with a diameter of 10 pm were drilled in glass by ultrasonic vibration cutting using a microtool fabricated by wire electrodischarge grinding. The workpiece was vibrated in order to realize high-precision tool rotation. Cutting was performed in the ductile regime at a depth of cut of 0.05 pm, leaving neither fractures nor cracks around the rim of the hole. The application of ultrasonic vibrations resulted in (1) a decrease in the required cutting force, (2) an extension of the tool life, (3) an increase in the permissible penetration and tool length and (4) smoother machined surfaces. Keywords: Ultrasonic vibration cutting, Microhole, Glass
1 INTRODUCTION It is well known that hard and brittle materials can be cut in the ductile regime at a small depth of cut. This phenomenon is called ductile-regime cutting and many studies of applications of the technique such as the realization of mirror surfaces by cutting have been reported [ I ] [2] [3]. However, the application of ductileregime cutting to the fabrication of concave microshapes with a high aspect ratio is impossible since single-point monocrystalline diamond tools, which are widely used in experiments on ductile-regime cutting, have large and low-aspect-ratio dimensions. Therefore, the present authors fabricated high-aspectratio concave microshapes by ductile-regime cutting using a microtool manufactured by wire electrodischarge grinding (WEDG) [4]. As a result, we succeeded in drilling microholes in silicon with a diameter of less than 10 pm [5]. Glass is another hard and brittle material of significant industrial value. Compared to silicon, the isotropic characteristics of glass impose a limitation on the shapes obtainable by chemical etching. In addition, the laser processing technique is not effective for transparent materials. Considering these facts, mechanical machining methods using a solid tool are more useful for the machining of glass than for silicon. Therefore, in this study, we selected glass as the workpiece material and investigated the drilling of microholes by cutting. The achievable machining rate of ductile-regime cutting is low due to the low penetration and the small depth of cut, which is usually less than 1 pm. A large depth of cut or high penetration increases the cutting force, leading to tool breakage or transition of the cutting mode from the ductile regime to the brittle regime. Assistance by ultrasonic vibration is a useful technique for the reduction of the cutting force. The cutting force is reduced by the application of ultrasonic vibration during the drilling of holes in duralumin, where the tool was vibrated [6]. However, there has been no attempt
to apply the technique to the ductile-regime cutting of microholes in brittle materials. Furthermore, large tool rotation run-out occurs due to the massive assembly of ultrasonic transducer and related parts required for vibration. This is undesirable for the drilling of microholes, where high-precision tool rotation is required. Installation of the vibration assembly on the workpiece is an effective technique for the elimination of this problem and which enables for the selection of a high-precision rotation tool spindle mechanism. In this paper, we investigate the ductile-regime cutting of microholes with a diameter of 10 pm in glass and the utilization of ultrasonic vibration of the workpiece. 2
EXPERIMENTAL METHOD
2.1 Microtool Cemented carbide was chosen as the tool material because its hardness, toughness and electrical conductivity properties enable it to cut glass, endure the cutting force and enable for tool fabrication by WEDG, which is an electrodischarge machining (EDM) method. WEDG is highly applicable to the fabrication of micropins because diameters as small as 3 pm can be realized. The tool diameter and length were defined by the holes to be drilled. Figure 1 shows the shape of a microtool and scanning electron microscope (SEM) micrographs of a 17 pm-diameter and 50 pm-long tool with a semi-circular cross section. The cutting edge roundness and wedge angle are approximately 0.5 pm and 70", respectively. Although the tool surface is covered with craters generated by electrodischarge and the cutting edge is not straight or as smooth as that of a tool fabricated by precise polishing, ductileregime cutting can be realized, as shown in the next chapter.
(d) R: Gap between wire electrode and tool material in WEDG process plus radius of wire electrode
Tool length
Wedge angle
Y
/
Cutting edge
Figure 1: (a) Overview (b) close-up view parallel to the cutting edge (c) close-up view normal to the cutting edge of a 17 pm-diameter microtool with a wedge angle of 70" and cutting edge radius of 0.5 pm and (d) tool shape. 2.2 Experimental arrangement for cutting In the previous study, the cutting and tool fabrication were performed on a WEDG machine [5]. It was impossible to measure the cutting force or to vibrate the workpiece, since the machine did not have the functionality for such operations. Therefore, a microultrasonic machining (MUSM) [7] setup shown in Figure 2 was employed in the present study. This setup was originally designed for MUSM by workpiece vibration; however, ultrasonic vibration cutting by workpiece vibration is possible using a microtool designed for cutting. The setup has three numerically controlled axes driven by stepping motors and leadscrew, with a step feed
of 0.05 pm. The workpiece was attached to the end face of a bolted Langevin type ultrasonic transducer with double-sided adhesive tape. It was vibrated vertically at a frequency of 40 kHz. The amplitude was 0.8 pm in all experiments. Since the tool diameter and depth of cut are very small in ductile-regime cutting using a microtool, the cutting force is also so small that accurate measurement using a dynamometer is a difficult process. Therefore, an electronic balance, which is commonly used for weighing samples in chemical experiments, was employed. It has a minimum increment of 10 mgf (approximately 0.1 mN) and a time resolution of 50 ms. The transducer holder, which holds the transducer at its nodal point of vibration, was placed on the balance. The tool spindle mechanism, which consists of a Vshaped bearing, mandrel and DC motor, was as employed in the WEDG machine for the fabrication of the microtool. Rotation run-out was less than 0.5 pm and rotation speed was 3000 rpm [8]. 2.3 Experimental procedure Figure 3 illustrates the experimental procedure. First, a 300 pm-diameter cemented carbide pin was placed in the mandrel and clamped and the mandrel placed onto the V-shaped bearing of the WEDG machine. The mandrel holding the microtool was placed onto the Vshaped bearing of the MUSM setup. High-precision rotation of the mandrel was maintained because the bearings were of the same type. The workpiece was vibrated and the rotating mandrel moved in a vertical motion. The initial contact between the tool and the workpiece was detected by the electronic balance. The critical depth of cut of brittle-ductile transition (d,) is an important parameter in ductile-regime cutting and the depth of cut has to be equal to or less than d,. Since the tool was fed at a step feed of 0.05 pm, the depth of cut was considered to be 0.05 pm. If the cutting mode is found to be the ductile regime, d, is determined to be 0.05 pm or larger. Borosilicate glass, which is widely used as optical glass, was selected as the workpiece. Cutting oil was
Figure 3: Experimental procedure
not used 3
EXPERIMENTAL RESULTS
3.1 Machining examples Figure 4 shows microholes with a diameter of 10 pm and a depth of 20 pm, which were drilled at a penetration of 0.05 p d s . These holes are the smallest holes drilled using the cutting tool. The holes in Figures 4 (a) and 4 (b) were drilled without and with vibration, respectively. Neither fractures nor cracks are observed around the rims of both holes, indicating that cutting was carried out in the ductile regime. This result suggests that drilling microholes by ductile-regime cutting is possible in glass as well as silicon. The influence of ultrasonic vibration on the appearance of the hole was not observed. Since the cutting mode was found to be in the ductile regime at a depth of cut of 0.05 pm, d, of borosilicate glass in drilling is determined to 0.05 pm or larger. 3.2 Cutting force Figure 5 shows the thrust cutting force during drilling at a penetration of 0.05 p d s and drilling depth of 20 pm using a 9.5 pm-diameter tool both with and without vibration. The cutting force was recorded once every second. In both experiments, the cutting force gradually increased until the drilling depth reached approximately 5 pm, after which it was maintained within the range 25-30 mN without vibration and at approximately 10 mN with vibration. This indicates that the use of vibrations reduced the cutting force by 6070%. Such a result enables for an increase in the penetration and tool length. 3.3 Tool life One of the problems associated with the use of microtools is the short tool life caused by tool breakage due to their small dimensions. Furthermore, cemented carbide is inferior to steel with regard to fracture toughness, thereby further reducing tool life. Therefore, the extension of tool life is of prime
importance if significant increases in productivity are to be realized. A decrease in the cutting force will result in the extension of the tool life. Figure 6 shows the relationship between tool life and penetration when holes are consecutively drilled using a microtool with a diameter of 9 pm and a length of 25 pm at a drilling depth of 20 pm. Here, tool life is defined as the total machining time to tool breakage. Wthout vibration, no hole was completely drilled at a penetration of 0.1 p d s . Tool life at a penetration of less than 0.1 pm/s was less than 60 min. Conversely, drilling was possible at a penetration of 0.2 p d s by the use of vibrations and tool life was increased to 160 min at a penetration of 0.025 p d s . This result shows that both penetration and tool life can be increased by applying vibrations to the workpiece. Figure 7 shows microholes drilled in the experiment of Figure 6 with vibration at a penetration of 0.125 p d s , at which 40
Without vibration
35 h
30 v
25 2 P 20 a .E 15 $ 10 a,
With vibration
Y
5 5 10 15 Drilling depth (prn)
0
20
Figure 5: Thrust cutting force (tool diameter = 9.5 pm, penetration = 0.05 pm/s)
E.-- 120 .!5 100a,
0 0
I-
80-
0 0
6040
-
0
20 0
0
I
0
-
v
0.1 0.2 Penetration (pm/s)
Figure 6: Relationship between tool life and penetration (tool diameter = 9 pm, tool length = 25 pm, drilling depth = 20 pm).
Figure 4: Microholes with a diameter of 10 pm and depth of 20 pm drilled (a) without vibration and (b) with vibration (penetration = 0.05 pm/s).
Figure 7: Microholes consecutively drilled with vibration using a single tool (tool diameter = 9 pm, penetration = 0.125 pm/s, drilling depth = 20 pm).
drilling was impossible without vibration. The influence of high penetration on the appearance of the hole was not observed and all holes were drilled without fractures or cracks around the rim. The relationship between tool life and tool length is illustrated in Figure 8. Tool life was measured at a penetration of 0.05 pm/s and drilling depth 5 pm smaller than tool length for tools with a diameter of 9 pm. The tool life of tools shorter than 35 pm was approximately doubled by the application of vibrations. Furthermore, the application of vibrations enabled for drilling using tools of lengths up to 40 pm. Note that tools longer than 30 pm broke soon after contact with the workpiece when vibrations were not used. 3.4 Machined surface To investigate the influence of ultrasonic vibration on the machined surface, holes were drilled using the same tool both with and without vibrations and the surfaces observed by SEM. The first hole was drilled with vibrations and the second one without, at a penetration of 0.05 pm/s using a tool with a diameter of
120 100 h
.L 80
E
v
0
With vibration
1
22 pm. Figure 9 shows the SEM micrographs of the bottom surfaces of the holes. Figure 9 (a) shows the surface without vibration. Concentric trenches that mirror the profile of the cutting edge of the tool are clearly observed. There are approximately three trenches per 1 pm in the radial direction. However, the surface with vibration, which is shown in Figure 9 (b), is much smoother than that without vibration. Since the hole in Figure 9 (b) was drilled before the one in Figure 9 (a), the effects of tool wear can be eliminated. One possible reason for the absence of the trenches may be that cutting was performed intermittently. 4 CONCLUSION Microholes with a diameter of 10 pm were drilled in glass by ultrasonic vibration cutting and the workpiece was vibrated to realize high-precision tool rotation. Cutting was performed in the ductile regime at a depth of cut of 0.05 pm. Neither fractures nor cracks were observed around the rim of the hole whether the workpiece was vibrated or not. The measurement of cutting force by an electronic balance showed that cutting force was reduced by 6070% at a vibration amplitude of 0.8 pm compared to that without vibration. Reducing the cutting force resulted in an increase in the tool life, penetration and tool length, thereby improving machining rate and machinable depth. The observation of machined surfaces by SEM showed that the surface with vibration was smooth, while concentric trenches that mirror the profile of the cutting edge clearly remained on the surface of the hole drilled without vibration. 5
2
O 0 20
l 25
,
;
/
.
I
30 35 40 45 Tool length (pm)
50
Figure 8: Relationship between tool life and tool length (tool diameter = 9 pm, penetration = 0.05 pm/s, drilling depth = 5 pm smaller than tool length).
IOpm Figure 9: Bottom surfaces of holes (a) without vibration and (b) with vibration (penetration = 0.05 pm/s).
REFERENCES Mizutani, K., Tanaka, Y., 1987, Lengths of Crack Extension and Upper Limits of Depth of Cut Preventing Crack Extension in Orthogonal Cutting of Ceramics -Their Estimation Using the Stress Field of the Expansion of a Cylindrical Tube-, Journal of the Japan Society of Precision Engineering, 53/11:1758-1764. Blake, P., Scattergood, R., 1990, Ductile Regime Machining of Germanium and Silicon, Journal of the American Ceramic Society, 73/4:949-957.
Nakasuji, T., Kodera, S., Hara, S., Matsunaga, H., Ikawa, N., Shimada, S., 1990, Diamond Turning of Brittle Materials for Optical Components, Annals of the CIRP, 39/1:89-92. Masuzawa, T., Fujino, M., Kobayashi, K., Suzuki, T., 1985, W r e Electrodischarge Grinding for Micromachining, Annals of the CIRP, 34/1:431434. Egashira, K., Mizutani, K., Microdrilling of Monocrystalline Silicon Using a Cutting Tool, Precision Engineering. Accepted for publication, in press. Onikura, H., Ohnishi, O., Feng, J., Kanda, T., Morita, T., 1996, Effects of Ultrasonic Vibration on Machining Accuracy in Microdrilling, Journal of the Japan Society of Precision Engineering, 62/5:676680. Egashira, K., Masuzawa, T., 1999, Microultrasonic Machining by the Application of Workpiece Vibration, Annals of the CIRP, 48/1:131-134. Sato, T., Mizutani, T., Kawata, K., 1985, ElectroDischarge Machine for Micro-Hole Boring, National Technical Report, 31/5:105-113.