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Improvement of Mechanical Strength of Micro Tools by Controlling Surface Characteristics
Improvement of Mechanlcal Strength of Mlcro Tools by Controlllng Surface Characterlstlcs H. Ohmorl(2), K. Katahlra, Y. Uehara, Y. Watanabe, W. Lln RlKEN (The Institute of Physlcal and Chemlcal Research), Sattama, Japan
Abstract Mlcro tools requlre exceptlmally hlgh surface characterlstlcs. controllable to the nanometer level. In thls research. a desk-top machlne capable of achlevlng excellent surface quality and machlnlny accuracy In mlcro machlnlng was developed, and mlcro tools of varlous shapes were prepared uslng the n e w developed machlne. The machlne successfully prepared pyramldal mlcro tools wlth tlps of 2pm. The surface of the prepared mlcro tools was observed closely uslng some advanced Instruments, and the fracture strength of the mlcro tools was evaluated by lndentatlon tests uslng a nanmlndentatlon testlng Instrument. These observatlons showed a clear and qwntttatke correlatlon betwean the nanometer-level surface quality and mechanlcal strength. In another test, thln metal sheets were punched uslng the hbrlcated mlcro tool. The resuttlng holes were found to be of excepUonally hlgh quality. Keywords: Mlcro tool, Surface quality, Mechanlcal strength
I INTRODUCTION Mlnlaturbatlon and w1gM redudlon of portable audlovlsual equlpment, p a l m t q computers, and cellular phones hiwe been urgently pursued. Thls has Increased demand for the mlnldurbatlon of hlghly fundlonal electronlc devlms, optlcal devlces, and mechanlcal components that compose these apparatuses, as well as for hlgh dlmenslonal accuracy of these parts. Hlgh dlmenslonal accuracy Is Increaslngly dllAcutt to achleve, as lt Is Influenced by surface roughness and wwlness. Improvement h surface quality Is deslred as part of the Increaslng demands for mechanlcal components, along wlth electrical and optkal devlces. In recent years, In order to meet these demands, varlous machlnlng methods hiwe been proposed, such as mkro grlndlng, cuttlng. mlcro electrlc dlscharge machlnlng, and beam machlnlng [1)[8]. As the resutts, the manhfacture of mlcrostructures and mlcro tools on the order of several mlcrons has been realhed. When tools are mlnlidurbed, the surface mlcrostrudure and materlal texture begln to f l e d the mechanlcal propertles of the components [9], thus undeslrably Increaslng the structural sensttMty. In partlcuhr, mlcro tools used for mlcrmpunchlng and mlcrmcuttlng requlre mechanlcal strength sulAclent to wlthstand the load durlng machlrrlng. If the surface lntegrltles of the mlcro tool are poor, a rough surface may act as a fracture orlgln, degradlng the strength of the tool. Furthermore, machlnlng objects uslng mlcro tools are used In order to achleve a hlghquality surface Rnlsh. H o w v e r , poor surface lntegrltles of the mlcro tool MI1 hcrease the Mctlonal reslstance durlng machhlng, naturally causlng degradatlon of the surface characterlstlcs of the object. Therefore. mlcro tools requlre exceptlonally hlgh surface charaderlstlcs, controllable to the nanometer level. Our research w s almed at produclng mlcro tools uslng preclslon grlndlng process, and In the process we developed a specltlc machlne for that purpose, as wll as controlllng and optlmhlng the machlnlng process condltlons In order to Improve the surface characterlstlcs.
Furthermore, In order to asmrtaln the erects of the surface characterlstlcs on the mechanlcal strength of the tool ttseK, we used a nanmlndentatlon tester and conducted rupture tests on the mlcro tools. We also used mlcro tools to conduct punchlng tests on thln metal sheets, to determlne the actual erects of the surface characterlstlcs on the product performance. 2
DEYELOPMENT OF THE MACHINE FOR MICRO TOOL MACHINING Flgure 1 shows the ederlor of the machlne developed specmcally for mlcro tool machlnlng. The machlne 1s deslgned to be extremely compact, and cm be used on a desk-top [lo]. Thls machlne has three Ilnear axes, X, Y, and 2 . In the processlng method, the work stage moves In the X and Y dlrectlms wlth respect to the outer circumference of the grlndlng wheel, whlch Is med. Thls movement accompllshes both feedlng and notchlng of the workplece, Rnlshlng the workplem to the requlred shape. Thls processlng method enables productlon of not only cyllndrlcal shapes, but also, K deslred, columnar shapes of square or elllptlcal cross-sections, or pyramld shapes h d n g comers. In addttlon, because the machlne has an Electrowlc In Process Dresslng system [11][12], the grlndng wheel surface characterlstlcs are constantly malntalned at the optlmum level, and the machlnlng reproduclbiity Is outstandlng. Flgure 2 shows a typlcd mlcro tool poduced uslng thls machlne. The mlcro tool 1s deslgned for use In punchlng, and Is m d e of a cemented carblde alloy. Based on the Rgure, Rner mlcro tool machhlng Is successfuly achleved, however K the wrface chracterlstlcs are examlned through an AFM, the state of the tool shank and that of the tlp are notlceably dlllerent, as shown In Flgure 3. With thls In mlnd, WB narrowed the mxhlnlng parameters govemlng the surface characterlstlcs of the mlcro tool to three parameters. for our testlng purposes. These Include depth of cut, mesh s k e ol grlndlng M e e l . and the number of processes from the rough machhlng to the Rnlshlng machlnlng. Then, uslng the abovedascrlbad
machine, which offers superb control of those parameters, we attempted to define machining conditions that would allow higher quality surface characteristics.
Controlling these machining conditions made it possible to significantly reduce the machining load and enabled machining of distinctive tools like those shown in Figures 7 (a) and (b), which feature smooth, uniform surface characteristics over the entire tool. Figure 7 (a) shows an ultra precise tool with a tip diameter of approximately 2vm, while Figure 7 (b) shows a tool with an extremely large aspect ratio. The figures also indicate that the material was stably removed in the ductile mode. In addition, the state of the tool shank and that of the tip were almost the same. Table 1 summarizes the machining conditions under which the shape shown in Figure 7 (a) was produced. Obtaining the optimum shape and surface characteristics will depend on the material selected for the tool. In the future, we will be working to produce micro tools with even greater strength and higher quality by using materials such as fine-grain sintered super-hard alloy and binderless alloy.
Figure 1: External view of developed machine for micro tool machining.
Figure 2: Overview of a typical micro tool (micro cylindrical shaft) produced using developed machine.
(b) Depth of cut of 0.5vm (a) Shank part
(b) Tip part
Figure 4: Effect of depth of cut on obtained surface characteristics.
Figure 3: AFM micrographs of ground surface (#I ,200). INVESTIGATION OF THE OPTIMUM CONDITIONS FOR MACHINING MICRO TOOLS Figure 4 shows the differences in surface characteristics obtained when a grinding wheel having a mesh size of # I ,200 was used for machining and the depth of cut was changed between 1vm and 0.5vm. The whole shapes of the machined tools were exactly the same, and care was taken to measure each at largely the same location. As indicated by this figure, in comparison with the surface having a depth of cut of Ipm, the surface with a depth of cut of 0.5pm was extremely smooth. At the same time, Figure 5 shows the results of SEM observation of the surface when machining was performed with the grinding wheel mesh size being changed to #1,200 and then to #4,000. As seen in this figure, increasing the mesh size of the grinding wheel being used from #1,200 to #4,000 produced a corresponding increase in the surface quality. Figure 6 shows the differences in the surface characteristics when processing was carried out twice from rough to finishing machining (#325, #4,000) and when processing was carried out four times (#325, # I ,200, #2,000, #4,000). In this case as well, increasing the number of processes produced superior surface characteristics. 3
(b) #4,000 (a) # I ,200 Figure 5:Effect of grinding wheel mesh size on obtained surface characteristics (pyramid shape micro tool).
(a) Two processing steps
(b) Four processing steps
Figure 6: Effect of number of processes on obtained surface characteristics.
extremely strong influence on the mechanical strength of the tool. Figure 10 shows the ruptured surface of the micro tool finished with the #1,200 mesh size grinding wheel examined via SEM. The rupture initiated from the mark left by the tool grinding. These results also show that minute grinding marks, which cause no problems when machining at the normal millimeter (macro) level, can serve as the crack initiation points for ruptures when machining at the micro level, suggesting that sufficient attention is required in order to avoid degradation of the strength of the tool.
Figure 8: Overview of micro tool rupture strength evaluation system using nano-indentation tester.
(b) Extremely large aspect ratio micro tool Figure 7: Overviews of produced micro tools under optimum machining conditions. Table 1: Experimental conditions.
I Workoiece I Cemented carbide allov Grinding wheel Grinding conditions Electrolytic dressing conditions
I
Cast iron bonded diamond wheels (wheel mesh size; #325, #1,200, #2,000, #M,OOO) Wheel rotation: 20,000 min-', Depth of cut: 0.5vm Open voltage: 30V, Peak current: I A , Pulse timing (on/off): 2/2ps, Pulse wave: square
4
THE RELATIONSHIP BETWEEN THE MICRO TOOL SURFACE CHARACTERISTICS AND THE RUPTURE STRENGTH This paragraph describes the results of testing conducted to determine the rupture strength of the micro tool using a nano-indentation tester, in order to identify the effects of the surface characteristics on the mechanical strength properties. As shown in Figure 8, we set a tool in place that would produce contact with the indentation probe at a position 5pm from the tip of the micro tool, and measured the indentation load and indentation depth. Here, the indentation depth was assumed to be equivalent to the amount of deformation of the micro tool. The surfaces of the measured samples had been finished using different grinding wheel mesh sizes ( # I ,200 and #M,OOO). The surface characteristics were those described in Figure 5 in the preceding paragraph. Figures 9 (a) and (b) show the results of nano-indentation testing conducted on materials finished with grinding wheel mesh sizes of #1,200 and #M,OOO, respectively. The line araoh showina the normal force and deformation of micro"too1 obtainedu from this testing indicates stable curves for both materials, which confirms the aopropriateness of the testina. Based on the fiaures. the , , , rupture strength (the indentation load at t h e point of rupture) is 177mN for the material finished with a grinding wheel mesh size of #1,200, and an extremely high value of 242mN for the mesh size of #4,000, indicating that differences in the surface characteristics exert an
(b) ##4,000 Figure 9: Relationship between normal force and deformation of micro tool.
Figure 10: Ruptured surface of micro tool finished with #1,200 mesh size grinding wheel.
5
PUNCHING TESTS CONDUCTED ON THIN METAL SHEETS In order to determine the effects of the micro tool surface characteristics on the actual performance of the tool, we conducted punching tests on thin platinum sheets, using micro tools manufactured in this testing. The overview during the testing is shown in Figure 11. The tests were carried out by securing the micro tool in a tool holder on a five-axis ultra-precision machine, and punching the thin metal sheet that was the object being machined at a speed of 10 mmlmin. For the test, we used two types of micro tools, with surfaces finished using different grinding wheel mesh sizes (#I ,200 and #4,000). Figure 12 shows the results of the punching test. The holes punched with the #4,000 finished material were extremely smooth micro-holes with sharp corner edge, however holes punched with the # I ,200 finished material, which produces a rougher surface, showed a significant amount of deformation of the metal sheet around the hole. This suggests that when the tool with the rough surface was used, a large machining resistance, i.e. the friction coefficient operating on the tool and the metal sheet, occurred during punching. In the future, we plan to conduct more in-depth studies using machining such as micro milling on these tools, to examine the effects of differences in surface characteristics on the finished surface of the object being machined, and on the lifetime of the tool.
Figure 11: Overview during punching tests
(a) #4,000
(b) # I ,200 Figure 12: Results of the punching test
6 SUMMARY In the present study, we developed a specific machine designed to create micro tools using micro grinding machining. The machine was controled and the machining process conditions were optimized so as to improve the surface characteristics. The surface characteristics of the prepared micro tools were controllable at the nanometer level by controling and optimizing the machining process conditions. Thus, we produced a micro tool having a extremly precise shape, measuring 2pm in diameter at the tip, and having an extremely large aspect ratio. Furthermore, we succeeded in demonstrating that degradation of the surface characteristics at the nano level strongly affects the mechanical strength properties of the tool, as well as the practical machining performance.
ACKNOWLEDG MEN1 In closing, the authors would like to express our sincere appreciation to NEXSYS Co. for their invaluable assistance in testing. REFERENCES Toenshoff H.K., Friemuth T., and Becker J.C., 2002, Process Monitoring in Grinding, Annals of the CIRP, 51/21551-571. Onikura H., Ohnishi O., and Take Y., 2000, Fabrication of Micro Carbide Tools by Ultrasonic Vibration Grinding, Annals of the CIRP, 49/1: 257260. Takeuchi Y., Maeda S . , Kawai T., and Sawada K., 2002, Manufacture of Multiple-Focus Micro Fresnel Lenses by Means of Nonrotational Diamond Grooving, Annals of the CIRP, 51/1: 343-346. Masuzawa T., and Kimura M., 1991, Electrochemical Surface Finishing of Tungsten Carbide Alloy, Annals of the CIRP, 40/1: 199-202. Masuzawa T., Kuo C.L., and Fujino M., 1994, A Combined Electrical Machining Process for Micronozzle Fabrication, Annals of the CIRP, 43/1: 189-192. Vasile M., Friedrich C., Kikkeri B. and McElhannon R., 1996, Micron-scale Machining: Tool Fabrication and Initial Results, Journal of Precision Engineering, 2/3: 180-186. Friedrich C., and Vasile M., 1996, The Micromilling Process for High Aspect Ratio Microstructures, Microsystem Technologies, 2/3: 144-148. Friedrich C., and Kithiganahalli B., 1994, Deflection Compensation Model for the Machining of Microshafts, Proceeding of 1994 ASPE Annual Conference: 461-464. Lonardo P.M., Lucca D.A., and Chiffre L.De, 2002, Emerging Trends in Surface Metrology, Annals of the CIRP, 51/2: 701-723. Uehara Y., Ohmori H., Yamagata Y., Lin W., Kumakura K., Morita S . , Shimizu T. and Sasaki T., 2001, Development of Small Tool by Micro Fabrication System Applying ELlD Grinding Technique, Initiatives of precision engineering at the beginning of a millennium, Kluwer academic publishers: 491-495. Ohmori, H., and Nakagawa, T., 1995, Analysis of Mirror Surface Generation of Hard and Brittle Materials by ELlD (Electrolytic In-Process Dressing) Grinding with Superfine Grain Metallic Bond Wheels, Annals of the CIRP, 44/1: 287-290.
Abstract Mlcro tools requlre exceptlmally hlgh surface characterlstlcs. controllable to the nanometer level. In thls research. a desk-top machlne capable of achlevlng excellent surface quality and machlnlny accuracy In mlcro machlnlng was developed, and mlcro tools of varlous shapes were prepared uslng the n e w developed machlne. The machlne successfully prepared pyramldal mlcro tools wlth tlps of 2pm. The surface of the prepared mlcro tools was observed closely uslng some advanced Instruments, and the fracture strength of the mlcro tools was evaluated by lndentatlon tests uslng a nanmlndentatlon testlng Instrument. These observatlons showed a clear and qwntttatke correlatlon betwean the nanometer-level surface quality and mechanlcal strength. In another test, thln metal sheets were punched uslng the hbrlcated mlcro tool. The resuttlng holes were found to be of excepUonally hlgh quality. Keywords: Mlcro tool, Surface quality, Mechanlcal strength
I INTRODUCTION Mlnlaturbatlon and w1gM redudlon of portable audlovlsual equlpment, p a l m t q computers, and cellular phones hiwe been urgently pursued. Thls has Increased demand for the mlnldurbatlon of hlghly fundlonal electronlc devlms, optlcal devlces, and mechanlcal components that compose these apparatuses, as well as for hlgh dlmenslonal accuracy of these parts. Hlgh dlmenslonal accuracy Is Increaslngly dllAcutt to achleve, as lt Is Influenced by surface roughness and wwlness. Improvement h surface quality Is deslred as part of the Increaslng demands for mechanlcal components, along wlth electrical and optkal devlces. In recent years, In order to meet these demands, varlous machlnlng methods hiwe been proposed, such as mkro grlndlng, cuttlng. mlcro electrlc dlscharge machlnlng, and beam machlnlng [1)[8]. As the resutts, the manhfacture of mlcrostructures and mlcro tools on the order of several mlcrons has been realhed. When tools are mlnlidurbed, the surface mlcrostrudure and materlal texture begln to f l e d the mechanlcal propertles of the components [9], thus undeslrably Increaslng the structural sensttMty. In partlcuhr, mlcro tools used for mlcrmpunchlng and mlcrmcuttlng requlre mechanlcal strength sulAclent to wlthstand the load durlng machlrrlng. If the surface lntegrltles of the mlcro tool are poor, a rough surface may act as a fracture orlgln, degradlng the strength of the tool. Furthermore, machlnlng objects uslng mlcro tools are used In order to achleve a hlghquality surface Rnlsh. H o w v e r , poor surface lntegrltles of the mlcro tool MI1 hcrease the Mctlonal reslstance durlng machhlng, naturally causlng degradatlon of the surface characterlstlcs of the object. Therefore. mlcro tools requlre exceptlonally hlgh surface charaderlstlcs, controllable to the nanometer level. Our research w s almed at produclng mlcro tools uslng preclslon grlndlng process, and In the process we developed a specltlc machlne for that purpose, as wll as controlllng and optlmhlng the machlnlng process condltlons In order to Improve the surface characterlstlcs.
Furthermore, In order to asmrtaln the erects of the surface characterlstlcs on the mechanlcal strength of the tool ttseK, we used a nanmlndentatlon tester and conducted rupture tests on the mlcro tools. We also used mlcro tools to conduct punchlng tests on thln metal sheets, to determlne the actual erects of the surface characterlstlcs on the product performance. 2
DEYELOPMENT OF THE MACHINE FOR MICRO TOOL MACHINING Flgure 1 shows the ederlor of the machlne developed specmcally for mlcro tool machlnlng. The machlne 1s deslgned to be extremely compact, and cm be used on a desk-top [lo]. Thls machlne has three Ilnear axes, X, Y, and 2 . In the processlng method, the work stage moves In the X and Y dlrectlms wlth respect to the outer circumference of the grlndlng wheel, whlch Is med. Thls movement accompllshes both feedlng and notchlng of the workplece, Rnlshlng the workplem to the requlred shape. Thls processlng method enables productlon of not only cyllndrlcal shapes, but also, K deslred, columnar shapes of square or elllptlcal cross-sections, or pyramld shapes h d n g comers. In addttlon, because the machlne has an Electrowlc In Process Dresslng system [11][12], the grlndng wheel surface characterlstlcs are constantly malntalned at the optlmum level, and the machlnlng reproduclbiity Is outstandlng. Flgure 2 shows a typlcd mlcro tool poduced uslng thls machlne. The mlcro tool 1s deslgned for use In punchlng, and Is m d e of a cemented carblde alloy. Based on the Rgure, Rner mlcro tool machhlng Is successfuly achleved, however K the wrface chracterlstlcs are examlned through an AFM, the state of the tool shank and that of the tlp are notlceably dlllerent, as shown In Flgure 3. With thls In mlnd, WB narrowed the mxhlnlng parameters govemlng the surface characterlstlcs of the mlcro tool to three parameters. for our testlng purposes. These Include depth of cut, mesh s k e ol grlndlng M e e l . and the number of processes from the rough machhlng to the Rnlshlng machlnlng. Then, uslng the abovedascrlbad
machine, which offers superb control of those parameters, we attempted to define machining conditions that would allow higher quality surface characteristics.
Controlling these machining conditions made it possible to significantly reduce the machining load and enabled machining of distinctive tools like those shown in Figures 7 (a) and (b), which feature smooth, uniform surface characteristics over the entire tool. Figure 7 (a) shows an ultra precise tool with a tip diameter of approximately 2vm, while Figure 7 (b) shows a tool with an extremely large aspect ratio. The figures also indicate that the material was stably removed in the ductile mode. In addition, the state of the tool shank and that of the tip were almost the same. Table 1 summarizes the machining conditions under which the shape shown in Figure 7 (a) was produced. Obtaining the optimum shape and surface characteristics will depend on the material selected for the tool. In the future, we will be working to produce micro tools with even greater strength and higher quality by using materials such as fine-grain sintered super-hard alloy and binderless alloy.
Figure 1: External view of developed machine for micro tool machining.
Figure 2: Overview of a typical micro tool (micro cylindrical shaft) produced using developed machine.
(b) Depth of cut of 0.5vm (a) Shank part
(b) Tip part
Figure 4: Effect of depth of cut on obtained surface characteristics.
Figure 3: AFM micrographs of ground surface (#I ,200). INVESTIGATION OF THE OPTIMUM CONDITIONS FOR MACHINING MICRO TOOLS Figure 4 shows the differences in surface characteristics obtained when a grinding wheel having a mesh size of # I ,200 was used for machining and the depth of cut was changed between 1vm and 0.5vm. The whole shapes of the machined tools were exactly the same, and care was taken to measure each at largely the same location. As indicated by this figure, in comparison with the surface having a depth of cut of Ipm, the surface with a depth of cut of 0.5pm was extremely smooth. At the same time, Figure 5 shows the results of SEM observation of the surface when machining was performed with the grinding wheel mesh size being changed to #1,200 and then to #4,000. As seen in this figure, increasing the mesh size of the grinding wheel being used from #1,200 to #4,000 produced a corresponding increase in the surface quality. Figure 6 shows the differences in the surface characteristics when processing was carried out twice from rough to finishing machining (#325, #4,000) and when processing was carried out four times (#325, # I ,200, #2,000, #4,000). In this case as well, increasing the number of processes produced superior surface characteristics. 3
(b) #4,000 (a) # I ,200 Figure 5:Effect of grinding wheel mesh size on obtained surface characteristics (pyramid shape micro tool).
(a) Two processing steps
(b) Four processing steps
Figure 6: Effect of number of processes on obtained surface characteristics.
extremely strong influence on the mechanical strength of the tool. Figure 10 shows the ruptured surface of the micro tool finished with the #1,200 mesh size grinding wheel examined via SEM. The rupture initiated from the mark left by the tool grinding. These results also show that minute grinding marks, which cause no problems when machining at the normal millimeter (macro) level, can serve as the crack initiation points for ruptures when machining at the micro level, suggesting that sufficient attention is required in order to avoid degradation of the strength of the tool.
Figure 8: Overview of micro tool rupture strength evaluation system using nano-indentation tester.
(b) Extremely large aspect ratio micro tool Figure 7: Overviews of produced micro tools under optimum machining conditions. Table 1: Experimental conditions.
I Workoiece I Cemented carbide allov Grinding wheel Grinding conditions Electrolytic dressing conditions
I
Cast iron bonded diamond wheels (wheel mesh size; #325, #1,200, #2,000, #M,OOO) Wheel rotation: 20,000 min-', Depth of cut: 0.5vm Open voltage: 30V, Peak current: I A , Pulse timing (on/off): 2/2ps, Pulse wave: square
4
THE RELATIONSHIP BETWEEN THE MICRO TOOL SURFACE CHARACTERISTICS AND THE RUPTURE STRENGTH This paragraph describes the results of testing conducted to determine the rupture strength of the micro tool using a nano-indentation tester, in order to identify the effects of the surface characteristics on the mechanical strength properties. As shown in Figure 8, we set a tool in place that would produce contact with the indentation probe at a position 5pm from the tip of the micro tool, and measured the indentation load and indentation depth. Here, the indentation depth was assumed to be equivalent to the amount of deformation of the micro tool. The surfaces of the measured samples had been finished using different grinding wheel mesh sizes ( # I ,200 and #M,OOO). The surface characteristics were those described in Figure 5 in the preceding paragraph. Figures 9 (a) and (b) show the results of nano-indentation testing conducted on materials finished with grinding wheel mesh sizes of #1,200 and #M,OOO, respectively. The line araoh showina the normal force and deformation of micro"too1 obtainedu from this testing indicates stable curves for both materials, which confirms the aopropriateness of the testina. Based on the fiaures. the , , , rupture strength (the indentation load at t h e point of rupture) is 177mN for the material finished with a grinding wheel mesh size of #1,200, and an extremely high value of 242mN for the mesh size of #4,000, indicating that differences in the surface characteristics exert an
(b) ##4,000 Figure 9: Relationship between normal force and deformation of micro tool.
Figure 10: Ruptured surface of micro tool finished with #1,200 mesh size grinding wheel.
5
PUNCHING TESTS CONDUCTED ON THIN METAL SHEETS In order to determine the effects of the micro tool surface characteristics on the actual performance of the tool, we conducted punching tests on thin platinum sheets, using micro tools manufactured in this testing. The overview during the testing is shown in Figure 11. The tests were carried out by securing the micro tool in a tool holder on a five-axis ultra-precision machine, and punching the thin metal sheet that was the object being machined at a speed of 10 mmlmin. For the test, we used two types of micro tools, with surfaces finished using different grinding wheel mesh sizes (#I ,200 and #4,000). Figure 12 shows the results of the punching test. The holes punched with the #4,000 finished material were extremely smooth micro-holes with sharp corner edge, however holes punched with the # I ,200 finished material, which produces a rougher surface, showed a significant amount of deformation of the metal sheet around the hole. This suggests that when the tool with the rough surface was used, a large machining resistance, i.e. the friction coefficient operating on the tool and the metal sheet, occurred during punching. In the future, we plan to conduct more in-depth studies using machining such as micro milling on these tools, to examine the effects of differences in surface characteristics on the finished surface of the object being machined, and on the lifetime of the tool.
Figure 11: Overview during punching tests
(a) #4,000
(b) # I ,200 Figure 12: Results of the punching test
6 SUMMARY In the present study, we developed a specific machine designed to create micro tools using micro grinding machining. The machine was controled and the machining process conditions were optimized so as to improve the surface characteristics. The surface characteristics of the prepared micro tools were controllable at the nanometer level by controling and optimizing the machining process conditions. Thus, we produced a micro tool having a extremly precise shape, measuring 2pm in diameter at the tip, and having an extremely large aspect ratio. Furthermore, we succeeded in demonstrating that degradation of the surface characteristics at the nano level strongly affects the mechanical strength properties of the tool, as well as the practical machining performance.
ACKNOWLEDG MEN1 In closing, the authors would like to express our sincere appreciation to NEXSYS Co. for their invaluable assistance in testing. REFERENCES Toenshoff H.K., Friemuth T., and Becker J.C., 2002, Process Monitoring in Grinding, Annals of the CIRP, 51/21551-571. Onikura H., Ohnishi O., and Take Y., 2000, Fabrication of Micro Carbide Tools by Ultrasonic Vibration Grinding, Annals of the CIRP, 49/1: 257260. Takeuchi Y., Maeda S . , Kawai T., and Sawada K., 2002, Manufacture of Multiple-Focus Micro Fresnel Lenses by Means of Nonrotational Diamond Grooving, Annals of the CIRP, 51/1: 343-346. Masuzawa T., and Kimura M., 1991, Electrochemical Surface Finishing of Tungsten Carbide Alloy, Annals of the CIRP, 40/1: 199-202. Masuzawa T., Kuo C.L., and Fujino M., 1994, A Combined Electrical Machining Process for Micronozzle Fabrication, Annals of the CIRP, 43/1: 189-192. Vasile M., Friedrich C., Kikkeri B. and McElhannon R., 1996, Micron-scale Machining: Tool Fabrication and Initial Results, Journal of Precision Engineering, 2/3: 180-186. Friedrich C., and Vasile M., 1996, The Micromilling Process for High Aspect Ratio Microstructures, Microsystem Technologies, 2/3: 144-148. Friedrich C., and Kithiganahalli B., 1994, Deflection Compensation Model for the Machining of Microshafts, Proceeding of 1994 ASPE Annual Conference: 461-464. Lonardo P.M., Lucca D.A., and Chiffre L.De, 2002, Emerging Trends in Surface Metrology, Annals of the CIRP, 51/2: 701-723. Uehara Y., Ohmori H., Yamagata Y., Lin W., Kumakura K., Morita S . , Shimizu T. and Sasaki T., 2001, Development of Small Tool by Micro Fabrication System Applying ELlD Grinding Technique, Initiatives of precision engineering at the beginning of a millennium, Kluwer academic publishers: 491-495. Ohmori, H., and Nakagawa, T., 1995, Analysis of Mirror Surface Generation of Hard and Brittle Materials by ELlD (Electrolytic In-Process Dressing) Grinding with Superfine Grain Metallic Bond Wheels, Annals of the CIRP, 44/1: 287-290.