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Development of a micro diamond grinding tool by compound process
Journal of Materials Processing Technology 209 (2009) 4698–4703
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Development of a micro diamond grinding tool by compound process Shun-Tong Chen a,∗ , Ming-Yi Tsai b , Yun-Cheng Lai a , Ching-Chang Liu a a b
Department of Mechatronic Technology, National Taiwan Normal University, Taiwan, 162, Sec. 1, He-ping East Rd., Taipei, 106, Taiwan Department of Mechanical Engineering, National Chin-Yi University of Technology, Taiwan, 35, Lane 215, Chung-Shan Rd. Sec. 1, Taiping City, Taichung County, 411, Taiwan
a r t i c l e
i n f o
Article history: Accepted 6 October 2008 Keywords: Micro-EDM Diamond tool Composite electroforming
a b s t r a c t This study presents a novel micro-diamond tool which is 100 m in diameter and that allows precise and micro-grinding during miniature die machining. A novel integrated process technology is proposed that combines “micro-EDM” with “precision composite electroforming” for fabricating micro-diamond tools. First, the metal substrate is cut down to 50 m in diameter using WEDG, then, the micro-diamonds with 0–2 m grain is “plated” on the surface of the substrate by composite electroforming, thereby becoming a multilayer micro-grinding tool. The thickness of the electroformed layer is controlled to within 25 m. The nickel and diamond form the bonder and cutter, respectively. To generate good convection for the electroforming solution, a partition designed with an array of drilled holes is recommended and verified. Besides effectively decreasing the impact energy of the circulatory electroforming solution, the dispersion of the diamond grains and displacement of the nickel ions are noticeably improved. Experimental results indicate that good circularity of the diamond tool can be obtained by arranging the nickel spherules array on the anode. To allow the diamond grains to converge toward the cathode, so as to increase the opportunity of reposing on the substrate, a miniature funnel mold is designed. Then the distribution of the diamond grains on the substrate surface is improved. A micro-ZrO2 ceramic ferrule is grinded to verify the proposed approach. The surface roughness of Ra = 0.085 m is obtained. It is demonstrated that the micro-diamond grinding tool with various outer diameters is successfully developed in this study. The suggested approach, which depends on machining applications, can be applied during the final machining. Applications include dental drilling tools, precision optic dies, molds and tools, and biomedical instruments. © 2009 Published by Elsevier B.V.
1. Introduction Currently, light-weight, thin, short and small products are typically fabricated in large numbers since they use minimal materials, generate minimal waste and noise, consume little energy, are cost-effective and attractive, in addition to accelerating chemical reactions. Such micro-products are applied widely to 3C components, precise tools, medical instruments, communications systems, and are utilized by the national defense industry among others. These micro-parts and structures are required to develop and fabricate low-cost tools that are micro-sized and have increased wear resistance. Some conditions limit traditional machining, for example, machined feature sizes that may exceed a mini-meter. Substantial amounts of mechanical energy or thermal energy are employed to remove material. However, a machining mechanism cannot be applied to micro-parts and structures due to thermal stress, thermal deformation and the surface degenerating layer of the machined material.
∗ Corresponding author. Tel.: +886 2 23923105x853; fax: +886 2 23929449. E-mail address: [email protected] (S.-T. Chen). 0924-0136/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2008.10.055
This study presents a novel, practical and low-cost approach that integrates micro-Electron Discharge Machining (micro-EDM) with precision composite electroforming to develop a micro-grinding tool. The developed micro-tool is utilized to machine hard and brittle materials, such as optical glass, fine ceramics, and tungsten carbide, and for micro-cutting or nano-grinding (Gatzen and Maetzig, 1997). To increase the strength of the proposed microgrinding tool, the tool shaft is made of tungsten carbide in ultra-fine particles. Its diameter is reduced to only 50 m by micro-EDM. The tool shaft surface provides a reposed base for the floating diamonds. Nickel ions and diamond grains are combined via an electrochemical mechanism to a composite electroformed layer that has two phases: the continuous phase of the metal bonder made of pure nickel, and the discontinuous phase of grinding grain which is fixed with a micro-diamond. The combined force of the diamond can be enhanced by nickel due to ions piling up one by one. Additionally, micro-tool strength is increased. The fixed diamond with the nickel layer for cementing forms the cutter. To increase the diamonds per unit area and the amount of exposed cutting edge thereby improving the cutting properties and tool life (Semba and Sato, 2000), a diamond grain 0–2 m in diameter is employed. This approach partially solves the prob-
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lem of micro-miniaturizing a tool and increases the lifetime of the tool. 2. Experimental hardware 2.1. Developed tabletop machining center The experiment is carried out in two stages. First, the micro-tool shaft is cut using micro-EDM. Second, the micro-diamond grain is “plated” on the shaft by composite electroforming. The work material of fine tungsten carbide, 300 m in diameter is cut down to the required dimensions using the developed tabletop machining center (Fig. 1(a)). The machining center with 10 nm resolution, due to its modularized design, can perform general micro-milling, micro-EDM, micro-high speed milling, micro-EDM milling, and micro-measurement on-line, as well as micro-wire-EDM. However, this study focuses only on micro-EDM. The metal substrate is easily machined to a very small diameter using the designed WEDG (Wire Electro Discharge Grinding) (Masuzawa et al., 1985; Chen et al., 2006) in addition to producing accurate circularity and cylindricity (Fig. 1(b)). 2.2. Design of the miniature electroforming tank To achieve a good dispersive effect for the diamond grains in the electroforming liquid, so that a uniform composite electroforming layer is acquired, a proposed miniature composite working tank with a perfect convectional design is constructed (Fig. 2). The electroforming solution is pumped out of the tank’s bottom and delivered back into the tank through the top using a micro-DC motor. The liquid entrance is below the liquid surface to eliminate excessive spraying and turbulent flow. The design has the following advantages. It is simple and inexpensive and provides an airtight circulation space for creating a non-directional con-
Fig. 2. Proposed miniature electroforming tank.
vectional liquid, thereby increasing the displacement of nickel ions and dispersion of diamond grains. Consequently, the concentration of the electroforming solution can be kept stable with no change during the whole experiment. Most importantly, the micro-diamond grains can be fully dispersed in the liquid without being aggregated together, which enhances the surface quality of the micro-electroformed tools. Furthermore, the design of the miniature tank effectively saves the electroforming solution and diamonds. A partition that resists flushing, minimizes turbulent flow and improves deposition efficiency in the cathode is designed. This partition in the working tank separates the zone of the flushing liquid from the depositing diamond. The liquid is guided to the electroforming zone through a micro-holes array and the gap between the partition’s plate and tank’s bottom. Subsequently, some of the liquid flows upward and circulates within the working tank, and some flows out of the working tank temporarily to outside circulating. Therefore, the energy with which the solution impacts the microsubstrate is reduced, and the diamond grains are then uniformly dispersed in the tank. 3. Experimental procedures 3.1. Influence of the anodic shape As the average diameter of the micro-grinding tool is only 150 m, its form accuracy, especially the circularity, is very important. The form and dimensional accuracies of the machined micro-hole are affected during machining when micro-tool circularity is poor. Since the substrate is kept static during electroforming, the shape of pure nickel in the anode affects the correct form of the electroformed diamond tool. The shapes, including (a) single block, (b) double blocks, (c) holed block, and (d) spherules array, of the nickel in the anode is applied and tested individually. Under the same experimental parameters, the best circularity is achieved when using the spherules array. Due to the volume of the nickel spherule is small enough so that it can provide a good dissolution rate and keep a low dissolution voltage. Thus the nickel ions can have a good and uniform precipitation in the anode. With the convectional design for the electroforming solution, the nickel ions have good fluidity and are steadily and non-directionally deposited on the substrate. As a result, good circularity of the micro-tool can be achieved with (d). Therefore the spherules array of pure nickel in the anode is employed throughout the experiment. 3.2. Experiment for the best electroforming zone
Fig. 1. Micro-metal substrate machining: (a) developed tabletop hybrid machining center and (b) principle of WEDG.
The amount of diamond grains will differ with different electroforming zones in the tank. Laddered and horizontal electroforming
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substrate can be obtained. Fig. 3(b) displays the distribution of current density on the substrate and the result of the appearance of the product. A phenomenon worth noting is that the electroformed substrate is shaped like a mushroom head at the free end of the substrate. It causes poor form accuracy on the microdiamond tool. This mushroom head results from the current being collected at the free end of the micro-metal substrate, especially at the sharp corner. Consequently, the strongest current density is located there. There are three approaches used to prevent the current crowding effect in this study. They are lengthening the distance between the cathode and anode, filleting on the endcorner of the substrate, and using a non-conducting material such as PMMA to disconnect from the strongest current density (refer to Fig. 6). Fig. 4 presents comparisons of the amount of diamonds. Only some diamonds are plated onto the substrate in zones A, B and D. It is reasoned that this phenomenon results from the undue flush generated in these zones. In addition, a non-fine and nonsmooth texture is produced on the surface of electroformed layer in zones A and B due to there is an overly small distance between the two zones and the anode. The excessive current density causes the rough surface on the electroformed layer to occur. A comparatively good result for the amount and distribution rate of diamonds exists in zone C as that zone has the lowest flush of electroforming liquid and is proper distance away from the anode. Consequently, the chances of catching and fixing the diamonds are increased. Hence the C zone is selected for the main electroforming zone in all experiments. 3.3. Micro-tool shaft machining Fig. 3. Identifying the best electroforming zone: (a) substrate placed horizontally and (b) distribution of current density.
is tested to identify the proper electroforming zone (Fig. 3(a)). The four micro-pillared metal substrates, including A, B, C and D, are lined up and placed horizontally on the conducting post in the cathode, and the interval between the pillars is 30 mm. Through circulation of the electroforming solution in the tank, the contents of the diamond grains which the diamonds drop onto the
To increase the amount of diamond grains and strength of the micro-grinding tool, tungsten carbide with ultra-fine particles is used as the tool shaft material. The outer diameter of the grinding tool is designed from 100 m to 200 m; hence the shaft diameter is machined by WEDG (refer to Fig. 1(b)) to between 50 m and 150 m to electroform the abrasives on a single side up to a 25 m thickness, which provides quantities of sufficient diamond grains for fine finishing. The substrates of the micro-grinding tools have various shapes—(1) straight, (2) fluted and (3) tapered (Fig. 5).
Fig. 4. Comparison of amounts of diamond grains.
S.-T. Chen et al. / Journal of Materials Processing Technology 209 (2009) 4698–4703
Fig. 5. Various micro-tool shafts after micro-EDM.
3.4. Effect of the funnel mold The plating process facilitates the deposition of metal ions on the cathode surface with a diameter of 50 m. Conversely, to fixing of the micro-diamond grains on the metal cathode is based on probability. The diamonds can be fixed on the substrate because the constructed base allows the flowing diamonds to settle down progressively by means of the nickel ions during the electrochemical reaction. The diamonds may be covered continuously by nickel ions once the diamond grain is fixed. Via a continuous process, the diamonds are joined with the substrate permanently when the coverage range exceeds the diamond radius. This phenomenon is due to the fact that there is only a physical relationship not a chemical reaction between nickel ions and diamond grains. The combination of nickel ions and diamonds is not controlled by a chemical reaction unless the diamond is processed using a special process.
Fig. 6. Design and test of the miniature funnel mold.
Fig. 7. Comparison of diamond grain content with and without the funnel mold.
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Fig. 8. A proper interval between grains resulted in chip pocket.
imental results. Few diamond grains are plated on the substrate without the funnel mold. Conversely, the amount of diamond grains is sufficient and the grains become uniformly distributed, and more diamonds are covered by the nickel layer, in particular when the funnel mold is utilized. Experimental results verify that the miniature funnel mold increases the concentration and distribution of diamond grains. Fig. 8(a) shows the finished surface of a micro-grinding tool. These diamond grains are electroformed non-directionally into the nickel layer, some of diamonds are partially visible on the electroformed surface, and some are fully embedded into the electroformed layer. Due to the nickel ions piling up one by one, a good combined strength between the micro-diamond grain and the nickel layer can be achieved. Notably, an interval of 2–3 m between these plated diamond grains provides a chip pocket that
Consequently, a strategy is proposed in which the miniature funnel mold increases the concentration and the fixing probability of diamond grains. The funnel mold is located on the cathode. The mold’s entrance is designed as an inner tapered hole through which diamond grains flow, increases gradually when scattered diamond grains drop slowly from the top to the bottom of the working tank. Finally, the substrate becomes surrounded by numerous diamond grains since many diamond grains collect around the cathode. Therefore the probability of fixing diamond grains onto the substrate base increases markedly (Fig. 6). More importantly, a kind of non-electrically conductive material such as PMMA is used to make the funnel mold so that separate from the high current density. As a result, an effect of point on the cathode is greatly reduced. To enhance the displacement of nickel ions and the distribution of diamond grains around the cathode, the proposed miniature funnel mold is partitioned into two sections by a gap. The gap is adjustable to allow the diamond grains and nickel electroforming solution to exit from the gap. In accordance with this experiment the proposed strategy can enhance the amount and the distribution rate of the diamond grains on the cathode. Fig. 7 shows the exper-
Fig. 9. Various finished micro-diamond grinding tools: (a) finished various microdiamond tools and (b) close-up view of the electroformed diamond layer.
Fig. 10. Verification of the fine grinding with the micro-diamond grinding tool: (a) illustration of the micro-grinding, (b) finish-grinding the inner taper surface and (c) surface roughness on the inner taper surface.
S.-T. Chen et al. / Journal of Materials Processing Technology 209 (2009) 4698–4703
can temporarily to store chips to avoid obstructing and blunting the cutting edge of the grinding tool (Fig. 8 (b)). 4. Verify of the proposed approach Continuing the process in Fig. 5(a), the four kinds of micro-metal substrates are fabricated in accordance with the above-mentioned conditions. Before conducting an electroforming process, it is essential that the interface of the substrate undergo various processes including ultrasonic vibration cleaning, alkaline degreasing, electrolysis cleaning, acid cleaning, acid dipping and acid activation. In addition, the composition, temperature, pH, and the additives of the electrolyte should be rigorously controlled. Fig. 9(a) displays various finished micro-grinding tools. These tools are used for the fine grinding of micro-holes, slots, and other applications. Fig. 9(b) is a magnification of the electroformed surface. Since the experimental parameters are strictly controlled, the electroformed layer with a thickness of 25 m can be accurately obtained. As for the electroformed diamonds, they belong to a multilayer structure. In addition, the amount and distribution of the diamond grains are quite satisfactory. A ferrule which is made of ZrO2 ceramic is finely grinded to verify the feasibility of the proposed method. The micro-ferrule and the micro-tapered grinding tool are tightly clamped by the microchucks. The revolutions of the micro-workpiece and micro-tool are at 500 and 100,000 rpm, respectively, besides opposite rotating in direction. In addition, the micro-diamond grinding tool is slowly fed along the inner taper direction. Two kinds of different microdiamond tools in grain diameter are employed to compare the grinding surfaces. The diamond grain-diameters of the micro-tool are 15–20 m and 0–2 m respectively. Each cutting depth is accurately controlled within 1 m during fine grinding process after rough grinding. The illustration of the micro-grinding is depicted in Fig. 10(a). The finish-grinding inner taper surface is shown and the close-up view of the SEM photograph for the comparison between the rough and fine grinding is displayed in Fig. 10(b). Noticeably, a well surface roughness is obtained when using fine grinding. The surface roughness, inspected by a 3D surface profiling system (SNU Precision Company, Korea) is given in Fig. 10(c), and is Ra = 0.085 m.
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5. Conclusions A micro-grinding tool with an outside diameter of 100–200 m is presented in this study. First, an appropriate surface roughness and outer diameter of the tungsten carbide substrate is fabricated by micro-EDM. The partition in the composite electroforming tank sufficiently mixes the electroforming solution and saves diamond grains. To increase the quantity of the electroformed diamond grains on the substrate, a novel miniature funnel mold is devised. By means of the proposed composite electroforming tank and funnel mold, the nickel ions and diamond grains are smoothly and tightly “plated” onto the substrate surface. The diamond grains are abundantly and uniformly distributed on the micro-grinding tool. The circularity of the micro-grinding tool can be improved, reducing the post re-machining process when using nickel spherules array in the anode. Additionally, a proper interval between diamond grains is produced automatically making a chip pocket for temporarily storing chips. The developed technology is successful from grinding the precision ceramic component, and can be applied quickly and cost-effectively when fabricating miniature and precise dies, tools and parts. It is believed that the proposed approach can make a substantial contribution to the precision machining industry. Acknowledgements The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC95-2622-E-003-005-CC3. This portion of the work is supported by the Office of R&D, National Taiwan Normal University and HONGIA Industry Co., Ltd. In addition, our gratitude also goes to the Academic Paper Editing Clinic, NTNU. Their help is gratefully acknowledged. References Chen, S.T., Liao, Y.-S., Lin, C.-S., 2006. Development of the integrated micro machining system. J. Chin. Soc. Mech. Eng. 27 (6), 619–625. Gatzen, H.H., Maetzig, J.C., 1997, Nanogrinding, Precision Eng. 21, 134–139. Masuzawa, T., Fujino, M., Kobayashi, K., Suzuki, T., 1985. Wire electro-discharge grinding for micro-machining. CIRP Ann. 34 (1), 431–434. Semba, T., Sato, H., 2000. Development of electroformed diamond tool with fine grains covered with metal oxide coating. CIRP Ann. 49 (1), 157–160.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Development of a micro diamond grinding tool by compound process Shun-Tong Chen a,∗ , Ming-Yi Tsai b , Yun-Cheng Lai a , Ching-Chang Liu a a b
Department of Mechatronic Technology, National Taiwan Normal University, Taiwan, 162, Sec. 1, He-ping East Rd., Taipei, 106, Taiwan Department of Mechanical Engineering, National Chin-Yi University of Technology, Taiwan, 35, Lane 215, Chung-Shan Rd. Sec. 1, Taiping City, Taichung County, 411, Taiwan
a r t i c l e
i n f o
Article history: Accepted 6 October 2008 Keywords: Micro-EDM Diamond tool Composite electroforming
a b s t r a c t This study presents a novel micro-diamond tool which is 100 m in diameter and that allows precise and micro-grinding during miniature die machining. A novel integrated process technology is proposed that combines “micro-EDM” with “precision composite electroforming” for fabricating micro-diamond tools. First, the metal substrate is cut down to 50 m in diameter using WEDG, then, the micro-diamonds with 0–2 m grain is “plated” on the surface of the substrate by composite electroforming, thereby becoming a multilayer micro-grinding tool. The thickness of the electroformed layer is controlled to within 25 m. The nickel and diamond form the bonder and cutter, respectively. To generate good convection for the electroforming solution, a partition designed with an array of drilled holes is recommended and verified. Besides effectively decreasing the impact energy of the circulatory electroforming solution, the dispersion of the diamond grains and displacement of the nickel ions are noticeably improved. Experimental results indicate that good circularity of the diamond tool can be obtained by arranging the nickel spherules array on the anode. To allow the diamond grains to converge toward the cathode, so as to increase the opportunity of reposing on the substrate, a miniature funnel mold is designed. Then the distribution of the diamond grains on the substrate surface is improved. A micro-ZrO2 ceramic ferrule is grinded to verify the proposed approach. The surface roughness of Ra = 0.085 m is obtained. It is demonstrated that the micro-diamond grinding tool with various outer diameters is successfully developed in this study. The suggested approach, which depends on machining applications, can be applied during the final machining. Applications include dental drilling tools, precision optic dies, molds and tools, and biomedical instruments. © 2009 Published by Elsevier B.V.
1. Introduction Currently, light-weight, thin, short and small products are typically fabricated in large numbers since they use minimal materials, generate minimal waste and noise, consume little energy, are cost-effective and attractive, in addition to accelerating chemical reactions. Such micro-products are applied widely to 3C components, precise tools, medical instruments, communications systems, and are utilized by the national defense industry among others. These micro-parts and structures are required to develop and fabricate low-cost tools that are micro-sized and have increased wear resistance. Some conditions limit traditional machining, for example, machined feature sizes that may exceed a mini-meter. Substantial amounts of mechanical energy or thermal energy are employed to remove material. However, a machining mechanism cannot be applied to micro-parts and structures due to thermal stress, thermal deformation and the surface degenerating layer of the machined material.
∗ Corresponding author. Tel.: +886 2 23923105x853; fax: +886 2 23929449. E-mail address: [email protected] (S.-T. Chen). 0924-0136/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2008.10.055
This study presents a novel, practical and low-cost approach that integrates micro-Electron Discharge Machining (micro-EDM) with precision composite electroforming to develop a micro-grinding tool. The developed micro-tool is utilized to machine hard and brittle materials, such as optical glass, fine ceramics, and tungsten carbide, and for micro-cutting or nano-grinding (Gatzen and Maetzig, 1997). To increase the strength of the proposed microgrinding tool, the tool shaft is made of tungsten carbide in ultra-fine particles. Its diameter is reduced to only 50 m by micro-EDM. The tool shaft surface provides a reposed base for the floating diamonds. Nickel ions and diamond grains are combined via an electrochemical mechanism to a composite electroformed layer that has two phases: the continuous phase of the metal bonder made of pure nickel, and the discontinuous phase of grinding grain which is fixed with a micro-diamond. The combined force of the diamond can be enhanced by nickel due to ions piling up one by one. Additionally, micro-tool strength is increased. The fixed diamond with the nickel layer for cementing forms the cutter. To increase the diamonds per unit area and the amount of exposed cutting edge thereby improving the cutting properties and tool life (Semba and Sato, 2000), a diamond grain 0–2 m in diameter is employed. This approach partially solves the prob-
S.-T. Chen et al. / Journal of Materials Processing Technology 209 (2009) 4698–4703
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lem of micro-miniaturizing a tool and increases the lifetime of the tool. 2. Experimental hardware 2.1. Developed tabletop machining center The experiment is carried out in two stages. First, the micro-tool shaft is cut using micro-EDM. Second, the micro-diamond grain is “plated” on the shaft by composite electroforming. The work material of fine tungsten carbide, 300 m in diameter is cut down to the required dimensions using the developed tabletop machining center (Fig. 1(a)). The machining center with 10 nm resolution, due to its modularized design, can perform general micro-milling, micro-EDM, micro-high speed milling, micro-EDM milling, and micro-measurement on-line, as well as micro-wire-EDM. However, this study focuses only on micro-EDM. The metal substrate is easily machined to a very small diameter using the designed WEDG (Wire Electro Discharge Grinding) (Masuzawa et al., 1985; Chen et al., 2006) in addition to producing accurate circularity and cylindricity (Fig. 1(b)). 2.2. Design of the miniature electroforming tank To achieve a good dispersive effect for the diamond grains in the electroforming liquid, so that a uniform composite electroforming layer is acquired, a proposed miniature composite working tank with a perfect convectional design is constructed (Fig. 2). The electroforming solution is pumped out of the tank’s bottom and delivered back into the tank through the top using a micro-DC motor. The liquid entrance is below the liquid surface to eliminate excessive spraying and turbulent flow. The design has the following advantages. It is simple and inexpensive and provides an airtight circulation space for creating a non-directional con-
Fig. 2. Proposed miniature electroforming tank.
vectional liquid, thereby increasing the displacement of nickel ions and dispersion of diamond grains. Consequently, the concentration of the electroforming solution can be kept stable with no change during the whole experiment. Most importantly, the micro-diamond grains can be fully dispersed in the liquid without being aggregated together, which enhances the surface quality of the micro-electroformed tools. Furthermore, the design of the miniature tank effectively saves the electroforming solution and diamonds. A partition that resists flushing, minimizes turbulent flow and improves deposition efficiency in the cathode is designed. This partition in the working tank separates the zone of the flushing liquid from the depositing diamond. The liquid is guided to the electroforming zone through a micro-holes array and the gap between the partition’s plate and tank’s bottom. Subsequently, some of the liquid flows upward and circulates within the working tank, and some flows out of the working tank temporarily to outside circulating. Therefore, the energy with which the solution impacts the microsubstrate is reduced, and the diamond grains are then uniformly dispersed in the tank. 3. Experimental procedures 3.1. Influence of the anodic shape As the average diameter of the micro-grinding tool is only 150 m, its form accuracy, especially the circularity, is very important. The form and dimensional accuracies of the machined micro-hole are affected during machining when micro-tool circularity is poor. Since the substrate is kept static during electroforming, the shape of pure nickel in the anode affects the correct form of the electroformed diamond tool. The shapes, including (a) single block, (b) double blocks, (c) holed block, and (d) spherules array, of the nickel in the anode is applied and tested individually. Under the same experimental parameters, the best circularity is achieved when using the spherules array. Due to the volume of the nickel spherule is small enough so that it can provide a good dissolution rate and keep a low dissolution voltage. Thus the nickel ions can have a good and uniform precipitation in the anode. With the convectional design for the electroforming solution, the nickel ions have good fluidity and are steadily and non-directionally deposited on the substrate. As a result, good circularity of the micro-tool can be achieved with (d). Therefore the spherules array of pure nickel in the anode is employed throughout the experiment. 3.2. Experiment for the best electroforming zone
Fig. 1. Micro-metal substrate machining: (a) developed tabletop hybrid machining center and (b) principle of WEDG.
The amount of diamond grains will differ with different electroforming zones in the tank. Laddered and horizontal electroforming
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substrate can be obtained. Fig. 3(b) displays the distribution of current density on the substrate and the result of the appearance of the product. A phenomenon worth noting is that the electroformed substrate is shaped like a mushroom head at the free end of the substrate. It causes poor form accuracy on the microdiamond tool. This mushroom head results from the current being collected at the free end of the micro-metal substrate, especially at the sharp corner. Consequently, the strongest current density is located there. There are three approaches used to prevent the current crowding effect in this study. They are lengthening the distance between the cathode and anode, filleting on the endcorner of the substrate, and using a non-conducting material such as PMMA to disconnect from the strongest current density (refer to Fig. 6). Fig. 4 presents comparisons of the amount of diamonds. Only some diamonds are plated onto the substrate in zones A, B and D. It is reasoned that this phenomenon results from the undue flush generated in these zones. In addition, a non-fine and nonsmooth texture is produced on the surface of electroformed layer in zones A and B due to there is an overly small distance between the two zones and the anode. The excessive current density causes the rough surface on the electroformed layer to occur. A comparatively good result for the amount and distribution rate of diamonds exists in zone C as that zone has the lowest flush of electroforming liquid and is proper distance away from the anode. Consequently, the chances of catching and fixing the diamonds are increased. Hence the C zone is selected for the main electroforming zone in all experiments. 3.3. Micro-tool shaft machining Fig. 3. Identifying the best electroforming zone: (a) substrate placed horizontally and (b) distribution of current density.
is tested to identify the proper electroforming zone (Fig. 3(a)). The four micro-pillared metal substrates, including A, B, C and D, are lined up and placed horizontally on the conducting post in the cathode, and the interval between the pillars is 30 mm. Through circulation of the electroforming solution in the tank, the contents of the diamond grains which the diamonds drop onto the
To increase the amount of diamond grains and strength of the micro-grinding tool, tungsten carbide with ultra-fine particles is used as the tool shaft material. The outer diameter of the grinding tool is designed from 100 m to 200 m; hence the shaft diameter is machined by WEDG (refer to Fig. 1(b)) to between 50 m and 150 m to electroform the abrasives on a single side up to a 25 m thickness, which provides quantities of sufficient diamond grains for fine finishing. The substrates of the micro-grinding tools have various shapes—(1) straight, (2) fluted and (3) tapered (Fig. 5).
Fig. 4. Comparison of amounts of diamond grains.
S.-T. Chen et al. / Journal of Materials Processing Technology 209 (2009) 4698–4703
Fig. 5. Various micro-tool shafts after micro-EDM.
3.4. Effect of the funnel mold The plating process facilitates the deposition of metal ions on the cathode surface with a diameter of 50 m. Conversely, to fixing of the micro-diamond grains on the metal cathode is based on probability. The diamonds can be fixed on the substrate because the constructed base allows the flowing diamonds to settle down progressively by means of the nickel ions during the electrochemical reaction. The diamonds may be covered continuously by nickel ions once the diamond grain is fixed. Via a continuous process, the diamonds are joined with the substrate permanently when the coverage range exceeds the diamond radius. This phenomenon is due to the fact that there is only a physical relationship not a chemical reaction between nickel ions and diamond grains. The combination of nickel ions and diamonds is not controlled by a chemical reaction unless the diamond is processed using a special process.
Fig. 6. Design and test of the miniature funnel mold.
Fig. 7. Comparison of diamond grain content with and without the funnel mold.
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Fig. 8. A proper interval between grains resulted in chip pocket.
imental results. Few diamond grains are plated on the substrate without the funnel mold. Conversely, the amount of diamond grains is sufficient and the grains become uniformly distributed, and more diamonds are covered by the nickel layer, in particular when the funnel mold is utilized. Experimental results verify that the miniature funnel mold increases the concentration and distribution of diamond grains. Fig. 8(a) shows the finished surface of a micro-grinding tool. These diamond grains are electroformed non-directionally into the nickel layer, some of diamonds are partially visible on the electroformed surface, and some are fully embedded into the electroformed layer. Due to the nickel ions piling up one by one, a good combined strength between the micro-diamond grain and the nickel layer can be achieved. Notably, an interval of 2–3 m between these plated diamond grains provides a chip pocket that
Consequently, a strategy is proposed in which the miniature funnel mold increases the concentration and the fixing probability of diamond grains. The funnel mold is located on the cathode. The mold’s entrance is designed as an inner tapered hole through which diamond grains flow, increases gradually when scattered diamond grains drop slowly from the top to the bottom of the working tank. Finally, the substrate becomes surrounded by numerous diamond grains since many diamond grains collect around the cathode. Therefore the probability of fixing diamond grains onto the substrate base increases markedly (Fig. 6). More importantly, a kind of non-electrically conductive material such as PMMA is used to make the funnel mold so that separate from the high current density. As a result, an effect of point on the cathode is greatly reduced. To enhance the displacement of nickel ions and the distribution of diamond grains around the cathode, the proposed miniature funnel mold is partitioned into two sections by a gap. The gap is adjustable to allow the diamond grains and nickel electroforming solution to exit from the gap. In accordance with this experiment the proposed strategy can enhance the amount and the distribution rate of the diamond grains on the cathode. Fig. 7 shows the exper-
Fig. 9. Various finished micro-diamond grinding tools: (a) finished various microdiamond tools and (b) close-up view of the electroformed diamond layer.
Fig. 10. Verification of the fine grinding with the micro-diamond grinding tool: (a) illustration of the micro-grinding, (b) finish-grinding the inner taper surface and (c) surface roughness on the inner taper surface.
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can temporarily to store chips to avoid obstructing and blunting the cutting edge of the grinding tool (Fig. 8 (b)). 4. Verify of the proposed approach Continuing the process in Fig. 5(a), the four kinds of micro-metal substrates are fabricated in accordance with the above-mentioned conditions. Before conducting an electroforming process, it is essential that the interface of the substrate undergo various processes including ultrasonic vibration cleaning, alkaline degreasing, electrolysis cleaning, acid cleaning, acid dipping and acid activation. In addition, the composition, temperature, pH, and the additives of the electrolyte should be rigorously controlled. Fig. 9(a) displays various finished micro-grinding tools. These tools are used for the fine grinding of micro-holes, slots, and other applications. Fig. 9(b) is a magnification of the electroformed surface. Since the experimental parameters are strictly controlled, the electroformed layer with a thickness of 25 m can be accurately obtained. As for the electroformed diamonds, they belong to a multilayer structure. In addition, the amount and distribution of the diamond grains are quite satisfactory. A ferrule which is made of ZrO2 ceramic is finely grinded to verify the feasibility of the proposed method. The micro-ferrule and the micro-tapered grinding tool are tightly clamped by the microchucks. The revolutions of the micro-workpiece and micro-tool are at 500 and 100,000 rpm, respectively, besides opposite rotating in direction. In addition, the micro-diamond grinding tool is slowly fed along the inner taper direction. Two kinds of different microdiamond tools in grain diameter are employed to compare the grinding surfaces. The diamond grain-diameters of the micro-tool are 15–20 m and 0–2 m respectively. Each cutting depth is accurately controlled within 1 m during fine grinding process after rough grinding. The illustration of the micro-grinding is depicted in Fig. 10(a). The finish-grinding inner taper surface is shown and the close-up view of the SEM photograph for the comparison between the rough and fine grinding is displayed in Fig. 10(b). Noticeably, a well surface roughness is obtained when using fine grinding. The surface roughness, inspected by a 3D surface profiling system (SNU Precision Company, Korea) is given in Fig. 10(c), and is Ra = 0.085 m.
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5. Conclusions A micro-grinding tool with an outside diameter of 100–200 m is presented in this study. First, an appropriate surface roughness and outer diameter of the tungsten carbide substrate is fabricated by micro-EDM. The partition in the composite electroforming tank sufficiently mixes the electroforming solution and saves diamond grains. To increase the quantity of the electroformed diamond grains on the substrate, a novel miniature funnel mold is devised. By means of the proposed composite electroforming tank and funnel mold, the nickel ions and diamond grains are smoothly and tightly “plated” onto the substrate surface. The diamond grains are abundantly and uniformly distributed on the micro-grinding tool. The circularity of the micro-grinding tool can be improved, reducing the post re-machining process when using nickel spherules array in the anode. Additionally, a proper interval between diamond grains is produced automatically making a chip pocket for temporarily storing chips. The developed technology is successful from grinding the precision ceramic component, and can be applied quickly and cost-effectively when fabricating miniature and precise dies, tools and parts. It is believed that the proposed approach can make a substantial contribution to the precision machining industry. Acknowledgements The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC95-2622-E-003-005-CC3. This portion of the work is supported by the Office of R&D, National Taiwan Normal University and HONGIA Industry Co., Ltd. In addition, our gratitude also goes to the Academic Paper Editing Clinic, NTNU. Their help is gratefully acknowledged. References Chen, S.T., Liao, Y.-S., Lin, C.-S., 2006. Development of the integrated micro machining system. J. Chin. Soc. Mech. Eng. 27 (6), 619–625. Gatzen, H.H., Maetzig, J.C., 1997, Nanogrinding, Precision Eng. 21, 134–139. Masuzawa, T., Fujino, M., Kobayashi, K., Suzuki, T., 1985. Wire electro-discharge grinding for micro-machining. CIRP Ann. 34 (1), 431–434. Semba, T., Sato, H., 2000. Development of electroformed diamond tool with fine grains covered with metal oxide coating. CIRP Ann. 49 (1), 157–160.