GRINDING OF MICROSTRUCTURES IN BRITTLE MATERIALS WITH MULTIPLE MICROPROFILED GRINDING WHEELS Denkena, B.1; Köhler, J.1; Kästner, J.1; Hahmann, D.1 1 Institute of Production Engineering and Machine Tools Leibniz Universität Hannover Garbsen, Germany INTRODUCTION by microgrinding and the use of this material for The manufacturing of complex shaped example for the manufacturing of micro air microstructures on microsystem parts in brittle guides. Crossed microstructures were machined materials provides a high geometrical accuracy by two orthogonal paths (FIGURE 1). Several . This requirement can only be met by using microstructures were generated parallel in microgrinding processes. There, metal bonded grinding with multiple micro profiled grinding diamond grinding tools offer a remarkable wheels in contrast to micro grinding with single potential due to their favorable wear behavior dicing blades. Thus, process economy is . However, the dressing of complex enhanced in grinding with multiple microprofiled microprofiles on these tools is not possible using grinding wheels. mechanical dressing methods because of a high Both structures in FIGURE 1 are machined with dresser wear and high dressing forces. Thus, the same material removal rate. One structure is Electro contact discharge dressing (ECDD) is machined with a low infeed and a high feed rate used for the microprofiling of the grinding and the other with a high infeed and a low feed wheels. ECDD is a numerically controlled rate. dressing process offering the possibility to create the geometry and topography of the grinding wheel simultaneously at negligible dressing forces . The power of an electric circuit thermally removes the metal bond of the grinding wheel in ECDD [4,5]. This paper presents investigations about the influence of the workpiece material and the grinding process parameters on the chipping at the edge of microstructures. Furthermore, strategies to reduce the microstructure dimensions and increase the flank angle of the microstructures are shown. ECDD is used for the dressing of the grinding wheels prior to the grinding experiments. FIGURE 1. The effect of the process parameters GRINDING WITH MULTIPLE MICROPRO- on the chipping at the edges of microstructures FILED METAL BONDED GRINDING WHEELS The micro ECDD process is used to generate The SEM-micrographs show that grinding with a multiple microprofiled grinding wheels. high infeed and a low feed rate produces larger Afterwards, the grinding of microstructures is chipping at the edge than grinding with a low investigated with these tools. Thereby, the infeed and a high feed rate. This is contrary to dressing and grinding operations were carried the results known for surface grinding. The out on the same grinding machine. The contact geometrical contact length is in the range of the conditions in grinding are influenced by the tool structure length for an infeed of 1 μm and a feed properties like grain size and concentration as rate of 1000 mm/min. However, the geometrical well as the process parameters. Among others contact length is about ten times longer than the the contact conditions are described by the chip structure length for an infeed of 100 μm and a thickness hcu and the geometrical contact length feed rate of 10 mm/min. The gaps between the lg. Aluminum oxide samples are chosen for the microstructures machined in the first grinding investigations, due to the favorable machinability path results in an interrupted cut in the second profile grinding process. Thus, the maximum However, the flat top of the 200 μm high contact length is shorter than the geometrical microstructure with an edge length of about contact length if the geometrical contact length 40 μm shows brittle material breakout. The exceeds the microstructure dimensions. chipped area is close to the size of several Considering this, the normal and tangential ceramic material grains (dK ≈ 15 µm). This single grain forces, calculated from force means, that the workpiece material properties, measurements, are significantly higher in e.g. the grain size and the fracture toughness, grinding with an infeed of 100 μm and a feed have a significant influence on the chipping at rate of 10 mm/min. This is the main reason for the edge of the microstructures. the increased edge break-out. The minimum workpiece microstructure dimensions are limited by the flute width of the grinding wheel profile if the microstructures are machined with only one tool path. The structure size can be reduced if a shift strategy with two parallel tool paths with the offset Δz is used (FIGURE 2). There, two cases are distinguished. The bar width of the grinding wheel profile is larger than the flute width in the first case. Then, the microstructure dimensions are reduced proportionally to the offset between the two tool paths. In the second case, the bar width of the grinding wheel profile is smaller than the flute width, like shown in FIGURE 2. Then, the microstructure dimensions are reduced until the offset between the tool paths according the bar width of the grinding wheel profile. The microstructures machined in the first tool path FIGURE 3. The limitation of minimum micro- are segmented in the second tool path if the structure dimensions by the material properties offset exceeds the bar width of the grinding wheel profile. Thus, in the following the edge quality of microstructures machined in aluminum oxide (dK < 1 µm), zirconium oxide and cemented carbide (dK < 1 µm) is analyzed. FIGURE 2. Reduction of microstructure dimensions by grinding with shift strategy FIGURE 4. The influence of the workpiece The detailed SEM-micrograph in FIGURE 3 material on the dimensions of the edge chipping shows a small microstructure, which was machined by the segmentation of a larger All samples are machined with the same microstructure, produced in the first tool path. process parameters except the total infeed ae,ges. The samples differ in the initial state of the perpendicular profile flanks with advanced material prior and after sintering beside the dressing and grinding strategies. material itself. The zirconium oxide and cemented carbide are fine grained and have a GRINDING WITH UNDERCUT PROFILES low porosity. The aluminum oxide has coarse A grinding wheel with an undercut profile is used grains in the range of about 15 μm and a higher for grinding microstructures with perpendicular porosity. The fracture toughness of zirconium flanks. Only one side of the grinding wheel oxide is twice as high and the fracture microprofile is dressed with an undercut to toughness of cemented carbide is up to five clarify the influence of the undercut profile on the times higher than for aluminum oxide. The SEM- machined microprofile shape. The undercut micrographs show that the edge quality is higher profile is dressed in two steps. The feed for zirconium oxide and cemented carbide direction of the micro electrode is perpendicular (FIGURE 4). The chipping for aluminum oxide to the grinding wheel surface in the first step. reaches more than 10 μm. The chipping is about The electrode is fed under the tilt angle γ in the 3 μm for zirconium oxide and less than 2 μm for second step to generate the undercut cemented carbide. The edge quality increases (FIGURE 6). The tilt angle γ is set in the range of with increasing fracture toughness and 15° to 30°. The dimensions of the decreasing material grain size. Thus, the further microstructures machined with two crossed tool reduction of the microstructure dimensions by paths are not the imprint of the grinding wheel using different offset Δz or higher infeed and the microprofile. They are generated by the increase of the height to width ratio is projected grinding wheel microprofile resulting investigated at cemented carbide. from the grinding wheel rotation. Thus, two flanks of the machined microstructures are expected to be perpendicular when grinding with the undercut microprofiled grinding wheel described above. FIGURE 5. Minimum microstructure dimensions in cemented carbide FIGURE 6. Influence of the undercut profile on the microstructure flanks FIGURE 5 shows the minimum microstructure dimensions machined in cemented carbide. Both FIGURE 6 shows a machined microstructure. structures were machined parallel. The The flanks machined with an undercut profile are microstructure top is reduced to a sharp tip with perpendicular except the lower third of the a radius of about 3 μm at a structure height of microstructure. The flanks machined with a about 400 μm. However, the flanks of the standard grinding wheel profile have a flank structures are not perpendicular due the grinding angle below 90°. The detailed SEM micrographs wheel micro profile. The dressing process is not of the corners machined without and with able to generate perpendicular profile flanks. undercut profile underline the significant The flank angle of the microprofiles, dressed on difference in the shape of the edges. The corner the grinding wheel, is about 78°. Thus, further machined without undercut profile shows a investigations were carried out to gain smooth crossover from the top area to the flanks. The corner machined with undercut about 400 µm and a tip radius of about 3 µm are profile shows a sharp crossover from the top ground. These micro tips point out a further area to the perpendicular flanks. Thus, grinding limitation of the microstructure geometry by the with an undercut profile enables the machining grinding wheel microprofile. Perpendicular flanks of perpendicular microstructure flanks and sharp cannot be ground at the microstructures due to edges. the flank angle below 90° at the dressed Further experiments have been carried out to microprofiles. Thus, grinding with an undercut machine square shaped microstructures with profile was investigated to grind perpendicular higher aspect ratios. Therefore, a shift strategy microstructure flanks. In this case, the shape of in grinding was used. The best results, shown in the machined microstructure depends on the FIGURE 7 were gained by an offset of 200 µm projected grinding wheel microprofile resulting between the two grinding paths. The total height from the grinding wheel rotation. Square shaped of the machined structure is about 450 µm. The protruded microstructures with a minimum edge square shaped part of the microstructure is length of about 60 µm and an aspect ratio of about 350 µm high and has an edge length of about 6 were ground with the combination of about 60 µm. This results in an aspect ratio of undercut profile and shift grinding strategy. about 6. ACKNOWLEDGEMENTS These research works have been conducted in the scope of the collaborative research centre (SFB) 516 “Design and Manufacturing of Active Microsystems” in the particular project B3 “Micromachining”. REFERENCES  Hesselbach, J.; Hoffmeister, H.-W.; Hlavac, M.: Micro-Ginding - Efficient Technique for Microstructuring Hardened Steels. In: Production Engineering - Research and Development (WGP), Volume 12, Number 1, pp. 1-4, 2005.  Klocke, F., Klink, A., Schneider, U:: Electrochemical oxidation analysis for dressing bronze-bonded diamond FIGURE 7. Microstructure with perpendicular grinding wheels. In: Production flanks and high aspect ratio Engineering - Research and Development (WGP), Volume 1, RESULTS AND CONCLUSIONS Number 2, pp. 141-148, 2007. The investigations show that several  Xie, J.; Tamaki, J.; Kubo, A.; Iyama, T.: microstructures can be machined parallel by the Application of electro-contact discharge use of multiple microprofiled grinding wheels. dressing to a fine-grained diamond The process parameters have a significant grinding wheel. In: Journal of the Japan influence on the edge of the machined Society for Precision Engineering, microstructures. The best edge quality is gained 67(11), 2001, pp. 1844-1849. at a high feed rate and a low infeed.  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