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					  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
[1]. 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
[2]. 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 [3]. 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
                                                      [1]  Hesselbach, J.; Hoffmeister, H.-W.;
                                                           Hlavac, M.: Micro-Ginding - Efficient
                                                           Technique        for      Microstructuring
                                                           Hardened      Steels.    In:   Production
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                                                      [2]  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   [3]  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.
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Furthermore, the material properties influence             Hahmann, D.: Development in the
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microprofile. The microstructure dimensions can            electro    discharge      dressing.     In:
be further reduced by shift grinding with two                                        th
                                                           Proceedings of the 9 International
parallel grinding paths with the offset Δz. With           conference of the euspen, 2009,
shift grinding strategy micro tips with a height of        pp. 146-149.

				
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