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Case history Research Centre Karlsruhe:
Odyssee through the world of clamping technology

Clamping chucks with highest precision requirements
Vacuum clamping technology for machining of micro-structured workpieces

The Research Centre Karlsruhe is one of the largest scientific and engineering research organizations in Germany employing approx 4500 people. Work is carried out here not only in different areas of basic research but also on developments relevant to products, emphasis on which is underlined by numerous cooperations with partners in science and business.

One of the main subjects of the research centre Karlsruhe is microsystem technology. Over 200 employees are currently working on short and mid-term transfer of research results into marketable products. Part of their work also involves the development of processes for economic manufacture of micro workpieces.

Microsystem technology products are found in nearly all areas of our daily life. Micro workpieces are for instance built into airbags and anti blocking systems, hearing aids, heart pacemakers, mobile telephones or CD players. The Institute for Microstructure Technology at the Research Centre Karlsruhe manufactures amongst other things highly accurate forming tools, which are used for making several hundred micro spectrometers.
The original material used for such accurate tools is 100 µm thin polymer slices, which ensue after several manufacturing procedures. „Needless to say that for a high-value product optimal basic requirements are essential,“ explained Marco Heiler, responsible for ultra-precise machining of the original material for forming tools „because an initial inaccuracy affects all following manufacturing processes and leaves traces on all of the micro workpieces which are later made with the tool“. The polymer slices may therefore only have very low deviation regarding plane parallelism and must be machined on an ultra precise milling machine. The clamping method has a large influence on the quality of the slices achieved during this process, and it proved to be extremely problematical in this particular case.
The specialist looked for a clamping method for machining the polymer slices, which on the one hand improved quality and on the other hand speeded up and simplified the working process. „I was sure that it must be possible to clamp the thin parts, without leaving any traces behind,“ so Marco Heiler, who on his search finally arrived at the Metapor chucks from Witte. „We had already tried various solutions, but with every clamping method new problems arose.“
First of all a conventional brass vacuum chuck with 1mm holes was used. Principally this worked well, but for parts less than 600µm it proved unsuitable. The holes left traces on the thin, soft polymer slices and they were unusable.

Those kind of traces were avoided by using an insert made of sinter metal. Deforming of workpieces on vacuum chucks with an insert made out of porous sinter bronze is impossible because there are no T-slots or holes. This clamping method left no traces behind, but it had other disadvantages. „After every change of chuck we had to mill over the clamping surface with diamonds in order to achieve the desired plane parallelism“, explained Marco Heiler, „the parts must lie absolutely flat and even to axis.“ However, during machining the particles being milled off were pressed into the sinter metal, so that after a few applications the clamping capacity became considerably limited.

In addition to that three parts were being clamped simultaneously and because of the uneven clamping force no uniform results could be achieved. The slices could therefore no longer be used for manufacturing processes.
On the continuing search for a suitable clamping method trials with material similar to the slices took place. „If chuck and workpiece have the same material properties, no traces should ensue“ thought Marco Heiler. A plexiglass clamping fixture was manufactured, in which there were drilled holes of 0,5mm diameter. This fixture was never used, because the mechanical machining, during which hundreds of holes were drilled, caused the plastic block to deform and it could no longer be used.
Finally they turned to the Bleckede company Witte Gerätebau who, with six different vacuum clamping systems and numerous pump units with different characteristics and capacities, are considered to be the market leaders regarding vacuum clamping technology.