F. Kahleyss, K. Weinert, University of Dortmund, Dortmund, Germany; D. Biermann, Technische Universität Dortmund, Dortmund, Germany
The milling of freeform surfaces with ball-shaped head cutters places high demands on machining processes. In contrast to parallel shaft cutters these tools enable an inclined cut and thus allow optimal process parameters at all times during the machining process. On the other hand, due to its continuously changing contact zone and hence an ever changing material removal rate 5-axis milling leads to a varying strain on the tool which might result in tool breakage. Downsizing these processes to a micro-structured level intensifies this problem due to very thin tool shafts and thus contains several challenges, especially if machining NiTi shape memory alloys. As a result of their special properties, these alloys belong to the group of materials with poor machinability. Earlier works have shown that effective machining parameters for this material lie within a very narrow window. If chosen to high or to low the result is poor machining quality and high tool wear. This behaviour is combined with very difficult chip breaking which further adds to the high demands of the machining process. The paper presents a systematic approach to the analysis of 5-axis micro-milling of NiTi shape memory alloys. It clarifies how results obtained from 3-axis micro-milling can be transferred to 5-axis processes and gives the optimal parameter values for the machining of NiTi. In a last step these values are applied to processes which create micro-structured cavities with undercuts. Medical implants consisting of NiTi with this type of structured surface are assumed to allow somatic cells an optimal connation. Therefore it is necessary to master simultaneous 5-axis processes with high process reliability and reasonable tool wear.
Summary: Due to their special properties, NiTi shape memory alloys belong to the group of materials with poor machinability. After increasing the values of the cutting parameters by a factor of 10 - 40 in earlier projects compared to the first suggestions in literature, the work at hand analyzes the process of simultaneous 5-axis micromilling. It aims at giving important clues at how to machine microstructured parts especially in biomedical applications.
