1 Department of Management Engineering, Technical University of Denmark2 Department of Mechanical Engineering, Technical University of Denmark
This thesis is aimed at developing the micromilling process, investigating the effects of miniaturization on surface topography as well as workpiece surface accuracy, through experimental investigation and modelling of surface topography and cutting forces. Being the focus on the application of micromilling as tooling technology for replication techniques, the work material selected for the experimental investigations was hardened tool steel. The realization of the micromilling process was achieved with a conventional 3 axis milling centre, provided with a high speed attached spindle. The use of such equipment, in connection with micro end mills of 200 ƒÝm in diameter, required the development of a method for optimal control of the axial depth of cut. Size effects on surface topography were investigated through experimental investigation and comparison with reference models, developed and validated for milling at conventional size. The non perfect scalability of surface topography was verified. The non perfect scalability of the tool geometry affects the cutting forces. Particularly the cutting edge radius is not downscaled with the same scaling factor as the diameter. Such size effect was effectively modelled and included in an original cutting force model for ball nose end milling. A modification of existing theory for modelling of cutting forces is then proposed, in order to take into account the actual geometry of the cutting process at micro scale. A number of measurement techniques for geometrical characterization of micro end mills have been considered. The limitations of conventional measuring instruments, as optical CMMs, have been demonstrated. Measurements based on 3D SEM images are seen as a forthcoming possibility. The capabilities of the process were verified by manufacturing an injection moulding mould insert with micro features. The satisfactory accuracy achieved indicates that micromilling can effectively be used as a tooling technology in the process chain for micro injection moulding.