Methods for Characterisation of Pressing Die Surfaces

Licentiate thesis, 2008

Design and manufacture of dies and moulds is a critical aspect of many production systems since the manufacturing and try-out of new dies and moulds often is essential in determining the lead-time and quality of a new production system. In the automotive industry, many new car models are introduced each year and for each of these models, a new set of pressing dies has to be designed and manufactured. In manufacturing of pressing dies, the effects of manufacturing processes on functional performance are not fully understood. One of the reasons for this is the lack of effective methods for characterisation. In the work described in this thesis, experimental work is conducted to establish such methods. First, the focus is on methods for obtaining measurement data. Second, the focus is on methods for relating the measurement data to functional performance. In contrast, the technique commonly used today for characterisation is visual and tactile inspection, with the help of reference surfaces, by an experienced operator. In this work, effective 3D measuring strategies were established for pressing die surfaces, including the use a surface replication technique. It was also found that machined surfaces are more homogeneous than manually polished surfaces and therefore require fewer measurements to give representative results. Furthermore, it was established that in production, a 2D profiler can be used for quality control of a finish milled steel surface, in regard to surface roughness, if an appropriate measuring strategy is used and limits for the evaluated parameters first are established for each combination of cutting tools, cutting data, workpiece material etc. In addition, using Rz (average peak-to-valley profile roughness) or Rp (highest peak) is better than using Ra (arithmetical mean deviation) if surface finish appearance is to be assessed. The method employed in this study, using scale-sensitive fractal analysis, for evaluating functional correlations and designing filters was very usable and gave good results. This method can surely be applied to other types of surfaces and functional correlations. Also, there is a correlation between the surface roughness parameter Sq (root-mean-square deviation of the surface) and the surface finish appearance of pressing dies and this correlation is stronger in certain wavelengths on the surface.