Improvement on µ-milling process

Concerning the micromilling processes, and related to the expected improvements defined by the end-user Createch-Medical, the work has been focused on the avoidance of geometrical defects and process quality instabilities generated by the appearance of wool wear during the machining process. In the case of the geometrical defects induced by the appearance of the tool wear, IK4-Ideko has developed a methodology for the accurate on-machine tool shape measurement using a non-contact laser device from BLUM-Novotest, linked to the analysis of the actual surface machined by the tool. These measurements have been validated by their comparison to high resolution confocal measurements, obtaining uncertainty values as high as 0,5 µm. Besides, once identified the actual tool geometry and the surface shape to be machined with it, a tool wear compensation method has been developed IK4-Ideko too, based on simple geometrical models developed for the machined part. The application of this method has been evaluated by analysing the strains induced on a fitting part machined with a worn tool when it is clamped. Results show that the developed method can significantly reduce the deviations induced by the worn tools and help avoiding the defect generation on parts designed for accurate fitting. Regarding the variation of the process state due to the appearance of tool wear, the monitoring and statistical treatment of process signals has been tackled by Brunel and Leuphana universities, in order to define a frame where an in-line evaluation of the process state could be done. After the analysis of several measurable process signals and different data treatment techniques, a good correlation was found between the appearance of tool wear and the increase of a wavelet coefficient from the cutting force. Once identified the signal features with high correlation with the tool wear, a monitoring scheme has been defined in order to enable the in-line evaluation of the cutting forces. By the use of a piezoelectric dynamometer, a charge amplifier and an acquisition board, it is possible to evaluate in real time the cutting force signals. Finally, a software tool has been developed by Leuphana University for the evaluation of the obtained signals. This tool can compare the present signals with previously recorded ones in order to identify excessive tool wearing or process instabilities, helping the user on the decision making related to the process control so excessive waste of time and materials can be avoided.