Influence of cutting speed and tool wear on vibrations and process stability when turning Inconel 718 with PCBN tools

Abstract

Results of the study of cutting speed and tool wear influence on the vibrations and their nature during machining superalloy Inconel 718 with low cBN content (cBN-L) and binderless pcBN (bcBN) tools are presented. Cutting speed was varied from 2 to 14 m/sec while other cutting conditions were constant. The process evolution was studied with the wavelet transform of tool acceleration spectra. Behaviour of the dynamic system was found to vary with the cutting speed and the specifics of tool wear. It is shown that machining with bcBN tools was accompanied by appearance of significant vibration when cutting speed exceeded 5 m/sec, but the machining with cBN-L was characterised by the suppression of vibrations over the entire speed range. This was related to the difference of tool microgeometry development during the course of wear. The wear of bcBN tools, in contrast to cBN-L, was characterised by merger of crater and flank wear and formation of negative clearance angle. Such difference results in an increase of cutting forces 2-4 times as compared to cBN-L tools. Estimation of dynamic stability by 0-1 test confirmed the process instability when turning with bcBN tools at cutting speeds higher than 8 m/sec which concurs with chatter appearance.