Influence of the cutting tool geometrical defects on the dynamic behaviour of machining

Most of the studies on stability in machining are based on the following assumption: the cutting tool is geometrically perfect. However in practice, the defects in the positions of the cutting edges can be important, in particular for long tool holder, and thus perturb significantly the cutting process. The paper aims to show the influence of these defects on the stability of the machining and the raising of chatter phenomenon. This study is based on numerical simulations of an industrial milling operation involving a long flexible tool holder. It was achieved using a proper time domain simulator of the milling process, based on an accurate geometrical modelling of the tool, the machined surface and the material removal. This simulator computes accurately the machined surface geometry and roughness as well as the cutting forces and the amplitude of the cutting tool vibrations. The simulations of the industrial case have highlighted two mains results. First of all, the conventional stability criteria are improper to predict the stability of tools with defaults. Thus, for this case, we propose a different indicator of stability that furthermore estimates the level of the chatter in case of instable-machining. Secondly, the numerical results show that the geometrical defects of the tool increase the stability of the cutting process. But in such cases, the using of chatter free cutting conditions is no longer a guaranty of a good machined surface.