Stable tongues induced by milling tool runout

High material removal rates and performances are required for modern milling operations, which may trigger self-excited chatter vibrations. Such undesired vibrations cause unacceptable machined surface quality and premature deterioration of the cutting tool. After many decades of research and successful industrial solutions to this problem, some unexpected phenomena still arise, which put in doubt the effectiveness of well-known chatter theories and of the associated predictive numerical methods. Specifically, runout is a typically ignored consequence of inaccurate fixing of the tool, which has essential impact on the actual cutter-workpiece engagement and on the machined surface quality. The unequal engagement of cutter teeth change the dynamical behavior radically and prevent the application of classical simplifications in the modeling of milling processes. Moreover, in addition to the kinematically different teeth cycle-paths, the coexisting forced vibrations induce early fly-over effects of cutting edges creating new stability boundaries close to the resonant oscillations. This paper presents the underlying principles of this experienced phenomenon related to tool runout and its stabilization effect on chatter vibrations. Focusing on conventional milling cutters, the paper breaks with the widely held assumption that forced vibration has negligible effect on stability in the presence of tool runout. Initial laboratory experiments validate this tool irregularity induced phenomenon and industrial tests demonstrate the technical relevance of the results.