Design criteria for grinding machine dynamic stability

Surface grinding is one of the oldest and most widely used machining process: to date, there are still few alternatives available for producing smooth and flat surfaces, satisfying both technical and economic constraints. The quality of a workpiece resulting from a grinding process is strongly influenced by the static and dynamic behavior of the mechanical system, composed by machine tool, wheel, fixture and workpiece. In particular, the dynamic compliance of the machine at wheel-workpiece interface may cause vibrations leading to poor surface quality. Starting from the analysis of process-machine interaction according to self-excited vibrations theories (the most relevant), this paper outlines a path for surface grinding machines design, focused on the identification of the most critical dynamic eigenmodes both in terms of dynamical parameters and geometry (vibration direction). The methodology is based on the application of Nyquist stability criterion for MIMO systems. Firstly, the methodology distinguishes between a limitation mainly ascribable to regenerative chatter and one ascribable to closed-loop eigenmodes properties. In this latter case, it will be shown that stability properties are strongly influenced by the shape and orientation of the elliptical movement of the wheel entailed by the limiting eigenmode (that, in general, is complex). Such an analysis can be also exploited to provide some indications guiding machine structural modifications. Finally, the approach is demonstrated on a couple of grinding machine variants via FE modeling.