Abstract: A six-degree-of-freedom (DOF) model is presented for the study of the bifurcation of the machine-tool spindle-bearing system in the paper. The dynamics of machine-tool spindle system supported by ball bearings can be described by a set of second order nonlinear differential equations with piecewise stiffness and damping due to the bearing clearance. Numerical results show when the inner race touches the bearing ball with a low speed, grazing bifurcation occurs. The solutions of this system evolve from quasi-periodic to chaotic orbit, from period doubled orbit to periodic orbit, and from periodic orbit to quasi-periodic orbit through grazing bifurcations. In addition, the route of the period-doubling bifurcation to chaos and the tori doubling process to chaos which usually occurs in the impact system are also observed in this spindle-bearing system. These researches rich our understanding to chaos and promote the investigation into nonlinear dynamics theory in the spindle-bearing system and application.