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The performance improvement of the computer numerically controlled (CNC) vertical milling machine is constrained by limited understanding of the dynamic behaviors of the spindle system. In order to study the dynamic characteristics of the CNC vertical milling machine spindle system under unbalanced force, the lumped mass method is applied to propose a novel dynamic model. In this comprehensive model, Hertz contact, radial clearance and time-varying stiffness of rolling bearings are considered. According to the contact behaviors of rolling bearings supporting the spindle, restoring force is a nonsmooth and nonlinear function of displacements and bearing radial clearance. Then, governing differential equations of the eighteen-degree-of-freedom system are derived using the Lagrange equation. In addition, the effectiveness of this dynamic model is verified by experiments. The influences of spindle speed and …