This paper presents a non-contact excitation method for evaluating the dynamic stiffness of a rotating spindle. The spindle response to an excitation force is measured, and frequency response functions (FRFs) are obtained. Based on the measured FRFs, dynamic uncertainty and its effect on cutting stability are investigated. Regenerative forces are generated using displacement feedback with a time lag element, and a closed-loop excitation test is executed automatically. The stability map obtained from the closed-loop test and the stability charts calculated from the FRFs are compared, and the uncertainty of the spindle dynamics during operations is clarified. (c) 2015 CIRP.