1. Introduction
The motorized spindle plays more and more important role in high-speed machining. The level and performance of CNC machine tool are decided and evaluated by a technology level of motorized spindle, a performance of motorized spindle. Then it is necessary and significant to find efficient and convenient ways for the research of motorized spindle. As the FEM (finite element method) has become an essential solution technique in many areas of engineering concerned static and dynamic process, the FEM is utilized in the design of motorized spindle bearing system. J.K. Choi analyzed the static and dynamic characteristics of motorized spindle bearing system by using FEM [1]. The thermal characteristics of the motorized spindle bearing system with a tilting axis were investigated using the FEM to improve the performance of the motorized spindle bearing system [2]. A qualitative power flow model was presented to characterize the power distribution of a high-speed motorized spindle [3]. Hongqi Li [4] constructed a thermal model of motorized spindle based on Timoshenko beam theory using the FEM and considered shear deformation, material and bearing damping, and the spindle/tool-holder interface. Jenq-Shyong Chen [5] analyzed the centrifugal-forceinduced characteristic variations of the shaft/bearing and draw bar mechanism performance at dynamic state by using FEM. Xu Min investigated the thermal characteristic of spindle bearing system [6]. Chi-wei Lin first developed a design flow chart to represent the overall spindle design problems. Based on this flow chart, eight design parameters were identified. And a design sensitivity analysis of these eight design parameters was then conducted based on an integrated FEM model to investigate their influence on the natural frequencies of the spindle system [7]. Lida Zhu et al. developed a spindle bearing system with a finite element model for simulation to investigate the dynamic characteristics [8]. TThe dynamic characteristics of the axis of the motorized spindle which is a very important part of motorized spindle are closely related to the performance of motorized spindle. So it is significant to investigate and analyze the dynamic characteristics of the axis of the motorized spindle.