Abstract: For the problem of the workpiece waviness, the relationship between the spindle unbalance and waviness phenomenon is researched. The measured result of the workpiece is decomposed and restructured by the wavelet transform, then, the power spectral density is used to analyze the spectrum energy of the signal in each scale decomposed by wavelet transform, and the corresponding frequency is obtained. The modal information of the spindle system is calculated by two methods. The main errors features is extracted with the frequency information of the spindle system, it includes the natural frequency and rotating frequency of the spindle, and the spectrum of the alternating current interference of the motor, the basic power frequency is 50 Hz and its multiple integers, sometimes also includes sub-harmonic signal. The extracted features from the workpiece is agree with the unbalance frequency of the spindle system and the alternating current interference of the motor, this explains the spindle unbalance is the main reason for the waviness, and provide a identification basis for the improvement of the machining accuracy.

Key words: Power spectral density, Spindle unbalance, Ultra-precision turning, Wavelet transform, Waviness