Abstract: The machine tool spindle radial motion error is measured online under the condition of workpiece being manufactured．The measured object is cylinder workpiece instead of the reference bar．The profile error of workpiece is separated by error separation technique to realize the online measurement of spindle radial motion error．Wavelet packet algorithm based on self-adapting threshold is used to get rid of the noise produced when workpiece is manufactured．At the same time，the measurement system frequency domain characteristic is analyzed from two aspects, which are harmonic suppression and general frequency domain feature．

Key words: Denoise, Error separation technique, On-line test Motion error, Wavelet packet algorism Self-adapting threshold