In positioning of machine tool table, the table over travels some length x_OR by the inertia after the moment of positioning signal input. The x_OR, called over run in this paper, is one of the most important factors determining the positioning accuracy of machine tool table. The other factor is the dispersion of x_OR caused by fluctuation of frictional force in slide way. This dispersion of x_OR may give the limitation of positioning accuracy. Representing the feed drives of machine tool by the spring mass system with one degree of freedom, the over run length x_OR and its dispersion are analysed by digital simulation. The analysis shows that the over run length x_OR in conventional machine tools may be about 5-50 μm. It can be minimized by increasing the natural frequency of feed drives or decreasing the feed velocity. The dispersion of x_OR is about 0.5-5 μm, and it can be reduced by decreasing the fluctuation of frictional force in slide way.

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