In order to further improve the precision of metal parts machining by means of CNC machine tools, which is based on the principle of semi closed loop and to reduce the positioning error caused by the backlash and thread pitch error in the process of mechanical and electrical transmission, a novel methodology for positioning accuracy measurement is proposed. In this paper, a set of high resolution grating sensors and corresponding display instruments were installed to measure the actual position of the worktable, aiming to compare the data between the grating sensor and the numerical control unit to form a deviation matrix. An initial mathematical model can be established in the form of fitting polynomial equation with higher order, showing that the trend of the original curve is oscillatory and divergent. To improve this undesirable situation effectively, a general interpolation formula for nonequidistant points has been derived and put into practice. According to the derived parabolic interpolating calculations, the parameters located in the backlash piecewise pitch control unit can be modified to promote the positioning precision. Compare to the traditional static testing method, the newness of the approach can be used to examine and regulate the positioning accuracy of the worktable in dynamic process.