Study on the motion errors of NC surface grinding machine : method for measuring the motion errors under the circular interpolation

New method for measuring the circular interpolation errors of high precision NC grinding machine is proposed. The measuring device consists of Capacitance Ball Probes, CBP, and capacitive displacement meter. A precision steel ball A is set on the table of the NC surface grinding machine and the other ball B is mounted at the end of the wheel head.While the ball B circles the ball A, the capacitance change due to the motion errors is detected. Since the obtained data are also affected by the floating capacity, which is generated among ball A and wheel head etc., the clearance between two balls, d, has to be set at a very small value, such as d=10μm. When auxiliary poles are set beside the ball A, the floating capacity can be effectively suppressed. Two-orientation method is also proposed to improve the accuracy in measurement. In this method, two sets of data are obtained before and after the change in direction of each ball, and the differences in data between the two are Fourier transformed. The true motion errors are then determined by phase operation. The calculation error originated from this method is assessed and the effectiveness of this process is confirmed by the computer simulation.