An application of real-time error compensation on an NC twin-spindle lathe

A unique form of the error synthesis model, which combines both the geometric and thermal errors of the NC twin-spindle lathe, is given by the homogeneous coordinate transformation method. Estimating the variation of the slopes of the error profiles enables the separation of the geometric and thermal errors. A set of strategies is presented for modeling different types of thermal error components according to their properties. The temperature field of the machine is monitored on-line by 16 thermal sensors. Based on measured temperatures and the coordinates of the slides, pre-established empirical models estimate the thermal error components in real-time. The kinematic model then synthesizes the estimated thermal and geometric error components to determine the planar error under the current machine conditions. Based on the determined planar error, the computer sends out the compensation signals to the machine’s controller. The controller cyclically picks up the compensation signals approximately every 10 ms and corrects the planar error by shifting the reference origin of the slides. In the compensation cutting test, the effectiveness of this method was verified, the size variations of the workpieces could be reduced from 60 to 14 μm, and the taper could be reduced from 50 to 15 μm/cm.