| Abstract: | Sub-micron precision machining requires very precise position and speed control of the motion of the machine tool axes. The accuracy of coordinated position control determines the profile accuracy of a part in contour machining, while the accuracy of speed control is the most significant factor in the resulting sub-surface damage that may occur in contour or non-contour machining. In high precision machining of brittle materials, it is desirable that the chip removal process be in the ductile regime of the material. The fundamental hypothesis is that if ductile regime chip removal is not accomplished, sub-surface damage will occur on the machined part. Although this can be partially corrected by removing the damaged layer by polishing, that is a very slow manual and costly operation. Therefore, it is desirable to machine such parts at the ductile regime to avoid sub-surface damage. Such a chip removal process requires precise control of feed rates at extremely slow speeds. This motion control problem is difficult due to the large friction and the unpredictable nature of the friction at very low speeds.The standard proportional-integral-derivative (PID) type servo control algorithms are not capable of delivering the desired precision in motion control. The friction must be accurately compensated for by the real-time control algorithm. This requires an accurate means of predicting the friction on-line. To this end, off-line experiments, designed with statistical considerations of factors affecting friction, were conducted. The data collected from these experiments were analyzed to understand the friction characteristics and to develop appropriate model structures for on-line predictions. |
