Development of an analyzing and tuning technology for improving circularity of cone-frustum motions of five-axis CNC machine tools

摘要

The machining of a cone frustum as specified on the ISO 10791-7 standard is a widely accepted test to evaluate the performance of five-axis machining centers. By measuring the cross-sectional circularity of the cone, the contouring accuracy of a five-axis CNC machine tool can be assessed. A reduction of circularity is directly related to the improvement of the contouring accuracy in circular motions. The contouring accuracy can also be assessed by measuring the circularity of the circular motion path described by the tool center point of a five-axis CNC machine tool. A better circular tool center path entails an enhancement on the circularity of the traversal sections of the cone frustum. The majority of CNC actuators are AC servomotors that are precisely controlled via two closed loops: one for position and one for velocity. The circular contouring accuracy of a five-axis CNC machine tool can be improved by adjusting the loop gains and time constant of the velocity loop integration. Although higher gains enable the servomotors to attain the desired positions faster, the resulting contouring accuracy may not be satisfactory. Furthermore, excessively high gains could cause vibrations. The objective of this research is to establish a frequency domain approach for tuning the control parameters for each motion axis such that their frequency responses are matched within a range of frequencies, reducing the circularity value of the cross sections of a cone frustum. Results demonstrate the effect of tuning the control loop parameters based on the frequency response of each axis. Case studies were conducted to determine the variation in the circularity value when the control parameters of each axis were tuned to obtain similar frequency responses to the linear and rotary axes within a frequency range. Rates of improvement greater than 80% were achieved after using the proposed method to tune the control parameters of each motion axis.