The Measurement of Motion Errors of NC Machine Tools and Diagnosis of their Origins by Using Telescoping Magnetic Ball Bar Method

Abstract

A method to measure the motion errors of NC machine tools and diagnosis of their origins have been studied by using the telescoping magnetic ball bar method. The motion errors during circular interpolation motion were measured by detecting the relative distance between the spindle nose and the workpiece on the table using an elastically connected bar. The motion errors, i.e. relative distance between two balls, were plotted in polar coordinates. These errors were obtained for both clockwise and counter-clockwise motions and plotted in XY, YZ, and ZX planes. The results show that the method has sufficient accuracy and is convenient for evaluating the motion accuracy of NC machine tools. A comparison of the polar plots of the measured error motion with the out-of-roundness errors for the machined circular plate shows good agreement. Various error origins were then classified into several groups based on the motion error traces, and theoretical analyses were made to find out the trace pattern typical to each error origin. A method has been developed to identify error origins of each machine by extracting characteristic patterns from the obtained motion error traces, while taking mechanical structure and servo control factors into consideration. By this method many NC machine tools have been diagnosed. The results show that the origins of each machine's motion errors were accurately identified.