PIECS - a software program for machine tool Process-Intermittent Error Compensation

Abstract

This report documents software, called PIECS, that performs process-intermittent error compensation for a turning center. The program is a part of a larger three loop control architecture that includes a real-time geometric-thermal error compensation loop and a postprocess loop. In process-intermittent error compensation, a part is measured by on-machine gauging after a semifinish cut which uses the same cutting parameters (speed, feed, and depth of cut) as are used in the finish cut, to reproduce process-dependent errors such as cutting-force induced tool or part deflection. During gauging a touch-trigger probe signal indicates that the part surface has been contacted. The coordinates of the points are then transformed to the part coordinate system and compared to the corresponding nominal coordinates so that errors may be determined. The error vector is defined as having its head at the measured coordinates of the gauged point and its tail at the nominal coordinate for that point. Since the philosophy chosen in this program is to compensate process-intermittent errors by changing the position and orientation of features, least squares curve fitting through the ends of the error vectors is used to determine the adjusted tool path curve. The compensation curve becomes the tool path for the corresponding feature for the finish cut. The adjusted position and orientation of the feature are thus determined. Based on this information, errors may be compensated using either of two options: (1) an error specification file may be used to compensate the detected errors in real time by adjusting the machine-tool servo commands or (2) an adjustment of the programmed tool path can be made by changing the part program. The report includes a description of the program algorithm, the input and output data sets as well as descriptions of each of the C-programming language functions that compose PIECS. A listing of the program is included in the appendix.

A spline algorithm for modeling cutting errors on turning centers

“…Compensation of process related errors based on process-intermittent measurements and modeling has shown in the past that the procedure can correct errors on parts with linear features (see references [8] through [10] Table 1. 18…”