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.
This paper presents an analysis of part fabrication errors (part dimension and form deviations) in closed-loop machining systems. Modeling methods are used to situate error components in the context of other error components to clarify the effects of error compensation. Several novel concepts are introduced to make complex relationships between error components easily comprehensible. Inspection data are considered for one gauging point at a time. The part's physical surface detected by a touch-trigger probe during inspection is held as a fixed reference. When errors are calculated from measurements and used to locate the nominal surface with respect to the detected surface, this derived location of the nominal surface depends on the accuracy of the measuring instrument. If a probe on a machine tool is used to inspect the part, any discrepancy in the location of the derived "nominal surface" (when compared to the location of the nominal surface in the part coordinate system used during machining) reflects the machine tool geometric errors. A method of organizing the error components was developed to take advantage of this fact and discern the effects of machine tool geometric errors among other errors. Another useful concept introduced is the hypothetical uncompensated surface. Inspection would detect the surface at this location if the part were machined without error compensation. This and other hypothetical locations are used to illustrate the requirements for machining with error compensation to produce a detected surface that approaches the nominal location. The concepts explained should help with interpreting inspection data, decomposing errors into components, distinguishing the contributions of error compensation adjustments, and locating nominal surfaces for error computations.
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