This report describes principles and methodology for compensating geometric errors of machined parts due to machining processes. The error tendencies are identified by on-machine inspection of the part before the finish cut. Tool path adjustments compensate errors anticipated in the finish cut. The methodology was validated on a prototype system implemented using the Process-Intermittent Error Compensation Software (PIECS) on a turning center. Process-intermittent gauging of a semifinished part is directed by the machine tool controller, thus detecting only errors due to tool setting, tool wear, tool deflection or part deflection. These process errors are distinguished from errors caused by machine tool geometry, which are identifiable only by inspection independent of the machine tool. For process errors, an error vector is defined as having its head at the measured coordinates of a gauged point, and its tail at the nominal coordinate for that point. Least squares curve fitting through the heads of the error vectors is used to determine the error profile, from which the adjusted tool path for the finish cut is derived. Errors on a feature with a nominally straight profile are compensated by adjusting the feature''s position, orientation and scale. Spline functions are used to fit errors on curved parts. At an intersection between an arc profile and a line profile, boundary conditions are imposed to maintain any required tangency between features. In the prototype system, errors may be compensated using either of two options. The first option makes use of an error specification file containing the error profile for each feature. The tool position is monitored during finish cut machining, and machine tool servo commands are adjusted in real time to control the tool path. Or, alternatively, new tool path values can be calculated and inserted in a template to create a new part program to compensate errors for the finish cut.
Citation: NIST Interagency/Internal Report (NISTIR) - 6811Report Number:
NIST Pub Series: NIST Interagency/Internal Report (NISTIR)
Pub Type: NIST Pubs
error, error compensation, machine tool, part, part program, process-intermittent inspection, quality control, real time, tool path