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Understanding Part Fabrication Errors in Closed-Loop Machining Systems



Herbert T. Bandy, Lawrence A. Welsch


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 compensation adjustments, and locating nominal surfaces for error computations.The paper also presents a description of how to structure error and error compensation data into distinct classes in closed-loop machining systems.
Journal of Research (NIST JRES) - 6876
Report Number


closed-loop machining, machine tool geometric errors, nominal surface, on-machine inspection, part errors, post-process inspection, process errors, software engineering, Unified Modeling Language


Bandy, H. and Welsch, L. (2002), Understanding Part Fabrication Errors in Closed-Loop Machining Systems, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD (Accessed April 20, 2024)
Created June 1, 2002, Updated February 19, 2017