ELSEVIER Automated measurement and compensation of thermally induced error maps in machine tools Narayan Srinivasa* and John C. Ziegertt *The Beckman Institute, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, IL, USA and tDepartment of Mechanical Engineering, University of Florida, Gainesville, FL, USA In this paper, a direct method of machine tool calibration is adopted to mode/ and predict thermally induced errors in machine tools. This method uses a laser ball bar (LBB) as the calibration instrument and is implemented on a two-axis computerized numerical control turning center (CNC). Rather than individually measuring the parametric errors to build the error model of the machine, the total positioning errors at the cutting tool and spindle thermal drifts are rapidly measured using the LBB within the same experimental setup. Unlike conventional approaches, the spindle thermal drifts are derived from the true spindle position and orientation measured by the LBB. A neural network is used to build a machine model in an incremental fashion by correlating the measured errors with temperature gradients of the various heat sources during a regular thermal duty cycle. The machine model devel- oped by the neural network is further tested using random thermal duty cycles. The performance of the system is also evaluated through cutting tests under various thermal conditions. A substantial improvement in the overall accuracy was obtained. © Elsevier Science Inc., 1996 Keywords: laser ball bar; spindle thermal drifts; total positional errors; neural network; error compensation cutting tests Introduction Improvement of machine tool accuracy is an essen- tial part of quality control in manufacturing pro- cesses. Because of a continuous demand for preci- sion machined parts, various methods for error compensation before and during the machining process are continually being investigated. These methods must be capable of providing the correct quality control actions to the machine tool by ac- tively monitoring and correcting the errors attrib- utable to manufacturing processes rather than pas- sively inspecting machined parts. These errors originate from numerous sources including geo- metrical errors of the structural elements, kine- Address reprint requests to Dr. Narayan Srinivasa, the Beckman Institute, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, IL 61801, USA. Precision Engineering 19:112-132, 1996 © Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010 matic errors related to motion of these elements, static and dynamic deflections of the workpiece, and thermally induced errors. Of these sources, thermally induced errors are thought to be the larg- est contributor to overall machine inaccuracy, con- tributing as much as 70% of the total positioning error. 1 Thermal errors arise attributable to thermal de- formations of the machine elements caused by heat sources that exist within the structure, includ- ing the leadscrew bearings and nuts, axis drive motors, spindle, friction on the way surfaces, heat generated by the cutting process, and the flow of coolant and lubricating oil. Thermal errors can be mitigated in two different ways. The first method minimizes the problem by thermally stabilizing the machine. 2-4 This is accomplished by using good design practice, which isolates the major heat sources from the positioning elements, and then 0141-6359/96/$15.00 PII S0141-6359(96)00042-6