Validation of Thermal Errors Compensation Models for Different Machine Tool Structures Via Test Pieces
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Validation of thermal errors compensation models for different machine tool structures via test pieces Otakar Horejš, Martin Mareš, Lukáš Havlík 1. Introduction 3. Six-spindle automatic lathe Although real-time software compensations of thermal errors exist, The test pieces can be also employed for the evaluation of the majority of these models are not sufficiently validated in real machine tool thermal errors. This application is illustrated on a six- finishing operations using a test piece. The accuracy and spindle automatic lathe. The basic scheme of the test piece, robustness of the developed models are mostly examined using manufactured by a specific technology, is depicted in Figure below. non-cutting measurements. In contrast, machining tests can be more intuitively understood to evaluate the machine’s accuracy for typical machine tool users. Moreover, the machined test piece represents a suitable method to verify thoroughly the industrial applicability of the developed thermal errors compensation model for machine tool builders. The paper presents the authors’ recent research on these issues. Specific test pieces for different machine tool structures are illustrated (test pieces for a gantry-type 5-axis milling centre with a rotary table, a six-spindle automatic lathe and a vertical turning lathe). 2. Gantry-type 5-axis milling centre The test piece material was an aluminium alloy covered with an The diameter denominated as D1 of the test piece is manufactured eloxal coating for better visibility of cutting tool imprints on the at the first machining position (MP), the diameter denominated as surface caused by thermal error at the TCP in Z-direction. The D2 of the test piece is manufactured at the second MP, etc. Test process for creation of the test piece was divided into two cyclically pieces were machined only in certain time intervals during the repeated phases: machining tool imprints in a column and calibration measurement (from 5 min to 30 min). Thus, several sets conducting a test with simultaneous spindle rotation and of test pieces represent the time axis of the thermal displacements movements along the Z axis. One column of tool imprints was of the machine in the X-direction. Subsequently, the manufactured machined with a programmed Z tool positioned. Thereafter, the test sets of test pieces were gauged on a CMM in an air-conditioned with simultaneous spindle rotation and movements in the Z axis room. Obtained thermal displacements in X-direction are applied as was carried out (2 hours). The procedure was repeated until the input data for thermal error compensation model development. entire matrix of imprints was machined (10 columns; 18 hours). The Application of the compensation model led to a 65% reduction in complete matrix would have contained only three rows in the ideal the thermal displacements in the X-direction for all tested case (no thermal error). The vertical scale of the thermal test piece machining positions of the six-spindle automatic lathe in provides a visual imprint of the TCP thermal deformations under comparison with the uncompensated state. the specific test with simultaneous spindle rotation and movements in the Z axis. Reduction of thermal error in the Z-direction using 4. Vertical turning lathe thermal compensation based on transfer functions (TFs) was as Another test piece was machined to validate the thermal errors high as 92% compared with the uncompensated state. compensation model of a vertical turning lathe with a rotary table diameter of 3,000 mm. The machined test piece proved that thermal error in the Z-direction is 7-fold lower with compensation compared to the uncompensated state of the vertical turning lathe. 5. Summary As there is no international standard specially dedicated for test pieces which are able to capture the machine tools thermal errors, specific test pieces were proposed for different machine tools structures. Different test pieces are employed due to the fact that the tested machine tools structures have naturally different thermal behaviour. Thus, each of the tested machine tools requires a specific test piece to evaluate thermal errors at the TCP. Manufactured test pieces prove that the developed compensation models based on TFs significantly minimize the thermal errors at the TCP. Test pieces are suitable tool for visualization, quantification and evaluation of thermal errors of machine tools. Department of Production Machines and Equipment Research Center of Manufacturing Technology | RCMT CZECH TECHNICAL UNIVERSITY IN PRAGUE Horská 3 | 128 00 Prague 2 | Phone: +420 221 990 914 | www.rcmt.cvut.cz FACULTY OF MECHANICAL ENGINEERING.