Modeling and simulation of surface morphology abnormality of 'S' test piece machined by five-axis CNC machine tool

摘要

In this paper, a convenient way to detect machining precision of five-axis CNC machine tool is suggested in theory. In a general way, NAS979 is cut to test machine tool; however, it fails to evaluate the combination motions of rotary axes sufficiently. Therefore, a novel S-shape test part, called 'S' test piece, has been presented to demonstrate the machine tool's capabilities. As a new test specimen, 'S' test piece has some advantage to exhibit the machining precision of five-axis machine tool. There are some visible marks related to performance of machine tool experimentally, however, the reasons for these abnormal marks are uncertain theoretically, the performance of the servo system may be one of the causes. In order to figure out the definite cause of the abnormal morphology, a simulated platform with the servo system is set up to amplify and the normal errors that come from the tracking of axes are presented. The simulation results of the abnormal morphology of 'S' test piece is provided. And the surface quality is evaluated by the peak-to-peak value (Vpp). There are obvious marks in four special regions of 'S' test piece that simulated with poor performance servo system, and these marks are invisible in the surface of 'S' test piece that simulated with good performance servo system. Vpp in these four regions changes greater than the other regions of 'S' test piece. The Vpp that simulated by the poor performance servo system is about 15 times larger than the error simulated by the optimized performance servo system in these four special regions. While, Vpp of other regions is essentially invariant. Then, the machining experiments of 'S' test piece are conducted with the standard suggested process. The abnormal morphology of machined 'S' test piece is so obvious that it can be observed by the naked eyes, without any test equipment. And the result of the experiment is consistent with the simulation result, which means that tracking errors of servo system have direct influence on surface morphology abnormality, and the surface quality of 'S' test piece could display the dynamic performance of the servo system intuitively in theory. As a standard comparison object, the surface quality of NAS979 test piece is analyzed through the same platform with the poor performance servo system, the largest Vpp is about 0.00022 mm, one eightieth smaller than 'S' test piece at least. And machining experiment is also carried out with the poor performance machine tool, the surface is so smooth that unexpected texture cannot be observed by the naked eyes, it should be test by the special measurements. The simulation results and experiment results both show that the surface quality of 'S' test piece is hugely worse than NAS979. Besides, there are several special regions of 'S' test piece to exhibit the surface texture waving with certain parameters. In a word, 'S' test piece is high effectively to exhibit the dynamic performance of the servo system of five-axis machine tool.