| With the rapid development of modern manufacturing technology, CNC machine tools have become indispensable devices, and high-speed high-precision five-axis CNC machine applications have become an important symbol to show the development of the technological level and high-tech manufacturing. In high speed and precision machining complex parts, kinematic performance of the five-axis CNC machine tools becomes key indicators both affecting the quality of the workpiece and evaluating equipment. In order to guarantee the CNC machining accuracy, the implementation of work accuracy test becomes important means for machine tool accuracy test and dynamic performance evaluation.From the view of the CNC machine work accuracy testing standard, the formation mechanism of contour error in five-axis CNC machining, the machining accuracy affected by CNC machine control servo characteristics and kinematic performance of the control system were classified. In this paper an axis velocity model is presented based on multi-axis motion look-ahead algorithm, and a specimen cutting error model related to dynamic indicators of the CNC machine tool is suggested. The formation mechanism of abnormal marks at particular specimen in high speed and precision machining is proposed, and a new way for evaluation of the CNC machine tool dynamic performance becomes viable, and a theoretical support for evaluation of dynamic performance of CNC machine tools by cutting results at designed surface is shown. This paper mainly carries out the following several aspects in research and achievements are established correspondingly.(1) The formation mechanism of contour error on a workpiece in five-axis CNC machining is shown firstly. In this paper the main sources of error in machining with five-axis CNC are analyzed, the formation mechanism of the motion error is discussed and the specimen’s contour error is constructed. In particular, the introduction of the rotation axis in five-axis machine tool leads to that the trajectory of the tip becomes more complex. By the use of multi-body movement theory, homogeneous coordinate transformation theory, mapping relationship between the axis following errors and the contour errors of the complex-shaped surfaces has been deduced for the five-axis CNC machining.(2) Servo characteristics are the main factors affecting the uniaxial following error. According to structural parameters of the translation axis and rotate axis including servo driver and mechanical mechanism, two forms of kinetic models of servo system are established. In this paper, servo performance indicators of CNC machine tool affecting each axis tracking error are investigated, especially the rotation axis servo characteristics are more sensitive. A circular trajectory is schemed as usually used in working accuracy test for investigating the multi-axis motion performance. The two cases of circular trajectories generated by translational and rotational axis synchronization motion, or two rotary axes synchronization motion are analyzed. Results on the contour error from the axis servo system parameters including stiffness, gain, servo time constants, backlash etc and mismatch between the parameters are analyzed.(3) Every axis error influenced by the dynamic indicators of the CNC control system is investigated. Tracking error of each axis has closely related to the axis moving instructions. The instrcution speed will directly give the tracking feed-rate for every axis but the axis moving instructions accepted by servo system will be adjusted by the interpolation algorithm, acceleration and deceleration control mode and parameters, and look-ahead algorithm. Therefore, the axes instruction is greatly different from G code instructions. It is unsuitable to simply decompose into each coordinate axis command by used the specimen formation mechanism. This paper employs feed speed programming model composed of a dynamic programming and multi axes feed rate calculation method. It is used to calculate every axis instruction in the case of high-speed high-precision machining of complex-shaped surface, which involves a huge of tiny segments, many turning points. A complete contour error model of the multi-axis CNC machine control system kinematic characteristics is constructed to analyze affecting weights of control system acceleration, jerk, maximum speed and other parameters on the axis feed speed and trajectory control parameters.(4) In this paper the two specimens including the standard NAS specimen and the "S" Specimen are introduced for work accuracy test simulation and cutting experiment. The harm of CNC machine dynamic indicators on its machining accuracy is researched. Based on CNC machine simulation of cutting specimens NAS, dynamic performance indicators shown on specimen processing are categorized. The process of "S"-shaped specimen side milling is taken as the research object in this paper, the surface properties of the specimen is shown geometrically, and the machine tools G code characteristics are listed. Meanwhile, surface milling speed variety related to CNC machine acceleration mode and other control parameters has been exhibished. The formation of tracking error is obtained by servo feed system model, and an abnormal milling mark is constructed simulatedly. Through simulation and experiments, the dynamic index action law on "S"-shaped specimen flank milling abnormal marks is suggested. It provides a clue for specimen structure design on the CNC machine tool working accuracy test, and the dynamic performance evaluation by using the specimen cutting.The issue comes from the state 04 major science and technology projects "The demonstration application of the domestic high grade CNC machine tools in typical aircraft structural parts machining ", item number is 2010ZX04015-011. The research results are partly used for verifying machining performance, guiding machine maintenance and optimizing the processing scheme. Meanwhile, the research results can provide a theoretical basis for the development of five-axis CNC machining dynamic accuracy test specifications, the development of high-speed high-precision five-axis CNC machine tools and its dynamic performance analysis. |
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