Discussion on Testing Method of Working Accuracy of Virtual Axis CNC Machine Tools

The traditional numerical control machine is designed according to the Cartesian coordinate system, and is a cantilever structure that is connected in series and cascaded and nested. The error between the drive chains is cumulative. Virtual axis CNC machine tools (also called parallel machine tools, six-legged machine tools, virtual axis machine tools or new agile manufacturing and machining centers) are new members of the machine tool family. They have abandoned the traditional fixed rail tool guidance method and used six degrees of freedom in parallel. The mechanism (Stewart platform) is its structural body, so it is theoretically possible to machine arbitrarily complex surface parts (such as impellers, molds, etc.). In addition, the coupling of the movement effects of the telescoping poles of this new machine tool has the potential for high accuracy. Machine tool accuracy tests usually include geometric accuracy, position accuracy, and job accuracy tests. At present, no standard has been seen in ISO organization or a country for the accuracy testing of virtual axis CNC machine tools. This article refers to the content of the traditional numerical control machine tool precision test, and combines the characteristics of the virtual axis CNC machine tool, draws up its work accuracy inspection program. 1 Inspection of the Working Accuracy of Traditional CNC Machine Tools As regards the inspection of the working accuracy of traditional CNC machine tools, the United States NAS979 (National Aerospace Standards) has formulated the “Circular-Rhombus-Square” test piece cutting standard (referred to as the trial cutting method) standard, and similar in China. The standard. Table 1 is the method of testing the accuracy of CNC universal tool milling machine with test cutting method. From Table 1, we can see that the test cut method verifies the straightness of the machine tool along the X coordinate in four ways: the perpendicularity of the X, Y, and Z coordinates with each other: the straightness, roundness of the NC interpolation, and the hole in the XY plane. Position accuracy and other items. In addition, this method determines the corresponding size of the cutting test piece according to the stroke range of each coordinate of the CNC machine tool. This rule is due to the linear mapping between the error and stroke of each coordinate of the traditional machine tool. Therefore, it is reasonable to use a small-sized test piece to inspect the working accuracy of a large stroke. The test cutting method is used to test the working accuracy of the machine tool, which requires high test conditions for the accuracy of the test piece, long experimental period, and because most of the cutting motion is performed along the XY plane, the precision of the XZ and YZ planes is not evaluated, so that the traditional numerical control cannot be evaluated. All the properties of the machine. The latest international machine tool inspection standards recommend replacing the NAS 979 test method with the ball bar test method to perform rapid and effective inspection of the machine tool's working accuracy. 2 Development of Verification Test Plan for Virtual Axis CNC Machine Tools

Machine tool model diagram

The difference between a virtual axis CNC machine tool and a traditional CNC machine tool is that the machining error is a complex non-linear mapping of the length error of each telescopic rod and the center position error of the hinge. Therefore, an appropriate working accuracy inspection scheme needs to be developed. Although in the traditional CNC machine tool work accuracy test, the ball bar test method replaces the test cut method more, but because the error diagnosis software provided with the ball bar instrument can not be directly used for the virtual axis numerical control machine tool work precision test, so the test cut Law became the preferred method here. Theoretical Basis for Formulating a Virtual Machine The model of the virtual axis CNC machine tool is shown in the figure. The error expression is ∆Li=li·∆ai-li·∆oo1-li·(R'Bi)-li·(R∆Bi). Where Li is the rod length vector of each telescopic rod, its corresponding error is ∆Li; li=Li/Li: ∆ai is the static platform hinge point absolute error: ∆oo1 is the distance between origin and static coordinate system distance error: R For any moment, the attitude transformation matrix between the dynamic and the static coordinate system, when the attitude of the moving platform is unchanged, R=E (unit matrix), R' is the error corresponding to R: Bi is the relative coordinate of the hinge point of the moving platform, ∆Bi is its The corresponding error. From the above equation, we can see that due to the nonlinear mapping relationship between the cutter pose error and the extension rod length error of the virtual axis CNC machine tool, the accuracy of the small-size test specimen can not accurately reflect the working accuracy of the large-size working space, so we need to choose and the working space of the machine tool. Test specimens of similar size are cut to evaluate their working accuracy. Inspection of Working Accuracy of Virtual Axis CNC Machine Tools Fixed posture work accuracy test When the virtual axis CNC machine tool is making a pose motion, only three position variables in the variables describing the dynamic platform pose change with the tool path, and the attitude angle is always the same. Table 2 shows the inspection plan for the attitude accuracy of the virtual axis CNC machine tool in the working space Ø300mm×300mm. Variable attitude inspection accuracy test Virtual axis When the NC machine tool changes attitude, the attitude of the moving platform continuously changes with the tool path. This greatly changes the working space of the machine tool compared to the fixed attitude, and the larger the change in the attitude angle, the smaller the working space of the machine tool. The ensuing question is: how to determine the size and detection means of the test piece when the machine tool is changed to the attitude accuracy test using the test cut method. The document JB/T 17421.2-2000 "General Practices for Machine Tool Inspection - Part 2: Determination of CNC Axis Positioning Accuracy and Repeated Positioning Accuracy" particularly elaborates the negotiation points between suppliers/manufacturers and users on the accuracy testing of traditional numerical control machine tools. . The author suggests that according to its principle, the relevant parties should negotiate and formulate inspection clauses for changing the attitude of variable-axis CNC machine tools as part of the inspection of the accuracy of such machine tools. 3 Conclusions Work accuracy is an important measure of machine performance. Although there are strict standards for the accuracy of CNC machine tools through the test of cutting tests of specimens, the test cutting method applicable to the inspection of the working accuracy of virtual axis CNC machine tools is still in the exploratory stage. This paper analyzes the inspection accuracy standard of traditional CNC machine tools. Based on the virtual difference between the virtual axis CNC machine tools and its essential features, a new type of virtual axis CNC machine tool fixed attitude testing accuracy test plan is proposed. Suggestions for the testing of variable attitude work accuracy are also provided for relevant researchers. reference.