1、Development of a multi-step measuring method for motion accuracy of NC machine tools based on cross grid encoderAbstractMachining accuracy is directly influenced by the quasi-static errors of a machine tool. Since machine errors have a direct effect on both the surface finish and geometric shape of
2、the finished work piece, it is imperative to measure the machine errors and to compensate for them. A revised geometric synthetic error modeling, measurement and identification method of 3-axis machine tool by using a cross grid encoder is proposed in this paper. Firstly a revised synthetic error mo
3、del of 21 geometric error components of the 3-axis NC machine tools is developed. Also the mapping relationship between the error component and radial motion error of round work piece manufactured on the NC machine tools are deduced. Aiming to overcome the solution singularity shortcoming of traditi
4、onal error component identification method, a new multi-step identification method of error component by using the cross grid encoder measurement technology is proposed based on the kinematic error model of NC machine tool. Finally the experimental validation of the above modeling and identification
5、 method is carried out in the 3-axis CNC vertical machining center. The entire error components have been successfully measured by the above method. The whole measuring time of 21 error components is cut down to 12 h because of easy installation, adjustment, operation and the characteristics of non-
6、 contact measurement. It usually takes days of machine down time and needs an experienced operator when using other measuring methods. Result shows that the modeling and the multi-step identification methods are very suitable for on machine measurement.Keywords: Cross Grid Encoder； Geometric motion
7、error； Modeling Sequential identification； CNC Machine tool1 IntroductionNC machine tools are one of the most important components in modern manufacturing facilities and high-performance machines are required. Machine-tool motion accuracy has a significant influence on the quality of the machining o
8、perations and, therefore, the development of a current measurement and evaluation methods has become a significant research subject. Generally, there are three major types of errors, which are known as geometric, thermal and cutting-force induced errors. However, geometric errors make up the major p
9、art of the inaccuracy of a machine tool. Thus modeling and identification of geometric errors are the key steps in error measurement and compensation for machine tools. The style of error model and accuracy of error identification greatly affect the precision of error measurement and compensation.Fo
10、r NC machine tools, the radial error of circular motion is the comprehensive function result of all the error components of link: worktable, sliding table and main spindle block. How to identify each single error component from the radial error is the key to find the error sources. Therefore a great
11、 amount of work, over the last decade, has gone into this area. Studies on machine accuracy tests have been carried out by many authors.Week 1 developed a method using a laser beam and a four quadrant photodiode to measure the radial error-motion of a rotating table of a gear hobbing machine, and th
12、e parallelism between the rotating axis and a linear guide-way. Thus, Zhang et al. 2 developed a displacement method to assess the 21 error components based on laser interferometer. Charles Wang 3 developed this measuring method using laser interferometer, named Laser Doppler Displacement Meter (LDD
13、M), and invented a new error identification methodstepped volumetric diagonal measuring method based on this measurement system. A hybrid on-line and off-line method for identifying machine geometric error components was presented by Ni and Wu 4. The method utilizes a multi-degree-of-freedom optical
14、 laser system, to simultaneously measure multiple geometric errors. I was awa et al. developed a circular motion measurement device that employs an optical fiber-type laser displacement interferometer 5. The capability of the measurement instrument was demonstrated for not only circular motion but a
15、lso positioning accuracy measurement in a two-dimensional plane.Chen et al. 6 and Florussen 7 gave a displacement measurement approach for machine geometric error assessment in 2001 respectively. Lee and Yang 8 measured the geometric errors in a miniaturized machine tool using capacitance sensors in
16、 2005.The double ball bar (DBB) method 9 and 10 is widely used for dynamic circle path measurement of NC machine tools. The DBB method seems to be an excellent measurement method, because it is simple and versatile, and has been adopted as circular test method in standards and directives for inspecting machine tools, such as ISO 230-4 ,VDI/DGQ 3441 and GB/T 17421-4. Furthermore, multi-axis NC machine tools have recently come into wide use. Therefore, accur
