| Nowadays,ultra precision machining technology has been applied not only in the field of sophisticated national defence and aerospace,but also in many aspects of national economy,and its application is rapidly expanding.Ultra precision machine tool is necessary technology to promote the development of ultra precision machining technology,assembly errors of ultra precision machine tool and manufacturing errors of key parts will cause spatial geometric errors,and will further force the movement axes deviated from the ideal position.Which plays a significant role in affecting the comprehensive performance and machining accuracy of ultra precision machine tool,and ultimately leads to the reduction of machining quality.The machining accuracy of machined workpiece is an important index to indirectly evaluate the comprehensive performance of ultra precision machine tool,the higher requirements of machining accuracy of workpiece are put forward in modern manufacturing industry,which provides a great challenge for the comprehensive performance and machining accuracy of ultra precision machine tool.At present,the machining accuracy of the three-axis ultra precision turning machine tool is slightly low in China,which has much room for improvement.How to rapidly improve the machining accuracy of three-axis ultra precision turning machine tool becomes an urgent problem to be solved.Therefore,error compensation method of machining accuracy of three-axis ultra precision turning machine tool was well studied in this dissertation,including the geometric error model,parameterization of error model,sensitivity analysis of geometric error and on-machine error compensation method.Firstly,the low-order body array for each axis of the machine tool was obtained based on multi-body system,and the topological relation of the geometric structure of the three-axis ultra precision turning machine tool was described by the low-order body array.The geometric errors of linear slideways and spindle were discussed,the geometric error model of general axis was established by homogeneous transformation matrix method,and the volumetric error model of three-axis ultra precision turning machine tool was obtained.According to the most practical measurement and evaluation method mentioned in the national standards(GB/T 17421.1,GB/T 17421.2,GB/T 7235)and international standards(ISO 230-1,ISO 230-2,ASME B89.3.4)for geometric errors,combined with the actual measurement equipments owned by the laboratory,the practical and commonly used measurement methods were adopt to measure all 21 geometric errors of three-axis ultra precision turning machine tool,which calibrates the vuales of geometric errors of three-axis ultra precision turning machine tool,and finally makes the volumetric error model parametric.Secondly,in order to quantify the effect of each geometric error on the machining accuracy of three-axis ultra precision turning machine tool and find out which of the numerous geometric error in a three-axis ultra precision turning machine tool has the greatest influence on the machining inaccuracy,a variance-based Sobol global sensitivity analysis method was used to conduct sensitivity analysis of the geometric errors of three-axis ultra precision turning machine tool.Monte Carlo sampling method was used for the geometric errors to obtain the sampling matrices,Sobol method was employed to investigate the sensitivity index of geometric errors with respect to the volumetric error model and its error components along X,Y,Z direction,the crucial geometric errors that have a greater effect on the machining accuracy were identified according to the sensitivity analysis results.In addition,global sensitivity analysis was conducted to the machining accuracy of some typical workpiece profile to find out the crucial geometric errors affecting the accuracy.Sensitivity analysis of geometric error provided a criterion for evaluating whether the geometric error was crucial for the machining accuracy,and laid a theoretical foundation for error compensation,error adjustment as well as error distribution.Thirdly,an on-machine measurement and error compensation system based on chromatic confocal probe was developed to overcome the disadvantage of introducing system error by offline compensation method,and the hardware structure,software design and its integration with the control system of the three-axis ultra precision turning machine tool were discussed.A chromatic confocal probe was serving as the measure execution unit,and a high precision stage form PI(Physik Instrumente)was used to adjust the position of measurement probe along the vertical direction.The calibration method of spatial relative position based on dual-standard spheres was proposed to ensure the position accuracy of the on-machine measurement(OMM)system relative to the center line of spindle in the process of repeated installations,OMM system held the capacity to achieve non-contact two-dimensional measurement and three-dimensional reconstruction of workpiece.Comparing the results obtained by OMM method with that of a typical high precision measuring equipment,the effectiveness and measurement accuracy of the OMM method were verifird.Finally,some identified crucial geometric errors were compensated according to the sensitivity analysis result of geometric errors,and the effectiveness of compensation method was verified by experiment.An on-machine error compensation method for the form error of workpiece was further studied,the influecnces of tool radius and tool decentring on machining accuracy were discussed in detail,and the corresponding compensation methods with respect to tool radius and tool decentring were put forward.The development and integration of on-machine compensation software was carried into the numerical control system of three-axis ultra precision turning machine tool,the regular and repeatable form error caused by various factors was compensated by modifying the program code in subsequent interval and multi-axis synchronous machining according to the OMM result of form error.A series of on-machine error compensation experiment were conducted with respect to flat,convex spherical,sinusoidal wave structure as well as the concave spherical surface with tool de-centring,the experimental results showed that the on-machine error compensation method can further improve the form error of above workpieces,the form errors of flat and spherical surface were greatly improved after twice on-machine error compensation,and the compensation efficiency achieved more than 60% with respect to the initial form error,the form error of acceptance standard workpiece for three-axis ultra precision turning machine tool can sufficiently meet the acceptance requirement after compensation.After integrating the on-machine error compensation system,three-axis ultra precision turning machine tool can achieve the integration function of precision machining,on-machine measurement and on-machine error compensation. |
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