Research On Measurement, Identification And Modeling Of Geometric And Thermal Errors Of Five-axis CNC Machine Tools

The machining accuracy of the CNC machine tools is one of the most important indicators for the performance evaluation of the machine tools. Therefore, it is of great significance to research on how to conveniently, rapidly, and accurately calculate the volumetric errors of machine tools for error compensation. This dissertation is supported by the National Natural Science Foundation of China (No.51175461) and National S&T Major Project of China (No.2009ZX04014-026). According the research missions of the projects and current research situations, the measurement and modeling technique of the volumetric errors of a five-axis machine tool was further researched in the dissertation.The innovation of the dissertation is that on the basis of the existing theory and achievements, a double optical path measurement method for the volumetric error of a five-axis machine tool, a ball-bar combined path measurement method for the volumetric errors of the rotational axis of five-axis machine tools and a ball-bar measurement of the thermal errors for the spindles of a five-axis machine tool were presented. Moreover, the mathematical expression modeling of volumetric errors based on NURBS curve and surface was researched. Finally, the error compensation experiments of a five-machine tool were conducted combined the volumetric error modeling of the machine tools and the measurement results (error parameters). The main contents and research results can be concluded as follows:Firstly, the local coordinate system and the definitions of the volumetric errors parameters of machine tools were introduced, and volumetric error modeling of CNC machine tools based on multi-body system theory was established, and then the volumetric error modeling of an RRTTT-type five-axis machine tool was deduced. In addition, the differential operator was used to separate the homogenous transformation matrix of the volumetric errors, which was greatly simplified the volumetric error modeling.A Double Optical Path Measurement Method (DOPMM) was proposed to measure the volumetric errors of the five-axis machine tools, including the volumetric error measurement of the translational axis and the rotational axis. And then the experiment of DOPMM was conducted on a five-axis machining center. It was found that it can not only identify the error parameters of translational axis without the solution of the linear equations, but also identify all the6error parameters of the rotational axis by simple algebraic operation without volumetric error modeling.A Combined Path Measurement Method (CPMM) based on ball bar was presented to measure the volumetric errors of a rotational axis of machine tools. The identified process of the volumetric error parameters were deduced, including the axial run-out error, radial run-out errors, angular position error, yaw error and pitch error. And the error parameters of the A-axis and C-axis of a five-axis machining center with a titling rotary table was measured and identified. The results showed that identified values by the CPMM were consistent with that by DOPMM. Therefore, our CPMM is sensible and reliable.A Ball-bar Measurement of Spindle Thermal Errors (BMSTE) was presented for a five-axis machine tool. With the help of space path measurement function of ball bar, two measurement paths, formed by the two rotational axis moving respectively, can be measured with just one installation, and least square method was used to process the measurement data to identify the spindle thermal errors, including one axial thermal elongation and two thermal drifts. The comparison experiment between our BMSTE and the five-point method mentioned in ISO230-3was conducted. The results show that our BMSTE is feasible and effective.A Mathematical Expression Method of Volumetric Errors (MEMVE) based on NURBS was proposed. Lease square method was used to calculate the control points of the NURBS curve and surface, and the impactions of NURBS parameters on the precision of the curve and surface fitting were analyzed, such as parameterization methods, number of control points, knot vectors and weights. Therefore, an optimal method based on genetic algorithm (GA) was presented to optimize the knot vectors and weights for mathematical expression modeling of the volumetric errors. The corresponding volumetric errors of any position point among the workspace of the machine tools can be fast obtained by searching the expression modeling based on binary search method. Finally, the specific applications of the curve and surface modeling of volumetric errors based on NURBS were introduced for the five axis machine tools.Combined the volumetric error modeling of machine tools, the application of the error measurement results and the curve and surface mathematical models of volumetric errors on the error compensation was researched. Brought in the compensation strategy of Jacobian matrix, simplified modeling of volumetric error based on the differential operators and the Jacobian matrix of differential kinematics were combined to calculate the compensation values of each motional axis and correct the actually executed values of the machine tools for the error compensation. The feasibility of the compensation strategy was verified by the simulation and experiment. And the machining experiments of a five-axis machining center were conducted. The results show that the identification results of error parameters are effective and the mathematical expression modeling has the advantage of the universality and efficiency.