Investigation Into Accuracy Design And Error Compensation Of High-precision Horizontal Machining Centers

This dissertation deals with the key issues relevant to error modeling, accuracy distribution, error identification and 3D error compensation of precision horizontal machine tools, and its application to the design, measurement and compensation of a "box-in-box" type 4-axis horizontal CNC machining center. The following contributions have been made.Geometric error modeling. A general approach for geometric error modeling of CNC machine tools is established based on screw theory and multi-body system theory. The proposed error model allows the geometric source errors affecting the compensatable and uncompensatable pose accuracy to be separated in an explicit manner, providing machine designers with an informative guideline to taking proper measures for enhancing the pose accuracy via accuracy distribution and error compensation.Accuracy analysis and distribution. Utilizing interval analysis method, the sensitivity analysis index is proposed to describe the influence of the geometric source errors on the pose accuracy of the machine tool quantitatively. The relationship between the assembly errors of guide rails and the motion errors of carriage is established based on error averaging method and Monte Carlo simulation. For accuracy distribution, an interval optimization model is introduced with the pose accuracy requirements as the constraint, and the assembly errors of guide rails as the design variables, which aims to obtain the maximum feasible interval of the design variables. The proposed methodology has been used in the accuracy design of a precision horizontal machine tool.Error measurement and identification. A rapid geometric error identification method with good robustness is presented using double ball bar. The polynomial model is used to represent geometric errors, and the linear mapping model between geometric errors and DBB length errors is established. Meanwhile, a measurement scheme which needs only one mounting of the double ball bar is applied to perform a large space measurement, and ridge regression method is adopted to identify all of the geometric errors. The results of simulation and experiment verify feasibility and effectivity of the proposed identification methods.Error compensation. 3D error compensation strategy is introduced. The machine tool workspace is divided into many rectangular prisms, and vertices of the prisms are taken as the observation points, whose positional errors are obtained using laser interferometer. For any given position, the compensated values can be calculated based on the measured position errors using the interpolated algorithm. The proposed method has been successfully applied to a precision machine tool. With 3D compensation, the roundness error of DBB can get reduction of 34%, and the contour error of the test piece can get reduction of 31%.