Comprehencive Compensation And Application For Multi-error Elements On Cnc Machine

This paper is sponsored by the important National Science&Technology Specific Projects of“Top Grade CNC Machine Tools and Basic Manufacturing Equipment”,“The National NaturalScience Funds of China” and “Specialized Research Fund for the Doctoral Program of HigherEducation”. This paper focuses on the multi-error elements which influent the machiningaccuracy of NC machine tools. The multi-error detection, the multi-error modeling, and themulti-error compensation were studied in this research.The main contents of this paper are as follows:⑴The multi-error elements analysis. To study the multi-error elements and its effect on themachining accuracy, various error elements (including thermal error, cutting force-induced error,tool wear-induced error, and original geometric error) were analyzed firstly in this paper; and amulti-error elements table was established based on the analysis of the multi-error elements.Using the multi-error elements table, the error elements of CNC machine tool can be quicklyidentified.⑵Establishing the comprehensive mathematical model and unified mathematical model forfour types NC machining center. According to the multi-error elements analysis and themulti-error elements table, the comprehensive mathematical model was established based on thehomogeneous coordinate transformation theory for four types of NC machining center. And theunified mathematical model for four types NC machining center was further established based onthe singular function. The unified mathematical model provides a theoretical basis for themulti-error elements detection and error modeling of CNC machine tool.⑶A new modeling method was put forward based on the orthogonal polynomial for thegeometric error of NC machine tool. The orthogonal polynomial-based modeling method canavoid the prediction model form errors, and all the modeling process can be implemented in theorthogonal polynomial calculation table. To analyze the error distribution, the MATLAB wasused in this study. According to theoretical analysis, the volume error of machine tool is relatedto the position of X, Y and Z-axis. That is to say, even if only X-axis moving, the geometric erroralso each are not identical when the Y or Z-axis at the different position. So, the comprehensivemathematical model must be established even if only single-axis compensating.⑷The temperature field and thermal deformation were analyzed based on the ANSYS, theanalysis results showed that, the thermal deformation of machine tools lags behind the machinetemperature, the lag time can be calculated by the Fourier number of non steady heat conduction.The lag time can be quantitatively analyzed by calculating the diffusion time of thermal perturbation. This conclusion can effectively predict the time to achieve the maximum thermaldeformation of machine tools after shutting down the machine. An error modeling method wasproposed based on the orthogonal polynomials and the least squares method for thermal errorand spindle thermal drift error. The advantage of this modeling method is to make full use ofgeometric error modeling results, thermal error model under different temperature can beobtained through rotating the geometric error model, the efficiency and precision of modeling isgreatly improved.⑸For the cutting force-induced error, the modal and harmonic response analysis wasimplemented based on the ANSYS. The analysis results showed that, the high order vibrationtype plays a decisive role in dynamic characteristics of machine tools, the frequency of therotating parts must far away from any of the each-order natural frequency. To avoid the naturalvibration of the machine tool, the spindle speed should be limited. A cutting force-induced errormodeling method was proposed based on the motor current detection. This error modelingmethod can make full use of the real-time motor current function of CNC system.⑹To realize the real-time error compensation, an error compensation system was designedbased on the external coordinates offset function and the single-chip microcomputer. Thereal-time error compensation system can exchange data with CNC through the PMC and canrealize the error compensation using the external coordinate offset function of CNC. An automodeling software was developed based the orthogonal polynomials and the least square torealize the real-time the geometric error and thermal error modeling. Using the error modelingsoftware, the modeling efficiency was greatly improved. A virtual error compensation systemwas designed based on the VB and the BP neural network to realize the error compensationunder the different cutting conditions and working conditions. A clustering error compensationmethod was proposed based on the Newton interpolation. Using the clustering errorcompensation method can realize the error compensation for a group of workpiece.⑺The CNC machining center, the CNC lathe and the heavy Longmen grining machine wereused to implement the error compensation experiment to verification the effect of the real-timeerror compensation system. Application of the real-time error compensation system, the accuracyof the machine tool was greatly improved. A group of shafts were used to verification thecompensation effect of the virtual error compensation system and the clustering errorcompensation strategy. Using the compensation method, the machining accuracy of these shaftswas greatly improved, and this error compensation method can be used in low cost CNCmachine tools to achieve better machining accuracy.