| Five-axis CNC machine tools provide greater productivity,better flexibility,and less fixture time than three-axis machining centers,this is because the cutting tool can approach the workpiece from any direction.However,the two rotary axes bring in additional geometric errors,such as squareness and parallelism between a rotary axis and a translational axis,which leads to larger volumetric errors than three-axis machine tools.This research focuses on the volumetric accuracy improvements of five-axis machine tools in the means of error compensation.First,the error components of translational and rotary axes are measured and identified,then the volumetric error model is modeled to calculate the position and vecter errors along toolpaths,finally the volumetric errors are compensated by calculating new NC codes for the toolpaths.With the support of National Science Foundation Projects,Chinese National Science and Technology Key Special Projects,National Key Scientific Instrument Development Project and China Scholarship Council,this research conducted lots of tests on a Quaser UX600 rotary-table five-axis machine from the laboratory of Prof.Yusuf Altintas in the University of British Columbia.In addition,another three machine tools also play crucial roles during the research.These machines are one five-axis machine tool from Shenyang Machine Tool Plant.In this research,on one hand an error identification method for position-independent geometric errors(PIGEs)and position-dependent geometric errors(PDGEs)for rotary axes was proposed,on the other hand volumetric error modeling,compensation models and a virtual CNC simulation platform based on the Screw theory were developed.The main contents of this thesis are as follows:(1)This theis did research on the error measurement and identification of rotary axes and three double ball bar(DBB)based measuring methods are proposed for measuring PIGEs separately,measuring PDGEs separately and measuring PIGEs and PDGEs simultaneously.Measuring PIGEs separately: Three measuring patterns are proposed,in which the translational axes are kept stationary and only two rotary axes move to obtain a circular trajectory.In this way,the effects of translational axes are totally excluded,and the measurement accuracy is improved.Motion equations,describing how the A-axis and C-axis move simultaneously to realize a circular trajectory,are presented.Measuring PDGEs separately: For each rotary axis,five measuring patterns are designed,in which the ball bar is sensitive to one direction throughout the measuring process.In the measurement,the ball bar functions as a high-precision displacement sensor with a single degree of freedom.Based on the ball bar readings,analytical solutions for 10 PDGEs,except for two angular position errors,are simply deduced.Measuring PIGEs and PDGEs simultaneously: ballbar tests in tangential,radial and axial directions were conducted.Eight PIGEs and 12 PDGEs were simultaneously identified and the analytical solutions were obtained.Finally,experimental verification of this approach is conducted on a five-axis machine tool.The results confirm that the method provides precision results of PIGEs and PDGEs for rotary axes.(2)This thesis did research on the volumetric error of five-axis machine tools,the volumetric error model was proposed based on Screw Theory and a generalized volumetric error model for five-axis machine tools were developed.Screw Theory can effectively model the forward and inverse kinematics model of five-axis machine tools.Besides,the concept of error twist was introduced and all the error components were modeled.It was verified that the volumetric error model obtained from Screw Theory was exactly the same with the one from the traditional homogeneous transformation.A detailed generalized kinematic model for three-and five-axis machine tools is built.It can be applied to 4 types of three-axis machine tools and 12 types of five-axis machine tools.By simply inputting the translational and rotary type numbers of the specific machine tool,the final kinematic model can be obtained without further human interventions.In addition,numerical modeling methods for error components,such as positioning error and straightness error,were developed.(3)This thesis did research on compensation methods for volumetric errors of five-axis machine tools.A iterative compensation strategy was proposed based on the inverse kinematics calculation via Screw Theory.A software was developed based on Virtual CNC to simulate the volumetric error compensation on five-axis machine tools.In this software,after the machine configuration is selected and all the numerical model of error components were input,the volumetric error of the corresponding machine along the toolpath can be predicted as well as calculating the compensation values.This simulation platform can be fit for five-axis machine tools with any configuration.The results from this simulation software verified the proposed compensation method is correct and effective.(4)Experimental tests were conducted to verify the volemetric error measuring method,modeling theory and compensation strategy.Many machine tools were used for experimental verifications.The main tests were conducted on the Quaser UX600 five-axis machine.The experimental tests indicated that the error measuring,modeling and compensation method for five-axis machin tools were effective and it can dramatically improve the accuracy of machine tools. |
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