Research On The Influence Of The Stiffness And Its Degeneration Of The Key Joint Surfaces Of High Precision Machine Tools On The Accuracy And Its Retention Of Machine Tools

With the advancement of technology,the precision requirements of CNC machine tools are getting higher and higher.Only considering the influence of geometric errors on CNC machine tools can not meet the accuracy analysis requirements of high-precision machine tools.The force-induced deformation error and thermal deformation error in the machine tool have gradually become an important part of the accuracy analysis of high-precision machine tools.As the foundation of equipment manufacturing,CNC machine tools contain multiple functional components.The joint characteristics between components,that is,the contact characteristics of the joint surface are very complicated and usually have nonlinear stiffness characteristics.And the stiffness of the key joint in the high-precision machine tool has an important influence on the accuracy and accuracy retention of the machine tools.Therefore,the research on the stiffness modeling and degradation characteristics of the key joint surface of high-precision machine tools is of great theoretical value and practical significance for the comprehensive accuracy modeling and analysis of machine tools.The purpose of this paper is to establish a precision model of the high-precision machine tools with comprehensive geometrical errors and joint force-induced deformation errors.The contact characteristics of the key joint surfaces,which are the fixed joint surface and the rolling joint surface,are mainly researched.The corresponding stiffness models of the joint surfaces are established and applied to precision modeling and analysis of the CNC machine tools.Moreover,the stiffness decay of the fixed joint surface under creep is studied.The stiffness decay comprehensive model of the fixed joint surface is established.Then,the influence of the creep of the fixed joint surface on the precision retention of the machine tool is analyzed with the established model.The following innovative research has been carried out in the course of this project:(1)The contact characteristics of the key rolling joint surfaces of high-precision machine tools are researched and applied to modular precision modeling of high-precision machine tools considering the comprehensive deformation of all rolling joint surfaces.The motion systems in the machine tools are divided into two categories: linear feed systems and spindle rotary systems.The rolling joints of the two modules are studied separately.Based on the coordinate transformation method and Hertz theory,the contact characteristics of different types of rolling joints are analyzed.The nonlinearity of each rolling joint surface and the coupling between different rolling joints are also studied.Finally,the modular precision model of CNC machine tools is established,and the accuracy and efficiency of precision analysis of CNC machine tools can be improved.(2)An efficient and accurate contact modeling method of rough joint surface based on macroscopic deformation instead of microscopic interaction is proposed.Based on the traditional GW model,the influence of the interaction between the asperities on the contact stiffness of the rough joint surface is studied with Green function and the exhaustive method.The distribution law of the deformations of asperities considering the asperity interaction is also researched.Then,the comparison between the results of the contact stiffness of the rough joint surface considering the macroscopic deformation and the microscopic interaction respectively are carried out.According to the comparison,an efficient and accurate contact modeling method of rough joint surface based on macroscopic deformation instead of microscopic interaction is proposed.The method can both guarantee the analysis accuracy and greatly reduce the calculation amount in the analysis process.Therefore,the analysis efficiency can be effectively improved,which is of great significance for studying the contact characteristics of the rough joint surface.(3)The change characteristics of the joint stiffness caused by the creep of the asperities on the rough joint surface are studied from the micro point of view and the stiffness degradation model of the joint surface is established.In this paper,the creep model of the asperities on the rough joint surface is established based on the Creep Mechanics and the geometric constraint relationship between the asperities.In order to solve the problem that the stress of each asperity changes with time,the discrete analysis method is used.The correctness of the discrete analysis method is indirectly verified by studying the influence of different unit time step on the calculation results.In this paper,the model is used to analyze the stiffness degradation of bolted joint considering the creep of joint surface and to analyze the dynamic and static accuracy retention of high-precision machine tools.The main contents of this paper are as follows:(1)The geometric accuracy modeling method of high precision CNC machine tools is studied.On the basis of sorting out and studying the existing geometric error modeling methods,this paper focuses on the multi-body system error analysis method.Moreover,the method is applied to the geometric accuracy modeling of five axis CNC machine tools.Then,the influence of the geometric error on the accuracy of high-precision machine tools.(2)The rolling joints in the key motion system of CNC machine tools are taken as the research object to analyze the contact characteristics of each rolling joint,and the comprehensive model of deformation errors of the rolling joints in each key motion system is established.Based on Hertz theory and coordinate change method,the stiffness modeling and research of the key rolling joints in CNC machine tools are carried out.Moreover,the coupling characteristics between different rolling joint surfaces are considered.Therefore,the modular stiffness model of joint surfaces is established.The proposed modeling method is verified by the finite element method.(3)The stiffness model of bolt joint considering the interaction of asperities is established.On the basis of the traditional GW contact model,the distribution law of the deformation of the asperities after considering the interaction is studied by exhaustive method.It is found that the deformation of the asperities still approximately satisfies the normal distribution.Based on the data analysis and the research of elastic theory,a new micro contact model of rough interface is proposed.The proposed model can calculate and analyze the stiffness of rough interface efficiently and accurately.Based on the proposed model,the stiffness model of bolt joint considering the interaction of asperities is obtained.(4)An accuracy model of high-precision machine tools with comprehensive geometrical errors and joint force-induced deformation errors is established and is applied to research the influence of the stiffness of the key joint surface on the accuracy of the high-precision machine tools.The stiffness models of the joint surfaces established in this paper are synthetically modeled.Meanwhile,the influence of geometric errors on the accuracy of machine tool is also considered.Therefore,a comprehensively accuracy model of high-precision machine tools is established,which integrates geometric errors and joint surface deformation errors.Moreover,the influence of the parameters of joint surfaces on the accuracy of CNC machine tool is studied.In addition,a quasi-static equivalent model of the milling force is also proposed in this paper,in order to connect the accuracy model of the machine tools established in this paper with the dynamic load under the actual working condition.The milling test of typical machine tools is carried out to achieve the force data.Then,the test data is analyzed.Therefore,the equivalent of the dynamic and static load is realized according to the data analysis and the equivalent criterion for equal mean deflection of the point of action(ie the tip)under dynamic and static loads.(5)The influence of creep on the stiffness of the fixed joint surface is studied and the stiffness model of the fixed joint surface is established.Based on the aging theory,the creep of a single asperity is studied.Then,the creep model of the rough surface is established based on the geometric constraint relationship between the asperities on the two rough surfaces.The creep model of the rough surface is solved and analyzed by the discrete analysis method in order to study the influence of creep on the stiffness of the fixed joint surface.Finally,a stiffness decay model of the bolt joint is established combined with the creep of the bolt and the connectors.(6)The influence of the stiffness degradation of the fixed joint on the accuracy retention of the machine tool is studied from the dynamic and static aspects,respectively.The deformation of the fixed joint caused by creep under the preload is studied to analyze the change of the machine tool accuracy caused by the creep of the joint.Therefore,the influence of the stiffness degradation of the fixed joint on the accuracy retention of the machine tool is studied from static aspect.Moreover,the guidance scheme of the bolt preload sequence is proposed.In addition,the analysis of the stiffness degradation characteristics of the bolted joint caused by creep is achieved with the proposed model.Then,the influence of the joint stiffness degradation on the dynamic performance of the machine tool is studied.The results show that the joint stiffness degradation has little effect on the dynamic performance of the machine tool.