| With the development of aviation industry,automobile industry,manufacturing,shipbuilding and marine engineering,the CNC machine tools are developing in the direction of precision,high speed and complex direction.In order to exert the advantages of high speed machining of CNC machine tools,high speed feed system must be designed.The positioning accuracy of micron level must be guaranteed.Double screw driven system was an efficient driving method,which has a better dynamic performance than the traditional drive,so it has been widely used.Dual drive feed table that in the case of large acceleration in the moving can easily excited vibration.It would produce a lot of heat,to make the feed system producing deformation,which affectd the positioning accuracy of the feed system.The current methods were usually based on the mechanical and thermal properties of the dual drive feed system for analysis and design,respectively.In this paper,the dual drive feed system as the research object,the analysis of the dynamic model,the vibration model and thermodynamic model based on thermal-mechanical coupling effect,which provided theoretical basis and effective guidance of multi-objective optimization design of the dual drive feed system.In order to study the dynamic characteristics of the dual screw driven feed system,a hybrid modeling method was proposed.The multi DOF model of dual drive feed system was established,which combined the influence of the screw nut pair,the support bearing and the linear rolling guide rail.Based on the Lagrange equation,the stiffness,mass and displacement formula of double drive feed system were obtained,and the inherent frequency was obtained by simulation.The influence of axial stiffness and the quality and position of the working table was analyzed,and the validity of the dynamic model was proved by experiments.Due to the source of heat and vibration,the boundary conditions were difficult to determine.The temperature field distribution and thermal equilibrium time,thermal modal,were analyzed by finite element method combined with testing.By the analysis of the temperature field,the temperature of each measuring point on the lead screw was changed to a nonlinear change with the increased of the running time and the feeding rate.The time of the lead screw to heat balance increases with the increase of the feed rate,and the temperature of the heat balance becomes larger.On the basis of these,a test method for measuring the thermal errors of the feed system was designed.According to the test datas of 5 thermal sensitive points,the mapping function of thermal error on time,coordinate position and temperature were constructed,and the integrated prediction model of thermal errors were established.In view of the situation that the feed system was affected by the force and heat,the axial dynamic parameter test model of binding surface of ball screw pair with the thermal mechanical coupling effect was established.The axial dynamic characteristic parameters of ball screw were influenced by the axial load and preload.Due to the temperature rise of the screw nut,the pre-tightening force of the lead screw was reduced.The axial dynamic stiffness of the lead screw was reduced by the increase of the axial load,which caused by higher temperature.On this basis,in order to study the internal mechanism of thermal contact coupling effect and the dynamic performance of the feed system,numerical analysis method for simulation of thermal mechanical coupling was discovered.It was concluded that the natural frequency of the feed system becomes smaller,the amplitude increases,especially the first order natural frequency was reduced by 7.14%and the amplitude was increased by 25%.Considered the effect of thermal-mechanical coupling,the multi-objective optimization model was established for the dual drive feed system,and the structure parameters of the feed system were optimized.The multi-objective optimization model is established with the objective function of the light mass of the working table,the optimal coupling mechanics performance and the first order natural frequency of the feed system.Then,a multi-objective model was built.Design variables were selected by sensitivity analysis to reduce the difficulty of building the objective function.Based on the finite element method,the basic data of the two order response surface model was obtained by the orthogonal design,and the multi objective optimization design objective function was established.The Pareto optimal solution set was obtained by using the NSGA-II algorithm.According to the design principle of the optimal coupling and the optimal mechanical properties of lightweight,dynamic parameters to select the size of the working table,the first order natural frequency of the optimized working table was increased by 9.28%,the vibration amplitude was reduced by 25%,and the mass was reduced by 0.3%.The problems of mass and dynamic static and dynamic performance were solved by NSGA-II algorithm,which provided a new idea and method for the design of the machine tool's feed system. |
PDF Full Text Request