| With the rapid development of the equipment manufacturing industry,the positioning precision of CNC machine tools has attracted more and more attention of the researchers in the whole world.The ball screw feed drive system is a key subsystem of CNC machine tools,and its thermal characteristics determines largely the positioning precision of the worktable of machine tools.This dissertation explored the key common technology for improving the positioning accuracy of CNC machine tools,which was supported by the National Science and Technology Major Project of China(Grant No.2013ZX04011-011)and the Nation Science Fund(Grant No.51375081,51775094).Taking a domestic lathe as the research object,the thermal characteristics of ball screw feed drive systems was investigated thoroughly in the paper.Firstly,the experimental system was set up to measure temperature and themal errors of the ball screw feed drive system.Secondly,using the adaptive FEM integrated with the Monte Carlo method,the algrothm was proposed to indentify the flux of the multiple varying and moving heat sources and determine the distributions of the temperature and thermal error for the ball screw feed drive system.Then by virtue of the measued temperatures and the results of the FEM simulation,the model of the temperature difference relationship between the measured surface point and the contact surface of a kinecmatic pair was put forward to predict the contact surface temperature.Accoording to the experimental results of the 45° slant feed system during the descending process of the worktable,the phenomenon of the working face change for the screw-nut pair and its effect on the positioning error were discussed.Finally,numerical and analytical models were established to predict the thermal errors of the ball screw,and the thermal error compensation control tests were carried out to verify the accuracy of the numerical and analytical models.The remarks are stressed as follows.(1)The experimental system was set up to measure the distributions of the temperature and the thermal erros for a 45° slant no-preload and a horizontal preload ball screw feed systems,respectively.In the meantime,the torque current of servo motor(TCOSM)and the position of worktable were obtained from FANUC numeric control system by means of the FOCAS1 functions.The effects of the feed velocity and the preload on the thermal characteristics and the TCOSM of the ball feed system were conduct to obtain the variations of temperature rise,thermal error and zero shift with time and position of worktable.(2)The FEM integrated with the Monte Carlo method was applied to indentifying the heat flux of heat sources in the ball screw feed system.By virtue of the experimental data,the varations of heat generation rates for the two bearings,moving screw-nut and two guides with time were determined during the working process of the feed system.According to the results of the FEM simultation,the temperature difference between the measured surface pint and the contact surfaces of the kinematic pair was described as a function of the running time and the feed velocity,of which the parameters were determined by fitting the data of the FEM simultaiton.Combining the experiment results with that of the FEM simaltion,the time-varying mechanisms of the friction and the screw deformation were analyzed to ascertain the effect of the feed velocity and the preload on the distributions of the temperature rise and the thermal deformation for the ball screw feed system,which elucidated the mechanism of fluid-soild-heat coupling between the contact surfaces of the kinematic pair.(3)The orientation variation of the TCOSM was analyzed to find the phenomenon of the positioning surface change(PSC)for the ball-nut-screw pair during the down working process of the 45° slant feed system,which resulted in the fluctuation of positioning error for the worktable.Using the experimental results,the effects of the feed velocity and the viscosity varation of the lubricating oil film with temperature on the screw stress were systematically analyzed to ascertain the positioning surface of the ball-nut-screw pair.Based on the power distribution of the servomotor,the the critical condition of the PSC was deduced,which provided a method of predicting the PSC.(4)Using the measued surefuace temperatures of the two bearings and the nut flange,the numeric prediction method of the temperature and thermal error for the screw shaft was proposed with the finite difference of heat conduct equation for one-dimension rod.For a one-dimension rod with single heat source,the analytical solution of partial differential equation of heat conduct was deduced with separation of variables.Then the analytical model of predicting the temperature and thermal error for the screw shaft with multiple heat sources was established with the superposition principle.The parameters of the numeric and analytical models were determined by fitting the experimental data.The two models were verified by the FEM simulation data and the experimental reuslts.(5)The scheme of the thermal error compensation control was proposed with the above numeric and annlytical models,respectively.The tests of the thermal error compensation control were conducted in the 45° slant no-preload and the horizontal preload ball screw feed systems to verify the accuracy and the robusness of the compensation control method,respectively... |
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