Study On Heat Characteristics And Thermal Error Compensation Of NC Machine Tools

When a NC machining tool is working, heat disturbances at internal and external thermal resource cause a non-even temperature field in the machining tool. At the same time, the coefficients of thermal expansion are different, the thermal stress and thermal deformation also emerge in parts of machining tool. The correct position relationship between tool and work is destroyed, and the accuracy of machining tools is come down, so the thermal error comes into being. In the process of precise machining, the thermal error of the mechanical process system covers 40-70% of the total machine error, the thermal error of machining tool covres 60-80% of thermal error. The thermal deformation has become the major error in precision and extra-precision machine.In this thesis, a vertical machine centers is our research object. A large of tests state that friction heat of the spindle bearings is the major thermal resource. In spindle box ,it is also the major reason that leads to the thermal error of spindle. The contact deformation and load distribution between balls and rings have been researched with the theory of Hertz contact; Applying the kinematics principle, the ball motions have been analyzed, and formulas of the velocity have also been build. The formulas of friction force and torque have been build with mathematics inference and experiment. At last, on the basis of the friction torque and velocity, the thermal resource model of spindle bearings has been build.The process factors that affect spindle bearing’s friction heat have been analyzed, and many tests have done to verify the result. The regular that the interference between bearings and spindle and spindle sleeves affects the thermal deformation error has been researched according to the thermal stress theory. Afer improving the assemble technology and decreasing the interference, temperature raise in key point and thermal error all are cut down. These methods verify the regular correct.Before thermal error compensates in a NC machining center, we must determine the key temperature measurement points. The points are more , the input information about the thermal character is more precise, but the cost for temperature measurement is more expensive, and the compensation system deal with the temperature information is more difficulty. So the least measurement points must be selected to express the thermal character information as full as possible. The fuzzy group and correlation analysis are made used of to optimizate the key points. At first some initia temperature measurement points in spindle box are selected as optimization base. Secondly these points are divided into several groups by fuzzy group theory. At last we select point from every group, them are accept or reject according to the result of correlation analysis. The error compensation has verified that the selected points are satisfied with the accuracy require of machining center.The following thing is to model relationship between the thermal error and temperature raise of key points. In this thesis multiple linear regression is applied to build model, and the experimental test in the vertical machining center has already stated that the effect is very ideal. Based on multiple linear regression, the piecewise linear regression is lead into build the thermal error model. The simulation result states it can make thermal error further descend.The thermal model based RBF(radial base function) neural netwoks has been built. The measurement data on the vertical machining center has been analysed. The result states that the model can be more closed to the real thermal model and reduce the effect of thermal error.On the basis of the machining tool coordinate system and work coordinate system, the thermal coordinate system and cool coordinate have put forward. All cases in machine process are deeply analyzed with these concepts. The methods that solve all questions in most abnormal cases also are list.