Research On Thermal Error Modeling And Compensation Method Of Screw For Vertical Machining Center

The machining error caused by thermal deformation accounts for 40%-70%of the total machining error.Thermal error is an important factor affecting the machining accuracy of parts.An important method to improve the machining accuracy of machine tools is thermal error modeling and compensation.Relevant scholars have done a lot of research and achieved rich results.At present,improving the prediction accuracy of thermal error model and the stability of compensation are the key and difficult points to be solved in the research of thermal error.This paper takes the Z-axis lead screw of vertical machining center as the research object,studied the mechanism of the thermal error of the lead screw based on the operating conditions,the thermal error modeling method under different fixed modes and the stability compensation method.The relationship between the electric control data and thermal error of the CNC system is analyzed,which reflects the operating conditions of the machine tool.According to the theory of heat transfer and thermodynamics,the formation mechanism and influencing factors of lead screw thermal error are analyzed,and the experimental data collection and analysis under variable working conditions are carried out.The thermal deformation law and causes of the lead screw under different working conditions are analyzed through the experimental method,and the correlation between the thermal error of the lead screw under different working conditions and the electronic control data of the NC system is determined.Aiming at the problem of insufficient adaptability of thermal error model caused by the change of thermal characteristic parameters of fixed-support type lead screw during operation,a thermal error modeling method of lead screw based on thermal characteristic analysis and data fusion is proposed.According to the theory of heat transfer,the mathematical model of the thermal characteristics of the screw nut is established,and a parameter identification method of the thermal error model based on improved particle swarm optimization algorithm(IPSOA)is proposed.The identification parameters include heat flux density,forced convection coefficient and heat conductivity coefficient.Furthermore,the electronic control data of the numerical control system is introduced,and the thermal error model of the fixed-support type lead screw suitable for different operating conditions is established.The test results show that the prediction residual error of this method for three groups of different operating conditions is within 7.2μm.Aiming at the problem that the prediction accuracy of the thermal error model is not high due to the bidirectional thermal deformation of the fixed-preload lead screw,a data-driven modeling method of the thermal error of the lead screw is proposed.This method proposes a thermal error prediction algorithm based on the binary linear function coefficients k_t,b_t values,considers the time-delay characteristics caused by the thermal elastic effect of the lead screw fully,uses the temperature data of multiple times in history and the electronic control data of the NC system as the input characteristics,and establishes a fixed-preload lead screw thermal error prediction model based on the Gated Recurrent Unit time sequential neural network and attention mechanism(GRU-Attention).The test results show that the prediction residual error of this method is within 8.5μm.The evaluation method of thermal error compensation and stability compensation method of lead screw are proposed.Aiming at the problem that the thermal error compensation is invalid due to the poor consistency of the thermal error data of the lead screw,an evaluation index and method for the thermal error compensation of the lead screw are proposed.In order to solve the problem that the compensation value is abnormal due to the external environment interference during the implementation of thermal error compensation,a stability compensation method based on mean filter and threshold compensation is proposed.The integrated development and test verification of the thermal error compensation function of the lead screw are completed.Test results on the fixed-support lead screw:the thermal error of the test piece can be reduced from 31μm before compensation to 4μm after compensation;The test results on the fixed fixed-preload lead screw:the average pass rate of the parts with key dimensions in the Z direction of the mobile phone case increased from 50.0%before compensation to 96.8%after compensation.The above results show that the thermal error modeling and compensation method proposed in this paper is effective.