Investigation Of Static And Dynamic Accuracy Performances Of High-precision Machine Tool With Thermo-mechanical Coupling Effect

High-speed and high-precision CNC(computerized numerical control)machine tool plays a great role in the development of electronics manufacturing industry and the supporting ability of electronic information industry.Recently,with the electronic products of information,communication,medical,and aerospace becoming high-end,high density and high reliability,higher requirements for the precision,efficiency and thermal stability of the CNC machine tool are put forward.Thermally induced error is an important factor to affect the machining accuracy of the ultra-precision,which can account for as much as 70% of the total machining error.Therefore,understanding of the mechanism causing the thermal error in practical engineering is of great significance to solve the common issues of the high precision machine tool in low thermal stability.The thermal dynamic behavior of the machine tool is a comprehensive embodiment of complicated thermal stress,nonlinear d istortion and thermo-mechanical interaction between the components induced by the varying heat source.Hence,the thermal issue should never be fixed within an isolated system.This paper investigates the static and dynamic accuracy and their evolution law of the machine tool under coupled thermo-mechanical effect in a global view.The main contribution is as follows:The nonlinear stiffness of the linear rolling guides is analyzed.The initial stiffness model is established based on Hertz contact theory,which considers the nonlinear elastic deformation and the preload effect.The results show that the initial stiffness of the guide in upper and lower raceways exhibit opposite change tendencies with the external load.The equivalent heat flux generated in the contact rolling interface is obtained by the calculation of the sliding friction,elastic hysteresis friction,and spinning friction energy.Meanwhile,the analytical wear model is established based on Achard wear theory.With the numerical simulation of the thermo-mechanical distortion and wear depth analysis of the linear guide the final stiffness model is established,and the results show that the final stiffness of upper and lower raceways present different decreasing trends with the combined effects of frictional heat transfer,thermo-elastic contact distortion and wear.The stiffness model is validated by LMS vibration tests,the natural vibration frequency shifts of the carriage from(2.36-6.58)k Hz to(2.05-5.66)k Hz due to the decrease of the contact stiffness are successfully detected,and the more accurate rigid mode frequency expressions are developed based on the proposed stiffness model.The measured and analytical frequencies show maxmium percentage error of 15.2% between them.The results also show that the thermal and wear effects on the entire contact stiffness variation of linear guides during operation is temporal.Thermal effect on the machine frame is investigated to analyze the thermal and static accuracy performances of the large scale mineral casting frame under environmental thermal drifts.The measurement of flatness and linear model analysis are performed to verify the reliability of the new mineral casting replacing the traditional materials used in the high precision machine tool.The results show that the static stiffness of the mineral casting bed can meet the precision requirement of 50μm of a machine tool frame structure.Compared with traditional materials,its dynamic performance is superior to cast iron,mode feature yield a little to the granite.The frequency of lower mode is higher than 90 Hz,which can maintain dynamic stability with typical excited frequencies in machine tools.The temperature field equations of the mineral casting frame are derived by finite element method.The numerical modeling of heat transfer and coupled thermo-mechanical combined with laser interferometer tests are performed to quantify the nonlinear deformation of the mineral casting frame with the effects of external thermal fluctuations,convection and thermo-mechanical coupling.The results show that the stable shop floor environment where a large scale frame is located can be of paramount importance for maintaining the dimensional accuracy of the structure under ambient thermal drifts.Thermal effect on the feed system is investigated to quantify the thermal and dynamic behaviors of a high precision worktable under complex thermal conditions.The dynamical differential equations of the feed system with hybrid support are established,which reveal the great importance of the gravity-center drive for its dynamic performance.The coupled thermo-mechanical model is established,which considers the combined heat source of internal heat generated by the motor and linear guide and environmental temperature variation.The complete feed system is included in the model to avoid the worktable isolated from the whole assembly system.The dynamic analysis is performed to quantify the nonlinear distortion and yawing motion of the worktable.The presented numerical model is validated by the measurement of yawing err of the worktable with laser interferometer.The results show that the motion errors of the worktable increase with temperature rise,the average yawing error can reach up to 5.12-10.67μm/m with a temperature variation of 4°C-8°C.Hence,to obtain an ideal dynamic performance of the worktable for high precision machining,the room temperature fluctuations should be controlled within 4°C.In addition,it also can be deduced from the measurement of the motor temperatures and the analysis of the heat flux generated by linear guide that the thermally induced motion errors of the worktable mainly come from the environmental thermal fluctuations not the internal heat sources of the system.The mechanism causing the variation of the machining accuracy of a high precision machine tool under real operating conditions is investigated with a global view.The coupled thermo-mechanical model of the whole machine is established to quantify the temperature field and the nonlinear distortion of the machine structure under the effects of combined heat sources fluctuations,thermal contact conductance,and thermo-mechanical coupling.The results show that the worktable presents second order bending with the temperature rise of 5°C,which greatly degrades its motion accuracy.The dynamic positional accuracy of TCP(Tool center point)under the combined effects of internal and external heat sources and coupled thermo-mechanical is predicted by coupled thermo-mechanical modeling of the dynamic impact of the moving structure.Both of the numerical and experimental results suggest that the hole registration accuracy decreases with temperature rise,and most of the drilling axes cannot reach the standard value of 25.4μm with the temperature rise of 5°C.The CPK(Process Capability Index)values and the varying law of the hole registration accuracy under different thermal conditions are obtained by microdrilling tests,which again demonstrates that the shop floor environment where a machine tool located is extremely important for the accuracy of manufacturing.