Research On The Thermal Behavior And The Thermal Structure Optimization Of The Ultra-precision Fly Cutting Machine Tool

Ultra-precision fly cutting is the best method to get large-scale and high precision KDP crystals. With the characteristics of soft-brittle and deliquescent, the KDP crystal is the internationally accepted difficult-to-machine material. Meanwhile, the KDP crystal is the key optical component in inertial confinement fusion program, which is related to national defense and nuclear power. Therefore, the new technology about the ultra-precision fly cutting machine tool used to manufacture KDP crystal is blockaded for our country. The machining precision of the ultra-precision fly cutting machine tool is influenced by its structure, static performance, dynamic performance and thermal performance. However, more than 50% of the overall geometrical inaccuracies of workpieces are caused by the thermal error. Meanwhile, the thermal deformation caused by the internal and external heat sources is the main source of the thermal error. Therefore, in this paper, the influence of the internal and external heat sources on the thermal error of the machine tool is studied. Based on the analysis, the reasonable thermal structure optimization methods are proposed in order to improve the thermal performance of the ultra-precision fly cutting machine tool.Firstly, in the ultra-precision fly cutting machine tool, the temperature rise caused by the internal heat sources is a heat-fluid-solid coupling process, which is related to the power loss of the motor and the viscous power dissipation of the internal fluid. Therefore, for the hydrostatic spindle system, the finite volume element method is proposed by combining the finite volume method and the finite element method, by which the heat-fluid-structure coupling model of the hydrostatic spindle system is built. By the finite volume element method, the interaction among temperature rise, oil film thickness and thermal deformation is studied and the influence of temperature rise on the stiffness, carrying capacity and eccentricity of the hydrostatic spindle is studied. The temperature field calculating accuracy of the hydrostatic spindle system is improved by the finite volume element method. For the aerostatic spindle system, the temperature rise during the whole machine process is obtained by the finite volume method. The internal heat source analysis will lay a good foundation for the thermal analysis of the ultra-precision fly cutting machine tool.Secondly, the calculating accuracy of the thermal analysis is influenced directly by the contact surfaces in the machine tool, so the finite element fractal method is proposed by combining the finite element method and the fractal theory, by which the dynamic mathematical model of the thermal contact resistance is built. Then the comprehensive finite element model of the ultra-precision fly cutting machine tool is built with consideration of the internal heat source, the thermal contact resistance, the stiffness of the spindle and the preload of the bolts. By the comprehensive finite element model, the dynamic change of the contact pressure caused by the temperature rise is studied and the thermal contact conductivity is obtained with consideration of the interaction among temperature rise, thermal contact resistance and mechanical structure, which will improve the simulation accuracy of the comprehensive finite element model of the ultra-precision fly cutting machine tool effectively. Then the temperature rise and thermal deformation of the machine tool are calculated and the influence of the internal heat sources on the thermal error is obtained.Thirdly, based on the comprehensive finite element model of the ultra-precision fly cutting machine tool, the influence of the external heat sources on the thermal error is studied, which will provide theoretical basis for the temperature control of the external heat sources. The ambient air temperature change is decomposed into three parts which are macroscopic change, fluctuation and disturbance. Then the influence of the ambient air temperature macroscopic change amplitude, fluctuation period and amplitude and local disturbance on the thermal error is studied by imposing the ambient air temperature change in the shape of slope, sinusoid and triangular wave on the finite element model. Therefore, the optimal ambient air temperature control accuracy is obtained. Meanwhile, the temperature rise of the hydrostatic spindle and the aerostatic spindle with different inlet fluid temperature is obtained. Then the temperature distribution and thermal deformation of the ultra-precision fly cutting machine tool with different inlet fluid temperature are calculated, by which the influence of the inlet fluid temperature on the thermal error is studied and the optimal inlet fluid temperature is obtained.At last, in order to improve the machining precision and machining efficiency, the proper thermal structure optimization methods are proposed in this paper. For the hydrostatic machine tool, the influence of the thermal contact conductivity on the thermal error is studied, by which the thermal contact conductivity optimization method is proposed. By the thermal contact conductivity optimization method, the optimal distribution of the thermal contact conductivity is chosen, which decreases the thermal balance time of the hydrostatic machine tool. For the aerostatic machine tool, the thermal deformation of each component is studied, by which the thermal displacement decomposition and counteraction method is proposed. By the thermal displacement decomposition and counteraction method, the thermal displacement of the tool tip is decomposed into three parts which are the thermal deformations of the beam, the spindle and the fly cutting head. Then the structures of the beam and the fly cutting head are optimized, which decreases the thermal error of the aerostatic machine tool.