In the machining of a five-axis machine tool with a double turntable, the dynamic characteristics of the machine tool are affected by different spatial positions of workpieces. Predicting the dynamic characteristics of machine tools at different spatial positions can effectively improve the machining accuracy of workpieces. In the present study, a five-axis CNC machine tool (Tuopu VMC-C50, China) is considered as the research object. Firstly, the bearing stiffness model of the double turntable rotary system is established, and the influence of the joint stiffness of the turntable bearing on the dynamic characteristics of the five-axis machine tool table rotary system is studied. Based on the established three-dimensional model and rigidity of the sliding joint surface of the machine tool, the machine tool modes at 81 working positions are analyzed. Accordingly, the influence of different swinging positions of the double turntable on the dynamic characteristics of the five-axis machine tool is studied. Finally, based on the three-dimensional Kriging prediction model, variations of the natural frequency and vibration amplitude of the system in the machining space for different swing table angles are analyzed.
The geometric error of the rotation axis of a multi-axis machine tool is a highly sensitive error in the geometry error of the machine tool. Therefore, establishing an evaluation system for geometric errors of rotation axes is important for improving the machining accuracy of workpieces. First, a model for predicting the geometric error of the rotation axis was established based on the multi-body system theory, and then the mapping relationship between the geometric error of the rotation axis and each region with curvature changes in the S-shaped sample is identified. Next, based on the average value in seven different regions with curvature changes in the S-shaped sample, the weights of the mapping regions were calculated. Subsequently, the weights of the 12 geometric errors of the rotation axis (εx (A), εy (A), εz (A), (δx (A), (δy(A), (δz(A), (εx (C), εy (C), εz (C), (δx (C), (δy(C) and (δz(C)), and the weights of two rotation axes were identified to establish a comprehensive evaluation system for the geometric errors of rotation axes. Finally, an S-shaped sample was machined on the TuopuVMC-C50 five-axis CNC machine tool, and on-machine measurement experiments were conducted on two rotation axes with the established evaluation system. The results indicate that the relative error between the actual value and the theoretical value is less than 1.0%. Further, the comprehensive performance of the machine's rotation axes was evaluated step by step based on the theoretically calculated weight values.
The positioning accuracy and machining performance of the five-axis machine tool are significantly impacted by the dynamic characteristics of the rotary system of the two-axis rotary table machine. In this study, first, the stiffness of the joint bearing of the rotary table system is calculated, and the effect of bearing clearance and external load on the stiffness is analyzed. Second, considering the stiffness characteristics of the joint, the natural frequency and mode shapes of the turntable system are calculated. Finally, the influence of turntable angle and bearing stiffness on the dynamic characteristics of the turntable system is analyzed. The results show that the natural frequency of the rotary table system does not change obviously with the axial stiffness of the turntable bearing joint, but increases significantly with the increase in the radial stiffness. The first order natural frequency of the turntable decreases with the increase in the swing angle, and the change in the first order natural frequency is 77.43 Hz. The research results provide theoretical basis and guidance for machine tool design and use.
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