Research On Dynamic Characteristics Of Fixed Joint And Dynamic Modeling Of Machine Tools

Since the performance of machine tools improves while the competitive life cycle times reduced, the traditional design method which through realization, testing, modification and optimization of physical prototypes can not meets the demand of modern machine tools. Instead, the design method of modern machine tools which employs virtual prototype techenology is an efficient way to reach a high-level design. Using this method, the designers can model the structure of machine tools; estimate the structural statics and dynamics and modeficate the design variations until the performance requirements are achieved in design phase. However, the base of the virtual prototype technology is the accrurate structural model of machine tools, and the dynamic modeling of joints is the bottleneck and key of the accurate structural model of machine tools.The dissertation is focused on the modeling of joints in machine tools. Using finite element (FE) method, experimental modal analysis (EMA) method and optimization method, the following four issues are deeply investigated:modeling of joints, parameter identification, analysis of influence factors and applied techenology. The main work is liseted as follows:1. The dissertation presents a tapered finite element model of spindle-holder taper joints in machine tools together with its parameter identification methods. The dynamic characteristics of the tapered joints are presented by the stiffness matrix and damping ratio coefficient of the tapered finite element. Based on inverse relationship between the frequency response function (FRF) matrix and dynamic matrix, the parameters of the tapered finite element are identified from the tested FRF. The comparison among the FRFs obtained from the different FE models and the tested FRF is used to verify the effectiveness of the tapered finite element dynamic model.2. Based on Hertz contact theory and EMA, the proportional relationship between equivalent elastic modulus and stiffness matrix of bolted joint finite element with different material combination is deduced. The relationship is verified by the modeling of machine tools structures. The relationship reveales the influence of material on dynamics of bolt joints quanlititatively, and provides a convience for wide use of the finite element dynamic model of bolted joints.3. The dissertation presents the method for dynamic modeling of machine tools structure. The finite element model of bolted joints and the tapered joints is integrated into the FE software. The structural model of machine tools is established by systhesizing the model of the two fixed joints, the FE model of structural component and the equivalent spring model of movable joints. The application of this method to the modeling of CKX5680seven-five-axis machine tool, the assembly of bed and base of YK31320gear hobbing machine and XCM1600machine tools is used to verify the effectiveness of the modeling method.4. To improve the dynamic characteristics of multi-bolt connection structure in machine tools, the dissertation presents a method to determine the optimal parameters of the bolted joints interface. Based on research on influence factors of dynamic characteristics of bolted joints, the number and nominal diameter of bolts are recognized as design variables. Through the constraints of the design variables, the feasible solutions of design variables are determined. According to the allowable frequency loss factor, the optimal number and nominal diameter of bolts are determined. The method provides a feasible way to determine the optimal design variables for the interface of multi-bolt connection structure in machine tools.5. Taking XCM1600machine tools as an example, the influence of joints on the static compliance of machine tools is quanlitively evaluted. The equivalent spring model of machine tools on the force closed chain and distribution of static compliance are established. Based on the simulated results obtained from the FE models and the equivalent spring model of machine tools on the force closed chain, the influence of joints on the static compliance of machine tools is evaluted. It shows18.1532%static compliance in X and27.3615%in Z direction of the machine tools is originated form joints.This dissertation is focuses on the dynamic modeling and application of joints in machine tools. A tapered zero thickness finite element model together with its parameter identification method of tapered joints is proposed. The statistical model between material and the dynamics of bolted joints is developed. Based on the dynamic model and the relationship between the dynamic characteristic and size, preload and material property of bolted joint, the paraments of multi-element bolted joints are optimized. Based on the proposed dynamic model of the two fixed joints and cited spring model of movable joints, the modeling method of structure in machine tools is presented and verified. Using the FE and equivalent spring model of XCM1600machine tools, the influence of joints on the static compliance is evaluated quantitively.