Dynamic Modeling On Fixed Joint Interface In Mechanical Structure

With the development of high precision, high speed and intelligentization of NC machine tools, the need for characteristic indexes (especially dynamic ones) in NC machine tools is much higher. To high-grade NC machine tools, the dynamic property is a key of whole machine tools, therefore, the dynamic performance in machine tools becomes a novel research hot point. However,60～80% of the total dynamic stiffness and 90% of the total damping in a whole machine tool structure come from joint interfaces. Apparently, the dynamic characteristics of joint interfaces in machine tools affect the dynamic behaviors of a whole machine tool structure considerably. The dynamic characteristics of joint interfaces had been widely studied from theory and experiment for so many years. Dynamic modeling on joint interface plays an important part in studying the dynamic characteristics of joint interface. Now, it is difficult for current models of joint interfaces to satisfy the high-grade need of a set of joint interface in NC machine tools and there lacks a parameterized model because of the diversity of joint interface's type in NC machine tools and some parameters of describing joint interface model. Thus there urgently needs a parameterized establishing way with high precision. Some works in the manuscript are as follows.In the first, a dynamic parameterized modeling approach on fixed joint interface in machine tools was conducted using virtual material, i.e., (1) the fixed joint interface may be considered as a virtual material with the same cross area, (2) the fixed joint interface could be equaled that the virtual material is rigidly connected with two joint surface components and (3) the analytic model on fixed joint interface can be constructed if engineering data (joint area, joint force, joint surface roughness, joint components' elastic moduli and Poisson ratios, et al) are treated as parameters, thereby the model can solve the engineering practical problem about dynamic model.In the other, an analytic model of virtual material's elastic modulus, shear modulus, Poisson ratio and density was proposed. Using Hertz contact theory and fractal theory, the interaction between normal and tangential characteristics of fixed interface was taken into account. In view of commercial finite element software, the dynamic-mode inserting method for parameterized fixed joint interface could be exploited adopting virtual material, therefore, a new approach is provided about the high-accuracy dynamic modeling engineering application for a whole machine tool with mechanical structure.Thirdly, a method to estimate the fractal dimension and fractal roughness parameter is studied employing surface roughness. Structure function of surface profile was derived. An experimental technique was given to determine fractal dimension and fractal roughness parameter. Two approximate expressions were proposed about fractal dimension-surface roughness, and fractal roughness parameter-surface roughness. The experimental results reveal that fractal dimension and fractal roughness parameter have nothing to do with the sampling length. Decreasing the surface roughness increases the fractal dimension and reduces the fractal roughness parameter.Finally, the analytic solutions for virtual material were effectively verified. Four component joint interfaces selected as some research examples, on the basis of experimental results about test specimen, the theoretical mode shapes were compared with the experimental ones (considering modal assurance criterion), i.e., qualitative comparison of similar mode shape and quantitative comparison of the corresponding natural frequency. The theoretical mode shapes basicly agree well with the experimental results. The relative errors betweenn the theoretical natural frequencies and experimental ones are mostly less than 10%. Analyzing the results of aforementioned four component joint interfaces, the verification of the analytic method was assured when the whole machine tool's joint interface from CKX5680 seven-five axis lathe-milling equipment was selected as a study example. The theoretical mode shapes basically agree with the experimental ones. The relative errors betweenn the theoretical natural frequencies and experimental results are mostly less than 9 %.