| With the rapid development of science and technology, high-grade NC machine tools are developing forward high speed, high precision, high efficiency and multiple purposes. Precision CNC lathe and tuning center have become one of the main development directions of modern NC machine tools. The development of a precision CNC lathe requires repeated comparisons and tests of various programs, and it also needs a lot of technology investment and long development cycles. How to use available technology and methods to effectively improve its dynamic characteristics and machining performance has become an urgent problem to be solved in the domestication of precision CNC lathe.In order to achieve high precision, high speed and high efficiency of precision CNC lathe, the dissertation carried out researches on the cutting chatter, machine tool spindle units and thermal error compensation with the use of theoretical analysis, numerical simulation and experimental methods. The research is supported by the National Science and Technology Major Project"HTC3250?n precision CNC lathe and turning center". The main research contents are as follows.(1) Taken into account of the variation of transient chip and cutting angle, a dynamic model is proposed for the ex-circle computerized numerical control turning based on the regenerative chatter mechanism. The simulation results clearly show the nonlinear relation between the limited cutting width and the spindle speed, and the spindle speed has more effect on the chatter than other factors. The cutting stability increases with the increase of the equivalent stiffness or the damping ratio in main vibration system, but it will decline when the directional coefficient, cutting overlapping coefficient or cutting stiffness increases. And the shapes of the lobes are simultaneously changed. However, the natural frequency of the main vibration system has no effect on the shapes of the stability lobes, and the lobes integrally move rightward with its increase. Through the above research, the chatter stability limit prediction is realized, and these results provide theoretical basis for chatter suppression. On this basis, special procedures for turning chatter analysis are developed, the numerical calculation of the cutting stable region is realized for CNC cylindrical turning chatter.(2) By means of finite element method and modal test system, the dynamic and static characteristics of the machine tool and cutting tool system are analyzed and tested on HTC3250?n precision CNC lathe, Based on them, the conclusion is obtained that main body of cutting chatter is the tools system of the precision CNC lathe according to the cutting experiment. These outcomes provide dynamic and static data for the cutting chatter suppression. In the structure design of precision CNC lathe, the application of high stiffness and high damping structures is proposed to reduce and isolate the vibration. Hydrodynamic and hydrostatic motor spindle, resin concrete bed and air-spring vibration isolator are designed, it is proved that these structures can meet the practical requirements of the machine tool by the cutting experiments.(3) A attenuation coefficient model for regenerative chatter cutting process is established, and the variations of system chatter frequency and attenuation coefficient are obtained by simulating the variable spindle speed cutting. The result shows that the energy in the constant speed cutting is more than which is input to the system by the regenerative feedback in the variable spindle speed cutting. By means of cutting experiments, the model and the simulation are verified, and the method is feasible to suppress the chatter by variable spindle speed cutting. A second-order time series model of cutting force is established by time series analysis methods, and the method is proposed to predict the cutting chatter by on-line identification of chatter stable region. A method is proposed to search and control the cutting chatter stable region. It uses the online spindle speed regulation to search the region and make the machine work in the stable region, and the cutting chatter is avoided. The program is developed to predict the cutting chatter, search and control chatter stable region, and the chatter active control is realized.(4) The finite elements method and basic process are proposed on the multi-objective optimization design and the critical speed verification of high-speed and high-precision spindle. The three-dimensional finite element parameterized model of the spindle is built, which uses the spring damping element to simulate the hydro-hybrid bearing. Based on the FEA model, the spindle static stiffness, natural frequency and mode shape are obtained. By the harmonic response analysis, the modes are compared between the resonance condition and normal condition, and the spindle's danger points are identified and checked. On this basis, the multi-objective optimization design model is built, and the method is presented to extract the proportional coefficients by scanning the design variables. Based on the model and method, the volume and mode amplitude are optimized by first order optimization method. The spindle's Campbell graph is calculated based on the rotor dynamics, the critical speeds are checked according to the curves. On this basis, the spindle unit analysis system of the CNC lathe has been developed, and it can make the spindle design more convenient and reliable.(5) Based on the finite elements technology, the method is proposed to analyze spindle system thermal characteristics and optimize the thermal structure. The method is also presented to reduce their thermal deformations by changing the boundary conditions. The research is carried out on HTC3250?n precision CNC lathe, the thermal characteristics and thermal deformation of machine tools spindle are tested. The method is proposed to select the sensitive points of temperature measurement based on grey comprehensive relationship degree, and the model of thermal error compensation is built based on multiple linear regressions. The method is presented on the embedded thermal error compensation of spindle system. According to the principle of semi-closed loop feed-forward compensation, the thermal compensator is developed by means of the embedded technology, and it is based on the excursion function of external coordinate origin point of CNC system. The thermal error compensation is realized on the precision CNC lathe, and it improves the processing precision of precision CNC lathe. |
PDF Full Text Request