| Large computerized numerical control(CNC)gear profile grinding machine tools,which are widely used in gear production and processing of the heavy equipment field such as wind power,hydropower,ships,construction machinery and so on,is the high efficiency and precision equipment for the machining of large size and high precision gear.Moreover,it has a large size,a large number of parts and ancillary equipment,complex internal and external structure.The guide rail,slide block,screw,worm gear and other components produce a lot of frictional heat as well as motor heating,grinding heat between wheel and workpiece because of long time and continuous operation in the machining of large size gear workpiece.So the temperature of the machine tool will increase gradually and the uneven temperature field result in thermal deformation.Under the influence of these factors,the actual conjugate tooth surface formed by enveloping motion between grinding wheel and workpiece deviates from the theoretical envelope trajectory and results in the machining error between the actual and the theoretical tooth surface.The machining accuracy of the machine tools will be seriously affected under the wear error,the large manufacturing and installation error from large size and weight of the components and parts.Moreover,due to the high production cost and high precision requirements of the large size gear workpiece and the great risk of machining,the machining accuracy,accuracy and stability of machine tools are more demanding.Because of these,it has very important significance to research the geometric error of large CNC gear profile and the machining error of gear tooth surface,establish error models of manufacture geometric error and thermal geometric error,the relation model between comprehensive geometric error of the machine tools and the machining error of tooth surface,propose a new error compensation method for the reduction of machining error of large size gear workpiece,the greatly improvement of machining precision of the machine tools.Therefore,according to the structure and processing characteristics of machine tools such as large nine-axis and five-axis linkage CNC gear profile grinding machine tool SKMC-3000/20,this thesis analyzes and investigates the machining error mechanism of tooth surface and error compensation technique for the machine tools from the following aspects:(1)The precondition for the development of error compensation module and the reduction of tooth surface machining error is the establishment of geometric error model for the machine tool.Thirty three geometric error components are analyzed and the driven chain of the principal coordinate systems is established according to space kinematics principle based on the movement mode of main motion axis of the machine tool.The position error and rotation error of grinding wheel coordinate system relative to workpiece coordinate system by using the pace homogeneous coordinate transformation method combined with the topological structure of the machine tool.Then the geometric error models are acquired.After that,the error compensation models,which provide theoretical basis for the following geometric error detection and error compensation experiment,for the main motion axes are founded by establishing Jacobian matrix and decoupling the errors.(2)In order to find out the influence of relative position errors between wheel and wokpiece caused by the integrated geometric error on the machining error of tooth surface,the space conjugate contact lines which express the theory profile of gear face from the space homogeneous coordinate transformation matrix between wheel and wokpiece by using the basic principles of forming gear grinding and gear meshing.Then tooth surface error models contain the relative position errors and the contact envelope surface between wheel and workpiece are acquired according to the enveloping theory of spatial curved surface.The relationship model between the relative position errors and the tooth surface error,which reveals the association rule of them,is established through the evaluation of the tooth surface precision and accuracy to provide a theoretical basis for the prediction and error correction of the machining error of tooth surface.(3)The thermal steady state and deformation of the whole machine tool are analyzed by using the computer numerical analysis method.In view of the present situation of irregular appearance and a large number of heat sources of the machine tool,the thermal deformation is very difficult to be calculated from geometric mathematics.The distribution of heat sources and the thermal values of main components and parts are analyzed in this thesis based on the theory of heat transfer and mechanical thermal deformation.Then combined with the assembly relationship and material characteristics of the components and parts,the temperature field distribution and thermal deformation of the machine tool in thermal equilibrium state are obtained by using the finite element software,which provides references for the establishment of experiment platform between temperature and displacement,and the development of thermal error real-time compensation module.(4)From the association rule between the integrated geometric error and the tooth surface error,the critical factor for tooth surface machining accuracy is the relative position errors between wheel and workpiece along X and Y direction.Therefore,the experiment platform between temperature and displacement is established on a large CNC gear profile grinding machine tool based on the numerical simulation analysis results.The key temperature variables are optimized as independent variables based on fuzzy clustering algorithm,multiple linear regressions combining the least squares method to build thermal error model of the machine tool.And then,the thermal error compensation model is built based on the relational model between relative position error and tooth surface machining error,which reveals the relationships among thermal deformation,tooth surface machining error and temperature.(5)The embedded real-time compensationsoftware module for geometric error and thermal error is developed by using the second development platform of HMI Programming Package based on the error analysis,the error compensation model of the machine tool and the functional characteristics of numerical control system SINUMERIK 840 D sl.And the compensation experiment was carried out on a large five axis CNC gear profile grinding machine tool.The in-situ detection result of tooth surface show that tooth profile deviation is decreased greatly after error compensation,and tooth surface machining accuracy is improved significantly.It indicates that the error compensation model accuracy and the compensation effect of software module are high enough to improve the machining accuracy and efficiency of the machine tool.The adopted theoretical and technical methods provide useful reference for error compensation and accuracy improvement of other types' machine tools.Accordingly,the manufacture geometric error modeling,thermal geometric error modeling and error compensation theory of large CNC gear profile grinding machine tools are proposed,and the association rules between the integrated geometric error of the machine tools and the machining error of tooth surface are revealed by theoretical analysis,finite element technology and field experiment.Then error compensation experiment is carried out by using the developed embedded error real-time compensation software module.The in-situ detection result shows that the machining accuracy of tooth surface is improved significantly,and the decrease trend of the tooth surface error agrees with the theoretical model.Above all,the adopted theory,principle,mathematical algorithm and essential thoughts,the established error model and the developed error compensation module have important guiding significance and good practical value for accuracy improvement,structural optimization,tooth surface accuracy prediction and online error compensation of large CNC gear profile grinding machine tools. |
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