Research On Static Accuracy Design Method And Error Measurement Technology In Large Dimension For Heavy-duty Machine Tools

Heavy-duty CNC machine tools are the key equipment in the fields of energy power,aerospace,national defense and military,automobile manufacturing,etc.The accuracy level of machine tools directly determines the processing precision,efficiency and reliability of large key parts.The system establishment of design theory is significant for improving the manufacturing level of heavy-duty machine tool products.The static accuracy design,which plays an important role in achieving accuracy target and cost control,is an important part of heavy-duty machine tool design.However,the current accuracy design method of machine tools does not take into account the specialty of flexibility resulted from the structure of large dimension,especially the influence of structural deformation error caused by gravity on static accuracy.As a result,the static accuracy of heavy-duty machine tools still needs to be ensured by the subsequent assemblies and adjustments,and the advantages of accuracy design in machine tool design cannot be taken.To solve the above problem,the static accuracy design method involving static error modeling,gravity deformation error modeling,error measurement in large dimension and static error budget is studied in this dissertation,aiming to provide some guidance on constructing the accuracy design method and theoretical system for heavy-duty machine tools.Considering the structural flexibility of large-dimension and large-span heavy-duty machine tools,a static error modeling method based on the multi-flexible body system theory is proposed.The machine tool is abstracted into a multi-flexible body system composed of elastic bodies and hinges.The coupling relationship between the geometric error in the rigid body motion and the elastic deformation error of the flexible body is revealed by the homogeneous transformation matrix.A mathematical model is established to describe the relationship between the static error components of motion axes and the spatial error of the tool tip under the interaction of the geometric error and the elastic deformation error.It provides a theoretical basis for the static accuracy design method.The correctness and accuracy of the proposed method are verified by the static error modeling and working accuracy tests of the heavy-duty vertical machining center.In order to accurately model the gravity deformation error components in the static error model,the influence of the gravity on the deformation error of the heavy-duty machine tool structure is quantitatively analyzed.Based on the spatial beam-spring combination super element,the whole stiffness model of the heavy-duty vertical machining center is established.The equivalent compliance coefficient of cantilever structure is used to derive the stiffness matrix of the spatial beam element with box-shaped variable cross-section structure.The stiffness characteristics of contact interfaces are simulated by the spring element.The gravity deformation error component in each static error component is calculated by the stiffness model,which lays a theoretical foundation for the establishment of non-rigid body error constraints in the subsequent static accuracy design method based on the gravity deformation error modeling.Accurate measurement for error in large dimension is the key to verifying the correctness and accuracy of the multi-flexible body system based static error model.For the measurement point layout in the nine-line error identification of machine tools,a Monte Carlo simulation based method is proposed to calculate the recommended measuring spacing of measurement points.This method takes the measurement efficiency into account under the accuracy requirement of error measurement in large dimension,and guides and standardizes the distribution of measurement points.Considering that different station configurations have certain influences on the measurement accuracy of the sequential multilateration method of laser tracker,a configuration optimization for laser tracker stations based on the global error magnification factor is proposed.The mathematical models of laser visibility constrained by the structure of heavy-duty machine tools and the high-precision receiving range of the retro-reflector are constructed so that the optimized configuration of laser tracker stations is practical for engineering application under the premise of meeting the measurement accuracy requirement.The verification experiment shows that the optimized configuration of laser tracker stations has high measurement accuracy.Combined with the results of the above research works,a static accuracy design method for heavy-duty machine tools based on the gravity deformation error modeling is proposed.The static error distribution problem of heavy-duty machine tools is converted into a nonlinear constrained multi-objective optimization problem under the worst condition.By introducing the structural parameters of the machine components into the design variables,the gravity deformation error is optimized from the perspective of structural flexibility while balancing the relative cost and robustness objectives,and the design margin of the geometric error components is improved.Considering that there may be no optimal solution for the multi-objective optimization problem of error distribution,according to the structural characteristics of heavy-duty machine tools,a calculation method of the gravity deformation of large-span beams for anti-deformation compensation is proposed,which provides a theoretical basis for adjusting the constraint of gravity deformation error in the optimization problem.Finally,the heavy-duty vertical machining center is taken as an example to describe the main implementation steps of the static accuracy design method for heavy-duty machine tools.The optimal distribution result shows that the accuracy design method proposed in this dissertation can fully consider the influence of gravity deformation error,which can help to guide the detailed design of machine components and reduce unnecessary assembly adjustments.