Theoretical And Numerical Studies And Control Software Development Of Continuous Forming For Surface Parts

The surface parts, which have the characteristics of light quality, good streamline, highbearing capacity, etc., are widely used in various industrial fields. With the development ofeconomy and the progress of society, the demand for surface parts shifts from mass productionfor single product to small batches or single-piece production for wide-variety of products. Dieforming is an important method for forming surface parts. However, each part needs a kind ofdie, and it has the disadvantages of long development and debugging cycle. When forminglarge scale surface parts, the volume of the die forming device is large and the manufacturingcosts are high. Therefore, it has great theoretical significance and utility value for studying thenovel surface parts forming technology.Roll bending is a traditional continuous forming method. Continuous forming employsthree flexible rolls instead of straight rolls of roll bending, which is a rapid and efficientforming technology. It is suitable for forming large scale parts with doubly curved surface. Theresearch on continuous forming has made some progress, but there are many urgent problemsto be solved. For improving the engineering application value, the continuous forming theory,accuracy of forming surface part, finite element model of numerical simulation and formingprocess control need to be further studied. In this paper, the double bending theory of sheetmetal in continuous forming process was analyzed, and the transverse and longitudinal shapecontrol models were established. Then, the control software of continuous forming process wasdeveloped.The main contents and conclusions in this paper are as follows:1. According to the deformation history of sheet metal in continuous forming process, thesurface forming mechanism was provided, which is a combination of flat plate transverselybending and cylindrical surface longitudinally bending. In continuous forming process, first thesheet metal is bent in transverse direction in local deformation region, then the sheet metal isbent in longitudinal direction by top roll, finally the sheet metal is changed into doubly curvedsurface part by the rotation of the flexible rolls. Therefore, the formed part needs to consider the one-direction springback and the influence between transverse and longitudinal bending. Basedon the three different material models, the bending moment and springback ratio of singlebending was given. The influence between transverse and longitudinal bending was analyzed.The results show that the difference of transverse radius before and after springback decreaseswith the decrease of longitudinal curvature, and the longitudinal relative springback ratio is anexponential decline curve with the increase transverse radius.2. The principle of continuous forming for different types of surface parts was described.According to the feature of swept surface, the mathematic equation of formed surface wasgiven. Based on the transverse springback and the influence of longitudinal bending ontransverse curvature, the axis equations of the top and bottom flexible rolls and the adjustingvalue of each control point can be obtained by compensating the equation of transverse sectionline of target surface, and then the transverse shape control model was established. Consideringthe unloading stage and the influence of transverse bending on longitudinal curvature, therelation between the downward displacement of top roll and the longitudinal target curvaturecan be acquired, and the longitudinal shape control model was established. The numericalsimulations were carried out by the control theory and method provided in this paper. It isfound that the quality and accuracy of the formed part is good.3. The modeling method with continuous flexible roll was put forward. The contact linebetween flexible rolls and sheet metal is smooth, which can avoid impression and improve thesurface quality. The distribution of stress and strain in that model is uniform, and the accuracyof the formed part is high. The flexible roll adjusting process was simulated, and the influenceof the number of control point on adjusting shape and forming precision were studied. Theadjusting shape approaches the target shape with the increase of the number of control point.And the forming precision also increases with the increase of the number of control point. Dueto the adjusting errors of flexible roll, a compensation method was proposed and the adjustingerrors decreased after compensation, which was verified through numerical simulations.4. The influence of different materials and thickness of sheet metal on reaction forces onflexible rolls was analyzed. The results show that the reaction forces on flexible roll increasewith the increase of the elastic modulus and yield strength of material and the plate thickness.The reaction forces on top flexible roll are larger than those on bottom flexible rolls. Theinfluence rules of material, plate thickness, downward displacement of top roll, shape offlexible roll and center distance between bottom rolls on forming accuracy of sheet metal werestudied. The results show that the forming errors decrease with the decrease of elastic module,decrease with increase of thickness of sheet metal, decrease with the decrease of the downwarddisplacement of top roll, decrease with the decrease of transverse radius of curvature andincrease with the increase of the center distance of the two bottom flexible rolls. The correlationship curve between transverse and longitudinal bending was obtained. When thedownward displacement of top roll remains the same, the longitudinal curvature increase withthe decrease of the transverse radius of curvature. When the shape of flexible roll remainsunchanged, the transverse radius of curvature increase with the increase of the downwarddisplacement of top roll.5. Based on the control theory and method established above, the continuous formingcontrol software is developed on VC++platform. The control software was divided into foursections: surface constructing module, parameter setting module, computing module and graphdisplaying module. The first module can reconstruct the target surface by importing the digitaldata points. The second module can be used for inputting the device parameters, materialproperty and dimension of sheet metal. The third module can calculate the downwarddisplacement of top roll and the adjusting value of each control point. The fourth module candisplay the continuous forming process with the help of OpenGL.6. The computer control system of continuous forming process was developed bycombining the control software with the experimental set. The numerical simulations andforming experiments of typical surface parts were carried out, and the results show that theexperimental results agree well with the simulated results. The shape control databases ofsurface parts with positive and negative Gaussian curvatures were established. The differenttypes of doubly curved surface parts were formed by continuous forming process controlsystem. The smoothness and dimensional accuracy of the saddle and torus surface parts wereanalyzed, and the results demonstrate that the surface quality and precision of surface partsformed by continuous forming technology meet the requirement.