Research On Equipment And Forming Technology Of Flexible Rolling For Three Dimensional Sheet Metal Parts

Surface flexible rolling (SFR) is a novel method to form3D sheet metal parts and to someextent, it can be considered as the combination of the traditional rolling and Multi-pointadjusting technology. The basic principle of SFR is to employ two bendable working rolls,whose strength and stiffness in radial direction are sufficient enough, as the forming tool. As therolls rotating,3D sheet metal parts will be formed continuously. Through the different workingroll bending shapes and roll gap distribution modes, the typical formed parts such as convexand saddle surface with different shapes will be formed. The critical content of SFR is thestructural design and shape adjusting of the bendable working rolls and in this study, the idea ofworking roll assembly is put forward and linear axis is selected as the working rolls and wireflexible shaft ground is used as the supporting rolls. Compared with the forming method above,SFR realizes the continuum and flexibility of forming for3D sheet metal parts and has a highvalue in the aspects of engineering application and academic. The specific research contentsand conclusions are summarized as followings,Theory research and forming equipment development of SFR:Surface flexible rolling employs two integral bendable working rolls, which still can rotatearound their axis despite in the state of being bended, as the forming tool. The bending shape ofworking rolls is regulated by the adjusting units. In the forming process, the bending of workingrolls causes the plate to produce a bending deformation in the transversal direction; that thenon-uniform roll gap makes an uneven compression in the thickness direction of sheet metalcauses the different elongations in the longitudinal direction. As the rolls rotating, the plate isfeeding constantly and3D sheet metal parts will be formed finally. The forming mechanism ofthe formed parts in the processing process was analyzed and the roll gap adjusting method ofconvex and saddle surface parts was explained; the controlling theory and method of SFR wasestablished; the calculating method of bending radius was showed when the roll shape is arc;the characteristics of SFR was introduced.The design scheme, main structure and parameters of the SFR forming equipment were introduced; the overall structure and the design of key parts including the adjusting unitassembly whose copper set are replaceable, working roll assembly and gearbox assembly, etc.were accomplished; the design scheme of the assembly of working rolls were showed and theselecting process of working roll and supporting rolls was introduced. Linear axis meets therequirements of bendable flexible rolls and has a smooth and hard surface, so it is used as theworking rolls. The flexibleness and transitivity of wire flexible shaft are favorable, so the wireflexible shaft is selected as the supporting rolls; the critical parts including the big nut, threebeams and guiding pillar, etc. in the forming equipment were checked and the result is thatthese components can satisfy the design demand.Forming technology exploration and experimental study of SFR:The adjusting modes of the working rolls were summarized, including gradual and oncemode. The gradual one is applied when the adjusting amount is large and the rolls needregulating many times to finish the target shape. The once one is applied when the amount issmall and need adjusting only once; the driving mode including single-end and both-end of theworking rolls were studied and in the both-end mode, the working roll rotates uniformly andproduces no lagging; three typical technology of SFR were put forward, namely, once, gradualand closed-loop technology. In the once mode, the working rolls are regulated once toaccomplish the processing. In the gradual mode, the roll shape needs regulating many times tofinish the forming. In the closed-loop mode, the roll shape can be regulated in the formingprocess according to the forming requirements and feedback information and this mode requiresa very high degree of controlling automation.The fine experimental parts has been obtained, including convex and saddle surface parts;the experimental parts were sensed by3D binocular optical sensing device and the results weremade into surface by means of the reverse engineering and based on the surface, the formingresults were analyzed; the impacts of forming parameters on the shapes of the formed partswere compared and analyzed, the parameters include roll gap height, bending radius and sheetmetal thickness; the forming defects in the SFR forming process were researched, the defectsinclude middle wave, side wave and stripe pressure mark.Numerical simulation of SFR:The theories in the model building process were explained, including explicit dynamics, continuum mechanics and elastic-plastic material constitutive relations, etc.; the model schemeof the working roll discretion was put forward, namely, the whole working roll is dispersed byseveral short rigid rollers and in the simulation process, the rollers are rotating around their axisand simulating the working roll rotation appropriately; the relations between the localcoordinate system of the rigid short rollers and the global one of the overall model wereexplained and the approach of vector synthesis was applied to solve the deviation problem ofthe short rollers moving down in the local one; finally, the numerical simulation results, whichare in accordance with the experimental ones, were displayed.The different parameter influences on the shape and stress-strain of the formed parts werestudied and the parameters are including roll gap height, bending radius, forming speed andfriction. Along with the roll gap height decreasing, the two main curvatures of the formed partare increasing. Along with the bending radius of the working rolls, the longitudinal curvature isincreasing and the transversal one is decreasing. Along with the forming speed increasing, thelongitudinal curvature decreases and the transversal decreases a little. Along with the increasingof the friction, the longitudinal and transversal curvatures are both decreases. The research ofparameter impact on the shape of formed parts will supply the theoretical foundation for theworking roll regulation and part shape controlling. Generally speaking, the stress distribution ofthe stress in the SFR is uniform and stress-strain distributions will change specificallyaccording with the parameters changing; the numerical simulation method was employed tostudy the shape error and the thickness changing.The load calculating method in the numerical simulation was investigated; based on thenumerical simulation method, the forming load was analyzed and the load changing trends inthe directions x, y and z have been obtained; the load in z-direction is the major source of plateplastic deformation and the one in y-direction can be considered approximately as the power todrive the plate to move forward; the influences of roll gap height, working roll bending radius,friction factor and materials on the z-direction load were analyzed; the characteristic of the loadin the SFR was analyzed and the load is composed of the rolling force and elastic force. Therolling force causes the sheet metal to be deformed plastically and the elastic one causes theplate to be bended in the transversal direction; the load calculating method was introducedsystematically; the load values calculated by the theory and that of the simulation were compared and analyzed.