Research On Rolling Forming Method Of 3D Surface Parts Based On Non-uniform Roll Gap

Three-dimensional(3D)surface parts are widely used in aerospace,automobile manufacturing,shipbuilding,architectural design,artwork design and other fields with the advantages of light weight,strong load-carrying capacity,good streamline,attractive appearance,flexible and changeable shape.Flexible rolling forming is a new continuous forming process for manufacturing curved surface parts.The local transverse bending of sheet metal is caused by the bending action of forming roll,while the local longitudinal bending deformation of sheet metal is produced by the uneven roll gap distribution.Rotation of forming rolls drives sheet metal continuous feeding,and continuous local bending deformations accumulate into integral deformation,thus forming surface parts with double curvatures.The rolling method for forming 3D surface parts has the following advantages: The contact area between the forming tool and the sheet is greatly reduced by changing the whole forming to the local forming,and thus the forming load is greatly reduced compared with the traditional die forming;The production efficiency is significantly improved by changing the reciprocating or intermittent forming to the continuous rotary forming;The forming tool is a continuous smooth roll,the surface quality of the forming part is better;The sheet metal has large plastic deformation and thus the springback is small in the forming process;The equipment is simple and small,so the manufacturing cost is low;Using a bendable flexible roller can form a variety of roll gaps,it can realize flexible forming of 3D surface parts with different types and specifications.Therefore,the rolling method for forming 3D surface parts has broad application prospects.In this paper,the rolling process of curved surface parts is analyzed theoretically and the formulas for calculating the longitudinal bending deformations of two typical surface parts,convex surface part and saddle surface part,are derived.The roll gap design methods for rolling two typical curved surface parts are given,and the rolling methods using flexible rolls and variable cross-section rolls are studied respectively.The research contents and conclusions in this paper include the following several aspects:(1)The forming mechanism of two typical curved surface parts is analyzed.Based on the mathematical description of three-dimensional shape,volume invariance as well as the neglect of sheet metal transverse spread,the formulas for calculating the longitudinal bending deformation of two typical curved surface parts are derived.According to the geometric relationship of roll gap,sheet size and the target surface shape,roll gap functions of two typical curved surface parts,height functions of flexible roll midlines and shape adjustment functions are inversely deduced.In addition,the method for calculating the positions of the shape adjustment units is given.(2)According to the characteristics of contact deformation zone between flexible rolls and sheet metal during the flexible rolling of two typical curved surface parts are analyzed.The contact deformation zone length is very small,and it is close to the line contact state.Flexible rolls have little restraint on sheet metal,and the factors such as the weight of sheet metal or the vibration of forming equipment easily cause the deformation instability of sheet metal swinging up and down.Therefore,a flexible rolling method using auxiliary rolls is proposed to stabilize the rolling process,and the differences of position change of the tail end face of sheet metal,stress and strain distribution,section line curvature distribution and shape error distribution between the two flexible rolling methods with and without auxiliary rolls are analyzed by numerical simulation.The results show that the rolling process can be stabilized by adding auxiliary rolls,and the quality of the formed parts can be improved.(3)The transverse and longitudinal deformations of two typical curved parts are analyzed on the basis of the flexible rolling method using auxiliary rolls.The results show that the longitudinal elongation of the sheet depends on the rolling reduction in the thickness direction.While the ultimate wide spread of sheet metal is the result of the combination of rolling reduction in thickness direction and transverse bending deformation.The effects of sheet size,bending radius of roll gap midline as well as the maximum rolling reduction on the longitudinal bending deformation of sheet metal are analyzed.The results show that the larger the sheet thickness or the sheet width is,the larger the bending radius of the longitudinal centerline of the two kinds of formed parts is.While the larger the bending radius of the roll gap midline or the maximum rolling reduction is,the smaller the bending radius of the longitudinal centerline of the two kinds of formed parts is.These numerical simulation results are in good agreement with the theoretical ones,which verify the rationality of the deduced formulas for calculating the longitudinal bending deformation of two typical curved surface parts.(4)The method of rolling 3D surface parts using variable cross-section rolls is studied.The finite element model for rolling curved surface parts using variable cross-section rolls is established and the rolling process of two typical curved surface parts is simulated.The results show that the surfaces of the two kinds of formed parts are smooth and the forming effects are good.The effects of roll diameter,speed ratio and sheet metal import angle on the rolling forming of curved surface parts are analyzed.These forming parameters have less influence on the transverse bending deformation of sheet metal,but greater influence on the longitudinal bending deformation of sheet metal.By reasonably controlling these forming parameters,such as asynchronous rolling at different speeds or setting sheet metal import angle on the rolling forming process,the forming range can be enlarged and the 3D curved surface parts with smaller bending radius can be obtained.