The CAD Software Development Of Controlling The Continuous Flexible Forming Process

The continuous flexible forming technology is a new type of sheet metal processingmethod, which is first proposed by the Multi-point forming (MPF) technology developmentcenter of Jilin University and corresponding experimental device was developed. Theinnovation of this forming method combines the Multi-point forming with the traditional rollbending forming，with flexible roll instead of the rigid roll in traditional roll bendingmachine. The shape of the flexible roll is controlled by a series of ruled basic body whichcan be adjustable in height. The top flexible roll was pressed down to contact the sheet metaland drive the three flexible roll to rotate，the sheet metal will produce continuous feeding andbent plastic deformation in both horizontal direction and longitudinal direction。Three-dimensional surface parts can be processed by the flexible bending forming，so this formingmethod overcome the limitation of traditional roll bending forming which only can processsimple two-dimensional surface parts. Besides， the surface quality and precision ofthree-dimensional surface parts formed out by continuous flexible forming machine satisfiedthe requirement. All in all，this new type of sheet metal processing method demonstrates agood value of practical application.In order to automatically control the process of the continuous flexible forming, it isvery important to develop corresponding CAD software. Based on the theory of cubicuniform b-spline curves and finite elements, this article researched of the mathematics modelof horizontal direction and longitudinal direction during the continuous flexible formingprocess. The main contents are as follows：(1) Developed the CAD software of continuous control of flexible forming process andresearched on the method of establishing the3-D model of adjusting institutions, flexiblerolls and ruled target forming surface.First of all, based on the programming environment of Visual C++6.0, designed theCAD software interface of controlling the continuous flexible forming process。Secondlyaccording to the structural characteristics of adjusting institutions, flexible rolls and ruledtarget surface and combined with the OpenGL drawing functions， the specific method of building these models was given, and then improve these models display effect by usinglighting technology, integration technology, and anti-aliasing techniques in OpenGL.Besides, all models can be rotated, translated and zoomed by using a mouse duringoperations t, this make the software more usable and convenience.(2) Researched on the reconstruction algorithm and the pretargeting of target surface.In order to achieve the conversion of CAD model, researched on the all the triangledatas of constructing target surface contained in INP and STL files, and set the interface toread the INP and STL format files in software. The INP format file contains the topology ofall triangle meshes，so the target surface can be reconstructed directly by reading thecorresponding node data. For the STL format file，it need to use the binary sort to remove ofredundant nodes, and then reconstructed the topology between face，edge and vertex.，basedon the topologythe target surface can be reconstructed. Finally，the target surface need totransform to the default coordinate system in software. Based on the topology relationship，determined four angle points of target surface and calculated the approximate normal vector.And then, using the geometric transformation theory of computer graphics to rotate andtransform the target surface，make approximate normal of vector target surface and the z axisof default coordinate system in software coincidence. The workpiece pretargeting can becompleted。(3) Normal vector calculation method of particular point on target surface.In order to let the reconstruced surface smoothly show in software， you need tocalculate the normal vector of each node. For the ruled surface which hasAnalytical expression (including implicit and explicit)，the normal vector in these points canbe got by calculating the cross product of two different vectors wich are tangent to thesurface. For the surface reconstructed by triangular mesh, the normal vector of trianglepatch contained the point can be firstly calculated，and then averaged these vectors。Finally， normalized this vector and approximately took it as the normal vector at thespecific point.(4) Researched the method of establishing lateral shaping model of flexible roll in theflexible bending process.In order to establish the shaping mathematical model of flexible roll in the flexiblebending process. First of all，a series of equidistant lines was intercepted from the trianglemesh of target surface along the longitudinal direction, and then the coordinate value ofcontrol points on these equidistant lines were calculate out by using finite element theory.Based on the coordinate value of control points on these equidistant lines and the theory ofthe cubic uniform b-spline curve, the polygon vertices’ coordinate value can be calculated out. Using the multiple knots to control the curve’s boundary conditions, the curve can bemade to pass through the polygon’s first and end point its control points and tangent to theedges at first and end. Finally，the shaping mathematical model of flexible roll in the flexiblebending process can be established based on the vertices of polygon.(5) For ideal elastic-plastic materials，longitudinal control model in the flexible bendingprocess was established.The equidistant lines’ mathematic model along the target surface’s lateral direction alsocan be established according to the method of establishing lateral shaping model, and theneach equidistant line was discreted into countless curves. Afrer using the least-squaresmethod to calculate the average of the curves’radius with the same horizontal coordinate，theaverage can be taken as the longitudinal radiu of curvature. Finally，based on the idealelastic-rebound formula，the longitudinal radiu of surface parts before rebound can becalculated out， and the longitudinal control model in flexible bending process wasestablished according to the geometric relationship between the the longitudinal curvature ofpart and the downward displacement of top flexible.