An investigation into the behaviour of sheet metal workpieces when subjected to simultaneous double bending
Document Type Article
Simultaneous double bending, where two 90° bends are formed in a single workpiece, in close proximity to each other and in alternate directions, are common in the metal forming industry, especially so in small production batches where press brakes and their associated tooling is extensively used. This bending process, resulting in what is commonly called a ‘joggle’ bend, is executed using specially shaped forming dies. The geometry of these dies determines the geometry of the bend and also the amount of sliding and /or stretching that takes place in the workpiece during the forming process. This in turn determines the forces and therefore the stresses experienced by the dies, and the strains experienced by the workpieces.
The objectives of this research were as follows: • To determine by experiment the relationship between the displacements of metal forming dies, the forces that they are subjected to at particular displacements and the geometry of the forming dies. • To observe and record, via the use of etched grid circles, the strain distribution on the surfaces of chosen experimental test pieces • To compare the experimental results with results predicted by Finite Element Analysis (FEA) using Deform PC Pr, a commercially available elasto-plastic capable FEA package. • To determine, using the same FEA package, the elastic stresses in the dies and the plastic strain in the workpieces at various stages throughout the forming process. • Having considered both experimental and analytical data, to determine whether general conclusions can be drawn that have not been observed previously and which would allow for better understanding and control of the process In order to complete the objectives, a test rig was designed and built: forming dies of various profiles were manufactured; experimental work was conducted and compared with FE predictions. A number of conclusions were noted that can improve the forming process and component design. The principle conclusions are: • Interference of strain fields on adjacent bends is a function of geometry of the die, with no interference occurring at die step heights of 10.5 mm and above for workpiece materials of 3 mm thickness, whether aluminium or steel. No useful experimental or finite element work was done on material thicknesses other than 3mm. • Workplace thinning occurs when the die step height is less than twice the material thickness. • Incremental force required for incremental strain in the workpiece is a function of displacement and has a continually rising characteristic. • Incremental force required for incremental strain in the workpiece is also a function of the geometry of the die, and the prolife of the applied force – displacement curve changes significantly with die geometry. • It is difficult to identify a relationship between die step height and total energy required for deformation, for workpieces make from eith