Tooling ideas to combat springback.
Springback is an ever-present metalforming phenomenon--the annoying elastic recovery of the part after it leaves the die. A 90-degree bend can spring back as much as 5 degrees. If you haven't compensated for this, you may have just made a useless part.
There are three popular ways to combat springback: sitffening the part by adding gussets or corner ribs (a design change), or the production techniques of overbending and coining.
Figure 1 shows a simple V-bend die for right-angle bends. The only difference from a standard 90-degree die set is the shaping of both the punch and female die to an angle less than 90 degrees to compensate for springback. To benefit from this compensation, large unsymmetrical parts should be supported on their longer legs.
Figure 2 shows a scheme for bending up the shorter leg while supporting the longer leg on a pinned ejector element that is slightly tilted for springback compensation.
Figure 3 is a similar approach where the shorter leg is bent downward and a strong compression spring compensates both for springback and variations in stock thickness.
In channel bending, Figure 4 shows the simplest approach: a standard female die (with or without the ejector pad shown) with the male punch slightly tapered (negatively) on both sides. In another variation, Figure 5, the female die is normal and the punch has a slight concave bottom surface matching the curvature of the ejector or pressure pad. The workpiece bulges upward during forming, but when removed from the tool, straightens itself out and draws in the side angles to compensate for bending stresses.
In cases where greater accuracy and uniformity are required, it is necessary to resort to special dies where both the punch and female die have unusual features. In all these following cases, the punch has undercut shapes as in Figure 4, and the female die has movable sides that bend the two sides of the workpiece against the tapered flanks of the punch at the end of the ram stroke.
There are several possibilities for practically achieving this mobility in the female die. In Figure 6, the sides pivot, and strong compression springs cause them to close in, yet allow for a slight opening at the start of the bending operation. In Figure 7, the sides are horizontally sliding members driven by adjustable plungers in the punch holder. Or cam-acting inclined-plane plungers in the punch can accomplish similar lateral motion.
In Figure 8, the two female dies are of a rocking design, actuated by the bottoming of the movable pad. And finally, in Figure 9, the die slides are rotating members that have a return movement affected by springs or counterweights (gravity).
Springback can be eliminated by taking advantage of coining or "setting,' the material in the bending zone. One of the simplest tool designs is shown in Figure 10. The female die is absolutely normal; its opening angle equals the final desired workpiece angle. The male punch has a small protruding nose that provides the coining action.
In another approach, Figure 11, the punch is almost normal; it has a slightly smaller angle, while the female die angle equals the desired workpiece angle. The root of the female die opening has a radius equal to slightly less than the sum of the punch-nose radius and the material thickness. This deficiency produces pronounced coining in the bending zone that eliminates springback.
Photo: Figure 1.
Photo: Figure 3.
Photo: Figure 2.
Photo: Figure 4.
Photo: Figure 5.
Photo: Figure 6.
Photo: Figure 7.
Photo: Figure 8.
Photo: Figure 9.
Photo: Figure 10.
Photo: Figure 11.
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|Publication:||Tooling & Production|
|Date:||Feb 1, 1984|
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