Justin TalianekCreative Approaches to Common Press Brake Challenges, Part 2: Managing Feature Deformation
August 1, 2024Comments
Encountering problems managing deformation while forming on a press brake? Don’t despair. Increase throughput by applying creative approaches to feature-deformation bending problems.
Encountering problems managing deformation while forming on a press brake? Don’t despair. Increase throughput by applying creative approaches to feature-deformation bending problems. This primer explores the mechanisms behind part deformation and how to prevent a problem before it arises. Identifying potential problem features early will grant more robust resolutions.
Bend-Line Danger Zone
A common problem a press brake operator encounters is deformation of a feature that resides too close to the bend line. This could be a hole, window or even the part edge. A feature in proximity to the die space could negatively affect the bend. As a piece of sheet metal folds, the bend line will tend to flow toward the area of least resistance.
Typically, a form’s material edge sits perpendicular to the bend line. As this material-edge angle changes, it creates a pulling effect on the dimension and a pucker on the bend’s exterior. This is known as a “blowout;” the area of material that resists forming because it lacks support during forming. Depending on the severity of the blowout and the amount of surrounding supporting material, this also could cause the tooling to deflect.
Handling Holes
When a designer creates a sheet metal part intended to be mounted to an object, inevitably, it will have holes. The holes may be tapped or designed for a bolt. Maybe they’re only plug-weld holes. In Fig. 1, 12-gauge stainless steel with a ½-in.-dia. hole dimensioned ⅛ in. from the bend line was bent in a 0.984-in.-thick 88-deg. V die. The hole has deformed substantially as a result. Some hole deformation can be acceptable, depending on the application, but when it’s not, a fabricator must find a resolution.
Pilot. One such fix might be “piloting,” or reducing the hole size during blanking and then drilling it out to size after forming. In Fig. 2, the hole diameter has been reduced to ¼ in., using the same V die as in Fig. 1. This reduced the hole’s interference within the die space. If slight deformation occurs, it likely will be removed when the hole is drilled out to size. Keep in mind that this adds another downstream process, and some holes might not be accessible post-forming. This is just one tool in the belt.
Bend-line slit. Another possible solution is “bend-line slitting,” a relief cut on the bend line. The idea is to insert a weak point purposefully where the material should form on the bend line instead of the feature. This keeps the bend line from pulling and eliminates the pucker. Fig. 3 shows a slit longer than required. The slit width only must be as wide as the feature. Again, this was bent in a 0.984-in.-thick, 88-deg. V die. Slits may not be acceptable in the finished part, thus welding and cleaning the area may be necessary, which adds downstream processes. 
Another example of a suitable alternative for a bend-line slit, is when the tooling ends before the material edge. Often, return flanges can pose interference issues with the tooling. Horn punches, or ear tools, are cut away to allow access to some of these shapes (Fig. 4). But what if they are not on hand or they do not match the tooling geometry? Simply slit the bend line from the material edge to the end of the tool. This also works for lower-die gaps when the return flanges descend.
The example in Fig. 5 shows a 0.630-in.-thick, 88-deg. V die, which minimized the amount of deformation but did not eliminate it. Tooling geometry also affects the amount of blowout. A larger shoulder-radius die or an acute die will allow the material to slip earlier in the forming process, worsening any potential blowout.
Use sacrificial backer. No rotary cam die available? The operator can try using scrap material as a “sacrificial backer,” placing a blank placed under the workpiece intended to add support through the bending cycle (Fig. 6). The backer must have strength greater than or equal to the workpiece, or the backer will tear and the workpiece will deform. Using a different material or thickness may result in varying degrees of springback between the workpiece and backer. Ideally, use a backer of the same material length, thickness, grade and temper as the work material. A larger-than-normal V opening may be required to account for the increased thickness of the part and backer. In the example, a 16-gauge stainless-steel part with a short flange would not come around without support from a backer. The backer is the same material as the workpiece. The punch nose radius measures 0.236 in. and the 88-deg. V die is 0.787 in. thick.
Unslitting, silhouetting, tabbing. These all are terms for intentionally not cutting feature lines in the blanking process to keep the skeleton attached to the workpiece for additional support during forming (Fig. 7). Lines running perpendicular can be cut without worry about deformation. Lines running parallel to the bend line are not cut. Depending on the amount of allowable deformation, miters and radii may be cut. 
Press brake operators can prevent deformation on a machined surface by using a sacrificial blank to take up the space (Fig. 8). With a machined surface on top of the part, the blank will act as a punch wrapper, making the pushing radius equal to the punch nose plus the blank’s material thickness. With a machined surface on the bottom of the part, the blank will act more as a sacrificial backer and support the forming process, as long as it bridges the die gap. For step-bending, a set of C clamps may seem like a good idea but the bends of the workpiece and sacrificial blank may elongate at different rates. Double-sided tape can help keep the blank and workpiece in unison while providing a small amount of give to cover the spread.
