A poorly designed part can frustrate your press brake technician, create inconsistent parts, lower part quality, increase scrap rates, and just like that … your profits are gone. The good news is that it does not need to be that way. With a little forethought and knowledge of basic hydraulic press brake design rules, many common forming issues will magically disappear.
Minimum Flange Height
What is the minimum flange height possible in a given part? This depends on the style of bend, die opening, die radius, punch nose radius, and forming method.
In a straightforward bend (see Figure 1), the flange height should be at least two times the material thickness plus the inside bend radius. This works under perfect conditions, using the correct die-width-to-bend-radius relationship. Circumstances may vary among different bending jobs, but this rule for minimum flange height still holds. Note that this does not address the minimum over a specific die width, a different but related topic that I’ll cover in a future column.
The die opening and the die radius (see Figure 2) play a significant role when the flange height is at or near its minimum. Press brake die radii can have either a sharp edge or a compound-radius edge.
Sharp-edge die radii are designed to catch short flanges. The compound-radius corners are designed to reduce the amount of die marking found on the part after forming, caused by kinetic friction between the sheet metal and the die. As the punch applies force, it pushes the material into the die space to create the bend. That same downward force is being applied at the die radius. As the part is pushed into the die space, it drags the sheet over the die radius; that dragging motion can scar the material surface.
If your die has a compound or large radius on both corners of the die, the formula—Minimum flange = 2Mt + Ir—may no longer be valid. Why? Because the sheet edge might not catch the edge of the die, and the workpiece probably will snap down into the die space. If the part does catch by chance, the bend may not be acceptable as every bend will be slightly different from the last. If you need to bend very short flanges, you usually will need a die with a sharp radius.
One option might be to go to a narrower die width, where the compound radius might catch the flange and make the bend. But you have other issues to deal with, tonnage being the primary one. You could end up bottoming the punch in the smaller die, which will change the inside bend radius, the bend allowance, and the bend deduction.
A perfect bend is where the inside bend radius is equal to the material thickness. This is where the 8x rule for die selection applies: The die width or opening is equal to eight times the material thickness. You might be able to go to a 6x die opening safely, which can allow the die to catch the flange edge and make the bend. However, you need to ensure you know how to go to a 6x die width safely, because improper use or an overtonnage situation can damage the tooling and the press brake and, even worse, possibly injure the operator.
If you form a flange that’s tapered down to the bend line, you’ll see a significant amount of distortion in the bend (see Figure 3). Once the taper hangs out over the die space, the flange is no longer supported by the die shoulder and die radius. The material that is supported by the die radius will bend upward as it should. However, the flange’s tapered, unsupported area will bend downward from the die radius to the bend’s center, opposite from what is needed. This creates a convex area along the bend and causes the part to twist under the punch nose. This twisting will taper the bend angle and the height dimension of the flange, usually throwing the workpiece out of tolerance.
You could use a smaller die opening, but that would only limit the deformed area’s size, so using a smaller die width is not the best option.What will work is modifying the part design so that the taper’s inner edge ends at a point that’s two times the material thickness plus the radius dimension (see Figure 4).
However, in some cases you cannot change the design, so what can be done? Another option might save the day. Wrapping is one way to correct a flange blowout. Again, the problem we face is that part of the flange is bending up and part of the flange is bending down, causing various dimensional and angular issues.
When this happens, you may be able to add a piece of material to the forming operation. The wrap material needs to be the same type or stronger, equal to or thicker than the workpiece material, and, of course, be a full piece with no taper. You’ll need to use some common sense when picking a thickness for the wrap as there is no fixed rule.
Once you decide the wrap material and thickness, you can use a little math to calculate a new combined thickness. Add the wrap material thickness to that of the workpiece thickness, then calculate a new die opening capable of forming the new combined thickness. You’ll be bending two pieces of material at the same time, and your bend calculations need to account for this (see Figure 5).
Forming Method Options
You probably air-form (or air-bend) your parts, which is a good thing if you understand how it works. Air forming floats the inside bend radius as a percentage of the die opening. Unfortunately, the forming method can make the workpiece susceptible to twisting and distortion in flanges that are too short.
Bottoming and coining are different, as they stamp the inside bend radius into the material. Additionally, these methods are associated with a tight inside bend radius and the punch having a small nose radius. This creates a sharp-bend situation, typically something you want to stay away from when air forming.
With coining, of course, creating a sharp inside radius is the point, and it is an option for bottom bending. To some extent these forming methods can reduce the amount of distortion found in a tapered flange by forcing the material deeper into the die. Still, this is not the best option as there will still be some taper and blowout of the bend, even after you spend time tweaking the back- and sidegauges.
Do not forget that performing bottoming or coining increases forming tonnage significantly. Also know that if you do change from air forming to bottoming or coining, you will also need to accommodate for the change in your bend allowance and bend deduction.
Mind the Flange Height
Note again that all this relates to the minimum flange dimension for a given part, not the minimum flange possible over a given die opening, a related but different subject that I will be taking up in a future column.
Regardless, when designing parts, keep your flange height at least two times the material thickness plus the inside bend radius, especially if the flange tapers off. A part redesign might not be possible, but the two options discussed here—wrapping and choosing an alternative bending method—might save the day.
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