March 7, 2006
It is all too easy to ruin a tool or upset a ram if bottom bending is done incorrectly, which is why many manufacturers do not recommend bottom bending when using their equipment or tooling. Understanding V-die selection and the effects of your decisions should be first and foremost in any bending operation, including bottom bending.
Liability! It's more than just a word. It is the single reason that manufacturers of press brakes and press brake tooling maintain that you cannot bottom bend with their tooling or machines.
It is all too easy to ruin a tool or upset a ram if the process is done incorrectly. Ram upset occurs when the natural deflection of the ram and bed becomes permanent. So who can blame manufacturers for not recommending bottom bending? Then again, can you afford not to use this time-honored method of forming sheet metal?
Whether you are bottom bending or air forming, understanding V-die selection and the effects of your decisions are first and foremost to any bending operation.
The effects of V-die selection vary, depending on the press brake's bending method. In air forming, the inside radius is produced as a percentage of the V-die opening. Arguably, fewer skills are required to air form than to bottom bend—correctly. Air forming—once set up—requires patience and the attentiveness to check and adjust the bend angle and dimensions. These constantly changing bend angles and dimensions often are made worse by poor punch selection. The punch-to-material relationship also affects bottom bending.
Improper V-die selection creates situations in which the press brake or the punch and die set is damaged.
For as long as press brakes have existed, the rule of thumb for calculating the correct V-die opening has been that it should be eight times the material thickness.
But is that true?
Yes, but only if the material thickness is equal to the punch radius. This rule of thumb loses validity in all other cases.
For example, if you apply the "eight times" rule to calculate the V-die opening for 0.050-inch material, you should need a 0.400-in. opening; fair enough, but will that produce a 3.000-in. inside radius? Not likely (bump forming excepted).
By using the following formula, you can achieve a geometrically perfect V die:
V-die opening = (Outside radius x 0.7071) x (a factor).
Wonderful, you say, but this isn't rocket science. Well, to some extent it is rocket science, because as you deviate from a perfect V-die opening, the situation changes, especially as the die width increases.
As you form a bend, material is drawn into the V die early in the bending process, creating some extra radius between the punch tip and the V-die shoulders. As the punch continues into the V die, the V tightens and eventually matches the punch radius.
As you form a bend, material is drawn into the V die early in the bending process, creating some extra radius between the punch tip and the V-die shoulders (see Figure 1). As the punch continues into the V die, the V tightens and eventually matches the punch radius.
When a larger-than-optimal V die is used, the bending process produces a larger, additional inside radius, creating an angular illusion in the finished bend. The effect is the same whether you are air forming or bottoming.
This is when bottoming becomes dangerous. If you have ever watched the bottoming process take place, you've seen the material bend beyond 90 degrees, and then touch off the punch face before being forced back by pressure—in essence, forming the springback out of the bend.
When you air form, you make a 90-degree bend by forcing the material to 90 degrees plus the springback. When the pressure is released, the spring-back pops the material back to the 90-degree angle.
When you bottom bend, continued downward pressure forces the material back to 90 degrees. But when a V die is too large, the larger radius manifests itself as a bend angle. No matter how much pressure you apply, the part still will not be forced to a 90-degree angle.
Using a V die that is too large is the single most common reason bottom bending damages equipment or causes press brake ram upset. This happens because the operator exceeds the allowable tonnage while trying to correct the overbend by trying to force the bend to the specified angle.
Every material has some amount of springback. When bottom bending, you can compensate for springback in two distinct ways. For both of these two methods to work correctly, you must use the correct V-die opening.
The first way to compensate for springback is by altering the punch angle. For example, the H-series grade of aluminum has 1.5 to 2 degrees of springback. To bottom bend this material, you would need an 88-degree punch and a 90-degree bottom (the set angle) to produce a "bottomed" bend. By using the punch angle that bends the material to 92 degrees before it is forced back to 90 degrees, you can compensate for having too much punch angle.
You also can deal with springback by using a 90-degree sharp punch and pinching action at the bottom of the bend. A sharp bend is a bend in which the punch tip radius is narrower than 63 percent of the material thickness. This forms a small ditch at the center of the inside radius that you can pinch at the bottom of the bend to relieve the springback when the relationship is sharp.
Despite the problems associated with larger-than-necessary V dies, using them can be advantageous in some situations. As odd as it may seem, there are some advantages to using an improper V-die width that can be employed both for air forming and bottom bending.
For example, one grade of plated mild steel has a slightly larger springback factor than regular mild steel—as much as 1 degree—because of the plating. Because a too-large V die creates a radius that manifests itself as angle when you measure it, it can provide you with the consistency of the bottom bend when the correct punch angle is not readily available.
If you bottom bend correctly, you can do it safely and produce a top-quality part. However, bottom bending is not as easy as it may seem at first. It does require an understanding of the process, the correct application of tonnage, and the effects of over- and undersized V dies.
So, bottom bending still can be considered a liability. That's why this author does not recommend bottom bending. (Wink, wink!) For those who do bottom bending, it is not a panacea. But when it's the right thing to do ... it's the thing to do.
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