July 11, 2002
There's no reason you can't form sharp, deep boxes with a press brake consistently. You just have to be familiar with what your tooling can and can't do under certain circumstances.
One of the more common questions asked in day-to-day press brake operations is "Can I form a box that deep?"
If you select the wrong tool, the side of the box will crash into the ram. This can produce the wrong bend angle or prevent the forming of tight corners. In most cases, it causes a back break — a forming phenomenon in which the material surrounding the bend takes on a radius (bend) in the opposite direction of the required bend, usually on the same side of the bend as the interference.
I'm going to take this opportunity to discuss the different types of tooling available for deep-box forming, the effects of springback on it, and the reasons why you must consider the ram's width. We'll also discuss power flow and stacking toolholders.
Two different types of tooling are used in a standard press brake for deep-box forming: a standard tool set that typically makes punch angles of 90 degrees, 88 degrees, and 85 degrees (henceforth referred to as 45 degree tools) and a 30/60 tool, in which one die face angle is 30 degrees and the other is set at 60 degrees (see Figure 1).
Unbalanced tooling (30/60) tilts the part being formed 15 degrees relative to the ram compared to a standard 45-degree tool set. This tilting allows a deeper box to be formed within the same tool height, measured from punch tip to ram bottom, as a 45-degree tool set (see Figure 2).
Unbalanced tools do introduce some risk of damage to the part or the tool from side thrust if extra care isn't taken to ensure proper alignment.
Compare the two charts in Figure 3. Chart A is used to estimate the minimum tool height requirements when using a standard 45/45-degree tool set. Chart B is used in the same manner, except that the data is for a 30/60 unbalanced tool set.
By comparing the two charts you can see that the unbalanced tool set can greatly increase the depth of the box to be formed in a given die space.
These charts are only estimations of the maximum depth. As you might have guessed by now, there is a much more accurate way to calculate the maximum depth. But to do so, we need to take springback into account.
Springback is another important aspect to consider, especially with a deep-box forming.
Springback is defined as a material's natural tendency to return to its unbent position. This is caused by the tension between the material's expansion on the outside of the radius and the compression on the inside (see Figure 4).
Besides the usual springback considerations for a normal bend, the width of the ram needs to be taken into account with deep-box forming to find the true minimum tool height required. (For example, Figures 3a and 3b give only rough estimates of minimum tool height. Springback is not even a consideration in these charts. You simply pick the next tallest set.)
The reason the width of the ram needs to be considered is that springback requires the material to be bent past the required bend angle. When the material is released from the bending pressure, it can spring back to a freestanding angle.
The amount of springback can differ by several degrees, depending on inside radius, material thickness, and the kind of material you're using. For example, cold-rolled carbon steel in light gauges has 0.5 degree to 1 degree of springback; aluminum 1.5 degrees to 2 degrees; and 304 stainless 2 degrees to 2.5 degrees (assuming a 1-to-1 relationship between the material thickness and the inside bend radius).
To avoid the problem of encountering the ram before achieving the bend angle, some quick calculations are in order.
Minimum tool height is calculated in the following manner, with only a single variable change between a standard 90-degree tool (balanced) and the 30/60 unbalanced tool (see Figure 1). This means the minimum required tool height can be expressed as follows for balanced tools:
Last but not least, you can achieve extra tool height by stacking toolholders.
Machines that were designed to use European precision tools more often than not come with toolholders that most U.S.-designed machines do not. Whether the holders are custom-made or precision-ground, the trick to making them work safely is to check the power flow through the ram, the holder, and the tool (see Figure 4).
One note of caution: Not all holders have been designed with power flow in mind when it comes to how they stack together. For your own sake, check the power flow! Not doing so can lead to tool crashes or a very nasty accident, all because the tool flexed off center.
With just a little forethought, you too can form some very nice-looking deep boxes on a press brake. Just watch your tooling and your springback, and you should be fine.