Deep box forming is one of the most common and hardest problems that you will encounter while operating a Press Brake.

Is this box too deep to form? How do I know? What tools do I use? When the final bend is made will the box close without crashing into the ram of the press?

There are some tricks to solving this dilemma and we begin with learning some new terms: Balanced and unbalanced. All of the tooling discussed elsewhere in this book are generally considered to be Balanced tooling. A Balanced tool has the same tool angle on each side of the tool, for example, a 90° punch and die set would have angles of 45° on each face of the tool.

An Un-Balanced tool has different angles on each side of the tool. The punch and die set will still have 90° of bend angle, but, one side is set at 30-degrees and the other is 60-degrees.

Figure 2 shows a balanced set, and figure 3 an unbalanced set. The idea being, you tip the box and get a taller box into a shorter space. So, just how tall of a box can you form on a press brake?

For that, we start with two somewhat ancient charts, figures 4 and 5.

These charts have been around a very long time, are fractional and will get the part done with clearance. But, being fractional they are just general guidelines. It is, however, quite easy to calculate the absolute minimum tool height with a little more accuracy.

The first step is to find out the total amount of clearance required for a given part. There are a couple of ways this can be done; you could consult charts or use a little right angle trigonometry

Look closely at the picture in figure 6; you see the two right angle triangles? They both have a common center line, that of the punch. These triangles are shown with the punch removed for clarity.

“a” is the hypotenuse which we will solve for, “b” is the Adjacent Side and “c” is the Opposite side of the lower triangle in figure 6.

For this triangle, we need to determine what information is available for use in the calculations. First, we know that angles “B” and “C” are 45° and angle “A” is 90°. So to solve we only need one more piece of information, the dimension of the side.

Assuming that the angle is 90° or less and you are using a standard balanced tool (45°on a side) and a box depth of 10.000 inches, the first triangle would be solved as follows:

If the Box depth is 10-inches deep at 90° the hypotenuse will calculate to
14.142-inches using the formula: c/Sine C.

10 / Sine 45 = 14.142

This takes care of the lower triangle; now for the upper one. There are again many ways that we could arrive at the answer, but the easiest is to measure the ram width and divide that by two. Because the angles of the triangle are again 45° and we then know that one-half of the ram is equal to sides “b” and “c” of the triangle. In this case, the hypotenuse is irrelevant. If the ram is 2.500 then sides “b” and “c” will be 1.250

There is one more thing to consider, we still need to allow for [glossary]springback[/glossary]. So instead of just dividing the ram by two just multiply by .563. This will allow for enough springback clearance to form most materials.

This means the minimum required tool height for a balanced tool, figure 7, can be expressed as follows:

The flange depth = b
The tool angle (45° ) = B
Ram width = Rw

Allowable minimum height = ( b / cosine B ) + (Rw * .563)

When extra clearance is required is when we would consider the use of a 30/60 unbalanced punch and die. It is best to avoid using an unbalanced tool except when you have no other choice because unbalanced tools develop a large amount of side-thrust causing stress in the press brake.

The formula we use to solve for an unbalanced tool is the same as it would be for a standard balanced tool. The top triangle will still be a 45° right angle triangle and the formula is solved just as balanced tools are. With the unbalanced tool, the only part of the formula that changes is the tool angle, angle “B”; everything else will be the same.

This means that the minimum required tool height for an unbalanced tool can be expressed as follows:

The flange depth = b
The tool angle (30° ) = B
Ram width = Rw

Allowable minimum height = ( b / cosine B ) + (Rw * .563)

Press Brakes – Other tools and methods, courtesy of Asma LLC and looks at topics ranging from offset bending to the subjects covered in this chapter.

Press Brakes – Other tools and methods
Courtesy of Asma LLC

After reviewing this material you should now be able to:

1. Define balanced vs unbalanced tooling.
2. Explain why fractional charts are not the best option.
3. Calculate absolute minimum tool heights.
4. Explain why a box brake may be a better option than a press brake.
5. Explain how unbalanced tooling can adversely affect the press brake.

Top of the page: Deep Box Bending

Next chapter: Gauging and Fixturing