Discovering the limits of press brake tooling

THE FABRICATOR® AUGUST 2000

May 30, 2001

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One of the most important aspects of press brake forming is tooling selection. What are the tools capable of? What kinds of loads can they withstand?

One of the most important aspects of press brake forming is tooling selection. What are the tools capable of? What kinds of loads can they withstand?

Image
Figure 1:
A standard straight press brake punch withstands more tonnage per foot than the press brake itself withstands.

Some press brake tooling manufacturers give a maximum allowable tonnage per foot for their tooling; others do not. This author often has recommended that fabricators never exceed the manufacturer's maximum allowable tonnage, but if the tooling isn't rated, how does the user know how much tonnage is too much?

Straight Punch Tonnage

Exactly how much tonnage per foot a given tool will take depends on material type, tool geometry, and tool hardness, but estimating the maximum allowable tonnage for that tool is possible. As an example, a standard straight press brake punch (see Figure 1) will withstand far more tonnage per foot (meter) than the press brake itself can withstand.

The maximum tonnage that a press brake can withstand safely can be stated as a rule of thumb: Never apply full machine tonnage over a length less than 60 percent of the distance between the side frames (see Figure 2).

For example, regardless of the machine's full tonnage rating, if the bed is 10 feet between the side frames, a full load could be applied uniformly only to forming lengths of 6 feet or more. Using full tonnage over a shorter length would cause a small section to be overloaded, resulting in some degree of upset to the bed, ram, and tooling.

Image
Figure 2:
Full machine tonnage should not be applied over a length less than 60 percent of the distance between the side frames.

Considering all the different variables such as material type and geometry, predicting the maximum tonnage capacity for a given tool is difficult. On the other hand, specialty tooling aside, a straight punch with a matched standard die can withstand greater tonnage than it would take for a machine upset (permanent deformation).

For a 250-ton press brake with 14 feet between the side frames as an average with a 60 percent minimum working area for full tonnage (avoiding upset), maximum tonnage is calculated as follows:

60% x 14 feet = 8.4 feet
250 tons/8.4 feet = 29.76 tons per foot
When rounded up, the maximum tonnage per foot under full load is 30 U.S. tons per foot. To find the equivalent metric tons, the following equation can be used:
U.S. tons/1.102 = metric tons
Maximum machine tonnage capacity can be expressed as:
Tonnage/(Distance between the side frames x 60%)
Image
Figure 3:
Standard throat depths for gooseneck punches generally begin with about 20 percent of total width past center being removed.

Assuming the same relationship of press brake tonnage to side frame length, 30 tons per foot is the maximum pressure that ever can be applied to tooling to avoid damage to the press brake for working bend lengths less than 60 percent of the side frame distance. Tonnage capabilities also should be halved in sectionalized tooling to avoid spot damage caused by overtonnage.

Gooseneck Punch and V-die Tonnage

Although there are variations, standard throat depths for gooseneck punches generally begin with about 20 percent of the total tool width (thickness) past center being removed (see Figure 3). This leaves only 30 percent of the total tool body for transferring the tonnage to the bend.

A 70 percent (20 percent past center) throat depth produces a 30 percent drop in tonnage capacity from that of a straight tool. For any increase in throat depth there will be a corresponding decrease in tool capacity (see Figure 4).

V-die Tonnage

For a V die, as long as the same principles are applied, the maximum tonnage is 30 tons per foot under maximum load (half that tonnage per foot for a sectionalized set). Acute tooling maximums also should be halved to 15 tons or less to avoid splitting the die or damaging the press brake.

Image
Figure 4:
When throat depth is increased, tool capacity decreases correspondingly.

Calculating Total Forming Tonnage

Forming tonnage can be computed quickly and accurately. Charts can be inaccurate (whether for bend deductions or tonnage) because they do not take into account forming methods or the types of bends being performed. Just like bend deductions, the tonnage can be predicted every time.

Tonnage per inch for 60,000 pounds per square inch (PSI) mild steel can be expressed as:

Tons per inch = [(575 x material thickness2)/die width]/12
This is the pressure required to bend a 1-inch piece of mild steel with a 60,000-PSI tensile strength in the bottom die width that the user has selected. The total tonnage required can be found by multiplying this number by the number of inches in a given bend.
Does the required tonnage exceed the capabilities of the tool to withstand the force? The user must either consult the tooling fact sheet to determine the tonnage per inch that the tool will handle or calculate it manually.
The maximum tooling tonnage per inch times the number of inches to be formed equals total allowable tonnage for that particular tool, which for the example 250-ton brake should never exceed 30 U.S. tons.
Different types of material also have an effect on the required tonnage. Because of variances in the tensile strength of different materials, the formula for pressure is incomplete. As in most cases, the basic formula is grounded in the tensile strength of cold-rolled steel, or about 60,000 PSI.

To discuss the factors used in the following formulas, a baseline reference is required. In this instance, American Iron and Steel Institute (AISI) 1035, the most common type of cold-rolled steel used, has been given a factor value of 1.

The factors, or multipliers, for various materials are listed here. For any material type that has not been given a factor value, a comparison of tensile strengths will allow an educated guess of its factor value:

  • 304 stainless = 1.4 to 6
  • Aluminum 6061 T6 = 1.28
  • Cold-rolled steel = 1.00
  • Aluminum 5052 H32 = 0.50
In developing a tonnage value, the forming method is the last aspect requiring a factor. Again, multiplying the final tonnage by the method factor results in a true total tonnage. Those factors are:
  • Air form = 1 (baseline factor)
  • Bottom bending = 1.5 to 5
  • Coining = 10 times and greater
Using the formula previously determined:
  • Tons per inch = {[(575 x material thickness2)/die width]/12} x the material factor
  • Material type = 304 stainless steel
  • Material factor = 1.4
  • Material thickness = 0.050
  • Punch radius = 0.030
  • Die width = 0.236
  • Bend length = 6.375
  • Tons per inch = {[(575 x 0.0502)/ 0.236]/12} = 0.507 ton per inch
  • Tonnage per length (baseline) = 0.507 x 6.375 (bend length) = 3.235 tons
  • Tonnage with material factored in = 3.235 x 1.4 (material factor) = 4.530 tons
  • Total tonnage = 4.530 (tonnage with material factored in) x 5 (method factor for bottom bending) = 22.65 tons
Image
Figure 5

For bend lengths of 60 percent or greater of the bed between the side frames, 22.65 is an acceptable applied tonnage if the maximum applied tooling tonnage is 30 tons per foot. At the same time, if sectionalized tooling or shorter pieces of tooling are used (less than 60 percent of the bed), 22.65 tons is an unacceptable pressure.

Dividing that final tonnage number by 12 determines the allowable tonnage per inch the tool will handle. Multiplying the answer by the total bend length will provide the maximum allowable tonnage over the bend length. This value must be greater than the required tonnage value. Note that the maximum required tonnage doesn't happen all at once; it builds up along a curve (see Figure 5). The graph shows that 80 percent of the total tonnage is developed in the first 20 degrees of bend angle.

Even with a small bend angle, the pressure on the tooling and equipment can be great.

Conclusion

Even if the manufacturer does not rate a tool for the tonnage it can handle, the tooling user can make an educated guess at what it should be.

For both personal safety and the longevity of the press brake, a general deflection factor for minimum tooling length at maximum tonnage application of 60 percent can be used in the formula. For more accuracy, the user should ask the press brake's manufacturer to explain how the deflection of a given machine is rated and then substitute those values in the formula.



ASMA LLC

Steve Benson

President
ASMA LLC
2952 Doaks Ferry Road N.W.
Salem, OR 97301-4468
Phone: 503-399-7514
Fax: 503-399-7514
Steve Benson, a member and former chair of FMA's Precision Sheet Metal Technology Council, is the president of Asma LLC.

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