# Discovering the limits of press brake tooling

*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?

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.

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 = 0.60 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, use 1.102 for the conversion factor:
- U.S. tons ÷ 1.102 = metric tons
- The maximum machine tonnage capacity can be expressed as:
- Tonnage/(Distance between the side frames x 0.60)

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.

Figure 3:Standard throat depths for gooseneck punches generally begin with about 20 percent of total width past center being removed. |

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**).

Throat Depth | Tool Capacity (Percent) | Tons per Foot |
---|---|---|

Straight Tool (No Throat) | 100 | 30 |

70 | 70 | 21 |

75 | 50 | 15 |

80 | 30 | 9 |

85 | 20 | 6 |

Fig. 4When throat depth is increased, tool capacity decreases correspondingly. |

## 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.

## 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 thickness^{2}x ^{___________________________________}Die Width | ) | ÷ 12 |

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 a value of 1. The factors, or multipliers, for various materials (M) are listed here. For any material type that has not been given a factor, a comparison of tensile strengths allows 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

- Air form = 1 (baseline factor)
- Bottom bending = 1.5 to 5
- Coining = 10 times and greater

- Material type = 304 stainless steel; the material factor (M) = 1.4
- Material thickness = 0.050
- Die width = 0.236
- Bottom bending = 1.5 to 5

Tons per Inch = | 575 x Material Thickness^{2}x ^{___________________________________}Die Width | ) | ÷ 12 x M x P | ||

Tons per Inch = | 575 x 0.050^{2}x ^{___________________________________}0.236 | ) | ÷ 12 x 1.4 x 5 | ||

Tons per Inch = 3.5531 |

To find the total tonnage needed, multiply the tons per inch by the part length in inches:

- Total Tonnage = Tons per Inch x Part Length
- Total Tonnage = 3.5531 x 6.375 in.
- Total Tonnage = 22.65 tons

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.

Figure 5The bending pressure doesn't build up gradually or evenly. The press brake develops most of the bending force in the early stages of the bend. |

## 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.

### Steve Benson

ASMA LLC

Salem, OR 97301-4468

Fax: 503-399-7514

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#### The FABRICATOR

*The FABRICATOR* is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. *The FABRICATOR* has served the industry since 1971.