64 The FABRICATOR August 2013

By Steve Benson

Selecting the press brake and tooling for a particular job involves much more than just making sure there’s enough brute force.

Make one miscalculation, and pieces of tooling can fl y while, at the same time, you may exceed the ram center load, causing the press brake ram and bed to bend. If you don’t believe this can hap-pen, look at almost any high-mileage mechanical press brake and count the number of shims be-tween the die and the bed.

To avoid these problems, take a look at your ton-nage and load limits.

How Much Tonnage Does a Job Require?Start by looking at a standard tonnage chart (see Figure 1) that relates the workpiece thickness with the width of the die opening. Where the two inter-sect gives you the approximate tonnage per foot required for the job, based on 60,000-PSI tensile cold-rolled steel.

Tonnage charts can be inaccurate, though. So

to fi nd the tonnage a job requires quickly and ac-curately every time, consider using the following formula, which solves for tonnage per inch when air forming 60,000-PSI-tensile AISI 1035 cold-rolled steel over a given die width: Tonnage per inch = {[(575 × Material thickness2) / Die width] / 12}. Multiply this value by the bend length, and you’ll get the total tonnage required for the application.

Still, in any job shop, material types vary, and each has its own tensile strength. Tensile strength is defi ned as the ability of a material to bear weight without breaking or being pulled apart under a smooth load, not a sudden impact.

To incorporate di� erent materials into the ton-nage calculation, you include a material factor in the tonnage formula. Here are some common ma-terial factors:

• AISI 1035 cold-rolled steel = 1• 304 Stainless = 1.4 to 6 • Aluminum 6061 T6 = 1.28 • Aluminum 5052 H32 = 0.50We use the 60,000-PSI-tensile AISI 1035 (the

most common type of cold-rolled steel used) as a baseline and so give it a value of 1. To obtain a factor for a specifi c material, you can perform a simple comparison of tensile strengths, working with 60,000-PSI tensile as the baseline. For example, if you’re working with a material with a specifi ed maximum tensile strength of 120,000 PSI, divide that by 60,000 to get 2, which becomes your mate-rial factor in the tonnage formula.

Considering all this, here is the tonnage calcula-tion you can use to determine the amount of ton-nage you need for a job:

Tonnage per inch = {[(575 × Material thickness2) / Die width] / 12} × Material factor

Total tonnage needed for the job = Tons per inch × Length of bend in inches

� ese formulas apply to air forming. If you’re bottoming or coining, you need to take this into account. Bottoming and coining tonnage calcula-

Why tonnage matters

Never—ever—exceed the load limits for your tooling or press brake

Figure 1 Tonnage charts relate workpiece thicknesses with the width of the die opening. Note that tonnage charts can be inaccurate for the job at hand.

Thickness of Metal

Width of Female Die OpeningApproximate pressure in tons per linear foot required to make a 90-degree air bend in mild steel (60,000 PSI tensile)

Gauge Dec. 1⁄4" 5⁄16" 3⁄8" 1⁄2" 5⁄8" 3⁄4" 7⁄8" 1" 11⁄8" 11⁄4" 11⁄2" 2" 21⁄2" 3" 31⁄2" 4" 5" 6" 7" 8" 10" 12"20 0.036 2.6 2.2 1.6 1.2 1.018 0.048 3.5 2.8 2.1 1.7 1.316 0.060 5.3 3.7 2.8 2.2 1.714 0.075 5.5 4.6 3.5 3.0 2.5 2.113 0.090 6.4 5.5 4.3 3.6 3.2 2.812 0.105 9.2 6.9 6.2 5.0 4.3 3.9 3.111 0.120 10.1 8.0 7.0 6.1 5.3 4.3 2.910 0.135 10.3 8.7 7.8 6.9 5.7 3.99 0.150 11.9 9.8 8.8 7.0 5.0 3.77 0.188 16.9 13.9 11.2 8.3 6.7 4.9

1⁄4" 0.250 27.5 22.1 15.0 11.6 9.6 7.9 6.75⁄16" 0.312 39.2 26.5 19.3 15 12.5 10.4 7.73⁄8" 0.375 42.7 31.2 23.8 19.5 16.3 12.4 9.6

7⁄16" 0.437 45.5 35.2 28.5 24.4 17.4 15.0 11.51⁄2" 0.500 48.5 39.5 33.2 24.6 19.5 16.1 13.45⁄8" 0.625 65.5 57.9 42.8 33.1 27.3 23.3 17.03⁄4" 0.750 92.3 68.1 53.0 36.2 36.2 26.9 21.07⁄8" 0.875 103.1 79.9 52.3 52.3 39.2 31.2

1 1.00 112.1 90.4 75.5 55.7 43.7

DANGER ZONE

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August 2013 The FABRICATOR 65

tions are rough approximations at best, because they can vary depending on the machine operator. But these bending methods do increase required tonnage substantially. If you’re bottoming, you need to multiply your calculated tonnage per inch by at least a factor of 5; if you’re coining, the factor can be 10 or even more.

Also note that the maximum tonnage doesn’t happen all at once; it builds along a steep curve. Under close observation, you will notice that 80 percent of the total tonnage is developed in the first 20 degrees of bend, so even a small bend angle can produce enormous pressures on the tooling and equipment.

There’s also more tonnage to consider than just what it takes to bend the sheet. Workpiece size also can affect tonnage requirements. When you are working with a small part, this force is extremely small. But for larger parts, the force can be signifi-cant. Supporting the material—be it with a lot of manpower, material lifters on the brake itself, or a crane—can help mitigate this effect.

Whether you are using a tonnage chart or cal-culating the tonnage requirements manually, you need to know how to apply the information. The job’s tonnage requirements should not exceed the tonnage capacity of the press brake or the tooling.

Press Brake Tonnage CapacityBefore continuing, we need to define some terms, the first being deflection. Deflection is the tempo-rary deformation of the press brake’s ram and bed that occurs naturally under a load (see Figure 2). When the pressure is removed, so is the deflection present under load, and the ram and bed return to their original positions. In other words, deflection is normal and to be expected.

The second term is upset, which occurs when you exceed the maximum deflection of the press brake ram. When this happens, the ram and bed stay permanently deflected (bent); once upset, the press brake ram and the bed become difficult and expensive to repair.

Most press brakes are designed for centerline loading; that is, they are designed for work in the center of the press brake bed. The ram power comes from the cylinders on either side of the bed,

60%

When Reversed

Loses Center Maintains Center

Figure 2 Deflection is the temporary deformation of the press brake ram and bed that occurs naturally under a load. An overtonnage situation can push a brake beyond its deflection limit, causing upset, where the ram and bed stay in a permanently deflected, or bent, state.

Figure 3 Generally, you should never apply full machine tonnage over an area that’s less than 60 percent of the distance between the side frames.

Figure 4 Standard American tooling (left) is reversible as far as pressure flow is concerned, but it can lose its center in the die. Note the small space that appears between the punch and right-hand die face. European-style tooling (on the right) is completely reversible as far as centers are concerned, but the pressure flow changes, as shown in Figure 5.

Figure 5 European-style tooling can be reversed, but the pressure flow changes. As shown on the right, the pressure flows past the ram and onto the mounting bolts. This creates a very dangerous situation.

Make one miscalculation, and pieces of tooling can fly while, at the same time, you may exceed the ram center load, causing the press brake ram and bed to bend.

If you don’t believe this can happen, look at almost any high-mileage mechanical press brake and count the number of shims between the die and the bed.

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