Why Does a Press Brake's Tonnage Table Sometimes Read Differently for the Same Part on Two Machines From Different Builders?

Because tonnage charts are built from each manufacturer's own die geometry assumptions, and a chart from one builder doesn't necessarily transfer to another builder's tooling even at the same nominal V-opening and material.

The chart isn't measuring the material alone

A press brake tonnage chart looks like it's telling you a property of the material. Bend this thickness of this alloy and it takes this much force. It's actually telling you the force needed for a specific die opening, a specific punch radius and a specific target inside bend radius, all bundled into one number. Change any of those and the number changes, even with identical material.

Different brake builders don't all use the same V-opening-to-thickness ratio as their standard. One builder's chart might be built around roughly an 8:1 opening-to-thickness ratio for a baseline recommendation, another around a slightly different ratio for the same nominal thickness. That difference alone shifts the tonnage number before you account for anything else.

Punch radius and material assumptions differ too

Charts also bake in an assumed punch nose radius and an assumed material tensile strength. A chart built around a standard mild steel tensile spec reads low for the same nominal thickness in a higher-strength alloy. A chart built around a more generous punch radius calls for less tonnage than one assuming a sharper radius, since a sharper punch concentrates force into a smaller contact area.

Two brakes with dies that look nominally the same, same V-width printed on the chart, can call for genuinely different tonnage if the underlying assumptions about radius and material strength weren't identical when each builder generated their table.

What actually matters for your job

The chart is a starting point, not a substitute for checking your actual setup.

Confirm the die opening you're using matches what the chart assumes. Measure it, don't read it off a label that might not match the physical tooling in the brake right now.

Confirm the punch radius in use, especially if tooling has been swapped between jobs or borrowed from another brake.

Check the material spec sheet against what the chart assumes for tensile strength. Stainless needs noticeably more tonnage than mild steel at the same thickness.

The practical habit

If you're moving a job between two brakes from different builders, don't assume the old chart's tonnage number carries over just because the die opening looks the same on paper. Verify the actual die opening and punch radius on the machine you're running today. If the numbers genuinely don't match what the job was originally proved out on, run a test bend and check the resulting inside radius and angle before committing a full sheet. A tonnage mismatch that's too low stalls the ram partway through the stroke. One that's too high risks overloading tooling that wasn't rated for it. Neither is something you want to discover on a production batch.