Why Does Coining a Bend Need So Much More Tonnage Than Air Bending?

Coining forces the material to fully conform to the punch and die shape at the bottom of the stroke, which means driving well past the point where the material is just bending. Air bending only pushes the material partway into the die and lets it spring into shape on its own, so the force needed is a fraction of what coining takes for the same angle. Shops commonly see coining tonnage run several times higher than air bending on the same setup.

What's actually different about the two methods

In air bending, the punch tip contacts the material and pushes it into the V-opening without the material ever touching the die walls at full depth. The bend angle is a function of how far the punch travels, and the same die opening can produce a range of angles depending on punch depth. Springback still happens, and the operator or the machine's programmed offset compensates for it.

Coining, sometimes called bottom bending, drives the punch all the way down until the material is compressed between the punch and the die surfaces.

That's what makes the bend angle and radius extremely repeatable and mostly immune to the material variation that causes springback in air bending. It's also what demands the tonnage: you're not just forming the bend, you're plastically deforming the material against a hard stop on both sides.

Where the extra tonnage actually comes from

Once the material contacts the die at the bottom of a coining stroke, further ram travel doesn't bend more material, it compresses what's already there. That compression resists much harder than the initial bending does, the same way it takes far more force to flatten a metal part than to bend it partway. A press brake sized correctly for air bending a given thickness can be badly undersized if someone tries to run the same job as a coined bend without checking the machine's rated tonnage first.

Why anyone bothers with coining at all

Air bending is cheaper on tonnage but leaves the exact angle dependent on the material's springback, which varies with alloy, temper, and even coil to coil. Coining trades that tonnage cost for angle repeatability that doesn't depend on guessing springback correctly. For a part where bend angle accuracy matters more than tonnage cost, coining is worth it. For most production work, air bending with a verified springback allowance gets there for less.

What this means before you run a job

Don't assume a press brake's air-bending tonnage rating applies if the job calls for bottom bending or coining.

Check the tonnage chart for the specific method, material, and thickness combination you're running, and confirm the machine is actually rated for it, not just capable of reaching the stroke depth. Running a machine past its rated tonnage on a coined bend is a mechanical failure waiting to happen, not just a bad part.