Why Does a Die Opening Chosen for One Bend Radius Sometimes Not Work for a Slightly Different Radius in the Same Material?

Because the inside bend radius on an air-bent part is determined by the V-die opening, not by the punch nose radius. Change the die opening a little and the resulting inside radius changes with it, even though nobody touched the punch.

Air bending doesn't copy the punch

On a bottoming or coining operation, the part takes the shape of the tooling because the punch drives the material fully into the die. Air bending is different. The sheet only ever touches the die shoulders and the punch tip, and it bridges the gap between them without being forced to the die's exact profile. The inside radius that results is roughly a fixed fraction of the die opening, commonly cited as somewhere around one-sixth of the V-width for typical air bending. It has very little to do with the punch's nose radius, within reason.

That means two setups running the same punch but different die openings will produce visibly different inside radii on the same material and thickness. If a print calls out a specific inside radius, the die opening is what has to match that spec, not the punch.

Where this actually bites people

A shop swaps a die for a job with a slightly thicker sheet, and picks the next size up in the die set without checking what radius it produces. The tonnage works out fine, the angle holds, and the part still looks right by eye. Then it fails inspection because the inside radius drifted outside the drawing's tolerance band. Nothing was "wrong" mechanically. The die just wasn't the one that produces the called-out radius.

The same problem shows up when a shop keeps one die per V-opening but runs several materials and thicknesses through it, assuming radius is mostly a function of thickness. Radius as a fraction of die opening holds reasonably well across similar material families, but a big enough change in material behavior, especially springback, shifts the number.

What to check before running the job

Look at three things together before committing tooling to a job: the material and thickness, the die opening, and the radius the drawing actually calls out. If the drawing specifies an inside radius, work backward from that radius to the die opening you need, don't just grab whatever die is already in the machine because the tonnage happens to fit. A tonnage chart tells you the die will survive the job. It doesn't tell you the part will match the print.

If a print doesn't specify an inside radius at all, most shops default to something close to the material thickness, which keeps the part inside a normal air-bending range without forcing an unusual die choice.

The practical takeaway

Radius and die opening are the same decision, not two separate ones. Pick the die for the radius you need first, then confirm tonnage and angle work with that choice, not the other way around.