Does a Press Brake Die's Radius Have to Match the Part's Inside Bend Radius Exactly?

No. The die's punch radius sets a floor for the inside bend radius you can get in air bending, but the actual radius that shows up in the part depends on material and how far into the V you push it. A certain amount of variance run to run is normal.

Why air bending doesn't lock the radius

In air bending, the punch doesn't bottom out in the die opening. It presses the sheet down until it springs back to the target angle. The resulting inside radius ends up roughly proportional to the die's V-opening rather than the punch tip radius itself, often somewhere around 16-20% of the V-opening, though that fraction shifts with material. Softer aluminum sits toward the small end. Harder stainless drifts larger. Two different punch radii can produce nearly the same actual bend radius if the V-opening and material match, and the same punch can give different radii across different materials.

Coining is the exception. Bottom the punch fully into the sheet with enough tonnage and the material conforms to the tool geometry, so the inside radius does lock to the punch radius. That's a different process with its own tonnage math, not something you get by accident on a standard air-bend setup.

What actually causes the variance you're seeing

If a part's inside radius is drifting more than expected, check three things before blaming the tooling.

Material lot changes matter first. Yield strength varies batch to batch, especially in cold rolled steel. A softer lot bends to a slightly tighter radius at the same tonnage and stroke.

V-opening selection relative to thickness matters too. The V-opening should scale with material thickness, typically around 6 to 10 times thickness depending on the material. Run the same die on a thicker sheet than it's rated for and the radius grows along with the extra springback.

Ram depth repeatability is the third culprit. On an older hydraulic brake without tight depth control, small drift in ram travel per stroke shows up directly as radius variance.

When you should actually change dies

Swap dies when the drawing calls for an inside radius tighter than your V-opening can produce in air bending, or when you're coining and the punch radius itself doesn't match the callout. Don't swap dies because two parts in the same material came out a few thousandths apart. That's the normal spread of the process, and chasing it with tooling changes usually just moves the variance somewhere else.

The practical takeaway

Treat the punch radius as a floor for the inside bend radius, not a precise target. Digiforge holds bend angle to ±0.5° on press-brake work. If a print calls out a tight inside radius, check whether it's realistic for the material and V-opening combination before assuming the die is wrong.