Why Does My Bent Part Come Back With a Different Inside Radius Than the Drawing Called Out?

Because the inside bend radius on a formed part is a function of the tooling, specifically the V-die opening and the punch nose radius, not a value the shop dials in freely. If your drawing specified a radius that doesn't match a die combination the shop actually has, you get whatever the closest available tooling produces.

The radius comes from the die, not from a setting

On a press brake, the punch nose radius sets a floor on the inside bend radius, and the V-die opening determines how much the material wraps around that punch before springback lets it relax. A given material thickness paired with a given die opening produces a fairly predictable inside radius (shops keep charts for this), but it isn't infinitely adjustable. You can't ask for a 0.5 mm inside radius on 3 mm plate with a die sized for that thickness and expect to get it. The material will bridge across the die opening and form a larger radius no matter what the print says.

This is different from machining, where a radius is cut directly by a tool path and can be whatever you specify within reason. In forming, the radius is an emergent property of tooling selection, material thickness, and to a lesser degree the material's own springback behavior. If the drawing radius doesn't correspond to an actual die in the shop's inventory, the operator picks the nearest one and you get its radius instead, whether or not anyone flagged the substitution.

What causes the biggest mismatches

Three things account for most of the surprises:

Drawing radius smaller than any practical die for the thickness. A common mistake is copying a radius spec from a thinner part onto a thicker one without checking whether it's still achievable. As a rule of thumb, minimum inside radius scales with thickness. Thin sheet can take a tight radius, thick plate can't, and pushing a small radius on thick material also raises the risk of cracking on the outside of the bend.

Springback wasn't accounted for. Material relaxes slightly after the punch retracts, opening the angle back up a bit and easing the radius. Different alloys spring back different amounts, and if the tooling was set up assuming one material's springback behavior and the part actually ran in a stiffer alloy, the resulting radius (and angle) drifts from what a first-pass setup predicted.

The CAD model used a generic default radius. A lot of sheet metal modeling defaults to a placeholder inside radius until someone changes it. If nobody updated it to match the shop's actual tooling, the drawing shows a radius that was never achievable to begin with.

What to do about it

Ask the shop what die openings they run for your material and thickness before finalizing the drawing, or at minimum, specify the radius as a reference dimension rather than a hard tolerance if you don't know the shop's tooling. If a mating part or a critical fit depends on that inside radius, say so explicitly and expect a conversation about what's achievable rather than assuming the number on the print is automatically buildable.

At DigiForge, sheet metal forming holds a bend angle tolerance of ±0.5° once the part is set up on a die that matches the material and thickness, but that's a statement about angle repeatability, not a promise to hit an arbitrary radius. If your design tolerates a range of inside radii rather than one exact number, note that on the drawing. It gives the shop room to use tooling that's actually on hand instead of guessing at your intent.

The short version: pick a radius your material thickness can actually form, or don't specify one at all and let the shop's standard tooling set it.