Bend radius: Bend radius is the radius of the inside curve of a bend in sheet metal, set by the tooling and by the material and thickness being bent.
Sheet metal cannot be folded to a perfectly sharp corner. In air bending on a press brake, the inside radius comes mostly from the width of the die opening. A common starting point for mild steel is an inside radius about equal to the material thickness. Harder materials and some aluminium tempers need a larger radius, or they crack on the outside of the bend.
What it means for the price
Designing to the radius the shop’s standard tooling produces avoids special tools and extra setups. A drawing that asks for an unusual radius may force custom tooling or a different bending method, which adds cost. The radius also changes the flat pattern, so a wrong assumption gives flanges that come out long or short and parts out of tolerance.
Example: a 2 mm steel bracket designed with a 2 mm inside radius suits standard tooling; the same bracket drawn with a 0.5 mm radius may need a sharper punch and risk cracking. The figures are an example.
Common confusions
The inside radius is not the outside radius, which is larger by one material thickness. Drawings should say which one they mean. The radius and the K-factor together decide the flat length. Read design for sheet metal.
Go deeper: Design for sheet metal: bend radii, reliefs, hole distances and flanges