Design for Brake Press Forming — DFM Guide

Bend Relief

Any bend that terminates at an edge of the material requires a bend relief — a notch cut into the flat blank at the end of the bend line. Without it, the material tears and bulges at the bend terminus. Relief width: material thickness. Relief length: inside bend radius + material thickness, extending past the bend line. Always include bend reliefs in your laser-cut flat pattern.

Minimum Flange Length

The minimum flange that a press brake can reliably form is 4x material thickness (some sources say 3x, but 4x gives you margin). On 0.125" steel, minimum flange is 0.500". Shorter flanges require special tooling (gooseneck, acute, or offset dies) and add cost. If your design needs a 0.250" return on 0.125" material, consider a hem instead of a bend.

Hole-to-Bend Clearance

Holes, slots, and other features must be kept away from bend lines. Minimum clearance: 2.5x material thickness from the edge of the feature to the bend centerline. On 0.125" material, that's 0.313" minimum. Features closer than this distort during forming — holes become oblong, slots warp, and tapped holes lose thread integrity.

Bend Sequence & Access

Parts with multiple bends must be formable in a sequence that doesn't create interference between previously formed flanges and the tooling. This is a common DFM problem — a four-bend box that looks simple in CAD may be impossible to form if the third bend prevents the punch from reaching the fourth. Design with forming sequence in mind, or let the shop suggest modifications.

Tonnage Awareness

Tonnage = (575 × material thickness² × bend length × tensile strength) / die opening. A 10' bend in 0.250" mild steel requires approximately 60 tons of force. Most shops have brakes from 60 to 300 tons. If your part requires a long bend in thick material, verify that the shop has sufficient tonnage. Under-tonnage results in incomplete bends; over-tonnage damages tooling.

Flat Pattern & K-Factor

When a sheet bends, the inside compresses and the outside stretches. The neutral axis (where neither compression nor tension occurs) is not at the center — it shifts inward. The K-factor (typically 0.33–0.50) describes this shift and determines the flat pattern dimensions. Use K=0.42 for mild steel air bending as a starting point. Provide the flat pattern DXF, not just the 3D model.