K-factor

When sheet metal is bent, the outside stretches and the inside compresses. Somewhere in the thickness there is a line that does not change length: the neutral axis. The K-factor is the ratio between that line’s distance from the inner face and the sheet thickness, usually between 0.3 and 0.5.

How it works

CAD uses the K-factor to compute bend allowance and, from it, the flat pattern size. The value depends on material, thickness, inside radius and press brake tooling (die opening). That is why the right K-factor is the one from the shop that will bend the part: a generic value can miss a flange by tenths of a millimeter, which adds up on parts with several bends.

Practical example

A 2 mm mild steel box with four bends came out 1.2 mm too long overall. The press brake team measured a test strip, the K-factor was changed from 0.44 to 0.38 in CAD, and the new flat pattern hit the dimension on the first try.

Common mistakes

• Using CAD’s default K-factor for every material and thickness.
• Changing the press brake die without reviewing the flat pattern.
• Drawing an inside radius smaller than the tooling can form.