K-factor

The K-factor is a decimal value, typically between 0.3 and 0.5, that describes where the neutral axis sits within a sheet as it bends. When metal folds, the outer surface stretches and the inner surface compresses, but somewhere in the middle lies a line of constant length. The K-factor is the ratio of that neutral axis position to the total sheet thickness. It answers a practical question every press brake operator faces: how much material do I actually consume when I fold this part?

Material properties and bend geometry both influence K-factor. Harder alloys like spring steel or stainless tend toward 0.4 to 0.5; annealed aluminum often sits at 0.33 to 0.38. Bend angle, inside radius, and thickness all play a role. A tight inside radius pushes the neutral axis outward, raising the K-factor; a wide radius lowers it. A 90-degree bend in mild steel sheet with an inside radius equal to the material thickness typically runs around 0.43 to 0.45.

Calculating flat pattern length

The K-factor appears in the bend allowance formula: BA = (π/180) × (inside radius + K-factor × thickness) × bend angle. This value, in millimeters or inches, is subtracted from the sum of the two leg lengths to find the flat pattern dimension. Underestimate K and your finished part will be too long; overestimate and it pinches short. Tolerance stacks quickly with multiple bends, so accuracy matters. Many shops derive K-factors empirically by test-bending scrap and measuring the result rather than relying on handbook values.

Modern CAM software for nesting and unfolding requires a K-factor input. Press brake controllers and CNC turret punch software do the same. Some systems use K-factor; others use a bend deduction (BD) or a bend allowance (BA) value instead, which amount to the same calculation expressed differently. The term K-factor originated in punch press work and gained wider use as sheet metal fabrication became more computerized.

Real variation exists. A part may need micro-adjustment after the first bend because K-factor changes with temperature, work-hardening, and tooling wear. Experienced fabricators maintain reference charts for their common materials and frequently used bend radii, and they run qualification bends before committing expensive blanks to production.