A sheet metal shop needed to cut flat blanks for a 2"×4"×2" U-channel in 14 gauge mild steel (0.0747"). Two 90° bends with a 0.125" inside radius. The estimator used IronKit's Bend Deduction & K-Factor Calculator to get the exact flat blank length before programming the CNC turret punch. Using K=0.38 from the Machinery's Handbook air-bend table (r/t = 1.67, mid-range bracket), the flat blank came out to 7.686" — 0.314" shorter than the naive sum-of-legs answer of 8.000". Without the BD calculation, every blank would be over-length and require rework.
Frequently Asked Questions
Why is flat blank length shorter than the sum of leg lengths?
Because the bend doesn't consume a full "step" of material — it consumes a bend allowance (the arc along the neutral axis), which is less than the two tangent extensions that make up the OSSB on each side. The difference between the two tangent extensions and the BA is the bend deduction. On a 90° bend in 14ga mild steel at 0.125" radius, each BD is 0.157" — so two bends cost 0.314" total from your blank length.
What is K-factor and why does it matter?
K-factor is the ratio of the neutral axis distance from the inner surface to the material thickness. It governs where the neutral axis (the material's stretch-zero line) actually sits during bending. K=0.38 means the neutral axis is 38% of the way through the 0.0747" thickness, or 0.028" from the inner surface. If you used K=0.50 (centerline approximation, which many older textbooks recommend), your flat blank would come out 0.312" — 0.009" different per bend. Small error per bend, but it compounds on multi-bend parts.
How much does springback affect this part?
About 2.5° per bend for mild steel at r/t = 1.67. To hit a true 90° angle, set the press brake to 87.5°. The elastic recovery brings the part back to 90° after the die lifts. This is a starting point — your specific machine, tooling, and material heat can shift it by ±1°. Run a test piece and measure before setting up production.
How does IronKit calculate flat blank length?
Flat blank = sum of legs − total BD. BA = (π/180) × angle × (radius + K × thickness). OSSB = tan(angle/2) × (radius + thickness). BD = 2 × OSSB − BA. K-factor is looked up from the Machinery's Handbook table by material + r/t ratio. You can override K per bend if you've measured your own tooling.