K-factor, springback, and how a flat DXF becomes a 90° fold that lands inside tolerance, without three trial parts and a frustrated foreman.
The neutral fibre sits 0.42–0.45 of thickness in mild steel. Cold-rolled is harder, K is lower. Pick a K-factor to suit your material, not the textbook average.
Bend to 90°, release, the part holds at 88°. Compensate by over-bending 2°. Aluminium springs back more than steel, calibrate the brake per material.
Inside radius smaller than the thickness cracks the outer fibre on most cold-rolled grades. If the drawing says R0 on a 2 mm bend, it's a die break, not a bend.
A 200-ton brake with 6 punches and 6 V-dies changes setup in 8 minutes. With one tool, every bend on a new part is a 30-minute reset. Workshops bill setup time.
Typical air-bend values for common sheet materials at 90° fold, V-die 8× thickness.
| Material | K-factor | Springback (°) |
|---|---|---|
| Mild steel (DKP) | 0.42–0.45 | 1–2° |
| S235JR structural | 0.42 | 2° |
| Galvanised steel | 0.40–0.42 | 1–2° |
| Stainless 304 | 0.40 | 3–4° |
| Stainless 316 | 0.38–0.40 | 3–4° |
| Aluminium 5052 | 0.45 | 3–5° |
| Aluminium 6061-T6 | 0.43 | 5–8° |
The brake operator can't fix a bad CAD drawing. Design for the brake from day one.
Bend allowance is the arc length along the neutral fibre. Bend deduction is the math shortcut for flat-pattern length. SolidWorks asks for one, AutoCAD for the other. Mix them and your flat pattern is 2 mm off.
Bend perpendicular to the rolling direction and the metal stretches easily. Bend parallel and it cracks at half the safe radius. Mark grain on every flat pattern that ships to the shop.
V-die 6× thickness for accurate 90° folds. 8× for general use. Wider die = lower force but less angle control. Narrow die = more force, sharper angles, more springback.
Some bend orders trap the part inside the die. The operator hits the next bend and the back-gauge can't reach. Walk the sequence through the brake operator before releasing the DXF.
Air bending leaves 1–4° springback. Bottoming (coining) eliminates it but needs 4–5× the press force. Use only on critical geometry, frames, mounting tabs, hinges.
Our press brake has a 6-tool auto tool changer, setup between parts is 8 minutes, not 30. We hold ±0.2° on production folds and ±0.1° on instrument-grade work.
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