Sheet Metal Bending — K-Factor, Allowance & Cost Rules
Bending is the cheapest forming operation in metal — one hit of the press makes a feature machining would need an hour to cut. The rules below keep it cheap and make it right.
Quick answer: Bending costs $0.50–2 per bend + setup amortized by batch. Flat pattern length = outside dimensions adjusted by bend allowance (K-factor ≈ 0.38–0.44 for air bending). Design rules: inside radius ≥ 1× thickness, flanges ≥ 3–4× thickness + radius, keep holes ≥ 2.5× thickness from bend lines. Fewer bends = linear cost savings.
The flat pattern and K-factor
A bent part's flat length isn't the sum of its outside dimensions — the metal stretches outside the bend and compresses inside, with a neutral axis in between. The K-factor (neutral axis position ÷ thickness, ~0.38–0.44 for air bending) feeds the bend allowance formula: BA = (R + K×T) × angle. Get it wrong and holes land off-position past the bend. CAD handles it; quote requests should say 'flat pattern included' and watch the shop relax.
Springback
- Metal springs back 0.5–3° after bending (more in high-strength alloys and stainless).
- Brakes compensate by overbending — that's why production tools cost more setup.
- Minimum inside radius: ≥ 1× thickness for steel, 1.5× for aluminum (cracks below that).
- Bottoming/coining reduces springback but multiplies tonnage and tooling cost.
Design rules that keep bends cheap
- Flange length ≥ 3–4× thickness plus bend radius — shorter flanges need special tooling.
- Holes and slots ≥ 2.5–3× thickness from the bend line (they distort in the deformation zone).
- Bend relief cuts at interior corners prevent tearing — small slots, cheap insurance.
- One direction of bends per setup: every flip/re-orientation adds handling minutes.
The cost chain
Material (the cost calculator) → cutting (laser calculator) → bending (this calculator's numbers) → finishing. Bends price linearly — $0.50–2 each — so a 4-bend redesign of a 8-bend part halves forming cost before anything else changes. The best sheet metal designers are cheap, and it shows on every quote.