Bend radius cheat sheet: minimum radii and the k-factors to start from
Two numbers set up every fold — the smallest inside radius a material will take without cracking, and the k-factor that turns the 3D part back into a flat blank. Here are the standard values we start from, and how we nail the exact one on your job.
Two numbers set up a bend
Every fold comes down to two questions. How tight can the inside radius be before the outside of the bend cracks? And where does the metal neither stretch nor compress — the neutral axis — so we can work out how long the flat blank has to be? The first is the minimum bend radius; the second is the k-factor. Get both right and the part folds to size the first time.
Minimum inside radius
The tighter you bend, the more the outer fibres stretch — push past what the material allows and the bend cracks. As a safe starting point for a 90° cold air bend, across the grain:
| MATERIAL | MIN INSIDE RADIUS | NOTE |
|---|---|---|
| Mild steel (CR) | ≈ 1.0 × t | Down to ~0.5 × t when soft |
| Stainless 304 / 316 | ≈ 1.0 × t | Springs back more — see N-06 |
| Aluminium 5052 | ≈ 1.0 × t | Fairly forgiving |
| Aluminium 6061-T6 | ≈ 2–3 × t | Crack-prone; anneal for tighter |
A safe default across everything we cut is an inside radius equal to the thickness (r = t) — reuse one radius on a part and we tool it once.
What the k-factor is
When metal bends, the inside squashes and the outside stretches; somewhere between them a layer does neither — the neutral axis. The k-factor is how far that layer sits from the inside face, as a fraction of thickness (so it runs 0 to 0.5). It’s not a shop secret — it follows the radius-to-thickness ratio and how hard the bend is formed:
| R / t | AIR BEND k | METHOD | TYPICAL k |
|---|---|---|---|
| < 1 | ≈ 0.33 | Air bending | 0.33 – 0.45 |
| 1 – 3 | ≈ 0.40 | Bottoming | ≈ 0.42 |
| > 3 | ≈ 0.45 – 0.50 | Coining | ≈ 0.50 |
Tighter bends push the neutral axis inward (lower k); larger radii and harder forming push it toward the middle (higher k).
Turning it into a flat length
With the radius and the k-factor you can size the flat blank. The arc that wraps the bend — the bend allowance — is: BA = (π/180) × angle × (R + K·t). Add the straight legs to the bend allowances and you have the developed length to cut.
From the book to your bench
These are the standard numbers we start from — the same ones in any fabrication handbook. The exact k a bend lands at also depends on the V-die we tool up and the specific batch of metal, so on a job where the flat length has to be right, we bend a test coupon on the real tooling, measure it, and calibrate the k-factor to that. That’s the difference between a cheat sheet and a part that fits: the table gets you close; the test bend gets you there.
- 01 Material and thickness Sets the minimum radius and the starting k.
- 02 The inside radius you want Or let us pick one that tools cleanly (r = t is safe).
- 03 Whether the flat length is critical If it is, we test-bend and calibrate before we run the batch.
Need the exact numbers for your part? Send the material and thickness — we’ll size the radius, calibrate the k-factor on a test coupon, and hand back a flat pattern that folds to spec.