Sheet Metal
Sheet Metal Bending: K-Factor and Bend Allowance Basics
Flat patterns only unfold correctly when the bend math is right. Here is what K-factor means and why your supplier asks about it.
Key takeaways
- Sheet metal stretches on the outside of a bend and compresses on the inside.
- K-factor locates the neutral axis, where length does not change.
- Typical K is 0.33 to 0.50 depending on material and bend radius.
- If the flat pattern is wrong, every bend after the first one is wrong too.
The problem in one sentence
When you bend a flat sheet, the outside of the bend gets longer and the inside gets shorter. The finished part is not the same length as the flat blank you started with.
If you cut the blank to the finished dimensions, the part comes out short. That is why flat patterns need a correction, and that correction is bend allowance.
Three terms you need
Neutral axis
Somewhere inside the thickness there is a layer that neither stretches nor compresses. Its length stays the same before and after bending.
K-factor
K-factor is the position of that neutral axis as a fraction of the material thickness. A K-factor of 0.44 means the neutral axis sits 44% of the way from the inside face.
Bend allowance
Bend allowance is the length of the arc along the neutral axis. It is the amount you add to the flat pattern for each bend.
The formula
The standard formula is:
BA = (π / 180) × A × (R + K × T)
- BA = bend allowance, in millimetres
- A = bend angle in degrees
- R = inside bend radius
- K = K-factor
- T = material thickness
A worked example
Take a 2 mm mild steel sheet, a 90° bend, an inside radius of 2 mm and a K-factor of 0.42.
BA = (3.1416 / 180) × 90 × (2 + 0.42 × 2)
BA = 0.01745 × 90 × 2.84
BA = 4.46 mm
So each 90° bend adds about 4.46 mm of developed length to the flat pattern. Get this number wrong by 0.3 mm and a part with four bends is 1.2 mm out before you even start.
Typical K values
| Material | Typical K | Notes |
|---|---|---|
| Mild steel | 0.42 to 0.45 | Good starting point |
| Cold rolled steel | 0.42 to 0.44 | Common for enclosures |
| Stainless steel | 0.45 to 0.50 | More springback, higher K |
| Aluminium 5052 | 0.44 | Cracks at tight radii |
| Aluminium 6061 | 0.45 to 0.48 | Needs a larger radius |
| Copper and brass | 0.40 to 0.45 | Soft, easy to form |
These are starting values. Real K depends on the tooling, the bend radius, the grain direction and the press setup. Your supplier's number will beat any table.
What to send your supplier
- A STEP file of the finished part, not the flat pattern, unless they ask for the flat.
- The material grade and thickness, not just "2 mm steel".
- The bend radius you expect, or ask them to choose one for the material.
- Which dimensions are critical, and which are cosmetic.
- Any hardware, holes or cutouts that must not deform during bending.
Common mistakes
- Using one K-factor for every material and every radius.
- Forgetting that grain direction changes the minimum bend radius.
- Designing a bend tighter than the tooling can form, which forces a redesign after quoting.
- Exporting a flat pattern from CAD and assuming the shop will use it as-is.
Common questions
Why does my flat pattern not match the finished part?
Almost always a K-factor or bend deduction mismatch. Your CAD default probably differs from the shop's tooling.
What is the difference between bend allowance and bend deduction?
Bend allowance is the arc length added. Bend deduction is what you subtract from the sum of the two flange lengths. They describe the same correction from different directions.
Does laser cutting affect the bend?
Not much, but a rough cut edge can start a crack on a tight bend. Deburr before forming on thick or hard material.
The bottom line
Bend allowance is not a detail you can leave to luck. Send the finished geometry, tell the shop the material and radius, and let their tooling define the K-factor.