Tool
Sheet metal bend allowance
A bent part is cut from a flat blank, and the blank is shorter than the sum of the two legs. This calculates how much shorter — bend allowance, bend deduction and the flat length.
Both legs measured to the outside corner (the mould line).
Bend allowance
—
Bend deduction
—
Outside setback
—
Flat length
—
The three numbers, in order
Bend allowance (BA) = θrad × (R + K × T). It is the arc length of the neutral axis, where θ is the bend angle in radians, R the inside radius, K the K-factor and T the thickness.
Outside setback (OSSB) = (R + T) × tan(θ/2). It is how far the outside corner sticks past the tangent point.
Bend deduction (BD) = 2 × OSSB − BA. This is what you subtract from the sum of the outside legs to get the flat length: flat = leg1 + leg2 − BD.
With T = 2 mm, R = 2 mm, 90° and K = 0.4: BA = 1.5708 × (2 + 0.8) = 4.40 mm, OSSB = 4 × tan(45°) = 4.00 mm, BD = 8.00 − 4.40 = 3.60 mm, and a 50 + 30 part flattens to 76.40 mm.
K-factor by situation
| Situation | K-factor |
|---|---|
| R ≈ T, cold-rolled steel | 0.33 – 0.35 |
| R > 2T, any material | 0.40 – 0.45 |
| Soft aluminium, large radius | 0.45 – 0.50 |
| Stainless, tight radius | 0.30 – 0.35 |
A flat pattern is defined by the tooling. Send the finished dimensions and let the shop apply its own K — a pattern generated with someone else's K-factor is usually wrong.
Questions
What is bend allowance?
The length of the neutral axis through the bend — the arc that neither stretches nor compresses. It is what you add to the two straight legs to get the flat length.
What is bend deduction?
The amount you subtract from the sum of the outside dimensions to get the flat length. Bend deduction = 2 × outside setback − bend allowance.
What K-factor should I use?
0.33–0.35 for cold-rolled steel at a radius close to the thickness, up to 0.45–0.5 for a large radius or soft material. If you do not know the tooling, 0.4 is a workable starting point and your laser shop will apply its own.
Why does my flat pattern come out short?
Usually because the K-factor used was for a different material or radius. The flat pattern is defined by the tooling, not by the drawing — send the finished dimensions and let the shop apply its own K.
Does the bend angle change the allowance?
Yes. The allowance is proportional to the bend angle in radians: a 90° bend uses π/2, a 45° bend uses π/4.
More: sheet metal design rules ·all tools ·sheet metal fabrication ·request a quote