Quiz: Bend Allowance Calculator

Take a quiz and see if you really understand how sheet metal behaves during bending.

Bend allowance5.5083 mmthe arc length of the neutral axis through the bend — this is the material the bend consumes

Neutral axis position0.63 mm from the insideK = 0.42 — below the mid-thickness, because the outside stretches more than the inside compresses

Outside setback6.0622 mm(R + T) tan(θ/2) — from the bend line to the outside mould line

Bend deduction6.6161 mmtwice the setback less the allowance — subtract this from the sum of the outside flange lengths

Flat blank length63.384 mm40 + 30 − 6.616 — cut the blank to this and the finished part comes out to size

Material the bend consumes6.6161 mm9.45% of the summed flange lengths

Radius to thickness ratio1.333a typical bend

Outside bend radius3.5 mm

Minimum flange the tooling allows5.25 mma rough guide — a flange much shorter than this cannot be held on the die

At a K-factor of 0.335.2255 mm allowance-0.2827 mm difference

At a K-factor of 0.445.5711 mm allowance0.0628 mm difference

At a K-factor of 0.55.7596 mm allowance0.2513 mm difference

The formula

bend allowance = θ(π/180)(R + K·T)

Bending stretches the outside

When sheet metal is bent, the outside surface is in tension and stretches while the inside is in compression. Somewhere between them is a neutral axis whose length does not change, and that arc length is what the flat blank must supply.

The neutral axis does not sit at the mid-thickness. Because the outside stretches more than the inside compresses, it shifts towards the inside face — typically to about 0.44 of the thickness, which is what the K-factor records. Tight bends push it lower, towards 0.33; generous radii let it approach the geometric middle at 0.5.

Bend deduction is the practical number on a shop floor: measure the part by its outside dimensions, add the flange lengths, subtract the deduction, and that is the blank size. Bend allowance is the same physics expressed from the bend line instead.

A radius smaller than the material thickness risks cracking the outside surface, and how much smaller you can go depends on the alloy and on the grain direction — bending across the grain tolerates a tighter radius than bending along it. K-factors are also material and tooling specific, so a shop measuring its own is more reliable than any published table.