Question 1

What is the K-factor in sheet metal bending?

Accepted Answer

The K-factor is the ratio of the distance from the inside surface to the neutral axis, divided by the material thickness T. The neutral axis is the layer that neither stretches nor compresses during bending. Typical values run from about 0.3 to 0.5; 0.42 is a common default for many materials and air bending.

Question 2

How do I calculate the bend allowance?

Accepted Answer

Bend allowance BA = (π/180) × θ × (R + K × T), where θ is the bend angle in degrees, R is the inside bend radius, T is the thickness and K is the K-factor. It is the arc length of the neutral axis through the bend and is added between the flat sections.

Question 3

What is the difference between bend allowance and bend deduction?

Accepted Answer

Bend allowance (BA) is the developed length you add through the bend. Bend deduction (BD) is the amount you subtract from the sum of the outside leg lengths to get the flat blank: BD = 2 × (R + T) × tan(θ/2) − BA. The flat length is therefore A + B − BD.

Question 4

How do I get the flat / blank length?

Accepted Answer

For a single bend with outside leg lengths A and B, the flat length is Flat = A + B − BD, where BD is the bend deduction. For multiple bends, compute the bend deduction for each bend and subtract all of them from the total outside length.

Question 5

Which K-factor value should I use?

Accepted Answer

The K-factor depends on the material, thickness, inside radius and bending method (air bending vs. coining). 0.42 is a reasonable starting point, but for accurate blanks you should calibrate K with a test bend on your actual material and tooling, then back-calculate it from the measured flat length.

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