How to find your K-factor from a test bend

What K is

When sheet bends, the inside is squeezed and the outside stretched; somewhere between them is a layer that keeps its length. K is where that layer sits, as a fraction of the thickness from the inside (SOLIDWORKS). Bricsys puts it typically between 0.3 and 0.5; the FAA handbook gives about 0.445 for its aircraft sheet, though its simpler formula uses ½ T. The bend uses up a length of flat sheet equal to that layer’s arc — the bend allowance, BA = π × (R + K × T) × θ ÷ 180, with θ the angle bent through in degrees.

1. Make the test piece

Cut a strip from the same material and thickness you will use, and measure its flat length carefully. Bend it on your own brake with your own tooling — the K you get belongs to that combination (Bricsys: it depends on the material, the bending type and the tools; PTC: test specimens are the most accurate way). A 90° bend is the usual choice.

2. Measure it

Measure both flanges to the outside — to the virtual sharp, where the two outside faces would meet (SOLIDWORKS). Measure the angle the strip actually ended at, after springback, with a protractor — do not assume the brake hit 90°. Measure the inside radius the bend took: the radius is measured to the inside surface (FAA), and in air bending it is set mainly by the die opening rather than the punch (Benson). And measure the real thickness, not the nominal.

The worked strip: 2.00 mm thick, cut 96.50 mm long, bent through 90°, inside radius 2.00 mm, both flanges measuring 50.00 mm to the outside.

3. Bend deduction

The flanges add up to more than the strip by exactly the bend deduction: BD = A + B − flat length (Bricsys) = 50.00 + 50.00 − 96.50 = 3.50.

4. Bend allowance

The bend deduction is twice the outside setback less the bend allowance (SOLIDWORKS), so BA = 2 × outside setback − BD. The outside setback is (R + T) × tan(θ ÷ 2) (FAA: setback = tan(½ bend angle) × (R + T)) — at 90° simply R + T = 4.00. So BA = 2 × 4.00 − 3.50 = 4.50.

5. Solve for K

Turn the bend-allowance formula round: K = (180 × BA ÷ (π × θ) − R) ÷ T = (180 × 4.50 ÷ (π × 90) − 2.00) ÷ 2.00 = 0.432. The vendors’ help pages give these relations forwards; working them backwards from a test strip is the procedure Javelin Technologies describes. It assumes the bend is a circular arc of radius R and that the thickness does not change.

How carefully to measure

An error in the cut length or either flange goes straight into the bend allowance, so K moves by the error divided by (angle in radians × thickness): on this strip, 0.10 mm moves K by about 0.032. The other measurements matter as much or more — on the same strip, reading the angle 2° off moves K by about 0.057, 0.10 mm of thickness by about 0.040, and a radius gauge 0.50 mm off by about 0.068. Use a caliper, a protractor and radius gauges, bend to 90° or more, and average two or three strips.

Two traps in the name

• DIN against ANSI. ANSI measures the neutral layer’s offset from the inside as a fraction of T; DIN measures it as a fraction of T ÷ 2 (PTC), so for the same layer a DIN K is twice the ANSI one. Check which your CAD system uses before you type a number in.
• The FAA’s “K”. In the aviation handbook, K is the setback factor tan(½ bend angle), not the neutral-layer position — at 90° it is 1. Same letter, different thing.

Common mistakes

• Measuring the flanges to the inside and using the outside-setback formula — or the reverse.
• Using the angle between the flanges instead of the angle bent through: a 90° bend leaves 90° between them, a 45° bend leaves 135°.
• Assuming the brake hit 90° instead of measuring the angle after springback.
• Taking the punch radius as the inside radius after air bending, or the nominal thickness as the real one.
• Reusing one K for another thickness, radius or die.

How this matches the tool

The K-factor calculator does these five steps for any strip, outside or inside flanges, and shows the measuring sensitivity beside the result. The worked numbers on this page were produced by its code when the page was built.

Sources

1. Dassault Systèmes, SOLIDWORKS 2022 Help, “K-Factor”: K is the position of the neutral sheet relative to the thickness; BA = π(R + KT)A/180
2. Dassault Systèmes, SOLIDWORKS 2022 Help, “Bend Allowance and Bend Deduction”: flat length = A + B − BD, measured from the virtual sharp; BD = twice the outside setback − BA
3. Bricsys, BricsCAD Mechanical Help, “K-Factor”: typically between 0.3 and 0.5; BD = B1 + B2 − unfolded length; depends on material, bending type and tools
4. PTC, Creo Elements/Direct 20.7 Help, “Allowance tables and formulas”: ANSI measures the neutral offset from T, DIN from T/2; test specimens are the most accurate way in your own shop
5. FAA-H-8083-31A, Aviation Maintenance Technician Handbook – Airframe, Vol. 1, ch. 4, pp. 4-59 to 4-64: bend radius measured to the inside surface; neutral axis about 0.445 T from the inside; its “K” is the setback factor tan(½ bend angle)
6. S. Benson, The Fabricator (2015), “Predicting the inside radius when bending with the press brake”: in air bending the inside radius is a percentage of the die opening
7. Javelin Technologies, “Calculating bend allowance, bend deduction, and K-factor” (2017): K-factor worked back from a measured test bend