K-factor in sheet metal

K is where the sheet neither stretches nor squeezes.

The K-factor is the position of the neutral axis inside a bend, written as a fraction of the sheet thickness. When sheet bends, the inside compresses and the outside stretches. Between them is a layer that does neither, and its distance from the inside surface, t, divided by the thickness, T, is the K-factor.

CAD uses that number to work out how much flat material each bend consumes, the bend allowance, and therefore how big to cut the blank. A K of 0.5 would put the neutral axis at mid-thickness. In real air bends it sits closer to the inside, so practical values are lower: every value in our production bend table falls between 0.32 and 0.48.

The one length that survives the bend.

Every fiber inside the bend changes length except one. The neutral axis keeps its flat length, so the flat blank equals the straight segments plus the neutral-axis arc through each bend.

One K, one radius, two ways to use them.

Bend allowance adds the neutral-axis arc to the straight lengths. Bend deduction subtracts from the outside leg lengths. Both come from the same K-factor and inside radius.

0.100″ 5052-H32, bent to 90°.

The same row the technical drawing guide develops its bracket from. Our table sets the inside radius at 0.126″ and K at 0.40, on a 20 mm die.

The K-factors we bend with.

59 production values from our bend table, grouped by the material family each one was calibrated for. Every row assumes a 90° air bend with the V-die shown; BA is calculated from the row, BD is the table value.

Source: Xeon NC production bend table (TRUMPF press brake tooling), the same data behind the bend allowance calculator and the bending ontology guide. Values are for the listed thickness and die. A different die changes the radius, and with it the K-factor and bend deduction. Find each die in the press brake tooling library.

Four things move the neutral axis.

A K-factor is only good for the conditions it was calibrated under. Change one of these and the flat pattern moves with it.

The calibrated values also do not follow a smooth curve. On the same 20 mm die, 0.080″ 5052 bends at K = 0.48 and 0.090″ at 0.37. Look up the row for your thickness instead of interpolating between neighbors.

Back out K from a test bend.

When you need a value our table does not cover, measure it. One bent coupon of known flat length gives you the bend deduction, and the formulas give you K.

Replace the default before you export.

Every sheet metal CAD system ships with a default K-factor. Set it, and the inside radius, from the row for your material and thickness before you make a flat pattern.

Then decide what you send. Upload the formed STEP when you can: we develop the flat pattern from the bend data for the die that runs your job. If you upload a flat DXF, the blank is cut as drawn, so the K-factor you used sets the finished flange lengths. See STEP vs. DXF and the Onshape STEP export guide.

K-factor, answered.