What Is K-Factor and Why Is My Sheet Metal Flat Pattern the Wrong Size?

K-factor is the fraction of material thickness where the neutral axis sits during a bend, and if the value your CAD software assumes doesn't match how the part actually bends, the unfolded flat pattern comes out too long or too short by a few tenths of a millimeter per bend. Across four or five bends that adds up fast, and the part won't fit the assembly it was designed for.

Why the neutral axis moves at all

When you bend sheet metal, the outside surface stretches and the inside surface compresses. Somewhere between the two there's a layer that neither stretches nor compresses, the neutral axis. In a flat, unbent sheet that layer sits right at the mid-plane, 50% of the thickness. Bend the material and that layer shifts toward the inside of the bend, because the inside compresses more than the outside stretches for a given radius.

K-factor is that shifted position expressed as a fraction of thickness. It's what your unfold calculation uses to convert bend angle and radius into flat length.

A K-factor of 0.33 means the neutral axis sits a third of the way in from the inside face. A K-factor of 0.5 means no shift at all, which is unrealistic for anything but a very generous radius. Most general sheet metal work lands somewhere around 0.33 to 0.45 depending on material, thickness, and bend radius. Every CAD package ships with a default that's a reasonable guess, not a promise.

Why the default guess fails you

The default K-factor in your CAD tool was tuned for some combination of material and radius that isn't necessarily yours. Thinner material, sharper radius, and harder alloys all push the real neutral axis position around. Use the default blindly on a tight-radius bend in a harder aluminum and your flat pattern runs long, because the software assumes more material got compressed than actually did and adds length back that isn't needed. Run stainless with a generous bend radius and you get the opposite error.

This is why bend deduction, not raw K-factor, is what actually gets used for flat pattern math in most shops. Bend deduction folds K-factor, radius, angle and thickness into a single correction factor derived from test bends in the real material, rather than a formula that only holds under ideal assumptions. If you're modeling with native sheet metal features, your software converts K-factor to bend deduction under the hood anyway, so the practical fix is the same either way. Use a bend allowance table calibrated to your actual material and radius, not the software default, whenever the fit matters.

What to do about it

For most brackets and enclosures where a bend line landing off by half a millimeter doesn't matter, the default is fine. For parts that mate to something else, or where multiple bends compound the error, get the real bend deduction for your material and gauge before modeling.

If you don't have test data, a shop that runs the material daily usually does. Ask before you finalize the flat pattern rather than after the first article comes back wrong.

DigiForge's sheet metal process runs cold rolled steel, galvanized steel, 5052 and 6061 aluminum, and 304 and 316 stainless, with a bend angle tolerance of plus or minus 0.5 degrees and a cut tolerance of plus or minus 0.13 mm. That tolerance is on the finished part, not the flat pattern. If your K-factor assumption is off by enough to matter, no amount of bend-angle accuracy on the brake fixes the final dimension. Model with a bend allowance that matches your actual material, and send STEP, STP, or DXF with the fold information intact rather than a flattened outline with no bend data.