What happens when metal bends
When you bend a strip of sheet, the outer surface has to travel a longer path than the inner surface, so it stretches; the inside compresses. Somewhere between the two is a layer that neither stretches nor compresses — the neutral axis. The length of the flat blank is set by the length of that neutral layer, not by the outside or inside dimensions of the finished part.
This is why you can't just add up the outside dimensions of a folded box and cut a blank that size — it will come out too long by a predictable amount at every bend. That amount is the bend allowance.
The K-factor, plainly
The neutral axis doesn't sit in the exact middle of the sheet; bending pushes it toward the inside of the curve. The K-factor is simply where it sits, as a fraction of the material thickness:
K = 0would mean the neutral axis is on the inner surface (never true).K = 0.5would mean dead centre.- Real materials land between about
0.3and0.5; a common default is0.44(ANSI) for steel and aluminium.
A higher K-factor means the neutral axis is closer to the middle, which means more material is "used up" in the bend, which means the flat blank comes out shorter. Get the K-factor wrong and every bent part is a millimetre or two off — invisible on the screen, obvious when it doesn't fit.
Bend allowance and the flat length
For each bend, the flat length of the bent region (the bend allowance) depends on four things: the bend angle, the inside radius, the material thickness, and the K-factor. Add up the flat lengths of the straight faces plus the bend allowance of each bend, and you have the total blank length. Do it for both directions and you have the flat pattern — the outline the laser or punch actually cuts.
Why the flat pattern is the whole point
The 3D model is what you design, but the shop cuts the flat. The flat pattern is the DXF that goes to the laser, plasma or punch, with the bend lines marked so the press brake operator knows where and which way to fold. If the flat is wrong, the part is scrap — so the flat pattern, not the pretty 3D view, is the real deliverable in sheet metal.
How Vernia handles it
Vernia builds sheet metal the way the commercial tools do: a Face from a sketch with a thickness, then Flange and Contour Flange for the bends. Each flange dialog carries the parameters that decide the bend — angle, inside radius, height, bend position and offset — with automatic corner relief so folds don't tear at the corners.
The K-factor defaults to 0.440 (ANSI) and can be changed per document to match your material and press. The flat pattern is a live, editable part, not a dead picture: change the base and the flat re-derives, keeping the holes and cuts you added. Bend lines are drawn on the flat, and one click exports a DXF with separate cut and bend layers, ready for the machine.