Is a flat pattern just the sum of the visible side lengths?

No. When metal bends, the outside of the bend stretches and the inside compresses, so the true flat length is measured along the neutral axis in between. A 100 mm by 100 mm bracket in 2 mm steel unfolds to slightly less than 200 mm, not exactly 200 mm.

What is bend allowance and K-factor?

Bend allowance is the extra length of material a bend consumes, calculated from the bend angle, the inside radius, the material thickness and the K-factor. The K-factor locates the neutral axis as a fraction of the thickness, typically 0.3 to 0.5, and shops usually have their own value dialled in for their press brake.

What file does a shop cut a flat pattern from?

Almost always a DXF, with the cut outline and holes as closed vector paths and any bend lines kept on a separate layer so they are folded rather than cut. Some shops also accept a DWG or a 3D STEP file of the formed part.

What is a flat pattern in sheet metal?

The part, unfolded flat

A flat pattern is a sheet metal part unfolded into a single flat shape, as if you peeled every bend back out to 180 degrees. It is the only geometry a cutting machine cares about, because on a sheet metal part you always cut first and bend second: the laser, punch or waterjet cuts the flat pattern out of the sheet, and the press brake folds it into shape afterwards.

Why it is not just the sum of the lengths

The trap is assuming the flat length equals the sum of the visible finished sides. It does not. When metal bends, the outside surface of the bend stretches and the inside surface compresses, and somewhere between them sits a layer that does neither, the neutral axis. The true flat length is measured along that line, so a 100 mm by 100 mm L-bracket in 2 mm steel unfolds to slightly less than 200 mm, because the bend itself consumes a little material. That difference is the bend allowance, calculated from the bend angle, inside radius, thickness and the K-factor, the position of the neutral axis expressed as a fraction of the thickness.

A flat pattern built on the wrong K-factor looks correct on screen and bends to the wrong final size. When it matters, ask the shop for the K-factor they use on their press brake, or let them calculate the flat pattern from your formed dimensions instead.

What shops actually cut from

Sheet metal cutting almost always runs from a DXF of the flat pattern, with the cut outline and holes as closed vector paths, and any bend lines kept on a separate layer so the machine folds them instead of cutting them. Some shops also accept a DWG, or a 3D STEP file of the formed part if the geometry is complex. See what is a DXF file and DXF vs DWG for the format details.

Getting a flat pattern from an existing part

If you have a formed bracket or enclosure but no drawing, you do not need to derive the bend allowance by hand. Our full walk-through, turning a photo of a sheet metal part into a manufacturing drawing, covers photographing the formed part, generating the flat pattern, and exporting a laser-ready DXF. Starting from a picture rather than a 3D model works the same way as converting any image to DXF.