STEP Folded Part

STEP (.step, .stp, ISO 10303) is the format mechanical CAD programs exchange solids in: SolidWorks, Inventor, Fusion, Solid Edge, Creo, CATIA, NX, FreeCAD and the sheet-metal CAM programs read it. File › Export › Export… › STEP Files (Folded Part) writes the drawing folded up, as the 3D view folds it, as a solid in millimetres, to check it fits the assembly it goes into or to hand to a customer's designer. A drawing whose industry is Sheet metal or HVAC offers it among its first formats.

What is written

One part, to the AP214 schema (automotive_design, ISO 10303-214, the one every CAD program reads). A drawing with dimensions or notes on its 3D model (DIM3D) is written to the AP242 schema instead (ap242_managed_model_based_3d_engineering, ISO 10303-242), with them as PMI: see Dimensions and notes.

A sheet-metal drawing (see Sheet metal and living hinges in 3D) is written as an exact solid: an advanced B-rep (ISO 10303-42, as AP214 and AP242 write it), one closed shell for each blank. Each flat's two faces are planes; each bend's inside and outside faces are cylinders round the bend's axis, the inside one its inside radius R; the sheet's edges are planes, round holes and arcs cylinders, Bézier edges exact B-spline surfaces. The bend's strip of the blank (its bend allowance) is bent round with its neutral layer (K of the way through) keeping its length, so the flanges come out at the flat pattern's outside sizes. An edge that crosses a bend at a slant (or curved) is written as a fine polyline close to the curve it takes, its side face a ruled B-spline surface.

Each bend folds to the angle and way its Bend Line note gives (UP 90° R2), else up for the Crease layer and down for the Reverse crease layer, to 90°, as the bend DXF reads them. The sheet's thickness is the drawing's T.

Anything else (a box's die line, a curved crease, the solids a drawing's layers are extruded to, wrapped or swept), and a sheet-metal part whose faces don't close up into a solid (a bend line that doesn't run across the blank), is written as the 3D view makes it: a faceted B-rep, closed shells of flat faces, the triangles the 3D view draws joined into flat faces wherever they lie in one plane. A part whose surface doesn't close is written as surfaces instead.

File › Import › Import 3D for Unfolding… reads a STEP file the other way: a sheet-metal part's exact solid is unfolded into its flat pattern, a carton's faces into a crease pattern. See Unfolding 3D Models.

Dimensions and notes

Each DIM3D is written as the CAx-IF recommends for AP242 PMI:

Semantic: a linear dimension as a dimensional_location ('linear distance') between two shape aspects, an angle as an angular_location, a radius or diameter as a dimensional_size; its value (in mm, or radians) as the nominal value of a shape_dimension_representation, its TOL as a plus_minus_tolerance. A note has no semantics.
Presentation: its lines, arrows and text as polylines (a geometric_curve_set in an annotation_curve_occurrence and draughting_callout) on an annotation_plane, in the plane the 3D page draws it.
Views: a draughting_model for each VIEW (those without one in "PMI"), with a camera from the usual view, related to the part's shape; each dimension tied to its callout.

The shape aspects aren't tied to faces of the solid, so a CAD program shows the dimensions and their values but may not highlight the faces they measure.

Notes

CAD programs measure and mate the exact solid, and can unfold it with their own sheet-metal tools. To change the part, change the drawing and export it again.
A faceted part's bends are sets of narrow flat faces.