PCB edge cuts from a 3D model

I’d like to explain how I use FreeCAD to create PCB edge cuts in KiCad.

I am starting from a STEP model of an off-the-shelf cast aluminum enclosure sold by Hammond. These are very popular for guitar pedals and other equipment. After selecting an enclosure that I think is the right size, I want to create a PCB edge cut by grabbing some of the geometry from the STEP model and extracting it into a SVG file. This creates the largest PCB that will fit in the enclosure. KiCad will let you import an SVG directly into the PCB edge cuts layer. And in Inkscape, the edge cuts SVG can be useful for understanding the 2D constraints of the design.

I was unsure of the correct way to do this. And I think I’m still a little unclear, because FreeCAD is a pretty sophisticated tool. But the approach that’s working for me right now is this:

1. In FreeCAD, open the STEP file of the enclosure.
2. Hit V, 3 for wireframe view, which makes it easier to select edges.
3. Control-click each of the edges you want.
4. Click the green Shape Binder button.
5. In the Data Tab, change the angle and axis of the Shape Binder so that it is parallel to the XY plane. Check by hitting 2 for the top view.
6. Make sure the Shape Binder is selected.
7. In the View Tab, under Display Options, make sure Display Mode is set to “Wireframe”. KiCad respects fills, and we don’t want the edge cut to have a fill.
8. Hit Control-E to Export and choose the flattened SVG file type. (I think “flattened” means projected onto the XY plane, which is why we had to rotate the object.)
9. In KiCad’s PCB Editor, hit Control-Shift-F to import graphics.
10. Choose the file you exported from FreeCAD.
11. Choose the Edge Cuts layer.

A limitation of this approach is that the SVG is all straight lines; there are no arcs. This seems to be a consequence of directly using the Shape Binder, which copies the objects from the STEP file. You’re stuck with whatever the STEP file uses, as far as I can tell.

Some PCB designs will need customized edge cuts. There is a way to edit the SVG before importing to KiCad. In FreeCAD, after creating the Shape Binder, you can create a Sketch and extract vertices or edges from the Shape Binder to use as constraints or reference points for a customized edge cut design. For example, if your design has two PCBs with a ribbon cable between them, you could use a Sketch to create a board based on the geometry of the enclosure but with a cutout for the ribbon cable.

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