printfins.com's breakaway support fins as a parametric object in FreeCAD: select a part, Add Support Fins, and a … fins mesh appears under it. Change the part (a Pad length, a fillet, its tilt) and the fins recompute to the new shape.

Same engine as the site and every plugin: the part is tessellated exactly the way the site tessellates a STEP (OpenCascade, 0.01 mm / 0.1 rad), then computeFins runs in V8 (vendored mini-racer) through the shared host, plugins/shared/py/supportfins_host.py. No fin geometry lives in the add-on.

Use

Select a part (a Part solid, a PartDesign Body or any feature in it, an App::Link, or a mesh), then Support Fins ▸ Add Support Fins (toolbar, or the Tools menu) in any workbench.
Bed = the document's XY plane at the part's lowest point: the part prints the way it sits in the model, z up. Tilt it with its Placement (or the Body's) to change the pose. Moving a group the part sits in (an App::Part) moves the fins too.
Settings are the fins object's properties, the site's settings with the site's defaults (generated from plugins/shared/engine/options.json). Ones the engine would ignore are hidden, as on the site; numbers are held to the engine's range. Report says what was placed and what wasn't reached; it's also printed to the Report view.
Auto update (on) recomputes with the part. Turn it off on a big part to keep editing snappy: the fins keep their last result, the Report says they're out of date, and Update Support Fins recomputes them (the selected ones, or all).
Export the part and its fins together (select both, File ▸ Export, .stl or .3mf).

Output is a mesh. Converting to a solid for STEP export comes later (local issue 016).

Install (for now, until the Addon Manager listing)

Download your computer's support-fins-freecad-<platform>.zip (printfins.com ▸ Plugins, or the plugins-latest release) and unzip it into the Mod folder of FreeCAD's user data folder (Help ▸ About ▸ Copy to clipboard shows it; FreeCAD 1.0 on macOS: ~/Library/Application Support/FreeCAD/Mod), so you get Mod/SupportFins/. Restart FreeCAD. python3 plugins/freecad/build.py --all builds those zips.

From a checkout:

python3 plugins/freecad/build.py --install ~/Library/Application\ Support/FreeCAD   # macOS

then restart FreeCAD. --install takes FreeCAD's user data folder (Help ▸ About shows it). The build vendors mini-racer for this machine's platform (FreeCAD 1.0 on macOS has no V8 or Qt WebEngine of its own, so this is the only runner).

Code

SupportFins/supportfins_freecad.py — the SupportFins object (Mesh::FeaturePython): tessellate the source, call the shared host, set the mesh and report.
SupportFins/supportfins_props.py — options.json ⇄ properties. No FreeCAD imports.
SupportFins/InitGui.py — the two commands, a global "Support Fins" toolbar (made once; delete it in Tools ▸ Customize and it stays deleted) and Tools-menu entries.
build.py — the add-on folder: add-on + shared host + options.json + engine bundle + vendored mini-racer.

Tests

python3 -m pytest -q plugins/freecad/tests/test_props.py          # anywhere; CI runs it
python3 plugins/freecad/build.py && \
  /Applications/FreeCAD.app/Contents/Resources/bin/freecadcmd plugins/freecad/tests/smoke.py
cat plugins/freecad/build/smoke.json                              # last line: PASS / FAIL

smoke.py runs the real FreeCAD headless (35 checks: fins under the part on its lowest point; following a length change, a tilt, a PartDesign feature and a moved App::Part; Auto update off/Update; STEP parity with the site, asked live from site_step.js via deno; Mesh, placed App::Part and App::Link sources; fins dropped into a placed group; settings reaching the engine, hidden when ignored, clamped to its range; errors in the Report; export of part + fins; save and reopen).