support-fins/plugins/prusa

Support Fins — PrusaSlicer 3.0 plugin

A native PrusaSlicer plugin companion to printfins.com, targeting the 3.0 plugin API (project.plugin 1.0.0). Bundle com.printfins.support-fins/ is one flat directory with a single menu entry:

• Support Fins → Add a Fin (add_fin.lua) — drops one breakaway support fin for a part printed tilted: a thin (1.2 mm) right triangle that stands under the part's sloped underside, the same shape as the 30°/45°/60° angled-print fins people place by hand (printables.com/model/1771718). Its slope runs from a low tip on the plate up to the top of a straight back edge, 0.2 mm under the part, on a thin foot with a round back end. A comb of tines runs along the slope: one-layer horizontal nubs, one bead (0.5 mm) wide, each top on the print preset's layer grid, that reach across the gap and 0.5 mm into the part at mid-layer (the site's old PROP.tineBite; the site's tines now stop at the surface), so each prints as one strand that fuses in and snaps clean (docs/FIN-SPEC.md). The top of the slope ends in a short flat (1.2 mm, a little more when steep), never a point. Params: Slope Angle (20–70°, default 45), Fin Height, a Gripping Tines toggle, and Tine Spacing (default 6 mm, the sway braces' printed spacing; at least 1 mm). Tines spread evenly from just above the foot to just under the top, about Tine Spacing apart along the slope (snapping to the layer grid can stretch a gap by up to a layer), so both ends always grip. Tines always keep at least one bare layer between them, so a tight spacing on a shallow slope never fuses two into a 2-layer band. A fin never gets fewer than 3 (the site's grip floor), so a huge spacing still leaves 3. A 25 mm 45° fin gets 7 at 6 mm, 18 at 2 mm, 4 at 12 mm.

  Sizing it: use the dialog's Fin Height (and Slope Angle), not the slicer's scale tool. Scaling stretches the tines too: at 2× they'd be two layers tall, off the layer grid, with double the gap, and they'd stop snapping clean. Add another fin rather than stretching one.

  Placing it: set Slope Angle to the angle of the part's underside. Turn the fin about Z only, so its slope climbs the same way as the underside, and slide it until the slope sits 0.2 mm under the part — e.g. put the low tip 0.2/sin(angle) (0.28 mm at 45°) out from where the underside meets the plate. The foot starts where the slope rises above it, so the part keeps the slope's own gap off it. Wide parts want a fin near each side.

  The tines overlap your part, which PrusaSlicer slices as a separate object, so each tine's 0.5 mm bite is extruded twice (a few hundredths of a mm³ per tine). If your version can merge the fin and part into one object, merging unions it instead.

  How the slope is built: the slope is one cube turned to the angle (rotate = {y = -angle}), a 1.5 mm rail whose top face is the slope. A stack of cube steps fills under it, every step corner inside the rail (one layer tall below where the rail starts, so the slope there is no rougher than the slicer's own layers). Each tine also reaches into the step behind it, so it holds on even if the rail misbehaves. The API's only triangle (make_prism) is isosceles with its apex centred (PrusaSlicer its_make_prism), so a vertical back edge would need a Negative cut, which the slicer draws as a grey box over half the fin.

Why placement is by hand — read before filming

The website is the real product: load any STL → rotate it into a stronger orientation → it finds the overhangs and contours breakaway fins to the part's underside → export an STL that prints support-free in any slicer. All of that is mesh analysis: it reads the model's triangles.

The 3.0 plugin sandbox cannot read a mesh. Verbatim from the API notes: "a mesh's geometry cannot be read back — Mesh exposes bounds() and translate(), not triangles," and there is no getter for the user's loaded object's geometry anywhere in ProjectApi.cpp. The API is generative (build parametric objects from make_cube/make_cylinder/…). So a plugin can't auto-detect an overhang, contour a fin to it, or snap onto a surface — it can only generate the fin and let you place it. Auto-fitting stays on the web. The honest on-camera line: "Prusa shipped plugins mid-build, so I brought the fin into the slicer — it can't see your model, so it can't auto-fit like the site, but it drops the real fin in and you set it under your overhang."

Install

Dev (no signing) — the fast loop: copy the bundle directory into PrusaSlicer's data dir:

cp -R .../plugins/prusa/com.printfins.support-fins "<data dir>/lua/"

<data dir> is the folder holding PrusaSlicer.ini. On the 3.0 alphas it is not the plain name — a 3.0.0-alpha11 Windows build used %APPDATA%\PrusaSlicer3-dev\; on macOS look under ~/Library/Application Support/. Don't guess; find the folder with PrusaSlicer.ini, or launch with --datadir. Create the lua/ subfolder if needed, then restart the slicer; the entry appears under Support Fins → Add a Fin.

Release (signed ZIP import): the convenient install path requires a signature:

PrusaSlicer plugin keygen -P author.private.pem -p author.public.pem
PrusaSlicer plugin sign   -P author.private.pem com.printfins.support-fins

The ZIP must hold the bundle's files at its root (manifest.json at the top level, not inside a com.printfins.support-fins/ folder), and importers need authorized_authors/matthewtrahan.pem (the author field names the key file).

Verify (must be done in a running 3.0 slicer — it can't be unit-tested)

1. Support Fins → Add a Fin appears; the dialog shows Slope Angle, Fin Height, the Gripping Tines toggle and Tine Spacing.
2. Generate with defaults → a 45° right triangle, 25 tall, its slope climbing toward +X with 7 small tines spread along it, a straight back edge, and a foot running from near the low tip to past the back edge. Preview of the intended shapes at 30/45/60°: ~/Downloads/support-fin-prusa-preview.png.
3. The slope must be smooth and flush. It is the one turned volume (a cube rotated about Y). If it tilts the wrong way or sits off the triangle, you'd see a plank sticking down or out and the stepped fill underneath. Report it with a screenshot.
4. Slice at your usual layer height and scrub the layer slider: each tine should show up on exactly one layer (never split across two). Change the layer height, generate a new fin, and check again — the tines follow the bed's print preset (a per-object layer height override isn't seen).
5. Set it under a tilted part (slope 0.2 mm under the underside), slice, print, bend the fin off — the tines should snap clean and leave faint dots.

All pieces are built in one flat object space via shapes.builder() (vendored from leotrax3d, MIT), which anchors every volume to the first and places each primitive by its bounding box, so nothing assumes where a primitive puts its origin.

Lint / local checks: run ./run-tests.sh from the plugins/prusa/ dir (needs lua/luac). It covers syntax (luac -p), the manifest JSON, the slicer's scan pass on a bare engine, and the fin arithmetic against a mock api (tests/add_fin_test.lua), which models the real semantics (corner-origin cubes, Z-axis cylinders, other_volumes translated relative to the main mesh and turned right-handed about their own origin, as the bundled temp tower's upright text shows, and the preset's layer grid). At 30/45/60° and the clamps it asserts the rail's top face is the slope, the fill stays under it with its notches inside the rail, the foot clears the part, and every tine is one layer on the grid, grips the fin and bites 0.5 mm into the part (also at a 0.3 layer over a 0.25 first layer, with the preset handing back strings like "150%", and with no preset at all). The fatal, silent trap the scan guards is any require/api call at file scope — the scan runs the whole file just to read info, on an engine with neither, so a hit there produces no menu entry and no error. Both files keep all api/require use inside functions (execute() for the plugin, method bodies for the module); keep it that way.

Notes / possible polish

• Material. Gap 0.2 is the site's PLA default (PROP). PETG wants bigger gaps; a material choice could follow the gap coupon's PETG print. (Tine reach isn't a material setting: tines end at the part's surface.)
• Distribution: optionally PR to leotrax3d/prusaslicer-plugins-unofficial for reach + its CI and signing/release workflow, keeping the canonical copy here.
• The earlier self-contained Overhang Test and Combined Support Demo entries were removed (git history keeps them) — the single Add-a-Fin is the product.