A native Onshape custom feature that adds Support Fins' designed-in breakaway supports to a part inside a Part Studio. It's a companion to printfins.com: same fins, same spec (docs/FIN-SPEC.md), but it runs on the CAD model itself instead of an STL.

Public Onshape document: Fin Supports, V1 (latest workspace)

What it does

Orient the part the way it will print, select it, and the feature adds the supports as new parts beside it (the original part is never modified):

Overhang ribs. Upside-down-T walls under each overhang. The rib top follows the underside a breakaway gap below it, necks to a 0.6 mm contact tip, and carries a comb of one-layer horizontal tines that fuse into the part. This is the web app's current default support.
Side bracing fins. Round-topped walls standing a gap off a flat side of a part tipped onto an edge or corner, gripping it with rows of tines (ported from the old fins.js geometry, since removed from the web tool). Off by default. Auto picks up to N faces facing apart, or you select the faces.
Bed pad. A thin oval under a part that touches the bed only along an edge or at a point, the same height as the rib flanges so the two merge flush.

Export the part together with its supports as one STL and slice with supports off, the same as a printfins.com export.

Why a FeatureScript

The web app has to work on triangle soup, so most of its engine is mesh plumbing: welding vertices, rebuilding adjacency, inside/outside tests, hand-built watertight solids. Onshape has a B-rep kernel, which removes most of that:

printfins.com (mesh)	This feature (B-rep)
Reorient the part, re-seat it on the plate	Pick a build plate (plane, face or mate connector); nothing moves
Overhang test per triangle	Overhang test per face against the build direction
Contour the rib top by sampling the mesh (`contourTop`, `settleTop`, …)	Sample the underside with kernel raycasts, then subtract a copy of the part grown by the gap, so the gap holds everywhere, including the flanks
Point-in-mesh parity tests	`qContainsPoint`, `evDistance`, booleans
Emit closed triangle solids	Sketch + extrude real solids; the slicer unions them with the part

It also covers issues #24 and #25 (STEP support): anything Onshape imports as a solid (STEP, Parasolid, native parts) works.

Install

Open the public document above.
In any Part Studio: toolbar Custom features → Add custom features → find the Fin Supports document → add Support-Fins FS.
Onshape pins your Part Studios to that version. When a new version is published, Onshape offers the update.

To work on the code instead, make a copy of the document, or paste supportFins.fs into a new Feature Studio. The icon is supportFins-icon.svg: upload it to the document and point the icon::import(...) line at it.

Use

Orient the part for printing (for example with a Transform feature). The Top plane is the build plate unless you pick another.
Support-Fins FS → select the part.
Print settings → set Layer height to the slicer's layer height (each tine is one layer, snapped to the layer grid), set Nozzle to your printer's nozzle (default 0.4 mm; tines and contact tips scale with it, walls only thicken, as on the site). Support gap is the site's (default 0.2 mm); for PETG try 0.3.
Optionally turn on Side bracing fins for a part balanced on an edge or corner.
Select the part and all its orange supports → Export → STL, as one file.

Every option has a hover tooltip. The full manual, with diagrams, the options, a troubleshooting table for every message the feature reports, and printing tips, is SupportFins_User_Guide.pdf.

Port notes

Geometry and constants follow the web engine and FIN-SPEC.md. Where this port differs:

Placement rules are a subset. Ported: 45° overhang test with a small slack on the threshold, 12 mm² minimum region, ribs running down-slope (or along the long axis when flat), rows at the max-unsupported-span pitch, a single rib on the lowest line of a large curved band, squat walls with a brim, edge-biased tine spacing with a 3-tine grip floor, and the side fin's 60° / 12 mm site separation. Not ported: splitRegion sub-patching, withLowTails, part-attached ("floor") supports, sway braces, wall cutouts and orientation scoring. The feature never picks the orientation; that stays the user's call, as on the site.
The rib tip steps rather than tapers. The tip is a 0.6 mm wall over the top 1.5 mm of a 1.0 mm stem, instead of profileHalf's linear neck.
One base height. Rib flanges, squat-wall brims and the pad share one height (0.6 mm, rounded to whole layers), so they meet without a step. Side-fin bases stay at the spec's 1 mm.
Side fins are sized to the face outline. The fin rises only as high as the face reaches across its whole length, so a tilted square face (a diamond) gets a fin in the middle rather than a plank beside a corner.
Supports are separate parts. Nothing is booleaned into the user's part. The tines and pad overlap it slightly, exactly as the web export does, and the slicer unions them.

Limits

Solid parts only. STL and other meshes import into Onshape as mesh bodies, which the feature can't process. For STLs, use printfins.com.
Overhangs over another part of the model rather than over the plate get no rib, the same as the web app. Use Add overhang faces to force one.
Side fins need a flat face at least 4 × 4 mm, leaning no more than the Max face lean (default 45°) from vertical, with open space beside it.
Very complex parts with large curved overhangs can take a few seconds to regenerate.

Status

Runs in Onshape (FeatureScript 2931) on test parts, including a tilted cube and a complex part with curved overhangs. Not yet print-tested: the geometry follows FIN-SPEC.md, but nothing generated by this feature has been sliced and printed yet. Reports from real prints are welcome.

Credits

Fin technique by Slant3D; engine, spec and constants from this repo by Matthew Trahan. FeatureScript port by Chris Lee, Southeast Expedition Medical, LLC. MIT License, same as the rest of the repo.

Claude Code (Opus 5.5) used to simplify the porting process and write much of the FeatureScript code.

README.md Unescape Escape

Support Fins — Onshape FeatureScript