Shark Support for UltiMaker Cura

WORK IN PROGRESS: MANY FEATURES MAY BE BROKEN (proof of concept, 0.2.1-wip)

Source-available · Non-commercial. Copyright Matthew Armstrong. Licensed under the
PolyForm Noncommercial License 1.0.0. No warranty, no liability: use at your own risk.

Shark Support adds thin shark-fin supports under big, flat, tilted faces. The typical case: you print a long
box tilted onto one long edge so it is strong along its length. Its base becomes a long 30–55° overhang that is
fine in theory but sags or curls over that length. Shark Support puts a row of fins under it:

• each fin is a thin vertical plate that follows the slope of the part, stops at 85 % of its height (the part bridges the last bit), has a blunt 5 mm flat top (no tiny, slow top layers) and a straight back down to the bed
• it only touches the part at small teeth, a dotted line of points every 3 mm that snaps off cleanly, starting right at the fin's low end. The first two teeth really touch (0.1 mm into the part) to hold the start of the overhang; the rest stop 0.15 mm short
• a crossbar foot under the tip (¼ of the fin's length, 4 mm high) and a 4 mm one-layer brim pad keep tall fins standing
• in Cura it is a button in the left toolbar: pick the face you want supported with the mouse, or press Auto for every big tilted flat face, and Remove all to clear them

Real screenshot: the plugin running inside Cura (5.15 development source, see Testing), with
fins under the 35° test box and the Shark Support panel open from its toolbar button.

Use it

1. Load your model and tilt it the way you will print it (Rotate tool, Lay flat, etc.).
2. Select it, then click the shark-fin button at the bottom of the left toolbar (under move, scale, rotate, mirror, per-model settings and support blocker). Its panel opens:

• Pick face, then click the underside face that needs support. Fins appear under that one face. Pick more faces the same way; picking a face again replaces its fins (handy after changing settings). Press the button again to stop picking. Turn the view so you can see the underside (a low side view works).
• Auto: fins under every big tilted flat face of the selected model(s), replacing their earlier fins.
• Remove all: removes every Shark fin object.
• Settings...: the numbers below.
  1. A message says how many fins were made. New objects called Shark fins (your model...) appear.
  2. Slice and print as usual. Normal support for that model is switched off (a per-model setting) so Cura does not add its own supports under the same face. You can change that in the settings.

The same actions are in the menu Extensions > Shark Support. If you move or rotate the model afterwards, use
Remove all and add the fins again: they are a separate object and do not follow the model.

Settings (Settings... in the panel, or Extensions > Shark Support > Settings...)

Setting	Default	What it does
Overhang angle from / to	20° / 70°	Auto puts fins under faces in this range (Cura's angles: 0° = wall, 90° = ceiling). A face you pick gets fins whatever its angle, as long as it faces down
Smallest face / Shortest slope	300 mm² / 15 mm	Auto leaves smaller faces alone (picked faces ignore this)
Fin spacing / Side margin	30 mm / 4 mm	Distance between fins across the face, and from its sides
Start height	4 mm	Fins start where the face is this high (low down it needs no help)
Thickness at bed / at top	2.4 / 1.2 mm	Fins taper; with a 0.4 mm nozzle that is 6 lines at the bed, 3 at the top
Gap between teeth	1 mm	The fin edge stays this far under the part except at the teeth
Tooth spacing / tip length	3 / 0.6 mm	The contact points. The first tooth starts at the fin's low end, the last sits at the tall end
Tip gap	0.15 mm	All teeth except the anchors stop this far under the part. Smaller = stronger contact, harder removal
Anchor teeth / overlap	2 / 0.1 mm	The first teeth from the low end reach this far into the part so they bond slightly and hold the start of the overhang. 0 turns it off
Top left free	15 %	The fin stops this far below the top of the face; the part bridges the rest. 0 runs it to the top
Blunt tip length	5 mm	The fin's flat top. No layer of the fin is shorter than this, so the printer never slows down for tiny top moves
Back lean	20°	Only with Lean the back out on
Crossbar length / height	0.25 × fin / 4 mm	The foot across the fin under its tip (a second one in the middle for fins over 100 mm)
Brim width / thickness	4 mm / first layer	One-layer pad around each fin and crossbar
Lean the back out to the bed	off	The back slopes out 20° instead of going straight down: wider base, but more plastic and time (test box: 1.97 m and 42 min instead of 1.68 m and 40 min). The command line also offers round and concave backs
Print fins as a Cura support mesh	off	Experimental: Cura then prints them with your support pattern and density

Extensions > Shark Support > Run self-check builds a test box in memory, runs the plugin's own code against it
(including face picking and the toolbar button),
and shows PASS/FAIL per check (also saved to shark_support_selfcheck.txt in your temp folder). Run it once after
installing.

Without Cura (PrusaSlicer, OrcaSlicer, anything)

The geometry code needs only numpy, so there is a command line tool:

python3 shark_cli.py part.stl                   # writes part_fins.stl
python3 shark_cli.py part.stl --spacing 25 --merged part_with_fins.stl
python3 shark_cli.py part.stl --back slope       # back leaning out instead of straight down
python3 shark_cli.py --help                     # every setting

The STL must already be oriented for printing (Z up, sitting on the bed). Import the fins STL as a second part at
the same position (PrusaSlicer: Add part; or slice the --merged file). The repository has examples in
examples/.

Real G-code: the merged test box sliced by PrusaSlicer 2.7.2 on the command line, showing every extrusion within
0.5 mm of the middle fin. The fin is solid, touches the part only at the teeth, stops below the top of the face
with a flat top, and goes straight down at the back.

What exists and what does not

Exists (0.2.1-wip)	Does not exist yet
Fins under flat tilted faces: picked with the mouse, or found automatically	Fins under curved or organic surfaces (it needs flat faces)
Fins cut at 85 %, blunt flat top, straight back, teeth every 3 mm, 2 anchor teeth, crossbar, brim pad	Painting exactly where on a face fins go
Cura toolbar button with panel, menu, settings dialog, self-check, undo	Fins that follow the model when it is moved
Installer that checks Cura's log; .curapackage	Live preview while editing settings
Command line tool for any slicer	PrusaSlicer / OrcaSlicer plugin (they have no Python plugin API)
Experimental "Cura support mesh" mode	A real print test (see below)

What was tested

Be aware of exactly what has and has not been tried:

• Geometry: 22 automatic tests (python3 tests/test_core.py): closed outward meshes, fins never reach into the part (only the anchor teeth reach in, exactly 0.1 mm; with anchors off every tip is 0.15 mm below it), boxes tilted 35° and 45°, turned on the bed, standing on a corner, the 85 % cut and 5 mm flat top, picking a face (and refusing a top face or a miss), triangle-soup input, spacing, long fins, no fins on a plain cube. A 196,000-triangle mesh took about 2 s on the development machine (not timed on a 7th-gen Core i3).
• Inside Cura: Cura could not be downloaded in the build sandbox, so it was run from Cura's and Uranium's source (5.15 development branch, plugin SDK 8.13) under a virtual display, with stand-ins only for Cura's C++ parts. There the plugin loaded, its toolbar button appeared and opened its panel, a real mouse click (sent with xdotool) on the box's underside made fins for that face, picking it again replaced them, Auto replaced those with automatic fins with the right per-object settings, the menu and dialog opened, the self-check passed, support-mesh mode and Remove all fins worked, and Cura's package manager listed it as installed, both after the installer and after Cura's own .curapackage route. Slicing in Cura was not tested (the slicing engine is one of the C++ parts that could not run).
• Slicing: the fins were sliced for real with PrusaSlicer 2.7.2 (command line), picture above. For the test box the fins take 1.68 m of filament and about 40 minutes on their own (PrusaSlicer defaults, 0.2 mm layers); the 0.1.2 fins, which ran to the top with a sharp tip, took 1.74 m and about 43 minutes. The zoom below shows the anchor teeth meeting the part's outer wall in the G-code while the other teeth leave a gap.

• Not tested: a real print, Cura 5.0–5.10 releases (face picking uses a different path there, written to match Cura 5.0's own support blocker but not run), Windows and Mac (launchers and installer), Flatpak Cura.

Research paper

Fin-ally Supported: Shark-fin supports for long, flat, tilted overhangs (version 1.0, first
published 19 October 2026) · PDF

Everything behind this plugin written up: the problem, what already exists (including a live patent on "fine point"
supports and how this differs), the method in enough detail to rebuild it, every mistake made on the way, the
numbers, a test plan, and a defensive disclosure of the method and its variations, dedicated to the public so
nobody can patent them. It is published on its own at
The-Dorkknight/fin-ally-supported; paper/ is a copy that
travels with the plugin. Text and figures: CC BY-SA 4.0. Infant sharks do, do, do support prints well.

How to cite: Armstrong, M. (2026). Fin-ally Supported: Shark-fin supports for long, flat, tilted overhangs. A
hobbyist's field guide and a defensive publication. Version 1.0, 19 October 2026.
https://github.com/The-Dorkknight/fin-ally-supported

More

• HOW_IT_WORKS.md: what the plugin reads, changes and never touches
• DESIGN_NOTES.md: research notes, decisions, measurements, ideas
• paper/: the research paper (Markdown and PDF)
• CHANGELOG.md · CONTRIBUTING.md · SECURITY.md · THIRD_PARTY.md

Not affiliated with or endorsed by UltiMaker. "Cura" and "UltiMaker" are names of their owners.

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Source code: The-Dorkknight/cura-shark-support on GitHub