SplitForge

Split 3D models into printable parts that fit back together.

Is the model too big for your printer, or would it print better in two halves? Drop the STL into SplitForge, choose where to cut, and it gives you two watertight parts with pegs, snap-fits or dowel holes already in place. It runs entirely in your browser: nothing is uploaded and nothing needs installing.

What it does

• Cuts cleanly. Both cut faces are filled and every part comes out as a closed, watertight solid, ready for any slicer. Sections with holes, and cuts that leave several separate pieces, are handled too.
• Places connectors where they fit. Connectors go inside the actual cut outline, clear of holes and edges, and each separate piece gets its own. If there isn't enough material behind a spot for the socket, that connector is skipped rather than punching through the far side.
• Exports parts ready to print. Each part is rotated onto the bed (cut face down, or pegs pointing up) and saved as STL. You can also download everything as one zip.
• Shows the details. Exploded, wireframe and X-ray views, the cut outline drawn on the plane, an overhang check in print orientation, and a watertightness check for both parts.

Connector types

Type	What you get	Good for
Peg	Chamfered pegs on one part, matching sockets in the other	Most prints. Strong and easy to line up
Snap	Pegs with a small bead that clicks into a groove	Parts you want to press together without glue (PETG, larger diameters)
Dowel	Holes in both parts plus separate pins to print	Printing both parts cut face down for the cleanest joint
None	Just the cut	Gluing, or adding your own joinery

Connectors can be laid out at the centre, the corners, the corners plus midpoints, or in a grid. You can set the diameter, depth, clearance and edge margin, and choose which part carries the pegs.

Getting started

1. Serve the folder with any static web server. For example, run python3 -m http.server and open http://localhost:8000.
2. Drop an STL onto the page, or click Try the sample model.
3. Pick the cut axis and drag Position, or type an exact height in mm.
4. Choose a connector type and pattern, and check the result in the viewport.
5. Open Export and download the parts, or Everything (.zip).

Viewport controls: drag to rotate, right-drag or Shift-drag to pan, scroll to zoom, double-click to reset. On a touchscreen, use one finger to rotate and two to pinch and pan.

Tips

• Clearance is the gap around each peg. 0.15–0.25 mm suits most FDM printers. Print a small test piece before committing to a big part.
• If connectors are skipped, the cut is too thin for the socket. Use a shallower or narrower connector, a smaller edge margin, or move the cut.
• Dowel mode lets both parts print cut face down, which gives the flattest mating surfaces.
• The model should be watertight. SplitForge warns you if it isn't, because a mesh with holes can leave gaps in the cut face.
• Light, dark and automatic themes are in the top bar, and the layout also works on phones and tablets.

Exports

Export	Contents
Part STLs	Each half as binary STL, optionally rotated onto the print bed
Dowel pins	One STL with every pin laid out ready to print
Everything (.zip)	Both parts, pins, connector positions and the OpenSCAD file
OpenSCAD	A parametric version that rebuilds the split from the original STL, so you can tweak it further
JSON / CSV	Connector positions in model coordinates

Development

There's no build step: index.html loads plain scripts from src/.

File	Purpose
src/stl.js	Binary and ASCII STL parsing, binary STL writing
src/split.js	Mesh welding, cutting, cap triangulation, connector placement and geometry, print orientation, overhangs. It has no DOM access, so it runs under Node
src/sample.js	The built-in sample model
src/renderer.js	WebGL2 viewport
src/exporters.js	STL, zip, OpenSCAD, JSON and CSV export
src/app.js	UI wiring
src/vendor/earcut.js	earcut 2.2.4 polygon triangulation (ISC licence)

The geometry tests split a range of shapes on every axis with every connector type. They check that each part is a closed, consistently oriented solid, that connectors stay inside the material, and that volumes add up. Run them with Node 18 or later:

npm test

Licence

SplitForge is free software under the GNU Affero General Public License v3.0 or later (AGPL-3.0-or-later). You can use, change and share it, including commercially. If you share a changed version, or run one as a service that other people use over a network, you must make its source code available under the same licence.