.slfy: an open format for engineering models that move and calculate

A .slfy file is one JSON document describing a 3D engineering model and its engineering:

parameters with ranges and units, shown as sliders
parts built from shapes (gears, pulleys, springs, bolts, lathe profiles, extrusions…) with boolean cuts, where any size can be a formula of the parameters
parts placed by relation: on a shaft, meshing with a gear, driven by a crank, through a Hooke’s joint
motion, forces, moments and dimensions attached to parts
calculations with their equations, worked in order, with limits and answers
charts sampled from formulas (bending moment diagrams, pump curves)
the question a model answers, for worked problems

glTF carries meshes; STEP carries exact CAD; neither carries the mechanism or the working. .slfy does.

Status: draft 0.1 (November 2026). Expect changes before 1.0. Website: https://slfy.solidify.studio.


Extension	`.slfy` (readers should also accept `.solidify.json`)
Media type	`application/vnd.solidify+json` (not yet registered with IANA)
Encoding	UTF-8 JSON
Schema	`schema/slfy-0.1.schema.json` (JSON Schema 2020-12), at `https://slfy.solidify.studio/schema/slfy-0.1.schema.json`
Specification	`SPEC.md`
Examples	`examples/`

The smallest file

{
  "format": "slfy",
  "version": "0.1",
  "model": {
    "id": "minimal-block",
    "title": "A block on a plate",
    "summary": "Two parts, one placed against the other.",
    "units": "mm",
    "parameters": [],
    "parts": [
      { "id": "plate", "name": "Base plate", "shapes": [{ "kind": "box", "sx": 0.3, "sy": 0.02, "sz": 0.2 }] },
      { "id": "block", "name": "Block", "mount": { "kind": "against", "target": "plate", "face": "+y" },
        "shapes": [{ "kind": "box", "sx": 0.1, "sy": 0.08, "sz": 0.1 }] }
    ]
  }
}

Examples

File	Shows
`minimal-block.slfy`	the least a file needs; placing by relation
`shaft-in-torsion.slfy`	parameters, formulas as sizes, motion, calculations with a limit, a chart
`spur-gear-pair.slfy`	gears, `mesh` mounts, a driven gear turning from its driver
`beam-question.slfy`	a worked question: givens, steps, answers, loads and reactions, a bending moment diagram
`helical-gear-pair.slfy`, `worm-reducer.slfy`, `open-differential.slfy`, `scotch-yoke.slfy`, `quick-return-mechanism.slfy`, `muff-coupling.slfy`	models from the Solidify library, reviewed by its team

Solidify opens them under Create → Open a model file (https://solidify.studio/#/create/open, arriving with its next release), and a link opens one straight in: https://solidify.studio/#/create/open?url=<https link to the file>. The example files on this site work as such links.

Checking a file

Any JSON Schema 2020-12 validator works, for example:

npx ajv-cli@5 validate --spec=draft2020 --strict=false -s schema/slfy-0.1.schema.json -d model.json

(Some tools pick a parser by extension; copy a .slfy to .json for them.) The schema checks structure; the specification also says how files are drawn and worked out, which a schema can’t.

Licences

The specification, schema and examples: CC BY 4.0 (LICENSE-SPEC). Code (later: the reference reader and viewer): Apache 2.0 (LICENSE). “Solidify” is the name of the product that created the format; the format itself is free for anyone to read and write.

Maintained by

Solidify (https://solidify.studio). The schema and examples are generated from the schemas Solidify itself reads and writes models with, so they stay in step with the app. Issues and suggestions are welcome.