The .slfy model format, version 0.1 (draft)

Status: draft, 2026-11-10. The key words MUST, SHOULD and MAY are used as in RFC 2119.

1. Scope

A .slfy file describes one engineering model: what its parts are and how they are shaped, how they are placed and move relative to each other, which sizes are adjustable, and the calculations and charts that go with it. A reader opens files; a viewer also draws them; a writer creates them.

Conformance levels

A viewer MUST state the level it supports. Any reader MUST NOT fail on a valid file because it does not support a higher level; it ignores what it does not use.

2. The file

UTF-8 JSON, one object:

Versions. A reader of MAJOR.x SHOULD open any file of the same MAJOR, ignoring fields it does not know. A newer MAJOR MAY change meanings; a reader SHOULD warn and try anyway. Before 1.0, minor versions MAY break.

Size. Files SHOULD stay under 2 MB. Lists have limits (see the schema): at most 32 top-level parts, 24 children per part, 32 shapes per part, 20 parameters, 20 calculations, 6 charts, 24 annotations.

3. Units and coordinates

• Geometry is always in metres, right-handed, Y up (as glTF). Angles in shapes, parts and mounts are radians, except where a field says degrees.
• model.units ("mm" or "m") is only the unit sliders are shown in. Parameters keep their own units; a size expression converts explicitly: a diameter slider d in mm gives a radius "d / 2000".
• Round shapes turn about their local Y axis (cylinders, tubes, cones, pulleys, gears, sprockets, tori, springs, lathe profiles). Every shape is centred on its own origin unless stated.

4. Expressions

Any field typed DimExpr is either a number or a string expression. Expressions are pure arithmetic: no assignments, no code. A reader MUST NOT evaluate them as program code.

Grammar (whitespace ignored):

expr   := term (("+" | "-") term)*
term   := power (("*" | "/") power)*
power  := factor ("^" power)?            // right-associative
factor := ("+" | "-") factor | number | name | name "(" args? ")" | "(" expr ")"
args   := expr ("," expr)*
number := decimal, optional exponent (1e-3)
name   := [A-Za-z_][A-Za-z0-9_]*

Names resolve, in order, to: pi or PI (π); in a chart series, x; a parameter id; an earlier calculation’s id, or its symbol when the symbol is itself a valid name.

Functions: sqrt pow hypot sin cos tan asin acos atan atan2 sinh cosh tanh exp log ln log10 abs min max floor ceil round step mac. log and ln are natural logarithms. step(v) is 1 when v ≥ 0, else 0 (Heaviside). mac(v, n) is the Macaulay bracket ⟨v⟩ⁿ: 0 when v ≤ 0, else vⁿ (n defaults to 1).

Errors. For sizes, a viewer MUST still draw the part: a division by zero, an unknown name, a root of a negative or a log of zero or less gives 0, and a size then falls back to the shape’s minimum. For calculations, the same cases make the step broken: shown as not workable (e.g. “—”), never as 0; later steps read a broken step as 0.

5. model

6. Parameters

{ id, label, symbol?, value, min, max, step, unit }. value is the starting value; a viewer MUST keep it within [min, max]. symbol is display text (it may be LaTeX-like, z_1).

7. Parts

7.1 Materials

castIron steel steelMatte steelDark chrome forged aluminium bronze brass copper bearing rubber friction gasket ceramic plastic wood paintGreen paintYellow paintRed paintGrey paintBlack paintBlue paintOrange galvanised leather nylon concrete crop straw glass oil hydraulicOil coolant fuel water soil grain. In 0.1 these are appearance keys. (Physical properties are planned for a later version.)

7.2 Mounts

A mount places a part relative to a target part (by id). Targets MUST be placed before the parts that mount on them; cycles are invalid. shift (Vec3Expr, metres, world axes) moves the result further. AxisDir is one of +x -x +y -y +z -z.

A part placed by a mount that transmits motion (mesh, slider, link, lever, follower, hooke, and coaxial on a turning shaft) moves because its target moves; it SHOULD NOT also have its own motion.

7.3 Motion

{ kind, axis, speed, amplitude?, phase? } on a driving part. axis is x, y or z in the part’s frame.

A viewer MAY play time slower than real time so motion is easy to follow, and SHOULD say so.

8. Shapes

Every shape has kind, and MAY have op (add default, subtract, intersect: applied to the shapes before it in the same part, in order), position and rotation (in the part’s frame), and a circular pattern: radialCount copies spaced evenly round the part’s Y axis at radius radialRadius, in the XZ plane.

Sizes below a shape’s minimum are raised to it (about 1–2 mm for most) so every part draws. A viewer meeting an unknown kind SHOULD draw a small placeholder block rather than fail.

9. Annotations

{ id, kind, label, … }, kind one of force, moment, dimension, label. Prefer anchor: { part, at?, along? }, which keeps the annotation on its part wherever the assembly puts it (at: center, a face +y…, or start/end of the part’s axis; along 0–1 picks a point along the axis). A force arrow ends on that point and acts along direction (default -y), length metres long; a moment turns about the part’s axis there; a dimension spans the part’s length. Without an anchor, from and to give the points directly. magnitude (DimExpr) is the live value shown with unit; value is fixed text.

10. Calculations

An ordered list of { id, label, symbol, formula, unit, latex?, description?, step?, answer?, limit? }. Each formula is evaluated in order, seeing the parameters and every earlier calculation (section 4). latex is display only. In a worked solution, step says in one sentence what the step does, and answer: true marks a result the question asks for. limit: { kind: "max" | "min", value, label? } flags the result when it is over (or under) the limit.

11. Charts

{ id, title, x: { label, unit, from, to }, y: { label, unit }, series: [{ id, label, formula }], samples? }. Each series formula is sampled at samples + 1 points (default 120; 8–400) from x.from to x.to, with x bound to the sample, plus parameters and calculations. At most 4 series per chart.

12. view

A viewer MAY frame the model itself instead (Solidify keeps the camera’s direction and fits the distance to the assembled model).

13. Extensions

extensions is an object whose keys are namespaced ("vendor.feature", e.g. "acme.fea"). A reader MUST ignore extensions it does not know, and SHOULD say that it did. Extensions MUST NOT change the meaning of core fields.

14. Security

A file is data. Expressions are arithmetic only (section 4) and MUST NOT be run as code. Readers SHOULD limit file size, list lengths and string lengths (the schema’s limits), and SHOULD treat every text field as plain text, never markup. A viewer opening files from links SHOULD only fetch over HTTPS.

15. Changes

• 0.1 (2026-11-10): first draft, from the model specification Solidify has used since 2026.