One coordinate unit becomes one inch

A coordinate value of one does not say whether a part measures one inch or one millimetre. The source STL here spans exactly one coordinate unit along each axis. The experiment assigns inches, then exports a 3MF that explicitly records millimetres.

How this was measured

The interpretation step leaves the coordinates unchanged. The exact relationship of one inch to 25.4 millimetres gives an independent expected extent of 25.4 mm. The preparation exporter scales the interpreted geometry, writes the millimetre unit in 3MF, and rereads the output to check its dimensions and topology. The source file stays unchanged.

Recorded output from this experiment
Measurement	Value
before / triangles	4
before / dimensions	1 × 1 × 1
before / units	Not recorded
before / open Edges	0
before / inconsistent Edges	0
before / non Manifold Edges	0
before / degenerate Triangles	0
before / components	1
interpreted / triangles	4
interpreted / dimensions	1 × 1 × 1
interpreted / units	in
interpreted / open Edges	0
interpreted / inconsistent Edges	0
interpreted / non Manifold Edges	0
interpreted / degenerate Triangles	0
interpreted / components	1
after / min	0 × 0 × 0
after / max	25.4 × 25.4 × 25.4
after / size	25.4 × 25.4 × 25.4
output Unit	mm

What the result does not establish

Nothing in the STL proves that inches were intended. That is an explicit choice for this example. Assigning a different interpretation would describe a different physical size, whereas converting units preserves the chosen physical size. A receiving application still needs to respect the unit recorded in 3MF.

Try these files

These samples were created by this project and released under CC0. They contain no visitor files. Sizes and SHA-256 hashes identify the exact bytes measured in this experiment.

One coordinate unit carries no physical interpretation. (STL, 284 bytes)
SHA-256
89ad231af173585bdb04427d0052f48155576861f43d94b458580dd26b3f7a1b
A one-inch tetrahedron exported with millimetres recorded. (3MF, 1,600 bytes)
SHA-256
06cde09030204c43ec0c1bb34bd6090d421f4a95a6f08a400aaa5c06d0f7aec6

Use this result

Why an STL opens at the wrong size

Versions and reproduction

Evidence generated 2026-09-05T21:13:29.140Z. The checked-in generator rebuilds the sample collection and measurements with one command:

node --experimental-strip-types scripts/build-workbench-evidence.ts

CAD source generation: scripts/build-workbench-cad.py with cadquery-ocp 7.9.3.1.1; checked-in analytic cylinder outputs. The STEP and IGES outputs are checked in so ordinary evidence rebuilds do not require Python.

@astrojs/cloudflare: 14.3.0
@cloudflare/containers: 0.3.7
astro: 7.3.1
fflate: 0.8.3
jose: 6.2.12
occt-import-js: 0.0.23
rhino3dm: 8.32.2
three: 0.185.1

Geometry source and generator SHA-256 hashes:

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