Check mass properties

A STEP subject is measured from its exact BRep and a mesh from its triangles; the claim is the same either way, and the result names the source it used.

1. Claim volume, area and centre

const part = await loadModel({ source: 'plate-with-through-bore.step', format: 'step' });

expectGeo(part)
  .toHaveVolume({ value: 30429.2 });
expectGeo(part)
  .toHaveSurfaceArea({ value: 6871.24 });
expectGeo(part)
  .toHaveCenterOfMass({ point: [0, 0, 10] });

import { readFile } from 'node:fs/promises';import { expectGeo } from 'geospec';import { createModelLoader } from 'geospec/model';import { Engine } from 'geospec-engine/node';const engine = new Engine();const loadModel = createModelLoader({ engine, readSource: async (source) => new Uint8Array(await readFile(source as string)) });const part = await loadModel({ source: 'plate-with-through-bore.step', format: 'step' });expectGeo(part)  .toHaveVolume({ value: 30429.2 });expectGeo(part)  .toHaveSurfaceArea({ value: 6871.24 });expectGeo(part)  .toHaveCenterOfMass({ point: [0, 0, 10] });await loadModel.dispose();engine.close();

The plate is 40 × 40 × 20 mm with a 10 mm bore: 32,000 mm³ less the bore's 1,570.8 mm³. Set the volume's value to 32000 and it fails by that much: Volume is 30429.20367320504 (brep evidence), which does not satisfy 32000 within 0.02.

2. Give mass a density

const part = await loadModel({ source: 'plate-with-through-bore.step', format: 'step' });

expectGeo(part)
  .toHaveMass({ value: 82.16 });
expectGeo(part)
  .toHaveMass({ value: 82.16, density: 0.0027 });

import { readFile } from 'node:fs/promises';import { expectGeo } from 'geospec';import { createModelLoader } from 'geospec/model';import { Engine } from 'geospec-engine/node';const engine = new Engine();const loadModel = createModelLoader({ engine, readSource: async (source) => new Uint8Array(await readFile(source as string)) });const part = await loadModel({ source: 'plate-with-through-bore.step', format: 'step' });expectGeo(part)  .toHaveMass({ value: 82.16 });expectGeo(part)  .toHaveMass({ value: 82.16, density: 0.0027 });await loadModel.dispose();engine.close();

No admitted subject carries a material, so a mass claim without density is refused with GEOSPEC_UNSUPPORTED_EVIDENCE, and its suggestion asks for one; in a script the refusal throws or raises GeoSpecAssertionError, as a failure does. The mass unit follows the density: grams per mm³ gives grams, so the plate in aluminium at 0.0027 g/mm³ weighs 82.16 g.

3. Read the evidence

A passed or failed claim reports the measured value, the expectation the engine applied and the source.

The mass claim on the plate measures as:

{
  "measured": 82.15884991765361,
  "normalizedExpected": { "density": 0.0027, "tolerance": 0.02, "value": 82.16 },
  "witnesses": { "density": 0.0027, "source": "brep", "tolerance": 0.02, "volume": 30429.20367320504 }
}

A failed claim carries GEOSPEC_MEASUREMENT_MISMATCH and says what it measured: Mass is 82.15884991765361 (brep evidence), which does not satisfy 82 within 0.02.

4. Switch to the mesh

Switch the model in step 1 to the gear and every number fails: the mesh measures 911.53 mm³ from its 624 triangles, its surface area of 910.80 mm² is the sum of the triangle areas, and the evidence source is mesh. A mesh approximates curved faces, so its numbers differ from the BRep of the same part; claim the numbers of the export you admitted. An open mesh still yields a volume and a centroid, so pair these claims with toBeWatertight from Check meshes.

Variations

A range instead of a value. value: { greaterThan: 30000, lessThan: 31000 } passes on the plate; tolerance applies to value only. Set tolerances and budgets has the comparison rules.

Centre of mass on chosen axes. point: { z: 10 } constrains z and leaves x and y free. A failed claim lists each axis that missed under failures, with the actual and expected values.

A nominal volume. tolerance: 0 on value: 30429.2 fails, because the plate measures 30429.20367 mm³. State the value you computed and a tolerance that covers your rounding, not the engine's.

The reference. The measurement reference lists every field and the admitted geometry of each claim; Check dimensions covers the bounds the same evidence yields.