Pre-fabrication validation checklist

Work through this before telling a user a part is ready to fabricate. Each item names the failure it catches, because a checklist without consequences gets skipped.

1. Provenance

• [ ] The STEP was produced by gen.py from the current model source. Catches: a stale artifact that no longer matches the code you just edited.
• [ ] A *.manifest.json exists alongside it, and its source.sha256 matches the model file. Catches: silently editing an exported STEP, which makes the design unreproducible.
• [ ] The manifest's interfaces block lists every dimension a bundled standard covers, and its values are the ones the model computed after any --param override. Empty is correct only when nothing on the part mates with a bundled standard — and then every interface dimension is named as unchecked in the report instead. *Catches: a static INTERFACES list frozen at import, recording pre-override numbers; and an interface that silently escaped checking.*
• [ ] Every parameter in the model is named with units. *Catches: the bare 12.7 nobody can later identify as half an inch.*

python scripts/gen.py part_model.py --outdir out/

2. Geometry is sound

• [ ] is_valid is true. Catches: self-intersecting or non-manifold solids that slicers and CAM silently mangle.
• [ ] solid_count is what you expect — usually 1. Catches: a boolean that failed and left two disjoint lumps, or a feature floating free of the body.
• [ ] Volume is plausible for the part's size and wall thickness. Catches: a cavity modelled solid, or a subtract that did nothing.
• [ ] Every geometric requirement in the request is declared in checks() and passes — clear regions for what must pass through or fit in, material regions for what must remain, bbox bounds for stated size limits. *Catches: a recess that swallowed its screw seat, a pocket the mating part cannot enter, a beam corridor with a wall in it, a feature a fillet silently ate — all invisible to is_valid and the bounding box.*

python scripts/check.py facts out/part.step
python scripts/check.py geometry out/part.step --model part_model.py

3. Interfaces

• [ ] Every interface dimension has a written source: a standard ID, a vendor drawing, or a user measurement. None came from memory. Catches: the single most expensive failure mode in this skill.
• [ ] Every interface covered by a standard is declared in the model's interfaces() and passes check.py interfaces. Catches: an interface nobody checked because the outer bounding box could not see it.
• [ ] Features that receive a standardised component use intent: "envelope". Catches: a pocket sized to nominal, which does not guarantee clearance for conforming parts.
• [ ] Any standard entry marked verified: false was confirmed against the primary document, or the user was told it is unconfirmed. Catches: propagating a derived number as if it were read from the standard.
• [ ] Metric vs imperial is confirmed where both exist, and no expression mixes them. Catches: the 25.0 vs 25.4 mm grid error, which accumulates to 1.6 mm over four pitches (five hole centres).
• [ ] Interfaces not covered by any bundled standard — a vendor drawing, a measurement — were reported to the user as unchecked, with the number and its source. Catches: a silent gap where the automatic check simply had nothing to say.

python scripts/check.py interfaces out/part.manifest.json

# one dimension by hand, when it is not declared in the model
python scripts/check.py fit --standard <id> --intent envelope --clearance <mm> --value <dim>=<mm>

4. Fits and assembly

• [ ] Every mating dimension has a deliberate clearance chosen for the process. Catches: nominal-to-nominal fits, which do not assemble.
• [ ] Multi-part assemblies were checked for interference. Catches: parts that overlap in CAD and therefore cannot exist together.
• [ ] Rigid multi-hole mounting patterns have at least one slot. Catches: a four-hole bolt pattern binding on accumulated tolerance.

python scripts/check.py clearance out/a.step out/b.step --min 0.3

5. Manufacturability

• [ ] Minimum wall and feature sizes are within the chosen process (fabrication-limits.md).
• [ ] Print or machining orientation is stated, and load runs along layers, not across them.
• [ ] Threads use inserts or captive nuts rather than printed threads, unless coarse.
• [ ] Enclosed cavities have a drain path for resin, and support-free access where possible.
• [ ] Milled internal corners have relief for the tool radius.

6. Material

• [ ] Material is compatible with the cleaning agent, not only the sample. Catches: acrylic crazing on 70% ethanol; PLA distorting in an autoclave.
• [ ] Sterilisation method is stated and the material actually survives it.
• [ ] Anything contacting cells, tissue, or animals has a justified material, or contact is designed out. Catches: assuming a printed resin part is cell-safe.
• [ ] Optical requirements — autofluorescence, scatter, transmission — are addressed if the part is near a beam or a detector.

7. Visual review — mandatory

• [ ] A snapshot was rendered and read after the most recent generation.
• [ ] Confirmed in the image: features on the intended faces; mold/chip polarity where resolvable; every port, bore, and boss present, inside the body, and passing through; nothing consumed by a fillet; clear apertures unobstructed.

python scripts/snapshot.py out/part.step --out out/part.png

This step is never waived by the numeric checks passing. is_valid: true with a correct bounding box is fully consistent with a pocket cut on the wrong face or an inverted mold. Some errors are visible in the picture; small relief and hidden passages need measured gauges or sections. Never infer connectivity or fit from an outline alone.

8. Report

Give the user, explicitly:

1. Process and material, and why.
2. Every interface dimension with its source and tolerance.
3. Clearances chosen, and the fit class they came from.
4. What the snapshot showed — described, not merely "a snapshot was generated".
5. Every check that did not pass, and every dimension you could not verify.
6. A recommendation to print a test coupon of the critical interface before committing to the full part, whenever the design depends on a fit.

State the unverified items plainly. A part list with one honest "this dimension needs confirmation" is far more useful than a confident one that is silently wrong.