Lab Hardware Cad

Lab Hardware CAD

Design physical research hardware as parametric Python source, export STEP as the authoritative artifact, and verify the result both numerically and visually before anything is fabricated.

The hard part of lab hardware is almost never the geometry. It is that the part must mate with equipment whose dimensions are fixed by a published standard or a vendor drawing. A holder that is 0.5 mm too wide does not fit the plate reader; a channel with the wrong aspect ratio collapses during bonding; a mount whose bolt pattern is 25.4 mm instead of 25.0 mm will not reach the optical table. This skill exists to keep those numbers correct and checked.

When to use

Use for any request to design, model, or fabricate a physical part for a lab: chip, mold, mount, adapter, holder, rack, bracket, enclosure, jig, fixture, arena, or maze. Also use to inspect or modify an existing STEP file.

Do not use for finite-element analysis, computational fluid dynamics, molecular structure, or scientific plotting. Those are different skills.

Setup

uv venv --python 3.12 .venv-labcad
uv pip install --python .venv-labcad/bin/python "build123d==0.13.0" "matplotlib>=3.8"

build123d 0.13.0 requires Python >=3.11,<3.15 and pulls in the OpenCascade kernel through cadquery-ocp-novtk. The wheel is large; install once per project and reuse it.

All bundled scripts take --help. check.py standards runs without build123d installed.

Model files are executed, not parsed. gen.py, check.py, and snapshot.py import a *_model.py and call its build(), which runs arbitrary Python in the current environment. That is inherent to parametric CAD — the source is the design. Only run model files authored in this session or supplied by the user from a trusted location. If a model came from the internet, a shared drive, or an untrusted colleague, read it before running it and say that you did.

Required workflow

Follow these steps in order. Steps 5 and 6 are not optional, and step 6 is not waived by step 5 passing.

1. Route to a device family

Read the request, classify it, and load exactly one family reference. Do not load all four — they are long, and mixing conventions between families is a common source of error.

If the part genuinely spans two families — a microfluidic chip that bolts to an optical table — load the family that owns the critical interface, then read only the interface section of the second. State in your response which family you routed to.

2. Establish the interface dimensions before any geometry

Every part has at least one mating interface. Before writing code, write down for each interface:

• the source of the dimension: a published standard, a vendor drawing, or a user measurement;
• the nominal value and tolerance;
• the clearance or interference you intend, and why.

Look the number up in assets/standards.json or the family reference. Never write an interface dimension from memory. If the number is not in the standards file or the reference, ask the user for the vendor drawing or the measurement rather than guessing. A guessed interface dimension is the single most expensive failure mode in this skill.

A pocket receiving an external component is sized against that component's maximum material envelope — nominal plus its plus-tolerance — and only then given clearance. A shaft entering a hole instead uses the hole's minimum diameter; envelope does not model that case.

python scripts/check.py standards --list
python scripts/check.py standards --show slas-microplate-footprint

The bundled standard IDs (exact strings; do not guess variants): slas-microplate-footprint, slas-microplate-height, slas-microplate-flange, slas-well-positions-96, slas-well-positions-384, slas-well-positions-1536, cuvette-standard-10mm, optical-breadboard-metric, optical-breadboard-imperial, cage-system-30mm, sm1-lens-tube-thread.

If the part mates with nothing in this list, that is common and fine: declare no interfaces, and name every interface dimension with its source (user spec, vendor drawing, measurement) as unchecked in the report. Never declare against an unrelated standard to fill the gap — a fabricated declaration is worse than an honest "nobody checked this".

3. Choose the process before choosing the geometry

Read references/fabrication-limits.md. Process determines minimum wall, minimum feature, achievable tolerance, and whether the part survives autoclaving or contact with your solvent. Do not assume FDM can hold ±0.05 mm without calibration. Cell-contact SLA parts need a validated material and post-processing workflow plus assay-specific testing. Record the process and material in the model docstring.

4. Author a parametric model

Write <part>_model.py. The source is the authoritative artifact — never hand-edit an exported STEP file, and never regenerate from a mesh.

Requirements:

• Every dimension that a user might change is a module-level named constant with units in the name: bore_d_mm, wall_t_mm, post_h_mm. No bare numbers in the body except 0, 1, and 2.
• Expose build() -> Part. gen.py calls it.
• Group parameters into an INTERFACE block (dimensions fixed by a standard, annotated with the standard ID) and a DESIGN block (dimensions you are free to choose).
• Derive every computed dimension inside a function, never at module level, so --param overrides actually reach it.
• Declare an interfaces() function returning the dimensions the part must fit, each with its standard ID and intent. This is what makes the interface machine-checkable in step 5. intent is "envelope" when the feature must accept any conforming part (a pocket, bore, or slot — checked one-sided at maximum material condition plus your clearance) and "match" when comparing against the nominal band, expanded by any explicit allowance. This is not a conformance certificate. clearance is the total intended clearance in mm and must be non-negative. Declare only dimensions that constrain this part's mating features — a property of the mating equipment (a table's edge border, a typical plate thickness) is not an interface of yours. If no bundled standard applies, return [].
• Declare a checks() function of go/no-go gauges measured from the built solid: a clear region for everything that must pass through or fit in (screw shafts, beam corridors, the mating part at maximum material condition dropping into its pocket), a material region with a requirement-sized minimum volume for everything that must remain (a ridge, a ledge, a screw seat), and a bbox_* bound for every size limit the user stated. Map every geometric requirement in the request to one entry; these catch the errors that is_valid, the bounding box, and declared numbers cannot see. gen.py runs them on every generation and fails the build when one fails. Schema and worked examples: references/build123d-patterns.md.
• Put the process, material, and every interface source in the module docstring.

"""SLAS base-footprint fit coupon; confirm upper plate body/draft before a full holder.

Process: FDM, PETG, 0.2 mm layer.  Tolerance budget +/-0.3 mm.
Interfaces:
  - Plate pocket: ANSI/SLAS 1-2004 (R2012) footprint 127.76 x 85.48 mm, +/-0.50 overall.
  - Upper plate body, lid and stage mounting: not represented in this coupon.
"""
from build123d import *

# --- INTERFACE (fixed by standard; do not tune) ---
plate_l_mm = 127.76   # ANSI/SLAS 1-2004 nominal
plate_w_mm = 85.48    # ANSI/SLAS 1-2004 nominal
plate_tol_mm = 0.50   # ANSI/SLAS 1-2004; the pocket is sized to nominal + this
# --- DESIGN (free) ---
pocket_clearance_mm = 0.40   # per-side; FDM, see fabrication-limits.md
wall_t_mm = 3.0
floor_t_mm = 2.5
body_h_mm = 12.0


def pocket_mm() -> tuple[float, float]:
    """Pocket at the plate's maximum material condition plus clearance per side.

    A pocket sized from nominal can jam on conforming plates.
    """
    growth = plate_tol_mm + 2 * pocket_clearance_mm
    return plate_l_mm + growth, plate_w_mm + growth


def interfaces() -> list[dict]:
    """What this part must fit. `check.py interfaces` verifies every entry."""
    pocket_l, pocket_w = pocket_mm()
    return [
        {"feature": "plate pocket length", "standard": "slas-microplate-footprint",
         "dimension": "footprint_length", "value": pocket_l,
         "intent": "envelope", "clearance": 2 * pocket_clearance_mm},
        {"feature": "plate pocket width", "standard": "slas-microplate-footprint",
         "dimension": "footprint_width", "value": pocket_w,
         "intent": "envelope", "clearance": 2 * pocket_clearance_mm},
    ]


def checks() -> list[dict]:
    """Gauges measured from the built solid. Sized from the REQUIREMENT's numbers
    (plate MMC, the user's height limit), not from the pocket parameters, so a
    wrong parameter cannot shrink the gauge to match the wrong geometry."""
    depth = body_h_mm - floor_t_mm
    return [
        {"feature": "plate at MMC drops into the pocket",
         "clear": {"box": (plate_l_mm + plate_tol_mm, plate_w_mm + plate_tol_mm, depth),
                   "at": [(0.0, 0.0, floor_t_mm + depth / 2)]}},
        {"feature": "under 15 mm for the stage", "bbox_z": {"max": 15.0}},
    ]


def build() -> Part:
    pocket_l, pocket_w = pocket_mm()
    with BuildPart() as carrier:
        Box(pocket_l + 2 * wall_t_mm, pocket_w + 2 * wall_t_mm, body_h_mm,
            align=(Align.CENTER, Align.CENTER, Align.MIN))
        with Locations((0, 0, floor_t_mm)):
            Box(pocket_l, pocket_w, body_h_mm, mode=Mode.SUBTRACT,
                align=(Align.CENTER, Align.CENTER, Align.MIN))
    return carrier.part

See references/build123d-patterns.md for the builder-vs-algebra choice, the interfaces() contract, sketching, selectors, fillets, and threaded-insert bores.

5. Generate and run the checks

python scripts/gen.py carrier_model.py --outdir out/
python scripts/check.py facts out/carrier.step
python scripts/check.py interfaces out/carrier.manifest.json
python scripts/check.py geometry out/carrier.step --model carrier_model.py

gen.py also evaluates the model's checks() gauges against the solid it just built, prints each PASS/FAIL, records them in the manifest, and exits non-zero on a failure — so a part that violates its own declared geometry never silently becomes an artifact. check.py geometry re-runs the same gauges against the exported STEP. Repeat any gen.py --param overrides with check.py geometry --param; otherwise the gauges use the source defaults.

out/ is a scratch convention, not a requirement. When the user asked for deliverables in a specific place, generate there (--outdir .) or copy the STEP, manifest, and DXF to it before finishing — a deliverable that exists only inside out/ has not been delivered.

gen.py writes carrier.step (authoritative), carrier.stl (mesh preview and printing), and carrier.manifest.json recording the source hash, resolved parameters, declared interfaces, library versions, and measured bounding box, volume, and validity. The manifest is the provenance record — keep it with the artifact.

check.py facts reports is_valid, bounding box, volume, surface area, centre of mass, and solid count. A part that reports is_valid: false is broken geometry; fix the source before going further.

check.py interfaces evaluates every entry the model declared against the standards database and exits non-zero on failure. Be clear about what it does and does not verify: it checks the declared numbers — catching a transcribed dimension, the wrong standard, and nominal-instead-of-MMC sizing — but it never measures the built geometry, and a value computed from the same constants it is checked against passes with zero headroom by construction. Do not cite it as evidence the geometry is right; facts and the snapshot are the geometry checks. An empty declaration list passes: a part that mates with nothing in the bundled database has nothing to declare, and its interface dimensions are instead named as unchecked in the report. A manifest must contain an explicit interfaces list; a missing field or null is a malformed manifest, not evidence that interfaces were reviewed and none applied.

Use interfaces rather than check.py fit for anything internal — a pocket, bore, or slot does not appear in the part's outer bounding box, which is what fit measures. Reach for fit only to check one number by hand (--value footprint_length=129.06), or when the part's own outline is the interface, such as a gasket cut to a plate footprint.

For assemblies, check that parts do not interfere:

python scripts/check.py clearance out/carrier.step out/lid.step --min 0.3