Fabrication limits, tolerances, and materials

Read this before finalising any geometry. Process determines what geometry is possible; material determines whether the part survives the lab.

Process tolerances

Achievable tolerance and minimum feature size, as planning figures. Every number here depends on the specific machine, material, and operator. Use them to choose a process and to size a first article, then verify with a test coupon.

Two consequences that catch people:

• Holes print undersize on both FDM and SLA. A 6.0 mm modelled hole typically measures under 6.0 mm. Oversize functional bores, or plan to ream them.
• Internal corners cannot be sharp in milling. If a milled pocket must accept a square part, add corner relief cuts. (For a part with rounded corners the tool radius is harmless as long as it stays at or below the part's minimum corner radius — see the corner-radius rule in references/labware-adapters.md.)

Laser cutting

• Kerf direction is fixed by the physics, so get it right in the handover. The beam removes a strip of width k (~0.1–0.3 mm) centred on the drawn line. Cutting on the line therefore makes holes and internal cutouts come out oversize by ~k, and the part's outer outline undersize by ~k. Say which convention the DXF uses (on-the-line is the default assumption) and let the shop offset, or offset the geometry yourself and say so — never both.
• Put cut geometry on a named layer (one layer per operation: CUT, ENGRAVE). Shops key power and speed to layer or colour; geometry on layer 0 forces them to guess.
• Cut order matters: internal features before the outer outline, or the part shifts once it is freed from the sheet.
• Sheet stock is not its nominal thickness. "3 mm" acrylic commonly runs ~2.8–3.2 mm; slots sized for nominal will be loose or tight. For solvent-welded joints prefer cast acrylic over extruded — cleaner cut edge, less vapour crazing — and remember alcohols craze acrylic either way (see Chemical, below).
• Laser-cut edges are sharp and slightly tapered; call out deburring or flame-polishing for anything handled or animal-facing.

Fits and clearances

Nominal dimensions do not produce fits. Choose a clearance deliberately, per side:

Then remember the other part has tolerance too. When mating to a standardised component, design the receiving feature against the component's maximum material condition, not its nominal — a pocket sized from nominal fits only the smaller half of conforming parts. This is what intent: "envelope" enforces. Declare it in the model and check the manifest:

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

Or check a single number by hand:

python scripts/check.py fit --standard slas-microplate-footprint \
  --intent envelope --clearance 0.8 --value footprint_length=128.81

Threads and inserts

Printed threads are usually a mistake. Layer resolution is comparable to the thread pitch, so printed threads are weak, dimensionally unreliable, and shed particles.

In descending order of preference:

1. Heat-set threaded inserts — the standard solution for printed parts. Model a straight bore to the insert manufacturer's specified diameter (it varies by insert; get the datasheet) and provide enough surrounding wall, typically at least 2 mm.
2. Clearance hole plus a captive nut in a hex pocket. Reliable and cheap.
3. Tapping the printed material directly — acceptable for light, infrequently-assembled joints.
4. Printing the thread — only for coarse threads (roughly M6 and above), never for fine threads like the 0.635 mm pitch SM1 (see references/optomechanics.md).

Orientation and anisotropy

For FDM especially, orientation is a design decision, not a printing detail:

• Parts are substantially weaker across layers than along them. Orient so that load runs along layers, and state the intended orientation in the model docstring.
• Overhangs beyond roughly 45 degrees need support, and supported surfaces come out rough and dimensionally poor. If a surface is a sealing or mating face, orient it so it is not supported.
• Holes printed with their axis vertical are round; printed horizontally they come out with a drooped top. Teardrop or chamfer horizontal holes that must stay round.
• Every enclosed cavity needs a drain path in resin printing. See references/microfluidics.md.

Materials

Thermal

Assume a printed part is not autoclavable unless it is a verified high-temperature material. Offer chemical or gas sterilisation as the alternative, and check that against the solvent notes below.

Chemical

• Acrylic (PMMA) crazes on contact with alcohols, including 70% ethanol — a serious problem in a lab that disinfects everything with ethanol.
• Polycarbonate is attacked by many solvents and by some alkaline cleaners.
• PLA hydrolyses; it degrades in warm, wet, or repeatedly-cleaned service.
• PP, PTFE, PEEK have broad chemical resistance and are the safe choices for solvent contact.

Always ask what the part will be cleaned with, not just what it will contain. Cleaning agent compatibility is more often the failure than the sample.

Biocompatibility

• Uncured SLA resin is cytotoxic. Even nominally biocompatible resins require the manufacturer's full post-cure and wash protocol, and leachables can still affect sensitive cell assays.
• For anything contacting cells, tissue, or animals: prefer glass, medical-grade polymer, or PTFE for the contact surface, and use the printed part as a holder that does not touch the sample.
• "Biocompatible" on a resin datasheet refers to a specific certified process and application. It does not transfer to your printer, your cure schedule, or your assay. Say this rather than implying a printed part is cell-safe.

Optical

• Printed and milled surfaces scatter; they are not optical surfaces.
• Most printed resins autofluoresce, often strongly, which contaminates fluorescence readouts.
• Black is not automatically non-reflective.
• Where an optical surface is needed, use glass or a bonded film and model the holder around it.

Cost and lead-time reality

Mention these when recommending a process: FDM is hours and pennies; SLA is hours and modest cost; SLS and CNC are typically outsourced with days of lead time and much higher cost. A design that needs ±0.05 mm has committed the user to CNC — flag that trade before they discover it at quoting.

Before fabrication

Work through references/validation.md.