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fabrication-limits.md
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1# Fabrication limits, tolerances, and materials
3Read this before finalising any geometry. Process determines what geometry is possible; material
4determines whether the part survives the lab.
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.
Raw lines 1 to 56## Process tolerances
8Achievable tolerance and minimum feature size, as planning figures. **Every number here depends on
9the specific machine, material, and operator.** Use them to choose a process and to size a first
10article, then verify with a test coupon.
12| Process | Typical tolerance | Min wall | Min feature | Notes |
13| --- | --- | --- | --- | --- |
14| FDM | ±0.3 mm (often worse over 100 mm) | 1.2 mm (3 x 0.4 mm nozzle) | ~0.8 mm | Anisotropic: much weaker across layers. Porous. |
15| SLA / DLP | ±0.1 mm | 0.8 mm | ~0.3 mm | Better surface and detail. Resin choice dominates properties. |
16| SLS (nylon) | ±0.2 mm | 0.8 mm | ~0.5 mm | Often less anisotropic than FDM; no supports; porous surface. |
17| CNC milling | ±0.05 mm or better | 0.8 mm in metal | Set by tool diameter | Internal corners carry the tool radius — you cannot mill a sharp internal corner. |
18| Laser cutting | ±0.1 mm | n/a | Kerf ~0.1-0.3 mm | 2D only. Edge taper on thick stock. Kerf offset must be applied. |
20Two consequences that catch people:
22- **Holes print undersize** on both FDM and SLA. A 6.0 mm modelled hole typically measures under
23 6.0 mm. Oversize functional bores, or plan to ream them.
24- **Internal corners cannot be sharp in milling.** If a milled pocket must accept a square part,
25 add corner relief cuts. (For a part with *rounded* corners the tool radius is harmless as long
26 as it stays at or below the part's minimum corner radius — see the corner-radius rule in
27 `references/labware-adapters.md`.)
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.
| Process | Typical tolerance | Min wall | Min feature | Notes |
|---|---|---|---|---|
| FDM | ±0.3 mm (often worse over 100 mm) | 1.2 mm (3 x 0.4 mm nozzle) | ~0.8 mm | Anisotropic: much weaker across layers. Porous. |
| SLA / DLP | ±0.1 mm | 0.8 mm | ~0.3 mm | Better surface and detail. Resin choice dominates properties. |
| SLS (nylon) | ±0.2 mm | 0.8 mm | ~0.5 mm | Often less anisotropic than FDM; no supports; porous surface. |
| CNC milling | ±0.05 mm or better | 0.8 mm in metal | Set by tool diameter | Internal corners carry the tool radius — you cannot mill a sharp internal corner. |
| Laser cutting | ±0.1 mm | n/a | Kerf ~0.1-0.3 mm | 2D only. Edge taper on thick stock. Kerf offset must be applied. |
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.)
29### Laser cutting
31- **Kerf direction is fixed by the physics, so get it right in the handover.** The beam removes a
32 strip of width k (~0.1–0.3 mm) centred on the drawn line. Cutting on the line therefore makes
33 **holes and internal cutouts come out oversize by ~k, and the part's outer outline undersize by
34 ~k**. Say which convention the DXF uses (on-the-line is the default assumption) and let the shop
35 offset, or offset the geometry yourself and say so — never both.
36- **Put cut geometry on a named layer** (one layer per operation: `CUT`, `ENGRAVE`). Shops key
37 power and speed to layer or colour; geometry on layer 0 forces them to guess.
38- **Cut order matters:** internal features before the outer outline, or the part shifts once it is
39 freed from the sheet.
40- **Sheet stock is not its nominal thickness.** "3 mm" acrylic commonly runs ~2.8–3.2 mm; slots
41 sized for nominal will be loose or tight. For solvent-welded joints prefer **cast** acrylic over
42 extruded — cleaner cut edge, less vapour crazing — and remember alcohols craze acrylic either
43 way (see Chemical, below).
44- Laser-cut edges are sharp and slightly tapered; call out deburring or flame-polishing for
45 anything handled or animal-facing.
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.
47## Fits and clearances
49Nominal dimensions do not produce fits. Choose a clearance deliberately, per side:
51| Fit | FDM | SLA | CNC |
52| --- | --- | --- | --- |
53| Free-sliding (a plate dropping into a pocket) | 0.40 mm | 0.20 mm | 0.10 mm |
54| Located but removable by hand | 0.25 mm | 0.10 mm | 0.05 mm |
55| Press / interference | Coupon-specific | Coupon-specific | Use a toleranced shaft/hole fit |
57For a guaranteed free fit, receiving-feature CAD size minus its undersize tolerance
58must exceed the mating part's maximum size plus the required functional gap.
59A nominal allowance is not the guaranteed assembled clearance.
61Then remember the **other** part has tolerance too. When mating to a standardised component,
62design the receiving feature against the component's **maximum material condition**, not its
63nominal — a pocket sized from nominal does not guarantee clearance for conforming parts. This is what
64`intent: "envelope"` enforces. Declare it in the model and check the manifest:
66```bash
67python scripts/check.py interfaces out/part.manifest.json
68```
70Or check a single number by hand:
72```bash
73python scripts/check.py fit --standard slas-microplate-footprint \
74 --intent envelope --clearance 0.8 --value footprint_length=129.06
75```
Fits and clearances
Nominal dimensions do not produce fits. Choose a clearance deliberately, per side:
| Fit | FDM | SLA | CNC |
|---|---|---|---|
| Free-sliding (a plate dropping into a pocket) | 0.40 mm | 0.20 mm | 0.10 mm |
| Located but removable by hand | 0.25 mm | 0.10 mm | 0.05 mm |
| Press / interference | Coupon-specific | Coupon-specific | Use a toleranced shaft/hole fit |
For a guaranteed free fit, receiving-feature CAD size minus its undersize tolerance must exceed the mating part's maximum size plus the required functional gap. A nominal allowance is not the guaranteed assembled clearance.
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 does not guarantee clearance for 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.jsonOr check a single number by hand:
python scripts/check.py fit --standard slas-microplate-footprint \
--intent envelope --clearance 0.8 --value footprint_length=129.06Runs code python scripts/check.py :67
77## Threads and inserts
79**Printed threads are usually a mistake.** Layer resolution is comparable to the thread pitch, so
80printed threads are weak, dimensionally unreliable, and shed particles.
82In descending order of preference:
841. **Heat-set threaded inserts** — the standard solution for printed parts. Model a straight bore
85 to the insert manufacturer's specified diameter (it varies by insert; get the datasheet) and
86 provide enough surrounding wall, typically at least 2 mm.
872. **Clearance hole plus a captive nut** in a hex pocket. Reliable and cheap.
883. **Tapping the printed material directly** — acceptable for light, infrequently-assembled joints.
894. **Printing the thread** — only for coarse threads (roughly M6 and above), never for fine
90 threads like the 0.635 mm pitch SM1 (see `references/optomechanics.md`).
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:
- 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.
- Clearance hole plus a captive nut in a hex pocket. Reliable and cheap.
- Tapping the printed material directly — acceptable for light, infrequently-assembled joints.
- 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).
92## Orientation and anisotropy
94For FDM especially, orientation is a design decision, not a printing detail:
96- Parts are substantially weaker **across** layers than along them. Orient so that load runs
97 along layers, and state the intended orientation in the model docstring.
98- Overhangs beyond roughly 45 degrees need support, and supported surfaces come out rough and
99 dimensionally poor. If a surface is a sealing or mating face, orient it so it is not supported.
100- Holes printed with their axis vertical are usually more circular; printed horizontally they come out with a
101 drooped top. Teardrop or chamfer horizontal holes that must stay round.
102- **Every enclosed cavity needs a drain path** in resin printing. See
103 `references/microfluidics.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 usually more circular; 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
Raw lines 105 to 106107### Thermal
109| Material | Approximate service limit | Autoclave (121 °C)? |
110| --- | --- | --- |
111| PLA | ~50-60 °C | **No** — distorts well below autoclave temperature |
112| PETG | ~70-80 °C | No |
113| ABS / ASA | ~90-100 °C | Marginal, generally no |
114| Polypropylene | Grade/application dependent | Some molded labware is rated for 121 °C; printed parts need validation |
115| Nylon (SLS) | ~120-160 °C | Sometimes; verify per grade |
116| PEEK | Grade/load dependent | Often compatible; verify grade and cycles |
117| Stainless steel, aluminium, glass | Grade/coating dependent | Often compatible; verify corrosion and thermal-shock limits |
119**Assume a printed part is not autoclavable unless it is a verified high-temperature material.**
120Offer chemical or gas sterilisation as the alternative, and check that against the solvent notes
121below.
Thermal
| Material | Approximate service limit | Autoclave (121 °C)? |
|---|---|---|
| PLA | ~50-60 °C | No — distorts well below autoclave temperature |
| PETG | ~70-80 °C | No |
| ABS / ASA | ~90-100 °C | Marginal, generally no |
| Polypropylene | Grade/application dependent | Some molded labware is rated for 121 °C; printed parts need validation |
| Nylon (SLS) | ~120-160 °C | Sometimes; verify per grade |
| PEEK | Grade/load dependent | Often compatible; verify grade and cycles |
| Stainless steel, aluminium, glass | Grade/coating dependent | Often compatible; verify corrosion and thermal-shock limits |
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.
Raw lines 107 to 122123### Chemical
125- **Acrylic (PMMA)** crazes on contact with alcohols, including 70% ethanol — a serious problem in
126 a lab that disinfects everything with ethanol.
127- **Polycarbonate** is attacked by many solvents and by some alkaline cleaners.
128- **PLA** hydrolyses; it degrades in warm, wet, or repeatedly-cleaned service.
129- **PP, PTFE, PEEK** have broad chemical resistance, but require compatibility checks for the exact
130 chemical, concentration, temperature and exposure time; none is universally safe.
132Always ask what the part will be cleaned with, not just what it will contain. Cleaning agent
133compatibility is more often the failure than the sample.
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, but require compatibility checks for the exact chemical, concentration, temperature and exposure time; none is universally safe.
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.
Raw lines 123 to 134135### Biocompatibility
137- **Uncured SLA resin is cytotoxic.** Even nominally biocompatible resins require the
138 manufacturer's full post-cure and wash protocol, and leachables can still affect sensitive cell
139 assays.
140- For anything contacting cells, tissue, or animals: prefer glass, medical-grade polymer, or PTFE
141 for the contact surface, and use the printed part as a holder that does not touch the sample.
142- "Biocompatible" on a resin datasheet refers to a specific certified process and application. It
143 does not transfer to your printer, your cure schedule, or your assay. Say this rather than
144 implying a printed part is cell-safe.
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.
146### Optical
148- Printed and milled surfaces scatter; they are not optical surfaces.
149- Most printed resins **autofluoresce**, often strongly, which contaminates fluorescence readouts.
150- Black is not automatically non-reflective.
151- Where an optical surface is needed, use glass or a bonded film and model the holder around it.
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.
153## Cost and lead-time reality
155Mention these when recommending a process: FDM is hours and pennies; SLA is hours and modest cost;
156SLS and CNC are typically outsourced with days of lead time and much higher cost. For ±0.05 mm requirements, ask the supplier to confirm process capability and inspection
157method; CNC or post-machining may be needed.
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. For ±0.05 mm requirements, ask the supplier to confirm process capability and inspection method; CNC or post-machining may be needed.
Raw lines 153 to 158163## Review sources
165The dimensional tables are illustrative planning values, not supplier acceptance criteria.
167- Resin cure and biocompatibility are process-specific: <https://formlabs.com/support/Best-practices-for-post-curing-prints/>
168- Polypropylene labware sterilisation: <https://www.thermofisher.com/de/de/home/life-science/lab-plasticware-supplies/plastic-material-selection/polypropylene-pp-labware.html>
169- Grade/chemical/temperature compatibility: <https://documents.thermofisher.com/TFS-Assets/LCD/posters/LCD-Chemical-Resistance-Guide-GALSPCHEMRESPOSTER-EN.pdf>
Review sources
The dimensional tables are illustrative planning values, not supplier acceptance criteria.
- Resin cure and biocompatibility are process-specific: <https://formlabs.com/support/Best-practices-for-post-curing-prints/>
- Polypropylene labware sterilisation: <https://www.thermofisher.com/de/de/home/life-science/lab-plasticware-supplies/plastic-material-selection/polypropylene-pp-labware.html>
- Grade/chemical/temperature compatibility: <https://documents.thermofisher.com/TFS-Assets/LCD/posters/LCD-Chemical-Resistance-Guide-GALSPCHEMRESPOSTER-EN.pdf>