Design
Why 3D printed holes come out undersized, and how to dimension them
Holes print small because material shrinks, extrudes or cures into the bore. Add a process offset, and ream anything critical.
HenleyPrint3D engineering2 min readUpdated Published
Key takeaways
- Starting point: add 0.2 to 0.4 mm to FDM clearance holes, 0.1 to 0.3 mm in SLS, 0.1 to 0.2 mm in SLA.
- Use ISO 273 clearance sizes for fasteners instead of the thread diameter.
- For ±0.05 mm, print undersize and ream.
Suits
Designers dimensioning bolt holes, bores and pin fits on printed parts.
Worth knowing
Holes print 0.1 to 0.4 mm small, so add the process offset and ream anything critical.
Offset to add by process
| Process | Typical undersize | Add to diameter | Notes |
|---|---|---|---|
| FDM | 0.1 to 0.4 mm | 0.2 to 0.4 mm | Worse under 6 mm and on horizontal axes |
| SLA | 0.05 to 0.2 mm | 0.1 to 0.2 mm | Add a drain path for blind holes |
| SLS (PA12) | 0.1 to 0.3 mm | 0.1 to 0.3 mm | Holes under 1.5 mm need drilling |
| PolyJet | 0.05 to 0.15 mm | 0.1 mm | Support material must be cleared |
Fastener clearance holes (ISO 273)
| Thread | Fine | Medium | Coarse |
|---|---|---|---|
| M3 | 3.2 mm | 3.4 mm | 3.6 mm |
| M4 | 4.3 mm | 4.5 mm | 4.8 mm |
| M5 | 5.3 mm | 5.5 mm | 5.8 mm |
| M6 | 6.4 mm | 6.6 mm | 7.0 mm |
| M8 | 8.4 mm | 9.0 mm | 10.0 mm |
Order parts through our SLS 3D printing or FDM 3D printing services, and see PA12 nylon SLS printing.
Frequently asked questions
Why is my 3D printed hole smaller than the CAD model?
Faceted STL geometry, extruded bead width in FDM, light scatter in SLA and powder growth in SLS all push material into the hole. The effect is usually 0.1 to 0.4 mm on diameter.
How much should I add to a hole for a bolt?
Start from the ISO 273 clearance size for the bolt, for example 5.5 mm for M5, then add 0.2 to 0.4 mm for FDM, 0.1 to 0.3 mm for SLS or 0.1 to 0.2 mm for SLA.
Can 3D printing hold a hole to ±0.05 mm?
Not straight off the printer. Print the hole 0.3 to 0.5 mm undersize and ream it, or bore it, to the final size.
Do small holes shrink more than large ones?
Proportionally, yes. A fixed bead width or cure bleed is a bigger fraction of a 3 mm hole than of a 30 mm one, and holes below about 1.5 mm in SLS are often too small to clear.
Should I tap threads into printed plastic?
For repeated assembly, use heat-set or press-in inserts sized to the insert datasheet. Tapped plastic threads wear quickly.
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