Expect about plus or minus 0.2 mm on most features. Leave 0.3 to 0.4 mm of clearance per side for parts that slide together, 0.1 to 0.2 mm for a snug fit, and make holes about 0.2 mm larger than the pin or screw that goes in them.
Why printed parts aren't exact
Our printers lay down plastic in thin layers from a heated nozzle. The plastic shrinks a little as it cools, and round holes are built from many short straight segments. The result is very repeatable, but small features come out slightly different from the CAD model.
Clearance cheat sheet
| Fit | Gap per side | Example |
|---|---|---|
| Loose slide | 0.4 to 0.5 mm | Lid that lifts off easily |
| Slip fit | 0.2 to 0.3 mm | Knob on a shaft, sliding drawer |
| Snug / light press | 0.1 to 0.15 mm | Cap that stays put by friction |
| Press fit | 0 to 0.05 mm | Pin pressed in, test first |
Holes
Holes printed vertically tend to come out slightly undersized. Add about 0.2 mm to the diameter, or plan to run a drill bit through for a precise fit. For screws, model a clearance hole and let the screw cut its own thread, or use a heat-set insert for parts that get taken apart often.
Threads
Coarse threads print well, such as tube caps and jar lids. Fine machine threads below about M6 are better tapped or handled with inserts.
Snap fits
Make snap arms long and thin so they flex instead of crack, and print them in PETG rather than PLA. PLA tends to snap the first time. See PLA vs PETG vs TPU vs ASA.
Test small first
If the fit is critical, print a small test piece with just the mating feature. It is quick and costs little, and it saves reprinting a large part.
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