Tolerances and clearances for 3D printed fits
Tolerance is the difference between a part that fits and a paperweight. Printers vary, but FDM behaviour is predictable enough that a few rules cover most cases.Start with your printer's real number
Print a 20 mm cube and a 10 mm hole test. If the cube is 20.2 and the hole is 9.7, your printer adds about 0.2 mm per side on outer walls and removes about 0.3 mm from holes. Most well-tuned machines land near these values, and SpeakCAD's defaults assume them.
Clearance table (per side, FDM, PLA/PETG)
| Fit | Clearance | Use |
|---|---|---|
| Press fit (no movement) | 0.1 to 0.2 mm | Pegs, bearings, magnets |
| Light press / snug | 0.2 to 0.3 mm | Lids that should stay on, knobs on D-shafts |
| Sliding fit | 0.4 to 0.6 mm | Drawers, sliding lids, pistons |
| Free / loose | 0.8 mm+ | Hinges, hanging parts |
| Printed threads | 0.3 to 0.4 mm on the profile | M8 and larger |
Snap fits
A cantilever clip with 0.2 mm of interference and a 45° lead-in engages firmly in PLA and PETG. Longer arms flex more: aim for an arm length at least 6 times its thickness. Print arms lying flat so the bend is along the layers.
Bearings and magnets
608 bearings (22 mm outer) go into a 22.2 mm pocket for a firm press. 6 mm round magnets fit a 6.2 mm hole. Add a 0.3 mm chamfer at the mouth to guide them in.
Shafts and bores
A 6 mm shaft that must not spin in a knob: 5.9 to 6.0 mm bore plus a flat matching the D profile. A 6 mm shaft that must rotate freely: 6.4 to 6.6 mm bore.
Test the fit before the full print
The numbers above are averages. To find your printer's own number without measuring anything, use Fit Check: SpeakCAD cuts a five-minute slice of the fit from your model, you report too tight, perfect or too loose, and the clearance your printer needs is saved on your account for every later part.
Describing fits to SpeakCAD
Say what you want to happen, not just the number: "press fit on a 6 mm shaft", "lid slides on smoothly", "magnets 6 x 2 mm pressed in". The AI applies the clearance and states the final dimensions in its reply. If a test print is off, quote the measured gap and ask for the correction; re-downloading after a tolerance fix is free.