Tolerance and clearance guide
How big a gap to leave between parts that fit together. Pick the fit you want and the size of the part you already have, and get the size to model for the other one.
Suggested gaps
Gaps are per side, so the hole is bigger than the peg by twice the figure. Start in the middle of the range, then go tighter or looser once you've tried it on your printer.
| Fit | FDM | Resin |
|---|---|---|
| Press fit | 0.00 to 0.05 mm | 0.00 to 0.03 mm |
| Friction fit | 0.10 to 0.15 mm | 0.05 to 0.08 mm |
| Sliding fit | 0.20 to 0.25 mm | 0.10 to 0.15 mm |
| Loose fit | 0.30 to 0.40 mm | 0.15 to 0.25 mm |
| Print in place | 0.30 to 0.50 mm | 0.20 to 0.35 mm |
| Printed thread | 0.20 to 0.30 mm | 0.10 to 0.15 mm |
Holes for metric bolts
Clearance holes from ISO 273, for a bolt that passes through. To screw a bolt straight into plastic, model the tap size: printed holes come out a little small, so the bolt cuts its own thread. For heat-set inserts, use the hole size the insert's maker lists, as it varies by brand.
| Bolt | Close clearance | Normal clearance | Screw into plastic |
|---|---|---|---|
| M2 | 2.20 mm | 2.40 mm | 1.60 mm |
| M2.5 | 2.70 mm | 2.90 mm | 2.05 mm |
| M3 | 3.20 mm | 3.40 mm | 2.50 mm |
| M4 | 4.30 mm | 4.50 mm | 3.30 mm |
| M5 | 5.30 mm | 5.50 mm | 4.20 mm |
| M6 | 6.40 mm | 6.60 mm | 5.00 mm |
| M8 | 8.40 mm | 9.00 mm | 6.80 mm |
Why parts don't fit
Holes print small
Curves are exported as flat facets, and the outer wall squashes inwards slightly. Small holes lose the most. Export STLs at a finer resolution, or use your slicer's hole compensation setting.
The first layer spreads
The squashed first layer bulges outwards ("elephant's foot") and makes the bottom of a part too big. Add a small chamfer to bottom edges, or turn on elephant's foot compensation.
Direction matters
Holes printed on their side come out slightly oval, with a drooping top. Print holes that must be round upright, or model a teardrop or flat top so the roof doesn't sag.
Test before the big print
Print a small test with a few pegs and holes stepped by 0.05 mm, and note which fits feel right. It takes ten minutes and saves reprinting a six-hour part.
Common problems
Print not coming out right? These guides list the likely causes and fixes, easiest first.