If two 3D-printed parts are meant to fit together, there has to be the right gap between them — a tolerance. Design them exactly the same size and they'll never fit. Here are the clearances that work in practice and why FDM needs a little extra thought.
What is a tolerance?
A tolerance is the deliberate clearance between two surfaces that meet. It isn't about sloppiness — every manufacturing method has some deviation, and you need room for parts to actually go together.
Think of clearance as the total difference in diameter or width. If you want 0.2 mm all the way around a pin, make the hole 0.4 mm larger in diameter than the pin.
Three common fits
Guideline values for FDM, adjust as needed:
- Press fit (tight): ~0.1–0.2 mm clearance. Parts hold without glue but take some force. Good for pins, plugs and things that should stay put.
- Sliding fit: ~0.3–0.4 mm clearance. Parts move against each other without binding. Good for lids, drawers and moving joints.
- Loose fit: ~0.5 mm or more. Easy to assemble and take apart, forgiving of dimensional variation. Good for parts opened often or assembled by non-experts.
Why FDM differs
FDM lays down molten plastic layer by layer, which affects dimensions in several ways:
- Nozzle width: Details smaller than the line width (often 0.4 mm) are hard to reproduce accurately.
- Elephant's foot: The bottom layer can squish out slightly, so holes get tighter and pins wider near the base.
- Shrinkage: Plastic contracts as it cools, more in some materials than others.
The result is that holes often come out slightly smaller than drawn and outer dimensions slightly larger. Account for that when you design.
How to design it right
- Put the tolerance on the right side: Make holes larger rather than pins smaller — it's easier to predict.
- Test before you commit: Print a small test piece with a few different clearances before a big batch.
- Mind the orientation: Holes printed lying down often turn out slightly oval. A drill or reamer afterwards gives dimensionally accurate holes.
- Chamfer the edges: A small chamfer on the entry makes parts much easier to fit together.
Unsure? Ask us
Tolerances are where an experienced eye saves the most time. If you don't know what clearance your design needs, describe how the parts should fit and we'll suggest values. We can also adjust the file for you.
Read how to export STL files correctly first, or upload your model for an instant quote — and we'll help you get the fit exactly right.