Clearance and tolerance: making parts that actually fit
Why a 10 mm peg will not go into a 10 mm hole, how much gap to leave, and how to work out the right number for your own printer.
This is the first thing that surprises nearly everybody. You model a peg 10 mm across and a hole 10 mm across, you print them, and the peg will not go in. Nothing is broken. On the screen those two numbers were identical; in plastic they never are.
Why the gap is needed
A printer lays down molten plastic which spreads slightly and then shrinks slightly as it cools. Holes tend to come out a little undersized, pegs a little oversized, and both by an amount that depends on the machine, the material, the speed and the temperature of the room. Two parts that are exactly the same size in the model are therefore an interference fit in reality.
The gap you deliberately leave between two parts is called clearance. It is not a mistake or a fudge; it is part of the design, and stating it is what turns "it should fit" into engineering.
Numbers to start from
- 0.1 mm — a press fit. Goes together with force and stays put. Good for a pin you never want to come out.
- 0.2 mm — a snug fit. Goes together by hand, does not rattle.
- 0.4 mm — a free fit. Turns and slides easily. The usual starting point for moving parts.
- 0.6 mm and over — loose. Fine for a lid you want a child to open one-handed, too loose for anything that should feel solid.
Clearance is the difference in diameter, not the gap on each side. A 10 mm peg in a 10.4 mm hole has 0.4 mm of clearance and 0.2 mm of space on each side.
Finding your own number
The figures above are a starting point, not an answer. The honest way to get your number is to print a test: one plate with a peg and several holes at 10.1, 10.2, 10.3, 10.4 and 10.6 mm, then see which one feels right. That takes twenty minutes once and saves every project afterwards.
Teaching it
Clearance is a good first taste of designing for a real process rather than an ideal one. It has no single correct value, it can be defended with a reason, and being wrong is instantly obvious in a way that is interesting rather than discouraging — the parts either fit or they do not, and the fix is a number.
Do it, don't just read it
This pairs with lessons inside the Studio, where the checkpoints tick because the design really has that property, and un-tick if you undo the work:
- Lesson 7 — A Hole That Fits
- Lesson 8 — Clearance and Fit
- Lesson 11 — Second Print
Read next
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What is 3D printing?
How a 3D printer turns a design on a screen into an object you can hold — explained for children, parents and teachers, with no jargon.
Starting out
How to design your first 3D model
A first design from nothing to a saved file: choose a shape, give it real sizes, check it, and export it. About twenty minutes.
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