Choosing fits: clearance, transition, interference
Two parts that meet have a relationship, and that relationship has a name: the fit. A pin in a hole, a lid on a box, a bearing in its seat — each one is either meant to slide, to locate snugly, or to lock together by force. Pick the wrong one and a part that should rotate freely seizes, or a press-fit you designed falls apart in your hand. The fit is a design decision you make from what the joint is for, not a number you guess at the end.
This article is about that decision. The actual numbers — how many tenths of a millimetre to leave on an FDM print — live in Real-world printed clearances; here we reason about intent.
Three families, chosen by function
Every fit falls into one of three families, and you choose between them by asking one question: does this joint need to move, to stay put, or to never come apart?
- Clearance fit — the shaft is always smaller than the hole, so there is a gap. The parts slide or rotate. Use it for a hinge pin, a drawer runner, a bolt passing through a clearance hole, a lid that drops onto a box.
- Transition fit — the gap is near zero; the parts locate precisely with little or no play but come apart by hand. Use it for a dowel pin aligning two halves of an enclosure, or a cap that should sit centred but still pull off.
- Interference (press) fit — the shaft is larger than the hole, so it must be forced in and stays by friction. Use it for a bearing seat, a knob on a shaft, a heat-set insert boss (see Designing for heat-set inserts).
The machining reference, and why it still helps
Mechanical engineering already solved this with the ISO fit system — a hole
letter and a shaft letter that together name a fit for a given diameter. You
will never type H7/g6 into a slicer, but the table is a useful reasoning
layer: it tells you the relative tightness the rest of the world considers a
sliding fit versus a press fit, so you start from a sane intent rather than a
random gap.
| Fit | Family | Typical use |
|---|---|---|
| H9 / d9 | loose clearance | rough running, dirt-tolerant pivots |
| H7 / g6 | close clearance | sliding shafts, located but free to move |
| H7 / k6 | transition | locating pins, light keying |
| H7 / p6 | interference | press-fit bearings, permanent assembly |
Read it as a ladder of intent: d9 is "definitely loose," g6 is "slides but
no slop," p6 is "driven in with a press." On a milled aluminium part those
correspond to clearances measured in single microns. That is the catch.
On FDM, everything shifts looser
A machine shop holds ±0.01 mm without trying. An FDM printer does not come close — squish, shrinkage, line width and ooze all conspire, and your real tolerance band is closer to ±0.1–0.2 mm. So every fit in that table effectively shifts toward "looser" when you print it.
How to actually use this
Start from the function, name the family, then convert to a printable gap. A hinge that must swing → clearance. Two enclosure halves that must align but open → transition. A bearing that must never spin in its pocket → interference, sized generously.
The family is the durable decision; it does not change with your printer, material, or nozzle. The number does. That is the next article's job: Real-world printed clearances turns each of these intents into a concrete gap in millimetres you can put in your model today.