Holes, slots & clearances

Why vertical holes print small, the clearances that actually fit first try, and when the right answer is a drill bit.

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Holes print small. All of them.

Three things shrink a printed hole: the polygon faceting of your exported mesh (an STL circle is really a 32-sided shape drawn inside the true circle), the way extruded plastic bulges inward on concave curves, and a little elastic squish as perimeters are laid. Stack them up and a vertical Ø5.0 mm hole lands around Ø4.8.

So design holes 0.2 mm oversize when the diameter matters, or just tell us the pin or screw that has to fit — the analysis on every quote flags holes near common fastener sizes.

The smallest hole we'll promise is Ø2.0 mm printed vertically. Anything smaller, model it anyway and plan to drill it out — plastic drills in seconds.

Why every printed hole measures under CAD

−0.2 mm

typical undersize

what a vertical hole loses in print

Ø2.0 mm

smallest reliable hole

printed vertical

+0.2 mm

oversize in CAD

when the diameter matters

Horizontal holes are ovals in denial

A hole printed through a vertical wall is built from stacked layers, and the top of the bore is an unsupported bridge. Gravity sags it; the result is slightly oval and rough on top. Under Ø8 mm it's usually fine. Over that, give the top of the hole a teardrop point or a 45° chamfer so it self-supports, or orient the part so the hole prints vertically.

If a horizontal hole has to be round and precise — a bearing seat, a shaft bore — model it 0.3 mm under and drill or ream to size after printing.

Horizontal bores: round sags, teardrop doesn't

Clearances that fit first try

These are per-side clearances between mating printed parts — the gap you model between a peg and its hole, a lid and its box, a slide and its rail. They assume both halves are printed; against machined metal you can roughly halve them.

The four clearances that cover almost every mating part

Press0.10–0.15 mm

Feels like needs a mallet, stays put

Use it for pins, bushes, one-time assembly

Snug0.20 mm

Feels like firm push, no rattle

Use it for lids, covers, snap housings

Sliding0.30 mm

Feels like moves freely, minimal slop

Use it for drawers, slides, mating prints

Loose0.50 mm

Feels like obvious play, never binds

Use it for hinge pins, dusty environments

FitClearance / sideFeels likeUse it for

Press0.10–0.15 mmneeds a mallet, stays putpins, bushes, one-time assembly

Snug0.20 mmfirm push, no rattlelids, covers, snap housings

Sliding0.30 mmmoves freely, minimal slopdrawers, slides, mating prints

Loose0.50 mmobvious play, never bindshinge pins, dusty environments

When two printed parts mate, put the whole clearance in one part and keep the other nominal — two half-clearances are twice as hard to debug.

Slots beat holes for anything adjustable

Where a bolt position doesn't need to be exact, use a slot. Slots absorb printer tolerance, your tolerance, and next month's design change all at once. Width the bolt diameter plus 0.5 mm, length whatever adjustment you want, and add a washer-sized flat around it so the bolt head doesn't dig into layer lines.

Counterbores print cleanly facing up or down; countersinks (45°) self-support nicely upside down. If a fastener recess prints over a hole, the slicer bridges it — a 0.4 mm sacrificial skin you drill through is the tidy trick, and we'll add one if we see the geometry calls for it.