Clearance, transition and press fits in FDM

Once you accept that FDM parts will not hit an exact nominal dimension, the practical question becomes how much clearance to build in between two mating parts so that they work reliably every time, regardless of which end of the tolerance band each part lands on. This guide gives concrete clearance ranges by fit type, grounded in what we actually run in production.

Free running fits

A free running fit is used where one part rotates or slides against another with no intent to ever tighten up, such as a shaft turning inside a printed bushing, or a lever pivoting on a printed pin. We recommend 0.3 to 0.5 mm of clearance per side for these, more toward 0.5 mm on rougher materials like ABS or on larger diameters where roundness deviation adds up. This is deliberately generous because a running fit that occasionally binds is worse than one that is slightly loose.

Sliding and locating fits

Sliding fits, where a part moves along a controlled path but is not meant to spin freely, such as a drawer in a housing or a cover that slides into place, work well at 0.2 to 0.3 mm clearance per side. Locating fits, meant to register one part against another for alignment rather than movement, such as a boss dropping into a socket, can go tighter at 0.1 to 0.2 mm because a small amount of controlled friction on assembly is acceptable or even desired.

Transition, press and interference fits

A transition fit sits close to zero clearance, roughly 0.0 to 0.1 mm, and is meant to be assembled by hand with noticeable but manageable force. This is common for parts that should stay together during handling but still be separable if needed, for example service covers. Below that, true press or interference fits use negative clearance, meaning the male feature is modelled larger than the female feature by roughly 0.1 to 0.2 mm, so the parts must be forced together and generate real interference stress.

Press fits are commonly used to seat metal threaded inserts, bushings or bearings into a printed part. Because FDM parts are not fully solid and layer adhesion is weaker than the bulk material in the layer direction, we check wall thickness around a press fit bore carefully, thin walls can crack under interference stress rather than gripping the inserted part.

Clearance for printed-in-place moving assemblies

Printed-in-place mechanisms, where hinges, ball joints or telescoping sections are printed already assembled without support material trapped inside the joint, need more clearance than an assembled-afterward pair of parts, typically 0.3 to 0.6 mm depending on material and joint size. The extra margin accounts for the fact that the two surfaces were in close proximity during the entire print and any minor over-extrusion, oozing or first layer effects at the interface can reduce clearance more than on parts printed separately and joined later.

Why a zero clearance CAD model always binds

A very common mistake is modelling two mating parts with exactly matching nominal dimensions, on the assumption that the printer will reproduce the model faithfully enough for them to fit. In practice this always results in a joint that either does not go together at all or requires forcing the parts with a tool, because the combined effect of oversize outer features and undersize inner features described in the XY and Z accuracy guide works against you at exactly this interface. Zero clearance in CAD is not a neutral choice, it is effectively a light interference fit once the parts are printed.

Material dependence and post-print adjustment

Clearance requirements shift with material. ABS and ASA parts, with their higher shrinkage and slightly rougher surface from higher print temperatures, generally need clearance toward the top of each range above. PLA, PETG and carbon-filled materials, with lower shrinkage and typically smoother extrusion, can often sit toward the bottom of the range. Flexible materials such as TPU behave differently again, effective clearance is partly absorbed by material compliance rather than geometry alone.

For a one-off part or a small batch, a light manual adjustment such as sanding a bore or filing a slot is a reasonable fallback if a fit comes out tighter than intended. For serial production this is not a strategy, since it does not scale and introduces part-to-part variation of its own. If a fit needs to be consistent across a production run, we set the clearance and confirm it on first article measurement rather than planning around manual fettling.