The intentional gap between mating parts ensuring they can be assembled without binding, accounting for dimensional tolerances, surface roughness, and thermal expansion.
Assembly clearance = sum of all tolerance contributions (RSS or worst-case), plus allowance for surface roughness and thermal expansion differentials. For FDM: typical minimum assembly clearance is 0.3–0.5 mm per side (0.6–1.0 mm on diameter for cylindrical fits). Tolerance stack-up analysis (RSS method) predicts worst-case assembly conditions.
Design clearances per function: loose assembly (0.5 mm per side), sliding fit (0.3 mm per side), print-in-place (0.4–0.6 mm per side). Iterate: print, measure, adjust. For multi-part printed assemblies, identify the critical dimension chain and minimize the number of contributing parts.
Insufficient clearance in complex assemblies is the most common FDM fit failure. Post-printing, high Ra surfaces occupy effective clearance: factor in Ra/2 per side. Parts printed in different orientations have different dimensional errors; do not assume symmetric clearance.
Related terms: Clearance Fit, Tolerance, Thermal Expansion, Hole Sizing for 3D Printing, Press Fit
| Field | engineering |
|---|---|
| Also called | fitment gap, mating clearance, assembly gap |
| Source | EDi Brain: rules-core |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert