Tolerance Calculator — Clearance for Mating Parts

Tolerance Formulas (shaft-basis)

• Shaft size = Nominal + Shrinkage compensation − Elephant's foot
• Hole size = Nominal + Shrinkage compensation + Design clearance + Hole enlargement
• Design clearance = the gap on diameter you want between the parts (positive for a clearance/transition fit, negative for an interference/press fit so the hole prints smaller than the shaft)
• Hole enlargement = Printer accuracy + extra hole compensation (FDM holes print undersize, so the CAD hole is opened up to land on size)
• Shrinkage comp. = Nominal size × Material shrinkage rate (applied to both parts, since for two parts in the same material it largely cancels)

This uses a shaft-basis approach (ISO 286): the shaft stays at the reference size (nominal) and the hole is varied to carry the fit clearance. For a clearance fit the hole is opened up so the parts slide; for an interference (press) fit the design clearance is negative, so the hole is smaller than the shaft and the parts must be forced together. Printer accuracy enlarges the hole only — it is the FDM correction for holes printing small, not a margin doubled onto both parts. Always print a test piece, because every printer is slightly different.