Fit Calculator — Press, Transition & Clearance Fits

For standard ball bearings pressed into a 3D printed housing, use FN1 (light press fit). The slight interference ensures the bearing stays in place. For bearings that need to be removable, use LT2 (tight transition).

Clearance fits (RC classes) have a positive gap — the printed hole is larger than the printed shaft, so the parts slide or rotate freely (this tool targets roughly 0.15mm for a close fit up to 0.45mm for an easy fit). Press fits (FN classes) are interference fits: the printed shaft is LARGER than the printed hole (a negative gap, around -0.1 to -0.2mm here), so they must be forced together and hold by friction.

FDM holes print 0.1-0.3mm undersize and shafts print oversize because the nozzle path adds material on the inside of holes and the outside of pegs. To hit the intended fit, this tool enlarges the modelled hole and shrinks the modelled shaft by your calibration tolerance. Because the same amount is added to the hole and removed from the shaft, it cancels out of the realised gap — the target fit is unchanged, only the dimensions you actually model change. Set the tolerance from a printed test coupon for your machine and filament.

Yes, transition fits are actually ideal for many 3D printed assemblies. They allow hand-assembly but hold position well. The slight material flexibility of most 3D print materials helps create a forgiving but secure fit.