Press Fit Calculator — Interference Fit Dimensions

Press Fit Calculations

• Realised hole = Insert diameter − Target interference (the hole MUST end up smaller than the insert)
• Design hole (draw this in CAD) = Realised hole + Hole-shrink compensation
• Effective interference = Target interference for the insert type (bearing 0.05, bushing/heat-set 0.10, magnet 0.05, pin 0.08 mm)
• Min wall = max(2 mm, Insert diameter × 0.3)
• Insertion force = P × π × d × L × μ, with contact pressure P from the Lamé thick-wall cylinder equation (P = E·δ / (d·((1+k²)/(1−k²) + ν)))

FDM holes print undersize, which on its own would make a press fit too tight. To get a repeatable fit you measure that shrink once on a calibration print and the calculator draws the CAD hole slightly larger so the printed hole lands on the target. The realised hole is always smaller than the insert — that interference is what grips the part. Heat-set inserts are the exception: the boss recess is sized a touch under the insert OD and the heated insert melts the plastic to lock in.