Press Fits and Bearing Fits

TL;DR

For printed press fits, don’t guess a hole size: pick slip vs light press vs heavy press, then confirm it with a small gauge print using the same orientation and perimeters. When installing a bearing, press only on the ring being seated (outer ring into a housing, inner ring onto a shaft) to avoid damaging the bearing.

What a press fit is (and why FDM makes it tricky)

A press fit holds by controlled interference: the insert OD is slightly larger than the printed seat ID, so the plastic stretches and grips by friction. In FDM, the printed side is usually the variable side: holes often print undersized, layer lines make strength direction-dependent, and many plastics relax (creep) under constant stress. Reliable press fits come from designing for a specific assembly force and failure mode, not from targeting a nominal diameter.

What changes press-fit behavior the most

Interference/clearance

Insert OD minus printed ID; tiny changes can double the press force and cracking risk.

Hole printing bias

FDM holes tend to come out smaller and less round than modeled; small holes are worst.

Orientation / anisotropy

If layers create a "seam" around the boss, the boss can split along layers like a pre-cut crack.

Perimeters & wall thickness

Hoop stress is carried by shells; thicker bosses and more perimeters resist splitting.

Material & creep

PLA is stiff and can crack; PETG/nylon are tougher but can creep and loosen over time.

Extrusion/flow & temp

Over-extrusion and excess heat shrink holes and raise press force; drift causes print-to-print variation.

Post-processing method

Reaming/boring/drilling to final size is often the most repeatable way to hit a bearing seat in FDM.

Choose the fit type (based on service needs and risk)

• Slip fit: drops in by hand; use when you need easy service or you will retain it with a plate, clip, screws, or adhesive.
• Light press fit: firm thumb pressure or gentle arbor press; good for alignment and modest retention without stressing the print.
• Heavy press fit: needs a vise/arbor press and careful support; use only with a thick boss and a clear load path, and accept higher risk of splitting or bearing damage.

Bearing fits: how to press without ruining the bearing

Ball bearings are sensitive to how you load them during installation. Apply force only to the ring that is being interference-fitted: press on the outer ring when seating into a housing, and press on the inner ring when fitting onto a shaft. If you press through the balls (loading one ring while supporting the other), you can dent the raceways and the bearing will feel gritty or bind. Also support the printed boss close to the pocket during pressing so it can’t flex and ovalize the seat.

Design patterns that make printed press fits predictable

• Add a lead-in chamfer (or small taper) on the seat to start straight and reduce plastic shaving.
• Increase boss OD and add fillets where the boss meets the body; sharp inside corners concentrate stress and start cracks.
• Use more perimeters (and enough top/bottom layers) around the seat; don’t rely on sparse infill for press-load strength.
• Consider a split-clamp seat (slit + screw clamp) when you want adjustable retention without high interference.
• For repeatable bearing pockets, design for a finishing step (ream/bore/drill) rather than depending on as-printed diameter alone.

Quick validation workflow (gauge before the real part)

1. Print a small gauge coupon: a short boss/pocket with the same wall thickness, perimeters, and print orientation as the final part.
2. Test with the real hardware and note: assembly force, whether it goes in straight, and whether it stays tight after 10–30 minutes (creep can show early).
3. Change one variable at a time: modeled ID/OD, flow/extrusion multiplier, or your finishing step (ream/drill).
4. Record the winning dimensions with the filament, nozzle, and profile used so you can repeat it later.

Common problems and first fixes