Bearing fit chart for 3D printing

Off-the-shelf ball bearings are precision parts: a 608 bearing outer diameter is 22.000 mm ±0.010 mm. Printed plastic is not: a 22 mm hole, as-modeled, lands anywhere from 21.5 to 22.1 mm depending on shrinkage, extrusion width, and curvature. Match that bore to that bearing and you get a fit nobody ordered. This page provides the actual CAD dimensions for common metric bearings, derived from ISO 286 fit classes adapted for FDM process error.

Press fit bearing seats (outer race, printed housing)

Press fit creates a permanent mechanical joint: the bore is smaller than the bearing outer diameter, so assembly requires force and the interference holds it in place. The nominal clearance is negative — interference, not clearance. For FDM, that interference is small enough (0.057–0.084 mm diametral) that it falls entirely inside a typical machine's repeatability band (±0.080 mm). The fit will hold when it works, but it will not work consistently part to part. Print test coupons with a range of diameters before committing to press fits in production.

Nominal Clearance is the designed functional clearance — here, interference — from ISO 286 scaled by the tolerance unit i. Negative means the hole ends up smaller than the bearing. Model Hole is the dimension to draw in CAD. Model − OD is the correction you are applying: draw the bore that much larger than the bearing and, after the material contracts and the perimeter closes in, the printed hole lands on the interference in column three. It compensates for shrinkage (PLA contracts 0.30%) and for FDM hole undersize, which is driven by extrusion width and curvature.

Transition fit: removable but located

Transition fit sits between clearance and interference: light contact, assemblable by hand with a tap from a mallet, and removable without destruction. Use this for alignment pins, jig bushings, or any joint that must be located but may need disassembly. The nominal clearance is small and positive (0.046–0.067 mm), roughly 80% of the FDM repeatability band.

The difference between press and transition is roughly 0.10–0.15 mm: small in absolute terms, but the boundary between a joint that holds and one that does not.

Material shrinkage effects

The table above uses PLA (0.30% linear contraction). Higher-shrink materials require larger modeled holes to hit the same as-printed target. At 42 mm OD (6004 / 6806), the spread from PLA-CF (0.15%) to PP (1.50%) is 0.58 mm — more than the original interference itself.

If you swap materials mid-project, run the calculator again. Do not reuse the PLA number for ABS or nylon and expect the same fit.

Three checks before committing a bearing housing

Use the table as a calculated starting dimension, then separate the three questions that are easy to mix together: is the bearing the size you bought, can your process repeat the seat, and can the housing carry the stress? The numbers on this page are model outputs, not a claim that every printer will measure the same way.

1. Measure the bearing. Record the actual outer diameter at several points and use the largest repeatable reading as the datum. A nominal 22 mm bearing is a catalogue size, not a substitute for checking the part in your hand.
2. Choose the fit from the job. Use a press seat only when the bearing must stay permanently. For a serviceable housing, start with the transition values and compare them with the FDM repeatability band. If the fit is tighter than the process can repeat, print a coupon before the full housing.
3. Check the load path. Keep the wall around the bore thick enough for the interference, add a lead-in chamfer, and use a ductile material for larger seats. If the purchased part is a shaft rather than a bearing, use the separate metal-shaft bore guide because the correction is one-sided.

For small bores, the hole compensation guide explains why a fixed “add 0.2 mm” rule breaks down. It also shows how to record a calibration coupon without presenting a calculated value as a measured result.

Minimum wall thickness for press fits

Interference creates hoop stress in the housing. If the wall is too thin, the stress exceeds yield and the boss splits or creeps open. A safe starting point for press fits is wall ≥ 0.25 × OD. At 16 mm that is 4 mm; at 42 mm it is 10.5 mm. Thinner walls can work in ductile materials (PETG, ABS, PC) with generous infill and slow press rates, but they will not work reliably in PLA above roughly 30 mm OD.

Frequently asked

Why does the model dimension grow faster than the OD?

It does not, and that is the useful part. Shrinkage is proportional to size, so a 42 mm bore contracts more than a 16 mm one in absolute terms. But the OD grows 2.6× (16 → 42 mm) while the correction grows only 1.14× (0.274 → 0.312 mm), because the curvature penalty that dominates small bores has largely faded by 42 mm. Practically: the correction is close to constant across the whole bearing range, roughly 0.27–0.32 mm. It is the fit that scales, not the compensation.

Can I use these numbers for a printed shaft pressing into a purchased bearing?

No. The geometry is reversed: you are now printing the shaft (male feature) to press into a fixed-dimension hole (the bearing bore). Shaft oversize is smaller than hole undersize, and shrinkage subtracts from the shaft instead of adding to the hole. Run the calculator with parts = shaft only and datum = hole. For reference, a printed 8 mm shaft (688 or 608 bore) needs to be modeled at 7.959 mm to hit –0.045 mm interference; modeling it at 8.0 mm produces a loose fit.

What if my bearing OD is not in the table?

Use the fit calculator. Enter the OD, select press fit, set datum = shaft (because the bearing is the purchased part) and parts = hole only. The calculator solves the same equations this table came from.

The calculator says my fit is inside the repeatability band. Should I give up?

Not necessarily, but temper expectations. A fit tighter than ±0.080 mm will not be consistent part to part on a typical consumer FDM machine. Some prints will hold; others will rattle or split. If you need reliability, either step to a looser fit class (transition instead of press) or tighten the machine (better motion system, lower layer height, careful calibration). Test coupons are mandatory.