Bearing seats and precision bores in FDM parts

Bearings, bushings and precision shafts are where hole design stops being a tolerance question and becomes a structural one. A hole that is a few tenths off on a cosmetic bracket is a non-issue. The same error in a bearing seat means the bearing spins loose in its housing, or worse, the printed boss around it cracks during assembly. This guide covers how to design those seats so they hold.

Press fit versus slip fit

A press fit relies on the printed bore being slightly smaller than the bearing or bushing outer diameter, so the part elastically grips it after assembly. For standard ball bearings in printed housings we typically model the bore 0.05 to 0.15 mm under the bearing outer diameter, depending on material stiffness. PC and PA12-CF tolerate a tighter press because they are stiffer and less prone to creep, while PLA and PETG need to stay toward the looser end because they will slowly relax and lose grip over weeks under sustained stress.

A slip fit, by contrast, is designed to let a shaft or bushing rotate or slide freely, and is used where the printed part is a housing rather than the bearing surface itself, for example around a metal bushing that does the actual sliding. Here the bore is modelled 0.2 to 0.4 mm larger than the mating part, enough for smooth movement without excessive play. Never rely on a printed surface as the direct sliding surface for a rotating shaft under continuous load: layer lines wear unevenly and the coefficient of friction against bare metal is unpredictable. Use a bushing or bearing instead.

Seat depth and shoulder design

A bearing needs enough engagement depth in its seat to stay aligned under load. As a rule of thumb, aim for a seat depth of at least 60 to 80 percent of the bearing width for radial loads, and design a shoulder, a printed step that the bearing seats against, rather than relying purely on friction along the bore to hold it in place axially. The shoulder should be at least 1 mm wide and printed as a clean 90 degree step, not a chamfer, so it presents a flat stop face.

Print the shoulder side down or at least oriented so the step forms cleanly, since an overhanging shoulder printed without support can sag and lose its flatness. In most housing designs this means printing the bore vertical with the shoulder near the bottom of the print, which also happens to be the best orientation for bore roundness as covered in the horizontal versus vertical holes guide.

Avoiding a press fit that cracks the boss

A common failure mode is a boss or housing wall around a bearing bore that splits during press-in, usually along a layer line running parallel to the bore axis. This happens when the hoop stress generated by the press fit exceeds the layer adhesion strength of the material at that point. It shows up more often on small, thin-walled bosses in brittle materials like PLA or unfilled PETG than on parts with generous wall thickness.

The fix is a combination of wall thickness, geometry and material choice. Keep at least 1.5 to 2 times the bore diameter of solid wall thickness around a press fit bore where possible. Where that is not possible because of space constraints, break up the hoop stress with a few short reinforcing ribs running radially outward from the boss, which redirect load away from a single failure plane. On parts that see repeated assembly and disassembly, or where the press fit force is high, PC or PA12-CF hold up noticeably better than PLA.

Sacrificial ribs and compliant seats

For applications where the exact bore diameter is uncertain until a sample is measured, a useful trick is a compliant seat: instead of a plain cylindrical bore, add three or four small internal ribs, each a fraction of a millimetre proud of the nominal bore surface, spaced evenly around the circumference. The bearing presses these ribs flat on insertion, which absorbs dimensional variation between prints without needing the whole bore to be perfectly toleranced, and it also reduces the total contact area, lowering the insertion force needed.

This approach works well for one-off prototypes and low volume runs where iterating the exact bore tolerance is not worth the time. For higher volume series production we generally recommend nailing down the compensation value through a short test batch instead, since it gives a more consistent result print to print.

Reaming a printed bore to size

When a bore tolerance is tighter than what printing alone can guarantee, print the hole intentionally undersize, typically 0.3 to 0.5 mm smaller than final, and ream it to the finished diameter afterward. Reaming produces a round, smooth bore regardless of the small variations that come from the printing process, and is a standard step for precision bearing seats in our workflow when the application calls for it.

This is covered in more depth, including drill bit selection and tapping, in the post-processing guide in this cluster. As a design rule, always leave a note on the drawing or model specifying which bores are intended to be reamed, so the undersize is not mistaken for a modelling error during quality checks.