The Hole Shrinkage Phenomenon
Every mechanical engineer learns this the hard way: model a 22.0 mm circular pocket for a standard 608 skate bearing, slice the file, print it, and the bearing refuses to seat because the printed hole measures 21.6 mm.
Why does this happen? FDM toolpaths approximate circles with polygonal line segments, and molten polymer cools under surface tension, pulling inward toward the center of curvature. This causes internal holes to consistently print undersized by 0.2 to 0.4 mm.
Designing Bearing Pockets That Fit
When modeling pockets for press-fit components:
- CAD Clearance Offset: For light hand press-fits, expand nominal hole diameter by +0.15 mm to +0.25 mm for PLA/PETG. For tight mechanical interference fits, model the diameter at +0.05 mm over bearing outer diameter.
- Lead-in Chamfer: Always include a 0.5 mm x 45° chamfer at the mouth of the pocket to guide the bearing outer race square during pressing.
- Shoulder Relief Groove: Cut a 0.3 mm undercut groove where the pocket floor meets the cylindrical wall so bearing corner radii seat flat.
Compliance Ribs: The Engineering Shortcut
Instead of relying on rigid, unforgiving cylindrical walls, design three or four tiny internal crush ribs (0.3 mm wide x 0.2 mm high) inside the pocket. When the metal bearing is pressed into place, the ribs deform elastically, taking up printer dimensional variation while providing firm concentric grip.
