Why Tolerances Matter More in 3D Printing CAD software shows perfect geometry, but 3D printers don't produce perfect parts. Depending on your printer, material, and settings, actual dimensions can be 0.1mm to 0.3mm off from your model. For static parts this rarely matters — but for gears, bores, and anything that needs to rotate or slide, getting clearance wrong means a part that's stuck solid or too loose to function. General Clearance Guidelines These are starting points, not absolute rules — always test on your own printer: - Press-fit parts (should stay together): 0.05–0.1mm smaller hole than shaft - Sliding fit (moves but no wobble): 0.15–0.25mm clearance - Loose rotating fit (gears, bearings): 0.3–0.4mm clearance - Bolt/screw holes: add 0.2–0.3mm to the bolt's nominal diameter FDM printers typically print holes slightly smaller than designed and shafts/pins slightly larger, due to how the nozzle traces the outer wall. Keep this in mind when setting bore diameters on gears or motor mounts. Applying This to Gears Gear teeth are especially sensitive to tolerance issues: - If two gears mesh too tightly, they'll bind and strip teeth under load. - If clearance is too loose, you'll get backlash — noticeable play when reversing direction. - A good starting backlash allowance for FDM-printed gears is 0.1–0.15mm per tooth flank, added on top of the standard involute profile. Applying This to Motor Mounts Motor mounts (like NEMA 17 or NEMA 23 plates) need tight tolerances on the bolt pattern but slightly looser tolerances on the center bore: - Bolt holes: keep clearance minimal (0.1–0.2mm) so the motor doesn't shift under vibration. - Center bore: leave 0.2–0.3mm so the motor shaft or coupler seats without needing to be forced in. How to Test Your Printer's Tolerance Print a simple test file with holes and pegs at different clearances (0.1mm, 0.2mm, 0.3mm, 0.4mm) and see which ones fit the way you expect. Every printer behaves slightly differently based on nozzle diameter, calibration, and material shrinkage — testing once saves hours of failed prints later. Designing With the Right Clearance From the Start Instead of manually adjusting every dimension, parametric scripts let you set clearance as an input variable — so you can dial in the exact fit your printer needs without redrawing the part. FusioScripts' gear and motor mount scripts include adjustable bore and clearance parameters for exactly this reason — check the Gears and Motors & Mounts categories to get started.