Hole Tolerances
FDM printers consistently produce holes smaller than their nominal diameter. The cause is a combination of thermal contraction and the circular interpolation method used by most motion systems. The standard compensation for drone hardware:
- M2 clearance hole: model at 2.4mm (nominal 2.0mm)
- M3 clearance hole: model at 3.3mm (nominal 3.0mm)
- M4 clearance hole: model at 4.4mm (nominal 4.0mm)
- M3 self-tapping hole in PETG: model at 2.7mm (the plastic threads)
- M3 self-tapping in ASA: model at 2.6mm (slightly harder, less give)
These values assume 0.4mm nozzle and standard resolution printing. Run a calibration print first — a simple plate with 5 holes of increasing diameter — to verify your specific machine's offset before committing to a complex assembly.
Press Fits and Slip Fits
For bearings, motor shafts, and mechanical assemblies:
- Shaft slip fit (smooth rotation): model hole at shaft_diameter + 0.4mm
- Shaft press fit (needs force to assemble): model hole at shaft_diameter + 0.1mm in PETG, + 0.0mm in ASA
- Bearing outer race press fit: model hole at bearing_OD − 0.1mm (PETG will compress slightly)
- Bearing outer race slip fit: model hole at bearing_OD + 0.15mm
- Snap fit engagement: 0.3–0.5mm interference for PETG, 0.2–0.3mm for ASA (stiffer)
Printed Thread Profiles
Printed threads require different profiles from standard ISO thread specifications because the FDM printing process cannot resolve the sharp V-groove thread form. Use rounded thread profiles (ISO metric but with 0.1mm corner radii on all thread flanks) and increase the thread pitch clearance:
- M3 external (printed bolt): reduce outer diameter by 0.3mm from nominal
- M3 internal (printed nut/boss): increase inner diameter by 0.3mm from nominal
- Thread engagement depth: minimum 1.5× thread pitch for functional threads in PETG
For most drone assembly applications, captured nut pockets are more reliable than printed threads. Design an M3 hex nut pocket (6.4mm across flats, 2.6mm deep) and use standard steel hardware — stronger, more reliable, and easier to disassemble for maintenance.
Material-Specific Adjustments
PETG: use the baseline values above. PETG has moderate thermal contraction and consistent shrinkage. ASA: reduce all hole offsets by 0.05mm (ASA shrinks slightly more than PETG). Nylon/PA: add 0.1mm to all hole clearances — nylon absorbs moisture and swells slightly after printing, which closes up tight fits over time. TPU: compress-fit tolerances do not apply — TPU deforms to fill gaps. Design TPU pockets to nominal dimensions with 0.0mm clearance for camera units and snap-in components.