Designing Parts for FDM: Tolerances, Overhangs and Supports

Designing Parts for FDM: Tolerances, Overhangs and Supports The best way to reduce print failures is to design for your process. FDM has specific constraints that differ from injection molding or machining. Here's what matters. Tolerances FDM parts come out slightly larger than designed due to material expansion. Account for this with clearance between mating parts. These are starting points — calibrate for your printer with a fit test print. Overhang Rules FDM prints layer-by-layer. Each layer must be supported by the one below. Solutions for steep overhangs: Chamfer instead of fillet at base edges Orient part to minimize overhangs Split part and glue/assemble Add support in slicer (tree supports preferred) Bridging Bridges (horizontal spans between two points) can print without support up to: PLA: 60–80 mm reliably PETG: 40–60 mm ABS: 30–50 mm Improve bridging: fast fan, slow speed, straight bridging (not curved) Wall Thickness Minimum printable wall: 0.4 mm (1 perimeter at 0.4 mm nozzle) Recommended minimum: 0.8 mm (2 perimeters) Functional parts: 1.2 mm+ (3 perimeters) Holes Holes print undersized. For accurate holes: Add 0.1–0.2 mm to hole diameter in CAD Or drill/ream to final size after printing Vertical holes print better than horizontal holes Threads M3 and smaller: use heat-set inserts — printed threads strip easily M5 and larger: can self-tap carefully, but heat-sets are still better Design for hex nut captures instead of printed threads for critical connections

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