Designing Parts for FDM: Tolerances, Overhangs and Supports
fdmdesigncadtolerancesoverhangs
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.
| Fit Type | Clearance to Add |
|---|---|
| Loose/sliding fit | 0.3–0.4 mm per side |
| Snug fit | 0.15–0.2 mm per side |
| Press fit | 0.05–0.1 mm per side |
| Tight (glued) | 0.0 mm — design exact |
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.
| Angle | Printable? |
|---|---|
| 0–45° from vertical | Yes — no support needed |
| 45–60° | Maybe — depends on material and cooling |
| 60–90° (horizontal) | No — needs support or redesign |
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