Design
How much tolerance should you add to a 3D printed part?
The right answer depends on the part, but the bigger principle is simple: do not model a printed part like a perfect nominal machined fit and expect it to snap into reality without thought.
Design
Start with practical clearance, not perfect nominal CAD
Printed parts usually need designed-in clearance for mating features, slots, holes, and assemblies.
A nominal CAD-to-CAD fit is rarely the safest assumption in FDM.
- Sliding fits need more clearance than cosmetic nest fits.
- Critical holes often print undersize and may need drilling or reaming.
- Assemblies should plan for reality, not just mathematical contact.
Design
Think in terms of the function of the fit
Ask whether the feature should slide, press, locate, capture, or simply clear. Those are very different jobs and should not all be modeled the same way.
- A decorative slip fit can tolerate more looseness than a locating boss.
- Repeated assemblies deserve more design margin than one-time snap fits.
- Tight features are worth testing when they matter most.
Key takeaways
What matters most
- Design for clearance intentionally instead of modeling perfect nominal fits.
- Fit type matters: sliding, locating, and cosmetic fits should not all be handled the same way.
- Critical assemblies benefit from testing or planned post-processing.