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.
For early prototypes, it is usually better to leave enough clearance to learn from the assembly than to make the first version so tight that every result depends on sanding or force.
- 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 locating feature may need one controlled face and clearance everywhere else. A cover may only need to sit cleanly. A bracket slot may need alignment forgiveness more than a perfect round hole.
- 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.
Design
Where tighter fits deserve a test
Snap fits, press fits, captive tabs, hinge pins, and printed threads all deserve extra caution. They can work, but they are sensitive to material, orientation, layer height, and the exact geometry of the mating part.
If the part depends on a tight feature, consider ordering a small test piece or splitting the risky feature into a revision before committing to a larger set.
- Test small fit coupons when a feature controls the success of the whole order.
- Use slots, reliefs, or adjustment features when the mating hardware is uncertain.
- Plan post-processing for critical drilled or reamed holes.
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.
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