How to set up the correct tolerances in 3D printed static assemblies

I already wrote about the recommended tolerances when prototyping moving assemblies in a single print. What happens when you need to test static applications that require tight interaction between parts?

3D printed samples to test tolerances in an static assembly

The first factor you have to keep in mind is that printing components independently allows tighter tolerances, as the risk of welding the parts (if printed together) is eliminated. The second factor to consider is the fact that you will need to assemble the components. Design the parts in a way that you can assemble them. Adding guides, locks and threads will help to keep the parts in place once assembled.

In order to determine the best space between components I printed three variations of an interacting part, with different tolerances: 0.10 mm (4 thou), 0.15 mm (6 thou), and 0.20 mm (8 thou). Then, I tried to assemble the components to see what gap allows the best fit between components. This is what I found:

• 0.1 mm (4 thou) gap: Too tight. The components don’t fit together, even when applying some force.
• 0.15 mm (6 thou): Fits properly. It is not necessary to force the parts to fit together.
• 0.20 mm (8 thou): Loose. The components move freely.

Based on these results, my recommendation when prototyping static assemblies in an Objet 3D printer is to use a 0.15 mm (6 thou) tolerance when printing parts independently for later assembly.