The first version of the planetary gearbox as 3d printed from Shapeways required a fair amount of post-machining to get all the pieces to fit together. I wanted to get to a point where I could just order some parts and have a reasonable expectation of them mostly working out of the box. To make that happen, I’d need to get a better understanding of where the tolerances were coming from.
Understanding the problem
Shapeways provides a fair amount of documentation on the processes and accuracy you can expect generally. Most of this is detailed in “Design rules and detail resolution for SLS 3D printing“, however the results there have some limitations. Primarily, they are only applicable to the specific geometries tested. Shrinkage is qualified as +- 0.15% of the largest dimension, and is likely influenced by the exact printed geometry. Secondarily, in the documented tests, the designers had full control over the part alignment in the print. The standard shapeways platform does not allow you to orient parts, you are at the whim of their technicians where the Z axis will end up.
For the gearbox, I had numerous fit points that needed to have controlled tolerances. The input and output bearing both needed a press fit for both sides. The internal gear for the planetary gearing needed a press fit, and the front and back shells also have a lip which would be more rigid if the fit was snug.
My solution? Print slight variants of the relevant pieces of each fit point with each radial dimension printed in increments of 0.1mm.
For each part, I broke out the calipers to measure the as printed size, and also attempted manual press fits of each part. I didn’t manage to put any identifying features on each of the prints, which probably annoyed the Shapeways technicians and made my life a bit harder. I just assumed that the sizes came back in increasing order despite the part number markings, which I’m pretty sure were incorrect. This resulted in the following table:
The second version of the gearbox had many other changes in addition to these, but this let me get a lot closer to the correct fit on the full assembly.