Advice for making a very large ring with very smooth surface

From what I’ve found, it seems like 3D printers don’t do ball bearings well, but they can do something similar. 3D printing works by melting filament and depositing it on something, like the build plate or the previous layer, working from the bottom to the top. Spheres don’t work particularly well due to the way they go from a point at the bottom to a large circle in a very short vertical distance. Normally that can be handled by using supports, but that doesn’t work when the spheres are inside the piece.

Some people smarter than me have spent time designing cylindrical bearings, which are more 3D printing friendly. There are a lot of models, but I’ve seen:
Open sided
Closed sided

I’ve printed one of the Closed sided models, using the loose tolerance models (tolMax). Even so, I had to do a bit of work with an xacto knife before they spun smoothly, and it needed a bit of a wear in to get really smooth. Because I used the tolMax, the bearings have a lot of play. I have not attempted the tight tolernace (tolMin) models yet. I have a bit of a problem with elephant foot that I need to address before I attempt that, but it’s mostly only a problem for prints this this.

I highly recommend going through the 3D Print Guide to dial in your printing accuracy before attempting anything that requires tight tolerance. It wouldn’t hurt to try several of the newer benchmark standards like the Print In Place Engine to make sure you’re happy before attempting your bearings.

Aside from the printer, the circles will only be as circular as your models. All the 3D modeling software actually does points and faces instead of true arcs. I use TinkerCad, and it only allows up to 64 facets on the cylinders. Better software will get you better models, but I don’t have any experience with them. Even if the software did allow true circles, the Slicer (Luban) breaks everything up into line segments too. At small diameters, it doesn’t matter much, but at 300mm, you’ll probably notice the flat spots. I don’t feel like dusting off my geometry equations to figure out the circumference difference, but I’m guessing it’ll be in the ~0.1mm range. If that’s too much deviation, you’ll want to print it slightly oversize and with thicker walls, then sand it down to be a true circle. I would definitely do a lot of test prints to make a 1mm tall circle before I attempted the entire bearing structure.

For the bevel gears, I think the model facets won’t be as much of a problem as for the bearing. But YMMV.