One of the post-machining operations required for the BE8108 gearbox was reducing the outer diameter of the internal gear. Stock internal gears seem to come with a large amount of outer material, likely because they are intended for stationary process control applications. For a mobile environment, a 100 tooth gear with a 50 mm pitch diameter is somewhat useless when it has 70 mm outer diameter.
I ended up turning this down on a lathe using a custom 3D printed mandrel.
The mandrel is just a 3 inch cylinder intended to be gripped in the lathe 4 jaw chuck, followed by a spur gear that mates with the internal gear. Given it was in plastic, I just printed a regular spur gear, which would nominally have some slop and backlash. However, because of printing tolerances, it actually required a press to install the gear onto. Given that, I made a separate removal fixture, which is just a hollow cylinder that can be used to press the result back off the mandrel.
This was my very first unsupervised lathe operation, and so as to not melt my plastic fixture, I ended up settling on 50um cut depths at 800RPM as a decently optimal speed. That still took about 40 minutes of turning per gear, although I was getting better and faster near the end.
Finally, a video and some pictures of the result: