A few months ago I started working for a new company which is designing and manufacturing automation and robot lines. I got two old, replaced harmonic drive gears from our service technician. They were used in industrial robots and got too much play. The cross roller bearings inside the bearings have worn out. These gears are great because normally they are really accurate, have a high gear ratio, are compact and fairly rigid if new (not like mine).
One wave generators (elliptical bearing in the center) was full of metal chips as it was thrown in a chip tray. I had to disassemble the whole bearing and clean it. It was really difficult to get all the balls back between the two rings. The outer ring had to be deformed to get them all in place. Unfortunately I didn't take any pictures of assembling/disassembling.
As you can see on my first sketch above, I will probably make a 4th and 5th axis for a CNC mill. Mine is too small, so I may also have to make a new, bigger one ; )