That's a complex marble machine.

I have some experience with inspecting gear profiles, and this fella is a hands-on guess and test type.
He's manually adjusting the locations of the gears, but there are a million other issues that could contribute to process variation.
For example: concentricity issues with the gear axis relative to any given bearing surfaces (on each individual tooth).
It's a nightmare of machined surfaces of custom gears.

I'm impressed with the machines reliability; given his methodology.