@regorlas had a pretty clever hack for measuring the rpms of the metal lathe: a USB battery bank with little flashlight built in was clamped in the lathe jaws and an led acting as a photodiode was attached to an oscilloscope and placed so that the light beam from the battery bank swept across it with every turn.
Each time the light shined on the led, the oscilloscope detected a pulse and with that we could measure the period for each rotation and turn that into rpms with some math
We used that to figure out the speeds for all of the possible belt and gearing combinations, which will be a useful reference for the future