If the lathe is proper variable speed, i.e. using a three phase motor and an inverter, I would suspect the inverter. Many of these have an option to run isolated from the earth but if not, and parts of the inverter make contact with anything earthed, they will trip the RCD, due to earth leakage, not an overcurrent situation.
One other thing, if it is an inverter controlled variable speed, be very wary of using a Mega, as they can wreck the electronics.
As the previous poster has asked, has anything else recently been changed?