To be correct this is about accurately machining a sleeve added over a motor spindle.
The motor is to power a steam raising blower for a model steam boiler. It is just a small 12V motor and will drive a lightweight centrifugal fan.
I made the brass mount and pushed it onto the serrated spindle, then to get the next level of accuracy I took an extremely light cut of this brass spindle whilst powering the motor. Do not power the lathe or even be tempted to - remove the power to the lathe as it is too easy to just hit the power button by mistake.
The motor torque is very low, so cuts have to be super fine, thus meaning the motor and brass sleeve have to be very close to perfect to start with.