If i may ask a dumb question, how do you accurately concave the commutator facing side of the brush ?

Purely a curiosity question only - i have no intention of using a dynamotor ever.

I recall using an ARR-15 some decades back, and i definitely recall the "dynamotor noise floor". Which made a noisy background when listening to a game show from BBC Kuching Malaysia in the 5 MHz band. ( Few ppl will remember that. )