I am not keen on VFD control from my own experience as low down torque was missing. I have heard that using an Inverter duty motor helps a lot but not gone that route as I fitted servos instead.
One thing that I had thought about doing was fitting a stepper motor instead of the air motor/wheel and that may be something to think about as it should be fairly easy to set up and writing a spindlespeed macro would be relatively straight forward (even for me
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Only problem I would see is you would have to have the motor running as you adjusted the speed as I think the stepper would stall if the speed was being reduced, certainly harder to turn the hand wheel if the motor is not running. Also may have to "home" the stepper on each start up of your machine, but something to think about.
Hood