I remember trying 12mm drill with 5mm pilot hole in steel on my mill ... at 2150rpm as I couldn't be bothered to change it. That was fun, but right at the limit of the motor.

Anyway I've concluded that there's no way to get mach3 to read the actual motor rpm with this method, so instead you're going to have to use the following formula I just worked out to convert.

Spindle rpm = (u*x-v*Kv*L)/(u-L)

Or rearranged for x:

x=(rpm*(u-l)+v*kv*l)/u

Where:
u=max duty cycle, so in my case 8.5.
L=neutral duty cycle, 6.7 for me.
v=voltage to motor
Kv=motor rpm/volt
x=rpm mach 3 reads out
rpm=actual spindle rpm
(formula simplifies nicely once you've got the values, tell me what they are and I'll do it for you in Excel if you like)

For that formula to work you need to set the 'Max speed' in 'Spindle Pulleys' dialogue to:
v*Kv*100/L

So for instance with the 7700kv motor I was testing with earlier, on 11.1v ... that's 11.1*7700*100/8.5 = just over a million rpm. Putting that value in means that if you put the correct max rpm of your motor in the spindle speed, then the motor will spin at that speed. It's only true for the max speed though, for other speeds you need to use the other formula.

More to follow, my posts are getting long...