I realize this is a valid point, but I'm always surprised that folks
concentrate on CFM so much. I built my own compressor from dumped parts.
Probably about 4 CFM into a 20 gallon tank.

I just grind for a minute then go get a drink of water and beat the dust out of
my pants as it charges again.

GTO(John)

I'm more worried about the air supply you will need. The die
grinders vary from about 7cfm to more than 15 cfm. Cheaper ones
seem to take more air. And 15 cfm at 90 psi is a LOT of air.
Think 30 amp/240 volt circuit.