It doesn't seem to be possible to easily control the spindle speed of the standard F120E unlike the high speed version where speed is controlled by a dc voltage.
I've been searching the web for a solution to this problem but so far without success! I'm sure that somewhere I've seen a reference to replacing the controller board but can't now find the reference.
Alternatively I suppose it would be possible to use the high speed motor board which does use a dc voltage to control the speed.
I'd be grateful for any ideas.