Spindle RPM control

CNC Idea!
RPM control of a spindle motor (controller) with a frequency to voltage
converter by using a spare axis CNC control program step output.
The rate is set in any block with the F word as needed.
The direction bit can control on/off/disable if wanted.
The FVC output will substitute for the pot in a typical PWM full wave
rectifier controller such as used in the Sherline machines.
The KBE controllers can be used in this manner.
This idea sort of is a poor mans constant velocity control or speed change
for various reasons in special cases.

Is this viable, plausible or practical?
I have not identified a VFC chip as yet. Do they exist?
RichD