CNC: Spindle speed control

Hi there,

After doing some research about CAD/CAM/CNC I have one unresolved
question remaining:

All of the systems/information I have seen so far seems to disregard
spindle speed control via g-code which seems funny to me as the
spindle speed besides the feed rate is an important factor in
cutting/turning (I'm still a early beginner in metal cutting).

Is there a feasable and financially reasonable solution (I own a
Sherline mill and lathe)?

Malte