Do you mean variable speed control that would allow speed control of the spindle independent of the carriage feed? That can't be done with just a vfd?
Yes. I don't think that's the underlying question though although perhaps I'm overthinking it.
I've been working on a code upgrade for my ELS that let's it put out a PWM pulse rate on DB25-1 compatible with that little MACH3 Cheap Chinese BoB.
The software is far enough along that the some of the buttons on the keypad serve as Spindle ON and Spindle Speed but as yet don't produce an output.
My ELS can already report SFM. Set the X value to the radius and select SFM instead of RPM and it reports the surface speed. Now as you move outwards of course the SFM increases because more material passes by the tip at the same RPM. But my ELS is designed to always start cutting at X=0.000.
Like setting the dial at 0.000 and running a pass, measuring diameter and then deciding 0.005 needs to come off. Easier to then move in 0.005" from zero and reset it to zero than to remember that it's at -0.017 from other passes and we have to in our head add -.005 which is then -0.022. And remember if we want to do a spring pass to back it out, and return it to ... what was that again... oh yeah -0.0023. oops.. Now it's 0.002" under size. Returning to the 0.000" set point is human factors design. Our brains work better that way.
To achieve and ideal surface finish the point is to maintain a set surface speed by varying the RPM while facing off at a specific pitch if you will. To do that one needs to control the spindle speed and have a facing off from BEGIN to END along with a distance per rev. So you have to calibrate the tool tip so you know where it is relative to the spindle center line. That introduces a whole new element of CNC like operations.
Although the infrastructure is in place in my ELS I've never done it (nor been pushed to do it) because although my South Bend VFD can accept 0-10V I don't have a powered cross feed on it (at least not directly).
My Gingery has powered cross feed but I don't have a variable speed spindle. So hard to test something.
Anyway, most people if they want to go that far just put CNC on their lathe, learn the bits needed and can do way more.