The other approach is of course to only automate the axis and leave the manual start stop speed alone. To use you turn the spindle on manually then activate the cnc program. When the program is done turn off the spindle. I’m doing this on my ELS conversion to start. Later it is possible to add start/stop and speed to the control by using a vfd interfaced to the control. At least the rocketronics ELS does this. I saw at John Danmyers he has spindle control too on his equipment with his ELS solution.
On reason to add spindle speed control to your control is then the control can achieve constant radial velocity under the cutter by speeding up the spindle as the cutter traverses the part. I’m thinking about lathes here.
Yes. Project #42 for my ELS was to connect the RS232 port on the ELS to the ModBus port on the VFD and control the spindle that way. The infrastructure is there now even including ALT buttons on the numeric keypad for spindle control.
The biggest issue with constant SFM with facing is you have to know where the cutter is relative to the lathe centerline. For CNC Lathe systems with automatic tool changers it's not such a big deal to install a tool and just like on a mill have a new X axis position. But manual users who like the features of an ELS but don't like CNC will at the drop of a hat grab a wrench and loosen, then twist, and then tighten the QC Tool holder. Now the tool tip is at a new X location.
For facing you can just move away from the headstock, probe in and reset the zero. But turning it's harder. Now it's touch off the work. Measure it. Enter it as diameter or divide by two and enter in radius. My ELS has the SFM mode so as long as the X is correct you can see SFM instead of RPM and tweak the VFD as you face the work. But nothing is automated.
As you can see, it gets convoluted really quickly.