do you have a servo driver that can work in two independent modes step/dir (position), and analog input (velocity mode)? take a look at the video, I did it by introducing ``A `` as standard axis for changing the position, and the motor rotates like a classic lathe through the gcode M3 ``S``command by analog signal.
I saw your lathe. Yes, my servo could work with analog and step/dir. But my board 7i95T hasn't analog output. But this is not a problem (my spindle works via S3, M codes fine). Main issue for now is "when the spindle rotates in velocity mode pos-cmd change (stepgen module) it's value, but axis A (joint and pid modules) have old one". From LinuxCNC perspective Axis A have no action, but stepgen "somehow" changed it's position (because there is another module spindle and spindle.pid who send commands for velocity). To avoid this issue I'm making reset BEFORE using Axis A, and rotate to 0.0 AFTER using Axis A (so stepgen pos-cmd became 0.0, joint became 0.0, pid.a became 0.0). Then I making reset (block pos-cmd to change it's value) and start working with Spindle (velocity mode). So only in this case pid.a == joint == stepgen.pos-cmd == 0.0 EVEN is stepgen is rotating via Spindle commands.
So if you have tips how I could switch back to Velocity mode without rolling back to pos-cmd == 0.0 -> please give an advice.
How to fix out of sync joint/pid.a-axis pos-cmd with stepgen, when spindle/pid.spindle works with the same stepgen. I feel it looks like the problem should be resolved in 1-2 rows in HAL/INI file.
. in my case, i bypassed it with only one relay that changes the mode on the driver via the M command M64, M65. but since you don't have an analog output, unfortunately really don't know how to help. 
