I've just done this to my MX3660 BOB and driver. The thread is here https://forum.linuxcnc.org/38-genera...-output-solved have a read through and then see how it stacks up to yours. On mine in the end I removed the whole POT circuit board and wired mine straight to the wires. If you have a spare output you can get the spindle to reverse too incase you want to tap.

Jools