So here is my analysis of whats going on so far:

1. Your Controller VI may not control the spindle at all. Nothing. It may only want to control the axis motion.

2. You have a standard on/off switch, estop mechanism as well. We can ignore these for now.

3. You will need to figure out exactly what your reversing switch does. Does it send a control signal to the board saying "forward/rearward motion" or does it actually swap the polarity on the motor inputs. Seems like it might be control signals based on how the motor is wired up. You should look up its part number if you can find it, and check its behavior with a meter to see what it does.

4. The hardest thing will be dealing with the potentiometer that controls spindle speed. It may have its resistance measured by the RPM display board and send a digital signal back to the controller to adjust the spindle speed.

Can you send a picture of the back of the RPM display?

Thanks for your patience.