Hi LerninCNC,
You can even add a cheap "single-relay board" design for Aduino if you want to implement M4 for spindle counterclockwise spin if you use the unused pins (for the unused 5th axis) to drive a Aduino relay board.
The 5th axis direction pin is jumper selectable and can be used for either the onboard relay or external direction control – it cannot be used for both.
The BOB board must be powered by 12-24V otherwise the optically isolated inputs will not work.
USB connection is NOT required
A 5 Volt connection is required for the BoB and the USB connection from the PC is the easiest method.
And (as mentioned by Reuelt) a 12 – 24 Volt supply will also be required to power the inputs and the analogue output of the BoB.
I think it is better to switch the DC side of your spindle.
As already mentioned by ZASto this is extremely bad advice.
Ideally, using the BoB relay to switch the AC side of the motor controller via an AC switching solid state relay would produce far less electrical noise and cause less problems than taking any high voltage AC or high current DC to the BoB.
Hope this helps,
Tweakie.