What you need to avoid is a ground loop, where there are two or more paths that any current in the ground circuit can take. This is because AC mains and high frequency currents from VFDs, inductive electronic components like relays and transformers generate alternating magnetic fields. This can induce stray currents in signal conductors of sufficient magnitude to cause false signals where a ground loop allows a current to flow.

Star grounding is the easy way to avoid this problem. Your backplane can act as a star point.

Shielding should be grounded at one end only.

The AC and DC grounds need not be separated if the DC generating devices are not earthed on the AC side. Sometimes (if they have an earth wire) they are internally connected, but not often.

I have the Machine and control box grounds connected and the BOB ground connected to the machine frame and spindle, so that the touch plate returns a 0v signal. I have no problems with that arrangement.

Cheers,

Rob