Strike CNC (My first CNC router)

[JAZZCNC]

Its not connected? Seems a bit silly. I was going to ground the spindle with that and also use it to complete the circuit for the tool height probe too. How did you ground yours?

Mine hisn't grounded and it doesn't need to be.! The VFD takes care of the EARTH side just make sure the VFD is EARTHED and it will be fine.
You could if you wanted belt and braces safety introduce a EARTH wire direct to the EARTH point in the control box but this is very different to DC ground the I/O's use.?
If the spindle shorts internally the VFD will trip so power will be cut and 99% of time this is fine.!! But if for some unlucky reason it didn't then the earth would leak back to the PCB trip so it does make it slightly safer.! . . . BUT. . .It can also work against you in some case, esp with a VFD, by using the long wire has an antena sending out electrical noise.?

Dont confuse the AC earthing to DC grounding that the I/O pins use.? Yes effectively they both end up at the same place but they are done very differently.
When your probing your complelting the DC circuit from the PP I/O pins which is low voltage low amps. If the machine frame is EARTHED and the spindle hisn't isolated thru bearings etc then often the circuit will complete because like I say the ultimately end back up at the same place EARTH.! . . Often and I'm 99% sure will be the case for you the spindle will return to Earth and allow the DC input circuit to complete.! . . . Again if you want belt n braces approach or for some reason the spindle is isolated then use a wire DIRECT from the DC negative pin of the BOB input NOT the EARTH of the control box and agian to avoid noise issues just use a temporey crocodile cliptype affair.! (BUT I'm 99.9 sure you won't need too)

EDIT: Do recommend you run a short as possible EARTH wire from star point in control box to the machine frame just in case of AC short of any kind. This will also more than likely be enough to allow the negative side of the DC input a path back to complete circuit.