Grounding and probe

[john swift]

double insulated equipment with no earth / ground connection can help avoid problems with ground loops when multiple pieces of equipment are interconnected
but can be present a shock hazard when they use switch mode power supplies
( the leakage current from one supply may be safe but several supplies can add up and be a shock hazard )

I would connect an earth connection to the cncrouter frame
more detail of your system is needed
what are you using as the spindle ?

you may get away with the static discharge but if it finds its way into the control electronics via the limit switches for example it can damage sensitive electronic devices

John