Which circular connector for spindle

[Doddy]

I've yet to see the GX-16 style connector with a female chassis / male floating plug combo. Save yourself the effort and search for a different, appropriate connector (RS Components have an Australian operation - I guess they supply the public as in the UK?). Run 4 core shielded cable, introduce the earth to one of the cores and (probably) adapt the spindle to carry the earth to the spindle enclosure. Conventional wisdom is to bond the shield at the VFD side to a common/star earth point, particularly if running the cable form close to other more EM-susceptible wiring.

The machine and control box should be equipotential - achieved by exporting the earth from the bit that plugs into the wall (I guess the control box) to each conductive, exposed component. If you isolate them then you introduce the risk of electric shock. So, ideally you'd run an earth strap from the machine to the earth stud on the control box. More practically, you might take the earth from the cabling to the spindle and bond that to the machine body - and where the machine is constructed from multiple mechanical elements - to bond each of these with an earth strap. Most people find some compromise in this utopian solution for their own machines if they are not looking to certify their product safe for use.