Grounding and probe

[m_c]

Any kind of metal frame should be connected to earth. It's a basic safety requirement for industrial machines.
John mentions double insulated equipment, but you would really struggle to create a CNC router that could meet double insulated requirements.


However, grounding and earthing can be a bit of a complex issue, but the basic issue is you want to avoid ground loops. That is where the same wire is connected to ground at both ends. An example might be a bit CY signal cable where both ends of the screen have been connected to ground, which can result in current flowing through the screen causing interference in the internal wiring instead of protecting the internal wiring from interference.


My usual wiring practise is to have a central star ground/earth point within the control box (I personally use earth DIN terminals that connect wiring to the DIN rail, but a single stud to the case will also work), which is connected to earth. I then connect all the PSU 0V to the same terminals, so all supplies are connected to earth. I then ensure any shields are connected at only one end (either directly to the case if terminating using glands at the case, or a drain wire to the nearest gnd point - there is no need to run all screen drain wires back to the star point).
Also, if you want to really minimise interference problems, spend a bit extra on a BOB that uses 12 or 24V control wiring.