Just for clarity - the conversation on the phone as I recall was related to exporting an earth to the machine, not to provide shielding (after all, we're providing galvanic isolation with the opto-isolator board). For this (and broadly aligned with Jazz's advice), provide an earth line back from the machine to a central earth point on the control box connected to the protective earth provided through the IEC (mains) connector.

Screens (rather than earths) I'm not going to get excited about.