How do I best probe earth ground?

[markz]

In order to sense the metal stock in my milling machine I've been attaching a 6HC IO connection between my spindle and my (electrically isolated) bed. When the bit hits metal it shows a connection and works like any other probe / switch.

I was maintaining my mill and realized that the AC spindle wiring ran earth ground to the spindle - which seems like a really good idea. It has been working fine with the 6HC ground line connected to the spindle and 6HC input pin connected to the bed.

So the question: is connecting the IO ground to earth ground a bad thing here? Should I use an optoisolator or connect IO ground somewhere else or ?

Thank you.