Hi, I'm installing my smooth stepper into my cnc enclosure and it's going to be mounted onto Perspex using nylon spacers. It will be powered by the USB cable only and the only other connection will be the parallel port cable into Port1.

So as Perspex is prone to static and the smooth stepper board is electrically isolated I was wondering if it would be better if I put the grounding jumper on and ran an earthing wire from the mounting hole to the enclosure ground?

However if the smooth stepper is earthing back through the USB cable then I might create an unwanted earthing loop by attaching another wire?

So should I ground the smooth stepper because of the perspex mounting or not?

No, I would not reccomend bonding the SS to earth ground. Sounds like it would be a better idea to figure out how to bleed of any built of static charge from the enclosure or use a diffrent enclosure.