Budget CNC routers like the OMIO lineup are amazing tools, but often that low price comes at the cost of some corners being cut. The VFD in the omio control box comes ungrounded, and the inexpensive water cooled spindles they use can sometimes have their ground pin floating from the factory.
If a ground fault in the spindle were to occur, the frame would become energized by the VFD output. Since in the default setup the spindle is isolated from the mains input, no leakage current would be detected, and that fault would not trip a GFCI.
These issues have been known within the DIY CNC community, and are all easily fixed by ensuring the ground pin is connected to the spindle casing, and properly grounding the VFD. While a fault like this is very much a worst case scenario, proper grounding is best practice and could potentially save someone from serious injury.
