As you may have probably read there are two methods used, one is total isolation the other is grounding or making common to ground all parts of the system and their respective power supply commons (-ve's).
Regardless of what method you opt for, your PC MB is generally not isolated from your spindle.
The first thing I would look at with a Chinese mill is, is the spindle motor frame and VFD grounded to the star point?
The Earth Ground star connection should be set up at a convenient point in the enclosure or cabinet and the Service ground should terminate there.
Make sure your conductors to the spindle are at least tightly twisted if separate conductors.
Power supply -VE's are referred to as common, although often the term ground is used, which adds to the confusion.