Hi,
there are two ways to approach EMI (electromagnetic interference).
Things like shielding cables, RC filters on signal lines are all variations on the theme of 'decreasing the sensitivity
of signal circuits to EMI'. This is not the best, or perhaps the first approach.
The other way is to apply line reactors and/or filters to the VFD/spindle so that no EMI is generated. Then all signal
circuits operate is a low noise environment.
What sort of spindle are you using?
Craig