Y Stepper turns when spindle is on

[magicniner]

A long extension simply helps to attenuate high frequency energy - the same can be had with a filtered inlet .

But the test I suggested with a long extension is free, as is continuing to use it if you can't find a filter that suits your budget :D

What's interesting from your descriptions of the problem is that this sounds to be a logic-level issue, either the BoB interface or the stepper / opto interfaces. This is either a poorly design board, really poor PSU, or some fault on board (there shouldn't be any substantial influence from the spindle). One thing to consider is that the spindle under load is likely to produce more EM noise than unloaded.

Is there an echo in here? ;-)