Thanks guys :-)
I have tried an industrial EMI filter
https://www.phoenixcontact.com/onlin...ary=usen&tab=1 but the problem persist.. I have connected the filter also to the PC supply and then to the CNC controller supply but the issue still there. unfortunately I only had 1 filter so I couldn't try having each power inputs through its own filter. Next step for me is to re-verify all grounding connections, shields bonding and then I will remove the spindle cable from the chain duct and route it in a metallic flexible conduit if I can find one.
I was also thinking about using an USB isolator but at this point it may be cheaper to get an ethernet cnc controller card as suggested by Roger.
Anyhow, your help is much appreciated, let's see how it goes.
Valter