Tying the grounds of multiple devices together over USB cables is likely to create a ground loop. Grounding is supposed to be a tree, rooted at the stake in the ground out by the electric meter. If you don't have that, every loop becomes an antenna. That's why braids are usually grounded only at one end.

This matters when you have something as RF noisy as a plasma cutter nearby.

If you have a scope, you can start looking at signals to see where there's RF hash. Then you can try putting ferrite inductors around various power and data cables to see if it helps. Often it does.

There are USB opto isolator cables.

I'd really like to see a big red switch on that setup to turn everything off. If somebody like a first responder ever needs to shut that thing off, it's not at all obvious which plug to pull first.

(I used to use the rather antiquated plasma cutter at the original TechShop. It never started properly. Plasma cutters start by creating an arc between electrode and the nearby nozzle. That makes some plasma, which is then blown out to the workpiece. Once there's plasma from electrode to workpiece, the nozzle is cut out of circuit. The one at TechShop had a consumable copper electrode and a consumable copper nozzle cap. The electrode is supposed to get all the wear, because the nozzle cap is only involved when the arc starts. TechShop's machine often failed to cut the nozzle cap out of circuit, and it consumed nozzle caps faster than electrodes. TechShop sold nozzle caps.)

I do this stuff for a living, and have an enormous amount of experience with it. There are worse things than ground loops, and USB cables with broken shields are one of them.

Ferrites do nothing for low-frequency impulse noise, which is what this type of issue usually comes down to. They are there to pass FCC/CE tests, and that's it.

I got noise out of 5V TTL encoder signals with clamp-around ferrites around the motor power leads. The encoder input was picking up the servomotor power.

The big problem here is that you can't do much about the source. A plasma arc started with audio-frequency AC (10KHz?) is going to emit RF. Not clear what the waveform looks like. If it's a square wave, which it probably is, there will be higher harmonics. Fortunately, it's only AC only during start; the cutter shifts to DC once the arc has been struck. Otherwise those things would be a huge interference headache whenever they were on.

> Once there's plasma from electrode to workpiece, the nozzle is cut out of circuit.

How is this achieved?

On my Parkside machine there is only one positive and one negative terminal on the power supply, so unless there's some active electronics in the torch handle, I don't think there is anyway to switch off the pilot arc circuit?

Is that a "scratch start" hand-held plasma torch, where you start the arc by briefly touching the workpiece? Those don't need an automatic start system. CNC machines don't usually start that way, since they usually lack enough force feedback to briefly touch the workpiece. So they have a fancier auto start system.

No it's not, my machine does have a pilot arc. It was extremely cheap though so quite plausible that it is made badly.

I'm unclear on whether the pilot arc is just always fully there, or if maybe there is a resistor in the pilot arc return which makes current preferentially flow through the main arc.

FWIW (and I know I didn't mention this) the cable I was originally using for the 6040 CNC router is a "double shielded" one which I bought in the past specifically for EMI reasons, but it wasn't immune to the pilot arc ignition.

I haven't tested it to see how good the shielding is but I haven't had any EMI-related disconnections since switching to that cable. (Other than the plasma cutter ones).