ESD causing CNC machine to stop

[scott216]

I have some ESD questions I thought would be best asked on this forum verses a CNC or woodworking forum. I have a ShopBot CNC router that mostly cuts wood.  When the dust collection system is running, static electricity builds up on the dust collection hose and once in a while the CNC just stops as a result.  There's quite a bit of advice on how to reduce this, but I'm skeptical about some of it and wanted to see what people in the EE community thought.  The dust collection hose is 5" diameter flexible PVC with a spiral steel wire embedded in the PVC that provides structural support for the hose. 

First common recommendation is to run a bare copper wire through the inside of the hose.  On the CNC side of the hose the wire shouldn't be connected to anything metal on the machine (because ESD going into the CNC machine tool can then travel to the electronics circuitry). On the dust collector side of the hose the copper wire is connected to ground.  This seems somewhat reasonable, but since the PVC is a pretty good insulator, wouldn't this only help in the immediate area of the copper wire and not the whole hose?

Another recommendation is to cut away a bit of the hose that encases the metal spiral wire and connect this to ground also.  In my opinion, in order for this to help, wouldn't the static electricity that's building up inside the hose have to jump through the PVC to discharge through the spiral wire?  Maybe this is happening; I don't know how easily the static electricity can do this.

The recommendation that makes the most sense to me (which I haven't tried yet), is to get an anti-static dust collection hose and connect it to ground.  There's one that made from urethane and the specs say "Permanently anti-static wall according to ISO 8031 electrical and surface resistance < 10⁹ Ω." 

Finally, the ESD problem seems to be worse when cutting MDF versus wood.  When cutting MDF the dust created is more like a cloud of very fine dust verses lots on tiny wood chips.  Do you think the ESD can propagate through this dust cloud more easily than wood chips and this is why the problem seems to be worse with MDF?  Or maybe fine MDF dust just creates more static-electricity build-up than wood.

I appreciate any insights you can provide.

[NiHaoMike]

Try turning off the lights to see where the ESD discharges are happening. I think grounding the spiral would be a good first move. Also raise the humidity if it's really low.

[PlainName]

When cutting MDF the dust created is more like a cloud of very fine dust verses lots on tiny wood chips.  Do you think the ESD can propagate through this dust cloud more easily than wood chips and this is why the problem seems to be worse with MDF?

Dunno about the router but that sounds like an explosive event waiting to happen.

[Siwastaja]

Dust explosion is a real danger. There is a reason why vacuum cleaner parts are made with conductive fillers, basically "ESD safe" plastics. If you DIY your own parts from standard PVC pipe, it will violate this design principle.

So start your fixes from there. Adding some sort of conductive surface treatment of the pipe would be best, but running a few metal wires inside could do the trick, too.