HF plasma cutter arc start

[mikeward2121]

I attached my plasma cutter to the z axis of my cnc table and proceeded to run a test. When Mach 2 called for the plasma cutter to turn on the z stepper went nuts. Is there a way to shield the plasma cutter from the cnc system so the high frequency arc start of the plasma cutter doesn't interfer?

[Torchhead]

Using HF start plasma with a low level logic (PC signals, step & dir, etc) close by is a challenge. You need a good buffered breakout on the parallel port. You need to avoid ground loops like the plague. The only common ground between the PC and the plasma unit and table should be back at the main breaker box. You have to get the HF plasma chassis and anything it touches grounded back to a close local LOW IMPENDANCE ground. The more surface area of the ground conductor the better and the closer it is to the equipment the better. Each piece on the table side needs a separate run to one grounding spot.

All of the inputs and outputs associated with the plasma need to be isolated. Any input needs opto isolation with it's own floating supply. Ouputs (torch switch, etc) need to be opto isolated or relay isolated.

Don't run control wires next to plasma wires or motor outputs close to either. If you use servos differential drives and receivers to cancel noise are essential.

Ferrite clamps on the computer cables can help but are not enough alone.

Place your plasma unit on the opposite side of the table from your PC and control electronics.

[Perp]

Is this for all plasma units, or is the HF unit most likely to have problems. I was considering a good Miller (like a Spectrum 3080) and would like to know if it will have these problems too. sorry for the hyjack

[Al_The_Man]

Commercial Plasma Gantrys that operate with Hypertherm or Thermadyne use a machine torch that have a large braid shield covering all the torch conductors including the H.F. Ignition wire.
This is important when long torch cables are run in cable carriers etc on the side of the Gantry, also many use a method of sinking a ground rod at the table where all large main ground conductors and table terminate, the braid shield is also connected to this point.
The HF unit puts out alot of E.M. energy.
Al.

[Perp]

I just realized you guys are talking about High Frequency (HF), all this time i was thinking Harbor Frieght (HF). that is why I made the comment about I wanted to get a good Plasma Unit. Sorry for dumbing down this post like that.
Doug

[KKamel]

I have used my table with 2 different plasmas both have hf start and seen no problem i even run unshielded cables since my table aint complete yet. This is what i have, after the lpt port i put a buffer to boost the voltage out from the port. Its basicly a hex inverter(ie 7404) or something i had laying around.
Only time i have had interferance is when i accedently lay the cables upon the cutting table.


Dunno if it helps, a chip like above should cost next to nothing.

[Torchhead]

Typically HF start causes problems more with inputs than a buffered output. A 7404 does not have sufficient drive to run the G201 drives but is better than nothing. Properly grounded an HF plasma can work. Poor grounding or ground loops can make you life miserable.

[Kevin Taylor]

I always thought this could be a problem I'v had my cnc mill stop when I was tig weldinh six foot away for no aperent reason A frend of mine had his garage door oppener keep starting up when he was welding one night shelded cables would be a must I must say Kevin

[the mask]

hi all
iry to ground evey thing but i still have problem

[Torchhead]

Originally Posted by the mask hi all iry to ground evey thing but i still have problem

Improper grounding is as bad as no grounding. There are a LOT of variables and factors involved and it is a lot like asking how to fly.....you have to know about each structure in the setup. If you have more than one ground path to a system you have a potential ground loop. The further away you are from a low impedance ground spot the more the ground becomes an antenna. There are entire textbooks devoted to grounding and EMI/RFI suppression. Even some Engineers do not understand the concepts. There is a difference between a circuit "common" and a "ground", unfortunately the two terms get interchanged. If the circuit designer did not design the circuit to have a central low noise common, then it's difficult to make it part of a "star" grounding system. Grounding is part science and part art!

TOM Caudle
www.CandCNC.com