Hi freq can send out signals if not calibrated or shielded properly.  The FCC regulates it.  There is a famous story that circulates about a TIG unit at Boeing that went wacko.  It caused a robotic rivet drilling machine to drill random holes in the wrong locations in aircraft wings until it was properly shielded.

Hi Kix!

I know more than once it damaged a few "cheapo" electronic watches I wore when I was welding Aluminum, and forgot to take them off, and store them at some distance from the welding area. Btw, I really learned once from experience when I was still wet behind the ears, to always wear "cheapo" electronic watches when I damaged a not so "Cheapo" electronic watch... Man do I miss that watch! :( :( :(

Hey, It does'nt always happen right away but inevitably the Hi Freq will damage your electronic watch unless you remember to take them off and, store them at safe distance from the weld area - around a 20ft. radius distance works for me. :) :) :) Of course, this is just from my own experience! ;)

Respectfully,
Henry

Kix and to all.....simply put any excited radio, uhf, vhf etc. can induce electrical current in any conductor that is subjected to it....the amount of voltage induced and therefore current potential is directly related to the strength of the signal.   This is a post from another thread where it was asked if standard high frequency could cause any damage to CNC circuitry.  

post: http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=74997;hl=high%20freq#pid74997

"Tom  I could not tell you exactly what freq the arc start will be on DCEN but definitely on the majority of older machines it will be the same as the constant high freq but will last only say .5-1sec.    The problem here in lies with the amount of current flowing thru a given conductor with this high frequency superimposed over it.....more current equals more wattage therefore more transmission power available thru the open air to be captured by other conductors like circut boards in CNC equipment.   Most CNC equipment built after 1980 or so (please dont make me pull out NEC code) is rated for a certain amount of shielding capability on the controls.  Modern stuff is well shielded against interference.  The problem specifically occurs when there is a reasonable wattage (think power) radio freq transmitter is nearby.  The power (wattage) is high enough to induce emf (electro motive force) in anything conductive nearby.  This in turn can induct  current in said conductors and cause failures.  Most electronic circuts built on the principals of the transistor only require .7 volts or less to function.  A high wattage radio transmitter can easily induce more voltage than .5 volts in a nearby conductor.  Therefore high freq ....it can shut it off or do any function inadvertently that can occur by the push of button on its controls any other device which functions on a solid state circuitry built on the premiss's of less then a volt unless its properly shielded by modern technology. 

A great example of this is the cheap cd boombox attached to my toolbox...it works fine most of the time but when I am playing a cd sometimes when welding aluminum (only aluminum) it will shut completely off an arc start....it laks quality shielding   Well imagine having two or more hours into a program on a cnc and the welder strikes an arc and its GONE....That is a reality Tom Cooper....on newer machines I do not see it happening.......but anything pre 1985  you betcha.....and even on modern machines if the shop electrical supply is improperly setup...a good backfeed could cause the same result.  Look in the CNC cabinets do you see coaxial wire and aluminum strips on the cabinets.

Now as far as making shielding to prevent such I say look at the common coax cable built from space technology......a conductor inside a shield which is also part of the signal.    The reason that shield works is the woven strands...geometry....opposite directions but still connected....ok you see the idea...it has to do with magnetic poles and how radiation can can conduct itself.    If the shielding is aligned 90 degrees against (each other' itself":molecularly" then it will cancel itself out in propagation (wattage) from the source.  It works the same essentially with an induced signal ......the cross conductors eat away the influence of the signal power wise.  So in other words a ferrous conductor woven in the traditional basket weave method makes a powerful shield against any high freq or other radio signal now matter what the wattage for all intents and purposes.

So in summery I can say without question the fears of your machinists are grounded in hard scientific fact.....check to make sure their controls are safe before venturing into their space."

I hope that sheds a bit of light into the mystery
Best Regards
Tommy