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Hi B&D, I guess the best discription of using high frequency(HF) for welding is it's used to extend the arc length, the distance from the electrode to the base metal, without contaminating the electrode with molten metal from the weld puddle. HF-start(HFs), is useful when welding SS/CU, etc; metals and you don't want arc start tracks left on the surface of the surrounding base metal. In this case the HF will assist the starting arc to jump an open distance without touching the base metal. After the arc is established then the HFs is turned off automaticaly via a timed circuit. There are usually two HFs adjustments, one for arc intensisity (jump distance) and one for HF shut off delay. HFs is very useful on using a Tig(GTAW) torch without a remote foot control or also called scratch start Tig. Even if you have a remote foot control, there are times when it's not handy to use it but still use HFs. HFs can also be used on other welding proceses, mainly SMAW(stick) but really isn't useful for GMAW(Mig). HF-continious(HFc), is best suited for Tig welding Aluminum. In this process were using AC-HFc. The AC is used to clean the base metal as it's being welded by alternating the current thru the base metal thus cleaning it to receive the filler metal. To keep the electrode away from the molten puddle we add HFc and this allows the tungsten electode arc length to be increased so the tungsten does not become contaminated with the molten metal as the current alternates. Once the tungsten electrode becomes contaminated with the molten metal then the arc becomes very difficult to control as it causes the arc to be scattered inconsistantly on the base metal surface. I'm sure there's many more uses for both HFs and HFc on automatic welding. I welded some 1/2" thick CU brew kettles with manual wet Tig at 360amps with a 1200� preheat using HFs. About the largest size stick electrode that can easly manualy used is 1/4" as the molten puddle is huge and can be difficult to control. I've used some 1/4" 7018 to put the cover pass on some new D9 Cat track grouser bars that were worn down. The grouser bar caps were about 1-1/2" thick and about 24" long if I remember correct, about 40yrs ago. I do remember how hot the electrode holder was even with putting the holder in a bucket of water and waring two gloves the holder was still hot by the time you burnt a rod. Usually 400amps plus is used mostly by automaic welding processes. T_Bone
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