I have found Iggy's homemade tig welder project and am quite
impressed:

http://igor.chudov.com/projects/Home...o-AC-Inverter/

I was wondering if anyone can help me out a little. I recently
acquired an Lincoln Idealarc 300 amp tig welder which already is an AC/
DC welder with High Freq arc start. The DC side is fine for what I
do, but I wanted to improve on the AC side by converting it to a
square wave welder. For now all I would like to do is to make it a
squarewave w/ variable frequency. Has anyone made an suggesful
homemade squarewave AC welder?

I have also discovered this design:

http://www3.telus.net/public/a5a26316/TIG_Welder.html

Any help is much appreciated.

Cheers!