I don't have much occasion to do gas welding. My TIG welder does all my work that a gas torch would be used for. I hardly ever use my torch unless I am reducing a car to pieces small enough to fit in the metal recycling bin or cutting out bulkhead parts for ocean liners. I might get one of those Cobalt lenses just to see if there is a diff in the stuff I do. I see that Seattle dale was having some trouble mig welding some 3/8 together. What is the amperage output of your machine? I would think that you should have at least a 200 or 250 amp machine to do that big of material easily. The thickness combined with the sheer mass of that material make for a very cold weld. 18" of 3/8" steel is a big heat sink. I know that when I do anything over 5/16 with my 160 amp machine I am looking for a bigger unit. Use straight CO2 gas, It welds a little hotter. Also try using the .035" E-70S-6 wire. It cleans a little better on dirty stuff. That is all I keep in my migs at the shop anymore.You may want to preheat the steel some too, It really does help on the thicker stuff. Sometimes just getting it up to 70 or 80 degrees can make a difference. On "T" joints in thicker material when using a marginally powered welder I like to keep the torch straight up or angled back slightly from the direction of travel.(pushing the bead). I usually turn the heat as high as it will go and back off of the wire speed a bit. That seems to dig into the base metal a little better. I then oscillate the tip side to side in a "C" motion so that I dwell a little more in the root of the weld and wash some bead up the sides of the joint. this makes a nice concave bead with a minimum of undercut. In doing any buttwelding on thick stuff I "V" grind my joint at about 30*. Buttwelding usually requires a little more wire and "pulling" the torch. I vary from "straight up" to leaning about 15 or 20* towards the direction of travel. That works pretty good for me. Thin stuff I usually leave a gap of the thickness of material or a little less. (1/8" gets a .100" gap, .250" gets approx .200" or "V"d slightly.) Sometimes on the thicker stuff I will do several passes to build up the bead. That is a really common way when stick welding to build up a bead, so why not use it when mig welding? Hope this helps.
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