TIG sheet metal to bar

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I'm trying to fix a commercial mower basket for a friend.  I need to weld 20G sheet metal to a hollow bar that's probably 1/16" thickness walled.  On standard polarity, when I step on the pedal enough to melt the hollow bar, I blow through the sheet metal.  I had my machine set at 150 amp max and was able to control the amperage pretty well.  Is there a trick to welding dissimilar thickness metals together?  I tried to focus the arc on the heavier metal and kinda "weave" the arc into the sheet metal.  I may bust out my old 110v MIG for this.   After putting everything away, I almost tripped over it.  Probably the best job possible for the little machine.
Reply:I welded .030 thick steel floor to the .125 thick round bars of my rollcage with no issues. I did use silicone bronze for the filler, just started the bead on the tube and pushed it onto the sheet metal.150Amps is a little extreme and you may not have the pedal control you should, 75 amps should be lots and you will have much better pedal control, use some filler bronze or steel to kill the heat a bit and then you can push ity around a bit to contact the sheet metal
Reply:You're way hot.  Here's how I set up for those jobs (and we do a lot of  them).  Get a piece of 16ga scrap and start at about 40 amps and go up just until you can get a good wet puddle.  That's your maximum setting for this job NOT your minimum.  Then proceed as you were, get your puddle going on the 16ga and wash it over onto the sheet metal.Everyone's technique is different but I could make that weld pretty easy at 55-60amps.  You have to be patient with thin stuff.
Reply:A good rule of thumb with steel tig is 1 amp per .001 of material. So for 1/8" (.125) you will want 125 amps plus maybe a small amount to get the bead started, so 130-150 would be a good setting for 1/8" not 1/16".  For 1/16" ( .0625) the suggested 70 amps would be a better choice.Keep in mind that it's easier to control the amps with the pedal if you are closer to your use range. Setting the machine at 150 and trying to use 60 amps means you will need to press down the pedal a bit less than 1/2 way roughly. On the other hand you need to push the pedal almost completely down to get the same amps if the machine is set at 70. Adjusting 2 or 3 amps with this setting is much easier because there is more throw on the pedal vs the higher amp setting.As said you can start the bead on the thicker piece and wash it over onto the thinner piece. Knowing what sort of joint configuration you are trying to do would help. A T joint with the thin edge as the vertical on the T will be tough because of how thin the 20ga is, especially since you are learning.As you mentioned this is where the small 1110v migs work best many times, especially if you don't have a lot of skills yet. Mig is usually a much easier process than tig on thin material for newer welders..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Changing your tungsten grind, and diameter may help concentrate and direct the heat better.  What grind and diameter are you using?  I would recommend 1/16" tungsten for these metal thicknesses, ground to a sharp, pencil shape (with possible small flat at tip).  If you are using too large or too blunt of a tungsten relative to the amount of current require for the weld, the arc will jump and move around on you.Also, pay attention to the angle you are pointing your tungsten.  You want to "aim" it where you want to direct the heat, which would mean, aim it pointing into the thicker surface.  Also, you can also move the torch sideways somewhat to bias the heat more towards one or the other pieces being joined.  If you are welding a "Tee" or "lap" joint, the piece you are melting the edge of needs less heat than the piece that you are melting in the middle.Finally, make sure you have a very tight fit-up, especially where you are trying to start your tack weld.  If you get the pieces close enough so as to actually be touching, you may find you can melt them together (so the puddles flow the pieces together) without adding any filler rod at all.  It will be more challenging to weld thin material if the gap between them is very large.  I would shoot for a gap no thicker than your filler rod diameter, if possible.  You will also be able to weld with less heat input (and less oxidation) if you have a tight fit-up, smaller filler rod, etc (smaller deposited weld bead)Once you get a tack weld, it will be easier to weld starting on the tack and moving along the weld seam, because the existing weld deposit will help conduct heat more evenly between the two pieces.  If you find the thinner piece is melting back while progressing the weld bead, watch your torch angle.Last edited by jakeru; 07-26-2011 at 03:43 PM.