Sheet metal backing

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I am welding 20 Ga sheet metal, using clamps that leave about .040 gap between the pieces. I can weld them but I get a lot of wire and junk on the backside. I need to have the inside nice also. I have tried aluminum and copper as a backer. Both work to some degree but do stick pretty good sometimes. Is there anything else I could try? Tia, Wireburner
Reply:I use copper plate for backer when I can. If you can eliminate the gap, you can get a better weld. You don't need much penetration on 20 ga sheet!! Also, less sophisticated machines seem to have a harder time with the really thin stuff. My Miller 175 kinda sucks at thin sheet, even when I use .023 filler wire. But, it does the job.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:So your gap is larger than the thickness of the piece your welding?  Thats why your having problems.  Close the butt and weld away and I bet you'll still be burning through.You wouldnt weld 1/4" plate with a 3/8" open root would you?Have we all gone mad?
Reply:Your using as much heat as an O/A torch would useAnd may not be concentrating on where you are welding and the wire is shooting through.Yes a copper backer can smoothen things out considerably, but you should not need that much heat.Work several areas spreading stitch welds out on the sheet metal center outward to tack.Try to weld at the same positons for all the welds and they will penetrate and also look good without pinholes to start rust.And because the weld is larger than the thickness of metal their is upset as the weld cools.If you can reach the other side,while still quite hot put a hammer or dolly behind the weld and accurately flatten your weld crown.I will see if I have a demostration panel to help show you.
Reply:Thanks for the help. I am using a Hobart 175. The clamps I am using are small but they set the distance. I am doing various projects, some things are just drill holes or screw holes, not much I can do about the size. Wireburner