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Thin-on-thick lap joint

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  • Thin-on-thick lap joint

    I'm curious what some of you would use for welding a lap joint, where you have a thinner (1/8") piece lying on a thicker (3/8") piece. I can normally run pretty nice beads in most positions and joints here in my home shop. But recently I was working on project where I had an old tank with 3/8" wall and several 2-inch holes in it where valves had been removed.

    I had some 4x4 inch squares of 1/8" plate which I used to weld over the holes, to "patch" them. I asked some pretty experienced welders what rod to use, etc. and got a BIG range of answers. Some suggested dropping to 3/32, others suggested staying with 1/8. There was absolutely no consensus.

    I ended up using 1/8" 7018AC (I have an AC welder). It was a mess. Worst beads I've ever laid. Looked worse than chicken poop. Seems like the thin plate got melted into the bead and there were big blobs and big gaps and everything else.

    I think I should have used 3/32 6011.... but I wondered what you guys would have done.

  • #2
    I probably would have used it as an excuse to buy a bigger MIG welder, since the only welder I have now is a Hobart Handler 140!

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    • #3
      You're making me want to play with some A/C stick to see what happens.. I always use D/C.

      is it possible this old tank had something in it that's contaminating your process?


      get it fairly clean for 7018. otherwise use 6011.. but it's hotter and hornier, so you might have bigger problems

      7014 runs really smooth (on D/C anyway) and doesn't penetrate deeply.. try it too

      Can you weld the inside of the 2" hole? So that the edge you're lapping at is the thicker material?
      Mike

      sigpic WHEELED VEHICLE SERVICE SINCE 1960

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      • #4
        If what you are doing is not working you have to try different.

        If all you can do is make short nice welds do that. You might have tried putting something on top of the thin piece to try and pull some heat out of it. Keep a shorter arc on the thin side. There are many ways to skin the cat and people offer what seems to work best for them.

        Are you sure they were ac rods and what was your amperage? It sounds like you just need more practice like I do.

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        • #5
          Originally posted by Ruark View Post
          I'm curious what some of you would use for welding a lap joint, where you have a thinner (1/8") piece lying on a thicker (3/8") piece. I can normally run pretty nice beads in most positions and joints here in my home shop. But recently I was working on project where I had an old tank with 3/8" wall and several 2-inch holes in it where valves had been removed.

          I had some 4x4 inch squares of 1/8" plate which I used to weld over the holes, to "patch" them. I asked some pretty experienced welders what rod to use, etc. and got a BIG range of answers. Some suggested dropping to 3/32, others suggested staying with 1/8. There was absolutely no consensus.

          I ended up using 1/8" 7018AC (I have an AC welder). It was a mess. Worst beads I've ever laid. Looked worse than chicken poop. Seems like the thin plate got melted into the bead and there were big blobs and big gaps and everything else.

          I think I should have used 3/32 6011.... but I wondered what you guys would have done.
          How old was the 7018AC, and was it properly stored? Were you dragging the rod? What cleaning did you do?

          Also, what are you going to use this tank for?

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          • #6
            Dumb question.......where are you centering the bead? I'd keep the puddle on the thick piece and just feather it into the thin piece. If you are putting too much heat into the thin piece, it will burn though and make a mess.

            7018AC, 6013 and 7014 are all acceptable for that application. I have limited experience welding with AC, but with DC all run well. My personal preference is 7014 though. Plus, no storage issues with the unused electrodes.
            "never argue with an idiot; he'll bring you down to his level, and win by experience"

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            • #7
              Thanks for all the replies. I was using 7018ac @ 125 amps. I was centering the bead right on the edge of the thin piece.

              The tank is an old propane tank I'm making into a roller. The holes are where various valves were removed. The metal was brushed down very clean. I think the issue was the thin plate overheating. I need to work on this with some scrap; the edge-feathering approach described might be the answer.

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              • #8
                Just do what mike landrich said, his is good advice.

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                • #9
                  Originally posted by Ruark View Post
                  Thanks for all the replies. I was using 7018ac @ 125 amps. I was centering the bead right on the edge of the thin piece.

                  The tank is an old propane tank I'm making into a roller. The holes are where various valves were removed. The metal was brushed down very clean. I think the issue was the thin plate overheating. I need to work on this with some scrap; the edge-feathering approach described might be the answer.
                  The issue could be zinc as many tanks are galvanized. Besides wire brushing the tank, grind a little and see if you get a better weld.
                  "never argue with an idiot; he'll bring you down to his level, and win by experience"

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