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Thread: Lap weld help

  1. #1
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    Lap weld help

    I'm mig welding a sump to the bottom of a gas tank on a race car. The tank is 20 ga and the sump is 16 ga. I'm not a pro and don't have much experience welding sheet metal. This is a lap joint weld with the 16 ga on top being welded to the 20 ga. I set my machine (Hobar Handler 190) for the 20 ga material and tuned it in on some 20 ga scrap. I can do tacks and run beads with no problem on the scrap but when I try to run tacks on the real deal I'm getting really crappy results. I'm concerned about burning through the thinner material but think maybe I should be setting up the machine for the heavier material. My intent is to do overlapping tacks all around. Any suggestions to get me back on track would be appreciated.

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    Re: Lap weld help

    DO NOT LAP WELD! You're liable to get some pretty serious burns on your legs (or worse!) Stick to welding on a bench. Much safer.

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    Re: Lap weld help

    If it is supposed to be leak free, then overlapping tacks will not work. Practice more and use heat sinks.

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    Re: Lap weld help

    Quote Originally Posted by rblowbucks View Post
    I'm mig welding a sump to the bottom of a gas tank on a race car. The tank is 20 ga and the sump is 16 ga. I'm not a pro and don't have much experience welding sheet metal. This is a lap joint weld with the 16 ga on top being welded to the 20 ga. I set my machine (Hobar Handler 190) for the 20 ga material and tuned it in on some 20 ga scrap. I can do tacks and run beads with no problem on the scrap but when I try to run tacks on the real deal I'm getting really crappy results. I'm concerned about burning through the thinner material but think maybe I should be setting up the machine for the heavier material. My intent is to do overlapping tacks all around. Any suggestions to get me back on track would be appreciated.
    On a serious note -

    Make sure you have a good fit-up. Gaps will lead to a lot of trouble welding it.

    Make sure you material is very clean. No paint between the parts, wire brush or sand everything, wipe it down the acetone.

    Pay attention to this next part :

    Focus the heat on the 16 gauge.

    I do almost exclusively stick welding... I don't remember the last time I even plugged in my MIG welder.

    With stick, I'd "whip" over to the thinner part, then back. Spending more time on the thicker metal.

    With MIG, I'd say instead of aiming for the root of the joint, bias towards the 16 gauge material. Does that makes sense?

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  6. #5
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    Re: Lap weld help

    I'll give it a try tomorrow if I get a chance. All metal was prepped with a flapper wheel. Does wiping down with acetone really make much of a difference?

    I'll try to find some scrap 16 ga and mock it up with the scrap 22 ga I have and practice it first. Also got some grinding to do to get rid the mess I already made.

    Thanks.

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    Re: Lap weld help

    Quote Originally Posted by rblowbucks View Post
    I'll give it a try tomorrow if I get a chance. All metal was prepped with a flapper wheel. Does wiping down with acetone really make much of a difference?

    I'll try to find some scrap 16 ga and mock it up with the scrap 22 ga I have and practice it first. Also got some grinding to do to get rid the mess I already made.

    Thanks.
    Wiping down with acetone won't make the big a difference. Not a bad idea, though. If I'm worried about how a weld will come out, it gets an acetone wipe-down.

    Make sure you weld "thick to thin". Thick can handle more heat than the thin. Focus 20% more heat on the 16 gage than the 20 gage.

    Good luck! Have fun!

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  8. #7
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    Re: Lap weld help

    When lap welding, you generally should start the puddle on the bottom piece, then, after you have a puddle there, "wash" it over to fuse with the top piece, because on the bottom piece you are starting a puddle in the "middle" of the metal (where the heat has two directions to wick away from the puddle), whereas on the top piece, you are melting an edge, which has only one direction for the heat to wick away. Edges always melt away (blow out) faster than puddles started in the middle of a piece of metal, so if you try to start your puddle on the top piece, the edge will melt away before ever fusing to the bottom piece.

    Not sure whether your project has the thinner metal on the bottom or the top, but that's the rule of thumb for when you're welding two pieces of equal thickness...can you better describe the joint you're trying to weld (or attach a sketch showing the relative thicknesses of the two pieces)?

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    Re: Lap weld help

    Mig brazing , or O/A brazing , are possibilities . Brazing tends to flow, and fill voids .
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  11. #9
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    Re: Lap weld help

    also make sure that there isnt a slight coating or plating on either piece to keep them from rusting.
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  12. #10
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    Re: Lap weld help

    Gas tanks can explosive.
    The only time I work on tanks was new. Even new tanks can be explosive.
    I use per heat to burn off any oil on all parts to keep down the explosive .
    I have know to many welders that was kill by welding on tanks. A few months in my county a welder was welding on a water tank and it explosived killing the welder.

    FYI a full tank is safer a empty is a killer.

    Dave

    Quote Originally Posted by rblowbucks View Post
    I'm mig welding a sump to the bottom of a gas tank on a race car. The tank is 20 ga and the sump is 16 ga. I'm not a pro and don't have much experience welding sheet metal. This is a lap joint weld with the 16 ga on top being welded to the 20 ga. I set my machine (Hobar Handler 190) for the 20 ga material and tuned it in on some 20 ga scrap. I can do tacks and run beads with no problem on the scrap but when I try to run tacks on the real deal I'm getting really crappy results. I'm concerned about burning through the thinner material but think maybe I should be setting up the machine for the heavier material. My intent is to do overlapping tacks all around. Any suggestions to get me back on track would be appreciated.

  13. #11
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    Re: Lap weld help

    16 and 20 gauge material is going to be super hard to weld with the MIG and make it water tight. Plus steel of that thickness is going to want to move and warp. Since it is a lap weld, it is perfect for brazing. And since its not structural I think brazing would be plenty strong. Oxy-Acetylene torch or TIG brazing would be my choice.
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  15. #12
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    Re: Lap weld help

    You didn't mention wire size. That would be cake with .024 wire and certainly doable with .030 wire.

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  17. #13
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    Re: Lap weld help

    No data on what type welding being done.

    Dave

    Quote Originally Posted by M J D View Post
    You didn't mention wire size. That would be cake with .024 wire and certainly doable with .030 wire.

  18. #14
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    Re: Lap weld help

    Wire feed is the ultimate 16 ga weapon.

  19. #15
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    Re: Lap weld help

    Quote Originally Posted by smithdoor View Post
    Gas tanks can explosive.
    The only time I work on tanks was new. Even new tanks can be explosive.
    I use per heat to burn off any oil on all parts to keep down the explosive .
    I have know to many welders that was kill by welding on tanks. A few months in my county a welder was welding on a water tank and it explosived killing the welder.

    FYI a full tank is safer a empty is a killer.

    Dave
    How can a new tank be explosive and why would pre-heating an explosive tank be safer? Also, how the hell does a water tank explode from welding? The welding caused the water pressure to increase so much/fast that the tank couldn’t withstand?

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    Re: Lap weld help

    Quote Originally Posted by Ambull01 View Post
    How can a new tank be explosive and why would pre-heating an explosive tank be safer? Also, how the hell does a water tank explode from welding? The welding caused the water pressure to increase so much/fast that the tank couldn’t withstand?
    New tank may have some preservatives that can volatilize from heat and produce vapors that can ignite, Pre-heat? beats me. I prefer steam, then CO2 purge.
    I remember the water tank explosion (one of those big city types) IIRC the wells were pumping methane or hydrogen bubbles that built up in the air space.

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  22. #17
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    Re: Lap weld help

    Quote Originally Posted by 12V71 View Post
    New tank may have some preservatives that can volatilize from heat and produce vapors that can ignite, Pre-heat? beats me. I prefer steam, then CO2 purge.
    I remember the water tank explosion (one of those big city types) IIRC the wells were pumping methane or hydrogen bubbles that built up in the air space.
    All of the fuel tanks I've welded were used. I drain the fuel (usually gasoline), wash the tank out, fill it with water, then light a torch and stick it in the tank while it's draining. That way, even if there is some sort of gasoline "residue" in the tank, the vapors get burned off before they're build up. Plus it acts as a nice bit of preheat.

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  23. #18
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    Re: Lap weld help

    There were a couple of thread about potential exploding farm implement from build up of hydrogen.

  24. #19
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    Re: Lap weld help

    Generally, on a lap joint, you really want to run the puddle on the "lower" part, and let the metal wash onto the "upper" part. In other words, let the puddle flow into the knife edge gently, as you concentrate the heat on the flat portion.

    Name:  splicing tubing10.jpg
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    Name:  splicing tubing11a.jpg
Views: 125
Size:  122.2 KB This was when my HH190 still ran decent. But anyways........you can see how the filler was washed up against the lip, but didn't eat into the lip. All the heat was concentrated away from the knife edge.

    Being as you're welding two different thicknesses.............you have a problem. Mainly, the thicker metal is on "top".......where the knife edge is.

    Theoretically, you should set your volts for the thicker material, and keep the bulk of the heat on that material, and allow the puddle to wash into the edge. But you can't do that here without possibly burning through the 20ga. You can't run your wire on the 20ga.

    As a last resort............set your volts for the 16ga, run the gun on the 16ga, and allow the puddle to melt the knife edge into the lighter material. Your gun would be directly over the lap, or slightly biased towards the 16ga, then let the metal run over the lip onto the 20ga. Your wire should be hitting the 16ga, not the 20ga.

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    Re: Lap weld help

    Well I was planning on tackling my truck rear quarter panel and frame repair but hearing about exploding gas tanks has given me pause lol.

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    Re: Lap weld help

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  27. #22
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    Re: Lap weld help

    That just water 💧
    I house water heater can exposed too.

    I saw on the news it was only 40 miles from me.
    Note the guy was not a welder

    Dave

    Quote Originally Posted by Broccoli1 View Post

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    Re: Lap weld help

    Quote Originally Posted by Broccoli1 View Post

    wow that’s sad
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