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Thread: Welding technique question

  1. #1
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    Welding technique question

    Hi to all,
    Thanks in advance! I have been following Tim's restoration as well as the posts from Fishcop on restoration. I have posted several times as well. Here is the question of the day: Over the weekend, I cut out the bad part of the rear passenger side floor and fitted a piece for replacement. As was described in my welding class, I used a spot weld to hold the piece in place, then went back around to fill in. Well, this worked OK in some places where the butt weld was up against thicker original metal. BUT where the edges were thin, it blew holes right through the sheet metal. I was using the "B" range and about 3 on the wire speed on my Lincoln 140 mig welder with 75/25 Ar / CO2. Then, I tried to close the holes with the welder. This seemed to work OK, except that I ended up with about a 1/4 inch of weld that needed to be ground down. When I put a strong light on the top, and look up underneath, it looks like swiss cheese. Even after grinding for a while, the weld looked like a lunar landscape. Obviously, I am doing something wrong. I can do an OK bead on thicker metal, and I can do an OK bead on a butt weld on 18 GA practice material.

    Do I need to cut out more of the old metal?

    Use a lower voltage setting?

    Abandon the idea of a continuous weld around the patch and just fill the holes in with fiberglass?

    Create a "lip" and punch holes for spot welding? this means weld-thru primer plus fiberglass finish.

    I need to improve my technique before moving on to the more complex repairs.

    Thanks.

    Rick

  2. #2
    Senior Member uai's Avatar
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    Hi Rick,

    get some pieces of 1.5x1/4 copper (bend them in various radiuses) and have them pressed on the other side during welding.
    You can also try to position the copper piece with some kind of magnetic device if you haven't got a third arm or somebody who can help you.
    This will greatly improve your welding as the copper transports the temperature away and you won't burn holes.
    Apart from that it's practice practice practice or even welding with a WIG or the old fashioned way with Acetylene.
    Cheers

    Uli

  3. #3
    Member bboss's Avatar
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    +1 on the above.

    You can make a backup plate that works fine by cutting off a scrap of copper water pipe and flattening it so it's double thickness.

    Ideally you would cut away all of the original metal that is rusted enough to be measurably thinner. Doing this, along with having really good fit-up on the patch, makes the welding much easier.

    I bet if you practice some more and use the backup plate when needed, you should not need to resort to lap welds or fiberglass.
    Brian Boss
    71 914-6 2.2E conversion
    71 911T mostly S spec____________
    Early S Member # 2758___________

  4. #4
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    +1 on the copper back up, you can fill holes with the copper behind it. on the practice try using some 20 & 22 gauge to test your skills on the thin areas, 18 is thicker than most metal on the whole car and as you have seen much easier to weld.
    You can also add a spot of mig weld for a second then let it cool and hit it again beginning to build up material when closing up holes, I dont think you mentioned wire size, I like .023" for the body /sheet metal repair, even .032" is easy to stick through thin areas when starting the weld.

    Good luck

  5. #5
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    Perhaps try one of these:

    http://www.eastwood.com/magnetic-plu...orizontal.html

    I haven't tried it myself, but will be doing so soon.
    '73 RoW 911T (project)
    '77 911S
    Early911SReg #2945

  6. #6
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    Thanks! I found a three inch length of 1 inch diameter copper pipe and smashed it flat with a hammer. I used this as a back-stop for the little piece that I welded into the side of the longitudinal where the back floor pan joins the side. It worked great! Not nearly as much blow-thru. I was able to run the welder on "A" and 2.5 on the wire speed. I used my little ceramic magnets to hold everything in place while I did the first tack welds. The copper was good also to provide a ground for the insert piece (even if the ceramic magnet is holding both pieces it does not provide sufficient ground back to the welder)

    Thanks for the guidance.

    Rick

  7. #7
    Senior Member Fishcop's Avatar
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    I was about to chime in with the copper backing plates... but everyone is on it!

    Blow-through can also occur if your wire speed is too high... better to use a medium setting and slower wire and just do short "dabs".
    John Forcier
    EarlyS #1987
    1968 911 Race Car "Grun Hilda"
    1969 S/T interpretation "Blau Healer"
    Restoration Saga

  8. #8
    I cartainly agree with all of the comments. I suggest that you first use the "dabs" on the thinner (previously rusted) metal and play with wire speed and heat until you stop getting blow throughs. When you accomplish a spot or two of new weld on the thinner metal you can begin to attach it to the thicker sheet metal. Once you have that bridge, it is much easier to work up a bead, as long as you take it easy with the old sheet metal using "dabs" in advance of your work. You will still get blow throughs but with patience you will get the job done.

    Jerry G

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