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Thread: flange or lap joints

  1. #1
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    Default flange or lap joints

    I have seen a lot of sheet metal panel replacement done using this method and wondered what is acceptable and what is not.
    I hear some people say its a no no. yet I read here and in other discussion forums that it is common practice.

    please discuss.

  2. #2
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    Ahhh yes, the flange, lap or butt weld debate. It is the one and only debate that has equalled what is the meaning of life debate.

    Here is a butt weld with backing example. No, it is not mine, but damn he did a good job. I just swiped it off the net.



    A little out of focus but here is a better picture at what it is. A strip of metal is plug welded to the back side to put this backing hanging under the panel so you can then "butt weld" the two pieces together. Of course you really aren't "butt welding" it but you get the idea. You are welding down into the backing AND the two pieces together at the same time.



    A flange is where you put a little step in the metal so you can lay another over the top and its no higher.






    Continued on next post.

  3. #3
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    A "Lap" is where you simply lay the metal over the other and weld along the seam.


    The pinch weld is one that is well, "pinched". The metal is pinched together and welded via the STRSW the Squeeze Type Resistance Spot Welder. Like this one on the hood of my 1959 Rambler.


    Now, what you will hear out in the crazy world of autobody is that you don't want to use any of these because they are "breeding grounds for rust" because moisture can get between them. Ok, I can accept that, it sure can happen if it isn't seam sealed. But the one thing that cracks me up about this notion is that the ENTIRE CAR is held together with these exact weld joints! That's right, every single car made for the last 75 years has it's ENTIRE BODY welded together with these joints. Manufacturing processes changed about then, though before that back as long as 100 years it was basically the same.

    Let me make this clear, EVERY SINGLE CARS BODY is assembled with dozens of pieces of sheet metal joined by these very joints, lap, pinch and flange. EVERY SINGLE PART OF EVERY SINGLE CAR MADE! So tell me, what is one more going to hurt?

    So, that is my rant on the crazy notion that it is a rust trap.

    It really comes down to what are your expectations for the car? Are you after a nut job grease pencil marks on the firewall kinda restoration? Then butt weld everything and make it as close as it was from the factory. If it is "just a car" to have fun with on weekends, any of these welds on your patch panels are going to be just fine.

    Brian

  4. #4
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    Brian said
    Let me make this clear, EVERY SINGLE CARS BODY is assembled with dozens of pieces of sheet metal joined by these very joints, lap, pinch and flange. EVERY SINGLE PART OF EVERY SINGLE CAR MADE! So tell me, what is one more going to hurt?
    Even Corvettes ?

    Mooch

  5. #5
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    Yes even Corvettes! Depending on the year, all of them after 1962 for example have a metal "cage" that many of the body parts are bonded to. That metal cage (for that matter the frame under the body) is held together with lap, pinch, and flange welds.

    Brian




  6. #6
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    Quote Originally Posted by MARTINSR View Post
    Yes even Corvettes! Depending on the year, all of them after 1962 for example have a metal "cage" that many of the body parts are bonded to. That metal cage (for that matter the frame under the body) is held together with lap, pinch, and flange welds.

    Brian



    That is really pretty though . No wonder they are so light .

    Mooch

  7. #7
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    They're not light! A 1966 Vette weighs more than a Chevelle!

    Brian

  8. #8
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    thank you MARTINSR for the definitions. I am sorry if I brought up a sore subject.
    floors seem to be my bone of contention, some say a lap or flange joint is not accepable and only a true butt joint should be done so both top and bottom look un-repaired.

    I understand if a 100 point restoration is involved all bets are off but, where and when is this type of joint allowed and not allowed?

    I have installed 1/4 panels using the lap joint method and all went well. other people say its a true butt weld or nothing.

  9. #9
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    Honestly, on the floor, again, if you are after a super job and want the satisfaction of knowing you did the best job possible, butt weld it. But in the real world, no one is ever going to know. For that matter if you do a nice lap weld just like the factory did down there (most floors are lap welded) no one will know the difference! I'm sorry, people get all anal about this stuff and honestly, if you do a lap weld that looks just like the factory ones on the floor no one is going to know one from the other unless they are a pretty serious specialist in your particular car. And if someone is looking at it that hard, well then the car is a valuable car and maybe you should butt weld it.

    But for your average "restoration" driver/parking lot show car, butt welds on the floor are a serious overkill in my opinion.

    Brian

  10. #10
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    My concerne is not strength, I know that all types of connections are used
    on inner bodies, what I am more interested in, is which joint won't
    telegraph through the paint later down the road from temperature expansions.
    I've seen to many panel connections that do just that.
    On the outside of the car where paint magnifies even the slightest change.
    Body filler can be used with no problems when it's featheredged out but
    try and fill a groove with it and it will telegragh most every time.
    SO which joint is the best defense against that? I would think butt weld,
    but what if welding is not an option? I think I've seen lap joints telegraph.

  11. #11
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    Quote Originally Posted by JCCLARK View Post
    My concerne is not strength, I know that all types of connections are used
    on inner bodies, what I am more interested in, is which joint won't
    telegraph through the paint later down the road from temperature expansions.
    I've seen to many panel connections that do just that.
    On the outside of the car where paint magnifies even the slightest change.
    Body filler can be used with no problems when it's featheredged out but
    try and fill a groove with it and it will telegragh most every time.
    SO which joint is the best defense against that? I would think butt weld,
    but what if welding is not an option? I think I've seen lap joints telegraph.
    The only time I've had problems with joints showing through the paint is when the metal is different thicknesses or I've used metal bonding materials and didn't weld. When either of these two are used and the seam is across the face of the panel and the metal gets hot from the sun you could see the seam. Whenever we use metal bonding products we make sure that the seams are under a molding or at the edge where the panel bonds to the car, we never put a bonded seam across the face of a panel that will show.

    The only time we overlap metal before welding is when we do floors because it's rarely seen. On almost all of our repairs we use the backing strip method which creates a seam that is similar to a butt weld.

  12. #12
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    Len, I went to Toyota Training and they say no backing, straight up butt weld on quarter panels and rockers (outer) for instance. I came back from that training and started butt welding everything. I was amazed at how easy it was. I had put up a road block in my mind that this 22 gauge steel couldn't be butt welded with a MIG, balderdash! I was butt welding it with the MIG on every job in no time!

    JC, I'm with you on the ghost lines, I have even seen it with a butt weld. I am not sure, the weld being thicker was my guess. I even remember thinking maybe I didn't wait for the metal to cool after grinding the weld flat and the thicker metal of the weld retained the heat and then the filler kinda "cooked" right there and even though it was blocked flat it changed the how dense it was or something. Anyway, I have had a few that drove me nuts when you could see a ghost line when done. I am thinking if you can't grind the front and back side of the butt weld you are very likely going to see something unless it is under a pile of filler.

    Brian

  13. #13
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    Quote Originally Posted by MARTINSR View Post
    They're not light! A 1966 Vette weighs more than a Chevelle!

    Brian
    I was talking about the new ones with the alum frames ,etc .

    LS6 is always pointing out how much heavier a new Mustang is compared to his Vette .

    Mooch

  14. #14
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    Back before I started glueing everything I used pop-rivets.
    Big mistake, even where I covered with a heavy coat of Duraglass
    and finish filler, later on, after in the sun a summer or two,
    you could see very slightly, every rivet telegragh in the clear coat.
    I didin't even notice it untill I started blocking the clear for a repair,
    then every rivet showed, even with all that filler on top.
    I glue everything now, and that has helped, but still, when a hard transition line
    is covered with filler, I can sometimes find it when loooking at it just right.
    Usually after it's been aged a year or more. It doesn't show the first year or two.It's so slight, and only at certain temps, most never notice it.

  15. #15
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    A new Vette weighs 3200 lbs.
    A Mustang 3600 lbs

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