auto body sheet metal welding, butt or lap?

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  #1  
Old 11-08-06, 07:24 PM
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Question auto body sheet metal welding, butt or lap?

I have a '67 Mustang coupe and I'm replacing the pass side quarter-panel skin.
My question is I want to butt weld the the new skin on and need to know what should I set the gap at between the skin and the body ? and how do I hold the gap so the seams are lined up.
The weld line is about 5 feet long along the top of the body line from the tail light to the door jamb.

Some people say I should lap weld but I think because of the contour, the seams wouldn't line up.

What do you think ?
 
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Old 11-09-06, 06:02 AM
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assuming you can access the back of the panel they do make clamps that hold the panel on while you tack weld it in place might check with your auto body store im sure they probably will have them with a gap of about 1/8 inch
there is probably other ways of installing the panel such as lap weld or possibly even panel adhesive but both would require filler to get it to look right where as if you have a good welder and the skills you will not be able to tell a panel was ever replaced with a butt weld.
 
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Old 11-09-06, 06:14 AM
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I've always used a lap joint. If you take and crimp the old metal to make a flange, then the new piece can lay on it flush. When it isn't feasable to clamp it I use a few pop rivets to hold it in place while I braze.
 
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Old 11-09-06, 03:03 PM
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I too use an air operated crimper attachment for my air chisel but I have only used it for flat straight sections I want to piece in.

I also have fenders to replace on my son's '65 Beaumont SD which is the Canadian equivalent of a Chevelle SS.
I held the new fender against the old sheet metal and it fit so well that it would be way less work to weld it as a lap and fill than to do a butt joint.
Not sure what the pro's would do in my and Mackey's case but even with a mig doing a butt would seem to be difficult to weld and have the metal not warp.
 
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Old 11-09-06, 04:59 PM
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Unhappy

Thanks for all of the responses.
I do have a mig welder to use and the fender skin is contoured so I can't use a crimp tool.
My first thought was to do a lap weld then looked into a butt weld and then grind it down but if I screw it up, I'll wreck the job.
I'm OK with the mig welder but not great at it.
Mig welding sheet metal is a real developed skill.

I've done quater-panels with pop rivets before and pleased with the results after I finished the body work was done but my pop rivets days are over.

Still not sure what to do.
 
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Old 11-09-06, 07:07 PM
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Mig welding sheet metal is not all that difficult, especially if you have the extra thickness of a lap joint.

Not sure how your mig is set up but you need to use gas rather that flux core wire.
Thiner wire works better and if you practice on some scrap material of the same thickness you shouldn't have any trouble.
Hardest thing is getting the machine set up right.
 
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Old 11-10-06, 12:34 PM
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Don't they make a special crimper that makes the ideal lap on a panel edge? It simply offsets the first half-inch or so of the edge to tuck under the adjoining panel, making the two main surfaces flush.
 
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Old 11-10-06, 01:27 PM
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Originally Posted by Kestas
Don't they make a special crimper that makes the ideal lap on a panel edge?
Yes they do. Gregg mentioned the air powered one and I have an old manual one - something like a large pliers that creates the flange.
 
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Old 11-10-06, 02:24 PM
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I've been around awhile but my autobody hobby was started after "vintage tools" were the thing to have.

Here is the type I have that attaches to an air chisel:
http://www.sjdiscounttools.com/sgt91625.html

Here is another type with the hammer built in:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=41696
 
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Old 11-10-06, 03:44 PM
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Question

Yea, my mig welder does use sheild gas.
Again, I don't think I can use a crimping tool along a contoured body line. The joint is located along the top of the 1/4 panel and it slopes down.
Have you used a crimp tool on a contoured line before.
 
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Old 11-10-06, 04:24 PM
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No, the crimper would make a mess of sharp contours.

I am going to lay my panel on top of the existing fender and use a lap to join it.
For me I would have better success doing it this way and feathering in the joint with filler.
I know this is not how the pros would do it but some of my projects done this way have lasted a very long time.
You just have to make sure if you do it like this that you seal the back side to prevent water from getting in.
 
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Old 11-10-06, 08:46 PM
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That's what concerns me, cracking and corrosion at the lap and that's why I started to consider a butt weld.
The drivers's side 1/4 panel had been repaced before & lap welded and it was badly craked. But it was like that long before I got the car so I don't know who or how well the job was done. I eventually replaced the full 1/4 panel & inner & outer wheel housings.
The full 1/4 panel goes up into the roof panel so no obvious joints to repair and that joint is short & crimped at the roof and leaded in.
But the passenger side looked pretty good until I started poking around and discovered alot of rust so here I am trying to figure out what is the best method with this side. Oh boy.
 
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