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Hi Guys
I have been asked a few times about Butt Welds and let me clarify some of what I have posted before.
OK to start with when butt welding the tighter the gap the better success you will have with the finnish work. At one time (not to long ago) I believed if you had some gap it would relieve some of the warpage from the heat of the weld, when actually caused more damage (warpage) to the panel. The reason for this I believe is the weld is allowed to shrink more with less resistance from the adjoining panels and allow more travel area.
The other area that I would like to refresh on is the (butt weld clamp) for hold the panels together while welding. The clamp will also cause a gap which I have an easy remedy for, when you have decided were the clamps will be placed during your dry run or trial fit, you can mark there location and width with a Sharpie. When there off you can grind a slot in the panel were you marked. the amount of the width and thickness of the clamp. This will allow the panel to be clamped and still maintain a perfect panel to panel fit.

This will also help with burning threw, which is easy when there is a larger gap between the two adjoining panels. Now its not impossible to weld on the edge of a panel with a wider gap, you just need to lower your heat on welder and keep the spot welds short and quick without sacrificing proper penetration on each edge. This takes practice and its a good idea to try a test panel to get your heat and speed down before attempting the actual panel you are looking to join.
 

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I never did understand the desire to add a gap to a butt weld. You do get full penetration if you do that, but if you have the welder set up correctly you'll get full penetration anyhow. You are right on about the shrinkage. Shrinkage is what causes warpage, not just heat. As you weld, the edges of the metal expand and finally melt into the weld bead. That melting relieves the stresses induced by the thermal expansion. But then when the bead and surrounding metal cools, it shrinks smaller than it was before. To realize this, take a sheet of metal and weld along one edge. You'll see it pucker up as the weld shrinks. Or take a round disc of metal, and weld all the way around the edge. It will turn into a shallow "bowl". The way to return it to the original shape is to stretch the weld. If that makes sense, it will help you visualize what's going on when you weld a flat replacement panel. Don't try to shrink a bulge....stretch the weld!

For the best welds, you should fit them fairly tightly, set the welder to get full penetration, then weld spot welds every few inches. Then make spot welds between them, and keep doing that until I have welds about every 1"-1.5". Then I start running beads and usually make 1-1.5" beads using a pulsing technique to avoid blowing through. These pulses may be 1/4" long welds, then when the "red" goes away I hit it again. I place these beads scattered along the seam while the others cool, and try to hammer the welds some while they are hot to stretch them back out. At this point you might see some warpage beginning to show due to shrinkage. I then grind the weld bead down a little, then hammer the seam again until the warpage disappears. Then I go back and extend the beads, again hammering and grinding. Don't over-hammer the bead or you'll stretch it, causing more problems. Watch the surrounding metal and the "bulges" and you'll know when to stop. Also, be sure the metal is cool when you're looking for bulges and deciding how much to hammer for final finishing.

For panel alignment, I just make a small v-notch on each panel edge, then install a small screw or a 1/8" or 5/32" Cleco with a washer on each side of the panels. That holds the panels in alignment, it's cheap, and it's easy to fill the small hole made by the screw or Cleco after the rest of the panel is welded.

If you keep the warpage under control from the beginning, and throughout the welding of the entire seam, you will save yourself a lot of work and aggravation trying to get it straight after it's all wavy. Hope that helps!:D
 

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Does anyone have any pics of a panel repair before the weld is ground down? I am practicing with a MIG on scrap 18Ga, and can't seem to get a bead down. I can connect the dots with tack welds, but I don't want to do that with my '55 . Any tips or pics would be great, as is the detailed instructions above.
On a similar note, some prefer oxy-acetylene welding others MIG for sheet metal. I have heard both camps claim that there is less distortion, I know there are a few guys on this forum who do a lot of welding of auto panels, which is the preferred method?
 

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First, I agree with the above discussion on gaps.

There's nothing wrong with connecting the dots. But, once you get down to 3/8"-1/2" between tacks, you can weld between them continuously. Try to master the pulsing technique that Chevynut posted about above. And since you'll be putting more heat in the panel with these, it's important to let the welds cool before doing more.

While oxy-acetylene welding works fine if you have the skill, I think that the mig is better for the novice. It's too easy to put too much heat in the panel with the torch unless you've developed the skill.
 

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a lap joint is always the best if you can arrive at that. However if you cant ,and must butt the two it will be fine as long as you are hot. If you have the chance to mig weld any panels you should always run them down hill. running down hill welds arent as hot as pouring the coals to it in the flat position. If you must weld in the flat position clamp a thick peice of alluminum behind what ever it is youre welding and it will **** the heat out of youre steel panels and will not bow and warp as bad. I hope this helps. Oh yeah one more thing if you mig the panels use .023 wire because the weld puddle will be less wide and therefore you will have less heat. James
 

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I will try the vertical instead of the flat I have been playing with on the MIG, and keep trying to refine the 1/4" pulse. I have .025 in my Lincoln, would switching to .023 help? (if so, I'm there)
I was over at a buddy's house Monday playing around with his TIG (Miller Syncrowave 350), after doing some stainless fusion welding this got me back to considering a torch for the sheet metal. I am certainly a novice at all welding, so I will need to practice up on iether method just want to do what is right for my repair work. I have a suspicion the 12"x12" practice metal I am using will have less of a tendancy to warp than the quarter patches I need to put in, let me know if I am wrong on this. The gas weld looked much better than the MIG and I had good penetration, but again I am a novice.
I appreciate the good info people are sharing. :tu
 

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as a general rule the smaller diameter the wire the less heat it takes to melt it therefore on thin metal you will be a whole lot better off using 023 wire but you still need to use shielding gas 75 argon with25 co2 a tig will get too hot if you are les experienced although the syncrowave is a great machine!! miller in my opinion is by far the best but for sheet metal work on a car body stay with the mig process. james
 

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While a lap joint may be better for structural welding it is not better for sheetmetal repairs. The lap joint will increase the stiffness of the joint area tremendously so if there is a high area or a low area after welding (and they will be there), it is very difficult to straighten it with a hammer and dolly, or any other means. The other problem is that the backside has a crevice for potential water and dirt entrapment which could result in future rust problems.

Tig welding is an alternative, though just like using a torch, it requires a higher skill level. One problem with tig welding is that it's difficult to coordinate your body, the torch, the filler, and the foot pedal or finger control unless you have a lot of experience. Relatively easy on the workbench, relatively hard on the car itself. Tig is also less forgiving than mig to impurities like surface rust, which you may have on the backside of the metal you're patching.
 

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rick you are right about the structural ways of thinking but if you butt 2 peices of metal together like on a quarter panel or something and then grind it smooth theres nothing there to hold it together anymore if you seam seal the back of the lap joint after you are finished i dont see how that would be bad at all
 

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I thought when you have proper penetration, the weld and panel material will basically melt together. So, with a butt weld you do have good structural integrity. Correct me if I'm wrong.
TJ
 

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rick you are right about the structural ways of thinking but if you butt 2 peices of metal together like on a quarter panel or something and then grind it smooth theres nothing there to hold it together anymore if you seam seal the back of the lap joint after you are finished i dont see how that would be bad at all
I have made butt welds and ground both sides, and you can't tell the resulting piece from a sheet of steel. You have to have good penetration, not just a surface bead. That's the "right" way of fixing panels, IMO.
 

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i have replaced a lot of panels and only use butt weld . i have seen cars with lap welds and on a hot day the body looks like a jigsaw puzel ,you can see every seam.
 

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I can..

i have replaced a lot of panels and only use butt weld . i have seen cars with lap welds and on a hot day the body looks like a jigsaw puzel ,you can see every seam.
"identify" w/ that statement!!
Many yrs ago, we added a steel, Z28 hood scoop, to a 55 hood. Used the "lap" method.. NOT nice when the hood got heated up!!:eek:
 

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"identify" w/ that statement!!
Many yrs ago, we added a steel, Z28 hood scoop, to a 55 hood. Used the "lap" method.. NOT nice when the hood got heated up!!:eek:
you have 2 thickness's of metal plus weld.. heat makes the metal move , the 2 thickness move at different rates ..put a lap joint in hot sun you will see it.
 

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Thanks for the insight on the lap weld, I was already sold on butt welds, but it is good to have an example that can be seen. For instance some may discount the higher possibility of rust with the fact that they use a weld through primer and then seal the back side, etc.
 

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Thanks for the insight on the lap weld, I was already sold on butt welds, but it is good to have an example that can be seen. For instance some may discount the higher possibility of rust with the fact that they use a weld through primer and then seal the back side, etc.
If you are truly trying to make an invisible weld tig is the best way to go
because you can hammer weld and finish the weld until it disappears, you can't do this as well with a mig weld its too hard to hammer weld it. The hammer welding is what removes the shrinkage and stretches the panels back flat.
And all welds will have less warpage with no gap, filling a gap creates more heat which means warpage. Gene:):)
 

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what is the technique for hammering the welds? is there a particular dolly or dollys that should be used? what should one watch for while hammering the welds?
 
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