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I think learning how to weld sheet metal properly is one of the most important things to learn.
Until recently acquiring a tig welder, I had to mig weld my panels because the mig was the only welder I had. A mig weld is a much more brittle weld and over hammering will lead into a cracked weld. However it is possible to produce a nice piece using your mig welder.
Here is a fender I mig welded.
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I think if you are using a mig it is key to have the panel fitting perfectly first, to reduce the need of extensive hammering. Don't ask me how I know .
I will show you the method I used for welding the fender,on some scrap pieces of metal.
I start off by tacking in 3/4 inch increments.
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I start in the center of the panel and work outward alternating sides with each tack. After every two or three tacks,I grind down the proud of the weld,using a cut-off wheel.
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Next I will re-stretch the HAZ of the weld (heat effected zone) This is the blued area around the weld. All I do is using the dolly on method I give each tack a few light hits with my body hammer. On dolly stretches metal off dolly shrinks metal.
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Now that I have the panel all tacked in place, I start filling in between the 3/4 inch tacks.
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After I fill in the weld I grind down the proud of the weld and re-stretch the Haz of the weld the same as I have shown with the tacks.When welding between the tacks I still start in the center and alternate sides.
When welding in I place a tack let off of the stinger then I lay another tack 1/2 on the previous tack and so on and so on.When placing each tack you have to almost burn through before letting off of the stinger.This is to insure a full penetration weld.This will take some getting used to.Here is a picture of the back side of the weld.
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Here is a picture of the panel welded up.
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I also welded a panel up using the same principals except I did not re-stretch the haz of the weld.Here is the result.
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I think it is pretty obvious which method works better.
This is just the method I found to work the best for me, I would love to see other methods if you do it differently.
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Johnny Arial