Mig welding tips & tricks for Sheet Metal - Wed Apr 23 2014 06:12 PM
I had a request for some info on planishing Mig welds, so I'd thought I'd post it here for all. I sometimes find it more difficult to explain something by starting with one of the last parts of a process, so we'll start at the beginning.
When looking at your welds from the back side, you want just a little bit of weld proud coming through the back side. This gives you something on both sides to planish (stretch) with hammer and dolly. If you still see the joint between the two sheets on the back side, it is a cold weld and is likely not flowing well into the parent metal but sitting up high on top. I will add that once you weld a cold weld, you can't fix it by continuing to add weld upon weld. I call this mud dauber syndrome. If the weld is too cold, it needs to be ground out and be re-welded.
Some hints to start with..
Welding sheet metal with a Mig involves what I refer to as "dot" welding. One zap at a time.
Before you even get to welding, this starts with panel fitment, you want gaps as absolutely tight as you can possibly manage. Every time you weld a mig dot, it is going to shrink in a circumferential fashion, pulling from all sides. If you leave a gap, there is a better chance than not that the panel is going to shrink together more as the gaps pull together. On low crown panels such as quarters, door skins, HOODS, this means that as they shrink the gap together, it is removing some of the crown that supports that panel. Planishing out welds is enough work as it is without adding having to compensate for panel movement in the mix. This is a sure fire recipe for a nice big oil can in the middle of the panel. So, tight gaps will eliminate some of this work. The panel will still shrink at the weld, but it will be nothing like you'll see with panel movement.
Welder set up... You should see a full penetration weld after each dot, meaning a slight bit on the back side as well, no panel joint visible. Use practice pieces of same material thickness, but insure you don't lay it on a steel welding table as that will become a heat sink. I like to duplicate conditions that will be present on the vehicle, so the practice piece should be in mid-air with no backing, just like most panels on the truck. So I normally tack practice panels together with a couple tacks and then span it across the jaws of an open vise. This should give you as close as possible what you'll experience when you do start in on the good stuff on the truck. Now undoubtedly you will have a good chance that you'll blow some holes. If you have the heat set high enough for full penetration and are blowing holes, then add just a bit more wire feed speed. If there isn't enough filler going in for the heat applied, it will burn away the parent metal. So try more feed speed before jumping on the "too much heat" chicken little. Once you get a happy medium, if you see your welds are too large, both front and back, try to limit weld sizing with shorter zap times. I'll also add that you should trim the little weld blob that forms at the end of the wire before performing each weld. This gives the machine better continuity for the electrical connection, for a more consistent start, each and every time. Consistency in every part of the process, from fit up to final planishing, gives us a more consistent panel in the end.
Once your panel fitment is perfect, and you're ready to weld in the panel, you need to tack one end, skip a few inches, and tack again, skip same distance, and continue from one end of the weld seam to the other, aligning the surface of the two panels as you go. Many people will tell you to skip around to keep heat buildup down, and I have been one of those. But to demonstrate why this process needs to be clarified, if you were welding a lower door patch (for instance) and tacked either end, you have a much better chance of panel misalignment than if you move progressively across the panel. Panel misalignment will result in dips, buckles, etc. that will not come out. So the more we can do for better panel fitment will reduce the need for excessive filler usage. Continue with the same sequence until the full length of the weld has been "tacked" in place. Then go back and planish each weld dot one at a time, in the same sequence as they were put in. Then use a 3" cutoff wheel to grind down the welds to just above panel surface. This gets them out of the way for planishing the next sets of weld dots. After completing the grinding, weld dots in between the first sets, repeat the planishing and grinding phase. When the weld dots are spaced at about 2" apart or less, revert from welding between the previous to overlapping the weld dots somewhere between 1/3 to 1/2. The overlapping will help to prevent missed spots and help to eliminate pin holes. Weld one cycle, planish, grind, repeat. On a weld seam I like to save the sanding flush for the end. If you sand flush after each individual dot, you will have a better chance of inadvertent sanding of the parent metal to the sides of the weld, resulting in thinning the panel.
Here is a video I did on grinding plug welds, which will be the same process, except for where we stop at the end. A plug weld we go ahead and clean up to the finish, for weld dots as part of a weld seam we leave final clean up to the end, as described above. But this should better show the grinding process I use.
https://www.youtube.com/watch?v=V2WHT_zMOE8
When looking at your welds from the back side, you want just a little bit of weld proud coming through the back side. This gives you something on both sides to planish (stretch) with hammer and dolly. If you still see the joint between the two sheets on the back side, it is a cold weld and is likely not flowing well into the parent metal but sitting up high on top. I will add that once you weld a cold weld, you can't fix it by continuing to add weld upon weld. I call this mud dauber syndrome. If the weld is too cold, it needs to be ground out and be re-welded.
Some hints to start with..
Welding sheet metal with a Mig involves what I refer to as "dot" welding. One zap at a time.
Before you even get to welding, this starts with panel fitment, you want gaps as absolutely tight as you can possibly manage. Every time you weld a mig dot, it is going to shrink in a circumferential fashion, pulling from all sides. If you leave a gap, there is a better chance than not that the panel is going to shrink together more as the gaps pull together. On low crown panels such as quarters, door skins, HOODS, this means that as they shrink the gap together, it is removing some of the crown that supports that panel. Planishing out welds is enough work as it is without adding having to compensate for panel movement in the mix. This is a sure fire recipe for a nice big oil can in the middle of the panel. So, tight gaps will eliminate some of this work. The panel will still shrink at the weld, but it will be nothing like you'll see with panel movement.
Welder set up... You should see a full penetration weld after each dot, meaning a slight bit on the back side as well, no panel joint visible. Use practice pieces of same material thickness, but insure you don't lay it on a steel welding table as that will become a heat sink. I like to duplicate conditions that will be present on the vehicle, so the practice piece should be in mid-air with no backing, just like most panels on the truck. So I normally tack practice panels together with a couple tacks and then span it across the jaws of an open vise. This should give you as close as possible what you'll experience when you do start in on the good stuff on the truck. Now undoubtedly you will have a good chance that you'll blow some holes. If you have the heat set high enough for full penetration and are blowing holes, then add just a bit more wire feed speed. If there isn't enough filler going in for the heat applied, it will burn away the parent metal. So try more feed speed before jumping on the "too much heat" chicken little. Once you get a happy medium, if you see your welds are too large, both front and back, try to limit weld sizing with shorter zap times. I'll also add that you should trim the little weld blob that forms at the end of the wire before performing each weld. This gives the machine better continuity for the electrical connection, for a more consistent start, each and every time. Consistency in every part of the process, from fit up to final planishing, gives us a more consistent panel in the end.
Once your panel fitment is perfect, and you're ready to weld in the panel, you need to tack one end, skip a few inches, and tack again, skip same distance, and continue from one end of the weld seam to the other, aligning the surface of the two panels as you go. Many people will tell you to skip around to keep heat buildup down, and I have been one of those. But to demonstrate why this process needs to be clarified, if you were welding a lower door patch (for instance) and tacked either end, you have a much better chance of panel misalignment than if you move progressively across the panel. Panel misalignment will result in dips, buckles, etc. that will not come out. So the more we can do for better panel fitment will reduce the need for excessive filler usage. Continue with the same sequence until the full length of the weld has been "tacked" in place. Then go back and planish each weld dot one at a time, in the same sequence as they were put in. Then use a 3" cutoff wheel to grind down the welds to just above panel surface. This gets them out of the way for planishing the next sets of weld dots. After completing the grinding, weld dots in between the first sets, repeat the planishing and grinding phase. When the weld dots are spaced at about 2" apart or less, revert from welding between the previous to overlapping the weld dots somewhere between 1/3 to 1/2. The overlapping will help to prevent missed spots and help to eliminate pin holes. Weld one cycle, planish, grind, repeat. On a weld seam I like to save the sanding flush for the end. If you sand flush after each individual dot, you will have a better chance of inadvertent sanding of the parent metal to the sides of the weld, resulting in thinning the panel.
Here is a video I did on grinding plug welds, which will be the same process, except for where we stop at the end. A plug weld we go ahead and clean up to the finish, for weld dots as part of a weld seam we leave final clean up to the end, as described above. But this should better show the grinding process I use.
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