Need help on welding sheet-metal...

I think I am slowly learning that this is the case. The small test pieces want to "oil-can" as I think it is referred too (kind of like the metal has some back and forth "spring" in it) even after the initial few spot weld tacks. I did tacks on the 2 long sides of the piece below, and even though everything was initially lined up perfectly and fit very snug, the side closest to the bottom of the picture, as you can see, started oil-canning on the outside of the patch and wouldn't line up. No amount of planishing on the other weld dots seemed to straighten anything back out....I actually had to planish the area outside of the patch where it was oil-canning to get it to line back up. View attachment 10766


I responded to this question previously, but only responded with a "fix it like this". I was remiss, as someone will always retain a process better if you explain the WHY IT HAPPENED and not just DO IT THIS WAY. So to that end, looking at the picture below, take note of the red arrows. Here we have an interior patch that is going to have weld dots around the perimeter. When each of these weld dots cools off, they shrink, pulling at the surrounding metal. So the excess metal you see at the lower end in the form of a pucker is due to the sides being pulled toward the center of the patch. On a car, this would happen in a similar way, but more than likely the crown would be pulled downward as well. We restore the original "flatness" of this defect by planishing all of the tacks until the pucker relaxes to it's previous state. Then grind them flush, and weld the next set of tacks in place, and repeat all over again; planish, grind, weld, repeat.


So that is the "why" the pucker showed up, the sides were pulled inward due to shrinking effects.



IMG_3341.JPG
 
This is why, as beginners, I previously wrote to reopen the gaps when you are tacking. If you start with the 4 corners and two in the middle, you will see the gap shrinking between the panels. When I go back in with a thin cutoff wheel, like .035 I can get at mcmaster and recreate the space that was there, the puckering goes away. Once you get technique down, it most probably will stop happening, but to get started, it helps to retain a gap for as many tacks as you can.
 
If a gap is there, and another is cut/opened up as the last one closes, there is nothing to keep the panels from continually pulling together. Major difference is you may not see it in a tell tale sign like the distortion that no-gap butt welding shows, but it is pulling together just the same. For the sample shown, with the end left un-tacked, continual opening of a gap on the sides only allows more pull and you will see more pucker as the panel pulls closer with each gap opened. Think this way, if you must continually open up a gap because things are getting tight, then it is obviously ALSO continually pulling together. On a crowned panel especially, you will see a loss of crown and it will result in a low spot. This is the benefit of an absolutely tight panel, you lose the least amount of crown during the welding process.

I think you are referring to the secondary distortion next to a weld that you get with the no gap weld. Basically when everything is nice and tight, the heating and expansion part of the weld process will cause the two panels to push against each other while the metal is in a more fluid state, causing some distortion. Then as it cools it shrinks and pulls, but some of that distortion remains, requiring some planishing to correct both the shrinking and weld distortion. At the end of the day, in respect to metalwork, removing this minor distortion is minimal compared to how much planishing would need to be done to restore the crown to the low area that forms in using the .035 gap theory.

Disclaimer: All of my ramblings here assume we are working toward a goal of as little filler as possible. If a person is less concerned about the amount of filler it takes to restore the crown, then adding a few low spots throughout your welding process MAY not be that much an issue. But in panels like hoods, trunk lids, and roof skins, that continual crown shape is what helps support the entire panel free from oil cans. Any low spot may serve as an interruption to that support.
 
Last edited:
when there are enough welds, it stops pulling together to the point of massive warpage.

I am not saying this is the right way to do it, I know first panel we did closed up and we just kept welding. There was alot of grinding, grinding thru weld since the panels were on top of eachother and there was no place left for the weld to go.

Dont have anything fancy, but panel ]v[ panel with v as filler stays flatter and wont separate when ground flat. When its ][ the novice grinds and panel separates.

I consider myself a novice.
 
when there are enough welds, it stops pulling together to the point of massive warpage.

I am not saying this is the right way to do it, I know first panel we did closed up and we just kept welding. There was alot of grinding, grinding thru weld since the panels were on top of eachother and there was no place left for the weld to go.

Dont have anything fancy, but panel ]v[ panel with v as filler stays flatter and wont separate when ground flat. When its ][ the novice grinds and panel separates.

I consider myself a novice.
To be fair, we are all novices. I try to learn something new with each new job.. If you're not learning, you're sitting still. (why do I hear that in a Ricky Bobby voice?)

First off, it doesn't sound as if you are planishing the welds. If that is the case, it would explain the massive warpage. If you are seeing a separation of the panel (visible joint) after grinding with the panels tight together, then your heat setting is too low. You should have a full penetration weld with each individual weld dot so you have weld proud on both sides, this lets you planish that weld dot by it's lonesome without affecting the rest of the panel. As pointed out elsewhere in this thread (or was it Chris' thread?) a hotter weld will typically get you a flatter weld for less grinding and less pin holes, and controlling the heat the panel actually sees by limiting trigger pull duration, you will actually have less HAZ showing. Just filling in a gap void may take less heat, may give less immediately visible distortion, but the biggie is that it also takes shape away from the panel as it closes tighter, you lose crown. On hoods, roof, etc, that equals oil canning in most cases.. Hey, I get it, a dozen different ways to get to town. I've even modified the way I initially started this "dot welding" method to overcome some issues that arose. But in the grand scheme of things, I respond to countless threads a year on here and other car forums where someone's MIG welding efforts have caused an oil can or a _________ (fill in the blank). In 99% of the cases, following what I have written in this thread resolves it. The process addresses the weld from start to finish, which begins at panel fitment and ends at planishing. It's all part of the weld process. Only focusing on the little bit of distortion that a tight butt weld may give is ignoring all the rest of the concerns, and is a recipe for oil cans.

If what you're doing is working for you, then that's all that matters. But I can't see how you can do that without losing crown or causing a low area. If it's in an area that doesn't rely as heavily on crown to hold the shape, then perhaps oil canning wont be an issue for you. I would also offer, if you're ever in the Southern MD area and want to do some weld practice to try it out firsthand, let me know, I'll be glad to help out. Afterward we can try TIG or even gas fusion welding and show you how enjoyable it is to do less grinding.
 
my son prefers tig now, to the point of too much. My eyes, each process has its place. You do not have to tig weld the jig we are making to hold up the fender or the workbench. Not even the rotiseree, although weld nuts are better tigged than migged. Welding a quarterpanel skin or wheel arch does not allow the block and hammer planishing approach. Even roof skins or hoods you cannot get behind all areas, mr thread with the firewall cannot get behind to planish properly without someone holding the dolly inside. So you can probably see we are "I'll take care of this myself" people so we need to alter the way that is considered more correct.
 
If he is getting more proficient using TIG welding, then you may have better luck with that. I will say, the majority of shrinking seen in welding is where you start and stop, so using a continuous weld (easier with TIG, near impossible using MIG when welding sheet metal) will give a gradual heating up and also gradual cooling down as you go across the panel for less shrinking. The shrinking is one of the major detractors of start-stop welding and why it's so important to planish afterward.

Food for thought, Regardless of process used, MIG, TIG or O/A, you'll likely see best results in tacking something in before attempting to fully weld. To show how clean a patch can be welded using TIG, nice tight butt joints will allow you to tack like this, then a no filler fusion weld will have absolutely the least amount of shrinking.

Fusion tacking, note the absence of color in HAZ...


On this one, fusion welded after tacking.

 
Back
Top