Help with welding sheet metal please!

M

mitch_04

Learnin'
I go back and forth, one day my sheet metal repairs go nice and smooth, the next day they sink at the weld and I feel like throwing things, heavy things. My process is this...

1. Lay new metal over old metal, clamp or screw in place.
2. Cut through both layers of metal to ensure cuts match
3. Remove old metal, clamp new metal with vice grips, panel clamps, whatever fits
4. Tack a couple tacks just to hold it in place
5. Go through and bump metal until it lines up, then tack.
6. Put a tack every 2 or 3 inches using above step.
7. Put tacks in between previous tacks, repeat until I have 1/4" lengths to finish weld.
8. Finish weld 1/4" lengths.
9. Smooth large pieces of weld with edge of cut-off wheel.
10. Finish smooth with 36 grit roloc disc.

The welds sink in, in some spots, up to 1/8". I try to bump them out as much as possible, but welds don't seem to like to move. I tried to get a good pic, after bumping, but it doesn't really show it well.







Any helpful ideas?
 
All welds including the tacks shrink. The shrink has to be controlled by stretching using hammer on dolly on the welds and heat effected zone adjacent to welds. Best done as you go to keep the panels under control. That is what I think may be going on from your description of the process.
Access to the back of the weld is essential and some times difficult.
 
Like above, planish your welds with a dolly and hammer to stretch them back out as you go, as you strike an arc the steel gets hot and stretches then as it cools it draws in on itself.
 
3x ^^^^^

Did you ever notice how all your gaps start to change after a couple tacks? Even after just one tack, you will notice the gaps changing. As soon as the gaps change, you know you have shrinkage. Planish the weld area until your gaps return to normal.

As for your door, as long as you have access to the inside, there shouldn't be too much reason to get excited. You will possibly need a helper to hold your dolly on the outside so you can stretch the shrink back out from the inside. The key will be to work the entire length of the weld evenly, slowly raising the whole area rather than beating one spot out to where it is close and then moving down. The shrink happened evenly, a little at a time over that entire length - so this is how you should stretch it.

Overall, nice work there! The shrink just got away from you. Everyone else learned the way you just did...
 
on a great day welding sheetmetal sucks . it is time consuming and a real pia unless you do it daily. just stay after it .
 
Oh what fun! I'm a rookie at this and often have trouble as well. Sometimes I can't tell when I need to shrink or stretch. Your weld probably wont bump out because like others said, you need to stretch the weld.

In areas where I can't push hard enough with the dolly, I end up sinking the weld even more because I'm stretching it the wrong way. Sometimes I hammer from the back if I can, on dolly so the force of the hammer pushes the weld back out.

It is time consuming, but then grind and planish as you go to keep everything in shape. I get about 2-3 tacks before I grind and planish. After tacked in, I weld about 1/4 inch at a time. After I get about 3/4-1", then I grind and planish back to shape. As I get better, I can maybe get away with welding longer sections at a time. But for now, if I go more than an inch or two, it gets to a point I cant recover from. I read on another forum to plan on 12" per hour to weld and planish. I spent 8 hours yesterday doing a 24" patch. ;)

Good luck!
 
So I should hammer directly on the weld with a dolly behind it as I am tacking the panel, as in after every tack? Or can I put a tack every 3 or 4 inches for the length of the panel, then come back and planish the welds I just did?

I had thought, as I was going along, that it didn't seem to be sinking too much. I was wrong.

Can the panel be too tightly butted? I have read before that you should have a small gap, like .030, and read elsewhere to go as tight as possible. In my experience tighter has seemed to work better, but I don't have a lot of experience either.
 
So I should hammer directly on the weld with a dolly behind it as I am tacking the panel, as in after every tack? Or can I put a tack every 3 or 4 inches for the length of the panel, then come back and planish the welds I just did?

I had thought, as I was going along, that it didn't seem to be sinking too much. I was wrong.

Can the panel be too tightly butted? I have read before that you should have a small gap, like .030, and read elsewhere to go as tight as possible. In my experience tighter has seemed to work better, but I don't have a lot of experience either.
 
mitch_04;35987 said:
I go back and forth, one day my sheet metal repairs go nice and smooth, the next day they sink at the weld and I feel like throwing things, heavy things.

The welds sink in, in some spots, up to 1/8". I try to bump them out as much as possible, but welds don't seem to like to move. I tried to get a good pic, after bumping, but it doesn't really show it well.

Any helpful ideas?
Click to expand...

Like the other guys have said, you have to stretch the welds to get them to move.
It makes a big difference where you decide to make the seam when welding. Its usually better to make the seam above the body line if possible, because the bigger and flatter the panel, the more shrinking and distortion you are going to get.

One of your good welding days was probably doing this. And you can see that there isn't much the metal can do, unless you really get it hot





This one has more crown to it, and that takes more abuse to make it sink.











You got more distortion on the front fender, but fortunately the lower part of the panels that are most likely to rust usually have a moderate crown to them, like your front fender.





It also looks like you're getting the metal too hot for mig welding, judging by the heat affected zone here, but some of the welders here would be able to analyze your welds a lot better than I can.


6c14b7a5-7534-4d84-8ba6-744d3912708d_zps725a3835.jpg
 
Zero seems to be the magic gap for myself (when possible) a gap causes sputtering and a need for too much filler material, your plan of welding the length in spots, I go about 6 inches apart, then planish is a good one, on dolly directly on the weld. Then make your next pass splitting the difference, and repeat. If you do a bunch of passes across the panel the. Try to planish your second round of weld are sucked in from the first, and the third round from the second..... Just be patient, after my quarters were fitted and trimmed and clamped up(never too many clamps, I like clecos and self tapping screws where applicable) I spent about a day on each welding them in to get them on the gnats ass, and it still took a dab of filler and some 2k.

- - - Updated - - -

Also forgot to add, floorpans and trunkpans and such are a great place to start with sheet metal welding. They end up hidden and take a little stress from working out your technique , also like shine said its a pain it the butt, even when I put it down for a few weeks I have to give myself a refresher.....
 
The picture with the large heat affected zone is a little misleading without knowing the story. The used fender arch that was supposedly good was much worse once I received it, I wasn't too happy. I took my copper tool, laid it behind it, and filled in the rusted out portions of the used arch. I welded from the backside, once smoothed I actually didn't have any problems.

The first picture was a piece that sets underneath the rear 1/4, just for structure. I was not worried at all about looks, just replacing structure.

The other pics (front fender patches behind wheel) went mostly well except the front of the passenger side one, I didn't have the metal as straight as it should have been before I welded. That's what being a rush gets you. However, I plan on slicing that section and bringing it back when I pull the fender to repair some more rust behind the fender.

Normally my HAZ is maybe the same size as the weld, on either side of the weld. I watch a lot of the builds that places like Ironworks, completely sectioning bodies, doing amazing custom body mods, and you can never see where a weld was. That's my inspiration and goal.
 
I have old eyes and they say things aren't quite as described.(maybe I am just blind )
It appears to me you are doing a lap weld and not a butt weld.
The outer piece of metal is going to move towards the rear metal flange when it is welded. The weld shrinks and pulls it toward the more rigid flanged panel.
I hate flange panels for that very reason. Unless the flange offset is exactly the same as the thickness outer piece it will happen every time.
My advice, get rid of the flange, do butt welds, hammer each weld as you go so that the original gap remains as you work .
 
butt weld with no gap is the best as it leaves less room for the weld to shrink and pull. When you're cutting through two panels at the same time for a perfectly aligned seam use an airsaw and hold it on a 45* angle if possible and your gap will be smaller. The larger the weld the more heat will be transfered and the more shrinkage and warpage will occur. You need a weld sized just enough to get good penetration showing on the backside. Holding a piece of copper on the backside will also cut down on warpage as the copper soaks up heat.
 
I am doing butt welds, I haven't done any flange welds. I believe what you are seeing is the "belt line" where a chrome strip goes through the body.

Thanks for the advice everyone!
 
Great discussion. I wish I had read it about 10 years ago. By trial and error I kinda figured out the no gap works best, but it's very hard for me to achieve. I guess I need better trimming tools and or skills, probably the skills, lol.

Does the rate of cooling of the steel affect it's shrinkage? Sometimes I blow off the panel after each tack, but I'm not sure if I'm helping anything, making things worse, or just wasting time and electricity to the compressor?
 
I do believe the rate of cooling affects shrinkage, but not because of bodywork experience. When I was growing up we had cheap pots and pans, one time I took one off the stove when I was done and rinsed it with cold water. It warped the hell out of the pan! However, the pans were just fine when left to air cool. Maybe it doesn't translate to bodywork, but I don't want to find out the hard way either!

I have heard of blowing the tacks cool before, but I do not know how effective it is.
 
The only time I cool a weld is when I want it to shrink. Before I bought a shrinker disc it worked kinda handy to shrink stretched metal(and I'm piss poor with a shrinking hammer and hate the way they look..)
 
jtfx6552;36097 said:
Does the rate of cooling of the steel affect it's shrinkage? Sometimes I blow off the panel after each tack, but I'm not sure if I'm helping anything, making things worse, or just wasting time and electricity to the compressor?
Click to expand...

Yep. I was taught to, and did for a long time, sit there with a bucket of water and quench the weld with each stitch by hitting it with a wrung out rag. So if i was welding in a patch, I would do a 3/8" stitch, then quench. Over and over. I was told this would keep the heat down in the panel, and reduce warpage. I've even seen it shown and discussed on many of these car rebuilding TV shows..
However, one day I went to go shave a marker light on a vehicle that was getting alot of shaving, and got all set up.. Except I couldn't find a clean rag. And it did irritate me to have to leave my gloves off in order to not ruin my gloves with the wet rag.. So I said "Screw it" and just went at it with no quenching..
Result, it worked BETTER. I think for two reason, one because although the heat by itself makes the area grow, then the weld itself causes it to locally shrink resulting in some bigger areas and some shrunken areas we know as warpage, the added quenching causes the area to grow then shrink even more, pulling in material in all directions. And you do this over and over and over again..
Where as if you don't cool it between stitches, even though the panel may rise to a higher overall temperature, it is not constantly expanding and shrinking.
You are also not constantly starting each weld with a cold joint. Once the area gets nice and warm at like 400 degrees, the welds deposit so much faster, that doing the half inch stitch requires less time on the power, meaning less heat input per inch of weld, which leads to less distortion.

Between letting them air cool and having a stud gun with the shrinker tip, you can get just about any level of warpage and oil canning out even if you dont have access to the backside of the panel to work it with a hammer and dolly.
 
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