But welding sheet metal

I've been working on a sheetmetal project where I'm making my hood into a cowl hood. This means I have to add some thin strips of metal to the existing hood. I cut and sanded the first part so it had zero gap when I butted it up to the edge of the hood. Then I started tacking it with the tig. By the third tack the metal started pull so badly that it overlapped. Next piece I put in I will add some shim stock spacers about 10thousands thick then the tacks should pull it tight. Anybody else experience this?

Yes, with any metal, but more pronounced with sheet metal, heating and metal shrinkage is something you have to work with as you fit pieces. It takes practice and skill to pull it off. I can't say I'm that good at it, but those that do autobody sure need to be. Generally you have two pieces that have a thin pie gap that closes up as the two parts are tacked together. There are different techniques to control shrinkage during welding. Some take shrinkage into account when making the parts, others go for the death by a million tacks and try to hold everything in place before welding. Skip stitch and or back step welding can help, too. Careful shrinking and stretching after the weld can take care of minor distortions. You can find all kinda of hot rod pros on YouTube that do crazy sheet metal manipulations, and all without any body filler!

When I am welding in sheet metal I try to fit the piece close to where it is going to go and then just work it into place with a body hammer as I tack around the edge. Once I get tacks all around the piece, I will place a couple tacks together and just move around the piece trying to control the heat to avoid warping it.

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Originally Posted by Rambozo

?... You can find all kinda of hot rod pros on YouTube that do crazy sheet metal manipulations, and all without any body filler!

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Those guys are the real master-crafters. I remember being horribly disappointed years ago when a certain custom bike show came on TV. A lot of the metal work was roughly mig'd together and the paint & body guys did all of the beauty work with filler and lots of sanding.

I'm getting pretty good with the body dolly wet but I may need to buy a metal shrinking hammer/dolly. Finished welding the first seam of 6 pieces today. I'm going to grind it smooth, do some dolly work and see how much better I need to be on the next seam.

I am getting better at tig welding sheet metal, but I have had a bit of practice from welding sheet metal in my trade for starters. The bigger sections of thin sheet are what I have trouble with still. I really get a kick out of watching my dad gas weld sheet metal when doing rust repairs on cars. Of course he's been doing it since the 1960's, and is more comfortable gas welding and using the torch to shrink metal than welding with any other process, since it comes more natural for him. MIG was just something that he had to learn for his trade in later years in order to keep up with technology, and he's good at it, but still better with oxy/acetylene. I think he will do good once I teach him the basics of tig welding. His main issue now is poor eyesight.

In the sheet metal trade I got used to welding and metal finishing everything nicely since it's usually exposed stainless (indoors), galvanized (outdoors/rooftops), or copper (outdoors). Especially the food service type items. No bondo allowed (haha). I always make sure to put the smallest tacks possible (get in and get out ASAP to minimize heat input), use as many vise grip clamps as I can (or use cleco's), and use a backing of some nature (usually copper) when possible. Fit up is everything. And of course, the tungsten needs to be sharp, and the filler rod needs to be small (diameter). The end result is what's important, and everyone does things a little different. You just have to figure out what works for you.

Put in my second piece today. Left a small gap and let the weld close it. It worked much better. I also put a lot more tacks and use compressed air to cool the panel after every 1/2" of weld. This project is going to take a lot longer than I thought!

Sounds like you've devised a solution. Please post some pictures of the work if you can!

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Originally Posted by Ron66

Put in my second piece today. Left a small gap and let the weld close it. It worked much better. I also put a lot more tacks and use compressed air to cool the panel after every 1/2" of weld. This project is going to take a lot longer than I thought!

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I forgot to mention, I usually leave at least a 1/16 gap around the edges. Moving around the piece if large enough will allow you to keep working without putting too much heat into one area causing warpage.



@youngnstudly
I thought all stainless or anything for that matter had to be back purged in the food industry for compliance.

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Originally Posted by Jason

@youngnstudly
I thought all stainless or anything for that matter had to be back purged in the food industry for compliance.

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Sorry for the confusion, I didn't mention anything about stainless in particular since I figured Ron66 wasn't building a cowl hood from stainless that he planned to eat off of:D. Plus I was speaking more of the metal finishing with the stainless. You are correct though, for the most part. However, you'd be surprised how many smaller HVAC shops in my trade don't back purge when welding stainless material for the food industry. Especially when doing quick repairs on already built items, or small jobs that the tight-wad restaurant owner wants the shop to build fast and cheap. If they think they can get away with not doing it, they'll try to. Yet another reason to eat at home!:O

Attachment 10765
I found that these work pretty good. They leave just a small gap.
I got mine at Harbor Freight.

Some pics of my progress
Attachment 10766
Attachment 10767

I got the same at Princess Auto the same as in Harbor Freight in Canada we have Princess Auto and the USA have Harbor Freight.
Attachment 10768Attachment 10769

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Originally Posted by Gman

Attachment 10765
I found that these work pretty good. They leave just a small gap.
I got mine at Harbor Freight.

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I have seen those on eastwood andhave wondered if they make things easier. Usually I just fit the piece and then use my triangle magnets to hold the piece while I tack it in place. Next time I am HF I might have to pick some of these up if they are not too expensive.

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Originally Posted by Jason

I have seen those on eastwood andhave wondered if they make things easier. Usually I just fit the piece and then use my triangle magnets to hold the piece while I tack it in place. Next time I am HF I might have to pick some of these up if they are not too expensive.

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Magnets work but with Tig they do cause problems but still looking for aluminum Magnets :O but no luck have to use the sheet metal clamps or vice grips.

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Originally Posted by Jason

I have seen those on eastwood andhave wondered if they make things easier. Usually I just fit the piece and then use my triangle magnets to hold the piece while I tack it in place. Next time I am HF I might have to pick some of these up if they are not too expensive.

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I use my triangle magnets for tacking up too. I do have the issues that Kempy mentioned sometimes with them, but there's no other way around it in certain situations. It's darn near impossible to tack something up that has a gap of any nature, while trying to use one hand to hold the part in place (say with pliers). And if you can't get to the backside of the part you're welding, vise grips are rendered useless. Cleco's can be used successfully in particular instances.

I guess I have never run into the issue with the magnets and TIG. I so also have clamps that I use some times and the magnets do not work for aluminum so I either clamp that or find some way to hold it whether its proping something against it or trying to hold it by other means while I tack it.

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Originally Posted by Kempy

Magnets work but with Tig they do cause problems but still looking for aluminum Magnets :O but no luck have to use the sheet metal clamps or vice grips.

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Will have to google that. Alumimum magnets??

Also, the magnets can cause problems with MIG as well. They gets us on occasion but rare.

What are these problems you guys speak of? I have never noticed any problems using them.

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Originally Posted by Jason

What are these problems you guys speak of? I have never noticed any problems using them.

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A strong magnet can and will attract the actual arc. So often the arc will wader away from where you are trying to point it. Similar to how a static electric charge can bend a stream of water.

To minimize warping/distortion when TIG welding sheetmetal, use techniques to minimize heat input. That means:
1. strive for as tight of a fit-up as possible (if tight enough so that autogenous welding can work, you can *really* minimize heat input),
2. use pulsing settings on the arc (either low or high speed pulsing can be helpful, but go with "dramatic" pulse settings - low background current/ratio, and low pulse duty cycle, relatively high peak current - to get the most benefit),
3. if back side is accessible have an assistant follow along with a copper spoon tool, (or perhaps rig something up clamped to the back side to suck the heat out,) etc. (A copper spoon tool also works *great* for filling holes!)
4. Use the right set-up for welding thin sheetmetal - adequately small diameter filler rod/wire, adequately small tungsten to crisply start and maintain the low welding current, short arc length, and weld with only as much amps as you need, trying to get a narrow bead width.
5. Do tack welds, stitch weld between the tacks, back step, etc. Don't let the heat affected zone grow too wide (on steel you can judge this by how much of the metal is "glowing".

If while tack welding you find the metal shrunk and pulled out of alignment (e.g., pieces overlapping) and if you have a hammer and dolly and accessible back side, consider hammering "on-dolly" on the tack that shrunk too much, to get it to expand and "undo" the shrinkage. This may not be all that hard to do, although planning to work with some shrinkage (e.g., that a joint will "zipper" and pull together during tack welding) is another OK approach. If you minimize the size of your tacks (make smaller diameter tacks), it will reduce the shrinkage.

It does take time to butt weld a long, thin sheetmetal joint with minimal warping with TIG, but doing it this way, you'll get much better penetration (and a strong weld) than with usual MIG techniques. MIG is another option if the penetration and strength don't matter as much and you prefer to get the job done faster.

Here's a related post:
http://www.everlastgenerators.com/fo...1333#post21333