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Old 10-24-2006, 08:00 AM
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Which way should welded metal warp?

This is probably an odd question...

A couple of weeks ago, there was an really good welding write-up...

"Basic of Basics" Welding - How do I repair body panels?

While I wasn't having problems getting the machine to tune, I had problems getting consistant penetration. I found that cutting the wire after each tack, and cutting it SHORT, greatly improved my welding and consequent penetration.

So, to the meat of the issue. I butt welded a patch panel in yesterday on the lower firewall of my impala (18 gage). I got it all fitted up, there was about a .020" gap in most places, a little bigger than that at one side.

After I got done welding the piece in place (did lots of little tacks and went slow), I ground down the welds and found that the seam had puckered OUT(convex), i.e. the seam was raised.

I guess I assumed that a welded seam would shrink inwards (concave) and would be a low spot. Did I do something incorrectly when welding or fitting the piece up that caused the seam to raise up? Or, is this exactly what happens and I now have to hammer/dolly the seam to get it straight?

If its a matter of hammer and dolly work now, would this be on-dolly or off dolly?

Thanks for the help!

Joe

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Old 10-24-2006, 09:06 AM
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Hi Joe,

The seam will move in whatever direction the shrink pulls it. It has to go somewhere. With a crown in the panel it will usually pull down, a flat panel can go either in or out.

You need to accomplish two things; moving the metal, and stretching the weld. Both require on dolly work. Off dolly will help you if you need to push out on a low spot and hammer down on a high spot to get the metal back where it was. But, depending on the size of the valley, the line between off and on dolly can be blurry. Technically you may be hitting off dolly even with the hammer directly over the dolly depending on how lumpy the surface is. Your first hit may be off dolly, your second hit in the same spot may be on dolly because you have hammer, metal and dolly contact as the metal gets smoother.... really, you should not concern yourself too much about the terms, just hit the metal. It will start to make sense to you with practice.

John
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Old 10-24-2006, 09:17 AM
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John, I have wondered about this too, is it shrunk or "stretched" more accurately has it expanded. I got to thinking when you have this butt of two pieces of metal and you tack it in the middle with the mig, the metal that is filling the tiny gap in the butt appears to "push" the two pieces of metal apart! And if you hammer on the mig weld before grinding it down FLAT on both sides it realy looks like it expands those panels apart.

At this point maybe it is stretched at that seam and not shrunk?

Brian
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Old 10-24-2006, 10:14 AM
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I am not going to attempt to give advice on body work here but from an old welders experience, I think it is somewhat of a mistake to simply say the metal has "shrunk" or "stretched" since it has probably done both. It is more correct to say the metal has "distorted" because there are going to be some areas that have shrunk(most) and some that have simply curved and stretched. The weld bead itself will certainly shrink causing distortion but the surrounding metal will have expanded from the heat and because of the distortion caused by the shrinkage and resulting "pull" on the surrounding metal it will not be able to return to it's original shape when it cools. This is what is happening when the gap MartinSr is talking about seems to get wider, it is being pushed apart. The metal is expanding from the heat thus the gap is getting wider it(the gap) is then being filled in with the weld bead which will not shrink enough to pull the metal back to it's original position so you wind up with the the area effectively expanded(stretched) even though the weld bead itself has actually contracted. I hope I am not being confusing but my point is that you have to stretch the areas that have shrunk while being mindful that other places are stretched too far.

Last edited by oldred; 10-24-2006 at 11:32 AM.
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Old 10-24-2006, 02:24 PM
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Hi Brian, and Oldred,

In my opinion, the metal has been shrunk and warped, not stretched. Stretching requires a much more serious amount of pull than a weld shrink can put on the sheet metal. It happens during the formation of a dent, or during hammer and dolly work.

If there is any stretching caused by a weld pulling the metal it is too small to be consequential in anything but a theoretical discussion. (I try to stay away from those). Use a micrometer to measure the thickness before and after welding. After careful filing, you will find the weld area is thicker than the metal near it, and none of the metal is thinner than before welding.

I don't recall experiencing panel gaps widening while welding, but if this is happening it is not what is commonly called stretching. The metal will expand during heating, but I would expect the gap would get smaller not wider, or the metal will move inward or outward to expand temporarily. As it cools it will contract/shrink down having more thickness and slightly less surface area than before. If you heated to the same temperature as welding without actually welding the two pieces together, you would expect the gap to be wider once it cooled. The edges would be melted making measurements difficult though. Welding will tighten up a gap by pulling the two pieces together and reducing surface area as the shrinking occurs. Hammer and dolly work on the weld expands the surface area of the weld while stretching/thinning the weld slightly. This can open a slight gap between panels if there was already one there before welding. The conundrum is, if the surface area is shrinking why is the gap closing? In a panel with a compound curve (shape) in it, the weld shrinks, the surface area gets smaller allowing the panels to pull together? I don't know. Might be too theoretical for my brain. But for practical purposes welding shrinks.

John
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Old 10-24-2006, 05:25 PM
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John, I should not have said "stretched" and that is not what I meant exactly. What I was trying to say is that the metal will expand due to heat and then because of the distortion caused by the weld bead it will not be able to contract back to it's original state thus it will be effectively "stretched". Also when welding a gap the gap will tend to separate and the smaller the parts the more pronounced this will be. Take a couple of strips about a foot long and lay them side by side with a tiny gap between them and try to weld them from end to end and you will find the gap will get too wide to weld before you get very far at all unless they are tacked so they can't spread apart. This is an extreme example and the problem is no where near this bad when welding panels but the principle is the same and this will explain the apparent contradiction when a panel bulges as if the metal has been stretched instead of shrinking.
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Old 10-24-2006, 08:49 PM
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Oh boy, this seperation of the gap that's been explained is the exact opposite of what I see happen when I do a butt weld from one end to the other. I sew up seams all the time from one end of a quarter panel to the other and as I cut and weld the gap closes up tight-same with any patchwork I've done. I've never seen a gap open up during welding unless there's some stress in the fit. Welding thin strips of metal can be decieving as the shrink on one end is so powefull it will bend through the width of the strip and if you keep welding from one end of the strip to the other the bend will eventually come back out. JMO
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Old 10-24-2006, 09:49 PM
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What I am describing is what MartinSr had noticed when he said it appears to push the metal apart. I know it seems like a contradiction but not if you think about it a bit. As you are welding the metal will expand from the heat, I think everyone already knows that, and this expansion will be quite a bit more than the shrinkage that will occur when the bead cools leaving the HAZ with both shrinkage and expansion (distortion). My point is that there is more to deal with than just shrinkage in the weld area and I was trying to explain why Modas had a bulge and not a shrunken in spot. I am not trying to disagree with anyone here and I certainly will not attempt to tell you guys how to do body work but after spending 38 years dealing with welding warpage trying to maintain bearing and shaft alignment along with a great deal of thin sheet welding on ducting I have a good understanding of what is causing the warpage and how to control it so I thought would offer my 2 cts. There is no doubt that properly welding a patch in a panel will lead to overall shrinkage but overheating and using the wrong skip technique or not skipping at all can very well lead to expanded areas along with the shrunken areas, distortion. As far as the seam being pushed apart I think you are missing what I am saying. The metal will try to spread apart and if it is two unrestrained strips it will do just that but if it is a large sheet such as a body panel this same principle will cause the metal to be forced up and/or down due to the expansion and the fact that it can not spread apart as the stress is trying to cause it to do, so there is a good deal more than just shrinkage going on here.
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Old 10-24-2006, 10:31 PM
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Didn't you know they all shrink in cold water!
Just don't let your dolley see it~
Ahhhrm.. anyway.. Lol.

When I do tac work like you described I usto cool it with a damp rag but I got away from that thinking down the road it may be trapping moisture and cause rust.
I use air to cool it down after each tac. I go in a different place every tac spreading it around as even as possible.
I normaly don't make panels to butt up to one another. I go in back of with the repair panel. It makes for a strong repair and your not grinding off all your welds you just put on. (also depends on the size of the panel your making.)

Here's a spot I did last week for a guy's mazda that had rusted through. There was no lip left. I cut it all out and made the pcs here. I put down epoxy primer and let that sit over night..
I had one coat of thin filler afterwards... Blocked it out and went right to basecoat/clear.

I've warped metal before.. even trying to be carefull.
Hope that helps.. ?
~Scott
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Old 10-25-2006, 07:55 AM
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First rule, ALL welds are shrink points!!!

Second rule, you can ruin a panel faster than you can imagine by shrinking for the wrong reason.

If you want to get better at working metal learn about how metal moves.
I read an old autobody book and experimented and read it again several times and bought a few metal working videos. Now you can rent the same expensive videos from Technical Video Rental.

For some explaination of metal and how it moves as I understand it see my website.

Metal working

Kevin
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Old 10-25-2006, 09:12 AM
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Hi Kevin,

Nice write up. One mild disagreement is with your second rule. Shrinking and stretching are both easy, and it is hard to really ruin a panel if you follow a few simple guidelines.

Everyone learns in slightly different ways, but in my case, I really started to get it once I stopped worrying about stretching or shrinking too much, and just did whatever it took to deal with what was in front of me.

When I am teaching metalshaping, I try to show how far past the conventional ideas/rules you can go without causing any harm. If metal is approached with a timid mind set, it is hard to accomplish much (for me anyway). I have found that just using a hammer and dolly to hit the metal a lot in one place, and observing that it is not ruined is a good starting point. Good smooth tools help. You can stretch a 1/2" high bulge in a panel, and shrink it back down, and you can shrink a flat spot and stretch it back up. This can be done with very simple tools.

If you use hammers that have not been dressed properly, or use too much heat to shrink, you can cause some surface damage, but this is not the proccess it is the technique or tool at fault.

I realize that some people prefer to sneak up on perfection, but I would rather jump on it and miss over and over until I get it. I seem to learn more this way.

John
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Old 10-25-2006, 09:27 AM
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Thanks, I think I have a better understanding of it now.

Brian
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Old 10-25-2006, 07:13 PM
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John,

The second rule is there to get people to stop and think before they do something that might make their problem worse.

Many do not have some of the basic knowledge we have. I hope to get them to stop and learn some more before they proceed. That is why I also try to point them to places where they can learn more. I know for me some of the shinking/ stretching stuff took a bit to sink in. I experimented some and after that my panel work improved significantly.

It is easy for us to shrink and un-shrink an area, but I think back to when I was starting out just a few years ago. I got a lot of bad advice from 'knowledgable' people that I know now are not as I first thought.

Granted, I am using a bit of a scare tactic, but I believe it warrants this tactic. I hope I get a few more guys reading up and learning better ways to approach the metal work.
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Old 10-25-2006, 07:34 PM
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you can find a lot of good info at www.metalmeet.com
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