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Old 12-30-2005, 10:25 PM
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cracked steel

I am working on a 1951 gmc truck. I have found several areas where the old steel has cracked. What is the best way to repair this? The metal is good but 50 years of rattling around has caused a few cracks. I would like to weld it back up but there is really no gap between the pieces and i have blown a few holes through allready in other areas. I have my mig turned down to 1 and the wire feed relatively slow but still blow trough sometimes. This is in a somewhat visable area. Help please.....

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Old 12-31-2005, 12:41 AM
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cracked where? body? frame? what?
robert.
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Old 12-31-2005, 02:53 AM
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When i have repaired cracks i drilled a hole at each end .030 to stop it from tearing further and then welded and of course grinding follows. But the hole in each end will stop the crack from getting longer.
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Old 12-31-2005, 07:17 AM
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If you weld with your settings too low, you will not get a good weld. Are you trying to weld a continuous bead? You might try just welding short zaps with the welder set hot enough to produce a flattish tack weld that gets good penetration all the way through to the other side. Let that area cool and don't weld there again until you can comfortably touch it with your bare hand.

John
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Old 12-31-2005, 09:19 AM
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this particular crack is on the lower end of the cowl(?) at a right angle to where the firewall starts. It is covered by the right side back of the hood when closed but is visable when the hood is open. See attached photo. On the other areas, I have just tried to spot type or tack weld but sometimes blow through. I like the idea of drilling a hole at the end of the crack. I read in another post that the trouble may be from the old metal being weakened.
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Old 12-31-2005, 06:58 PM
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It appears to be a stress crack to me. I would drill a small hole (1/16"-1/8") at the end of the crack to prevent it from growing, and then weld it up.

Aaron
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Old 12-31-2005, 07:48 PM
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welding sheet metal

When welding sheet metal the big problem is often burn through..the technques that work for structural steel and thicker materials often do not work well..

When welding something thin (like body metal) set your machine up using some thin scrap about like your fender or hood metal..then adjust the machine and technique to suit..

then when welding the hood it is a short "ZAAP' just make a tack..let that cool..move down a bit and do another "ZAAP"..and so on down the seam..when the seam is tacked well then work your way up the seam...backstep the weld..that is starting your arc say an inch or two away from a tack and welding back into the tack.then do it again move out an inch or two and weld back into the last short bead..Let the metal stay cool if you need to do some hammer and dolly work or use a wet rag that is fine..just do not overheat your metal.

Try that that should help you a bit..

OMT
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Old 01-01-2006, 10:35 AM
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First of all, don't sweat it, if you left that crack there it will still be there in twenty years. I have owned my 48 Chevy truck since 1974, it had the exact same crack then. I welded it up and it never came back. But honestly, I have seen that crack on just about every cab I have ever seen. Not that I have paid that much attention to them, but I do remember always seeing them

I don't believe in the hole drilling trick. The only time that would make any sense is a place where getting proper penatration is an issue, that certainly isn't the case here.

Follow OMT's recommendations for welding it and it will be gone forever.

Brian
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Old 01-01-2006, 10:47 AM
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The reason that I mentioned drilling a hole, is that it will stop the crack from spreading.The military used it on aircraft parts that had stress cracks, back when I was an aircraft machinist. It takes more stress to start a new crack at that hole, than to continue the original one on. Theoretically, welding it should stop the crack, but a little insurance never hurts.

Aaron
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Old 01-01-2006, 11:06 AM
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I am with you Aaron, I understand the theory and how it will work, but the metal at the end of the crack will be fused together after welding whether there is a hole drilled or not. After you have the metal all melted together at that point what is the difference if there use to be a hole there, the metal doesn't know there was.

Brian
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Old 01-01-2006, 11:52 AM
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Thanks for the advice, I really appreciate learning this stuff from you guys that have seen these challenges and lived to tell the tale!!!
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Old 01-01-2006, 12:19 PM
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Quote:
Originally Posted by fezz-man
I have my mig turned down to 1 and the wire feed relatively slow but still blow trough sometimes.
blowing thru the metal is usualy because your not feeding enough wire into the molten pool

when you blow a hole, you shouldnt adjust BOTH knobs down, you either turn UP the wire speed, or turn DOWN the heat

put turn the heat back up, set the wire to match, and use some scrap to adjust your wire speeds till you get the sound of frying bacon, you can just lay beads down on a junk piece of steel

after that, cut some pieces and try welding them when they are tightly butted together, slightl gapped apart, overlapped and @ 90* angles

as you find the right heat settings for each, right it down on a piece of paper and stick it to your welder, but honestly it wont take long before you dont have to look at it anymore
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Old 01-01-2006, 04:57 PM
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Quote:
Originally Posted by MARTINSR
I am with you Aaron, I understand the theory and how it will work, but the metal at the end of the crack will be fused together after welding whether there is a hole drilled or not. After you have the metal all melted together at that point what is the difference if there use to be a hole there, the metal doesn't know there was.

Brian
Brian, I can understand your questioning the hole part. I learned this when I was quite young in the military, and my real interest was in learning how to make parts and tools, not learning why we repaired things the way we did. The furthest I got on that subject was "why drill the hole?", and that is how it was explained to me in the books that were handed to me. I didn't understand it, since the parts were being welded by "certified" welders, then sent for NDI (Non Destructive Inspection) before the part was certified to be used. I figured if it is good enough for a mulit-million dollar aircraft (back in 70's), it will probably be good enough for an automobile, hopefully worth several thousand dollars. I have used that practice ever since.

Maybe one of the engineers on the board can answer that one. Maybe it's an outdated proceedure, but has always worked for me.

Aaron
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Old 01-01-2006, 05:21 PM
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Burn thru on old rusty sheet metal is a pretty common problem. Clean off the rust as much as possible and if you have access to the back side of the area to be welded you can place a copper heat sync up against the crack on the back side. Stitch weld in short increments letting the area cool between welds until complete. The weld will not stick to the copper and you won't burn thru as readily. Good luck on the truck. I wish I still had my 52.

Joe
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Old 01-02-2006, 11:01 AM
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Quote:
Originally Posted by adtkart
I figured if it is good enough for a mulit-million dollar aircraft
Aaron
Aaron, it certainly isn't going to hurt! And on an aircraft I am flying in, I hope they went to the outlimits of correct!

I know it WORKS, it will stop a crack that you can't weld or fuse or whatever. My brother has one of those fancy shmancy one piece counter tops that his teenage daughter put a hot pot on. The heat cracked the counter in a spider web design. He drilled a small hole at the end of every crack and it has not gotten any bigger.

I remember hearing that all the time back in the seventies when I was first starting in this business, I don't hear it much now. I wonder if you would still be instructed to do it these days?

Brian
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