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Old 09-05-2004, 02:21 PM   #1
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Join Date: Mar 2003
Location: South Mississippi
Posts: 36,373
Randy Ferguson's guide to welding sheetmetal

Here's a gem I picked up from a great site on metal working and the author was kind enough to let me copy it here.
You have to weld thin gauge sheetmetal much differently than you would heavier plate, even in the 1/8" range. You must weld a series of spot welds, with each one overlapping the last. I try to overlap by a 1/3 to 1/2 on each successive weld. You will also need to stop at regular intervals (not more than an inch) and stretch that area out before moving on, otherwise, it will have more distortion than you care to deal with.
You start by placing tack welds at 1" intervals to keep the two pieces in good alignment. (I'm only assuming you're butt welding, at this point.) You can then start the welding process. I prefer to start in the center of the seam and work out both directions toward the edge. You must stop and work the weld as you go though. To best do this, a 1/16" cut-off wheel works great for knocking down the proud weld bead. You want to leave just a tiny amount of weld above the surface, because you're going to work this down some as you hammer the weld to stretch the metal back out. You only work within the Heat Affected Zone or HAZ as we often refer to. This is the blued area around the weld. Do not leave the area when stretching a weld. Even though the surrounding area is distorted, it is still unharmed. It's just sucked in down some when the metal shrunk along the HAZ. It will pop right back into place when you stretch the weld seam. You stretch it by hammering on dolly, in other words, you place a dolly on the backside of your weld and hit it with a hammer, making sure your hammer blows are against the dolly (or post dolly if you have one) This will rapidly stretch the area drawn in by the heat and relieve the panel of the stress caused by the heat, removing the warpage. You can now add another series of adjoining spots and continue the process, jumping back and forth from side to side, working ever closer to the edges of the panel until finished. Once you've gotten the weld completed, you can go back and fine tune the weld seam work the panel with a slapper and dolly, producing a very smooth panel. I prefer to use a file to work down the final few thousandths of proud weld, rather than a grinder that will remove too much material. A shrinking disc will come in real handy too, to shrink any areas that you overstretch. I plan to replace a lower section of a '39 Ford fender tonight. I'll document the process and enter it here.
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