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Old 04-27-2021, 02:34 AM   #11
MP&C
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Location: Leonardtown, MD
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Re: Questions for the welding pros

You didn't discuss your weld process much in the first post, so I'll show what I do when MIG welding sheet metal...


Let's say we are installing a lower door skin to repair some rust. The repair panel has been trimmed for zero gap (or as close as we can), and the panel is tacked on, starting at one end, and working PROGRESSIVELY TOWARD the other. Don't skip around from one end to the other as you have a greater chance of misalignment in that you may have more metal trapped on one side than the other, which will result in a buckle from the excess metal once the welds bring it together. So I start at one end, tack, move an inch or so, align the panels together, make another tack, repeat. This insures the panels are correctly aligned as you work progressively from one end of the weld seam to the other.





Once done with the last tack, go back to the beginning and use a hammer and dolly to planish each weld dot, working in the same start to finish pattern. Now go back to the beginning and use a 3" cutoff wheel to grind the weld dots down to just above flush, both FRONT AND BACK, as this gets the weld prouds out of the way for both planishing the next sets of weld dots, and also removes all the excess weld that is going to change the heat load. A weld bead can be up to 4-5 times the thickness of the parent metal, grinding it down keeps the effective metal thickness the same throughout. I grind to just above panel height for a weld seam like this, and final cleanup with a 3" roloc sander will be used at the end to dress the seam to the parent metal. Here's a video that shows the grinding process, but as this is a plug weld it is dressed immediately following. Again, a full weld seam gets dressed at the end. Much less chance of taking away too much metal.

https://www.youtube.com/watch?v=V2WHT_zMOE8


Now that those welds are down and out of the way, let's add the next set. Here I'll overlap the last set of welds by about 1/3 to 1/2. Whatever the distance of your overlap, keep it consistent throughout.





This method helps to eliminate any missed areas like what occurred on the back side of your test piece. Again, start from the same spot you did before, overlap the first weld dot where you started, go the the next, overlap, repeat, until you get to the end. Then go back and repeat the planish from start to finish, then grind weld dots from start to finish, both FRONT AND BACK, and then repeat the overlap process again. Keep repeating the process until the weld seam has been finished, then use a roloc sander to dress the little bit of remaining weld to flush with parent metal, both front and back side.


If this sounds like a slow, monotonous process, yes it is, that is the intent. This promotes consistency in the welding process, from fitup through to final welds dressed. At no point is it necessary to cool the welds, by the time planishing and grinding are complete, the welds on the panel are plenty cool to the touch. Any artificial cooling serves no purpose.
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Robert
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