Plug Weld ? Need advice

I don't think it's fair to blame poorly trained craftsmen for the 5/16" recommendation. The conditions under which the welds are made are not always possible to control 100%, and coatings that cause weld contamination are either in or near the weld zone. Add to that the varying thickness of material all around the vehicle but especially in structurally reinforced areas, a recommendation of 5/16" starts to make sense. Personally I like to use 9/32 on my hole punch most of the time if there is room on the flange for it, but I don't go below 1/4" unless forced to.
 
One more plug weld question. When welding two different gauges of metal together, what's the best way to setup the welder. The filler panel is 20 ga but the hinges are somewhere are 16-14 gauge. Do you still go by the base metal? I'm going to get some practice metal and see what works.
 
For that scenario I normally put the hole in the thin piece and set the welder up for the thicker. Add your fill directly in the center of the hole, and allow the weld puddle to fill the hole. For this method I had used the letter A size that I described earlier.
 
MP&C;41731 said:
For that scenario I normally put the hole in the thin piece and set the welder up for the thicker. Add your fill directly in the center of the hole, and allow the weld puddle to fill the hole. For this method I had used the letter A size that I described earlier.
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Thanks Robert. I already put the holes in the hinges. I got some scrap metal that I'm going to practice on. If I have to I will weld up the holes in the hinges and drill the thin metal.
 
If the hole is in the thick piece, you have to develop a technique that has enough heat to fuse the thick piece without blowing through the thin base, which is probably one of the more difficult MIG techniques to learn. One method would be to set the machine for the thicker material, strike the arc very near the edge of the hole, rapidly weld the perimeter, cut the arc very briefly, then restart it to finish filling the center of the hole, again starting on the edge to avoid blowing through.
 
Thanks Crash. I cut some practice pieces today. I'm going to practice with it some this week. I just haven't had a lot of time to work on the car lately with all going on with the holidays. If I cant master it on practicing, I'll weld the hinges up, remove the panel and weld it from the backside.
 
I did some practice welding. It is nerve racking having the welder turn up this high on a piece of 20ga metal. I tried a practice piece tonight. The thin piece is 20ga, and the thicker is the closest I could find to the hinges, I even ground it down some. I think the thick piece is within .010. of the hinges. I want to post a few pics. Let me know what you guys think. The thick piece is 12ga and the hinges are somewhere around 13. I set the welder at 14ga then turn it up a little.







I have the panel mocked up with screws, but I still need to make sure that these hinges are in the correct locations. I have double checked the measurements against another project car here in the shop and I'm within a 1/16 of an inch on a couple of measurements and right on with the rest. My original plan was just to tack the hinges in the corners and test fit the lid. I'm worried about putting screw holes in the hinge area and then trying to weld it. I guess I could use screws on the two center holes of each hinge, then once I know the hinges are right tack them in, then close the hole from the bottom on the 20ga piece before welding the top center holes where the screws were used. I was also planning to use a copper backing when welding on the hinges. Maybe I'm over thinking all of this. Thanks for the help.
 
Using copper to back up the thin metal will aid in letting you burn a hot weld without a chance of blow through-your test welds look just fine though...penetration is good and the puddle flow on the top is fine.
 
Did some more practicing on 1/4 inch and 3/16 holes on 20ga metal. I actually have the welder set closer to 18 ga and maybe a little on the hot side for the 18ga. On the 3/16 hole #1, it was not hot enough, turned the heat up on # 2 and it did better. The welder sputtered a little but I think it was my weld thru. On # 3, I increased the wire speed just a tad. I think the setting for # 2 is ok for 3/16.



 
Penetration looks plenty good on 2&3 IMO but if you want to be sure you can do a destructive test. ICAR has a weld certification test that is pretty good-good enough that I had to retake my overhead portion of the test and I thought I was a good welder LOL... But you use their machines with no play time to get your heat and wire speed dialed in. anyhow they do a simple destructive test that is easy to duplicate. Make two sheetmetal test panels, one with your hole for the plug weld, make your weld and allow it to cool naturally. Start separating the two pieces of metal pulling each away from each other untill they each bend 90* at the weld. Clamp one in a vise and grab the other with pliers or similar and twist in one direction until they separate. To pass the test your weld will stay intact on the side you applied the weld to and tear out a hole in the base metal the size of the hole drilled. It is an eye opener
 
Bob, I tried that test. It worked great. I thought it would have been easier for me to separate, but I really had to wrestle with it. In the end I had a hole the size of the plug weld in the base metal. Thanks.
 
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