How to Fill a Gap When Welding
Kevin first addresses why you should avoid gaps altogether. It's always better to make sure your fit is right, with straight lines, smooth cuts and tight joints, getting your fit up and prep done first. He explains that a tight fit leads to a good weld structurally. That's not as important, perhaps, with something like art, but if you are building buildings or bridges, for example, it has to be a good joint. The weld will be X-rayed and inspected for soundness, porosity, gaps, etc. The space in a gap lets air come up and cause porosity because you're super-heating the air above and below the gap and have a puddle of molten metal that pulls the air up through the gap and into the weld. By not having a good, tight joint, you're causing more problems for yourself. Kevin says a good fit is just part of being a good welder.
Regardless, Kevin shows you how to fill a gap. He sets up two pieces of 1/4" plate with a V-gap, with one end fairly close and the other end wide. Again, he emphasizes that this is for aesthetic, not structural welding.
Using Longevity's ProMTS 200 because he loves the MIG welder in it, Kevin first gives the metal a few tacks so the steel won't move. Next he's ready to run a welding bead. He's got the MIG welder set at 23 volts for weld tacking, so he's going to turn it down a little to 21.5, but leave the wire feed as it is at 147. Kevin reminds us that, with this machine, that doesn't mean 147 inches a minute (he's not sure how Longevity measures wire speed), but it's about two-thirds of the way up.
Before Kevin starts welding, though, he addresses whether you can just run your bead from one end to the other. He explains he wouldn't. Because of the gap, he's found that if you try to just run a bead from one end to the other, one piece of metal will move away as you are welding, so the gap actually widens. Kevin says it's best to tack it on one end, tack it on the other, then he likes to hopscotch back and forth. He'll tack it in the middle, then run a short bead between two of the tacks, then go to another area and run a bead between two other tacks, etc., just filling it in as he works his way out. That helps spread out the heat more evenly and reduces warpage on thinner metals as well as prevents blow-outs. Kevin then runs the bead.
He says that that voltage worked well as he filled in the sections between the tacks. He also wove back and forth to fill in larger areas, "stealing" a little metal from each side of the gap. Then Kevin shows the first weld at 21.5 volts and another at 21.2. From the back you can see that both welds got good penetration all the way through.
A couple of other ways to fill gaps include straightening a coathanger and laying it inside the gap, getting a piece of thick copper -1/8" or 1/4" preferred - and using it as a backer because the steel won't stick to the copper. You can clamp or prop the copper, which gives the molten steel something to run up against as you are welding. Depending upon your design, you could even back it with another piece of steel and just weld it in.
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