Matt Joseph’s CarTech Blog Entry for
Sheet Metal Forming and Fabrication
A Tip for Successfully MIG Butt Welding Light Gage Sheet Steel
Making strong, attractive MIG butt welds in body panels is essential to doing great panel work, unless you’re blessed with an expensive TIG weldor and the skill and experience to use it effectively. The only other option for butt welding thin steel section is torch welding. Although this was common practice years ago, it remains incredibly skill intensive and carries with it a host of potential problems. It is, by now, almost obsolete.
For most of us MIG (properly, GMAW for Gas Metal Arc Welding) is the easiest and most effective way to join thin steel sections, but, like any other good thing, it has its own lurking snags and pitfalls.
If you have had, or are having, problems MIG butt welding light gage sheet steel materials, help is at hand.
There are several obvious factors involved in making successful MIG butt welds, like using good MIG tips in good condition, and using top quality welding wire, and the right mixture shielding gas. There is another big secret to success is simple, that is often overlooked, but critically important.
“And now, may I have the envelope, please?...thank you…and the answer is, [stage direction: heavy drum roll] FIT-UP”
“Oh, yeah,” you say, “But I already knew that. Thanksalot.”
Okay, you already knew something about that. For years I thought that I knew something about it, too, but it turned out that what I knew was mostly wrong. It was to craft the butt ends of material to fit-up as uniformly and closely as possible. That knowledge was only half correct, and it’s the wrong half that will get you into trouble every time. I know, because my MIG butt welds were awful.
Proper fit-up involves money. After all, these days, what doesn’t? Luckily, the amounts are literally just nickels and dimes.
Follow the Money
Over the years, dozens of people have asked me why their meticulously prepared MIG butt welds in light gage steel look like owl poop. In describing everything that they did right, they inevitably and lovingly refer to the great time and dedication that they lavished on preparing the fit-up of the metal butt ends. Still, somehow their welds were ragged, distorted, and just plain ugly.
The truth is that their butt end fit-ups were too close. They assumed that it was impossible to get fit-ups too consistent or too close, but that’s half dead wrong. The other half, perfectly consistent fit-up accuracy is okay, but often overrated. It is not particularly critical that you have micrometer-like accuracy in butt end fit-ups to weld them successfully. As with horse shoes and hand grenades, “close” will take you a long way. However, leaving enough fit-up gap is absolutely critical.
Two major things happen when fit-ups are too close. It becomes very difficult to follow a complex weld line in three dimensions, because the closely butted metal ends make it hard to see your weld line. This problem can be reduced by using a high temperature marker to indicate the weld line, but leaving sufficient gap takes care of the problem automatically.
More important, too close fit-ups cause welding problems, because welding heat makes the butt ends of the metal expand into each other. That slow collision occurs with enormous force, and leaves the metal no place to go but up or down, into the third dimension. That will give you buckled metal. Sometimes, this kind of stretch stays local in the weld area, but it has the potential to set up distortions all over a panel. Finding these warpages and swales can be difficult enough, but correcting them is in the class of a numbered Excedrin® headache.
Here are the nickel and dime gaps for butt end welding. They look large, but your weld puddle will travel down them easily.
Now, a word about that money that I mentioned, specifically those nickels and dimes. In 20 to 22-gage sheet metal, your fit-up gap for MIG butt welding should be no less than the thickness of an unworn dime, but it doesn’t hurt to be stingy with your money and generous with your fit-up gap. I tend to use the thickness of an unworn nickel, or even a few thousandths more than that.
Reasonably, you ask, “But at that fit-up gap distance, how am I going to keep my weld puddle from dropping out and burning my shoes, or feet, or both?” The answer is that you are not going to have to worry about that, because the MIG welding process will take care of it for you. When you weld down a reasonable gap with the correct wire speed, voltage and gas flow settings, and maintain a correct torch distance, capillary attraction will keep your weld puddle seated neatly between the butt ends of the metal that you are welding, with little effort on your part. It sounds wrong, but it works. Your burn-through and major distortion problems will become things of the past if you follow this advice.
The gap shown here is much too wide, but it demonstrates what is possible with wide gaps, without drop-out. Note that a gap this wide would take too long to level and finish to be practical.
Wide enough gap is half of this story. The other half is using discontinuous welds, an “on/off” approach with your arc. You can do this by triggering your gun on-and-off, or with a stitch timer that will automatically do the on/off switching for you. Good stitch weldors let you set the on and off times separately. Either way, your bead segments in 20 to 22-gage will be in the range of 1/8” (3/32” to 5/32”) long, somewhat simulating that lovely visual ripple look of good stick welding.
You can make these welds of great beauty with manual on/off triggering, but a stitch timer makes them even more perfectly, because it is more consistent than trigger switching, and because you avoid inadvertently moving your torch head when trigger each weld segment.
In practice, you weld your bead segment into your fit-up space for a second or so, then trigger or stitch time your torch off. Before the puddle has cooled much below orange, you trigger or stitch time your wire back on, and direct the wire into the center of the cooling puddle’s lower half, adding another bead segment. Capillary attraction and magnetism will draw the puddle uniformly into the fit-up space between the butt edges.
These three photos show to top side, bottom side and a cross section of a weld done with 3/32” butt end gap in 22-gage mild steel. This weld will be strong and easy to finish.
This technique allows cooling between short weld segments and controls and limits the overall heat in the weld area. That means less distortion and less chasing stretches around after you weld. It also makes it much easier for you to follow the weld line, because there is a generous gap to look at. It all but completely solves problems of puddle drop-out, and makes for welds that are consistently strong, uniform, and attractive. They also are easy to finish to panel level.
I realize that all of this sounds counter intuitive. If narrow fit-ups have problems of drop-out, distortion, and irregularity, it would seem logical that a larger fit-up distance would make these problems worse. The simple fact is that it greatly reduces them or makes them go away completely.
If this sounds impossible, all that I ask is that you try it. My experience tells me that this advice will put you on the road to mastering superior MIG butt welding of thin sheet metal sections.
And what do I ask in return? Well, after you no longer need your new fit-up gages, that nickel and the dime, put them into someone’s expired parking meter, you know, one of those “random acts of human kindness.” Who knows? It could be my expired meter.