2/2/2008: Sheetmetal Welding Tips

Plug Welding

Well, lately i’ve been doing a lot of plug welding. These welds are used in place of the original factory spot welds when replacing a panel (since I don’t have a spot welder).

Here’s a handy video I came across illustrating one way of doing a plug weld with a MIG welder. This technique works well for me as well.

In addition, I very much recommend this web page: Plug Welds and MIG Spot Welding. Really, the whole site is worth spending an evening reading. Great stuff.

My own observations on plug welding, thus far:

  • The welder should be set fairly hot, and the metal needs to be good and clean. I found it easiest to tune the settings on a piece of scrap of the same thickness. Focus on the back of the weld to make sure you have thorough penetration of the base metal.

  • Clamp well.. make sure the two panels are very tightly clamped together. Use a clamp on each side if you need to.

  • Hole size does matter. If the holes are too small, it will be very hard to get good penetration, because the weld pool will not be large enough to really melt into the base metal. I have an air powered punch/flange tool, but the holes it makes are far too small. I still use it to start with, but then enlarge the holes to 5/16” with a hand drill and unibit.

  • When going around the edge of the hole, make sure you get right up to the edge. If there’s any doubt, swirl around the circle again while it’s still molten- make sure that it’s fully connected to that top panel all the way around.

Butt Welding

There are plenty of tutorials out there (especially on metalmeet.com) about how to do a seamless butt weld. With a MIG welder, you basically want to fit the seam pretty tight.. leave a slight gap, but no more than 1/32” or so.

You often see advice to leave a wider gap for MIG than you would with TIG.. this is kind of bogus advise in my opinion. It leaves room for more shrinking and distortion, but it makes it harder to see it as it’s happening (and correct it). So I feel that a tighter joint is a better way to go.

Clamp it along the seam (it’s good to have lots of vice grip panel clamps!) and then start tacking it. Tack once every inch, but make sure you stretch each tack on dolly (grinding first if necessary) to undo any shrinking due to the heat. It’s pretty obvious where this is needed if you fit the seam tightly- it will start to pull together. stretch it, only in the HAZ (areas where the metal is discolored from the heat) to restore the even gap.

Once the whole seam is tacked every inch, go back and start put another tack mid-way between each of those tacks, and repeat until it’s filled in. Then flatten the welds with a grinder so they’re almost even with the base metal, and go back with the hammer and dolly and stretch the HAZ again until it’s even.

Here’s a video of this being done (by someone a lot more skilled than I am) with TIG.. Ignore the background noise..

For more tips, check out these MetalMeet threads. I think you’ll need to register there to see them though.. It’s totally worth it.