Oh, how I struggled to plug weld with a MIG welder!  I want to pass on what I learned so someone else can avoid my problems.

You can plug weld with a MIG welder to hold 2 layers of sheet metal together.  It is a replacement for the spot welding done at the factory, and according to many web sites, it is supposed to be stronger than spot welding.  The idea is you drill a hole through the upper layer of sheet metal, clamp the two layers together, then weld up the hole, thereby attaching the upper layer to the lower layer.

Simple enough, but I kept having the same problems time, after time, after time:

  • blow through.  I’d strike an arc and promptly blow through the lower layer of metal even at low heat
  • melting around the hole I drilled in the top layer of metal. I’d complete the weld and find that I had melted out an area around the hole in the upper layer of metal
  • the weld would look perfect but would not stick to the lower layer, and the joint would fall apart

I read everything and watched every video I could find on the internet, then tried every suggestion: lower heat, higher heat, moving the tip further from the metal…yet I still ran into these problems again and again.   Each web site usually provided just one suggestion but that is not very useful. A whole series of things are needed to get a good weld, not just one.  Finally,  I now only have a failure once in a while.

Here is how to avoid my problems:

  1. Clean the metal carefully.
  2. Drill 1/4″ holes for sheet metal up to 20AWG and if you are welding thin sheet metal to something thick like square tubing, drill a 5/16″ hole.
  3. Clamp both pieces very tightly together.
  4. Put your welder on a low setting. For my Lincoln MigPack 180, the heat setting is A and the wire feed is about 3.
  5. For really thin metal, clamp a piece of copper or aluminum to the back of the bottom sheet of metal.   Neither copper nor aluminum will weld to steel and both metals conduct excess heat readily from the plug weld.
  6. Ensure you have a perfect ground.
  7. Start your weld in the DEAD centre of the hole with the tip at 90 degrees to the metal, hesitate momentarily, then continue the weld in a circular motion to spread the puddle onto the upper layer of metal
  8. Allow the metal to cool completely before moving on to your next weld.
  9. Just before you work on the car, make a couple of practice welds on scraps of  metal

Here are details about each of the points I made above.

1. Clean the metal carefully

Clean the metal scrupulously before clamping it together, both sides if possible.  There can be no rust, no grease and no oil.

Rust does not conduct electricity like clean metal and the arc will strike to the nearest clean spot, which may not be the centre of the plug hole and that will ruin your weld.  It also does not conduct heat like the surrounding metal, making for an uneven weld.  Grind the surfaces.  I found that a flapper pad works best.

Oil and grease are insulators, and the arc will strike around them or not at all. So, use a degreaser on the pieces of metal before clamping them together.

 2. Drill 1/4″ holes for sheet metal up to 20AWG and if you are welding thin sheet metal to something thick like square tubing, drill a 5/16″ hole

The air-powered sheet metal punches you can buy at Princess Auto make a 3/16″ hole.  I  found this is not big enough no matter what the wire feed setting or voltage. The weld has to bond to the lower piece of sheet metal before it melts the upper piece.  With a 3/16 hole this happens occasionally.  A 1/4″ hole always ensure that this happens.

There is an exception.  If you are welding thin sheet metal to something much thicker like square tubing, it takes longer to get the thick metal hot.  I have found that a 5/16″ hole works best.

Get yourself a step drill bit for drilling the holes because it will quickly drill burr-free holes.

3. Clamp both pieces very tightly together

The two pieces have to be in intimate contact to transfer heat evenly to each other.  If they are not, one or the other will melt first and the result will be a burn-through.  Make sure there are no burrs before clamping.

You need to put clamps on either side of the hole. 1/4″ away from the hole is ideal. If you cannot get a clamp in place, use Cleco fasteners if you can afford them.  At 3.50 a pop in Canada, I cannot afford them, so I use #6 x1/2″ self drilling screws.

Put your welder on a low setting. For my Lincoln MigPack 180, the heat setting is A and the wire feed is about 3

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6.Ensure you have a perfect ground

If you have a magnetic welding clamp or one with smooth jaws, pick it up and fling it in the nearest garbage can.  At low voltage settings, these rarely connect well.  Get one with sharp alligator jaws and twist it so it bites into the metal.  Do this every time just before you start welding.

7. Start your weld in the DEAD centre of the hole with the tip at 90 degrees to the metal, hesitate momentarily, then continue the weld in a circular motion to spread the puddle onto the upper layer of metal

Starting the weld in the dead centre of the hole is absolutely critical to a good job.  This is one of the biggest problems I have had with plug welding.

Support your torch hand so it does not shake.

If you don’t have an auto-darkening helmet.  Get one.  Even at that, you may still not be able to see the tip of the wire properly.  Get a strong flashlight that clamps to the work and mount it so it illuminates the wire from behind.  If there is light streaming in from behind you over the mask you will not be able to see the wire tip.  Tape a piece of non-flammable cloth to the top of your helmet to block out the light entering from behind, or buy a helmet that does not allow the light to enter from behind your head.

The hesitation needs only to be momentary, just enough to allow things to cool off a bit. About 1/2 second is enough.  More than that will result in a weak weld.

8. Allow the metal to cool completely before moving on to your next weld

If you don’t the metal will buckle.  Using a blow gun with compressed air speeds things up enormously.

9. Just before you work on the car, make a couple of practice welds on scraps of  metal

You need to reset your brain each time you start welding.  It is important that your practice pieces are the same thicknesses of those on the car, and in the same position (horizontal or vertical) as those on the car.  Don’t bother trying to weld overhead.  Leave this to the really good guys.

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