MIG welding

Posted 2020-02-13

Recently, I took a MIG welding class at Artisan’s Asylum in Somerville, MA. I wanted to document what I learned so that I can refer back to it in the future, so here it is!

Safety

There are four primary hazards:

  1. Burns. You’re dealing with liquid metal, so the work piece will remain hot even after you finish a weld. Also, small balls of molten steel will fly away from the work area and can burn through clothing and footwear, or ignite flammable objects nearby. Precautions: welding jacket, welding gloves, face mask, eye protection. Scan the surrounding area for fire hazards before welding.
  2. Electric shock. MIG welding is an electrical process in which current is passed between the welding wire and the ground clamp. Precautions: don’t weld in damp areas.
  3. Radiation. Ultraviolet light emitted from the arc can damage eyesight. Precautions: welding face shield, preferably an auto-darkening one, though a small amount of exposure will occur due to the delay between the arc flash and the sensor activation, mostly a concern when you spend 40 hours a week welding.
  4. Asphyxiation. MIG welding typically uses a gas mix called C25, a mix of 75% argon and 25% carbon dioxide, in order to displace oxygen from the work area which would oxidize the molten steel and prevent a solid weld from forming. As the gas mix displaces oxygen, welding in an enclosed area could result in hypoxia. Precautions: weld in a well-ventilated area. The welding shop at A^2 has an exhaust system.

Mechanics

There are two primary controls on the welding machine: voltage (a proxy for temperature) and feed speed. There’s a chart on the welding machine with recommended settings for types and thicknesses of sheet metal. Some machines have an “auto feed” setting which tends to work fairly well.

Don’t weld anything galvanized, as the zinc will vaporize and can cross the blood-brain barrier causing neurological damage.

There are three common gauges of welding wire: 0.023”, 0.030”, and 0.035”. 030 is a good general purpose wire. When you switch out a reel, be sure that the feed rollers are fitted for the correct gauge. The number facing out on the reel is the groove in use, regardless of whether it’s actually on the same side of the number or the opposite side.

To reduce friction on the sleeve that holds the welding wire inside the gas cable, try to avoid sharp loops, similar to how a garden hose can kink.

You want to keep the nozzle about 3/8” away from the work piece.

After a weld, slag and ash may accumulate on the surface. This can be brushed off with a wire brush or ground down for a cleaner surface.

The welding tip is where the current is transferred from the electrode running down the cable into the welding wire. It’s relatively easy for the tip to become damaged and require replacement; it’s a 15-cent consumable part so not a big deal. To replace, pull off (don’t unscrew) the sheath from the end of the nozzle, unscrew the tip, and screw a new one in. The inner diameter of the sheath may also accumulate slag buildup over time which can easily be cleaned out using the tapered end of MIG welding pliers.

To warn people nearby, announce “welding” before starting a weld.

Two directions for welding: push and drag. Not much difference between them other than what you can see: drag allows you to see the bead you’re laying while push allows you to see where you’re going.

Tack Welds

Useful for tacking a piece in place before laying a bead. With about 3/8” of wire protruding from the tip, place the tip against the work surface, and depress the trigger for about 1 second.

Beads

A bead can be produced by dragging the end of the welding wire along the work piece. The bead should be about twice as wide as the work piece is thick; e.g. for 1/8” steel the bead should be about 1/4” wide. The width can be controlled by regulating the speed at which you drag the nozzle across the work surface.

A nicer bead can be laid by using a circular motion, which also produces the aesthetically pleasing waves.

The height of the bead should be fairly low, as the larger the angle between the sheet metal and the bead, the less sturdy the weld will be.

Fillets

This is basically just a bead laid to join two pieces at a right angle. Use the same basic technique, but you’ll be bumping then nozzle up against the side and bottom pieces in order to be close enough to the work area right at the join.

“Series of tacks”

Can be useful when welding very thin stock that welding a bead might melt right through. This works by using the thermal capacity of the previously welded tack to help absorb some of the heat. Place the wire right up against the previous tack for optimal heat dissipation.

Fill-in

You can fix a hole by placing a bunch of dots inside it. It won’t look super pretty, you’ll almost certainly need to grind it down afterwards, and it won’t be very strong. You could use it as an aesthetic (rather than structural) fix e.g. if it’ll be painted over afterwards.