Mig Welding 101
Wanna know what happens when you run out of, or forget to turn on the shielding gas? The right side was welded without the gas on and is full of pits, spatter, and soot. Not only does it look bad, but once you get up to speed with the welder, you'll be able to tell the gas is off by the sound it makes while welding. | 
Let's turn the autopilot off and see what happens with less voltage and wire speed. Note that the Auto-Set light went off. | 
When the wire speed is too slow, you'll hear and see "popping" as the wire arcs on the surface of the metal, but is burned up before more wire can be fed into the weld to keep it going. Here you can see the ball of melted wire that clumps on the tip of itself. Consequently, when the wire speed is too high, it will also cause "popping" that can be felt in the gun as well as seen and heard as too much wire is basically crammed into the weld and arcing as it melts, brakes, and arcs over and over. | 
Aside from the extremes, you can still make a weld with improper wire speed. From top to bottom, you can see too much to too little with the three in the middle being workable depending on your situation and your ability to react to it. | 
For most MIG welding situations there's no need to get carried away with trying to move the nozzle in a specific pattern until you get into thicker material and double passes. Welding in a steady, straight line has been shown up to now, and is also shown on the right butt and fillet welds. In the middle, is a basic back-and-forth weave that's used for bridging a bigger gap and welding a second pass. On the left, is one I use on thicker material and deep grooves, and is done in a continuous straight line, but welding about 1/2-inch before going back over it about 1/4-inch and repeating. | 
A second pass was done over the first weld here using a weave, while making sure the weld puddle traveled across the surface of the weld underneath and penetrating each piece of metal before weaving to the other side. So, even if you have a 140-amp, 115-volt MIG machine like this Miller, you can still weld thicker material above 3/16-inch; it'll just require a second pass. |
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