The ease of MIG Welding has caused a surge of popularity in the home and workshop markets. With a little instruction virtually anyone can be MIG welding with an hour's practice time. The more you practice, the easier it will be and the better your welds, but before you get over confident about your MIG welds, try cutting apart a seam you have made to check penetration and make the other tests (bending two butt welded sample plates in a vice for example) to see if the welds you are making are as strong as they should be.
One of the first few adjustments you will have to make to your MIG machine is to insert the wire and set the drive roller tension. The wire unrolls from the reel through a guide and over a motorised roller, which feeds the wire, through another guide and into the cable going into the torch. Lay your torch and cable out on the floor as straight as possible. Mount the wire spool into the machine, but be careful when cutting the end of the wire, which is usually bent over to lock into a hole at the side of the reel. Don't cut the end loose until you are ready to feed it.
Hold the cut end with some pliers and carefully file or sand the cut end until it is smooth. A sharply cut end may snag as it travels through the cable or torch. Insert the end into the guide, loosen and swing away the wire tension adjuster and feed the wire over the groove in the drive roller and on into the next guide, still holding the wire firmly most machines are set up to handle several sizes of wire, but you must be certain that you use the drive roller groove that is the right size for the wire you are using. Most rollers have two sizes on them and can be turned around on their shaft. When six inches or more of wire have been fed into the cable, put the wire tension adjuster back in place over the wire, keeping the wire centred in the groove. Turn on the welder and put the wire feed speed about one quarter of the way and pull the trigger. At this point, adjust the pressure on the wire tensioner until the rollers are feeding the wire, and then turn it 1/4-turn more. With the nozzle and contact tube unscrewed from the end of the torch, keep the torch cable straight and run the machine until the wire comes out of the torch. Slip the correct size contact tip over the wire and screw it in then attach the nozzle.
The final check on wire tension is to make sure the wire can "slip" if necessary. Eventually in your MIG practice, you will virtually weld the wire to the contact tube. When this happens and you are still pulling the trigger, the machines tries to feed more wire but it can't come out of the gun, so it jams up inside, in what is called a "birds nest". To prevent this, bend the wire to the side of your torch and put the nozzle right against the shop floor, simulating a stuck wire. While watching the drive mechanism inside the machine, pull the trigger. The drive rollers should have just enough tension to always pull the wire through, but not enough to prevent slippage in case of a jam.
Now you have to set up the bottle. With the bottle firmly secured, either on the back of your machine or chained against a wall attach the regulator to the bottle. Then attach the reinforced braided gas hose to the regulator with a hose clamp, connecting the gas to the machine. Keep the two regulator gauges protected from impact, and always keep the gas valve turned off when you are not using the machine. Turn the cylinder valve all the way, not just a few turns. You can check any of your gas connections for leaks using some soapy water solution as bubbles indicate leaks. Adjust your gas regulator to the proper flow rate (cubic feet per hour rate according to your instructions) for the material and wire you are working on.
Now you're ready to practice. Select some 1/8" steel, set your amperage according to the suggestions in your instructions, and set your wire speed a little higher than what's recommended. The wire speed is key to getting a good weld with proper heat and penetration. Tuning the speed is easy. Listen to the sound the welder is making as you make a straight bead. If the speed is too fast, most of the wire coming out will be red hot and there will be a loud crackling noise. Gradually adjust the speed down until you get a steady sizzling sound. The wire should now be melting right at the weld. Practice tuning in the wire speed at different power settings and metal thickness.
There are two torch-holding techniques for MIG welding, forehand and backhand. The first is when the torch is angled and moved such that when the weld is taking place ahead of the torches direction of travel. The backhand is when the weld takes place behind the torch. Most beginners find the back hand technique (sometimes called dragging the weld) works best on most steel projects, giving the welder a better view of the puddling action. In either style the torch is about 35 degrees to the work. If you find your self doing a vertical seam you can travel either up or down, but use the forehand technique (sometimes called pushing the weld) only when going up and the backhand technique going down, keeping the arc on the puddle's leading edge at all times for good penetration.
If you are working with materials under 1/8" thick, most steels can be MIG welded without bevelling the edges, but you should tack the parts together with a slight gap between them before finishing off the seam with a full weld. This assures good penetration. On 1/8" and thicker materials, the edges should be bevelled before tacking them. If you find that you are consistently burning through the material the heat setting is too high, or you are moving too slowly. If it is "cold" appearing and not penetrating, the amperage may be too low, or you may be travelling to fast. You will adjust your travel speed with the torch by watching how the puddle develops, moving the torch along at a slow, steady rate. One of the great advantages of the MIG process is that your torch is right down close to the work, and there is much more intense light, heat and spatter. You can get your head closer to the work to really see the puddle progress. Keep a sharp pair of wire cutting pliers on your welder at all times. You will use it when you start each time to maintain around 1/2" of wire sticking out of the gun.
If you attempting to weld thicker materials, or two edges that have irregular gaps, you can weave the arc back and forth from side to side as you travel, bridging the distance between the two edges.
Sometimes a start and stop technique works well for bridging gaps or filling holes. You watch the arc and weave from one side to the other, then pause (let go of the trigger) for a second with the torch in the same spot, allowing the puddle to solidify. Then pull the trigger again, building a new puddle onto the last one. To fill a hole, just keep making tack-welds around the edge of the hole until you have made it smaller. Let the metal cool, and then add an inner row of tacks inside the first ring until you close up the hole with filler metal. This is often done in filling sheet metal holes in bodywork, but MIG welding in a circle of new metal or the same thickness should fill any holes larger than a 1 pence piece.