[Mike Bradbury]

I am a welding instructor at a local college and there are several aspects of the GMAW process that knowing them will help you tremendously. Gas Metal Arc Welding is easy to learn but also easy to screw up. Defects like cold lap and undercut can seriously weaken the welds performance and are easy to miss if you are not paying attention to the process.

MIG welding with 75/25 gas is primarily going to be short circuit transfer, which means as the wire is feeding is actually coming into contact with the base metal and completing the circuit. Once it touches the base metal the electricity can then flow through the wire, but the wire cannot conduct the amount of current that is being supplied and it quickly reaches its melting point and melts off causing an arc. This is referred to as the electricity pinching off the end of the wire, this happens many times a second and produces a sizzling bacon sound when the machine is set correctly. If the wire is stubbing into the base metal the it might be that your wire feed is too high or there is something that is preventing the wire to make good contact. With this in mind you can see that where the arc occurs is where the heat is, this is why it is very important with anything other than thin sheet metal, manipulation is required to achieve a sound weld.

If you merely drag the gun along without "stitching" the joint then there is a high probability that one side or the other did not get as much penetration as the other resulting in a weakened weld. Some like the small circle technique while others prefer the "c" shaped progression, still others use the zig-zag method. Some joints weld better with different manipulations but ultimately it is going to what you are most comfortable with.

Setting the machine is very important to get good penetration, a machine set too low may give an OK looking bead but zero penetration. Learn to set the machine by listening to how it sounds and feels while you are welding, you will get the hang of it after a bit of practice. If the weld has a lot of spatter then try turning the volts down by a 1/2 volt at a time until it clears up, or clean your material better. If you are getting undercut, slow down and let the puddle fill in completely or turn up the wire feed a bit to get more wire in the weld. Over lap (looks like a beer belly) change your gun angle or speed up. Cold lap will be very evident when you stop welding and you lift your hood the weld will still be red hot but the base metal right next to it will not be. The edges (toes) of the weld should flow right into the base metal with out big ridges or deep undercuts.

The term stick out refers to the amount of wire that sticks out beyond the contact tip. This distance is very important because each wire has a specific distance that it performs best at. Most solid wires are going to be near 1/2 inch stick out. If you get closer the current increases, and further the current decreases. This swing is about 50 amps from 1/4 to 3/4 stick out, this is a big deal if you are trying to get good penetration and your stick out is too far. Also the further you get away the less gas shielding you will have.

There are many other aspect of the theory, but this shod get you moving in the right direction.
Mike