Spot welding is quickly becoming more popular in the welding industry, because it saves money on materials and does not require as much training as regular welding. It is a very efficient way to bond two pieces of metal. The most important key is to set the welder correctly so that it does not burn through both pieces of metal completely.
Clean, Stack and Clamp
The first step is to remove any oils or debris such as rust from the metal, especially where you intend to weld it. If there is a thin coating of oil on the pieces from the steel yard, simply use rubbing alcohol on a clean rag to remove. If there is rust or other oxidation present, a wire brush will usually do the trick. Next, stack the two pieces of metal as they are to be welded. Then, using locking pliers or welding clamps, lock them together so that they cannot move. Doing this takes out all chances of the metals shifting and becoming misaligned during the welding process.
This step is the one that takes the most practice and experience to get right. There is a chart inside the door of your welder which says where to set the current for each different thickness of metal it is capable of welding. The trick here is to burn all the way through the top piece of metal, and only about halfway through the next. Therefore, if you are spot welding two sheets of metal that are an eighth of an inch thick each, you will want to set the welder for somewhere between one-eighth and one-quarter inch welding. That way, the first sheet will be burnt through, but the second will not.
Now that the material is prepped and the machine is ready to go, throw on your welding hood and leather gloves and create the spot weld. Keep the tip of the MIG gun about a quarter-inch away from the surface of the metal, and pull the trigger. Do not move the gun as you would in regular welding. Rather, hold it still so that it penetrates the steel in one spot, creating a round weld. How long you leave the arc on will depend on the thickness of the material and the capabilities and settings of your machine, which is where the practice comes into play.