Although the subject of welding covers a lot of ground, it really doesn’t take a lot of technique or expensive equipment for the small (and sometimes not so small) jobs that the average home craftsman might tackle in his garage workshop. You’ll probably be working with steel about 1/16 to ¼ inch thick. (Leave aluminum welding to a professional until you learn to weld.) These jobs can be done well with an oxygen/acetylene welder, a small 110-volt arc-welder, or a wire-feed welder, often referred to as a MIG welder. And you can teach yourself how to weld in an afternoon. Of course, you can rent the equipment. But you might consider purchasing it because it’s relatively inexpensive. You can buy a small arc-welder for right around $120, while a mig welder or an oxy-acetylene welder will cost more. Today, oxy-acetylene welders are nearly obsolete (if you're thinking only in terms of welding). If you can afford to spend more, we recommend a mig welder (wire-feed).
What actually happens to work when you weld is very basic and simple. In essence, you’re melting two pieces of steel together. Plus you’re adding a little extra steel with the welding rod. The heat to melt the material comes from the flame of the oxygen-acetylene welder. Or it’s caused by the electricity arcing from the rod or wire to the work if you’re using an arc-welder or mig welder. Good technique simply means that you’ve learned to do this smoothly and in a way that results in good penetration and a thorough mixing of the molten metals. If you just melt the rod or wire and let it stick to the surface of the work, that’s bad technique… and the weld will not hold. Good technique comes with practice, and it really doesn’t take long to get the hang of it.
Naturally, you wear dark goggles to protect your eyes from the flame of a gas-welder. Wear a full-face mask and welding gloves when you’re arc-welding or mig welding to protect yourself from spattering metal ejected from the weld. An assistant should not look directly at the arc or the flame. Both are intense enough to cause serious eye burns. Make sure there are no flammable materials nearby.
Oxygen-Acetylene Welding:
We begin with oxy-acetylene welding because that's where the techniques were originally developed. If you understand this type of welding, you will have the basis for a good understanding of arc and mig welding.
Oxy-Acetylene welding equipment consists of two large tanks  (one containing the oxygen and the other containing the acetylene), a regulator assembly at the top of each tank, a pair of hoses leading from the regulators to the torch handle and the torch handle itself. The first thing to do is adjust the line pressure, which is controlled by the large wing nut on the front of each regulator. Close both valves on the torch handle (one controls the oxygen and the other controls the acetylene). Then slowly open the large valves at the top of the tanks. It’s a good practice to stand to one side when doing this because the sudden pressure could blow out the face of a defective gauge and send parts flying. Next, open one valve on the torch handle about a half turn, then screw the regulator wing nut in or out until the line pressure reads about 5psi. Close the valve, then adjust the other line to the same pressure.
Use a No. 2 or No. 3 welding tip, and begin by experimenting with a piece of scrap metal. The basic idea is to use the torch to create a small puddle of molten metal on the work, then put the end of the welding rod into the puddle. This melts the rod, causing it to mix with the molten metal, which bonds the two pieces together. If you do that correctly, you will have made a good weld. If you don’t apply enough heat to get the work to puddle, but instead quickly melt the rod onto the work, the weld will not hold.
If you apply too much heat to a single spot you’ll melt away the work entirely, leaving a large hole. Good technique involves adjusting the torch to the correct intensity for the thickness of the work, creating a good puddle, and moving the puddle along at the correct rate as you feed the correct amount of rod. Although we're describing gas welding, the same type of puddling must also occur with arc or mig welding. A feel for that comes only with practice.
No matter how you try, you cannot make a good weld unless you correctly adjust the torch. Figure 1 shows four types of flames. The difference between them is caused by the relationship between the amount of oxygen and acetylene, which is controlled by the two valves on the torch handle. The flame you’re after is the neutral flame which comes just as the acetylene feather of the carburizing flame disappears into the inner cone. The correct intensity is determined by the thickness of the work and by how rapidly you move the puddle along. Beginners tend to do better with a lower flame because it allows them to work more slowly.
Figures 2, 3 and 4 show how to hold and move the torch. Angle the flame toward the work with the rod coming in from the opposite direction. Once the puddle forms, move the torch in circular or semicircular patterns across the weld as you slowly advance the puddle. Continue to feed the rod, but don’t force it into the work or it’ll stick. When things start flowing correctly, you’ll know it’s right.
Arc Welding:
Like adjusting the intensity of the flame with gas-welding, with arc-welding  you’ll have to set the heat range by changing the amperage setting on the welder. This is done either with a dial or by plugging the hot lead into one of several fixed-amperage receptacles on the welder. As a general rule, use 5/32-inch rod and 75 amps on 1/8-inch material, changing to 100 amps for ¼-inch material. Thinner (1/16-inch) material is more difficult to weld and you’ll have to experiment with smaller rod and lower heat ranges (amp settings).
Although what you’re after is basically the same, technique is different with arc-welding. You have to establish an arc, which melts the work and the rod simultaneously and deposits the rod on the work. To establish an arc, simply scratch the end of the electrode (rod) across the work, just like a match. (With a mig welder, you simply hold the tip close to the work and pull the trigger.)
As soon as the spark jumps, move the tip of the electrode slightly away from the work, then move the arc over to the weld and begin welding. If you move the electrode too far away you’ll break the arc. If it’s too close, the electrode will stick to the work, which probably will happen several times before you get the hang of it. A good arc has a crisp, crackling sound.
It’s good practice to lead the arc with the electrode just a little so you can see the puddle develop. Keep in mind that the electrode continues to melt away so you’ll have to move it closer to the work as you weld. Move the electrode along the weld slowly, allowing the molten puddle to follow. If you move too rapidly the puddle will not fully develop and the weld will be weak. Figure 5 shows the arc weld graphically. Also, take a look at the videos showing tips for arc welding by following the links at the bottom of this page.
Mig Welding:
Mig welding is by far the easiest to learn. With a few minutes of practice, you'll be making professional welds. Mig welding is similar to arc welding, but the wire is automatically fed from the end of the gun at a  preset rate. A gas bottle provides gas, which is expelled from the end of the gun to shield the weld from the ambient atmosphere and avoid the oxidation caused by oxygen. You can make unshielded welds (without gas), but the welds will be highly oxidized which will make them weak and brittle. If your system is not equipped with gas, you must use flux cored wire to avoid highly oxidized welds. The key to good mig welding is to set the wire feed-rate and the electric arc intensity at the correct values for the material. You can do this only by experimentation. The size of the wire in mig welding is also important, and the tip must match the wire size. Follow the instructions that came with your machine. Your machine will probably come with tips and wire for thin-wall material (roughly 0.0625 to 0.120 inch thick). You might have to buy larger wire and the appropriate tip for thicker material. For additional information on setting the wire feed rate and the heat range, as well as techniques of applying the weld, take a look at the online videos linked below. The first two links are for mig welding, and the last two are for arc welding.
Recommendations:
If you're just starting out and you want to weld with the least investment in equipment, buy an inexpensive arc welder - about $120 and up. But good arc welding will take a little more practice than mig welding. If you can spend more, take a look at the HTP MIG 130 (the above link). The base price includes flux-cored wire (no gas system). Personally, I do not like flux-cored wire. Spend the extra money and get a gas bottle, a flowmeter, and hard wire (no flux core). You'll be much happier with the system. And you'll be making good welds quicker than you think - virtually from the outset. The videos linked below show how easy it is. The website linked from the HTP MIG 130 image (above, right) also has very good welding videos. I recommend that you watch them all before making your decision on which type of welder to purchase.
On Line Videos:
How to Setup a Mig Welder (Also see videos on website linked via the HTP image above.)
Mig Welding Techniques
Arc Welding, Beginner Tips
Arc Welding a Lap Joint:
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