Let me start with safety, DO NOT SKIP, YOU NEED TO READ THIS. The UV rays that are produced at the weld can and will cause damage to your eyes you know that. But did you know that it WILL damage to your skin as well? I have gotten "sun burn" from welding. I have only "peeled" from regular sunburn a few times in my life (I have a dark Portuguese complexion) I have peeled from MIG welding more! This was a long time ago, I would never let that happen again. Wear a light long sleeve cotton shirt at the very least. You want jeans and high top boots on too. Wear welding gloves, go down to the local auto parts or hardware store and get yourself some nice SOFT gloves. Some are made so stiff that it is hard to work in them. I got some at ACE hardware that are dang near sensual ?. Get a good helmet, I have a Cherokee that is only about $90.00. It is a hands free helmet that you open the shaded lens with your chin! I have had it for about 15 years without a problem. When open it gives you a full 4x5-inch or so CLEAR lens, unlike the "self darkening" helmets that are always shaded. You can flip up the lens and grind anytime you want. I want the full control of when to see through a shade or not. I highly recommend this helmet. Also, another thing that I have only begun using a year or so ago (I hate thinking about how long I didn't use it) a welding respirator. A 3M NOISH approved are available at ACE hardware for about $18.00. If you are welding with weld-thru or "E" coat primer you are making ZINC FUMES! And can get "zinc fume fever" VERY easily. Even when welding clean metal, you are still making fumes that are hazardous. Be sure the respirator fits under your helmet. The Cherokee helmet for instance doesn't have room for a cartridge mask so a single throwaway is all that will work. HEAR ME NOW BELIEVE ME LATER, PROTECT YOURSELF. Even if you don't care about the protection because you are such a bad-a$$, do it because you will produce a better weld. It is hard to lay a nice bead when you have a hot molten ball of steel in your shoe or pants. ?
I would like to start with this "disclaimer". These are tips in welding in the real world by real human beings that are not pro welders. A pro welding under controlled conditions would possibly disagree with some things here. That is fine if you get other input to better your skills, this is for basic understanding of MIG welding in the real world.
MIG welding is actually just a "controlled short". It is a short just like if you touch your two battery cables together on your car and it sparks. The MIG does the same thing, you just are in control of it. You are melting the metal with heat created by this "short".
Basic principles of MIG welding are this: You have VOLTAGE, the pressure that pushes electrons through a circuit. Then CURRENT, (same as amperage) the amount of electrons being pushed. And RESISTANCE this restricts electrons from flowing. The gas (Argon, CO2, or a mixture of both 75-25% is most common) is blowing away the impurities in the air around and on the surface of the weld. If there is a breeze you may need to up the pressure from the recommended 25-30 cubic feet per minute or 3-4 PSI.
What the heck does this mean?
The arc that is formed when the wire comes out of the gun and hits the metal is your "CIRCUIT" (or current path), The welder has to have enough voltage to keep the current flowing. You control these variables with the "heat" switch (VOLTAGE) the wire speed knob (CURRENT) and the "stick-out" and or "arc length" (RESISTANCE).
The MIG welder has to be "tuned" just like a spray gun or your carburetor on you car. That perfect balance between too hot a weld (blowing holes) and too cold a weld (not enough penetration) is where you want to be. Fortunately this balance is pretty wide for at least "normal" welding on your car.
"Hot" or "Cold" weld. I will refer to welds in this way to describe them. Extreme "Hot" would be heavy melting, puddling, burning holes. Extreme "Cold" or "Cool" would be not enough melting thus not enough penetration.
To find this balance, get a piece of scrape metal that is the same thickness as the metal you will be welding (or very close to it). Clamp your ground clamp to it and lay a bead on it. Start the bead with the MIG set at the recommended settings on your welder. You don't have to lay a bead to be proud of here, just weld. If you have someone to help this can be a big help, but if you don't go it alone, you'll need to learn to do it someday anyway. While you are laying a bead have your partner turn the wire speed (CURRENT) up and or down till you hear that perfect ZGHZGHZGHZGHZGHZGHZ and you will see in the bead that it is perfect. The weld will be laying out relatively flat with no "undercut" (at the edge of the weld there isn't a low spot where the weld has burned away the metal and not replaced it with melted wire). If your wire speed it too slow there will be gaps in sound then pops. Watch the weld and see if the weld is "crawling" up the wire, that is a dead give away speed is too slow. If it is too fast, it have faster pops as the wire is burning away and quickly hitting the metal arcing again. These sounds can be very subtle so it may take a while to learn the sound, have patience.
Now if you have to do this alone, just hold the gun one of your hands and have the other on the wire control, it is very awkward at first but you will be able to do it well in no time.
I have to clear up an "old husbands tale" about the sound of the weld. I have heard and read many times that your weld should sound like "bacon frying". Let me tell you right now, if your welds sound like bacon frying, you are laying some crappy welds! It should sound more like an electric buzzer "ZGHZGHZGHZGHZGHZGHZGZHZGH". If it sounds like bacon frying "ZGHZZ-POP-ZHZGHZ-POP-CRACKLE-ZHZGZ" You are doing something wrong.
So, what does this technical stuff about voltage and current have to do with learning to weld? Well if you understand WHAT is happening, it is easier to MAKE happen what you want.
If you are welding a little too "hot" and are blowing holes, you could adjust this simply by creating more RESISTANCE by lengthening the "stick-out" or distance that you are holding the tip of the gun from the surface being welded. So as you weld a bead, if you see the weld puddling too much and fear that it may blow through, you can back the tip off the metal a little and create more resistance. If you are welding thin metal you can start with a longer stick-out and produce a "cooler" weld also. Now, this has to be done with caution because if you rise up too much, the gas does not shield the weld leaving the weld porous.
The direction of travel and speed WILL also effect the "heat" of the weld. If you use the "drag" technique with the gun dragging away from the weld, it will be "cooler". If you are using the "push" technique with the gun pushing into the weld it will produce a "hotter" weld. So, if you were to start on the left side of a seam and with the gun leaning to the right at 45 degrees welding from left to right, this would be "dragging". If you were to start on the right side of the seam with the gun leaning to the right as before and welding left, to the left side of the panel, that would be "pushing" the weld. I find that when welding sheet metal if there is a burning through problem, changing to the pull technique will do the job most of the time.
Of course speed is obvious, the slower you go the hotter and flatter the weld. The faster you go the taller and cooler the weld. All of these techniques can be used with one and other through out even one seam (though you wouldn't likely change from push to pull or vice-versa) to control the weld. Ideally you would have the weld set up and cleaned so you wouldn't have to do this, but realistically you do have to change gun distance speed as you weld.
The first BIG tip I can give is to have a nice pair of angle wire cutters beside you at all times. You will want to cut the wire off at the proper length EVERY time you start a weld. This does two things, first of all it gives you the "stick-out" that you want every time. Second, it will give you a sharp tip to "pierce" the metal or weld-thru primer. Third, that little ball of metal on the end of your wire, it is oxidation! That is right, if you leave it there, you are pushing RUST into your nice new weld! I learned this tip from a certified pro welder (underwater even) and it totally changed the way I weld.
The second BIG tip is to have everything CLEAN and set up TIGHT for the weld. Like with painting, the preparation is KEY. Even the slightest grease, tar, paint, rust, etc. can cause BIG problems. As a rule the smaller lower the voltage your welder, the more critical this is. Clamp the pieces TIGHT, and keep the metal clean at least one inch from the weld with two or three inches preferably.
So lets weld a "lap" weld where a piece of sheet metal is laying over another and you will be welding the top piece to the one laying under it. I find that a pull technique with the gun at a 45 degree angle and pointing right straight at the seam the most effective. But the thing is, the upper sheet will burn MUCH easier than the bottom. The edge of that sheet has much less of a "heat sink" effect than bottom sheet. Sort of like starting to burn that log in your fireplace, if you start on the edge where it is thin it will start burning MUCH easier than if you started right in the middle right? Well, when you are welding this lap weld you want to start the weld on the bottom piece and even concentrate the weld on the bottom. This may sound funny but if you get a piece of paper and fold it so you have a "lap" seam you can see what I am going to tell you. Hold this seam flat with a few pieces of tape but leave it exposed so you can "weld" it with some white glue. Lay a bead of glue on the bottom paper right next to the edge of the upper paper just as I described above. When you get real close to the upper paper edge you will see the glue sort of "grab" on to the edge. You don't even have to move the bead all they way over to it, if you are very close and you just barely hit it with the glue, it will "hang on" to it. Continuing the bead down the edge with most of it laying on the bottom sheet, the edge of the bead will "grab" the edge of the paper, without any effort on your part.
Your weld bead will do the same thing. When you are concentrating on the bottom, harder to melt metal just move the bead over to touch the edge of the top metal and it will "grab on" to it. You can run the bead with most of the heat being directed on the bottom and just "grabbing" the top without blowing it away.
This goes for plug welding too. The size of the hole depends of a few factors, usually 5/16" is the norm. But sometimes you could go down to 3/16" or up to 3/8" depending on the thickness of the metal or how important the strength of the weld is. That sounds funny but if you are welding in a "backing" for a butt weld for instance, it is only being plug welded to hold it there till you lay the bead into it while welding the two adjacent pieces together. As before with lap welding you want to direct the weld into the "bottom" of the hole to hit the bottom piece of metal first, then melting it into the surrounding metal just like the lap weld. If you have perfectly prepared plug welds, you should easily be able to fill the hole with weld leaving the top almost flat. If you are ending up with a large hump, you need to raise your voltage, or wire speed to weld "hotter". Weld a number of tests before going on to your car. Weld as hot as you can without burning through and look under the panel to see your penetration If the weld is coming through the bottom producing a hump under it, the weld is too "hot". Either speed up, lengthen the arc or stick-out or lower voltage.
Hope this helps you produce better welds.