Article on The Internet Mini Encyclopædia
MIG stands for Metal Inert Gas. A wire is fed through the
welder wand automatically. The Arc is struck from the end of the wire, which
deposits the molten wire on the surface to be welded (which is earthed). The Gas
stops it oxidizing and also stops oxygen getting in the weld.
TIG stands for Tungsten Inert Gas. Basically the same but the welder uses a separate rod rather than feeding wire through the wand.
MIG is for iron & steel, whereas TIG can also used for steel or aluminium.
Because both forms of welding use an arc, there are different amperage welders depending on whether you are joining sheet stock or thicker metal. Witness a friend who tried to weld his mini with a MIG welder designed for bar steel - and ended up burning out the A-panel!
A weld should be strong and even. If the arc is not struck properly, this results in what is known as "chickenshit welding" because the wire/rod is unevenly applied due to the resistance, making the weld look like a load of chickens have spent the night there.
MIG = Metal Inert Gas
TIG = Tungsten Inert Gas
As you see both use an inert shielding gas. The purpose of which is twofold:
MIG use a metal central electrode that is used up as filler material. (The observant mind will now notice it is not possible to do a MIG weld without building up a mound)
TIG use a Thorium/Tungsten electrode that is not used up; unless you contaminate them, which you do a lot when low on experience. Then you have to grind them up before continuing, or your weld will be porous and weak because of inclusion of impurities (see also Tom Harvey's points on grinding when using TIG).
If you need filler material you add that using a rod as you do in Oxygen-Acetylene welding. Indeed TIG welding looks much like O-A welding, and some experience using this method is an advantage. The best looking TIG welds are the ones where no filler rod is used, the O-A welder will probably also recognize this from experience.
The shield gas is the same for MIG and TIG. I use Argon, as it is the one I found can be used for most of my purposes. Gas mixes will be better for some uses, but unusable for others.
The power source for a TIG welder must be continously variable (DC) while you weld. Some use a pedal, I use a thumb-dial on the torch. Without it, you will still produce a bright light, but then mostly from swearing. TIG without HF-start is an inconvenience, but as it drives up the price by quite a bit I suggest you do without it. If you plan on using TIG for aluminium, it must have AC which also drives up the price. I would hope that more hobbyists discover TIG, as that will certainly bring down the prices as it did with MIG. It will certainly result in better quality welds, as (with TIG) you usually get a good weld or a big hole, which is easy to tell apart.
The heat is much more local with TIG than with other methods, and also with a much finer control. This makes it possible to weld much closer to objects that do not like heat.
Using oxy with various steels
Oxywelding mild or sheet steel is fine, as you are aware. However, oxywelding of high-carbon, carbon/nitrogren treated steel (i.e. nitriding, tuftriding, etc), or iron castings or steel forging is out, either because it will either destroy the hardening or screw up the grain of the forgings, leading to massive stress raisers and reduced strength, apart from the difficult of successfully gas welding something 1/2 inch thick or more. For example, if you look in the yellow pages you can see there are many specialist engineering companies that advertise they can weld castings, this is done by one of several convoluted processes. They can also do stuff like spray new metal onto worn cranks and regrind them to original size (not cheap but neither are S cranks).
If you are brazing to a non structural type part, e.g. solving a heater-tap emergency or similar, then you are likely to get away with it, but relieving a 4-ton forging (i.e. front suspension lower arm) of all the strength it gained by being forged is IMHO not wise.
FWIW, most modern cars contain structural body panels that it is not permitted to weld as the composition of the steel is such that its strength is destroyed by heat. This is what to expect in the new Mini 2000 for the bits that are not made from plastic, I guess (sigh).
Tom Harvey also gave us this information for TIG welders:
"I am a self taught TIG welder. I had some experience with SMAW (stick welding) and MIG welding but wanted to get into TIG. I was religous about grinding my tungstens and keeping them clean.
I recently attended a welding school presented by one of the USA's foremost welding experts and he was actually taught by the processes inventor. This gentleman suggested that we try his method of not grinding tundstens. I was rather surprised ,as all of us in the class were, since we had always ground ours. I tried it and found my welds looked great. I passed the ASME section IX qualification tests for all positions carbon and stainless steel with unground tunstens.
This had made me a believer. The explanation behind not gringing is that the very end point of the tungsten is immediately burned off when the arc is started and of course the ability of the diameter tungsten to carry amperage is reduced since the actual area is smaller then the unground area.
I am not really sold on either way... to grind or not to grind. Just a idea for those of you TIG welders to try. One last note, all welders who passed ASME tests for aluminum, carbon or stainless all used unground tunstens."
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