Buying a welding machine on eBay

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What is welding?

Welding is the process of melting two metals together, at or above the melting temperature of the metals, so that they are metallurgically bonded.

How is it done?

There are many different types of welding, but the most common and relevant to Home and Lifestyle are Manual Metal Arc Welding (referred to hereon in as MMAW), Metal Inert Gas welding (referred to hereon in as MIG), Tungsten Inert Gas welding (referred to hereon in as TIG) and Oxygen Fuel welding.

MMAW is also know as "stick welding" and  "arc welding".
MMAW is the oldest and most basic form of arc welding. That said, it is also the cheapest to use as the machine is quite simple and the consumables are very inexpensive.
MMAW uses high current, low voltage electricity (usually AC) to form an electrical arc (electricity flowing through air, like lightning). MMAW uses a consumable electrode (the electrode gets used up). The electrode conducts the arc onto the work metal, melts in the process and forms filler metal. The electode is a thin metal stick that is placed into an electrode holder, which the operator holds. The welder is basically a large transformer, transforming mains power into low voltage (around 40 volts), high current (anywhere from 20 - 300 amps, depending on the size of the welder). The whole setup is one big electrical circuit, so a "ground clamp" is required to complete the circuit. The ground clamp clamps onto the workpiece, and represents the neutral terminal, if the electrode is the active. The arc must be manually started; that is, by striking the workpiece with the electrode like lighting a match. This takes a lot of practise to get right.
All metals react with oxygen in the air (oxidising - known as rust in iron or steel), and react much quicker when temperatures are elevated. MMAW overcomes this by including a coating on the electrode that vapourises along with the metal and forms a cloud of slag that keeps the oxygen out. This slag solidifies on the weld and is chipped away to see the weld.

MIG is a modern and sophisticated form of welding. Hence, MIG welding equipment is much more expensive than MMAW.
MIG welding works on the same principle of electrical arcs as MMAW, but has different ways of dealing with shielding and electrodes.
MIG uses a continous wire as an electrode. The wire is continously fed, at the rate at which it is used up into filler metal, from the machine up through the "umbilical cord" to the nozzle/handpiece. There is still a work clamp with MIG welding, but the major difference is the form of shielding. MIG, as its name suggests, uses an inert gas (such as argon) to shield the work area, keeping the oxygen out. The gas is stored in a cylinder (seperate to the welder) and flows into the welder, up the umbilical cord and out the nozzle, surrounding the wire electrode. This means that you can weld more reactive metals, such as aluminum. There is no slag to be chipped away in MIG welding. Gasless MIG welding means that gas is not used, rather a flux in the wire shields the weld. MIG is also easier to use; when you pull the trigger on the nozzle, the wire begins feeding, starting the arc for you, and the gas begins flowing. This makes MIG welding a much faster and easier process of welding - you simply "point and shoot".

TIG is the most modern welding technique listed here. TIG is somewhat similar to MMAW and MIG combined. TIG uses a non-consumable electrode, made out of tungsten. The weld is shielded like MIG welding. TIG, unlike the above two methods, may or may not involve filler metal. This means that for delicate engineering procedures (ie fan blades where balancing the mass is critical) TIG is ideal. When filler metal is applied, it is done so through a seperate filler metal stick. This makes TIG the hardest procedure to use, as it involves using both hands to weld (one for the handpiece and the other for the filler metal). Although TIG is hard to use, in good hands it produces the strongest, most visually attractive welds and can weld almost any engineering metal (stainless steel can only be welded with TIG).

Oxygen Fuel Welding
Oxygen fuel welding is the oldest technique of welding. It involves two tanks of compressed gas: oxygen and a fuel, usually acetelene. The fuel and oxygen is mixed together in the right ratio, then sent out the nozzle, where the volatile mix is ignited, forming a very hot flame. The flame is directed at the work piece until it melts. Filler metal is then added via a separate stick. Oxygen fuel welding can also cut metal, with the right nozzle. The equipment for oxygen fuel welding is usually hoses, a mixer and a variety of welding nozzles and cutting nozzles. Gas welding is much slower than the electrical methods, but can be used to weld thinner metals. The equipment usually includes cutting nozzles, so gas welding is quite versatile.

Which machine is right for me?

Firstly, look at your needs.
If you just want to weld mild steel, and do it cheaply, then MMAW is probably the way to go. MMAW is most suitable and will serve a home handyman well. MMAW welders can weld a large range of sizes of metal, and come very cheap. MMAW can also be used in windy conditions, where the shielding gases of MIG and TIG are blown away.
If you plan to do a lot of welding on a range of metals, and you want it to be easy and quick, MIG welding is optimum. Expect to be spending a larger amount of money than MMAW. MIG welders are ofter large and have large gas bottles, so hence are not as portable as MMAW.
If you have the patience to learn TIG welding, then you can weld almost any metal, and make it look good too. TIG equipment is also quite expensive.Bear in mind TIG can take a very long time before you master it, so for a home handyman, TIG generally is not suitable.
Gas welding equipment is not as cheap as MMAW, and on top of that there is gas bottles that cost. However gas welding is suitable for low volume welding, and can also be fitted to cut metal.

If you plan to do a lot of welding, look at the duty cycle of the welder. This states how long the welder can continously weld before it has to cool down. Ie a duty cycle of 50% means you can weld for 50% of the time.

Dont forget that while a larger electrical welder may look inviting, they almost always require 3 phase power or at least a 15 amp circuit. Read the fine details before you bid.

Safety First!

Welding is a very safe procedure if safety precautions are taken:

  • Always wear a welding mask when welding. The arc is as bright as the sun, but contains far more nasty rays like UV.
  • Always wear protective, non flammable, non synthetic clothes when welding. If you are welding for longer periods, or you are welding with high amperage, wear long sleeves as you can get "sunburnt" from the large amounts of UV rays.
  • Electric shock can kill. Be sensible.
  • The metal that has been welded may not look hot, but even after 10 minutes it can still be hot enough to burn you.
I hope I have enlightened you on some aspects of welding.

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