Metal-inert gas, or MIG, welding is one way to join separate pieces of metal. This semiautomated technique involves the use of a shielding gas to protect the weld from the air until it's complete and a wire feed that supplies electrode to the weld at a constant rate. There are two types of power sources you can use for MIG welding: constant voltage and constant current. Voltage is measured in volts, while current is measured in amps.
MIG welders heat the metal through use of an electrical arc, where current jumps an air gap between two electrodes. The voltage is the electric potential, and it's related to current by a famous formula called Ohm's law, which states that voltage equals current multiplied by resistance. A constant voltage system maintains the voltage at an unchanging level. This means that if resistance changes, the current will change also. This type of system uses the rate of wire electrode feed to control current.
Constant current systems are somewhat more unusual in MIG welding, because it's important to be able to control the wire feed rate. In this case, the device maintains an unchanging current, and the device automatically controls the voltage by controlling the rate of wire electrode feed. In general, this type of system works best for large-diameter electrodes and automated welding, since these circumstances don't require rapid changes to the wire feed rate.
Amps and Volts
You can use the controls on your MIG welder to change the voltage. As you'd predict from Ohm's law, increases in the voltage will increase the current also. Most MIG welders are direct current (DC) rather than alternating current (AC) because DC yields a more stable arc, whereas AC would briefly extinguish the arc at one point during the course of each half-cycle. Reducing the wire speed reduces the current, whereas increasing wire speed will boost it.
Correctly setting the wire feed speed -- and thus the current -- is important, because it affects the weld quality. The wire must be fed quickly enough to ensure welding occurs smoothly, but if the wire speed is too fast, the high current creates a low of heat and may lead to "blow-through" holes. This is especially true for thinner sheets of metal. The choice of voltage is usually dictated by the thickness of the metal; the user's guide for your device will suggest what levels you should use.
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