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MIG Welding Equipment, Gas Metal Arc Welding (GMAW). GMAW is most commonly referred to as “MIG” welding, and the following text will use “MIG” or “MIG welding” when referring to GMAW. MIG welding is a process in which a consumable, bare wire electrode is fed into a weld at a controlled rate of speed, while a blanket of inert argon gas shields the weld zone from atmospheric contamination. In addition to the three basic types of metal transfer that characterize the MIG welding process, there are several variations of significance.MIG Welding: Pulsed sprayPulsed spray MIG welding is a variation of the MIG welding process that is capable of all–position welding at higher energy levels than short-circuiting arc welding. The power source provides two current levels; a steady “background” level, which is too low to produce spray transfer; and a “pulsed peak” current, which is superimposed upon the background current at a regulated interval. The pulse peak is well above the transition current, and usually, one drop is transferred during each pulse. The combination of the two levels of current produces a steady arc with axial spray transfer at effective welding currents below those required for conventional spray arc welding. Because the heat input is lower, this variation in operation is capable of welding thinner sections than are practical with the conventional spray transfer.
Arc spot welding as metal arc spot welding is a method of joining similar to resistance spot welding and riveting. A variation of continuous gas metal arc welding, the process fuses two pieces of sheet metal together by penetrating entirely through one piece into the other. No joint preparation is required other than cleaning the overlap areas. The welding gun remains stationary while a spot weld is being made. Mild steel, stainless steel, and aluminum are commonly joined by this method.
Electrogas MIG welding. The electrogas (EG) variation of the MIG welding process is a fully automatic, high deposition rate method for the welding of butt, corner, and T-joints in the vertical position. The eletrogas variation essentially combines the mechanical features of electroslag welding (ESW) with the MIG welding process. Water-coded copper shoes span the gap between the pieces being welded to form a cavity for the molten metal. A carriage is mounted on a vertical column; this combination provides both vertical and horizontal movement. Welding head, controls, and electrode spools are mounted on the carriage. Both the carriage and the copper shoes move vertically upwards as welding progresses. The welding head may also be oscillated to provide uniform distribution of heat and filler metal. This method is capable of welding metal sections from 1/2 in. (13 mm) to more than 2 in. (5.08 an) in thickness in a single pass. Deposition rates of 35 to 46 lb (16 to 21 kg) per hour per electrode can be achieved.
MIG Welding Equipment
NOTE
Different types of MIG welding equipment are available through normal supply channels. Manuals for each type must be consulted prior to welding operations.
The MIG welding unit is designed for manual welding with small diameter wire electrodes, using a spool-on-gun torch. The unit consists of a torch (fig. 5-23), a voltage control box, and a welding contractor (fig. 5-24). The torch handle contains a complete motor and gear reduction unit that pulls the welding wire electrode from a 4 in. (102 mm) diameter spool containing 1 lb (0.5 kg) of wire electrode mounted in the rear of the torch.
Three basic sizes of wire electrode may be used: 3/32 in. (2.38 mm), 3/64 in. (1.19 mm), and 1/16 in. (1.59 mm). Many types of metal may be welded provided the welding wire electrode is of the same composition as the base metal. The unit is designed for use with an ac-dc conventional, constant-current welding power supply. Gasoline engine-driven arc welding machines issued to field units may be used as both a power source and a welding source.
MIG Welding Torch Nomenclature
MIG Welding Torch: Contact tube (fig. 5-23)The MIG Welding torch contact tube is made of copper and has a hole in the center of the tube that is from 0.01 to 0.02 in. (0.25 to 0.51 mm) larger than the size of the wire electrode being used. The contact tube and the inlet and outlet guide bushings must be charged when the size of the wire electrode is changed. The contact tube transfers power from the electrode cable to the welding wire electrode. An insulated lock screw is provided which secures the contact tube in the torch.
MIG Welding Torch nozzle and holder (fig. 5-23)The nozzle is made of copper to dissipate heat and is chrome-plated to reflect the heat. The holder is made of stainless steel and is connected to an insulating material which prevents an arc from being drawn between the nozzle and the ground in case the gun canes in contact with the work.
Inlet and outlet guide bushings (fig. 5-23). The bushings are made of nylon for long wear. They must be changed to suit the wire electrode size when the electrode wire is changed.
Pressure roll assembly (fig. 5-23). This is a smooth roller, under spring tension, which pushes the wire electrode against the feed roll and allows the wire to be pulled from the spool. A thumbscrew applies tension as required.
Motor (fig. 5-23). When the inch button is depressed, the current for running the motor comes from the 110 V ac-dc source, and the rotor pulls the wire electrode from the spool before starting the welding operation. When the trigger is depressed, the actual welding operation starts and the motor pulls the electrode from the spool at the required rate of feed. The current for this rotor is supplied by the welding generator.
Spool enclosure assembly (fig. 5-23). This assembly is made of plastic which prevents arc spatter from jamming the wire electrode on the spool. A small window allows the operator to visually check the amount of wire electrode remaining on the spool.
NOTE
If for any reason the wire electrode stops feeding, a burn-back will result. With the trigger depressed, the welding contactor is closed, thereby allowing the welding current to flow through the contact tube. As long as the wire electrode advances through the tube, an arc will be drawn at the end of the wire electrode. Should the wire electrode stop feeding while the trigger is still being depressed, the arc will then form at the end of the contact tube, causing it to melt off. This is called burn-back.
MIG Welding Contractor (fig. 5-24)The positive cable from the dc welding generator is connected to a cable coming out of the welding contactor, and the ground cable is connected to the workpiece. The electrode cable and the welding contactor cable are connected between the welding contactor and voltage control box as shown.
MIG Welding Argon gas hose (fig. 5-24)This hose is connected from the voltage control box to the argon gas regulator on the argon cylinder.
Electrode cable (fig. 5-24). The electrode cable enters through the welding current relay and connects to the argon supply line. Both then go out of the voltage control box and into the torch in one line.
Voltage pickup cable (fig. 5-24). This cable must be attached to the ground cable at the workpiece. This supplies the current to the motor during welding when the trigger is depressed.
Torch switch and grounding cables (fig. 5-24). The torch switch cable is connected to the voltage control box, and the torch grounding cable is connected to the case of the voltage control box.