248 Section 3 Gas Metal and Flux Cored Arc Welding Copyright Goodheart-Willcox Co., Inc. Gas Metal Arc Welding Principles In GMAW and FCAW, a welding arc is struck between the electrode and the base metal. The welder presses the trigger on the welding gun to start the process. Shielding gas begins to fl ow. Electrode wire feeds out of the gun and a welding arc is struck. The arc melts the base metal and forms a weld pool. Metal from the electrode melts and enters the weld pool. The gun is moved forward to keep the weld pool the correct size. Welding continues until the welder releases the trigger. After the welder releases the trigger, the electrode wire stops feeding and the arc also stops. Shielding gas fl ows for a short time after the welding stops to protect the weld and base metal as they cool. Preparing to Weld For GMAW or FCAW, begin by setting up the equipment. Connect the welding machine, wire feeder, welding gun, and shielding gas as described in Chapter 16. Check that all electrical, shielding gas, and water connections are tight and leakproof. Also, make sure that you are wearing the proper protective clothing for welding. After checking the equipment, decide on the type of welding you will be doing. Answer the following questions: • What type of base metal is to be welded? • How thick is the base metal? • In what position will the welding be done? • What type of metal transfer will be used? Select an electrode recommended for the type of base metal you will be welding. Refer to Chapter 16 for information regarding the type and diameter of GMAW or FCAW electrodes to use with that base metal. Install a spool of electrode wire on the hub of the wire feeder. Determine the type of drive rolls best suited for the type of electrode to be used. Select and install rolls with the correct groove diameter. Adjust the pressure setting on the rolls to feed the electrode wire properly. See Figure 17-1. Select a contact tube for the diameter of electrode wire you have chosen. Examine the contact tube. The hole in the center should not be worn or have spatter on the end. Install the contact tube into the gun. Attach the gas nozzle onto the end of the gun. Next, select the shielding gas and the fl ow rate recommended for the base metal you will be welding. Select the type of metal transfer to be used. Chapter 16 describes the type of shielding gas that should be used for various base metals and types of metal transfer. Figure 17-2 lists recommended fl ow rates for shielding gases. Set the recommended fl ow rate on the fl owmeter as discussed in Chapter 16. Self-shielded FCAW guns do not have a nozzle and no shielding gas is used. Finally, set the voltage and wire feed speed recommended for the type of metal transfer you have selected. Chapter 16 lists recommended voltages and wire feed speeds. Electrode Extension In GMAW and FCAW, the voltage set on the welding machine determines the arc length. The wire feed speed determines the current. The welder must control the contact tube-to-work distance or the electrode extension. Contact tube-to-work distance is the distance from the contact tube to the surface of the work. Electrode extension, or electrode stick-out, is the distance from the end of the contact tube to the end of the electrode. See Figure 17-3. The electrode must extend beyond the contact tube to preheat the electrode wire. Some preheating is Goodheart-Willcox Publisher Figure 17-1. Rotating the wire tension adjusting knobs will increase the pressure on the electrode wire as it passes through the rolls. Wire tension adjusting knobs