© COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON II
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Lesson 1
The Basics of Arc Welding
Lesson 2
Common Electric
Arc Welding Processes
Lesson 3
Covered Electrodes for Welding
Mild Steels
Lesson 4
Covered Electrodes for Welding Low Alloy Steels
Lesson 5
Welding Filler Metals for Stainless Steels
Lesson 6
Carbon & Low Alloy
Steel Filler Metals -
GMAW,GTAW,SAW
Lesson 7
Flux Cored Arc Electrodes Carbon Low Alloy Steels
Lesson 8
Hardsurfacing Electrodes
Lesson 9
Estimating & Comparing Weld Metal Costs
Lesson 10
Reliability of Welding Filler Metals
technology makes it possible to design motor speed controls that will produce the same speed, even though the load on the motor varies or the input voltage to the motor may fluctuate. 2.4.5.1 A limited amount of gas metal arc welding is performed with constant current type power sources.  In this case, the motor speed automatically varies to increase or decrease the wire feed speed as the arc length varies to maintain a constant voltage. 2.4.5.2 The wire feeder also controls the main contactor in the power source for safety reasons.  This assures that the welding wire will only be energized when the switch on the welding gun is depressed. 2.4.5.3 The flow of shielding gas is controlled by a solenoid valve (magnetic valve) in the wire feeder to turn the shielding gas on and off when the gun switch is actuated.  Most feeders utilize a dynamic breaking circuit to quickly stop the motor at the end of a weld to prevent a long length of wire protruding from the gun when the weld is terminated.  Most feeders have a burn-back circuit that allows the welding current to stay on for a short period of time after wire feeding has stopped, to allow the wire to burn back exactly the right amount for the next arc initiation. 2.4.5.4 The feed rolls, sometimes called drive rolls, pull the wire off the spool or reel, and push it through a feed cable or conduit to the welding gun.  These rolls must usually be changed to accommodate each different wire diameter, although some rolls are designed to feed a combination of sizes. 2.4.6 Welding Gun - The function of the welding gun, sometimes referred to as a torch, is to deliver the welding wire, welding current, and shielding gas to the welding arc.  Guns are available for semi-automatic operation and for automatic operation, where they are fixed in the automatic welding head. 2.4.6.1 Guns for GMAW have several characteristics in common.  All have a copper alloy shielding gas nozzle, that delivers the gas to the arc area in a nonturbulent, angular pattern to prevent aspiration of air.  The nozzle may be water cooled for semiautomatic welding at high amperage and for automatic welding where the arc time is of long duration.  Welding current is transferred to the welding wire as the wire travels through the contact tip or contact tube located inside the gas nozzle (Refer to Figure 9).  The hole in the contact tip through which the wire passes is only a few thousandths of an inch larger than the wire diameter.  A worn contact tip will result in an erratic arc due to poor current transfer.  Figure 15 shows a few different semiautomatic gun configurations that are commonly used for GMAW.