Unit 11 Preparing to Weld 61 Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. To strike an arc using the tap method, bring the electrode close to the location where the weld is to begin or continue. Lightly tap the electrode on the work. When the arc starts, hold a slightly longer-than-normal arc for about one second and then establish the proper arc length. It will require some practice to master the force required to start the arc by the tap method. If you apply too much force, the electrode will stick to the work. With- out enough force, the arc will not start. The tap method requires less area to strike the arc and will enable you to strike the arc without damag- ing the base metal around the joint. If the electrode should freeze (stick) to the work, break it loose by twisting or bending the electrode by sharply moving the holder. If this does not work, squeeze the clamp section of the electrode holder and release the electrode from the holder. Once the length of the arc is established, try to maintain the correct length as the electrode burns down. If the arc is too short, not enough heat will be generated to melt the parent metal and the rod may stick. If the arc is too long, excessive spattering may occur and an irregular bead with poor penetration will result. In general, use an arc length equal to or slightly less than the core diameter of the elec- trode. Since the arc length cannot be measured, practice running beads until you can tell when you are maintaining the correct arc length and welding speed. Many welders go by the sound of the arc to help them determine correct arc length. Note that arc welding is an electric circuit, which includes voltage, current, and a load (resistance). The arc length (as maintained by the space between the electrode and work- piece) can impact arc voltage and current. A longer arc length increases resistance, which can slightly lower current. Conversely, a shorter arc length can reduce resistance, which can slightly increase current. Therefore, changing arc length can impact the heat of the weld, rate of electrode melting, and the speed at which a weld can be made. Be aware of the interrelation of these dif- ferent aspects of arc welding. Maintaining the correct welding speed is important. Watch the pool of molten metal directly behind the arc. Do not watch the arc. The shape of the weld pool and the ridges formed when the molten metal solidifi es indicate cor- rect welding speed. See Figure 11-6. The ridges should be uniform and the edges straight. Start Establishing arc Arc length: slightly less than diameter of electrode Start Establishing arc Scratch Method Tap Method Figure 11-5. Methods for starting the arc. Goodheart-Willcox Publisher Straight edges Electrode Weld pool Evenly spaced ridges Figure 11-6. The completed arc bead should have straight edges, evenly spaced ridges, and uniform height. Goodheart-Willcox Publisher