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BACKHAND WELDING
BUTT JOINTS

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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Figure 15-35.—Types of welded joints. commonly used joints. Notice that the corner joint and butt joint are classified as groove welds, while the tee and lap joints are classified as fillet welds. Welding is always done in the flat position whenever possible. The puddle is much easier to control, and the welder can work longer periods without tiring. Quite often it is necessary to weld in the overhead, vertical, or horizontal position in equipment repair. The flat position is used when the material is to be laid flat or almost flat and welded on the topside. The welding torch is pointed downward toward the work. This weld may be made by either the forehand or backhand technique. The overhead position is used when the material is to be welded on the underside, with the torch pointed upward toward the work. In welding overhead, you can keep the puddle from sagging if you do not permit it to get too large or assume the form of a large drop. The rod is used to control the molten puddle. You should not permit the volume of flame to exceed that required to obtain a good fusion of the base metal with the filler rod. Less heat is required in an overhead weld because the heat naturally rises. The horizontal position is used when the line of the weld runs horizontal across a piece of work, and the torch is directed at the material in a horizontal or near horizontal position. The weld is made from right to left across the plate (for the right-hand welder). The flame is inclined upward at an angle of 45° to 65°, and the weld is made with a normal forehand technique. Adding the rod to the top of the puddle will prevent the molten metal from sagging to the lower edge of the bead. If the puddle is to have the greatest possible cohesion, it should not be allowed to get too hot. In a vertical weld, the pressure exerted by the torch flame must be relied upon to a great extent to support the puddle. It is important to keep the puddle from becoming too hot, and to prevent the hot metal from running out of the puddle onto the finished weld. It may be necessary to remove the flame from the puddle for an instant to prevent overheating, and then return it to the puddle. Vertical welds are begun at the bottom, and the puddle is carried upward with a forehand motion. The tip should be inclined from 45° to 60°, the exact angle depending upon the desired balance between correct penetration and control of the puddle. The rod is added from the top and in front of the flame with a normal forehand technique. Welded Joints The properties of a welded joint depend partly on the correct preparation of the edges being welded. All mill scale, rust oxides, and other impurities must be removed from the joint edges or surfaces to prevent their inclusion in the weld metal. You should prepare the edges to permit fusion without excessive melting, and you should take care to keep to a minimum the heat loss due to radiation into the base metal from the weld. A properly prepared joint will give a minimum of expansion on heating and a minimum of contraction on cooling. 15-26







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