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9/27/01 AC 43.13-1B CHG 1 (3) The torch can be shut off simply by closing the acetylene off first and allowing the gas remaining in the torch tip to burn out. Then turn off the oxygen valve. If the torch is not to be used again for a long period, the pressure should be turned off at the cylinder. The hose lines should then be relieved of pres sure by opening the torch needle valves and the working pressure regulator, one at a time, allowing the gas to escape. Again, it is a good practice to relieve the oxygen pressure and then the acetylene pressure. The hose should then be coiled or hung carefully to prevent damage or kinking. (4) Soft soldering is used chiefly for copper, brass, and coated iron in combination with mechanical seams; that is, seams that are riv eted, bolted, or folded. It is also used where a leak-proof joint is desired, and sometimes for fitting joints to promote rigidity and prevent corrosion. Soft soldering is generally per formed only in very minor repair jobs. This process is used to join electrical connections because it forms a strong union with low elec trical resistance. (a) Soft solder gradually yields under a steadily applied load and should not be used unless the transmitted loads are very low. It should never be used as a means of joining structural members. (b) A soldering iron is the tool used in sol dering. Its purpose is to act as a source of heat for the soldering operation. The bit, or work ing face, is made from copper since this metal will readily absorb heat and transmit it to the work. Figure 4-33 shows a wedge-shaped bit. FIGURE 4-33. Electric soldering iron. (c) To tin the soldering iron, it is first heated to a bright red, and then the point is cleaned (by filing) until it is smooth and bright. No dirt or pits should remain on its surface. After the soldering iron has been mechanically cleaned, it should be reheated sufficiently to melt solder and chemically cleaned by rubbing it firmly on a block of sal ammoniac (ammo nium chloride). Rosin flux paste may also be used. Solder is then applied to the point and wiped with a clean cloth. (d) A properly tinned copper iron has a thin unbroken film of solder over the entire surface of its point. (e) Soft solders are chiefly alloys of tin and lead. The percentages of tin and lead vary considerably in various solder, with a corre sponding change in their melting points, rang ing from 145-311°C (293-592°F). Half-and-half (50/50) solder is a general pur pose solder and is most frequently used. It contains equal proportions of tin and lead, and it melts at approximately 182 °C (360 °F). (f) The application of the melted solder re quires somewhat more care than is apparent. The parts to be soldered should be locked to gether or held mechanically or manually while tacking. To tack the seam, the hot copper iron is touched to a bar of solder, then the drops of solder adhering to the copper iron are used to tack the seam at a number of points. The film of solder between the surfaces of a joint must be kept thin to make the strongest joint. (g) A hot, well-tinned soldering copper iron should be held so that its point lies flat on the metal (at the seam), while the back of the cop per iron extends over the seam proper at a 45-degree angle, and a bar of solder is touched Par 4-88 Page 4-61 to the point. As the solderPDF Image | AFS-640
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