Traditionally, welding is defined as a process that joins metal  by melting or hammering the work pieces until they are  united together. With the right equipment and instruction,  almost anyone with some basic mechanical skill, dexterity,  and practice can learn to weld. There are three general types of welding: gas, electric arc,  and electric resistance. Each type of welding has several  variations, some of which are used in the construction of  aircraft. Additionally, there are some new welding processes  that have been developed in recent years that are highlighted  for the purpose of information.  This chapter addresses the welding equipment, methods,  and various techniques used during the repair of aircraft and  fabrication of component parts, including the processes of  brazing and soldering of various metals. Gas welding is accomplished by heating the ends or edges  of metal parts to a molten state with a high temperature  flame. The oxy-acetylene flame, with a temperature of  approximately 6,300 °Fahrenheit (F), is produced with a torch  burning acetylene and mixing it with pure oxygen. Hydrogen  may be used in place of acetylene for aluminum welding, but  the heat output is reduced to about 4,800 °F. Gas welding was  the method most commonly used in production on aircraft  materials under  3 ⁄ 16-inch in thickness until the mid 1950s,  when it was replaced by electric welding for economic (not  engineering) reasons. Gas welding continues to be a very  popular and proven method for repair operations.  Nearly all gas welding in aircraft fabrication is performed  with oxy-acetylene welding equipment consisting of: Electric arc welding is used extensively by the aircraft industry  in both the manufacture and repair of aircraft. It can be used  satisfactorily to join all weldable metals, provided that the  proper processes and materials are used. The four types of  electric arc welding are addressed in the following paragraphs.  Shielded metal arc welding (SMAW) is the most common type  of welding and is usually referred to as “stick” welding. The  equipment consists of a metal wire rod coated with a welding  flux that is clamped in an electrode holder that is connected by a  heavy electrical cable to a low voltage and high current in either  alternating current (AC) or direct current (DC), depending on  the type of welding being done. An arc is struck between the  rod and the work and produces heat in excess of 10,000 °F,  which melts both the material and the rod. The welding circuit  consists of a welding machine, two leads, an electrode holder,  an electrode, and the work to be welded.  [Figure 5-2] When the electrode is touched to the metal to be welded,  the circuit is complete and the current flows. The electrode  is then withdrawn from the metal approximately  1 ⁄ 4-inch to  form an air gap between the metal and the electrode. If the  correct gap is maintained, the current bridges the gap to form  a sustained electric spark called the arc. This action melts the  electrode and the coating of flux. As the flux melts, it releases an inert gas that shields the  molten puddle from oxygen in the air to prevent oxidation.  The equipment may be permanently installed in a shop, but  most welding outfits are of the portable type.  [Figure 5-1] 5-2 •  An appropriately-rated fire extinguisher. •  Special wrench for acetylene tank valve if needed. •  A flint or spark lighter. •  Welding goggles fitted with appropriate colored lenses. •  A welding torch with an internal mixing head, various  size tips, and hose connections. •  Two lengths of colored hose (red for acetylene and  green for oxygen) with adapter connections for the  regulators and torch. •  Acetylene and oxygen pressure regulators and cylinder  pressure gauges. •  Two cylinders, acetylene and oxygen. Shielded Metal Arc Welding (SMAW) Electric Arc Welding Gas Welding Types of Welding Figure 5-1.  Portable oxy-acetylene welding outfit. 0 200 100 300 0 200 100 300 0 200 100 300 0 200 100 300 Oxygen pressure regulator Acetylene pressure regulator Acetylene cylinder Torch Oxygen cylinder