A flashback is dangerous because it is the burning of gases  within the torch. It is usually caused by loose connections,  improper pressures, or overheating of the torch. A shrill  hissing or squealing noise accompanies a flashback, and  unless the gases are turned off immediately, the flame may  burn back through the hose and regulators causing great  damage and personal injury. The cause of the flashback  should always be determined and the problem corrected  before relighting the torch. All gas welding outfits should  have a flashback arrestor.  Cutting ferrous metals by the oxy-acetylene process is  primarily the rapid burning or oxidizing of the metal in a  localized area. This is a quick and inexpensive way to cut  iron and steel where a finished edge is not required. Figure 5-23  shows an example of a cutting torch. It has  the conventional oxygen and acetylene valves in the torch  handle that control the flow of the two gases to the cutting  head. It also has an oxygen valve below the oxygen lever on  the cutting head so that a finer adjustment of the flame can  be obtained. The size of the cutting tip is determined by the thickness of  the metal to be cut. Set the regulators to the recommended  working pressures for the cutting torch based on the tip size  selected. Before beginning any cutting operation, the area  should be clear of all combustible material and the proper  protective equipment should be worn by personnel engaged  in the cutting operation.  The flame for the torch in  Figure 5-23  is set by first closing  the oxygen valve below the cutting lever and fully opening the  oxygen valve on the handle. (This supplies the high-pressure  oxygen blast when the cutting lever is actuated.) The acetylene  valve on the handle is then opened and the torch is lit with a  striker. The acetylene flame is increased until the black soot  is gone. Then, open the oxygen valve below the cutting lever  and adjust the flame to neutral. If more heat is needed, open the  valves to add more acetylene and oxygen. Actuate the cutting  lever and readjust the preheat flame to neutral if necessary.  The metal is heated to a bright red color (1,400 °F–1,600 °F,  which is the kindling or ignition temperature) by the preheat  orifices in the tip of the cutting torch. Then, a jet of high- pressure oxygen is directed against it by pressing the oxygen  lever on the torch. This oxygen blast combines with the  red-hot metal and forms an intensely hot molten oxide that  is blown down the sides of the cut. As the torch is moved  along the intended cut line, this action continues heating  the metal in its path to the kindling temperature. The metal,  thus heated, also burns to an oxide that is blown away to the  underside of the piece. Proper instruction and practice provides the knowledge and  skill to become proficient in the technique needed to cut  with a torch. Hold the torch in either hand, whichever is  most comfortable. Use the thumb of that hand to operate the  oxygen cutting lever. Use the other hand to rest the torch on  and steady it along the cut line.  Begin at the edge of the metal and hold the tip perpendicular  to the surface, preheating until the spot turns bright red.  Lightly depress the cutting lever to allow a shower of sparks  and molten metal to blow through the cut. Fully depress the  cutting lever and move the torch slowly in the direction of  the intended cut.  Practice and experience allow the technician to learn how to  judge the speed at which to move the torch. It should be just  fast enough to allow the cut to penetrate completely without  excessive melting around the cut. If the torch is moved too fast,  the metal will not be preheated enough, and the cutting action  stops. If this happens, release the cutting lever, preheat the  cut to bright red, depress the lever, and continue with the cut. Shutting down the welding equipment is fairly simple when  some basic steps are followed: •  Turn off the flame by closing the acetylene valve on  the torch first. This shuts the flame off quickly. Then,  close the oxygen valve on the torch handle. Also, close  oxygen valve on cutting torch, if applicable.  •  If the equipment is not used in the immediate future  (approximately the next 30 minutes), the valves on  the acetylene and oxygen cylinders should be closed  and pressure relieved from the hoses. •  In a well-ventilated area, open the acetylene valve on  the torch and allow the gas to escape to the outside  atmosphere, and then close the valve. •  Open the oxygen valve on the torch, allow the gas to  escape, and then close the valve. Figure 5-23.  Cutting torch with additional tools. $^{ }$Oxy-acetylene Cutting Shutting Down the Gas Welding Equipment 5-14