Chapter 14 Oxyfuel Gas Cutting 415 open. Adjust the oxygen regulator screw to give the correct working pressure, as shown in Figure 14-12. Close the torch oxygen valves. 6. Open the torch acetylene valve fully. Adjust the acetylene working pressure on the regu- lator to the pressure shown in Figure 14-12. Close the torch acetylene valve. 7. Check the low-pressure gauges for any gradual increase in pressures. If they are increasing, the regulator seat may be leaking. If the pres- sure on either of the low-pressure gauges is increasing, shut the station down immediately. 8. The pressures are now adjusted, and the cutting torch is ready to be lighted. 9. To light the torch, open the torch oxygen valve 1/4 turn. Open the torch acetylene valve fully. Use a spark lighter to ignite the fuel gas. 10. Press down the oxygen cutting lever and adjust the torch acetylene valve until the preheating fl ames are neutral. 11. The torch is now ready for oxyacetylene cutting operations. 14.5 Using a Cutting Torch The cutting torch must be used carefully in order to create clean and accurate cuts. The tip must be in excellent condition and set to align properly with the cutting line. See Figure 14-6. The preheating fl ames must be correctly adjusted, and the cutting oxygen pressure must be correct. To cut, bring the tip of the inner cone of the preheating fl ames to the edge of the metal to be cut. The cutting torch should be held so that the inner cone of the preheat fl ames is about 1/16″ to 1/8″ (1.6mm to 3.2mm) from the surface of the metal being cut, as shown in Figure 14-13. As soon as a spot on the cutting line has been heated to a bright cherry red color, open the cutting oxygen valve all the way. The jet of oxygen coming through the center of the tip (oxygen jet) causes the heated metal to burn (oxidize) away, forming the kerf (cut). One of the best indications of a good cutting operation is the appearance of the slag stream at the bottom of the cut. The ideal slag stream passes directly through a plate less than 1″ (25.4mm) thick. Figure 14-11. Cross section of an injector-type cutting torch. The acetylene is drawn into the mixing chambers by the pulling action (suction) of the oxygen jet. This injector-type cutting torch is particularly adaptable for use with acetylene generators that operate under low pressure. Preheating mixture of oxygen and acetylene Acetylene Preheating oxygen Cutting oxygen Mixing chamber Injector nozzle Figure 14-12. Table of the approximate oxygen and acetylene pressures used for cutting steel with injector-type cutting torch. Oxyacetylene Cutting Parameters for Injector-Type Cutting Torch Metal thickness Preheat orifice drill size Cutting orifice drill size Oxygen regulator pressure Acetylene regulator pressure in mm psig kPa psig kPa 1/8–1/4 1/4–3/8 3/8–1/2 1/2–3/4 3/4–1 1 1/2–2 3.2–6.4 6.4 –9.5 9.5 –12.7 12.7–19.1 19.1 –25.4 38.1 –50.8 75 74 72 71 70 68 67 62 59 55 54 51 1 1 1 1 1 1 7 7 7 7 7 7 15–20 20–25 25–30 30–35 35–40 45–50 103–138 138–172 172–207 207–241 241–276 310–345