Chapter 15 Oxyfuel Gas Cutting 413 Copyright Goodheart-Willcox Co., Inc. Cutting tips can be one-piece or two-piece con- struction. One-piece tips are used for oxyacetylene cutting only. Two-piece tips are used for all other gas cutting, such as natural gas, propane, and LP gas mixes. For satisfactory service, tips must be kept in good condition. The orifi ce end must be clean. The tip sur- face must be clean and at a right angle to the orifi ce passages, so the preheat fl ames are properly shaped and aimed. The sealing face of the tip, (which con- tacts the torch body) must be clean and free from scratches, burrs, and nicks, or the joint may leak. Clean the tips as described in Chapter 14, Oxyfuel Gas Cutting Equipment and Supplies. Pro Tip Store extra tips in soft holders. A wooden block with holes drilled in it works well. 15.4 Using a Cutting Torch Carefully review the detailed, step-by-step procedure for turning on an oxyacetylene welding or cutting outfi t in Chapter 13. The note following Step 9 gives instructions on the need to depress the cutting oxygen lever while setting the oxygen working pressure. This is the only difference between turning on an oxyfuel cutting outfi t and turning on an oxyfuel welding outfi t. Setting the correct working pressure on each regu- lator is critical. See the table in Figure 15-9 to determine the proper oxygen and acetylene working pressures and cutting tip size needed based on the thickness of the metal to be cut. Ensure that the proper size tip is installed in the gun. To set the working pressure, open the acetylene torch valve one-half turn. Slowly turn in the acetylene regulator adjusting screw or knob until the low-pressure acetylene gauge indicates working pressure needed. Close the torch acetylene valve. Never use acetylene at a pressure above 15 psig (103 kPa). Oxyacetylene Cutting Parameters for Positive-Pressure Torch Metal thickness Cutting orifice diameter Approximate gas pressures Approximate travel speed in mm Drill size in mm Acetylene psi Oxygen psi in/min 1/8 3.2 60 0.040 1.02 3 10 18–22 1/4 6.4 60 0.040 1.02 3 15 14–18 3/8 9.5 55 0.052 1.32 3 20 12–16 1/2 12.7 55 0.052 1.32 3 25 10–14 3/4 19.1 55 0.052 1.32 4 30 8–12 1 25.4 53 0.060 1.52 4 35 8–11 1 1/2 38.1 53 0.060 1.52 5 40 6–8 2 50.8 49 0.073 1.85 5 45 5–7 Goodheart-Willcox Publisher Figure 15-9. Approximate tip sizes and oxygen and acetylene working pressures for cutting various sizes of steel sheet and plate with a positive-pressure cutting torch. This table is for a torch equipped with a tip that has four preheating orifices. For heavier or faster cutting, a larger diameter cutting orifice or tips with more than four preheating orifices may be used.