420 Modern Welding Copyright Goodheart-Willcox Co., Inc. Very dirty and rusty metal should be cleaned before the cutting operation begins. The impurities on the metal slow the cutting speed and may cause a rough and irreg- ular kerf. Figure 15-16 shows a number of completed cuts. The cause of each poor cut is explained in the caption. When cutting, the welder should stand in a com- fortable position that permits looking into the cut as it is being formed. In order to see into the kerf, the welder should move the torch away from his or her body rather than toward it. The cut can be made across the welder’s body from right to left or left to right. The welder must position his or her body to obtain a good view of the kerf. The torch is usually held with both hands for optimal control, Figure 15-17. Normally, the tip is perpendicular to the surface being cut. The end of the inner cone of the preheating fl ame should be held just above the metal surface. If the cutting tip has four, six, or more preheating ori- fi ces, one orifi ce should precede (lead) the cutting ori- fi ce. One orifi ce should follow the cut. The other ori- fi ces should be aligned to heat each side of the kerf equally, as shown in Figure 15-8. 15.5.1 Cutting Thin Steel Cutting steel that is 1/8″ (3.2 mm) or less in thick- ness requires the smallest cutting tip available. A tip with only a few preheat holes is often used. In addi- tion, the tip is usually pointed in the direction the torch is traveling. If even a small tip size seems too large, changing the tip angle to 70°–75° will help. See Figure 15-18. In effect, this increased angle increases the thickness of the metal being cut. On very thin metal, holding the tip near vertical produces too much preheating, and a poor cut results. When cut- ting very thin metal, many welders actually rest the edge of the torch tip on the base metal. Be careful to keep the end of the preheating fl ames’ inner cones just above the base metal. 1 2 3 4 5 6 7 8 9 American Welding Society Figure 15-16. Edge conditions resulting from oxyfuel gas cutting operations. (1) A good cut in 1″ (25 mm) plate. The edge is square, and the drag lines are essentially vertical and not too pronounced. (2) The preheat flames were too small for this cut, and the cutting speed was too slow, causing bad gouging at the bottom. (3) The preheating flames were too long, causing the top surface to melt over. The cut edge is irregular, and there is an excessive amount of adhering slag. (4) The oxygen pressure was too low, resulting in the top edge melting over because of the slow cutting speed. (5) The oxygen pressure was too high and the nozzle size too small, so control of the cut was lost. (6) The cutting speed was too slow, emphasizing irregularities of the drag lines. (7) The cutting speed was too fast, resulting in a pronounced break in the dragline, and an irregular cut edge. (8) The torch travel was unsteady, resulting in a wavy and irregular cut edge. (9) The cut was lost and not carefully restarted, causing bad gouges at the restarting point. Goodheart-Willcox Publisher Figure 15-17. A student is practicing oxyfuel gas cutting on thick 1 1/2″ (38 mm) mild steel.