Chapter 14 Oxyfuel Gas Cutting 419 Metal that is very dirty and rusty should be cleaned before starting the cutting operation. The impurities on the metal slow the cutting speed, and may cause a rough and irregular kerf. Figure 14-19 shows a number of completed cuts. The cause of each poor cut is explained in the caption. The torch motion to be used in cutting is a matter of the welder’s own experience. Usually, the welder moves the torch in a straight line, with no side-to- side motion. In some cases, the thickness of the metal requires an oscillating (side-to-side) motion in order to obtain the necessary width of cut. When cutting, the welder should stand in a comfortable 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 him or herself while cutting, rather than toward him or her. The cut may be made from right to left with a good view of the kerf. The torch is usually held with both hands for optimal control, as shown in Figure 14-20. 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. If the cutting tip has 4, 6, or more preheating orifi ces, one orifi ce should precede (lead) the cutting orifi ce. One orifi ce should follow the cut. The other orifi ces should be aligned to heat each side of the kerf equally. See Figure 14-6. Welders should wear safety boots with high tops, trousers without cuffs, and leggings to protect against fl ying molten slag. Welding leggings are pieces of leather that cover the shin and top of the shoe to protect against splashes of molten metal and sparks. (Trousers with cuffs must be covered to keep them from catching the white-hot metal slag as it drops from the cut.) Place a heat-resistant (refrac- tory) material container under the cut to catch the very hot liquid slag. 14.6.1 Cutting Thin Steel Cutting steel that is 1/8″ (3.18mm) or less in thickness requires the use of the smallest cutting tip available. A tip with only a few preheat holes is 1 2 3 4 5 6 7 8 9 Figure 14-19. Typical edge conditions resulting from oxyfuel gas cutting operations: (1) A good cut in 1″ (25.4mm) plate. The edge is square, and the drag lines are essentially vertical and not too pronounced. (2) The preheat fl ames were too small for this cut, and the cutting speed was too slow, causing bad gouging at the bottom. (3) The preheating fl ames 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 that 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. (American Welding Society) Figure 14-20. A student is practicing oxyfuel gas cutting on thick 1 1/2″ (38mm) mild steel.