Figure 4-19. - The effect of moving a cutting torch too rapidly across the work.
Figure 4-18. - Position of torch tip for starting a cut.
Hold the torch perpendicular to the work, with the inner cones of the preheating flames about 1/16 inch above the end of the line to be cut (fig. 4-18). Hold the torch in this position until the spot you are heating is a bright red. Open the cutting oxygen valve slowly but steadily by pressing down on the cutting valve lever.
When the cut is started correctly, a shower of sparks will fall from the opposite side of the work, indicating that the flame has pierced the metal. Move the cutting torch forward along the line just fast enough for the flame to continue to penetrate the work completely. If you have made the cut properly, you will get a clean, narrow cut that looks almost like it was made by a saw. When cutting round bars or heavy sections, you can save preheating time by raising a small burr with a chisel where the cut is to begin. This small raised portion will heat quickly, allowing you to start cutting immediately.
Once you start the cut, you should move the torch Slowly along the cutting mark or guide. As you move the torch along, watch the cut so you can tell how it is progressing. Adjust the torch as necessary. You must move the torch at the correct speed, not too fast and not too slow. If you go too slowly, the preheating flame melts the top edges along the cut and could weld them back together again. If you go too rapidly, the flame will not penetrate completely, as shown in figure 4-19. When this happens, sparks and slag will blow back towards you. If you have to restart the cut, make sure there is no slag on the opposite side.
Figure 4-20. - Recommended procedure for cutting thin steel. Notice that the two preheat flames are in line with the cut (kerf).
When cutting steel 1/8 inch or less in thickness, use the smallest cutting tip available. In addition, point the tip in the direction the torch is traveling. By tilting the tip, you give the preheating flames a chance to heat the metal ahead of the oxygen jet, as shown in figure 4-20. If you hold the tip perpendicular to the surface, you decrease the amount of preheated metal and the adjacent metal could cool the cut enough to prevent smooth cutting action. Many Steelworkers actually rest the edge of the tip on the metal during this process. If you use this method, be careful to keep the end of the preheating flame inner cone just above the metal.Continue Reading