Figure 4-21. - Progress of a cut in thick steel. A. Preheat flames are 1/16 to 1/8 inch from the metal surface. Hold the torch in this spot until the metal becomes cherry red. B. Move the torch slowly to maintain the rapid oxidation, even though the cut is only partially through the metal. C. The cut is made through the entire thickness; the bottom of the kerf lags behind the top edge slightly.
Steel, that is greater than 1/8 inch thick, can be cut by holding the torch so the tip is almost vertical to the surface of the metal. If you are right-handed, one method to cut steel is to start at the edge of the plate and move from right to left. Left-handed people tend to cut left to right. Either direction is correct and you may cut in the direction that is most comfortable for you. Figure 4-21 shows the progress of a cut in thick steel.
After heating the edge of the steel to a dull cherry red, open the oxygen jet all the way by pressing on the cutting lever. As soon as the cutting action starts, move the torch tip at a even rate. Avoid unsteady movement of the torch to prevent irregular cuts and premature stopping of the cutting action.
To start a cut quicker in thick plate, you should start at the edge of the metal with the torch angled in the opposite direction of travel. When the edge starts to cut, bring the torch to a vertical position to complete the cut through the total thickness of the metal. As soon as the cut is through the metal, start moving the torch in the direction of travel.
Two other methods for starting cuts are used. In the first method, you nick the edge of the metal with a cold chisel at the point where the cut is to start. The sharp edges of the metal upset by the chisel will preheat and oxidize rapidly under the cutting torch, allowing you to start the cut without preheating the entire edge of the plate. In the second method, you place an iron filler rod at the edge of a thick plate. As you apply the preheat flames to the edge of the plate, the filler rod rapidly reaches the cherry red temperature. At this point, turn the cutting oxygen on and the rod will oxidize and cause the thicker plate to start oxidizing.
It is more difficult to cut cast iron than steel because the iron oxides in cast iron melt at a higher temperature than the cast iron itself. Before you cut cast iron, it is best to preheat the whole casting to prevent stress fractures. Do not heat the casting to a temperature that is too high, as this will oxidize the surface and make cutting
Figure 4-22. - Torch movements for cutting cast iron.Continue Reading