Handbook-Manual Cutting

Before you attempt your first bevel cut, experiment for several minutes with an unlit torch and try to learn the best

way to move the torch along a straight line at a uniform angle. To hold both line and angle is of critical importance. You will probably find that the only way you can do this, for a straight bevel cut in plate, is to move both hands and torch as a unit, dragging your support hand along the surface of the plate. You can’t keep your support hand in a fixed position, as you may have found it possible to do in making a vertical cut. Once you feel confident about motion, light your torch. Adjust the preheat flames to maximum length (just short of the condition where a gap appears between the flames and the end of the nozzle.) Position the torch so that the preheat flames closest to the plate surface almost touch the plate. Don’t try to make the bevel right up to the lower edge of the plate on your first try; allow yourself some leeway. Make the cut at as steady a speed as possible; remember that you probably can’t cut quite as fast as in the previous exercise. If you find it difficult to observe the cutting reaction directly, keep your eye on the angle at which the stream of slag and sparks leave the lower sur- face of the plate. If you can complete your first bevel without losing it once along the way, feel that you’ve made a good start. Examine the surface of the bevel on the plate closely. If the top corner is badly melted over, perhaps your travel speed was a bit too low, or your flames a bit too strong. Make whatever adjustments you feel are necessary, and try another cut, moving back at least 1 cm from the original line of cut. Keep at it until you run out of plate or have made a cut that appears reasonably straight and uniform. Remember that it is the straightness of that lower edge, not the exact angle of bevel, or the smoothness of the cut surface, or the amount of melting at the top edge, which will be important when it comes to welding. Once you have achieved some skill in bevelling of flat plate, try bevelling a piece of pipe, preferably at least as large as 4-in. Schedule 40 (which has a wall thickness of about 6 mm.). Only occasionally will an operator be called on to bevel flat plate with a hand torch in preparation for welding, since virtually every shop has a cutting machine, large or small, which can do the work more precisely. In the fabrication of steel piping systems, regardless of the welding method to be used, the ability to make good bevel cuts by hand is almost invaluable. Much of the bevelling involved in piping system work can be done by special machines, but there will always be some jobs that can only be done by hand. Near the close of Chapter 13 there is a sketch (made from life) which will give you some good ideas about how to hold the torch. Time spend in learning how to bevel pipe will be time well spent.