• Radius. The radius (R) of the bend is always to the inside of the metal being formed unless otherwise stated. The minimum allowable radius for bending a given type and thickness of material should always be determined before you proceed with any bend allowance calculations. • Setback The setback (SB) is the distance from the bend tangent line to the mold point. In a 90-degree bend, SB = R + T (radius of the bend plus thickness of the metal). The setback dimension must be determined prior to making the bend because setback is used in determining the location of the beginning bend tangent line. BEND ALLOWANCE FORMULA By experimentation with actual bends in metals, aircraft engineers have found that accurate bending results could be obtained by using the following formula for any degree of bend from 1 to 180: (0.0173 x R + 0.0078 x T) Where R = the desired bend radius, x  N  =  B A T = the thickness of the material, and N = the number of degrees of bend. Refer to the NA 01-1A-1 for the appropriate bend allowance tables. CUTTING SHEET METAL Once a project has been laid out on the metal, the next step is to cut it to shape. The type of cutting equipment to be used depends primarily upon the type and thickness of the material. Another consideration is the size and number of pieces to be cut. A few relatively thin pieces of comparatively soft metal may be cut faster with hand-trimming methods. But for harder metals, faster output, and more professional results, machines designed for metal-cutting purposes are used. Machines used to cut sheet metal may be divided into two groups—manually operated and power operated. Each cutting machine has a definite cutting capacity that should never be exceeded. A few of the more common types that may be available to you have been described in the previous sections. BENDING SHEET METAL Straight-line bends and folds in sheet metal are ordinarily made on the cornice brake and bar folder; however, a considerable amount of bending is also completed by hand-forming methods. Hand forming may be accomplished by using stakes, blocks of wood, angle iron, a vise, or the edge of a bench. Bending Over Stakes Stakes are used to back up sheet metal to form many different curves, angles, and seams. Stakes are available in a wide variety of shapes, some of which are shown in figure 13-33. The stakes are held securely in a stake holder or stake plate, which is anchored in a workbench. The stake holder contains a variety of holes to fit a number of different types of shanks. Although stakes are by no means delicate, they must be handled with reasonable care. They should not be used as backing when you are chiseling holes or notches in sheet metal. Bending in a Vise Straight-line bends of comparatively short sections can be made by hand with the aid of wooden or metal bending blocks. After the part has been laid out and cut to size, you should clamp it along the bend line between two form blocks, which are held in a vise. The form blocks usually have one edge rounded to give the desired bend radius. See figure 13-34. By tapping lightly with a rubber, plastic, or rawhide mallet, bend the metal protruding beyond the bending block to the desired angle. You should gradually make the bend even. Start tapping at one end and work back and forth along the edge. Continue this process until the protruding metal is bent to the desired angle. If a large amount of metal extends beyond the bending blocks, you should maintain enough hand pressure against the protruding sheet to prevent the metal from bouncing. Remove any irregularity in the flange by holding a straight block of hardwood edgewise against the bend and striking it with heavy blows of a hammer or mallet. If the amount of metal protruding beyond the bending blocks is small, make the entire bend by using the hardwood block and a hammer. Curved flanged parts have mold lines that are either concave or convex. The concave flange is formed by stretching, while the convex flange is 13-20