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.

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.

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

Figure 13-33.-Stakes and stake plate.

formed by shrinking. Such parts are shaped with the aid of hardwood or metal form blocks. These blocks are made in pairs and specifically for the shape of the part being formed. Each pair conforms to the actual dimension and contour of the finished article. 

You should cut the material to be formed to size, allowing about one-quarter inch of excess material for trim. File and smooth the edges of the material to remove all nicks caused by the cutting tools. This reduces the possibility of the material cracking at the edges during the forming operation. Place the material between the form blocks and clamp it in a vise so that the material will not move or shift. Clamp the work as closely as possible to the particular area being formed to prevent strain on the form block and to keep the material from slipping.

Figure 13-34.-Preparation for straight bend by hand.

Concave surfaces are formed by stretching the material over a form block. See figure 13-35. You should use a plastic or rawhide mallet with a smooth, slightly rounded face to start hammering at the extreme ends of the part, and then continue toward the center of the bend. This procedure permits some of the material at the ends of the part to be worked into the center of the curve where it will be needed. Continue hammering until the metal is gradually worked down over the entire flange and flush with the form block. After the flange is formed, trim off the excess material and check the part for accuracy. Convex surfaces are formed by shrinking the material over a form block. See figure 13-36. You should use a wooden or plastic shrinking mallet and a backup or wedge block to start hammering at the center of the curve, and then work toward both ends. Hammer the flange down over the form by striking the metal with glancing blows at an angle of approximately 45 degrees. You should use a motion that will tend to pull the part away from the radius of the form block. The wedge block is used to keep the edge of the flange as nearly perpendicular to the form block as possible. The wedge block also lessens the possibility of buckling, splitting, or cracking the metal.

Another method of hand forming convex flanges is to use a lead bar or strap. The material, which is secured in the form block, is struck by the lead strap. The strap takes the shape of the part being formed and forces it down against the form block. One advantage of this method is the metal is formed without marring or wrinkling and is not thinned as much as it would be by other methods of hand forming. This method is also illustrated in figure 13-36. After the flange is formed by either method, trim off the excess material and check the part for accuracy.