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Stakes and stake plate
Bending on a Bar Folder

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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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. Bending on a Brake The easiest and most accurate method of making straight-line bends in a piece of sheet metal is to use a box and pan brake or a cornice brake. The use of these brakes is relatively simple. However, if they are not used correctly, the time and the work involved in computing the bend allowance and laying out the job, as well as the metal, are wasted. Before you bend any work that must have an accurate bend radius and definite leg length, the brake settings should be checked with a piece of scrap metal. To make an ordinary bend on a brake, you should place the sheet to be bent on the bed so that the bend line is directly under the upper jaw or clamping bar. Then, pull down the clamping bar handle.    This brings the clamping bar down to hold the sheet firmly in place. Next, set the stop for the proper angle or amount of bend. Finally, make the bend by raising the bending leaf until it strikes the stop. If more than one bend is to be made, bring the next bend line under the clamping bar and repeat the procedure. See figures 13-22 and 13-25. Figure 13-35.—Forming concave hand bend. 13-22







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