Cut the paper cover at the points where it would have to be stretched to fit, and attach additional pieces of paper with masking tape to cover the exposed portions. Die and rubber methods should be used where bends are to be made at right angles, which complicate the use of a brake. Inconel® refers to a family of nickel-chromium-iron super alloys typically used in high-temperature applications. Clamp the material in the form blocks. [Figure 14] In this method, a depression is made into the sandbag to take the shape of the hammered portion of the metal. The pit changes shape with the hammering it receives and must be readjusted accordingly. The shear angle on a punch should be from 2° to 3°, with a 1° clearance angle on the die. The procedure for bumping sheet metal parts on a sandbag follows certain basic steps that can be applied to any part, regardless of its contour or shape. Although the pattern does not lie flat, it gives a fairly accurate idea of the approximate shape of the metal to be cut, and the pieced-in sections indicate where the metal is to be stretched. Convex surfaces are formed by shrinking the material over a form block. With a single tool, create … Shaping of the form block requires particular care because the more nearly accurate it is, the less time it takes to produce a smooth, finished part. Locate and drill holes for alignment pins. It takes about twice as much pressure to shear or punch stainless steel as it does mild steel. Stretching means to lengthen or increase a particular area of metal while shrinking means to reduce an area. Enlarge the pilot holes to their completed hole dimension. Insert the sheet in the brake and bend the metal up approximately 20° to 30°. Using a round, soft-faced mallet, strike the flange directly over the V portion with light, even blows while gradually forcing it downward into the V. Begin at one end of the flange and form the curve gradually and evenly by moving the strip slowly back and forth, distributing the hammer blows at equal spaces on the flange. These blocks are made in pairs similar to those used for straight angle bends and are identified in the same manner. Remember, the purpose of bumping is to work the material into shape by stretching rather than forcing it into the form with heavy blows. In this process, crimp the flange that is to form the inside of the curve. [Figure 15]. If a sharp curve is made, the angle (cross section of the formed angle) closes slightly. Prepare several templates (patterns of the crosssection), as shown in Figure 13 so that the form can be checked for accuracy. Titanium can also be worked by grinding, drilling, sawing, and the types of working used on other metals. When hand drilling two or more titanium parts at the same time, clamp them together tightly. Clamp the material in the form blocks, and bend the exposed flange against the block. Roll forming may be accomplished cold on equipment designed for forming aluminum. Magnesium is the world’s lightest structural metal. Such parts are shaped with the aid of hardwood or metal forming blocks. If the forms are made of metal, use a softfaced mallet. [Figure 5]. It should also be curved slightly beyond 90° to allow for spring-back. Layouts should be made with a carpenter’s soft pencil because any marring of the surface may result in fatigue cracks. Trim the excess material away and file and polish. If the flange is to point toward the inside of the bend, the material must be shrunk. Lightening holes may be cut with a hole saw, a punch, or a fly cutter. While the V-block is more satisfactory because it is faster, easier, and affects the metal less, good results can be obtained by the shrinking block method. Drill bits that do not cut satisfactorily cause damage to the hole. Use the maximum drill rpm as illustrated in Figure 18. Too much clearance permits the metal to be drawn over the edge of the die and causes it to become work hardened, resulting in excessive strain on the machine. This reduces the possibility of the material cracking at the edges during the forming process. The following drilling procedures are recommended: Drill pilot holes in loose repair parts with power feed equipment before preassembling them. The edges of the forming block are rounded to give the correct radius of bend to the part, and are undercut approximately 5° to allow for spring-back of the metal. Strike the edge of the flange at a slight angle to keep the vertical flange from bending outward. In Figure 9, note that crimps are placed at equally spaced intervals to absorb material and cause curving, while also giving strength to the part. Bend the part up until the correct height of the joggle is attained. It can be recognized very readily by an experienced worker. Use a hand drill only when positive-power-feed drills are not available. Hand-Held Sheet Metal Edge Shapers. Drill the material on a backing plate, such as cast iron, which is hard enough to permit the drill bit to cut completely through the stock without pushing the metal away from the drill point. Lightening holes should never be cut in any structural part unless authorized. Shrinking With V-Block and Shrinking Block Methods. Cold working most magnesium alloys at room temperature is very limited, because they work harden rapidly and do not lend themselves to any severe cold forming. An overly heavy blow buckles the metal, so keep moving the flange across the V-block, but always lightly strike the spot directly above the V. Lay out a full-sized, accurate pattern on a sheet of paper or plywood and periodically check the accuracy of the curve. Stress relieves the part as follows: heat the part to a temperature above 1,250 °F (677 °C), but below 1,450 °F (788 °C).
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