Figure 4-159.  Forming a concave flange. Figure 4-160.  Forming a convex flange.  45° 1.  Cut the material to size, allowing about  1 ⁄ 4-inch excess  material for trim and drill holes for alignment pins. 2.  Remove all burrs (jagged edges). This reduces the  possibility of the material cracking at the edges during  the forming process. 3.  Locate and drill holes for alignment pins. 4.  Place the material between the form blocks and  clamp blocks tightly in a vise to prevent the material  from moving or shifting. Clamp the work as closely  as possible to the particular area being hammered to  prevent strain on the form blocks and to keep the metal  from slipping. Concave Surfaces Convex Surfaces Forming by Bumping Bend the flange on the concave curve first. This practice  may keep the flange from splitting open or cracking when  the metal is stretched. Should this occur, a new piece must  be made. Using a plastic or rawhide mallet with a smooth,  slightly rounded face, start hammering at the extreme ends  of the part and continue toward the center of the bend. This  procedure permits some of the metal at the ends of the part  to be worked into the center of the curve where it is needed.  Continue hammering until the metal is gradually worked down over the entire flange, flush with the form block. After  the flange is formed, trim off the excess material and check  the part for accuracy.  [Figure 4-159] Convex surfaces are formed by shrinking the material over  a form block.  [Figure 4-160]  Using a wooden or plastic  shrinking mallet and a backup or wedge block, start at the  center of the curve and work toward both ends. Hammer the  flange down over the form, striking the metal with glancing  blows at an angle of approximately 45° and with a motion  that tends to pull the part away from the radius of the form  block. Stretch the metal around the radius bend and remove  the buckles gradually by hammering on a wedge block. Use  the backup block to keep the edge of the flange as nearly  perpendicular to the form block as possible. The backup block  also lessens the possibility of buckles, splits, or cracks. Finally,  trim the flanges of excess metal, planish, remove burrs, round  the corners (if any), and check the part for accuracy. As discussed earlier, bumping involves stretching the sheet  metal by bumping it into a form and making it balloon.  [Figure 4-161]  Bumping can be done on a form block or  female die, or on a sandbag. 4-79