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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 

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 

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 

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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. 

## Forming by Bumping 

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. 

