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Either method requires only one form: a wooden block, a lead die, or a sandbag. The blister, or streamlined cover plate, is an example of a part made by the form block or die method of bumping. Wing fillets are an example of parts that are usually formed by bumping on a sandbag. 

## Form Block or Die 

The wooden block or lead die designed for form block bumping must have the same dimensions and contour as the outside of the blister. To provide enough bucking weight and bearing surface for fastening the metal, the block or die should be at least one inch larger in all dimensions than the form requires. 

Follow these procedures to create a form block: 

1. Hollow the block out with tools, such as saws, chisels, gouges, files, and rasps. 

2. Smooth and finish the block with sandpaper. The inside of the form must be as smooth as possible, because the slightest irregularity shows up on the finished part. 

3. Prepare several templates (patterns of the cross- section), as shown in Figure 4-161  so that the form can be checked for accuracy. 

4. Shape the contour of the form at points 1, 2, and 3. 

5. Shape the areas between the template checkpoints to conform the remaining contour to template 4. 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. 

After the form is prepared and checked, perform the bumping as follows: 

1. Cut a metal blank to size allowing an extra 1 ⁄ 2 to 1-inch to permit drawing. 

2. Apply a thin coat of light oil to the block and the aluminum to prevent galling (scraping on rough spots). 

3. Clamp the material between the block and steel plate. Ensure it is firmly supported yet it can slip a little toward the inside of the form. 

4. Clamp the bumping block in a bench vise. Use a soft- faced rubber mallet, or a hardwood drive block with a suitable mallet, to start the bumping near the edges of the form. 

5. Work the material down gradually from the edges with light blows of the mallet. Remember, the purpose of bumping is to work the material into shape by stretching rather than forcing it into the form with heavy blows. Always start bumping near the edge of the form. Never start near the center of the blister. 

6. Before removing the work from the form, smooth it as much as possible by rubbing it with the rounded end of either a maple block or a stretching mallet. 

7. Remove the blister from the bumping block and trim to size. 

