1 2 2 3 3 4 4 1 Templates for working the form block Form block Holddown plate Finished part Figure 4-161.  Form block bumping. 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.  4-80 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. 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. After the form is prepared and checked, perform the bumping  as follows: Form Block or Die