Hardwood Form Blocks V-Blocks Sheet Metal Holding Devices Sheet Metal Hammers and Mallets Sandbags Figure 4-69.  Sheet metal mallet and hammers. Figure 4-68.  Dollies and stakes. the crimp to prevent the material from creeping away, but  remains stationary while the crimp is hammered flat (being  shrunk). This type of crimping block is designed to be held  in a bench vise. Shrinking blocks can be made to fit any specific need. The  basic form and principle remain the same, even though the  blocks may vary considerably in size and shape. A sandbag is generally used as a support during the bumping  process. A serviceable bag can be made by sewing heavy  canvas or soft leather to form a bag of the desired size, and  filling it with sand which has been sifted through a fine  mesh screen. Before filling canvas bags with sand, use a brush to coat the  inside of the bag with softened paraffin or beeswax, which  forms a sealing layer and prevents the sand from working  through the pores of the canvas. Bags can also be filled with  shot as an alternative to sand. The sheet metal hammer and the mallet are metal fabrication  hand tools used for bending and forming sheet metal without  marring or indenting the metal. The hammer head is usually  made of high carbon, heat-treated steel, while the head of the  mallet, which is usually larger than that of the hammer, is  made of rubber, plastic, wood, or leather. In combination with  a sandbag, V-blocks, and dies, sheet metal body hammers  and mallets are used to form annealed metal.  [Figure 4-69] Most stakes have machined, polished surfaces that have been  hardened. Use of stakes to back up material when chiseling,  or when using any similar cutting tool, defaces the surface  of the stake and makes it useless for finish work. Hardwood form blocks can be constructed to duplicate  practically any aircraft structural or nonstructural part. The  wooden block or form is shaped to the exact dimensions and  contour of the part to be formed. V-blocks made of hardwood are widely used in airframe  metalwork for shrinking and stretching metal, particularly  angles and flanges. The size of the block depends on the work  being done and on personal preference. Although any type  of hardwood is suitable, maple and ash are recommended for  best results when working with aluminum alloys. In order to work with sheet metal during the fabrication  process, the aviation technician uses a variety of holding  devices, such as clamps, vises, and fasteners to hold the  work together. The type of operation being performed and  A shrinking block consists of two metal blocks and some  device for clamping them together. One block forms the  base and the other is cut away to provide space where the  crimped material can be hammered. The legs of the upper  jaw clamp the material to the base block on each side of  Shrinking Blocks 4-28