NOTE: Holes can also be enlarged by using a series  of step reamers. The chip chaser is designed to remove chips and burrs lodged  between sheets of metal after drilling holes for riveting.  [Figure 4-54]  Chip chasers have a plastic molded handle  and a flexible steel blade with a hook in the end. Sheet metal forming dates back to the days of the blacksmith  who used a hammer and hot oven to mold metal into the  desired form. Today’s aircraft technician relies on a wide  variety of powered and hand-operated tools to precisely bend  and fold sheet metal to achieve the perfect shape. Forming  tools include straight line machines, such as the bar folder and  press brake, as well as rotary machines, such as the slip roll  former. Forming sheet metal requires a variety of tools and  equipment (both powered and manual), such as the piccolo  former, shrinking and stretching tools, form blocks, and  specialized hammers and mallets.  [Figure 4-55] Tempered sheet stock is used in forming operations whenever  possible in typical repairs. Forming that is performed in the  tempered condition, usually at room temperature, is known  as cold-forming. Cold forming eliminates heat treatment and  the straightening and checking operations required to remove  the warp and twist caused by the heat treating process. Cold- formed sheet metal experiences a phenomenon known as  spring-back, which causes the worked piece to spring back  slightly when the deforming force is removed. If the material  shows signs of cracking during cold forming over small radii,  the material should be formed in the annealed condition.  Annealing, the process of toughening steel by gradually  heating and cooling it, removes the temper from metal,  making it softer and easier to form. Parts containing small  radii or compound curvatures must be formed in the annealed  condition. After forming, the part is heat treated to a tempered  condition before use on the aircraft. Construction of interchangeable structural and nonstructural  parts is achieved by forming flat sheet stock to make channel,  angle, zee, and hat section members. Before a sheet metal part  is formed, a flat pattern is made to show how much material  is required in the bend areas, at what point the sheet must be  inserted into the forming tool, or where bend lines are located.  Determination of bend lines and bend allowances is discussed  in greater detail in the section on layout and forming. The bar folder is designed for use in making bends or folds  along edges of sheets.  [Figure 4-56]  This machine is best  suited for folding small hems, flanges, seams, and edges to  be wired. Most bar folders have a capacity for metal up to 22  gauge in thickness and 42 inches in length. Before using the  bar folder, several adjustments must be made for thickness of  material, width of fold, sharpness of fold, and angle of fold.  The adjustment for thickness of material is made by adjusting  the screws at each end of the folder. As this adjustment is  Bar Folding Machine Forming Tools Chip Chasers Figure 4-54.  Chip chaser. Figure 4-55.  Hammer and mallet forming. Figure 4-56.  Bar folder. 4-21