Figure 4-37.  Die grinder. Figure 4-36. Files.  Figure 4-38.  Burring tools. Burring Tool Hole Drilling Die Grinder 4-14 The portion of the file on which the teeth are cut is called the  face. The tapered end that fits into the handle is called the  tang. The part of the file where the tang begins is the heel.  The length of a file is the distance from the point or tip to the  heel and does not include the tang. The teeth of the file do  the cutting. These teeth are set at an angle across the face of  the file. A file with a single row of parallel teeth is called a  single-cut file. The teeth are cut at an angle of 65°–85° to the  centerline, depending on the intended use of the file. Files  that have one row of teeth crossing another row in a crisscross  pattern are called double-cut files. The angle of the first set  usually is 40°–50° and that of the crossing teeth 70°–80°.  Crisscrossing produces a surface that has a very large number  of little teeth that slant toward the tip of the file. Each little  tooth looks like an end of a diamond point cold chisel. from forming on the teeth. File teeth can be cleaned with a  file card.  A die grinder is a handheld tool that turns a mounted  cutoff wheel, rotary file, or sanding disk at high speed.  [Figure 4-37]  Usually powered by compressed air, electric  die grinders are also used. Pneumatic die grinders run at  12,000 to 20,000 revolutions per minute (rpm) with the  rotational speed controlled by the operator who uses a hand-  or foot-operated throttle to vary the volume of compressed  air. Available in straight, 45°, and 90° models, the die  grinder is excellent for weld breaking, smoothing sharp  edges, deburring, porting, and general high-speed polishing,  grinding, and cutting.  Files are graded according to the tooth spacing; a coarse file  has a small number of large teeth, and a smooth file has a large  number of fine teeth. The coarser the teeth, the more metal is  removed on each stroke of the file. The terms used to indicate  the coarseness or fineness of a file are rough, coarse, bastard,  second cut, smooth, and dead smooth, and the file may be  either single cut or double cut. Files are further classified  according to their shape. Some of the more common types  are: flat, triangle, square, half round, and round. This type of tool is used to remove a burr from an edge of a  sheet or to deburr a hole.  [Figure 4-38] Drilling holes is a common operation in the airframe repair  shop. Once the fundamentals of drills and their uses are  learned, drilling holes for rivets and bolts on light metal is  There are several filing techniques. The most common is to  remove rough edges and slivers from the finished part before  it is installed. Crossfiling is a method used for filing the  edges of metal parts that must fit tightly together. Crossfiling  involves clamping the metal between two strips of wood  and filing the edge of the metal down to a preset line. Draw  filing is used when larger surfaces need to be smoothed and  squared. It is done by drawing the file over the entire surface  of the work. To protect the teeth of a file, files should be stored separately  in a plastic wrap or hung by their handles. Files kept in a  toolbox should be wrapped in waxed paper to prevent rust 