not difficult. While a small portable power drill is usually  the most practical tool for this common operation in airframe  metalwork, sometimes a drill press may prove to be the better  piece of equipment for the job. Portable power drills operate by electricity or compressed air.  Pneumatic drill motors are recommended for use on repairs  around flammable materials where potential sparks from an  electric drill motor might become a fire hazard. When using the portable power drill, hold it firmly with both  hands. Before drilling, be sure to place a backup block of  wood under the hole to be drilled to add support to the metal  structure. The drill bit should be inserted in the chuck and  tested for trueness or vibration. This may be visibly checked  by running the motor freely. A drill bit that wobbles or is  slightly bent should not be used since such a condition causes  enlarged holes. The drill should always be held at right angles  to the work regardless of the position or curvatures. Tilting  the drill at any time when drilling into or withdrawing from  the material may cause elongation (egg shape) of the hole.  When drilling through sheet metal, small burrs are formed  around the edge of the hole. Burrs must be removed to allow  rivets or bolts to fit snugly and to prevent scratching. Burrs  may be removed with a bearing scraper, a countersink, or  a drill bit larger than the hole. If a drill bit or countersink  is used, it should be rotated by hand. Always wear safety  goggles while drilling. Pneumatic drill motors are the most common type of drill  motor for aircraft repair work.  [Figure 4-39]  They are  lightweight and have sufficient power and good speed  control. Drill motors are available in many different sizes  and models. Most drill motors used for aircraft sheet metal  work are rated at 3,000 rpm, but if drilling deep holes or  drilling in hard materials, such as corrosion resistant steel  The drill press is a precision machine used for drilling holes  that require a high degree of accuracy. It serves as an accurate  means of locating and maintaining the direction of a hole that  is to be drilled and provides the operator with a feed lever  that makes the task of feeding the drill into the work easier.  Special drill motors that drill two holes at the same time are  used for the installation of nutplates. By drilling two holes  at the same time, the distance between the holes is fixed and  the holes line up perfectly with the holes in the nutplate.  [Figure 4-41] Right angle and 45° drill motors are used for positions that are  not accessible with a pistol grip drill motor. Most right angle  drill motors use threaded drill bits that are available in several  lengths. Heavy-duty right angle drills are equipped with a  chuck similar to the pistol grip drill motor.  [Figure 4-40] or titanium, a drill motor with more torque and lower rpm  should be selected to prevent damage to tools and materials. Figure 4-40.  Angle drill motors. Figure 4-39.  Drill motors. Figure 4-41.  Nutplate drill. Drill Press Pneumatic Drill Motors Two Hole  Right Angle and 45° Drill Motors Portable Power Drills 4-15