Standard twist drills are used to drill rivet holes. Table 13-2 specifies the size drill to be used with the various size rivets. Note that there is a slight clearance in each case. This prevents binding of the rivet in the hole.

Locations for the rivet holes should be center punched and the drilling done with a power drill, either electric or pneumatic. Electric drills constitute a fire hazard when you are drilling on or near an aircraft. The hazard is caused by the arcing of the brushes. Therefore, the pneumatic drill should be used. The center punch mark should be large enough to prevent the drill from slipping out of position, but must not be made with enough force to dent the surrounding material. All burrs must be removed by using a larger size drill or by using a deburring tool.

In aircraft construction, manufacturers are eliminating protruding-head rivets on the exterior surfaces. In fabricating stressed metal skin, all exposed rivet heads must be countersunk to lie flush with the outer surface of the skin. It is essential to provide an aerodynamically smooth surface. See figure 13-45.

Flush rivets are more difficult to install because the parts being riveted must be countersunk. Another hazard is the closeness of the rivet set to the metal during riveting. If considerable skill is not used, the metal will be damaged by the rivet set. Flush rivets are made with heads of several different angles, but the 100-degree rivet is standard for all Navy aircraft. The two methods used to countersink flush rivets are dimple and machine countersinking. In some instances, a combination of the two may be used; in other words, the top sheet of an assembly may be dimpled while the under sheet is machine countersunk.

Before driving any rivets, make sure all the holes line up perfectly, all the shavings and burrs have been removed, and the parts to be riveted are fastened securely together. It is important that the sheets be held firmly together near the area of the rivet being driven.

Table 13-2.-Drill Sizes for Various Size Rivets

To adjust the speed of the gun, place it against a block of wood. Never operate a rivet gun without resistance against the set. The vibrating action may cause the retaining spring to break, allowing the set to fly out.

A rivet set can be a deadly weapon. If a rivet set is placed in a rivet gun without a set retainer and the throttle of the gun is opened, the rivet set may be projected like a bullet. This may cause severe injury to a person or destruction of equipment.

Figure 13-45.-Incorrect countersinking.

The gun should be adjusted so the rivet can be driven in the shortest possible time, but you must take care not to drive the rivet so hard or in such a manner as to dimple the metal. Practice will enable you to properly adjust a gun for any type of work.

The rivet should be pushed into proper position and held there firmly, with the set of the rivet gun resting squarely against the rivet head. The bucking bar is held firmly and squarely against the protruding rivet shank. (In most instances, the bucking bar must be manipulated by another person, called the "bucker.") The gunner then exerts pressure on the trigger and starts driving. The gun must be held tightly against the rivet head, and it must not be removed until the trigger has been released. The bucker removes the bucking bar and checks the upset head after the gunner has stopped driving. A signal system is usually employed to develop the necessary teamwork, and consists of tapping lightly against the work. One tap may mean "not fully driven, hit it again"; two taps may mean "good rivet"; three taps may mean "bad rivet, remove and drive another. "

The upset head, often referred to as the bucktail, should be 1 1/2 times the original diameter of the shank in width and 1/2 times the original diameter in height, as shown in figure 13-46. If the head formed is narrower and higher than the dimensions given, more driving is necessary. If it is wider and shallower, it must be removed and replaced.