BEADING ROLLS.—Beading rolls are used for turning beads (grooves) on tubing, cans, and buckets. Beads may also be placed on sheet stock that is to be welded. There are several different types of beading rolls. Those shown in figure 13-41 are single bead rolls. When you are beading, the groove should not be made too deeply in a single rotation as this tends to weaken the metal. TURNING ROLLS.—Turning rolls are used for turning an edge to receive a stiffening wire. When you are turning an edge, rest the cylinder to be wired on the lower wheel and press against the gauge. The gauge is adjusted according to the size of wire to be used. With the work set in place, bring the upper roll down until it grips the metal. Turn the crank slowly while you are holding the metal so that the metal will feed into the rolls. Continue to press against the guide. After the first revolution, gradually raise the metal until it touches the outer face of the top roll. Remove the stock by raising the top roll. WIRING ROLLS.—Wiring rolls are used to finish the wired edges prepared in the turning rolls. To use the wiring rolls, you should adjust the top roll so that it is directly above the point on the lower roll where the beveled and flat surfaces meet, as shown in view A of figure 13-42. Adjust the guide to the position shown in view B, then bring the top roll down so that it will turn the edge of the metal as shown in view C. Remove the stock from the machine by raising the top roll. CRIMPING ROLLS.—Crimping rolls are used to make one end of a pipe smaller than the other so that two sections may be slipped together, one end into the other. A bead is placed on a pipe first, and then it is crimped. The bead forms a shoulder to keep the pipe from slipping too far into the adjoining section. BURRING ROLLS.—Burring is perhaps the most difficult operation to perform on a rotary machine. Before you place the work in the machine, Figure 13-42.—Wiring operation. make sure the cylinder or circular disc to be burred is cut or formed as perfectly round as possible. Then adjust the gauge on the machine so the space between the inside of the upper roll and the gauge is set to the width of the burr. Next, place the object between the rolls and against the gauge. Then you should lower the upper roll until it scores the material slightly. Turn the crank slowly to allow the metal to slide between thumb and fingers. Apply a slight upward pressure as the metal passes between the rolls. After the first revolution, lower the top roll and again pass the metal between the rolls. Repeat this process, raising the edge slightly with each complete revolution of the material, until the edge has been burred to the proper angle. RIVETING PROCEDURES You must use your knowledge, ability, and experience to plan an aircraft structural repair that involves riveting. Each rivet must be selected and driven in a precise manner to meet the riveting specification. Some of the specifications are rivet spacing and edge distance, diameter of the rivet hole, aerodynamic smoothness, and size of the rivet bucktail. These can be accomplished only through determination, practice, and accurate manipulation of all layout and riveting equipment. Rivet Selection The following rules should govern your selection and use of rivets: 1. Replacements must not be made with rivets of lower strength material unless they are larger than those removed. For example, a rivet of 2024 aluminum alloy should not be replaced by one made of 2017 aluminum alloy unless the 2017 rivet is a size larger. Similarly, when 2117 rivets are used to replace 2017 rivets, the next larger size should be used. 2. When rivet holes become enlarged, deformed, or otherwise damaged, you should use the next larger size as replacement. 3. Countersunk-head rivets should be replaced by rivets of the same type and degree of countersink, either AN426 or MS20426. 4. All protruding-head rivets should be replaced with universal-head rivets, either AN470 or MS20470. 5. Rivets less than three thirty-seconds of an inch in diameter should not be used for any structural parts, 13-26