Sliding valve Piston Set Blank rivet sleeve set Set retainer Exhaust Cylinder Throttle deflector Throttle lever Throttle valve Throttle tube Movement of air during forward stroke Bushing Movement of air during rearward stroke hegulator adjustment screw Air path FIGURE 5-45. Rivet gun nomenciature.
through the hole. If the hole is too large, the rivet will not fill the hole completely. When it is bucked, the joint will not develop its full strength, and structural failure may occur at that spot.
If countersinking is required, consider the thickalloy are drilled with greater accuracy by a drill having an included angle of 118° because the large angle of the drill has less tendency to tear or elongate the hole.
Center punch locations for rivet holes before beness of the metal and adopt the countersinking ginning the actual drilling. The center punch mark method recommended for that thickness. If dimpling is required, keep hammer blows or dimpling pressures to a minimum so that no undue work-hardening occurs in the surrounding area. Diameter Rivet Pilot Size Ream Size Drilling 3/32 3/32 (.0937)° 40 (.098) To make a rivet hole of the correct size, first drill 1/8 1/8 (.125) 30 (.1285) a hole slightly undersize. This is known as predrill- 5/32 5/32 (.1562) 21 (.159) ing, and the hole is called a pilot hole. Ream the 3/16 1/4 3/16 (.1875) 1/4 (.250) 11 (.191) F (.257) pilot hole with a twist drill of the correct size to get 5/16 5/16 (.3125) • (.316) the required dimension. Pilot and reaming drill 3/8 3/8 (.375) V (.377) sizes are shown in figure 5-46. The recommended clearance for rivet holes is from 0.002 to 0.004 in. "Note that ream size exceeds the maximum tolerance of .004 inch. This is permissible only if the next When drilling hard metals the twist drill should larger drill size happens to be so much larger than have an included angle of 118° and should be oper- the tolerance of .004 inch.
ated at low speeds; but for soft metals, use a twist drill with an included angle of 90° and it should be operated at higher speeds. Thin sheets of aluminum FIGURE 5-46. Pilot and reaming twist drill sizes.
acts as a guide and lets the drill grip or bite into the metal with greater ease. Make the center punch mark large enough to prevent the drill from slipping out of position, but punch lightly enough not to dent the surrounding material. Hold a hard, smooth, wooden backing block securely in position behind the hole locations when drilling.
Drilling is usually done with a hand drill or with a light power drill. Hold the power drill firmly with both hands. Extend the index and middle fingers of the left hand against the metal to act as a guide in starting a hole, and as a snubber or brake when the drill goes through the material. Before beginning to drill, always test the inserted twist drill for trueness and vibration by spinning the hand drill or running the motor freely and watching the drill end. If the drill wobbles, it may be because of burrs on its shank or because the drill is bent or incorrectly chucked. A drill that wobbles or is slightly bent must not be used because it causes enlarged holes.
Always hold the drill at right angles to the work, regardless of the position of the hole or the curvature of the material. Use an angle drill or drill extensions and adapters when access is difficult with a straight drill. Never tip the drill sideways when drilling or when withdrawing from the material because this causes elongation of the hole.
When holes are drilled through sheet metal, small burrs are formed around the edge of the hole. This is especially true when using a hand drill since the drill speed is slow and there is a tendency to apply more pressure per drill revolution. Remove all burrs with a burr remover before riveting.
Countersinking and Dimpling An improperly made countersink reduces the strength of a flush-riveted joint and may even cause failure of the sheet or the rivet head. The two methods of countersinking commonly used for flush riveting in aircraft construction and repair are the machine or drill countersinking, and dimpling or press countersinking. The proper method for any particular application depends on the thickness of the parts to be riveted, the height and angle of the countersunk head, the tools available, and accessibility.
As a general rule, use the drill countersink method when the thickness of the material is greater than the thickness of the rivet head, and use the dimpling method on thinner material. Figure 5-47 illustrates general rules for countersinking. Note in figure 5-47A that the material is quite thick and the head of the countersunk rivet extends only about halfway through the upper layer of metal.
Countersinking will leave plenty of material for gripping.
Preferred countersinking Permissible countersinking Unacceptable countersinking FIGURE 5-47. Countersinking.
In figure 5-47B, the countersunk head reaches completely through the upper layer. This condition is permissible but should be avoided.
In figure 5-47C, the head extends well into the second layer of material. This indicates that the material is thin and that most of it would be ground away by drill countersinking; therefore, dimpling is preferred. Dimpling will work best if the material is not over 0.040-in. thick.
Machine or drill countersinking is accomplished by a suitable cutting tool machined to the desired angle. The edge of the hole is cut away so that the countersunk rivet head fits snugly into the recess.
The resulting recess is referred to as the "well" or "nest."
During the process of machine countersinking, first drill the original rivet hole to the exact rivet size, as recommended in the table in figure 5-46 The limits within which the head of the rivet may extend either above or below the surface of the metal are close, 0.006 in. in most cases. Therefore, 170
perform the countersinking accurately, using equipment which is capable of producing results within the specified tolerance.
Hold the countersinking tool firmly at right angles to the material. Do not tip it. Tipping elongates the well and prevents the countersunk rivet head from fitting properly. Oversized rivet holes, undersized countersink pilots (in the case of the stop countersink), chattering caused by improper use of the countersink or by a countersink in poor condition, and a countersink not running true in the chuck of the drill are some of the causes of elongated wells.
Press countersinking or dimpling can be accomplished by either of two methods. Male and female die sets can be used, or using the rivet as the male die and the draw die as the female die is acceptable.
In either case, the metal immediately surrounding the rivet hole is pressed to the proper shape to fit the rivet head. The depression thus formed, as in machine countersinking, is known as the "well" or "nest."
The rivet must fit the well snugly to obtain maximum strength. The number of sheets which can be dimpled simultaneously is limited by the capacity of the equipment used. The dimpling process may be accomplished by the use of hand tools, by dies placed in a pneumatic squeeze or single shot riveter, or by using a pneumatic riveting hammer.
Dimpling dies are made to correspond to any size and degree of countersunk rivet head available. The dies are usually numbered, and the correct combination of punch and die to use is indicated on charts specified by the manufacturer. Both male and female dies are machined accurately and have highly polished surfaces. The male die or punch is cone shaped to conform to the rivet head and has a small concentric pilot shaft that fits into the rivet hole and female die. The female die has a corresponding degree of countersink into which the male guide fits.
When dimpling a hole, rest the female die on some solid surface, place the material on the female die, insert the male die in the hole to be dimpled, and then hammer the male die. Strike with several solid blows until the dimple is formed.
In some cases, the face of the male die is convex to allow for springback in the metal. Dies of this type are used to advantage when the sheet to be dimpled is curved. Some dies have flat faces and are principally used for flat work. Dimpling dies are usually made so that their included angle is 5° less than that of the rivet. This arrangement allows for springback of the metal, In die dimpling, the pilot hole of the female die should be smaller than the diameter of the rivet to be used. Therefore, the rivet hole must be reamed to the exact diameter after the dimpling operation has been completed so that the rivet fits snugly.
When using a countersink rivet as the male dimpling die, place the female die in the usual position and back it with a bucking bar, Place the rivet of the required type into the hole and strike the rivet with a pneumatic riveting hammer. This method of countersinking is often called "coin pressing." It should be used only when the regular male die is broken or not available.
Coin pressing has a distinct disadvantage in that the rivet hole must be drilled to correct rivet size before the dimpling operation is accomplished.
Since the metal stretches during the dimpling operation, the hole becomes enlarged and the rivet must be swelled slightly before driving to produce a close fit. Because the rivet head will cause slight distortions in the recess, and these are characteristic only to that particular rivet head, it is wise to drive the same rivet that was used as the male die during the dimpling process. Do not substitute another rivet, either of the same size or a size larger.
Thermo-Dimpling This type of dimpling consists of two processes, radius dimpling and coin dimpling. The major dif ference between radius and coin dimpling is in the construction of the female die. In radius dimpling a solid female die is used. Coin dimpling uses a sliding ram female die (figure 5-48) that makes this process superior.
During the coin dimpling process, the metal is coined (made to flow) into the contours of the dies so that the dimple assumes the true shape of the die.
The pressure exerted by the coining ram prevents the metal from compressing and thereby assures uniform cross sectional thickness of the sides of the dimple and a true conical shape.
• Coin dimpling offers several advantages. It improves the configuration of the dimple, produces a more satisfactory aerodynamic skin surface, eliminates radial and circumferential cracking, ensures a stronger and safer joint, and allows identical dies to be used for both skin and understructure dimpling.
The material being used is a very important factor to consider in any dimpling operation. Materials such as corrosion-resistant steel, magnesium, 171
Radius dimpling Coin dimpling FICURE 5-48. Radius and coin dimpling dies.
and titanium each present different dimpling problems.
The 2024-T aluminum alloy can be satisfactorily coin dimpled either hot or cold. However, cracking in the vicinity of the dimple may result from cold dimpling because of hard spots in the metal. Hot dimpling will prevent such cracking.
The 7075-T6 and 2024-T81 aluminum alloys are always hot dimpled. Magnesium alloys also must be hot dimpled because, like 7075-16, they have low formability qualities. Titanium is another metal that must be hot dimpled because it is tough and resists forming. The same temperature and dwell time used to hot dimple 7075-T6 is used for titanium.
Corrosion-resistant steel is cold dimpled because the temperature range of the heating unit is not high enough to affect dimpling.
The coin ram dimpling dies are designed with a number of built-in features. The faces of both the male and female dies are dished (the male concave and the female convex) at an angle of 2° on the pilot. This facilitates removal of the metal after the dimple has been made.
The female dimpling set has two parts: (1) The body, which is merely a counterpart of the male die; and (2) the coining ram, which extends up through the center of the conical recess of the body.
In forming a dimple, the metal is forced down into the female die by the male die. The metal first contacts the coining ram, and this supports the metal as it is forced down into the conical recess.
When the two dies close to the point where the forces of both are squeezing the material, the coining ram forces the metal back into the sharp corners of the dies.
When cold dimpling, the dies are used alone.
When hot dimpling, a strap or block heater is slipped over either or both dies and connected to an electric current.
The dies should be kept clean at all times and in good working order. It is advisable to clean them regularly with steel wool. Special precautions must be taken when the dies are in the machine. If the machine is operated with the dies in place but without material between them, the male die will enlarge and ruin the coining ram.
When possible, coin dimpling should be performed on stationary equipment and before the assembly of parts. However, many instances arise in which dimpling must be done after parts are assembled to other structures. In such cases, dimpling operations are performed by portable squeeze dimplers. Most squeezers may be used either for cold dimpling or, combined with a junction box, for hot dimpling.
There are dimpling applications in which it is not possible to accommodate any squeezer- or yoke-type equipment. Under these circumstances, it is necessary to use a pneumatic hammer and a bucking bar type of tool to hold the dimpling dies.
DRIVING RIVETS The methods of driving solid shank rivets can be classified into two types, depending on whether the riveting equipment is portable or stationary. Since stationary riveting equipment is seldom used in airframe repair work, only portable equipment that is used in hand, pneumatic, or squeezer methods is discussed here.
Before driving any rivets, be sure that all holes line up perfectly, all shavings and burrs have been removed, and that the parts to be riveted are securely fastened together.
Two men, a "gunner"' and a "bucker," usually work as a team when installing rivets. However, on some jobs the riveter holds a bucking bar with one hand and operates a riveting gun with the other.
When team riveting, an efficient signal system can be employed to develop the necessary teamwork.
The code usually consists of tapping the bucking bar against the work; one tap may meaninot fully seated, hit it again"; two taps may mean #good rivet"; three taps may mean "bad rivet, remove and drive another"; and so on.
Bucking Selection of the appropriate bucking bar is one of the most important factors in bucking rivets. If the bar does not have the correct shape, it will deform the rivet head; if the bar is too light, it will not give the necessary bucking weight, and the material may become bulged toward the shop head;
and, if the bar is too heavy, its weight and the bucking force may cause the material to bulge away from the shop head. Weights of bucking bars range from a few ounces to 8 or 10 lbs., depending upon the nature of the work. Recommended weights of bucking bars to be used with various rivet sizes are given in figure 5-49.
Rivet Approx.
Diameter Weight (In Inches) (In Pounds) 3/32 2 to 3 1/8 3 to 4 5/32 3 to 4½ 3/16 4 to 5 1/4 5 to 622 FIGURE 5-49. Recommended bucking bar weights.
Always hold the face of the bucking bar at right angles to the rivet shank. Failure to do this will cause the rivet shank to bend with the first blows of the rivet gun, and will cause the material to become marred with the final blows. The bucker must hold the bucking bar in place until the rivet is completely driven. If the bucking bar is removed while the gun is in operation, the rivet set may be driven through the material. Do not bear down too heavily on the shank of the rivet. Allow the weight of the bucking bar to do most of the work. The hands merely guide the bar and supply the necessary tension and rebound action.
Allow the bucking bar to vibrate in unison with the gun set. This process. is called coordinated bucking. Coordinated bucking can be developed through pressure and stiffness applied at the wrists; with experience, a high degree of deftness can be obtained.
Lack of proper vibrating action, the use of a bucking bar that is too light or too heavy, and failure to hold the bucking bar at right angles to the rivet can all cause defective rivet heads. A rivet going "clubhead" (malforming) can be corrected by rapidly moving the bucking bar across the rivet head in a direction opposite that of clubhead travel.
This corrective action can be accomplished only while the gun is in action and the rivet is partly driven. If a rivet shank bends at the beginning of the bucking operation, place the bar in the corrective position only long enough to straighten the shank.
Hand Driving Under certain conditions, it may be necessary to rivet by hand driving. Either of two methods can be used depending upon the location and accessibility of the work. In the one method, the manufactured head end of the rivet is driven with a hand set and hammer, the shank end is bucked with a suitable bucking bar. In the other method, the shank end of the rivet is driven with a hand set and a hammer, and the manufactured head is bucked with a hand set held in a vise or a bottle bar (a special bucking bar recessed to hold a rivet set). This method is known as reverse riveting. It is commonly used in hand riveting but is not considered good practice in pneumatic riveting.
When using either of the described methods, keep hammer strokes to a minimum. Too much hammering will change the crystalline structure of the rivet.
or the material around it, causing the joint to lose 173
some of its strength. Hold the bucking bar and rivet set square with the rivet at all times. Misuse of the rivet set and bucking bar will result in marring or scratching the rivet head or material, and may cause undue corrosion. This, in turn, will weaken the structure of the aircraft.
The diameter of a correctly formed shop head should be one and one-half times the diameter of the rivet shank, and the height should be about one-half the diameter.
Pneumatic Driving The procedure for pneumatic riveting is practically the same as for hand riveting. Preparation of the sheet, selection of rivets, and drilling of rivet holes are the same. In hand riveting, however, the pressure for bucking the rivet is applied using a hand set and hammer. In pneumatic riveting, the pressure is applied with a set and an air-driven hammer or gun.
To get good riveting results with a pneumatic rivet gun, follow these basic pointers:
(1) Select the right type and size of rivet gun and the correct rivet set for the size of rivet to be driven. Install the rivet set firmly, as shown in figure 5-50.
end of the set, causing it to bind in the barrel of the gun.
(3) Hold the rivet set at right angles to the work to prevent damage to the rivet head or the surrounding material as shown in figure 5-51. Upset the rivet with a medium burst from the rivet gun.
FICURE 5-51. Position of the set.
FIGURE 5-50. Installing rivet set.
(2) Adjust the speed of the riveting gun (vibrations per minute). Always press set firmly against a block of wood before pressing the trigger. Never operate the gun without resistance against the set because the vibrating action may cause the retaining spring to break, allowing the gun set to fly out of the gun. Also, free vibration may flare or mushroom the gun
(4) Remove the bucking bar and check the shop head of the rivet. It should be one and one-half times the diameter of the rivet in width and one-half times the rivet diameter in height. If the rivet needs further driving, repeat the necessary procedures to complete the job.
A small piece of adhesive tape applied to the cupped end of the rivet set often corrects an unsatisfactory cupped condition, which occasionally gives trouble in forming uniformly shaped rivet heads.
Squeeze Riveting The squeeze method of driving a rivet produces the most uniform and balanced type of shop head Each rivet is upset in a single operation; all rivets are headed over with uniform pressure; all heads are formed alike; and each rivet shank is sufficiently and uniformly expanded to completely fill each rivet hole. Squeeze riveters come equipped with pairs of end sets, each pair being designed for a particular job. Once the correct end set is selected 174
and the squeezer adjusted for a particular application, all the rivets will be driven uniformly, thus providing an efficient method of riveting.
Portable squeezers are particularly suited for riveting large assemblies where the tool must be moved in relation to the work. They are not too heavy and can easily be operated by one person. The preparation of the material for riveting with the squeeze riveter is the same as for hand or pneumatic riveting. For better results when using the squeeze riveter, observe these rules:
(1) Carefully select and insert suitable end sets to match the rivet being used. The importance of using the right end sets cannot be overemphasized. It is impossible to buck the rivet properly unless the correct pairs are used. Always be sure that the air is shut off or the squeezer is disconnected when inserting end sets.
(2) Adjust the squeezer cylinder pressure to obtain the correct pressure for the diameter of the rivet being used. Most squeezers are equipped with a blowoff valve to regulate the cylinder pressure. This unit governs the amount of air pressure allowed in the cylinder.
(3) Carefully regulate the gap to conform to the length of the rivet being used. Some squeezers are equipped with a gap regulator which controls the stroke of the plunger of a C-yoke squeezer, or the movement of the movable jaw of an alligator-yoke squæezer. For squeezers not equipped with a gap regulator, the gap can be adjusted by placing metal shims under the end sets of both jaws, or by using end sets of different lengths. On some types of squeeze riveters, the end set on the stationary jaw is held in place by an Allen screw, which allows regulation of the gap.
(4) Before using the squeezer on the work, test the cylinder pressure and gap for accuracy of adjustment on a piece of scrap material. The scrap material must be the same thickness as the material being used, and the rivets the same length and diameter.
(5) If the parts to be riveted are small and easily handled, mount the squeeze riveter in a bench vise or in a special clamp, and hold the part to be riveted in your hand.
Microshaving Sometimes it is necessary to use a microshaver when making a repair involving the use of countersunk rivets. If the smoothness of the material (such as skin) requires that all countersunk rivets be driven within a specific tolerance, a microshaver is used. This tool has a cutter, stop, and two legs or stabilizers, as shown in figure 5-52.
Marks Stop Cutting tool FICURE 5-52. Microshaver.
The cutting portion of the microshaver is located inside the stop. The depth of cut can be adjusted by pulling outward on the stop and turning it in either direction (clockwise for deeper cuts). The marks on the stop permit adjustments of 0.001 in.
If the microshaver is adjusted and held correctly, it will cut the head of a countersunk rivet to within
0.002 in. without damaging the surrounding material. Adjustments should always be made on scrap material. When correctly adjusted, the shaver will leave a small round dot about the size of a pinhead on the microshaved rivet.
RIVET FAILURES Generally speaking, the design of riveted joints is based on the theory that the total joint strength is simply the sum of the individual strengths of a whole group of rivets. It is then obvious that, if any one rivet fails, its load must immediately be carried by others of the group; if they are unable to carry this added load, progressive joint failure then occurs. Stress concentrations will usually cause one rivet to fail first; and careful analysis of such a 175
rivet in a joint will indicate that it has been too highly loaded, with the possibility that neighboring rivets may have partially failed.
Shear Failure Shear failure is perhaps the most common of rivet failures. It is simply a breakdown of the rivet shank by forces acting along the plane of two adjacent sheets, causing a slipping action which may be severe enough to cut the rivet shank in two. If the shank becomes loaded beyond the yield point of the material and remains overloaded, a permanent shift is established in the sheets and the rivet shank may become joggled.
Bearing Failure If the rivet is excessively strong in shear, bearing failure occurs in the sheet at the edge of the rivet hole. The application of large rivets in thin sheets brings about such a failure. In that case, the sheet is locally crushed or buckled, and the buckling destroys the rigidity of the joint. Vibrations, set up by engine operation or by air currents in flight, may cause the buckled portion to flutter and the material to break off close to the rivet head. If buckling occurs at the end of the sheet, a tear-out may result. In either case, replacement of the sheet is necessary.
Head Failure Head failure may result from complex loadings occuring at a joint, causing stresses of tension to be applied to the rivet head. The head may fail by shearing through the area corresponding to the rivet shank, or, in thicker sheets, it may fail through a prying action which causes failure of the head itself. Any visible head distortion is cause for replacement. This latter type of head failure is especially common in blind rivets.
Rivet Inspection To obtain high structural efficiency in the manufacture and repair of aircraft, an inspection must be made of all rivets before the part is put in service.
This inspection consists of examining both the shop and manufactured heads and the surrounding skin and structural parts for deformities. A scale or rivet gage can be used to check the condition of the upset rivet head to see that it conforms to the proper requirements. Deformities in the manufactured head can be detected by the trained eye alone. However, on flush rivets, a straightedge can be used as shown in figure 5-53.
Rivet gage Straightedge.
FIGURE 5-53. Tools used to gage rivets.
Some common causes of unsatisfactory riveting are improper bucking, rivet set slipping off or being held at the wrong angle, and rivet holes or rivets of the wrong size. Additional causes for unsatisfactory riveting are countersunk rivets not flush with the well; work not properly fastened together during riveting; the presence of burrs, rivets too hard, too much or too little driving; and rivets out of line.
Occasionally, during an aircraft structural repair, it is wise to examine adjacent parts to determine the true condition of neighboring rivets. In doing so, it may be necessary to remove the paint. The presence of chipped or cracked paint around the heads may indicate shifted or loose rivets. Look for tipped or loose rivet heads. If the heads are tipped or if rivets are loose, they will show up in groups of several consecutive rivets and will probably be tipped in the same direction. If heads which appear to be tipped are not in groups and are not tipped in the same direction, tipping may have occurred during some previous installation.
Inspect rivets known to have been critically loaded, but which show no visible distortion, by drilling off the head and carefully punching out the shank. If, upon examination, the shank appears joggled and the holes in the sheet misaligned, the rivet has failed in shear. In that case, try to determine what is causing the shearing stress and take the necessary corrective action. Flush rivets that show head slippage within the countersink or dimple, indicating either sheet bearing failure or rivet shear 176
failure, must be removed for inspection and replacement.
Joggles in removed rivet shanks indicate partial shear failure. Replace these rivets with the next larger size. Also, it the rivet holes show elongation, replace the rivets with the next larger size. Sheet failures (such as tear-outs, cracks between rivets, and the like) usually indicate damaged rivets, and the complete repair of the joint may require replacement of the rivets with the next larger size.
The general practice of replacing a rivet with the next larger size (1/32 in. greater diameter) is necessary to obtain the proper joint strength of rivet and sheet when the original rivet hole is enlarged.
If the rivet in an elongated hole is replaced by a rivet of the same size, its ability to carry its share of the shear load is impaired and joint weakness results.
REMOVING RIVETS When removing a rivet for replacement, be very careful so that the rivet hole will retain its original size and shape and replacement with a larger size rivet will not be necessary. If the rivet is not removed properly, the strength of the joint may be weakened and the replacement of rivets made more difficult.
When removing a rivet, work on the manufactured head. It is more symmetrical about the shank than the shop head, and there will be less chance of damaging the rivet hole or the material around it.
To remove rivets, use hand tools, a power drill, or a combination of both. The preferred method is to drill through the rivet head and drive out the remainder of the rivet with a drift punch. First, file a flat area on the head of any round or brazier head rivet, and center punch the flat surface for drilling.
On thin metal, back up the rivet on the upset head when center punching to avoid depressing the metal. The dimple in 2117-1 rivets usually eliminates the necessity of filing and center punching the rivet head.
Select a drill one size smaller than the rivet shank and drill out the rivet head. When using a power drill, set the drill on the rivet and rotate the chuck several revolutions by hand before turning on the power. This procedure helps the drill cut a good starting spot and eliminates the chance of the drill slipping off and tracking across the metal. Drill the rivet to the depth of its head, while holding the drill at a 90° angle. Be careful not to drill too deep because the rivet shank will turn with the drill and cause a tear. The rivet head will often break away and climo the drill, which is a good signal to withdraw the drill. If the rivet head does not come loose of its own accord, insert a drift punch into the hole and twist slightly to either side until the head comes off.
Drive out the shank of the rivet with a drift punch slightly smaller than the diameter of the shank. On thin metal or unsupported structures, support the sheet with a bucking bar while driving out the shank. If the shank is exceptionally tight after the rivet head is removed, drill the rivet about two-thirds of the way through the thickness of the material and then drive out the remainder of the rivet with a drift punch.
The procedure for the removal of flush rivets is the same as that just described except that no filing is necessary. Be very careful to avoid elongation of the dimpled or the countersunk holes. The rivet head should be drilled to approximately one half the thickness of the top sheet.
SPECIAL RIVETS There are many places on an aircraft where access to both sides of a riveted structure or structural part is impossible, or where limited space wil not permit the use of a bucking bar. Too, in the attachment of many nonstructural parts, such as aircraft interior furnishings, flooring, deicing boots, and the like, the full strength of solid shank rivets is not necessary.
For use in such places, special rivets have been designed which can be bucked from the front. They are sometimes lighter than solid shank rivets, yet amply strong for their intended use. These rivets are manufactured by several corporations and have unique characteristics that require special installation tools, special installation procedures, and special removal procedures. Because these rivets are often inserted in locations where one head (usually the shop head) cannot be seen, they are also called blind rivets.
The various types of mechanically expanded rivets, their fabrication, composition, uses, selection, and identification were discussed in Chapter 6, Hardware, Materials, and Processes, in the Airframe and Powerplant. Mechanics General Handbook, AC 65-9A. The installation techniques will be covered in this section.
Installation Tools The tools used to install self-plugging (friction lock) rivets depend upon the manufacturer of the rivet being installed. Each company has designed special tools which should always be used to ensure satistactory results with its product. Hand tools as well as pneumatic tools are available.
After selection or determination of the rivet to be used in any installation, the proper size twist drill must be determined. Generally, manufacturers recommend the following finish drill sizes for the common shank diameters (figure 5-54).
Be very careful when drilling the material. Hold the drill at right angles to the work at all times to keep from drilling an elongated hole. The blind rivet will not expand as much as a solid shank rivet. If the hole is too large or elongated, the shank will not properly fill the drilled hole. Com mon hand or pneumatic powered drills can be used to drill the holes. Some manufacturers recommend predrilling the holes; others do not.
Equipment used to pull the stem of the rivet, as previously stated, will depend upon the manufacturer of the rivet. Both manually operated and power-operated guns are manufactured for this purpose. Nomenclature for various tools and assemblies available depends upon the manufacturer.
Application and use of the equipment is basically the same. Whether the equipment is called hand tool, air tool, hand gun, or pneumatic gun (figure 5-55), all of these are used with but one goal, the proper installation of a rivet.
The choice of installation tools is influenced by several factors: The quantity of rivets to be installed, the availability of an air supply, the accessibility of the work, and the size and type of rivet to be installed. In addition to a hand or power riveter, it is necessary to select the correct "pulling head" to complete the installation tool.
Selection of the proper pulling head is of primary importance since it compensates for the variables of head style and diameter. Since your selection will depend on the rivets to be installed, you should consult the applicable manufacturer's literature.
• SELF-PLUGGING (FRICTION LOCK) RIVETS Self-plugging (friction lock) rivets are fabricated in two common head styles: (1) A protruding head similar to the AN470 or universal head, and (2) a 100° countersunk head. Other head styles are available from some manufacturers.
The stem of the self-plugging (friction lock) rivet may have a knot or knob on the upper portion, or it may have a serrated portion as shown in figure
5-56.
The sequence of steps to follow in the installation of self-plugging (friction lock) rivets is basically the same as that for solid shank rivets, but the methods and equipment vary. The following steps are typical of any installation:
(1) Select the rivet to be installed-determined by thickness of material to be riveted, strength desired in assembly, and los cation of installation (protruding or coun tersunk head).
(2) Drill the hole (s) -determine sizé of twist drill to be used, do not elongate rivet hole, remove burrs, and use a stop countersink if necessary.
CHERRYLOCK COUNTERSINKING DIMENSIONS -c.→ - 010R.
UTI Mir.
3/32 #40 #30 .129 -097 .100 .132 100° A6520426 HEAD 100° NAS1097 HRAD 90°U #20 „160 .164 Max.
# 10 .192 _196 F .256 261 .182 .176 BULBED CHERRYLOCK .228 .222 .195 .189 .173 -167 S1/8 #27 .143 -146 ·289 .283 .246 .240 .216 ·210 5/32 #16 .176 .180 .356 .350 .302 .296 .258 .252 3/16 #5 .205 .209 479 473 ·395 ,389 FIcURE 5-54. Cherry rivet installation data 178
CHERRY RIVET GUN HUCK RIVET GUN 120
HAND TOOL G36
HAND TOOL G700
HYROSHIF T TOOL 139-A
AIR TOOL 0784
HYDROSHIF T TOOL 352
AIR TOOL FIGURE 5-55. Self-plugging (friction lock) rivet guns.
Satisfactory Unsatisfactory Grip length too short for material thickness Undersize Oversize hole hole FICURE 5-57. Inspection of self-plugging (friction lock) riveta.
Protruding Countersunk head head FICURE 5-56. Self-plugging (friction lock) rivets.
(3) Install the rivet-make certain the rivet head is seated firmly, position the selected *uch tool on the rivet stem, pull rivet stem until the stem snaps, apply approximately 15 Ibs. of pressure to the end of the stem, and trim the stem flush with the rivet head. If aerodynamic smoothness is a factor, the stem can be shaved with a rivet shaver. Drill head only Inspection The inspection of installed self-plugging (friction lock) rivets is very limited. Often the only inspec tion that can be made is on the head of the rivet. It should fit tightly against the metal. The stem of the rivet should be trimmed flush with the head of the rivet whether it is a protruding head or a countersunk head.
If you can see the shop head side of the installed rivet, inspect it for the requirements illustrated in figure 5-57. When the rivet head is considered unsatisfactory, remove the rivet and install another in FICURE 5-58. Removal of self-plugging (friction lock) ita place. rivets.
Removal Procedures Self-plugging (friction lock) rivets are removed in the same manner as solid shank rivets except for the preliminary step of driving out the stem (figure 5-58). The following steps should be used in their proper sequence:
(1) Punch out the rivet stem with a pin punch.