Abstract:
Method and apparatus of back riveting construction for sections of aircraft structure utilizing the flush riveting wherein the back bar tooling for all the rivets in the entire section is a single piece of stationary tooling which conforms to the exterior shape of the aircraft section. The rivet gun is a single impact pneumatic gun with a rivet driving hammer which is offset from the axis of the gun&#39;s piston and impact pin to upset rivet under C-channel flanges.

Description:
The present invention relates to an apparatus and method for installing rivets in aircraft structures and more particularly to a hand-held single impact rivet gun used for back riveting in conjunction with back bar tooling. 
     BACKGROUND OF THE INVENTION 
     Structural joining processes used for aircraft airframe skin structure basically are either bonded or riveted. Aside from other engineering and manufacturing factors used for the selection of the appropriate joining process, bonded structure has superior aerodynamic qualities while riveted structure costs less to produce. A need for a joining process which is aerodynamically smooth and inexpensive as riveted structure has become a general goal in aircraft design. Devising a riveting process which can produce the smooth appearance of a bonded structure would satisfy both the esthetics, aerodynamics and manufacturing costs requirements. A riveting method to fulfill these needs is one which back drives the shank of the rivet on the inside surface of the aircraft structure so there are no protruding rivet heads on the exterior surface, while the head of the rivet is set in a countersunk hole so as to provide a flush and smooth exterior surface. 
     The standard riveting techniques used today involve one person holding the rivet with a bucking bar against the shank of the rivet while a second person with a rivet gun upsets the rivet on the exterior surface which sometimes deforms the sheet metal skin. Conventional rivet guns apply rapid impact strikes to the rivet head which sometimes overdrives the rivet causing a deformation to the skin. Single impact rivet guns can be accurately adjusted so the rivet is not over or under driven and there is no possibility of deforming the aircraft skin. Single impact rivet guns are obviously faster than the conventional rivet guns and have more precise control in upsetting the rivet. 
     The concept of a single impact rivet gun has been around for at least two decades, as illustrated in U.S. Pat. No. 4,039,034 to Wagner and U.S. Pat. No. 4,192,389 to Raman. In the first patent listed, the rivet gun is manually held while in the second patent the rivet gun is mounted in an overall structure which also holds the sections being riveted and the bucking bar on the opposite side of the rivet. Conventional riveting techniques involve a hand-held rivet gun with a bucking bar held on the opposite end of the rivet normally by a second person. The concept of back riveting, wherein the shank of the rivet is on the inside of the aircraft structure rather than the outside, is old as taught in U.S. Pat. No. 4,007,540 to Tyree and U.S. Pat. No. 2,312,554 to Jocques. 
     The concept of a single person riveting operation is generally old in the art, as taught by U.S. Pat. No. 2,559,248 to Harcourt, U.S. Pat. No. 4,967,947 to Sarh, U.S. Pat. No. 4,662,556 to Gidlund, and U.S. Pat. No. 4,759,109 to Mason et al. All of the last four mentioned patents teach a machine which holds the section of the aircraft being riveted as well as the backing bar device and the riveting gun in an automated unitary structure wherein the backing member moves with the rivet gun across the surface of the section being riveted. U.S. Pat. No. 2,312,554 to Jocques, previously mentioned, also teaches a single person riveting apparatus which again is a unitary structure like the above-mentioned four patents. In the above-mentioned patent to Sarh the bucking component and the riveting component are both mounted on a universal base wherein the bucking unit and the riveting unit are computer controlled for three axis movement in unison. 
     The concept of an offset rivet gun whereby the driving hammer for the rivet is offset from the axis of the piston and impact pin is taught in the above-mentioned patent to Jocques; however, it is not hand-held nor is it used to drive a rivet under an extending flange of a C-channel, as done in the present invention. 
     SUMMARY OF THE INVENTION 
     The rivet gun of the present invention is a hand-held pneumatic gun which back drives the rivets against a solid tooling surface having an identical shape of the aircraft section being fabricated. This process replaces the hand-held bucking bar for each rivet with a stationary tooling surface which provides a backing bar for all of the rivets within the section being fabricated. 
     Since the single stroke of the rivet gun is carefully limited by a stop, it upsets the shank of the rivet the precise amount necessary to swell the shank and upset the end of the shank to form a head and tightly contain the two or more sheets being joined. Driving the rivet with one controlled hit instead of several also provides a speed advantage over a traditional riveting and also greatly increases the exact amount of upset of the rivet. The sheet clamp up force can also be controlled with the present rivet gun in light of the built-in spring which requires the riveter to push the rivet gun against the rivet until the spring is fully compressed. Once the spring is fully compressed the trigger automatically unlocks and the rivet gun is armed and ready to fire. The spring compression that is generated clamps both the sheets being riveted together and reduces the possibility of a non-shear condition existing between the sheets. The actual rivet gun of the present invention is very similar to a nail gun used in house construction which performs the same basic task as a single impact rivet gun only with more stroke of the impact pin. 
     The principal object of the present invention is to provide a single person riveting method which utilizes a single impact rivet gun for back riveting against a fixed tooling surface. 
     A further object of the present invention is to provide a rivet gun having an offset driving hammer for reaching under the flange of a C-channel. 
     Another object of the present is to provide a single piece of stationary back bar tooling for backing all of the rivets in a section of aircraft being constructed. 
     Another object of the present invention is to provide a riveting technique which leaves the exterior aircraft surface smooth with no deformation of the skin and the rivets not visible. 
     Another object of the present invention is to provide a back riveting gun which precisely upsets all of the rivets so as to provide a maximum strength connection. 
     Further objects and advantages will be pointed out or will become evident in the following detailed description, claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of the riveting gun of the present invention with portions broken away to see the principal parts of the gun; 
     FIG. 2 is a side elevational view of the rivet gun in place on a C-channel with the rivet offset against stationary tooling  36 ; 
     FIG. 3 is a perspective view of the fixed tooling used in conjunction with the rivet gun; 
     FIG. 4 is a perspective view of the fixed tooling with the skin, C-channels and stringers clamped up for riveting; and 
     FIG. 5 is a perspective view of a riveted assembled section of aircraft structure removed from the tooling. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The riveting method of the present invention is accomplished through the use of riveting gun  10  shown in FIGS. 1 and 2 in conjunction with fixed tooling generally identified by numeral  36  shown in FIGS. 3 and 4. 
     The riveting gun  10  is a single impact gun including a housing  11  containing a piston  12  which is concentrically attached to impact pin  13 , which in turn drives hammer  30  to upset a back rivet  60 , as illustrated in FIG.  2 . Rod and hammer guide  26  is integral with sleeve  23  and slides in sleeve  22  which is integral with housing  11 . Located on the bottom of hammer guide  26  is a bearing surface  34  for engaging the surface being riveted. Located in spring chamber  16  as shown in FIG. 2 is a coiled compression spring which forces sleeve  23  along with the rod and hammer guide  26  downward against snap ring  21 . In both the FIGS. 1 and 2 positions, sleeve  23  is in its fully compressed position with trigger  24  ready to fire when depressed. 
     The locking mechanism for the rivet gun is accomplished through trigger lockout arm  18  which attaches to sleeve  23  through pin  17 , as best seen in FIG. 2 with the upper end of arm  18  pinned to the trigger  24 . When the bearing surface  34  of the rivet gun  10  is not pressed against a surface, sleeve  23  engages with snap ring  21  whereby link  18  locks the trigger from firing until the bearing surface  34  of the rivet gun is forced against the surface being riveted and the spring  15  is fully compressed to the position shown in FIG.  2 . The structure and circuitry for driving piston  12  against stop  20  is not shown since it is conventional and well known in the pneumatic nail driving art. The basic operation of this rivet gun with the exception of the offset driving hammer  30  is similar to pneumatic nail guns. While the trigger lockout mechanism in nail guns is intended for safety purposes, it is used for a different function with the present invention wherein the force required to unlock the gun when pressed against the surface being riveted is utilized to tightly hold the two members together while the rivet head is upset to provide an optimum strength connection. 
     The rod and hammer guide  26  illustrated in FIGS. 1 and 2 permits a rivet to be driven off-center from the impact pin  13  of the gun  10  so that C-channels  40 , as shown in FIG. 2, can be riveted to skin  39  even though there is an overhanging flange on the C-channel. Rod and hammer guide  26  includes a horizontal slot  28  extending across guide  26  which contains a pivotally mounted hammer  30  attached to guide  26  through pin  32 . Before the rivet gun can fire, the bearing surface  34  of the rivet gun must be pressed against the surfaces being riveted with sufficient force to fully compress spring  15  and arm firing trigger  24 . 
     The process just described is a method of back-riveting wherein the upset portion of the rivet is on the inside surface of the aircraft section  59  being fabricated with the bucking bar function being performed by back bars  44  and  48  which are holding countersunk rivets  60  flush with the skin of the aircraft section while the inside end of the rivet is being upset as shown in FIG.  2 . 
     The overall stationary tooling, generally represented by reference numeral  36 , as shown in FIG. 3, is used to build a section of an aircraft fuselage, as illustrated in FIG.  5 . The tooling  36  provides a backup bar function for all of the rivets in the section  59  being constructed. Typical aircraft sections, as shown in FIG. 5, include a series of C-channels  40  or Z-channels  66  spaced longitudinally along the fuselage section while a series of stringers  62  run normal thereto passing through openings  68  and C-channels  40 . The lower flange  64  of the C-channel is riveted to the skin  39  on approximately one-inch spacings along the entire length of the flange which are not shown in the drawing. The stringers  62  have an angle cross section with one flange  66  which lies flush with the aircraft skin  39  and is riveted along its full length with a similar spacing to the C-channel rivets. 
     The stationary tooling  36 , as symbolically shown in FIG. 2, is actually an elongated solid bar  44 , as shown in FIG. 3, having an arcuate surface which conforms with the curvature of the aircraft section at that particular station. The bar  44 , also referred to as back bar surface or bucking bar, mounts to tooling frame  50  through a pair of removable pins  58  at opposite ends of the back bar. When the various parts to be riveted are placed in the tooling  36 , as shown in FIG. 4, a series of hold down clamps  46  are utilized to hold the lower flange  64  of the C-channels tightly against the skin  38  so that there is no movement during the riveting operation. Hold down clamps  46  are mounted on hold down bars  52  which are positioned juxtaposed to back bar surfaces  44 . Also mounted on hold down bars  52  are series of locator clamps  54  one on each end of bar  52  which holds the web of the C-channels against the bars  52 . Positioned normal to the back bars  44  are another set of back bar tooling surfaces  48 , as shown in FIG. 3, which provide for the rivets in stringers  42 . These back bars  48  pass through opening  68 , as shown in FIG. 5, and bars  44  and their tooling surfaces are flush with those in back bars  44 . 
     Located on each end of the tooling section  36 , is a secondary pair of arcuate tooling surfaces  56  which support the ends of back bars  48  so there is no deflection of bars  48  during riveting. 
     FIG. 4 illustrates a section of stationary tooling  36  mounted on a frame  50  with the aircraft skin  38 , C-channels  40 , and stringers  42  clamped in place and ready for riveting. The rivets  60  have a countersunk flat head which matches a countersunk hole in the skin so that once riveted, they provide a smooth flat surface and once painted cannot be seen. The rivets are held in place in skin  30  prior to upsetting by a thin strip of adhesive tape, not shown, which is removed after riveting. 
     Due to upsetting the rivets on the back side and the precision amount of upsetting there is no chance for deforming the sheet metal skin due to over driving the rivet. The mechanical stop  20  in gun  10  limits the stroke of impact pin  13  which ensures that the rivets are driven consistently every time and prevents them from being overdriven. 
     OPERATION 
     Before the tooling  36  is loaded, the skin  38  is predrilled and countersunk for all the rivet holes and the countersunk rivets are placed in the skin with some type of thin adhesive tape which holds the rivet heads flush with the skin so as to prevent the rivets from falling out. The skin is then placed in the tooling  36  with the rivet heads resting against the back bars  44  and  48 . The C-channels  40  are clamped in place both against the tooling back bars  44  as well as against the hold down bars  52  through the action of hold down clamps  46  and locator clamps  54 . 
     Once all the C-channels  40  and stringers  42  are accurately clamped in place, the hand-held riveting gun  10  rivets the various C-channels and stringers to skin  38 . The riveting time over conventional riveting is substantially shortened for a variety of reasons, the first being the rivet gun only requires a single impact for each rivet and, secondly, there is no time delay while a second person positions a hand-held bucking bar against each rivet as it is being upset. The bearing surface  34  on the firing end of the rivet gun  10  is placed over the shank of the rivet which is protruding from the sandwiched parts. The operator applies a force to the rivet gun towards the hard tooling surface  36  which will compress internal spring  15  in the rivet gun. When the spring is fully compressed, the trigger automatically unlocks and the operator fires the gun which swells the shank end of the rivet and completes the installation with a single blow. With this system there is no deforming of the sheet material in the skin as the rivet gun impacts the rivet head. The operation of the rivet gun also facilitates the clamp up of the parts in conjunction with the clamps in the tooling which ensures structural integrity between all the parts being riveted. Building the aircraft sections in a precise tooling of this nature increases the dimensional accuracy in alignment of the sections as compared with the prior art methods. 
     While I have shown and described in considerable detail what is believed to be the preferred forms of the invention, it would be understood by those skilled in the art that the invention is not limited to such details, but might take various other forms within the scope of the following claims.