Patent Publication Number: US-6210084-B1

Title: Pressure foot assembly for clamping a joint

Description:
This application claims benefit of Provisional application Ser. No. 60/066,614, filed Nov. 26,1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clamping device, and more particularly, to a portable clamp used on a lap-joint. 
     2. Background of the Invention 
     Traditional manufacturing techniques for assembling components to produce large mechanical structures to a specified contour have relied on fixtured tooling techniques utilizing assembly jigs and templates to locate the parts correctly relative to one another. Unfortunately, this method often yielded parts outside of acceptable tolerance because of imperfections in the templates or changes in the fixtured tooling caused by temperature variations. 
     To solve the problems encountered by traditional techniques, a system and method for assembling components was developed that utilized spatial relationships between key features of subassemblies as represented by coordination holes drilled into the subassemblies using numerical part definition records. The subassemblies were made intrinsically determinate of the dimensions and contour of the assembly. 
     The use of key features to determine the dimensions and contour of an airplane fuselage section is shown in FIG.  1 . Here, a skin  20  has a plurality of stringers  22  and a plurality of shear ties  24  riveted thereon. A frame member  30  having a curved contour which is the same as the desired contour of the airplane fuselage is then riveted to the shear ties  24  and stringer clips  26 . 
     The stringers  22 , the shear ties  24  and the stringer clips  26  must be fastened to the fuselage skin  20  with extreme accuracy and consistency. Accuracy of parts manufacture ensures that the airplane will come together perfectly with no pre-stressed parts and no cosmetic imperfections. 
     Initially, a computer numerically controlled (CNC) machine tool performs machining operations on the skin  20 . Coordination holes are drilled in the skin  20  and the stringers  22 . Corresponding coordination holes are also drilled in the shear ties  24  and the stringer clips  26 . A final machining operation of edge routing is performed by a high speed routing end-effector to route the edges of the fuselage skin  20  to the correct dimensions, as specified by the original part definition data base, by accurately locating the edges of the skin relative to the coordination holes in the skin. 
     The stringers  22  are tack fastened to the skin  20  through their aligned coordination holes. Then the shear ties  24  and stringers  22  are drilled and riveted to the skin  20 . The stringer clips  26  are inserted at the correct location and are held in place while drilled and riveted to form a panel  34 . 
     The skin  20  also has a series of panel-to-panel coordination holes  32  drilled along the edge of the skin  20 . The panel-to-panel coordination holes  32  are used to position the panels  34  relative to each other. The panels  34  are still relatively flexible so the ultimate configuration is determined by the parts and their matched coordination holes. 
     The panel-to-panel coordination holes  32  are aligned on adjacent holes and sealant is applied between the facing surfaces of the panel edges. The panels  34  are aligned so that the panel-to-panel coordination holes  32  on adjacent panels  34  line up exactly and the two panels are fastened together at their adjacent edges by temporary cleco fasteners through the coordination holes. The panels are then drilled and riveted to permanently fasten them together to form a super panel  36 . 
     Coordination holes are drilled into the frames  30  and are aligned with the coordination holes in the stringer clips  26 . The frames  30  are fastened and their alignment determines the contour of the super panel  36 . Thus, the contour is independent of any hard tooling. Once the super panel  36  is formed, the temporary cleco fasteners holding the parts in position are replaced by permanent fasteners. 
     The super panels  36  are temporarily fastened using the panel-to-panel coordination holes  32  to form fuselage quarter panels which are in turn temporarily fastened to form a lower fuselage lobe  38 A and an upper fuselage lobe  38 B, as shown in FIGS. 2A and 2B. A floor grid  40  is aligned with the lower lobe  38 A using coordination holes, and is fastened in place. The fixture  44  does not determine the contour or dimensions of the fuselage. Instead, the coordination holes drilled into the floor grid  40  determines the cross-dimensions of the fuselage  42 . 
     Once the frame members  30  and lobe skin coordination holes  46  are all aligned and temporarily fastened with cleco fasteners, they are drilled to form the final fuselage section  42 , as shown in FIG.  2 B. The fuselage section  42  is then disassembled, de-burred, cleaned, and sealant is added. 
     After sealing, each super panel  36  is again aligned using the coordination holes. The overlapping portion of the panels  36 , a lap joint  48 , is shown in FIGS. 2B and 2C. Each lap joint  48  has a plurality of columns  50 , where each of the columns  50  has 3 rows of rivets  52 A-C. Two rivets of the rows  52 A and  52 C are for rivets that require a countersink as well as drilling. 
     The super panels  36  could be fastened to form a quarter panel by an assembly device, such as that described in U.S. Pat. No. 4,662,556 (the &#39;556 patent). However, the device described in the &#39;556 patent moves a working unit along a guide beam that is supported by two huge arc-shaped girders, and could not be used to form the lower or upper fuselage lobes  38 A and  38 B, respectively, because of its size and design. Simply put, the unit described in the &#39;556 patent or any variations thereof would not fit within the fuselage lobes  35 A and  38 B, and certainly not the fuselage assembly  42 . Attempts to redesign the assembly device discussed in the &#39;556 patent to handle larger portions of the fuselage assembly  42  have failed because of severe problems with vibration which interfered with the proper seating of fasteners such as rivets. Further, the assembly device discussed in the &#39;556 patent is not versatile and requires an expensive and heavy support structure. 
     Presently, the fuselage quarter panels  36  and, lower and upper lobes  38 A and  38 B, and the final fuselage assembly  42  are re-tacked into position after being filed, cleaned, and sealed. Then, the panels  36  are riveted together by hand, where one person stands on a platform (not shown) outside the fuselage, inserting and then pneumatically driving a rivet fastener while another person stands inside the fuselage, bracing a large bucking bar against a rivet shank and holding it in place by leaning against the bucking bar with his shoulder. Obviously, such a process presents a risk of injury. Further, the manual process results in rivets that were unevenly deformed, poorly seated, or riveted too close to an edge of the lap joint  48 . 
     Unfortunately, the manual process is dangerous, time-consuming, expensive and often leads to extensive rework. Consequently, there is a need in the art for a fastening system that speeds up production, ensures riveting and drilling accuracy, eliminates the required step of disassembling the entire fuselage to de-burr, clean and seal, and can be operated within the final fuselage assembly  42 . 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a pressure foot applies a clamping pressure to an area undergoing drilling or fastening operations on a joint formed by a first panel and a second panel. The pressure foot includes a front clamp for pressing against a front side of the area undergoing the drilling or fastening operations, and a carriage for moving the front clamp along a length and width of the joint formed by the first panel and the second panel. 
     According to another aspect of the invention, a pressure foot device applies a clamping pressure to a relatively small area of a lap joint formed by an overlap and temporary fastening of a first aircraft fuselage skin panel and a second aircraft fuselage skin panel. The pressure foot includes an outside end-effector movable along a length and width of an outside surface of the lap joint. The outside end-effector has a porthole clamp for surrounding the area of a lap joint to be drilled and applies pressure to the area during a drilling operation. The pressure foot also includes an inside end-effector movable along a length and width of an inside surface of the lap joint. The inside end-effector has a bucking bar positioned opposite to the porthole clamp and applying pressure onto the inside surface of the lap joint during the drilling operation. 
     According to yet another aspect of the invention, a method clamps and drills a lap joint formed by two panels. The method includes the steps of positioning a clamp foot to surround a section of an outside surface of the lap joint, applying a predetermined pressure to the section on the outside surface of the lap joint, drilling a hole into the section of the lap joint, and fastening the section of the lap joint. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, aspects, and advantages of the present invention will become better understood with regard to the following accompanying drawings where: 
     FIG. 1 is a perspective view of an assembled prior art super panel, showing skin, stringers, shear ties, stringer clips, and frame members; 
     FIG. 2A is a perspective view of a prior art fuselage lower lobe showing a floor grid; 
     FIG. 2B is a perspective view of a prior art completely assembled fuselage section; 
     FIG. 2C is a plan view of a prior art skin lap joint between two super panels; 
     FIG. 3 is an end view of a mini-riveter system of the present invention; 
     FIG. 4A is a side view of an index pin of the mini-riveter system; 
     FIG. 4B is a front view of the index pin; 
     FIG. 4C is a front view of a reflective head of the index pin; 
     FIG. 5 is a perspective view of external guide rails and an outside end-effector subsystem of the mini-riveter system; 
     FIG. 6 is a schematic diagram of a plurality of vacuum generators of the external guide rails; 
     FIG. 7 is a plan view of a contact portion, including vacuum seals of the primary guide rails of the external guide rails; 
     FIG. 8A is a perspective view from the upper left of the outside end-effector; 
     FIG. 8B is a perspective view from the lower left of the outside end-effector; 
     FIG. 8C is a perspective view from the upper right of the outside end-effector; 
     FIG. 8D is a perspective view from the lower right of the outside end-effector; 
     FIG. 8E is a perspective view of the bottom of the outside end-effector; 
     FIG. 9A is a perspective view of a pressure foot subassembly of the outside end-effector; 
     FIG. 9B is a side view of a frame and a mid-linkage of the pressure foot subassembly; 
     FIG. 10A is a bottom view of a fastener feed fingers of the outside end-effector; 
     FIG. 10B is a side view of the fastener feed fingers of the outside end-effector; 
     FIG. 11 is a perspective view of the inside end-effector and internal guide rails of the mini-riveter system; 
     FIG. 12A is a perspective view of the inside end-effector; 
     FIG. 12B is a perspective view of the bottom of the inside end-effector; 
     FIGS. 13A-13C are side views of a rivet protrusion sensor of the inside end-effector, where: 
     FIG. 13A shows a bucking bar at initial clamp-up; 
     FIG. 13B shows a bucking bar just prior to deformation; 
     FIG. 13C shows a bucking bar seated against a button upon completion of a fastening cycle; 
     FIG. 14A is a perspective view of a straight bucking bar; 
     FIG. 14B is a perspective view of a left-handed bucking bar; 
     FIG. 14C is a perspective view of a right-hand bucking bar; 
     FIG. 14D is a side view of a left-hand bucking bar inserted between a lap joint and a stringer; 
     FIG. 15 is a perspective view of a system cart including a control system of the mini-riveter system; 
     FIG. 16 is a flow chart showing a main operational routine implemented by a control processing unit (CPU) of the control system; 
     FIG. 17 is a flow chart showing a clamping and drilling subroutine invoked by the operational routine; and 
     FIG. 18 is a flowchart showing a fastening subroutine invoked by the operational routine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Definitions: 
     Airframe: the structural assembly that comprises the body of an airplane without wings or horizontal and vertical stabilizers; 
     Boelube: Cetyl alcohol, a nontoxic lubricant used for metal cutting; 
     Bucking Bar: A metal tool used to flatten the rivet&#39;s shank into a driven button during the riveting process. The bucking bar is used as an anvil to react the forces being driven into the rivet with a rivet gun, thus deforming the rivet; 
     Clamp-Up: Hold two or more pieces of the airframe together so that there are no gaps between the principal surfaces; the ability to hold the work pieces together; 
     Countersink Depth: Depth of countersink in a fastener hole; 
     Countersinking: Machining of a conical hole coaxial with a through-hole for purposes of accepting a fastener head that will be flush (i.e.; the same height as) with the material surrounding the hole; 
     Dwell Time: A period of time that is permitted to elapse as part of normal fastening operations: e.g.; for sealant squeeze out, for maintaining power to the rivet gun during rivet driving, etc; 
     End-Effector: A tool positioner with modules installed; 
     E-Stop (Emergency Stop): A software-independent stop signal that causes the system to stop immediately upon activation; 
     Fail-Safe: Incorporating some feature for automatically counteracting the effect of an anticipated possible source of failure; having no chance of failure; infallible, problem-free; 
     Fasteners: The generic term used to describe rivets and bolts; 
     Feed-hold: A software-controlled stop of the system at any point in the process; (Power to motors and drives should not need to be removed.) 
     Lap Joint: An area of overlap between two panels to be fastened, where columns of rivets are installed along a length of the lap joint and rows of rivets are installed along the width of the lap joint. 
     Machine Control Data (MCD): The program that is loaded into the controller that directs the operation of the MRS in performing the lap fastening process; 
     Modal: Numerical Control (N/C) operating modes that are maintained (latched) in an acting state for all subsequent operations until modified by another N/C command; 
     Module: An independently operable unit that is part of the total system. Examples are the drill/countersink module, the rivet drive and feed module; 
     Rc (Rockwell “C”): A standard method of measuring and designating the hardness of metals; 
     Rivet: A metal bolt or pin used to join two or more objects by inserting it through a hole in each object and then hammering the narrow end to form another head (or button); 
     Sealant: A durable, waterproof material applied to selected assemblies to prevent water from infiltrating and aiding in the corrosion of those assemblies; 
     Software Source Code: The editable software scripts that a software developer writes for a computer application; 
     Stay-Within Envelope: An imaginary envelope that the system, when mounted on guide rails installed on an airframe, must not extend beyond. 
     Swirl Marks: Marks into the surface of a material that is being drilled that are concentric with the hole. The cutaway material that is being expelled from the hole causes swirl marks; and 
     Workpiece, Component, Panel: Airframe or any structure or item that the system will perform elements of the fastening process on. 
     Mini-Riveter System 
     The present invention relates to a mini-riveter system capable of quickly and accurately fastening two panels at a lap joint without the use of large cumbersome machinery. 
     As shown in FIG. 3, the mini-riveter system  100  includes external guide rails  102 , supported by and positioned on an outside surface of the overlapped panels  110 , and an outside end-effector subsystem  104  movable along the external guide rails  102 , for clamping the panels  110 , drilling/countersinking the panels  110 , fastener feeding/insertion into the panels  110 , and driving a rivet to fasten the panels  110 . The mini-riveter system  100  also includes internal guide rails  106 , supported by and positioned on an inside surface of the panels  110 , and an inside end-effector subsystem  108 , movable along the internal guide rails  106 , for clamping the panels  110  and bucking a rivet to fasten the panels  110 , even when the fastener is obstructed by a feature attached to one of the panels  110 . 
     The mini-riveter system  100  is easily transportable on a control system cart  112 , as shown in FIG. 15, which supports a control system  114 . The system  100  is capable of being quickly installed onto the two panels  110  without special tooling support requirements. Further, the system  100  is compact enough that it can be easily installed and moved around the inside and outside of an airframe fuselage section. Finally, the system  100  is flexible enough to fasten individual panels, combinations of panels, subsets of an airframe fuselage, or an entire airframe fuselage. 
     The outside end-effector  104  and the inside end-effector  108 , as shown in FIG. 3, clamp down a portion of a lap joint  116  formed by the two panels  110  in a localized manner without interfering with other nearby operations. Further, the localized pressure extended during the clamp down prevents burring and keeps chips from falling between the lap joint  116 . Thus, the localized pressure allows the steps of sealing and drilling the lap joint  116  to be immediately followed by the step of fastening the lap joint  116 . This quick process replaces the former process of untacking drill components, filing them, cleaning them, sealing them and re-tacking and aligning them and then fastening the panels  110  together at the lap joint  116 . 
     The system  100  also offers a high degree of modularity, allowing quick and easy replacement of drills, countersinks, rivet guns, and bucking bars. This arrangement provides a high degree of flexibility and enables the system  100  to accommodate a large percentage of fastening tasks required on an air frame. 
     The small size and light weight of the mini-riveter system  100  makes it ideal for gang fastening, where multiple versions of the system  100  are installed at various positions along a larger mechanical structure, such as an airframe to conduct simultaneous operations on the same lap joint, or to conduct simultaneous operations on different lap joints of the structure/airframe fuselage. This capability of the system  100  significantly improves the production flow rate of an aircraft fuselage. 
     Unlike prior art fastening devices which home or zero their coordinate systems on a fixture, the mini-riveter system  100  is able to home on the coordination holes being used to align the two panels  110 . The use of the coordination holes to home the inside end-effector  104  and the outside end-effector  108  increases both the end-effectors&#39; accuracy, and by re-homing the inside and outside end-effectors  104  and  108 , respectively, at each of the coordination holes along a lap joint  116 , drift due to thermal change or fastener-induced growth is minimized. 
     Direct Index Pins: 
     The mini-riveter system  100  homes or zeros in on the same coordination holes used to align the overlapping panels  110  at the lap joint  116 . To accomplish this, direct index pins  120 , as shown in FIGS. 4A-4C, are installed in the coordination holes at the lap joint  116 . The direct index pins  120  include a protruding key  122 , having a threaded shank that snugly fits within the coordination holes, as well as a portion that extends from the outside surface of the lap joint  116 , having an outer lip  124  used to align the external guide rails, and recess  125  used to home the outside end-effector  104 . The direct index pins  120  also include a reflective head  126  that threadingly engages the shank of the protruding key  122 . The reflective head  126 , which extends out from the inside surface of the lap joint  116 , includes a reflecting square  128 , used to home the inside end-effector  108 . The reflecting square has a width w in parallel with the length of the lap joint  116 . 
     The mini-riveter system  100  establishes a positioning reference coordinate system relative to the index pins  120  installed in the lap joint  116 . The use of the index pins  120  allows the establishment of local coordinate points to re-zero both the outside end-effector  104  and the inside end-effector  108 . By periodically re-zeroing the end-effectors, the likelihood of improper positioning of holes and fasteners due to growth or distortion along the lap joint  116  is dramatically reduced. Also, by homing on the coordination holes, there is no need for a fixture to home the end-effectors. The use of a fixture which would reduce the overall advantages gained by aligning components with coordination holes. 
     External Guide Rails: 
     The external guide rails  102 , as shown in FIG. 5, include a primary rail  130 , a secondary rail  132 , and a plurality of rail ties  134 A- 134 C. The rail ties  134 A- 134 C are each aligned to the key  122  of one of the index pins  120 . Then, the rail ties  134  are coupled to the primary and secondary rails  130  and  132 , and are used to align the primary and secondary rails  130  and  132  to the lap joint  116 . 
     The primary rail  130  and the secondary rail  132  each have a tube portion  135 A and  135 B, respectively, for sliceable engagement with the outside end-effector  104 , as well as respective primary and secondary contact platforms  136 A and  136 B, for contact with the lap joint  116 . The tube portions  135 A and  135 B are mechanically coupled to their respective contact platforms  136 A and  136 B. 
     The contact platforms  136 A and  136 B each have a plurality of lips  138 A- 138 F, respectively, each extending toward the lap joint  116 . Each of the lips  138 A-F have a threaded hole  140 , used to align the rail with its respective one of the rail ties  134 A-C 
     Vacuum System: 
     The contact platforms  136 A and  136 B, as shown in FIG. 5, include a vacuum system having a plurality of vacuum generators  144 A-F. FIG. 6 is a schematic diagram of the vacuum generators  144 A- 144 C for the primary contact platform  136 A; where each of the generators  144 A- 144 C, respectively, has a vacuum pump  146 A- 146 C, a vacuum gauge  148 A- 148 C, and a vacuum switch  150 A- 150 C. The vacuum generators  144 A- 144 C are preferably PIAB™ generators (Part No. X 10). Each of the vacuum generators  144 A- 144 C is in pneumatic communication with corresponding rubber gasketed vacuum pads  152 A- 152 C, shown in FIG. 6, which are located on a side of the contact platform  136 A contiguous with the panels  110  forming the lap joint  116 . The secondary contact platform  136 B has identical vacuum pumps  146 D- 146 F, vacuum gauges  148 D- 148 F and vacuum switches  150 D- 150 F, as well as vacuum pads  152 D- 152 F. 
     The tube portions  135 A and  135 B are hollow and carry an air flow pressurized to approximately 90-100 psi. The air is supplied off the tube portions  135 A and  135 B via air taps (not shown) to the vacuum generators  144 A- 144 C, and  144 D- 144 F, respectively, of the primary and secondary platforms  136 A and  136 B. The positive air pressure supplied by the tube portions  135 A and  135 B expands in one or more orifice ejector nozzles (not shown) of the vacuum generators  14 A- 144 F, converting pressure and heat energy into motion energy. The compressed air jet increases speed rapidly, while the pressurized temperature of the air decreases, inducing a high vacuum flow, thereby creating a vacuum on a suction side of the vacuum pumps  146 A- 146 F. Both the primary rail  130  and the secondary rail  132  connect and operate in the same manner, where the vacuum generators  144 A- 144 F produce a vacuum in corresponding vacuum pads  152 A- 152 F. The vacuum pads are isolated from one another so if one of the pads  152 A- 152 F is lost, it will not affect the vacuum in the remaining pads. 
     As shown in FIG. 6, each of the vacuum generators  144 A- 144 F has a pneumatic logic circuit including three AND gates  154 A- 154 C, and three vacuum switches  150 A- 150 C. The logic circuit verifies that a vacuum has been produced by a particular vacuum generator. When the vacuum pads  152 A- 152 F have reached an acceptable level of vacuum, the pneumatic logic circuit creates and sends a vacuum present signal to the next vacuum generator  144 . Each logic circuit “AND&#39;s” the previous vacuum signal with the current vacuum signal and sends it on to the next vacuum generator  144 . The process repeats until the entire rail has been checked and the resulting signal is sent to the CPU  398  for processing, leading to a warning display or an E-stop system shutdown. 
     The vacuum applied by the vacuum generators  144 A- 144 F must be sufficient to couple the external guide rails  102  to the lap joint  116  while it is supporting the outside end-effector  104 , as shown in FIG.  5 . The coupling force to the panels  110  forming the lap joint  116  must be sufficient for the external rails  102  to transfer up to 700 lbs. of force generated by the outside end-effector  104  to the panels  110  during fastener operations. 
     The vacuum system allows the external guide rails  102  to be completely supported by the panels  110  forming the lap joint  116  without the need of a support fixture. This allows the system  100  to be brought to any part being worked on, even when a joint is located in an inconvenient area that would not admit fixturing or large automated machinery. Also, because the external rails  102  are vacuum coupled to the lap joint  116 , the external rails  102  follow the contour of the panels making up the joint, keeping the outside end-effector relatively normal to it. Further, since the vacuum pads  152 A- 152 F are made of neoprene or rubber, the panels are not damaged during fastener operations. 
     The removable rail ties  134 A- 134 C each include receptacles  156 A- 156 C for engaging a key  122  of a respective one of the index pins  120 . Once one of the receptacles  156 A- 156 C has been engaged with the key  122 , it positions the rail tie  134  in appropriate x,y coordinates relative to the lap joint  116 . As shown in FIG. 5, the rail tie  134 B, like each of the rail ties  134 A- 134 C has two hand-tightened bolts  158 A and  158 B, that threadingly engage the threaded holes  140  in the lips  138 B and  138 E of the primary rail  130  and secondary rail  132 , respectively. The bolts  158 A and  158 B properly locate the primary rail  130  and secondary rail  132  along the x-axis. Once corresponding bolts of another rail are engaged to their respective lips, the primary rail  130  and secondary rail  132  are also aligned along the y-axis. 
     The arrangement of the external guide rails  102  allows it to be entirely supported by the panels  110  forming the lap joint  116  using coordination holes  142  as reference points. The rail ties  134 A- 134 C are aligned with their respective coordination holes using the key  122  of the index pins  120 . The rail ties  134 A- 134 C are then fastened to the primary rail  130  and the secondary rail  132 , using features, i.e. the coordination holes, of the panels  110  as the only means of aligning the external rails  102  to the lap joint  116 . Thereafter, the vacuum is applied, causing the external rails  102  and the outside end-effector  104  to be entirely supported by the panels  110  forming the lap joint  116 . 
     Outside End-Effector Subsystem 
     The outside end-effector subsystem  104 , as shown in FIGS. 8A-8E, includes an outside end-effector engagement assembly for lifting the outside end-effector  104  and slidingly engaging the outside end-effector  104  onto the external rails  102 . The outside end-effector  104  also includes an outside tool positioning assembly for positioning modular component, such as drills and fasteners, relative to the indexing pins  120 . The outside tool positioning assembly includes an external position detection subassembly, for detecting the indexing pins  120  and for measuring the distance traveled by the outside end-effector  104  from the last homed position. The outside tool positioning assembly also includes an outside end-effector drive subassembly for moving the outside end-effector  104  along the x-axis on the external guide rails  102 , and a pressure foot subassembly  236  for clamping the lap joint at the area where a fastening operation is to occur. The outside end-effector  104  further includes a module movement subassembly  250  for positioning a drill/countersink module and a rivet drive/fastener feed module. 
     Outside End-Effector Engagement Assembly: 
     The outside end-effector engagement assembly, as shown in FIGS. 8A and 8D, includes a primary handle  200 , and a secondary handle  202 , which are used by an operator to lift the outside end-effector  104  onto the primary rail  130  and the secondary rail  132 . 
     The outside engagement assembly also includes a primary clamshell bearing system  204 , and a secondary clamshell bearing system  206 , as shown in FIG. 8D, for allowing the outside end-effector  104  to be installed or removed anywhere along the length, i.e. x-axis, of the external guide rails  102 . A primary pivot arm  208  of the primary bearing system  204  is opened or closed on the primary rail  130  by a primary air cylinder  212 , as shown in FIG.  8 D. In the same manner, a secondary pivot arm  210 , of the secondary bearing system  206  is opened or closed on the secondary rail  132  by a secondary air cylinder  214 , as shown in FIG.  8 C. 
     In a preferred embodiment, the primary and secondary pivot arms  208  and  210  can be locked closed to prevent the outside end-effector  104  from falling off the external guide rails  102  if the unit were to experience an air pressure loss condition. This is accomplished by using a locking air cylinder (not shown) to move a tool pin (not shown) through the primary and secondary arms  208  and  210 , respectively, and the main body  216  of the outside end-effector  104 . The tool pin keeps the pivot arms from opening when pressure is lost. An optional push button (not shown) located on the main body  216  allows the operator to operate the locking air cylinder at will. 
     Outside End-Effector Tool Positioning Assembly: 
     External Position Detection Subassembly: 
     The external position detection subassembly, as shown in FIG. 8A, includes a homing sensor  218 , and a final external position encoder  220 . 
     The homing sensor  218 , shown in FIG. 8A, is preferably a proximity sensor. When requested by the CPU  398 , the homing sensor  218  detects the gap  125  within the key  122  of the selected one of the index pins  120  being homed to and re-establishes, i.e. re-zeros, its x,y coordinate system based on the nearby detected index pin  120 . Preferably, when operating on an aircraft fuselage, the control system  114  will request the homing sensor  218  to locate an index pin  120  along the fuselage, i.e. re-zero, at every bay of the fuselage, where a bay is defined by two frames of the aircraft fuselage. By re-zeroing at every bay, inaccuracies from either fastener-induced growth of material or temperature variation can be significantly reduced. Thus, the outside end-effector  104  can maintain a high degree of positional accuracy by periodically re-calibrating its alignment based on the same structure of panels  110  forming the lap joint  116  which supports the outside end-effector  104 . 
     The final external position encoder  220 , as shown in FIGS. 8A and 8B, comprises a plurality of first wheels  222 A- 222 C which engage above and beneath the primary rail  130 . The first wheels  222 A- 222 C move when the outside end-effector  104  moves relative to the primary rail  130 . The external encoder  220  operates in a closed loop system reporting the position of the outside end-effector  104  to the control system  114  relative to the last homed position. 
     Outside End-Effector Drive Subassembly: 
     The outside end-effector drive subassembly for moving the outside end-effector  104  along the x-axis on the external guide rails  102 , as shown in FIGS. 8C and 8D, includes a first friction drive wheel  224 , a first friction air cylinder  226  for engaging the first friction drive wheel  224  to the primary rail  130 . The first friction drive wheel  224  is rotated by an x-axis servo-motor  228  which drives the outside end-effector  104  along the x-axis. Since the first friction drive wheel  224  has no gears or teeth, no damage will occur to the lap joint  116  or the mini-riveter system  100  if the outside end-effector  104  encounters an obstacle while traveling along the x-axis. Instead of burning out a motor or “chewing up” components, the first friction drive wheel  122  simply spins in place without causing any damage. The external encoder  220  reports the location of the outside end-effector  104  to the control system  114  which, in turn, deactivates the x-axis servo-motor  228  when a designated position is reached. 
     Pressure Foot Subassembly: 
     The pressure foot subassembly  230 , shown in FIGS. 8C-8E,  9 A and  9 B, applies a clamping pressure to a relatively small area of the lap joint  116  in support of fastening and drilling operations. 
     The pressure foot subassembly  230  includes a porthole clamp  232 , shown in FIG. 9A having an orifice  234  sized to allow passage of a drill, countersink, or fastening device. The porthole clamp  232  is pressed against a relatively small area of the lap joint  116  to apply pressure around an area to be drilled and fastened. Preferably, the porthole clamp  232  is steel hardened to at least Rc 65, and is polished to 16 Rhr or smoother to prevent scratches to the panels  110  during clamp-up. 
     The pressure foot  230  also includes a U-shaped frame  235 , shown in FIGS. 8E and 9B. A lower arm  238  of the frame  236  is coupled to the porthole clamp  232 . A mid-linkage  242  flexibly couples an end of an upper arm  240  and an end of the lower arm  238 . The mid-linkage  242  includes a joint  244 , which is in physical contact with a clamping air cylinder  246 . When the clamping air cylinder  246  is extended, the mid-linkage  242  and the U-shaped frame  236  are expanded, causing pressure to be applied between the outside end-effector  104  and the lap joint  116 . 
     A sensor  248 , as shown in FIG. 9B, is capable of detecting clamp-up forces applied to the lap joint  116  of up to 700 lbs. Preferably, the pressure foot  230  via the porthole clamp  232  is capable of providing a manually adjustable clamp-up pressure ranging from 100-500 lbs. For optimal results a pressure of 300 lbs. is applied. Further, in a preferred embodiment, the dwell time of the clamp  232  prior to drilling is between 1 and 20 seconds. 
     By applying clamp-up pressure to a localized region during drilling, there is no burring occurring between the panels  110  of the lap joint  116 . If an inner burr were produced and allowed to remain, it would greatly reduce the fatigue life of the panels  110 . Further, no chips or shavings are falling between the panels  110  of the lap joint  116 . Thus, the panels  110  need not be disassembled, filed/de-burred, cleaned, sealed, and then reassembled as previously required. The elimination of these steps affords a significant savings in time and cost. Further, the use of a clamp-up system that mounts on the parts/panels  110  being assembled is unique and allows a much more flexible clamp-up system. 
     The pressure foot subassembly  230 , as shown in FIG. 8E, is moved along the y-axis from row to row of rivets along the width of the lap joint  116  by a clamp air motor  248  and a clamp ball screw  250 . The position of the subassembly  230  is determined by a LVDT position measuring device  251  connected to the pressure foot  230 , as shown in FIG.  9 A. The control system  114  reads a signal produced by the LVDT device  251  to verify the position of the porthole clamp  232 . If the porthole clamp  232  is out of position, then an air valve (not shown) is actuated to drive the clamp air motor  248  which then moves the porthole clamp  232  into the correct position. 
     Module Movement Subassembly: 
     The outside end-effector  104  uses the module movement assembly  250 , shown in FIGS. 8C-8E to align a machine axis of a drill/countersink module  252  or a rivet drive/fastener feed module  254  with the orifice  234  of the porthole clamp  232  and the section or area of the lap joint  116  to be fastened. 
     The module movement assembly  250  includes an external module carriage  256  slidingly engaged with the main body  216  of the outside end-effector  104  along a linear bearing  258 . A module servo-motor  260  moves the drill/countersink module  252  and the rivet drive/fastener feed module  254  from a position where the drill/countersink module  252  was aligned to operate to a position where the rivet drive/fastener feed module  260  is aligned to operate, from row to row along a selected column of rivets. 
     Drill/Countersink Module: 
     The drill/countersink module  252 , as shown in FIGS. 8A-8E, prepares a position or area of the lap joint  116  for receiving a fastener by drilling and countersinking a hole at the position. The drill module  252  includes drill unit  262  which is pneumatically driven, and interchangeable. The drill unit  262  maybe interchanged with a different sized unit by removing it from a drill holder  264  which is horizontally fixed and vertically slidable relative to the external carriage  256  of the module movement assembly  250 . The drill unit  262  is removed from the drill holder  264  by unscrewing a quick release drill knob  266 , as shown in FIG.  8 A. 
     The drill unit  262  rotates an integral drill bit and countersink  268 , as shown in FIGS. 8C and 8E. The integral drill bit and countersink  268  allows the position of the lap joint  116  to be both drilled and countersunk with one plunge of the drill unit  262 . 
     The drill/countersink module  252  further includes first and second pneumatically powered drill plunging air cylinders  270  and  272 , respectively, coupled to the external carriage  256  of the module movement assembly  250  and the drill holder  264  for moving the drill unit  262  along the z-axis normal to the lap joint  116 . The drill module  252  includes a stop  274  to limit the motion of the integral drill bit and countersink  268  into the lap joint  116  to provide the desirable countersink depth. The stop  274  also acts as a fail safe, preventing overdriving of the drill bit and countersink  268  into the lap joint  116 . A Boelube reservoir  275 , shown in FIGS. 8A and 8C, provides lubricant during the drilling process to enhance hole quality and extend the life of the drill bit and countersink  268 . 
     Rivet Drive/Fastener Feed Module: 
     The rivet drive/fastener feed module  254 , as shown in FIGS. 8A-8E, loads a rivet/fastener into a hole drilled by the drill module  252  and then upsets the rivet in the hole in a manner that assures a high degree of accuracy, preventing rework. 
     The rivet module  284  includes a rivet drive unit  276 , which is pneumatically driven and interchangeable. The rivet drive unit  276  may be interchanged with a different drive unit, allowing the rivet module  254  to accommodate various fastener requirements. The interchange of the drive units is accomplished by removing the rivet drive unit  276  from a rivet drive holder  278 , which is horizontally fixed and vertically sliceable relative to the external carriage  256  of the module movement assembly  250 , and replacing it with a new rivet unit. The rivet drive unit  276  is removed from the rivet drive holder  278  by unscrewing first and second quick release rivet knobs  280 A and  280 B, respectively, as shown in FIG.  8 A. 
     The rivet module  254  further includes a first and second pneumatically powered rivet seating plunger  282  and  284 , respectively, as shown in FIG. 8D, coupled to both the rivet drive holder  278  and a cylindrical portion  279  of the external carriage  252  for moving the rivet unit  276  along the z-axis. The rivet drive unit  276  drives a rivet driver head (not shown) used to impact a head of the rivet, resulting in the deformation/upsetting of the rivet. The first and second rivet seating plunger  282  and  284  seat the rivet driver head against the head of the rivet to be upset. 
     The rivet module  254  also includes a fastener supply system. A plurality of rivets are sorted and queued by a vibratory bowl  286 , shown in FIG. 14, and pneumatically (using air pressure) fed to the rivet module  254  via feed tubes  288 A and  288 B. 
     The rivets delivered by the rivet feed tubes  288 A and  288 B are fed to a set of fastener feed fingers  290 , as shown in FIGS. 8E,  10 A and  10 B. The rivet fingers  290  are pneumatically powered to hold the rivet while it is inserted into the hole to be fastened. 
     As shown in FIGS. 10A and 10B, the feed fingers  290  include a circular structure  291 , having an inner orifice, where four fingers  292 A- 292 D are attached to a respective side of the inner orifice of the circular structure  291 . The feed fingers  290  lower the rivet into the hole to be fastened using the first and second pneumatic seating plungers  282  and  284 , respectively. 
     Interfaces: 
     The outside end-effector  104  also includes a plurality of electrical and pneumatic interfaces. For example, a plurality of pneumatic and electrical connections are located at bottom connectors  294 , shown in FIGS. 8D and 8E. The pneumatic bottom connectors  294  supply air to the air cylinders, pneumatic riveter and drill units discussed above. The electrical group of the bottom connectors  294  supply power to the above-discussed servo motors, and the power is distributed via an electrical service box  296 , shown in FIG.  8 A. Preferably, the bottom connectors are quick disconnects allowing the outside end-effector  104  to be easily moved, serviced, and installed. 
     Internal Guide Rails: 
     The internal guide rails  106 , as shown in FIG. 11, are positioned on the inside surface of the lap joint  116 . The internal guide rails  106  support the inside end-effector  108  and transfer forces generated by the inside end-effector  108  during fastening operations to the panels  110  forming the lap joint  116 . 
     The internal guide rails  106  include an upper rail  300  and a lower rail  302 . Each of the upper and lower rail  300  and  302 , respectively, includes an upper and lower tube portion  304 A and  304 B, for sliceable engagement with the inside end-effector  108 . The upper and lower-rail  300  and  302  also have an upper and lower bar portion  306 A and  306 B, which are mechanically coupled to the respective tube portion  304 A and  304 B. The upper and lower bar portions  306 A and  306 B are coupled to a plurality of upper and lower attachment brackets  308 A- 308 C, and  310 A- 310 C, respectively, as shown in FIG.  11 . 
     Attachment Brackets: 
     As shown in FIG. 11, the upper guide rail  300  is hung by the upper attachment brackets  308 A- 308 C by hooking the brackets  308 A- 308 C to a feature previously coupled to the inside surface of the panels  110  forming the lap joint  116 . In a similar manner, the lower guide rail  302  is stood upon the attachment brackets  310 A- 310 C. In one embodiment, as shown in FIG. 11, the present system is used within an aircraft fuselage section where the features include a plurality of stringers  311  positioned horizontally at intervals along the inside surface of the panels  110  and intersected by a plurality of frames  312  defining the bays within the fuselage section. 
     The attachment brackets  308 A- 308 C and  310 A- 310 C are hooked behind a T-shaped portion of the stringers  311  and adjacent to one of the frame members  312 . As shown in FIG. 11, the attachment brackets  308 A- 308 C and  310 A- 310 C are each clamped to the stringers  311  with respective circular plates  314 A- 314 F, and  315 A- 315 F, which contact a face of the stringers  311  and respective hooks  316 A- 316 F and  317 A-F, which reach behind the T-portion of the stringer  311 . Respective levers  318 A- 318 F and  319 A- 319 F draw the circular plates  314 A- 314 F,  315 A- 315 F and the hooks  316 A- 316 F,  317 A- 317 F together to lock both the upper and lower guide rails  300  and  302  onto their respective stringers  311 . 
     The attachment brackets  308 A- 308 C and  310 A- 310 C, as shown in FIG. 11, attach the internal guide rails  106  to the inside surface of the panels  110 , or airframe, forming the lap joint  116 . In the present embodiment, the stringers  311  and frame members  312  are aligned by coordination holes. Therefore, the internal guide rails  106  will benefit from the self aligned features coupled to the panel skins  110  and will, in turn, be aligned with the lap joint  116  without the need for externally supported fixturing. 
     In an alternative embodiment, the attachment brackets  308 A- 308 C and  310 A- 310 C may be varied in length or be adjustable in length of allow attachment to irregular features coupled on the inside surface of the panels  110 . If the inside surface has no features, then the above-described vacuum generators and pads could be used to replace the attachment brackets  308 A- 308 C and  310 A- 310 C. 
     The arrangement of the internal guide rails  106  allows an end-effector to be installed inside a fuselage or other restricted area which would not normally support a fixture or large mechanism required to accomplish the same task. 
     Inside End-Effector 
     The inside end-effector  108 , as shown in FIGS. 12A and 12B, includes an inside end-effector engagement assembly for allowing the inside end-effector  108  to slide along the internal guide rails  106 , an inside tool positioning assembly for accurately positioning bucking bar modules along an x′-axis (parallel to the inside guide rails  106 ) relative to the index pins  120  inserted in the lap joint  116 , and a rotational carriage assembly for moving the bucking bar modules along a y′-axis (perpendicular to the inside guide rails  106 ) relative to the index pins  120 . 
     Engagement Assembly: 
     The inside end-effector engagement assembly includes four inside standard bearings  320 A- 320 D, as shown in FIGS. 12A and 12B. The inside end-effector  108  is loaded at the outside end of the internal guide rails  106  by threading the internal guide rails  106  into the area defined by the standard bearings  320 A- 320 D. By locking the inside end-effector  108  to the internal guide rails  106  in this manner, the inside end-effector  108  is fail safe, and much lighter in weight than a unit locked in place with air cylinders. 
     Tool Positioning Assembly: 
     The tool positioning assembly includes an internal position detection subassembly, and an inside end-effector drive subassembly  336 . 
     Internal Position Detection Subassembly: 
     The internal position detection subassembly, as shown in FIG. 12B, includes an internal homing sensor  322  having first and second helium-neon lasers  324 A and  324 B, and respective first and second Charge Coupled Devices (CCD&#39;s)  326 A and  326 B. The first and second lasers  324 A and  324 B are directed toward the reflecting square  128  of the index pins  120  and their beams are parallel and spaced a distance just short of the width of the reflecting square  128 , between 1 and 5 mm, preferably 3 mm. Accordingly, as the inside end-effector is moved along the length of the lap joint  116  when both the first and second CCD&#39;s  326 A and  326 B simultaneously read their respective laser beams as being reflected by the reflecting square  128 , the inside end-effector  108  has been homed to a zero position on the x′, y′ coordinate system defining the inside surface of the lap joint  116 . Preferably, the determination that the inside end-effector  108  has been homed is made by the control system  114 . 
     The internal position detection subassembly also includes an internal final position encoder  328 , shown in FIG. 12B, which determines the distance Δx′ that the inside end-effector  108  has traveled along the internal guide rails  106  from the last measured home position, as defined by the index pins  120 . 
     The internal encoder  328 , as shown in FIGS. 12A and 12B, includes a two-wheel detector  330  that moves relative to the upper guide rail  130 , where the number of rotations and hence the distance traveled by the detector  330  is indicated by a signal to the control system  114  and is used to determine the position of the inside end-effector  108 . As shown in FIGS. 12A and 12B, the two-wheel detector  330  is engaged with the upper rail  300  using a detector air cylinder  332  which, when activated, pivots an arm  334  causing the two-wheel detector  330  to move against the upper rail  300 . 
     Inside End-Effector Drive Subassembly: 
     The inside end-effector drive subassembly  336 , as shown in FIG. 12A, moves the inside end-effector  108  along the internal guide rails  106 . The inside drive subassembly  336  includes an internal friction drive wheel  338  which is driven by an x′ axis servo-motor  340 . The use of the internal friction drive wheel  338  eliminates problems encountered when using gears or teeth. If the inside end-effector  108  were to encounter an obstacle, the internal friction drive wheel  338  would simply spin in place without causing any damage to either the inside end-effector  108  or the internal guide rail  106 . 
     The internal friction drive wheel  338  is engaged with the upper rail  300  by a second drive air cylinder  342  which, when activated, pivots a drive arm  344 , causing the internal friction wheel  338  to move up against the upper rail  300 . 
     Rotational Carriage Assembly: 
     The rotational carriage assembly of the outside end-effector  108  rotates a left-hand (LH) bucking bar  350  and a right-hand (RH) bucking bar  352 , as shown in FIG. 12B, relative to an inside frame  354  and the upper and lower guide rails  300  and  302 , respectively. 
     Bucking Bar Modules: 
     The rotational carriage assembly includes a LH bucking bar module  356  and a RH bucking bar module  358 , as shown in FIGS. 12A and 12B. 
     Both the LH bucking bar module  356  and the RH bucking bar module  358  include LH and RH quick release knobs  360 A and  360 B, respectively, as shown in FIG. 12A, allowing the two bucking bars to be easily interchanged with bucking bars having bucking dies of different shapes, sizes, and materials suited to a particular task. With this arrangement, the bucking bars can be easily swapped on the fly. 
     Further, the LH bucking bar module  356  and the RH bucking bar module  358  include a LH retract/extend cylinder  362 A, and a RH retract/extend cylinder  362 B, respectively. The LH and RH retract/extend cylinders  362 A and  362 B are pneumatically driven, and respectively cause the LH bucking bar  350  and the RH bucking bar  352  to move along the Z′ axis normal to the lap joint  116  on the inside surface of the panels  110 . 
     Protrusion Sensor: 
     The LH and RH bucking bar modules  356  and  358 , respectively, also include a LH protrusion sensor  364 A and RH protrusion sensor  364 B, as shown in FIGS. 12A,  12 B and  13 A-C, which are used to measure the length of the shank of a rivet  372  protruding from the inside surface of the lap joint  116 . 
     The LH and RH bucking bar modules  356  and  358  move the respective LH and RH bucking bars  350  and  352  along the z′ axis to three basic positions. In a first position, the LH and RH bucking bars  350  and  352 , respectively, are fully retracted to clear away from obstructive features attached to the inside surface of the panels  110 , allowing the inside end-effector  108  freedom of movement. In the second position, as shown in FIG. 13A, one of the bucking bars  350  and  352  is clamped against the inside surface of the panels  110  against an area to be fastened prior to and during a drilling operation. During this operation, the protrusion sensors  364 A and  364 B measure a distance (d1) from a fixed sensor component  368 A and  368 B. In the third position, one of the bucking bars  350  and  352  is driven against a shank  366  of a rivet  372  inserted into the newly drilled hole used to fasten the position of the lap joint  116 . Here, the protrusion sensor  364 A measures a distance (d2) from the fixed sensor component  368 A. The two values (d1) and (d2) are sent to the control system  114 , which processes this information to determine the length of the shank  366  protruding from the inside surface. The length of the shank  366  is compared against a table value of rivet lengths to determine whether the proper rivet has been installed in the hole and, if so, whether it is in tolerance. 
     As shown in FIG. 13C, the LH protrusion sensor  364 A continues to monitor the length (d3) of the shank, as it is deformed into a button  370 . In a preferred embodiment, the signal from the LH protrusion sensor  364 A is processed by the control system  114  to determine when a proper sized button has been formed (i.e., d3=proper button size indicated by table) and to immediately stop the rivet driver unit  276  from upsetting the rivet. This feedback system ensures a properly sized and seated rivet for each fastening operation. 
     The operation of the RH bucking bar module  358  and the RH protrusion sensor  364 B operate in an identical manner to the LH bucking bar module  356  and the LH protrusion sensor  364 A, as described above and shown in FIGS. 13A-13C. 
     Bucking Bar Dies: 
     The LH and RH bucking modules  356  and  358 , respectively, hold and position the LH and RH bucking bars  350  and  352 . Either of the LH or RH bucking modules  356  and  358 , respectively, can hold and position a straight bucking bar  371 , as shown in FIGS. 13A-13C, and  14 A. The straight bucking bar  371  can be swapped with either the LH or RH bucking bars  350  and  352 , when the inside end-effector  108  is upsetting a rivet, such as the rivet  372  shown in FIG. 13B, that is not obstructed by a T-shaped portion  374  of the stringer  311 . The straight bucking bar  371  has a die with a first gap  376  for receiving a drill bit during the drilling operation. The alignment of the first gap  376  and the drill bit extends the life of the drill bit and countersink  268  as well as the straight bucking bar  371 . 
     To solve the problem of fastening obstructed rivets, such as a top rivet  378  shown in FIG. 13A, the LH and RH bucking modules  356  and  358 , respectively, cause the LH and RH bucking bars  350  and  352 , respectively, to rotate behind the T-shaped portion  374  of the stringer  311 , as shown in FIG.  14 D. The LH and RH bucking bars  350  and  352  each include a LH and RH aluminum arms  380 A and  380 B, and LH and RH “L-shaped” bucking dies  382 A and  382 B, as shown in FIGS. 12B,  14 B, and  14 C. The LH and RH “L-shape” of the bucking dies  382 A and  382 B allow the dies to slide behind an obstruction, such as the T-portion  374  of the stringer  311 . The bucking dies  382 A and  382 B may have double offsets built therein, where one offset is for getting behind frames and the other offset is forgetting behind the stringers  311 . 
     Conventional bucking dies are formed from steel. Unfortunately, when the L-shaped dies are formed from steel, the rivets formed using these dies are severely clinched (i.e. clubfoot) buttons. Further, unusually long drive times are needed to upset the rivet. To counter these problems, it was determined that a thin section  384 A and  384 B of the bucking dies  382 A and  382 B, as shown in FIG. 12B, was vibrating an unacceptable amount during riveting operations. After the problem was identified, solutions were attempted using finite element analysis, data gathering observations, and configuration variation. As a result, it was determined that a material having a density of between 14.3-14.5 G/cm 3  was required. Further, the material should have a compressive strength of 650,000 psi, a minimum transverse rupture of 420,000 psi and a hardness of 72-74 Rc. Accordingly, the L-shaped bucking dies  382 A and  382 B are preferably formed using Tungsten Carbide™ from the Carbide Corporation which meets the above requirements. More preferably, a Tungsten Carbide™ grade CD-337 or ISO code G-20 or C-code C-11 is used to form the LH and RH bucking dies  382 A and  382 B. Tungsten Carbide™ has twice the density of steel and has almost twice the strength. By using Tungsten Carbide™ as the material forming the LH and RH bucking dies  382 A and  382 B, respectively, the clinching problem was eliminated and drive times were reduced to normal. This material could be used to improve the riveting process any time a die must undergo torsion or other torque-induced distortion during rivet deformation, including the manual process. 
     Rotational Turret Subassembly: 
     The rotational carriage assembly of the inside end-effector  108  includes a rotational turret subassembly for rotating the LH and RH bucking bars  350  and  352 , respectively, along an a-axis, which rotates about the z′ axis. The LH and RH bucking bars  350  and  352  are rotated and extended so that the L-shaped bucking dies  382  and  382 B, respectively, can be positioned between the rivet shank  366  and an obstruction, such as the stringer  310 , shown in FIG.  14 D. Rotation of the turret subassembly in effect moves the rivet bucking dies  382 A and  382 B to a pre-selected position (x′, y′) by rotating the LH and RH bucking bars  350  and  352  on the a-axis. 
     The rotational turret subassembly, as shown in FIG. 12B, includes a turret bearing  386 , which allows rotation of the LH and RH bucking bar modules  356  and  358 , which are mounted to a rotating support  388 , relative to the inside frame  354 . The rotation of the rotating support  385  is driven by a rotational servo-motor  390 , as shown in FIG.  12 A. 
     The position of the rotating support  388  and hence the LH and RH bucking bars  350  and  352  is monitored and reported to the control system  114  by a rotational encoder  392 , as shown in FIG.  12 B. When a selected one of the bucking bars  350  and  352  has reached its predetermined position, the CPU  398  shuts off the rotational servo-motor  390  and proceeds with a drilling or fastening operation. 
     The inside end-effector  108  includes a plurality of pneumatic and electrical connections  394 . Preferably, these connections are quick disconnects, allowing the easy installation and removal of the inside end-effector  108 . 
     Control System Cart 
     The mini-riveter system  100  includes the control system cart  112 , as shown in FIG. 15, which includes the vibratory bowl  286  for supplying fasteners, the control system  114  including a CPU  398  and display  400 . The control system cart  112  also includes an electrical power supply  402  and an air/pneumatic source  404 . The cart  112  is designed to transport the inside end-effector  108  and the outside end-effector  104  to a work area with minimal effort, and begin operations with a nominal compliment of operators. The cart  112 , has the capabilities to perform all of the required operations for fastening the lap joint  116 , including process checking/verification even before the mini-riveter system  100  is loaded onto the aircraft fuselage. 
     Operations: 
     In the first embodiment, the mini-riveter system  100  is used to fasten two overlapping skin panels  110  forming a lap joint  116 . Initially, each of the panels  110  is cleaned and the overlapping surface of the panels  110  are treated with a sealant. The panels  110  forming the lap joint  116  are then temporarily fastened with cleco fasteners in at least two points using coordination holes as means for alignment. The panels  110  may also be temporarily fastened to other panels to form part of a temporarily fastened fuselage assembly section. 
     Once a fuselage has been tacked together, an operator inserts index pins  120  into at least three coordination holes postioning the key portion  122  to protrude from the outside surface of the lap joint  116 , and positioning the reflective head  126  to protrude from the inside surface of the lap joint  116 . External rails  102  are then positioned and aligned to the index pins  120  with the three rail ties  134 A-C. Once properly aligned, air pressure is applied via the tube portions  135 A and  135 B of the primary rail  130  and the secondary rail  132  to the vacuum generators  144 A-F which generate a vacuum between the panels  110  and the rails, holding them in position. The outside end-effector  104  is then lifted onto the external guide rails  102  using the primary handle  200  and the secondary handle  202 . The first and second clamshell bearing systems  204  and  206  are then closed by the activation of the primary and secondary air cylinders  212  and  214  locking the outside end-effector  104  into sliding engagement with the external guide rails  102 . 
     The internal guide rails  106  are installed onto the inside surface of the panels  110  forming the lap joint  116  by positioning the upper and lower attachment brackets  308 A-C and  310 A-C adjacent to the frames within the fuselage and hooked behind the T-shaped portion of parallel stringers coupled to a respective one of the panels  110  forming the lap joint  116 . The upper and lower attachment brackets  308 A-C and  310 A-C are then locked into place by tightening the levers  318 A-F and  319 A-F associated with each of the hooks  316 A and  317 A-F. This step roughly ensures that the internal guide rails  106  are properly aligned on the x′ and y′ axes on the inside surface of the lap joint  116 . 
     Once the internal guide rails  106  have been properly installed and generally aligned, the inside end-effector subsystem  108  is slid onto the ends of the internal guide rails  106  and then properly homed to the first of the index pins  120  using its reflecting square  128 . Then, the outside end-effector  104  is homed to the recess  125  of the key  122  of a first of the index pins  120 , thereby independently aligning both the outside end-effector  104  and the inside end-effector  108 . 
     The mini-riveter system  100  is directed to drill, countersink, and then rivet a plurality of columns within the lap joint  116 , where each column consists of three rows of rivets. First, the outside end-effector  104  mini-riveter system  100  is driven from the home position or its last known position, to a distance along the x-axis upon which the selected column lies. Next, the pressure foot subassembly  230  is driven along the y-axis to the middle row to be fastened and then is pressed against the lap joint, applying pressure of between 100 and 500 lbs. The inside end-effector  108  is driven an identical distance along its x′ axis to mirror the position of the outside end-effector  104 . Then, one of the LH, RH or straight bucking bars  350 ,  352 , or  371  is extended and rotated to an x′ and y′ position, such that it mirrors the position of the porthole clamp  237  of the outside end-effector  104 . Further, the first gap portion  376  of the bucking bar is positioned along the z′ axis to match the z-axis defining the machine axis along which the drill unit  262  will operate and a pressure of between 100 and 500 lbs. is exerted on the inside surface of the lap joint  116  by the bucking bar. 
     The external carriage  256  holding both the drill/countersink module  252  and the rivet/fastener feed module  254  is moved to align the drill bit and countersink  268  along the y axis. Next the drill module  256  is activated and moved along the z axis until a hole and countersink having the proper dimensions have been drilled within the lap joint  116 . After the drill is retracted, the external carriage  256  moves the riveter/fastener feed module  254  along the y axis into position in alignment with the newly drilled hole. The fastener feed module  254  loads a selected rivet into the rivet feed fastener fingers  292 A-D. Then, the inside end-effector  108  backs off the bucking bar while the fastener fingers  292 A-D load the selected rivet into the newly drilled hole. The driver of the rivet module  254  is then seated against the head of the rivet, and the bucking bar is moved towards the inside surface until it contacts the shank of the rivet. The rivet is held in place by the driver head of the pneumatic riveting unit  276 . The rivet is then upset by a series of pneumatically induced pulses from the driver head of the riveting unit  276  until it is properly seated. 
     One of the rivet protrusion sensors  364 A and  364 B compares the length of the rivet shaft to the length of the desired rivet to ensure that the proper rivet was loaded before allowing the driving sequence, and then monitors the deformation of the shank to ensure that the riveting process ceases once a desired button has formed. The bucking bar and the pressure foot subassembly  230  are then released and moved to a new row. This process is repeated until each of the three rows within the column has been drilled, countersunk and properly riveted. Then, the inside and outside end-effector  104  and  108  respectively are moved along the x and x′ axes respectively for positioning along a new column. This process is repeated until the entire lap joint  116  has been properly fastened. 
     The above-described process may be used for a plurality of mini-riveter systems used simultaneously on different bays of an aircraft fuselage. In this embodiment, one set of operators can operate two or more systems by setting up a second system while a first system is performing an operation on a lap joint. In this manner production flow rates can be greatly increased without increasing manpower requirements. 
     FIG. 15 shows a series of program instructions coordinated by the CPU  398  of the control system  114  to direct the mini-riveter system  100  during positioning, drilling, and fastening operations. Flow charts from which source code can be written by one skilled in the art are illustrated in FIGS. 15-17. 
     Referring to FIG. 15, a main routine  500 , which is executed by the CPU  398  begins at step  502  by requesting an input of data, including the x and y, as well as the x′ and y′ position of a fastener on a particular row and column of the lap joint  116 , as well as the position where the fastening process commences and the number of fasteners to be used. Next, in step  504 , the CPU  398  determines whether the next position to be fastened is that of a middle row fastener. If not, then the CPU  398  proceeds to step  506  and sets a flag “middle row required first,” and returns to step  502 , where it instructs the mini-riveter system  100  to move to the next designated position. If the CPU  398  determines in step  504  that the selected rivet position is a middle fastener position, it then proceeds to step  510 , where it checks if a hole has already been drilled in that position. If a hole has been drilled, then the CPU  398  proceeds to step  512 , and sets a flag “no double drilling” and returns to step  502 . However, if a hole had not already been drilled, the CPU  398  then proceeds to step  514  and checks whether the proper drilling/countersink module and rivet/fastener feed modules had been installed. If not, the CPU  398  proceeds to step  516  and begins a holding loop, as well as setting a flag “change modules.” However, if the proper modules have been installed, then the CPU  398  proceeds to step  518  and checks whether the outside end-effector  104  needs to be homed. If yes, the CPU  398  proceeds to step  520  and instructs the outside end-effector  104  to home to the nearest of the index pin  220 . If the homing step is not required, then the CPU  398  proceeds to step  522 , which invokes the hole drilling subroutine  550 . 
     In the first step  552  of the hole drilling subroutine  550 , shown in FIG. 16, the CPU  398  directs the outside and inside end-effectors  104  and  108  respectively to move along the x and x′ axis, respectively, to the position to be drilled. Next, in step  554 , the CPU  398  moves the porthole clamp  232  of the pressure foot assembly  230  along the y axis, while the bucking bar is moved and rotated to a mirror position on the y′ axis. Next, in step  556 , the CPU  398  directs the pressure foot assembly  230  to apply a force onto the lap joint  116  for a specified dwell time, which is selected in step  558 . Then, the CPU  398  proceeds to step  560 , where it moves external the module carriage  256  to position the drill/countersink module  268  to the desired position along the (x,y) axes of the lap joint  116 . The CPU  398  then proceeds to step  562 , where it directs the application of Boelube to the area to be drilled. After step  562 , the CPU  398  proceeds to step  564 , where it instructs the drilling/countersink module  268  to travel along the y axis to a specified point for properly drilling and countersinking the hole. Then, the CPU  398  proceeds to step  566  and optionally directs the application of air pressure to the area to remove any drill chips. Next, the CPU  398  proceeds to step  568 , where it directs the inspection of the hole. The CPU  398  then proceeds to step  570 , where it ends the subroutine  550 , and returns to the main routine  500 . 
     Once the hole drilling subroutine  550  has been completed, the CPU  398  proceeds to step  524  of the main routine  500  and invokes the fastening subroutine  600 . 
     In the first step  602  of the fastening subroutine  600 , shown in FIG. 17, the CPU  398  directs the external module carriage  256  to position the rivet driver/fastener feed module  254  to place it in alignment with the newly drilled hole. Next, the CPU  398  proceeds to step  624  and directs the fastener feed system to load a rivet into the assembly&#39;s finger units  292 A-D. The CPU  398  then proceeds to step  626 , where it directs the inside end-effector assembly  108  to un-clamp the bucking bar, which was applying pressure to the inside surface of the lap joint  116 . In step  628 , the CPU  398  backs off the bucking bar to a standby position, and the fastener feed fingers  292 A-D install the rivet into the newly drilled hole. The CPU  398  then proceeds to step  630 , where it directs the rivet head protrusion sensor to measure the length of the shaft protruding from the inside surface of the lap joint  116 . From there, the CPU  398  proceeds to step  632 , where it compares the measured length of the shank protrusion with a tabular range of values allowable for the selected rivet to ensure that the correct rivet was loaded into the hole. If the CPU  398  determines that an improper type of rivet was loaded into the hole or that the rivet has an abnormal shank, it then proceeds to step  634  and sets a flag and stop further work. However, if the rivet is determined to be of the proper type and size, then the CPU  398  proceeds to step  636  and directs the pneumatic riveter unit  276  to begin bucking the rivet. The CPU  398  then proceeds to step  638 , where it continues to monitor and the protrusion sensor  364 A and  364 B to determine if the deformed shank has formed a proper button of a specified height. If the button is still too large, the CPU  398  may direct the riveting process to continue until the proper button height has been obtained. If the proper button height cannot be obtained after checking its height in step  640 , the CPU  398  will proceed to step  642  and set a flag and stop the system  100 . However, if the CPU  398  determines that the button height falls within proper tolerances, it ends the subroutine and proceeds back to step  524  of the main routine  500 . 
     Once the fastening subroutine  600  has been completed, the CPU  398  of the control system  114  proceeds to step  526  of the main routine  500 , where it checks to see whether another fastening operation is to occur or whether it is the last fastener on the lap joint  116 . If the CPU  398  determines that the last fastener has not yet been installed, then it proceeds to step  528  and moves the mini-riveter system  100  to the next desired position and returns to step  502 . However, if the CPU  398  determines that this was the last fastener operation to occur on the lap joint  116 , then it proceeds to step  530  and displays an instruction on display  400  to remove the mini-riveter system from the bays being operated on. 
     The mini-riveter system  100  is easy to set up and use, and requires only a small amount of manpower and man hours to set up and operate. Further, the mini-riveter system  100  can fit into areas heretofore inaccessible by drilling and fastening machines, due to its ability to be supported and aligned by the components it is fastening and because of its small size. Preferably the entire mini-riveter system  100  does not exceed 200 lbs., where the end-effectors are designed to weigh less than 40 lbs. and the tracks even less. Further, the mini-riveter system is small in size and was designed not to exceed an envelope of 17″ along the y and y′ axes by 24″ along the z and z′ axes. This same design concept, where a small, light weight end-effector is supported and indexed relative to the parts being assembled can be used in many other areas of part assembly. 
     Except as otherwise disclosed herein, the various components shown in outline or block form are individually well-known and their internal construction and operation is not critical, either to the making or the using of this invention. 
     While the detailed description above has been expressed in terms of specific examples, those skilled in the art will appreciate that many other configurations could be used to accomplish the purpose of the disclosed inventive apparatus. Accordingly, it will be appreciated that various equivalent modifications of the above-described embodiments may be made without departing from the spirit and scope of the invention. Therefore, the invention is to be limited only by the following claims.