Abstract:
A method of manufacturing, using a riveting system, is operable to join two or more workpieces with a rivet. In another aspect of the present invention, a self-piercing rivet is employed. Still another aspect of the present invention employs an electronic control unit and one or more sensors to determine a riveting characteristic and/or an actuator characteristic.

Description:
This application is a divisional of 10/300,317, filed Nov. 20, 2002 now U.S. Pat. No. 7,024,270 which is a continuation of U.S. patent application Ser. No. 09/824,872, filed on Apr. 3, 2001, now issued as U.S. Pat. No. 6,502,008, which is a divisional of U.S. patent application Ser. No. 09/358,751, filed on Jul. 21, 1999, now issued as U.S. Pat. No. 6,276,050, which is a continuation-in-part of U.S. patent application Ser. No. 09/119,255, filed on Jul. 20, 1998, now abandoned, which claims priority to German Application No. DE 197 31 222.5, filed on Jul. 21, 1997; all of which are incorporated by reference herein. 

   BACKGROUND 
   This invention relates generally to riveting and more particularly to a riveting system and a process for forming a riveted joint. 
   It is well known to join two or more sheets of metal with a rivet. It is also known to use self-piercing rivets that do not require a pre-punched hole. Such self-piercing or punch rivet connections can be made using a solid rivet or a hollow rivet. 
   A punch rivet connection is conventionally formed with a solid rivet by placing the parts to be joined on a die. The parts to be joined are clamped between a hollow clamp and the die. A plunger punches the rivet through the workpieces such that the rivet punches a hole in the parts thereby rendering pre-punching unnecessary. Once the rivet has penetrated the parts to be joined, the clamp presses the parts against the die, which includes a ferrule. The force of the clamp and the geometry of the die result in plastic deformation of the die-side part to be joined thereby causing the deformed part to partially flow into an annular groove in the punch rivet. This solid rivet is not deformed. 
   Traditionally, hydraulically operated joining devices are used to form such punch rivet connections. More specifically, the punching plunger is actuated by a hydraulic cylinder unit. The cost of producing such joining devices is relatively high and process controls for achieving high quality punch rivet connections has been found to be problematic. In particular, hydraulically operated joining devices are subject to variations in the force exerted by the plunger owing to changes in viscosity. Such viscosity changes of the hydraulic medium are substantially dependent on temperature. A further drawback of hydraulically operated joining devices is that the hydraulic medium, often oil, has a hydroscopic affect thereby requiring exchange of the hydraulic fluid at predetermined time intervals. Moreover, many hydraulic systems are prone to hydraulic fluid leakage thereby creating a messy work environment in the manufacturing plant. 
   When forming a punch connection or joint with a hollow rivet, as well as a semi-hollow rivet, the plunger and punch cause the hollow rivet to penetrate the plunger-side part to be joined and partially penetrate into the die-side part to be joined. The die is designed to cause the die-side part and rivet to be deformed into a closing head. An example of such a joined device for forming a punch rivet connection with a hollow rivet is disclosed in DE 44 19 065 A1. Hydraulically operating joining devices are also used for producing a punch rivet connection with a hollow rivet. 
   Furthermore, rivet feeder units having rotary drums and escapement mechanisms have been traditionally used. Additionally, it is known to use linear slides to couple riveting tools to robots. 
   It is also known to employ a computer system for monitoring various characteristics of a blind rivet setting system. For example, reference should be made to U.S. Pat. No. 5,661,887 entitled “Blind Rivet Set Verification System and Method” which issued to Byrne et al. on Sep. 2, 1997, and U.S. Pat. No. 5,666,710 entitled “Blind Rivet Setting System and Method for Setting a Blind Rivet Then Verifying the Correctness of the Set” which issued to Weber et al. on Sep. 16, 1997. Both of these U.S. patents are incorporated by reference herein. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a riveting system is operable to join two or more workpieces with a rivet. In another aspect of the present invention, a self-piercing rivet is employed. A further aspect of the present invention uses a self-piercing rivet which does not fully penetrate the die-side workpiece in an acceptable joint. Still another aspect of the present invention employs an electronic control unit and one or more sensors to determine a riveting characteristic and/or an actuator characteristic. In still another aspect of the present invention, an electric motor is used to drive a nut and spindle drive transmission which converts rotary actuator motion to linear rivet setting motion. In yet another aspect of the present invention, multiple rivet feeders can selectively provide differing types of rivets to a single riveting tool. Unique software employed to control the riveting machine is also used in another aspect of the present invention. A method of operating a riveting system is also provided. 
   The riveting system of the present invention is advantageous over conventional devices in that the present invention employs a very compact and mechanically efficient rotational-to-linear motion drive transmission. Furthermore, the present invention advantageously employs an electric motor to actuate the riveting punch thereby providing higher accuracy, less spilled fluid mess, lower maintenance, less energy, lower noise and less temperature induced variations as compared to traditional hydraulic drive machines. Moreover, the electronic control system and software employed with the present invention riveting system ensure essentially real time quality control and monitoring of the rivet, riveted joint, workpiece characteristics, actuator power consumption and/or actuator power output characteristics, as well as collecting and comparing historical processing trends using the sensed data. 
   The riveting system and self-piercing hollow rivet employed therewith, advantageously provide a high quality and repeatable riveted joint that is essentially flush with the punch-side workpiece outer surface without completely piercing through the die-side workpiece. The real-time characteristics of the rivet, joint and workpieces are used in an advantageous manner to ensure the desired quality of the final product. Furthermore, the performance characteristics may be easily varied or altered by reprogramming software set points, depending upon the specific joint or workpiece to be worked upon, without requiring mechanical alterations in the machinery. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic view showing the preferred embodiment of the riveting system of the present invention; 
       FIG. 2  is a partially diagrammatic, partially elevational view showing the preferred embodiment riveting system; 
       FIG. 3  is a perspective view showing a riveting tool of the preferred embodiment riveting system; 
       FIG. 4  is an exploded perspective view showing the nut and spindle mechanism, punch assembly, and clamp of the preferred embodiment riveting system; 
       FIG. 5  is an exploded perspective view showing the gear reduction unit employed in the preferred embodiment riveting system; 
       FIG. 6  is a cross sectional view, taken along line  6 — 6  of  FIG. 3 , showing the riveting tool of the preferred embodiment riveting system; 
       FIG. 7  is an exploded perspective view showing a receiving head of the preferred embodiment riveting system; 
       FIG. 8  is a cross sectional view showing the receiving head of the preferred embodiment riveting system; 
       FIG. 9  is a cross sectional view, similar to  FIG. 6 , showing a first alternate embodiment of the riveting system; 
       FIG. 10  is a partially fragmented perspective view showing a rivet feed tube of the preferred embodiment riveting system; 
       FIG. 11  is an exploded perspective view showing a feeder of the preferred embodiment riveting system; 
       FIGS. 12   a – 12   f  are a series of cross sectional views, similar to that of  FIG. 6 , showing the self-piercing riveting sequence of the preferred embodiment riveting system; 
       FIGS. 13   a – 13   e  are a series of diagrammatic and enlarged views, similar to those of  FIG. 12 , showing the self-piercing riveting sequence of the preferred embodiment riveting system; 
       FIGS. 14 and 15  are diagrammatic views showing the control system of the preferred embodiment riveting system; 
       FIGS. 16 and 17  are graphs showing force versus distance riveting characteristics of the preferred embodiment riveting system; 
       FIGS. 18   a – 18   d  are software flow charts of the preferred embodiment riveting system; and 
       FIG. 19  is a partially diagrammatic, partially side elevational view showing a second alternate embodiment riveting system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 and 2 , a joining device for punch rivets, hereinafter known as a riveting system  21 , includes a riveting machine or tool  23 , a main electronic control unit  25 , a rivet feeder  27 , and the associated robotic tool movement mechanism and controls, if employed. Riveting tool  23  further has an electric motor actuator  29 , a transmission unit, a plunger  31 , a clamp  33  and a die or anvil  35 . Die  35  is preferably attached to a C-shaped frame  37  or the like. Frame  37  also couples the advancing portion of riveting tool  23  to a set of linear slides  39  which are, in turn, coupled to an articulated robot mounted to a factory floor. A linear slide control unit  41  and an electronic robot control unit  43  are electrically connected to linear slides  39  and main electronic control unit  25 , respectively. The slides  39  are actuated by a pneumatic or hydraulic pressure source  45 . 
   The transmission unit of riveting tool  23  includes a reduction gear unit  51  and a spindle drive mechanism  53 . Plunger  31 , also known as a punch assembly, includes a punch holder and punch, as will be described in further detail hereinafter. A data monitoring unit  61  may be part of the main controller  25 , as shown in  FIG. 2 , or can be a separate microprocessing unit, as shown in  FIG. 1 , to assist in monitoring signals from the various sensors. 
   Reference is now made to  FIGS. 3 ,  5  and  6 . A main electrical connector  71  is electrically connected to main electronic control unit  25 , which contains a microprocessor, a display screen, indicator lights, and input buttons. Connector  71  is also electrically connected to the other proximity switch sensors located in riveting tool  23 . Electric motor  29  is of a brushless, three phase alternating current type. Energization of electric motor  29  serves to rotate an armature shaft, which in turn, rotates an output gear  73 . Electric motor  29  and gear  73  are disposed within one or more cylindrical outer casings. 
   Reduction gear unit  51  includes gear housings  75  and  77  within which are disposed two different diameter spur gears  79  and  81 . Various other ball bearings  83  and washers are located within housings  75  and  77 . Additionally, removable plates  85  are bolted onto housing  75  to allow for lubrication. Spur gear  79  is coaxially aligned and driven by output gear  73 , thus causing rotation of spur gear  81 . Adapters  87  and  89  are also stationarily mounted to housing  77 . 
     FIGS. 4 and 6  show a nut housing  101  directly connected to a central shaft of spur gear  81 . Therefore, rotation of spur gear  81  causes a concurrent rotation of nut housing  101 . Nut housing  101  is configured with a hollow and generally cylindrical proximal segment and a generally enlarged, cylindrical distal segment. A load cell  103  is concentrically positioned around proximal segment of nut housing  101 . Load cell  103  is electrically connected to a load cell interface  105  (see  FIG. 3 ) which, in turn, is electrically connected to monitoring unit  61  (see  FIG. 1 ). Sensor interface  105  is an interactive current amplifier. Load cell  103  is preferably a DMS load cell having a direct current bridge wherein the mechanical input force causes a change in resistance which generates a signal. Alternately, the load cell may be of a piezo-electric type. 
   A rotatable nut  111 , also known as a ball, is directly received and coupled with a distal segment of nut housing  101  such that rotation of nut housing  101  causes a simultaneously corresponding rotation of nut  111 . Ball bearings  113  are disposed around nut housing  101 . A spindle  115  has a set of external threads which are enmeshed with a set of internal threads of nut  111 . Hence, rotation of nut  111  causes linear advancing and retracting movement of spindle  115  along a longitudinal axis. A proximal end of a rod-like punch holder  121  is bolted to an end of spindle  115  for corresponding linear translation along the longitudinal axis. A rod-like punch  123  is longitudinally and coaxially fastened to a distal end of punch holder  121  for simultaneous movement therewith. 
   An outwardly flanged section  125  of punch holder  121  abuts against a spring cup  127 . This causes compression of a relatively soft compression spring  128  (approximately 100-300 newtons of biasing force), which serves to drive a rivet out of the receiver and into an initial loaded position for engagement by a distal end of punch  123 . A stronger compression spring  141  (approximately 8,000–15,000 newtons of biasing force) is subsequently compressed by the advancing movement of punch holder  121 . The biasing action of strong compression spring  141  serves to later return and retract a clamp assembly, including a clamp  143  and nose piece, back toward gear reduction unit  51  and away from the workpieces. 
   A main housing  145  has a proximal hollow and cylindrical segment for receiving the nut and spindle assembly. Main housing  145  further has a pair of longitudinally elongated slots  147 . A sleeve  149  is firmly secured to punch holder  121  and has transversely extending sets of rollers  151  or other such structures bolted thereto. Rollers  151  ride within slots  147  of main housing  145 . Longitudinally elongated slots  153  of clamp  143  engage bushings  155  also bolted to sleeve  149 . Thus, rollers  151  and slots  147  of main housing  145  serves to maintain the desired linear alignment of both punch holder  121  and clamp  143 , as well as predominantly prevent rotation of these members. Additional external covers  157  are also provided. All of the moving parts are preferably made from steel. 
   Referring to  FIGS. 6 and 15 , a spindle position proximity switch sensor  201  is mounted within riveting tool  23 . A spring biased upper die and self-locking nut assembly  203  serves to actuate spindle position proximity switch  201  upon the spindle assembly reaching the fully retracted, home position. A plate thickness proximity switch sensor  205  is also mounted within riveting tool  23 . An upper die type thickness measurement actuator and self-locking nut assembly  207  indicate the positioning of clamp  143  and thereby serve to actuate proximity sensor  205 . Additional proximity switch sensors  281  and  283  are located in a feed tube for indicating the presence of a rivet therein in a position acceptable for subsequent insertion into the receiver of riveting tool  23 . These proximity switches  201 ,  205 ,  281  and  283  are all electrically connected to main electronic control unit  25  via module  601 . Furthermore, a resolver-type sensor  211  is connected to electric motor  29  or a member rotated therewith. Resolver  211  serves to sense actuator torque, actuator speed and/or transmission torque. The signal is then sent by the resolver to main electronic control unit  25 . An additional sensor (not shown) connected to electric motor  29  is operable to sense and indicate power consumption or other electrical characteristics of the motor which indicate the performance characteristics of the motor; such a sensed reading is then sent to main electronic control unit  25 . 
     FIGS. 7 and 8  best illustrate a receiver  241  attached to a distal end or head of riveting tool  23  adjacent punch  123 . An upper housing  243  is affixed to a lower housing  245  by way of a pair of quick disconnect fasteners  247 . A nose piece portion  249  of the clamp assembly is screwed into lower housing  245  and serves to retain a slotted feed channel  251 , compressibly held by elastomeric O-ring  253 . A pair of flexible fingers  255  pivot relative to housings  243  and  245 , and act to temporarily locate a rivet  261  in a desired position aligned with punch  123  prior to insertion into the workpieces. Compression springs  262  serve to inwardly bias flexible fingers  255  toward the advancing axis of punch  123 . Furthermore, a catch stop  263  is mounted to upper housing  243  by a pivot pin. Catch stop  263  is downwardly biased from upper housing  243  by way of a compression spring  265 . A suitable receiver is disclosed in EPO patent publication No. 09 22 538 A2 (which corresponds to German Application No. 297 19 744.4). 
     FIG. 10  illustrates a feed tube  271  having end connectors  273  and  275 . End connector  273  is secured to receiver  241  (see  FIG. 8 ) and connector end  275  is secured to feeder  27  (see  FIG. 2 ). Feed tube  271  further includes a cylindrical outer protective tube  277  and an inner rivet carrying tube  279 . Inner tube  279  has a T-shaped inside profile corresponding to an outside shape of the rivet fed therethrough. Feed tube  271  is semi-flexible. Entry and exit proximity switch sensors  281  and  283 , respectively, monitor the passage of each rivet through feed tube  271  and send the appropriate indicating signal to main electronic control unit  25  (see  FIGS. 2 and 15 ). The rivets are pneumatically supplied from feeder  27  to receiver  241  through feed tube  271 . 
     FIG. 11  shows the internal construction of SRF feeder  27 . The feeder has a stamped metal casing  301 , upper cover  303  and face plate  305 . Feeder  27  is intended to be stationarily mounted to the factory floor. A storage bunker  307  is attached to an internal surface of face plate  305  and serves to retain the rivets prior to feeding. A rotary bowl or drum  309  is externally mounted to face plate  305 . It is rotated by way of a rotary drive unit  311  and the associated shafts. A pneumatic cylinder  313  actuates drive unit  311  and is controlled by a set of pneumatic valves  315  internally disposed within casing  301 . An electrical connector  317  and the associated wire electrically connects feeder  27  to main electronic control unit  25  by way of module  601  (see  FIGS. 2 ,  14  and  15 ). 
   A pneumatically driven, sliding escapement mechanism  319  is mounted to face plate  305  and is accessible to drum  309 . A proximity switch sensor  321  is mounted to escapement mechanism  319  for indicating passage of each rivet from escapement mechanism  319 . Proximity switch  321  sends the appropriate signal to the main electronic control unit through module  601 . Rotation of drum  309  causes rivets to pass through a slotted raceway  323  for feeding into escapement  319  which aligns the rivets and sends them into feed tube  271  (see  FIG. 10 ). 
     FIG. 9  shows a first alternate embodiment riveting system. The joining device or riveting tool has an electric motor operated drive unit  401 . Drive unit  401  is connected to a transmission unit  402  which is arranged in an upper end region of a housing  425 . Housing  425  is connected to a framework  424 . 
   A drive shaft  411  of drive unit  401  is connected to a belt wheel  412  of transmission unit  402 . Belt wheel  412  drives a belt wheel  414  via an endless belt  413  which may be a flexible toothed belt. The diameter of belt wheel  412  is substantially smaller than the diameter of belt wheel  414 , allowing a reduction in the speed of drive shaft  411 . Belt wheel  414  is rotatably connected to a drive bush  415 . A gear with gear wheels can also be used instead of a transmission unit  402  with belt drive. Other alternatives are also possible. 
   A rod  417   a  is transversely displaceable within the drive bush  415  which is appropriately mounted. The translation movement of rod  417   a  is achieved via a spindle drive  403  having a spindle nut  416  which cooperates with rod  417   a . At the end region of rod  417   a , remote from transmission unit  402 , there is formed a guide member  418  into which rod  417   a  can be introduced. A rod  417   b  adjoins rod  417   a . An insert  423  is provided in the transition region between rod  417   a  and rod  417   b . Insert  423  has pins  420  which project substantially perpendicularly to the axial direction of rod  417   a  or  417   b  and engage in slots  419  in guide member  418 . This ensures that rod  417   a  and  417   b  does not rotate. Rod  417   b  is connected to a plunger  404 . Plunger  404  is releasably arranged on rod  417   b  so that it can be formed according to the rivets used. A stop member  422  is provided at the front end region of rod  417   b . Spring elements  421  are arranged between stop member  422  and insert  423 . Spring elements  421  are spring washers arranged in a tubular portion of guide member  418 . Guide member  418  is arranged so as to slide in a housing  425 . The joining device is shown in a position in which plunger  404  and clamp  405  rest on the parts to be joined  407  and  408 , which also rest on a die  406 . 
   In a punch rivet connection formed by a grooved solid rivet, the rivet is pressed through the parts to be joined  407  and  408  by plunger  404  once the workpieces have been fixed between die  406  and hold down device/clamp  405 . Clamp  405  and plunger  404  effect clinching. The rivet then punches a hole in the parts to be joined, after which, clamp  405  presses against these parts to be joined. The clamp presses against the die such that the die-side part to be joined  408  flows into the groove of the rivet owing to a corresponding design of die  406 . The variation of the force as a function of the displacement can be determined by the process according to the invention from the power consumption of the electric motor drive  401 . For example, during the cutting process, plunger  404  and, therefore also the rivet, covers a relatively great displacement wherein the force exerted by plunger  404  on the rivet is relatively constant. Once the rivet has cut through the plunger side part to be joined  407 , the rivet is spread into die  406  as the force of plunger  404  increases. The die side part to be joined  408  is deformed by die  406  during this procedure. If the force exerted on the rivet by plunger  404  is sustained, the rivet is compressed. If the head of the punch rivet lies in a plane of the plunger-side part to be joined  407 , the punch rivet connection is produced. The force/displacement curve can be determined from the process data. With a known force/displacement curve which serves as a reference, the quality of a punch connection can be determined by means of the measured level of the force as a function of the displacement. 
   The drive unit, monitoring unit and the spindle drive can have corresponding sensors for picking up specific characteristics, the output signals of which are processed in the monitoring unit. The monitoring unit can be part of the control unit. The monitoring unit emits input signals as open and closed loop control variables to the control unit. The sensors can be displacement and force transducers which determine the displacement of the plunger as well as the force of the plunger on the parts to be joined. A sensor which measures the power consumption of the electric motor action drive unit can also be provided, as power consumption is substantially proportional to the force of the plunger and optionally of the clamp on the parts to be joined. 
   In this alternate embodiment, the speed of the drive unit can also be variable. Owing to this feature, the speed with which the plunger or the clamp acts on the parts to be joined or the rivet can be varied. The speed of the drive unit can be adjusted as a function of the properties of the rivet and/or the properties of the parts to be joined. The advantage of the adjustable speed of the drive unit also resides in the fact that, for example, the plunger and optionally the clamp is initially moved at high speed to rest on the parts to be joined and the plunger and optionally the clamp is then moved at a lower speed. This has the advantage of allowing relatively fast positioning of the plunger and the clamp. This also affects the cycle times of the joining device. 
   It is further proposed that the plunger and optionally the clamp be movable from a predeterminable rest position that can be easily changed through the computer software. The rest position of the plunger and optionally of the clamp is selected as a function of the design of the parts to be joined. If the parts to be joined are smooth metal plates, the distance between a riveting unit which comprises the plunger and the clamp and a die can be slightly greater than the thickness of the superimposed parts to be joined. If a part to be joined has a ridge, as viewed in the feed direction of the part to be joined, the rest position of the riveting unit is selected such that the ridge can be guided between the riveting unit and the die. Therefore, it is not necessary for the riveting unit always to be moved into its maximum possible end or home position. 
   A force or a characteristic corresponding to the force of the plunger, and optionally of the clamp, can be measured in this alternate embodiment during a joining procedure as a function of the displacement of the plunger or of the plunger and the clamp. This produces a measured level. This is compared with a desired level. If comparison shows that the measured level deviates from the desired level by a predetermined limit value in at least one predetermined range, a signal is triggered. This process control advantageously permits qualitative monitoring of the formation of a punch connection. 
   This embodiment of the process also compares the measured level with the desired level at least in a region in which clinching is substantially completed by the force of the plunger on a rivet. A statement as to whether a rivet has been supplied and the rivet has also been correctly supplied can be obtained by comparing the actual force/displacement trend with the desired level. The term ‘correctly supplied’ means a supply where the rivet rests in the correct position on the part to be joined. It can also be determined from the result of the comparison whether an automatic supply of rivets is being provided correctly. 
   The measured level is also compared with the desired level at least in a region in which the parts to be joined have been substantially punched by the force of the plunger on a rivet, in particular a solid rivet, and the clamp exerts a force on the plunger-side part to be joined. This has the advantage that it is possible to check whether the rivet actually penetrated the parts to be joined. 
   According to this embodiment of the process, the measured level is compared with the desired level, at least in a region in which a rivet, in particular a hollow rivet, substantially penetrated the plunger-side part to be joined owing to the force of the plunger and a closing head was formed on the rivet. It is thus also possible to check whether the parts to be joined also have a predetermined thickness. A comparison between the measured level and the desired level is performed, at least in a region in which a closing head is substantially formed on the rivet, in particular a hollow rivet, and clinching of the rivet takes place. It is thus possible to check whether the rivet ends flush with the surface of the plunger-side part to be joined. 
   Returning to the preferred embodiment,  FIGS. 12   a – 12   f  and  FIGS. 13   a – 13   e  show the riveting process steps employing the system of the present invention. The preferred rivet employed is of a self-piercing and hollow type which does not fully pierce through the die-side workpiece. First,  FIGS. 12   a  and  13   a  show the clamp/nose piece  249  and punch  123  in retracted positions relative to workpieces  501  and  503 . Workpieces  501  and  503  are preferably stamped sheet metal body panels of an automotive vehicle, such as will be found on a conventional pinch weld flange adjacent the door and window openings. The robot and linear slides will position the riveting tool adjacent the sheet metal flanges such that nose piece  249  and die  35  sandwich workpieces  501  and  503  therebetween at a target joint location. It is alternately envisioned that a manually (non-robotic) moved riveting tool or a stationary riveting tool can also be used with the present invention. 
     FIG. 12   b  shows clamp/nose piece  249  clamping and compressing workpieces  501  and  503  against die  35 . Punch  123  has not yet begun to advance rivet  261  toward workpieces  501  and  503 . At this point, the plate thickness proximity switch senses the thickness of the workpieces through actual location of the clamp assembly; the plate thickness switch sends the appropriate signal to the main controller. Next, punch  123  advances rivet  261  to a point approximately  1  millimeter above the punch-side workpiece  501 . This is shown in  FIGS. 12   c  and  13   b . If the workpiece thickness dimension is determined to be within an acceptable range by the main electronic control unit then energization of the electric motor further advances punch  123  to insert rivet  261  into punch-side workpiece  501 , as shown in  FIG. 13   c , and then continuously advances the rivet into die-side workpiece  503 , as shown in  FIGS. 12   d  and  13   d . Die  35  serves to outwardly deform and diverge the distal end of rivet  261  opposite punch  123 . 
     FIG. 12   e  shows the punch subsequently retracted to an intermediate position less than the full home position while clamp/nose piece  249  continues to engage punch side workpiece  501 . Finally, punch  123  and clamp/nose piece  249  are fully retracted back to their home positions away from workpieces  501  and  503 . This allows workpieces  501  and  503  to be separated and removed from die  35  if an acceptable riveted joint is determined by the main electronic control unit based on sensed joint characteristics. As shown in  FIG. 13   e , an acceptable riveted joint has an external head surface of rivet  261  positioned flush and co-planar with an exterior surface of punch-side workpiece  501 . Also, in an acceptable joint, the diverging distal end of rivet  261  has been sufficiently expanded to engage workpiece  503  without piercing completely through the exterior surface of die-side workpiece  503 . 
   A simplified electrical diagram of the preferred embodiment riveting system is shown in  FIG. 14 . Main electronic control unit  25 , such as a high speed industrial microprocessor computer, having a cycle time of about 0.02 milliseconds purchased from Siemons Co., has been found to be satisfactory. A separate microprocessor controller  61  is connected to main electronic control unit  25  by way of an analogic input/output line and an Encoder2 input which measures the position of the spindle through a digital signal. Controller  61  receives an electric motor signal and a resolver signal. The load cell force signal is sent directly from the tool connection  105  to the main electronic control unit  25  while the proximity switch signals (from the feeder, feed tube and spindle home position sensors) are sent from the tool connection  71  through an input/output delivery microprocessor module  601  and then to main electronic control unit  25 . Input/output delivery microprocessor module  601  actuates error message indication lamps  603 , receives a riveting start signal from an operator activatable switch  605  and relays control signals to feeder  27  from main electronic control unit  25 . An IBS/CAN gateway transmits data from main electronic control unit  25  to a host system which displays and records trends in data such as joint quality, workpiece thickness and the like. Controller  61  is also connected to a main power supply via fuse  607 . 
     FIG. 16  is a force/distance (displacement) graph showing a sequence of a single riveting operation or cycle. The first spiral spring distance range is indicative of the force and displacement of punch  123  due to light spring  128 . The next displacement range entitled hold down spring, is indicative of the force and displacement generated by heavy spring  141 , clamp  143  and the associated clamping nose piece  249 . Measurement of the sheet metal/workpiece thickness occurs at a predetermined point within this range, such as 24 millimeters from the home position, by way of load cell  103  interacting with main electronic control unit  25 . In the next rivet length range, the rivet length is sensed and determined through load cell  103  and main electronic control unit  25 . The middle line shown is the actual rivet signature sensed while the upper line shown is the maximum tolerance band and the lower line shown is the minimum tolerance band of an acceptable rivet length for use in the joining operation. If an out of tolerance rivet is received and indicated then the software will discontinue or “break off” the riveting process and send the appropriate error message. 
     FIG. 17  shows a force versus distance/displacement graph for the rivet setting point. The sensed workpiece thickness, the middle line, is compared to a prestored maximum and minimum thickness acceptability lines within the main electronic control unit  25 . This occurs at a predetermined distance of movement by the clamp assembly from the home position or other initialized position. The rivet length (or other size or material type) signature is also indicated and measured. Load cell  103  senses force of the clamp assembly and punch assembly. The workpiece thickness is determined by comparison of a first sensed force value at a preset displacement versus a preprogrammed force value at that location. Subsequently sensed force values are also compared to preset acceptable values; these subsequent sensed force values are indicative of rivet size and joint quality characteristics. The computer is always on-line with the tool and process in a closed-loop manner. This achieves a millisecond, real time control of the process through sensed values. 
     FIGS. 18   a – 18   d  show a flow chart of the computer software used in the main electronic control unit  25  for the preferred embodiment riveting system of the present invention. The beginning of the riveting cycle is started through an operator actuated switch, whereafter the system waits for the spindle to return to a home position. From a prestored memory location, a rivet joint number is read in order to determine the prestored characteristics for that specific joint in the automotive vehicle or other workpiece (e.g., joint number 16 out of 25 total). Thus, the workpiece thickness, rivet length, rivet quality and force versus distance curves are recalled for comparison purposes for the joint to be riveted. 
   Next, the software determines if a rivet is present in the head based upon a proximity switch signal. If not, the feeder is energized to cause a rivet to be fed into the head. The spindle is then moved and the workpiece is clamped. The plate or workpiece thickness is then determined based on the load cell signals and compared against the recalled memory information setting forth the acceptable range. If the plate thickness is determined to be out of tolerance, then the riveting process is broken off or stopped. If the plate thickness is acceptable for that specific joint, then the rivet length is determined based on input signals from the load cell. If the punch force is too large, too soon in the stroke, then the rivet length is larger than an acceptable size, and vice versa for a small rivet. The riveting process is discontinued if the rivet length is out of tolerance. 
   The spindle is then retracted after the joint is completed. After the spindle is opened or retracted to the programmed home position, which may be different than the true and final home position, indicator signals are activated to indicate if the riveted joint setting is acceptable (OK), if the riveting cycle is complete (RC), and is ready for the next rivet setting cycle (reset OK). It should also be appreciated that various resolver signals and motor power consumption signals can also be used by second microprocessor  61  to indicate other quality characteristics of the joint although they are not shown in these flow diagrams. However such sensor readings would be compared against prestored memory values to determine whether to continue the riveting process, or discontinue the riveting process and send an error signal. Motor sensor readings can also be used to store and display cycle-to-cycle trends in data to an output device such as a CRT screen or printout. 
     FIG. 18   d  shows a separate software subroutine of error messages if the riveting process is broken off or discontinued. For example, if the plate thickness is unacceptable, then an error message will be sent stating that the setting is not okay (NOK) with a specific error code. Similarly, if the rivet length was not acceptable then a not okay setting signal will be sent with a specific error code. If another type of riveting fault has been determined then another rivet setting not okay signal will be sent and a unique error code will be displayed. 
   Another alternate embodiment riveting system is illustrated in  FIG. 19 . A robotically controlled riveting tool  801  is essentially the same as that disclosed with the preferred embodiment. However, two separate rivet feeders  803  and  805  are employed. Rivet feeders  803  and  805  are of the same general construction as that disclosed with the preferred embodiment, however, the rivet length employed in the second feeder  805  is longer (such as 5 millimeters in total length) than that in the first feeder  803  (such as a total rivet length of 3 millimeters). Each feeder  803  and  805  transmits the specific length rivets to a selector junction device  807  by way of separate input feed tubes  809  and  811 . Selector device  807  has a pneumatically actuated reciprocating slide mechanism which is electrically controlled by a main electronic control unit  813 . When main electronic control unit  813  recalls the specific joint to be worked on, it then sends a signal to selector device  807  as to which rivet length is needed. Selector device  807  subsequently mechanically feeds the correct rivet through a single exit feed tube  815  which is connected to a receiver  817  of riveting tool  801 . 
   Thus, a single riveting tool can be used to rivet multiple joints having rivets of differing selected sizes or material characteristics without the need for complicated mechanical variations or multiple riveting tool set ups. The software program within main electronic control unit  813  can easily cause differing rivets to be sent to the single riveting tool  801 , while changes can be easily made simply by reprogramming of the main electronic control unit. This saves space on the crowded assembly plant line, reduces mechanical complexity and reduces potential failure modes. 
   The accuracy of riveting, as well as measurements in the preferred embodiment, are insured by use of the highly accurate electric servo motor and rotary-to-linear drive mechanism employed. For example, the rivet can be inserted into the workpieces with one tenth of a millimeter of accuracy. The control system of the present invention also provides a real time quality indication of the joint characteristics, rather than the traditional random sampling conducted after many hundreds of parts were improperly processed. Thus, the present invention achieves higher quality, greater consistency and lower cost riveted joints as compared to conventional constructions. 
   While various embodiments have been disclosed, it will be appreciated that other configurations may be employed within the spirit of the present invention. For example, the spindle and punch holder may be integrated into a single part. Similarly, the nose piece and clamp can be incorporated into a single or additional parts. Belleville springs may be readily substituted for compression springs. Additional numbers of reduction gears or planetary gear types can also be used if a gear reduction ratio is other than that disclosed herein; however, the gear types disclosed with the preferred embodiment of the present invention are considered to be most efficiently packaged relative to many other possible gear combinations. A variety of other sensors and sensor locations may be employed beyond those specifically disclosed as long as the disclosed functions are achieved. Additionally, analog or other digital types of electronic control systems, beyond microprocessors, can also be used with the riveting tool of the present invention. The electronic control units of the monitor and delivery module can be part of or separate from the main electronic control unit. It is also envisioned that more than two workpiece sheets can be joined by the present invention, and that the workpieces may be part of a microwave oven, refrigerator, industrial container or the like. While various materials and dimensions have been disclosed, it will be appreciated that other materials and dimensions may be readily employed. It is intended by the following claims to cover these and any other departures from the disclosed embodiments which fall within the true spirit of this invention.