Abstract:
A friction welding system includes a first spindle and a second spindle. The first spindle and the second spindle securely locate a first part and a second part, respectively. The first spindle defines a first axis. The second spindle defines a second axis. A tailstock fixture is disposed along the first and second axes to securely locate a third part. A motor rotates the first and second spindles. A controller controls the motor and the angular orientation of the first and second spindles. The first spindle is moveable along the first axis. The second spindle is movable along the second axis. The first part and the second part can contact the third pat while rotating to effect two separate fiction welds. The controller controls the rotational position of the first spindle and the second spindle upon completion of the weld.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 60/697,070, filed Jul. 6, 2005, the entire contents of which are hereby expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to a friction welder and, more specifically, to a friction welder capable of welding three work-pieces together along two weld interfaces to form a single component.  
       BACKGROUND OF THE INVENTION  
       [0003]     Friction welding machines are generally known in the art. In a friction weld, heat is generated by rubbing two parts together until the material at the interface between the two work-pieces reaches a plastic state. The two parts are then forged together under pressure to finalize the weld and expel gases, thus forming a single component having an integral bond. A friction weld can typically be formed in a very short period of time compared to more conventional arc welding methods, and thus friction welds are less labor intensive, more uniform and more cost effective than conventional methods. Friction welders are especially well-suited for welding round bars, tubes, or other generally round shapes to one another, or for welding round parts to flat plates, disks, gears, etc. The friction welding process may be used used to produce automotive drive shafts, automotive air bag canisters, gear shafts, engine valves, and other parts, and in other applications in which a high quality weld is desired.  
         [0004]     On one known friction welder, a first part or work-piece is mounted to a rotating chuck or spindle assembly, while a second part or work-piece is mounted to a stationary chuck or tailstock. A drive motor accelerates the rotating chuck to a desired speed, and the parts are then forced together under pressure, such that the friction between the two parts produces enough heat to produce a material flux. The parts are then forged together under pressure, which expels gas and produces a fine grain weld.  
         [0005]     Some automotive drive shafts are made using the friction welding process. Typically, a first yoke and a second yoke are welded to the opposite ends of a central tube. This process is typically performed in two steps. Ideally, the yolks are located approximately orthogonal to one another. Therefore, after the first yoke has been welded to the central tube, one welding the second yoke to the central tube the orientation of the second yoke relative to the first yoke needs to be controlled. This orientation may be controlled using an orientation system. One such orientation system can be found in U.S. Pat. No. 5,858,142, the entire disclosure of which is incorporated by reference herein and which is assigned to the assignee of the present disclosure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a perspective view of a friction welding system assembled in accordance with the teachings of the present invention.  
         [0007]      FIG. 1A  is an elevational view in schematic of a driveshaft formed from three individual work-pieces using the system of  FIG. 1 .  
         [0008]      FIG. 2  is a fragmentary view and perspective of a support table for supporting the friction welding system of  FIG. 1 .  
         [0009]      FIG. 3  is a cross-sectional taken along line  3 - 3  of  FIG. 2 .  
         [0010]      FIG. 4  is an enlarged fragmentary view in perspective of a slide table.  
         [0011]      FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 4 .  
         [0012]      FIGS. 6A-6F  are enlarged cross-sectional views in schematic taken at an interface between either one of the rotating work-pieces and the fixed work-piece and illustrating an exemplary weld sequence.  FIGS. 6A-6F  also illustrate the axial alignment between the rotating work-piece and the fixed work-piece when the weld cycle is complete despite potential axial runout or mis-alignment between the work-pieces experienced during the weld sequence.  
         [0013]      FIG. 7  is a schematic illustration of a friction welding control system incorporating the teachings of the present invention.  
         [0014]      FIG. 8  is a pow chart of an exemplary main control program used to control the friction welder system illustrated in  FIG. 1 .  
         [0015]      FIG. 9  is a schematic diagram of the amplifier circuit of the control loop shown in  FIG. 8 .  
         [0016]      FIG. 10  is a spindle profile curve in graphic form which indicates the desired spindle speed as a function of time during the entire weld process.  
         [0017]      FIG. 11  is an enlarged schematic view of a first work-piece secured in a spindle and another work-piece secured in a center clamp. 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  illustrates an exemplary friction welder  10 . The friction welder  10  includes a first spindle assembly  12 , a second spindle assembly  14 , and a center clamp assembly  15 . The first spindle assembly  12  includes a rotatable spindle  12   a  having a collet  12   b  (the collet  12   b  is obscured in  FIG. 1  but is similar to collet  14   b  shown in  FIG. 1 ). The spindle assembly  14  includes a rotatable spindle  14   a  having a collect  14   b . The collets  12   b  and  14   b  may be conventional, and are arranged so that the collet  12   b  secures a first work-piece  16  to the spindle  12   a , while the collet  14   b  secures a second work-piece  18  to the spindle  14   a . The first and second work-pieces  16  and  18  are visible in  FIG. 1A . The first work-piece  16  includes transverse axis  16   a  extending into the plane of the Figure, while the second work-piece  18  includes a transverse axis  15   a  extending vertically in  FIG. 1A . The center clamp assembly  15  includes a pair of claps  15   a  and  15   b , which are arranged to secure a third work-piece  20 . In the disclosed example, the first and second work-pieces  16  and  18  are yokes of the type commonly employed on drive shafts, while the third work-piece  20  is a shaft or tube. Accordingly, using the disclosed friction welder  10 , the first, second and third work-pieces may be assembled to form a drive shaft  22 . As will be explained in greater detail below, the orientation of the work-piece  16  relative to the work-piece  18  preferably is controlled such that the orientation of the transverse axis  16   a  of the work-piece  16  relative to the transverse axis  18   a  of the work-piece  18  is controlled. It will be understood that, in many applications, these transverse axes  16   a ,  18   a  will be oriented orthogonal relative to one another in the finished drive shaft  22 . Additionally, as will be described in more detail below, the final positioning of the first and second work-pieces  16 ,  18  relative to the third work-piece  20  can be accurately controlled during the weld process such as to control the final length of the drive shaft  22  within a predetermined tolerance. The first and second spindle assemblies  12  and  14 , along with the individual clamps  15   a  and  15   b  of the center clamp assembly  15 , are mounted to a table  24 .  
         [0019]     Referring still to  FIG. 1 , the clamp  15   a  includes two individual pieces  25   a  and  25   b . The clamp  15   a  includes an actuator  26 , and the actuator is mounted to the clamp  15   a  such that, by actuating the actuator  26 , the individual pieces  25   a  and  25   b  can be separated or brought together as desired in order to release or secure the third work-piece  20  in the clamp  15   a . Similarly, the clamp  15   b  includes a pair of individual pieces  28   a  and  28   b , and also includes an actuator  30 . The actuator  30  is mounted to the clamp  15   b  such that, by actuating the actuator  30 , the individual pieces  28   a  and  28   b  can be separated or brought together as desired in order to release or secure the third work-piece  20  in the clamp  15   b . In accordance with the disclosed example, both spindle assemblies  12  and  14  are oriented along or parallel to an X axis. The actuators  26  and  30  are oriented parallel to a Z axis. Further, in the disclosed example, the axes of each of the individual work-pieces  16 ,  18  and  20  preferably are oriented along the X axis. One or both of the clamps  15   a  and  15   b  may be adjustably mounted to the table  24 . In the disclosed example, the clamp  15   a  is adjustably mounted on a set of rails  32  oriented parallel to the X axis, such that the distance between the clamp  15   a  and the clamp  15   b  can be adjusted. The precise location of the individual pieces  25   a ,  25   b  and  28   a ,  28   b  of the center clamps  15   a  and  15   b  can be controlled along the Y and Z axes using suitable shims.  
         [0020]     The spindle assembly  12  includes a pair of guide rails  34  which extend to the clamp  15   a . A pair of actuators  36   a  and  36   b  are mounted to the spindle assembly  12 , such that, upon actuating the actuators  36   a  and  36   b , the spindle assembly  12  is movable in a direction parallel to the X axis, such that the spindle assembly  12  can be moved closer to the clamp  15   a . Similarly, the spindle assembly  14  includes a pair of guide rails  38  which extend to the clamp  15   b . A pair of actuators  40   a  and  40   b  are mounted to the spindle assembly  14 , such that, upon actuating the actuators  40   a  and  40   b , the spindle assembly  14  is movable in a direction parallel to the X axis, such that the spindle assembly  14  can be moved closer to the clamp  15   b . Accordingly, it will be appreciated that during the weld process, the clamps  15   a  and  15   b  are held stationary and secure the third work-piece  20 , while the rotating spindle assemblies  12  and  14  are movable along the X axis so as to bring the rotating first and second work-pieces  16  and  18  disposed in the spindle assemblies into contact with the third work-piece  20  secured by the clamps  15   a  and  15   b  of the center clamp assembly  15 .  
         [0021]     A drive motor  42  (not shown in  FIG. 1  but visible in  FIG. 3 ) is mounted to the table  24 , and includes a drive train  44  that (also not shown in  FIG. 1  but visible in  FIG. 3 ) operatively engages each of the spindle assemblies  12  and  14  in order to transmit rotation of the drive motor  42  to the spindle assemblies  12  and  14  in order to rotate the spindle assemblies. In the disclosed example, the drive train  44  includes a drive belt  46  engaging a pulley  48  on the spindle  12   a  of the spindle assembly  12 , and also includes a drive belt  50  engaging a pulley  52  on the spindle  14   a  of the spindle assembly  14 . Preferably, in order to protect the drive motor  42  and various components of the drive train  44 , a rollup cover  54  may be provided at each end of the table  24 . The rollup cover  54  is connected to the adjacent spindle assembly  12  or  14  so that the cover  54  pays out from a supply roll in response to movement of the relevant spindle assembly. Similarly, a protective bellows  56  or other suitable cover may be provided between each spindle assembly  12  or  14  and the center clamp assembly  15 .  
         [0022]     Referring now to  FIGS. 2 and 3 , the table  24  is shown. A top side  58  of the table  24  includes a pair of openings  60  and  62 . The openings  60  and  62  are sized to permit portions of the drive train  44 , for example the drive belt  46  and the drive belt  50 , to extend upwardly from an interior of the table  24  in order to engage the relevant spindles  12   a  and  14   a . Moreover, the openings  60  and  62  are long enough to permit the movement of the spindle assemblies  12  and  14  along the X axis such that the drive belts  46  and  50  will not encounter any interference. The table  24  may also include adjustable feet  64  to permit leveling of the table  24  on the floor or other support surface. An actuator  65  may be provided in order to move the clamp  15   a  relative to the clamp  15   b  along the rails  32 .  
         [0023]     As shown in  FIG. 3 , the drive motor  42  is disposed inside the table  24 , and operates to simultaneously rotate the spindles  12   a  and  14   a  via the drive train  44 . Only a portion of the drive train  44  is visible in  FIG. 3  (the drive belts  48  and  50 , and their associated pulleys, are visible in  FIG. 1 ). The motor  42  includes an output shaft  74 , and a drive sprocket  76  or other suitable pulley is mounted to the shaft  74 . A drive belt  78  connects the drive sprocket  76  to a second drive sprocket  80 . In this example, the first and second drive sprockets  76 ,  80  have the same diameter, although it is possible to use different diameters in order to change the gear ratio. The drive sprocket  80  engages a drive shaft  82  that is rotatably mounted within the table  24 . In the disclosed example, the drive shaft  82  is not a single piece and does not extend the length of the table  24 . Instead, the drive shaft  82  includes a right shaft  84  mounted to a right end of the drive shaft  82 , and further includes a left shaft  86  mounted to a left end of the drive shaft  82 . The right shaft  84  and left shaft  86  are coupled to the drive shaft  82  by suitable coupling assemblies, which are identified by reference numeral  88 . Consequently, rotation of the drive shaft  82  rotates both the right shaft  84  and left shaft  86 . Preferably, the shafts  82 ,  84  and  86  are supported by suitable bearings  90  mounted to the table  24 . In this example, the right shaft  84  includes a splined right end  91  and the left shaft  86  includes a splined left end  93 . Alternatively, the left and right ends  91  and  93  could include gears. As a further alternative, a single-piece drive shaft  82  could be used that the length of the table  24 . As used herein, the term drive shaft encompasses both a single drive shaft and a plurality of shafts coupled together.  
         [0024]     A control system  92  is operatively coupled to the drive motor  42  in order to direct operation of the motor  42 , including controlling starting, stopping, the rotational speed, and the angular orientation, during operation of the friction welder  10 . The control system  92 , using feedback from the motor  42 , can read the speed at which the motor  42  is rotating and direct the motor to adjust its speed if necessary. Additionally, the control system  92  may be operatively coupled to the actuators  36   a ,  36   b ,  40   a ,  40   b  coupled to the spindle assemblies  12 ,  14 , as well as transducers (identified by reference numeral  249  in  FIG. 7 ) for monitoring and controlling the position of the first and second work-pieces  16 ,  18 . The control system  92  can be a personal computer, a PC-compatible industrial computer, a programmable logic controller, a combination of the two, or any other structure that can direct the operation the motor  42  and the actuators  36   a ,  36   b ,  40   a ,  40   b.    
         [0025]     Referring now to  FIGS. 4 and 5 , a portion of the drive train  44  for driving the spindles  12   a  and  14   a  is shown. The portion of the drive train  44  that drives the spindle assembly  12  is shown, although it will be appreciated that the portion of the drive train that drives the spindle assembly  14  may be substantially similar. A slide table  94  includes a pair of guides  100  which are sized and shaped to engage the rails  32  that slidably support the spindle assembly  12 . The second spindle assembly  14  also includes a slide table  96  ( FIG. 1 ), which may be substantially similar to the slide table  94  of  FIGS. 4 and 5 . A gear and bearing assembly  102  is mounted to an underside of the slide table  94 . The gear and bearing assembly  102  includes a central aperture  103  that is adapted to engage the splined end  91  of the right shaft  84 . The gear and bearing assembly  102 , along with the central aperture  103 , are arranged so that as the slide table  94  moves along the rails  32 , the splined end  91  of the right shaft  84  slides through the central aperture  103 . The gear and bearing assembly  102  also includes a lower drive gear or pulley  104  and may also include an idler pulley  126 . The drive belt  46  engages both pulleys  104  and  126 , and also engages the pulley  48  carried by the spindle  12   a . Accordingly, while the drive shaft is rotating, the slide table  94  and hence the entire spindle assembly  12  can slide along the rails  32  without interrupting the operation of the drive train  44  and without interrupting the rotation of the spindle  12   a . Suitable bearings are provided, such as bearings  106  that support the pulley  104 , and bearings that support the pulley  126 . The pulley  104  may include a set of teeth  108  or serrations in order to ensure that rotation of the pulley  104  is transmitted into movement of the drive belt  46 . The idler pulley  126  may be mounted to a slide plate  128  to permit adjustment of the tension on the drive belt  46 . Suitable slots  120  and fasteners  122  can be provided to permit adjustment, with the slots extending generally parallel to a Z axis ( FIG. 1 ). A pair of locator bolts  124  may be mounted to the slide plate  128 , with the locator bolts bearing against a side of the slide plate  94 . Rotation of the locator bolts  124  pushes the slide plate  128  in the Z direction, thereby altering the tension on the drive belt  46 .  
         [0026]     The spindle assembly  12  can be mounted to the slide plate  94  using known fasteners such as bolts and holes  98  in the slide plate  94 . A set of locator blocks  130  may be disposed on the slide plate  94  in suitable recesses (not shown). In certain conditions that will be described herein, the location of the spindle assembly  12  relative to the slide plate  94  may require adjustment. Accordingly, shims  132  may be provided, and the shims  132  may be inserted between a lower potion of the spindle assembly  12  and top portion of the slide plate  94 . Thus, it will be appreciated that the position of the spindle  12   a  of the spindle assembly  12  can be adjusted in the Y and Z directions.  
         [0027]     Referring now to  FIGS. 6A-6F , the alignment of the first work-piece  16  relative to the third work-piece  20  is shown. It will be understood that, when the first work-piece  16  is disposed in the collett  12   b  of the spindle  12   a , and axis of the first work-piece  16  might not be precisely aligned with the rotational axis of the spindle  12   a . This possible misalignment may create a certain amount of runout, which is represented in each of  FIGS. 6A-6F  by the distance between the axis  134  (the axis of the first work-piece  16 ) and the axis  136  (the axis of the third work-piece  20 ). In other words, as shown in  FIG. 6A , the axis  134  might not line up with the axis  136 , and thus the axes  134  and  136  are not coaxial. The same situation can occur between the second work-piece  18  and the other end of the third work-piece  20 . The user can use the shims  132  (described above with respect to  FIGS. 4 and 5 ) to adjust the position of the spindle assembly  12  in the Y and Z directions, which effectively adjusts the position of the axis  134  relative to the position of the axis  136 . The user can also perform this shimming process in a similar manner with respect to the second spindle assembly  14 .  
         [0028]     However, despite adjustments, the axis  134  of the first work-piece  16  may not be precisely aligned with the rotational axis of the spindle  12   a  for a number of reasons. First, the collet  12   b  may not secure the first work-piece  16  in a position such that the axis  134  of the first work-piece is coaxial with a rotational axis  135  of the spindle  12   a  (shown in  FIG. 11  and which is the misalignment situation described above), and also may not secure the first work-piece  16  in a position such that the axis  134  of the first work-piece  16  is precisely parallel to the rotational axis  135  of the spindle  12   a . Such a situation is illustrated schematically in  FIG. 11 . Thus, the first work-piece  16  may not revolve around its own axis  134 , and may instead rotate in a path  138  outlined in  FIG. 6B  (this path of rotation is exaggerated for ease of understanding). Thus, as the first work-piece  16  rotates during the weld process, it will follow the path  138  shown in  FIGS. 6B-6F . As can be seen, there is only a single angular orientation—or a narrow range of possible angular orientations—in which the axis  134  of the first work-piece  16  is aligned with, or at least most closely aligned with (within an acceptable tolerance), the axis  136  of the third work-piece  20 . As is shown in  FIGS. 6B through 6F , when the axes  134  and  136  are misaligned, this misalignment can be determined by rotating the spindle  12   a  and measuring the misalignment using known methods. Using this process, the user can determine which rotational position of the spindle  12   a  results in the smallest misalignment. This rotational spindle position is then the desired spindle orientation. Further, once the user is able to determine the smallest difference, the user can then adjust the position of the spindle  12   a  relative to both the Y and Z axes as discussed above. Thereafter, using the control system described herein, it is then possible to complete the weld process with the spindle  12   a  stopped in the desired spindle orientation. In other words, in order to ensure that the weld process is completed with the least amount of misalignment between the axis  134  of the first work-piece  16  in the axis  136  of the third work-piece  20 , the control system  92  must be used so that rotation of the first work-piece  16  stops at the desired spindle orientation when the weld process is finished.  
         [0029]     Both spindles  12   a  and  14   a  rotate at the same time and in the same direction by virtue of their connection to the drive shaft  82  of the drivetrain  44 . Further, both spindle assemblies  12  and  14  can be adjusted relative to the Y and Z axes independently. Consequently, as long as the first and second work-pieces  16  and  18  have the proper starting orientation relative to one another, then the first and second work-pieces  16  and  18  will have the same ending orientation relative to one another, by virtue of the fact that both spindles  12   a  and  14   a  are driven by the same drivetrain  44 . Moreover, by controlling the angular orientation of the spindles  12   a  and  14   a  at the end of the weld process, both spindles  12   a  and  14   a  will stop at the desired spindle orientation.  
         [0030]     As shown in  FIG. 7 , the control system  92  includes a computer  226  or PLC (or both) which is operatively connected to a motion controller  228  and at least one transducer  249 . In one embodiment, the at least one transducer  249  includes a pair of transducers that may include, for example, position sensors adapted to detect the position of the spindle assemblies  12 ,  14 . The transducers  249  therefore in one embodiment would be disposed on the table  24  or directly on the spindle assemblies  12 ,  14 . The motion controller  228  is operatively connected to a power amplifier  230 , the drive motor  42  which includes a tachometer  234 , and position sensor  236 . The motion controller  228 , power amplifier  230 , drive motor  42 , tachometer  234 , and position sensor  236  together form a control loop  240 . The drive motor  42  is preferably a variable speed drive motor commonly employed in the art, and the tachometer  234  and position sensor  236  are likewise commonly employed in the art. Preferably, the position sensor  236  is calibrated to measure the angular position of the output shaft  74  as it rotates about its axis in increments of a rotation, and position sensor  236  converts the detected position to an actual position command  237 . The position sensor  236  also tracks the actual number of rotations during each of the weld phases, such as the actual acceleration, pre-heat, heat and forge rotations, respectively, as discussed below. Preferably, each complete rotation of the output shaft  74  can be broken into a thousand discrete angular positions. Based on a number of material variables input by the operator, such as the material weight, dimensions, and thickness of first, second and third parts, the host computer  226  generates a desired spindle profile (shown in  FIG. 10 ) which represents the desired rotational speed of the output shaft  74  at any moment during the weld cycle. The desired final angular position of the first work-piece  16  and second work-piece  18  relative to the third work-piece  20  is input into the computer  226  via an input register  238  and is communicated to motion controller  228 . The operator inputs the material variables mentioned above into the host computer  226 , which then calculates the desired total number of spindle rotations required between the actual starting position and the desired final position. The total number of desired rotations includes the desired acceleration rotations, the desired pre-heat rotations, the desired heat rotations, and the desired forge rotations.  
         [0031]     The tachometer  234  generates a signal which indicates the actual speed (see  FIG. 9 ) of the drive motor  42 , while the position sensor  236  (see  FIG. 7 ) generates a signal which indicates the actual angular position of the output shaft  74 . Based on the desired final position and the actual position, the motion controller  228  generates a motion command  254  or speed signal which is communicated to the power amplifier circuit  230  and then to drive motor  42 . Thus, a control loop  240  is formed which continuously generates feedback regarding the actual speed and the actual position of the output shaft  74 , which matches the actual speed and position of the first work-piece  16 . Ideally, actual speed closely approximates desired speed, while actual position closely approximates the desired position. The desired position, which is generated by the host computer  226  as explained below, represents the desired angular position of the output shaft  74  relative to its axis of rotation at any particular point in time during the weld cycle. Any differences between actual speed and/or position and desired speed and/or position are corrected by the control loop  240  as discussed in greater detail below.  
         [0032]     Referring now to  FIG. 9 , the amplifier circuit  230  includes summation node or junction  258  which sums the difference between the speed signal  254  and the actual speed  235 . The junction  258  generates a difference signal  259 , which is communicated to velocity amplifier  260 , which in turn generates a current command signal  262 . Current command signal  262  is communicated to summation node or junction  264 , which sums the difference between current command signal  262  and current feedback signal  266  from motor  42 . Junction  264  generates a difference signal  265 , which is communicated to amplifier  268 , which is connected to the drive motor  42 .  
         [0033]      FIG. 8  shows a flow chart of the weld cycle employing orientation control in accordance with the friction welder  10  disclosed herein. Upon commencement or start  282  of the weld cycle, the computer  226  performs a series of pre-weld calculations  293  stored in output register  270 . The values for each of the output variables depend on a number of variables programmed into the input register  238 . The input variables include, for example, the type of material to be welded, the weight of the rotating work-piece, and the geometric or size properties of the work-pieces to be welded together. The input register  238  also includes the desired final angular orientation between the work-pieces relative to their common axis, the lengths of the first and second work-pieces  18 ,  20 , respectively, the length of the third work-piece  20 , and the desired length for the finished product. The computer  226  obtains values based on input values and performs calculations to determine the parameters of the weld process, including the number of forge rotations required for the spindle to stop at the desired angular position at the calculated forge force level.  
         [0034]     When the operator initiates the start command  282 , the computer  226  generates the spindle profile curve  320  shown in  FIG. 10 , and also sets the start position of slide table  94  so that the total travel of the slide table  94  will match the desired upset distance. Before the spindle rotation begins, a subroutine  289  causes the motion controller  228  to designate the position of the output shaft  74  a setpoint or “home” mark and communicates a go command to the motion controller  228 , which in turn communicates the speed signal  254  to the drive motor  42 , and absent any positional errors detected by subroutine  289 A, commencing the rotation of the output shaft  74 .  
         [0035]     As shown in  FIG. 10 , the first phase of the weld cycle is the acceleration phase  290 , during which the output shaft  42  is accelerated to a desired rotational speed  253 . During acceleration phase  290 , subroutine  292  (see  FIG. 8 ) via control loop  240  constantly compares the actual spindle acceleration rotations, in increments of 1/4000th of a revolution, to the desired spindle acceleration rotations as dictated by the spindle profile  320  for that particular moment during the acceleration phase  290 . While the increments have just been described as including 1/4000th of a revolution, alternative embodiments may include any rotational increments including, for example, 1/1000th, 1/10,000th, or any other increment capable of serving the principles of the present disclosure. The motion controller  228  makes the necessary speed adjustments via speed signal  254  as required, and the comparison by subroutine  292  continues until the acceleration phase  290  is complete. Subroutine  292  typically triggers the completion of the acceleration phase by monitoring the total spindle rotations for that phase, but may also be programmed to trigger the end of the first phase  290  based on elapsed time.  
         [0036]     Upon completion of another subroutine  292 A checking for errors and any necessary in-process corrections, a signal is sent to computer  226  which indicates that the second phase  296  is about to commence. Phase  296 , which commences at a time indicated by time T 1  in  FIG. 10 , includes both a pre-heat phase  296 A and a heating phase  296 B. Phase  296 B terminates when the material at the interface between the first work-piece  16  and the third work-piece  20  has reached a plastic state, which should coincide with the completion of the desired pre-heat rotations and the desired heating rotations, and which signals the end of phase  296 . At the beginning of phase  296 , the output shaft  74  is rotating the spindles  56  at the desired rotation or weld speed, and the motion controller  228  via control loop  240  maintains the rotation the output shaft  74  at this desired speed. During the pre-heat stage  296 A, the computer  226  sends a force command  285  to the actuators  36   a ,  36   b , which moves the spindle assembly  12  and brings the first work-piece  16  into contact with the third work-piece  20 . Generally, simultaneously, the actuators  40   a ,  40   b  move the spindle assembly  14  and bring the second work-piece  18  into contact with the third work-piece  20 . The first and second work-pieces  16 ,  18  are brought into contact with the third work-piece  20  at the pre-heat pressure force level  279 . Subsequently, at stage  296 B the actuators  36   a ,  36   b ,  40   a ,  40   b  cause the first and second parts  18 ,  20  to be continuously forced against the third work-piece  20  at a specific heat pressure force level  284 . The fiction between the first and second work-pieces  18 ,  20  against the third work-piece  20  immediately begins to heat the interface between the parts at the commencement of stage  296 A, and the heating continues through stage  296 B. During phase  296 , subroutine  298  via control loop  240  constantly compares the actual pre-heat rotations, in increments of 1/4000th of a revolution, to the desired pre-heat rotations, plus the desired number heating rotations to the actual heating rotations as dictated by the spindle profile  320  for that particular moment during phase  296 . When subroutine  298  detects that the total heating rotations have been completed with the material at the work-piece interface reaching a plastic state, subroutine  298  indicates the completion of phase  296  by sending a signal to computer  226 .  
         [0037]     Phase  296  is followed by a forge phase  300  which commences at time T 2 , and which terminates when the desired forge rotations have been completed and the spindle rotation has stopped, which occurs at time T 3 . During forge phase  300 , the output shaft  74  decelerates in accordance with profile curve  320 . Forge phase  300  is in turn followed by a dwell phase  302  in which the three parts  18 ,  20 ,  22  are maintained under pressure as the material at the interfaces cools, with phase  302  terminating at time T 4 . At the initiation of the forge phase  300 , motion controller  228  begins decelerating the output shah  74 , and subroutine  301  via control loop  240  constantly compares the desired forge rotations, in increments of 1/4000th of a revolution, to the actual forge rotations as dictated by the spindle profile  320  for that particular moment during phase  300 , and motion controller  228  makes the necessary speed adjustments via speed signal  254 . The comparison by subroutine  301  continues until the forge phase  300  is complete at time T 3 , at which point the output shaft  74  has stopped and the spindles  12   a ,  14   a  are at the desired final position. Also during the forge phase  300 , as the output shaft  74  begins to slow down, computer  226  sends a signal to the actuators  36   a ,  36   b ,  40   a ,  40   b,  which causes an increase in pressure between first work-piece  16  and third work-piece  20 , and between the second work-piece  18  and the third work-piece  20 , up to the forge force level  283 .  
         [0038]     When output shaft  74  stops, computer  226  measures the actual travel of the actuators  36   a ,  36   b ,  40   a ,  40   b  and compares the actual upset length to the desired upset length and determines if the actual upset is within bounds. Subroutine  310  monitors the time under forge pressure, and sends a signal to computer  226  when the dwell time is complete, which occurs at time T 4 . At time T 4 , the forge pressure is released and the weld cycle is complete. Finally, motion controller  228  reports any final positional errors to computer  26 , which can be communicated to the operator. Once again, the orientation may be controlled using an orientation system of the type found in commonly assigned U.S. Pat. No. 5,858,142, the entire disclosure of which is incorporated by reference herein.  
         [0039]     In this example a single drive shaft extends the length of the table and drives both the spindle  12   a  and the spindle  14   a  using a single drive motor. It has been found that such a design is robust and can accurately drive both spindles  12   a ,  14   a  relative to each other and also produce the driving force necessary to produce the weld. This has proved especially useful in materials difficult to friction weld such as aluminum. By using a single shaft to drive both spindles, the relationship between the first spindle  12   a  and the second spindle  14   a  is directly controlled.  
         [0040]     In use of the friction welder  10 , a user inserts the first work-piece  16  into the spindle assembly  12  and inserts the second work-piece  18  in the second spindle assembly  14 . In this particular example, the first and second parts  18 ,  20  are yokes for a drive shaft. As is known, yokes are required to be angularly disposed 90° from each other along the drive shaft. Thus, a user will place the second work-piece  18  in the second spindle assembly  14  such that this orientation is achieved. Because the spindles  12   a  and  14   a  of the first and second spindle assemblies  12  and  14  are operatively coupled through the drive train  44 , any rotation of either of the spindles  12   a  and  14   a  will result in an equal rotation of the other spindle. Thus, this relative angular orientation between the first work-piece  16  and the second work-piece  18  is maintained throughout the welding process.  
         [0041]     The third work-piece  20  is placed in the center clamp assembly  15 . To ensure that a quality weld is achieved, the first work-piece  16  is aligned with the third work-piece  20  by shimming the spindle assembly  12  as outlined above, so that the axis  134  of the first work-piece  16  is aligned with the axis  136  of the third work-piece  20 . This process is repeated with the second work-piece  18  so as to align the axis of them second work-piece  18  with the axis  136  of the third work-piece  20 . However, because the first work-piece  16  and/or the second work-piece  18  might not be perfectly aligned with the third work-piece  20  and at least some spindle orientations, the axis  134  of the first work-piece  16  may not remain aligned with the axis  136  of the third work-piece  20  at all spindle orientations while the first work-piece rotates  16  in the spindle assembly  12 . However, because the desired spindle orientation has been determined as outlined above, as long as the spindle is stopped at the desired spindle orientation the axes  134  and  136  of the first work-piece  16  and the third work-piece  20  will be properly aligned (within an appropriate tolerance). The same holds true for the alignment of the second work-piece  18  and the third work-piece  20 . During the welding process, the control system  92  constantly monitors the rotational position of the spindles to ensure that the spindles stop in the desired spindle orientation.  
         [0042]     Referring now to  FIG. 11  the spindle  12   a  of the spindle assembly  12  includes the rotational axis  135 . As is shown, the axis  134  of the first work-piece  16  might not be positioned in precise alignment with the axis  135  of spindle  12   a . This misalignment may be one cause of the runout illustrated in  FIGS. 6A-6F . However, by rotating the spindle  12   a  through a number of possible positions, such as, for example, four positions located in four rotational quadrants, the user may determine which rotational position results in the smallest misalignment, and may easily determine whether that smallest misalignment falls within acceptable tolerance. The size of the acceptable tolerance will vary in accordance with the end application of the welded work-pieces, and determining the exact size of the tolerance for the end application is a design consideration and may be determined by those of skill in the art. The rotational position of the spindle  12   a  that results in the smallest misalignment may be the desired spindle position, and may be both the starting point in the finishing point for the spindle during the weld process.  
         [0043]     In another example, a first motor drives the first spindle assembly and a second motor drives the second spindle assembly. Both the first motor and the second motor are controlled by a controller to ensure that the first and second spindles are being controlled relative to each other. In such a set up the controller can control the individual motors independently. As such, if the first work-piece  16  and the second work-piece  18  have different material properties, they may require a different weld process, i.e., higher forge force, faster revolutions, or the like. The controller can ensure that the final positions of the first part and the second part are the desired positions.  
         [0044]     As mentioned above, the controller  226 , in one embodiment, may be operatively coupled to the spindle assemblies  12 ,  14 , as well as a pair of transducers  249 . In the weld process described herein, the computer  226  measures the actual travel of the actuators  36   a ,  36   b ,  40   a ,  40   b  and compares the upset length to a desired upset length ad determines if the actual upset length is within acceptable bounds or tolerances. More specifically, the computer  226  may be in substantially continuous communication with the transducers  249  to substantially continuously monitor the position of the spindle assemblies  12 ,  14 . So configured, the friction welder  10  disclosed herein may be used to accurately and consistently control the final length of the final product, which includes a drive shaft  22  in the example disclosed hereinabove.  
         [0045]     In performing length control, the computer  226  may use the lengths of the first, second and third work-pieces  16 ,  18 ,  20 , as well as the final desired length of the drive shaft  22 . In standard operations, the desired final length will be known and input into the input register  238  by the operator. Additionally, the lengths of each of the first, second and third work-pieces  16 ,  18 ,  20  may independently be known, for example, through a pre-measuring process. In such a case, these values may also be entered into the input register  238  by the operator. However, the friction welder  10  could also perform a calibration process prior to beginning the weld process described above.  
         [0046]     Such a calibration process would be conducted subsequent to the operator inserting the work-pieces  16 ,  18 ,  20  into the friction welder  10 , but prior to beginning the weld process. With the work-pieces  16 ,  18 ,  20  secured into their respective spindles  12   a ,  14   a  and clamp assembly  15 , the operator would instruct the computer  226  to perform calibration. First, the computer  226  would instruct the actuators  36   a ,  36   b ,  40   a ,  40   b  to begin driving the first and second work-pieces  16 ,  18  toward the third work-piece  20 . During this period, the computer  226  constantly monitors the transducers  249  and therefore the position of the spindle assemblies  12 ,  14 . In one embodiment, for example, the computer  226  may take a positional reading from the transducers  249  every 1/1000th of a second. It should be appreciated, however, that these readings could be taken at nearly any frequency capable of serving the principles of the disclosure. From these readings, the computer  226  can calculate and monitor the rates at which each of the first and second work-pieces  16 ,  18  are traveling toward the third work-piece  20 . Once the first and second work-pieces  16 ,  18  abut the third work-piece  20 , their travel rates will drop to zero and the computer will instruct the actuators  36   a ,  36   b ,  40   a ,  40   b  to cease operation. At this point, the computer  226  takes a reading from the transducers  249 . This reading identifies the precise location of each of the spindle assemblies  12 ,  14  and enables the computer  226  to calculate an initial overall length of the combined work-pieces  16 ,  18 ,  20 . The computer  226  stores each of these values.  
         [0047]     Based on this initial overall length, the computer  226  would determine if the combined work-pieces  16 ,  18 ,  20  are sufficiently dimensioned to produce a final work-piece  22  having a final desired length within predetermined tolerances. For example, the initial overall length may be too short or too long to undergo an effective or desirable friction weld process. In conducting this determination, the computer  226  considers the initial overall length, the desired final length, and an average amount of length loss, for example, during the weld process. The computer  226  subtracts the average amount of length loss from the initial overall length to define a maximum final length. The computer  226  compares this maximum final length with the desired final length. If the computer  226  determines that the maximum final length is less than the desired final length within predetermined tolerances, the computer  226  issues a notification to the operator that the final product may not meet the dimensional specifications, thereby allowing the operator to substitute one or more of the work-pieces  16 ,  18 ,  20  with a different work-piece that would allow the tolerances to be met. In an alternative form, the computer  226  may even notify the operator of which of the three work pieces  16 ,  18 ,  20  needs replacement. In another form, the machine  10  may be automated and, therefore, may automatically replace one or more of the work pieces  16 ,  18 ,  20  without notifying the operator at all. However, if the maximum desired length is greater than or equal to the desired final length within predetermined tolerances, the computer  226  instructs the actuators  36   a ,  36   b ,  40   a ,  40   b  to back the first and second work-pieces  16 ,  18  away from the third work-piece  20  and begin the weld process.  
         [0048]     As mentioned above, in some circumstances, the maximum final length may be much greater than the final desired length, thereby defining a combination of work-pieces  16 ,  18 ,  20  too long to undergo an effective or desirable weld process. This may be because the welding process or quality of the weld may be compromised if too much material must be removed. In this situation, the computer  226  may alert the operator or automatically substitute one or more of the work-pieces  16 ,  18 ,  20 .  
         [0049]     After completing the calibration process, the computer  226  would then perform the weld process, as described above, with the additional feature of monitoring the length of the product. Specifically, during the friction weld process, the computer  226  continuously monitors the positions of the spindle assemblies  12 ,  14  via the transducers  249 . The computer  226  also continuously compares the current position of the spindle assemblies  12 ,  14  to the stored position of the spindle assemblies  12 ,  14  that was detected during the calibration process and associated with the initial overall length of the combined work pieces  16 ,  18 ,  20 . Therefore, while the interfaces between the first and third work-pieces  16 ,  20  and the second and third work-pieces  18 ,  20  reach a plastic state during the heating phase  296 B of the friction weld process described above with reference to  FIG. 10 , the computer  226  can closely monitor the change in length of the combined work-pieces  16 ,  18 ,  20  and adjust the process accordingly. For example, although the interfaces between the various work-pieces may be sufficiently plasticized to accommodate the transition from the heating phase  296 B to the forge phase  300 , as identified in  FIG. 10  and describe above, if the computer  226  determines that the overall length of the product is not within the predetermined tolerances, the computer  226  may prolong the heating phase  296 B by continuing to instruct the actuators  36   a ,  36   b ,  40   a ,  40   b  to force the first and second work-pieces  16 ,  18  into the third work-piece  20 . This will further dispose of material at the interfaces and decrease the final overall length of the drive shaft  22 . Through continued monitoring of the transducers  249 , the computer  226  can then determine when the overall length falls within the predetermined tolerances. Upon this occurring, the computer  226  can control the friction welder  10  to transition to the forge phase  300  and complete the weld.  
         [0050]     While the length of the final product has been described as being controlled by adjusting the time that the actuators  36   a ,  36   b ,  40   a ,  40   b  apply force to the first and second work-pieces  16 ,  18 , the computer  226  may control the final length by adjusting other parameters such as the amount of pressure or force applied by the actuators  36   a ,  36   b ,  40   a ,  40   b , the rotational velocity of the first and second spindles  12   a ,  14   a  and, therefore, the first and second work-pieces  16 ,  18 , or any other parameter associated with the machine  10  and capable of serving the disclosed purpose,  
         [0051]     Further yet, while the length-control process has been described as being based primarily on the continuous monitoring of the positions of the spindle assemblies  12 ,  14 , in an alternate form, the computer  226  may perform a pre-weld calculation to determine a weld process control algorithm for producing a final product meeting the desired final length within predetermined tolerances. This pre-weld calculation may be based on the initial overall length of the work-pieces, historical weld data, weld parameter calculations, or other information associated with the material, the final product, or the machine being used. Historical weld data may include, for example, average material loss, average beat generation, average weld strength, average time ranges for completing the welds, or any other useful information that may be recorded and stored for subsequent use. The weld parameter calculations may include calculations approximating velocity profiles, force profiles and time ranges, for example, based on the particular properties of the material used, the sizes of the work-pieces  16 ,  18 ,  20  or any other information.  
         [0052]     In a further alternative situation, during the weld process, a material defect in one or more of the work pieces  16 ,  18 ,  20  may cause the overall length of the work-pieces to rapidly and unexpectedly deteriorate. The computer  226 , through continuous monitoring of the transducers  249 , can identify this and adjust the weld process accordingly. For example, the computer  226  may adjust the rotational velocity of the first and second work-pieces  16 ,  18  or the movement of the spindle assemblies  12 ,  14  in an effort to reach the final desired length.  
         [0053]     As stated above, if the computer  226  determines during the calibration process that the initial overall length of the work-pieces  16 ,  18 ,  20  is insufficient to undergo the friction weld process and meet the desired final length, the computer  226  may notify the operator to enable the operator to substitute one or more of the work-pieces  16 ,  18 ,  20  for different work-pieces. Alternatively, however, in some circumstances, the operator may determine to continue with the weld process although the computer  226  indicates that the initial overall length may be insufficient. In this case, the computer  226  would instruct the friction welder  10  to proceed with the weld process. During the weld process, however, the computer  226  may still continuously monitor the positions of the spindle assemblies  12 ,  14 . During this continuous monitoring, the computer  226  may determine that by an adjustment of the weld process, the final desired length may be achieved. For example, if the computer  226  determines that the overall work-piece length is approaching the final desired length, the computer  226  may increase the rotational velocity of the first and second work-pieces  16 ,  18  to more quickly transition between the heating phase  296 B and the forge phase  300 . This determination by the computer  226  may be dependent on the type of material being friction welded, the geometry and/or the size and weight. Nevertheless, the computer  226  actively pursues a product having a desired final length within predetermined tolerances.  
         [0054]     Accordingly, it should be appreciated that while this length control process has been described as being implemented in conjunction with the orientation control process described above, the friction welder  10  disclosed herein may perform the length control process independently of the orientation control process. Furthermore, it should be appreciated that the friction welder  10  disclosed herein may be utilized to accurately and consistently orient the axes of multiple components, as well as accurately and consistently control the length of multi-component products such as the drive shaft  22  described hereinabove.  
         [0055]     The foregoing description is not intended to limit the scope of the invention to the precise form disclosed. It is contemplated that various changes and modifications may be made by those skilled in the art without departing from the spirit and scope of the invention.