turn now to the drawings and , initially , to fig1 which illustrates a typical work station 20 utilizing releasable clamping devices 22 embodying the present invention . as indicated in fig1 a plurality of the clamping devices 22 are mounted on a work table 24 at spaced locations for purposes of holding an elongated workpiece 26 supported on a work platform 28 which is integral with , but extends along and above , the work table 24 . as seen in fig1 and , even more clearly , in fig2 the workpiece 26 being held by the clamping devices 22 is of a complex shape , a typical instance being a stringer employed in the construction of modern aircraft wings having a contoured airfoil . as illustrated , the workpiece 26 includes an upright web 30 and laterally disposed upper and lower flanges 32 , 34 projecting outwardly from the web at vertically spaced locations . each clamping device 22 includes a c - clamp 36 for releasably , firmly , holding the workpiece 26 on an upper surface of the work platform 28 . each c - clamp 36 includes a distal member 38 extending to a tip end 40 , a proximal member 42 , and a bight member 44 interconnecting the distal member and the proximal member . in a preferred design , the longitudinal axis of the distal member and the longitudinal axis of the bight member form an acute angle . this relationship aids in the operation of the invention in a manner which will be explained below . in a manner to be described , c - clamp 36 is moved between a withdrawn position distant from the workpiece 26 as indicated by dashed lines in fig2 and a clamping position as indicated by solid lines . it will be appreciated that were the c - clamp 36 to merely follow a circular path between the withdrawn position and the clamping position , the tip end 40 would necessarily strike the upper flange 32 of the workpiece 26 . for this reason , it is necessary that the c - clamp 36 follow a modified path between its withdrawn position and its clamping position in a manner which will be described . it is also noteworthy that the workpiece 26 is generally representative of the initial form assumed when placed on the work platform 28 . at the conclusion of machining operations , it may actually assume a shape as indicated by dashed lines in fig2 and indicated by reference numeral 26a . thus , the c - clamp 36 must initially be able to firmly hold the workpiece 26 and , after the course of the operations thereon , be capable of firmly holding the workpiece 26a . the clamping device 22 must , therefore , be capable of holding the workpiece throughout the duration of time that operations are being performed on it . the mechanism which assures movement of the c - clamp 36 in the path just indicated will now be described . a pair of support plates 46 are flanged at 48 for fixed attachment by means of bolts 50 to the underside of the work platform 28 . the support plates 46 lie in parallel , spaced apart planes and each is provided with a shaped cam slot 52 which extends generally downwardly from the platform 28 . the cam slots 52 in the support plates 46 are identically sized and shaped and are positioned in parallel side - by - side relationship . an actuator 54 which is preferably a pneumatic cylinder is pivotally mounted as at 56 to the work table 24 at a location laterally spaced from the centerline of the work platform 28 . an extremity of an actuating rod 58 associated with the actuator 54 is pivotably connected by means of a pin 62 to one end of a bell crank 60 which , in turn , is pivotably mounted by means of a pin 64 fixed to , and extending between , the support plates 46 . an end of the bell crank 60 opposite that carrying the pin 62 is pivotally connected by means of a pin 66 to one end of a link 68 . the other end of the link 68 is pivotably mounted on a follower pin 70 which is fixed on a bifurcated end of the proximal member 42 of the c - clamp 36 and projects from opposite sides thereof . the projecting parts , or extremities , of the follower pin 70 are slidably engaged with the shaped cam slot 52 . a second follower pin 72 is also fixed to the proximal member 42 of the c - clamp 36 and projects from opposite sides thereof at a location spaced a short distance away from the follower pin 70 . as in the instance of the follower pin 70 , the projecting parts , or extremities , of the follower pin 72 are also slidably engaged with the shaped cam slot 52 . thus , it will be appreciated that the c - clamp 36 lies generally in a plane intermediate and parallel io to the support plates 46 and that the follower pins 70 , 72 are integral with the c - clamp and have parallel axes which are generally perpendicular to the c - clamp and project from opposite sides of the c - clamp for slidable reception with the cam slots 52 . the cam slot 52 is most clearly illustrated in fig5 . it includes a first run 74 which extends a substantially straight first distance and a second run 76 extending a substantially straight second distance and angularly disposed relative to the first run . the second run 76 is substantially upright , that is , perpendicular to an upper surface of the work platform 28 and the first and second runs mutually subtend an obtuse angle . a third run 78 intermediate the first and second runs 74 , 76 extends along an arc whose center is located outside of a region 80 subtended by the obtuse angle between the first and second runs . the cam slot 52 includes a fourth run 82 which communicates with the second run 76 at a location distant from the third run . the fourth run extends generally parallel to the second run 76 but laterally offset from the second run by reason of a jog 83 in the cam slot 52 . the fourth run 82 extends to a lower terminus 86 which is at an extreme distance from an upper terminus 84 of the first run 74 . as seen particularly well in fig3 and 4 and to a lesser extent in some of the other views , covers 88 are provided which are substantially coextensive with the outer surface of the support plates 46 . the covers 88 serve primarily to overlie the shaped cam slot 52 to prevent any interference with the movement therein of the follower pins 70 , 72 and undesirable entry of any foreign material . suitable fasteners 90 are employed for securing the covers 88 on the support plates 46 . with particular attention now to fig6 the operation of the invention resulting in desired travel of the c - clamps 36 will now be described . when the clamping device 22 is in a quiescent state , the actuating rod 58 is in a retracted position under the influence of the actuator 54 . in this position of the actuating rod 58 , the bell crank 60 assumes an extreme clockwise position about the pin 62 such that the c - clamp assumes a fully retracted position as indicated by a reference numeral 36a . in this position , the follower pin 72 engages the terminus 84 of the first run 74 ( fig5 and 6 ). as the actuator 54 causes the actuating rod 58 to move toward an extended position , it moves the c - clamp 36 to successive arbitrarily illustrated positions indicated , respectively , by reference numerals 36b , 36c , and 36d in fig6 . the position of the c - clamp 36 indicated by reference numeral 36d in fig6 is not intended to indicate an actual position of the c - clamp 36 but only a position which it could attain if it were not obstructed by the workpiece 26 or by the work platform 28 . because the workpiece 26 and the work platform 28 are actually present , the c - clamp 36 will actually assume a final , clamping , position in engagement with the lower flange 34 of the workpiece in the manner indicated in fig2 . lost motion is provided for by the actuator 54 . thus , with extension of the actuating rod 58 , the tip end 40 is advanced to a position proximate the upper flange 32 . then , by reason of the third run 78 , the tip end 40 is caused to skirt around an outermost extremity of the upper flange . the relationship between the distal member 38 and the bight 44 whereby together they subtend on an acute angle is of substantial added benefit in assuring that the tip end 40 will avoid striking the upper flange 32 of the workpiece 26 . when the tip end 40 is thereby positioned in a plane lower than that of the upper flange 32 , continued extension of the actuating rod 52 is effective to move the follower pins 70 , 72 into and along the second run 76 of the cam slot 52 . this continued motion is effective to draw the tip end 40 of the c - clamp 36 downwardly and into firm engagement with the upper surface of the lower flange 34 . with continued operation of the actuator 54 , the cam followers 70 , 72 , continue to advance along the cam slot 52 until the cam follower 70 moves into engagement with the lower terminus 86 in the fourth run 82 . by reason of the shape of the fourth run 82 , the tip end 40 of the c - clamp 36 is caused to move , first , a short distance toward the web 30 of the workpiece 26 , then into gripping engagement with the lower flange 34 of the workpiece . the purpose of the jog represented by the fourth run 82 at the lowermost region of the cam slot 52 is to maintain the position of the tip end 40 of the clamp as close as possible to the web 30 of the workpiece 26 during the terminal movement of the c - clamp 36 . placing the tip end 40 of the c - clamp 36 as close as possible to the centerline of the web 30 results in the application of the maximum possible clamping force to the workpiece 26 . the clamping devices 22 at the work station 20 can be operated in a number of different ways , as desired . for example , they can be operated in unison so that they all operate together to clamp the workpiece 26 . in another instance , selected opposed pairs of clamping devices 22 can be operated to hold the workpiece at one location while others are retracted to enable milling , grinding , drilling , or other desirable operations to be performed in the region whereat those clamping devices would normally be engaged with the workpiece . in still another instance , all of the clamping devices 22 may be employed to engage the flange 34 on one side of the workpiece while all of the clamping devices on the other side of the workpiece will be in their withdrawn positions . the present invention , as just described , is considered to be a significant advance , automating the clamping of complex shaped workpieces which previously had to be attended to manually . it is particularly valuable because of the ability of the clamping devices 22 to accommodate a whole range of sizes and shapes of workpieces 26 . because of this , it is not necessary to make a wholesale change of the work table 24 in order to accommodate each new sized or shaped workpiece . fig7 is illustrative of the clamping device 92 which was previously known to the inventor . it was because of the substantial drawbacks in the clamping device 92 that the present invention was devised . in fig7 the clamping device 92 is seen to include a c - clamp 94 having a proximal member 96 , a distal member 98 terminating at a tip end 100 and a bight member 102 joining the proximal member and the distal member . it is noteworthy that the distal member 98 and the bight member 102 are substantially perpendicular to one another . a bell crank 104 , similar to the bell crank 60 , is mounted in a suitable manner so as to pivot about a fixed pin 106 and is pivotally connected by a pin 108 to one end of a link 110 . the other end of the link 110 is pivotally attached to a follower pin 112 fixed to the proximal member 96 and projecting outwardly therefrom . a second follower pin 114 is spaced from , but parallel to the follower pin 112 and also projects from the surfaces of the proximal member 96 . a pair of parallel spaced apart support plates 116 ( only one is illustrated ) depend from and are attached to a longitudinally extending work platform 118 . each of the support plates 116 is provided with a shaped cam slot 120 which has only two runs therein : a first generally straight run 122 which is angularly disposed relative to a second straight , generally upright second run 124 . the follower pins 112 , 114 are slidably engaged with the cam slot 120 . when the bell crank is moved by a suitable actuator ( not shown ) to the point at which the cam follower 114 engages a terminus 126 of the run 122 , the c - clamp 94 is in its withdrawn position with the tip end 100 distant from the workpiece 26 . however , as the bell crank 104 is operated to draw the follower pin 112 toward a terminus 128 of the second run 124 , the c - clamp 94 passes through its position indicated by solid lines to its final clamped position indicated by the notation 94a . while the prior art clamping device 92 is adequate for purposes of clamping workpieces of relatively simple shape , it is clearly seen in fig7 that it is unable to satisfy the clamping requirement for a workpiece of more complex shape such as the workpiece 26 . in the instance depicted in fig7 it is seen that the follower pin 114 is just leaving the run 122 and entering the run 124 , the follower pin 112 already well into the run 124 . as the c - clamp 94 is moved in the direction of an arrow 130 , the tip end 100 has followed an arcuate path which would adequately move down and into engagement with the lower flange 34 of the workpiece 26 , in the absence of the upper flange 32 . however , in the presence of the flange 32 , it would strike that flange and be incapable of reaching the lower flange without harm to itself or to the flange 32 . it is only by reason of the shaped cam slot 52 of the present invention , with the aid of the revised shape of the c - clamp 36 , that clamping of the workpiece 26 can be achieved . while a preferred embodiment of the invention has been disclosed in detail , it should be understood by those skilled in the art that various other modifications may be made to the illustrated embodiments without departing from the scope of the invention as described in the specification and defined in the appended claims .