Abstract:
A servo motor in die tapping unit provides built in shock protection for the motor by fixing a shock plate to the servo motor and floatingly mounting the shock plate to a base structure attachable to a stripper plate in a press. Sets of opposing springs allow movement of the shock plate relative the base plate to relieve shock loads when the press is operated, the shock plate guided on dowel pins fixed to the base structure. The shock plate, and tapping unit and base structure form a self contained assembly for mounting together in the press.

Description:
BACKGROUND OF THE INVENTION 
   This invention concerns in die tapping devices which are used to tap holes in workpieces being formed by dies installed in presses. 
   Such tapping devices have often been operated by the motion of the press, as described in U.S. Pat. No. 6,547,496. Since the tap drive is dependent on the sinusoidal press motion, significant limitations on tapping speed results, increasing the cycle time for the process. 
   Thus, electrical servo motor drives have been developed for in-die tapping units in which servo motors mounted to the tapping unit are used to drive the tap, as described in copending U.S. application Ser. No. 10/417,428, filed on Apr. 15, 2003. 
   The tap drive is made independent of the press motion by the use of servo motors, and this allows driving of the tap at maximum speed to reduce cycle times. 
   A significant problem is created by mounting a servo motor to a tapping unit in that the servo motor is thereby subjected to shock loading when the stripper plate impacts the workpiece and fixture, and suddenly decelerates the servo motor, shortening the service life of the motor. 
   Typically, spring mounted stripper plates carry the tapping units and the stripper plate springs have been relied on to reduce the shock loading of servo motors. 
   However, the stripper plates comprise a separately supplied component from the tapping unit, and the stripper plate springs are not designed specifically to adequately attenuate shock loading of the associated servo motor in this application. 
   The user of this equipment must therefore attempt to design proper stripper springs to reduce shock loading of the motors to acceptable. This necessity is often neglected to the detriment of the service life of the servo motors. 
   According, it is an object of the present invention to provide an effective shock protection for servo motor driven die tapping units which does not rely on stripper plate springs to reduce shock or require a special design of the stripper plate springs to provide shock protection. 
   SUMMARY OF THE INVENTION 
   The above object and other objects which will become apparent upon a reading of the following specification and claims are achieved by providing built in shock protection combined with the servo motor tapping unit itself. In a first embodiment, a shock plate to which the tapping unit is fixed, extends horizontally over an aligned parallel base structure attached to an associated stripper plate, the shock plate floatingly mounted to the base structure by opposing resiliently deflectable elements. 
   A series of upwardly projecting guide pins are fixed at one end to the base structure and slidably received in the shock plate to guide relative vertical movement of the shock plate and tapping unit. 
   Two sets of opposed compression springs act to resiliently position the shock plate closely spaced above the base plate, the spring rates and spacing of the shock plate set to reduce the maximum shock loading to a predetermined acceptable level. This floating mount allows limited relative movement between shock and base plates when impacting of a stripper plate occurs during press cycling. This attenuates the shock loading of the tapping unit mounted on the shock plate to a level where damage to the servo motor is avoided. 
   In a second, vertically oriented motor embodiment, the servo motor is attached to a shock plate. A tooling housing is located spaced beneath the shock plate and is directly attached to a stripper plate. A series of pins guide movement of the shock plate relative to the tooling housing, which mounts a tap drive and holder assembly. 
   A first set of compression springs are interposed between the shock plate and housing recessed in pockets in the shock plate, compliantly resisting downward movement of the shock plate. 
   A second set of compression springs are received in a respective bore in the shock plate and are each compressed beneath a washer secured by a machine screw to the base structure engaged against a rim at the bottom of a respective shock plate bore. The second set of springs compliantly resists upward movement of the shock plate, such that the shock loading when the stripper impacts the workpiece and/or fixture and when the stripper reverses direction, is greatly attenuated. 
   In a third embodiment, a housing comprising the base structure has a portion which extends alongside the servo motor axis and a tool drive and holder assembly is mounted to be offset to the servo motor axis. A shock plate is interposed between the motor and one end of the housing. 
   In all of these embodiments, the spring rates, number of springs and shock plate spacing from the base are designed for mass of the particular tapping unit to properly attenuate the shock loading to a predetermined safe maximum level. 
   The assembly of the tapping unit, shock plate base structure, and springs is installed as an assembly onto the stripper plate to minimize the burden on the user and to insure proper protection for the servo motor. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a pictorial view of a first embodiment of a tapping unit having built in shock protection assembly installed onto a stripper plate, with press platens and a workpiece also shown. 
       FIG. 2  is an enlarge pictorial view of the tapping unit shock plate assembly shown in  FIG. 1 . 
       FIG. 3  is a side elevational view of the tapping unit shown in  FIGS. 1 and 2 . 
       FIG. 4  is a fragmentary view of the tapping unit shown in  FIG. 3  showing the internal details of the tap drive and holder components. 
       FIG. 5  is a pictorial view of a second embodiment of a tapping unit according to the present invention, also showing in phantom lines the outline of press platens and a stripper plate. 
       FIG. 6  is a partially sectional elevation view of the second embodiment shown in  FIG. 5 . 
       FIG. 7  is a pictorial enlarged view of the second embodiment with the shock plate and transmission housing shown in phantom lines to reveal the springs and guide pins mounting the shock plate to the transmission housing. 
       FIG. 8  is an elevational view of the components shown in  FIG. 7 . 
       FIG. 9  is a pictorial view of a third embodiment of a tapping unit according to the invention. 
       FIG. 10  is a vertical sectional view through the lower part of the tapping unit shown in  FIG. 9 . 
       FIG. 11  is an end view of the shock plate and transmission housing components of the tapping unit shown in  FIGS. 9 and 10 . 
       FIG. 12  is a view of the section  12 - 12  taken in  FIG. 11 . 
       FIG. 13  is a view of the section  13 - 13  taken in  FIG. 11 . 
       FIG. 14  is a view of the section  14 - 14  taken in  FIG. 11 . 
   

   DETAILED DESCRIPTION 
   In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
   Referring to the drawings and particularly  FIGS. 1-4 , an upper platen  10  and lower platen  12  of a press is shown with a workpiece  14 , such as an elongated strip formed with holes to be tapped, extending across a fixture  16  installed on the lower platen  12 . 
   A tapping unit  18  for tapping the holes formed in the workpiece  14  is mounted together with shock plate  30  and base structure  32  on a stripper plate  20  movably suspended on supports  22  with gas springs  24  urging the plate  20  to a down position on the supports  22 . 
   After the press is operated to bring the stripper plate  20  against the workpiece  14  on the fixture  16 , the tapping unit  18  is operated to advance and rotate the tap in a hole in the workpiece  14  in the well known manner. 
   As seen in  FIG. 2 , the tapping unit  18  is comprised of a horizontally oriented servo motor  26  having a transmission housing  28  attached at one end, which motor and housing in turn is mounted to a horizontally extending shock plate  30  underlying the motor  26  and housing  28  which is fixed to the shock plate  30  with suitable fasteners (not shown). 
   The shock plate  30  in turn is floatingly mounted on a base structure comprising a plate  32  to form a tapping unit with built in shock protection. This assembly is affixed to the stripper plate  20  with screws  21  when being installed in a press. 
   The shock plate  30  is floatingly mounted at a predetermined space above the base plate  32  by opposing resiliently deflectable elements comprising two opposing sets of four springs each. 
   The springs  34  in an upper set are received in a respective counterbore pockets  36  and compressed therein by a washer  38  and machine screw  40  received in base plate  32  to urge the shock plate  30  downwardly. 
   The opposing springs  42  of a lower set are received in respective lower counterbore pockets  44  and engaged with the undersurface of shock plate  30  to urge the same upwardly, counteracting the springs  34  so that the shock plate  30  thus floats above the base plate  32  with a predetermined gap between the shock plate  30  and base plate  32 . 
   A set of four dowel pins  46  are press fit into holes in the base plate  32  and extending vertically and slidably received into bushing sleeves  48  fit in holes in the shock plate  30 . This guides the vertical motion of the shock plate  30  induced by shocks experienced by the tapping unit assembly during press operation. 
   The transmission housing  28  contains bevel gears  50 A,  50 B driven by the output shaft  52  of the servo motor  26 . A polygon drive shaft  54  is rotated by gear  50 B which in turn drives a lead screw  60  threaded into a bushing  56  nonrotatably held in a bore  58  in the housing  28  by a key  59 . The lead screw  60  mounts a tap holder  62  releasably holding a tap  64 . The lead screw  60  advances axially when rotated to advance and rotate the tap  64  when tapping a hole in the well known manner. A safety spring  66  allows the bushing  56  to be retracted if the tap  64  is blocked from advancing, as could happen if the hole to be tapped is not formed properly or is absent. A nose piece  65  holds tap holder  62  to transmission housing  28 , and serves as a pilot registration for concentric alignment of tap  64  to the hole to be tapped in the workpiece  14 . 
   Thus, excessive shock loading of the servo motor  26  is prevented by being mounted to the floating shock plate  30  which is spaced above the base plate  32  by the springs  34 ,  42 . The springs  34 ,  42  are matched to the mass of the tapping unit, and the tapping unit, shock plate and base plate installed as a single package with built in shock prevention so as to not require any shock prevention measures to be undertaken by the user. 
   The following formulas have been used to insure that a predetermined maximum deceleration is not exceeded: 
   
     
       
         
           
             
               
                 
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   m=Mass of motor 
   V=Velocity of press 
   N B =Number of springs on bottom 
   NT=Number of springs on top 
   K=Spring stiffness 
   Δx=Gap between bottom of shock plate and base structure 
   a=Acceleration 
   The number of springs and their rate and the gap between the shock plate and base can be varied to insure that the maximum deceleration will not exceed a predetermined maximum value, usually under 5 g&#39;s. 
   In one example, four springs in each set having a spring rate of 143 pounds per inch, a press speed of 12 inches/second, and a gap of 0.06 inches produced a deceleration of less than 4 g&#39;s for the servo motor used. 
     FIGS. 5-8  show a second embodiment of the invention featuring a servo tapping unit  68  in which a servo motor  70  is vertically oriented within a press, mounted on a stripper plate  72  suspended from the press upper platen  10  as in the first described embodiment. 
   A base structure comprising a housing  74  is attached to the stripper plate  72  which supports the servo motor  70  with an interposed shock plate  76 . 
   The interposed shock plate  76  is floatingly supported at a spaced location above the base housing  74  by two opposing sets of four springs each. A first set of springs  78  are each received in a respective upwardly facing counterbore  80  in the shock plate  76 , compressed beneath a headed screw  82  threaded into a hole in the top of the base  74  to act to urge the shock plate  76  downwardly. 
   A second set of four springs  84  are each received in a downwardly facing counterbore  86  in the shock plate  76  compressed against the upper surface of the base housing  74  to urge the shock plate upwardly. 
   A set of dowels  88  are press fit in holes in the base housing  74  and slidably received in bushing sleeves  90 . This guides any movement of the shock plate  76  due to shock loading by press operation causing impacting of the stripper plate  72  on the workpiece  14  and fixture  16 . 
   Suitable tap holder and drive components  94  as shown are mounted within the base housing  74 . 
   These include a polygonal drive element  93  having a square drive end received in a square hole in a tap holder plug  95  threaded in a lead screw sleeve  97  keyed to be nonrotatable but able to axially advance against the force of safety spring  99  if the tap  64  cannot advance. 
   A nose piece  92  holds a tap holder to base housing  74 , and serves as a pilot registration for the concentric alignment of tap  64  to the hole to be tapped in workpiece  14 . 
     FIGS. 9-14  show a third embodiment of a tapping unit with shock protection according to the invention. 
   In this embodiment, a vertically oriented servo motor  96  is mounted on a base structure comprising a housing  98  with a floating interposed shock plate  100 . 
   A tap holder and drive housing  101  is mounted on a housing section offset horizontally from the axis of the servo motor  96 . 
   The shock plate  100  is resiliently float mounted above the housing  98  as in the other embodiments with two opposing sets of four springs. A first set of four springs  102  are received in respective upward facing counterbores  104 . The springs  102  are compressed against end walls at the bottom of counterbores  104  by the heads of screws  106  threaded into the base housing  98  to urge the shock plate  100  downwardly. A set of spacer-bushings  108  limit the extent of compression of the springs  102 . 
   A second opposing set of four springs  110  are received in downwardly facing bores  112  in the shock plate  100  compressed against the upper surface of the base housing  98  to urge the shock plate  100  upwardly, balancing the effect of the springs  102  so that the shock plate  100  with the servo motor  96  floats above the base housing  98 . 
   A set of four dowel pins  114  are press fit into holes in the base housing  98  at their lower ends projecting up and slidable in bushings  116  carried in bores in the shock plate  100 . This guides the vertical movement of the shock plate and servo motor  96  induced by shock loading when the press is operated, as in the above described first and second embodiments. 
   The base housing  98  contains a gear  118  driven by the servo motor output shaft  120  which drives an idler gear  122 , driving a gear hub  124 . Gear hub  124  rotates a tap holder assembly  126  housed within a cover  128  and annular extension  129  fixed onto the base housing  98  with bolts  131 . 
   An outer sleeve  136  has a threaded engagement with internal threads  138  of the cap  128 . A keyway  140  and key  142  establish a rotational connection with the gear hub  124  while allowing axial movement. 
   This causes downward advance of the tap holder assembly  126  and a tap  130  secured therein with an included tap holder  132  carrier by an inner sleeve  144 . 
   A safety spring  134  is interposed between the outer sleeve  136  and inner sleeve  144  to allow the outer sleeve  136  to move down even if the inner sleeve  144  cannot advance for some reason.