Abstract:
The upper platen of a press is driven by a ballscrew that is rotated by an electric motor. The ballscrew is suspended from the crown of the press, and the force on the ballscrew shaft when the upper platen is driven into the workpiece causes the crown to flex upward. The flexure of the crown is sensed by a position transducer mounted on a cantilever support. The position transducer generates a signal that can be used to determine the tonnage force developed by the press.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to developing a signal representative of the force exerted by a mechanically driven press through the use of a position sensing transducer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Presses used for metal forming operations such as hemming are well known in the art. In order to accommodate automotive body panels, hemming presses have a footprint on the order of 125 square feet, and are 16-20 feet tall. Such presses typically use a hydraulic cylinder to raise and lower the upper platen of the press, and exert a force on the workpiece exceeding 200,000 pounds. High tonnage presses are normally driven by a hydraulic cylinder, but facilities that use such presses are desirous of replacing the hydraulic drive on a press with a mechanical drive such as a ballscrew. A mechanical drive does not use hydraulic fluid, will not develop hydraulic fluid leaks, and does not present hydraulic fluid disposal problems. As a result, mechanical drives are more environmentally friendly than hydraulic drives. Mechanical drives have other advantages over hydraulic drives; a mechanical drive consumes less energy than a hydraulic drive, a mechanical drive is quieter in operation than a hydraulic drive, a mechanical drive is more reliable than a hydraulic drive, and a mechanical drive can be designed with positive positioning and positive position holding features.  
           [0003]    An electric motor with an encoder can be controlled to rotate a prescribed number of turns in an attempt to produce a predicted force on a workpiece. The exact force that is actually produced however is dependent on a number of variables such as the operating temperature of the press and the dies, the temperature of the workpiece, the amount of wear on the press and the dies, the weight of the dies, any high spots on the workpiece, two panels stuck together, debris and other variables that are impossible to predict or control.  
           [0004]    Thus, it would be desirable to provide a mechanism that would provide an indication of the magnitude of the force or the tonnage that is actually developed by a mechanically driven press. The developed tonnage could be displayed on a gauge or on a screen and monitored by an operator to ensure that the tonnage on a workpiece is not too high or too low, ensuring part repeatability and quality. In the case of a hydraulically driven press, it is a simple matter to couple a pressure transducer to the hydraulic fluid used to drive the press platen, and the tonnage exerted by the platen will be related to the pressure of the hydraulic fluid as sensed by the pressure transducer. In the case of a mechanically driven press however, there is no hydraulic fluid, and a different method of monitoring and reading the tonnage exerted by the press has to be devised.  
           [0005]    It would accordingly be desirable to provide a tonnage monitor for a mechanically driven press that would provide an indication of the tonnage exerted by the press during press operation.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0006]    The invention provides a tonnage monitor for a mechanically driven press. An electric motor is used to drive a ballscrew shaft that is suspended from the crown of the press. The upper press platen is mounted on the ballscrew nut, and rotation of the ballscrew shaft causes the upper press platen to be driven up and down. The crown of the press is designed to flex as the upper press platen applies increasing force against the workpiece, and the flexure is sensed by a transducer and used to provide an indication of the tonnage developed by the press.  
           [0007]    It is accordingly an object of the invention to provide a press with a mechanical drive that includes a tonnage monitor to provide an indication of the tonnage developed by the press.  
           [0008]    It is another object of the invention to provide a press that is driven by a ballscrew suspended from the crown of the press in which the deflection of the crown that occurs during operation of the press is used to provide an indication of the tonnage force developed by the press.  
           [0009]    It is another object of the invention to provide a mechanically driven press in which a displacement transducer is used to measure deflection of the crown of the press in order to develop a signal representative of the tonnage force that is developed by the press. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiment and accompanying drawings in which:  
         [0011]    [0011]FIG. 1 is a perspective view of a mechanically driven press according to the invention showing the upper platen of the press in the Open position;  
         [0012]    [0012]FIG. 2 is a detail view of the press of FIG. 1 showing the crown of the press when the upper platen of the press in the Open position;  
         [0013]    [0013]FIG. 3 is a detail view of the press of FIG. 1 showing the crown of the press when the upper platen of the press is in the Closed position exerting a force on the lower press platen;  
         [0014]    [0014]FIG. 4 shows a load cell in a test fixture used to calibrate the output of a displacement transducer; and,  
         [0015]    [0015]FIG. 5 is a graph showing the relationship of the tonnage developed by the press to the voltage output of the displacement transducer. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    Turning now to the drawings, FIG. 1 shows a mechanically driven press generally designated by the reference numeral  10 . The press comprises a lower base  12  that supports a lower platen  13 . A lower die  14  is positioned on the lower platen  13  and global change the lower die  14  is configured to support a workpiece  16 . Two vertical columns  18  are mounted on diagonally opposite corners of the lower base, and the two corner posts support the crown  19  of the press. The crown of the press is a box beam comprising a top plate  21  and a bottom plate  22  spaced from one another by two side plates  23  (only one shown). Two end plates  24  (only one shown) are provided one each at each end of the crown  19  to close the ends of the box beam and to prevent the introduction of foreign matter to the interior of the beam. Each corner post houses a vertical tie bar  25  (best seen in FIG. 2) that extends from the base of the press and extends out of the top plate  21  of the crown. A tie bar nut  27  is threaded onto the top of each tie bar  25  to secure the crown  19  of the press to the respective corner post  18 . The tie bar nuts  27  are tightened to preload the corner posts  18  with a combined preload force that is greater than the largest force that will normally be exerted by the press on a workpiece.  
         [0017]    An electric motor  30  that is used to drive the press is mounted on a support plate  32  that is cantilevered from a spacer block  33 . The spacer block  33  is positioned in the middle of the crown  19 , equally spaced from the two tie bar nuts  27 . A mounting bracket  35  is attached to the cantilevered end of the support plate  32  remote from the spacer block  33 , and a displacement transducer  36  such as a Linear Variable Differential Transformer (LVDT) is mounted to the support plate  32  by the bracket  35 . The LVDT  36  comprises a body and a spring loaded probe  38  that extends from one end of the body. The tip of the spring loaded probe  38  is in contact with the top plate  21  of the crown of the press. The tip of the probe  38  will remain in contact with the top plate  21  of the crown of the press in the event that the crown of the press moves relative to the body of the LVDT. A signal lead  39  couples a signal from the LVDT  36  to an output display on the control panel of the press (not shown).  
         [0018]    The electric motor  30  that drives the press includes a built-in encoder  41  and has an output shaft that is coupled to a clutch  42 . The clutch  42  is coupled to a gearbox  44  and a brake  46  is mounted on the side of the gearbox that is opposite the clutch  42 . The output of the gearbox  44  is coupled to a vertical drive shaft  50  that extends out of the bottom of the gearbox. The vertical driveshaft  50  from the gearbox is keyed to a drive socket  51  formed on the upper end of a thrust shaft  52 . Two thrust bearings  54  are used to mount the thrust shaft  52  for rotation in the crown  19  of the press. The thrust shaft  52  and the thrust bearings  54  isolate the gearbox  44  from the compressive forces that the ballscrew generates as the upper platen and die are driven by the ballscrew into the lower platen and die. A mounting flange  56  on the lower end of the thrust shaft  52  is attached by bolts to an upper gimbal joint  58  that is connected to the shaft  59  of a ballscrew. A ballscrew nut  61  is threaded on the lower end of the ballscrew shaft  59 , and the ballscrew nut is connected by a lower gimbal joint  62  to the upper platen  64  of the press. The upper platen  64  is formed with a vertical platen tube  65  that can receive the lower end of the ballscrew shaft when the platen is in the raised position. An upper die  67  is mounted on the underside of the upper platen  64 . In order to guide the vertical motion of the upper platen  64 , a hollow sleeve  68  is mounted on each corner of the upper platen and each hollow sleeve is in sliding engagement with one of four guide posts  70  that are mounted one each on the four corners of the press base.  
         [0019]    In operation, the electric drive motor  30  rotates the ballscrew shaft  59  to reciprocate the upper press platen  64  and upper die  67  between Open and Closed positions. The rotation of the motor  30  is monitored by the encoder  41  to rotate the ballscrew shaft the required number of revolutions to drive the upper press platen from the Open position to the Closed position and to exert the required force on the workpiece  16 . When the upper platen  64  is in the Open position as shown in FIG. 1, the upper platen is raised, and a workpiece  16  may be placed on or removed from the lower die  14  on the lower platen  13 . FIG. 2 is a detail view showing the crown  19  of the press and the LVDT  36  When the upper press platen is in the Open position. The top plate  21  of the crown of the press is flat, and the signal output of the LVDT  36  when the crown is in this condition can be calibrated to be zero.  
         [0020]    When the upper platen  64  is in the Closed position, the upper die is in contact with the workpiece  16 . Further rotation of the ballscrew shaft  59  after the upper platen and die have come into contact with a workpiece will increase the pressure on the workpiece, and the upper and lower dies will compressively form the workpiece into the desired configuration as is well known in the art. The increased pressure on the workpiece is transmitted by the ballscrew shaft  59  through the upper gimbal  58  to the thrust bearings  54 . The thrust bearings transmit the force on the ballscrew shaft  59  to the crown  19  of the press, causing the crown to flex upward into a curved shape as shown in FIG. 3.  
         [0021]    [0021]FIG. 3 is a detail view showing the crown  19  of the press and the LVDT  36  when the upper press platen is in the Closed position, exerting a force on the lower press platen. The force exerted by the upper platen on the lower platen causes the crown to flex upward into a curved shape. At the same time, the spring loaded probe  38  of the LVDT  36  extends out of the LVDT housing to follow the motion of the crown. The dotted line  72  shows the displacement of the center of the crown  19  of the press relative to the ends of the tie bars  25  when the crown is flexed upward into a curved shape. The motion of the spring loaded probe  38  causes the LVDT to develop a second output signal that is representative of the tonnage force exerted by the press.  
         [0022]    In actual practice, the LVDT used is a Schaevitz model GCD-SE. The LVDT has a signal output of 0-5 volts DC, and the spring loaded probe  38  has a stroke of 0.25 inches. In order to calibrate the output of the LVDT so that the output can be converted to the tonnage force exerted by the press, a load cell  75  can be mounted in the press as shown in FIG. 4. The load cell  75  is positioned on a test stand  74 , and the test stand  74  is placed on the base  12  of the press. An upper test fixture  76  is mounted on the upper platen  64 , in direct vertical alignment with the load cell  75 . The press motor is then energized to drive the upper test fixture  76  into the load cell  75 , and the load cell will measure the force exerted by the press. At the same time, the voltage output of the LVDT can be measured throughout the tonnage range of the press, and a calibration curve such as the curve  78  shown in FIG. 5 can generated. The constant slope of the curve  78  of FIG. 5 indicates that the relationship between the force measured by the load cell and the output voltage of the LVDT is constant.  
         [0023]    Once the curve  78  of FIG. 5 has been generated, the slope of the curve, in this case 58.78 tons/volt, can be used to convert the voltage signals from the LVDT  36  to an indication of the tonnage force generated by the press. The tonnage force signals can be displayed on an output display such as a video monitor on the control panel of the press in a manner well known to those skilled in the art. In tests using a press of the type shown in FIG. 1, the deflection of the crown of the press measured by the LVDT varied from zero to 0.080 inches as the tonnage of the press increased from zero to 100 tons.  
         [0024]    Having thus described the invention, various alterations and modifications will be apparent to those skilled in the art, which alterations and modifications are intended to be within the scope of the invention as defined by the appended claims.