Patent Publication Number: US-2013227999-A1

Title: Workpiece punch-molding method and workpiece punch-molding device

Description:
TECHNICAL FIELD 
     The present invention relates to a workpiece punch-molding method and a workpiece punch-molding device. 
     BACKGROUND ART 
     Hitherto, in molding a workpiece having a predefined shape from a metal plate, a mold punch depresses a punching position of the metal plate being held on a die from an upper side thereof to punch the workpiece into the predefined shape (for example, refer to Patent document 1). 
     A counter punch is disposed at a lower position opposite to the mold punch, capable of descending freely. When the mold punch is descended to punch the workpiece from the metal plate, the counter punch descends together with the workpiece while contacting the workpiece. At this moment, applying a counterweight to the workpiece from the counter punch causes a depressing force to be applied to the workpiece being sandwiched by the mold punch and the counter punch in the punching direction; therefore, in punching the workpiece it is possible to mold the workpiece into a desired thickness by adjusting the counterweight appropriately. 
     As a configuration of the counter punch, it may include a compression chamber for generating a pressurizing force from a fluid pressure of a pressurized oil or the like; accordingly, it is possible to adjust the counterweight appropriately (for example, refer to Patent document 2). 
     CITATION LIST 
     Patent Documents 
     
         
         Patent document 1: Japanese Patent Laid-open No. 2001-246428 
         Patent document 2: Japanese Patent Laid-open No. 2001-129620 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the mass production of a workpiece such as an automobile component or the like for example, the demand of the workpiece may fluctuate in different periods (seasons, months, days or the like) of a year. In punch-molding the workpiece according to the conventional device, it is possible to produce the workpiece effectively according to the demands by decreasing the punching speed of the device to restrain the amount of production in a period where the demand of the workpiece is low and by increasing the punching speed of the device to increase the amount of production in a period where the demand of the workpiece is high. 
     However, if the workpiece molded in the period where the amount of production is restrained by decreasing the punching speed of the device is compared with the workpiece molded in the period where the amount of production is increased by increasing the punching speed of the device, there arises a problem that the thickness of the latter one is slightly thinner than the thickness of the former one. 
     Moreover, in punch-molding the workpiece according to the conventional device, even though the counterweight is set to punch the workpiece to a desired thickness, as the punching speed increases gradually from the start of punching by the mold punch (specifically, for example the number of punching times per minute increases), there also arises a problem that the thickness of the workpiece becomes thinner than a desired thickness accordingly. 
     The present invention has been accomplished in view of the aforementioned problems, and it is therefore an object of the present invention to provide a workpiece punch-molding method and a workpiece punch-molding device capable of punching a workpiece to a predefined thickness at a high precision even though a punching speed of the workpiece fluctuates. 
     Solution to Problem 
     A workpiece punch-molding method for punching a workpiece having a predefined shape from a metal plate according to the present invention comprises: a punching step of depressing a punching position of the metal plate being held on a die to punch the workpiece into the predefined shape by a mold punch, the punching step being configured to include a counter punch pressing step of making a counter punch, which is disposed opposite to the mold punch and is configured to be freely movable in the vertical direction, to follow a punching motion of the mold punch while the counter punch is being pressed to contact the workpiece by a predefined counterweight, a counterweight setting step of generating a fluid pressure from a fluid pressure applying unit to be applied to the counter punch as the counterweight, and a depressurizing step of depressurizing the fluid pressure from the fluid pressure applying unit according to a descended position of the counter punch by a depressurizing unit, a data obtaining step of obtaining data denoting a relationship between a punching speed of the mold punch and a delay time of a depressurizing motion of the depressurizing unit relative to the punching motion of the mold punch being disposed prior to the punching step, and the depressurizing unit being actuated at an early timing on the basis of the data obtained by the data obtaining step in the depressurizing step. 
     In the punching step of the present invention, as the mold punch descends, the counter punch is pressed to contact the workpiece by the counter punch pressing step with a predefined counterweight from the lower side of the workpiece to punch the workpiece having a predefined shape from the metal plate to mold the workpiece into a predefined thickness. 
     The descending counter punch is applied with a fluid pressure from the fluid pressure applying unit in the counterweight setting step; according to the counterweight of this moment, the workpiece is molded into the predefined thickness while being descended. In other words, while the workpiece is descending in the punch, the counterweight is maintained constant by the fluid pressure from the fluid pressure applying unit with respect to the workpiece. 
     Thereafter, after the mold punch reaches the lowest position, it shifts to an ascending motion and moves away from the workpiece. As the mold punch is at the lowest position, it is necessary to release the counterweight applied from the counter punch to the workpiece; therefore, the depressurizing step is disposed to perform the depressurization of the fluid pressure applying unit through the depressurizing unit. 
     However, if the timing for releasing the counterweight applied by the counter punch is later than the timing where the mold punch reaches the lowest position, the counterweight would be applied to the workpiece in a very short time where the mold punch shifts to the ascending motion to make the workpiece thinner than the predefined thickness. Moreover, since the delay in timing for releasing the counterweight by the counter punch becomes greater as the punching speed by the mold punch on the workpiece becomes higher, such delay has been considered to be caused by a delay in the depressurizing motion of the depressurizing unit from the start of depressurization by the fluid pressure applying unit to the end of depressurization. 
     In the present invention, the data obtaining step is disposed, and the depressurizing unit is actuated by using the data obtained by the data obtaining step in the depressurizing step. Specifically, the delay time in the depressurizing motion of the depressurizing unit for each punching speed of the mold punch is understood according to the data obtained by the data obtaining step, the depressurizing motion of the depressurizing unit is actuated at an early timing advanced by the delay time occurred in the depressurizing motion of the depressurizing unit if the punch is performed at the present punching speed. Accordingly, it is possible to match the timing where the depressurization by the depressurizing unit ends with the timing where the mold punch reaches the lowest position, and consequently to avoid the delay of releasing the counterweight by the counter punch. Thereby, even though the punching speed is varied, it is possible to punch the workpiece into the predefined thickness at a high precision. 
     In the workpiece punch-molding method according to the present invention, it is preferable that a data storing step of storing the data obtained by the data obtaining step in a storing unit is disposed prior to the punching step, and the data stored in the storing unit by the data storing step is used to actuate the depressurizing unit in the depressurizing step. 
     By disposing the data storing step, it is possible to use the data stored in the storing unit in the depressurizing step in a subsequent punching step; thereby, it is possible to perform the punching step effectively without performing the data obtaining step every time before the punching step. 
     In the workpiece punch-molding method according to the present invention, it is preferable that the counterweight setting step is configured to include a punching speed obtaining step of obtaining the punching speed of the mold punch, and a fluid pressure adjusting step of adjusting the fluid pressure from the fluid pressure applying unit so as to depressurize the fluid pressure applied to the counter punch following the punching motion of the mold punch according to an increment on the punching speed obtained by the punching speed obtaining step. 
     Accordingly, as to be described later, even though the punching speed of the mold punch increases gradually, it is possible to maintain the counterweight at the predefined one; thereby, it is possible to punch the workpiece into the predefined thickness at a high precision. 
     A workpiece punch-molding method for punching a workpiece having a predefined shape from a metal plate according to the present invention comprises: a punching step of depressing a punching position of the metal plate being held on a die to punch the workpiece into the predefined shape by a mold punch, the punching step being configured to include a counter punch pressing step of making a counter punch, which is disposed opposite to the mold punch and is configured to be freely movable in the vertical direction, to follow a punching motion of the mold punch while the counter punch is being pressed to contact the workpiece by a predefined counterweight, and a counterweight setting step of generating a fluid pressure from a fluid pressure applying unit to be applied to the counter punch as the counterweight, and the counterweight setting step being configured to include a punching speed obtaining step of obtaining a punching speed of the mold punch, and a fluid pressure adjusting step of adjusting the fluid pressure from the fluid pressure applying unit so as to depressurize the fluid pressure applied to the counter punch following the punching motion of the mold punch according to an increment on the punching speed obtained by the punching speed obtaining step. 
     In the punching step of the present invention, the counter punch is descended to contact the workpiece in the counter punch pressing step, and at this moment, the counterweight set by the counterweight setting step is applied to the workpiece by the counter punch. 
     In the counterweight setting step, firstly, the punching speed of the mold punch is obtained by the punching speed obtaining step. Accordingly, the variation (mainly increment) on the punching speed of the mold punch is always understood from the start of punching. Thereafter, the fluid pressure from the fluid pressure applying unit is depressurized in relation to the increment on the punching speed by the fluid pressure adjusting step. 
     The fluid pressure adjusting step is based on the knowledge discovered by the inventors of the present application via various experiments. In the punch of a workpiece, the thickness of a workpiece punched immediately after the start of the punching by the mold punch is slightly thinner than the thickness of a workpiece punched after the punching speed of the mold punch has increased. And along with the increment of the punching speed of the mold punch, the thickness of the workpiece would become thinner. 
     Regarding this phenomenon, the inventors of the present application had performed various experiments emphasized on the relationship between the punching motion of the mold punch and the fluid pressure applied to the counter punch by the fluid pressure applying unit. The result is clear that if the number of punching times per minute (equivalent to the punching speed of the mold punch) is greater, the surge pressure (the maximum value of abrupt pressure fluctuations along with the current variations of a fluid) by the fluid of the fluid pressure applying unit increases in comparison with the case where the number of punching times is smaller, and resultantly, a counterweight greater than the desired counterweight is applied from the counter punch to the workpiece. 
     On the basis of the knowledge, when the punching speed of the mold punch increases gradually, the fluid pressure of the fluid pressure applying unit is decreased gradually in relation to the degree of increment of the punching speed, the influence on the counterweight by the surge pressure can be cancelled by a decreased amount of fluid pressure in the fluid pressure adjusting step. Therefore, even though the punching speed of the mold punch gradually increases, it is possible to maintain the counterweight at the predefined one; thereby, it is possible to punch the workpiece into the desired thickness at a high precision. 
     A workpiece punch-molding device for punching a workpiece having a predefined shape from a metal plate of the present invention comprises: a die for holding the metal plate, a mold punch for depressing a punching position of the metal plate being held on a die to punch the workpiece into the predefined shape, a counter punch disposed to be freely movable in the vertical direction with respect to the mold punch and configured to follow a punching motion of the mold punch while being pressed to contact the workpiece by a predefined counterweight, a fluid pressure applying unit configured to generate the counterweight by applying a fluid pressure to the counter punch, a depressurizing unit configured to depressurize the fluid pressure from the fluid pressure applying unit, a storing unit configured to store preliminarily obtained data denoting a relationship between a punching speed of the mold punch and a delay time of a depressurizing motion of the depressurizing unit relative to the punching motion of the mold punch, and a depressurizing motion controlling unit configured to actuate the depressurizing unit on the basis of the data in the storing unit so as to depressurize the fluid pressure from the fluid pressure applying unit at an early timing in relation to the delay time of the depressurizing motion. 
     According to the punching device of the present invention, when the counter punch descends by following the descent of the mold punch, a predefined counterweight is applied by the fluid pressure applying unit to the workpiece via the counter punch contacting the lower side of the workpiece. The workpiece is punched to the predefined shape and the predefined thickness from the metal plate by the counterweight. 
     Thereafter, after the mold punch reaches the lowest position, it shifts to an ascending motion and moves away from the workpiece. As the mold punch is at the lowest position, it is necessary to release the counterweight applied from the counter punch to the workpiece; therefore, the fluid pressure controlling unit actuates the depressurizing unit to depressurize the fluid pressure by the fluid pressure applying unit so as to release the counterweight. 
     The fluid pressure controlling unit is provided with the storing unit and the depressurizing motion correcting unit; thus, the depressurizing motion correcting unit advances the motion timing of the depressurizing unit according to the data stored in the storing unit. 
     The data stored in the storing unit is preliminarily obtained data denoting a relationship between the punching speed of the mold punch and the delay time in the depressurizing motion of the depressurizing unit relative to the punching motion of the mold punch. The depressurizing motion correcting unit understands the delay time in the depressurizing motion of the depressurizing unit for each punching speed of the mold punch according to the data stored in the storing unit, and starts the depressurization by the depressurizing unit at an early timing advanced by the delay time occurred in the depressurizing motion of the depressurizing unit if the punch is performed at the present punching speed. Accordingly, it is possible to match the timing where the depressurization by the depressurizing unit ends with the timing where the mold punch reaches the lowest position. 
     Consequently, it is possible to avoid the delay of releasing the counterweight by the counter punch as the mold punch is ascending; thereby, even though the punching speed is varied, it is possible to punch the workpiece into the predefined thickness at a high precision. 
     In the workpiece punch-molding device of the present invention, it is preferable that a fluid pressure adjusting unit configured to adjust the fluid pressure from the fluid pressure applying unit so as to depressurize the fluid pressure applied to the counter punch following the punching motion of the mold punch according to an increment on the punching speed of the mold punch is further provided. 
     Accordingly, as to be described later, even though the punching speed of the mold punch increases gradually, it is possible to maintain the counterweight at the predefined one; thereby, it is possible to punch the workpiece into the predefined thickness at a high precision. 
     A workpiece punch-molding device for punching a workpiece having a predefined shape from a metal plate of the present invention comprises: a die for holding the metal plate, a mold punch for depressing a punching position of the metal plate being held on the die to punch the workpiece into the predefined shape, a counter punch disposed to be freely movable in the vertical direction with respect to the mold punch and configured to follow a punching motion of the mold punch while being pressed to contact the workpiece by a predefined counterweight, a fluid pressure applying unit configured to generate the counterweight by applying a fluid pressure to the counter punch, and a fluid pressure adjusting unit configured to adjust the fluid pressure from the fluid pressure applying unit so as to depressurize the fluid pressure from the fluid pressure applying unit following the punching motion of the mold punch according to an increment on the punching speed of the mold punch. 
     The counter punch is configured to be applied by the fluid pressure from the fluid pressure applying unit so as to generate the desired counterweight. Further in the present invention, the fluid pressure from the fluid pressure applying unit is decreased, by the fluid pressure adjusting unit, according to the increment on the punching speed by the mold punch. 
     As mentioned above, the control by the fluid pressure adjusting unit on the fluid pressure by the fluid pressure applying unit in the punching motion of the mold punch is based on the knowledge discovered by the inventors of the present application via various experiments. Specifically, if the number of punching times per minute (equivalent to the punching speed of the mold punch) is greater, the surge pressure (the maximum value of abrupt pressure fluctuations along with the current variations of a fluid) by the fluid of the fluid pressure applying unit increases in comparison with the case where the number of punching times is smaller, and resultantly, a counterweight greater than the desired counterweight is applied from the counter punch to the workpiece. In order to alleviate the surge pressure at this moment, there has been considered to have an accumulator (capacitor) and the like connected immediately close to the fluid pressure applying unit. 
     However, it is not only required to have a space so as to dispose the accumulator immediately close to the counter punch and the fluid pressure applying unit but also required a large scaled accumulator so as to alleviate a relatively great surge pressure; therefore, it is disadvantageous to adopt the accumulator. 
     Whereupon in the present invention, the fluid pressure adjusting unit is disposed on the basis of the knowledge; thereby, as the punching speed by the mold punch increases gradually, the fluid pressure of the fluid pressure applying unit is being adjusted so as to reduce the counterweight of the counter punch in relation to the degree of increment of the punching speed, and consequently the influence caused by the surge pressure on the counterweight by the decreased fluid pressure can be cancelled. 
     Accordingly, the variation of the counterweight in relation to the increment of the punching speed by the mold punch can be prevented so as to punch the workpiece into the desired thickness at a high precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram schematically illustrating a structure of a punch-molding device according to an embodiment of the present invention; 
         FIG. 2  is an explanatory diagram illustrating punching motions of a workpiece; 
         FIG. 3  is a graph illustrating a relationship between a punching speed and a predefined oil pressure; and 
         FIG. 4  is a graph illustrating a relationship between a punching speed and a depressurizing motion. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As schematically illustrated in  FIG. 1 , a punch-molding device of the present embodiment is composed of a mold mechanism  1 , an oil pressure supplying unit  2  and a controller  3 . 
     The mold mechanism  1 , as the main parts thereof being illustrated in  FIGS. 2(   a ) to  2 ( d ), is composed of a lower mold  4  positioned below a metal plate X serving as a raw material, and an upper mold  5  positioned above the lower mold  4  through the intermediary of the metal plate X. The lower mold  4  includes a die  6  and a counter punch  7 , and the upper mold  5  includes a mold punch  8 . 
     The mold punch  8  has a shape corresponding to a punching shape of the plate X, and is driven to descend by an elevation drive unit (not shown). The mold punch  8  is descended to contact a punching position of the plate X being held on the die  6  and is further descended to depress the plate X toward a lower side of the die  6  so as to punch a workpiece W having a predefined shape. 
     The counter punch  7  is disposed opposite to the mold punch  8 , capable of moving freely in the vertical direction, and is biased upward by an oil pressure cylinder  9  (fluid pressure applying unit). Specifically, as to be described later, the counter punch  7  applies a counterweight to the workpiece W when subjected to an oil pressure supplied by the oil pressure cylinder  9 . 
     The mold punch  8  descends with a depressing force greater than the upward-pointing counterweight applied by the counter punch  7 . Thereby, as the mold punch  8  is punching the plate X, the counter punch  7  follows the descent of the mold punch  8  to descend while contacting the lower side of the plate X. 
     The oil pressure supplying unit  2  is configured to supply an oil pressure to the oil pressure cylinder  9  of the counter punch  7 , and as illustrated in  FIG. 1 , it is provided with an oil pressure pump  10  for generating the oil pressure, a proportion controlling relief valve  11  connected to the downstream of the oil pressure pump for adjusting the oil pressure supplied to the oil pressure cylinder  9 , and a changeover valve  12  (depressurizing unit) for switching the depressurization and oil pressure supply of the oil pressure cylinder  9 . 
     The proportion controlling relief valve  11  is an electromagnetic valve capable of setting a desired relief voltage according to an input electric current. By supplying the oil pressure via the proportion controlling relief valve  11  to the oil pressure cylinder  9 , the oil pressure of the oil pressure cylinder  9  is maintained at the predefined relief voltage at the moment when the counter punch  7  is descending; thereby, the counterweight of the counter punch  7  is maintained constant. The changeover valve  12  is an electromagnetic valve operating to select the depressurization and the oil pressure supply according to the input of a control signal. 
     As illustrated in  FIG. 1 , the controller  3  is configured to control the operations of the mold mechanism  1 , and a part thereof is disposed with a speed obtaining unit  13  configured to obtain a punching speed of the mold punch  8 , an oil pressure defining unit  14  configured to define an oil pressure applied to the oil pressure cylinder  9 , a first storing unit  15  configured to store data to be used by the oil pressure defining unit  14 , an oil pressure controlling unit  16  (fluid pressure adjusting unit) configured to control the oil pressure supplying unit  2  according to the oil pressure defined by the oil pressure defining unit  14 , and a second storing unit  17  configured to store data to be used by the oil pressure controlling unit  16 . 
     Further, the oil pressure controlling unit  16  includes, as a partial function thereof, a depressurizing motion controlling unit  18  (depressurizing motion controlling member) configured to depressurize the oil pressure cylinder  9  according to the switching by the changeover valve  12  on the basis of the data stored in the second storing unit  17 . 
     The speed obtaining unit  13  obtains the punching speed of the mold punch  8  (specifically, the number of punching times by the mold punch  8  per minute, for example) as the mold mechanism  1  is in operation. 
     The first storing unit  15  is preliminarily stored with a relationship between the number of punching times by the mold punch  8  per minute and the defined oil pressure (relief pressure) of the oil pressure cylinder  9  as the data. As illustrated in  FIG. 3 , in the data, the defined oil pressure decreases as the number of punching times by the mold punch  8  per minute increases (the increment of the punching speed). 
     The oil pressure defining unit  14 , with reference to the data in the first storing unit  15 , defines an adequate oil pressure in accordance with the punching speed obtained by the speed obtaining unit  13 . The oil pressure controlling unit  16 , through controlling the proportion controlling relief valve  11  of the oil pressure supplying unit  2 , applies the oil pressure defined by the oil pressure defining unit  14  to the oil pressure cylinder  9 . Moreover, the oil pressure controlling unit  16 , through controlling the changeover valve  12  of the oil pressure supplying unit  2 , switches the oil pressure supply and the oil pressure discharge (depressurization) of the oil pressure cylinder  9  at a predefined timing. 
     The second storing unit  17  is preliminarily stored with a correlation map denoting a correlative relationship between the number of punching times by the mold punch  8  per minute and a required time from the start of the changeover to the end of the changeover by the changeover valve  12  (the delay time from the input of the control signals instructing the depressurizing motion to the depressurized state). As illustrated by an example in  FIG. 4 , the correlation map denotes the positions of the mold punch  8  if one stroke of the mold punch for each number of punching times by the mold punch  8  in one minute is converted into the rotation angles of 360°. 
     Specifically, in the correlation map of  FIG. 4 , the top dead center at the moment where the mold punch  8  starts to descend is 0°, the bottom dead center at the moment where the mold punch  8  shifts from descending to ascending is 180°, and the top dead center at the moment where the mold punch  8  ends ascending is 360°. As illustrated by the correlation map in  FIG. 4 , in order to make the changeover valve  12  to finish the changeover to the depressurized state at the moment where the mold punch  8  is at the bottom dead center (180°), the input of the control signal for instructing the changeover valve  12  to perform the depressurization is issued at the moment where the mold punch  8  is at 161°, advancing the timing of inputting the control signal by the depressurizing motion controlling unit  18  for instructing the depressurizing motion by the delay time (in the present embodiment, the delay is 0.02 seconds). 
     In addition,  FIG. 4  illustrates the case where the timing of inputting the control signal to the changeover valve  12  at the moment where the number of punching times by the mold punch  8  in one minute is the maximum (160 times) is also applied to the moment where the number of punching times by the mold punch  8  in one minute is the minimum (50 times). As the data stored by the second storing unit  17 , it is acceptable to define different timings of inputting the control signal in accordance with each number of punching times by the mold punch  8  per minute. 
     By controlling the changeover valve  12  of the oil pressure supplying unit  2  according to the correlation map of  FIG. 4  stored in the second storing unit  17 , it is possible for the depressurizing motion controlling unit  18  of the oil pressure controlling unit  16  to match the timing where the oil pressure cylinder  9  ends the depressurization and the timing where the mold punch  8  reaches the lowest position so as to prevent the delay in releasing the counterweight by the counter punch  7 . 
     Hereinafter, the punch of the workpiece W by the punch-molding device having the abovementioned configuration will be described. 
     Prior to the punch of the workpiece W by the punch-molding device, the data denoting the relationship between the number of punching times by the mold punch  8  per minute and the defined oil pressure (relief pressure) of the oil pressure cylinder  9  is stored in the first storing unit  15 ; moreover, the data denoting the relationship between the punching speed of the mold punch  8  and the delay time of the switching motion of the changeover valve  12  relative to the punching motion of the mold punch  8  is obtained (data obtaining step), and the correlation map is calculated by using the obtained data and is stored in the second storing unit  17  (data storing step). 
     Thereafter, as illustrated by  FIG. 2(   a ), as the punch of the workpiece W by the punch-molding device is started, the plate X is held on the die  6  (plate disposing step). At the moment, the upper surface of the counter punch  7  is positioned at the same level as the upper surface of the die  6 , and an oil pressure is applied from the oil pressure supplying unit  2  to the oil pressure cylinder  9  as the counterweight according to the control by the oil pressure controlling unit  16 . 
     Next, as illustrated by  FIG. 2(   b ), the mold punch  8  descends to depress the punching position of the plate X downward (punching step). At the moment, the counter punch  7  follows the depressing of the mold punch  8  to descend while contacting the workpiece W with the counterweight applied thereupon (counter punch pressing step). 
     In addition, in the descending of the counter punch  7 , the oil pressure cylinder  9  configured to generate the counterweight is applied with the oil pressure in relation to the current punching speed of the mold punch  8  via the oil pressure supplying unit  2  according to the control of the oil pressure controlling unit  16  (counterweight setting step). 
     As the punching speed of the mold punch  8  increases, the abrupt fluctuation occurs in the operation oil inside the oil pressure cylinder  9 , which may cause a surge pressure to occur in the oil pressure cylinder  9  and consequently cause the counterweight to increase instantaneously. If the counterweight becomes greater than the predefined one due to the surge pressure at this moment, the workpiece W is subjected to the excessive press by the mold punch  8  and the counter punch  7  in the vertical direction to deform in the punching direction, the workpiece W can not be molded into the predefined thickness. 
     Thereupon, the influence caused by the surge pressure on the workpiece W is alleviated according to the control operation of the controller  3 . Specifically, the control operation of the controller  3  is described with reference to  FIG. 1 . Firstly, the speed obtaining unit  13  obtains the punching speed of the mold punch  8  (the number of punching times by the mold punch  8  per minute in the present embodiment) according to a speed signal output from the mold mechanism  1  in operation (punching speed obtaining step). Subsequently, the oil pressure defining unit  14 , on the basis of the data in the first storing unit  15  as illustrated in  FIG. 3 , extracts a defined oil pressure in accordance with the current number of punching times, and modifies the relief pressure of the proportion controlling relief valve of the oil pressure supplying unit  2  to the defined oil pressure. Accordingly, as the number of punching times increases (the punching speed increases), the oil pressure controlling unit  16  decreases the oil pressure of the oil pressure cylinder  9  in accordance with the degree of increment on the punching speed (fluid pressure adjusting step). 
     Thereby, even though the punch is performed at a speed occurred with the surge pressure, the surge pressure is cancelled by the depressurized pressure of the oil pressure cylinder  9 ; therefore, it is possible to maintain the counterweight at the predefined one, and consequently, it is possible to apply the desired counterweight to the workpiece W to perform the punch. 
     Next, as illustrated in  FIG. 2(   c ), at the moment where the mold punch  8  descends to the bottom dead center, the workpiece W having the predefined shape is being punched out and the workpiece W is being subjected to the depress by the mold punch  8  and the counter punch  7  in the vertical direction to deform in the punching direction so as to form the workpiece W having the predefined thickness. Thereafter, the mold punch  8  having reached the bottom dead center shifts to ascend, and meanwhile the oil pressure cylinder  9  of the counter punch  7  is depressurized (depressurizing step). 
     Here, the depressurization of the oil pressure cylinder  9  is performed through the input of a control signal from the depressurizing motion controlling unit  18  to the changeover valve  12  of the oil pressure supplying unit  2 . The control signal at the moment is input by the depressurizing motion controlling unit  18  into the changeover valve  12  at an early timing based on the correlation map stored in the second storing unit  17 , the timing where the depressurization of the oil pressure cylinder  9  ends matches with the timing where the mold punch  8  reaches the lowest position, the release of the counterweight by the counter punch  7  is performed at the best timing; even though the punching speed of the mold punch  8  is varied, it is possible to punch the workpiece W into the predefined thickness at a high precision. 
     As illustrated in  FIG. 2(   d ), as the mold punch  8  reaches the ascending position, the changeover valve  12  of the oil pressure supplying unit  2  is switched to the oil pressure supply state by the depressurizing motion controlling unit  18 ; the counter punch  7 , in accordance with the oil pressure supplied to the oil pressure cylinder  9 , ascends to push the workpiece W away from the die  6 ; thereby, one punching motion is completed. Subsequently, the above motions are repeated to perform the punch-molding of the workpiece W continuously. 
     As mentioned above, according to the present embodiment, even though the punching speed of the mold punch  8  is varied, the release of the counterweight by the counter punch  7  is performed at the best timing, it is possible to punch the workpiece into the predefined thickness at a high precision. As the number of punching times increases (the punching speed increases), the oil pressure controlling unit  16  decreases the oil pressure of the oil pressure cylinder  9  in accordance with the degree of increment on the punching speed; therefore, the influence caused by the surge pressure on the workpiece W can be inhibited, which makes it possible to punch the workpiece W at a high precision. 
     It should be noted that the numerical values in  FIG. 3  are illustrated merely as an example in the present embodiment; therefore, it should not be understood that the numerical values are defined appropriately according to the scale of the punch-molding device, the metal plate X serving as the raw material, the shape of the workpiece W and the like. 
     In the present embodiment, the number of punching times in one minute is used as the punching speed of the mold punch  8 ; however, the time unit for the number of punching times is not limited thereto. 
     In the present embodiment, there has been described such a configuration that the counter punch  7  is caused to generate the counterweight by the oil pressure of the oil pressure cylinder  9 ; however, it is acceptable to use another fluid or a compressed gas in addition to the oil pressure. 
     INDUSTRIAL APPLICABILITY 
     According to the workpiece punch-molding method and the workpiece punch-molding device of the present invention, it is possible to punch the workpiece into a predefined thickness at a high precision; thereby, it is useful in producing an automobile component or the like of a high quality. 
     DESCRIPTION OF REFERENCE NUMERALS 
     X: metal plate; W: workpiece;  6 : die;  7 : counter punch;  8 : mold punch;  9 : oil pressure cylinder (fluid pressure applying unit);  12 : changeover valve (depressurizing unit);  16 : oil pressure controlling unit (fluid pressure adjusting unit);  17 : second storing unit (storing unit);  18 : depressurizing motion controlling unit (depressurizing motion controlling member).