Patent Publication Number: US-6337042-B1

Title: Press machine and method of manufacturing pressed products

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a press machine and a method of manufacturing pressed products, and particularly to an improvement for reducing mutual interference between a plurality of sets of molds to enhance the processing accuracy. 
     2. Description of the Background Art 
     FIG. 6 is an explanation diagram showing the structure of a conventional press machine as a background of the invention. This machine  151  has a bottom base  71  installed on the floor, a pair of supports  75   a  and  75   b  uprightly provided on the bottom base  71 , and a top base  72  supported on the supports  75   a  and  75   b . The bottom base  71 , supports  75   a  and  75   b , and top base  72  fixedly coupled to each other form a frame stand  86 . A pair of fixed molds  73   a  and  73   b  are fixed on the bottom base  71 . Fixed on the top base  72  are a pair of a (first) servo motor  76   a  and a (second) servo motor  76   b.    
     The servo motors  76   a  and  76   b  are respectively in mesh with ball threads  77   a  and  77   b , which rotate to individually drive the ball threads  77   a  and  77   b  in the vertical direction. Moving molds  74   a  and  74   b  are fixed at the lower ends of the ball threads  77   a  and  77   b , respectively. 
     The moving molds  74   a  and  74   b  are located right above the fixed molds  73   a  and  73   b  to face the fixed molds  73   a  and  73   b , respectively. The servo motors  76   a  and  76   b  rotate in the normal rotation and reverse rotation directions to move the moving molds  74   a  and  74   b  in the mold-closing direction (i.e. downward) and in the mold-opening direction (i.e. upward). 
     The servo motors  76   a  and  76   b  are supplied with current (i.e., electric current) from a (first) servo amplifier  78   a  and a (second) servo amplifier  78   b , respectively. The servo amplifiers  78   a  and  78   b  are individually controlled by an amplifier controlling unit  85 , so that the magnitudes of currents supplied to the servo motors  76   a  and  76   b  are controlled individually. The amplifier controlling unit  85  includes a CPU  80  and a pulse generator  79 . 
     FIG. 7 is a block diagram showing the inside structure of the servo amplifier  78   a , which is representative of the servo amplifiers  78   a  and  78   b . The servo amplifier  78   a  is supplied with a directing value X 0  related to the operating position of the servo motor  76   a  (i.e. the rotating position of the rotor) from the pulse generator  79  and a measured value X related to the operating position of the servo motor  76   a  from an encoder  90 . 
     As shown in the timing chart of FIG. 8, the directing value X 0  is represented by the number of pulses along the time series. A normal rotation directing signal CW is outputted in pulse form when directing that the servo motor  76   a  should operate in the normal rotation direction, and a reverse rotation directing signal CCW is outputted in pulse form when directing that it should operate in the reverse rotation direction. The cumulative value of the difference between the number of pulses of the normal rotation directing signal CW and the number of pulses of the reverse rotation directing signal CCW corresponds to the directing value X 0  related to the operating position of the servo motor  76   a.    
     The rate of change of the directing value X 0  corresponds to the target value of the operating speed of the servo motor  76   a  (i.e. its rotating speed), which is proportional to the pulse frequency as shown in FIG.  8 . The encoder  90  outputs pulses of the same form in correspondence with the amount of operation of the servo motor  76   a  (i.e. the amount of rotation of the rotor). 
     Referring to FIG. 7 again, the subtracter  91  calculates the difference between the directing value X 0  and the measured value X and outputs the calculated value as a positional deviation ΔX. The amplifier  92  amplifiers the positional deviation ΔX. The subtracter  91  and the amplifier  92  form a position controlling unit. The F/V converter  97  converts the rate of time change in the measured value X, i.e., the frequency of the pulses representing the measured value X to a voltage signal. The subtracter  93  calculates the difference between the output signal from the amplifier  92  and the output signal from the F/V converter  97  and outputs the calculated value as a speed deviation ΔS. The amplifier  94  amplifies the speed deviation ΔS. The subtracter  93 , amplifier  94  and F/V converter  97  form a speed controlling unit. 
     The output signal from the amplifier  94  is inputted to a current amplifier  96 . The current amplifier  96  amplifies the input signal and supplies a current I proportional in magnitude to the input signal to the servo motor  76   a . Thus the current I is controlled so that the measured value X follows the directing value X 0  at speed proportional to the difference between the measured value X and the directing value X 0  . The CPU  80  shown in FIG. 6 executes arithmetic processing and the directing value X 0  is outputted through the pulse generator  79  on the basis of the value calculated in the arithmetic processing. The operation of the servo motor  76   a  is thus controlled. 
     FIG. 9 is a flowchart showing the procedure of the arithmetic processing performed by the CPU  80 . When the arithmetic processing is started, first, the processings in steps S 51  and S 52  are simultaneously executed. Specifically, the servo motors  76   a  and  76   b  are driven to return to the origin (the initial position). This processing is continued until they have returned to the origin (step S 53 ), and the process moves to steps S 54  and S 55  after it is finished. When they have returned to the origin, the moving molds  74   a  and  74   b  are positioned at the standby position separated above the fixed molds  73   a  and  73   b.    
     In the following steps S 54  and S 55 , the servo motors  76   a  and  76   b  are driven to perform weighting operation. Then the moving molds  74   a  and  74   b  move in the mold-closing direction to respectively hit on the fixed molds  73   a  and  73   b , and they are further pressurized for the press work. Steps S 54  and S 55  are simultaneously executed. These processes are executed until the press work is completed (step S 56 ). When the press work has been finished, the process moves to steps S 57  and S 58 . 
     In steps S 57  and S 58 , the servo motors  76   a  and  76   b  are driven to perform withdrawing operation. Then the moving molds  74   a  and  74   b  move in the mold-opening direction to return to the standby position. The steps S 57  and S 58  are carried out at the same time. These processes are continued until they return to the standby position (step S 59 ). When they have returned, the process moves to steps S 54  and S 55  again. The above-described processes are repeated to repeatedly carry out the press work. 
     FIG. 10 is a flowchart showing the internal flow in step S 54 , which is representative of steps S 54  and S 55 . Similarly, FIG. 11 shows a flowchart showing the internal flow in step S 57 , which is representative of steps S 57  and S 58 . FIG. 12 is a timing chart showing variations in the target value of the operating speed (i.e. the changing rate of the directing value X 0 ), the positional deviation ΔX, and the torque of the servo motor  76   a  that are caused in the weighting operation of step S 54  and the withdrawing operation of step S 57 . Now, referring to FIGS. 10 to  12 , the weighting operation and withdrawing operation of the machine  151  will be described. 
     When the weighting operation based on the processing in step S 54  is started, first, the moving mold  74   a  is driven to move in the mold-closing direction at high speed (step S 61 ). At this time, the target value of the operating speed first increases from zero, stays at a high value when the directing value X 0  reaches a given reference value, and then decreases when the directing value X 0  reaches another reference value. Subsequently, the target value of the operating speed is maintained at a low value (step S 62 ). 
     The reference values for defining the operating positions at which the target value of the operating speed is changed are previously set through teaching performed prior to the processing in FIG.  9 . The reference value defining the timing for changing from the high-speed moving operation based on step S 61  to the low-speed moving operation based on step S 62  is set so that the moving mold  74   a  is located at such a position that it does not abut on the fixed mold  73   a  when the directing value X 0  reaches that reference value. Hence the moving mold  74   a  moves at high speed from the standby position toward the fixed mold  73   a , whose speed decreases before it hits the fixed mold  73   a , and then the moving mold  74   a  moves at low speed toward the fixed mold  73   a . This reduces the impact produced when the moving mold  74   a  and the fixed mold  73   a  hits on each other. 
     The moving mold  74   a  hits on the fixed mold  73   a  at a certain point of time in the low-speed moving operation. While the moving mold  74   a  moves at speed approximately equal to the target value until it hits on the fixed mold  73   a , it cannot maintain the speed corresponding to the target value after hitting. Accordingly, after hitting, the positional deviation ΔX increases. Then the speed deviation ΔS increases accordingly and the current I increases. As a result, the torque of the servo motor  76   a  increases. That is to say, the moving mold  74   a  is pressurized against the fixed mold  73   a  with an increasing pressing force. 
     After that, when the directing value X 0  reaches another reference value, the operating-speed target value decreases toward zero. Then the process moves to step S 63  and the operating-speed target value is maintained at zero. That is to say, the directing value X 0  is held at a constant value. At this time, the moving mold  74   a  is pressed against the fixed mold  73   a  by a constant pressing force. The press work is carried out throughout from the beginning of pressing to the standing-still operation. The standing-still operation is ended when a previously set certain time has elapsed and the process moves to step S 57 . 
     In step S 57 , the moving mold  74   a  is driven to move at high speed in the mold-opening direction (step S 71 ). During this operation, the operating-speed target value first increases from zero, stays at high value when the directing value X 0  reaches a given reference value, and then decreases to zero when the directing value X 0  reaches another reference value. The number of pulses of the reverse rotation directing signal CCW outputted as the directing value X 0  in the high-speed withdrawing operation based on step S 57  is equal to the number of pulses of the normal rotation directing signal CW outputted in step S 61  (high-speed moving operation) and step S 62  (low-speed moving operation). Then the pressing force applied to the moving mold  74   a  is quickly released and thereafter the moving mold  74   a  returns to the standby position at high speed. 
     The conventional machine  151  operates as described above to realize efficient press work while reducing impact between the moving molds  74   a  and  74   b  and the fixed molds  73   a  and  73   b.    
     However, since the two fixed molds  73   a  and  73   b  and the two servo motors  76   a  and  76   b  are provided on the single frame stand  86 , the conventional machine  151  has the following problems. FIGS. 13 to  16  are timing charts used to explain the problems. In FIGS. 13 to  16 , the speeds (a) and (b) represent the moving speeds of the moving molds  74   a  and  74   b  and the loads (a) and (b) represent the pressing forces applied to the moving molds  74   a  and  74   b , respectively. 
     As stated above, the CPU  80  sends the directing value X 0  to the servo amplifiers  78   a  and  78   b  so that the moving molds  74   a  and  74   b  arrive at the fixed molds  73   a  and  73   b  at the same time in the weighting operation. However, because of deflections of the bases  71  and  72 , difference in capability between the servo motors  76   a  and  76   b , slight errors in the transmission mechanism from the servo motors  76   a  and  76   b  to the moving molds  74   a  and  74   b , and some other reasons, the moving molds  74   a  and  74   b  do not always arrive at the fixed molds  73   a  and  73   b  at the same time. 
     For example, as shown in FIG. 13, when the moving mold  74   a  arrives at the fixed mold  73   a  earlier than the moving mold  74   b  arrives at the fixed mold  73   b , the moving mold  74   b  arrives at the fixed mold  73   b  after the moving mold  74   a  has arrived at the fixed mold  73   a , in which case an excessive pressing force is applied to the moving mold  74   a  in the period before the pressing force to the moving mold  74   b  increases to a certain extent. This excessive load serves as a factor that reduces the processing accuracy in the pressing work. 
     Furthermore, using the machine  151  in a long time will cause deformation of the bases  71  and  72 , variations in the characteristics of the servo motors  76   a  and  76   b , wear of the transmission mechanism, and the like. Even if the simultaneous arrival is maintained, the deformation, variations, wear, etc. of the parts of the machine produced in long time use may cause inequality in pressing force between the moving molds  74   a  and  74   b , as shown in FIG.  14 . This inequality serves to reduce the accuracy of the press work, too. 
     Moreover, in the withdrawing operation, the moving molds  74   a  and  74   b  may separate from the fixed molds  73   a  and  73   b  at different points of time because of deflections of the bases  71  and  72 , difference in capability between the servo motors  76   a  and  76   b , slight errors in the transmission mechanism from the servo motors  76   a  and  76   b  to the moving molds  74   a  and  74   b , and other reasons. For example, when the moving mold  74   a  separates from the fixed mold  73   a  earlier than the moving mold  74   b  separates from the fixed mold  73   b  as shown in FIG. 15, an excessive pressing force is applied to the moving mold  74   b  in the period from when the moving mold  74   a  starts withdrawing to when the moving mold  74   b  withdraws to some extent. This excessive load serves as a factor that reduces the accuracy of the press work, too. 
     Further, in the machine  151 , the above-mentioned teaching is carried out individually to the two servo motors  76   a  and  76   b . Specifically, the reference values for the directing value X 0  directing the servo amplifier  78   a  and the reference values for the directing value X 0  directing the servo amplifier  78   b  are separately set. The CPU  80  sends the directing value X 0  individually to the servo amplifiers  78   a  and  78   b  while referring to the reference values set in this way. It is thereby attempted to improve the processing accuracy. 
     However, as shown in FIG. 16, when the reference values are set so that predetermined target load (pressing force) can be obtained through teaching (a) to the servo motor  76   a  and teaching (b) to the servo motor  76   b  that are separately performed, the pressing forces applied to the moving molds  74   a  and  74   b  may become lower than the target value in the following processing shown in FIG.  9 . This is caused because the magnitude of deflection (the amount of deflection) occurring in the bases  71  and  72  differs between when the pressing force is applied to one of the moving molds  74   a  and  74   b  and when it is simultaneously applied to both. 
     As described above, the conventional press machine in which a plurality of sets of molds are coupled to a common frame stand has the problem that improvement of pressing accuracy is hindered because of mutual interference between the plurality of sets of molds. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is directed to a press machine having a plurality of fixed molds and a plurality of motors provided on a common stand, wherein the plurality of motors individually drive moving molds respectively in pairs with the plurality of fixed molds to perform press work. According to the present invention, the press machine comprises: a plurality of amplifiers for passing current individually through the plurality of motors; and an amplifier controlling portion for individually controlling the plurality of amplifiers to realize weighting operation of moving the plurality of moving molds in mold-closing direction and pressing the plurality of moving molds respectively against the plurality of fixed molds and withdrawing operation of moving the plurality of moving molds in mold-opening direction. Each of the plurality of amplifiers comprises a control portion for calculating an amount of current to be passed through a corresponding one of the plurality of motors so that a measured value of operating position of the corresponding motor follows a directing value, and a torque control portion for sending the amount of the current calculated by the control portion to the corresponding motor while limiting the same so that torque of the corresponding motor does not exceed a limit value, wherein in the weighting operation, the amplifier controlling portion further advances the directing value for each of the plurality of amplifiers in the mold-closing direction after the torque reaches the limit value. 
     Preferably, according to a second aspect of the present invention, in the press machine, in the weighting operation, the amplifier controlling portion advances the directing value for each of the plurality of amplifiers in the mold-closing direction before the torque reaches the limit value and lowers rate of change in the directing value before corresponding pair of the moving and fixed molds come in contact. 
     Preferably, according to a third aspect of the present invention, in the press machine, in the weighting operation, the amplifier controlling portion raises up the rate of change in the directing value for each of the plurality of amplifiers after the torque reaches the limit value. 
     Preferably, according to a fourth aspect of the present invention, in the press machine, in the weighting operation, the amplifier controlling portion lowers the limit value for each of the plurality of amplifiers at the same time as lowering the rate of change in the directing value before the corresponding pair of the moving and fixed molds come in contact. 
     Preferably, according to a fifth aspect of the present invention, in the press machine, in the withdrawing operation, the amplifier controlling portion advances the directing value for each of the plurality of amplifiers in the mold-opening direction while maintaining the limit value until corresponding pair of the moving and fixed molds open by a given amount or more, and then raises the limit value. 
     A sixth aspect of the present invention is directed to a method of manufacturing pressed products, and the method manufactures the pressed products by performing press work by using the press machine. 
     According to the machine of the first aspect, the directing value is further advanced in the mold-closing direction after the torque reaches the limit value, so that the effect of mutual interference between the plurality of sets of molds can be absorbed to perform the press work with stable load. This enhances the accuracy of the press work. 
     According to the machine of the second aspect, while the directing value is advanced in the mold-closing direction in the weighting operation, the rate of change in the directing value is lowered before the molds come in contact, i.e., mold contact occurs, which improves the efficiency of the work while avoiding impact caused as the mold. 
     According to the machine of the third aspect, the rate of change in the directing value is raised up after the torque reaches the limit value, so that a state with highly stable load can be realized quickly. Accordingly, even if the plurality of sets of molds come in contact at different points of time, it is possible to more effectively avoid intensive application of excessive load to a part of the sets. 
     According to the machine of the fourth aspect, in the weighting operation, the limit value of the torque is lowered at the same time as the speed of movement of the moving molds is lowered before the mold contact, so that the load can be stabilized in the press work and the travel of the moving molds can be finished in shorter time, thus further improving the efficiency of the work. 
     According to the machine of the fifth aspect, in the withdrawing operation, the limit value of the torque is maintained until the molds open by a given amount or more, and then the limit value of the torque is raised. Accordingly, even if the plurality of sets of molds separate at different points of time, it is possible to more effectively avoid intensive application of excessive load to a part of the sets. 
     According to the manufacturing method of the sixth aspect, it is possible to obtain pressed products with excellent processing accuracy. 
     The present invention has been made to solve the above-described problems in the background art, and an object of the present invention is to reduce mutual interference between a plurality of sets of molds to provide a press machine and a pressed product manufacturing method with improved processing accuracy. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanation diagram showing the structure of a machine of a preferred embodiment. 
     FIG. 2 is an internal block diagram showing the servo amplifier of FIG.  1 . 
     FIGS. 3 and 4 are flow charts showing the procedure of arithmetic processing by the CPU in FIG.  1 . 
     FIG. 5 is a timing chart used to explain operation of the machine of FIG.  1 . 
     FIG. 6 is an explanation diagram showing the structure of a conventional machine. 
     FIG. 7 is an internal block diagram showing the servo amplifier of FIG.  6 . 
     FIG. 8 is a timing chart used to explain operation of the machine of FIG.  6 . 
     FIG. 9 is a flow chart showing the procedure of arithmetic processing by the CPU in FIG.  6 . 
     FIGS. 10 and 11 are flow charts showing the procedures of steps S 54  and S 57  of FIG. 9, respectively. 
     FIG. 12 is a timing chart used to explain operation of the machine of FIG.  6 . 
     FIGS. 13 to  16  are timing charts used to explain problems of the machine of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. Structure 
     FIG. 1 is an explanation diagram showing the structure of a press machine according to a preferred embodiment of the present invention. This machine  101  has a bottom base  1  installed on the floor, a pair of supports  5   a  and  5   b  uprightly provided on the bottom base  1 , and a top base  2  supported on the supports  5   a  and  5   b . The bottom base  1 , supports  5   a  and  5   b , and top base  2  fixedly coupled to each other form a frame stand  16 . A pair of fixed molds  3   a  and  3   b  are fixed on the upper surface of the bottom base  1 . 
     A pair of a (first) servo motor  6   a  and a (second) servo motor  6   b  are fixed on the top base  2 , which are located above the fixed molds  3   a  and  3   b , respectively. The servo motors  6   a  and  6   b  are respectively in mesh with ball threads  7   a  and  7   b , which rotate to individually drive the ball threads  7   a  and  7   b  in the vertical direction. Moving molds  4   a  and  4   b  are fixed at the lower ends of the ball threads  7   a  and  7   b , respectively. 
     The moving molds  4   a  and  4   b  are located right above the fixed molds  3   a  and  3   b  to face the fixed molds  3   a  and  3   b , respectively. The servo motors  6   a  and  6   b  rotate in the normal rotation and reverse rotation directions to move the moving molds  4   a  and  4   b  in the mold-closing direction (i.e. downward) and in the mold-opening direction (i.e. upward). 
     The servo motors  6   a  and  6   b  are supplied with (electric) current from a (first) servo amplifier  8   a  and a (second) servo amplifier  8   b , respectively. The servo amplifiers  8   a  and  8   b  are individually controlled by an amplifier controlling unit  15 , so that the magnitudes of currents supplied to the servo motors  6   a  and  6   b , i.e., the amounts of passed currents are controlled individually. The amplifier controlling unit  15  includes a CPU  10 , a pulse generator  9 , a pulse counter  11 , a DA converter  12  as a torque limit directing portion, and an AD converter  13  as a torque monitor. The amplifier controlling unit  15  realizes given operation of the moving molds  4   a  and  4   b  through the servo amplifiers  8   a  and  8   b  and the servo motors  6   a  and  6   b.    
     FIG. 2 is a block diagram showing the inside structure of the servo amplifier  8   a , which is representative of the servo amplifiers  8   a  and  8   b . The servo amplifier  8   a  is supplied with a directing value X 0  related to the operating position of the servo motor  6   a  (i.e. the rotating position of the rotor) from the pulse generator  9  and a measured value X related to the operating position of the servo motor  6   a  from an encoder  20 . The encoder  20  is constructed as a known rotary encoder, for example. Similarly to those in the conventional machine  151 , the directing value X 0  and the measured value X are both represented by the number of pulses along the time series as shown in FIG.  8 . 
     The subtracter  21  calculates the difference between the directing value X 0  and the measured value X and outputs the calculated value as a positional deviation ΔX. 
     The amplifier  22  amplifiers the positional deviation ΔX. The subtracter  21  and the amplifier  22  form a position controlling unit. An F/V converter  27  converts the rate of time change of the measured value X, i.e., the frequency of the pulses representing the measured value X to a voltage signal. The subtracter  23  calculates the difference between the output signal from the amplifier  22  and the output signal from the F/V converter  27  and outputs the calculated value as a speed deviation ΔS. The amplifier  24  amplifies the speed deviation ΔS. The subtracter  23 , amplifier  24  and F/V converter  27  form a speed controlling unit. The position controlling unit and the speed controlling unit are included in the control portion of the invention. 
     The output signal from the amplifier  24  is inputted to a current amplifier  26  through a torque detecting/limiting portion  25 . The current amplifier  26  amplifies the input signal and supplies a current I proportional in magnitude to the input signal to the servo motor  6   a . Thus the control portion functions to control the current I so that the measured value X follows the directing value X 0  at speed proportional to the difference between the measured value X and the directing value X 0  . 
     The torque detecting/limiting portion  25  detects the torque of the servo motor  6   a  through the current I, for example, and sends the detected value to the AD converter  13 . The torque detecting/limiting portion  25  also limits the input signal to the current amplifier  26  so that the detected value of the torque will not exceed a limit value of the torque indicated by the DA converter  12 . Specifically, when the magnitude of the output signal from the amplifier  24  does not exceed a value corresponding to the torque limit value, the torque detecting/limiting portion  25  sends the output signal from the amplifier  24  to the current amplifier  26  as it is, but when the magnitude of the output signal from the amplifier  24  exceeds the value corresponding to the torque limit value, it sends the value corresponding to the torque limit value to the current amplifier  26  in preference to the output signal from the amplifier  24 . 
     The measured value X outputted from the encoder  20  is inputted to the pulse counter  11 , too. The CPU  10  executes arithmetic processing on the basis of the measured value X inputted through the pulse counter  11  and the detected value of the torque inputted through the AD converter  13 . The directing value X 0  is then outputted through the pulse generator  9  and the torque limit value is outputted through the DA converter  12  on the basis of the value calculated in the arithmetic processing. 
     2. Operation 
     The CPU  10  executes the arithmetic processing along the procedure shown in FIG.  9 . However, unlike the conventional machine  151 , the machine  101  executes the arithmetic processing in the weighting operation and withdrawing operation according to the flow charts shown in FIGS. 3 and 4. FIG. 3 shows the internal flow of step S 54  as a representative of steps S 54  and S 55 , and FIG. 4 shows the internal flow of step S 57  as a representative of steps S 57  and S 58 . 
     FIG. 5 is a timing chart showing variations of the target value of the operating speed (i.e. the rate of change in the directing value X 0 ), a torque limit value, the positional deviation ΔX, and the torque of the servo motor  6   a  in the weighting operation and the withdrawing operation carried out on the basis of the processings shown in FIGS. 3 and  4 . Now, referring to FIGS. 3 to  5 , the weighting operation and the withdrawing operation of the machine  101  will be described. 
     When the weighting operation is started, first, the moving mold  4   a  is driven to move in the mold-closing direction at high speed (step S 1 ). During this operation, the target value of the operating speed first increases from zero and stays at a high value when the directing value X 0  reaches a given reference value. The target value of the operating speed then decreases when the directing value X 0  reaches another reference value. Subsequently, the target value of the operating speed is maintained at a constant low value when the directing value X 0  reaches still another reference value (step S 2 ). 
     Similarly to those in the machine  151 , the reference values for defining the operating positions at which the target value of operating speed is changed are previously set through teaching performed prior to the processing in FIG.  9 . The reference value defining the timing for changing from the high-speed moving operation based on step S 1  to the low-speed moving operation based on step S 2  is set so that the moving mold  4   a  is located at such a position that it does not abut on the fixed mold  3   a  when the directing value X 0  reaches that reference value. 
     Hence the moving mold  4   a  moves at high speed from the standby position toward the fixed mold  3   a , whose speed decreases before it hits on the fixed mold  3   a , and then the moving mold  4   a  moves at low speed toward the fixed mold  3   a . This reduces the travel time and also alleviates the impact produced when the moving mold  4   a  and the fixed mold  3   a  hit on each other. 
     At time t 1  at which the operation changes from the high-speed moving operation to the low-speed moving operation, the CPU  10  lowers the torque limit value outputted through the DA converter  12  from a maximum value M set before then to a lower limit value L. The limit value L is taught in advance as a value corresponding to the pressing force applied to the moving mold  4   a  in the press work. 
     The moving mold  4   a  hits the fixed mold  3   a  at a certain point of time in the lowspeed moving operation (at time t 2 ). While the moving mold  4   a  moves at speed approximately equal to the target value until it hits on the fixed mold  3   a , it cannot maintain the speed corresponding to the target value after hitting. Accordingly, after hitting, the positional deviation ΔX increases. Then the speed deviation ΔS increases accordingly and the current I increases. As a result, the torque of the servo motor  6   a  increases. That is to say, the moving mold  4   a  is pressurized against the fixed mold  3   a  through an increasing pressing force. 
     At a certain point of time in the period in which the pressing force is increasing (at time t 3 ), the torque of the servo motor  6   a  reaches the limit value L. The CPU  10  detects this through the AD converter  13  (step S 3 ) and then it raises up the rate of change in the directing value X 0 , i.e. the target value of the operating speed (step S 4 ). As a result, the directing value X 0  rapidly changes in the mold-closing direction, and the positional deviation ΔX rapidly increases accordingly. However, the torque stays at the limit value L because of the function of the torque detecting/limiting portion  25 . Accordingly the moving mold  4   a  is pressed by a constant pressing force corresponding to the limit value L. 
     When the directing value X 0  reaches a further reference value (at time t 4 ), the pressing-in movement operation based on step S 4  is ended and the process moves to step S 5 , and the operating-speed target value is maintained at zero. That is to say, the directing value X 0  is held at a certain value. In this standing-still operation, the moving mold  4   a  is continuously pressed against the fixed mold  3   a  by the constant pressing force corresponding to the limit value L. Press work is carried out throughout from the beginning of pressing to the standing-still operation. When a previously set certain time has elapsed (at time t 5 ), the standing-still operation ends and the withdrawing operation is started. 
     When the withdrawing operation is started, the operating-speed target value is set to a negative large value, e.g. a value whose magnitude is equal to that of the operating-speed target value in the pressing-in movement operation in step S 3  and whose sign is inverted. As a result, the moving mold  4   a  is driven to move at high speed in the mold-opening direction (step S 11 ). Hence the torque of the servo motor  6   a  rapidly decreases. 
     At a certain point of time in the press-in releasing movement operation (at time t 6 ), the moving mold  4   a  separates from the fixed mold  3   a . At this time, the torque of the servo motor  6   a  becomes zero and then the pressing force applied to the moving mold  4   a  also becomes zero. The directing value X 0  further changes in the mold-opening direction to reach still another reference value (at time t 7 ), and then the press-in releasing movement operation ends and the high-speed withdrawing operation based on the processing in step S 12  is started. The reference value defining the timing for changing from the press-in releasing movement operation to the high-speed withdrawing operation (time t 7 ) is set so that the moving mold  4   a  is at a location separated by a given distance or more from the fixed mold  3   a  when the directing value X 0  reaches this reference value. 
     At time t 7 , the torque limit value is raised from the limit value L to the maximum value M. In the high-speed withdrawing operation after time t 7 , the operating-speed target value increases from zero in the mold-opening direction, stays at a high value when the directing value X 0  reaches a given reference value, and then decreases to zero when the directing value X 0  reaches another reference value. Thus the moving mold  4   a  returns to the standby position (at time t 8 ). 
     The number of pulses of the reverse rotation directing signal CCW outputted as the directing value X 0  in the high-speed withdrawing operation is equal to the number of pulses of the normal rotation directing signal CW outputted in step S 1  (high-speed moving operation) and step S 2  (low-speed moving operation). Then the pressing force applied to the moving mold  4   a  is quickly released and the moving mold  4   a  returns to the standby position at high speed. 
     3. Advantages 
     In contrast with the conventional machine  151 , the machine  101  operating as described above has the following advantages. First, the directing value X 0  is further advanced in the mold-closing direction in the pressing-in movement operation after the torque has reached the limit value L, so that the moving molds  4   a  and  4   b  can be pressed by the constant pressing force corresponding to the limit value L even if the intervals between the moving molds  4   a  and  4   b  and the fixed molds  3   a  and  3   b  vary due to mutual interference between the two sets of molds. Specifically, even when a factor to vary the pressing force occurs due to mutual interference between the two sets of molds, it is possible to absorb its effect and perform the press work with stable load. This enhances the accuracy of the press work. 
     Also, since the pressing-in movement is performed at high speed, it is possible to quickly realize the highly stable pressing force. Particularly when the moving molds  4   a  and  4   b  arrive at the fixed molds  3   a  and  3   b  at different points of time, this more effectively avoids the problem of application of excessive load to the mold that has arrived earlier. 
     Moreover, the limit value of the torque is lowered as the operation changes from the high-speed moving operation to the low-speed moving operation and the limit value of the torque is raised as the operation changes from the press-in releasing movement operation to the high-speed withdrawing operation, which enables stable load to be exerted in the press work and reduces the time required for the moving molds  4   a  and  4   b  to travel, thus enhancing the efficiency of the work. 
     Further, since the press-in releasing movement operation is performed with the torque limit value maintained at low value and the torque limit value is raised after the molds are opened by a given distance or more, it is possible to effectively avoid the problem even when the moving molds  4   a  and  4   b  separate from the fixed molds  3   a  and  3   b  at different points of time. 
     4. Modifications 
     (1) The description above has shown the machine in which two sets of molds are coupled to the common frame stand  16 . However, the present invention can generally be applied in a form in which a plurality of sets of molds are coupled to a common frame stand  16 . 
     (2) The transmission mechanism for transmitting the power from the servo motors  6   a  and  6   b  to the moving molds  4   a  and  4   b  is not limited to the ball threads  7   a  and  7   b , but other mechanism such as belt may be adopted instead. In the invention, the wording “the operating position of the motor” is not limited to the rotating position of the rotor, but it may be something else generally related to the operation of the motor, such as the amount of movement of the ball threads  7   a  and  7   b , for example. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.