Abstract:
A control method for an NC machine tool is provided which ensures a high precision of the contouring control, and allows for extension of the drill life, and reduction in machining time. The control method comprises: generating an operation command signal on the basis of the machining program and a time constant; generating a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system ( 106 ) from the generated operation command signal by a position loop gain; generating an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system ( 106 ) from the generated velocity command signal by a velocity loop gain; and controlling a drive motor of the feed drive system ( 106 ) on the basis of the generated electric current command signal for driving thereof, wherein a machining mode is determined from the machining program and, if the machining mode is a drilling mode, at least one of the operation command signal, the velocity command signal and the electric current command signal is modified when the feed drive system ( 106 ) is driven to be retracted opposite to a drilling feed direction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of controlling the operation of a feed drive system and a spindle of an NC machine tool and, more particularly, to a control method which effectively reduces the wear and abrasion of a drill edge during a drilling with a drill. 
     2. Description of the Prior Art 
     In recent years, NC machine tools have been designed so that a spindle and a feed drive system operate at higher speeds and at higher accelerations. It is known that the higher-speed and higher-acceleration operation extends the drill life. This is supposedly because the drill of such a high-speed and high-acceleration machine tool is subjected to a slow feed process for a shorter period of time when retracted opposite to a drilling feed direction, and a drill edge is kept in friction contact with the bottom of a drilled hole for a shorter period of time than in a conventional-speed NC machine tool. This will be explained in detail with reference to FIG.  14 . FIG. 14 shows velocity fluctuations in the feed rate of the drill in the vicinity of the bottom of the hole in the high-speed and high-acceleration machine tool (solid line) and in the conventional machine tool (broken line) Provided that the drill edge is in friction contact with the bottom of the hole when the feed rate is within a range of ±2 m/min, the friction contact lasts for about 0.13 second in the case of the high-speed and high-acceleration machine tool and for 0.26 second in the case of the conventional machine tool, as shown in FIG.  14 . In the conventional machine tool, the period during which the drill edge is kept in friction contact with the bottom of the hole is longer by 0.13 second than in the high-speed and high-acceleration machine tool, so that the drill life is short. 
     The operations of the spindle and the feed drive system are typically controlled by a controller as shown in FIG.  15 . As shown, the controller  100  includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , and a spindle controlling section  105 . The machining program storing section  101  stores a machining program preliminarily created. The program analyzing section  102  analyzes the machining program stored in the machining program storing section  101  to pick out commands concerning the rotation of the spindle, and the feed rate and feed position of the feed drive system from the machining program, and then sends a command signal concerning the rotation of the spindle to the spindle drive controlling section  105  and command signals indicative of the feed rate and feed position of the feed drive system  106  to the command generating and distributing section  103 . 
     The spindle drive controlling section  105  controls a spindle drive system  107  according to the received command signal for driving thereof. The command generating and distributing section  103  determines target feed positions at regularly spaced time points for the operation of the feed drive system  106  on the basis of the received command signals and a predetermined time constant to generate operation command signals indicative of the respective target feed positions, and then transmits the operation command signals one after another to the feed drive controlling section  104 . The feed drive controlling section  104  generates a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system  106  from a received operation command signal by a position loop gain K p . Then, the feed drive controlling section  104  generates an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system  106  from the generated velocity command signal by a velocity loop gain K v . The feed drive controlling section  104  further generates an output by multiplying a deviation of a present drive electric current signal fed back from the feed drive system  106  from the generated electric current command signal by an electric current loop gain K I , and then transmits the output as a drive command signal to the feed drive system  106 . The operation of the feed drive system  106  is controlled on the basis of the received drive command signal. Although the single feed drive system is shown in FIG. 15, machine tools such as machining centers generally have a plurality of feed drive systems  106 , and the command generating and distributing section  103  and the feed drive controlling section  104  are provided for each of the plurality of feed drive systems  106 . 
     The NC machine tool (e.g., machining center) is adapted to perform a variety of machining operations such as end milling, boring, reaming and milling. In particular, the end milling is generally employed for contouring control, so that it is important to precisely control the respective feed drive systems  106  in order to achieve the contour of great precision. Even if the respective feed drive systems  106  are simultaneously driven, parameters including the position loop gains K p , the feed velocity loop gains K v  and the cutting feed time constants for the respective feed drive systems are set at the same levels in order to prevent reduction in the precision of the contouring control. In the aforesaid high-speed and high-acceleration machine tool, the machining operations are each performed at a spindle rotation speed of 20,000 to 30,000 m −1  or greater and at a feed rate of 10 to 20 m/min or greater, so that the time constant is set at such a level that the feed drive systems  106  are each driven at an acceleration of lower than 0.1 G during the machining operation. 
     In the conventional machine tool, the parameters for the feed drive systems  106  are thus set mainly for the contouring control. Therefore, the parameter settings are not necessarily optimized for the drilling in order to reduce the stagnant time of the drill in the bottom of the drilled hole for reduction of the wear and abrasion of the drill and for extension of the drill life. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a control method for an NC machine tool, which ensures a high precision of the contouring control, and allows for extension of the drill life, and reduction in machining time. 
     In accordance with the present invention to achieve the aforesaid object, the present control method for an NC machine tool comprises generating an operation command signal on the basis of a machining program and a time constant, generating a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system from the generated operation command signal by a position loop gain, generating an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system from the generated velocity command signal by a velocity loop gain, and controlling a drive motor of the feed drive system on the basis of the generated electric current command signal, wherein a machining mode prescribed in the machining program is determined and, if the machining mode is a drilling mode, a predetermined operation modification value is added to the operation command signal to generate the velocity command signal and then the electric current command signal, and the drive motor of the feed drive system is driven and controlled on the basis of the generated electric current signal, when the feed drive system is driven to be retracted opposite to a drilling feed direction. 
     With this arrangement, where the machining mode is the drilling mode, the predetermined operation modification value is added to the generated operation command signal and the generation of the velocity command signal is based on the resulting operation command signal when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than a rapid feed rate to be employed in a non-drilling, so that a slow feed period or a stagnant time during which the drill stays in a drilled hole can be reduced when the feed direction is reversed. Thus, a drill edge is in friction contact with the bottom of the drilled hole for a shorter period of time. This extends the drill life and, in addition, reduces a machining time. After completion of every drilling, the added operation modification value is canceled. 
     In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, a predetermined velocity modification value is added to the generated velocity command signal to generate the electric current command signal and the drive motor of the feed drive system is driven and controlled on the basis of the generated electric current signal, when the feed drive system is driven to be retracted opposite to the drilling feed direction. 
     With this arrangement, the predetermined velocity modification value is added to the generated velocity command signal and the generation of the electric current command signal is based on the resulting velocity command signal when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than the rapid feed rate to be employed in the non-drilling as described above, so that the stagnant time of the drill can be reduced when the feed direction is reversed. Thus, the drill edge is in friction contact with the bottom of the drilled hole for a shorter period of time. This extends the drill life and, in addition, reduces the machining time. After completion of every drilling, the added velocity modification value is canceled. 
     In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the feed drive system is driven and controlled by employing a position loop gain having a greater value than a position loop gain to be employed in the non-drilling mode when the feed drive system is driven to be retracted opposite to the drilling feed direction. 
     With this arrangement, the velocity command signal is generated by employing the position loop gain which has a greater value than the position loop gain to be used in the non-drilling mode when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than the rapid feed rate to be employed in the non-drilling as described above, so that the stagnant time of the drill can be reduced when the feed direction is reversed. Thus, the same effects as described above are provided. After completion of every drilling, the position loop gain is reset to an ordinary value. 
     In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the feed drive system is driven and controlled by employing a velocity loop gain having a greater value than a velocity loop gain to be employed in the non-drilling mode when the feed drive system is driven to be retracted opposite to the feed direction. 
     With this arrangement, the electric current command signal is generated by employing the velocity loop gain which has a greater value than the velocity loop gain to be used in the non-drilling mode when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than the rapid feed rate to be employed in the non-drilling as described above, so that the stagnant time of the drill can be reduced when the feed direction is reversed. Thus, the same effects as described above are provided. After completion of every drilling, the velocity loop gain is reset to an ordinary value. 
     In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, a bell-shaped time constant is employed as the time constant when the feed drive system is driven in the drilling feed direction, and a linear time constant is employed as the time constant when the feed drive system is driven in a retracting direction. 
     With this arrangement, the operation command signal is generated by employing the bell-shaped time constant when the feed drive system is driven in the drilling feed direction, and by employing the linear time constant when the feed drive system is driven in the retracing direction. The linear time constant allows for quicker rise of the feed rate than the bell-shaped time constant. Therefore, the traveling speed of the drill can be increased when the feed drive system is driven to be retracted opposite to the drilling feed direction. As a result, the stagnant time of the drill can be reduced when the feed direction of the drill is reversed. Thus, the same effects as described above are provided. After completion of every drilling, the time constant is reset from the linear time constant to the bell-shaped time constant. 
     In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the feed drive system is driven and controlled by reducing the time constant to be used when the feed drive system is driven in the drilling feed direction to a value smaller than a time constant to be used when the feed drive system is driven in the non-drilling mode. The term “time constant” herein means a rise time to be elapsed until the feed drive system reaches a predetermined feed rate. 
     With this arrangement, the time constant having a smaller value than the time constant to be employed in the non-drilling mode is employed for the generation of the operation command signal when the feed drive system is driven in the drilling feed direction. Therefore, the feed drive system is stopped at the bottom of the drilled hole at a high deceleration rate, and then its traveling direction is reversed. Thus, the stagnant time of the drill in the bottom of the hole can be reduced, so that the drill edge is in friction contact with the bottom of the hole for a shorter period of time. This extends the drill life and reduces the machining time. After completion of every drilling, the time constant is reset to an original value. 
     In the aforesaid control method for driving and controlling the drive motor of the feed drive system, and in a control method for controlling the rotation speed of a spindle according to the machining program, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the rotation speed of the spindle is decelerated when the feed drive system is driven to be retracted opposite to the drilling feed direction. 
     With this arrangement, the spindle is rotated at a lower rotation speed when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, a distance during which the drill edge is kept in friction contact with the bottom of the drilled hole is reduced when the feed drive system is driven to be retracted, whereby the drill life can be extended. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a controller to be employed for implementing a control method according to a first embodiment of the present invention; 
     FIG. 2 is a block diagram illustrating a controller to be employed for implementing a control method according to a modification of the first embodiment; 
     FIGS. 3 and 4 are graphs for explaining effects provided by the controller shown in FIG. 2; 
     FIG. 5 is a block diagram illustrating a controller to be employed for implementing a control method according to a second embodiment of the present invention; 
     FIG. 6 is a block diagram illustrating a controller to be employed for implementing a control method according to a third embodiment of the present invention; 
     FIG. 7 is a block diagram illustrating a controller to be employed for implementing a control method according to a modification of the third embodiment; 
     FIG. 8 is a graph for explaining effects provided by the controller shown in FIG. 7; 
     FIG. 9 is a block diagram illustrating a controller to be employed for implementing a control method according to a fourth embodiment of the present invention; 
     FIG. 10 is a block diagram illustrating a controller to be employed for implementing a control method according to a fifth embodiment of the present invention; 
     FIG. 11 is a graph for explaining effects provided by the controller shown in FIG. 10; 
     FIG. 12 is a block diagram illustrating a controller to be employed for implementing a control method according to a sixth embodiment of the present invention; 
     FIG. 13 is a block diagram illustrating a controller to be employed for implementing a control method according to a seventh embodiment of the present invention; 
     FIG. 14 is a graph for explaining fluctuations in feed rate measured in the vicinity of the bottom of a drilled hole during a drilling performed according to a conventional control method; and 
     FIG. 15 is a block diagram illustrating a conventional controller for controlling an NC machine tool. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Several embodiments of the present invention will hereinafter be described with reference to the attached drawings. 
     An explanation will be given to a first embodiment of the present invention. FIG. 1 is a block diagram schematically illustrating the construction of a controller according to the first embodiment. The controller  1  according to this embodiment includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a reversal recognition section  2 , an operation modification section  3 , and an operation modification value storing section  4 . 
     The machining program storing section  101  stores a machining program preliminarily created. The program analyzing section  102  is adapted to analyze the machining program stored in the machining program storing section  101  to pick out a command concerning a feed position from the machining program, and to transmit a command signal indicative of the feed position to the command generating and distributing section  103 . 
     The command generating and distributing section  103  is adapted to determine target feed positions at regularly spaced time points for the operation of a feed drive system  106  on the basis of the received command signal and a time constant to generate operation command signals indicative of the respective target feed positions, and to transmit the operation command signals one after another to the feed drive controlling section  104 . 
     The feed drive controlling section  104  is adapted to generate a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system  106  from a received operation command signal by a position loop gain K p , to generate an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system  106  from the generated velocity command signal by a velocity loop gain K v , to generate an output by multiplying a deviation of a present driving electric current signal fed back from the feed drive system  106  from the generated electric current command signal by an electric current loop gain K I , and to transmit the output as a drive command signal to the feed drive system  106 . The operation of the feed drive system  106  is controlled on the basis of the received drive command signal. 
     The reversal recognition section  2  starts the following process upon recognition of a drilling start command picked out from the machining program by the program analyzing section  102 . The reversal recognition section  2  is adapted to output a process implementation signal to the operation modification section  3  upon recognition of an operation command which is generated by the command generating and distributing section  103  for commanding to start the retraction of the feed drive system  106 , and to output a process cancellation signal to the operation modification section  3  upon recognition of an operation command which is generated by the command generating and distributing section  103  for commanding to end the retraction of the feed drive system  106 . The reversal recognition section  2  ends this process upon recognition of a drilling termination command picked out from the machining program by the program analyzing section  102 . 
     The operation modification value storing section  4  stores a predetermined operation modification value. The operation modification section  3  is adapted to read the operation modification value from the operation modification value storing section  4  upon reception of the process implementation signal applied from the reversal recognition section  2 , to add the operation modification value to an operation command signal generated by the command generating and distributing section  103  for modification of the operation command signal, and to input the modified operation command signal to the feed drive controlling section  104 . The operation modification section  3  is further adapted to cancel the operation modification value added for the modification (through subtraction) upon reception of the process cancellation signal applied from the reversal recognition section  2 , and to input the resulting operation command signal to the feed drive controlling section  104 . 
     In the controller  1 , the program analyzing section  102  picks out commands on the rotation of a spindle, the feed rate and feed position of the feed drive system  106 , and the like from the machining program. Of these commands, the commands on the feed rate and feed position of the feed drive system  106  are transmitted to the command generating and distributing section  103 . If the drilling start command such as defined by a so-called G-code is present in the machining program, the drilling start command is transmitted to the reversal recognition section  2 . 
     Upon reception of the drilling start command signal, the reversal recognition section  2  starts the operation modification process, and monitors operation command signals generated by the command generating and distributing section  103 . Upon recognition of the operation command for commanding to start the retraction of the feed drive system  106 , the reversal recognition section  2  outputs the process implementation signal to the operation modification section  3 . Upon reception of the process implementation signal, the operation modification section  3  reads the operation modification value from the operation modification value storing section  4 , and modifies the operation command signal generated by the command generating and distributing section  103  by adding the operation modification value to the operation command signal. Then, the modified operation command signal is inputted to the feed drive controlling section  104 . 
     Thus, the feed drive controlling section  104  successively generates a velocity command signal, an electric current command signal and a drive command signal on the basis of the modified operation command signal. The operation of the feed drive system  106  is controlled on the basis of the drive command signal finally generated. In the controller  1  according to this embodiment, the feed drive system  106  travels a greater distance per unit time for the retraction thereof in the drilling than in the non-drilling. Therefore, a rapid feed operation can be achieved more quickly for the retraction of the feed drive system in the drilling than in the non-drilling. 
     Upon recognition of completion of the retracting operation, the reversal recognition section  2  transmits the process cancellation signal to the operation modification section  3 . Upon reception of the process cancellation signal, the operation modification section  3  cancels the operation modification value added for the modification. That is, the modification value is subtracted from an operation command signal currently generated by the command generating and distributing section  103 , and the resulting operation command signal is inputted to the feed drive controlling section  104 . Thus, the feed drive controlling section  104  successively generates a velocity command signal, an electric current command signal and a drive command signal on the basis of the operation command signal resulting from the subtraction. The operation of the feed drive system  106  is controlled on the basis of the finally generated drive command signal. Thus, the operation modification value added for the modification is canceled. The reversal recognition section  2  ends the process upon recognition of the drilling termination command picked out from the machining program by the program analyzing section  102 . 
     In the controller  1  according to this embodiment, the rapid feed operation can be achieved more quickly for the retraction of the feed drive system in the drilling than in the non-drilling as described above. Therefore, the slow feed period or the stagnant time of the drill can be reduced when the feed direction of the feed drive system is reversed. As a result, the drill edge is kept in friction contact with the bottom of a drilled hole for a shorter period of time, whereby the drill life can be extended and the machining time can be reduced. 
     In general, the feed drive system  106  includes a feed mechanism having a ball screw thread and a ball nut. Where the controller is directed to high precision of the contouring control, the controller has the function of compensating for a backlash between the ball screw thread and the ball nut. With this function, a backlash value is added to a target feed position generated as an operation command signal by the command generating and distributing section  103  when the feed direction of the feed drive system  106  is reversed from one direction to the other direction, whereby the feed drive system  106  is moved in excess by the backlash value. Therefore, the aforesaid operation modification process can be performed by utilizing the backlash compensating function. FIG. 2 illustrates a controller which is capable of performing the operation modification process by utilizing the backlash compensating function. 
     As shown in FIG. 2, the controller  5  includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a backlash compensation section  6 , a parameter rewriting section  7  and a parameter storing section  8 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103  and the feed drive controlling section  104  have the same constructions as those described above. 
     The parameter storing section  8  stores a predetermined backlash compensation value. The backlash compensation section  6  is adapted to read the backlash compensation value from the parameter storing section  8  upon recognition of an operation command which is generated by the command generating and distributing section  103  for commanding to reverse the feed direction of a feed drive system  106 , to add the backlash compensation value to an operation command signal generated by the command generating and distributing section  103 , and to input the resulting operation command signal to the drive controlling section  104 . The backlash compensation section  6  and the parameter storing section  8  serve for the backlash compensating function. 
     The parameter rewriting section  7  enables the aforesaid operation modification process. More specifically, the parameter rewriting section  7  is adapted to rewrite or replace the backlash compensation value with the operation modification value (which is greater than the backlash compensation value) in the parameter storing section  8  upon recognition of the drilling start command picked out by the program analyzing section  102 , and to rewrite again the operation modification value with the original backlash compensation value upon recognition of the drilling termination command picked out by the program analyzing section  102 . 
     In the controller  5 , the feed drive system  106  is controlled to be moved in excess by the backlash value by the backlash compensation section  6  for elimination of the backlash when the feed direction of the feed drive system  106  is reversed in the non-drilling. When the feed drive system  106  is retracted from the bottom of a drilled hole in the drilling, the feed drive system  106  is controlled to be moved in excess by the operation modification value rewritten by the parameter rewriting section  7 . FIG. 3 is a graph illustrating fluctuations in the feed rate of the feed drive system  106  and the position of the drill edge observed in the vicinity of the bottom of the drilled hole when the driving operation is compensated with the backlash value (in this case 0 μm) stored in the parameter storing section  8 , and FIG. 4 is a graph illustrating fluctuations in the feed rate of the feed drive system  106  and the position of the drill edge observed in the vicinity of the bottom of the drilled hole when the driving operation is modified with the operation modification value (in this case 300 μm) rewritten by the parameter rewriting section  7 . As can be seen from these graphs, the feed drive system  106  is retracted from the bottom of the hole at a higher acceleration, and the stagnant time of the drill in the bottom of the hole is reduced by modifying the operation command signal with the greater operation modification value. 
     In the controller  5 , the rapid feed operation can be performed at a higher speed for the retraction of the feed drive system  106  in the drilling than in the non-drilling as in the controller  1 . Therefore, the slow feed period or the stagnant time of the drill can be reduced when the feed direction is reversed. As a result, the drill edge is in friction contact with the bottom of the drilled hole for a shorter period of time, whereby the drill life can be extended and the machining time can be reduced. 
     To demonstrate the effects of the control method of this embodiment, the following test was performed. With the use of a coated carbide solid drill with an oil hole (MDW085MHK available from Sumitomo Denko Co., Ltd.) having a diameter of 8.5 mm, a workpiece of FC250 was drilled at a cutting speed of 150 m/min at a feed rate of 0.4 mm/min for formation of holes each having a depth of 26.5 mm. Where the backlash compensation process was performed with a backlash compensation value of 0 μm (which is equivalent to a case where the operation modification process was not performed), the drill life was expired when 312 holes were formed through the drilling. On the contrary, where the operation modification process was performed with an operation modification value of 300 μm, the drill life was expired when 624 holes were formed through the drilling. Thus, the control method of this embodiment extended the drill life. 
     Next, an explanation will be given to a second embodiment of the present invention. FIG. 5 is a block diagram schematically illustrating the construction of a controller to be employed for implementing a control method according to the second embodiment. As shown, the controller  10  according to this embodiment includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a reversal recognition section  2 , a position loop gain changing section  11 , and a position loop gain storing section  12 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103 , the feed drive controlling section  104  and the reversal recognition section  2 , which have the same constructions as those of the controller  1  of FIG. 1 according to the first embodiment, are designated by the same reference characters, and no detailed explanation will be given thereto. 
     The position loop gain storing section  12  stores a position loop gain K p  for the drilling which is set at a level higher by the greatest possible degree than a position loop gain K p  for the non-drilling. The position loop gain changing section  11  is adapted to read the position loop gain K p , from the position loop gain storing section  12  upon reception of the process implementation signal applied from the reversal recognition section  2  to update the position loop gain K p  for use in the driving operation to be performed by the feed drive controlling section  104 , and to reset the updated position loop gain K p  to the original value upon reception of the process cancellation signal applied from the reversal recognition section  2 . 
     In the controller  10 , when the command generating and distributing section  103  generates an operation command for commanding to start the retraction of the feed drive system  106  from the bottom of a drilled hole, the process implementation signal is outputted from the reversal recognition section  2  to the position loop gain changing section  11 , and the position loop gain K p  to be used by the feed drive controlling section  104  is changed to the position loop gain K p  for the drilling by the position loop gain changing section  11 . Thus, the feed drive system  106  is driven to be retracted at a feed rate higher than a rapid feed rate to be employed in the non-drilling. Thus, the controller  10  reduces the stagnant time of the drill when the feed direction of the drill is reversed to the retracting direction, and provides the same effects as provided by the aforesaid controller  1 . 
     Upon completion of every drilling, the process cancellation signal is outputted from the reversal recognition section  2  to the position loop gain changing section  11 , and the position loop gain K p  to be used by the feed drive controlling section  104  is reset to the original value by the position loop gain changing section  11 . 
     Next, an explanation will be given to a third embodiment of the present invention. FIG. 6 is a block diagram schematically illustrating the construction of a controller to be employed for implementing a control method according to the third embodiment. As shown, the controller  15  according to this embodiment includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a reversal recognition section  16 , a velocity modification section  17 , and a velocity modification value storing section  18 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103  and the feed drive controlling section  104 , which have the same constructions as those of the controller  1  of FIG. 1 according to the first embodiment, are designated by the same reference characters, and no detailed explanation will be given thereto. 
     The reversal recognition section  16  is adapted to start an operation modification process upon recognition of the drilling start command picked out by the program analyzing section  102 , to output a process implementation signal to the operation modification section  3  upon recognition of an operation command which is generated by the command generating and distributing section  103  for commanding to start the retraction of a feed drive system  106 , and to end the operation modification process upon recognition of the drilling termination command picked out by the program analyzing section  102 . 
     The velocity modification value storing section  18  stores a predetermined velocity modification value. The velocity modification section  17  is adapted to read the velocity modification value from the velocity modification value storing section  18  upon reception of the process implementation signal applied from the reversal recognition section  16 , and to input the velocity modification value to the feed drive controlling section  104  in which the velocity modification value is added to a velocity command signal generated by multiplying an operation command signal by the position loop gain K p  for modification of the velocity command signal. 
     In the controller  15 , when the command generating and distributing section  103  generates the operation command for commanding to start the retraction of the feed drive system  106  from the bottom of a drilled hole, the process implementation signal is outputted from the reversal recognition section  16  to the velocity modification section  17 . Upon reception of the process implementation signal, the velocity modification section  17  adds the velocity modification value to the velocity command signal generated by the feed drive controlling section  104  for the modification of the velocity command signal. In the feed drive controlling section  104 , a drive command signal is generated on the basis of the modified velocity command signal. The operation of the feed drive system  106  is controlled on the basis of the drive command signal thus generated. Accordingly, the feed drive system  106  is driven to be retracted at a feed rate higher than the rapid feed rate to be employed in the non-drilling, so that the stagnant time of the drill can be reduced when the feed direction is reversed to the retracting direction. Thus, the controller  15  provides the same effects as those provided by the aforesaid controller  1 . 
     In the meanwhile, a known problem associated with a feed drive system of a common machine tool is that a projection is liable to be formed on a workpiece due to an error occurring in positional control at the change over of a quadrant during an arc cutting operation. The positional control error is supposedly attributable to a delay in generation of a torque of a servo motor for overcoming a static friction generated at the reversal of the feed direction of the feed drive system due to the frictional resistance of the feed drive system. A similar error is caused by a lost motion which occurs due to torsion of a ball screw. Therefore, a controller which is directed to high precision of contouring control has a compensation function (so-called lost motion compensation function) for temporarily increasing the feed rate of the feed drive system  106  at the reversal of the feed direction to eliminate the follow-up delay. This function is realized by adding a compensation value to a velocity command signal or temporarily increasing a velocity loop gain to increase the feed rate of the feed drive system  106  when the feed direction of the feed drive system  106  is reversed from one direction to the other direction. The aforesaid process for adding the modification value to the velocity command signal can be performed by utilizing the lost motion compensation function. FIG. 7 illustrates a controller which is capable of performing this process by utilizing the lost motion compensation function. 
     As shown in FIG. 7, the controller  20  includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a lost motion compensation section  21 , a parameter rewriting section  22  and a parameter storing section  23 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103  and the feed drive controlling section  104  have the same constructions as those described above. 
     The parameter storing section  23  stores a predetermined lost motion compensation value. The lost motion compensation section  21  is adapted to read the compensation value from the parameter storing section  23  upon recognition of an operation command which is generated by the command generating and distributing section  103  for commanding to reverse the feed direction of the feed drive system  106 , and to input the compensation value to the drive controlling section  104 , in which the compensation value is added to a velocity command signal generated therein. 
     The parameter rewriting section  22  is adapted to rewrite the lost motion compensation value stored in the parameter storing section  23  with a lost motion compensation value for the drilling which is set higher by the greatest possible degree than a lost motion compensation value for the non-drilling upon recognition of the drilling start command picked out by the program analyzing section  102 , and to rewrite again the lost motion compensation value with the original value upon recognition of the drilling termination command picked out by the program analyzing section  102 . 
     In the controller  20 , the lost motion compensation section  21  increases the feed rate of the feed drive system  106  by adding the lost motion compensation value to the velocity command signal for elimination of stick-slip and the lost motion whenever the feed direction of the feed drive system  106  is reversed in the non-drilling. When the feed drive system  106  is retracted from the bottom of a drilled hole in the drilling, the feed drive system  106  is driven on the basis of the lost motion compensation value for the drilling rewritten by the parameter rewriting section  22  thereby to reach the rapid feed rate more quickly than in the non-drilling. FIG. 8 is a graph illustrating a fluctuation in the position of the drill observed in the vicinity of the bottom of the drilled hole when the feed operation is feedback-controlled by a servo motor with a lost motion compensation value of 0 (broken line) and a fluctuation in the position of the drill observed in the vicinity of the bottom of the drilled hole when the feed operation is feedback-controlled by a servo motor with a lost motion compensation value of 100 rewritten by the parameter rewriting section  22  (solid line). As can be seen from the FIG. 8, the feed drive system  106  can be retracted from the bottom of the hole at a higher acceleration, and the stagnant time of the drill in the bottom of the hole is reduced by employing the greater lost motion compensation value. 
     In the controller  20 , the rapid feed operation can be performed at a higher feed rate for the retraction of the feed drive system  106  in the drilling than in the non-drilling as in the controller  1 . Therefore, the slow feed period or the stagnant time of the drill can be reduced when the feed direction of the drill is reversed. As a result, the period, when the drill edge is in friction contact with the bottom of the drilled hole, is short, whereby the drill life can be extended and the machining time can be reduced. 
     To demonstrate the effects of the control method of this embodiment, the following test was performed. With the use of a coated carbide solid drill with an oil hole (MDW085MHK available from Sumitomo Denko Co., Ltd.) having a diameter of 8.5 mm, a workpiece of ADC12 was drilled at a cutting speed of 500 m/min at a feed rate of 0.5 mm/min for formation of holes each having a depth of 25.5 mm. Where the feed drive system was controlled with a lost motion compensation value of 0 at the reversal thereof, the drill life was expired when 208 holes were formed through the drilling. On the contrary, where the feed drive system was controlled with a lost motion compensation value of 100 at the reversal thereof, the drill life was expired when 312 holes were formed through the drilling. Thus, the control method of this embodiment extends the drill life. 
     Next, an explanation will be given to a fourth embodiment of the present invention. FIG. 9 is a block diagram schematically illustrating the construction of a controller to be employed for implementing a control method according to the fourth embodiment. The controller  25  according to this embodiment includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a reversal recognition section  2 , a velocity loop gain changing section  26 , and a velocity loop gain storing section  27 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103 , the feed drive controlling section  104  and the reversal recognition section  2 , which have the same constructions as those of the controller  1  of FIG. 1 according to the first embodiment, are designated by the same reference characters, and no detailed explanation will be given thereto. 
     The velocity loop gain storing section  27  stores a velocity loop gain K v , for the drilling which is set at a level higher by the greatest possible degree than a velocity loop gain K v  for the non-drilling. The velocity loop gain changing section  26  is adapted to read the velocity loop gain K v  from the velocity loop gain storing section  27  upon reception of a process implementation signal applied from the reversal recognition section  2  to update the velocity loop gain K v  for use in the process to be performed by the feed drive controlling section  104 , and to reset the updated velocity loop gain K v  to the original value upon reception of a process cancellation signal applied from the reversal recognition section  2 . 
     In the controller  25 , when the command generating and distributing section  103  generates an operation command for commanding to start the retraction of the feed drive system  106  from the bottom of a drilled hole, the process implementation signal is outputted from the reversal recognition section  2  to the velocity loop gain changing section  26 , and the velocity loop gain K v  to be used by the feed drive controlling section  104  is changed to the velocity loop gain K v  for the drilling by the velocity loop gain changing section  26 . Thus, the feed drive system  106  is driven to be retracted at a feed rate higher than the rapid feed rate for the non-drilling. Therefore, the controller  25  reduces the stagnant time of the drill when the feed direction of the drill is reversed to the retracting direction, and provides the same effects as those provided by the aforesaid controller  1 . 
     Upon completion of every drilling, the process cancellation signal is outputted from the reversal recognition section  2  to the velocity loop gain changing section  26 , and the velocity loop gain K v  to be used by the feed drive controlling section  104  is reset to the original value by the velocity loop gain changing section  26 . 
     Next, an explanation will be given to a fifth embodiment of the present invention. FIG. 10 is a block diagram schematically illustrating the construction of a controller to be employed for implementing a control method according to the fifth embodiment. As shown, the controller  30  includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a rapid-feed time constant rewriting section  31  and a rapid-feed time constant storing section  32 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103  and the feed drive controlling section  104 , which have the same constructions as those of the controller  1  of FIG. 1 according to the first embodiment, are designated by the same reference characters, and no detailed explanation will be given thereto. 
     The rapid-feed time constant storing section  32  stores a time constant to be employed when the command generating and distributing section  103  determines a target feed position of the feed drive system  106  for a rapid feed operation. The command generating and distributing section  103  is adapted to calculate the target feed position for the rapid feed operation on the basis of the time constant stored in the rapid-feed time constant storing section  32 . High-speed and high-acceleration machine tools generally employ a bell-shaped time constant as shown in FIG. 11 to alleviate a shock exerted at the start of the driving of a feed drive system. 
     The rapid-feed time constant rewriting section  31  stores a bell-shaped time constant (which provides an acceleration-deceleration pattern indicated by a broken line in FIG. 11) and a linear time constant (which provides an acceleration pattern indicated by a solid line in FIG.  11 ), and is adapted to rewrite the time constant stored in the rapid-feed time constant storing section  32  with the linear time constant upon recognition of the drilling start command picked out by the program analyzing section  102 , and to rewrite again the linear time constant with the bell-shaped time constant upon recognition of the drilling termination command picked out by the program analyzing section  102 . 
     In the controller  30 , the time constant stored in the rapid-feed time constant storing section  32  is rewritten with the linear time constant by the rapid-feed time constant rewriting section  31  when the drilling start command is picked out by the program analyzing section  102 , and the rapid feed operation of the feed drive system  106  is controlled on the basis of the linear time constant. With the linear time constant, the feed drive system  106  is controlled to reach the rapid teed rate more quickly than with the bell-shaped time constant as shown in FIG.  11 . Therefore, the feed drive system  106  is driven to be retracted at a feed rate higher than the rapid feed rate in the non-drilling, and the stagnant time of the drill can be reduced when the feed direction of the drill is reversed to the retracting direction. Thus, the controller  30  provides the same effects as those provided by the aforesaid controller  1 . 
     Next, an explanation will be given to a sixth embodiment of the present invention. FIG. 12 is a block diagram schematically illustrating the construction of a controller to be employed for implementing a control method according to the sixth embodiment. As shown, the controller  35  includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a cutting feed time constant rewriting section  36  and a cutting feed time constant storing section  37 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103  and the feed drive controlling section  104 , which have the same constructions as those of the controller  1  of FIG. 1 according to the first embodiment, are designated by the same reference characters, and no detailed explanation will be given thereto. 
     The cutting feed time constant storing section  37  stores a time constant to be employed when the command generating and distributing section  103  determines a target feed position of the feed drive system  106  for a cutting feed operation. The command generating and distributing section  103  is adapted to calculate the target feed position for the cutting feed operation on the basis of the time constant stored in the cutting feed time constant storing section  37 . The cutting feed time constant is generally set at a level optimized for the precision in the desired contour. 
     The cutting feed time constant rewriting section  36  stores a time constant set at such a level that the feed drive system  106  is accelerated or decelerated at the highest possible rate for the drilling, and is adapted to rewrite the time constant stored in the cutting feed time constant storing section  37  with the cutting feed time constant for the drilling upon recognition of the drilling start command picked out by the program analyzing section  102 , and to rewrite again the time constant with the original time constant upon recognition of the drilling termination command picked out by the program analyzing section  102 . 
     In the controller  35 , the time constant stored in the cutting feed time constant storing section  36  is rewritten with the time constant for the drilling by the cutting feed time constant rewriting section  37  when the drilling start command is picked out by the program analyzing section  102 , and the drilling feed operation of the feed drive system  106  is controlled on the basis of the time constant for the drilling. The feed drive system  106  is decelerated at a high deceleration rate corresponding to the time constant for the drilling when the feed drive system  106  reaches the bottom of a drilled hole, so that the stagnant time of the drill in the bottom of the hole can be reduced. Thus, the controller  35  provides the same effects as those provided by the aforesaid controller  1 . 
     Next, an explanation will be given to a seventh embodiment of the present invention. FIG. 13 is a block diagram schematically illustrating the construction of a controller to be employed for implementing a control method according to the seventh embodiment. As shown, the controller  40  according to this embodiment includes a machining program storing section  101 , a program analyzing section  102 , a command generating and distributing section  103 , a feed drive controlling section  104 , a reversal recognition section  2 , a spindle drive controlling section  41 , a velocity modification section  42 , and a velocity modification value storing section  43 . The machining program storing section  101 , the program analyzing section  102 , the command generating and distributing section  103 , the feed drive controlling section  104  and the reversal recognition section  2 , which have the same constructions as those of the controller  1  of FIG. 1 according to the first embodiment, are designated by the same reference characters, and no detailed explanation will be given thereto. 
     The velocity modification value storing section  43  stores a deceleration value as a modification value set at such a level that the speed of the spindle drive system  107  is reduced as much as possible. The velocity modification section  42  is adapted to read the deceleration value from the velocity modification value storing section  43  to input the deceleration value to the spindle drive controlling section  41  upon reception of a process implementation signal applied from the reversal recognition section  2 , and to input a command to the spindle drive controlling section  41  to cancel a deceleration process or increase the speed of the spindle drive system  107  by a value equivalent to the deceleration value upon reception of a process cancellation signal applied from the reversal recognition section  2 . The spindle drive controlling section  41  is adapted to control and drive the spindle drive system  107  on the basis of a signal received from the program analyzing section  102 , and to accelerate or decelerate the rotation speed of the spindle drive system  107  on the basis of the acceleration or deceleration value received from the velocity modification section  42 . 
     In the controller  40 , when the command generating and distributing section  103  generates an operation command for commanding to start the retraction of the feed drive system  106 , the process implementation signal is outputted from the reversal recognition section  2  to the velocity modification section  42 . Upon reception of the process implementation signal, the deceleration value is read out of the velocity modification storing section  43  by the velocity modification section  42 , and outputted to the spindle drive controlling section  41 . The spindle drive controlling section  41  decelerates the rotation speed of the spindle drive system  107  on the basis of the inputted deceleration value. In the controller  40 , the rotation speed of the spindle, i.e., the drill, is thus decelerated when the feed direction of the feed drive system  106  is reversed for the retraction thereof. Therefore, a distance during which the drill edge is kept in friction contact with the bottom of a drilled hole is reduced, whereby the drill life can be extended. 
     Upon recognition of completion of the retracting operation, the reversal recognition section  2  transmits the process cancellation signal to the velocity modification section  42 . Upon reception of the process cancellation signal, the velocity modification section  42  cancels the deceleration process, i.e., inputs a command to the spindle drive controlling section  41  for commanding to increase the rotation speed of the spindle drive system  107  by a value equivalent to the deceleration value. Thus, the rotation speed of the spindle drive system  107  is reset to a rotation speed to be employed for the cutting operation. Upon recognition of the drilling termination command picked out by the program analyzing section  102 , the reversal recognition section  2  ends the process. 
     While the present invention has thus been described in detail by way of the specific embodiments thereof, the embodiments are not intended to limit any other conceivable embodiments of the invention. The aforesaid processes are each implemented alone in the embodiments, but may be implemented in combination.