Patent Publication Number: US-10315282-B2

Title: Control device and control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-085978 filed on Apr. 25, 2017, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a control device and a control method for controlling a machine tool so as to drill a workpiece, in accordance with a machining program. 
     Description of the Related Art 
     Long chips generated during drilling work may cause a tool or a machined surface to be damaged. To deal with this problem, Japanese Laid-Open Patent Publication No. 02-036009 discloses a step feed process (peck drilling) in which a tool (drill) is reciprocated (moved forward and backward) in the axial direction to perform drilling in order to cut up long chips generated during drilling. 
     SUMMARY OF THE INVENTION 
     However, in Japanese Laid-Open Patent Publication No. 02-036009, each time a tool is advanced by a predetermined distance to drill a workpiece, the tool is retracted in a direction opposite to the machining direction, and thus a number of continuous drilling steps are required to complete the entire process of drilling the workpiece. Consequently, problems occur that the machining time is increased and the life time of the tool is shortened. 
     It is therefore an object of the present invention to provide a control device and a control method which make it possible to cut up long chips generated during drilling, shorten the processing time, and prevent the life time of the tool from being shortened. 
     According to a first aspect of the present invention, there is provided a control device for controlling a machine tool so as to drill a workpiece based on a machining program, wherein the machine tool includes a rotary tool configured to drill the workpiece and an axis feed motor configured to move the rotary tool in the axial direction, and the control device includes: 
     a motor control unit configured to, when drilling the workpiece, control the axis feed motor so that the rotary tool is axially moved by intermittently decelerated feed in which the feed rate of the rotary tool is alternately switched between a first feed rate and a second feed rate that is lower than the first feed rate, under the condition in which the following relational equations are satisfied: 
                     F   1     ⁢     t   1       +       F   2     ⁢     t   2         =       F   S     ⁡     (       t   1     +     t   2       )         ;   and                 t   2     =       1   S     ⁢     (         F   1         F   1     -     F   2         +   C     )             
where S is the rotational speed of the rotary tool; F S  is a feed rate specified in accordance with the machining program; F 1  is the first feed rate; F 2  is the second feed rate; t 1  is a first rate moving time for which the rotary tool is moved at the first feed rate in one switching cycle; t 2  is a second rate moving time for which the rotary tool is moved at the second feed rate in one switching cycle; C is a constant; and F 1 &gt;F S &gt;F 2 &gt;0, and C≥0.
 
     According to a second aspect of the present invention, there is provided a control method for controlling a machine tool so as to drill a workpiece based on a machining program, wherein the machine tool includes a rotary tool configured to drill the workpiece and an axis feed motor configured to move the rotary tool in the axial direction, and the control method includes: 
     a motor control step of, when drilling the workpiece, controlling the axis feed motor so that the rotary tool is axially moved by intermittently decelerated feed in which the feed rate of the rotary tool is alternately switched between a first feed rate and a second feed rate that is lower than the first feed rate, under the condition in which the following relational equations are satisfied: 
                     F   1     ⁢     t   1       +       F   2     ⁢     t   2         =       F   S     ⁡     (       t   1     +     t   2       )         ;   and                 t   2     =       1   S     ⁢     (         F   1         F   1     -     F   2         +   C     )             
where S is the rotational speed of the rotary tool; F S  is a feed rate specified in accordance with the machining program F 1  is the first feed rate; F 2  is the second feed rate; t 1  is a first rate moving time for which the rotary tool is moved at the first feed rate in one switching cycle; t 2  is a second rate moving time for which the rotary tool is moved at the second feed rate in one switching cycle; C is a constant; and F 1 &gt;F S &gt;F 2 &gt;0, and C≥0.
 
     According to the present invention, it is possible to cut up or break up long chips generated during drilling work and shorten the machining time of drilling work. In addition, since the machining is continuously performed without retracting the rotary tool, it is possible to prevent the life time of the rotary tool from being shortened. 
     The above and other objects features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a machine tool to be controlled by a control device according to an embodiment of the present invention; 
         FIG. 2  is a graph for explaining axis feed by intermittently decelerated feed of the present embodiment; 
         FIG. 3  is a graph showing an example in which machining time is shortened compared to the ordinary axis feed, by use of the axis feed by the intermittently decelerated feed of the present embodiment; 
         FIG. 4  is a diagram for explaining a rotational angle required for a tool to cut up a long chip by axis feed by intermittently decelerated feed; and 
         FIG. 5  is a functional block diagram of the control device shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A control device and a control method according to the present invention will be detailed hereinbelow by describing preferred embodiments, with reference to the accompanying drawings. 
       FIG. 1  is a schematic configuration diagram of a machine tool  12  to be controlled by a control device (e.g., numerical controller)  10  of the present embodiment. The machine tool  12  machines an unillustrated target object (workpiece) with a tool (e.g., a cutting tool such as a drill)  22  attached to a spindle  20  thereof. The machine tool  12  includes the spindle  20 , a spindle head  24  for rotating the spindle  20  about a Z-axis parallel to the Z-direction (vertical direction), a column  26  for moving the spindle head  24  in the Z-direction, a table  28  disposed under the spindle  20  (negative Z-direction) and configured to support the workpiece, and a table drive unit  30  for moving the table  28  in the X- and Y-directions. Here, it is assumed that the X-direction, the Y-direction, and the Z-direction are ideally orthogonal to each other. 
     By movement of the spindle  20  in the Z-direction and movement of the table  28  in the X- and Y-directions, the machine tool  12  can drill a hole at a desired position on the workpiece and also perform three-dimensional machining on the workpiece. Here, it is assumed that the gravity acts in the negative Z-direction. 
     The tool  22  is held by a tool holder  32 . The tool holder  32  is attachable to and detachable from the spindle  20  so that the tool  22  is attached to the spindle  20  via the tool holder  32 . By inserting the tool holder  32  into a mounting hole (not shown) formed at the front end of the spindle  20 , the tool  22  is attached to the spindle  20 . The tool  22  rotates together with the spindle  20  about the Z-axis parallel to the Z-direction. 
     The machine tool  12  is configured as a machining center in which the tool  22  attached to the spindle  20  can be changed by an automatic tool changer  34 . The automatic tool changer  34  has a turret tool magazine  36 . The tool magazine  36  has a plurality of grips  36   a  arranged in the circumferential direction. Each of the multiple grips  36   a  removably holds a tool  22  via the tool holder  32 . The tool holder  32  is attachable to and detachable from the grip  36   a , and the tool  22  is attached to the grip  36   a  via the tool holder  32 . Examples of the tool  22  may include non-rotating tools, drills, end mills, milling cutters, and the like. 
     A Z-axis driving mechanism for moving the spindle head  24  relative to the column  26  in the Z-axis direction parallel to the Z-direction is coupled to the spindle head  24 . The Z-axis driving mechanism has a Z-axis servomotor  60  (see  FIG. 5 ) and an unillustrated power conversion mechanism (ball screw, nut, etc.) that converts rotational motion of the Z-axis servomotor (axis feed motor)  60  into linear motion and transmits the linear motion to the spindle head  24 . Further, the spindle  20  is rotated about the Z-axis parallel to the Z-direction by driving of a spindle motor  62  (see  FIG. 5 ) provided in the spindle head  24 . Further, the tool magazine  36  is rotated (turned) by an unillustrated turning motor. Driving of the Z-axis servomotor  60  and the spindle motor  62  is controlled by the control device  10 . 
     The column  26  and the table drive unit  30  are supported on a base  40 . The table drive unit  30  includes Y-axis slider members  42 , a saddle  44 , and X-axis slider members  46 . The saddle  44  is supported so as to be movable in the Y-direction with respect to the base  40  via the Y-axis slider members  42 . The table  28  is supported so as to be movable in the X-direction with respect to the saddle  44  via the X-axis slider members  46 . 
     An unillustrated Y-axis driving mechanism for moving the saddle  44  relative to the base  40  in the Y-axis direction parallel to the Y-direction is coupled to the saddle  44 . Similarly, an unillustrated X-axis driving mechanism for moving the table  28  relative to the saddle  44  in the X-axis direction parallel to the X-direction is coupled to the table  28 . The Y-axis driving mechanism has a Y-axis servomotor and a power transmission mechanism (ball screw, nut, etc.) for converting rotational motion of the Y-axis servomotor into linear motion and transmitting the linear motion to the saddle  44 . The X-axis driving mechanism has an X-axis servomotor and a power transmission mechanism (ball screw, nut, etc.) for converting rotational motion of the X-axis servomotor into linear motion and transmitting the linear motion to the table  28 . Driving of the Y-axis servomotor and the X-axis servomotor is controlled by the control device  10 . 
     The machine tool  12  is provided with a splash guard  48  that covers a machining area  12   a  in the machine tool  12  to thereby prevent chips (machining chips, cutting chips) generated during machining from scattering around. The machine tool  12  may be provided with a nozzle (not shown) for ejecting cutting fluid toward the tool  22  during machining. 
     When the machine tool  12  is controlled so as to drill a workpiece, the control device  10  controls the Z-axis servomotor  60  such that the tool (drill)  22  is axially fed (axially moved) by intermittently decelerated axis feed. 
       FIG. 2  is a graph for explaining the axis feed of the tool (drill)  22  by intermittently decelerated axis feed. The following description will be made on the assumption that the tool  22  is a drill (rotary tool) for drilling a hole. In  FIG. 2 , the horizontal axis shows time and the vertical axis shows the position of the drill  22  with respect to the Z-direction. In  FIG. 2 , the solid line shows movement of the drill  22  by intermittently decelerated axis feed, and the dashed-dotted line shows movement of the drill  22  when the drill  22  is axially fed by a normal axis feed (i.e., when the drill  22  is axially fed at a constant feed rate F S ). Here, the constant feed rate F S  is a feed rate specified in accordance with the machining program. 
     As shown in  FIG. 2 , in accordance with the intermittently decelerated axis feed, the drill  22  is axially fed such that the feed speed (moving speed) is alternately switched between the first feed rate F 1  and the second feed rate F 2 . At this time, t 1  denotes a moving time (machining time) for which the drill is moved (machines a workpiece) at the first feed rate F 1  in one switching cycle, and t 2  denotes a moving time (machining time) for which the drill is moved (machines a workpiece) at the second feed rate F 2  in one switching cycle. In the following description, the moving time t 1  is referred to as the first rate moving time t 1 , and the moving time t 2  is referred to as the second rate moving time t 2 . 
     That is, the drill  22  is moved at the first feed rate F 1  for the first rate moving time t 1 , then the drill  22  is moved at the second feed rate F 2  for the second rate moving time t 2 , and thereafter the drill  22  is again moved at the first feed rate F 1  for the first rate moving time t 1 . In the above manner, the cycle is repeated. 
     The first feed rate F 1 , the second feed rate F 2  and the feed rate F S  have the relationship of F 1 &gt;F S &gt;F 2 &gt;0. In this manner, since the drill  22  is continuously advanced to machine a workpiece without retraction, it is possible to prevent the life time of the drill  22  from being shortened. Here, the direction in which the drill  22  moves down (in the negative Z-direction) toward the workpiece is defined as the positive direction. 
     The moving time from a machining start position Z 0  to a machining end position Z 1  in a case that the drill  22  is axially moved by the intermittently decelerated axis feed is denoted by T′, while the moving time from the machining start position Z 0  to the machining end position Z 1  in a case that the drill  22  is axially moved at a constant feed rate F S  is denoted by T. In order to make the time T′ equal to or shorter than the time T, the following relational equation (1) should be satisfied.
 
 F   1   t   1   +F   2   t   2   =F   S ( t   1   +t   2 )  (1)
 
     When this relation is satisfied, the time T′ is shorter by ΔT than the time T, depending on the distance from the machining start position Z 0  to the machining end position Z 1 , as shown in  FIG. 3 . That is, the machining time becomes shorter than the time T by ΔT. 
     In order to cut up long chips generated during drilling by intermittently decelerated axis feed of the drill  22 , it is necessary to satisfy the following relational equation (2). Here, as shown in  FIG. 4 , θ is a rotational angle of the drill  22  required for the drill  22  to rotate from when the feed rate is switched from the first feed rate F 1  to the second feed rate F 2  until when a chip generation surface G 2  formed by the cutting edge of the drill  22  that is axially fed to drill a workpiece at the second feed rate F 2  and at a rotational speed S intersects a chip generation plane G 1  formed by the cutting edge of the drill  22  that is axially fed to drill the workpiece at the first feed rate F 1  and at the rotational speed S. The long chip can be cut off when the generation plane G 2  intersects the generation plane G 1 . Here, it is defined that f 1 =F 1 /S, f 2 =F 2 /S. 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     2 
                     ⁢ 
                     
                       π 
                       ⁡ 
                       
                         ( 
                         
                           
                             f 
                             1 
                           
                           
                             
                               f 
                               1 
                             
                             - 
                             
                               f 
                               2 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The time t 2  required for the drill  22  to rotate by the rotation angle θ can be given by the following relational equation (3). 
     
       
         
           
             
               
                 
                   
                     t 
                     2 
                   
                   = 
                   
                     θ 
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       S 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Accordingly, the following relational equation (4) can be derived from the relational equations (2) and (3). 
                     t   2     =       1   S     ⁢     (       F   1         F   1     -     F   2         )               (   4   )               
where f 1 =F 1 /S, f 2 =F 2 /S.
 
     Actually, due to elastoplastic deformation of chips, the rotational angle required for breaking the chips may become longer than θ expressed in the relational equation (2). Taking this into consideration, t 2  can be given by the following relational equation (5). Note that C is 0 or more (C≥0). When C is greater than 0 (C&gt;0), C becomes a predetermined constant and is given as a parameter. 
     
       
         
           
             
               
                 
                   
                     t 
                     2 
                   
                   = 
                   
                     
                       1 
                       S 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             F 
                             1 
                           
                           
                             
                               F 
                               1 
                             
                             - 
                             
                               F 
                               2 
                             
                           
                         
                         + 
                         C 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     From the above, the first feed rate F1, the second feed rate F 2 , the first rate moving time t 1 , and the second rate moving time t 2  that satisfy the relational equations (1) and (5) should be determined. Information (predetermined pieces of prescribed information) necessary to determine F 1 , F 2 , t 1 , t 2  based on the relational equations (1) and (5) is given as arguments and/or parameters in the machining program. In the present embodiment, the information (predetermined pieces of prescribed information) necessary for determining F 1 , F 2 , t 1 , t 2  is given as arguments and parameters. Specifically, as the necessary information, t 1  is given as an argument, and α (=F 2 /F 1 ) as a parameter. It should be noted that the rotational speed S and the feed rate F S  are specified by the machining program as usual. 
       FIG. 5  is a functional block diagram of the control device  10  in a case where the machine tool  12  is controlled so as to perform drilling on a workpiece. The control device  10  includes a program storage unit  50 , a program analysis unit  52 , a parameter storage unit  54 , an information determining unit  56 , and a motor control unit  58 . The control device  10  is made up of a computer having a processor such as a CPU, a storage medium for storing data, and the like. 
     The program storage unit  50  stores therein a machining program. The program analysis unit  52  reads and analyzes the machining program stored in the program storage unit  50  and converts the program into computer-recognizable command values. The command values are output to the motor control unit  58 . The command value for specifying the rotational speed S of the spindle motor  62  (drill  22 ) and the command value for specifying the feed rate F S  of the drill  22  are output also to the information determining unit  56 . It should be noted that the program analysis unit  52  may not necessarily output to the motor control unit  58  the command value for specifying the feed rate F S  of the drill  22 . Further, the program analysis unit  52  also outputs information (command values) indicating the first rate moving time t 1  given as an argument to the information determining unit  56 . 
     For example, the block of the drilling cycle command for instructing a drilling work included in the machining program can be represented as: 
     G81.1 Z-20.0 R2.0 F800 xx yy. 
     “G81.1” is a command G-code indicating that this block is a block of a drilling cycle command. “Z-20.0” indicates the machining end position Z 1  of the drilling work. Since “Z-20.0” indicates the depth from the machining surface of the workpiece, the depth is 20.0 mm in the negative Z-direction from the machining surface. “R2.0” indicates the machining start position Z 0  of the drilling work. Since “R2.0” indicates the height from the machining surface in the positive Z-direction, the height from the machining surface is 2.0 mm. If the machining start position Z 0  is set at the machining surface, there is concern that the workpiece and the drill  22  might interfere with each other before the drill  22  starts rotating, so that the machining start position Z 0  is displaced from the height position of the machining surface. 
     “F800” indicates the feed rate F S . “xx yy” is an argument. In the present embodiment, the first rate moving time t 1  is given as an argument. The first rate moving time t 1  is also converted into computer-recognizable information through the program analysis unit  52 . 
     The machining program has a block of rotation command for specifying the rotational speed S of the spindle motor  62  (drill  22 ), and the block of the rotation command is also analyzed by the program analysis unit  52 . 
     The parameter storage unit  54  stores therein parameters. In the present embodiment, since α (=F 2 /F 1 ) is given as a parameter, the parameter storage unit  54  stores information indicating α. The information indicating α stored in the parameter storage unit  54  is read by the information determining unit  56 . 
     The information determining unit  56  acquires the necessary information (α, t 1 ), and based on the acquired necessary information (α, t 1 ) and the relational equations (1) and (5), determines four pieces of information, namely, information indicating the first feed rate F 1 , information indicating the second feed rate F 2 , information indicating the first rate moving time t 1 , and information indicating the second rate moving time t 2 . The information determining unit  56  determines the acquired t 1 , as it is, as the first rate moving time, and calculates the information (F 1 , F 2 , t 2 ) other than the first rate moving time t 1 , to thereby determine the first feed rate F 1 , the second feed rate F 2 , and the second rate moving time t 2 . 
     For example, F 1 , F 2 , t 2  calculated using necessary information (α, t 1 ) and relational equations (1) and (5) are expressed by the following relational equations. The information determining unit  56  outputs the determined four pieces of information (F 1 , F 2 , t 1 , t 2 ) to the motor control unit  58 . 
     
       
         
           
             
               
                 F 
                 1 
               
               = 
               
                 
                   ( 
                   
                     
                       
                         t 
                         1 
                       
                       + 
                       
                         t 
                         2 
                       
                     
                     
                       
                         t 
                         1 
                       
                       + 
                       
                         α 
                         ⁢ 
                         
                             
                         
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                           t 
                           2 
                         
                       
                     
                   
                   ) 
                 
                 ⁢ 
                 
                   F 
                   S 
                 
               
             
             , 
             
               
 
             
             ⁢ 
             
               
                 F 
                 2 
               
               = 
               
                 α 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   F 
                   1 
                 
               
             
             , 
             
               
 
             
             ⁢ 
             
               
                 t 
                 2 
               
               = 
               
                 
                   1 
                   S 
                 
                 ⁢ 
                 
                   ( 
                   
                     
                       1 
                       
                         1 
                         - 
                         α 
                       
                     
                     + 
                     C 
                   
                   ) 
                 
               
             
           
         
       
     
     Note that the information indicating C (constant) may be held in advance in the information determining unit  56  or may be stored in the parameter storage unit  54 . When the information on C is stored in the parameter storage unit  54 , the information determining unit  56  also acquires information on C from the parameter storage unit  54 . 
     Here, since it is conceivable that the first feed rate F 1  exceeds the use condition of the drill  22 , it is preferable to set a limit such as an upper limit on the value of “F 1 /F S ”. When the value of F 1 /F S  is limited to the upper limit, it is necessary to recalculate F 2  and t 2  accordingly. 
     In performing drilling, the motor control unit  58  controls the Z-axis servomotor  60  so that the drill  22  is axially advanced by intermittently decelerated feed in which the feed rate is alternately switched between the first feed rate F 1  and the second feed rate F 2  under the condition that the relational equations (1) and (5) are satisfied. At this time, the motor control unit  58  controls the Z-axis servomotor  60  based on the four pieces of information (F 1 , F 2 , t 1 , t 2 ) determined by the information determining unit  56 , without using the command values based on “F800” within the aforementioned block of drilling cycle commands. 
     The motor control unit  58  controls the spindle motor  62  based on the command value of the rotation number S analyzed by the program analysis unit  52 . 
     [Modifications] 
     In the above embodiment, α (=F 2 /F 1 ) and t 1  are used as the information (predetermined multiple pieces of prescribed information) necessary for determining F 1 , F 2 , t 1 , t 2 , but the present invention should not be limited to this. For example, the information of necessity may be α (=F 2 /F 1 ) and t 2 . In this case, α may be given as a parameter, and t 2  may be given as an argument in the program. 
     Alternatively, α (=F 2 /F 1 ) and Q 1  (=F 1 t 1 ) may be used as the necessary information. In this case, α may be given as a parameter, and Q 1  may be given as an argument in the machining program. Here, Q 1  is the machining distance by which the drill  22  is moved at the first feed rate F 1  in one switching cycle. In this case, the information determining unit  56  calculates t 1 , t 2 , F 1 , F 2  to thereby determine the first rate moving time t 1 , the second rate moving time t 2 , the first feed rate F 1 , and the second feed rate F 2 . The calculated t 1 , t 2 , F 1 , F 2  are expressed by the following relational equations. 
     
       
         
           
             
               
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             ⁢ 
             
               
                 F 
                 1 
               
               = 
               
                 
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                 F 
                 2 
               
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                 α 
                 ⁢ 
                 
                     
                 
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                   1 
                 
               
             
           
         
       
     
     Alternatively, α (=F 2 /F 1 ) and Q 2  (=F 2 t 2 ) may be used as the necessary information. In this case, α may be given as a parameter, and Q 2  may be given as an argument in the machining program. Here, Q 2  is the machining distance by which the drill  22  is moved at the second feed rate F 2  in one switching cycle. Furthermore, at least two of the information of F 1 , F 2 , t 1 , t 2  may be given as necessary information. In short, the necessary information may be any information as long as the information is information required to calculate F 1 , F 2 , t 1 , t 2  by using the relational equations (1) and (5). 
     Technical Concepts Obtained from the Embodiments 
     The technical concepts that can be grasped from the above embodiments and Modifications 1, 2 will be described below. 
     &lt;First Technical Concept&gt; 
     The control device ( 10 ) for controlling the machine tool ( 12 ) so as to drill a workpiece based on the machining program includes the motor control unit ( 58 ) configured to, when drilling the workpiece, control the axis feed motor ( 60 ) of the machine tool ( 12 ) so that the rotary tool ( 22 ) of the machine tool ( 12 ) is axially moved by intermittently decelerated feed in which the feed rate of the rotary tool is alternately switched between the first feed rate (F 1 ) and the second feed rate (F 2 ) that is lower than the first feed rate (F 1 ), under the condition in which the relational equations (1) and (5) are satisfied. 
     As a result, it is possible to cut up long chips generated during drilling work and to shorten the machining time of drilling work. Further, since the machining is continuously performed without retracting the rotary tool ( 22 ), it is possible to prevent the life time of the rotary tool ( 22 ) from being shortened. 
     The control device ( 10 ) may further include the information determining unit ( 56 ) configured to determine, based on predetermined multiple pieces of prescribed information and the relational equations (1) and (5), four pieces of information (F 1 , F 2 , t 1 , t 2 ) including information indicating the first feed rate (F 1 ), information indicating the second feed rate (F 2 ), information indicating the first rate moving time (t 1 ), and information indicating the second rate moving time (t 2 ). The motor control unit ( 58 ) may be configured to control the axis feed motor ( 60 ) based on the four pieces of information (F 1 , F 2 , t 1 , t 2 ) so that the rotary tool ( 22 ) is axially moved by the intermittently decelerated feed when drilling the workpiece. 
     Thus, the four pieces of information (F 1 , F 2 , t 1 , t 2 ) can be determined using multiple pieces of prescribed information and the relational equations (1) and (5). Accordingly, it is possible to axially feed the rotary tool ( 22 ) by intermittently decelerated feed in which the feed rate is alternately switched between the first feed rate (F 1 ) and the second feed rate (F 2 ) so as to satisfy the relational equations (1) and (5). As a result, it is possible to cut up or break up long chips generated during drilling work and to shorten the machining time of drilling work. Further, since the machining is continuously performed without retracting the rotary tool ( 22 ), it is possible to prevent the life time of the rotary tool ( 22 ) from being shortened. 
     The multiple pieces of prescribed information may include information indicating a ratio between the first feed rate (F 1 ) and the second feed rate (F 2 ) and information indicating the first rate moving time (t 1 ) or the second rate moving time (t 2 ). This enables the information determining unit ( 56 ) to determine the four pieces of information (F 1 , F 2 , t 1 , t 2 ). 
     The multiple pieces of prescribed information may include information indicating a ratio between the first feed rate (F 1 ) and the second feed rate (F 2 ) and information indicating a machining distance (Q 1 ) by which the rotary tool is moved at the first feed rate (F 1 ) in one switching cycle or a machining distance (Q 2 ) by which the rotary tool is moved at the second feed rate (F 2 ) in one switching cycle. This enables the information determining unit ( 56 ) to determine the four pieces of information (F 1 , F 2 , t 1 , t 2 ). 
     At least one of the multiple pieces of prescribed information may be given as a parameter separately from the machining program. At least one of the multiple pieces of prescribed information may be given as an argument in the machining program. 
     &lt;Second Technical Concept&gt; 
     The control method for controlling the machine tool ( 12 ) so as to drill a workpiece based on the machining program includes the motor control step of, when drilling the workpiece, controlling the axis feed motor ( 60 ) of the machine tool ( 12 ) so that the rotary tool ( 22 ) of the machine tool ( 12 ) is axially moved by intermittently decelerated feed in which the feed rate of the rotary tool is alternately switched between the first feed rate (F 1 ) and the second feed rate (F 2 ) that is lower than the first feed rate (F 1 ), under the condition in which the relational equations (1) and (5) are satisfied. 
     As a result, it is possible to cut up or break up long chips generated during drilling work and to shorten the machining time of drilling work. Further, since the machining is continuously performed without retracting the rotary tool ( 22 ), it is possible to prevent the life time of the rotary tool ( 22 ) from being shortened. 
     The control method may further include the information determining step of determining, based on predetermined multiple pieces of prescribed information and the relational equations (1) and (5), four pieces of information (F 1 , F 2 , t 1 , t 2 ) including information indicating the first feed rate (F 1 ), information indicating the second feed rate (F 2 ), information indicating the first rate moving time (t 1 ), and information indicating the second rate moving time (t 2 ). The motor control step may be configured to control the axis feed motor ( 60 ) based on the four pieces of information (F 1 , F 2 , t 1 , t 2 ) so that the rotary tool ( 22 ) is axially moved by the intermittently decelerated feed when drilling the workpiece. 
     Thus, the four pieces of information (F 1 , F 2 , t 1 , t 2 ) can be determined using multiple pieces of prescribed information and the relational equations (1) and (5). Accordingly, it is possible to axially feed the rotary tool ( 22 ) by intermittently decelerated feed in which the feed rate is alternately switched between the first feed rate (F 1 ) and the second feed rate (F 2 ) so as to satisfy the relational equations (1) and (5). As a result, it is possible to cut up or break up long chips generated during drilling work and to shorten the machining time of drilling work. Further, since the machining is continuously performed without retracting the rotary tool ( 22 ), it is possible to prevent the life time of the rotary tool ( 22 ) from being shortened. 
     The multiple pieces of prescribed information may include information indicating a ratio between the first feed rate (F 1 ) and the second feed rate (F 2 ) and information indicating the first rate moving time (t 1 ) or the second rate moving time (t 2 ). This enables the information determining step to determine the four pieces of information (F 1 , F 2 , t 1 , t 2 ). 
     The multiple pieces of prescribed information may include information indicating a ratio between the first feed rate (F 1 ) and the second feed rate (F 2 ) and information indicating a machining distance (Q 1 ) by which the rotary tool is moved at the first feed rate (F 1 ) in one switching cycle or a machining distance (Q 2 ) by which the rotary tool is moved at the second feed rate (F 2 ) in one switching cycle. This enables the information determining step to determine the four pieces of information (F 1 , F 2 , t 1 , t 2 ). 
     At least one of the multiple pieces of prescribed information may be given as a parameter separately from the machining program. At least one of the multiple pieces of prescribed information may be given as an argument in the machining program. 
     While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood that variations and modifications can be effected thereto by those skilled in the art without departing from the scope of the invention as defined by the appended claims.