Abstract:
A machine tool includes a clamping mechanism for clamping/unclamping rotation of a rotary table. A load is measured when an unclamping command is output and then the rotary table is rotated after a set time set in the timer passes, and the set time of the timer is changed in response to a magnitude of the measured load. In this way, it is possible to prevent the clamping mechanism from being damaged due to a subsequent command given before unclamping ends, and extend a life span of a component.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a machine tool such as a machining center, which includes a rotary table, and more particularly, to a machine tool having a function of automatically and optimally setting an unclamp timer in a clamping mechanism of a rotary table. 
         [0003]    2. Description of the Related Art 
         [0004]    A rotary table such as an index table has been widely used in a machine tool such as a machining center. In particular, in various types of dividing mechanisms (for example, a dividing mechanism using a servo motor of a turret tool post) for performing a dividing operation of a workpiece in the machine tool, a clamping mechanism has been used such that a location is fixed after a turning and dividing operation. In the clamping mechanism, not only a motor or control but also the clamping mechanism is an important factor to perform a highly reliable dividing operation. When another turning and dividing operation is performed again after performing a turning and dividing operation in the clamping mechanism, a turning operation is performed after the clamping mechanism is unclamped. 
         [0005]    In general, the clamping mechanism in the rotary table is operated by air pressure or oil pressure. In the clamping mechanism, a subsequent command needs to be given after unclamping is completely performed. For this reason, there is a need to verify whether unclamping is completely performed or not. Whether unclamping is completed or not has been verified using a dedicated sensor, or an unclamping operation has been determined to be completed when a certain period of time passes after a command to perform unclamping is given. 
         [0006]    Japanese Patent Application Laid-Open No. 2010-17830 discloses a technology related to a detector that detects a deterioration state of a clamp spring in a mechanism for fixing and releasing a tool by an urging force of the clamp spring with respect to a main spindle in a machine tool. 
         [0007]    Japanese Patent Application Laid-Open No. 2007-7735 discloses a technology related to unclamping verification means for determining that a fixing member is in an unclamped state by detecting a position in a turning direction of a turning member by position detection means in a clamping mechanism of a machine tool. 
         [0008]    When a conventional dedicated sensor for verification of unclamping is provided, or when an unclamping operation is verified using some kind of a sensor of the technologies disclosed in Japanese Patent Application Laid-Open Nos. 2010-17830 and 2007-7735, mentioned above, the unclamping operation may be reliably performed. However, there is a difficulty in adjusting a sensor, and a separate sensor needs to be provided, and thus there is concern about cost increase. 
         [0009]    In a case in which an unclamping operation is determined to be completed when a certain period of time passes after a command to perform unclamping is given, a period of time from when a command to perform unclamping is given from a numerical controller or the like until unclamping is mechanically performed is presumed, and unclamping is regarded to be completed when the period of time passes. Then, a period of time (predetermined time) from when a command to perform unclamping is given until unclamping is completed is set in a timer, and the timer is configured to receive a subsequent command after waiting for the afore-mentioned predetermined time from the instant when the command to perform unclamping is given. 
         [0010]      FIG. 4  illustrates timing charts showing an unclamping operation. 
         [0011]    In  FIG. 4 , (a) illustrates an unclamping command which is turned ON at a time t 1  and turned OFF at a time t 3 . A period of time from time t 1  to time t 3  is set by an unclamp timer. In addition, (c) illustrates that an interlock is configured to be locked between time t 1  and time t 3  during which the unclamp timer operates. (b) illustrates that a state of a mechanical clamp is changed from a clamped state to an unclamped state by receiving the unclamping command turned ON at time t 1 , and the change to the unclamped state is completed at a time t 2 . 
         [0012]    Herein, a setting time (from time t 1  to time t 3 ) of the unclamp timer is set to be longer than an actual unclamping time (from time t 1  to time t 2 ) such that unclamping ends. For this reason, even when unclamping ends (at time t 2 ) after the unclamping time passes, the interlock is in the locked state and thus another command may not be executed while the unclamp timer operates (that is, from time t 1  to time t 2 ). For this reason, a subsequent command is executed after the unclamp timer ends (that is, after time t 3 ). 
         [0013]    On the contrary, it may take a long time after the unclamping command is received until unclamping mechanically ends due to degradation over time of a component or degradation of grease in a piston part of a clamping mechanism when quality of air is poor or when the clamping mechanism operates by air pressure. In this instance, when it takes an excessively long time until unclamping ends, if original setting of the unclamp timer is unchanged, there is concern that a subsequent command is executed before the mechanical unclamping is completed. In addition, after the unclamping command is given, a rotation operation command is likely to be received. Thus, when a subsequent rotation operation command is started before an actual unclamping operation is completed, there is concern that the clamping mechanism is damaged and thus reliability of the clamping mechanism is degraded. 
       SUMMARY OF THE INVENTION 
       [0014]    In this regard, the present invention has been conceived in view of the above-mentioned problems, and an object of the present invention is to provide a machine tool capable of extending a life span of a component by preventing damage of a clamping mechanism that clamps rotation of a rotary table included in the machine tool by adjusting setting of an unclamp timer. 
         [0015]    A machine tool according to the present invention includes a rotary table for attaching a workpiece, a motor for driving the rotary table, a clamping mechanism for clamping rotation of the rotary table, and a timer for setting a time taken until a subsequent command is made available after an unclamping command for releasing a clamping operation of the clamping mechanism to a set time, and further includes a load measurement unit for measuring a load of an output shaft of the motor, a reference load storage unit for storing a reference load of the rotary table in advance, a provisional load storage unit for measuring and storing, as a provisional load, a load obtained when an unclamping command of the rotary table is output and the rotary table is rotated after the set time set in the timer passes, a load comparison unit for comparing the provisional load stored in the provisional load storage unit with the reference load stored in the reference load storage unit, and a timer changing unit for changing the set time of the timer when the provisional load exceeds the reference load as a result of comparison of the load comparison unit. 
         [0016]    The reference load storage unit may measure a load obtained during rotation of the rotary table in a period at which an unclamped state is reliably obtained after the unclamping command is output and then a predetermined time passes, and store a maximum value of the measured load. 
         [0017]    The provisional load storage unit may measure a load obtained during rotation of the rotary table in a period after the set time set in the timer passes, and store a maximum value of the measured load. 
         [0018]    The load comparison unit may be configured to determine whether or not the provisional load falls within a predetermined ratio with respect to the reference load, and the timer changing unit may be configured to change the set time of the timer based on a result of the load comparison unit. 
         [0019]    According to the present invention, it is to possible to provide a machine tool capable of extending a life span of a component by preventing damage of a clamping mechanism that clamps rotation of a rotary table included in the machine tool by adjusting setting of an unclamp timer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above and other objects and features of the present invention will be apparent from the following description of embodiments with reference to accompanying drawings, in which: 
           [0021]      FIG. 1  is a cross-sectional view of an index table (rotary table) having a direct drive mechanism used for a machine tool according to the present invention; 
           [0022]      FIGS. 2A and 2B  are flowcharts illustrating procedures of setting of an unclamp timer; 
           [0023]      FIG. 3  is a timing chart in setting of the time of unclamping timer; and 
           [0024]      FIG. 4  illustrates timing charts showing an unclamping operation. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    A description will be given of an index table (rotary table) having a direct drive mechanism used for a machine tool according to the present invention using  FIG. 1 . 
         [0026]    A housing  3  is fixed to a case  2 . In the housing  3 , a shaft  1  is rotatably supported through main bearings  4   a  and  4   b . A stator  5   a  of a motor is fixed to the case  2 . On the other hand, a rotor  5   b  of the motor is attached to a rotor holder, which is fixed to the shaft  1 , and rotatably supported with respect to the case  2 . 
         [0027]    Further, an encoder  6 , a cylinder  7  and a rear plate  8  are all fixed to the case  2 . A brake disk  9  is coupled with the shaft  1  and rotatable with respect to the case  2 . A piston  10  can operate forward and backward in the cylinder through seal materials  11   a ,  11   b  and  11   c . All the seal materials  11   a ,  11   b  and  11   c  prevent chips or cutting fluid from entering from outside. In addition, the piston  10  is urged to operate in a direction in which clamping is performed by a spring  12 . 
         [0028]    When rotation of the rotary table is clamped, a flow passage of air is controlled by an electromagnetic valve such that compressed air flows into an air chamber  13   a  for clamping. When rotation of the rotary table is unclamped, the flow passage of air is controlled by the electromagnetic valve such that compressed air flows into an air chamber  13   b  for unclamping. 
         [0029]    When a clamping command is input to the index table, compressed air flows into the air chamber  13   a  for clamping, and thus the piston  10  moves to a side of the brake disk  9 , and the brake disk  9  is interposed between the piston  10  and the rear plate  8 . As a result, the shaft  1  is put in a non-rotatable state. On the contrary, when an unclamping command is input to the index table, compressed air flows into the air chamber  13   b  for unclamping, and thus the piston  10  moves in an opposite direction with respect to the brake disk  9 , and the brake disk  9  is released from the piston  10 . As a result, the shaft  1  is put in a rotatable state. 
         [0030]    It takes a predetermined time to complete the clamping and unclamping operations. It is possible to detect that unclamping is completed using a displacement sensor, a force sensor, and a flow sensor. In case where any of such mechanisms are not provided, a predetermined time that elapses after the unclamping command is given is set on the assumption that unclamping is completed when the set time elapses. Hereinafter, the set time that elapses after the unclamping command is given will be referred to as an “time period of unclamp timer”, and a time that elapses until a mechanism is actually unclamped will be referred to as an “unclamping time”. 
         [0031]    A time period of the unclamp timer is a time that is set on the assumption that unclamping is completed. Thus, in the time period of the unclamp timer, another command is prevented from being executed by putting an interlock in a locked state. For this reason, a subsequent command is executed after the time period of the unclamp timer ends. 
         [0032]    Incidentally, when the time period of the unclamp timer is set to be long, unclamping is reliably completed, and thus reliability may be maintained. However, it takes time to shift from the unclamping operation to a subsequent operation. On the contrary, when the time period of the unclamp timer is set to be short, an operation subsequent to the unclamping operation can be quickly executed. However, an actual unclamping time increases due to degradation over time or the like. When the actual unclamping time becomes longer than the time period of the unclamp timer, a subsequent operation is executed before unclamping is completed. Thus, there is concern that the brake disk  9  comes into contact with the piston  10  or the rear plate when the subsequent operation is executed. 
         [0033]    For this reason, the time period of the unclamp timer needs to be appropriately set. Next, a description will be given of a method of optimizing the time period of the unclamp timer using a load detection function of the index table in the present embodiment. 
         [0034]    When the shaft  1  is rotated in a state in which unclamping is mechanically completed, a load received by the motor corresponds to inertia of a workpiece or a jig fixed on the rotary table. However, when the shaft is rotated in a state in which unclamping is not mechanically completed, a load resulting from the brake disk  9  coming into contact with the piston  10  or the rear plate is detected in addition to the inertia. In the present embodiment, the time period of the unclamp timer is set by taking into consideration a fact that a load received by the motor increases in the state in which unclamping is not completed. 
         [0035]    Next, a description will be given of a method of setting the time period of the unclamp timer in detail based on  FIGS. 2A ,  2 B and  3 .  FIGS. 2A and 2B  are flowcharts illustrating procedures of setting the time period of the unclamp timer, and  FIG. 3  is a timing chart in setting of the time period of the unclamp timer. Hereinafter, the flowcharts of  FIGS. 2A and 2B  will be described for each step.
       (Step S 100 ) An optimization program for a time period of unclamping timer is initiated by an operator or the like to start an operation of automatically setting the unclamping timer.   (Step S 101 ) A value t of the unclamp timer, a maximum load Pmax corresponding to a limit value of a load during rotation, a ratio r used when a provisional load is compared with a reference load, a maximum timer value tmax corresponding to a limit value of a value of the unclamp timer, and a maximum number N of adjustments of the time period of unclamp timer are set.   (Step S 102 ) A value n is set to 0.   (Step S 103 ) A process of storing the reference load is started (Process (a) of  FIG. 3 ).   (Step S 104 ) An unclamping command is given.   (Step S 105 ) A time long enough to allow unclamping to be achieved without fault, which exceeds the value t of the unclamp timer, is waited.   (Step S 106 ) A predetermined rotation operation command is given.   (Step S 107 ) A maximum value of a load obtained during a rotation operation is stored as a reference load P0 in a memory.   (Step S 108 ) Whether P0 exceeds Pmax or not is determined. The operation proceeds to step S 109  when P0 exceeds Pmax (YES), and proceeds to step S 200  when P0 does not exceed Pmax (NO).   (Step S 109 ) Alarm that urges maintenance is output.   (Step S 200 ) A process of storing a provisional load is started (see Process (b) of  FIG. 3 )   (Step S 201 ) A clamping command is given.   (Step S 202 ) An unclamping command is given after a predetermined time.   (Step S 203 ) A predetermined rotation operation command is given with respect to a table.   (Step S 204 ) A maximum value of a load obtained during the rotation operation after a set time t which is set in the unclamp timer passes is stored in the memory as a provisional load P1.   (Step S 300 ) It is determined whether a value (P1/P0) obtained by dividing the provisional load P1 by the reference load P0 is greater than a predetermined value r or not. The operation proceeds to step S 400  when the value (P1/P0) is greater than the predetermined value r (YES), and proceeds to step S 500  to terminate automatic setting of the unclamp timer when the value (P1/P0) is not greater than the predetermined value r (NO).   (Step S 400 ) It is determined whether or not n is greater than or equal to the maximum number N of adjustments, or whether or not t is greater than or equal to the limit value tmax of the value of the unclamp timer. The operation proceeds to step S 501  when either is satisfied (YES), and proceeds to step S 401  when neither is satisfied (NO).   (Step S 401 ) The value t is updated by adding a predetermined value Δt to t.   (Step S 402 ) The value n is updated by adding 1 to the value n.   (Step S 501 ) An alarm that urges maintenance such as replacement of a clamping mechanism or the like is output.       
 
         [0056]      FIG. 3  illustrates a timing chart of a function of automatically setting the unclamping time. 
         [0057]    In  FIG. 3 : 
         [0058]    Process (a) is a reference load storing process, in which the unclamping command is given, and then the rotation operation command is given after elapse of an unclamp timer time sufficient to perform unclamping, and a maximum value obtained during a rotation load operation is set to P0; 
         [0059]    Process (b) is a provisional load storing process, in which a maximum value obtained during the rotation load operation after elapse of the unclamp timer time t is set to P1. In this case, unclamping does not end within the unclamp timer time t due to degradation over time or the like of a component. Thus, P1 is greater than the predetermined ratio r of P0. In this case, the unclamp timer time is corrected; and 
         [0060]    Process (c) is a provisional load storing process after correction of the unclamp timer time, in which the unclamp timer time is corrected to be longer when compared to a case of the provisional load storing process (b). In this way, unclamping ends within the unclamp timer time t, and thus the maximum value obtained during the rotation load operation becomes almost equal to a value of the reference load P0 stored in the reference load storing process. At this time, setting of the unclamp timer time is ended. 
         [0061]    In the present embodiment, it is determined whether the provisional load P1 is greater than the predetermined ratio r of the reference load P0, and the unclamp timer time t is adjusted by increasing the unclamp timer time t by a predetermined time Δt when the provisional load P1 is greater than the predetermined ratio r of the reference load P0. However, depending on lengths of the predetermined time Δt, a value of the unclamp timer time t may be increased more than necessary. For this reason, after the unclamp timer time t is adjusted by being increased in stages, a predetermined changed time may be set to a small value, and the unclamp timer time t may be adjusted to an optimum unclamp timer time by being decreased in stages. In addition, although the present embodiment has been described using the index table installed in the machine tool as the rotary table, the present embodiment may be similarly applied to a cradle-type index table supported from an opposite side of the table.