Patent Publication Number: US-2021178546-A1

Title: Machine tool and vibration estimation method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Applications No. 2019-227061 filed on Dec. 17, 2019 and No. 2020-048618 filed on Mar. 19, 2020, the contents all of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a machine tool for estimating abnormal vibration of a spindle as well as relating to a vibration estimation method. 
     Description of the Related Art 
     Japanese Laid-Open Patent Publication No. 2008-132558 discloses an abnormality detection method in which data on vibration arising during machining is acquired from a vibration sensor attached to a tool so as to detect abnormal vibration based on the acquired vibration data. 
     SUMMARY OF THE INVENTION 
     However, in the above abnormality detection method, it is necessary to attach a vibration sensor for acquiring vibration data concerning the tool, and the number of parts of the machine tool accordingly increases. 
     It is therefore an object of the present invention to provide a machine tool and a vibration estimation method capable of reducing the number of parts. 
     The first aspect of the present invention resides in a machine tool, including: a spindle unit configured to rotatably support a spindle; a moving mechanism configured to move the spindle unit; a motor configured to drive the moving mechanism; a motor control unit configured to control the motor; and an estimation unit configured to estimate that abnormal vibration is likely to have occurred in the spindle when an amplitude of a signal indicating a driving state of the motor falls out of a predetermined allowable range. 
     The second aspect of the present invention resides in a vibration estimation method for estimating abnormal vibration of a spindle of a machine tool, the machine tool including a spindle unit configured to rotatably support the spindle, a moving mechanism configured to move the spindle unit, a motor configured to drive the moving mechanism, and a motor control unit configured to control the motor, the vibration estimation method including: an acquisition step of acquiring a signal indicating a driving state of the motor; and an estimation step of estimating that abnormal vibration is likely to have occurred in the spindle when an amplitude of the signal acquired at the acquisition step falls out of a predetermined allowable range. 
     The third aspect of the present invention resides in a machine tool, including: a spindle unit configured to rotatably support a spindle; a moving mechanism configured to move the spindle unit; a motor configured to drive the moving mechanism; a motor control unit configured to control the motor; and an estimation unit configured to estimate that abnormal vibration is likely to have occurred in the spindle when an intensity of a frequency component, contained in a signal indicating a driving state of the motor, that has the same frequency as a rotation frequency of the spindle, exceeds a predetermined threshold. 
     The fourth aspect of the present invention resides in a vibration estimation method for estimating abnormal vibration of a spindle of a machine tool, the machine tool including a spindle unit configured to rotatably support the spindle, a moving mechanism configured to move the spindle unit, a motor configured to drive the moving mechanism, and a motor control unit configured to control the motor, the vibration estimation method including: an acquisition step of acquiring a signal indicating a driving state of the motor; and an estimation step of estimating that abnormal vibration is likely to have occurred in the spindle when an intensity of a frequency component, contained in the signal acquired at the acquisition step, that has the same frequency as a rotation frequency of the spindle, exceeds a predetermined threshold. 
     According to the aspects of the present invention, it is possible to detect abnormal vibration without providing any vibration sensor, and hence reduce the number of parts. 
     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 diagram showing a machine tool of the present embodiment; 
         FIG. 2  is a graph showing a signal waveform of the positional deviation of a motor; 
         FIG. 3  is a graph showing a signal waveform of the positional deviation of the motor, including a period with no abnormal vibration occurring and a period with abnormal vibration occurring, with a set allowable range being superposed; 
         FIG. 4  is a flowchart showing a flow of vibration estimation process; 
         FIG. 5  is a diagram showing how the allowable range is expanded in periods of acceleration and deceleration of a moving mechanism during machining; 
         FIG. 6  is a graph showing a signal waveform of the positional deviation of the motor, including a period with an abnormal vibration occurring, together with a set allowable range, a first expanded range, and a second expanded range; and 
         FIG. 7  is a graph showing a signal waveform obtained by transforming the signal of the positional deviation of a motor (time domain signal) into a frequency domain signal, together with a set threshold TH. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will be described below, in detail, with reference to the accompanying drawings. 
     Embodiment 
       FIG. 1  is a schematic diagram showing a machine tool  10  of the present embodiment. The machine tool  10  machines a workpiece with a tool. Specific examples of the machine tool  10  include a lathe machine and a machining center. The machine tool  10  includes a spindle unit  12 , a moving mechanism  14 , a control device  16 , and a vibration estimation device  18 . 
     The spindle unit  12  is a structural portion that rotatably supports a spindle  12   a , and includes the spindle  12   a  and a housing  12   b  having a bearing attached thereto for supporting the spindle  12   a . A spindle motor or the like for rotationally driving the spindle  12   a  is attached to the housing  12   b . The spindle unit  12  is a portion corresponding to a headstock (spindle stock) when the machine tool  10  is a lathe machine, and is a portion corresponding to a spindle head when the machine tool  10  is a machining center. The spindle unit  12  is mounted on the moving mechanism  14 . 
     The moving mechanism  14  is a mechanism for moving the spindle unit  12 . The moving mechanism  14  may be a linear movement mechanism having an axis, or may be a linear movement mechanism having no axis. The moving mechanism  14  moves the spindle unit  12  in a first direction in which the spindle  12   a  extends, a second direction orthogonal to the first direction in a plane, or a third direction orthogonal to each of the first direction and the second direction. The control device  16  may define a machine coordinate system in which the first direction is the Y-axis direction, the second direction is the X-axis direction, and the third direction is the Z-axis direction. 
     The control device  16 , based on a machining program and machining conditions, controls the machine body including the spindle  12   a  and the moving mechanism  14 . The machining program and machining conditions are stored in an unillustrated storage unit of the control device  16 . The machining program defines the positions of a tool and a workpiece at a time of machining the workpiece. The machining conditions are conditions for machining the workpiece, and specifically include the moving speed of the moving mechanism  14 . The control device  16  includes a motor  20  that drives the moving mechanism  14 , a plurality of sensors  22  that detect physical quantities related to the motor  20 , and a motor control unit  24  that controls the motor  20 . 
     The motor control unit  24  gives a command to the motor  20  so that the physical quantity fed back from each of the multiple sensors  22  becomes a target value. When the motor  20  drives the moving mechanism  14  in accordance with the command, the moving mechanism  14  and the spindle unit  12  mounted on the moving mechanism  14  move in the first direction, the second direction, or the third direction. 
     The vibration estimation device  18  is provided outside the control device  16  and exchanges various information with the control device  16 . The vibration estimation device  18  may be provided inside the control device  16 . The vibration estimation device  18  estimates abnormal vibration of the spindle  12   a  based on a signal indicating the driving state of the motor  20  during machining. The driving state of the motor  20  include at least one of the positional deviation, the speed deviation, the acceleration deviation, the jerk deviation, the electric current value, the speed, the acceleration, and the jerk, of the motor  20 . In the present embodiment, the positional deviation of the motor  20  is adopted as an index of the driving state of the motor  20 . The vibration estimation device  18  includes an acquisition unit  26 , an estimation unit  28 , and a notification unit  30 . 
     The acquisition unit  26  acquires a signal indicating the driving state of the motor  20  from the motor control unit  24 .  FIG. 2  is a graph showing a signal waveform of the positional deviation of the motor  20 . The positional deviation of the motor  20  is obtained as a signal waveform whose amplitude varies periodically with the passage of time. 
     The estimation unit  28  monitors the varying amplitude of the signal indicating the driving state of the motor  20 .  FIG. 3  is a graph showing a signal waveform of the positional deviation of the motor  20  including a period with no abnormal vibration and a period with an abnormal vibration, together with a set allowable range AR. The amplitude of the positional deviation of the motor  20  has such a relationship that the amplitude becomes greater when an abnormal vibration is occurring than when no abnormal vibration is occurring. This relationship applies to the amplitude of the speed deviation, the acceleration deviation, the jerk deviation, the electric current value, the speed, the acceleration, or the jerk, of the motor  20 , in the same manner as the amplitude of the positional deviation of the motor  20 . 
     The estimation unit  28  compares the amplitude of the signal indicating the driving state of the motor  20  with the upper and lower limits of the predetermined allowable range AR. Here, when the amplitude of the signal indicating the driving state of the motor  20  is within the allowable range AR, the estimation unit  28  estimates that no abnormal vibration is occurring in the spindle  12   a . The case where the amplitude is within the allowable range AR is a case where the amplitude is below the upper limit of the allowable range AR and above the lower limit of the allowable range AR. 
     On the other hand, when the amplitude of the signal indicating the driving state of the motor  20  falls out of the allowable range AR, the estimation unit  28  estimates that the spindle  12   a  is likely to be abnormally vibrating. The case where the amplitude falls out of the allowable range AR is a case where the amplitude is above the upper limit of the allowable range AR or below the lower limit of the allowable range AR. The estimation unit  28  may estimate that abnormal vibration may be occurring in the spindle  12   a  when a period during which the amplitude of the signal indicating the driving state of the motor  20  is out of the allowable range AR exceeds a predetermined time period. The period during which the amplitude is out of the allowable range AR is a period during which the positive peak and the negative peak of the periodically changing amplitude are continuously kept out of the allowable range AR. 
     When it is estimated that abnormal vibration is likely to have occurred in the spindle  12   a , the notification unit  30  issues a notification of the estimation result. The notification unit  30  may issue a notification to the effect that the spindle  12   a  has been estimated to have a likelihood of abnormal vibration, by controlling at least one of a display unit, a speaker, and a light emitting unit. At least one of the display unit, the speaker, and the light emitting unit may be provided in the vibration estimation device  18 , in the control device  16 , or in an external device of the vibration estimation device  18 , other than the control device  16 . 
     Next, as to the vibration estimation method of estimating abnormal vibration of the spindle  12   a , a vibration estimation process of the vibration estimation device  18  will be described.  FIG. 4  is a flowchart showing a flow of vibration estimation process. 
     At step S 1 , the acquisition unit  26  acquires a signal indicating the driving state of the motor  20  from the motor control unit  24 . When the signal indicating the driving state of the motor  20  is acquired, the vibration estimation process proceeds to step S 2 . 
     At step S 2 , the estimation unit  28  compares the amplitude of the signal acquired at step S 1  with the upper and lower limits of the predetermined allowable range AR. When the amplitude is within the allowable range AR, the estimation unit  28  estimates that no abnormal vibration is occurring in the spindle  12   a . In this case, the vibration estimation process returns to step S 1 . On the other hand, when the amplitude falls out of the allowable range AR, the estimation unit  28  estimates that the spindle  12   a  is likely to be abnormally vibrating. In this case, the vibration estimation process proceeds to step S 3 . 
     At step S 3 , the notification unit  30  issues a notification to the effect that it has been estimated that abnormal vibration is likely to have occurred in the spindle  12   a . When the notification that abnormal vibration is likely to have occurred is issued, the vibration estimation process proceeds to step S 4 . 
     At step S 4 , the motor control unit  24  stops the motor  20  that is driving the moving mechanism  14 . When the motor  20  is stopped, the vibration estimation process ends. 
     [Modification] 
     (Modification 1) 
       FIG. 5  is a diagram showing how the allowable range AR is expanded in the periods of acceleration and deceleration of the moving mechanism  14  during machining. In machining, there are two types of periods, i.e., a tool contact movement period during which the moving mechanism  14  moves with the tool being in contact with the workpiece, and a tool non-contact movement period during which the moving mechanism  14  moves with the workpiece and the tool being located away from each other. The motor control unit  24  controls the motor  20  so that the speed of the moving mechanism  14  during the tool non-contact movement period is higher than during the tool contact movement period. Therefore, when the moving mechanism  14  is accelerating or decelerating during machining, the amplitude of the signal indicating the driving state of the motor  20  tends to increase even though it is not an abnormal vibration. 
     Therefore, in this modification, the estimation unit  28  expands the allowable range AR when the moving mechanism  14  is accelerating or decelerating during machining. As a result, it is possible to suppress occurrence of an erroneous estimation that abnormal vibration may be occurring even though no abnormal vibration has actually occurred. 
     Note that the estimation unit  28  may recognize when the moving mechanism  14  is accelerating or decelerating during machining, based on a command output from the motor control unit  24  to the motor  20 . Further, the estimation unit  28  may analyze the machining program and the machining conditions stored in the storage unit of the control device  16  to thereby recognize when the moving mechanism  14  is accelerating or decelerating during machining, based on the analysis result. 
     (Modification 2) 
     The estimation unit  28  may suspend estimation when the moving mechanism  14  is accelerating or decelerating during machining. As a result, as in the case of Modification 1, it is possible to suppress occurrence of an erroneous estimation that abnormal vibration may be occurring even though no abnormal vibration has actually occurred. 
     (Modification 3) 
     When it is estimated that abnormal vibration is likely to have occurred in the spindle  12   a , the motor control unit  24  may stop the motor  20  at a higher deceleration rate as the degree to which the amplitude deviates from the allowable range AR is greater. In other words, the motor control unit  24  stops the motor  20  at a higher deceleration rate as the degree to which the amplitude falls out of the allowable range AR to deviate from the allowable range AR is greater. As a result, the higher the possibility of abnormal vibration, the more quickly the movement of the moving mechanism  14  is stopped, and thus it is possible to prevent the workpiece from being machined. 
       FIG. 6  is a graph showing a signal waveform of the positional deviation of the motor  20  including an occurrence of abnormal vibration, together with a set allowable range AR, a first expanded range EAR 1 , and a second expanded range EAR 2 . In Modification 3, for example, the first expanded range EAR 1  larger than the allowable range AR and the second expanded range EAR 2  larger than the first expanded range EAR 1  are specified in advance. When the positive peak and the negative peak of the amplitude fall within the first expanded range EAR 1 , the motor control unit  24  sets a first deceleration rate (“low”) as the deceleration rate at which the motor  20  is stopped, and stops the motor  20  at the set first deceleration rate. When the positive peak and the negative peak of the amplitude fall out of the first expanded range EAR 1  but within the second expanded range EAR 2 , the motor control unit  24  sets a second rate (“middle”) higher than the first deceleration rate, as the deceleration rate at which the motor  20  is stopped, and stops the motor  20  at the set second deceleration rate. When the positive peak and the negative peak of the amplitude fall out of the second expanded range EAR 2 , the motor control unit  24  sets a third deceleration rate (“high”) higher than the second deceleration rate, as the deceleration rate at which the motor  20  is stopped, and stops the motor  20  at the set third deceleration rate. 
     As shown in  FIG. 6 , the motor control unit  24  may increase the deceleration rate for stopping the motor  20 , in a stepwise manner according to the degree to which the amplitude falls out of the allowable range AR to deviate from the allowable range AR. As a result, the higher the possibility of abnormal vibration becomes, the more quickly the movement of the moving mechanism  14  can be stopped, and thus it is possible to prevent the workpiece from being machined. 
     (Modification 4) 
     The estimation unit  28  may monitor the intensity of a specific frequency component in the signal indicating the driving state of the motor  20  during machining. In this modification, the estimation unit  28  performs processing such as FFT (Fast Fourier Transform) on the signal indicating the driving state of the motor  20  during machining, and transforms the time domain signal into the frequency domain signal (frequency spectrum) that represents the contents of the frequency components. 
       FIG. 7  is a graph showing a signal waveform (signal spectrum) obtained by transforming a signal of the positional deviation of the motor  20  (a time domain signal) into a frequency domain signal, together with a threshold TH. In the signal waveform in the frequency domain, the intensity (amplitude) at the time when an abnormal vibration is occurring tends to be greater than the intensity (amplitude) at the time when no abnormal vibration is occurring. This relationship also applies to cases where the signal of any of the speed deviation, the acceleration deviation, the jerk deviation, the electric current value, the speed, the acceleration, and the jerk, of the motor  20  is transformed into the signal in the frequency domain, in the same manner as the case where the positional deviation of the motor  20  is transformed into the signal in the frequency domain. 
     The estimation unit  28  compares the intensity of a frequency component, in the transformed frequency domain signal, that has the same frequency as the rotation frequency of the spindle  12   a , with the predetermined threshold TH. When the intensity of the frequency component having the same frequency as the rotation frequency of the spindle  12   a  is equal to or lower than the threshold TH, the estimation unit  28  estimates that the spindle  12   a  is not subjected to abnormal vibration. Here,  FIG. 7  shows a case where the intensity of the frequency component (the portion enclosed by the chain lines in the graph) having the same frequency as the rotation frequency of the spindle  12   a  does not exceed the threshold TH. 
     On the other hand, when the intensity of the frequency component having the same frequency as the rotation frequency of the spindle  12   a  is greater than the threshold TH (when it exceeds the threshold TH), the estimation unit  28  estimates that the spindle  12   a  is likely to have abnormal vibration. The estimation unit  28  may estimate that abnormal vibration is likely to be occurring in the spindle  12   a  when a period during which the intensity of the frequency component having the same frequency as the rotation frequency of the spindle  12   a  is more than the threshold TH exceeds a predetermined time period. 
     In the flow of the vibration estimation process of this modification, step S 2  in  FIG. 4  is partially changed. That is, at step S 2 , the estimation unit  28  transforms the signal (time domain signal) acquired at step S 1  into the frequency domain signal, and compares, based on the transformed signal, the intensity of the frequency component having the same frequency as the rotation frequency of the spindle  12   a  with the predetermined threshold TH. Here, when the intensity is equal to or less than the threshold TH, the estimation unit  28  estimates that no abnormal vibration is occurring in the spindle  12   a . In this case, the vibration estimation process returns to step S 1 . On the other hand, when the intensity exceeds the threshold TH, the estimation unit  28  estimates that the spindle  12   a  is likely to have abnormal vibration. In this case, the vibration estimation process proceeds to step S 3 . 
     In this way, also in this modification, abnormal vibration can be detected without providing a vibration sensor, as in the above embodiment. Further, in this modification, the presence or absence of abnormal vibration can be determined only for the vibration generated by the rotation of the spindle  12   a.    
     It should be noted that this modification can be combined with any of the above modifications  1  to  3 . That is, the estimation unit  28  may increase the threshold TH or suspend estimation when the moving mechanism  14  is accelerating or decelerating during machining. Further, when it is estimated that the spindle  12   a  is likely to be abnormally vibrating, the motor control unit  24  may stop the motor  20  at a higher deceleration rate as the degree to which the intensity exceeds the threshold TH to deviate from the threshold TH becomes greater. Moreover, when it is estimated that the spindle  12   a  is likely to be abnormally vibrating, the motor control unit  24  may increase the deceleration rate at which the motor  20  is stopped, in a stepwise manner according to the degree to which the intensity exceeds the threshold TH to deviate from the threshold TH. 
     (Modification 5) 
     The above embodiment and modifications may be arbitrarily combined as long as no technical inconsistency occurs. 
     [Invention that can be Grasped from the Above] 
     The first to fourth aspects of the invention are described below as inventions that can be grasped from the above embodiment and modifications. 
     &lt;First Aspect of the Invention&gt; 
     The first aspect of the invention resides in a machine tool ( 10 ). The machine tool ( 10 ) includes: a spindle unit ( 12 ) configured to rotatably support a spindle ( 12   a ); a moving mechanism ( 14 ) configured to move the spindle unit ( 12 ); a motor ( 20 ) configured to drive the moving mechanism ( 14 ); a motor control unit ( 24 ) configured to control the motor ( 20 ); and an estimation unit ( 28 ) configured to estimate that abnormal vibration is likely to have occurred in the spindle ( 12   a ) when the amplitude of a signal indicating the driving state of the motor ( 20 ) falls out of a predetermined allowable range (AR). 
     This configuration makes it possible to detect abnormal vibration without providing any vibration sensor, and hence reduce the number of parts of the machine tool ( 10 ). 
     The estimation unit ( 28 ) may be configured to expand the allowable range (AR) when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This configuration makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The estimation unit ( 28 ) may be configured to suspend estimation when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The machine tool ( 10 ) may further include a notification unit ( 30 ) configured to, when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ), issue a notification to the effect that it has been estimated that abnormal vibration is likely to have occurred in the spindle. This configuration makes it possible to prompt the operator to inspect the abnormal vibration. 
     The motor control unit ( 24 ) may be configured to stop the motor ( 20 ) when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ). This configuration makes it possible to prevent the workpiece from being machined when it is estimated that abnormal vibration is likely to be occurring. 
     The motor control unit ( 24 ) may be configured to stop the motor ( 20 ) at a higher deceleration rate as the degree to which the amplitude falls out of the allowable rang (AR) to deviate from the allowable range (AR) is greater. With this configuration, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     The motor control unit ( 24 ) may be configured to increase the deceleration rate at which the motor ( 20 ) is stopped, in a stepwise manner according to the degree to which the amplitude falls out of the allowable range (AR) to deviate from the allowable range (AR). With this configuration, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     The driving state of the motor ( 20 ) may be represented by at least one of the positional deviation, the speed deviation, the acceleration deviation, the jerk deviation, the electric current value, the speed, the acceleration, and the jerk, of the motor ( 20 ). 
     &lt;Second Aspect of the Invention&gt; 
     The second aspect of the invention resides in a vibration estimation method for estimating abnormal vibration of a spindle ( 12   a ) of a machine tool ( 10 ), the machine tool including a spindle unit ( 12 ) configured to rotatably support the spindle ( 12   a ), a moving mechanism ( 14 ) configured to move the spindle unit ( 12 ), a motor ( 20 ) configured to drive the moving mechanism ( 14 ), and a motor control unit ( 24 ) configured to control the motor ( 20 ). The vibration estimation method includes: an acquisition step (S 1 ) of acquiring a signal indicating the driving state of the motor ( 20 ); and an estimation step (S 2 ) of estimating that abnormal vibration is likely to have occurred in the spindle ( 12   a ) when the amplitude of the signal acquired at the acquisition step (S 1 ) falls out of a predetermined allowable range (AR). 
     This method makes it possible to detect abnormal vibration without providing any vibration sensor, and hence reduce the number of parts of the machine tool ( 10 ). 
     The estimation step (S 2 ) expands the allowable range (AR) when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This method makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The estimation step (S 2 ) may suspend estimation when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This method makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The vibration estimation method may further include a notifying step (S 3 ) of, when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ), issuing a notification to the effect that abnormal vibration is likely to have occurred in the spindle ( 12   a ). This method makes it to prompt the operator to inspect the abnormal vibration. 
     The vibration estimation method may further include a stopping step (S 4 ) of stopping the motor ( 20 ) when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ). This method makes it possible to prevent the workpiece from being machined when it is estimated that abnormal vibration is likely to be occurring. 
     The stopping step (S 4 ) may stop the motor ( 20 ) at a higher deceleration rate as the degree to which the amplitude falls out of the allowable range (AR) to deviate from the allowable range (AR) is greater. With this method, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     The stopping step (S 4 ) may increase the deceleration rate at which the motor ( 20 ) is stopped, in a stepwise manner according to the degree to which the amplitude falls out of the allowable range (AR) to deviate from the allowable range (AR). With this method, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     &lt;Third Aspect of the Invention&gt; 
     The third aspect of the invention resides in a machine tool ( 10 ). The machine tool ( 10 ) includes: a spindle unit ( 12 ) configured to rotatably support a spindle ( 12   a ); a moving mechanism ( 14 ) configured to move the spindle unit ( 12 ); a motor ( 20 ) configured to drive the moving mechanism ( 14 ); a motor control unit ( 24 ) configured to control the motor ( 20 ); and an estimation unit ( 28 ) configured to estimate that abnormal vibration is likely to have occurred in the spindle ( 12   a ) when the intensity of the frequency component, contained in a signal indicating the driving state of the motor ( 20 ), that has the same frequency as the rotation frequency of the spindle ( 12   a ), exceeds a predetermined threshold (TH). 
     This configuration makes it possible to detect abnormal vibration without providing any vibration sensor, and hence reduce the number of parts of the machine tool ( 10 ). 
     The estimation unit ( 28 ) may be configured to increase the threshold (TH) when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This configuration makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The estimation unit ( 28 ) may be configured to suspend estimation when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This configuration makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The machine tool ( 10 ) may further include a notification unit ( 30 ) configured to, when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ), issue a notification to the effect that it has been estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ). This configuration makes it to prompt the operator to inspect the abnormal vibration. 
     The motor control unit ( 24 ) may be configured to stop the motor ( 20 ) when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ). This configuration makes it possible to prevent the workpiece from being machined when it is estimated that abnormal vibration is likely to have occurred. 
     The motor control unit ( 24 ) may be configured to stop the motor ( 20 ) at a higher deceleration rate as the degree to which the intensity exceeds the threshold (TH) to deviate from the threshold (TH) is greater. With this configuration, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     The motor control unit ( 24 ) may be configured to increase the deceleration rate at which the motor ( 20 ) is stopped, in a stepwise manner according to the degree to which the intensity exceeds the threshold (TH) to deviate from the threshold (TH). With this configuration, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     The driving state of the motor ( 20 ) may be represented by at least one of the positional deviation, the speed deviation, the acceleration deviation, the jerk deviation, the electric current value, the speed, the acceleration, and the jerk, of the motor ( 20 ). 
     &lt;Fourth Aspect of the Invention&gt; 
     The fourth aspect of the invention resides in a vibration estimation method for estimating abnormal vibration of a spindle ( 12   a ) of a machine tool ( 10 ), the machine tool including a spindle unit ( 12 ) configured to rotatably support the spindle ( 12   a ), a moving mechanism ( 14 ) configured to move the spindle unit ( 12 ), a motor ( 20 ) configured to drive the moving mechanism ( 14 ), and a motor control unit ( 24 ) configured to control the motor ( 20 ). The vibration estimation method includes: an acquisition step (S 1 ) of acquiring a signal indicating the driving state of the motor ( 20 ); and an estimation step (S 2 ) of estimating that abnormal vibration is likely to have occurred in the spindle ( 12   a ) when the intensity of the frequency component, contained in the signal acquired at the acquisition step (S 1 ), that has the same frequency as the rotation frequency of the spindle ( 12   a ), exceeds a predetermined threshold (TH). 
     This method makes it possible to detect abnormal vibration without providing any vibration sensor, and hence reduce the number of parts of the machine tool ( 10 ). 
     The estimation step (S 2 ) may increase the threshold (TH) when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This method makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The estimation step (S 2 ) may suspend estimation when the moving mechanism ( 14 ) is accelerating or decelerating during machining. This method makes it possible to suppress occurrence of an erroneous estimation that abnormal vibration is likely to be occurring even though no abnormal vibration has actually occurred. 
     The vibration estimation method may further include a notifying step (S 3 ) of, when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ), issuing a notification to the effect that abnormal vibration is likely to have occurred in the spindle ( 12   a ). This method makes it to prompt the operator to inspect the abnormal vibration. 
     The vibration estimation method may further include a stopping step (S 4 ) of stopping the motor ( 20 ) when it is estimated that abnormal vibration is likely to have occurred in the spindle ( 12   a ). This method makes it possible to prevent the workpiece from being machined when it is estimated that abnormal vibration is likely to have occurred. 
     The stopping step (S 4 ) may stop the motor ( 20 ) at a higher deceleration rate as the degree to which the intensity exceeds the threshold (TH) to deviate from the threshold (TH) is greater. With this method, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     The stopping step (S 4 ) may increase the deceleration rate at which the motor ( 20 ) is stopped, in a stepwise manner according to the degree to which the intensity exceeds the threshold (TH) to deviate from the threshold (TH). With this method, the higher the possibility of occurrence of abnormal vibration becomes, the more quickly the movement of the moving mechanism ( 14 ) can be stopped, and it is possible to prevent the workpiece from being machined. 
     The present invention is not particularly limited to the embodiment described above, and various modifications are possible without departing from the essence and gist of the present invention.