Machine tool, numerical control device, and vibration suppression method

Abstract

To provide a machine tool, a numerical control device, and a vibration suppression method, which can suppress vibration.SOLUTION: A machine tool includes: a rotary shaft which rotates a workpiece or a tool for machining the workpiece; a vibration sensor which detects vibration generated in the tool or the workpiece during machining; and a numerical control device which controls an operation of the rotary shaft. The numerical control device has a sensor information acquisition section which acquires the number of rotation of the rotary shaft and frequency and amplitude of vibration a plurality of times, an arithmetic section which calculates a target number of rotation as a target value of the number of rotation on the basis of the number of rotation and the frequency acquired by the sensor information acquisition section a plurality of times, a first control section which controls the operation of the rotary shaft a plurality of times so that the number of rotation becomes the target number of rotation, and an extraction section which extracts the number of rotation corresponding to the smallest amplitude from a plurality of target numbers of rotation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a machine tool, a numerical control device, and a vibration suppression method. [Background technology]

[0002] Machine tools process workpieces using tools mounted on the spindle, for example. The numerical control device (hereinafter also called the NC (Numerical Controller)) of the machine tool outputs commands to the spindle and controls its operation. For example, if chatter vibration occurs between the tool and the workpiece during cutting using a machine tool, chatter patterns will appear on the machined surface of the workpiece, degrading the quality of the machined surface. In addition, the tool's lifespan may be shortened or the tool may break. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2007-44852 [Patent Document 2] Japanese Patent Publication No. 2015-217500 [Overview of the project] [Problems that the invention aims to solve]

[0004] The objective is to provide a machine tool, a numerical control device, and a vibration suppression method that can suppress vibrations. [Means for solving the problem]

[0005] The machine tool according to this embodiment comprises a rotating shaft for rotating a workpiece or a tool for machining a workpiece, a vibration sensor for detecting vibrations generated in the tool or workpiece during machining, and a numerical control device for controlling the operation of the rotating shaft, wherein the numerical control device includes a sensor information acquisition unit that acquires the rotational speed of the rotating shaft, as well as the frequency and amplitude of vibration multiple times, a calculation unit that calculates a target rotational speed, which is a target value for the rotational speed, multiple times based on the rotational speed and frequency acquired by the sensor information acquisition unit, a first control unit that controls the operation of the rotating shaft multiple times so that the rotational speed becomes the target rotational speed, and an extraction unit that extracts the rotational speed corresponding to the smallest amplitude from the multiple target rotational speeds. [Brief explanation of the drawing]

[0006] [Figure 1] This block diagram shows an example of the configuration of a machine tool according to the first embodiment. [Figure 2] This figure shows an example of a stability limit diagram according to the first embodiment. [Figure 3] This figure shows an example of the relationship between the limiting depth of cut and acceleration. [Figure 4] This is a flowchart showing an example of the operation of a machine tool according to the first embodiment. [Figure 5A] This figure shows an example of estimating the stable rotational speed according to the first embodiment. [Figure 5B] This figure shows an example of estimating the stable rotational speed according to the first embodiment. [Figure 5C] This figure shows an example of estimating the stable rotational speed according to the first embodiment. [Figure 6] This figure shows an example of the calculation results for the stable rotational speed in a machine tool according to the first embodiment. [Figure 7] This figure shows an example of the calculation results for the stable rotational speed in a machine tool using the first comparative example. [Figure 8] This figure shows an example of a stability limit diagram according to the second embodiment. [Modes for carrying out the invention]

[0007] Embodiments of the present invention will be described below with reference to the drawings. These embodiments are not limiting to the present invention. The drawings are schematic or conceptual, and the proportions of each part may not necessarily be the same as those of actual objects. In the specification and drawings, elements similar to those described above with respect to previously shown drawings are denoted by the same reference numerals, and detailed explanations are omitted as appropriate.

[0008] (First Embodiment) Figure 1 is a block diagram showing an example of the configuration of a machine tool 100 according to the first embodiment. The machine tool 100 comprises a spindle 1, a spindle motor 2, an encoder 3, a vibration sensor 4, a numerical control device 5, a servo control device 6, and a display device 7.

[0009] The spindle 1, acting as the axis of rotation, is equipped with a tool 11 for machining the workpiece W, and rotates the tool 11. In the example shown in Figure 1, the tool 11 is used for end mill cutting of the workpiece W. Therefore, the tool 11 moves in the X-axis and Z-axis directions while rotating, cutting the workpiece W. In this case, the workpiece W is fixed. Note that in the example shown in Figure 1, the movement axes for moving the spindle 1 along the X-axis and Z-axis are omitted. Also, the number of teeth on the tool 11 is, for example, 4. Note that the number of teeth on the tool 11 is just an example.

[0010] The spindle motor 2 receives a servo command from the servo control unit 6 and rotates the spindle 1.

[0011] The encoder 3, acting as a rotation speed sensor, detects the rotation speed of the spindle 1 (spindle rotation speed). The encoder 3, for example, detects the rotational position of the spindle motor 2's axis. The rotation speed is calculated from this rotational position.

[0012] The vibration sensor 4 detects vibrations generated in the tool 11 or workpiece W during machining. The vibration sensor 4 is, for example, positioned to contact the workpiece W. The position of the vibration sensor 4 is preferably close to the machining point. The vibration sensor 4 may also be provided on the base on which the workpiece W is placed or fixed. The vibration sensor 4 is, for example, an acceleration sensor. However, it is not limited to this, and the vibration sensor 4 may also be a linear scale that detects the position (displacement) of the spindle 1 or the moving axis that moves the workpiece W (sensorless monitoring). The vibration sensor 4 may also be provided on the spindle 1, for example. The following describes the case in which vibration occurs in the tool 11. However, it is not limited to this, and the vibration generated in the tool 11 may also be a vibration generated in the workpiece W. Therefore, even when the workpiece W is prone to vibration, such as when the workpiece W is a thin-walled workpiece, the machine tool 100 can be used.

[0013] The numerical control device 5 controls the operation of the spindle 1. The numerical control device 5 calculates the rotational speed that suppresses chatter vibrations that occur in the tool 11 or workpiece W during machining. This suppresses chatter vibrations. Chatter vibrations are large vibrations that occur between the tool 11 and the workpiece W. Details of chatter vibrations will be explained later.

[0014] The numerical control device 5 includes a sensor information acquisition unit 51, a calculation unit 52, a control unit 53, and an evaluation unit 54.

[0015] The sensor information acquisition unit 51 obtains the rotational speed n0 (min) from the encoder 3 and the vibration sensor 4. -1 ), as well as the vibration frequency f0 (Hz) and amplitude a0 are acquired. More specifically, the sensor information acquisition unit 51 acquires the rotational speed, frequency, and amplitude multiple times. The sensor information acquisition unit 51 sends the rotational speed, frequency, and amplitude to the calculation unit 52.

[0016] The calculation unit 52 calculates the target rotational speed, which is the target value of the rotational speed, based on the rotational speed, frequency, and amplitude. More specifically, the calculation unit 52 performs the target rotational speed calculation multiple times. Further details about the target rotational speed will be explained later, with reference to Figure 4.

[0017] The control unit (first control unit) 53 controls the movement of the spindle 1 so that the rotational speed becomes the target rotational speed. More specifically, the control unit 53 controls the movement of the spindle 1 multiple times. For example, the control unit 53 sends a servo control command to the servo control unit 6 to change the rotational speed. Thus, the numerical control device 5 controls the spindle 1 at the target rotational speed to machine the workpiece W.

[0018] The evaluation unit 54 evaluates the rotational speed modified by the control unit 53 based on the amplitude. More specifically, the evaluation unit (extraction unit) 54 extracts the rotational speed corresponding to the smallest amplitude from a plurality of target rotational speeds. More specifically, the evaluation unit 54 extracts the initial rotational speed, which is the initial value of the rotational speed, and the rotational speed corresponding to the smallest amplitude from a plurality of target rotational speeds. This makes it possible to obtain a rotational speed that can suppress vibration. Details of the evaluation of rotational speed based on amplitude will be explained later with reference to Figures 3 and 4.

[0019] Furthermore, the evaluation unit 54 sends the extracted rotational speed to the control unit 53 so that the spindle 1 can be controlled with the extracted rotational speed. The control unit 53 controls the operation of the spindle 1 so that the rotational speed becomes the rotational speed extracted by the evaluation unit 54.

[0020] Furthermore, the evaluation unit 54 stores the target rotational speed and the amplitude acquired by the sensor information acquisition unit 51 in a storage unit (not shown) so as to correspond to each other. The storage unit is provided, for example, within the numerical control device 5.

[0021] More specifically, the sensor information acquisition unit 51, the calculation unit 52, and the control unit 53 perform a series of operations a predetermined number of times, including acquiring rotational speed and frequency, calculating target rotational speed, controlling the movement of the spindle 1, and acquiring amplitude. The evaluation unit 54 extracts the rotational speed corresponding to the smallest amplitude from a number of target rotational speeds corresponding to the predetermined number of times. The predetermined number of times is, for example, two.

[0022] The servo control unit 6 receives a servo control command from the control unit 53 and controls the spindle motor 2. This changes the rotation speed.

[0023] The display device 7 includes a display control unit 71 and a display unit 72.

[0024] The display control unit 71 displays the determination results of the calculation unit 52, etc., on the display unit 72.

[0025] Furthermore, the sensor information acquisition unit 51, calculation unit 52, control unit 53, and evaluation unit 54 that constitute the numerical control device 5 may be implemented by a single CPU (Central Processing Unit), or they may each be implemented by separate CPUs. Also, the servo control unit 6 may be part of the numerical control device 5.

[0026] Next, I will explain chatter vibration.

[0027] Chatter vibration is one type of machining defect. Chatter vibration can be broadly divided into two types: forced chatter vibration and self-excited chatter vibration. In self-excited chatter vibration, once vibration occurs, it often develops into a large vibration. Self-excited chatter vibration causes many problems, such as deterioration of the machined surface and damage to the machine structure. Suppressing chatter vibration leads to improvements in machining accuracy and machining efficiency, for example.

[0028] Self-excited chatter vibrations include regenerative types, where vibrations from a previous rotation remain as surface irregularities, causing variations in the cutting thickness during the current cutting operation, and modal coupling types, where multiple vibration modes influence each other, leading to larger vibrations. In both regenerative and modal coupling types, the vibrations grow larger due to instability in the feedback loop, which consists of the transmission characteristics of the cutting process and the machine structure. To suppress self-excited chatter vibrations, it is necessary to stabilize the system.

[0029] One countermeasure against self-excited chatter vibration is the development of a stability limit diagram, which analyzes the machinable range by performing a stability analysis of the transfer function of the machine structure and the cutting process in advance. This method is very convenient because it does not require changes to the machine structure and allows for prediction of machining conditions in advance. However, it requires measurements for each tool 11 and workpiece W, and the measurements require a certain level of expertise. Furthermore, the stability limit diagram created based on the measurement results may differ significantly from the actual machining results. For application to actual machines, there is a need for a countermeasure that is easy to implement and can be used by operators without specialized knowledge.

[0030] One countermeasure that satisfies these conditions is spindle speed variation technology. There are mainly two methods for this technology. The first method is spindle speed variation (CSSV, Continuous Spindle Speed ​​Variation). CSSV is a technology that suppresses chatter vibration by varying the rotation speed of spindle 1 in a sinusoidal or triangular wave manner. The second technology is spindle speed adjustment (DSST, Discrete Spindle Speed ​​Tuning). DSST is a technology that avoids chatter vibration by changing the rotation speed to a region where the stability range increases, based on the principle of the stability limit diagram.

[0031] Next, we will explain the stability limit diagram for self-excited chatter vibration.

[0032] FIG. 2 is a diagram showing an example of a stability limit diagram according to the first embodiment. The vertical axis of the graph shown in FIG. 2 indicates the depth of cut. The horizontal axis of the graph shown in FIG. 2 indicates the rotational speed. FIG. 2 shows the rotational speed dependence of the critical depth of cut a lim . Chatter vibration occurs when machining is performed at a depth of cut greater than the critical depth of cut a lim .

[0033] FIG. 2 shows, for example, a stability limit diagram in self-excited chatter vibration in milling. Milling can be represented by a block diagram consisting of a cutting process and the transmission characteristics of the machine structure. By solving this block diagram, the following equations (1) to (4) regarding the critical depth of cut a lim , the phase difference ε, the rotational speed n, etc. can be derived. a lim =-(Λ R / K t )×{1+(Λ I / Λ R ) 2} (Equation 1)

Number

Number

Number

[0034] Using the above equations, the stability limit diagram shown in FIG. 2 can be obtained. The stability limit diagram shows the characteristics of chatter vibration, and the critical depth of cut a lim at which chatter vibration occurs before machining, the chatter frequency fc Furthermore, the phase difference ε can be determined. In addition, by selecting the rotational speed in the stable region called the stable pocket, it is possible to select conditions with a wide machining limit. In Figure 2, the stable pocket is the limit depth of cut a lim This is the region where the coefficient of rotation increases rapidly. Furthermore, the rotational speed near the stable pocket is called the stable rotational speed.

[0035] Next, we will explain how to avoid chatter vibrations using DSST.

[0036] Chatter vibration can be avoided by selecting an appropriate rotational speed from the stability limit diagram, and DSST is a technology that uses this principle to avoid chatter vibration.

[0037] First, the rattle frequency f c The relationship between rotational speed n and the rotational speed n is expressed by Equation 5. 60f c / (Z×n)=k+(ε / 2π) (Equation 5) Equation 5, derived from Equation 3, shows the angular frequency ω of the chatter vibration. c The frightening frequency f c (f c =ω c This can be obtained by converting it to ( / 2π).

[0038] In the stability limit diagram obtained by equations 1 to 4, the limit cut is greatest when, for example, the phase difference ε = 0. Calculating with ε = 0, we can derive equations 6 and 7 below.

number

[0039] DSST allows us to obtain, for example, machining conditions A2 after changing the rotational speed from the initial machining conditions A1 shown in Figure 2. The depth of cut for machining conditions A1 is the limiting depth of cut a lim It is larger than that. On the other hand, machining condition A2 is the limit depth of cut a lim It is smaller than that. Therefore, chatter vibration can be suppressed by changing the rotational speed to the rotational speed of machining condition A2.

[0040] Here, machining conditions B1 and B2 shown in Figure 2 have a larger depth of cut compared to machining conditions A1 and A2. For machining conditions B1 and B2, the limit depth of cut is a lim It is larger than that. Therefore, even if the rotational speed is changed to the rotational speed of machining condition B2, chatter vibration will continue. In other words, depending on the depth of cut, chatter vibration may not be suppressed by simply changing the rotational speed.

[0041] Next, the limit depth of cut a lim Let's explain the relationship between amplitude and amplitude.

[0042] Figure 3 shows the limiting depth of cut a lim This figure shows an example of the relationship between and acceleration AC. The limit cut depth a is shown in Figure 3. lim This is the result of the cut-in limit test. The acceleration AC shown in Figure 3 is the detection result of the acceleration sensor as vibration sensor 4. Note that the amplitude shows the same trend as the acceleration AC. Furthermore, acceleration AC shows the acceleration at a cut-in depth of 10 mm. Most of the acceleration AC results shown in Figure 3 are for the limit cut-in depth a lim The machining is being performed with a depth of cut exceeding the limit.

[0043] The horizontal axis of the graph shown in Figure 3 represents the rotational speed. The vertical axis on the left side of the graph shown in Figure 3 represents the limiting depth of cut a. lim This shows the corresponding axial cut. The vertical axis on the right side of the graph shown in Figure 3 is acceleration (m / s²). 2This shows the graph. Also, the vertical axis on the right has its axis direction reversed. Therefore, the acceleration AC decreases as you move upwards on the paper and increases as you move downwards on the paper.

[0044] As shown in Figure 3, the limit depth of cut a lim It can be seen that a similar rotational speed dependence trend exists between and acceleration AC. For example, the limiting depth of cut a lim The number of minutes increases rapidly at 3675 min. -1 and 3800 min -1 At a rotational speed of a, the acceleration AC decreases sharply. That is, the depth of cut is the limiting depth of cut a. lim Even when it exceeds this value, the acceleration AC, i.e., the amplitude, is strongly suppressed near the stable rotational speed. Furthermore, the above relationship is based on the depth of cut being the limiting depth of cut a lim It can also be seen when it exceeds a certain threshold.

[0045] Limit cutting depth a lim By utilizing the above relationship between rotational speed and acceleration AC (amplitude), it is possible to evaluate whether the rotational speed is near the stable rotational speed based on the amplitude. Therefore, even when the depth of cut is large and chatter vibration occurs, it becomes possible to search for the stable rotational speed.

[0046] Next, we will explain a method for finding a stable rotational speed (vibration suppression method) that combines DSST and rotational speed evaluation using amplitude.

[0047] Figure 4 is a flowchart showing an example of the operation of the machine tool 100 according to the first embodiment.

[0048] First, the calculation unit 52 assigns 1 to the number of repetitions N and calculates the override value (%)n rate0 Substitute 100 (S10). The override value is a command value that indicates the percentage change from the reference rotational speed.

[0049] Next, the sensor information acquisition unit 51 detects the chatter vibration (S20). The sensor information acquisition unit 51 obtains the amplitude (dB) a0 and rotational speed (min -1 )n0, initial rotation speed nh The amplitude a0 and frequency f0 are obtained, for example, by performing FFT (Fast Fourier Transform) analysis on vibration data from vibration sensor 4. h This is a command value that is set in advance by the operator.

[0050] Next, the calculation unit 52 calculates k0 + v0 as the wavenumber k0 and phase difference v0 (S30). k0+v0=60f0 / (Z×n0) (Equation 8) Furthermore, the wavenumber k0 is the integer part of the right-hand side of Equation 8, and the phase difference v0 is the decimal part of the right-hand side of Equation 8.

[0051] Next, the calculation unit 52 determines whether the chatter vibration is regenerative chatter (S40). The calculation unit 52 determines whether the phase difference v0 satisfies equation 9. More specifically, the calculation unit 52 determines whether chatter vibration is occurring based on the acquired rotational speed and frequency. v LL <v0<v UL (Formula 9) Here, the lower limit of judgment v LL This is the lower limit for determining whether regenerative chatter vibration is present. Lower limit v LL This can be set arbitrarily, for example, 0.4. Judgment upper limit v UL This is the upper limit for determining regenerative chatter vibration. UL This value can be set arbitrarily, for example, to 0.9.

[0052] If the phase difference v0 does not satisfy equation 9 (NO in S40), the calculation unit 52 determines that the chatter vibration is forced chatter vibration. The display control unit 71 causes the display unit 72 to display that the chatter vibration is forced chatter vibration (S50).

[0053] On the other hand, if the phase difference v0 satisfies equation 9 (YES in S40), the calculation unit 52 determines whether the number of repetitions N is 1 or not (S60).

[0054] If the number of repetitions N is 1 (YES in S60), the evaluation unit 54 substitutes the amplitude a0 into the storage amplitude a1, the rotation speed n0 into the storage rotation speed n1, the frequency f0 into the storage frequency f1, and the storage override value n rate1 Override value n rate0 Substitute this value (S70).

[0055] In other words, the evaluation unit 54 determines the override value n in step S10. rate0 The amplitude a0 acquired by the sensor information acquisition unit 51 is stored in the storage unit in association with the other information.

[0056] Next, the calculation unit 52 calculates the change rotation speed (min -1 The calculation unit 52 calculates the target rotational speed (S80). In other words, if chatter vibration occurs, the calculation unit 52 calculates the target rotational speed n. The changed rotational speed n is expressed by equation 10. n = 60f0 / {Z × (k0 + ud + v)} (Equation 10) Here, the search range ud is a parameter that indicates whether to increase (accelerate) or decrease (deceleration) the rotational speed of the spindle 1. The change phase difference v can be set arbitrarily.

[0057] Furthermore, the calculation unit 52 calculates the override value n based on the changed rotation speed n. rate0 Substitute the value into the override value n. rate0 This is expressed by equation 11. n rate0 =100+{(nn h ) / n h ×100} (Formula 11)

[0058] Furthermore, the calculation unit 52 substitutes, for example, zero for the change phase difference v shown in equation 10. The calculation unit 52 substitutes zero or 1 for the search range ud shown in equation 10. Note that the search range ud is set in advance by the operator. If the search range ud is zero, the change rotation speed n directly above the rotation speed n0 is obtained. If the search range ud is 1, the change rotation speed n directly below the rotation speed n0 is obtained.

[0059] Next, the calculation unit 52 determines whether the variation in rotational speed is within a range that can be varied (S90).

[0060] The calculation unit 52 sets the override limit n limit The override upper limit n is calculated. limit This is expressed by equation 12. n limit =(n-n0) / n0×100 (Equation 12)

[0061] The calculation unit 52 sets the override limit n limit Determine whether or not equation 13 is satisfied. -n ML <n limit <+n PL (Formula 13) Here, the upper limit of variation n ML This is the upper limit of the negative override fluctuation. Fluctuation limit n ML This can be set arbitrarily, for example, 20%. The upper limit of variation is n. PL This is the upper limit of the positive override fluctuation. Fluctuation limit n PL This can be set arbitrarily, for example, to 20%.

[0062] Override limit n limit If the condition does not satisfy equation 13 (NO in S90), the display control unit 71 displays on the display unit 72 that the rotation speed change limit has been reached (S100).

[0063] Override limit n limit If equation 13 is satisfied (YES in S90), the calculation unit 52 determines whether the search range ud is 1 or not (S110).

[0064] If the search range is 1 (YES in S110), the calculation unit 52 assigns 0 to the search range ud (S120). On the other hand, if the search range ud is 0 (NO in S110), the calculation unit 52 assigns 1 to the search range ud (S130).

[0065] After step S120 or step S130, the control unit 53 overrides the value n rate0The rotational speed of spindle 1 is changed accordingly. The delay timer dT is the time from when the rotational speed of spindle 1 is changed until step S150 is executed. By setting a delay timer dT to wait until the machining stabilizes, chatter vibration can be detected more accurately. The delay timer dT can be set arbitrarily, for example, to 1 second.

[0066] Next, the calculation unit 52 substitutes N+1 for the number of repetitions N (S150). That is, it adds 1 to the number of repetitions N. After that, steps S20 to S60 are executed again. Note that the amplitude a0, rotational speed n0, and frequency f0 obtained in step S20 when the number of repetitions N is 2 or more are detected values ​​obtained after the rotational speed of the spindle 1 has been changed.

[0067] If the number of repetitions N is not 1 (NO in S60), the evaluation unit 54 determines whether the storage amplitude a1 is greater than the amplitude a0 (S160). That is, the evaluation unit 54 compares the amplitude before and after the change in the rotational speed of the spindle 1.

[0068] If the storage amplitude a1 is greater than the amplitude a0 (YES in S160), the evaluation unit 54 substitutes the amplitude a0 for the storage amplitude a1, the rotational speed n0 for the storage rotational speed n1, the frequency f0 for the storage frequency f1, and the storage override value n rate1 Override value n rate0 Substitute the values ​​(S170). On the other hand, if the storage amplitude a1 is less than or equal to the amplitude a0 (NO in S160), then the storage amplitude a1, storage rotation speed n1, storage frequency f1, and storage override value n rate1 It will not be updated.

[0069] In other words, the evaluation unit 54 compares the amplitude acquired by the sensor information acquisition unit 51 with the amplitude stored in the memory unit. If the amplitude acquired by the sensor information acquisition unit 51 is smaller than the amplitude stored in the memory unit, the evaluation unit 54 causes the memory unit to update the target rotational speed and the amplitude acquired by the sensor information acquisition unit 51.

[0070] After step S160 or step S170, the calculation unit 52 determines whether the number of repetitions N is 3 or not (S180).

[0071] If the number of repetitions N is not 3 (NO in S180), step S80 is executed again.

[0072] Furthermore, in steps S110 to S130, the search range ud is inverted from 0 to 1 or from 1 to zero each time the number of repetitions N changes. That is, the calculation unit 52 performs the target rotational speed calculation multiple times so that the rotational speed alternately accelerates and decelerates. This makes it possible to search for a stable rotational speed near the rotational speed at the start of the search for a stable rotational speed.

[0073] On the other hand, if the number of repetitions N is 3 (YES in S180), the control unit 53 stores the override value n rate1 The rotational speed of spindle 1 is changed accordingly (S190). Storage override value n rate1 This is the override value of the rotational speed corresponding to the smallest amplitude a0 among the three amplitudes a0 acquired in step S20. Machining of the workpiece W continues at the changed rotational speed of spindle 1.

[0074] Next, we will explain the verification results of the method shown in Figure 4.

[0075] Figures 5A to 5C show examples of estimating the stable rotational speed according to the first embodiment. The horizontal axis of the graphs shown in Figures 5A to 5C represents the rotational speed (min -1 ) and the same values ​​are shown in Figures 5A to 5C respectively. 2 This shows the vertical axis of the graph shown in Figure 5B, which represents the rattle frequency f. c The value is shown in Hz. The vertical axis of the graph shown in Figure 5C represents the phase difference (rad).

[0076] As can be seen from Figure 5B, the rattle frequency f c It changes periodically with changes in rotational speed. Also, the rattle frequency f c 2925min -1, 3075 min -1 , and 3225 min -1 changes rapidly in the vicinity.

[0077] As can be seen from FIG. 5C, periodicity is also observed in the change in the phase difference due to the change in the rotational speed. Also, the phase difference changes rapidly in the vicinity of 2925 min -1 , 3075 min -1 , and 3225 min -1 in the vicinity. This is similar to the characteristics of self-excited chatter vibration. From this, it can be seen that the vicinity of 2925 min -1 , 3075 min -1 , and 3225 min -1 is the stable rotational speed.

[0078] As can be seen from FIG. 5A, in the change in the actual amplitude due to the change in the rotational speed, a decrease is also seen in the vicinity of 2925 min -1 , 3075 min -1 , and 3225 min -1 in the vicinity. From this, it is inferred that the vicinity of 2925 min -1 , 3075 min -1 , and 3225 min -1 is the stable rotational speed.

[0079] FIG. 6 is a diagram showing an example of the calculation result of the stable rotational speed in the machine tool 100 according to the first embodiment. The horizontal axis of the graph shown in FIG. 6 is the initial rotational speed n h (min -1 ). The vertical axis of the graph shown in FIG. 5 is the optimum rotational speed (min -1 ). The optimum rotational speed is the rotational speed output by the method for obtaining the stable rotational speed.

[0080] FIG. 6 shows the simulation results. The simulation is for the initial rotational speed n -1 from 3025 min -1 to 3125 min h under 5 conditions.The study was conducted on [target]. The values ​​between plots were calculated by linear approximation from the preceding and succeeding values. By using the algorithm (method) shown in Figure 4, the rotational speed of spindle 1 was calculated to be the stable rotational speed (3075 min). -1 and 3225min -1 It can be seen that the values ​​converge to around ). From this, the stable rotational speed could be calculated using the method shown in Figure 4. In the simulation shown in Figure 6, the number of repetitions N is 3, which corresponds to N=1 to 3 shown in Figure 4. Therefore, it is possible to obtain a stable rotational speed with a small number of repetitions N.

[0081] As described above, according to the first embodiment, the evaluation unit 54 extracts the rotational speed corresponding to the smallest amplitude from a plurality of rotational speeds (target rotational speeds). This makes it possible to find the rotational speed at which vibration is reduced (stable rotational speed). Furthermore, it is possible to search for a stable rotational speed even when chatter vibration is continuing.

[0082] Furthermore, the machine tool 100 is not limited to end milling, but may also be used for boring, turning, and other operations. In the case of turning, the rotating axis rotates the workpiece W. In this case, for example, the workpiece W is cut by feeding the tool at a constant speed.

[0083] Furthermore, an encoder 3 may not be provided. For example, in an inverter-controlled synchronous motor, feedback may not be performed. In this case, assuming that the main shaft 1 is rotating at the command value, the command value may be used as the rotational speed n0. That is, the sensor information acquisition unit 51 acquires the command value as the rotational speed n0.

[0084] (Comparative example) Figure 7 shows an example of the calculation results for the stable rotational speed in a machine tool according to the first comparative example. The first comparative example differs from the first embodiment in that the rotational speed is not evaluated using amplitude.

[0085] In the first comparative example, the evaluation unit 54 is not provided.

[0086] Figure 7 shows 2900 min -1 3250 min -1 Initial rotations n between h The results of calculating the stable rotational speed using DSST, which does not evaluate rotational speed using amplitude, are shown below. As shown in Figure 7, most of the rotational speeds calculated in the first comparative example were the stable rotational speed (3075 min) estimated from Figures 5A to 5C. -1 and 3225min -1 It deviates from the standard. This is because, due to machining with a constant axial depth of cut of 14 mm, it deviates from the limit depth of cut depending on the rotational speed, resulting in a chatter frequency f that differs from the theory of the stable limit diagram. c This is thought to be due to the occurrence of vibrations. Therefore, it is thought that the rotational speed calculated in the first comparative example did not converge to the estimated stable rotational speed.

[0087] In the first comparative example, a stable rotational speed can be obtained when the depth of cut is small, as shown in machining conditions A1 and A2 in Figure 2, but a stable rotational speed cannot be obtained when the depth of cut is large, as shown in machining conditions B1 and B2.

[0088] In contrast, in the first embodiment, the limit cutting depth a, as explained with reference to Figure 3, is lim Based on the relationship between the amplitude and acceleration AC, the rotational speed is evaluated using the amplitude. This allows for obtaining a stable rotational speed even when the depth of cut is large and chatter vibration occurs. In other words, a stable rotational speed can be obtained regardless of the depth of cut.

[0089] As a second comparative example, it is sometimes possible to obtain a stable rotational speed by repeatedly changing the rotational speed little by little and checking the amplitude, without performing DSST. However, because it is unknown at what rotational speed the amplitude becomes small, it may take a long time to repeatedly change the rotational speed in small increments. Also, there is a possibility that the machining will be completed before a stable rotational speed is obtained.

[0090] In contrast, in the first embodiment, the target rotational speed is calculated using the frequency obtained by analyzing the vibration data of the vibration sensor 4. This makes it possible to obtain a stable rotational speed in a shorter time.

[0091] (Second Embodiment) Figure 8 shows an example of a stability limit diagram according to the second embodiment. The second embodiment differs from the first embodiment in that the cutting depth is changed after the rotational speed has been changed to the extracted rotational speed.

[0092] The control unit (second control unit) 53 controls the operation of the spindle 1 to reduce the depth of cut of the tool 11 to the workpiece W until the amplitude falls below a predetermined value. The predetermined value is a value that is set in advance and can be set arbitrarily. This makes it possible to automatically obtain a depth of cut that can suppress chatter vibration. That is, the control unit 53 controls the operation of the spindle 1 to reduce the depth of cut of the tool 11 to the workpiece W during machining at the rotational speed extracted by the evaluation unit 54 until the state in which chatter vibration occurs is changed to a state in which chatter vibration does not occur. The control unit 53 may, for example, reduce the depth of cut based on the determination result of whether or not chatter vibration is present.

[0093] As shown in Figure 4, for example, machining conditions C2 after changing the rotational speed can be obtained from the initial machining conditions C1 shown in Figure 8. Machining condition C3 is a condition in which the depth of cut is reduced from machining condition C2. The depth of cut for machining condition C2 is the limiting depth of cut a lim It is larger than the limiting depth of cut a. lim It is smaller than that. Therefore, chatter vibration can be suppressed by changing the rotational speed to the rotational speed of machining condition C3.

[0094] Furthermore, the depth of cut for machining condition C3 shown in Figure 8 is greater than the depth of cut for machining condition A2 shown in Figure 2. This is because, at stable rotational speeds, chatter vibration can be suppressed with a smaller reduction in the depth of cut. In addition, even when the optimal depth of cut for stable machining is unknown in an unfamiliar machining process, it becomes possible to find appropriate machining conditions in a shorter time.

[0095] Alternatively, instead of the control unit 53, the operator may reduce the depth of cut until chatter vibration is suppressed. In this case as well, a depth of cut that can suppress chatter vibration can be obtained.

[0096] As in the second embodiment, the depth of cut may be changed after the rotational speed is changed. The machine tool 100 according to the second embodiment can obtain the same effects as the first embodiment.

[0097] At least a portion of the machine tool 100, numerical control device 5, and vibration suppression method according to this embodiment may be configured as hardware or as software. If configured as software, a program that implements at least some of the functions of the machine tool 100, numerical control device 5, and vibration suppression method may be stored on a recording medium such as a flexible disk or CD-ROM, loaded into a computer, and executed. The recording medium is not limited to removable ones such as magnetic disks or optical disks, but may also be a fixed recording medium such as a hard disk drive or memory. Furthermore, the program that implements at least some of the functions of the machine tool 100, numerical control device 5, and vibration suppression method may be distributed via a communication line such as the Internet (including wireless communication). In addition, the program may be encrypted, modulated, or compressed and distributed via a wired or wireless line such as the Internet, or stored on a recording medium.

[0098] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of symbols]

[0099] 100 Machine tool, 1 Spindle, 11 Tool, 2 Spindle motor, 3 Encoder, 4 Vibration sensor, 5 Numerical control device, 51 Sensor information acquisition unit, 52 Calculation unit, 53 Control unit, 54 Evaluation unit, 7 Display device, 71 Display control unit, 72 Display unit, W Workpiece, a lim Limit cutting depth, a0 amplitude, a1 storage amplitude, n change rotation speed, n0 rotation speed, n1 storage rotation speed, n rate0 Override value, n rate1 Storage override value, n h Initial rotation speed, f0 frequency, f1 storage frequency, f c String of references: frequency, k0 wavenumber, v0 phase difference, ud search range, N number of repetitions.

Claims

1. A rotating shaft for rotating the workpiece or the tool used to process the workpiece, A vibration sensor for detecting vibrations generated in the tool or workpiece during machining, A numerical control device that controls the movement of the aforementioned rotating shaft, Equipped with, The numerical control device is A sensor information acquisition unit that acquires the rotational speed of the rotating shaft, as well as the frequency and amplitude of the vibration multiple times, A calculation unit that performs calculations multiple times on the target rotational speed, which is a target value for the rotational speed, based on the rotational speed and frequency acquired by the sensor information acquisition unit, A first control unit that controls the movement of the rotating shaft multiple times so that the rotation speed becomes the target rotation speed, An extraction unit that extracts the rotational speed corresponding to the smallest amplitude from a plurality of target rotational speeds, It has, The calculation unit performs the calculation of the target rotational speed multiple times so that the rotational speed alternately accelerates and decelerates, in a machine tool.

2. The sensor information acquisition unit, the calculation unit, and the first control unit perform a series of operations a predetermined number of times, including acquiring the rotational speed and frequency, calculating the target rotational speed, controlling the movement of the rotation axis, and acquiring the amplitude. The machine tool according to claim 1, wherein the extraction unit extracts the rotational speed corresponding to the smallest amplitude from the target rotational speeds corresponding to the predetermined number of times.

3. The machine tool according to claim 2, wherein the extraction unit extracts the rotational speed corresponding to the smallest amplitude from an initial rotational speed which is an initial value of the rotational speed and a plurality of target rotational speeds.

4. The machine tool according to claim 1, wherein the first control unit controls the movement of the rotating shaft so that the rotation speed becomes the rotation speed extracted by the extraction unit.

5. The machine tool according to claim 4, wherein the numerical control device further comprises a second control unit that controls the movement of the rotating shaft so as to reduce the depth of cut of the tool into the workpiece until the amplitude acquired by the sensor information acquisition unit falls below a predetermined value while the workpiece is being processed at the rotational speed extracted by the extraction unit.

6. The machine tool according to claim 1, wherein the extraction unit stores the target rotational speed and the amplitude acquired by the sensor information acquisition unit in a storage unit so as to correspond to each other.

7. The machine tool according to claim 1, wherein the extraction unit causes the storage unit to update the target rotational speed and the amplitude acquired by the sensor information acquisition unit if the amplitude acquired by the sensor information acquisition unit is smaller than the amplitude stored in the storage unit.

8. The machine tool according to claim 1, wherein the calculation unit determines whether chatter vibration is occurring based on the acquired rotational speed and frequency, and if chatter vibration is occurring, calculates the target rotational speed.

9. A numerical control device that controls the movement of a rotating shaft that rotates a workpiece or a tool used to process the workpiece, A sensor information acquisition unit that acquires the rotational speed of the rotating shaft, as well as the frequency and amplitude of vibrations generated in the tool or workpiece during machining, multiple times. A calculation unit that performs calculations multiple times on the target rotational speed, which is a target value for the rotational speed, based on the rotational speed and frequency acquired by the sensor information acquisition unit, A first control unit that controls the movement of the rotating shaft multiple times so that the rotation speed becomes the target rotation speed, An extraction unit that extracts the rotational speed corresponding to the smallest amplitude from a plurality of target rotational speeds, Equipped with, The calculation unit is a numerical control device that performs calculations for the target rotational speed multiple times so that the rotational speed alternately accelerates and decelerates.

10. A vibration suppression method for a machine tool comprising: a rotating shaft for rotating a workpiece or a tool for machining the workpiece; a vibration sensor for detecting vibrations generated in the tool or workpiece during machining; and a numerical control device for controlling the operation of the rotating shaft, The rotational speed of the rotating shaft and the vibration frequency are acquired by the sensor information acquisition unit, the target rotational speed is calculated by the calculation unit based on the rotational speed and frequency acquired by the sensor information acquisition unit, the operation of the rotating shaft is controlled by the first control unit so that the rotational speed becomes the target rotational speed, and the amplitude of the vibration is acquired by the sensor information acquisition unit. This process is repeated multiple times. From a plurality of target rotational speeds, the rotational speed corresponding to the smallest amplitude is extracted by the extraction unit. It is equipped with the following: The vibration suppression method comprises a calculation unit that performs calculations for the target rotational speed multiple times so that the rotational speed alternately accelerates and decelerates.

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