Control device for machine tools

Abstract

The present application provides a control device of a machine tool, which can perform optimal adjustment of a variation condition and can obtain stable chatter suppression effect. The control device of the machine tool includes: a variation instruction calculation unit that calculates a variation instruction based on a speed instruction of a spindle motor in the machine tool and a variation condition for varying a rotation speed of the spindle motor, generates a speed control instruction for controlling a speed of the spindle motor based on the speed instruction and the variation instruction; a speed deviation determination unit that determines whether a speed deviation indicating a difference between the speed instruction and an actual speed of the spindle motor within a predetermined period is within a first allowable range; and a condition changing unit that changes the variation condition when the speed deviation is outside the first allowable range.

Description

Technical Field

[0001] This invention relates to a control device for machine tools. Background Technology

[0002] Previously, techniques were known to suppress regenerative self-excited chatter generated during cutting by periodically varying the spindle speed of a machine tool (for example, see Patent Document 1).

[0003] In the machine tool described in Patent Document 1, if chatter occurs, at least one of the average speed, amplitude, and period of the machine tool's rotating axis is changed by altering predetermined parameters.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2012-91283 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] In such machine tools, to achieve stable chatter suppression, it is necessary to appropriately adjust the variation conditions when the spindle motor speed changes. However, due to the complexity of setting these variation conditions, it is difficult to adjust them on-site during machining. Therefore, a machine tool control device capable of optimally adjusting the variation conditions and achieving stable chatter suppression is sought.

[0009] Methods for solving problems

[0010] The machine tool control device of the present invention includes: a change command calculation unit that calculates a change command based on a speed command of a spindle motor in the machine tool and change conditions for changing the rotational speed of the spindle motor, and generates a speed control command for controlling the speed of the spindle motor based on the speed command and the change command; a speed deviation determination unit that determines whether a speed deviation, representing the difference between the speed command and the actual speed of the spindle motor within a predetermined period, is within a first allowable range; and a condition modification unit that changes the change conditions if the speed deviation is outside the first allowable range.

[0011] The machine tool control device of the present invention includes: a change command calculation unit that calculates a change command based on a speed command of a spindle motor in the machine tool and change conditions for changing the rotational speed of the spindle motor, and generates a speed control command for controlling the speed of the spindle motor based on the speed command and the change command; a torque command determination unit that determines whether a torque command for the spindle motor within a predetermined period is within a second allowable range; and a condition modification unit that changes the change conditions if the torque command is outside the second allowable range.

[0012] The machine tool control device of the present invention includes: a change command calculation unit that calculates a change command based on a speed command of a spindle motor in the machine tool and change conditions for changing the rotational speed of the spindle motor, and generates a speed control command for controlling the speed of the spindle motor based on the speed command and the change command; a speed deviation determination unit that determines whether a speed deviation between the speed command and the actual speed of the spindle motor within a predetermined period is within a first allowable range; a torque command determination unit that determines whether a torque command for the spindle motor within the predetermined period is within a second allowable range; and a condition modification unit that changes the change conditions when the speed deviation is outside the first allowable range and the torque command is outside the second allowable range.

[0013] Invention Effects

[0014] According to the present invention, optimal adjustment of the varying conditions can be made, and a stable flutter suppression effect can be obtained. Attached Figure Description

[0015] Figure 1 This is a diagram showing an outline of the machine tool according to this embodiment.

[0016] Figure 2 This is a flowchart illustrating the processing flow of the motor control device in this embodiment.

[0017] Figure 3 This is a diagram illustrating an example of speed commands, change commands, and change conditions.

[0018] Figure 4 This is a diagram illustrating an example of speed deviation and the first permissible range.

[0019] Figure 5 This is a diagram illustrating an example of torque command and a second permissible range. Detailed Implementation

[0020] Hereinafter, an example of an embodiment of the present invention will be described. Figure 1This is a diagram showing an outline of the machine tool 1 according to this embodiment. The machine tool 1 is a device for performing predetermined machining operations such as cutting under the control of the numerical control device 2.

[0021] Machine tool 1 is equipped with a motor control device 10 for controlling the motor. In addition, the motor control device 10 includes a change command calculation unit 11, a speed control unit 12, a current control unit 13, a current detection unit 14, a speed deviation determination unit 15, a torque command determination unit 16, and a condition modification unit 17.

[0022] The purpose of the electric motor control device 10 is to suppress regenerative self-excited chatter generated during cutting operations on the machine tool 1. Here, chatter refers to the continuous vibration generated between the tool and the workpiece in the machine tool. Chatter is broadly classified into forced chatter and self-excited chatter based on its primary cause.

[0023] Forced chatter is generated by a forced vibration source. On the other hand, self-excited chatter can occur even without a specific vibration source, when the dynamic characteristics of the machine tool overlap with the cutting process and a certain condition is met.

[0024] In self-excited chatter, regenerative self-excited chatter is chatter caused by variations in chip thickness. To suppress regenerative self-excited chatter, a strategy is needed to maintain a constant chip thickness by adjusting the tool rotation speed.

[0025] Typically, techniques are known to suppress regenerative self-excited chatter generated during cutting by periodically varying the spindle speed of a machine tool.

[0026] In order to stably suppress regenerative self-excited chatter, it is necessary to make appropriate adjustments to the variation conditions when the speed of the spindle motor changes. However, due to the complexity of the condition settings, it is difficult to make adjustments on the machining site.

[0027] Here, the main causes of regenerative self-excited chatter in the spindle motor can be identified as 1) insufficient torque of the spindle motor and 2) insufficient following ability of the spindle motor.

[0028] Regarding insufficient torque of the spindle motor, due to the saturation of the spindle motor's torque command, the actual speed amplitude of the spindle motor decreases, which unintentionally reduces the chatter suppression effect and produces chatter.

[0029] The torque command of the spindle motor includes both the load torque of the spindle motor (including inertia, cutting load, etc.) and the acceleration / deceleration torque generated due to speed variations. Therefore, in order to adjust the variation conditions when the spindle motor speed changes, it is difficult for the machine tool operator to determine whether the torque command is saturated.

[0030] In addition, regarding the insufficient following ability of the spindle motor, due to the insufficient frequency response of the spindle motor, the actual speed amplitude of the spindle motor is reduced, which will unintentionally reduce the chatter suppression effect and produce chatter.

[0031] The frequency response of a spindle motor varies depending on several factors, such as the machine tool's structure (e.g., motor capacity, control gain, load inertia, etc.). Therefore, in order to adjust the variation conditions of the spindle motor speed, it is difficult for the machine tool operator to determine whether the motor's following ability is insufficient.

[0032] The motor control device 10 of this embodiment effectively suppresses such regenerative self-excited flutter by performing the control shown below.

[0033] The variable command calculation unit 11 calculates the variable command based on the speed command of the spindle motor 18 in the machine tool 1 and the variable conditions for changing the speed of the spindle motor 18, and generates a speed control command based on the speed command and the variable command.

[0034] Specifically, the variation command calculation unit 11 calculates a variation command including the variation conditions based on the speed command of the spindle motor 18 and the variation conditions. Here, the variation conditions include the variation amplitude rate for changing the amplitude of the speed command and the variation frequency rate for changing the frequency of the speed command. In addition, the variation conditions can be arbitrarily set by the user as parameters, or they can be preset values.

[0035] Then, the variable command calculation unit 11 superimposes the speed command onto the calculated variable command to generate a speed control command for controlling the speed of the spindle motor 18. That is, the speed control command includes both the speed command and the variable command.

[0036] The speed control unit 12 calculates the speed deviation, representing the difference between the speed command and the actual speed, based on the speed command output from the numerical control device 2 and the actual speed feedback signal of the spindle motor 18 output from the speed detection unit 19 (e.g., an encoder). Then, the speed control unit 12 generates a torque command by performing proportional-integral (PI) control on the speed deviation and outputs the torque command to the current control unit 13. Additionally, the speed control unit 12 outputs the calculated speed deviation to the speed deviation determination unit 15.

[0037] The current control unit 13 generates a voltage command for driving the spindle motor 18 based on the torque command output from the speed control unit 12 and the actual current feedback signal output from the current detection unit 14, and outputs the voltage command to the spindle motor 18. Additionally, the current control unit 13 outputs the torque command to the torque command determination unit 16.

[0038] The current detection unit 14 detects the current value of the spindle motor 18 and outputs the detected current value as an actual current feedback signal to the current control unit 13.

[0039] The speed deviation determination unit 15 determines whether the speed deviation, which represents the difference between the speed command and the actual speed of the spindle motor 18 within a predetermined period, is within a first allowable range. Here, the predetermined period may be, for example, one cycle or half a cycle of a variation period used to change the rotational speed of the spindle motor 18. Furthermore, the speed deviation determination unit 15 monitors the speed deviation for each cycle or half cycle of the variation period and determines whether the speed deviation is within the first allowable range.

[0040] In addition, when the speed deviation determination unit 15 determines whether the speed deviation exceeds the first allowable range if the speed deviation is outside the first allowable range.

[0041] The torque command determination unit 16 determines whether the torque command for the spindle motor 18 within a predetermined period is within the second allowable range. Here, the predetermined period may be, for example, one cycle or half a cycle of the variation cycle used to change the speed of the spindle motor 18. Furthermore, the torque command determination unit 16 monitors the torque command for each cycle or half cycle of the variation cycle and determines whether the torque command is within the second allowable range.

[0042] In addition, the torque command determination unit 16 determines whether the torque command exceeds the second allowable range if the torque command is outside the second allowable range.

[0043] When the speed deviation is outside the first permissible range, the condition modification unit 17 changes the modification conditions. Specifically, when the speed deviation is outside the first permissible range, the condition modification unit 17 changes the amplitude rate and / or frequency rate of the variation conditions.

[0044] Furthermore, when the speed deviation exceeds the first permissible range, the condition modification unit 17 reduces the amplitude variation rate and / or frequency variation rate. On the other hand, when the speed deviation is less than the first permissible range, the condition modification unit 17 increases the amplitude variation rate and / or frequency variation rate.

[0045] Furthermore, when the torque command is outside the second permissible range, the condition modification unit 17 changes the variation conditions. Specifically, when the torque command is outside the second permissible range, the condition modification unit 17 changes the variation amplitude rate and / or variation frequency rate.

[0046] Furthermore, when the torque command exceeds the second permissible range, the condition modification unit 17 reduces the amplitude variation rate and / or frequency variation rate. On the other hand, when the torque command is less than the second permissible range, the condition modification unit 17 increases the amplitude variation rate and / or frequency variation rate.

[0047] Furthermore, the condition modification unit 17 can also change the modification conditions when the speed deviation is outside the first allowable range and the torque command is outside the second allowable range.

[0048] Figure 2 This is a flowchart illustrating the processing flow of the motor control device 10 in this embodiment.

[0049] In step S1, the change command calculation unit 11 calculates a change command including the change conditions based on the speed command of the spindle motor 18 and the change conditions.

[0050] In step S2, the variation command calculation unit 11 superimposes the speed command onto the variation command calculated in step S1 to generate a speed control command for the spindle motor 18.

[0051] Figure 3 This is a diagram illustrating an example of speed commands, change commands, and change conditions. For example... Figure 3 As shown, the variation command has a variation amplitude and a variation frequency, which are superimposed on the speed command. The variation amplitude is calculated based on the speed command and the variation amplitude rate, and the variation frequency is calculated based on the speed command and the variation frequency rate.

[0052] like Figure 3 As shown, the varying amplitude and frequency are calculated using the following formula.

[0053] Variable amplitude [min] -1 ] = Speed ​​command [min -1 × (variable amplitude rate [%) × 100)

[0054] Frequency of change [Hz] = (speed command [min]) -1 ] / 60)×(Variation frequency [%)×100)

[0055] in addition, Figure 3 The change instructions in the text have a triangular wave pattern, but are not limited to this. For example, change instructions can also have patterns such as sine waves, square waves, and rectangular waves.

[0056] return Figure 2 In step S3, the speed deviation determination unit 15 monitors the speed deviation for each cycle or half cycle of the variation period and determines whether the speed deviation is within the first allowable range.

[0057] If the speed deviation is within the first allowable range (yes), the process proceeds to step S4. On the other hand, if the speed deviation is outside the first allowable range (no), the process proceeds to step S5.

[0058] Figure 4This is a diagram illustrating an example of speed deviation and the first permissible range. In Figure 4 In this context, the change cycle is calculated by taking the reciprocal of the change frequency in the change instruction. Figure 4 In the example shown, the first allowable range is set to 0 to 300 min. -1 The first allowable range can be set arbitrarily by the user, or a preset value can be used. Alternatively, the first allowable range can also be calculated based on the maximum speed of the spindle motor 18 and the speed command from the numerical control device 2.

[0059] In addition, Figure 4 In the example shown, the speed deviation determination unit 15 makes a determination by comparing the maximum value of the speed deviation, which is an absolute value, with a first allowable range, but it is not limited to this. For example, the speed deviation determination unit 15 may also make a determination by comparing the average value of the speed deviation over a predetermined period with the first allowable range.

[0060] return Figure 2 In step S4, the torque command determination unit 16 monitors the torque command for each cycle or half cycle of the variation period and determines whether the torque command is within the second allowable range. If the torque command is within the second allowable range (yes), the process ends. On the other hand, if the torque command is outside the second allowable range (no), the process proceeds to step S6.

[0061] Figure 5 This is a diagram illustrating an example of torque command and a second permissible range. In Figure 5 In the example shown, the second allowable range is set to 60% (lower limit) to 90% (upper limit) of the absolute value of the torque command. The second allowable range can be set arbitrarily by the user or a preset value can be used.

[0062] In addition, Figure 5 In the example shown, the torque command determination unit 16 makes a determination by comparing the maximum value of the torque command, which is an absolute value, with a second allowable range, but it is not limited to this. For example, the torque command determination unit 16 may also make a determination by comparing the average value of the torque command over a predetermined period with the second allowable range.

[0063] return Figure 2 In step S5, the speed deviation determination unit 15 determines whether the speed deviation exceeds the first allowable range. If the speed deviation exceeds the first allowable range (yes), the process proceeds to step S7. If the speed deviation does not exceed the first allowable range (no), the process proceeds to step S8.

[0064] In step S6, the torque command determination unit 16 determines whether the torque command exceeds the second allowable range. If the torque command exceeds the second allowable range (yes), the process proceeds to step S7. If the torque command does not exceed the second allowable range (no), the process proceeds to step S8.

[0065] In step S7, the condition changing unit 17 reduces the amplitude rate and / or frequency rate of the change as the change condition.

[0066] In step S8, the condition changing unit 17 increases the amplitude rate and / or frequency rate of the change as the change condition.

[0067] Here, in steps S7 and S8, the change factor of the variable amplitude rate and / or variable frequency rate is shown in the following formula, or it can be calculated based on the current change factor, the maximum value of the speed deviation, and the maximum value of the first allowable range of the speed deviation.

[0068] Change ratio = (Current change ratio) + {(Current change ratio) - (Maximum speed deviation) / (Maximum speed deviation within the first permissible range) × (Current change ratio)}

[0069] In addition, as shown in the following formula, the change ratio of the amplitude rate and / or frequency rate can also be calculated based on the current change ratio, the maximum value of the torque command, and the maximum value of the second allowable range of the torque command.

[0070] Change ratio = (Current change ratio) + {(Current change ratio) - (Maximum value of torque command) / (Maximum value of the second allowable range of torque command) × (Current change ratio)}

[0071] Furthermore, the change factor for the amplitude rate and / or frequency rate can be arbitrarily set by the user as a parameter, or a preset change factor can be used. Additionally, in Figure 2 In the flowchart shown, step S4 is executed after step S3, but alternatively, step S3 can be executed after step S4. In this case, the processes of step S5 and step S6 are swapped.

[0072] As described above, the motor control device 10 of this embodiment includes: a change command calculation unit 11, which calculates a change command based on the speed command of the spindle motor 18 in the machine tool 1 and change conditions for changing the rotational speed of the spindle motor 18, and generates a speed control command for controlling the speed of the spindle motor 18 based on the speed command and the change command; a speed deviation determination unit 15, which determines whether the speed deviation, which represents the difference between the speed command and the actual speed of the spindle motor 18 within a predetermined period, is within a first allowable range; and a condition modification unit 17, which changes the change conditions when the speed deviation is outside the first allowable range.

[0073] Therefore, the motor control device 10 can change the variation conditions according to the speed deviation of the spindle motor 18. Thus, the motor control device 10 can adjust the variation conditions to the best suitability of the spindle motor 18, and obtain a stable chatter suppression effect.

[0074] Furthermore, the variation conditions include a variation amplitude rate for varying the amplitude of the speed command and a variation frequency rate for varying the frequency of the speed command. When the speed deviation falls outside the first permissible range, the condition modification unit 17 changes the variation amplitude rate and / or the variation frequency rate. Thus, the motor control device 10 can change the variation amplitude rate and / or the variation frequency rate according to the speed deviation of the spindle motor 18. Therefore, the motor control device 10 can appropriately adjust the variation conditions.

[0075] Furthermore, if the speed deviation determination unit 15 determines whether the speed deviation exceeds the first allowable range when the speed deviation is outside the first allowable range. If the speed deviation exceeds the first allowable range, the condition modification unit 17 reduces the amplitude variation rate and / or frequency variation rate. If the speed deviation is less than the first allowable range, the condition modification unit 17 increases the amplitude variation rate and / or frequency variation rate.

[0076] Therefore, the motor control device 10 can increase or decrease the amplitude rate and / or frequency rate of the change based on the speed deviation of the spindle motor 18. Thus, the motor control device 10 can appropriately adjust the changing conditions.

[0077] In addition, the motor control device 10 includes: a change command calculation unit 11, which calculates a change command based on the speed command of the spindle motor 18 in the machine tool 1 and the change conditions for changing the rotation speed of the spindle motor 18, and generates a speed control command for controlling the speed of the spindle motor 18 based on the speed command and the change command; a torque command determination unit 16, which determines whether the torque command for the spindle motor 18 within a predetermined period is within a second allowable range; and a condition modification unit 17, which changes the change conditions when the torque command is outside the second allowable range.

[0078] Therefore, the motor control device 10 can change the variation conditions according to the torque command of the spindle motor 18. Thus, the motor control device 10 can adjust the variation conditions of the load and torque margin of the spindle motor 18 to achieve a stable chatter suppression effect.

[0079] In addition, the variation conditions include a variation amplitude rate for varying the amplitude of the speed command and a variation frequency rate for varying the frequency of the speed command. When the torque command is outside the second permissible range, the condition modification unit 17 changes the variation amplitude rate and / or the variation frequency rate.

[0080] Therefore, the motor control device 10 can change the amplitude rate and / or frequency rate according to the torque command of the spindle motor 18. Thus, the motor control device 10 can appropriately adjust the variation conditions.

[0081] Furthermore, if the torque command determination unit 16 determines whether the torque command exceeds the second allowable range when the torque command is outside the second allowable range. If the torque command exceeds the second allowable range, the condition modification unit 17 reduces the amplitude variation rate and / or frequency variation rate. If the torque command is less than the second allowable range, the condition modification unit 17 increases the amplitude variation rate and / or frequency variation rate.

[0082] Therefore, the motor control device 10 can increase or decrease the amplitude rate and / or frequency rate according to the torque command of the spindle motor 18. Thus, the motor control device 10 can appropriately adjust the variation conditions.

[0083] In addition, the motor control device 10 includes: a change command calculation unit 11, which calculates a change command based on the speed command of the spindle motor 18 in the machine tool 1 and the change conditions for changing the rotational speed of the spindle motor 18, and generates a speed control command for controlling the speed of the spindle motor 18 based on the speed command and the change command; a speed deviation determination unit 15, which determines whether the speed deviation between the speed command and the actual speed of the spindle motor 18 within a predetermined period is within a first allowable range; a torque command determination unit 16, which determines whether the torque command for the spindle motor 18 within a predetermined period is within a second allowable range; and a condition modification unit 17, which changes the change conditions when the speed deviation is outside the first allowable range and the torque command is outside the second allowable range.

[0084] Therefore, the motor control device 10 can change the variation conditions according to the speed deviation and torque command of the spindle motor 18. Thus, the motor control device 10 can optimally adjust the variation conditions of the spindle motor 18's following ability and the load and torque margin of the spindle motor 18, thereby achieving a stable chatter suppression effect.

[0085] The embodiments of the present invention have been described above, but the motor control device 10 can be implemented by hardware, software, or a combination thereof. Furthermore, the control method performed by the motor control device 10 can also be implemented by hardware, software, or a combination thereof. Here, implementation by software means implementing it by reading and executing a program on a computer.

[0086] Programs can be stored and provided to a computer using various types of non-transitory computer-readable media. Non-transitory computer-readable media encompasses various types of tangible storage media. Examples of non-transitory computer-readable media include magnetic recording media (e.g., hard disk drives), optical-magnetic recording media (e.g., optical discs), CD-ROMs (Read Only Memory), CD-Rs, CD-R / Ws, semiconductor memories (e.g., mask ROMs, PROMs (Programmable ROMs), EPROMs (Erasable PROMs), flash memory ROMs, and RAMs (random access memory)).

[0087] Furthermore, the described embodiments are preferred embodiments of the present invention, but the scope of the present invention is not limited to these embodiments. Various modifications can be made without departing from the spirit of the present invention.

[0088] Explanation of reference numerals in the attached figures

[0089] 1. Machine tools

[0090] 2. Numerical control device

[0091] 10. Motor control device

[0092] 11. Change Instruction Calculation Unit

[0093] 12 Speed ​​Control Unit

[0094] 13 Current Control Unit

[0095] 14 Current Detection Section

[0096] 15. Speed ​​Deviation Judgment Unit

[0097] 16 Torque Command Determination Unit

[0098] 17. Conditions Change Department

[0099] 18 spindle motors

[0100] 19. Speed ​​Detection Department.

Claims

1. A control device for a machine tool, characterized in that, The machine tool's control device includes: The variable command calculation unit calculates a variable command based on the speed command of the spindle motor in the machine tool and the variable conditions for changing the speed of the spindle motor, and generates a speed control command to control the speed of the spindle motor based on the speed command and the variable command. The speed deviation determination unit determines whether the speed deviation between the speed command and the actual speed of the spindle motor within a predetermined period is within a first allowable range. as well as The condition modification unit changes the variation conditions when the speed deviation is outside the first allowable range, thereby suppressing chatter caused by variations in chip thickness during the cutting process of the machine tool. The first allowable range is calculated based on the maximum speed of the spindle motor or the speed command from the numerical control device. The variation conditions include a variation amplitude rate for varying the amplitude of the speed command and a variation frequency rate for varying the frequency of the speed command. The change factor of the amplitude variation and the frequency variation are calculated based on the current change factor, the maximum value of the speed deviation, and the maximum value of the first allowable range of the speed deviation.

2. The control device according to claim 1, characterized in that, If the speed deviation is outside the first allowable range, the condition change unit changes the variable amplitude rate and / or the variable frequency rate.

3. The control device according to claim 2, characterized in that, If the speed deviation is outside the first allowable range, the speed deviation determination unit determines whether the speed deviation exceeds the first allowable range. If the speed deviation exceeds the first allowable range, the condition modification unit reduces the amplitude variation rate and / or the frequency variation rate. If the speed deviation is less than the first allowable range, the condition change unit increases the variable amplitude rate and / or the variable frequency rate.

4. A control device for a machine tool, characterized in that, The machine tool's control device includes: The variable instruction calculation unit calculates a variable instruction based on the speed instruction of the spindle motor in the machine tool and the variable conditions for changing the speed of the spindle motor, and generates a speed control instruction for controlling the speed of the spindle motor based on the speed instruction and the variable instruction. The torque command determination unit determines whether the torque command for the spindle motor within a predetermined period is within a second allowable range; and The condition modification unit changes the variation conditions when the torque command is outside the second allowable range, thereby suppressing chatter caused by variations in chip thickness during cutting on the machine tool. The second permissible range is set to 60% to 90% of the absolute value of the torque command. The variation conditions include a variation amplitude rate for varying the amplitude of the speed command and a variation frequency rate for varying the frequency of the speed command. The change ratios of the amplitude variation rate and the frequency variation rate are calculated based on the current change ratio, the maximum value of the torque command, and the maximum value of the second allowable range of the torque command.

5. The control device according to claim 4, characterized in that, If the torque command is outside the second allowable range, the condition changing unit changes the amplitude rate and / or the frequency rate.

6. The control device according to claim 5, characterized in that, If the torque command is outside the second allowable range, the torque command determination unit determines whether the torque command exceeds the second allowable range. If the torque command exceeds the second permissible range, the condition modification unit reduces the amplitude variation rate and / or the frequency variation rate. If the torque command is less than the second allowable range, the condition change unit increases the amplitude variation rate and / or the frequency variation rate.

7. A control device for a machine tool, characterized in that, The machine tool's control device includes: The variable instruction calculation unit calculates a variable instruction based on the speed instruction of the spindle motor in the machine tool and the variable conditions for changing the speed of the spindle motor, and generates a speed control instruction for controlling the speed of the spindle motor based on the speed instruction and the variable instruction. The speed deviation determination unit determines whether the speed deviation between the speed command and the actual speed of the spindle motor within a predetermined period is within a first allowable range. The torque command determination unit determines whether the torque command for the spindle motor within a predetermined period is within a second allowable range; and The condition modification unit changes the variation conditions when the speed deviation is outside the first allowable range and the torque command is outside the second allowable range, thereby suppressing chatter caused by variations in chip thickness during the cutting process of the machine tool. The first allowable range is calculated based on the maximum speed of the spindle motor or the speed command from the numerical control device. The second permissible range is set to 60% to 90% of the absolute value of the torque command. The variation conditions include a variation amplitude rate for varying the amplitude of the speed command and a variation frequency rate for varying the frequency of the speed command. The change ratios of the amplitude variation rate and the frequency variation rate are calculated based on the current change ratio, the maximum value of the speed deviation, and the maximum value of the first allowable range of the speed deviation; or the change ratios of the amplitude variation rate and the frequency variation rate are calculated based on the current change ratio, the maximum value of the torque command, and the maximum value of the second allowable range of the torque command.

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