Numerical control device and control method for numerical control device

Abstract

To provide a numerical control device which can calculate machine structural vibration properties with good accuracy, and a control method for the numerical control device.SOLUTION: A CPU of a numerical control device sets processing conditions different from one another. Vibration data of generated vibration is measured for each processing condition in a relationship between a processing condition when processing work-piece and a machine structural compliance (S7). The CPU analyzes a frequency characteristic for each measured vibration data (S9). The CPU determines a chatter frequency at which chatter vibration occurs, for each analysis result (S11). The CPU calculates a phase difference of chatter vibration for each determined chatter frequency and processing condition (S13). The CPU calculates a phase of the machine structural compliance for each calculated chatter phase difference (S15). The CPU derives vibration properties which are a relationship between an attenuation ratio and a machine structural resonance frequency for each calculated compliance phase and determined chatter frequency (S24).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a numerical control device and a control method for a numerical control device. [Background technology]

[0002] Patent Document 1 discloses a machine tool that displays a stability limit diagram and indicates a stable rotational speed range. The machine tool uses a vibration sensor to detect vibrations associated with machining a workpiece. The machine tool detects the occurrence of chatter vibrations from the vibrations detected by the vibration sensor and detects the chatter vibration frequency. The machine tool calculates an optimal rotational speed based on the chatter vibration frequency and the rotational speed of the spindle. The machine tool calculates an unstable rotational speed at which machining will become unstable based on preset phase information that is predicted to make machining unstable. The machine tool creates a stability limit diagram between the optimal rotational speed and the unstable rotational speed and displays it on a display. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-200848 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned machine tool calculates the unstable rotational speed assuming that the chatter vibration frequency is the same as the resonance frequency of the machine structure that can cause chatter vibration, which can lead to errors in the stability limit diagram that is created, etc. Therefore, there is a problem in that the machine tool cannot accurately calculate the optimum rotational speed, etc. based on the vibration characteristics of the machine.

[0005] An object of the present invention is to provide a numerical control device and a control method for a numerical control device that can accurately calculate the vibration characteristics of a mechanical structure, etc. [Means for solving the problem]

[0006] The numerical control device of claim 1 is characterized in comprising: setting means for setting different machining conditions when machining a workpiece; vibration data measuring means for measuring vibration data of vibrations generated due to the relationship between the machining conditions and compliance of a mechanical structure for each of the machining conditions set by the setting means; frequency characteristic analysis means for analyzing frequency characteristics for each of the plurality of vibration data measured by the vibration data measuring means; chatter frequency determining means for determining a chatter frequency at which chatter vibration occurs for each of the plurality of analysis results of the frequency characteristic analysis means; phase difference calculating means for calculating a phase difference of the chatter vibration for each of the machining conditions corresponding to the chatter frequency determined by the chatter frequency determining means; phase calculating means for calculating a phase of the compliance of the mechanical structure for each phase difference of the chatter vibration calculated by the phase difference calculating means; and vibration characteristic derivation means for deriving vibration characteristics, which are the relationship between the damping ratio of the mechanical structure and the resonant frequency of the mechanical structure, for each phase of the compliance calculated by the phase calculation means and the chatter frequency determined by the chatter frequency determining means. The numerical control device derives the relationship between the damping ratio and the resonance frequency, i.e., the vibration characteristics, based on the compliance phase and the chatter frequency. Therefore, the numerical control device can accurately calculate the vibration characteristics of the machine tool. Furthermore, by calculating the compliance phase, the numerical control device can also handle cases where the compliance includes multiple resonance frequencies, and can obtain an accurate relationship between the resonance frequency and the damping ratio. Therefore, the numerical control device can accurately calculate multiple vibration characteristics based on multiple vibration data.

[0007] The numerical control device of claim 2 may further include a first calculation means for calculating the damping ratio and the resonance frequency based on the vibration characteristics derived by the vibration characteristics derivation means. Therefore, by acquiring a plurality of vibration data, the numerical control device can calculate the damping ratio and the resonance frequency based on the vibration characteristics, a plurality of chatter frequencies, and a plurality of compliance phases.

[0008] The numerical control device of claim 3 may further include a stable rotation speed calculation means for calculating a stable rotation speed for suppressing the chatter vibration based on at least the resonance frequency out of the damping ratio and the resonance frequency calculated by the first calculation means. Thus, the numerical control device can identify a stable rotation speed for suppressing the chatter vibration, which is a self-excited vibration.

[0009] The numerical control device of claim 4 may further include a relative transfer function calculation means for calculating a relative transfer function, which is a relative form of the transfer function, based on the damping ratio and the resonance frequency calculated by the first calculation means. Therefore, the numerical control device can calculate the form of the transfer function more accurately by calculating the relative transfer function based on the calculated damping ratio and resonance frequency.

[0010] The numerical control device of claim 5 may further include forced vibration prediction means for predicting forced vibration based on at least the resonance frequency of the resonance frequency and the damping ratio calculated by the first calculation means. Therefore, the numerical control device can predict forced vibration based on at least the resonance frequency, as well as chatter vibration, which is self-excited vibration. Therefore, the numerical control device can determine a spindle rotation speed that is unfavorable for forced vibration.

[0011] The numerical control device of claim 6 may further comprise a relative stability limit diagram creating means for creating a relative stability limit diagram, which is a relative form of the stability limit diagram, based on the relative transfer function calculated by the relative transfer function calculating means. Therefore, since the numerical control device can create the relative stability limit diagram based on the calculated relative transfer function, it is possible to accurately obtain the relative value of the stability limit between the rotation speed on the horizontal axis and the critical cutting-in amount on the vertical axis of the stability limit diagram.

[0012] The numerical control device of claim 7 may further include a display means for displaying at least one of the damping ratio and the resonance frequency calculated by the first calculation means on a display unit, thereby enabling the numerical control device to present the calculated damping ratio and resonance frequency to a user.

[0013] The numerical control device of claim 8 may further include a display unit that displays the stable rotation speed calculated by the stable rotation speed calculation unit on a display unit. Thus, the numerical control device can present the identified stable rotation speed to a user.

[0014] The numerical control device of claim 9 may further comprise a display means for displaying the relative transfer function calculated by the relative transfer function calculation means on a display unit, so that the numerical control device can present the calculated relative transfer function to a user.

[0015] The numerical control device of claim 10 may further include a display means for displaying the relative stability limit diagram created by the relative stability limit diagram creating means on a display unit. Thus, the numerical control device can present the created relative stability limit diagram to a user.

[0016] The numerical control device of claim 11 may further include a machining condition acquisition means for acquiring the machining conditions, a phase difference setting means for setting a compliance phase difference between the phase of the compliance in the current machining operation and the phase of the compliance in the next machining operation, and a setting means for setting the next machining conditions based on the damping ratio pre-stored in a storage unit or the damping ratio calculated by the first calculation means, the current machining conditions acquired by the machining condition acquisition means, and the compliance phase difference set by the phase difference setting means, and the vibration data measurement means for measuring the vibration data during machining based on the next machining conditions set by the setting means. Therefore, the numerical control device measures the vibration data by machining a workpiece based on the set next machining conditions. This allows the numerical control device to more accurately calculate the damping ratio and the resonant frequency.

[0017] The control method of the numerical control device of claim 12 includes a setting step of setting different machining conditions when machining a workpiece, a vibration data measuring step of measuring vibration data of vibrations generated due to a relationship between the machining conditions and compliance of a machine structure for each of the machining conditions set in the setting step, a frequency characteristic analysis step of analyzing frequency characteristics for each of a plurality of pieces of vibration data measured in the vibration data measuring step, a chatter frequency determination step of determining a chatter frequency at which chatter vibration occurs for each of a plurality of analysis results of the frequency characteristic analysis step, and The numerical control device is characterized by comprising: a phase difference calculation step of calculating a phase difference of the chatter vibration for each of the machining conditions corresponding to the chatter frequency determined in the frequency determination step, a phase calculation step of calculating a phase of compliance of the mechanical structure for each of the phase differences of the chatter vibration calculated in the phase difference calculation step, and a vibration characteristic derivation step of deriving vibration characteristics that are a relationship between the damping ratio of the mechanical structure and the resonance frequency of the mechanical structure for each of the phases of the compliance calculated in the phase calculation step and the chatter frequency determined in the chatter frequency determination step. By executing the above steps, the numerical control device can achieve the same effect as the numerical control device described in claim 1. [Brief explanation of the drawings]

[0018] [Figure 1] 2 is a block diagram showing the electrical configuration of the machine tool 1. FIG. [Figure 2] FIG. 10 is a flowchart illustrating a vibration characteristic acquisition process. [Figure 3] 10A is a diagram showing the absolute value of compliance with respect to frequency, and FIG. 10B is a diagram showing the phase f2 of compliance with respect to frequency. [Figure 4] (a) is a diagram showing the relative stability limit diagram, (b) is a diagram showing the chatter frequency fc, and (c) is a diagram showing the phase difference ε of chatter vibration. [Figure 5] (a) is a graph showing the resonance frequency fn and damping ratio ζ, (b) is the stable rotation speed of the spindle, and (c) is the rotation speed of the spindle with large forced vibration. [Figure 6](a) is a plot of the calculation results of the chatter frequency fc and compliance phase γ corresponding to the spindle rotation speed, and (b) is a plot of the calculation results of the chatter frequency fc and phase γ. [Figure 7] (a) is a graph showing the cutting force Fx1 in the time domain, (b) is a graph showing the Fourier transform result Fx1' of the cutting force Fx1, and (c) is a graph predicting the Fourier transform result X1 of the vibration displacement in the next machining. [Figure 8] (a) is a graph showing the cutting force Fx2 in the time domain, (b) is the Fourier transform result Fx2' of the cutting force Fx2, and (c) is a graph showing the predicted Fourier transform result X2 of the vibration displacement in the next machining. DETAILED DESCRIPTION OF THE INVENTION

[0019] An embodiment of the present invention will be described. A numerical control device 29 shown in Fig. 1 controls the operation of a machine tool 1 to perform cutting processing on a workpiece (not shown) held on the upper surface of a table (not shown). The left-right direction, front-rear direction, and up-down direction of the machine tool 1 are the X-axis direction, Y-axis direction, and Z-axis direction, respectively.

[0020] The configuration of machine tool 1 will be described with reference to FIG. 1. Machine tool 1 is a vertical machine tool that performs machining (e.g., drilling, tapping, side machining, etc.) on a workpiece held on, for example, a table top surface by moving a tool attached to a spindle extending in the Z-axis direction in the X-axis, Y-axis, and Z-axis directions. Machine tool 1 is equipped with a spindle mechanism, a spindle movement mechanism, a tool changer, and other components (not shown). The spindle mechanism includes a spindle motor 51 and rotates the spindle to which a tool is attached. The spindle movement mechanism further includes a Z-axis motor 52, an X-axis motor 53, and a Y-axis motor 54, and moves the spindle along each of the X, Y, and Z feed axes relative to the workpiece supported on the top surface of a table (not shown). The tool changer includes a magazine motor 55 and drives a tool magazine (not shown) that stores multiple tools, thereby replacing the tool attached to the spindle with another tool. An acceleration sensor 41 is provided in the spindle mechanism and detects vibrations generated in machine tool 1.

[0021] The electrical configuration of the machine tool 1 will be described. The machine tool 1 is equipped with an operation panel 15, a numerical control device 29, drive circuits 51A to 55A, a spindle motor 51, a Z-axis motor 52, an X-axis motor 53, a Y-axis motor 54, a magazine motor 55, etc. The operation panel 15 is mounted on a cover (not shown). The operation panel 15 is equipped with an input unit 15A and a display unit 15B. The input unit 15A is used by the user to input various operation settings and the like into the machine tool 1. The display unit 15B displays various setting screens for selecting a machining program, setting the machining conditions for the machining program, etc. The user sets various operations of the machine tool 1, the machining conditions for the workpiece, etc. by operating the input unit 15A while checking the display unit 15B.

[0022] The numerical control device 29 comprises a CPU 31, a ROM 32, a RAM 33, a storage device 39, and an interface 35. The CPU 31 controls the machine tool 1. The ROM 32 stores a program for performing the vibration characteristic acquisition process described below. The RAM 33 temporarily stores various data, etc. The storage device 39 stores a machining program for processing a workpiece, etc. The storage device 39 stores a mass coefficient m, a damping coefficient c, a stiffness coefficient k, etc. as parameters used in the vibration characteristic acquisition process described below. The storage device 39 also stores a damping ratio ζ, a resonance frequency fn, etc. described below. An acceleration sensor 41 transmits vibration data of the spindle mechanism to the CPU 31.

[0023] Input unit 15A and display unit 15B are connected to CPU 31 via interface 35. CPU 31 is connected to drive circuits 51A to 55A via interface 35. Drive circuits 51A to 55A are connected to control targets: spindle motor 51, Z-axis motor 52, X-axis motor 53, Y-axis motor 54, and magazine motor 55. Spindle motor 51, Z-axis motor 52, X-axis motor 53, Y-axis motor 54, and magazine motor 55 are equipped with encoders 51B to 55B.

[0024] Encoders 51B-55B detect the rotational positions and the like of the drive shafts of spindle motor 51, Z-axis motor 52, X-axis motor 53, Y-axis motor 54, and magazine motor 55, and output the detection results to drive circuits 51A-55A. CPU 31 can detect the rotational position of spindle motor 51 based on the detection results of encoder 51B by drive circuit 51A. CPU 31 can detect the X-, Y-, and Z-axis coordinate values of Z-axis motor 52, X-axis motor 53, and Y-axis motor 54 based on the detection results of encoders 52B-54B by drive circuits 52A-54A. The X-, Y-, and Z-axis coordinate values are spindle position information. CPU 31 controls each drive circuit 51A-54A based on the position information and the like to machine the workpiece.

[0025] The vibration characteristics acquisition process of this embodiment will be described with reference to Figures 2 to 5. When the user operates the operation panel 15 to set the vibration characteristics acquisition mode, the CPU 31 reads out a program stored in the ROM 32 and executes the vibration characteristics acquisition process (see Figure 2). When executing the vibration characteristics acquisition process, the CPU 31 determines whether the user has set the machining conditions (S1). The machining conditions include the rotation speed of the spindle, the number of teeth of the tool, the radial cutting depth, the diameter of the tool, etc., and are stored in the storage device 39.

[0026] Among the machining conditions, the rotation speed of the tool is defined in the machining program, but it can also be adjusted with a separate dial (not shown). For example, in the first machining, the rotation speed n1 is 4250 [min -1 ]. The radial cutting depth is not stored in advance and must be input by the user. In this embodiment, the timing for inputting the radial cutting depth is before the execution of machining, but it may also be input immediately before the execution of the calculation process described below. Also, the tool diameter is stored in advance and does not need to be input. Note that if the tool diameter is not stored in the storage device 39, it can be input by the user.

[0027] As will be described later, in order to measure multiple chatter vibrations within the same machining program, the user may set other different machining conditions in the process of S1. For example, the rotation speed n2 may be set to 4750 [min -1 Note that the second machining conditions may be derived from the first machining, as in the processes of S45 to S51 described below. In other words, the CPU 31 may set the machining conditions for the second and subsequent machining operations in advance, or may derive them by calculation.

[0028] The CPU 31 determines whether or not a machining program selected by the user has been accepted from among the machining programs stored in the storage device 39 (S3). If it is determined that a machining program has not been accepted (S3: NO), the CPU 31 waits until a machining program is accepted. The user operates the input unit 15A to input a machining program and presses the decision button (not shown).

[0029] When a machining program is accepted by the user operating input unit 15A (S3: YES), CPU 31 interprets each line of the accepted machining program and starts machining the workpiece (S5). During machining, machine tool 1 may vibrate. The vibration occurs due to the relationship between the machining conditions, the compliance of the machine structure, and the workpiece. Note that the machine structure is a concept that includes not only the machine structure of machine tool 1, but also tools, workpieces, jigs, etc.

[0030] The CPU 31 measures vibration data during machining using an acceleration sensor 41 provided on the spindle mechanism (S7). The CPU 31 analyzes the frequency characteristics of the acquired vibration data using a Fourier transform or the like (S9). The CPU 31 determines whether chatter vibration is occurring (S10). Note that, as an example, the CPU 31 determines that chatter vibration is occurring when there is a frequency component with the largest vibration amplitude that is not synchronized with the rotation frequency or the cutting edge passing frequency. Chatter vibration can be a regenerative type, a mode coupling type, or the like.

[0031] If it is determined that chatter vibration is not occurring (S10: NO), the CPU 31 determines whether or not machining of the workpiece is complete (S41). If it is determined that machining is not complete (S41: NO), the CPU 31 continues machining of the workpiece and executes the processes of S7 to S10.

[0032] If it is determined that chatter vibration is occurring (S10: YES), the CPU 31 determines the chatter frequency fc of the chatter vibration from the frequency characteristics of the analyzed vibration (S11). The CPU 31 calculates the phase difference ε of the chatter vibration using equation (1-1) (S13). The CPU 31 performs the calculation by substituting the determined chatter frequency fc, the spindle rotation speed n defined in the machining program, the input number of tool teeth N, and the chatter vibration phase difference ε into equation (1-1). The spindle rotation speed n may be an actual measured value during machining.

number

[0033] Next, the CPU 31 calculates the compliance phase γ using equation (1-2) (S15). The equation used in this embodiment is disclosed in Eiji Shamoto, "Technical Commentary: Mechanism of Chatter Vibration Generation and Suppression in Cutting," Electric Steel Manufacturing, Vol. 82, No. 2, 2011, pp. 143-155. Equation (1-2) expresses the relationship between the compliance phase γ and the chatter vibration phase difference ε. The CPU 31 substitutes the calculated chatter vibration phase difference ε into equation (1-2) and solves for the compliance phase γ. Note that equation (1-2) is only an example of an equation expressing the relationship between the compliance phase γ and the chatter vibration phase difference ε, and other equations may be used. For example, the number of cutting edges of the tool, the component force ratio (the ratio between the radial and tangential cutting forces), the cutting start angle, and the cutting end angle may be used.

number

[0034] Equation (1-3) shows the relationship between the compliance phase γ, chatter frequency fc, damping coefficient c, mass coefficient m, and stiffness coefficient k.

number

[0035] Equation (1-4) shows the relationship between the damping coefficient c, damping ratio ζ, mass coefficient m, and stiffness coefficient k. The damping ratio ζ is a parameter that represents the damping of vibrations in a mechanical structure, and is a value between 0 and 1.

number

[0036] Equation (1-5) is obtained by substituting equation (1-4) for the damping coefficient c in equation (1-3).

number

[0037] Equation (1-6) is derived by rearranging equation (1-5). Specifically, the right-hand side of equation (1-6) is obtained by dividing the numerator and denominator of the right-hand side of equation (1-5) by the mass coefficient m.

number

[0038] Equation (1-7) is the relational expression between the resonance frequency fn, the mass coefficient m, and the stiffness coefficient k. If the stored damping ratio ζ is not used, an approximate resonance frequency fn may be used.

number

[0039] Equation (1-8) is derived by substituting equation (1-7) into equation (1-6). Here, the chatter frequency fc and compliance phase γ in equation (1-8) are known because they were calculated in the processes of S11 and S15. Equation (1-8) shows the relationship between the damping ratio ζ and the resonance frequency fn of the mechanical structure, i.e., the vibration characteristics. In equation (1-8), for example, if either the damping ratio ζ or the resonance frequency fn is known, the other can be calculated. In this embodiment, which will be described later, both the damping ratio ζ and the resonance frequency fn can be calculated.

number

[0040] Equation (1-9) is obtained by rearranging equation (1-8) so that the right-hand side is set to 0. Since the compliance phase γ, chatter frequency fc, and damping ratio ζ are known, equation (1-9) can be regarded as a quadratic equation related to the resonance frequency fn.

number

[0041] Equation (1-10) is obtained by solving equation (1-9) for the resonance frequency fn.

number

[0042] The CPU 31 derives the vibration characteristics, which are the relationship between the damping ratio ζ and the resonance frequency fn, i.e., equations (1-8) to (1-10) (S24). The CPU 31 uses equation (1-11) to derive the vibration characteristics, which are the relationship between the damping ratio ζ and the resonance frequency fn (S24). Equation (1-11) is a relational expression obtained by solving equation (1-8) for the damping ratio ζ. The chatter frequency fc and compliance phase γ obtained in the processes of S11 and S15 are substituted into equation (1-11) to derive the vibration characteristics, which are the relationship between the damping ratio ζ and the resonance frequency fn. The CPU 31 stores the derived vibration characteristics in the storage device 39 in association with machining conditions such as the rotational speed of the spindle.

number

[0043] Next, the CPU 31 determines whether or not a plurality of vibration characteristics are stored (S25). If it is determined that a plurality of vibration characteristics are not stored (S25: NO), the CPU 31 determines whether or not machining of the workpiece is complete (S41). If it is determined that machining of the workpiece is not complete (S41: NO), the CPU 31 continues machining of the workpiece and executes the processes of S7 to S10. In this case, if chatter vibration does not occur again (S10: NO), the CPU 31 continues machining of the workpiece until the machining program is completed, and then completes machining of the workpiece.

[0044] If chatter vibration occurs again within the same machining program (S10: YES), the CPU 31 executes the processes of S11 to S15. In this case, if two or more machining conditions are set in the process of S1, chatter vibrations with different chatter frequencies may occur within the same machining program. If chatter vibrations with different chatter frequencies occur, the CPU 31 again calculates the vibration characteristics, which are the relationship between the damping ratio ζ and the resonance frequency fn, using equation (1-11) (S24). Since the vibration characteristics have already been stored in the storage device 39 (S25: YES), the CPU 31 proceeds to the process of S29.

[0045] The CPU 31 calculates the damping ratio ζ and the resonance frequency fn based on two of the multiple vibration characteristics stored (S29). The CPU 31 solves simultaneous quadratic equations with two unknowns for the two vibration characteristics (by substituting the obtained values into equation (1-11)) with the damping ratio ζ and the resonance frequency fn as variables. As a result, the CPU 31 calculates the damping ratio ζ and the resonance frequency fn. For example, when the phase γ of the first compliance is −134° and the chatter frequency fc is 114 Hz, and the phase γ of the second compliance is −177° and the chatter frequency fc is 144 Hz, the damping ratio ζ and the resonance frequency fn are 0.0128 and 112.6 Hz (see FIG. 5(a)).

[0046] The CPU 31 calculates a relative transfer function (see FIG. 3) based on the calculated damping ratio ζ and the calculated resonant frequency fn (S31). Here, the relative transfer function has a horizontal axis representing frequency, and a vertical axis representing the absolute value of compliance (see FIG. 3(a)), which is an unknown value, and the phase of compliance (see FIG. 3(b)), which is an accurate value. In other words, the calculated relative transfer function has an unknown absolute value (magnitude) of compliance on the vertical axis, but can accurately represent the phase. The CPU 31 performs calculations by substituting the acquired damping ratio ζ and resonant frequency fn into the transfer function of the second-order lag system. As shown in FIG. 3(a), the resonant frequency f1 of the compliance peaks near 112.6 Hz, which is the calculated resonant frequency fn. As shown in FIG. 3(b), the phase γ of the compliance changes sharply near 112.6 Hz, which corresponds to the resonant frequency fn.

[0047] The CPU 31 creates data capable of plotting a relative stability limit diagram (FIG. 4(a)) based on the calculated relative transfer function (S33). A relative stability limit diagram refers to the shape of a stability limit diagram relative to the relative value of the critical axial depth of cut Ap. The relative stability limit diagram also refers to the shape of a stability limit diagram in which the horizontal axis indicates the exact value and the vertical axis indicates the unknown numerical value. In the relative stability limit diagram shown in FIG. 4(a), the vertical axis indicates the critical axial depth of cut Ap and the horizontal axis indicates the rotation speed of the spindle. The curve Th indicates the threshold value of the critical axial depth of cut Ap corresponding to the rotation speed. In other words, the curve Th indicates the threshold value for whether chatter vibration occurs. Chatter vibration occurs when machining is performed in the region above the curve Th, and chatter vibration does not occur when machining is performed in the region below the curve Th. The curve Th indicates the threshold value for whether chatter vibration occurs when machining is performed in the region above the curve Th. -1 , 1250mm -1 , 1650mm -1 , 2300mm -1 , 3300mm -1 , 6000mm -1 Fig. 4(b) shows the chatter frequency fc for each spindle rotation speed, and Fig. 4(c) shows the phase difference ε of chatter vibration for each spindle rotation speed.

[0048] The CPU 31 calculates the stable rotation speed of the spindle (S35). The CPU 31 determines the stable rotation speed of the spindle, for example, from the shape of the relative stability limit diagram (FIG. 4(a)). Specifically, from the data that can be depicted, it determines the range of rotation speeds in which the critical axial cutting depth Ap is greater than a predetermined threshold value. As shown in FIG. 4(a), the range of stable rotation speed of the spindle calculated by the CPU 31 is, for example, 2000 to 2200 min -1 , 3000~3500min -1 , 5300~6000min -1 (See FIG. 5(b)). When the spindle is rotated within the stable rotation speed range, the CPU 31 can set a large critical axial depth of cut Ap while suppressing the occurrence of chatter vibration. The CPU 31 may obtain the outline of the stability limit diagram from the resonance frequency fn. The CPU 31 may also obtain the relative stability limit diagram from the damping ratio ζ and the resonance frequency fn. In this case, the CPU 31 can create a stability limit diagram with a more accurate relative relationship on the vertical axis compared to when the stability limit diagram is obtained from only the resonance frequency fn.

[0049] The CPU 31 predicts forced vibration, which is an example of chatter vibration (S37). The CPU 31 predicts forced vibration using equation (1-12). Here, k1 indicates an integer, and N indicates the number of teeth of the tool. Information about the number of teeth of the tool is stored in the storage device 39 in the process of S3. For example, if the calculated resonance frequency fn is 112.6 Hz and the number of teeth of the tool is 1, the rotation speed of the spindle where forced vibration is large is 6756 [min -1 ], 3378 [min -1 ], 2252 [min -1 ] can be calculated (see FIG. 5(c)). Therefore, the user can set the machining conditions so as to avoid the above rotation speeds where the forced vibration is expected to be large.

number

[0050] The CPU 31 displays, for example, the resonance frequency fn (see FIG. 5(a)), the damping ratio ζ (see FIG. 5(a)), the relative transfer function (see FIG. 3), the relative stability limit diagram (see FIG. 4), the stable rotation speed (see FIG. 5(b)), the rotation speed at which forced vibration is large (see FIG. 5(c)), the mathematical expression of the relative transfer function, etc. on the display unit 15B (S39). Note that if the information cannot be displayed on one screen of the display unit 15B, the CPU 31 may switch the display. In other words, the CPU 31 may display at least one of the resonance frequency fn, the damping ratio ζ, the relative transfer function, the relative stability limit diagram, the stable rotation speed of the spindle, and the predicted rotation speed at which forced vibration is large on the display unit 15B.

[0051] On the other hand, if it is determined that the machining of the workpiece has been completed (S41: YES), the CPU 31 determines whether to change the machining conditions and machine the workpiece (S43). Whether to perform the next machining may be determined by the user or may be determined in advance. If two or more machining conditions are set in the process of S1 and the processes of S29 to S39 have already been performed, the process may be terminated. If it is determined that machining will be performed under the next machining conditions (S43: YES), the CPU 31 acquires the damping ratio ζ stored in the storage device 39 (S45). When calculating the rotation speed n2 for the second machining, the CPU 31 uses, for example, the damping ratio ζ stored in advance in the storage device 39. When calculating the rotation speed for the third or subsequent machining operations in the process of S45, the CPU 31 uses the damping ratio ζ calculated in the process of S29.

[0052] The CPU 31 acquires the spindle rotation speed defined in the current machining program (S47). For example, in the second machining, the rotation speed n1 is acquired. The CPU 31 sets the compliance phase difference Δγ (S49). The compliance phase difference Δγ may be set in advance by the user, or a predetermined value may be stored in the storage device 39. The compliance phase difference Δγ predicts the difference in the compliance phase γ that will change in the next cutting from the compliance phase γ obtained in the current cutting.

[0053] The CPU 31 sets the spindle rotation speed n2 for the next machining of the workpiece (S51). The rotation speed n2 for the next machining is calculated using equations (1-13) to (1-15). In this case, the rotation speed n2 for the next machining is calculated so that the compliance phase γ2 becomes γ1 + Δγ and the chatter frequency fc becomes the chatter frequency fc2. In equation (1-15), N indicates the number of teeth of the tool. The CPU 31 substitutes the rotation speed n1 from the previous machining, the compliance phase γ1 of chatter vibration, the chatter frequency fc1, etc. into equations (1-13) to (1-15). In this way, the CPU 31 obtains the rotation speed for the next machining.

number

[0054] The CPU 31 then performs machining of the workpiece at the calculated spindle rotation speed n2 (S5). By rotating the spindle at the rotation speed n2 to cut the workpiece, it is predicted that chatter vibrations with a different chatter frequency fc will occur compared to the previous machining. If it is determined that chatter vibrations have not occurred (S10: NO), the CPU 31 continues machining the workpiece as described above. On the other hand, if it is determined that chatter vibrations have occurred in the second or subsequent machining operations (S10: YES), the CPU 31 executes the processes of S11 to S15 to calculate the chatter frequency fc, chatter vibration phase difference ε, and compliance phase γ at different rotation speeds. For example, the CPU 31 calculates the chatter frequency fc2, chatter vibration phase difference ε2, and compliance phase γ2 based on the vibration data for the rotation speed n2.

[0055] As described above, the CPU 31 derives the vibration characteristics using equation (1-11) (S24). If multiple vibration characteristics are not stored (S25: NO), the CPU 31 proceeds to S41. On the other hand, if multiple vibration characteristics are stored (S25: YES), the CPU 31 performs calculations for the damping ratio ζ and the resonance frequency fn by solving the simultaneous quadratic equations with two unknowns for any two vibration characteristics (S29). In the process of S29, the CPU 31 may perform calculations from multiple vibration characteristics obtained within the same machining program as described above, or may solve the simultaneous quadratic equations with two unknowns for multiple vibration characteristics obtained from multiple different machining programs and multiple or a single currently obtained vibration characteristic. The CPU 31 performs the processes of S31 to S39 based on the calculated damping ratio ζ and the resonance frequency fn. The CPU 31 determines whether machining of the workpiece with the changed rotational speed has been completed (S41).

[0056] If it is determined that the machining of the workpiece has not been completed (S41: NO), the CPU 31 returns the process to S7 and continues machining the workpiece. On the other hand, if it is determined that the machining of the workpiece has been completed (S41: YES), the CPU 31 determines whether or not to perform machining under the next machining conditions (S43). If the CPU 31 determines that the next machining will be performed (S43: YES), the CPU 31 proceeds to S45. If it determines that the machining will not be performed under the next machining conditions (S43: NO), the CPU 31 ends the process.

[0057] In the above description, a case has been described in which compliance includes one resonance frequency ωn as shown in Fig. 3(a), but the above embodiment is also applicable to a case in which two resonance frequencies are included in the mechanical structure of the machine tool 1. The calculation of the two resonance frequencies will be described below.

[0058] For example, the CPU 31 executes the processes of S10 to S15 multiple times, measures chatter vibrations six times (S10: YES), and calculates the chatter frequency of each chatter vibration (S11).The CPU 31 calculates the compliance phase for each of the six chatter frequencies (S15).

[0059] In this case, CPU 31 performs a regression analysis, which will be described later, on the compliance phase. CPU 31 may calculate two resonance frequencies from the results of the regression analysis. Here, the two calculated resonance frequencies are approximately equal to the two resonance frequencies possessed by the mechanical structure of machine tool 1. Therefore, CPU 31 can calculate two resonance frequencies from the results of measuring chatter vibration six times in total. Furthermore, CPU 31 can indicate the relationship between one of the two resonance frequencies and damping ratio ζ by substituting one of the acquired resonance frequencies into, for example, equation (1-10).

[0060] As described above, in the vibration characteristic acquisition process, the CPU 31 can set different machining conditions. The CPU 31 measures vibration data for each machining of a workpiece performed based on the set machining conditions. The CPU 31 analyzes the frequency characteristics of the vibration data of the machining vibration for each measured vibration data. The CPU 31 determines the chatter frequency fc for each analysis result of the analyzed frequency characteristics. The CPU 31 calculates the chatter vibration phase difference ε for each chatter frequency fc corresponding to each vibration data and for each machining condition. The CPU 31 calculates the compliance phase γ for each of the multiple chatter vibration phase differences. The CPU 31 derives multiple vibration characteristics, which are the relationship between the damping ratio ζ and the resonance frequency fn, i.e., multiple equations (1-11).

[0061] The CPU 31 derives multiple vibration characteristics, which are the relationship between the damping ratio ζ and the resonance frequency fn, based on multiple chatter frequencies fc1, fc2 and multiple compliance phases γ1, γ2. By executing machining of the workpiece under multiple machining conditions, the CPU 31 can derive multiple vibration characteristics, which are the relationship between the damping ratio ζ and the resonance frequency fn, based on multiple chatter frequencies fc and multiple compliance phases γ. Furthermore, by calculating the compliance phase γ, the CPU 31 can also handle cases where the compliance includes multiple resonance frequencies, and can obtain an accurate relationship between the resonance frequency fn and the damping ratio ζ.

[0062] The CPU 31 calculates the damping ratio ζ and the resonance frequency fn based on the derived vibration characteristics. Therefore, by acquiring multiple vibration data, the CPU 31 can calculate the damping ratio ζ and the resonance frequency fn based on the vibration characteristics, multiple chatter frequencies fc1, fc2, and multiple compliance phases γ.

[0063] The CPU 31 identifies a stable rotation speed for suppressing chatter vibrations based on at least the resonance frequency fn out of the calculated damping ratio ζ and the resonance frequency fn. Therefore, the CPU 31 can identify a stable rotation speed for suppressing chatter vibrations, which are self-excited vibrations.

[0064] The CPU 31 calculates a relative transfer function, which is the relative form of the transfer function, based on the calculated damping ratio ζ and the calculated resonance frequency fn. Therefore, the CPU 31 can accurately present the form of the transfer function by calculating the relative transfer function based on the calculated damping ratio ζ and the calculated resonance frequency fn. In other words, the CPU 31 can accurately calculate the relative transfer function.

[0065] The CPU 31 predicts forced vibration based on at least the resonance frequency fn of the calculated damping ratio ζ and resonance frequency fn. Therefore, the CPU 31 can predict forced vibration based not only on chatter vibration, which is self-excited vibration, but also on at least the resonance frequency fn of the resonance frequency fn and the damping ratio ζ. Therefore, the CPU 31 knows the rotation speed of the spindle that is unfavorable for forced vibration. Therefore, the CPU 31 can predict the rotation speed at which forced vibration is large / small based not only on chatter vibration, which is self-excited vibration, but also on at least the resonance frequency fn of the resonance frequency fn and the damping ratio ζ.

[0066] The CPU 31 creates a relative stability limit diagram, which is a relative form of the stability limit diagram, based on the calculated relative transfer function. Therefore, the CPU 31 can create a rough outline of the stability limit diagram based on the calculated relative transfer function, and can accurately obtain the relative value of the stability limit between the rotational speed on the horizontal axis of the stability limit diagram and the critical axial cutting-in amount Ap on the vertical axis. Therefore, the CPU 31 can accurately obtain the rough outline of the stability limit diagram and its relationship with the horizontal axis.

[0067] The CPU 31 displays at least one of the calculated damping ratio ζ and the resonance frequency fn on the display unit 15B, so that the CPU 31 can present the damping ratio ζ and the resonance frequency fn to the user.

[0068] The CPU 31 displays the identified stable rotation speed on the display unit 15B, so that the CPU 31 can present the identified stable rotation speed to the user.

[0069] The CPU 31 displays the calculated relative transfer function on the display unit 15B, so that the CPU 31 can present the calculated relative transfer function to the user.

[0070] The CPU 31 displays the created relative stability limit diagram on the display unit 15B, so that the CPU 31 can present the created relative stability limit diagram to the user.

[0071] The CPU 31 acquires the current machining conditions for machining the workpiece. The CPU 31 acquires the damping ratio ζ stored in advance in the storage device 39 or the calculated damping ratio ζ. The CPU 31 sets a compliance phase difference Δγ between the compliance phase γ1 in the current machining and the compliance phase γ2 in the next machining. The CPU 31 sets the machining conditions for the next machining based on the acquired damping ratio ζ, the set machining conditions, and the set compliance phase difference Δγ. The CPU 31 measures vibration data when machining the workpiece based on the set next machining conditions. The CPU 31 measures multiple vibration data by machining the workpiece based on the set next machining conditions (S7). The CPU 31 analyzes multiple chatter vibrations from the measured multiple vibration data (S11 to S15) and calculates vibration characteristics (S24). This allows the CPU 31 to more accurately calculate the damping ratio ζ and the resonance frequency fn.

[0072] The present invention is not limited to the above embodiment. The machine tool 1 in the above embodiment is a vertical machine tool with a spindle extending in the Z-axis direction, but the present invention can also be applied to a horizontal machine tool with a spindle extending horizontally. The above embodiment can also be applied to lathes and other machines, and can be applied to any type of machine tool. The machine tool 1 in the above embodiment supports a workpiece on the table top surface for machining. However, for example, a rotary table that can rotate 180° may be used instead of the table. While the CPU 31 is provided in the numerical control device 29, this is not limiting, and a separate CPU may be provided with a function for executing vibration characteristic acquisition processing.

[0073] The vibration data was acquired using acceleration sensor 41 provided in the spindle mechanism, but vibration data may also be measured by combining time information with sound, servo feedback information, a speed sensor, a displacement sensor, or the unevenness (roughness) of the surface of the workpiece after machining. Acceleration sensor 41 may be provided in a location other than the spindle mechanism where it can measure vibrations of machine tool 1. In this case, acceleration sensor 41 may be provided in the feed mechanism, for example, to detect vibrations of the feed mechanism.

[0074] The vibration characteristic acquisition process does not necessarily have to be performed by a user, but may be performed automatically at a predetermined cycle. Furthermore, the calculation of the vibration characteristics may be performed while the machining program is being executed and the workpiece is being machined. Furthermore, once the stable rotation speed is calculated, the rotation speed may be automatically changed to the stable rotation speed during machining. Furthermore, the number of teeth does not necessarily have to be input by a user, and may be estimated from vibration analysis results or other known estimation methods.

[0075] In the above embodiment, the resonant frequency fn is calculated assuming that the damping ratio ζ is stored in the storage device 39. However, this is not a limitation. For example, if the storage device 39 stores the resonant frequency fn, the CPU 31 may calculate the damping ratio ζ by reading out the resonant frequency fn and substituting the resonant frequency fn for the acquired vibration characteristics. In the above embodiment, the calculation of the vibration characteristics is completed before the workpiece is machined, but this is not a limitation. For example, the calculation method of the above embodiment can be applied even when the completion of machining is determined between steps S11 and S39 and the calculation is completed after the machining is completed. In the above embodiment, the relative stability limit diagram was described with reference to the case where the critical axial depth of cut Ap is the stability limit. However, in machining such as face milling, the tool is prone to vibration in the radial direction. In this case, the critical radial depth of cut may also be the stability limit.

[0076] In the above embodiment, the resonance frequency fn and the damping ratio ζ were calculated using equation (1-11) based on the results of multiple trials, but this is not limited to this. The CPU 31 can also calculate the resonance frequency fn without using the damping ratio ζ. In the following explanation, the same points as in the above embodiment will be omitted and differences will be mainly explained. For convenience of explanation, the following explanation will be given using a case where the workpiece was machined twice.

[0077] By substituting the vibration data acquired in the first and second machining operations into equation (1-2), the following equations (2-1) and (2-2) are calculated.

number

[0078] The following equation (2-3) divides the left and right sides of equation (2-2) by equation (2-1).

number

[0079] The following equation (2-4) is calculated by transforming equation (2-3). Specifically, equation (2-4) is obtained by dividing the numerator and denominator on the right side by the mass coefficient m.

number

[0080] Here, the following equation (2-5) is obtained by substituting the relational expression of the resonance frequency fn, stiffness coefficient k, and mass coefficient m in equation (1-7) into equation (2-4).

number

[0081] The following equation (2-6) is found by treating equation (2-5) as a quadratic equation for fn and solving it for the resonance frequency fn. Two solutions to the quadratic equation can be obtained, but the solution that includes the + term among the terms operated with ± is considered valid. Here, the resonance frequency fn of the machine tool 1 can be found by substituting the compliance phases γ1 and γ2 and the chatter frequencies fc1 and fc2 into equation (2-6). Therefore, by using equation (2-6), the CPU 31 can calculate the resonance frequency fn without using the damping ratio ζ.

number

[0082] In the above embodiment, the CPU 31 calculates the damping ratio ζ and the resonance frequency fn by solving simultaneous quadratic equations with two unknowns using Equation (1-11), but this is not limiting. The CPU 31 may also calculate the resonance frequency fn using regression analysis. A method for determining the resonance frequency fn and the damping ratio ζ using regression analysis will be described below with reference to FIG. 6(b).

[0083] FIG. 6(a) shows the calculation results when chatter vibration occurs in, for example, seven machining operations. FIG. 6(a) shows the calculation results of chatter frequencies fc1 to fc7 and compliance phases γ1 to γ7 corresponding to machining at rotational speeds n1 to n7. As an example, the rotational speed n3 in the third machining operation is 6350 min -1 The chatter frequency fc3 is 1062.7 Hz, and the compliance phase γ3 is 170.807 deg.

[0084] The CPU 31 plots the chatter frequencies fc1 to fc7 and the compliance phases γ1 to γ7 on a graph with the compliance phase γ on the vertical axis and the chatter frequency fc on the horizontal axis (see FIG. 6(b)). The CPU 31 performs a regression analysis on the plotted points to draw a curve cal1. From the drawn curve cal1, the CPU 31 determines that the optimized resonance frequency fn is 856 Hz and the optimized damping ratio ζ is 0.08.

[0085] In the regression analysis, the relational expression of the vibration characteristics is expressed as a polynomial by Taylor expansion or the like, and multiple regression analysis is performed. Another regression analysis method may use the steepest descent method with the damping ratio ζ and the resonance frequency fn as variables.

[0086] Alternatively, without using regression analysis, the CPU 31 may limit the resonance frequency fn and the damping ratio ζ within a certain range and comprehensively draw curves while varying the resonance frequency fn and the damping ratio ζ. The CPU 31 selects the curve that is closest to the plotted point. In other words, the CPU 31 may calculate the damping ratio ζ and the resonance frequency fn from the curves obtained using each method, or may read them from the plotted results.

[0087] The CPU 31 may use a histogram to calculate the resonance frequency fn and the damping ratio ζ. Alternatively, the CPU 31 may calculate two quadratic equations by substituting, for example, the first measurement data and the second measurement data into Equation (1-9). In this case, the CPU 31 can calculate the resonance frequency fn and the damping ratio ζ because there are two equations with the resonance frequency fn and the damping ratio ζ as two variables. The CPU 31 may solve the simultaneous quadratic equations with two unknowns by appropriately combining the calculated vibration characteristics (for example, the first and third times, or the first and seventh times in FIG. 6(a)). The CPU 31 may calculate the damping ratio ζ and the resonance frequency fn for all possible combinations and use the average value, median value, or the like as the true value.

[0088] In the above embodiment, the forced vibration is predicted using equation (1-12), but this is not limiting. The CPU 31 may predict the forced vibration based on the damping ratio ζ and the resonance frequency fn. In this case, the CPU 31 can also predict the relationship between the forced vibration generated at other rotation speeds and how many times it will increase.

[0089] Furthermore, for example, if the machining conditions of feed rate, rotation speed, number of tool teeth, tool diameter, axial depth of cut, and radial depth of cut are known, the CPU 31 can predict forced vibration using the following method. Among the machining conditions, the CPU 31 acquires the tool diameter, number of tool teeth, tool helix angle, axial depth of cut, which is the depth of cut in the axial direction of the spindle, radial depth of cut, which is the depth of cut in the radial direction perpendicular to the axial direction, and the component force ratio, which is the ratio between the radial and tangential cutting forces F. However, it is preferable that the cutting force F here be a force obtained by subtracting the edge force (smoothing force) generated when smoothing the workpiece due to the roundness of the cutting edge. For example, in Figure 7, the machining conditions are down-cutting, feed rate of 240 mm / min, and rotation speed of 1000 min. -1 The number of cutting edges of the tool is 4, the tool diameter is 10 mm, the axial cutting depth is 5 mm, and the radial cutting depth is 2 mm. In addition, in Figure 8, the cutting conditions are as follows: cutting direction is down cut, feed rate is 240 mm / min, rotation speed is 1687 min -1The number of tool teeth is 4, the tool diameter is 10 mm, the axial depth of cut is 5 mm, and the radial depth of cut is 2 mm.

[0090] As shown in Figures 7(a) and 8(a), the CPU 31 obtains the cutting forces Fx1 and Fx2 in the time domain. The cutting forces Fx1 and Fx2 are forces in the X-axis direction. As shown in Figures 7(b) and 8(b), the CPU 31 performs, for example, a fast Fourier transform on the cutting forces Fx1 and Fx2 to obtain Fourier transform results Fx1' and Fx2' of the cutting forces Fx1 and Fx2. The CPU 31 multiplies the Fourier transform results Fx1' and Fx2' of the cutting forces Fx1 and Fx2 by a relative transfer function (see Figure 3), respectively, to obtain Fourier transform results X1 and X2 of the vibration displacement shown in Figures 7(c) and 8(c). The Fourier transform results of the vibration displacement in Figures 7(c) and 8(c) show the displacement predicted for the next machining operation, and peaks can be observed around 80 Hz and 110 Hz, respectively. Figure 7(c) shows the analysis results when the resonance frequency fn and cutting conditions do not match. The Fourier transform result X1 of the vibration displacement has a small peak, but forced vibration occurs at 80 Hz. On the other hand, Figure 8(c) shows the analysis results when the resonance frequency fn and cutting conditions match. The Fourier transform result X2 of the vibration displacement has a large peak near 110 Hz, indicating that forced vibration is large. Therefore, the CPU 31 can predict forced vibration in the X-axis direction. Therefore, the CPU 31 can predict not only chatter vibration, which is self-excited vibration, but also forced vibration based on the relative transfer function and cutting conditions.

[0091] The CPU 31 that executes the process of S7 is an example of the vibration data measuring means of the present invention. The CPU 31 that executes the process of S9 is an example of the frequency characteristic analysis means of the present invention. The CPU 31 that executes the process of S11 is an example of the chatter frequency determining means of the present invention. The CPU 31 that executes the process of S24 is an example of the vibration characteristic derivation means of the present invention. The CPU 31 that executes the process of S26 is an example of the first acquisition means of the present invention. The CPU 31 that executes the process of S28 is an example of the resonance frequency calculation means of the present invention. The CPU 31 that executes the process of S31 is an example of the relative transfer function calculation means of the present invention. The CPU 31 that executes the process of S33 is an example of the relative stability limit diagram creation means of the present invention. The CPU 31 that executes the process of S37 is an example of the forced vibration prediction means of the present invention. The CPU 31 that executes the process of S47 is an example of the machining condition acquisition means of the present invention. The CPU 31 that executes the process of S49 is an example of the phase difference setting means of the present invention. The CPU 31 that executes the process of S51 is an example of the setting means of the present invention. [Explanation of symbols]

[0092] 1: Machine tool 29: Numerical control device 31: CPU 32:ROM 33:RAM 39: Storage device fc, fc1, fc2, fc3, fc4, fc5, fc6, fc7: Chatter frequencies ε, ε1, ε2, ε3, ε4, ε5, ε6, ε7: Phase difference γ, γ1, γ2, γ3, γ4, γ5, γ6, γ7: compliance phases fn: Resonance frequency Δγ: Phase difference of compliance

Claims

1. a setting means for setting different machining conditions when machining a workpiece; a vibration data measuring means for measuring vibration data of vibrations occurring due to the relationship between the machining conditions and the compliance of a machine structure for each of the machining conditions set by the setting means; a frequency characteristic analysis means for analyzing frequency characteristics for each of the plurality of vibration data measured by the vibration data measurement means; a chatter frequency determination means for determining a chatter frequency at which chatter vibration occurs for each of a plurality of analysis results of the frequency characteristic analysis means; a phase difference calculation means for calculating a phase difference of the chatter vibration for each of the machining conditions corresponding to the chatter frequency determined by the chatter frequency determination means; a phase calculation means for calculating a phase of compliance of the mechanical structure, which is expressed by equation (1), for each phase difference of the chatter vibration calculated by the phase difference calculation means; a vibration characteristic derivation means for deriving vibration characteristics, which are a relationship between a damping ratio of the mechanical structure and a resonance frequency of the mechanical structure, for each of the compliance phases calculated by the phase calculation means and the chatter frequencies determined by the chatter frequency determination means. (Note that in equation (1), γ represents the compliance phase, fc represents the chatter frequency, c represents the damping coefficient, m represents the mass coefficient, and k represents the stiffness coefficient.)

2. a first calculation means for calculating the damping ratio and the resonance frequency based on the vibration characteristics derived by the vibration characteristics derivation means; 2. The numerical control device according to claim 1, further comprising:

3. a stable rotation speed calculation means for calculating a stable rotation speed for suppressing the chatter vibration based on at least the resonance frequency out of the damping ratio and the resonance frequency calculated by the first calculation means; 3. The numerical control device according to claim 2, further comprising:

4. a relative transfer function calculation means for calculating a relative transfer function in which the value related to compliance has a graph shape relative to the graph shape of the accurate transfer function by substituting the damping ratio and the resonance frequency calculated by the first calculation means into a transfer function of a second-order delay system; 3. The numerical control device according to claim 2, further comprising:

5. forced vibration prediction means for predicting forced vibration based on at least the resonance frequency out of the resonance frequency and the damping ratio calculated by the first calculation means; 3. The numerical control device according to claim 2, further comprising:

6. a relative stability limit diagram creating means for creating a relative stability limit diagram in which the value of the critical depth of cut, which indicates the critical depth of cut at which chatter vibration occurs during machining of the workpiece, is in a form relative to the form of an accurate stability limit diagram, based on the relative transfer function calculated by the relative transfer function calculating means; 5. The numerical control device according to claim 4, further comprising:

7. a display means for displaying at least one of the damping ratio and the resonance frequency calculated by the first calculation means on a display unit; 3. The numerical control device according to claim 2, further comprising:

8. a display means for displaying the stable rotation speed calculated by the stable rotation speed calculation means on a display unit; 4. The numerical control device according to claim 3, further comprising:

9. a display means for displaying the relative transfer function calculated by the relative transfer function calculation means on a display unit; 5. The numerical control device according to claim 4, further comprising:

10. a display means for displaying the relative stability limit diagram created by the relative stability limit diagram creating means on a display unit; 7. The numerical control device according to claim 6, further comprising:

11. a processing condition acquisition means for acquiring the processing conditions; a phase difference setting means for setting a compliance phase difference between the phase of the compliance in the current machining and the phase of the compliance in the next machining; the setting means sets next machining conditions based on the damping ratio stored in a storage unit in advance or the damping ratio calculated by the first calculation means, the current machining conditions acquired by the machining condition acquisition means, and the compliance phase difference set by the phase difference setting means; The vibration data measuring means measures the vibration data when machining is performed based on the next machining conditions set by the setting means.

3. The numerical control device according to claim 2.

12. a setting step of setting different machining conditions when machining a workpiece; a vibration data measuring step of measuring vibration data of vibrations occurring due to a relationship between the machining conditions and compliance of a machine structure for each of the machining conditions set in the setting step; a frequency characteristic analysis step of analyzing frequency characteristics for each of the plurality of vibration data measured in the vibration data measurement step; a chatter frequency determination step of determining a chatter frequency at which chatter vibration occurs for each of a plurality of analysis results of the frequency characteristic analysis step; a phase difference calculation step of calculating a phase difference of the chatter vibration for each of the machining conditions corresponding to the chatter frequency determined in the chatter frequency determination step; a phase calculation step of calculating a phase of compliance of the mechanical structure expressed by equation (1) for each phase difference of the chatter vibration calculated in the phase difference calculation step; a vibration characteristic derivation step of deriving vibration characteristics, which are a relationship between a damping ratio of the mechanical structure and a resonance frequency of the mechanical structure, for each of the compliance phases calculated in the phase calculation step and the chatter frequencies determined in the chatter frequency determination step. (Note that in equation (1), γ represents the compliance phase, fc represents the chatter frequency, c represents the damping coefficient, m represents the mass coefficient, and k represents the stiffness coefficient.)

Images