NUMERICAL CONTROL SYSTEM

Abstract

Numerical control system (2, 2a, 2b, 2c) comprising the following: - a detection unit (4) for obtaining a cutting force generated in a machine tool (1); - a control unit (9) for calculating a control value depending on a cutting condition and for controlling a feed drive mechanism of the machine tool (1); - a storage unit (5) for dynamic property information for storing dynamic properties of a tool or a workpiece in relation to the cutting condition; - a storage unit (6) for machining process information for storing information representing a machining process occurring between the tool and the workpiece in relation to the cutting condition; - a countermeasure determination unit (7) to calculate, when a machining defect is detected from the cutting force or a state of the feed drive mechanism of the machine tool (1), a plurality of deviation degrees for possible reasons for the machining defect and to calculate the calculated deviation degrees and thereby determine a reason for the machining defect whose occurrence has been detected; and - a correction value calculation unit (8) for calculating, depending on the reason for the processing defect determined by the countermeasure determination unit (7), a correction value related to the control value and for outputting the correction value to the control unit (9).

Description

Technical area

[0001] The present invention relates to a numerical control system for controlling a machine tool. State of the art

[0002] A machine tool is a machining device for performing machining using a tool to apply force or energy to a workpiece. Specifically, a cutting tool for performing machining brings its cutting edge, which moves at high speed, into contact with the workpiece, creating shear fractures on the surface of the workpiece and thereby removing unnecessary parts of the workpiece.

[0003] Various types of machining defects can occur during cutting operations. Causes include chatter vibration, tool bending, tool wear, and friction in the feed drive mechanism.

[0004] Chatter vibrations are classified into two types. One is self-excited chatter vibration, in which vibrations occurring in a machined part develop or propagate through the machine structure and its machining process. The other is forced chatter vibration, in which vibrations generated by intermittent cutting generate vibrations in the machine structure. Both chatter vibration types reduce machining accuracy and tool life.

[0005] Tool bending is a phenomenon in which, while the tool edge is cutting a workpiece, the tool experiences cutting forces that bend it relative to its non-cutting state, resulting in a reduction in machining accuracy. Tool wear is a phenomenon in which the cutting edge of the tool repeatedly collides with the workpiece, causing it to become damaged. Consequently, tool wear reduces the quality of the machined surface and causes an increase in cutting force.

[0006] Friction in the feed drive mechanism is friction at a sliding portion of the feed drive system of a machine tool. When the friction characteristics in the feed drive mechanism change due to movement or aging of the feed drive system, this causes the relative position between the tool and the workpiece to deviate from the specified position.

[0007] Each of the above-mentioned phenomena is known to be a cause of machining defects, which impairs machine tool productivity. Regarding the above-described cause of machining defects, countermeasures have been proposed to prevent or reduce machining defects.

[0008] Patent Document 1 describes a technique in which, if it is determined that chatter vibrations have occurred, it is determined whether the chatter vibrations are of the self-excitation vibration type or the forced vibration type. Then, a stable spindle rotation speed at which chatter vibrations do not occur is calculated depending on the vibration type.

[0009] Patent Document 2 describes a technique in which the tool wear value is calculated in a time series depending on the machining state, and when the tool wear value is expected to exceed a threshold value and it becomes necessary to replace the tool, the cutting conditions are changed so that the current machining can be completed without replacing the tool.

[0010] Patent Document 3 describes a technique in which the spindle speed for reducing tool deflection is calculated from the amplitude at the time when the cutting force generated between the tool and the workpiece causes the workpiece to vibrate.

[0011] Patent Document 4 describes a technique in which the trajectory error caused by friction is corrected by correcting a torque command with a value obtained by multiplying the estimated friction force by the gain determined based on the gain characteristics of the transfer function relating the command position to the deviation position.

[0012] Patent Document 5 discloses a machine learning device that learns the failure prediction of a main shaft of a machine tool or a motor driving the main shaft. The machine learning device includes: a state observation unit that observes a state variable including at least one of data output from a motor controller controlling the motor, data output from a sensor detecting a state of the motor, and data output from a measuring device measuring a state of the main shaft or the motor; a determination data acquisition unit that acquires determination data for determining whether a failure has occurred in the main shaft or the motor or a failure degree; and a learning unit that learns the failure prediction of the main shaft or the motor according to a data set generated based on a combination of the state variable and the determination data.

[0013] Patent Document 6 discloses a vibration detection method for detecting whether chatter vibration is generated in a machine tool having a rotary shaft that rotates either a tool or a workpiece, and for detecting a type of generated chatter vibration based on vibration information generated when the rotary shaft generates the chatter vibration. When chatter vibration generation is detected, a parameter calculation device calculates a first frequency range and a second frequency range. A detection validity is calculated taking into account a rotational speed detection resolution and a frequency resolution, and based on the ratio of a frequency range of rotationally periodically forced chatter vibration to a frequency range of regenerative chatter vibration.Based on the determination validity, it is determined whether the generated chatter vibration is a regenerative chatter vibration, a rotationally periodically forced chatter vibration, or a chatter vibration periodically forced by groove passages.

[0014] Patent Document 7 discloses a numerical control unit for a machine tool that machines a workpiece by rotating a tool, the control unit comprising: vibration detection means for detecting vibrations generated by machining of the workpiece; detection means for detecting a physical quantity correlated with a phase difference between chatter vibration during previous machining and chatter vibration during this machining, based on a detection result of the vibration by the vibration detection means;a spindle speed calculation device for determining a resonance frequency of the machine tool by multiplying the physical quantity by a correction factor that varies depending on the physical quantity detected by the detection device, and for calculating the number of revolutions of the tool based on the resonance frequency; and a spindle speed control device for rotating the tool at a spindle speed calculated by the spindle speed calculation device. State of the artPatent documents Patent Document 1: Unexamined Patent Application Publication JP 2012- 213 851 A Patent Document 2: Unexamined Patent Application Publication JP 2012- 208 921 A Patent Document 3: Unexamined Patent Application Publication JP 2012- 540 615 A Patent Document 4: Unexamined Patent Application Publication JP 2015- 156 194 A Patent document 5: DE 10 2017 003 165 A1 Patent document 6: DE 10 2012 204 968 A1 Patent document 7: US 2015 / 0 306 720 A1 Summary of the inventionProblems to be solved by the invention

[0015] In the technique described in Patent Document 1, the spindle speed is calculated to suppress chatter vibrations. Therefore, when a worn tool is used or when the radial depth of the cut is too large, it is impossible to suppress chatter vibrations by changing the spindle speed.

[0016] In the technique described in Patent Document 2, the cutting conditions are changed so that the wear rate increased during the machining process does not exceed a certain limit. This can cause machining defects such as chatter vibration and tool bending, depending on the changed cutting conditions.

[0017] In the technique described in Patent Document 3, the spindle speed is calculated to reduce tool deflection. Therefore, in the case where the tool position errors are not caused by tool deflection but by friction in the feed drive mechanism, it is impossible to reduce the machining defect by changing the spindle speed.

[0018] In the technique described in Patent Document 4, torque commands are corrected to compensate for trajectory errors caused by friction. Therefore, in cases where cutting conditions such as spindle speed and feed rate are improper, trajectory errors can be reduced, but machining defects such as chatter vibration and tool bending will occur.

[0019] As described above, there are numerous reasons for machining defects. To address these, each of the conventional techniques narrows down the reasons for machining defects to specific types of reasons and provides countermeasures for them. Therefore, in the event that a machining defect occurs for a reason other than those narrowed down, it was impossible for any of the conventional techniques to mitigate the machining defects.

[0020] Accordingly, the present invention has been conceived in view of the problems described above, and provides a numerical control system capable of identifying, when a machining defect occurs, the main cause of the machining defect and thereby providing an appropriate countermeasure for the identified cause. Solving the problems

[0021] The numerical control system according to the present invention includes: a detection unit for obtaining a cutting force generated in a machine tool; a control unit for calculating a control value depending on a cutting condition and controlling a feed drive mechanism of the machine tool; a dynamic characteristic information storage unit for storing dynamic characteristics of a tool or a workpiece in relation to the cutting condition; a machining process information storage unit for storing information representing a machining process occurring between the tool and the workpiece in relation to the cutting condition;a countermeasure determination unit for calculating, when it is detected from the cutting force or a state of the feed drive mechanism of the machine tool that a machining defect has occurred, a plurality of deviation degrees for possible causes of the machining defect, and calculating the calculated deviation degrees to thereby determine a cause of the machining defect whose occurrence has been detected; and a correction value calculation unit for calculating, depending on the cause of the machining defect determined by the countermeasure determination unit, a correction value based on the control value, and outputting the correction value to the control unit. Advantages of the invention

[0022] When a machining defect occurs in a machine tool, the numerical control system according to the present invention is capable of determining the cause of a detected machining defect from a plurality of types of reasons for machining defects. Short description of the drawings Fig. 1 is a block diagram of a numerical control system according to Embodiment 1 of the present invention. Fig. 2 is a configuration diagram of a control unit of the numerical control system according to Embodiment 1 of the present invention. Fig. 3A and Fig. 3B shows a flowchart showing the processing of the numerical control system according to Embodiment 1 of the present invention. Fig. 4 is a stability limit graph according to Embodiment 1 of the present invention. Fig. 5 is an enlarged graph of a part of the stability limit graph according to Embodiment 1 of the present invention. Fig. 6 is an enlarged graph of a part of the stability limit graph according to Embodiment 1 of the present invention. Fig. 7 is a block diagram of a numerical control system according to Embodiment 2 of the present invention. Fig. 8 is a block diagram of a correction value calculation unit of the numerical control system according to Embodiment 2 of the present invention. Fig. 9 shows an example of a tool wear control model according to Embodiment 2 of the present invention. Fig. 10 is a block diagram of a numerical control system according to Embodiment 3 of the present invention. Fig. 11 is a block diagram of a numerical control system according to Embodiment 4 of the present invention. EmbodimentsEmbodiment 1

[0023] In the description of the present invention, the causes of detected machining defects are referred to as machining defect causes. Specifically, the machining defect causes include chatter vibration, tool bending, tool wear, and friction in the feed drive mechanism, which will be described in more detail below.

[0024] Chatter vibrations are vibrations that occur during cutting operations. Chatter vibrations can reduce machining accuracy and affect tool life. Chatter vibrations are classified into self-excited chatter vibrations and forced chatter vibrations. Self-excited chatter vibrations are exponentially amplified by the cutting process itself. Forced chatter vibrations are caused by vibrations of the machine structure with the cutting force.

[0025] Tool bending is a phenomenon in which the tool edge is bent by absorbing a cutting force, so that the machining accuracy is deteriorated.

[0026] Tool wear is a phenomenon in which the cutting edge of a tool repeatedly contacts the workpiece, causing it to become worn. Tool wear reduces the quality of the machined surface and causes an increase in cutting force.

[0027] Friction in the feed drive mechanism is friction at a sliding portion of the feed drive system of a machine tool. When the friction characteristics in the feed drive mechanism change due to movement or aging of the feed drive system, this causes the relative position between the tool and the workpiece to deviate from the specified position.

[0028] The numerical control system according to Embodiment 1 detects the occurrence of a machining defect in a machine tool. When detecting the occurrence of a machining defect, the numerical control system calculates respective deviation degrees for machining defect causes. The numerical control system then compares the deviation degrees and determines a machining defect cause that results in the largest deviation degree, and adds a correction to the control value corresponding to the determined machining defect cause.

[0029] Fig. 1 is a block diagram of a numerical control system according to Embodiment 1 of the present invention. In Fig. 1, the machine tool 1 has a tool 10 that rotates at high speed. The cutting edge of the tool 10 comes into contact with the workpiece 11 and cuts it. The tool 10 is, for example, a face milling cutter or a drill.

[0030] A sensor 3 is mounted in the machine tool 1, for one or both of the feed drive mechanism and the table. The sensor 3 outputs a sensor signal to the numerical control system 2. Sensors 3 can be mounted for a plurality of feed drive mechanisms. Here, the feed drive mechanisms include a servo shaft and the spindle.

[0031] The sensor 3 measures a force generated in the machine tool 1, an acceleration, a speed, a position, or the like. A force sensor, an acceleration sensor, a speed sensor, or a displacement sensor is used for the sensor 3, depending on the measurement object. The sensor 3 may be an encoder attached to a feed drive mechanism motor of the machine tool 1 or a linear scale attached to a guide mechanism of the machine tool 1. The sensor 3 can measure the angular acceleration, angular velocity, or angle of a rotary feed drive mechanism.

[0032] Sensor 3 may be a combination of a plurality of the sensors described above. The measuring devices may be mounted along the directions of a plurality of feed drive mechanisms and measure the position, velocity, and acceleration related to each mechanism movement.

[0033] Depending on a cutting condition that has been set internally, the numerical control system 2 controls the feed drive mechanism states of the machine tool 1, such as the position, speed, and current of the feed drive mechanism, so that a workpiece is machined. Here, the cutting condition is a combination of conditions such as the tool shape, tool material, workpiece shape, workpiece material, machining path, cutting speed for each path, the feed speed for each path, the axial depth of cut for each path, and the radial depth of cut for each path.

[0034] The numerical control system 2 includes a detection unit 4, a dynamic characteristic information storage unit 5, a machining process information storage unit 6, a countermeasure determination unit 7, a correction value calculation unit 8, and a control unit 9. The detection unit 4 receives signals from the sensors 3.

[0035] The dynamic property information storage unit 5 stores dynamic property information and outputs the dynamic property information to the countermeasure determination unit 7. The machining process information storage unit 6 stores machining process information and outputs the machining process information to the countermeasure determination unit 7.

[0036] The countermeasure determination unit 7 determines a reason for the machining defect that has occurred. The correction value calculation unit 8 calculates a correction value depending on the reason for the machining defect. The control unit 9 updates a control value based on the correction value received from the correction value calculation unit 8.

[0037] Fig. 2 is a configuration diagram of the control unit 9 of the numerical control system 2 according to Embodiment 1 of the present invention. Fig. 2 shows the components of the control unit 9, and also shows the countermeasure determination unit 7, the detection unit 4, the correction value calculation unit 8, and a motor 19. A correction value acquisition unit 13 acquires the correction value calculated by the correction value calculation unit 8 and outputs the correction value to a command generation unit 14 and a friction compensation unit 17. The command generation unit 14 receives signals from the detection unit 4 and generates a position command while monitoring the state of the feed drive mechanism of the machine tool 1.

[0038] The command generation unit 14 also generates the position command based on a cutting condition 12 and outputs the position command to a position control unit 15. The position control unit 15 controls the position of the motor 19 based on the position command and outputs a speed command to a speed control unit 16. Based on the speed command, the control unit 16 generates a torque command for controlling the speed of the motor 19 and outputs the torque command to a current control unit 18.

[0039] The friction compensation unit 17 estimates the friction force generated at the feed drive mechanism, generates a correction torque command from the estimated friction force, and outputs the correction torque command to the current control unit 18. The current control unit 18 generates a current command based on the torque command and the correction torque command and outputs the generated current command to the motor 19. The motor 19 is driven by the current command.

[0040] Cutting condition 12 here is a combination of conditions, including the following: the shape of tool 10, the material of tool 10, the shape of workpiece 11, the material of workpiece 11, the machining paths, the cutting speed for each path, the feed rate for each path, the axial depth of cut ap for each path, and the radial depth of cut for each path. Cutting condition 12 may contain a condition other than those listed above. Cutting condition 12 may also be only a subset of the conditions listed above.

[0041] The detection unit 4 is described. The machine tool 1 outputs the state of the feed drive mechanism and sensor signals to the detection unit 4. The detection unit 4 outputs the state of the feed drive mechanism of the machine tool 1, as well as measurement signals including information about the cutting force generated in the machine tool 1, to the control unit 9 and the countermeasure determination unit 7.

[0042] If the sensor 3 is a force sensor, the detection unit 4 outputs time-series waveform signals as its measurement signals. If the sensor 3 is an acceleration sensor, a speed sensor, or a displacement sensor, the detection unit 4 may include a cutting force observer so that a cutting force estimated by the cutting force observer is output as a measurement signal. For example, the cutting force observer is expressed by formula (1). [Formula 1] Fcut=r×K×I−M×a−Ffric

[0043] In formula (1) the following notations apply: Fcut: estimated cutting force; r: conversion coefficient for converting a force from the rotational direction to the linear motion direction; K: torque constant; I: current; M: mass of the feed drive mechanism; a: acceleration of the feed drive mechanism; Ffric: friction force. The friction force Ffric is the friction force generated at the sliding area of ​​the mechanical component.

[0044] The mechanical component is, for example, a bearing. Formula (1) shows an example of an observer for a single inertial mass. If the feed drive mechanism has multiple inertial masses, one observer can be used for the multiple inertial masses.

[0045] Next, the dynamic property information storage unit 5 will be described. The dynamic property information storage unit 5 internally stores dynamic property information corresponding to the cutting condition 12 and outputs the dynamic property information to the countermeasure determination unit 7. The dynamic property information is information used to calculate a relative displacement value between the tool 10 in a cutting process and the workpiece 11. The dynamic property information changes depending on the cutting condition 12 and the state of the feed drive mechanism obtained at the time the dynamic property information is acquired.

[0046] More specifically, the dynamic property information includes the dynamic compliance of the system consisting of the tool 10 and the workpiece 11. The dynamic property information may include a combination of the following: a mass equivalent to the tool 10 and the workpiece 11, a damping factor equivalent thereto, and a spring constant equivalent thereto. The dynamic property information may be obtained experimentally in advance or may be calculated by finite element analysis or the like.

[0047] Next, the machining process information storage unit 6 will be described. The machining process information storage unit 6 internally stores machining process information associated with the cutting condition 12 and outputs the information to the countermeasure determination unit 7. The machining process information is information used to calculate a cutting force generated between the tool 10 and the workpiece 11 when the tool 10 cuts the workpiece 11. The machining process information may be a measured value obtained in an experiment or a value calculated using finite element analysis or the like.

[0048] An example of machining process information is described below. The machining process information is a combination of specific cutting forces and edge forces, which are used to calculate cutting forces using formulas (2) to (4). The edge forces are the cutting forces when the tool cutting value is zero. [Formula 2] Ft=Kct×Ac+Ket [Formula 3] Fr=Kcr×Ac+Ker [Formula 4] Fz=Kcz×Ac+Kez

[0049] In formulas (2) to (4) F t , F r and F z the cutting force in the tool rotation direction, the cutting force in the tool radial direction, or the cutting force in the tool axial direction. K ct , K cr and K czare the specific cutting force in the tool rotation direction, the specific cutting force in the tool radial direction, and the specific cutting force in the tool axial direction, respectively. K et , K er and K ez are the edge force in the tool rotation direction, the edge force in the tool radial direction, and the edge force in the tool axial direction, respectively. Ac is the cutting cross-sectional area. The cutting cross-sectional area Ac changes from moment to moment depending on the tool rotation angle.

[0050] Next, the countermeasure determination unit 7 will be described. The countermeasure determination unit 7 acquires dynamic characteristic information output from the dynamic characteristic information storage unit 5, machining process information output from the machining process information storage unit 6, cutting conditions 12 output from the control unit 9, measurement signals output from the detection unit 4, and a state of the feed drive mechanism output therefrom.

[0051] Based on the cutting forces and the state of the feed drive mechanism, the countermeasure determination unit 7 determines whether a machining defect has occurred. To determine the occurrence of a machining defect, the countermeasure determination unit 7 predetermines a vibration threshold or a trajectory error threshold. The countermeasure determination unit 7 compares the threshold with the measurement signal and determines that a machining defect has occurred if the measurement signal exceeds the threshold.

[0052] The countermeasure determination unit 7 can convert the measurement signals, which are time-series waveforms, into frequency-domain components. If a vibration component in the frequency domain exists that exceeds the threshold value, the countermeasure determination unit can determine that a machining defect has occurred. The threshold value can be changed depending on the type of cutting condition 12.

[0053] The following are concrete examples. For example, the countermeasure determination unit 7 compares the vibration amplitude of the measurement signal with the vibration threshold. As another example, the countermeasure determination unit 7 may compare a trajectory error obtained from the state of the feed drive mechanism with the trajectory error threshold.

[0054] The countermeasure determination unit 7 can also calculate the actual machining path from the feed drive mechanism. If the trajectory error between the commanded machining path specified by a cutting condition and the actual machining path exceeds the trajectory error threshold, the countermeasure determination unit can determine that a machining defect has occurred. The actual machining path is a trajectory along which the tool actually moved. The commanded machining path is a trajectory specified for the tool in the cutting condition.

[0055] If the countermeasure determination unit determines that a machining defect has occurred based on the measurement signal or the state of the feed drive mechanism, the countermeasure determination unit 7 calculates respective deviation degrees for the machining defect causes. Specifically, the countermeasure determination unit 7 calculates a deviation degree D1 for self-excited chatter vibrations, a deviation degree D2 for forced chatter vibrations, a deviation degree D3 for tool deflection, a deviation degree D4 for tool wear, and a deviation degree D5 for friction in the feed drive mechanism.

[0056] It is conceivable that the deviation degree expresses the difference between a reference machining state and the machining state at the time a machining defect is detected, and that it expresses the degree of deviation from the reference machining state. Formula (5) is an example of the deviation degree. [Formula 5] Dn=|Xn−An||Bn−An|(n=1,⋯,5)

[0057] The indices n correspond to individual machining defect causes. n = 1 indicates self-excited chatter vibrations. n = 2 indicates forced chatter vibrations. n = 3 indicates tool bending. n = 4 indicates tool wear. n = 5 indicates friction in the feed drive mechanism.

[0058] The machining state variables Xn represent various types of quantities corresponding to the causes of machining defects. These include the spindle speed for self-induced chatter vibration, the spindle speed for forced chatter vibration, the radial depth of cut for tool deflection, the feed speed for tool wear, and a friction correction value for the friction in the feed drive mechanism. However, the machining state variables corresponding to the respective machining defect causes are not limited to those listed above.

[0059] An is a machining state variable reference, which represents a reference value of the machining state variable for performing stable machining without a machining defect. Bn is a machining state variable limit value, which represents a machining state variable at which it is impossible to realize a machining operation specified in the cutting condition. The machining state variable references An and the machining state variable limit values ​​Bn represent different types of quantities corresponding to the index n of the machining defect reasons.

[0060] The machining state variable references An and the machining state variable limit values ​​Bn are, respectively, the spindle speed for self-excited chatter vibration, the spindle speed for forced chatter vibration, the radial depth of cut for tool deflection, the feed speed for tool wear, and a friction correction value for the friction in the feed drive mechanism. It should be noted that the machining state variables corresponding to the machining defect causes are not limited to those listed above.

[0061] The countermeasure determination unit 7 selects the largest deviation degree from the self-excited chatter vibration deviation degree D1 to the friction deviation degree D5 in the feed drive mechanism, and selects the machining defect reason corresponding to the selected largest deviation degree. It thereby determines the machining defect reason as a countermeasure signal.

[0062] Next, the correction value calculation unit 8 will be described. The correction value calculation unit 8 receives the countermeasure signal output from the countermeasure determination unit 7 and calculates a correction value depending on the machining defect cause determined by the countermeasure determination unit 7, and then outputs the correction value to the control unit 9. The correction value is the value for correcting the control value calculated by the control unit 9. The control value is a state value of the machine tool 1 controlled by the control unit 9 so that machining is performed based on the cutting condition.

[0063] For example, in the case where the countermeasure determining unit 7 outputs a countermeasure signal indicating that a machining defect has been caused by self-excited chatter vibrations or forced chatter vibrations, a correction value for the spindle speed is calculated.

[0064] Since self-excited chatter vibrations and forced chatter vibrations are different types of machining defects, the spindle speed correction value for self-excited chatter vibrations is not necessarily the same as that for forced chatter vibrations. If the countermeasure determination unit 7 outputs a countermeasure signal indicating that the machining defect is caused by tool deflection, the correction value calculation unit 8 calculates a correction value for the radial depth of the cut.

[0065] If the countermeasure determination unit 7 outputs a countermeasure signal indicating that the machining defect is caused by tool wear, the correction value calculation unit 8 calculates a correction value for the feed speed, which is a control value, or generates a tool change command. If the countermeasure determination unit 7 outputs a countermeasure signal indicating that the machining defect is caused by friction in the feed drive mechanism, the correction value calculation unit 8 calculates a correction value for the correction torque command.

[0066] Next, the control unit 9 will be described. The control unit 9 obtains the state of the feed drive mechanism from the detection unit 4 and then controls the operation of the motor 19, which is provided with the feed drive mechanism of the machine tool 1, depending on a cutting condition preset within the control unit 9. When a correction value is received from the correction value calculation unit 8, the control unit 9 corrects the control value.

[0067] The control unit 9 also outputs the internally specified cutting condition 12 to the countermeasure determination unit 7.

[0068] Fig. 3A and Fig. 3B shows a flowchart showing the processing in the numerical control system 2 according to Embodiment 1 of the present invention. Fig. 3A and Fig. 3B shows a sequence of processing steps performed by the numerical control system 2 in a flowchart. Referring to Fig. 3A and Fig. 3B describes the processing by means of the numerical control system 2.

[0069] First, reference is made to Fig. 3A.

[0070] In step S11, based on the measurement signal and the state of the feed drive mechanism obtained from the detection unit 4 and the cutting condition 12, the countermeasure determination unit 7 determines whether or not a machining defect has occurred.

[0071] If it is determined that a processing defect has occurred (YES in S11), the countermeasure determination unit 7 proceeds to step S12. If it is determined that a processing defect has not occurred (NO in S11), the countermeasure determination unit 7 performs step S11 again.

[0072] In step S12, the countermeasure determination unit 7 obtains the measurement signal, the state of the feed drive mechanism, and the cutting condition 12 from the control unit 9. The countermeasure determination unit 7 also obtains dynamic characteristic information from the dynamic characteristic information storage unit 5. Furthermore, the countermeasure determination unit 7 obtains machining process information from the machining process information storage unit 6.

[0073] In steps S13 and S14, the countermeasure determination unit 7 calculates deviation degrees for all machining defect causes. Note that steps S13-1 to S13-5 are collectively referred to as steps S13, and steps S14-1 to S14-5 are collectively referred to as steps S14.

[0074] The numerical control system 2 according to embodiment 1 calculates all deviation degrees sequentially. However, all deviation degrees can also be calculated in parallel. The order for calculating the deviation degrees is also not limited to the order in Fig. 3A shown sequence.

[0075] In step S13-1, the countermeasure determination unit 7 uses the measurement signal, the state of the feed drive mechanism, the cutting condition 12, the dynamic characteristic information, and the machining process information to calculate a stability limit of the self-excited chatter vibration. Next, the stability limit is described, which is known from the technical document below. Fig. 4 shows a stability limit graph according to Embodiment 1 of the present invention (Non-Patent Document 1: Shamoto Eiji, “Mechanism and Suppression of Chatter Vibrations in Cutting,” DAIDO Steel Technical Journal, “DENKI-SEIKO,” 2011, Vol. 82, No. 2).

[0076] The vertical axis in Fig. 4 shows axial depths ap of the incision. The horizontal axis in Fig. 4 shows spindle speeds S. In Fig. 4 gives the limit ap limThe axial depth of the incision defines the boundary between a stable area (ASP) and an unstable area (AUP). The limit value (ap) lim The axial depth of the cut is the stability limit for the self-excited chatter vibration. Within the stable range ASP in Fig. 4 no self-excited chatter vibration occurs.

[0077] However, self-excited chatter vibration occurs within the unstable range AUP. In other words, if the axial depth ap of the notch exceeds the limit ap lim the axial depth of the incision, then self-excited chatter vibrations occur.

[0078] The limit ap lim the axial depth of the cut can be calculated using the known method disclosed in Non-Patent Document 1. An exemplary method for calculating the threshold value ap limthe axial depth of the notch is shown below. According to Non-Patent Document 1, the limit value ap lim the axial depth of the incision using formula (6), formula (7) and formula (8). [Formula 6] aplim=−ΛRKct(1+(ΛIΛR)2) [Formula 7] ε=π−2 tan−1(ΛIΛR) [Formula 8] S=60ωcN(2kπ+ε)(k=0,1,2⋯)

[0079] In formula (8) ω c is the chatter vibration frequency, and N is the number of tool edges. Λ R and Λ I contained in formula (6) and formula (7) are the real part and the imaginary part of A, respectively. A can be obtained by solving the equation in formula (9). [Formula 9] det[[I]+Λ[A0][G(iωc)]]=0

[0080] In formula (9), [I] is the unit matrix, and i is the imaginary unit. [A0] is the cutting coefficient matrix. The cutting coefficient matrix [A0] is determined from the radial depth of the cut, the tool rotation angle, and the machining process information. The radial depth of the cut, the tool rotation angle, and the machining process information can be calculated from the cutting condition 12.

[0081] [G(iω c )] denotes a dynamic property matrix on the chatter vibration frequency ω c , which denotes the dynamic compliance obtained from its dynamic property information and expressed with a transfer function. The dynamic property matrix [G(iω) c)] can be calculated as a transfer function of a secondary vibration system using an equivalent mass, an equivalent damping factor, and an equivalent spring constant included in the dynamic property information.

[0082] The limit ap lim The axial depth varies unevenly when the spindle speed S changes. Upwardly convex regions are called stable pockets sp. In the stable pockets sp, the spindle speed S is stable compared to the regions outside the stable pockets.

[0083] Each of Fig. 5 and Fig. 6 is an enlarged graph of a part of the stability limit graph of Embodiment 1 of the present invention. In Fig. 5 shows a stable pocket sp1. In Fig. 6 shows a stable pocket sp2 and a stable pocket sp3. In step S14-1, the countermeasure determination unit 7 calculates a deviation degree for self-excited chatter vibrations.

[0084] A method for calculating the deviation degree D1 for self-excited chatter vibrations is presented using Fig. 5. A spindle speed S1 is the spindle speed corresponding to the peak value or tip of the stable pocket sp1. A spindle speed S2 is the spindle speed at the time a machining defect is detected. An axial depth ap2 of the cut is the axial depth of the cut corresponding to the spindle speed S2.

[0085] A limit value S lim1 The spindle speed is a spindle speed at the stability limit, whose value on the vertical axis corresponds to the axial depth ap2 of the cut. A limit value S lim2 of the spindle speed is another spindle speed at the stability limit, the value of which on the vertical axis corresponds to the axial depth ap2 of the cut.

[0086] If the spindle speed obtained in step S12 is S2, as shown in Fig. 5, the countermeasure determination unit 7 is capable of calculating the deviation degree D1 for self-excited chatter vibrations by using the spindle speed S1 and the limit value S lim1 sets the spindle speed to the machining state variable reference A1 or the machining state variable limit B1.

[0087] Next, a description will be given of a method for calculating the degree of deviation in the case where the stability limit value and the spindle speed at the time when the machining defect occurs are in the range shown in Fig. 6. The spindle speed S3 is the spindle speed at the time a machining defect is detected. The spindle speed S4 is the spindle speed corresponding to the peak value or the tip of a stable pocket sp2.

[0088] The spindle speed S5 is the spindle speed corresponding to the peak value or tip of a stable pocket sp3. The axial depth ap3 of the notch is the axial depth of the notch corresponding to the spindle speed S3. The axial depth ap4 of the notch is the axial depth of the notch corresponding to the spindle speed S4. The axial depth ap5 of the notch is the axial depth of the notch corresponding to the spindle speed S5.

[0089] The spindle speed limit S lim3 is the spindle speed at the stability limit corresponding to the axial depth ap3 of the cut. In Fig. 6, the axial depth ap4 of the notch is smaller than the axial depth ap3 of the notch, and the axial depth ap5 of the notch is greater than the axial depth ap3 of the notch. Therefore, the countermeasure determination unit 7 cannot find a path from the axial depth ap3 of the notch to reach the stable pocket sp2 simply by changing the spindle speed.

[0090] As in Fig. 6, in such a case, the countermeasure determination unit 7 selects a stable pocket whose apex position corresponds to a spindle speed closest to the spindle speed S3, from stable pockets whose axial depth ap5 of the notch is greater than the axial depth ap3 of the notch. Fig. 6, the countermeasure determination unit 7 calculates a deviation degree D1 for the self-excited chatter vibration with respect to a stable pocket sp3 closest to the spindle speed S3.

[0091] That is, in the in the in Fig. In the situation shown in Figure 6, the countermeasure determination unit 7 specifies the spindle speed S3 in the machining state variable X1 of the self-excited chatter vibration at the time a machining defect is detected. Then, the countermeasure determination unit 7 specifies the spindle speed S5 on the machining state variable A. 1 vor , and specifies the spindle speed limit S lim3 to the machining state variable limit value B1 to calculate the deviation degree D1 for self-excited chatter vibration.

[0092] In the case of Fig. 5 the following applies: Since the processing state variable limit B1 is the limit S lim1 the spindle speed, generates the spindle speed S which is less than the limit S lim1the spindle speed, self-excited chatter vibrations, and the deviation degree D1 of the self-excited chatter vibration becomes greater than one. In the event that the spindle speed S is greater than the limit value S lim1 the spindle speed, the deviation degree D1 for self-excited chatter vibration becomes less than one. This means that if the deviation degree D1 for self-excited chatter vibration is greater than one, self-excited chatter vibration occurs, which is the cause of machining defects.

[0093] In step S13-2, the countermeasure determination unit 7 calculates a displacement force response of the machine structure using the measurement signal and the dynamic characteristic information. Specifically, the countermeasure determination unit 7 calculates displacements in the machine structure in the frequency domain from the product of the cutting force obtained from the measurement signal and the dynamic compliance obtained from the dynamic characteristic information.

[0094] In step S14-2, the countermeasure determination unit 7 calculates a deviation degree D2 for forced chatter vibration based on formula (5). In formula (5), the countermeasure determination unit 7 inputs the machine structure displacement (calculated in step S13-2) at the time a machining defect is detected into the machining state variable X2 of the forced chatter vibration at the time a machining defect is detected. Here, the machining state variable reference A2 is zero, and the machining state variable limit B2 is the displacement limit of the machine structure. The displacement limit of the machine structure is a limit value up to which displacement of the machine structure is permitted.

[0095] In case the machining state variable X2 of the forced chatter vibration at the time when a machining defect is detected is equal to the machining state variable reference A2 of the forced chatter vibration, the deviation degree D2 for forced chatter vibration becomes zero.

[0096] If the machining state variable X2 of the forced chatter vibration at the time a machining defect is detected is equal to the machining state variable limit value B2, the deviation degree D2 for forced chatter vibration becomes one. The larger the difference between the machining state variable X2 of the forced chatter vibration at the time a machining defect is detected and the machining state variable reference A2, the larger the deviation degree D2 for forced chatter vibration becomes.

[0097] The larger the difference between the machining state variable limit value B2 and the machining state variable reference value A2, the smaller the deviation degree D2 for forced chatter vibration becomes. The deviation degree D2 for forced chatter vibration is calculated using the difference between the machining state variable reference value A2 and the machining state variable limit value B2 as a reference, thus providing a value for the displacement of the machine structure at the time a machining defect is detected.

[0098] In step S13-3, the countermeasure determination unit 7 calculates the relationship between the radial depth of the cut and the tool deflection value from the cutting condition 12, the state of the feed drive mechanism, the dynamic characteristic information, and the machining process information. The countermeasure determination unit 7 calculates the tool deflection values ​​to the moments during one revolution of the tool 10 using Formula (10), and detects the maximum tool deflection value during the one revolution of the tool 10 as a tool deflection value δ corresponding to the radial depth of the cut. Formula (10) is a calculation formula for the tool deflection value δ. [Formula 10] δ=Fr×l33×E×It

[0099] In formula (10), δ is the tool deflection value; F ris the cutting force in the tool radial direction; l is the tool projection length; E is the Young's modulus of the tool; and I t is the secondary moment of the tool cross-section. The cutting force F r in the tool radial direction can be calculated from formula (3) using the machining process information and the cutting condition 12. While the tool makes one revolution, the cutting force F r in tool radial direction depending on the cutting cross-sectional area A c .

[0100] The tool projection length l and the Young's modulus E of the tool can be obtained from cutting condition 12. The secondary moment I t of the tool cross-section can be calculated from the tool shape. The secondary moment I tof the tool cross-section can be calculated from formula (11) assuming that the tool cross-section is a circle with diameter d. The secondary moment It of the tool cross-section can also be calculated by performing a finite element analysis on the shape of the tool 10. [Formula 11] It=π×d464

[0101] Without using formula (11) in step S13-3, the countermeasure determination unit 7 may calculate dynamic deflections of the tool by using an equation of motion of a cantilever when the tool 10 is assumed to be a continuum. In this case, the countermeasure determination unit 7 assumes a solution for the tool deflection value δ whose maximum value is among those obtained from the equation of motion for a period during which the tool 10 rotates a predetermined number of times.

[0102] In step S14-3, the countermeasure determination unit 7 calculates a tool deflection deviation degree D3 based on Formula (5). When calculating the tool deflection deviation degree D3, the countermeasure determination unit 7 sets the value δ in the radial depth of the notch at the time a machining defect is detected to the machining state variable X3 of the tool deflection at the time a machining defect is detected in Formula (5).

[0103] Also, for the calculation, the countermeasure determination unit 7 in formula (5) gives zero (which indicates the state without tool bending) and a tool bending limit value d lim to the machining state variable reference A3 or the machining state variable limit B3. Here, the tool deflection limit d limthe maximum tool deflection value with which the workpiece 11 can be machined in dimensions within the desired accuracies.

[0104] When a tool deflection deviation degree D3 is calculated using the calculation method described above, the tool deflection deviation degree D3 has the following characteristics. If the machining state variable X3 of the tool deflection at the time a machining defect is detected is equal to the machining state variable reference A3, the tool deflection deviation degree D3 becomes zero. If the machining state variable X3 of the tool deflection at the time a machining defect is detected is equal to the machining state variable limit value B3 of the tool deflection, the tool deflection deviation degree D3 becomes one.

[0105] The larger the difference between the machining state variable X3 of the tool deflection at the time a machining defect is detected and the machining state variable reference A3, the larger the tool deflection deviation degree D3 becomes. The larger the difference between the machining state variable limit value B3 of the tool deflection and the machining state variable reference A3 of the tool deflection, the smaller the tool deflection deviation degree D3 becomes.

[0106] In other words, the tool bending deviation degree D3 expresses the difference between the machining state variable X3 of the tool bending at the time when a machining defect is detected and the machining state variable reference A3 of the tool bending, with reference to the difference between the machining state variable limit value B3 of the tool bending and the machining state variable reference A3 of the tool bending.

[0107] In step S13-4, the countermeasure determination unit 7 obtains the specific cutting force K ct in the tool rotation direction at the time when a machining defect is detected, or the edge force K et in the tool rotation direction at the time a machining defect is detected, from the machining process information. In another example of obtaining K ct and K etthe countermeasure determination unit 7 can determine in advance a specific cutting force K ct in tool rotation direction and an edge force K et in the tool rotation direction, where the forces are in the condition without tool wear, and then it can use formula (12) and formula (13) and calculate the specific cutting force K ct in tool rotation direction and the edge force K et in the tool rotation direction depending on the tool usage time. [Formula 12] Kct=α×tn×Kct0 [Formula 13] Ket=β×tn×Ket0

[0108] In formula (12) and formula (13), α and β are weighting coefficients; t is the tool usage time; n is an ordinal number representing a wear progression value; K ct0 is a specific cutting force in the tool rotation direction in a state in which the tool is not worn; and Ket0 is the edge force in the tool rotation direction in a state where the tool is not wearing. The countermeasure determination unit 7 acquires or calculates specific cutting forces and edge forces in the tool radial direction and tool axial direction using the same method as described above.

[0109] In step S14-4, the countermeasure determination unit 7 calculates a tool wear deviation degree D4 based on formula (5). More specifically, the countermeasure determination unit 7 performs the calculation of formula (5) with respect to each parameter of the specific cutting force K. ct in tool rotation direction, the specific cutting force K cr in tool radial direction and the specific cutting force K cz in the tool axial direction, and it takes the largest degree out of these in all directions for the tool wear deviation degree D4.

[0110] Instead, the countermeasure determination unit 7 may also perform a calculation of formula (5) with respect to each parameter of the edge force K et in tool rotation direction, the edge force K er in tool radial direction and the edge force K ez in the tool axial direction and take the largest degree of these in all directions as the tool wear deviation degree D4.

[0111] For example, in the case where the specific cutting force K ct in the tool rotation direction is the largest specific cutting force among these in all directions, in formula (5) the countermeasure determination unit 7 gives the specific cutting force K ct in the tool rotation direction, the specific cutting force K ct0 in the tool rotation direction in a state without tool wear and the limit value K ctthe specific cutting force in the tool rotation direction in X4, the machining state variable reference A4 of the tool wear or the machining state variable limit value B4 of the tool wear, and it calculates the tool wear deviation degree D4.

[0112] As another example, for the case where the edge force K et in the tool rotation direction is the largest edge force among those in all directions, in formula (5) the countermeasure determination unit 7, the edge force K et in tool rotation direction, the edge force K etin the tool rotation direction in a state without tool wear and the limit value Ket of the edge force in the tool rotation direction into the machining state variable X4 of the tool wear at the time when a machining defect is detected, the machining state variable reference A4 of the tool wear and the machining state variable limit value B4 of the tool wear, respectively, and it calculates the tool wear deviation degree D4.

[0113] Since the countermeasure determination unit 7 calculates the tool wear deviation degree D4 as described above, if the machining state variable reference X4 of the tool wear at the time when a machining defect is detected is equal to the machining state variable reference A4 of the tool wear, the tool wear deviation degree D4 becomes zero. If the machining state variable X4 of the tool wear at the time when a machining defect is detected is equal to the machining state variable limit value B4 of the tool wear, the tool wear deviation degree D4 becomes one.

[0114] The larger the difference between the machining state variable X4 of tool wear at the time a machining defect is detected and the machining state variable reference A4, the larger the tool wear deviation degree D4 becomes. The larger the difference between the machining state variable limit value B4 of tool wear and the machining state variable reference A4, the smaller the tool wear deviation degree D4 becomes. In step S13-5, the countermeasure determination unit 7 calculates the machining position of the machine tool 1 from the state of the feed drive mechanism.

[0115] In step S14-5, the countermeasure determination unit 7 calculates a friction deviation degree D5 in the feed drive mechanism based on Formula (5). In Formula (5), the countermeasure determination unit 7 inputs the machining position at the time a machining defect is detected (calculated in step S13-5), the machining position specified in the cutting condition, and the machining position limit value into the machining state variable X5 of the friction in the feed drive mechanism at the time a machining defect is detected, the machining state variable reference A5 of the friction in the feed drive mechanism, and the machining state variable limit value B5 of the friction in the feed drive mechanism, respectively. Here, the machining position limit value is a machining position at which the trajectory error assumes its maximum value.

[0116] Since the friction deviation degree D5 in the feed drive mechanism is calculated as described above, in the case where the machining state variable X5 of the friction in the feed drive mechanism at the time when a machining defect is detected is equal to the machining state variable reference A5 of the friction in the feed drive mechanism, the friction deviation degree D5 in the feed drive mechanism becomes zero.

[0117] In the case that the machining state variable X5 of the friction in the feed drive mechanism at the time when a machining defect is detected is equal to the machining state variable limit value B5 of the friction in the feed drive mechanism, the friction deviation degree D5 in the feed drive mechanism becomes one.

[0118] The greater the difference between the machining state variable X5 of the friction in the feed drive mechanism at the time a machining defect occurs and the machining state variable reference A5 of the friction in the feed drive mechanism, the greater the friction deviation degree D5 in the feed drive mechanism becomes. The greater the difference between the machining state variable limit value B5 of the friction in the feed drive mechanism and the machining state variable reference A5 of the friction in the feed drive mechanism, the smaller the friction deviation degree D5 in the feed drive mechanism becomes.

[0119] It will be Fig. 3B is referred to.

[0120] In step S15, the countermeasure determination unit 7 compares the degrees of the self-excited chatter vibration deviation degree D1 with the friction deviation degree D5 in the feed drive mechanism, which were determined in step S14-1 to step S14-5 according to Fig. 3A, and selects the largest deviation degree among them. The countermeasure determination unit 7 outputs a "processing defect reason" corresponding to the largest deviation degree as a countermeasure signal to the correction value calculation unit 8.

[0121] In steps S16, the correction value calculation unit 8 calculates a correction value for avoiding the machining defect.

[0122] If the countermeasure determination unit 7 outputs "self-excited chatter vibration" as the cause of the machining defect, the correction value calculation unit 8 calculates a correction value obtained by multiplying the spindle speed S2 by a first coefficient in step S16-1. In step S18-1, the control unit 9 updates the spindle speed depending on the correction value calculated in step S16-1.

[0123] If the countermeasure determination unit 7 outputs "forced chatter vibration" as the cause of the machining defect, the correction value calculation unit 8 calculates a correction value obtained by multiplying the spindle speed S2 by a second coefficient in step S16-2. In step S18-2, the control unit 9 updates the spindle speed based on the correction value calculated in step S16-2.

[0124] If the countermeasure determination unit 7 outputs "tool deflection," the correction value calculation unit 8 calculates a correction value obtained by multiplying the radial depth of the cut at the time a machining defect is detected with a third coefficient in step S16-3. In step S18-3, the control unit 9 updates the radial depth of the cut depending on the correction value calculated in step S16-3.

[0125] If the countermeasure determination unit 7 outputs "tool wear," the correction value calculation unit 8 determines whether the tool wear deviation degree D4 is less than one in step S16-4. If the tool wear deviation degree D4 is one or less, the cause of the machining defect cannot be eliminated even if the feed rate is changed. Therefore, the correction value calculation unit 8 outputs a tool replacement command instead of the correction value in step S17-2.

[0126] If the tool wear deviation degree D4 is less than one in step S16-4, the correction value calculation unit 8 calculates a correction value obtained by multiplying the feed speed at the time a machining defect is detected by a fourth coefficient in step S17-1. In step S18-4, the control unit 9 changes the feed speed depending on the correction value calculated in step S17-1.

[0127] If the countermeasure determination unit 7 outputs "friction in the feed drive mechanism," the correction value calculation unit 8 calculates a correction value obtained by multiplying the correction torque command at the time a machining defect is detected by a fifth coefficient in step S16-5. In step S18-5, the control unit 9 changes the correction torque command depending on the correction value calculated in step S16-5.

[0128] The first to fifth coefficients used by the correction value calculation unit to calculate correction values ​​can be determined by conducting machining experiments in advance. It is also possible to use the first to fifth coefficients calculated from a cutting condition when no machining defect occurs.

[0129] Steps S18-1 to S18-5 are collectively referred to as step S18. As described so far, in step S18, the control unit 9 changes a control value set therein using the correction value received from the correction value calculation unit 8. When the tool replacement command is output as a correction value in step S17-2, a tool replacement instruction can be displayed to the worker.

[0130] As described above, in the numerical control system 2 according to Embodiment 1, the countermeasure determination unit 7 calculates the deviation degree D n (n is an integer from 1 to 5) when determining that a machining defect has occurred. The cutting conditions, cutting forces, dynamic characteristic information, machining process information, and the feeding drive mechanism are used to calculate the deviation levels.

[0131] Based on the calculated deviation levels, the reason why a machining defect has occurred is then determined, and the correction value is calculated with respect to the determined machining defect reason.

[0132] However, it should be noted that the calculation formula used for the deviation degrees is not limited to formula (5). For example, formulas (14) to (15) can be used. [Formula 14] Dn=|Xn−An|2|Bn−An|2 (n=1,⋯,5) [Formula 15] Dn=e|Xn−An|e|Bn−An| (n=1,⋯,5)

[0133] The absolute value of the difference between the processing state variable X n at the time when a machining defect is detected, and the machining state variable reference A n indicates how far the processing state variable X nat the time when a machining defect is detected, deviates from the machining state variable reference An, which is a reference value.

[0134] Therefore, it is appropriate that a formula for calculating the degree of deviation results in a larger degree of deviation, since the absolute value of the difference between the processing state variable X n at the time when a machining defect is detected, and the machining state variable reference A n becomes larger. It is also appropriate that in the properties of the deviation degree calculation formula, a larger difference between the machining state variable limit value B n and the processing state variable reference A n a smaller degree of deviation D n results.

[0135] In Embodiment 1, the machining defect reason is determined by selecting the largest of all calculated deviation degrees. A deviation calculation formula such as Formula (16) can be used so that the smallest Dn of all deviation degrees is selected to determine the machining defect reason. [Formula 16] Dn=|Bn−An||Xn−An| (n=1,⋯,5)

[0136] In the event that a machining defect occurs, the numerical control system 2 according to Embodiment 1 can determine more accurately than conventional numerical control systems which reason is the main reason or which type of machining defect has occurred from the five types of machining defects including self-excited chatter vibration, forced chatter vibration, tool bending, tool wear, and friction in the feed drive mechanism.

[0137] Since the numerical control system 2 according to Embodiment 1 is also capable of identifying which machining defect has occurred, the type of correction control can be changed depending on the type of machining defect cause. This makes it possible to prevent or reduce machining defects with regard to a variety of machining defect causes.

[0138] The deviation levels according to embodiment 1 have the following common properties. With regard to all deviation levels D n from n = 1 to n = 5, the degree of deviation Dn is zero in the case in which the processing state variable X n at the time when a processing defect occurs, equal to the processing state variable reference A n and the degree of deviation D n becomes one if the processing state variable X nat the time when a machining defect occurs is equal to the machining state variable limit Bn.

[0139] Using the difference between the machining state variable limit B n and the processing state variable reference A n The degree of deviation D serves as a reference scale n as a measured value that quantitatively represents the degree of deviation of the processing state variable X n at the time when a machining defect is detected, from the machining state variable reference A n indicates.

[0140] By determining the degrees of deviation D nBy calculating the difference between a machining state variable at the time a machining defect is detected and the machining state variable reference An corresponding to the normal state, a possible process can be quantitatively compared with the differences that occur with other types of machining defects. This makes it possible to more accurately determine which machining defect has occurred.

[0141] In the numerical control system 2 according to Embodiment 1, the countermeasure determination unit 7 selects a machining defect cause from among self-excited chatter vibration, forced chatter vibration, tool bending, tool wear, and friction in the feed drive mechanism. However, the machining defect causes are not limited to these five types. Nor are the machining defect causes fixed to these five types, and the causes may be four or fewer or six or more.

[0142] In the present embodiment 1, the machining defect reason is selected based on the largest deviation degree. However, a plurality of machining defect reasons may also be selected. In addition to the machining defect reason corresponding to the largest deviation degree, the countermeasure determination unit 7 may also select a machining defect reason corresponding to the second largest deviation degree and perform correction for the plurality of machining defect reasons.

[0143] The numerical control system 2 according to Embodiment 1 is configured to internally store the dynamic characteristic information and the machining process information. However, the dynamic characteristic information and the machining process information input to the countermeasure determination unit 7 may also be stored outside the numerical control system 2.

[0144] For example, the dynamic characteristic information, the machining process information, and the cutting condition may be stored in a database external to the numerical control system 2, so that the countermeasure determination unit 7 obtains the dynamic characteristic information, the machining process information, and the cutting condition via a communication device such as a telecommunication network.

[0145] In Embodiment 1, the numerical control system 2 controlling the machine tool 1 for cutting is described. However, the application of the present invention is not limited to cutting machine tools. The present invention can also be applied to other types of machining, such as cutting, drilling, grinding, polishing, rolling, forging, and bending. That is, in other types of machining, it is also possible to obtain a machining state variable and a deviation degree for each of its machining defect causes, and compare the deviation degrees to determine a machining defect cause. Embodiment 2

[0146] In Embodiment 1, it is described that after selecting a machining defect reason, the correction value is calculated by multiplying the control value corresponding to the machining defect reason by a coefficient. In the present Embodiment 2, a configuration is described in which the correction value calculation unit switches rules for changing the control depending on the determination results for a machining defect reason.

[0147] Fig. 7 is a block diagram of a numerical control system according to Embodiment 2 of the present invention. In Fig. 7 are components that are identical or equivalent to those of embodiment 1 shown in Fig. 1 are assigned the same reference numerals. A description will be given focusing on the parts that characterize the present embodiment 2.

[0148] In the same manner as the countermeasure determination unit 7 according to Embodiment 1, a countermeasure determination unit 7a receives the state of the feed drive mechanism, the dynamic characteristic information, and the machining process information, and performs internal processing in the same manner as the countermeasure determination unit 7 in Embodiment 1.

[0149] The countermeasure determination unit 7a outputs “a machining deficiency reason” internally selected by the countermeasure determination unit 7a as a countermeasure signal to a correction value calculation unit 8a, which will be described later.

[0150] In addition to the countermeasure signal, the countermeasure determination unit 7a outputs the dynamic characteristic information, the machining process information, the cutting condition, a feed drive mechanism state signal, and the measurement signal to the correction value calculation unit 8a. The correction value calculation unit 8a receives the countermeasure signal from the countermeasure determination unit 7a and then selects a control rule depending on the type of machining defect cause selected by the countermeasure determination unit 7a, and outputs a correction value calculated depending on the selected control rule to the control unit 9.

[0151] Fig. 8 shows a block diagram of the correction value calculation unit 8a. In Fig. 8, the countermeasure determination unit 7a and the control unit 9 are shown in addition to the components of the correction value calculation unit 8a. The correction value calculation unit 8a includes a signal acquisition unit 20, a self-excited chatter vibration control unit 21, a forced chatter vibration control unit 22, a tool deflection control unit 23, a tool wear control unit 24, a friction control unit 25, and a correction value output unit 26.

[0152] The signal acquisition unit 20 receives the countermeasure signal and determines the machining defect cause selected by the countermeasure determination unit 7a. If it is determined that self-excited chatter vibrations are the machining defect cause, the signal acquisition unit 20 outputs a countermeasure signal to the self-excited chatter vibration control unit 21.

[0153] If it is determined that forced chatter vibrations are the cause of the machining defect, the signal detection unit 20 outputs a countermeasure signal to the forced chatter vibration control unit 22. If it is determined that tool deflection is the cause of the machining defect, the signal detection unit 20 outputs a countermeasure signal to the tool deflection control unit 23.

[0154] If it is determined that tool wear is the cause of the machining defect, the signal detection unit 20 outputs a countermeasure signal to the tool wear control unit 24. If it is determined that friction in the feed drive mechanism is the cause of the machining defect, the signal detection unit 20 outputs a countermeasure signal to the friction control unit 25.

[0155] The self-excited chatter vibration control unit 21 uses the measurement signal, the feed drive mechanism state signal, the cutting condition, the dynamic characteristic information, and the machining process information to calculate a spindle speed S and an axial depth ap of the cut for suppressing self-excited chatter vibration.

[0156] Specifically, the self-excited chatter vibration control unit 21 calculates a stability limit for self-excited chatter vibration using the method described in step S13-1 of Embodiment 1, and selects a stable pocket sp that is closest to the spindle speed at the time a machining defect is detected. Then, in the selected stable pocket, the self-excited chatter vibration control unit 21 calculates the maximum axial depth ap of the notch and the spindle speed S corresponding to the calculated maximum axial depth ap of the notch as correction values.

[0157] The forced chatter vibration control unit 22 uses the measurement signal, the feed drive mechanism state signal, the cutting condition, the dynamic characteristic information, and the machining process information to calculate a spindle speed S for suppressing forced chatter vibration. Specifically, the forced chatter vibration control unit calculates a machine structure vibration displacement |[G(iω)] · [F(iω)]| from a dynamic characteristic matrix [G(iω)] and a cutting vector [F(iω)] at an arbitrary frequency ω.

[0158] Then, the forced chatter vibration control unit 22 selects such a frequency ω6 at which the vibration displacement becomes smaller than the displacement limit value, calculates the spindle speed S6 by using formula (17), and outputs the calculated spindle speed S6 as a correction value. [Formula 17] S6=30×ω6π

[0159] The tool deflection control unit 23 uses the cutting condition, the feed drive mechanism state signal, the dynamic characteristic information, and the machining process information, and calculates a spindle speed S and a radial depth ae of the cut for suppressing the tool deflections.

[0160] Specifically, by using the method described in step S13-3 in Embodiment 1, the tool deflection control unit 23 calculates tool deflections each corresponding to a combination of a spindle speed S and a radial depth ae of the cut. For the spindle speed S7 and the radial depth ae1 of the cut, the tool deflection control unit 23 then selects combinations of the spindle speed S and the radial depth ae of the cut, each combination resulting in a tool deflection value smaller than the tool deflection limit value.

[0161] From the selected combinations, the tool deflection control unit 23 finds a combination that gives a maximum material removal rate MMR obtained from formula (18) and outputs the combination as the correction values. [Formula 18] MMR=ƒ×N×S7×ap×ae1

[0162] In the above formula, f is the feed rate per cutting edge, N is the number of tool edges, S7 is the spindle speed, ap is the axial depth of cut, and ae1 is the radial depth of cut. Note that the combination of spindle speed S and radial depth ae of cut, which yield the maximum material removal rate, can be obtained through numerical simulations.

[0163] The tool wear control unit 24 uses the machining process information and a tool wear control model set internally in advance, and calculates a feed speed for suppressing a machining defect caused by tool wear. Fig. 9 shows an example of a tool wear control model according to Embodiment 2 of the present invention.

[0164] In Fig. 9 the vertical axis indicates feed speeds F. The horizontal axis indicates specific cutting forces K ct in the tool rotation direction. Fig. 9 shows the relationship between the specific cutting force K ct in the tool rotation direction and the feed speed F, which are to be specified, whereby the specific cutting force K ct by K ct1 , which represents the power at the time when it is unused, until K ct6changed, which represents the force at the time when the tool life is reached.

[0165] The tool wear control unit 24 uses the Fig. 9 shown tool wear control model for calculating a feed speed F corresponding to the specific cutting force K ct in the tool rotation direction, obtained from the machining process information, and thereby outputs the calculated feed rate F as a correction value. However, it should be noted that the tool wear control model is not based on the Fig. 9 is limited.

[0166] A mathematical model can also be used which, as a function of the specific cutting force K ctin the tool rotation direction is used to calculate the feed rate. It should also be noted that the spindle speed S, the axial depth ap of the cut, or the radial depth ae of the cut can be used instead of the feed rate.

[0167] The friction control unit 25 has friction models corresponding to the acceleration / deceleration positions of the feed drive mechanism and its turning positions. The friction control unit 25 uses the feed drive mechanism status signal to select a friction model corresponding to the current position of the feed drive mechanism and outputs it as a correction value.

[0168] The correction value output unit 26 outputs to the control unit the correction value calculated by one of the following: self-excited chatter vibration control unit 21, forced chatter vibration control unit 22, tool deflection control unit 23, tool wear control unit 24, and friction control unit 25.

[0169] As described so far, in the present Embodiment 2, the correction value calculation unit 8a switches the control rules depending on the machining defect cause determined by the countermeasure determination unit 7a. That is, instead of directly selecting a correction value depending on the machining defect cause, the numerical control system 2a selects a control rule and thereby calculates a correction value depending on the machining state variable.

[0170] Therefore, in the case where a machining defect has occurred, the numerical control system 2a according to the present embodiment 2 selects a control rule from among them according to the self-excited chatter vibration, the forced chatter vibration, the tool deflection, the tool wear, and the friction in the feed drive mechanism, and thereby the machining defects are reduced with high accuracy. Embodiment 3

[0171] In the description of Embodiments 1 and 2, a machining defect reason is determined based on deviation degrees. In the present Embodiment 3, the description is made according to a configuration in which the selection of a machining defect reason is corrected based on a worker's input in addition to the deviation degrees.

[0172] Fig. 10 is a block diagram of a numerical control system 2b according to Embodiment 3 of the present invention. In Fig. 10 are components that are identical or equivalent to those listed in Fig. 7 according to Embodiment 2 are assigned the same reference numerals. Accordingly, the description will be made here focusing on the parts related to the present Embodiment 3.

[0173] The numerical control system 2b includes: the detection unit 4, the dynamic characteristic information storage unit 5, the machining process information storage unit 6, a countermeasure determination unit 7b, the correction value calculation unit 8, the control unit 9, and an input unit 27. The machine tool 1 includes the sensor 3 and the tool 10, and the workpiece 11 is arranged on the tool.

[0174] The input unit 27 receives the operator's inputs and outputs selection modification coefficients Q1 to Q5 to the countermeasure determination unit 7b, which will be described later. The operator inputs priority levels P1 to P5 for self-excited chatter vibrations, forced chatter vibrations, tool bending, tool wear, and friction in the feed drive mechanism, respectively, to the input unit 27.

[0175] Depending on the priority levels P1 to P5 input by the operator, the input unit 27 outputs the selection modification coefficients Q1 to Q5 corresponding to the self-excited chatter vibrations, the forced chatter vibrations, the tool deflection, the tool wear, and the friction in the feed drive mechanism, respectively. For example, the operator inputs an integer of one or greater than the priority levels P1 to P5. The selection modification coefficients Q1 to Q5 can be determined, for example, using formula (19). In formula (19), n = 1 to 5. [Formula 19] Qn=PnP1+P2+P3+P4+P5 (n=1,⋯,5)

[0176] However, it should be noted that the default range for priority levels P1 to P5 is not limited to the above. Furthermore, priority levels P1 to P5 directly input by the worker can be used as selection modification coefficients Q1 to Q5.

[0177] The Countermeasure Determination Unit 7b receives the following: the dynamic property information output from the dynamic property information storage unit 5, the machining process information output from the machining process information storage unit 6, and the cutting condition output from the control unit 9.

[0178] The countermeasure determination unit 7b also receives the measurement signal from the detection unit 4, the feed drive mechanism state signal output therefrom, and the selection modification coefficients Q1 to Q5 output from the input unit 27. If it is determined from the measurement signal or the state of the feed drive mechanism that a machining defect has occurred, the countermeasure determination unit 7b selects a machining defect cause and outputs the selected result as the countermeasure signal to the correction value calculation unit 8.

[0179] In addition to the countermeasure signal, the countermeasure determination unit 7b outputs the following: the dynamic characteristic information, the machining process information, the cutting condition, the feed drive mechanism status signal, and the measurement signal. In the process of selecting a machining defect cause, the countermeasure determination unit 7b uses the same method as that used by the countermeasure determination unit 7a to determine the deviation degree D. n to calculate the deviation degrees L1 to L5, which are degrees before the priority adjustment and correspond to the processing defect reasons.

[0180] Then, the countermeasure determination unit multiplies the deviation degrees L1 to L5 before the priority adjustment by the selection modification coefficients Q1 to Q5, respectively, and obtains deviation degrees D n = L n × Q n(n = 1 to 5); and then the countermeasure determination unit selects a processing defect reason according to the largest deviation degree.

[0181] As described above, in the present embodiment 3, the countermeasure determination unit 7b obtains deviation degrees D n = L n × Q n (n = 1 to 5) that are modified based on inputs from the worker. Accordingly, the numerical control system 2b according to the present embodiment 3 is capable of preferentially taking effective measures based on the worker's determination. Embodiment 4

[0182] In Embodiment 3, how the selection of a machining defect reason is modified based on a worker's input is described. In the present Embodiment 4, a configuration is described in which the selection of a machining defect reason is corrected based on a machining history.

[0183] Fig. 11 is a block diagram of a numerical control system 2c according to Embodiment 4 of the present invention. In Fig. 11 are components that are identical or equivalent to those listed in Fig. 7 of Embodiment 2 are assigned the same reference numerals. Accordingly, the description will be made here focusing on the parts related to the present Embodiment 4.

[0184] The numerical control system 2c includes: the detection unit 4, the dynamic characteristic information storage unit 5, the machining process information storage unit 6, a countermeasure determination unit 7c, the correction value calculation unit 8, the control unit 9, and a machining history storage unit 28. The machine tool 1 includes the sensor 3 and the tool 10, and the workpiece 11 is arranged on the tool.

[0185] The machining history storage unit 28 records the results of the machining defect reasons selected by the countermeasure determination unit 7c, which will be described later, and outputs a selection modification coefficient from the internally recorded history of the selected machining defect reasons to the countermeasure determination unit 7c.

[0186] More specifically, the machining history storage unit 28 stores the number of times H1 to H5 by which the countermeasure determination unit 7c has selected the self-excited chatter vibrations, the forced chatter vibrations, the tool bending, the tool wear, and the friction in the feed drive mechanism, respectively, in the past, and calculates machining history-based selection modification coefficients Rn from Formula (20). [Formula 20] Rn=HnH1+H2+H3+H4+H5 (n=1,⋯,5)

[0187] The countermeasure determination unit 7c receives the dynamic characteristic information, the machining process information, and the cutting condition output from the dynamic characteristic information storage unit 5, the machining process information storage unit 6, and the control unit 9, respectively. The countermeasure determination unit also receives the measurement signal output from the detection unit 4, the feed drive mechanism state signal output therefrom, and the machining history-based selection modification coefficients Rn output from the machining history storage unit 28.

[0188] In the case where it is determined from the measurement signal or the feed drive mechanism state signal that a machining defect has occurred, the countermeasure determining unit selects a machining defect reason and outputs the selected result to the correction value calculating unit 8 as a countermeasure signal.

[0189] In addition to the countermeasure signal, the countermeasure determination unit 7c outputs the dynamic characteristic information, the machining process information, the cutting condition, the feed drive mechanism state signal, and the measurement signal to the correction value calculation unit 8. Furthermore, the countermeasure determination unit 7c outputs the countermeasure signal to the machining history storage unit 28.

[0190] In the process of selecting a machining defect reason, the countermeasure determination unit 7c uses the same method as that used by the countermeasure determination unit 7a for calculating the deviation degree Dn to calculate deviation degrees J1 to J5 before the history-based modification for the machining defect reasons.

[0191] The countermeasure determination unit 7c multiplies the deviation degrees before the history-based modification by the processing history-based selection modification coefficients R1 to R5, respectively, and obtains the deviation degrees D n = J n × R n (n = 1 to 5). The countermeasure determination unit then selects a processing defect reason according to the largest deviation degree.

[0192] As described above, in the present Embodiment 4, the countermeasure determination unit 7c obtains deviation degrees modified based on the machining history. Accordingly, the numerical control system 2c according to the present Embodiment 4 is capable of taking effective measures based on the priorities indicated by the machining history of the machine tool 1.

[0193] The description has been made for Embodiments 1 to 4. However, the scope of the present invention is not limited to these embodiments. The embodiments can be appropriately modified within the scope of the present invention. The above-described embodiments can also be combined with each other in other practical forms, if appropriate. List of reference symbols 1 machine tool 2, 2a, 2b, 2c Numerical control system 4 Detection unit 5 Storage unit for dynamic property information 6 Storage unit for machining process information 7, 7a Countermeasures Determination Unit 7b, 7c Countermeasure determination unit 8, 8a Correction value calculation unit 9 Control unit 10 tools 11 Workpiece 12 Cutting condition 27 Input unit 28 Editing history storage unit D1 Deviation level for self-excited chatter vibrations D2 Deviation level for forced chatter vibrations D3 Deviation degree for tool bending D4 Deviation degree for tool wear D5 Friction deviation degree in the feed drive mechanism X n Processing state variable

Claims

[1] Numerical control system (2, 2a, 2b, 2c) comprising: - a detection unit (4) for obtaining a cutting force generated in a machine tool (1); - a control unit (9) for calculating a control value depending on a cutting condition and for controlling a feed drive mechanism of the machine tool (1); - a dynamic property information storage unit (5) for storing dynamic properties of a tool or a workpiece in relation to the cutting condition; - a machining process information storage unit (6) for storing information representing a machining process occurring between the tool and the workpiece in relation to the cutting condition; - a countermeasure determination unit (7) for, when it is detected from the cutting force or a state of the feed drive mechanism of the machine tool (1) that a machining defect has occurred, calculating a plurality of deviation degrees for possible causes of the machining defect and calculating the calculated deviation degrees and thereby determining a cause of the machining defect whose occurrence has been detected; and - a correction value calculation unit (8) for calculating, depending on the cause of the machining defect determined by the countermeasure determination unit (7), a correction value related to the control value and for outputting the correction value to the control unit (9). [2] The numerical control system (2, 2a, 2b, 2c) according to claim 1, wherein, when it is detected that a machining defect has occurred, the countermeasure determination unit (7) obtains machining state variables corresponding to the possible causes of the machining defect based on at least one of the dynamic property information, the machining process information, the cutting force, the cutting condition, and the state of the feed drive mechanism, and calculates respective deviation degrees for the possible causes of the machining defect using the cutting condition values ​​corresponding to the possible causes of the machining defect. [3] The numerical control system (2, 2a, 2b, 2c) according to claim 1 or 2, wherein the countermeasure determination unit (7) selects N degrees of deviation in descending order of magnitude from the degrees of deviation (N < M (N is an integer equal to or greater than one, and M is the number of degrees of deviation)) and determines that N possible causes of the machining defect corresponding to the selected N degrees of deviation are each a cause of the detected machining defect. [4] Numerical control system (2, 2a, 2b, 2c) according to one of claims 1 to 3, wherein the correction value calculation unit (8) has a control law for calculating a correction value for the cause of the machining defect. [5] The numerical control system (2, 2a, 2b, 2c) according to any one of claims 1 to 4, further comprising an input unit capable of inputting characteristics for possible causes of the machining defect, wherein the countermeasure determination unit (7) modifies the deviation degrees on the basis of the received priorities. [6] The numerical control system (2, 2a, 2b, 2c) according to any one of claims 1 to 5, further comprising a machining history storage unit for storing a machining history, wherein the countermeasure determination unit (7) modifies the deviation degrees based on the machining history. [7] The numerical control system (2, 2a, 2b, 2c) according to any one of claims 1 to 6, wherein the plurality of possible causes of the machining defect include one of the following: self-excited chatter vibrations, forced chatter vibrations, tool bending, tool wear, and friction in the feed drive mechanism.

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