JP2020042347A - Machine tool and tool abnormality determination method - Google Patents

Abstract

To provide a machine tool and a tool abnormality determination method capable of easily and accurately detecting an abnormality of a tool even when a tool or cutting conditions change.SOLUTION: A machine tool according to a present embodiment includes: a drive mechanism for driving a control object that moves a workpiece or a tool and rotating a tool having a plurality of blades for cutting the workpiece; a motor for operating the drive mechanism; a first position sensor; a second position sensor; a current control unit for controlling a supply current to the motor; a servo control unit that outputs a torque command that determines the supply current to the current control unit; and a numerical control unit that calculates a processing force for the workpiece, which is the control object, on the basis of the position information of the motor, the position information of the control object, and the torque command. The numerical control unit calculates an amplitude of the processing force for each frequency, and determines that an abnormality occurs in the tool on the basis of the ratio of the amplitude at the first frequency, which is the rotation frequency of the tool multiplied by the number of blades, to the amplitude at the second frequency, which is an integral multiple of the rotation frequency of the tool.SELECTED DRAWING: Figure 2

Description

  Embodiments according to the present invention relate to a machine tool and a tool abnormality determination method.

  A machine tool processes a workpiece using a tool mounted on a spindle, for example. A numerical controller (hereinafter, also referred to as an NC (Numerical Controller) device) of a machine tool outputs a command to a spindle, controls operation of the spindle, and monitors torque of the spindle, that is, a load current of the motor. The NC device detects an abnormality of a tool used for machining a workpiece by monitoring a torque of a spindle.

  For example, when a machine tool first processes a workpiece with a certain tool, the NC device monitors the torque of the spindle from the start of rotation of the spindle to a steady cutting through a no-load state before cutting. The NC device stores the torque of the main shaft during steady cutting in the first machining as a reference torque. In the subsequent machining, when the torque of the main spindle during steady cutting becomes larger than the stored reference torque by a certain degree or more, the NC device determines that an abnormality has occurred in the tool.

JP 2015-143969 A

  However, the value of the reference torque changes depending on the type of tool. Therefore, it is necessary to store the reference torque for each tool. Further, when the cutting conditions such as the spindle speed, the feed speed, and the cutting depth change, the value of the reference torque also changes. Therefore, when the reference torque is stored, there is a problem that it is difficult to correctly detect the abnormality of the tool if the cutting condition changes for each processing.

  Therefore, the present invention has been made to solve the above problems, and provides a machine tool and a tool abnormality determination method capable of easily and accurately detecting an abnormality of a tool even when a tool or cutting conditions are changed. It is.

  The machine tool according to the present embodiment drives a control object that moves a work or a tool, and a drive mechanism that cuts the work while rotating a tool having a plurality of blades that cuts the work, and a motor that operates the drive mechanism, A first position sensor for detecting the position of the control target, a second position sensor for detecting the position of the motor, a current control unit for controlling the supply current to the motor, and a torque command for determining the supply current to the current control unit A numerical value for calculating a machining force for a work to be controlled based on a servo control unit to be output, and position information of a motor obtained from a first position sensor, position information of a control target obtained from a second position sensor, and a torque command. The numerical control unit calculates the amplitude of the machining force for each frequency, the amplitude at a first frequency obtained by multiplying the rotation frequency of the tool by the number of blades, and the rotation frequency of the tool Based on the ratio of the amplitude and the second frequency of an integral multiple, it is determined that an abnormality has occurred in the tool.

FIG. 2 is a diagram illustrating an example of a configuration of a motor and a drive mechanism of the machine tool according to the first embodiment. FIG. 2 is a block diagram illustrating an example of a configuration of a servo unit, a driving mechanism, and a numerical control device (NC device) according to the first embodiment. 4 is a graph showing a cutting force according to the first embodiment. 5 is a graph showing the amplitude of a cutting force with respect to a frequency. FIG. 4 is a flowchart illustrating an example of a tool abnormality determination operation according to the first embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. This embodiment does not limit the present invention.

  The drawings are schematic or conceptual, and the proportions and the like of each part are not always the same as actual ones. In the specification and drawings, the same reference numerals are given to the same elements as those described above with respect to the already-explained drawings, and the detailed description will be appropriately omitted.

(1st Embodiment)
FIG. 1 is a diagram illustrating an example of a configuration of the motor 14 and the drive mechanism 2 of the machine tool according to the first embodiment. The motor 14 is connected to the drive mechanism 2 via the coupling 3. The drive mechanism 2 includes a bed 20, a support bracket 22, a nut 23, a ball screw 25, a bearing 26, a linear guide 27, and a table 28.

  The bed 20 is fixed to the main body of the machine tool, and mounts other components of the drive mechanism 2. The support bracket 22 is fixedly arranged on the bed 20, and is configured to support both ends of the ball screw 25 via bearings 26. The ball screw 25 is connected to the motor 14 by the coupling 3 and is configured to rotate according to the rotation of the shaft of the motor 14. The nut 23 is formed with a screw hole so as to be screwed with a screw portion of the ball screw 25, and can move in the axial direction of the ball screw 25 as the ball screw 25 rotates. A table 28 to be controlled is fixed to a nut 23 and supported by a linear guide 27. The table 28 moves in the axial direction (the direction of arrow A) of the ball screw 25 and the linear guide 27 as the nut 23 moves. Thus, the machine tool can convert the rotational movement of the motor 14 into the linear movement of the table 28.

  The table 28 carries the work and moves the work relative to the bed 20 in order to cut the work with a tool. In this case, the control target is a moving object such as the table 28 and a work. Alternatively, instead of the table 28, a tool motor (not shown) for rotating the tool, a chuck for mounting the tool on the tool motor, and a spindle head on which the tool is mounted may be used. In this case, the control target is a moving object such as the table 28, a tool motor, a chuck, and a tool. The tool is a member that processes a work, and is, for example, a drill, an end mill, or the like. Drills, end mills, and the like have at least one blade for cutting a work, and can cut the work with the blade while rotating.

  FIG. 2 is a block diagram illustrating an example of a configuration of the servo unit 1, the drive mechanism 2, and the numerical control device (NC device) 4 according to the present embodiment. The NC device 4 includes an HMI (Human-Machine Interface) 40, a program supply unit 41, a program analysis unit 42, a trajectory generation unit 44, a cutting force estimation unit 45, a memory 46, an error detection unit 47, A frequency analysis unit 48.

  The HMI 40 is an interface between the operator and the numerical controller 4, and is, for example, a touch panel display. The operator inputs or selects a machining program in the HMI 40. Further, the HMI 40 indicates the operating state of the machine tool to the operator, or displays the occurrence of an error in response to an error signal from the error detection unit 47.

  The program supply unit 41 obtains the machining program stored in the memory 46 and supplies the machining program to the program analysis unit 42. The memory 46 is, for example, a read only memory (ROM), a random access memory (RAM), a hard disc drive (HDD), or a solid state drive (SSD). The memory 46 stores a system program for controlling the entire machine tool, a machining program, machine tool parameters, and various data. The RAM area of the memory 46 is also used as a load area or a work area when executing a system program or the like.

  The program analysis unit 42 analyzes the machining program obtained from the memory 46 and creates trajectory generation data. In the analysis of the machining program, for each block of the machining program, the coordinates of the target position at which the moving unit (for example, the table, the spindle head, etc.) of the drive mechanism 2 is moved, and the target moving speed of the moving unit are obtained. The block is a basic unit of the machining program, and indicates a command for one line, for example. One block indicates a command for a basic operation such as a linear movement, an arc movement, and a start / stop of rotation of a spindle. The trajectory generation data is created for each block and transferred to the trajectory generation unit 44.

  The trajectory generation unit 44 generates a position command for each sampling cycle (control cycle of the servo unit 1) based on the trajectory generation data. The trajectory generating unit 44 outputs a position command to the servo control unit 12 of the servo unit 1. The memory 46, the program analysis unit 42, the trajectory generation unit 44, the cutting force estimation unit 45, the error detection unit 47, and the frequency analysis unit 48 inside the NC device 4 are realized by one CPU (Central Processing Unit). And may be realized by individual memories or CPUs.

  The servo controller 12 outputs a torque command to the current controller 13 in each control cycle according to the position command. The control cycle is, for example, about 1 msec. The torque command is a command for determining a supply current to the motor 14. The current control unit 13 supplies a current according to the torque command to the motor 14. The motor 14 is provided with a rotary encoder 60. The rotary encoder 60 as a second position sensor detects rotation of the shaft of the motor 14 and measures displacement of the rotation position of the shaft of the motor 14. The rotary encoder 60 feeds back a change in the rotational position of the shaft of the motor 14 to the servo control unit 12. Hereinafter, information on the rotational position of the shaft of the motor 14 is referred to as “motor position”, and the feedback thereof is also referred to as “motor position feedback”.

  The motor 14 drives the drive mechanism 2 as described with reference to FIG. Thereby, the table 28 moves along the linear guide 27.

  The drive mechanism 2 is provided with a linear encoder 50 connected to the table 28. The linear encoder 50 as a first position sensor measures a change in the linear position of the table 28. The linear encoder 50 feeds back the change of the linear position of the table 28 to the servo control unit 12. Hereinafter, the information on the linear position in the table 28 is referred to as “scale position”, and the feedback thereof is also referred to as “scale position feedback”.

  The servo control unit 12 receives feedback from the linear encoder 50 and the rotary encoder 60 and generates a torque command so as to reduce an error with respect to a command position for controlling the scale position and the motor position.

ここで、ｒｅｆが右上添え字として付されたＩａ（以下、Ｉａ＿ｒｅｆという）は、トルク指令から得られる電流指令値である。ｘｔはスケール位置であり、その上のドットは２回の時間微分を示す。θｍはモータ位置（回転角）であり、その上のドットは２回の時間微分を示す。Ｔｆｒｉｃは、摩擦トルクである。Ｆｆｒｉｃは、テーブル２８のＡ方向への直動に対する摩擦力である。Ｊｒはモータからボールねじまでの慣性モーメントである。Ｋｔはモータ１４のトルク定数である。Ｒは回転系と直動系との変換係数である。Ｍｔは、ワークおよびテーブル２８等の移動物の質量である。 Further, the torque command, the scale position, and the motor position are also fed back to the cutting force estimation unit 45. The cutting force estimating unit 45 calculates a cutting force for the workpiece to be controlled based on the torque command, the scale position, and the motor position. The cutting force as the processing force may be, for example, a force for pressing the drill or the end mill against the work when the drill or the end mill cuts the work or a force applied to the work by the rotating drill or the end mill. The cutting force estimating unit 45 may calculate the cutting force Fcut by calculating Expression (3) or Expression (4) in Patent Document 1.

Here, Ia (hereinafter referred to as Ia_ref) with ref appended as an upper right suffix is a current command value obtained from a torque command. xt is the scale position, and the dot above it indicates two time derivatives. θm is the motor position (rotation angle), and the dot on it indicates two time differentiations. Tfric is a friction torque. Ffric is a frictional force with respect to the linear movement of the table 28 in the A direction. Jr is the moment of inertia from the motor to the ball screw. Kt is a torque constant of the motor 14. R is a conversion coefficient between the rotation system and the linear motion system. Mt is the mass of a work and a moving object such as the table 28.

ここで、推定値を示すパラメータには、「ハット」が付されている。ｇｃｕｔは、エラー検出部４７内のフィルタの遮断周波数である。各パラメータの右下添え字ｎは、ノミナル値を示す。 Further, by using the friction torque Tfric and the frictional force Ffric estimated in advance and using a low-pass filter for suppressing high-frequency noise, the estimated value of the cutting force Fcut is represented by Expression (4).

Here, the parameter indicating the estimated value is indicated by “hat”. gcut is a cutoff frequency of a filter in the error detection unit 47. The subscript n at the lower right of each parameter indicates a nominal value.

  Of the parameters included in Expressions 3 and 4, parameters other than the current command value Ia_ref obtained from the torque command, the scale position xt, and the motor position θm are stored in the memory 46. The memory 46 may be built in the NC device 4 or may be provided outside thereof. The parameters may be received from outside the machine tool.

  The cutting force estimating unit 45 calculates the cutting force on the workpiece by applying the torque command, the scale position, and the motor position to Expression 3 or Expression 4. The cutting force estimating unit 45 receives the torque command, the scale position feedback, and the motor position feedback for each sampling cycle (control cycle) of the servo control unit 12, and calculates this cutting force. As described above, the cutting force estimation unit 45 can estimate the cutting force for each control cycle without using an additional sensor such as a force sensor.

  The frequency analysis unit 48 analyzes the frequency of the cutting force estimated by the cutting force estimation unit 45 and calculates the amplitude (frequency component) of the cutting force for each frequency. The frequency analysis unit 48 receives the cutting force from the cutting force estimation unit 45 for each control cycle of the servo control unit 12, obtains the data points of the cutting force necessary for the frequency analysis, and then performs the frequency analysis of the cutting force. The frequency analysis unit 48 analyzes the frequency of the cutting force at each sampling cycle of the cutting force. The cutting force sampling period is a period during which data points of the cutting force necessary for one frequency analysis are acquired. The control cycle is, for example, about 1 millisecond, and the sampling cycle of the cutting force is, for example, about 1 second. In this case, the frequency analysis unit 48 performs the frequency analysis using the data points of about 1000 cutting forces.

  For example, a tool having at least one blade, such as an end mill, cuts substantially the same portion of the work by the number of blades every one rotation. As each blade of the tool cuts the work, a peak of the cutting force appears. For example, the number of blades of the tool (the number of blades that cut the same portion of the work every time the tool makes one rotation) is M, and the rotation speed of the main shaft that rotates the tool is N (rotation / minute). In this case, the peak of the cutting force is observed P = M × N / 60 times per unit time of one second. That is, the cutting force of each blade appears substantially at a frequency P (Hz) during processing. Hereinafter, this frequency P is referred to as a fundamental frequency P (first frequency). When the frequency analysis unit 48 analyzes the frequency of the cutting force, a large amplitude of the cutting force appears at the fundamental frequency P (see FIG. 4).

  If any of the blades of the tool is missing or worn, the waveform of the cutting force changes, and the amplitude of the cutting force for each frequency also changes. In the present embodiment, the abnormality of the tool is determined from the change in the amplitude of the cutting force.

  The error detector 47 determines a tool abnormality based on a change in the amplitude of the cutting force. The error detection unit 47 determines a tool abnormality in real time based on the cutting force for each sampling cycle of the cutting force of the frequency analysis unit 48. The determination of the abnormality of the tool will be described later with reference to FIGS.

  When a tool error is detected, the error detection unit 47 outputs a stop signal to the trajectory generation unit 44 and outputs an error signal to the HMI 40. Upon receiving the stop signal, the trajectory generation unit 44 stops updating the position command. Accordingly, the operations of the motor 14 and the drive mechanism 2 stop. At the same time, when receiving the error signal, the HMI 40 displays a tool abnormality on the display. Thereby, the user can know the abnormality of the tool.

  Next, an abnormality determination method will be described with reference to FIGS.

  In the present embodiment, the tool has, for example, eight blades (the number of blades of the tool M = 8), and the rotation speed N of the main shaft is 300 (rotation / minute). In this case, the fundamental frequency P at which the cutting force of each blade appears is P = M × N / 60 = 8 × 300/60 = 40 (Hz). The rotation frequency Q of the main shaft is Q = N / 60 = 300/60 = 5 (Hz). The basic frequency P is also a frequency (P = M × Q) obtained by multiplying the number of teeth M of the tool by the rotation frequency Q of the main shaft.

  FIGS. 3A and 3B are graphs showing the cutting force according to the present embodiment. FIG. 3A shows the cutting force when the blade of the tool has almost no chipping. FIG. 3B shows the cutting force when one of the eight blades is missing. The vertical axis indicates cutting force (N (Newton)), and the horizontal axis indicates time. The graphs of FIGS. 3A and 3B show the cutting force for about one second, which is the sampling period of the cutting force. The periodic peak of the cutting force in FIG. 3A appears every time each blade of the tool cuts the workpiece. The period AP between adjacent peaks corresponds to the fundamental frequency P (40 Hz) and is about 0.025 seconds (1 / P). The period AQ includes eight periods AP and corresponds to the rotation frequency of the main shaft (cycle of one main shaft rotation) Q (5 Hz), and is about 0.2 second (1 / Q). Note that the scale of the vertical axis is the same between FIGS. 3A and 3B.

  As shown in FIG. 3A, when there is almost no defect in the blade of the tool, a peak of the cutting force is observed each time each blade of the tool cuts the work. The peak of the cutting force appears at a period of 1 / P almost every period AP. Also, eight peaks corresponding to the number M of blades of the tool appear during the period AQ indicating the cycle (1 / Q) of one revolution of the main spindle.

  In FIG. 3A, the cutting force is slightly different for each blade. This is because all the blades of the tool are not in the same state even when there is almost no wear or breakage like the blade in the initial state (new). For example, if the distance from the center of rotation of the tool to the tip of each blade is slightly different for each blade, the cutting amount of each blade is also slightly different.

  As shown in FIG. 3B, when one of the eight blades is missing, a waveform of a cutting force different from that in FIG. 3A appears. For example, as compared with FIG. 3A, the amplitude of the waveform of the cutting force is larger, and the height of each peak of the cutting force is also greatly different. This is because the change in the cutting force due to breakage also affects the cutting force of the other blades, and the load applied to each of the remaining seven blades is different. It is similar to the graph of FIG. 3A in that the waveform of the cutting force appears continuously in the cycle (1 / Q) of the period AQ indicating one revolution of the spindle.

FIGS. 4A and 4B are graphs showing the amplitude of the cutting force with respect to the frequency. FIG. 4A shows the amplitude of the cutting force when the blade of the tool has almost no defect, and corresponds to FIG. 3A. FIG. 4B shows the amplitude of the cutting force when one of the eight blades is missing, and corresponds to FIG. 3B. The vertical axis indicates the amplitude of the cutting force. The vertical axis indicates, for example, 20 log ₁₀ (Fcut) (dB) which is a decibel value of the cutting force Fcut. The horizontal axis indicates frequency. Note that the scale of the vertical axis is the same between FIG. 4A and FIG. 4B. The vertical axis may indicate the cutting force Fcut (N) instead of the decibel value of the cutting force Fcut.

  As shown in FIGS. 4A and 4B, the amplitude of the cutting force at the fundamental frequency P (40 Hz) is called FP. Further, assuming that m is an integer of 1 or more, the amplitude of the cutting force at a second frequency (Q × m) that is an integral multiple of the rotation frequency Q (5 Hz) of the main shaft is referred to as Fm. Since the frequency P, which is the number of teeth (M times) of the rotation frequency Q of the main shaft, is the cutting frequency, m is a positive integer less than the number of teeth M. For example, when the number of blades M is 8, m is an integer of 1 to 7. In other words, the range of the integer m is a range in which a frequency (Q × m) that is an integral multiple of the rotation frequency Q of the main shaft is lower than the fundamental frequency P. For example, in FIGS. 4A and 4B, frequencies that are integer multiples of the rotation frequency Q (5 Hz) of the main shaft and less than the fundamental frequency P (40 Hz) are 5 Hz (5 × 1) to 35 Hz (5 Hz). × 7).

  A frequency higher than the fundamental frequency P is a frequency that is not directly related to cutting by the blade. Further, among the frequencies lower than the fundamental frequency P, frequencies other than Q × m are also frequencies that are not directly related to cutting by the blade. Therefore, here, attention is paid to the amplitudes F1 to F7 and FP of the cutting force at the frequency Q × m and the basic frequency P. In other words, in the tool abnormality determination, the frequency range that is an integer m times the rotation frequency Q of the main shaft is set to be less than the basic frequency P.

  Here, when there is almost no defect in the blade of the tool, a large peak FP appears in the amplitude of the cutting force at the basic frequency P (for example, 40 Hz) as shown in FIG. On the other hand, peaks F1 to F7 appear in the amplitude of the cutting force also at the cutting frequency Q × m of each blade (for example, 5 Hz, 10 Hz, 15 Hz, 20 Hz, 25 Hz, 30 Hz, and 35 Hz). However, the peaks F1 to F7 are relatively small (smaller than the peak FP). That is, a small peak of the amplitude of the cutting force appears even at a frequency Q × m that is an integral multiple of the rotation frequency Q (5 Hz) of the main shaft. This is because the blades of the tool are not all in the same state even when there is almost no loss. As described with reference to FIG. 3A, even when there is no chipping, the cutting amount of each blade is slightly different, and the waveform of the cutting force has a periodicity of 1 / Q. Accordingly, a slight peak appears in the amplitude of the cutting force even at an integral multiple (5 Hz to 35 Hz) of the rotation frequency Q of the main shaft.

  When one of the eight blades is missing, as shown in FIG. 4B, at a cutting frequency Q × m of each blade (for example, 5 Hz, 10 Hz, 15 Hz, 20 Hz, 25 Hz, 30 Hz, and 35 Hz). Large peaks F1 to F7 appear in the amplitude of the cutting force. These peaks F1 to F7 are larger than those in the case where the blade of the tool in FIG.

  Further, at the fundamental frequency P (40 Hz), a large peak appears in the amplitude of the cutting force as in FIG. The amplitude of the cutting force at the basic frequency P shown in FIG. 4B is substantially the same as the amplitude of the cutting force at the basic frequency P shown in FIG.

  As described above, when one blade has a defect, the amplitudes F1 to F7 of the cutting force at the second frequency Q × m (m = 1 to 7), which is an integral multiple of the rotation frequency Q of the main shaft, increase. On the other hand, the amplitude FP of the cutting force at the fundamental frequency P (P = Q × M) is substantially the same regardless of the presence or absence of a blade defect. Therefore, in this embodiment, the abnormality of the tool is determined using the ratio between the cutting force amplitudes F1 to F7 at the second frequency Q × m and the cutting force amplitude FP at the basic frequency P.

  FIG. 1 is referred to again. The frequency analysis unit 48 uses a DFT (Discrete Fourier Transform) to perform each frequency Q × m (for example, 5 Hz, 10 Hz, 15 Hz, 20 Hz, 25 Hz, 30 Hz, or an integer multiple of the rotation frequency Q (for example, 5 Hz) of the spindle). The amplitudes F1 to F7 and FP of the cutting force at 35 Hz) and the basic frequency P (40 Hz) are calculated.

  The number of blades M of the tool and the rotation speed N of the spindle required for calculating the basic frequency P and the rotation frequency Q of the spindle are respectively stored in the memory 46 and described in the machining program. For example, when the user sets a tool to be used for processing a workpiece by the HMI 40, the number of blades M of the tool is read from the memory 46 to the frequency analysis unit 48. In addition, for example, the program analysis unit 42 analyzes the machining program and sends the obtained rotation speed N of the spindle to the frequency analysis unit 48. Thereby, the frequency analysis unit 48 can calculate the fundamental frequency P and the rotation frequency Q of the main shaft, and can calculate the amplitudes F1 to F7 and FP of the cutting force by DFT. The maximum value of m may be calculated by M-1.

The error detector 47 calculates a ratio R7 between the amplitude FP of the cutting force and the average value (average of the peak values) of the amplitudes F1 to F7 of the plurality of cutting forces. The ratio R7 is represented by Expression 1.
R7 = {(F1 + F2 + F3 + F4 + F5 + F6 + F7) / 7} / FP (Formula 1)
When Expression 1 is generalized using the integer m, the ratio Rm between the amplitude FP of the cutting force and the average value of the amplitudes F1 to Fm of the plurality of cutting forces is expressed by Expression 2.
Rm = {(F1 + F2 +... + Fm) / m} / FP (Equation 2)

  The ratio R7 indicates a variation in the cutting force of the plurality of blades. If there is no variation, that is, if all the blades are ideally in the same state (the same cutting force), no peak in the amplitude of the cutting force appears at a frequency that is an integral multiple of the rotation frequency Q of the spindle. In this case, the average value of the amplitudes F1 to F7 of the plurality of cutting forces is a small value, and the ratio R7 is also a small value. On the other hand, if there is a variation, that is, if some of the blades are defective, the waveform of the cutting force has a periodicity of 1 / Q. In this case, a peak of the amplitude of the cutting force appears at a frequency Q × m that is an integral multiple of the rotation frequency Q of the main shaft. Therefore, the average value of the amplitudes F1 to F7 of the plurality of cutting forces becomes a large value, and the ratio R7 also becomes a large value.

  Therefore, the error detection unit 47 compares the ratio R7 with a certain threshold. If the ratio R7 is less than a certain threshold, the error detection unit 47 determines that the tool is normal. On the other hand, if the ratio R7 is equal to or greater than a certain threshold, the error detection unit 47 determines that an abnormality has occurred in the tool. A certain threshold is, for example, 0.9.

  For example, if the blades of the tool wear almost uniformly as a whole, the ratio R7 does not change much. Therefore, even if the blade wears due to cutting, the ratio R7 remains below a certain threshold, and the error detection unit 47 determines that the tool is normal.

  On the other hand, when a specific blade is missing or extremely abraded compared to other blades, as described above, the peak of the amplitude of the cutting force appears at a frequency that is an integral multiple of the rotation frequency Q of the main shaft. When the ratio R7 increases and exceeds a certain threshold, the error detection unit 47 determines that the tool is abnormal.

  Note that the error detection unit 47 uses the average value of the amplitudes F1 to F7 of the cutting force in calculating the ratio R7. FIGS. 3 (B) and 4 (B) show a case where one of the eight blades is broken, but the period of the waveform of the cutting force depends on the number of blades to be broken, the position of the broken, and the like. Change. Therefore, which of the plurality of cutting force amplitudes F1 to F7 has the larger cutting force amplitude depends on the manner of the defect. Therefore, by using the average value of the amplitudes F1 to F7 of the plurality of cutting forces, it is possible to accurately detect the abnormality of the tool not only for one blade but also for other defect patterns.

  Further, the error detection unit 47 may be provided with a timer function, measure a period in which the ratio R7 is equal to or greater than a certain threshold, and use the period for abnormality determination. In this case, the error detection unit 47 determines that an error has occurred in the tool when a certain period or more has elapsed continuously after the ratio R7 has exceeded a certain threshold. Accordingly, it is possible to prevent the error R from erroneously detecting that the tool has an abnormality because the ratio R7 temporarily exceeds a certain threshold value due to noise of the cutting force or the like.

  A certain threshold and a certain period are set, for example, as parameters. In this case, information on a certain threshold and a certain period is stored in the memory 46 and read out by the error detection unit 47. Note that a certain threshold and a certain period may be set in the machining program. In this case, information on a certain threshold value and a certain period may be analyzed by the program analysis unit 42 and sent to the error detection unit 47 for error detection.

  FIG. 5 is a flowchart illustrating an example of the tool abnormality determination operation according to the present embodiment.

  First, during machining of a workpiece, the cutting force estimating unit 45 estimates a cutting force (S10). Next, the frequency analysis unit 48 performs a frequency analysis of the cutting force at a frequency that is an integral multiple of the rotation frequency Q of the main shaft and at the basic frequency P using the DFT (S20). The frequency analysis unit 48 calculates the amplitudes F1 to F7, FP of the cutting force.

  Next, the error detection unit 47 calculates the ratio R7 from the amplitudes F1 to F7 and FP of the cutting force (S30). Next, the error detection unit 47 determines whether the ratio R7 is equal to or greater than a certain threshold (S40). If the ratio R7 is less than a certain threshold (NO in S40), the error detection unit 47 determines that the tool is normal, and the NC device 4 continues monitoring the cutting operation and the cutting force.

  If the ratio R7 is equal to or greater than a certain threshold (YES in S40), the error detection unit 47 determines that an error has occurred in the tool (S50). In this case, the error detection unit 47 transmits a stop signal and an error signal to the trajectory generation unit 44 and the HMI 40, respectively. Thereby, the trajectory generation unit 44 stops updating the position command, and the HMI 40 displays an error on the display (S60).

  Thus, the NC device 4 according to the present embodiment calculates the ratio R7 between the amplitude FP of the cutting force at the fundamental frequency P and the amplitudes F1 to F7 of the cutting force at a frequency that is an integral multiple of the rotation frequency Q of the main shaft. In addition, when the ratio R7 is equal to or greater than a certain threshold, the NC device 4 determines that an abnormality has occurred in the tool. As described above, when any of the blades of the tool is broken, the amplitudes F1 to F7 of the cutting force greatly change. As a result, the NC device 4 according to the present embodiment can automatically and accurately detect the loss or wear of each blade of the tool.

  In addition, the magnitude of the cutting force or the torque command also changes depending on the cutting conditions such as the type of tool, the number of revolutions of the spindle, the feed speed, and the depth of cut. If the error detection unit 47 determines the abnormality of the tool based on the magnitude of the cutting force and the torque command at the time of the first machining, if the tool and the cutting conditions change from those at the time of the original machining, the abnormality of the tool is correctly performed. Is difficult to detect.

  On the other hand, the machine tool according to the present embodiment calculates the ratio R7 between the amplitude FP of the cutting force and the amplitudes F1 to F7 of the cutting force, and determines the abnormality of the tool based on the ratio R7. When the type of the tool or the cutting condition changes, not only the magnitude of the cutting force amplitudes F1 to F7, but also the magnitude of the cutting force amplitude FP similarly changes. Therefore, even if the type of the tool or the cutting conditions change, the ratio R7 hardly changes. Thus, it is not necessary to consider changes in tools and cutting conditions. Further, when the tool is normal at the time of the first machining or the like, there is no need to store the amplitude of the reference cutting force and set a certain threshold based on the amplitude of the reference cutting force.

  FIGS. 3B and 4B show a case where one blade is broken. However, the error detection unit 47 can similarly detect an abnormality when a plurality of blades are broken. .

  Further, according to the present embodiment, no additional components (such as additional sensors) are required. Therefore, the machine tool according to the present embodiment is excellent in miniaturization and can keep costs low.

  The frequency analysis unit 48 may perform frequency analysis using FFT (Fast Fourier Transform) instead of DFT. In this case, as shown in FIGS. 4A and 4B, the frequency analysis unit 48 calculates the waveform of the amplitude of the cutting force in a wide frequency band. The error detection unit 47 obtains the amplitude of the cutting force at the fundamental frequency P and a frequency that is an integral multiple of the rotation frequency Q of the spindle from the waveform of the amplitude of the cutting force, and calculates the ratio R7. However, the frequency analysis unit 48 according to the present embodiment can calculate the frequency for frequency analysis based on the number of blades M of the tool and the rotation speed N of the main shaft. Accordingly, the calculation time when using DFT is shorter than the calculation time when using FFT, so it can be said that it is preferable to use DFT.

  Further, the error detection unit 47 according to the first embodiment does not use, for example, a frequency several times the basic frequency P or more. Therefore, the frequency analysis unit 48 may have a low-pass filter function or a low-pass filter circuit (for example, a moving average filter), and may cut a frequency component of the cutting force that is several times or more the fundamental frequency P among the cutting forces. . For example, when the basic frequency P is 40 Hz, the frequency components above 200 Hz may be cut.

  Instead of the cutting force, a parameter used for calculating the cutting force may be used for determining a tool abnormality. For example, in Equations (3) and (4), when the term of the acceleration obtained by time-differentiating xt twice greatly contributes to the cutting force, the acceleration is used to determine the abnormality of the tool instead of the cutting force. In this case, the cutting force estimation unit 45 calculates the acceleration from the scale position of the linear encoder 50. The frequency analysis unit 48 may analyze the frequency of the acceleration and calculate the amplitude of the acceleration for each frequency. In this case, the error detection unit 47 may determine that an abnormality has occurred in the tool using the amplitude of the acceleration as in the case of the cutting force. In this case, since the motor position and the torque command are not required, the abnormality of the tool can be determined more easily.

  Further, the error detection unit 47 according to the present embodiment uses the ratio between the cutting force amplitude FP and the average value of the plurality of cutting force amplitudes F1 to F7 in calculating the ratio R7, but using another method. Is also good. For example, the ratio R7 may be a ratio between the amplitude FP and the amplitude of any one of the amplitudes F1 to F7. In this case, the error detection unit 47 may use the ratio between the amplitude FP of the cutting force and the maximum amplitude, the minimum amplitude, or the intermediate amplitude among the amplitudes F1 to F7 to calculate the ratio R7. Further, the ratio R7 may be a ratio between the amplitude FP and the average value of the amplitudes of a plurality of arbitrary cutting forces among the amplitudes F1 to F7. In this case, for example, the ratio R7 may be R7 = {(F1 + F3) / 2} / FP.

  At least a part of the tool abnormality determination method in the machine tool according to the present embodiment may be configured by hardware or software. When configured with software, a program that implements at least a part of the function of the tool abnormality determination method may be stored in a recording medium such as a flexible disk or a CD-ROM, and read and executed by a computer. The recording medium is not limited to a removable medium such as a magnetic disk or an optical disk, but may be a fixed recording medium such as a hard disk device or a memory. Further, a program that implements at least a part of the function of the tool loss determination method may be distributed via a communication line (including wireless communication) such as the Internet. Furthermore, the program may be distributed in an encrypted, modulated, or compressed state via a wired or wireless line such as the Internet, or stored in a recording medium.

  Although several embodiments of the present invention have been described, these embodiments are provided by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in other various forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and equivalents thereof.

14 motor, 2 drive mechanism, 4 NC device, 40 input display section, 40 HMI, 41 program supply section, 42 program analysis section, 44 trajectory generation section, 45 cutting force estimation section, 46 memory, 47 error detection section, 48 frequency Analysis department

Claims (6)

A drive mechanism that drives a control object that moves a work or a tool, and that cuts the work while rotating the tool having a plurality of blades that cut the work,
A motor for operating the driving mechanism;
A first position sensor for detecting a position of the control target;
A second position sensor for detecting a position of the motor;
A current control unit that controls a supply current to the motor,
A servo control unit that outputs a torque command for determining the supply current to the current control unit,
Numerical value for calculating a machining force of the control target on the workpiece based on the position information of the motor obtained from the first position sensor, the position information of the control target obtained from the second position sensor, and the torque command. And a control unit,
The numerical control unit calculates the amplitude of the machining force for each frequency, the second at the first frequency obtained by multiplying the rotation frequency of the tool by the number of the plurality of blades and an integer multiple of the rotation frequency of the tool. A machine tool that determines that an abnormality has occurred in the tool based on a ratio between the amplitude and the amplitude at two frequencies.   The machine tool according to claim 1, wherein the second frequency is lower than the first frequency. The second frequency is a frequency obtained by multiplying the rotation frequency of the tool by m, where m is an integer of 1 or more,
The machine tool according to claim 1, wherein the ratio is a ratio between the amplitude at the first frequency and an average value of the amplitudes at a plurality of second frequencies corresponding to each integer m.   The machine tool according to any one of claims 1 to 3, wherein the numerical control unit determines that an abnormality has occurred in the tool when the ratio is equal to or greater than a threshold.   The machine tool according to claim 4, wherein the numerical control unit determines that an abnormality has occurred in the tool when a period in which the ratio is equal to or greater than the threshold is equal to or greater than a predetermined period. A drive mechanism that drives a control object that moves a work or a tool and that cuts the work while rotating the tool having a plurality of blades that cut the work, a motor that operates the drive mechanism, and a motor that drives the motor. A tool abnormality of a machine tool including a current control unit for controlling a supply current, a servo control unit for outputting a torque command for determining the supply current to the current control unit, and a numerical control unit for controlling the servo control unit A determination method,
The numerical control unit, based on the position information of the motor and the position information of the control target and the torque command, to calculate the machining force of the tool on the workpiece,
The numerical control unit calculates the amplitude of the processing force for each frequency,
The numerical controller is based on a ratio of the amplitude at a first frequency obtained by multiplying the rotation frequency of the tool by the number of the plurality of blades and the amplitude at a second frequency that is an integral multiple of the rotation frequency of the tool. A tool abnormality determination method, comprising: determining that an abnormality has occurred in the tool.

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