Maintenance method for internal grinding machine, internal grinding system, and method for manufacturing ring-shaped parts

Abstract

To provide an internal grinder maintenance method which appropriately determines grind stone replacement timing.SOLUTION: An internal grinder maintenance method includes: a process in which a driving current of a motor for moving a rotating grind stone in a cutting direction is measured at a spark-out time, and driving current data at the spark-out time is collected; and a process in which replacement timing of the grind stone is determined on the basis of change in the driving curent data over time at the spark-out time.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present application relates to a maintenance method for internal grinding machines, an internal grinding system, and a method for manufacturing ring-shaped components.

Background Art

[0002] Internal grinding is a processing method that holds a workpiece by a chuck or the like mounted on the spindle stock of a grinding machine, and grinds the inner surface of the workpiece having a hollow cylindrical shape (including a ring shape) through the rotational motion of the grinding wheel, the reciprocating motion of the mount, the rotational motion of the workpiece, and the like. In this specification, a grinding machine capable of performing such internal grinding is referred to as an "internal grinding machine".

[0003] Conventionally, the grinding wheel in an internal grinding machine has been replaced when the type of workpiece is changed, or when a defect occurs in the grinding process. Examples of defects in the grinding process include abnormalities such as increased variation in the machined dimensions of workpieces, occurrence of chipping, and generation of abnormal noise during grinding. Further, regardless of whether a defect actually occurs, the grinding wheel may be replaced when the usage time of the grinding wheel or the number of processed workpieces reaches a predetermined value.

[0004] For example, when manufacturing a rare-earth sintered ring magnet, the hollow cylindrical magnet is subjected to outer peripheral surface grinding, length direction grinding, and internal grinding. In internal grinding, the radial dimension of the grinding wheel needs to be smaller than the inner diameter dimension of the ring magnet. As rare-earth sintered ring magnets become more compact, a large load is applied to the thin grinding wheel during internal grinding, and the wear of the grinding wheel tends to increase. When the degree of wear and deterioration of the grinding wheel increases, the aforementioned defects are more likely to occur in the grinding process. In addition, the wear and deterioration of the grinding wheel leads to further processing instability.

[0005] Patent Document 1 and Patent Document 2 disclose techniques for monitoring vibration during internal grinding and estimating the wear and deterioration of the grinding wheel based on the vibration state.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-171871 [Patent Document 1] Japanese Patent Publication No. 2008-290203 [Overview of the project] [Problems that the invention aims to solve]

[0007] To monitor vibrations during the internal grinding process, it is necessary to attach vibration-detecting sensors to the internal grinding machine. Since vibrations have degrees of freedom in three directions within three-dimensional space, detection requires, for example, the use of multi-axis acceleration sensors with three or more axes, and the analysis of the resulting data becomes complex.

[0008] The applicant investigated measuring the drive current (load current) of a motor used to rotate a grinding wheel and determining the degree of wear (deterioration) of the grinding wheel based on the change in the measured drive current over time. However, it was found that the current of the motor that rotates the grinding wheel does not change much over time even when the grinding wheel is worn or deteriorated, making it difficult to use this method to determine when to replace the grinding wheel.

[0009] The present invention provides a maintenance method for an internal grinding machine, an internal grinding system, and a method for manufacturing ring-shaped parts that can solve these problems. [Means for solving the problem]

[0010] In exemplary and non-limiting embodiments, the maintenance method for an internal grinding machine of the present disclosure includes the steps of: measuring the drive current of a motor that moves a rotating grinding wheel in the cutting direction at the time of spark-out and collecting the drive current data at the time of spark-out; and determining the timing for replacing the grinding wheel based on the change in the drive current data at the time of spark-out over time.

[0011] In one embodiment, the process includes causing the grinding wheel to reciprocate along a rotation axis, and the timing for replacing the grinding wheel is determined based on the characteristic quantities of the vibration waveform of the drive current generated in response to the reciprocating motion of the grinding wheel at the time of spark-out.

[0012] In one embodiment, the feature quantity includes at least one of the amplitude, average value, and maximum value of the vibration waveform of the drive current.

[0013] In one embodiment, when the value of the feature quantity exceeds a predetermined value, the grinding wheel is replaced.

[0014] In one embodiment, the relationship between the feature quantity and the replacement timing of the grinding wheel is defined by a trained model generated by machine learning, which uses the drive current data collected at the time of spark-out for multiple grinding wheels as training data.

[0015] In one embodiment, the timing for replacing the grinding wheel is determined based on a plurality of parameters, including, in addition to the change over time of the drive current data at the time of spark-out, the number of workpieces that have been internally ground by the grinding wheel, and at least one of the total time of the internal grinding.

[0016] In one embodiment, the grinding wheel has diamond abrasive grains on its surface.

[0017] In a non-limiting and exemplary embodiment, the internal grinding system of the present invention is an internal grinding system for grinding the internal surface of a hollow cylindrical workpiece extending along a central axis, comprising: a first motor for rotating a grinding wheel around a rotation axis parallel to the central axis; a second motor for moving the grinding wheel in a cutting direction perpendicular to the rotation axis; a third motor for moving the grinding wheel in a traverse direction parallel to the rotation axis; a control device for controlling the operation of the first motor, the second motor and the third motor; and an ammeter for measuring the drive current of the second motor at the time of spark-out, when the movement of the grinding wheel by the second motor stops and the rotating grinding wheel grinds the internal surface of the hollow part of the workpiece, wherein the control device stores the drive current of the second motor at the time of spark-out in a memory device.

[0018] In one embodiment, the system further includes a maintenance information providing device that presents the user with information regarding the timing of replacement of the grinding wheel based on the drive current of the second motor during the spark-out event.

[0019] In one embodiment, the maintenance information providing device presents to the user the characteristic quantities of the vibration waveform of the drive current data generated in response to the reciprocating motion of the grinding wheel along the traverse direction during the spark-out.

[0020] In one embodiment, the maintenance information providing device determines the timing for replacing the grinding wheel based on the feature quantities.

[0021] In one embodiment, the feature quantity includes at least one of the amplitude, average value, and maximum value of the vibration waveform of the drive current.

[0022] In one embodiment, the maintenance information providing device presents the user with information regarding the replacement of the grinding wheel when the value of the feature quantity exceeds a predetermined value.

[0023] In some embodiments, the maintenance information providing apparatus determines the content of the information related to the replacement time of the grinding wheel or the time to present the information to the user, with respect to the relationship between the feature value and the replacement time of the grinding wheel, by a trained model generated by machine learning using the drive current data at the time of spark-out collected for a plurality of grinding wheels as learning data.

[0024] In some embodiments, the grinding wheel has diamond abrasive grains on its surface.

[0025] In a non-limiting exemplary embodiment, the method for manufacturing a ring-shaped component of the present invention includes a step of preparing a hollow cylindrical workpiece extending along the central axis direction, an inner surface grinding step of machining the inner surface of the hollow portion of the workpiece with a grinding wheel, and a step of obtaining the ring-shaped component from the workpiece. In the inner surface grinding step, the grinding wheel is rotated around a rotation axis parallel to the central axis by a first motor, the grinding wheel is moved in a cutting direction intersecting the rotation axis by a second motor, the inner surface of the hollow portion is pressed by the rotating grinding wheel, the movement of the grinding wheel by the second motor is stopped, and the drive current of the second motor is measured during spark-out in which the inner surface of the hollow portion is ground by the rotating grinding wheel.

[0026] In some embodiments, the replacement time of the grinding wheel is determined based on the measured value of the drive current.

[0027] In some embodiments, the workpiece is a rare-earth sintered ring magnet.

[0028] In some embodiments, the grinding wheel has diamond abrasive grains on its surface. Effects of the Invention

[0029] According to embodiments of the present invention, it is possible to provide a maintenance method for an internal grinding machine, an internal grinding system, and a method for manufacturing ring-shaped parts. Specifically, the timing for replacing the grinding wheel can be appropriately determined based on the current flowing through the cutting motor during spark-out in the internal grinding process. [Brief explanation of the drawing]

[0030] [Figure 1] This is a perspective view showing an example of the arrangement between a hollow cylindrical workpiece 10 and a grinding wheel 20 during internal grinding. [Figure 2A] This is a schematic top view showing an example of the configuration of the internal grinding machine 100 in this embodiment. [Figure 2B] This is a schematic side view showing an example of the configuration of the internal grinding machine 100 in this embodiment. [Figure 3] (a) to (d) are schematic cross-sectional views showing each stage of the internal grinding process. [Figure 4] (a) to (d) are other schematic cross-sectional views showing each stage of the internal grinding process. [Figure 5] (a) is a schematic cross-sectional view showing the stage at which spark-out begins in the internal grinding process, and (b) is a schematic cross-sectional view showing the stage at which spark-out is completed. [Figure 6] (a) is a schematic graph showing the relationship between traverse amount and time, and (b) is a schematic graph showing the relationship between cut amount and time. [Figure 7] This graph shows an example of the drive current of the second motor M2 when performing internal grinding on a single workpiece using a grinding wheel. [Figure 8] (a), (b), (c), and (d) are graphs showing examples of the drive current flowing through the second motor M2 when the number of workpieces internally ground with a grinding wheel is 1, 200, 5000, and 17000, respectively. [Figure 9](a), (b), (c), and (d) are enlarged graphs showing the drive current flowing through the second motor M2 during spark-out in Figures 8(a), (b), (c), and (d), respectively. [Figure 10A] This graph shows the correlation coefficient between the number of internal grinding operations and the statistical quantities (maximum value, range, σ) related to the drive current of the cutting axis motor (second motor M2). [Figure 10B] This graph shows the correlation coefficient between the number of internal grinding operations and the statistical quantities (maximum value, range, σ) related to the drive current of the grinding wheel spindle motor (first motor M2). [Figure 11] (a), (b), (c), and (d) are graphs showing examples of the drive current flowing through the first motor M1 when the number of workpieces that have been internally ground with a grinding wheel is 1, 200, 5000, and 17000, respectively. [Figure 12] (a), (b), (c), and (d) are graphs showing examples of the drive current flowing through the third motor M3 when the number of workpieces internally ground with a grinding wheel is 1, 200, 5000, and 17000, respectively. [Figure 13] This is a block diagram showing an example of the hardware configuration of a control device. [Modes for carrying out the invention]

[0031] Embodiments of the present invention will be described below. However, unnecessarily detailed descriptions may be omitted. For example, detailed descriptions of already well-known matters and redundant descriptions of substantially identical configurations may be omitted. This is to avoid the following description becoming unnecessarily verbose and to facilitate understanding by those skilled in the art. The inventors provide the accompanying drawings and the following description so that those skilled in the art can fully understand this disclosure, and not to limit the subject matter described in the claims. In the following description, components having the same or similar function are denoted by the same reference numerals.

[0032] The following embodiments are illustrative, and the technology of this disclosure is not limited to these embodiments. For example, the numerical values, shapes, materials, steps, order of steps, and display screen layouts shown in the following embodiments are merely examples, and various modifications are possible as long as they do not create a technical inconsistency. Furthermore, it is possible to combine one embodiment with another as long as it does not create a technical inconsistency.

[0033] First, a basic explanation of internal grinding will be given with reference to Figures 1, 2A, and 2B. Following this, a maintenance method for the internal grinding machine and an embodiment of the internal grinding system according to the present invention will be described. Finally, an example of performing internal grinding on a rare-earth sintered ring magnet will be described as an embodiment of the manufacturing method for ring-shaped parts according to the present invention.

[0034] Figure 1 is a perspective view showing an example of the arrangement between a hollow cylindrical workpiece 10 and a grinding wheel 20 during an internal grinding process. For reference, the figure shows mutually orthogonal X, Y, and Z axes.

[0035] In the illustrated example, the hollow cylindrical workpiece 10 is positioned with its central axis parallel to the Z-axis direction. As will be described later, the workpiece 10 is held on the spindle of an internal grinding machine. The grinding wheel 20 has an abrasive wheel 20A, which is made of a large number of abrasive grains formed from a high-hardness material such as diamond and electroplated by plating, and a rigid shaft 20B that supports the abrasive wheel 20A. The grinding wheel 20 is inserted inside the hollow cylinder of the workpiece 10 and performs internal grinding by rotating and pressing against the inner surface 12 of the hollow cylinder. The rotation axis of the grinding wheel 20 moves a predetermined amount little by little in the direction of "cutting" (X-axis direction) as internal grinding progresses. During internal grinding, grinding fluid is supplied to the machined surface for cooling, sludge removal, and to reduce the machining load by improving lubrication.

[0036] The radial size of the grinding wheel 20 must be smaller than the radial size (inner diameter) of the hollow cylinder of the workpiece 10. For example, if the inner diameter of the workpiece 10 is 20 mm, an abrasive wheel 20A with an outer diameter of about 16 mm can be used. In such a case, the outer diameter and length of the rigid shaft 20B may be, for example, about 15 mm and 100 mm, respectively. If the inner diameter of the workpiece 10 becomes smaller, it becomes necessary to use an even thinner rigid shaft 20B. For example, when performing internal grinding on a hard workpiece such as a rare-earth sintered ring magnet, the force acting on the grinding wheel 20 during internal grinding (machining load) is large, and the thin rigid shaft 20B will bend.

[0037] During internal grinding, the grinding wheel 20 may perform a "traverse" motion, reciprocating along a direction parallel to its axis of rotation (the Z-axis direction). This traverse motion ensures uniformity of the grinding amount in the Z-axis direction. The workpiece 10 itself also rotates around its central axis. Preferably, the rotation direction of the workpiece 10 and the rotation direction of the grinding wheel 20 are opposite.

[0038] The depth of cut of the grinding wheel 20, that is, the amount of movement of the grinding wheel 20 in the X-axis direction, defines the thickness of the area to be removed from the inner surface of the hollow cylinder of the workpiece 10. The position of the grinding wheel 20 in the X-axis direction when the cut is stopped is determined according to the target inner diameter dimension of the workpiece 10 after internal grinding.

[0039] Figure 2A is a schematic top view showing an example configuration of the internal grinding machine 100 in this embodiment, and Figure 2B is a schematic side view of the internal grinding machine 100. The internal grinding machine 100 is included in the internal grinding system 1000 according to this disclosure. The internal grinding system 1000 may also include a maintenance information providing device, which will be described later, in addition to the internal grinding machine 100.

[0040] A hollow cylindrical workpiece 10, which is the workpiece (object to be machined), is set in the internal grinding machine 100. In the illustrated example, the hollow cylindrical workpiece 10 is mounted in the internal grinding machine 100 so that its central axis is parallel to the Z-axis direction.

[0041] The internal grinding machine 100 includes a first motor M1, a second motor M2, and a third motor M3. The first motor M1 rotates the grinding wheel 20 around a rotation axis R1 parallel to the Z-axis direction. The second motor M2 moves the grinding wheel 20 in the cutting direction (X-axis direction) perpendicular to the rotation axis R1. The third motor M3 moves the grinding wheel 20 in the traverse direction (Z-axis direction) parallel to the rotation axis R1. The rotational motion of the output shafts of the second motor M2 and the third motor M3 is converted into linear motion by a device such as a ball screw. The positions and number of these motors M1 to M3 are not limited to the illustrated example. In particular, the second motor M2 and the third motor M3 may be positioned differently from the illustrated example depending on the structure of the device that converts rotational motion into linear motion.

[0042] The first motor M1, the second motor M2, and the third motor M3 are sometimes called the "grinding wheel axis motor," the "cutting axis motor," and the "traverse axis motor," respectively. The internal grinding machine 100 also includes a fourth motor M4 that can rotate the workpiece 10 around a rotation axis parallel to the Z axis. The workpiece 10 is fixed to the output shaft of the fourth motor M4 via a workpiece holder (workpiece holding device) 42 such as a collet chuck.

[0043] These motors M1 to M4 could each be, for example, a servo motor. A servo motor is an AC synchronous motor equipped with a rotational position detection sensor, such as a rotary encoder. A motor generally comprises a stator and a rotor. The torque of a motor is controlled by adjusting the magnitude of the current (drive current) flowing through the coils of the stator. In this embodiment, the rotational position, speed, and torque of each of the motors M1 to M4 can be controlled to desired values ​​by known control methods, using feedback of measured, estimated, or commanded values ​​of the drive current, rotational position, and rotational speed of each motor.

[0044] The internal grinding machine 100 includes a control device 60 that controls the operation of the first motor M1, the second motor M2, the third motor M3, and the fourth motor M4 (see Figure 2B). The control device 60 includes a computer, which will be described later, and is electrically connected to actuators other than motors M1 to M4, and various sensors (ammeters and position sensors). In Figure 2B, for convenience, the control device 60 is shown to be located inside the base 50, but the location of the control device 60 and other components is not limited to the illustrated example. Furthermore, some or all of the functions of the control device 60 may be performed by a computer or other device located separately from the internal grinding machine 100. In particular, operations such as saving current measurement values ​​and calculation processing necessary for maintenance, which will be described later, may be performed by a computer (a control device located outside the grinding machine) separate from the computer (control device for grinding operation) that the internal grinding machine 100 uses to perform normal grinding operations.

[0045] In this embodiment, the control device 60 includes a servo amplifier for driving motors M1 to M4. Although not shown, motor drive circuits that supply current to motors M1 to M4 are connected to each of motors M1 to M4 and control motors M1 to M4 according to command values ​​from the control device 60. Each of motors M1 to M4 may be a motor module that incorporates various electrical circuits such as a motor driver, DC-DC converter, and inverter.

[0046] A first motor M1, which rotates the grinding wheel 20 around a rotation axis R1 parallel to the Z-axis, is mounted on a cutting frame 24. In the illustrated example, the cutting frame 24 is supported on a traverse frame 32 so as to be able to move in the X-axis direction along a cutting axis rail 26X. The position of the cutting frame 24 in the X-axis direction is controlled by a second motor M2 on the traverse frame 32. The drive current of the second motor M2 can control the position, movement speed, and pressing force of the grinding wheel 20 in the X-axis direction against the inner surface of the hollow cylinder of the workpiece 10. The drive current of the second motor M2 may also be called the "cutting axis current".

[0047] The traverse movable frame 32 is supported on the base 50 so that it can move along the traverse axis rail 34Z in the Z-axis direction (traverse axis direction). The position of the traverse movable frame 32 in the Z-axis direction is controlled, for example, by a third motor M3 provided on the base 50. The drive current of the third motor M3 can control the position and movement speed of the grinding wheel 20 in the Z-axis direction. In Figure 2A, there are two third motors M3, but there may be only one third motor M3. The drive current of the third motor M3 may be called the "traverse axis current".

[0048] A fourth motor M4, which can rotate the workpiece 10 around a rotation axis parallel to the Z-axis, is supported by a workpiece frame 44 on a base 50. During internal grinding, the rotation of the workpiece 10 by the fourth motor M4 causes the position of the area in contact with the grinding wheel 20 on the inner surface of the hollow cylinder of the workpiece 10 to move, making it possible to grind the entire circumferential surface of the inner surface of the hollow cylinder of the workpiece 10. If the workpiece 10 is not rotated, it is necessary to perform a planetary motion to rotate the rotation axis R1 of the grinding wheel 20 around the central axis of the workpiece 10.

[0049] As will be described later, in the internal grinding process performed using the internal grinding machine 100 described above, the first motor M1 rotates the grinding wheel 20 around a rotation axis R1 parallel to the central axis, and the second motor M2 moves the grinding wheel 20 in the cutting direction (X-axis direction) intersecting the rotation axis R1. Then, the rotating grinding wheel 20 presses against the inner surface of the hollow part of the workpiece 10. After stopping the movement of the grinding wheel 20 by the second motor M2, a "spark-out" process is performed in which the inner surface of the hollow part is ground with the rotating grinding wheel 20. Details of the "spark-out" process will be described later.

[0050] In this embodiment, the internal grinding machine 100, or internal grinding system 1000, is equipped with an ammeter 28 that measures the drive current (cutting axis current) of the second motor M2 during the internal grinding process when the movement of the grinding wheel 20 by the second motor M2 stops and the rotating grinding wheel 20 grinds the inner surface of the hollow part of the workpiece 10 during a "spark-out" event. The control device 60 is configured to store the drive current of the second motor M2 during a spark-out event in the storage device 240. The display device 220 displays information such as changes in drive current and grinding wheel replacement as needed.

[0051] The storage device 240 and the display device 220 may also be located within the maintenance information provision device described later.

[0052] In the embodiments of this disclosure, the reason for measuring the drive current of the second motor M2 during spark-out is not for the feedback necessary for the motor control described above, but to obtain data or information necessary for the maintenance of the grinding wheel 20. Details of this point will be described later.

[0053] Next, the internal grinding process will be explained in detail with reference to Figures 3 to 6.

[0054] Figures 3(a) to 3(d) and 4(a) to 4(d) are other schematic cross-sectional views showing each stage of the internal grinding process.

[0055] First, as shown in Figure 3(a), the grinding wheel 20 is inserted into a position where it contacts the inner surface 12 of the workpiece 10. As shown in Figures 3(b) to 3(d), the grinding wheel 20, which rotates around a rotation axis parallel to the Z-axis, moves in the X-axis direction while performing a reciprocating (traverse) motion in the Z-axis direction. As mentioned above, the movement of the grinding wheel 20 in the X-axis direction is the "depth of cut," and the amount of movement defines the amount of cut. As internal grinding progresses, the amount of cut increases, and the inner diameter of the workpiece 10 expands.

[0056] As shown in Figures 4(a) to 4(d), when the rigid shaft 20B is deflected by the machining load acting on the grinding wheel 20 during internal grinding, the amount of grinding on the inner surface of the workpiece 10 differs depending on the position in the Z-axis direction (Z-coordinate value). As highlighted in Figures 4(c) and 4(d), the rigid shaft 20B deflects, and the inner diameter of the workpiece 10 decreases as the Z-coordinate value increases.

[0057] In this embodiment, to ensure that the inner diameter of the workpiece 10 has a constant value regardless of the Z coordinate value, after stopping the cutting, a "spark-out" is performed, in which internal grinding is continued while maintaining the position of the grinding wheel 20 in the X-axis direction. "Spark-out" refers to the state in which the cutting of the grinding wheel 20 is stopped because the inner diameter of the workpiece 10 has reached or approached the target dimension. However, strictly speaking, in the hollow cylinder of the workpiece 10, the inner diameter has reached the target dimension at the position close to the second motor M2 that rotates the grinding wheel 20, but has not reached the target dimension at the other end farther from the second motor M2. Here, at the position close to the second motor M2, if the sharpness of the grinding wheel has decreased, the inner diameter may be close to the target dimension but not reached. In other words, this means that there is a difference in the inner diameter dimension between the position close to the second motor M2 and the position farther away from it. This is because grinding progresses relatively quickly at the base of the grinding wheel 20 in proportion to the depth of cut, while grinding progresses more slowly near the tip of the grinding wheel 20 due to the elastic deflection of the rigid shaft 20B. However, if the depth of cut is stopped during spark-out, grinding of the remaining material continues, and the inner diameter of the workpiece 10 becomes the same within the tolerance range regardless of location. For this reason, spark-out is sometimes referred to as "zero cut".

[0058] Figure 5(a) is a schematic cross-sectional view showing the stage at which spark-out begins during the internal grinding process, and Figure 5(b) is a schematic cross-sectional view showing the stage at which spark-out is completed. Due to the elastic force of the rigid shaft 20B, the rigid shaft 20B attempts to return to a straight state against the machining load. The remaining machining portion of the workpiece 10 (the region where the inner diameter was relatively small) is ground during spark-out, and uniformity of the inner diameter dimension is achieved. At the stage shown in Figure 5(b), the deflection of the rigid shaft 20B is almost eliminated. In the stages of Figure 5(a) and Figure 5(b), the position of the base of the rigid shaft 20B in the X direction is the same.

[0059] Figure 6(a) is a schematic graph showing an example of the relationship between the traverse amount (Z-axis coordinate value) and time during internal grinding, and Figure 6(b) is a schematic graph showing the relationship between the depth of cut (X-direction coordinate value) and time. In the example in Figure 6(a), the traverse amount changes in a triangular wave shape as time progresses, but the traverse motion is not limited to this example. The traverse amount may change in a square wave, sinusoidal wave, or any other shape as time progresses.

[0060] As can be seen in Figure 6(b), the depth of cut (X-coordinate value) is kept constant during the spark-out period. Furthermore, in the illustrated example, the traverse motion is also performed during the spark-out period.

[0061] In this embodiment, the drive current of the second motor M2 is measured from the start to the completion of spark-out. The second motor M2 needs to generate a torque of a magnitude sufficient to counteract the machining load of internal grinding in order to maintain the depth of cut (X-coordinate value) of the rotating grinding wheel at a constant position. For this reason, the magnitude of the drive current of the second motor M2 fluctuates, reflecting the fluctuations in the machining load of internal grinding. The inventors have found that the change in machining load that occurs during the process in which the deflection of the grinding wheel 20 changes from the stage in Figure 5(a) to the stage in Figure 5(b) depends on the degree of wear and deterioration of the grinding wheel 20. They then conceived that by measuring the drive current of the second motor M2 during spark-out, it is possible to estimate the degree of wear and deterioration of the grinding wheel 20 and determine when to replace the grinding wheel 20.

[0062] Figure 7 is a graph showing an example of the drive current of the second motor M2 when internal grinding of a single workpiece using a certain grinding wheel. The horizontal axis shows the elapsed time from the start to the end of the internal grinding process, and the vertical axis shows the magnitude (measured value) of the drive current of the second motor M2. In this example, the internal grinding process takes approximately 26 seconds for the cut and approximately 15 seconds for the spark-out. The reason why the magnitude of the current is fluctuating is because traversing occurred, as shown in Figure 6(a). More specifically, the magnitude of the force (load) in the direction of the cut that the grinding wheel receives from the workpiece changes according to the change in the amount of traversing. As shown in Figure 5(a), the grinding wheel 20 flexes, so the inner diameter of the workpiece 10 tends to decrease as the coordinate position in the Z-axis direction increases. As a result, at positions where the amount of traversing is relatively large, the load that the grinding wheel 20 receives from the workpiece 10 increases. During spark-out, the depth of cut needs to be kept constant, and to achieve this, the second motor M2 needs to generate torque corresponding to the load the grinding wheel 20 receives from the workpiece 10. Since traverse movement also occurs during spark-out, the current flowing through the second motor M2 during spark-out will also increase or decrease in accordance with the vibration of the traverse amount.

[0063] Figures 8(a), (b), (c), and (d) are graphs showing examples of the drive current flowing through the second motor M2 when the number of workpieces internally ground by the grinding wheel 20 is 1, 200, 5000, and 17000, respectively.

[0064] Figures 9(a), (b), (c), and (d) are enlarged graphs showing the drive current flowing through the second motor M2 during spark-out in Figures 8(a), (b), (c), and (d), respectively.

[0065] As shown in Figures 8 and 9, it was found that the current flowing through the second motor M2 during spark-out depends on the degree of wear and deterioration of the grinding wheel 20. For example, the average and maximum values ​​(higher envelope) of the current flowing through the second motor M2 during spark-out decrease with the passage of grinding time (time shown in Figure 9) as the wear and deterioration of the grinding wheel 20 progresses. This is thought to be because the grinding load during spark-out changes sensitively depending on the wear and deterioration state of the grinding wheel 20. As mentioned above, even during spark-out, the second motor M2, which is the cutting axis motor, controls the position by pressing the workpiece 10 with the grinding wheel 20. For this reason, the second motor M2 needs to generate torque. The magnitude of this torque is proportional to the magnitude of the drive current of the second motor M2.

[0066] Comparing Figure 9(a) and Figure 9(b), no significant difference is observed in the current waveform. In contrast, comparing Figure 9(b) and Figure 9(c), it can be seen that the average and maximum values ​​(higher envelope) of the current waveform decrease with time elapsed since the start of internal machining. Furthermore, comparing Figure 9(c) and Figure 9(d), an increase in the amplitude of the current waveform's vibration is observed. This increase in amplitude is due to the deterioration of the grinding wheel, which leads to a larger change in the load on the motor during traverse. In this experimental example, it was confirmed that wear and deterioration of the grinding wheel were significant with approximately 17,000 workpieces being internally ground, and it was found that it is preferable to replace the grinding wheel when the current waveform in Figure 9(c) changes to the current waveform in Figure 9(d).

[0067] Figure 10A is a graph showing the correlation coefficient between the number of internal grinding operations and statistical quantities (maximum value, range, σ) related to the drive current of the cutting spindle motor (second motor M2). "Maximum value" is the maximum value of the drive current of motor M2, "Range" is the difference between the maximum and minimum values, and "σ" is the standard deviation. The correlation coefficient was calculated using known statistical analysis processing for both the cutting phase (between 2.5 seconds and 25 seconds after the start of machining) and the spark-out phase (between 27 seconds and 40 seconds after the start of machining). Figure 10B is a graph showing a similar correlation coefficient obtained for the drive current of the grinding wheel spindle motor (first motor M1).

[0068] As shown in Figure 10A, the correlation coefficient between the maximum current flowing through the cutting axis motor (second motor M2) at spark-out and the number of machining operations is approximately 0.72, which is a large value. Furthermore, the standard deviation σ does not exceed 0.5. This indicates that the maximum current flowing through the cutting axis motor (second motor M2) at spark-out is a parameter that reflects the increase in the number of machining operations and is an appropriate indicator for determining the replacement timing. On the other hand, as shown in Figure 10B, the correlation between the maximum current flowing through the grinding wheel spindle motor (first motor M1) at spark-out and the number of machining operations is also 0.62, which is a relatively high correlation. However, in the case of the grinding wheel spindle motor, the correlation coefficient between the standard deviation σ of the current flowing at spark-out and the number of machining operations is also a high value of approximately 0.75. This means that the fluctuation and variability of the current flowing through the grinding wheel spindle motor (first motor M1) at spark-out increases with the increase in the number of machining operations. Therefore, it is difficult to determine the timing for replacing the grinding wheel based on the change in current flowing through the motor of the grinding wheel shaft (first motor M1) when spark-out occurs.

[0069] Figures 11(a), (b), (c), and (d) are graphs showing examples of the drive current flowing through the first motor M1 that rotates the grinding wheel 20 when the number of workpieces internally ground by the grinding wheel 20 is 1, 200, 5000, and 17000, respectively.

[0070] From Figure 11, it can be observed that the waveform of the current flowing through the first motor M1 changes depending on the wear and deterioration state of the grinding wheel 20. However, according to the inventors' research, it was difficult to find a characteristic quantity that is effective in determining when to replace the grinding wheel 20.

[0071] Figures 12(a), (b), (c), and (d) are graphs showing examples of the drive current flowing through the third motor M3 that performs the traverse operation when the number of workpieces internally ground by the grinding wheel 20 is 1, 200, 5000, and 17000, respectively.

[0072] As can be seen from Figure 12, the waveform of the current flowing through the third motor M3 does not depend on the wear and deterioration state of the grinding wheel 20.

[0073] As is clear from the above experiments, it is possible to extract characteristic quantities indicating the wear and deterioration state of the grinding wheel from the current flowing through the second motor M2, which is the cutting shaft motor, when it sparks out. Based on these characteristics, it is possible to determine or estimate when the grinding wheel should be replaced.

[0074] The maintenance method for an internal grinding machine in the completed embodiment of the present invention includes the steps of: measuring the drive current of a motor that moves a rotating grinding wheel in the cutting direction at the time of spark-out and collecting drive current data at the time of spark-out; and determining the timing for replacing the grinding wheel based on the change in the drive current data at the time of spark-out over time.

[0075] As mentioned above, the maintenance method of this disclosure makes it easier to extract feature quantities such as waveform amplitude when the grinding wheel is subjected to a step of reciprocating motion along the rotation axis (traverse). It is considered that it will be easier to determine when to replace the grinding wheel based on the feature quantities of the vibration waveform of the drive current generated in response to the reciprocating motion of the grinding wheel during spark-out. As mentioned above, it is desirable that the feature quantities include at least one of the amplitude, average value, and maximum value of the vibration waveform of the drive current, in which case, for example, the grinding wheel should be replaced when the value of the feature quantity exceeds a predetermined value.

[0076] The relationship between the above features and the timing of grinding wheel replacement can be defined by a trained model generated by machine learning, which uses drive current data collected during spark-out for multiple grinding wheels as training data. The trained model generated by machine learning includes a general-purpose computer, a computer program that defines the machine learning model, and a set of parameter values ​​obtained through learning. Such a trained model performs predictive maintenance using features extracted from the current during spark-out.

[0077] In addition to the change in drive current data over time during spark-out, the timing for replacing the grinding wheel may be determined based on multiple parameters, including at least one of the number of workpieces internally ground by the grinding wheel and the total internal grinding time. For example, when both conditions are met—the number of workpieces internally ground exceeds 15,000 and the amplitude of the drive current data during spark-out increases beyond a predetermined value—it is possible to notify the user that it is time to replace the grinding wheel.

[0078] The internal grinding system 1000 in this embodiment may further include a maintenance information providing device that presents the user with information regarding the timing of replacement of the grinding wheel 20 based on the drive current of the second motor M2 at the time of spark-out. Preferably, this maintenance information providing device is configured to present the user with characteristic quantities of the vibration waveform of the drive current data generated in accordance with the reciprocating motion of the grinding wheel 20 along the traverse direction at the time of spark-out.

[0079] The maintenance information provider may be configured to determine the timing of grinding wheel replacement based on feature quantities. Preferably, the feature quantities include at least one of the amplitude, average value, and maximum value of the vibration waveform of the drive current. The maintenance information provider may be configured to present information regarding grinding wheel replacement to the user when the value of the feature quantity exceeds a predetermined value. Furthermore, the maintenance information provider may be configured to determine the content of the information regarding grinding wheel replacement timing or the timing of its presentation to the user based on a trained model generated by machine learning using drive current data collected from multiple grinding wheels at the time of spark-out as training data, relating the feature quantities to the timing of grinding wheel replacement.

[0080] If the control device 60 is implemented by a computer, the maintenance information provision device may also be implemented by the control device 60. Alternatively, the maintenance information provision device may be implemented by a server computer on the cloud, or it may be configured to display maintenance information on a mobile terminal owned by the user.

[0081] Figure 13 is a block diagram showing an example of the hardware configuration of the control device 60 in this embodiment. The control device 60 includes an input device 210, a display device 220, a communication interface 230, a storage device 240, a processor 250, a ROM (Read Only Memory) 260, and a RAM (Random Access Memory) 270. These components are interconnected via a bus 280.

[0082] The input device 210 is a device for converting user instructions into data and inputting it into a computer. The input device 210 may be, for example, a keyboard, mouse, or touch panel.

[0083] The communication interface 230 is an interface for data communication between the control unit 60 and external devices. For example, the communication interface 230 can perform wired communication compliant with USB, IEEE 1394 (registered trademark), or Ethernet (registered trademark). The communication interface 230 can perform wireless communication compliant with the Bluetooth (registered trademark) standard and / or the Wi-Fi (registered trademark) standard. Both standards include wireless communication standards that utilize the 2.4GHz frequency band.

[0084] The storage device 240 is, for example, a semiconductor memory, a magnetic storage device, or an optical storage device, or a combination thereof. An example of an optical storage device is an optical disc drive. An example of a magnetic storage device is a hard disk drive (HDD) or a magnetic tape recording device.

[0085] The processor 250 is one or more semiconductor integrated circuits, also known as a central processing unit (CPU) or microprocessor. The processor 250 sequentially executes a computer program stored in the ROM 260, which describes the set of instructions necessary for the grinding process, including internal grinding, to achieve the desired processing.

[0086] In this embodiment, the processor 250 of the control device 60 can function as a maintenance information provider by executing a predetermined algorithm based on a program. This maintenance information provider provides the user with information regarding the timing of replacement of the grinding wheel 20 based on the drive current of the second motor M2 at the time of spark-out. Note that the function of the maintenance information provider does not necessarily have to be realized by the control device 60 of the internal grinding machine 100. For example, an external computer (e.g., a personal computer, a mobile terminal device, a cloud computer) that acquires drive current data from the control device 60 via wired or wireless communication may function as part or all of the maintenance information provider.

[0087] If the maintenance information provision device is implemented by an external computer or the like, the storage of drive current data may be performed by the maintenance information provision device.

[0088] The term "Processor 250" is broadly interpreted to include FPGAs (Field Programmable Gate Arrays), GPUs (Graphic Processor Units), ASICs (Application Specific Integrated Circuits), or ASSPs (Application Specific Standard Products) equipped with a CPU.

[0089] ROM260 can be, for example, writable memory (e.g., PROM), rewritable memory (e.g., flash memory), or read-only memory. ROM260 stores programs that control the operation of the processor. ROM260 does not have to be a single recording medium; it can be a collection of multiple recording media. Some of these collections may be removable memory.

[0090] RAM270 provides a workspace for temporarily unpacking the control program stored in ROM260 during boot-up. RAM270 does not need to be a single recording medium; it can be a collection of multiple recording media.

[0091] A method for manufacturing a ring-shaped part can be carried out using an internal grinding system 1000 having the configuration described with reference to Figures 2A and 2B. The method for manufacturing a ring-shaped part according to this disclosure includes the steps of: preparing a hollow cylindrical workpiece extending along the central axis; an internal grinding step of processing the inner surface of the hollow portion of the workpiece with a grinding wheel; and obtaining a ring-shaped part from the workpiece. In the internal grinding step, a first motor rotates the grinding wheel around a rotation axis parallel to the central axis, a second motor moves the grinding wheel in a cutting direction intersecting the rotation axis, the rotating grinding wheel presses against the inner surface of the hollow portion, the movement of the grinding wheel by the second motor is stopped, and the drive current of the second motor is measured at the time of spark-out when the inner surface of the hollow portion is being ground with the rotating grinding wheel. Based on the measured value of the drive current, the timing for replacing the grinding wheel can be determined.

[0092] According to this method for manufacturing ring-shaped parts, when the workpiece is a rare-earth sintered ring magnet, rare-earth sintered ring magnets can be manufactured with good yield by using grinding wheels that are replaced at appropriate timings. [Explanation of Symbols]

[0093] 10.. Work 20... Grinding wheels 24. Cutting Mobile Stand 26X... Notched shaft rail 28...Ammeter 32...Traverse mobile platform 34Z...Traverse Axle Rail 42..Work holder 44. Work stand 50... base 60... Control device 220...Display device 240...Storage device 100···Internal grinding machine 1000...Internal grinding system R1... Rotation axis M1...First motor M2...Second motor M3...the third motor M4...the fourth motor

Claims

1. A maintenance method for an internal grinding machine, A step of measuring the drive current of a motor that moves a rotating grinding wheel in the cutting direction at the time of spark out, and collecting the drive current data at the time of spark out, A step of determining the replacement timing of the grinding wheel based on the change in the drive current data during the aforementioned spark-out, Maintenance methods for internal grinding machines, including those mentioned above.

2. The process includes causing the grinding wheel to perform a reciprocating motion along the axis of rotation, A method for maintaining an internal grinding machine according to claim 1, wherein the timing for replacing the grinding wheel is determined based on the characteristic quantities of the vibration waveform of the drive current generated in accordance with the reciprocating motion of the grinding wheel during the spark-out.

3. The maintenance method for an internal grinding machine according to claim 2, wherein the characteristic quantity includes at least one of the amplitude, average value, and maximum value of the vibration waveform of the drive current.

4. A method for maintaining an internal grinding machine according to claim 3, wherein the grinding wheel is replaced when the value of the feature exceeds a predetermined value.

5. The maintenance method for an internal grinding machine according to any one of claims 2 to 4, wherein the relationship between the feature quantity and the timing of replacement of the grinding wheel is defined by a trained model generated by machine learning using the drive current data at the time of spark out collected for a plurality of grinding wheels as training data.

6. A method for maintaining an internal grinding machine according to any one of claims 1 to 4, wherein the timing for replacing the grinding wheel is determined based on a plurality of parameters, including, in addition to the change over time of the drive current data at the time of spark out, the number of workpieces that have been internally ground by the grinding wheel, and at least one of the total time of internal grinding.

7. The method for maintaining an internal grinding machine according to any one of claims 1 to 4, wherein the grinding wheel has diamond abrasive grains on its surface.

8. An internal grinding system for grinding the inner surface of a hollow cylindrical workpiece that extends along the central axis, A first motor rotates the grinding wheel around a rotation axis parallel to the central axis, A second motor moves the grinding wheel in a cutting direction perpendicular to the rotation axis, A third motor moves the grinding wheel in a traverse direction parallel to the rotation axis, A control device for controlling the operation of the first motor, the second motor, and the third motor, When the movement of the grinding wheel by the second motor stops and the rotating grinding wheel grinds the inner surface of the hollow part of the workpiece during spark-out, an ammeter is used to measure the drive current of the second motor, Equipped with, The control device stores the drive current of the second motor at the time of the spark out in a storage device. An internal grinding system that determines the replacement timing of the grinding wheel based on the change in the drive current over time.

9. The internal grinding system according to claim 8, further comprising a maintenance information providing device that presents to the user information regarding the timing of replacement of the grinding wheel based on the drive current of the second motor during the spark-out.

10. The internal grinding system according to claim 9, wherein the maintenance information providing device presents to the user the characteristic quantities of the vibration waveform of the drive current data generated in accordance with the reciprocating motion of the grinding wheel along the traverse direction during the spark-out.

11. The internal grinding system according to claim 10, wherein the maintenance information providing device determines the timing of replacement of the grinding wheel based on the characteristic quantity.

12. The internal grinding system according to claim 10 or 11, wherein the feature quantity includes at least one of the amplitude, average value, and maximum value of the vibration waveform of the drive current.

13. The internal grinding system according to claim 12, wherein the maintenance information providing device presents the user with information regarding the replacement of the grinding wheel when the value of the feature quantity exceeds a predetermined value.

14. The internal grinding system according to claim 10 or 11, wherein the maintenance information providing device determines the content of information regarding the replacement timing of the grinding wheel or the timing to present it to the user, based on the relationship between the feature quantity and the replacement timing of the grinding wheel, using a trained model generated by machine learning, which uses the drive current data at the time of spark out collected for a plurality of grinding wheels as training data.

15. The internal grinding system according to any one of claims 8 to 11, wherein the grinding wheel has diamond abrasive grains on its surface.

16. A method for manufacturing a ring-shaped part, The process involves preparing a hollow cylindrical workpiece that extends along the central axis, An internal grinding process in which the inner surface of the hollow portion of the workpiece is processed with a grinding wheel, A step of obtaining a ring-shaped part from the aforementioned workpiece, Includes, In the aforementioned internal grinding process, The first motor rotates the grinding wheel around a rotation axis parallel to the central axis. The second motor moves the grinding wheel in a cutting direction intersecting the rotation axis, and the rotating grinding wheel presses against the inner surface of the hollow section. The movement of the grinding wheel by the second motor is stopped, and the drive current of the second motor is measured during the spark-out phase when the rotating grinding wheel grinds the inner surface of the hollow section. A method for manufacturing a ring-shaped component, which determines the replacement timing of the grinding wheel based on the change in the drive current over time.

17. The method for manufacturing a ring-shaped component according to claim 16, wherein the workpiece is a rare-earth sintered ring magnet.

18. The method for manufacturing a ring-shaped part according to claim 17, wherein the grinding wheel has diamond abrasive grains on its surface.

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