Workpiece centering method and machine tool

Abstract

To provide a centering method of a workpiece which enables more accurate centering without using skilled technique, and to provide a machine tool.SOLUTION: A workpiece W is held by a workpiece holding part 25 provided at a tip of a spindle 15. An eccentricity of the workpiece W is measured over an entire periphery of the workpiece W with the spindle 15 being rotated. The spindle 15 is rotated so that an impact application part 27 is located at a rotation angle corresponding to a maximum eccentricity of the workpiece W based on the eccentricity of the workpiece W. The impact application part 27 applies an impact to the workpiece W only for a time of 0.01 seconds or shorter per time.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a workpiece centering method and a machine tool.

Background Art

[0002] Conventionally, machine tools that perform turning processing or grinding processing while a workpiece rotates are known. In such machine tools, particularly when performing high-precision machining, after the workpiece is mounted on the spindle, a centering operation is required to align the center of the workpiece with the rotation center of the spindle. In particular, for machining that requires submicron geometric accuracy, centering of 1 μm or less is required.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As one of the main centering methods, there is a method using a high-precision workpiece holding portion. However, since such a method depends on the centripetal accuracy of the workpiece holding portion and the state of the gripping portion, in many cases only a centering accuracy of about several micrometers can be expected during mass production.

[0005] On the other hand, there also exists a method of manually aligning the core using a dial gauge or the like after gripping the workpiece until the alignment falls within an allowable value. However, when performing alignment of 1 micrometer or less, high skill is required for the timing and strength of the striking that is the alignment means, which tends to cause a reduction in work efficiency.

[0006] The present disclosure provides a workpiece centering method and a machine tool that are capable of performing higher-precision centering without using skilled techniques.

Means for Solving the Problems

[0007] Embodiments of this disclosure relate to the following [1] to [5].

[0008] [1] A method for centering a workpiece, comprising the steps of: gripping the workpiece with a workpiece holding part provided at the tip of a spindle; measuring the eccentricity of the workpiece around its entire circumference while rotating the spindle; rotating the spindle based on the eccentricity of the workpiece so that an impact application part is positioned at a rotation angle corresponding to the maximum eccentricity of the workpiece; and applying an impact to the workpiece with the impact application part for a time of 0.01 seconds or less per impact.

[0009] [2] The method for centering a workpiece according to [1], wherein the impact-applying unit impacts the workpiece a number of times corresponding to the maximum eccentricity.

[0010] [3] The method for centering a workpiece according to [1] or [2], wherein, after the step of applying an impact to the workpiece with the impact-applying unit, the step of measuring the eccentricity of the workpiece again around the entire circumference of the workpiece while rotating the main spindle.

[0011] [4] A machine tool comprising a spindle, a workpiece holding unit provided at the tip of the spindle for gripping a workpiece, an eccentricity measuring unit for measuring the eccentricity of the workpiece around its entire circumference while rotating the spindle, an impact applying unit for applying impact to the workpiece, and a control unit for controlling the rotation of the spindle and the impact applying unit, wherein the control unit rotates the spindle so that the impact applying unit is positioned at a rotation angle corresponding to the maximum eccentricity of the workpiece, based on the eccentricity of the workpiece measured by the eccentricity measuring unit, and the impact applying unit applies impact to the workpiece for a time of 0.01 seconds or less per impact.

[0012] [5] The machine tool according to [4], wherein the impact-applying unit impacts the workpiece a number of times corresponding to the maximum eccentricity. [Effects of the Invention]

[0013] According to this embodiment, more precise centering can be achieved without the need for skilled techniques. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a cross-sectional view showing a machine tool according to one embodiment. [Figure 2] Figure 2 is a schematic diagram showing a part of a machine tool according to one embodiment. [Figure 3] Figure 3 is a flowchart showing a method for centering a workpiece according to one embodiment. [Figure 4] Figure 4(a) is a graph showing the relationship between the amount of eccentricity during centering and the load in a workpiece centering method according to one embodiment, and Figure 4(b) is a graph showing the relationship between the amount of eccentricity during centering and the load in a workpiece centering method according to a comparative example. [Figure 5] Figures 5(a) and 5(b) are graphs showing the change in the maximum eccentricity of a workpiece when a workpiece alignment method according to one embodiment is implemented. [Modes for carrying out the invention]

[0015] The embodiments will be described in detail below with reference to the drawings. The following figures are schematic representations. Therefore, the size and shape of each part are exaggerated as appropriate to facilitate understanding. Furthermore, it is possible to modify and implement the designs as appropriate without departing from the technical concept. In the following figures, the same parts are denoted by the same reference numerals, and some detailed explanations may be omitted. In addition, the numerical values ​​such as dimensions and material names of each component described in this specification are examples of embodiments and are not limiting; they can be selected and used as appropriate. In this specification, terms that specify shapes and geometric conditions, such as parallel, orthogonal, and perpendicular, include not only their strict meanings but also substantially the same state. Also, for the convenience of explanation, the terms "up" or "down" may be used, but the up and down directions may be reversed.

[0016] Further, in the present embodiment, the "X-axis" refers to an axis that is perpendicular to the vertical direction and perpendicular to the rotation axis of the main spindle, and the "Y-axis" refers to an axis that is parallel to the vertical direction. The "Z-axis" refers to an axis that is parallel to the rotation axis of the main spindle.

[0017] Hereinafter, a machine tool according to an embodiment will be described with reference to the drawings. First, the configuration of the machine tool according to the present embodiment will be described. Figure 1 is a schematic diagram showing the machine tool according to the present embodiment.

[0018] As shown in Figure 1, the machine tool 10 according to the present embodiment is a machine that cuts a workpiece W with a tool 21. In this machine tool 10, the workpiece W rotates about a rotation axis Az parallel to the Z-axis, and the tool 21 abuts against the rotating workpiece W, whereby the workpiece W is machined. The tool 21 may be a turning tool such as a cutting bite.

[0019] The machine tool 10 includes a main spindle 15, a workpiece holding unit 25, an eccentricity measurement unit 26, an impact applying unit 27, and a control unit 30. The workpiece holding unit 25 is provided at the tip of the main spindle 15 and grips the workpiece W. The eccentricity measurement unit 26 measures the eccentricity of the workpiece W over the entire circumference of the workpiece W while rotating the main spindle 15. The impact applying unit 27 applies an impact to the workpiece W. The control unit 30 controls at least the rotation of the main spindle 15 and the impact applying unit 27.

[0020] The machine tool 10 further includes a machine body 11 that holds the tool 21 and the workpiece W. The machine body 11, for example, rotates the workpiece W about the rotation axis Az, and relatively moves the tool 21 and the workpiece W so as to move them toward and away from each other. The orientation of the rotation axis Az, the direction of relative movement between the tool 21 and the workpiece W, as well as the presence or absence of movement of the tool 21 and the workpiece W and the direction thereof, may be set as appropriate.

[0021] In the present embodiment, the workpiece W is rotated around the rotation axis Az parallel to the Z axis as described above. Further, the tool 21 and the workpiece W move relative to each other in the Z-axis direction and the X-axis direction. More specifically, the tool 21 moves in the X-axis direction, and the workpiece W moves in the Z-axis direction. Thereby, for example, the surface on the positive side in the Z-axis direction and / or the outer peripheral surface around the axis parallel to the Z axis of the workpiece W is cut.

[0022] The machine main body 11 has a base 12, a Z-axis moving table 13, and a headstock 14. The Z-axis moving table 13 is capable of linear movement (horizontal movement) on the base 12 in the Z-axis direction with high precision by an air slide or the like. The headstock 14 is mounted on the Z-axis moving table 13. The headstock 14 is provided with a main spindle 15 and a main spindle motor 16. The main spindle 15 is arranged parallel to the Z axis. The main spindle motor 16 generates a driving force that rotates the main spindle 15 around the rotation axis Az. A workpiece holding portion 25 is attached to the main spindle end of the main spindle 15 (the end portion on the side where machining is performed). The workpiece holding portion 25 grips the workpiece W at a position opposite to the main spindle 15. The Z-axis moving table 13 is driven in the Z-axis direction by a feed screw 17, and the feed screw 17 is rotationally driven by a Z-axis motor 18. With such a configuration, the workpiece W is rotated around the axis parallel to the Z axis by the main spindle 15 together with the workpiece holding portion 25. Further, the workpiece W moves in the Z-axis direction together with the workpiece holding portion 25.

[0023] In the direction facing the workpiece holding portion 25, an X-axis moving table 19 movable in the X-axis direction, and a tool post 22 mounted on the X-axis moving table 19 are provided. A tool 21 is attached to the tool post 22. The X-axis moving table 19 is driven in the X-axis direction by an X-axis motor having a feed screw and a rotation detector, in substantially the same manner as the Z-axis moving table 13. With such a configuration, the tool 21 moves in the X-axis direction together with the tool post 22.

[0024] The control unit 30 controls at least the rotation of the spindle 15 and the impact application unit 27. The control unit 30 may be configured to include, for example, an NC device and a driver (e.g., a servo driver) not shown. The control unit 30 controls the rotational speed of the spindle 15 (spindle motor 16). The control unit 30 further controls the speed and position of the Z-axis moving table 13 (Z-axis motor 18) and the X-axis moving table 19 (X-axis motor).

[0025] The control of the spindle 15 may be, for example, feedback control based on the detected value of a rotation detector (not shown) that detects the rotation of the spindle motor 16, or it may be open-loop control without feedback control. The control of the Z-axis mobile table 13 and the X-axis mobile table 19 is, for example, closed-loop control that performs feedback control based on the position detection of these mobile tables. However, depending on the accuracy required for the feed mechanism of the machine tool 10, it may be semi-closed-loop control that performs feedback control based on the detected value from the motor encoder, or it may be open-loop control without feedback control.

[0026] The machining accuracy of the machine tool 10 may be set as appropriate. For example, the machine tool 10 may be capable of machining at the nano-level. For example, the positioning accuracy of the Z-axis moving table 13 and the X-axis moving table 19 may be 0.1 nm or more and 1 nm or less. Also, the rotational speed accuracy of the spindle 15 may be 0.01 rpm. Examples of workpieces W to be cut by such an ultra-precision machining device include glass or resin lenses, or molds for forming lenses.

[0027] Referring to Figure 2, the configuration of the spindle 15 and the area around the workpiece W of the machine tool 10 will be further explained.

[0028] As described above, the machine tool 10 includes a spindle 15, a workpiece holding unit 25, an eccentricity measuring unit 26, an impact applying unit 27, and a control unit 30.

[0029] The spindle 15 is rotatable around the rotation axis Az and is also movable in the Z-axis direction by the Z-axis moving table 13. The spindle motor 16 (Figure 1) and the Z-axis motor 18 (Figure 1) are each connected to the control unit 30 via the CNC device 31 by a first control signal line 36. The CNC device 31 is a device that numerically controls the amount of movement and movement speed of the spindle 15 using a computer. Rotation commands and rotational position information are transmitted and received between the control unit 30 and the spindle motor 16. Position commands and position information are transmitted and received between the control unit 30 and the Z-axis motor 18.

[0030] The workpiece holding part 25 is provided at the tip of the spindle 15 and moves integrally with the spindle 15. The workpiece holding part 25 grips the workpiece W. It is desirable that the workpiece holding part 25 has a smooth suction surface 25a on the workpiece W side in order to stably grip the workpiece W. The workpiece holding part 25 may be a chuck such as a vacuum chuck that holds the workpiece W by sucking gas (e.g., air) from the surrounding area through the suction surface 25a. The workpiece holding part 25 holds the workpiece W by sucking gas with a vacuum device (not shown) to the suction surface 25a.

[0031] The workpiece W has a measuring surface Sa, an action surface Sb, and a surface to be attracted Sc. The measuring surface Sa is the surface for measuring the eccentricity by the eccentricity measuring unit 26. The action surface Sb is the surface to which impact for centering can be applied by the impact applying unit 27. The surface to be attracted Sc is the surface facing the workpiece holding unit 25. The surface to be attracted Sc is preferably a smooth surface in order to allow the workpiece W to be stably gripped by the workpiece holding unit 25.

[0032] The eccentricity measuring unit 26 measures the eccentricity of the workpiece W around its entire circumference while the main spindle 15, workpiece holding unit 25, and workpiece W are rotating. The eccentricity measuring unit 26 is positioned opposite the measuring surface Sa of the workpiece W. The eccentricity measuring unit 26 may be a displacement sensor. The displacement sensor may be a contact type or a non-contact type. The displacement sensor may be electrical or optical. It is desirable that the displacement sensor has a displacement output with little time delay as the workpiece W rotates, and sufficient resolution to confirm the required alignment accuracy. In this case, the eccentricity measuring unit 26 is located above the workpiece W in the vertical direction, but is not limited to this. The eccentricity measuring unit 26 may be located on the positive side or the negative side of the X-axis direction relative to the workpiece W.

[0033] The eccentricity measurement unit 26 is electrically connected to the control unit 30 by a second control signal line 37. The eccentricity data of the workpiece W measured by the eccentricity measurement unit 26 is sent to the control unit 30 by the second control signal line 37.

[0034] The impact-applying unit 27 applies impact to the working surface Sb of the workpiece W. The impact-applying unit 27 may be, for example, an impact hammer. The impact-applying unit 27 has the ability to apply impact in a short time of 0.01 seconds or less. The lower limit of the time for which the impact-applying unit 27 applies impact is not particularly limited, but may be, for example, 0.001 seconds or more. Furthermore, it is desirable that the impact-applying unit 27 has the ability to apply impact with an amplitude of 0.01 μm or less. The lower limit of the amplitude for which the impact-applying unit 27 applies impact is not particularly limited, but may be, for example, 0.001 μm or more.

[0035] The impact application unit 27 is connected to the control unit 30 via a third control signal line 38 through the I / O controller 32 and the piezo controller 33. The I / O controller 32 receives input signals from the control unit 30 and transmits control signals to the piezo controller 33. The piezo controller 33 is a controller for driving and controlling the impact application unit 27. The piezo controller 33 drives and controls the impact application unit 27 according to the control signals input from the I / O controller 32. The piezo controller 33 may, for example, apply a drive voltage with a waveform and pulse width corresponding to a pulse signal to the piezo element of the impact application unit 27. The control unit 30 sets the number of impacts and the impact amplitude by the impact application unit 27 and sends an impact start signal to the impact application unit 27.

[0036] Next, the operation of this embodiment (workpiece centering method) will be described.

[0037] First, the workpiece W is gripped by the workpiece holding portion 25 provided at the tip of the main spindle 15 (step S1 in Figure 3). At this time, the suction surface 25a of the workpiece holding portion 25 comes into contact with the suction surface Sc of the workpiece W, and the workpiece W is stably gripped.

[0038] Next, the control unit 30 rotates the spindle 15 (step S2 in Figure 3). During this time, the control unit 30 transmits a rotation command to the spindle motor 16 via the first control signal line 36 and the CNC device 31. Upon receiving the rotation command, the spindle motor 16 rotates the spindle 15, and the workpiece W held by the workpiece holding unit 25 also rotates. While the spindle 15 is rotating, rotation position information is transmitted and received from the spindle motor 16 to the control unit 30. The rotation position information includes the current rotation angle of the spindle 15 (spindle indexing angle). The control unit 30 receives the rotation position information of the spindle 15.

[0039] Next, the control unit 30 measures the eccentricity of the workpiece W around its entire circumference while rotating the spindle 15 (step S3 in Figure 3). During this time, the eccentricity measurement unit 26 measures the eccentricity of the rotating workpiece W. Specifically, the eccentricity measurement unit 26 measures the measurement surface Sa of the rotating workpiece W with or without contact to determine the eccentricity of the workpiece W. The eccentricity of the workpiece W refers to the distance from the rotation axis Az of the spindle 15 to each measurement surface Sa of the workpiece W. The eccentricity measurement unit 26 transmits the measured eccentricity to the control unit 30 via the second control signal line 37.

[0040] Next, the control unit 30 rotates the spindle 15 so that the impact application unit 27 is positioned at a rotation angle (spindle indexing angle) of the spindle 15 corresponding to the maximum eccentricity of the workpiece W, based on the eccentricity of the workpiece W (step S4 in Figure 3). Specifically, the control unit 30 transmits a rotation command to the spindle motor 16 via the first control signal line 36 and the CNC device 31. This rotation command includes a rotation angle of the spindle 15 such that the impact application unit 27 faces the position where the eccentricity of the workpiece W is maximum. The maximum eccentricity of the workpiece W refers to the difference in distance between the part of the measuring surface Sa of the workpiece W that is furthest from the rotation axis Az of the spindle 15 and the part that is closest to it.

[0041] Next, the control unit 30 issues a command to the impact application unit 27 to apply an impact (step S5 in Figure 3). During this time, the control unit 30 sets the number of impacts and the impact amplitude by the impact application unit 27 and transmits an impact start signal. The impact start signal is transmitted to the impact application unit 27 via the third control signal line 38 through the I / O controller 32 and the piezo controller 33. Upon receiving the impact start signal, the impact application unit 27 applies an impact to the working surface Sb of the workpiece W. The impact application unit 27 physically impacts the working surface Sb of the workpiece W one or a predetermined number of times. As a result, the workpiece W moves a small distance in the direction in which the working surface Sb approaches the rotation axis Az of the main spindle 15. Therefore, the maximum eccentricity of the workpiece W is reduced. The number of times the workpiece W is impacted can be calculated and determined by the control unit 30 according to the maximum eccentricity of the workpiece W.

[0042] The impact application unit 27 applies impact for a very short time, less than 0.01 seconds. By applying impact to the working surface Sb of the workpiece W for a very short time, less than 0.01 seconds, the undesirable effects of the stick-slip phenomenon can be minimized. This makes it possible to achieve precise alignment of, for example, less than 1 μm.

[0043] Subsequently, the control unit 30 rotates the spindle 15 and again measures the eccentricity of the workpiece W around its entire circumference (step S6 in Figure 3). During this time, the eccentricity measuring unit 26 measures the eccentricity of the rotating workpiece W. The eccentricity measuring unit 26 transmits the measured eccentricity to the control unit 30 via the second control signal line 37. The control unit 30 determines the maximum eccentricity of the workpiece W. If the maximum eccentricity of the workpiece W exceeds the target value, the control unit 30 rotates the spindle 15 again so that the impact applying unit 27 is positioned at a rotation angle corresponding to the maximum eccentricity of the workpiece (returning to step S4 in Figure 3). If the maximum eccentricity of the workpiece W is less than or equal to the target value, the centering operation is completed.

[0044] Next, we will explain why the impact-applying unit 27 enables minute alignment by applying impact to the working surface Sb of the workpiece W for a short time of 0.01 seconds or less.

[0045] Generally, when centering is performed using a static load, if the frictional force between the workpiece W and the workpiece holder 25 is less than or equal to the maximum static frictional force, the workpiece W will not move even if a load is applied to it. On the other hand, if the load on the workpiece W is gradually increased and exceeds the maximum static frictional force, it enters the dynamic friction region. At this time, the frictional resistance between the workpiece W and the workpiece holder 25 decreases rapidly, causing the workpiece W to move more than necessary (stick-slip phenomenon). In contrast, in this embodiment, it is possible to apply a load to the workpiece W that exceeds the maximum static frictional force and to apply an impact to the workpiece W for a sufficiently short time. This makes it possible to move the workpiece W minutely and perform minute centering.

[0046] The above effects will be further explained with reference to Figures 4(a) and 4(b).

[0047] Figure 4(a) is a graph showing the relationship between eccentricity and load during centering according to this embodiment. In Figure 4(a), the period from time T=t0 to t1 is the state before the centering operation is performed. During this time, the control unit 30 measures the eccentricity of the workpiece W around its entire circumference while rotating the spindle 15. The maximum eccentricity at this time is defined as δ1. Next, the control unit 30 rotates the spindle 15 so that the impact application unit 27 is positioned at the rotation angle of the spindle 15 (spindle indexing angle) corresponding to the maximum eccentricity δ1 of the workpiece W.

[0048] Next, the period from time T=t1 to t2 represents the state during the first centering operation. During this time, the control unit 30 issues a command to the impact application unit 27 to apply an impact. The impact application unit 27 also applies an impact to the working surface Sb of the workpiece W a predetermined number of times (four times in this example). The load applied by the impact application unit 27 exceeds the maximum static friction force between the workpiece W and the workpiece holding unit 25. At this time, the time for which the impact application unit 27 applies the impact is a short time of 0.01 seconds or less. In this case, because the time for which the impact application unit 27 applies the impact to the workpiece W is sufficiently short, even if the load applied by the impact application unit 27 exceeds the maximum static friction force, the workpiece W does not move more than necessary.

[0049] Next, the eccentricity of the workpiece W is measured between times T=t2 and t3. During this time, the control unit 30 rotates the spindle 15 and again measures the eccentricity of the workpiece W around its entire circumference. The maximum eccentricity at this time is denoted as δ2. The maximum eccentricity δ2 is smaller than the maximum eccentricity δ1 (δ1 > δ2). Then, the control unit 30 rotates the spindle 15 so that the impact application unit 27 is positioned at the rotation angle of the spindle 15 (spindle indexing angle) corresponding to the maximum eccentricity δ2 of the workpiece W.

[0050] Next, the second centering operation is performed between times T=t3 and t4. During this time, the control unit 30 issues a command to the impact application unit 27 to apply an impact. The impact application unit 27 also applies an impact to the working surface Sb of the workpiece W a predetermined number of times (twice in this example). The load applied by the impact application unit 27 exceeds the maximum static friction force between the workpiece W and the workpiece holding unit 25. At this time, the time for which the impact application unit 27 applies the impact is a short time of 0.01 seconds or less. In this case, because the time for which the impact application unit 27 applies the impact to the workpiece W is sufficiently short, even if the load applied by the impact application unit 27 exceeds the maximum static friction force, the workpiece W does not move more than necessary.

[0051] Next, the eccentricity of the workpiece W is measured between times T=t4 and t5. During this time, the control unit 30 rotates the spindle 15 and again measures the eccentricity of the workpiece W around its entire circumference. The maximum eccentricity at this time is defined as δ3. The maximum eccentricity δ3 is smaller than the maximum eccentricity δ2 (δ2 > δ3).

[0052] Thus, according to this embodiment, by sufficiently shortening the time that the impact-applying unit 27 applies impact, it becomes possible to adjust the workpiece W by moving it in very small increments. For this reason, adjustment of, for example, 1 μm or less can be easily achieved.

[0053] Figure 4(b) is a graph showing the relationship between eccentricity during centering and load in a comparative example. The comparative example shown in Figure 4(b) is substantially the same as the example shown in Figure 4(a), except that the impact application time of the impact application unit 27 is longer than in this embodiment (for example, about 0.1 seconds or more).

[0054] In Figure 4(b), the period from time T=t1 to t2 shows the state during the first centering operation. During this time, the impact unit 27 applies an impact to the working surface Sb of the workpiece W only once. The load applied by the impact unit 27 exceeds the maximum static friction force between the workpiece W and the workpiece holding unit 25. In this case, the impact time applied by the impact unit 27 is long, 0.1 seconds or more. In this case, because the impact time is long, the workpiece W moves significantly with a single impact from the impact unit 27.

[0055] Next, the eccentricity of the workpiece W is measured between time T=t2 and t3. The maximum eccentricity δ4 at this time is smaller than the maximum eccentricity δ1 mentioned above (δ1 > δ4).

[0056] Next, a second centering operation is performed between times T=t3 and t4. During this time, the impact application unit 27 applies an impact to the working surface Sb of the workpiece W only once. The load applied by the impact application unit 27 exceeds the maximum static friction force between the workpiece W and the workpiece holding unit 25. In this case, the impact application time by the impact application unit 27 is long, 0.1 seconds or more. In this case, because the impact application time is long, the workpiece W moves significantly with a single impact from the impact application unit 27.

[0057] Next, the eccentricity of the workpiece W is measured between times T=t4 and t5. The maximum eccentricity δ5 at this time is greater than the maximum eccentricity δ4 (δ4 < δ5). This is because, during the second centering operation, the impact application unit 27 applied impact to the workpiece W for a long time, causing the workpiece W to move more than necessary.

[0058] Thus, in the comparative example, because the impact-applying unit 27 applies impact for a long time, the workpiece W moves significantly with a single impact. For this reason, it is difficult to perform alignment of, for example, 1 μm or less, or a long time is required until alignment is completed.

[0059] Next, with reference to Figures 5(a) and 5(b), a specific example of actually performing the centering work according to this embodiment will be described.

[0060] First, as shown in Figure 5(a), before the centering operation (N=0), the maximum eccentricity (PV) of the workpiece W was 144.6 μm. Next, the centering operation (rough centering operation) according to this embodiment was performed twice (N=1, 2). As a result, the maximum eccentricity of the workpiece W decreased sequentially to 66.7 μm (N=1) and 24.0 μm (N=2).

[0061] Next, as shown in Figure 5(b), the centering operation (precise centering operation) according to this embodiment was performed five times (N=3, 4, 5, 6, 7). As a result, the maximum eccentricity of the workpiece W gradually decreased to 8.3 μm (N=3), 6.9 μm (N=4), 3.9 μm (N=5), 2.4 μm (N=6), and 1.6 μm (N=7).

[0062] As described above, according to this embodiment, the impact application unit 27 applies impact to the workpiece W for a time of 0.01 seconds or less per impact. In this way, because the time for which the impact application unit 27 applies impact to the workpiece W is sufficiently short, even if the load applied by the impact application unit 27 exceeds the maximum static friction force, the workpiece W does not move more than necessary. As a result, it becomes possible to center the workpiece W by moving it in very small increments, and high-precision centering (e.g., 1 μm or less) can be easily achieved. As a result, it becomes possible to center the workpiece W automatically without using skilled techniques.

[0063] Furthermore, according to this embodiment, the impact-applying unit 27 applies impact to the workpiece W a number of times corresponding to the maximum eccentricity of the workpiece W. This allows for fine adjustment of the amount of movement of the workpiece W, and suppresses the phenomenon of the eccentricity of the workpiece W fluctuating up and down (hunting) each time the impact-applying unit 27 applies impact.

[0064] Furthermore, according to this embodiment, after the impact application unit 27 applies an impact to the workpiece W, the eccentricity of the workpiece W is measured again around the entire circumference of the workpiece W while the spindle 15 is rotated. This makes it possible to confirm whether or not the maximum eccentricity of the workpiece W has reached or fallen below the target value.

[0065] The multiple components disclosed in the above embodiments and modifications can be combined as needed. Alternatively, some components may be removed from all the components shown in the above embodiments and modifications. [Explanation of symbols]

[0066] 10 Machine tools 11. Main body of the machine 12 base 13 Z-axis moving table 14 Headstock 15 Spindle 16. Main shaft motor 17 Lead screw 18 Z-axis motor 19 X-axis moving table 21 Tools 22. Tool rest 25 Workpiece holding section 26 Eccentricity measuring section 27 Impact-generating part 30 Control Unit

Claims

1. In the method for centering a workpiece, The process involves gripping the workpiece with a workpiece holding part provided at the tip of the main spindle, A step of measuring the eccentricity of the workpiece around its entire circumference while rotating the main spindle, A step of rotating the spindle so that the impact application part is positioned at a rotation angle corresponding to the maximum eccentricity of the workpiece, based on the eccentricity of the workpiece. The impact application unit comprises the step of applying an impact to the workpiece with an amplitude of 0.01 μm or less per impact for a duration of 0.01 seconds or less, A method for centering a workpiece, wherein the load applied to the workpiece by the impact-applying part exceeds the maximum static friction force between the workpiece and the workpiece holding part.

2. The method for centering a workpiece according to claim 1, wherein the impact-applying unit applies impact to the workpiece a number of times corresponding to the maximum eccentricity.

3. The method for centering a workpiece according to claim 1, wherein, after the step of applying impact to the workpiece with the impact-applying unit, the eccentricity of the workpiece is measured again around the entire circumference of the workpiece while rotating the main spindle.

4. The main shaft, A workpiece holding part is provided at the tip of the main spindle and grips the workpiece, An eccentricity measuring unit that measures the eccentricity of the workpiece around its entire circumference while rotating the main spindle, An impact application unit that applies impact to the workpiece, The system includes a control unit that controls the rotation of the main shaft and the impact application unit, The control unit rotates the main shaft so that the impact application unit is positioned at a rotation angle corresponding to the maximum eccentricity of the workpiece, based on the eccentricity of the workpiece measured by the eccentricity measuring unit. The impact application unit applies an impact to the workpiece with an amplitude of 0.01 μm or less per impact for a duration of 0.01 seconds or less. A machine tool in which the load applied by the impact-applying part to the workpiece exceeds the maximum static friction force between the workpiece and the workpiece holding part.

5. The machine tool according to claim 4, wherein the impact-applying unit applies impact to the workpiece a number of times corresponding to the maximum eccentricity.

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