Workpiece centering method and machine tool

Abstract

To provide a workpiece centering method and a machine tool capable of performing highly accurate centering without using a skilled technique, in particular, when a center position of a main spindle and a center position of a workpiece are offset.SOLUTION: A workpiece W is held by a workpiece holding portion 25 provided at a tip of a spindle 15. The amount of eccentricity of the workpiece W is measured for the entire circumference of the workpiece W while rotating the spindle 15, and an eccentric position of the workpiece W is measured while rotating the spindle 15. The workpiece W is positioned based on the amount of eccentricity of the workpiece W and the eccentric position of the workpiece W. An impact applying unit 27 applies impact to the positioned workpiece W.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] Conventionally, machine tools are known that perform turning or grinding while the workpiece is rotating. When performing particularly high-precision machining with such machine tools, after the workpiece is attached to the spindle, centering is required to align the center of the workpiece with the center of rotation of the spindle. In particular, centering to within 1 μm is necessary for machining that requires submicron geometric accuracy. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-82242 Summary of the Invention [Problem to be solved by the invention]

[0004] One of the main centering methods is to use a highly accurate workpiece holder. However, this method depends on the centering accuracy of the workpiece holder and the condition of the gripping part, so in mass production, centering accuracy of only a few microns can often be expected.

[0005] On the other hand, there is also a method in which after gripping the workpiece, manual alignment is performed using a dial gauge or the like until it is within the tolerance. However, when aligning to an accuracy of less than 1 micron, a high level of skill is required for the timing and strength of the impact, which is the alignment means, and this can easily lead to a decrease in work efficiency.

[0006] In more complex machining, the center of the spindle may be offset from the center of the workpiece. In this case, the probe reading must be adjusted so that it reaches a maximum when the workpiece rotation angle reaches a specific value, and the difference between the maximum and minimum readings of the probe approaches a desired value. However, performing such a process manually is particularly burdensome.

[0007] The present disclosure provides a workpiece centering method and machine tool that are capable of performing high-precision centering without the use of skilled techniques, particularly when the center position of the spindle and the center position of the workpiece are offset. [Means for solving the problem]

[0008] The embodiments of the present disclosure relate to the following [1] to [6].

[0009] [1] A method for centering a workpiece, comprising the steps of: gripping a workpiece with a workpiece holding portion provided at the tip of a spindle; measuring the eccentricity of the workpiece around the entire circumference while rotating the spindle; measuring the eccentric position of the workpiece while rotating the spindle; positioning the workpiece based on the eccentricity and eccentric position of the workpiece; and applying an impact to the positioned workpiece with an impact applying portion.

[0010] [2] The method for centering a workpiece according to [1], wherein the impact applying unit applies impact to the workpiece a number of times corresponding to the maximum eccentricity of the workpiece.

[0011] [3] A method for centering a workpiece according to [1] or [2], wherein after the step of the impact applying unit applying an impact to the workpiece, a step of measuring the eccentricity of the workpiece again around the entire circumference of the workpiece while rotating the main shaft is provided.

[0012] [4] A machine tool comprising: a spindle; a workpiece holding unit provided at the tip of the spindle and configured to grip a workpiece; an eccentricity measuring unit configured to measure the eccentricity of the workpiece around the entire circumference while rotating the spindle; a mark detecting unit configured to detect marks applied to the workpiece in order to measure the eccentric position of the workpiece; an impact applying unit configured to apply an impact to the workpiece; and a control unit configured to control the rotation of the spindle and the impact applying unit, wherein the control unit calculates the eccentric position and the amount of eccentricity of the workpiece based on the eccentricity of the workpiece measured by the eccentricity measuring unit and the position of the mark measured by the mark detecting unit, and the control unit performs a rotational positioning of the spindle based on the eccentric position and the eccentricity of the workpiece so that the impact applying unit is located at a rotation angle corresponding to the maximum eccentric position of the workpiece, and causes the impact applying unit to apply an impact to the workpiece.

[0013] [5] The machine tool according to [4], wherein the impact applying unit applies impact to the workpiece a number of times according to the maximum eccentricity of the workpiece.

[0014] [6] The machine tool according to [4], wherein the eccentricity measuring unit, the mark detecting unit, and the impact applying unit move linearly by a linearly moving member. [Effects of the Invention]

[0015] According to this embodiment, even when the center position of the spindle and the center position of the workpiece are offset, highly accurate centering can be performed without using skilled techniques. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view showing a machine tool according to an embodiment. [Figure 2] FIG. 2 is a schematic configuration diagram showing a part of a machine tool according to one embodiment. [Figure 3] 3(a) and 3(b) are views of the workpiece as seen from the direction of the rotation axis of the spindle. [Figure 4]FIG. 4 is a flowchart showing a workpiece centering method according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] Each embodiment will be described in detail below with reference to the drawings. The figures shown below are schematic illustrations. Therefore, the size and shape of each part are appropriately exaggerated for ease of understanding. Furthermore, appropriate modifications can be made within the scope of the technical concept. In the figures shown below, identical parts are denoted by the same reference numerals, and some detailed descriptions may be omitted. Furthermore, the numerical values, such as dimensions, and material names of each component described in this specification are examples of embodiments and are not limited thereto. They may be selected and used as appropriate. In this specification, terms specifying shapes or geometric conditions, such as parallel, orthogonal, and vertical, are used in their strict sense and also include substantially the same state. For convenience of explanation, the terms "upper" and "lower" may be used, but the up-down direction may be reversed.

[0018] In this embodiment, the "X-axis" is an axis perpendicular to the vertical direction and perpendicular to the rotation axis of the main shaft, the "Y-axis" is an axis parallel to the vertical direction, and the "Z-axis" is an axis parallel to the rotation axis of the main shaft.

[0019] A machine tool according to an embodiment will be described below with reference to the drawings. First, the configuration of the machine tool according to this embodiment will be described. Fig. 1 is a schematic diagram showing the machine tool according to this embodiment.

[0020] As shown in Fig. 1, machine tool 10 according to this embodiment is a machine that cuts a workpiece W using a tool 21. In this machine tool 10, the workpiece W rotates about a rotation axis Az parallel to the Z axis, and tool 21 comes into contact with the rotating workpiece W, thereby cutting the workpiece W. Tool 21 may be a turning tool such as a turning tool.

[0021] Machine tool 10 includes spindle 15, workpiece holding unit 25, mark detection unit 24, eccentricity measurement unit 26, impact application unit 27, and control unit 30. Workpiece holding unit 25 is provided at the tip of spindle 15 and grips workpiece W. Eccentricity measurement unit 26 measures the eccentricity of workpiece W over the entire circumference of workpiece W while rotating spindle 15. Mark detection unit 24 detects mark M applied to workpiece W in order to measure the eccentric position of workpiece W. Impact application unit 27 applies an impact to workpiece W. Control unit 30 controls at least the rotation of spindle 15 and impact application unit 27.

[0022] The machine tool 10 further includes a machine body 11 that holds a tool 21 and a workpiece W. The machine body 11, for example, rotates the workpiece W about a rotation axis Az and moves the tool 21 and the workpiece W relatively so as to move them closer to and away from each other. The orientation of the rotation axis Az, the direction of the relative movement of the tool 21 and the workpiece W, and whether or not the tool 21 and the workpiece W move and the direction of movement thereof may be set as appropriate.

[0023] In this embodiment, as described above, the workpiece W is rotated around the rotation axis Az parallel to the Z axis. Furthermore, the tool 21 and the workpiece W move relatively 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. As a result, the workpiece W is cut, for example, on the surface on the positive side in the Z axis direction and / or on the outer peripheral surface around an axis parallel to the Z axis.

[0024] The machine 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 (horizontal) movement in the Z-axis direction on the base 12 with high precision using 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 about the rotation axis Az. A workpiece holder 25 is attached to the main spindle end (the end on the side where machining is performed) of the main spindle 15. The workpiece holder 25 grips a workpiece W at a position opposite 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 driven to rotate by a Z-axis motor 18. With this configuration, the workpiece W is rotated around an axis parallel to the Z axis by the spindle 15 together with the workpiece holder 25. The workpiece W also moves together with the workpiece holder 25 in the Z axis direction.

[0025] In the direction facing the workpiece holder 25, there are provided an X-axis movable table 19 that is movable in the X-axis direction, and a tool rest 22 mounted on the X-axis movable table 19. A tool 21 is attached to the tool rest 22. The X-axis movable table 19 is driven in the X-axis direction by an X-axis motor that has a feed screw and a rotation detector, similar to the Z-axis movable table 13. With this configuration, the tool 21 moves in the X-axis direction together with the tool rest 22.

[0026] The control unit 30 controls at least the rotation of the spindle 15 and the impact applying 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 rotation speed of the spindle 15 (spindle motor 16). The control unit 30 also controls the speed and position of the Z-axis moving stage 13 (Z-axis motor 18) and the speed and position of the X-axis moving stage 19 (X-axis motor).

[0027] Control of spindle 15 may be, for example, feedback control based on the detection value of a rotation detector (not shown) that detects the rotation of spindle motor 16, or open loop control without feedback control. Control of Z-axis movable table 13 and X-axis movable table 19 is, for example, closed loop control that performs feedback control based on position detection of these movable tables. However, depending on the accuracy required of the feed mechanism of machine tool 10, they may be semi-closed loop control that performs feedback control based on the detection value from the motor encoder, or open loop control without feedback control.

[0028] The machining accuracy of machine tool 10 may be set appropriately. For example, machine tool 10 may be capable of nano-level machining. For example, the positioning accuracy of Z-axis moving stage 13 and X-axis moving stage 19 may be set to 0.1 nm or more and 1 nm or less. Furthermore, the rotation speed accuracy of spindle 15 may be set to 0.01 rpm. Examples of workpieces W that are cut by such ultra-precision machining equipment include glass or resin lenses, or molds for molding lenses.

[0029] The configuration around the spindle 15 of the machine tool 10 and the workpiece W will be further described with reference to FIG.

[0030] As described above, machine tool 10 includes spindle 15, workpiece holding unit 25, eccentricity measuring unit 26, mark detecting unit 24, impact applying unit 27, and control unit 30.

[0031] The spindle 15 is rotatable about a rotation axis Az and is movable in the Z-axis direction by a Z-axis moving stage 13 (Fig. 1). The spindle motor 16 (Fig. 1) and the Z-axis motor 18 (Fig. 1) are each connected to a control unit 30 via a CNC device 31 by a first control signal line 36. The CNC device 31 is a device that numerically controls the movement amount, movement speed, etc. of the spindle 15 using a computer. Rotation commands and rotation 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.

[0032] The workpiece holding unit 25 is provided at the tip of the spindle 15 and moves integrally with the spindle 15. The workpiece holding unit 25 grips the workpiece W. It is desirable that the workpiece holding unit 25 has a smooth suction surface 25a on the side facing the workpiece W so that it can stably grip the workpiece W. The workpiece holding unit 25 may be, for example, a chuck such as a vacuum chuck that sucks in surrounding gas (e.g., air) from the suction surface 25a to hold the workpiece W by suction. The workpiece holding unit 25 sucks in gas using a vacuum device (not shown), thereby suctioning and holding the workpiece W on the suction surface 25a.

[0033] The workpiece W has a measurement surface Sa, an action surface Sb, and an attracted surface Sc. The measurement surface Sa is a surface for measuring the amount of eccentricity using the eccentricity measuring unit 26. The action surface Sb is a surface to which an impact for centering can be applied by the impact applying unit 27. The attracted surface Sc is a surface facing the workpiece holding unit 25. It is desirable that the attracted surface Sc be a smooth surface so that the workpiece W can be stably gripped by the workpiece holding unit 25.

[0034] The workpiece W has a central axis Aw. The central axis Aw extends in the Z-axis direction and is arranged parallel to the rotation axis Az of the spindle 15. In this embodiment, the central axis Aw of the workpiece W is offset from the rotation axis Az of the spindle 15 (FIGS. 3(a) and 3(b)). Specifically, the central axis Aw of the workpiece W is offset in the X-axis direction and the Y-axis direction from the rotation axis Az of the spindle 15. However, the central axis Aw of the workpiece W may coincide with the rotation axis Az of the spindle 15.

[0035] A mark M is provided on the workpiece W in advance. The mark M is detected by the mark detection unit 24, as described below. The mark M is used when measuring the eccentric position of the workpiece W. The mark M may be present on a part of the measurement surface Sa of the workpiece W. The mark M may be decorated in a color such as black by printing or the like on the workpiece W. Alternatively, the mark M may be a recessed or protruding portion formed on the workpiece W.

[0036] The eccentricity measuring unit 26 measures the eccentricity of the workpiece W around its entire circumference while the spindle 15, workpiece holder 25, and workpiece W are rotating. The eccentricity measuring unit 26 is disposed in a position facing the measurement 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 an electrical type or an optical type. 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 positioned vertically above the workpiece W, but this is not limited to this. The eccentricity measuring unit 26 may be positioned on the positive side or negative side of the X-axis direction with respect to the workpiece W.

[0037] The eccentricity measuring unit 26 is electrically connected to the control unit 30 via a second control signal line 37. Data on the eccentricity of the workpiece W measured by the eccentricity measuring unit 26 is sent to the control unit 30 via the second control signal line 37.

[0038] The mark detection unit 24 detects the mark M provided on the workpiece W in order to measure the eccentric position of the workpiece W. The mark detection unit 24 is disposed at a position facing the measurement surface Sa of the workpiece W. The mark detection unit 24 may be a sensor that detects the position of the mark M. The mark detection unit 24 may be a contact sensor or a non-contact sensor. The mark detection unit 24 may be an electrical sensor or an optical sensor.

[0039] The mark detection unit 24 detects the mark M applied to the workpiece W while the spindle 15, the workpiece holder 25, and the workpiece W are rotating. The mark detection unit 24 is electrically connected to the control unit 30 by a fourth control signal line 39. A signal indicating that the mark detection unit 24 has detected the mark M is sent to the control unit 30 by the fourth control signal line 39.

[0040] The impact applying unit 27 applies an impact to the action 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 an impact in a short time of 0.01 seconds or less. There is no particular lower limit to the time for which the impact applying unit 27 applies an impact, but it may be, for example, 0.001 seconds or more. It is also desirable that the impact applying unit 27 has the ability to apply an impact with an amplitude of 0.01 μm or less. There is no particular lower limit to the amplitude for which the impact applying unit 27 applies an impact, but it may be, for example, 0.001 μm or more.

[0041] The impact applying unit 27 is connected to the control unit 30 via a piezo controller 33 and a third control signal line 38. The piezo controller 33 is a controller for driving and controlling the impact applying unit 27. The piezo controller 33 drives and controls the impact applying unit 27 in accordance with a control signal input from the control unit 30. The piezo controller 33 may, for example, apply a drive voltage having a waveform or pulse width in accordance with a pulse signal to the piezo element of the impact applying unit 27. The control unit 30 sets the number of impacts and impact amplitude to be applied by the impact applying unit 27, and sends an impact start signal to the impact applying unit 27.

[0042] The control unit 30 calculates the eccentric position and amount of eccentricity of the workpiece W based on the amount of eccentricity of the workpiece W measured by the eccentricity measuring unit 26 and the position of the mark M measured by the mark detecting unit 24. Furthermore, the control unit 30 rotates and positions the spindle 15 based on the eccentric position and amount of eccentricity of the workpiece W so that the impact applying unit 27 is positioned at a rotation angle corresponding to the maximum eccentric position of the workpiece W. Thereafter, the control unit 30 causes the impact applying unit 27 to apply an impact to the workpiece W.

[0043] The eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 may be integrated with one another. The eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 are attached to a first linear moving member 28 and a second linear moving member 29. The first linear moving member 28 moves the eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 in the Y-axis direction perpendicular to the rotation axis Az of the main shaft 15. The second linear moving member 29 moves the eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 in the X-axis direction perpendicular to the rotation axis Az of the main shaft 15.

[0044] 3(a) and 3(b) are views of the workpiece W as viewed from the direction of the rotation axis Az of the spindle 15. FIG. 3(b) shows the workpiece W rotated 180° from FIG. 3(a). As shown in FIGS. 3(a) and 3(b), when the workpiece W, which is positioned eccentrically with respect to the rotation axis Az, rotates, the first linear motion member 28 and the second linear motion member 29 move the eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 so that they follow the surface of the workpiece W. That is, even when the workpiece W rotates, the eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 are translated so that they are in contact with or close to the surface of the workpiece W. The first linear motion member 28 and the second linear motion member 29 may be pre-installed in the machine tool 10. The eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 may move independently of each other in the Y-axis and X-axis directions.

[0045] Next, the operation of this embodiment (the method for centering a workpiece) configured as described above will be described.

[0046] First, the workpiece W is gripped by the workpiece holder 25 provided at the tip of the spindle 15 (step S1 in FIG. 4). At this time, the suction surface 25a of the workpiece holder 25 comes into contact with the surface Sc of the workpiece W to be attracted, and the workpiece W is stably gripped.

[0047] Next, the control unit 30 rotates the spindle 15 (step S2 in FIG. 4). 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 15 is rotated by the spindle motor 16, and the workpiece W held by the workpiece holder 25 also rotates. While the spindle 15 is rotating, rotational position information is transmitted and received from the spindle motor 16 to the control unit 30. The rotational position information includes the current rotation angle of the spindle 15 (spindle index angle). The control unit 30 receives the rotational position information of the spindle 15.

[0048] Next, the control unit 30 measures the eccentricity of the workpiece W around the entire circumference thereof while rotating the spindle 15 (step S3 in FIG. 4). During this time, the eccentricity measuring unit 26 measures the eccentricity of the rotating workpiece W. Specifically, the eccentricity measuring unit 26 measures the measurement surface Sa of the rotating workpiece W in a contact or non-contact manner 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 measuring unit 26 transmits the measured eccentricity to the control unit 30 via the second control signal line 37.

[0049] Next, the control unit 30 measures the eccentric position of the workpiece W while rotating the spindle 15 (step S4 in FIG. 4). During this time, the mark detection unit 24 detects the mark M applied to the workpiece W. When the mark detection unit 24 detects the mark M, it sends a signal to the control unit 30 via the fourth control signal line 39. The control unit 30 stores the rotation angle of the spindle 15 when it receives the signal indicating that the mark M has been detected.

[0050] Next, the control unit 30 calculates the eccentric position of the workpiece W based on the position of the mark M. The eccentric position refers to the rotation angle of the spindle 15 at which the amount of eccentricity of the workpiece W is maximum, based on the position of the mark M. In this way, the control unit 30 calculates the eccentric position and amount of eccentricity of the workpiece W. In other words, the control unit 30 obtains information on the direction and amount of displacement of the workpiece W relative to the rotation axis Az of the spindle 15.

[0051] Next, the control unit 30 determines whether the maximum eccentricity of the workpiece W is within the range of the target value (step S5 in FIG. 4). The maximum eccentricity of the workpiece W refers to the difference in distance between the farthest and closest parts of the measurement surface Sa of the workpiece W from the rotation axis Az of the spindle 15. The target value has a target upper limit and a target lower limit. If the maximum eccentricity of the workpiece W is between the target upper limit and the target lower limit (if step S5 is Y), the impact applying unit 27 does not apply any further impact to the workpiece W, and the series of steps in the centering operation is completed.

[0052] The target lower limit value may be 0, but in more complex machining, there may be an offset between the rotation axis Az of the spindle 15 and the central axis Aw of the workpiece W. In such a case, the target lower limit value does not have to be 0.

[0053] Next, the control unit 30 determines the rotational position of the spindle 15 based on the eccentric position and the amount of eccentricity of the workpiece W so that the impact applying unit 27 is located at a rotation angle (spindle index angle) of the spindle 15 that corresponds to the maximum amount of eccentricity of the workpiece W (step S6 in FIG. 4). 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 that causes the impact applying unit 27 to face the position where the amount of eccentricity of the workpiece W is maximum.

[0054] Next, the control unit 30 controls the impact applying unit 27 to apply impacts to the workpiece W (step S7 in FIG. 4). During this time, the control unit 30 sets the number of impacts and impact amplitude to be applied by the impact applying unit 27 based on the eccentricity of the workpiece W, and transmits an impact start signal. The impact start signal is transmitted to the impact applying unit 27 via the third control signal line 38 and the piezo controller 33. Upon receiving the impact start signal, the impact applying unit 27 applies impacts to the action surface Sb of the workpiece W. The impact applying unit 27 physically applies impacts to the action surface Sb of the workpiece W a predetermined number of times, either once or multiple times. As a result, the action surface Sb of the workpiece W moves a small distance in a direction approaching the rotation axis Az of the main shaft 15. This reduces the maximum eccentricity of the workpiece W. The control unit 30 calculates and determines the number of impacts to be applied to the workpiece W according to the maximum eccentricity of the workpiece W.

[0055] The time for which the impact applying unit 27 applies the impact may be as short as 0.01 seconds or less. In this way, by applying an impact to the action surface Sb of the workpiece W by the impact applying unit 27 for only a short time of 0.01 seconds or less, the undesirable effects of the stick-slip phenomenon can be minimized. This enables precise alignment to, for example, 1 μm or less.

[0056] Thereafter, the control unit 30 again measures the eccentricity of the workpiece W for a part of or the entire circumference of the workpiece W while rotating the spindle 15 (step S8 in FIG. 4). 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 calculates the maximum eccentricity of the workpiece W.

[0057] If the maximum eccentricity amount of the workpiece W is not between the target upper limit value and the target lower limit value (if step S9 is N), the control unit 30 controls the impact applying unit 27 again to apply an impact to the action surface Sb of the workpiece W. In this case, the impact applying unit 27 physically applies an impact to the action surface Sb of the workpiece W a predetermined number of times, either once or multiple times.

[0058] If the maximum eccentricity of the workpiece W is between the target upper limit and the target lower limit (if step S9 is Y), the process returns to step S3 again, and then steps S4 and S5 are performed. In step S5, if the maximum eccentricity of the workpiece W is between the target upper limit and the target lower limit (if step S5 is Y), the centering operation is completed.

[0059] In each of the above steps, the eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 move linearly in the Y-axis direction and the X-axis direction by the first linear moving member 28 and the second linear moving member 29. This allows the eccentricity measuring unit 26, the mark detecting unit 24, and the impact applying unit 27 to follow the surface of the workpiece W.

[0060] Next, the reason why minute alignment is possible by the impact applying unit 27 applying an impact to the action surface Sb of the workpiece W for a short period of time of 0.01 seconds or less will be explained.

[0061] Generally, when centering is performed using a static load, if the frictional force between the workpiece W and the workpiece holder 25 is equal to or less than the maximum static frictional force, the workpiece W will not move even if a load is applied to the workpiece W. On the other hand, if the load on the workpiece W is gradually increased and exceeds the maximum static frictional force, the dynamic friction region is reached. 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 an impact to the workpiece W that exceeds the maximum static frictional force and for a sufficiently short time. This allows the workpiece W to be moved slightly, enabling fine centering.

[0062] As described above, according to this embodiment, the amount of eccentricity of the workpiece W is measured over the entire circumference of the workpiece W while rotating the spindle 15, and the eccentric position of the workpiece W is measured while rotating the spindle 15. Thereafter, the workpiece W is positioned based on the amount of eccentricity and the eccentric position of the workpiece W, and the impact applying unit 27 applies an impact to the positioned workpiece W. This allows for highly accurate centering without the need for skilled techniques, especially when the center position of the spindle 15 and the center position of the workpiece W are offset.

[0063] Furthermore, according to this embodiment, the impact applying unit 27 may apply an impact to the workpiece W for a time of 0.01 seconds or less per impact. In this way, the time for which the impact applying unit 27 applies an impact to the workpiece W is sufficiently short, so that even if the load applied by the impact applying unit 27 exceeds the maximum static friction force, the workpiece W will not move more than necessary. As a result, it is possible to center the workpiece W by moving it very slightly at a time, and high-precision centering (for example, 1 μm or less) can be easily achieved. As a result, it is possible to center the workpiece W automatically without the need for skilled techniques.

[0064] Furthermore, according to this embodiment, the impact applying unit 27 applies impact to the workpiece W a number of times according to the maximum eccentricity of the workpiece W. This allows the amount of movement of the workpiece W to be adjusted in small increments, and suppresses the phenomenon (hunting) in which the amount of eccentricity of the workpiece W fluctuates each time the impact applying unit 27 applies an impact.

[0065] Furthermore, according to this embodiment, after the impact applying unit 27 applies an impact to the workpiece W, the amount of eccentricity of the workpiece W is measured again for the entire circumference of the workpiece W while rotating the spindle 15. This makes it possible to confirm whether the maximum amount of eccentricity of the workpiece W has reached or exceeded the target value.

[0066] It is also possible to combine the multiple components disclosed in the above-described embodiments and modifications as needed, or to delete some of the components disclosed in the above-described embodiments and modifications. [Explanation of symbols]

[0067] 10 Machine tools 11 Machine body 12 Foundation 13 Z-axis moving table 14 Headstock 15 Spindle 16 spindle motor 17 Lead screw 18 Z-axis motor 19 X-axis moving table 21 Tools 22 Tool rest 24 Mark detection unit 25 Work holding part 26 Eccentricity measurement section 27 Impact applying section 28 First linear member 29 Second linear member 30 Control Unit

Claims

1. In the workpiece centering method, a step of gripping a workpiece with a workpiece holder provided at the tip of the spindle; a step of measuring the eccentricity of the workpiece over the entire circumference of the workpiece while rotating the spindle; measuring an eccentric position of the workpiece while rotating the spindle; a step of positioning the workpiece based on the amount of eccentricity of the workpiece and the eccentric position of the workpiece; and a step of applying an impact to the positioned workpiece by an impact applying unit.

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

3. 2. The method for centering a workpiece according to claim 1, further comprising a step of measuring the eccentricity of the workpiece again over the entire circumference thereof while rotating the spindle after the step of the impact applying unit applying an impact to the workpiece.

4. The main axis and a workpiece holding portion provided at the tip of the spindle and configured to hold a workpiece; an eccentricity measuring unit that measures the eccentricity of the workpiece over the entire circumference of the workpiece while rotating the main shaft; a mark detection unit that detects a mark provided on the workpiece in order to measure the eccentric position of the workpiece; an impact applying unit that applies an impact to the workpiece; a control unit that controls the rotation of the main shaft and the impact applying unit, the control unit calculates the eccentric position and the eccentric amount of the workpiece based on the eccentric amount of the workpiece measured by the eccentric amount measuring unit and the position of the mark measured by the mark detecting unit; The control unit rotates and positions the spindle so that the impact application unit is located at a rotation angle corresponding to the maximum eccentric position of the workpiece based on the eccentric position and the amount of eccentricity of the workpiece, and causes the impact application unit to apply an impact to the workpiece.

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

6. The machine tool according to claim 4 , wherein the eccentricity measuring unit, the mark detecting unit, and the impact applying unit are moved linearly by linearly moving members.

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