Abstract:
Provided is method of calibrating Y-axis direction position of contact tip of form measuring instrument including: table rotatable about Z-axis; contact tip capable of contacting workpiece; and contact tip driving means to drive contact tip in at least X- and Z-axis directions among X-, Y- and Z-axis directions perpendicular to one another. Method performs tracing measurement of inclined surface or inclined cylinder side surface which is part of workpiece obtained by inclining workpiece placed on table about Y-axis, or side surface of off-centered cylinder having center axis off-centered in X-axis direction by rotating surface to obtain measurement value at each angular position of rotation of table, obtains angular position of rotation at which smallest value among measurement values of tracing measurement is detected as angular position of rotation with smallest detected value, and adjusts Y-axis direction position of contact tip based on angular position of rotation with smallest detected value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims the benefit of priority from prior Japanese Patent Application No. 2009-120327, filed on May 18, 2009, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a form measuring instrument such as a roundness measuring machine which measures a displacement in synchronization with an angular position of rotation of a workpiece that is rotating about a specified rotation axis, and a calibration method and calibration program therefor. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventionally, there has been known a roundness measuring machine which receives a workpiece on its table, rotates the table, and makes a contact tip (e.g., a contact piece) scan the surface of the workpiece (see JPH5-231864A, JP2551698B). Such a roundness measuring machine rotates the table to detect a displacement of the contact tip in an X-axis direction and in a Z-axis direction. That is, the roundness measuring machine performs the measurement while locking the contact tip in a Y-axis direction. Therefore, in order to obtain a highly accurate measurement, it is important to calibrate the position of the contact tip in the Y-axis direction. 
         [0006]    Normally, calibration of the position of the contact tip in the Y-axis direction is performed in the following manner. 
         [0007]    First, a reference sphere is positioned at the center of rotation of the table, and a Y-axis direction adjustment screw of a detector holder is manually twisted while the contact tip is fixed in contact with the reference sphere. Then, the adjustment screw is stopped at a position where X-axis and Z-axis direction peaks of a level meter (X-axis and Z-axis direction peaks of the sphere) are observed. 
         [0008]    However, the above calibration method relies only upon human perceptions and tends to generate an error. The Y-axis direction error causes errors in the angular position and X-axis position of the workpiece and in the measured values, and such errors are greater at a position more proximal to the center of rotation of the table. 
         [0009]    For example, when there is a Y-axis direction error, a measurement of the flatness of an optical flat (having a cylindrical shape) which is performed by inclining the optical flat will result in that different inclination values will be detected at the center and at a position more distal, and that the shape of the top surface of the optical flat, which should be a flat shape by right, will be measured as a warped shape. Further, in a measurement of roundness or cylindricity, the smaller the diameter of the workpiece, at the farther position from the 0-degree position of the workpiece, the contact tip will gain contact with the workpiece. Thereby an error may be caused in an analysis calculation result, and the centering may not be converged. 
       SUMMARY OF THE INVENTION 
       [0010]    A method of calibrating a form measuring instrument according to the present invention is a method of calibrating a form measuring instrument for calibrating a position of a contact tip of the form measuring instrument in a direction along a Y-axis, the form measuring instrument including: a table for placement of a workpiece thereon, the table being rotatable about a Z-axis; the contact tip capable of contacting with the workpiece; and contact tip driving means configured to drive the contact tip in directions along at least an X-axis and the Z-axis among the X-axis, the Y-axis, and the Z-axis perpendicular to one another, the method comprising: performing tracing measurement of an inclined flat surface, a side surface of an inclined cylinder, or a side surface of an off-centered cylinder by rotating the surface to obtain a measurement value at each angular position of rotation of the table, the inclined surface and the side surface of the inclined cylinder each being a part of the workpiece obtained by inclining the workpiece placed on the table about the Y-axis, the off-centered cylinder having a center axis off-centered in the direction along the X-axis; obtaining an angular position of rotation of the table at which a smallest value among the measurement values obtained by the tracing measurement is detected, as an angular position of rotation with smallest detected value; and adjusting the position of the contact tip in the direction along the Y-axis based on the angular position of rotation with smallest detected value. 
         [0011]    A form measuring instrument according to the present invention comprises: a table for placement of a workpiece thereon, the table being rotatable about a Z-axis; a contact tip capable of contacting with the workpiece; contact tip driving means configured to drive the contact tip in directions along at least an X-axis and the Z-axis among the X-axis, a Y-axis, and the Z-axis perpendicular to one another; and a control unit operative to calibrate a position of the contact tip in a direction along the Y-axis, the control unit including: means operative to obtain an angular position of rotation of the table at which a smallest value among measurement values is detected as an angular position of rotation with smallest detected value, the measurement values being obtained by performing tracing measurement of an inclined flat surface, a side surface of an inclined cylinder, or a side surface of an off-centered cylinder by rotating the surface, the inclined surface and the side surface of the inclined cylinder each being a part of the workpiece obtained by inclining the workpiece placed on the table about the Y-axis, the off-centered cylinder having a center axis off-centered in the direction along the X-axis; and means operative to calculate an amount of adjustment by which the position of the contact tip in the direction along the Y-axis is adjusted based on the angular position of rotation with smallest detected value. 
         [0012]    A program for calibrating a form measuring instrument according to the present invention is a program for calibrating a form measuring instrument for calibrating a position of a contact tip of the form measuring instrument in a direction along a Y-axis, the form measuring instrument including: a table for placement of a workpiece thereon, the table being rotatable about a Z-axis; the contact tip capable of contacting with the workpiece; and contact tip driving means configured to drive the contact tip in directions along at least an X-axis and the Z-axis among the X-axis, the Y-axis, and the Z-axis perpendicular to one another, the program controlling a computer to: perform tracing measurement of an inclined flat surface, a side surface of an inclined cylinder, or a side surface of an off-centered cylinder by rotating the surface to obtain a measurement value, the inclined surface and the side surface of the inclined cylinder each being a part of the workpiece obtained by inclining the workpiece placed on the table about the Y-axis, the off-centered cylinder having a center axis off-centered in the direction along the X-axis; obtain an angular position of rotation of the table at which a smallest value among obtained measurement values is detected as an angular position of rotation with smallest detected value; and calculate an amount of adjustment by which the position of the contact tip in the direction along the Y-axis is adjusted based on the angular position of rotation with smallest detected value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is an appearance perspective diagram showing a schematic configuration of a form measuring instrument according to an embodiment of the present invention. 
           [0014]      FIG. 2  is an expanded diagram of a part of a displacement detecting unit  6 . 
           [0015]      FIG. 3  is a block diagram showing a configuration of an arithmetic processing unit  31 . 
           [0016]      FIG. 4  is a flowchart showing an operation of the form measuring instrument according to an embodiment. 
           [0017]      FIG. 5  are schematic diagrams showing step S 102  of  FIG. 4 . 
           [0018]      FIG. 6  is a schematic diagrams showing step S 103  of  FIG. 4 . 
           [0019]      FIG. 7  are schematic diagrams showing step S 104  of  FIG. 4 . 
           [0020]      FIG. 8  are schematic diagrams showing steps S 105  to  5107  of  FIG. 4 . 
           [0021]      FIG. 9  are schematic diagrams showing step S 108  of  FIG. 4 . 
           [0022]      FIG. 10  are schematic diagrams showing step S 110  of  FIG. 4 . 
           [0023]      FIG. 11  are schematic diagrams showing step S 111  of  FIG. 4 . 
           [0024]      FIG. 12  is a diagram showing an operation of the form measuring instrument according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    Next, an embodiment of the present invention will be explained with reference to the drawings. 
       Embodiment 
     Configuration of Form Measuring Instrument According to an Embodiment 
       [0026]    First, with reference to  FIG. 1 , the configuration of the form measuring instrument according to an embodiment will be explained.  FIG. 1  is an appearance perspective view of a form measuring instrument (roundness measuring machine) according to an embodiment. 
         [0027]    The form measuring instrument rotates a workpiece  4  formed of a rotating object about a specified rotation axis and measures a displacement of a surface of the workpiece  4  at its each angular position of rotation. 
         [0028]    As shown in  FIG. 1 , the form measuring instrument includes a measuring unit  1  and an arithmetic processing device  2  connected to the measuring unit  1  via a drive control unit  1   a.    
         [0029]    The measuring unit  1  includes a base mount  3 , a table  5  provided on the base mount  3  and on which a workpiece  4  is placed, a displacement detecting unit  6  configured to detect any displacement of the workpiece  4  placed on the table  5 , and an operation section  7  used for operating them. 
         [0030]    The table  5  drives a disk-shaped stage  11  to rotate by means of a rotation drive unit  12  that is positioned below the stage  11 , thereby to rotate the workpiece  4  placed on the stage  11 . Adjustment knobs  13  are provided on the side surface of the rotation drive unit  12  at generally 90-degree intervals in the circumferential direction. Operating these adjustment knobs  13  allows for manual centering and leveling of the stage  11 . That is, the stage  11  is constructed to be adjustable in X-axis, Y-axis, and Z-axis directions perpendicular to one another. The stage  11  is also constructed such that it is centered and leveled by a control unit  41 , which will be described later. 
         [0031]    The displacement detecting unit  6  is constructed as follows. That is, a column  21  that extends upward stands on the base mount  3 , and a slider  22  is mounted on the column  21  in a way to be able to move an up and down (Z-axis) direction. A detector holder  23  is attached to the slider  22 . The detector holder  23  can be driven in a horizontal (X-axis) direction, and has a detector  25  provided at the end. A contact tip  24 , which can bring its tip into contact the workpiece  4 , is provided at the end of the detector  25 . The column  21 , the slider  22 , the detector holder  23 , and the detector  25  constitute a contact tip driving means. 
         [0032]    By moving the slider  22  and the detector holder  23  to scan (trace) the surface of the workpiece  4  in the X-axis direction or the Z-axis direction while rotating the table  5 , it is possible to the obtain an amount of displacement of the contact tip  24  at each position in the X-axis direction or the Z-axis direction as measurement data. 
         [0033]    As shown in  FIG. 2 , the detector holder  23  can rotate by 90 degrees about a rotation shaft  28  that extends in the X-axis direction, such that the detector  25  and the contact tip  24  can take a vertical posture shown in  FIG. 2  ( a ) and a horizontal posture shown in  FIG. 2  ( b ) in accordance with the form and measurement surface of the workpiece  4 . The position of the tip of the contact tip  24  in the Y-axis direction when the detector holder  23  is in the vertical posture can be adjusted by an adjustment screw  26  provided on a side surface of the detector holder  23 . The position of the tip of the contact tip  24  in the Y-axis direction when the detector holder  23  is in the horizontal posture can be adjusted by an adjustment screw  27  provided on an end surface of the detector holder  23 . 
         [0034]    The arithmetic processing device  2  acquires measurement data obtained by the displacement detecting unit  6 . The arithmetic processing device  2  includes an arithmetic processing unit  31  configured to execute arithmetic processing, an operation section  32 , and a display device  33 . The arithmetic processing device  2  is configured to be able to control the operation of the measuring unit  1  like the operation section  7  is. 
         [0035]    Next, with reference to  FIG. 3 , the configuration of the arithmetic processing unit  31  will be explained. As shown in  FIG. 3 , the arithmetic processing unit  31  mainly includes a control unit (CPU: Central Processing Unit)  41 , a RAM (Random Access Memory)  42 , a ROM (Read Only Memory  43 , an HDD (Hard Disk Drive)  44 , and a display control unit  45 . In the arithmetic processing unit  31 , code information and positional information entered from the operation section  32  are input to the control unit  41  via an I/F  46   a . The control unit  41  executes various processes in accordance with a macro program stored in the ROM  43  and various programs that are loaded onto the RAM  42  from the HDD  44  via an I/F  46   b.    
         [0036]    The control unit  41  controls the measuring unit  1  via an I/F  46   c  in accordance with a measurement execution process. The HDD  44  is a recording medium that stores various control programs. The RAM  42  stores various programs and provides a work area for various processes. The control unit  41  displays a measurement result, etc. on the display device  33  via the display control unit  45 . 
         [0037]    The control unit  41  reads out various programs from the HDD  44  and executes the following operation shown in  FIG. 4  by executing the programs. 
       [Operation of Form Measuring Instrument According to Embodiment] 
       [0038]    Next, with reference to the flowchart shown in  FIG. 4 , a method, according to an embodiment, of calibrating the position of the contact tip by using the form measuring instrument will be explained. The first half (steps S 101  to S 106 ) of  FIG. 4  shows a Y-axis calibration procedure for when the detector holder  23  is in the horizontal posture, and the latter half (steps S 101  to S 113 ) of  FIG. 4  shows a Y-axis calibration procedure for when the detector holder  23  is in the vertical posture. When performing the calibration to be described below, a human operator sets an optical flat  4   a  having a cylindrical shape on the stage  11  as the workpiece  4 . 
         [0039]    First, the control unit  41  sets the detector holder  23  in the horizontal posture (step S 101 ). Next, the control unit  41  scans the top surface of the set optical flat  4   a  by making the contact tip  24  trace the surface, and executes a leveling process on the optical flat  4   a  based on the measurement result (step S 102 ). The leveling process is a process of aligning the top surface of the optical flat  4   a  horizontally in an X-Y plane defined along the X-axis and the Y-axis, as shown in  FIGS. 5  ( a ) and ( b ). In the explanation of the present embodiment, the center C of the optical flat  4   a  is not necessarily required to fall on the center of rotation O of the stage  11 . 
         [0040]    Next, the control unit  41  inclines the stage  11  (or the optical flat  4   a ) about the Y-axis by a specified angle θ 1  as shown in  FIG. 6  (step S 103 ). Next, as shown in  FIGS. 7  ( a ) and (b), the control unit  41  makes the contact tip  24  trace the top surface of the optical flat  4   a  while rotating the stage  11  (or the optical flat  4   a ) about the Z-axis, and measures an amount of displacement Δ 1  of the contact tip  24  in the Z-axis direction at an angular position of rotation φ 1  (step S 104 ). It is desired that the contact tip  24  trace the top surface at a position more proximal to the center of rotation O. Then, the control unit  41  calculates an angular position of rotation that has the smallest value Δmin 1  of the amounts of displacement Δ 1  (such an angular position will be referred to as angular position of rotation with smallest detected value φmin 1 ) (step S 105 ). 
         [0041]    When there is an error in the position of the contact tip  24  in the Y-axis direction (when the contact tip  24  is not positioned on the X-axis), the amount of displacement Δ 1  of the contact tip  24  in the Z-axis direction at the angular position of rotation φ 1  will be, for example, as shown in  FIG. 8  ( a ). That is, during this measurement, for example, the optical flat  4   a  is inclined about the Y-axis such that a given portion of the optical flat  4   a  becomes the highest when that portion comes to the 180-degree position. Therefore, if the contact tip  24  is positioned on the X-axis, the amount of displacement Δ 1  takes the largest value when the angular position of rotation φ 1  is 180 degrees, while the amount of displacement Δ 1  takes the smallest value when the angular position of rotation φ 1  is 0 degree. However, when the position of the tip of the contact tip  24  is deviated from the origin in the Y-axis direction as described above, the angular position of rotation with smallest detected value φ 1  is observed at a position deviated from 0 degree, as shown in  FIG. 8  ( a ). In the illustrated example, the angular position of rotation with smallest detected value φmin 1  appears at near 54 degrees. This means that the tip of the contact tip  24  measures near the angular position of rotation φ 1  of 306 degrees when the stage  11  is at the angular position of rotation φ 1  of 0 degree, which means that the contact tip  24  is deviated to the negative side of the Y-axis direction. 
         [0042]    Subsequent to step S 105 , the control unit  41  calculates an amount of movement M 1  based on the angular position of rotation with smallest detected value φmin 1  (step S 106 ). The amount of movement M 1  is an amount of movement of the contact tip  24  in the Y-axis direction by which the angular position of rotation with smallest detected value φmin 1  will become 0 degree. The human operator corrects the Y-axis direction error of the contact tip  24  in the horizontal direction, by manually adjusting the adjustment screw  27  based on the amount of movement M 1 . 
         [0043]    Next, Y-axis calibration of the contact tip  24  in the vertical posture will be performed. The human operator places a cylindrical workpiece  4   b  instead of the optical flat  4   a  on the stage  11 . The control unit  41  determines whether or not the replacement of the workpieces  4  has been completed, based on information entered from the operation section  32  (step S 107 ). The cylindrical workpiece  4   b  is of high accuracy, and has a diameter smaller than that of the optical flat  4   a.    
         [0044]    When the cylindrical workpiece  4   b  is set, the control unit  41  rotates the detector  25  by 90 degrees to put the contact tip  24  in a posture to measure a vertical surface, scans the set cylindrical workpiece  4   b  by rotatively tracing upper and lower two positions of the side surface, and executes leveling and centering processes on the cylindrical workpiece  4   b  based on the result of the measurement (step S 108 ). The centering process is a process of aligning the axis of the cylindrical workpiece  4   b  with the Z-axis as shown in  FIGS. 9  ( a ) and ( b ). Next, the control unit  41  sets the detector holder  23  in the vertical posture (step S 109 ). 
         [0045]    Next, the control unit  41  places the workpiece  4  (cylindrical workpiece  4   b ) such that its axis is off-centered from the rotation axis O or such that the workpiece  4  is inclined (step S 110 ). For example, the control unit  41  off-centers the position of the axis of the cylindrical workpiece  4   b  from the Z-axis as shown in  FIG. 10  ( a ). Alternatively, the control unit  41  inclines the cylindrical workpiece  4   b  about the Y-axis such that the axis thereof is inclined from the Z-axis as shown in  FIG. 10  ( b ). 
         [0046]    Next, as shown in  FIGS. 11  ( a ) and ( b ), the control unit  41  makes the contact tip  24  trace the side surface of the cylindrical workpiece  4   b  while rotating the stage  11  (or the cylindrical workpiece  4   b ) about the Z-axis, and measures an amount of displacement Δ 2  of the contact tip  24  in the X-axis direction at an angular position of rotation φ 2  (step S 111 ). Next, the control unit  41  calculates the angular position of rotation that has the smallest value Δmin 2  of the amounts of displacement Δ 2  (such an angular position will be referred to as angular position of rotation with smallest detected value φmin 2 ) (step S 112 ). When there is an error in the position of the contact tip  24  in the Y-axis direction (when the contact tip  24  is not positioned on the X-axis), the amount of displacement Δ 2  of the contact tip  24  in the X-axis direction at the angular position of rotation φ 2  will be substantially the same as shown in  FIG. 8  ( a ). 
         [0047]    Next, the control unit  41  calculates an amount of movement M 2  based on the angular position of rotation with smallest detected value φmin 2  (step S 113 ). The amount of movement M 2  is an amount of movement of the contact tip  24  in the Y-axis direction by which the angular position of rotation with smallest detected value φmin 2  will become 0 degree. The human operator corrects the Y-axis direction error of the contact tip  24  in the vertical posture, by manually adjusting the adjustment screw  26  based on the amount of movement M 2 . 
       [Advantages of Form Measuring Instrument According to Embodiment] 
       [0048]    As described above, the form measuring instrument according to the embodiment adjusts the Y-axis based on the angular positions of rotation with smallest detected value φmin 1  and φmin 2 . That is, the form measuring instrument does not rely only upon human perceptions, and therefore can perform Y-axis direction adjustment highly accurately. Furthermore, the form measuring instrument according to the embodiment needs not measure the workpiece  4  entirely, and hence can perform centering in a short time. 
       Other Embodiments 
       [0049]    Though the embodiment of the form measuring instrument having been explained, the present invention is not limited to the embodiment described above, but various alterations, additions, substitutions, etc. can be made within the scope of the spirit of the invention. 
         [0050]    The embodiment described above is intended for a human operator to manually adjust an error of the contact tip  24  in the Y-axis direction based on the amounts of movement M 1  and M 2 . However, as shown in  FIG. 12 , after step S 106 , the control unit  41  may automatically adjust the Y-axis of the contact tip  24  in the horizontal posture by adjusting the adjustment screw  27  based on the amount of movement M 1  (step S 201 ). Also, after step S 113 , the control unit  41  may automatically adjust the Y-axis of the contact tip  24  in the vertical posture by adjusting the adjustment screw  26  based on the amount of movement M 2  (step S 202 ). 
         [0051]    In the embodiment described above, a spherical workpiece may be placed instead of the cylindrical workpiece  4   b.