JP2001201339A - Correcting method for straightness precision of measuring instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a correcting method for the straightness precision of a measuring instrument capable of solving the defect of the inversion method and capable of obtaining high-reliability measured data without improving an existing device. SOLUTION: This correcting method for the straightness precision of the measuring instrument 1 having linear motion mechanisms includes a straightness precision data calculating process in which a master work MW valued with shape data in advance in measured while a detector 7 is moved by the linear motion mechanism 4, 5 of the measuring instrument 1 and the valued shape data are subtracted from the master work measured data to obtain the straightness precision data of the linear motion mechanisms a work measured data calculating process in which a work is measured while the detector 7 is moved by the linear motion mechanisms of the measuring instrument 1 to obtain the work measured data, and a work shape arithmetic process in which the straightness precision data are subtracted from the work measured data to obtain the true value data of the work.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a straightness correction method for a measuring instrument. For example, the present invention relates to a straightness accuracy correction method for a measuring machine having a linear motion mechanism, such as a roundness measuring machine and a roughness measuring machine, for correcting the straightness accuracy of the linear motion mechanism.

[0002]

2. Description of the Related Art For example, when a work is measured on a measuring machine such as a roundness measuring machine or a roughness measuring machine provided with a linear motion mechanism as a guide (reference), the measured data is directly related to the shape data of the work. Since the mechanical accuracy (straightness) of the moving mechanism is included, it is necessary to separate and remove the straightness of the moving mechanism from the measurement data in order to extract the true work shape.

Conventionally, as a method of separating and removing the straightness accuracy of a linear motion mechanism from measurement data, Japanese Patent No. 2935603 has been proposed as a "roundness measuring device with a straightness measuring function". This is a turntable on which a straightness check gauge is mounted, a detector for detecting the surface position of one side of the straightness check gauge, and a column for holding the detector movably along the surface of the straightness check gauge. First storage means for storing data obtained from the detector as first data, and a straightness check gauge rotated by 180 ° about the rotation axis of the turntable, and the straightness is detected by the detector. Second storage means for storing data obtained by detecting a surface position on the same surface as one side surface of the check gauge as second data; and a column for storing data based on the first data and the second data. A calculating means for calculating a straightness error correction amount.

[0004]

SUMMARY OF THE INVENTION The aforementioned Japanese Patent No. 293
No. 5603 “Circularity measuring device with straightness measuring function” adopts a so-called inversion method, which is the first measurement (first measurement) and the measurement by rotating the turntable by 180 °. The second measurement is required, and the measurement trajectory must be exactly the same on the target work surface. That is, setting a work in measurement is extremely difficult and requires skill, and requires a considerable number of man-hours, such as the need to invert the position of the detector when performing two measurements. Further, in the guide of the detector, it is not always the same (for example, in the case of a roundness measuring device, the output of the arm holding the detector is not necessarily the same), and the error There is a problem that is easily generated.

SUMMARY OF THE INVENTION An object of the present invention is to provide a straightness correction method for a measuring instrument capable of solving the above-mentioned disadvantages of the inversion method and obtaining highly reliable measurement data without improving existing equipment. It is in.

[0006]

Therefore, a straightness accuracy correcting method for a measuring instrument according to the present invention employs the following configuration to achieve the above object. The straightness accuracy correction method according to claim 1 is a straightness accuracy correction method for a measuring machine having a linear motion mechanism, wherein the shape data is previously determined while moving the detector using the linear motion mechanism of the measuring machine. Measuring the attached master work, subtracting the valued shape data from the master work measurement data to obtain straightness accuracy data of the linear motion mechanism, and using a straight motion mechanism of the measuring machine. A work measurement data calculating step of measuring the work while measuring the work while moving the detector, and a work shape calculation step of calculating the true value data of the work by subtracting the straightness accuracy data from the work measurement data; It is characterized by having.

[0007] According to this configuration, in the straightness accuracy data calculation step, while the detector is moved using the linear motion mechanism of the measuring machine, the master work to which the shape data has been previously priced is measured, and the master work measurement is performed. The straightness data of the linear motion mechanism is obtained by subtracting the priced shape data from the data. In the work measurement data calculation step, the work is measured while moving the detector using the linear motion mechanism of the measuring machine, and the work measurement data is obtained.
In the work shape calculation step, the true value data of the work is obtained by subtracting the straightness accuracy data from the work measurement data. Therefore, in the straightness accuracy data calculation step, since only one measurement is required, the disadvantage of the inversion method, that is, the disadvantage of requiring skill and man-hour can be solved. Moreover, since the same problem of the detector guide is eliminated, a more accurate correction result can be obtained. In addition, since the method is for processing the workpiece measurement data, highly reliable measurement data can be obtained without performing special work on the measuring machine. Further, since the calculation of the straightness accuracy data and the correction process of the straightness accuracy data with respect to the workpiece measurement data can be performed by an application independent of the measuring machine, for example, the averaging process of the master workpiece measurement data (the Data processing (e.g., the process of calculating the average value from multiple masterwork measurement data obtained by repeatedly measuring under the same measurement conditions) to reduce variations in measurement data due to electrical noise and measurement environment. Highly accurate correction can be realized.

A straightness correction method according to a second aspect of the present invention is a straightness correction method for a measuring instrument having a linear motion mechanism, wherein the detector is moved using the linear motion mechanism of the measuring instrument.
A straightness accuracy data calculating step of measuring a master work to which shape data is previously priced, and subtracting the priced shape data from the master work measurement data to obtain straightness accuracy data of a linear motion mechanism; A straight-precision data storage step for storing the straight-precision data obtained in the calculation step, and work measurement data for measuring the work while moving the detector using the linear motion mechanism of the measuring machine, and obtaining the work measurement data. The calculation step, a work measurement data storage step of storing the work measurement data taken in the work measurement data calculation step, and the work measurement data and the straightness accuracy data stored in each of the storage steps are read out from the work measurement data. A work shape calculating step for obtaining true value data of the work by subtracting the straightness accuracy data And wherein the door.

According to this configuration, a straightness accuracy data storing step of storing straightness accuracy data and a work measurement data storage step of saving work measurement data are added to the first aspect. Obtain and store straightness accuracy data and workpiece measurement data at different points in time,
At any time (when necessary), it is also possible to obtain the true value data of the work by subtracting the straightness accuracy data from the work measurement data.

A straightness correction method according to a third aspect of the present invention is the straightness accuracy correction method for a measuring machine according to the first or second aspect, wherein the straightness accuracy data calculation step includes the step of: A prismatic master work is mounted on the work mounting surface of the pedestal by using a measuring jig having a reference sphere for setting an origin embedded in a state where the pedestal or the master work is partially exposed. Then, in this state, after positioning the detector of the measuring machine at the vertex of the reference sphere and setting the measurement origin, the master work is measured while moving the detector along one side of the master work by the linear motion mechanism. It is characterized by the following.

According to this configuration, the master work is measured using the measuring jig provided with the pedestal and the reference sphere for setting the origin, and at the time of the measurement, the measuring machine is attached to the reference sphere for setting the origin. After the detector is positioned and the measurement origin is set, the master work is measured while moving the detector along the measurement surface of the master work by the linear motion mechanism. Since the workpiece can be accurately moved to a predetermined measurement point, the master workpiece can be measured with high accuracy.

A straightness correction method according to a fourth aspect of the present invention is the straightness correction method for a measuring machine according to the third aspect, wherein the pedestal has a reference vertical surface orthogonal to the workpiece mounting surface. And the reference sphere is embedded in the reference vertical plane in a partially exposed state. According to this configuration, when correcting the straightness accuracy of the roundness measuring device (straightness accuracy of the column and the slider for vertically moving the detector up and down), the pedestal is positioned at the center of the rotary table of the roundness measuring device. Set the pedestal on the rotary table so that the reference vertical surface is located, and then place the master work on the work mounting surface of the pedestal so that one side of the master work matches the reference vertical surface of the pedestal. After placing, first
The measuring element of the detector is brought into contact with the reference sphere, and the position of the detector is adjusted so that the measuring element captures the vertices of the reference sphere both vertically and vertically. After setting this state as the measurement origin, the probe of the detector is brought into contact with a point on the surface of the master work that is a fixed distance away from the measurement origin, and the detector is moved upward (or downward) from this position. The vertical straightness of the entire length of the master work can be measured. Therefore, the master work can be positioned on the rotary table using the measuring jig, and the measuring point of the height can be set by bringing the measuring element of the detector into contact with the reference sphere. Can be positioned at a fixed position of the master work, and a locus from a fixed position of the master work can be measured.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. This embodiment is an example applied to a straightness accuracy correction method of a roundness measuring device. FIG. 1 shows a roundness measuring machine 1 of the present embodiment. The roundness measuring machine 1
A base 2; a turntable 3 arranged rotatably about an axis (Z-axis) perpendicular to one side (left side) of the upper surface of the base 2; A column 4 arranged in parallel with the axis, a slider 5 provided to be able to move up and down along the column 4 in the vertical direction (Z-axis direction), and a direction (X Arm 6 that can move forward and backward in the axial direction)
And a detector 7 attached to the distal end of the arm 6 so as to be adjustable in the Y-axis direction and having a probe 7A at the distal end.

The rotation angle data of the rotary table 3
The height position of the slider 5 (position data in the Z-axis direction), the amount of advance / retreat of the arm 6 (position data in the X-axis direction), the amount of adjustment of the detector 7 (position data in the Y-axis direction), and the like are not shown. It can be detected by means. The column 7 and the slider 5 move the detector 7 in the vertical direction (Z
A linear motion mechanism (guide) that moves up and down in the axial direction) is configured.

FIG. 2 shows a data processing device 21 for receiving and processing measurement data from the roundness measuring machine 1.
The data processing device 21 includes a control device 22 and a storage device 2
3 and a display device 24. Control device 2
2 correspond to FIGS. 3 to 3 according to a processing program stored in advance.
It has a function of executing each step (processing) shown in FIG. That is, when the roundness measuring device 1 measures a master work to which shape data has been previously priced, the master work measurement data is taken in, and the valued shape data is subtracted from the master work measurement data. A straightness data calculation step for obtaining straightness data of the linear motion mechanism, a straightness data storage step for storing the straightness data obtained in the straightness data calculation step (see FIG. 3), and a roundness measuring device 1 A work measurement data calculating step of taking in the work measurement data when the work is measured; a work measurement data storing step of storing the work measurement data taken in the work measurement data calculation step (see FIG. 4); The stored work measurement data and straightness accuracy data are read out, and the straightness accuracy data is read from the work measurement data. By subtracting the data comprises a function of executing the workpiece shape calculation step (see FIG. 5) to determine the true value data of the workpiece.

Next, the measuring method of the present embodiment will be described. (Straight-precision data calculation step and straight-precision data storage step) The roundness measuring device 1 is used to measure a prismatic master work MW (for example, optical flat) to which shape data has been priced in advance, and the master work is measured. After subtracting the priced shape data from the measured data to obtain straightness data of the linear motion mechanism, the straightness data is stored. Specifically, as shown in FIG. 3, in step (ST) 1, first, the master work MW is measured. After the master work MW is placed vertically on the rotary table 3, the tracing stylus 7A of the detector 7 is brought into contact with one side surface (measurement surface) of the master work MW, and in this state, the slider 5 is moved upward. (See FIG. 6). Then, the tracing stylus 7A of the detector 7 is displaced by the shape of the master work MW and the straightness of the linear motion mechanism.
The displacement amount is sent to the control device 22 of the data processing device 21 together with the position data in each axial direction. The control device 22
The position of the measurement surface of the master work MW is determined based on the position of the slider 5 in the Z-axis direction, the amount of movement of the arm 6 (the position in the X-axis direction), the amount of adjustment of the detector 7 (the position in the Y-axis direction), and the amount of displacement of the tracing stylus 7A. The master work measurement data is calculated and written into the storage device 23.

Next, in ST2, the straightness accuracy data is obtained by subtracting the pre-valued shape data from the master work measurement data. At this time, the shape data of the master work MW is measured by another high-precision measuring device and stored in the storage device 23 in advance. Accordingly, the master work measurement data and the priced shape data of the master work MW are read from the storage device 23, and the shape data is subtracted from the master work measurement data to obtain straightness accuracy data. Next, in ST3, the obtained straightness accuracy data is stored in the storage device 23.

(Work Measurement Data Calculation Step and Work Measurement Data Storage Step) The work W is measured using the roundness measuring device 1 and the work measurement data is obtained, and then the work measurement data is stored. Specifically, as shown in FIG. 4, the work W is first measured in ST11. After the work W is placed upright on the rotary table 3, the measurement is performed by bringing the tracing stylus 7A of the detector 7 into contact with the measurement surface of the work W (see FIG. 7). In this case, measurements such as roundness and cylindricity are performed. Next, in ST12, the work measurement data is stored. That is, the work measurement data measured in ST11 is
1, the work measurement data is stored in the storage device 23.

(Work shape calculation step) The work measurement data and straightness accuracy data stored in each storage step are read out, and the straightness accuracy data is subtracted from the work measurement data to obtain true value data of the work. Specifically, as shown in FIG. 5, the work measurement data is read from the storage device 23 in ST21. Next, in ST22, straight-precision data is read from the storage device 23. Next, ST
At 23, the true value data of the work is obtained by subtracting the straightness accuracy data from the read work measurement data.
Next, in ST24, the obtained true value data of the work is stored in the storage device 23. That is, in the storage device 23,
Since the true value data of the work whose straightness accuracy of the linear motion mechanism has been corrected is stored with respect to the work measurement data, highly reliable measurement data can be obtained.

According to the embodiment described above, in the straightness accuracy data calculation step, the master work MW to which the shape data has been previously priced is measured by using the roundness measuring device 1 and the master work measurement data is used. The straightness accuracy data of the linear motion mechanism of the roundness measuring machine is obtained by subtracting the priced shape data. Apart from this, in the work measurement data calculating step, the work W is measured using the roundness measuring machine 1. After the measurement, the work measurement data is obtained, and after these steps, in the work shape calculation step, the straightness accuracy data is subtracted from the work measurement data to obtain the true value data of the work.

Therefore, in the straightness accuracy data calculation step, since only one measurement is required, the disadvantage of the inversion method can be solved. Moreover, since the same problem of the detector guide is eliminated, a more accurate correction result can be obtained. Also,
Since this is a method for processing the work measurement data, highly reliable measurement data can be obtained without performing special work on the measuring machine. Further, since the calculation of the straightness accuracy data and the correction process of the straightness accuracy data with respect to the workpiece measurement data can be performed by an application independent of the measuring machine, for example, the averaging process of the master workpiece measurement data (the Data processing (e.g., the process of calculating the average value from multiple masterwork measurement data obtained by repeatedly measuring under the same measurement conditions) to reduce variations in measurement data due to electrical noise and measurement environment. Highly accurate correction can be realized.

Further, since the straightness accuracy data and the work measurement data are separately stored in the storage device 23, the straightness accuracy data and the work measurement data are obtained and stored at different points in time, and stored when necessary. By subtracting the straightness accuracy data from the work measurement data, the true value data of the work can be obtained.

In the above embodiment, the rotary table 3
The master work MW or the work W is directly placed on the table for measurement. For example, as shown in FIG. 8, the master work MW is placed on the turntable 3 via the measurement jig 11. It may be mounted. The measuring jig 11 has a pedestal 12. The pedestal 12 has a semi-cylindrical shape in which a short cylinder is cut in half from the center, and a work mounting surface 13 is formed on the upper surface thereof.
The reference sphere 15 is half-buried at the center position of.
The master work MW is fixed to the work mounting surface 13 by close contact (ringing).

In measuring the master work MW,
One side (measuring surface) of master work MW is reference vertical surface 1
After the master work MW is mounted on the work mounting surface 13 of the pedestal 12 so that the master work MW coincides with 4, the measuring element 7A of the detector 7 is first brought into contact with the reference ball 15, and the measuring element 7A is moved up and down and back and forth. In both cases, the position of the detector 7 is adjusted so as to catch the vertex of the reference sphere 15. After that, the height counter (Z
Set the axial position detector) to “0” (set the origin)
After that, the tracing stylus 7A of the detector 7 is moved to a point (for example,
(The lowermost position), the detector 7 is moved upward from this position, and the vertical accuracy of the entire length of the master work MW is measured.

If the straightness of the master work MW is measured using such a measuring jig 11, a locus of the master work MW from a fixed position can be measured. That is, the master work MW can be positioned on the rotary table 3 by using the measuring jig 11, and the origin in the height direction can be set by bringing the tracing stylus 7A of the detector 7 into contact with the reference sphere 15. Since the stylus 7A of the detector 7 can always be positioned at the fixed position of the master work MW, the trajectory of the master work MW from the fixed position can be measured.

Further, the use of the measuring jig 11 makes it possible to easily verify the straightness accuracy data obtained in the straightness accuracy data calculation step by the inversion method. For example, in the state shown in FIG. 9A (the same state as in FIG. 8), one side of the master work MW is measured, and the measurement data is stored. Next, as shown in FIG.
After rotating the rotary table 3 by 180 degrees, the measurement origin is set and measured in the same manner as described above, and the measurement data is stored. Even if the rotary table 3 is rotated by 180 degrees, one side of the master work MW is located on the center axis of the rotary table 3, so that one side of the master work MW can be rotated without requiring skill or man-hours. 3 can be located on the central axis. In this case, the detector 7
Of the tracing stylus 7A is set inward. Finally, the first measurement data measured first and the second measurement data measured next are calculated, and the master work MW is calculated.
And the straightness of the linear motion mechanism are obtained separately.

In the above-described embodiment, either the straightness accuracy data calculation step or the work measurement data calculation step may be performed first. For example, work W is measured first, work measurement data is obtained and stored, and later,
Measure the master work MW to obtain straightness accuracy data,
At any time, the workpiece measurement data may be corrected with straightness accuracy data. In addition, the correction result (true value data of the work) obtained by correcting the straightness accuracy data for the work measurement data is not limited to the case where it is saved as a file separate from the work measurement data. The correction result (true value data of the work) obtained by correcting the above may be replaced and stored as work measurement data.

In the measuring jig 11 described above, the reference sphere 15 for setting the origin is embedded in the reference vertical surface 14 of the pedestal 12. However, the present invention is not limited to this, and the reference sphere 15 may be embedded in the master work MW. A similar effect can be expected. Also,
In the above-described embodiment, the roundness measuring device has been described. However, the present invention is not limited to this, and can be applied to general measuring devices having a linear motion mechanism, such as a roughness measuring device.

Further, the present invention can also be applied to the correction of aging of the straightness accuracy of a measuring instrument as described below. In general, since a straightness accuracy standard (master work) usable as a master work is expensive, it is difficult for a general user to prepare for the purpose of calibrating a measuring machine.
In particular, a large straightness accuracy standard (in this embodiment, an optical flat) that can be used for straightness accuracy correction of a measuring machine having a long stroke is extremely expensive.

Therefore, only in the first time, straightness accuracy data (this is referred to as straightness accuracy data 1) is created by the master work described in the present embodiment, and immediately thereafter, straightness standards that are generally easily available (with arbitrary accuracy) can be obtained. It is not necessary to price: Prepare a quasi-master work, measure it to obtain work measurement data (this is referred to as work measurement data 1), subtract the straightness accuracy data 1 from this, and calculate the straightness standard (quasi-master work). ) Is obtained and stored in the storage device 23. That is, (work measurement data 1) − (straightness accuracy data 1) = true value data is obtained and stored in the storage device 23. Next, after a certain period (usually about one year), the straightness reference (quasi-master work) is measured again, and the true value data is subtracted from the work measurement data (this is referred to as work measurement data 2). New straight-accuracy data (this is straight-accuracy data 2
). That is, (work measurement data 2) − (true value data) = straight accuracy data 2 is obtained. Straightness data 2 obtained in this way
Is stored in the storage device 23 and used as straightness accuracy data until the next calibration.

According to such a method, measurement of the master work using the high-precision straightness standard need be performed only once, and thereafter, the inexpensive straightness standard (quasi-master work) is used.
As a result, high-accuracy straightness accuracy data can be obtained at any time, so that periodic straightness accuracy calibration of the measuring instrument can be performed at low cost.

[0032]

According to the straightness correction method for a measuring instrument of the present invention, the drawbacks of the inversion method can be eliminated and highly reliable measurement data can be obtained without improving existing equipment.

[Brief description of the drawings]

FIG. 1 is a perspective view of a roundness measuring instrument according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a data processing device of the embodiment.

FIG. 3 is a flowchart showing a straightness accuracy data calculation step and a straightness accuracy data storage step in the embodiment.

FIG. 4 is a flowchart showing a work measurement data calculation step and a work measurement data storage step in the embodiment.

FIG. 5 is a flowchart showing a work shape calculation step in the embodiment.

FIG. 6 is a diagram showing a relationship between a master work and a tracing stylus of a detector in a straightness accuracy data calculating step.

FIG. 7 is a diagram showing a relationship between a master work and a tracing stylus of a detector in a work measurement data calculating step.

FIG. 8 is a perspective view showing a state in which measurement is performed using a measuring jig in a straightness accuracy data calculation step.

9 is a diagram showing a procedure for verifying straightness accuracy data using the measuring jig of FIG. 8;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Roundness measuring machine 4 Column (element which comprises a linear motion mechanism) 5 Slider (element which comprises a linear motion mechanism) 7 Detector 11 Measurement jig 12 Pedestal 13 Work mounting surface 14 Reference vertical plane 15 Reference ball MW master work W work

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Atsushi Tsuruta 6-8-20 Hiroko Shinkai, Kure-shi, Hiroshima Mitutoyo Co., Ltd. (72) Inventor Tsukasa Kojima 1-1-1 Kita 7-Nishi, Kita-ku, Sapporo, Hokkaido No. 2 F term in System Technology Institute, Inc. (Reference) 2F062 AA04 AA51 AA55 AA57 DD12 DD23 DD32 EE01 EE62 FF04 FF17 HH01 HH13 JJ04 2F069 AA04 AA54 AA56 AA57 AA60 AA66 EE04 EE09 GG07 GG07 GG07 GG07 GG07 GG07 GG07 GG07 GG07 GG07 GG07 GG07 GG07

Claims (4)

[Claims] 1. A straightness correction method for a measuring machine having a linear motion mechanism, wherein a master work having shape data pre-valued is measured while moving a detector using the linear motion mechanism of the measuring machine. Then, by subtracting the priced shape data from the master work measurement data to obtain straightness accuracy data of a linear motion mechanism, and a straightness accuracy data calculation step, while moving the detector using the linear motion mechanism of the measuring machine A work measurement data calculating step of measuring the work and obtaining the work measurement data; and a work shape calculating step of calculating the true value data of the work by subtracting the straightness accuracy data from the work measurement data. A straightness correction method for measuring machines. 2. A straightness accuracy correction method for a measuring machine having a linear motion mechanism, wherein a master work having shape data pre-valued is measured while moving a detector using the linear motion mechanism of the measuring machine. Then, the straightness data calculation step of subtracting the valued shape data from the master work measurement data to obtain straightness data of the linear motion mechanism, and storing the straightness accuracy data obtained in the straightness data calculation step A straightness accuracy data storage step, a work measurement data calculation step of measuring a work while moving the detector using the linear motion mechanism of the measuring machine, and a work measurement data calculation step for obtaining the work measurement data, and a work measurement data calculation step A work measurement data storing step of storing the measured work measurement data, and the work measurement data and straightness accuracy stored in each of the storing steps. It reads over data, straightness accuracy correction method measuring machine, characterized in that it comprises a workpiece shape operation step of subtracting the straightness accuracy data from the work measurement data obtaining the true value data of the workpiece. 3. The straightness accuracy correction method for a measuring machine according to claim 1, wherein in the straightness accuracy data calculating step, a part having a pedestal having a work mounting surface and a part of the pedestal or the master work is provided. Using a measuring jig having a reference sphere for setting an origin embedded in an exposed state, a prismatic master work is placed on the work placement surface of the pedestal. The straightness accuracy of the measuring machine is characterized by measuring the master work while moving the detector along one side of the master work by a linear motion mechanism after positioning the detector of the measuring machine at Correction method. 4. The straightness correction method for a measuring machine according to claim 3, wherein the pedestal is formed in a semi-cylindrical shape having a reference vertical surface orthogonal to the workpiece mounting surface, and the reference vertical surface is formed on the base. A straightness correction method for a measuring instrument, characterized in that a reference sphere is embedded in a partially exposed state.