Abstract:
In the machine tool ( 10 ) pertaining to the present invention, an imaging device ( 33 ) takes an image of a tool ( 20 ) being moved in the feeding direction. Contour lines ( 51 ) are identified by means of the plurality of sets of image data generated from imaging. The movement trajectory ( 52 ) and the central axis ( 53 ) of the tool ( 20 ) are identified on the basis of the contour lines ( 51 ). When the movement trajectory ( 52 ) and the central axis ( 53 ) are offset, said offset can be used to correct the positioning of the tool ( 20 ) with the machine tool ( 10 ). As a result, the processing accuracy of a workpiece improves. Moreover, when the dimensions of a tool ( 20 ) that has a tilted posture are measured, it is possible to determine the actual tool diameter or the actual blade position in the tilted posture. The aforementioned blade position and tool diameter can be used to correct the positioning of the machine tool ( 10 ). Thus, the processing accuracy of the workpiece improves even more.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is the national stage application under 35 USC 371 of International Application No. PCT/JP2011/074847, filed Oct. 27, 2011, which claims the priority of Japanese Patent Application No. 2010-241406, filed Oct. 27, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to, for example, a method and apparatus which measure the cutting edge position, tool length, tool size, cutting edge shape, tool run-out, or other tool dimensions of a tool which is used in a CNC (computer numerical control) machine tool and to a machine tool. 
     BACKGROUND OF THE INVENTION 
     For example, in an NC machine tool, at the time of working a workpiece, a tool such as a drill bit or end mill which is attached to a spindle rotates while contacting the workpiece. The tool is fed in a predetermined feed direction in the NC machine tool. In improvement of the working precision of the workpiece, it is considered necessary to specify the center axis of the tool, which determines the center of rotation of the tool or the posture of the tool, and to set the tool to a desired posture.
     PLT 1: Japanese Patent Publication No. 2005-324300A   

     SUMMARY OF THE INVENTION 
     Technical Problem 
     For example, PLT 1 discloses a method of capturing an image of a shape of a tool which is attached to a spindle so as to measure the shape of the tool. With this method, the shape of a tool which is positioned at a predetermined position is measured. Therefore, it is not possible to measure offset between a center axial line of the tool and the feed direction. If such offset occurs, the working precision of the workpiece ends up falling. 
     The present invention was made in consideration of the above situation and has as its object the provision of a method of measurement and an apparatus for measurement of tool dimensions which can measure tool dimensions by a posture and movement path of the tool which is used in the working operation, which can further confirm in advance an offset between a coordinate system of the machine tool and a coordinate system of the measurement apparatus, and which can improve the working precision of the workpiece even more, and of a machine tool using the same. 
     Solution to Problem 
     To achieve the above object, according to the present invention, there is provided a method of measurement of tool dimensions which uses an image capture device which moves relative to a tool so as to capture an image of the tool and which uses the obtained image data to measure the dimensions of the tool, the method of measurement of tool dimensions, comprising the steps of capturing an image of the tool which moves along a predetermined feed direction by the image capture device at a plurality of positions and reading out a plurality of image data which are generated at the positions of the movement and specifying the contour of the tool at each of the plurality of the image data and using the specified plurality of contours as the basis to specify the movement path of the tool and the center axial line of the tool. 
     With this method of measurement of tool dimensions, the movement path of the tool may be specified by calculating the trend in the positions of a reference point, which has a predetermined positional relationship with the contour of the tool, on the plurality of the image data. The center axial line of the tool may be specified by calculating the average of a reference line, which has a predetermined positional relationship with the contour of the tool, on the plurality of the image data. 
     Such a method of measurement of tool dimensions may be further provided with a step of using the specified movement path and center axial line of the tool as the basis to specify at least one of a cutting edge position and tool size of the tool using the contour of the tool. 
     According to the present invention, there is provided an apparatus for measurement of tool dimensions which captures an image of a tool and uses the obtained image data to measure the dimensions of the tool, the apparatus for measurement of tool dimensions comprising an image capture device which captures an image of the tool which moves along a predetermined feed direction at a plurality of positions to generate a plurality of image data of the positions of the movement, and a processing device which specifies contour of the tool at each of the plurality of image data and uses the specified plurality of contours as the basis to specify a movement path of the tool and a center axial line of the tool. 
     According to the present invention, there is provided a machine tool which captures an image of a tool which is mounted on a spindle and uses the obtained image data to measure dimensions of the tool on the machine, comprising an image capture device which captures an image of the tool which moves relatively along a predetermined feed direction at a plurality of positions to generate a plurality of image data of the positions of the movement, and a processing device which specifies the contour of the tool at each of the plurality of image data and uses the specified plurality of contours as the basis to specify a movement path of the tool and a center axial line of the tool, and which uses the specified movement path of the tool as the basis to specify offset between a planar coordinate system of two perpendicular axes which are set in a machine tool and a planar coordinate system of two perpendicular axes which is set in a field of view of the image capture device and is linked with the planar coordinate system of the machine tool and controlling them so as to correct offset. 
     According to the present invention, it is possible to provide a method of measurement and an apparatus for measurement which enable the working precision of a workpiece to be improved much more and a machine tool using the same. It measures the angle formed by the movement path and the center axial line of the tool, that is, the relative angle, so can measure this angle without regard as to mounting error of an image capture device with respect to a machine tool. In the case of work where the center axial line of the tool is made to match the direction of the movement path of this tool, the NC device may be corrected so that this angle becomes zero. If working the workpiece by the vicinity of the vertex on the center axial line of the ball end mill, the peripheral speed of the cutting edge is small and the worked surface falls in quality. To solve this problem, the center axial line of the ball end mill is tilted with respect to the workpiece and the workpiece is worked by the part of the cutting edge with a large peripheral speed away from the vertex. At this time, whether the angle which is formed between the movement path of the tool and the center axial line has become a desired angle is measured. If not, the NC device may be corrected. In particular, a five-axis machine tool which has three perpendicular linear feed axes and two rotary feed axes enables the posture of the tool with respect to the workpiece to be changed, so correction is easy. 
     Further, according to the present invention, since it is possible to specify the movement path of the tool, it is possible to find the error in the movement path of the NC device, that is, the offset of the coordinate axes of the field of view of the measurement apparatus and the coordinate axes of the machine tool, and possible to correct this offset so as to match the measurement results of the tool dimensions with the coordinate system of the machine tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view which schematically shows the configuration of a machine tool according to one embodiment of the present invention. 
         FIG. 2  is a view which shows image data according to a specific example. 
         FIG. 3  is a flow chart which shows a flow of processing of a method of measurement of tool dimensions according to one embodiment of the present invention. 
         FIG. 4  is a view which shows image data according to another specific example. 
         FIG. 5  is a view which schematically shows processing which specifies a movement path and center axial line of a tool. 
         FIG. 6  is a view which schematically shows processing which specifies a tool size or cutting edge position etc. of a tool. 
         FIG. 7  is a view which schematically shows processing which specifies offset of a coordinate system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Below, embodiments of the present invention will be explained with reference to the drawings. 
       FIG. 1  is a view which schematically shows the configuration of a machine tool  100  according to one embodiment of the present invention. This machine tool  10  is provided with, not only mechanical parts which perform work on a workpiece (not shown), but also an NC device  12  which controls operation of the machine tool  10  and a dimension measurement apparatus  13  which is connected to the machine tool  10  and the NC device  12  and which measures the dimensions of a tool of a machine tool  10 . For the machine tool  10 , for example, a five-axis vertical machining center is used. In the present embodiment, the dimension measurement apparatus  13  is shown separate from the NC device  12 , but the dimension measurement apparatus  13  may also for example be assembled inside of the NC device  12 . 
     First, the mechanical configuration of the machine tool  10  will be explained. The machine tool  10  is set with an XYZ perpendicular three-axis reference coordinate system. The machine tool  10  is provided with a bed  15  which extends along a horizontal surface, that is, an XY plane, a table base  16  which is arranged on the bed  15 , and a column  17  which rises from the bed  15  at the rear of the table base  16  in the vertical direction parallel to the Z-axis. A workpiece (not shown) is fastened on the table base  16 . A spindle head  18  is supported on the column  17 . At the bottom end of the spindle head  18 , a tool  20  is attached facing downward via the spindle  19  in a detachable manner. The tool  20  is driven to rotate by a spindle motor (not shown) which is assembled inside the spindle  19 . Here, for the tool  20 , for example, a ball end mill is used. 
     In addition, the spindle head  18  is attached to the spindle stock (not shown) to be able to turn about an axial line parallel to the X-axis through an A-axis rotary feed mechanism. At the same time, the spindle head  18  is attached to the spindle stock to be able to turn about an axial line parallel to the Z-axis through a C-axis rotary feed mechanism. The rotary feed mechanism is, for example, comprised of a direct drive motor or servo motor. The spindle head  18  has built into it rotational angle detectors (not shown) which detect the rotational angles in the A-axis and C-axis rotational directions. The rotational angles which are read by these rotational angle detectors are output, that is, are fed back, to the NC device  12 . 
     The table base  16  is supported on the bed  15  to be able to move in the horizontal direction (X-axial direction) through a linear feed mechanism. On the other hand, the spindle head  18  is supported at the column  17  to be able to move in the vertical direction (Z-axial direction) and horizontal direction (Y-axial direction) through linear feed mechanisms. The linear feed mechanisms are, for example, provided with ball screws and servo motors which drive rotation of the ball screws. Note that, instead of the Y-axial direction movement of the spindle head  18 , the table base  16  may move in the Y-axial direction. Further, instead of a configuration where the spindle head  18  has an A-axis and C-axis, the configuration may be made one where the table base  16  has an A-axis and C-axis. 
     The bed  15  has built into it an X-axial position detector  21  which reads the relative position between the bed  15  and the table base  16  along the X-axis. The column  17  has built into it a Y-axial position detector (not shown) and a Z-axial position detector  22  which read the relative positions between the column  17  and the spindle head  18  along the Y-axis and the Z-axis. For these position detectors, for example, digital scales may be used. The relative positions which are read by these position detectors are specified by the coordinate values of the reference coordinate system. The read coordinate values are output, that is, are fed back, to the NC device  12 . 
     The above-mentioned rotary feed mechanisms and linear feed mechanisms are used to realize relative movement between the table base  16  and the tool  20 . The relative movement is realized based on later explained drive signals which are supplied from the NC device  12 . The tool  20  which rotates during relative movement contacts the workpiece at a predetermined working point. In this way, the workpiece is worked to a desired shape. 
     Next, the configuration of the NC device  12  will be explained. The NC device  12  is provided with a memory unit  24  which stores an NC program, a program analysis unit  25  which analyzes the NC program which is stored in the memory unit  24 , a movement command unit  26  which generates movement commands in accordance with the NC program which was analyzed by the program analysis unit  25 , and a servo control unit  27  which outputs drive signals to the servo motors of the machine tool  10  in accordance with movement commands which are output from the movement command unit  26 . The movement commands include, for example, indexing data of the working point of the workpiece and coordinate value data which shows the position of the spindle  19  corresponding to a working point after indexing. 
     Next, the configuration of the dimension measurement apparatus  13  will be explained. The dimension measurement apparatus  13  is provided with a dimension measurement unit  31  which is arranged on the table base  16 . The dimension measurement unit  31  is provided with a light source  32  and an image capture device  33  which faces the light source  32 . For the light source  32 , for example, a high luminance LED which outputs parallel light toward the image capture device  33  is used. The image capture device  33  is provided with a lens unit  34  and CCD (charge coupled device) image sensor  35 . The CCD image sensor  35  is, for example, a 2D image sensor. 
     The dimension measurement apparatus  13  is provided with an image adjustment device  36  which receives image data of a captured image from the image capture device  33  and a control device  37  which controls the operation of a light source device  32  and image capture device  33 . The above-mentioned CCD image sensor  35  outputs an analog image signal corresponding to an image which is formed on the light receiving surface at, for example, a frame rate of 30 to 60 frames per second. The analog image signal is converted to digital image data by an A/D converter which is built into the image capture device  33  and is output to the image adjustment device  36 . 
     The image adjustment device  36  performs shading correction, noise reduction, white balance adjustment, contour correction, contrast adjustment, and other image adjustment processing and binarizes the digital image data. The image adjustment device  36  stores the image data after image adjustment in the later explained frame memory. On the other hand, the control device  37  outputs a drive signal which controls movement or zoom of the image capture device  33  to the image capture device  33 . Note that, in the field of view of the image capture device  33 , an xy perpendicular 2-axis field-of-view coordinate system is set corresponding to the YZ plane of the above-mentioned reference coordinate system. The coordinate values of this field-of-view coordinate system are linked with the coordinate values of the reference coordinate system in each field of view of the positions after movement of the image capture device  33  in the YZ plane. 
     The dimension measurement apparatus  13  is provided with a memory device  41  which stores the dimension measurement program and tool data, a processing device  42  which performs various processing based on the dimension measurement program, and a frame memory  43  which stores the image data for each frame. In the processing, the dimension measurement program may be read out temporarily to a memory (not shown). Details of the dimension measurement program and tool data will be explained later. Note that, the dimension measurement program may, for example, be read from a FD (floppy disk) or CD-ROM or other portable memory medium to the memory device  41  or may be downloaded from a LAN or Internet or other such computer network to the memory device  41 . 
     The dimension measurement apparatus  13  is, for example, provided with a display device  44  which has a display screen which shows information of two brightness values for each pixel forming the image data which shows the image (silhouette) of a tool or coordinate values of the pixels and an input device  45  which, for example, inputs an instruction to the processing device  42  by designation of a predetermined position on the display screen. The display device  44  may, for example, be a LCD (liquid crystal display) panel or other flat display panel, while the input device  45  may, for example, be a touch panel or a keyboard, mouse, etc. A user, for example, can use a touch panel or a mouse to designate the direction of a contour line of the tool  20  on the image which is displayed on the display screen of the display device  44  or designate a measurement position on a contour line of the tool  20 . 
     Next, the method of measurement of the dimensions of the tool  20  by the machine tool  10  according to the present invention will be explained. Here, for example, the movement path of the tool  20  and the center axial line of the tool  20  are measured. The movement path of the tool  20  is the path of the movement of the tool  20  which is sent in a predetermined feed direction by the linear feed mechanism. The center axial line of the tool  20  is the center axial line of the tool  20  which matches with the center of rotation of the spindle  19 . In performing the processing for measurement of the dimensions of the tool  20 , the processing device  42  of the dimension measurement apparatus  13  reads out the dimension measurement program from the memory device  41  to, for example, the memory. In this way, the processing device  42  performs various processing based on the dimension measurement program. 
     First, the processing device  42  outputs a start signal to the NC device  12 . In response to receipt of the start signal, the NC device  12  outputs a drive command toward the machine tool  10 . As a result, in the machine tool  10 , the spindle  19  is positioned on the XY plane at a predetermined position between the light source  32  and the image capture device  33 . At the same time, the spindle  19  is, for example, positioned at a position of any rotational angle in the A-axial direction. In this way, the center axial line of the tool  20  perpendicularly intersects the Y-axis and the Z-axis on the YZ perpendicular two-axis planar coordinate system of the reference coordinate system. In this way, the tool  20  is set to a slanted posture. Here, the rotational angle is, for example, set to an angle by which the center axial line of the tool  20  rotates from a state parallel to the Z-axis toward a horizontal plane across 45 degrees in the A-axial direction. At this time, the tool  20  is driven to rotate about this center of rotation. 
     At the same time, the processing device  42  outputs a start signal to the control device  37  for starting the operation of the light source  32  and image capture device  33 . In response to receipt of the start signal, the control device  37  outputs a drive signal for driving the image capture device  33 . In this way, the image capture device  33  starts image capture. The image capture device  33  generates an analog image signal for each frame captured. The image data which is generated from this image signal is stored through the image adjustment device  36  in the frame memory  43  for each frame. Details of the image data will be explained later. 
     The tool  20  is made to move in a 45 degree feed direction based on the Y-axial direction linear movement and the Z-axial direction linear movement. The speed of the Y-axial direction linear movement and the speed of the Z-axial direction linear movement are set equal. That is, in setting the machine tool  10 , the A-axial direction rotational angle of the tool  20  and the feed direction angle of the tool  20  are set the same. Therefore, the movement path of the tool  20  is set to a direction which perpendicularly intersects the Y-axis and the Z-axis of the reference coordinate system which is set at the machine tool  10 . If the tool  20  enters the field of view of the image capture device  33 , image data is output for each frame capturing the tool  20  at each position during movement in the set feed direction. Note that, the movement of the tool  20  is stopped at the point of time when the tool  20  moves inside the field of view over a predetermined distance. The distance may be freely set. 
     At the light receiving surface of the CCD image sensor  35 , an image of a projection of a shadow of the tool  20  is formed by parallel light which is emitted from the light source  32 . The image data is comprised of a large number of pixels which specify an image inside of the field of view. As explained above, in the image data, two brightness values are specified for each pixel, so, for example, as shown in  FIG. 2 , in the field of view V specified by the image data, the dark pixels are specified as projected parts of the shadow of the tool  20 , while the bright pixels are specified as parts receiving the parallel light. In this way, the contour of the tool  20  is specified. 
       FIG. 3  is a flow chart which shows the flow of processing of the method of measurement of the dimensions of the tool  20  according to one embodiment of the present invention. At step S 1 , the processing device  42  reads out one frame of image data from the frame memory  43 . Here, image data in which the majority of the contour of the tool  20  enters the field of view V is read out. Based on the read out image data, at step S 2 , the processing device  42  detects the edge of the contour of the tool  20 . As explained above, the pixels are shown by two brightness values, so the edge is specified by dark pixels which adjoin bright pixels among the dark pixels which correspond to pixels of the image of the tool  20  in the field of view of the image. In this way, as clear from  FIG. 2 , at step S 3 , the processing device  42  specifies a contour line  51  of the tool  20  based on the extraction of a continuous plurality of dark pixels adjoining bright pixels. The contour line  51  is, for example, specified by a recurrence curve. 
     Next, the processing device  42  identifies the configuration of the contour line  51  at step S 3 . The tool  20  is a ball end mill, so, as shown in  FIG. 4 , for example, a circle  51   a  which is defined by the front end of the tool  20  and a rectangular shape  51   b  which is connected to the rear end of the circle  51   a  are identified. At step S 4 , in identification of the circle  51   a , it is sufficient that a semicircular component of the front end of the tool  20  be specified from the shape of the contour line  51 . In the specification of a rectangular shape  51   b , it is sufficient that the parallel components (possibly including shank) be specified from the shape of the contour line  51 . At step S 5 , the processing device  42  specifies a reference point which has a predetermined positional relationship with the circle  51   a  from the semicircular component of the circle  51   a . Here, the reference point is the center point P of the circle  51   a . At the same time, the processing device  42  specifies a reference line which has a predetermined positional relationship with the rectangular shape  51   b  from the parallel components of the rectangular shape  51   b . Here, the reference line is the axial line L of the rectangular shape  51   b  which extends in the longitudinal direction of the rectangular shape  51   b.    
     As explained above, inside the field of view V of the image capture device  33 , a field-of-view coordinate system is defined, so the coordinate values in the field-of-view coordinate system of the pixels which form the circle  51   a , center point P, rectangular shape  51   b , and axial line L are specified. Here, the axial line L is converted to a numerical formula based on the coordinate values which specify the rectangular shape  51   b . In this way, the image data with specified coordinate values is stored in the frame memory  43  at step S 6 . The coordinate value data which shows the coordinate values of the field-of-view coordinate system of the circle  51   a , center point P, rectangular shape  51   b , and axial line L may be stored in the frame memory  43  linked with the image data. In this way, the processing of steps S 1  to S 6  is completed. 
     Next, at step S 7 , it is judged if the number of frames which have finished being processed has reached the number of frames which are required for measurement of the dimensions. The number of frames which are required for measurement may be freely set. For measuring the dimensions of a tool  20 , that is, the movement path and center axial line, a plurality of frames is considered necessary, so the processing of the processing device  42  returns to step S 1 . Note that, the number of frames which are required in the present embodiment is, for example, set to 4. The processing device  42  repeats the processing from steps S 1  to S 6  for each frame. The tool  20  moves in the feed direction, so the position of the contour line  51  of the tool  20  changes with each frame. In this way, when the number of frames finished being processing reaches the number of frames required for measurement of the dimensions, the processing of the processing device  42  proceeds to step S 8 . Note that, when the movement path of the tool is straight, the necessary number of frames is the minimum 2, when the movement path is an arc, the necessary number of frames is the minimum 3, and when the movement path is any other, the necessary number of frames is the minimum 4. In either case, the larger the number of frames, the higher the measurement precision, but this is set in advance in accordance with the working conditions. 
     At step S 8 , the trend in the positions of the plurality of center points P specified for each plurality of frames is used as the basis to calculate the movement path of the tool  20 . As shown in  FIG. 5 , circles  51   a  which are specified for each frame are arranged based on the field-of-view coordinate system, and the coordinate values (xn, yn) of the center points Pn are specified. A recurrence curve which passes through all of the specified center points P is calculated. In this way, the calculated recurrence curve corresponds to the movement path  52  of the tool  20 . Next, at step S 9 , the average of the plurality of center axial lines L which are specified by the plurality of frames are used as the basis to calculate the axial line of the tool  20 . Specifically, a recurrence curve of an average of the plurality of center axial lines L is calculated. As shown in  FIG. 5 , the calculated recurrence curve corresponds to the center axial line  53  of the tool  20 . The calculated tool data which specifies the movement path  52  and center axial line  53  are stored in the memory device  41 . In the tool data, the coordinate values may be specified by conversion from the field-of-view coordinate system to coordinate values of the reference coordinate system. 
     As clear from  FIG. 5 , in the example of the present embodiment, the movement path  52  and center axial line  53  of the tool  20  do not match. The movement path  52  and the center axial line  53 , for example, intersect by an intersection angle α. Here, if predicated on the angle of the movement path  52 , that is, the angle of the feed direction, being 45 degrees as set, it is learned that the position of the rotational angle of the spindle  19  is off from 45 degrees of the setting. This offset ends up remarkably lowering the working precision of the workpiece. Therefore, the calculated intersection angle α may be used for mechanical correction of the rotational angle of the A-axial direction of the machine tool  10  or may be used for correction of the coordinate values for control of the position of the spindle  19  by the NC device  12 . In this way, in the machine tool  10 , it is sufficient that correction be performed so that the center axial line  53  and the movement path  52  match. Note that, when, as a result of measurement of the dimensions, the movement path  52  and the center axial line  53  match, this shows that the position of the rotational angle of the spindle  19  is as set. 
     In addition to the above measurement, in the dimension measurement apparatus  13 , the tool size and cutting edge position of the slanted posture tool  20  may also be measured. The processing device  24 , in the same way as above, uses the image data as the basis to specify components of the circle  51   a  and rectangular shape  51   b  from the contour line  51 . At this time, as shown in  FIG. 6 , for example, the tool size of the tool  20  may be measured in a direction perpendicularly intersecting the center axial line  53 . Further, the contact point Yi between the imaginary line YL which is parallel with the Y-axis of the reference coordinate system and the circle  51   a  may be measured as the cutting edge position of the tool  20  in accordance with the relative positional relationship of the tool  20  and the workpiece. On the other hand, the contact point Zi between the imaginary line ZL which is parallel with the Z-axis of the reference coordinate system and the circle  51   a  may be measured as the cutting edge position of the tool  20 . The tool length of the tool  20  may be calculated from the cutting edge position. In measurement of the tool size or cutting edge position, the measurement position on the contour line  51  of the tool  20  may be set in advance by dimension measurement software. As a result, the dimensions of the tool  20  can be measured automatically. 
     In the above way, according to the machine tool  10  according to the present embodiment, the image capture device  33  captures an image of the tool  20  during movement in the feed direction. The plurality of image data which is generated by image capture is used to specify the contour line of the tool  20 . The contour line  51  is used as the basis to specify the movement path  52  and center axial line  53 . If there is offset between the movement path  52  and the center axial line  53 , this offset can be used for correction of the positioning of the tool  20  at the machine tool  10 . As a result, the working precision of the workpiece is improved. Further, for example, if the dimensions of the slanted posture tool  20  are measured, the actual cutting edge position at the slanted posture and the tool size can be specified. The cutting edge position and tool size can be used for correction of the positioning of the machine tool  10 . As a result, the working precision of the workpiece is further improved. 
     In such a machine tool  10 , the user may manually designate the measurement position of the tool size or cutting edge position of the tool  20 . Based on the image data, the contour line  51  is displayed on the display screen of the display device  44 . On the display screen, for example, the measurement position on the contour line  51  may be displayed as an option. The user may specifically designate any measurement position on the contour line  51 . The measurement position may be designated, for example, on the display screen by a mouse. The measurement position is also designated by the contact position on the touch panel. In this way, in the same way as explained above, the processing device  42  can measure dimensions of the tool  20  including the tool size or cutting edge position, tool length, etc. in accordance with the designated measurement position. 
     Next, the processing for specifying the offset of the reference coordinate system of the machine tool  10  and the field-of-view coordinate system of the image capture device  33  will be explained. For example, as shown in  FIG. 7 , in the machine tool  10 , the tool  20  is made to move in a feed direction parallel to the Z-axis of the reference coordinate system. In the image capture device  33 , a movement path  52   a  parallel to the Z-axis is specified. At this time, if the image capture device  33  is accurately positioned with respect to the table base  16 , the movement path  52   a  is prescribed in parallel to the y-axis of the field-of-view coordinate system. As a result, the processing device  42  judges that no offset has occurred between the reference coordinate system and the field-of-view coordinate system. Note that, here the right angle in the machine tool  10  is in the allowable range. 
     On the other hand, as shown in  FIG. 7 , if the field-of-view coordinate system is offset by a predetermined rotational angle about the X-axis of the reference coordinate system, the moving field of view  52   a  is specified as not parallel with the y-axis of the field-of-view coordinate system. That is, the movement path  52   a  intersects the y-axis of the field-of-view coordinate system by a predetermined intersection angle. This intersection angle matches the rotational angle β of the field-of-view coordinate system which is defined about the X-axis of the reference coordinate system. The processing device  42  judges that offset has occurred between the reference coordinate system and the field-of-view coordinate system by exactly the amount of the rotational angle β. At this time, in the machine tool  10 , the specified rotational angle β may be used as the basis to adjust the mechanical attachment position of the image capture device  33  on the table base  16 . Note that, instead of adjustment of the mechanical attachment position, the specified rotational angle β may be used as the basis to, for example, correct the coordinate values between the NC device  12  and the dimension measurement apparatus  13 . 
     Further, in specifying the offset between the reference coordinate system and the field-of-view coordinate system, in the machine tool  10 , the tool  20  may be made to move in a feed direction parallel to the Y-axis. In the image capture device  33 , a movement path  52   b  parallel to the Y-axial direction of the reference coordinate system is specified. The angle of intersection of this movement path  52   b  and x-axis may be used as the basis to specify the rotational angle β in the same way as above. In the same way, in specifying either the reference coordinate system or the field-of-view coordinate system, the tool  20  may be made to move in a feed direction which intersects the Y-axis and the Z-axis. In the image capture device  33 , the movement path  52   c  of the direction perpendicularly intersecting the Y-axis and the Z-axis of the reference coordinate system is specified. At this time, the angle of the feed direction and the angle of the movement path  52   c  on the field of view of the image capture device  33  may be compared. It is also possible to use this movement path  52   c  as the basis to specify the rotational angle β in the same way as above. 
     In the above way, the offset between the reference coordinate system of the machine tool  10  and the field-of-view coordinate system of the image capture device  33  is specified based on the movement path  52   a  to  52   c  of the tool  20 . If the offset of the reference coordinate system and the field-of-view coordinate system is specified in this way, the image capture device  33  can be arranged at an accurate position with respect to the machine tool  10 . As a result, the image capture device  33  can accurately measure the dimensions of the tool  20 . Therefore, the offset which was calculated based on measurement of the dimensions can be used accurately for correction of the positioning of the machine tool  10 . The working precision of the workpiece is therefore improved. 
     In the above embodiments, the method of measurement and the apparatus for measurement of the tool dimensions of the present invention were explained using a vertical machining center as an example of the machine tool  10 , but the method of measurement and the apparatus for measurement of tool dimensions of the present invention can also be realized by, for example, a horizontal machining center or other machine tool. Further, the method of measurement and the apparatus for measurement of tool dimensions of the present invention were explained using a ball end mill as an example of a tool  20 , but the method of measurement and the apparatus for measurement of tool dimensions of the present invention can also be realized, for example, for a flat end mill or drill bit or other tool.