WO2010046754A1

WO2010046754A1 - Boring apparatus and hole machining method

Info

Publication number: WO2010046754A1
Application number: PCT/IB2009/007169
Authority: WO
Inventors: Kozo Takada; Shinichi Machigashira; Tetsuya Inokuchi
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2008-10-22
Filing date: 2009-10-20
Publication date: 2010-04-29
Also published as: JP2010099762A; JP5208669B2

Abstract

A boring apparatus in which a hole in a work (w) is machined by feeding a tool holder (34, 35) provided with a bit (102, 103, 104, 105) in an axial direction relative to the work(w), while rotating the tool holder (34, 35) about an axis, the boring apparatus including: a holder body(lθl); a first bit (102, 103, 104) that machines a hole in the work (W) in a first machining process; a second bit (105) that machines a hole in the work (W) in a second machining process that is performed after the first machining process; a measuring device (60) that measures the hole in the work (W) after the second machining process; and a correction unit (84) that corrects a radial position of the first bit (102, 103, 104) in the first machining process and a radial position of the second bit (105) in the second machining process on the basis of a measurement result in the measuring device (60).

Description

BORING APPARATUS AND HOLE MACHINING METHOD

BACKGROUND OF THE INVENTION

1. Field of the Invention [0001] The invention relates to a boring apparatus and a hole machining method.

2. Description of the Related Art

[0002] Conventional boring apparatuses are described, for example, in Japanese Patent Application Publication No. 6-285705 (JP-A-6-285705) and Japanese Patent

Application Publication No. 2002-307216 (JP-A-2002-307216). In the boring apparatus described in JP-A-6-285705, a precision finish machining bit is provided so that it can move in the radial direction with respect to a holder body. In the boring apparatus described in JP-A-2002-307216, a precision finish machining bit and an intermediate finish machining bit are provided so that they can move in the radial direction with respect to a holder body.

[0003] Generally, the configuration of the boring apparatus described in JP-A-6-285705 is used. In the boring apparatus of this type, although the tip attached to the precision finish machining bit wears out, the accuracy of precision finish machining can be maintained by correcting the protrusion amount of the precision finish machining bit from a holder body. However, the problem associated with the boring apparatus in which the intermediate finish machining bit is fixed is that the service life of the tip of the precision finish machining bit is shortened.

SUMMARY OF THE INVENTION

[0004] The inventors have conducted a comprehensive research of reasons why the service life of the tip of the precision finish machining bit is shortened, and the results obtained revealed the following causes. In the boring apparatus described in JP-A-6-285705, the precision finish machining bit is provided so that it can move in the radial direction with respect to a holder body. However, the intermediate finish machining bit is fixed to the holder body. When a work is machined, obviously not only the tip of the precision finish machining bit, but also the tip of the intermediate finish machining bit wears out. In other words, the machining diameter of the tip of the intermediate finish machining bit gradually decreases. By contrast, the tip of the precision finish machining bit is moved in the radial direction with respect to the holder body, thereby maintaining a constant diameter of precision finish machining.

[0005] As a result, the difference between the precision finish machining diameter and intermediate finish machining diameter gradually increases. In other words, the machining allowance (corresponds to cutting diameter) for the tip of the precision finish machining bit gradually increases. As a result, the wear of the tip of the precision finish machining bit increases. The inventors have found that this is the reason why the service life of the tip of the precision finish machining bit is shortened. In other words, the inventors have found that even if the tip of the intermediate finish machining bit wears out, the service life of the tip of the precision finish machining bit can be increased by inhibiting the variation amount of the machining allowance for the tip of the precision finish machining bit.

[0006] JP-A-2002-307216 describes a configuration in which a precision finish machining bit and an intermediate finish machining bit are provided so that they can move in the radial direction with respect to a holder body. More specifically, JP-A-2002-307216 indicates that the machining allowance for the precision finish machining bit and the machining allowance for the intermediate finish machining bit are easy to adjust. However, the correction of both bits is not described.

[0007] The invention provides a boring apparatus and a hole machining method that can extend the service life of a tip of a precision finish machining bit.

[0008] The first aspect of the invention relates to a boring apparatus in which a hole in a work is machined by feeding a tool holder provided with a bit in an axial direction relative to the work, while rotating the tool holder about an axis, including: a holder body; a first bit that is provided at an outer circumferential surface of the holder body movably in a radial direction of the holder body with respect to the holder body and that machines a hole in the work in a first machining process; a second bit that is provided at the outer circumferential surface of the holder body movably in the radial direction of the holder body with respect to the holder body and that machines a hole in the work in a second machining process that is performed after the first machining process; a measuring unit that measures the hole in the work after the second machining process; and a correction unit that corrects a radial position of the first bit in the first machining process and a radial position of the second bit in the second machining process on the basis of a measurement result in the measuring unit when machining a hole in a next work.

[0009] With the above-described configuration, bits of two kinds, namely, the first bit and second bit can move in the radial direction with respect to the holder body. Therefore, in a case where the first bit tip is worn out, the radial position of the first bit can be corrected, and in a case where the second bit tip is worn out, the radial position of the second bit can be corrected. Therefore, in a case where the second machining process is the precision finish machining process, the variation of allowance determined by the second bit tip in the precision finish machining process can be inhibited with respect to that in the conventional configuration. As a result, the service life of the second bit tip can be extended. [0010] Further, with the above-described configuration, the variation in allowance determined by the first bit tip that is caused by the work can be inhibited, and the variation in allowance determined by the second bit tip that is caused by the work can be also inhibited. Therefore, the first bit tip and the second bit tip can be set to almost identical service lives. In other words, with the above-described configuration, the first bit tip and the second bit tip can be replaced at the same time, thereby making it possible to shorten the equipment stoppage time.

[0011] Further, the work measurements are conducted only after the second machining process that is an after-process, and the radial positions of the first bit and second bit are corrected on the basis of the measurement results. In other words, the radial positions of the first bit and second bit are corrected in one measurement cycle. The correction of the radial position of the first bit can be conducted with high accuracy by measuring the work after the first machining process. However, in this case, two measurement processes are performed: one after the first machining process and the other after the second machining process. Accordingly, the time (cycle time) required for machining and measuring the work is extended. By contrast, with the above-described configuration, by conducting one-cycle measurements after the second machining process, it is possible to correct not only the second bit, but also the first bit. As a result, it is not necessary to increase the number of measurement cycles over that of the conventional configuration. Therefore, the extension of cycle time can be inhibited.

[0012] The correction of the first bit in the first machining process is conducted on the basis of measurement results obtained after the second machining process. However, because the wear amount of the first bit tip is approximately proportional to the wear amount of the second bit tip, the wear amount of the first bit tip can be estimated with a certain accuracy, provided that the wear amount of the second bit is measured. Therefore, a sufficient accuracy can be obtained by correcting the radial position of the first bit on the basis of measurement results obtained after the second machining process. In accordance with the invention, the radial position of the first bit means the position of the first bit in the radial direction of the holder body with respect to the holder body. The radial position of the second bit means the position of the second bit in the radial direction of the holder body with respect to the holder body.

[0013] In the boring apparatus according to the first aspect, the correction unit may determine an accumulated value of correction amount of the radial position of the first bit on the basis of an accumulated value of correction amount of the radial position of the second bit.

[0014] With such a configuration, the accumulated value of correction amount

I U of the radial position of the first bit can be a sufficiently adequate value. As mentioned hereinabove, the wear amount of the first bit tip is approximately proportional to the wear amount of the second bit. Further, the wear amount of the second bit tip corresponds to the accumulated value of correction amount of the radial position of the second bit. In other words, because the accumulated value of correction amount of the radial position of the first bit is determined by referring to the accumulated value of correction amount of the radial position of the second bit, the accumulated value of correction amount of the radial position of the first bit can be determined with sufficient accuracy, without directly measuring the wear amount of the first bit.

[0015] The accumulated value of correction amount of the radial position of the first bit as referred to herein is a difference between the radial position of the first bit in the initial state in which the first bit tip has not been worn out and the radial position of the first bit after correction. The accumulated value of correction amount of the radial position of the second bit as referred to herein is a difference between the radial position of the second bit in the initial state in which the second bit tip has not been worn out and the radial position of the second bit after correction.

[0016] The correction unit may set the accumulated value of correction amount of the radial position of the first bit lower than the accumulated value of correction amount of the radial position of the second bit.

[0017] With such a configuration, the first bit tip can be prevented from conducting machining in excess of the machining diameter that has to be machined by the second bit tip. If the machining conducted by the first bit tip exceeds the machining diameter that has to be machined by the second bit tip, the work cannot be formed to a target shape. In other words, the work is defective. Accordingly, the above-described feature can inhibit the production of defective works.

[0018] In the boring apparatus according to the first aspect, a machining process for a hole in the work may include a precision finish machining process of precision finish machining the hole in the work and an intermediate finish machining process performed immediately before the precision finish machining process, wherein the first machining process is the intermediate finish machining process, and the second machining process is the precision finish machining process.

[0019] With such a configuration, the bit in the precision finish machining process" and the bit in the intermediate finish machining process are corrected. As a result, the service life of the second bit tip in the finish machining process can be extended. *

[0020] In the boring apparatus according to the first aspect, a machining process of a hole in the work may include a precision finish machining process of precision finish machining the hole in the work, an intermediate finish machining process performed immediately before the precision finish machining process, and a rough machining process performed immediately before the intermediate finish machining process, wherein the first machining process is the intermediate finish machining process and the rough machining process, the second machining process is the precision finish machining process, and the first bit includes an intermediate finishing bit that machines the hole in the work in the intermediate finish machining process and a rough machining bit that machines the hole in the work in the rough machining process.

[0021] With such a configuration, the bit in the precision finish machining process, bit in the intermediate finish machining process, and bit in the rough machining process are corrected. As a result, the service life of the second bit tip in the precision finish machining process can be extended. The service life of the intermediate finish machining bit in the intermediate finish machining process can be also extended.

[0022] The second aspect of the invention relates to a hole machining method in which a hole in a work is machined by using a tool holder in which a bit is provided at an outer circumferential surface of a holder body and feeding the tool holder in an axial direction relative to the work, while rotating the tool holder about an axis, including: a first machining process of machining the hole in the work in use of a first bit that is provided at the outer circumferential surface of the holder body movably in a radial direction of the holder body with respect to the holder body; a second machining process of machining the hole in the work after the first machining process in use of a second bit that is provided at the outer circumferential surface of the holder body movably in the radial direction of the holder body with respect to the holder body; and a measuring process of measuring the hole in the work after the second machining process, wherein the first machining process includes a process of correcting a radial position of the first bit on the basis of a measurement result and machining a hole in the work when a next hole is machined in the work, and the second machining process includes a process of correcting a radial position of the second bit on the basis of a measurement result and machining the hole in the work when a next hole is machined in the work.

[0023] The second aspect corresponds to a machining method using the boring apparatus according to the above-described first aspect. In other words, the method according to the second aspect demonstrates an effect similar to that demonstrated by the boring apparatus according to the first aspect. Thus, the service life of the second bit tip is extended. Further, the first bit tip and second bit tip can be replaced at the same time. Therefore, the equipment stoppage time can be shortened. In addition, the increase in cycle time can be inhibited.

BRIEF DESCRIPTION OF THE DRAWINGS [0024] The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a front view of the boring apparatus of the embodiment; FIG 2 is a right side view of the boring apparatus of the embodiment;

FIG 3 is an enlarged 3A-3A sectional view of the configuration shown in FIG 1; FIG 4 is an enlarged 4B-4B sectional view of the configuration shown in FIG 3; FIG 5 illustrates a state in which the 5C-5C sectional view of the configuration shown in FIG 4 is turned to the right through 90°; FIG 6 is a 6D-6D sectional view of the configuration shown in FIG 4;

FIG. 7 is a block diagram of the control device 80 of the embodiment; FIG 8 is a flowchart illustrating a hole machining method using the boring apparatus of the embodiment;

FIG. 9 is a process diagram illustrating the machining processing performed by the machining control unit 82; and

FIG. 10 is a graph showing the relationship between the accumulated value of corrected amount and number of precision finish machining correction cycles, this graph illustrating the correction processing performed by the correction unit 84 constituting the control device 80.

DETAILED DESCRIPTION OF EMBODIMENTS

[0025] Specific embodiments of the boring apparatus and hole machining method in accordance with the invention will be described below with reference to the appended drawings.

[0026] <First Embodiment (Entire Configuration of the Boring Apparatus) The entire configuration of the boring apparatus of the first embodiment will be explained below with reference to FIGS. 1 and 2. FIG 1 is a front view of the boring apparatus. FIG. 2 is a right side view of the boring apparatus. In this case, a work W that is to be machined by the boring apparatus of the embodiment is a cylinder block of an engine, and the machining site thereof is a bore portion of the cylinder block. The object of machining is a four-cylinder cylinder block, and the boring apparatus is considered in which two cylinders can be machined at the same time.

[0027] As shown in FIGS. 1 and 2, the boring apparatus of the embodiment is mainly provided with a bed 10, a column 20, a main shaft head 30, a jig table 50, a measuring device 60, a measurement table 70, and a control device 80.

[0028] The bed 10 is formed as a thin plate by casting and fixed to the floor. The column 20 is provided with a column body 21, a ball screw for Z-axis feed (not shown in the figure), and a motor 22 for Z-axis feed. The column body 21 is formed by casting in an almost rectangular parallelepiped shape, installed vertically on the bed 10, and slidably supports the main shaft head 30. Two guide rails 21a are formed parallel to each other and so as to extend in the vertical direction at the side surface (front surface in FIG 1, left surface in FIG 2) of the column body 21. The ball screw for Z-axis feed is supported by the column body 21 so as to extend, in the vertical direction between the two guide rails 21a and be able to rotate about the Z axis (about vertical axis). The Z-axis feed motor 22 is disposed at the upper end of the column body 21 and coupled to the upper end of the ball screw for Z-axis feed so as to drive rotationally the ball screw for Z-axis feed. [0029] The main shaft head 30 is provided with a main shaft head body 31, first and second main shafts 32, 33, first and second tool holders 34, 35, first and second draw bars 36, 37, a rotation motor 38, a rotation transmission mechanism 39, first and second

• push-out motors 40, 41, and first and second push-out mechanisms 42, 43. The main shaft head 30 will be described below in greater detail. General features of the main shaft head 30 will be now explained.

[0030] The main shaft head body 31 is supported so that it can slide in the vertical direction with respect to the column body 21. The first and second main shafts 32, 33 are provided rotatably about the vertical axis (about Z axis) at the main shaft head body 31. The first and second tool holders 34, 35 are fixed to the lower ends of the first and second main shafts 32, 33. The first and second tool holders 34, 35 are provided with a holder body 101 and a plurality of bits 102 to 105 provided at the outer circumferential surface of the holder body 101. Tips 102c to 105c are attached to the bits 102 to 105. In other words, a bore portion (hole) is machined in the cylinder block W by feeding the tool holder 34, 35 in the axial direction of the cylinder block W, which is work, while rotating the tool holders about the respective axis. In this case, the rotation of the first and second main shaft 32, 33 is performed by the rotation motor 38 via the rotation transmission mechanism 39.

[0031] The precision finish machining bit 105 and intermediate finish machining bits 103, 104 are provided so that they can move in the radial direction of the holder body 101 with respect to the holder body 101. In other words, the precision finish machining bit 105 and intermediate finish machining bits 103, 104 can be corrected for the radial positions thereof with respect to the holder body 101 in accordance with the wear amount of the tips 105c, 103c, 104c. The radial movement of the precision finish machining bit 105 and intermediate finish machining bits 103, 104 is performed by driving the first and second push-out motors 40, 41 via the first and second draw bars 36, 37 and first and second push-out mechanisms 42, 43.

[0032] The jig table 50 is disposed on the bed 10, serves as the front surface side (front surface side in FIG. 1) of the column body 21, and is positioned below the main shaft head body 31. The jig table 50 is provided so that it can move in the X-axis direction between both ends of the column body 21 with respect to the bed 10. More specifically, the jig table 50 is provided for a ball screw of X-axis feed and a motor for X-axis feed. At the jig table 50, the work (cylinder block) subjected to pre-machining treatment and carried in to the boring apparatus is clamped. [0033] The measuring device 60 is constituted, for example, by a touch cell and provided adjacently to the column 20 on the bed 10. More specifically, the measuring device 60 is provided at the carry-out side of the cylinder block W in the column 20. The measuring device 60 can move in the direction (Z-axis direction) of approaching to or withdrawing from the upper surface of the bed 10. The measuring device 60 measures the inner diameter of the bore portion of the cylinder block W. More specifically, the measuring device measures the bore portions of the second cylinder and fourth cylinder from the left side in FIG. 1, from among the bore portions of four cylinders. In other words, the measuring device 60 performs measurements in the site machined by the first tool holder and the site machined by the second tool holder. [0034] The measurement table 70 is positioned on the bed 10 directly below the measuring device 60. In other words, the measurement table 70 is a table onto which the machined cylinder block W is loaded. The measurement table 70 is provided so that it can move in the X-axis direction with respect to the bed 10. More specifically, the measurement table 70 is provided with a ball screw for X-axis feed and a motor for X-axis feed.

[0035] The control device 80 controls the motor 22 for Z-axis feed that drives the main shaft head 30, the motor for Z-axis feed that drives the measuring device 60, the motor 38 for rotating the first and second main shafts 32, 33, motors 40, 41 for pushing out the bits 102 to 105, the motor for X-axis feed of the jig table 50, and the motor for X-axis feed of the measurement table 70. In other words, by controlling the motors, the control device 80 conducts alignment control of all the moving members. The detailed configuration of the control device 80 will be described below.

[0036] (Detailed Configuration of the Main Shaft Head 30) The detailed configuration of the main shaft head 30 will be described with reference to FIGS. 3 to 6 in addition to FIGS. 1 and 2. FIG. 3 is an enlarged 3A-3A sectional view of the

) configuration shown in FIG. 1. FIG. 4 is an enlarged 4B-4B sectional view of the configuration shown in FIG. 3. FIG. 5 illustrates a state in which the 5C-5C sectional view of the configuration shown in FIG 4 is turned to the right through 90°. FIG. 6 is a 6D-6D sectional view of the configuration shown in FIG. 4.

[0037] As described above, the main shaft head 30 is provided with the main shaft head body 31, first and second main shafts 32, 33, first and second tool holders 34, 35, first and second draw bars 36, 37, rotation motor 38, rotation transmission mechanism 39, first and second push-out motors 40, 41, and first and second push-out mechanisms 42, 43.

[0038] The main shaft head body 31 is formed by casting in a substantially rectangular parallelepiped shape. A ball screw nut (not shown in the figure) is formed at the rear surface (right surface in FIG. 2) of the main shaft head body 31. The ball screw nut is screwed onto the ball screw for Z-axis feed that is attached to the column body 21. In other words, where the motor 22 for Z-axis feed that is attached to the column body 21 is rotationally driven, the ball screw for Z-axis feed rotates and the main shaft head body 31 moves in the vertical direction with respect to the column body 21, following this rotation. In the main shaft head body 31, a through hole 31a extending in the vertical direction is formed in two (left and right) locations in the configuration shown in FIG 1. The pitch of the two through holes 31a is set to be twice as large as the pitch of the bore portions of the cylinder block W. The first and second main shafts 32, 33 have identical cylindrical shape and are rotationally inserted in respective through holes 31a of the main shaft head body 31, with the bearings being interposed therebetween.

[0039] The first and second tool holders 34, 35 are attached to the lower ends of the first and second main shafts 32, 33, respectively. In other words, the first and second tool holders 34, 35 rotate with respect to the main shaft head body 31, following the rotation of the first and second main shafts 32, 33. The first and second tool holders 34, 35 have identical configurations. Therefore, only the first tool holder 34 will be described below with reference to FIGS. 3 to 6.

[0040] The first tool holder 34 is constituted by the holder body 101, a rough machining bit 102, a first intermediate finish machining bit 103, a second intermediate finish machining bit 104, a precise finishing bit 105, first to third slide pieces 106 to 108, first to third movable pins 109 to 111, a rotation restricting pin 112, and a cap 113. [0041] As shown in FIGS. 5 and 6, the holder body 101 is formed in a cylindrical shape having a through hole along the central axis. A total of four notches 101a to 101d of a predetermined length are formed in the axial direction equidistantly in the circumferential direction at the outer circumferential surface of the holder body 101. The radial sectional shape of the notches 101a to 101d is an almost L-like shape toward the circumferential direction of the holder body 101, as shown in FIG. 4. A female thread is formed in the notches 101a to 101d. Through holes lOle to 101g passing through the surface of the notches 101b to 101d and the inner circumferential surface of the holder body 101 are formed in the notches 101b to 101d in three locations of the holder body 101. Further, a through hole 101h that passes through from the outer circumferential surface to the inner circumferential surface is formed in the upper site of the notch 101a where the through holes lOle to 101g have not been formed. In other words, the through holes lOle to 101g and through hole 101h are formed in different locations in the axial direction.

[0042] The rough machining bit 102 is provided with a tip 102c for conducting rough machining of the bore portion of the cylinder block W. More specifically, the rough machining bit 102 is configured by a bit body 102a, an attachment bolt 102b, and the tip 102c, as shown in FIG 5. The bit body 102a of the rough machining bit 102 is formed in a narrow rectangular parallelogram shape. The bit body 102a is disposed in the notch 101a and fixed to the holder body 101 by screwing the attachment bolt 102b into the female thread of the holder body 101. As shown in FIG 4, this bit body 102a can be accommodated in the notch 101a of the holder body 101. In other words, in a state in which the bit body 102a is attached to the holder body 101, the bit body is positioned on the inside of the circumscribed circle of the holder body 101. The tip 102c is fixed below the bit body 102a so as to protrude outward from the outer circumferential surface of the bit body 102a. In a state in which the bit body 102a is fixed to the holder body 101, the tip 102c protrudes radially outward from the circumscribed circle of the holder body 101. The diameter of machining with the tip 102c changes only due to the wear of the tip 102c. [0043] The first intermediate finish machining bit 103 is provided with a tip

103c for conducting intermediate finish machining after the rough machining of the bore portion of the cylinder block W has been completed. More specifically, as shown in FIG 6, the first intermediate finish machining bit 103 is configured by a bit body 103a, an attachment bolt 103b, and the tip 103c. The bit body 103a of the first intermediate finish machining bit 103 is formed as a whole in a narrow rectangular parallelogram shape and is provided with a flexible portion 103d. The flexible portion 103d is formed to be thinner than other zones by forming a groove. Because of deflection of the flexible portion 103d, one side of the flexible portion 103d can shift with respect to the other side. [0044] The bit body 103a is disposed in a notch 101b formed adjacently to the notch 101a, and the bit body is fixed to the holder body 101 by screwing the attachment bolt 103b in the female thread of the holder body 101. The position at the bit body 103a into which the attachment bolt 103b is inserted is above the flexible portion 103d. As shown in FIG 4, the bit body 103a can be accommodated in the notch 101b of the holder body 101. In other words, in a state in which the bit body 103a is attached to the holder body 101, the bit body is positioned inside the circumscribed circle of the holder body 101. The tip 103c is fixed below the flexible portion 103d of the bit body 103a so as to protrude outward from the outer circumferential- surface of the bit body 103a. In a state in which the bit body 103a is fixed to the holder body 101, the tip 103c protrudes radially outward from the circumscribed circle of the holder body 101. Further, the position of the radial outward end portion of the tip 103c moves radially outward when the flexible portion 103d is deflected. In other words, the diameter of machining with the tip 103c varies depending on the deflection amount of the bid body 103a and wear of the tip 103c. [0045] Similarly to the first intermediate finish machining bit 103, the second intermediate finish machining bit 104 is provided with a tip 104c for conducting intermediate finish machining after the rough machining of the bore portion of the cylinder block W has been completed. More specifically, as shown in FIG. 6, the second intermediate finish machining bit 104 is configured by a bit body 104a, an attachment bolt 104b, and the tip 104c. The bit body 104a of the second intermediate finish machining bit 104 is formed as a whole in a narrow rectangular parallelogram shape and is provided with a flexible portion 104d. The flexible portion 104d is formed to be thinner than other zones by forming a groove. Because of deflection of the flexible portion 104d, one side of the flexible portion 104d can shift with respect to the other side. [0046] The bit body 104a is disposed in a notch 101d formed at the rear surface side of the notch 101b adjacently to the notch 101a, and the bit body is fixed to the holder body 101 by screwing the attachment bolt 104b in the female thread of the holder body 101. The position at the bit body 104a into which the attachment bolt 104b is inserted is above the flexible portion 104d. As shown in FIG 4, the bit body 104a can be accommodated in the notch 101d of the holder body 101. In other words, in a state in which the bit body 104a is attached to the holder body 101, the bit body is positioned inside the circumscribed circle of the holder body 101. The tip 104c is fixed below the flexible portion 104d of the bit body 104a so as to protrude outward from the outer circumferential surface of the bit body 104a. In a state in which the bit body 104a is fixed to the holder body 101, the tip 104c protrudes radially outward from the circumscribed circle of the holder body 101. Further, the position of the radial outward end portion of the tip 104c moves radially outward when the flexible portion 104d is deflected. In other words, the diameter of machining with the tip 104c varies depending on the deflection amount of the bid body 104a and wear of the tip 104c. [0047] The precision finish machining bit 105 is provided with a tip 105c for conducting precision finish machining after the rough machining of the bore portion of the cylinder block W has been completed. More specifically, as shown in FIG. 5, the precision finish machining bit 105 is configured by a bit body 105a, an attachment bolt 105b, and the tip 105c. The bit body 105a of the precision finish machining bit 105 is formed as a whole in a narrow rectangular parallelogram shape and is provided with a flexible portion 105d. The flexible portion 105d is formed to be thinner than other zones by forming a groove. Because of deflection of the flexible portion 105d, one side of the flexible portion 105d can shift with respect to the other side. [0048] The bit body 105a is disposed in the notch 101c formed at the rear surface side of the notch 101a where the rough machining bit 102 is fixed and adjacently to the notches 101b, 101d, and the bit body is fixed to the holder body 101 by screwing the attachment bolt 105b in the female thread of the holder body 101. The position at the bit body 105a into which the attachment bolt 105b is inserted is above the flexible portion 105d. As shown in FIG. 4, the bit body 105a can be accommodated in the notch 101c of the holder body 101. In other words, in a state in which the bit body 105a is attached to the holder body 101, the bit body is positioned inside the circumscribed circle of the holder body 101. The tip 105c is fixed below the flexible portion 105d of the bit body 105a so as to protrude outward from the outer circumferential surface of the bit body 105a. In a state in which the bit body 105a is fixed to the holder body 101, the tip 105c protrudes radially outward from the circumscribed circle of the holder body 101. Further, the position of the radial outward end portion of the tip 105c moves radially outward when the flexible portion 105d is deflected. In other words, the diameter of machining with the tip 105c varies depending on the deflection amount of the bid body 105a and wear of the tip 105c.

[0049] The first to third slide pieces 106 to 108 have an L-shaped cross section, as shown in FIGS. 5 and 6. One side of the L-like shape of the first to third slide pieces 106 to 108 is mated with the lower end surface of the holder body 101, and the other side of the L-like shape of the first to third slide pieces 106 to 108 abuts against the inner circumferential surface of the holder body 101.

[0050] More specifically, the first slide piece 106 is disposed in a first reduced-diameter taper groove 202 of the below-described draw bars 36, 37 and therefore positioned in a location in which it closes an inner circumferential opening of the through hole lOle in the circumferential direction, as shown in FIG 6. The second slide piece 107 is disposed in a second reduced-diameter taper groove 203 of the below-described draw bars 36, 37 and therefore positioned in a location in which it closes an inner circumferential opening of the through hole 101f in the circumferential direction, as shown in FIG. 6. The third slide piece 108 is disposed in an enlarged-diameter taper groove 204 of the below-described draw bars 36, 37 and therefore positioned in a location in which it closes an inner circumferential opening of the through hole 101g in the circumferential direction, as shown in FIG 5.

[0051] The first to third movable pins 109 to 111 are inserted into the through - holes lOle to 101g formed in the holder body 101. These first to third movable pins 109 to 111 can move in the radial direction of the holder body 101 with respect to the holder body 101. The radial outward end of the first movable pin 109 abuts against the bit body 103a of the first intermediate finishing bit 103, and the radial inward end of the first movable pin 109 abuts against the first slide piece 106. The radial outward end of the second movable pin 110 abuts against the bit body 104a of the second intermediate finishing bit 104, and the radial inward end of the second movable pin 110 abuts against the second slide piece 107. The radial outward end of the third movable pin 111 abuts against the bit body 105a of the precision finishing bit 105, and the radial inward end of the third movable pin 111 abuts against the third slide piece 108.

[0052] The rotation restricting pin 112 is inserted and fixed in the through hole 101h. The radial inward end of the rotation restricting pin 112 protrudes radially inward from the inner circumferential surface of the holder body 101 and engages with a key groove 201 of the draw bars 36, 37. In other words, the rotation restricting pin 112 has a function of restricting the relative rotation of the holder body 101 and drawn bars 36, 37. The cap 113 is fixed to the lower end surface of the holder body 101 and covers the lower end side of the holder body 101.

[0053] The first and second draw bars 36, 37 are inserted into hollow portions of the first and second main shafts 32, 33, provided so that the draw bars cannot rotate relative to the first and second main shafts 32, 33, and provided so that the draw bars can move axially relative to the first and second main shafts 32, 33. The first and second draw bars 36, 37 have the same configuration. Accordingly, only the first draw bar 36 will be explained below with reference to FIGS. 3 to 6.

[0054] The first draw bar 36 is composed of a solid shaft member. The first draw bar 36 is inserted into the hollow portion of the first main shaft 32 and a hollow portion of the holder body 101. The key groove 201, first reduced-diameter taper groove 202, second reduced-diameter taper groove 203, and enlarged-diameter taper groove 204 are formed in the outer circumferential surface at the lower end side of the first draw bar 36.

[0055] As shown in FIG 5, the key groove 201 has a predetermined length in the axial direction and is formed so that the groove bottom of the key groove 201 is parallel to the axial direction. The key groove 201 is formed in a position corresponding to the inner circumferential opening of the through hole 101h that is formed in the holder body 101. The rotation restricting pin 112 is engaged with the key groove 201. In other words, by engaging with the key groove 201, the rotation restricting pin 112 restricts the relative rotation of the holder body 101 and draw bar 36. However, the holder body 101 having the rotation restricting pin 112 fixed thereto can move in the axial direction relative to the draw bar 36 through the length of the key groove 201.

[0056] As shown in FIG 6, the first reduced-diameter taper groove 202 is formed in a position shifted in phase by 90° in the circumferential direction from the key groove 201 so as to have a predetermined length in the axial direction from the lower end of the draw bar 36 and so that the groove bottom of the first reduced-diameter taper groove 202 reduces in diameter in the downward direction in FIG. 6. The first slide piece 106 is slidably inserted into the first reduced-diameter taper groove 202. The depth of the first reduced-diameter taper groove 202 is slightly larger than the radial thickness of the sliding portion of the first slide piece 106 at the lower end side of the draw bar 36 and is almost equal to the radial thickness of the sliding portion of the first slide piece 106 at the upper end side of the draw bar 36. In other words, the first slide piece 106 moves in the radial direction of the holder body 101 by following the axial movement of the draw bar 36 relative to the holder body 101.

[0057] More specifically, in a case where the draw bar 36 moves from top to bottom with respect to the holder body 101, the first slide piece 106 moves radially outward with respect to the holder body 101. In this case, the first slide piece 106 pushes the first movable pin 109 radially outward. As a result, the bit body 103a of the first intermediate finishing bit 103 is deflected radially outward. In other words, as a result of the above-descried operation, the position of the radial outward end of the tip 103c of the first intermediate finishing bit 103 moves radially outward.

[0058] In a case where the draw bar 36 moves from bottom to top with respect to the holder body 101, the first slide piece 106 moves radially inward with respect to the holder body 101. In this case, the first slide piece 106 pulls back the first movable pin 109 radially inward due to elastic recovery of the bit body 103a. In other words, as a result of the above-descried operation, the position of the radial outward end of the tip 103c of the first intermediate finishing bit 103 moves radially inward.

[0059] As shown in FIG 6 the second reduced-diameter taper groove 203 is formed in a position shifted in phase by 90° in the circumferential direction from the key groove 201 on the rear surface side of the first reduced-diameter taper groove 202 so as to have a predetermined length in the axial direction from the lower end of the draw bar 36 and so that the groove bottom of the second reduced-diameter taper groove 203 reduces in diameter in the downward direction in FIG 6. The shape of the second reduced-diameter taper groove 203 is identical to that of the first reduced-diameter taper groove 202. The second slide piece 107 is slidably inserted into the second reduced-diameter taper groove 203. The depth of the second reduced-diameter taper groove 203 is slightly larger than the radial thickness of the sliding portion of the second slide piece 107 at the lower end side of the draw bar 36 and is almost equal to the radial thickness of the sliding portion of the second slide piece 107 at the upper end side of the draw bar 36. In other words, the second slide piece 107 moves in the radial direction of the holder body 101 by following the axial movement of the draw bar 36 relative to the holder body 101. [0060] More specifically, in a case where the draw bar 36 moves from top to bottom with respect to the holder body 101, the second slide piece 107 moves radially outward with respect to the holder body 101. In this case, the second slide piece 107 pushes the second movable pin 110 radially outward. As a result, the bit body 104a of the second intermediate finishing bit 104 is deflected radially outward. In other words, as a result of the above-descried operation, the position of the radial outward end of the tip 104c of the second intermediate finishing bit 104 moves radially outward.

[0061] In a case where the draw bar 36 moves from bottom to top with respect to the holder body 101, the second slide piece 107 moves radially inward with respect to the holder body 101. In this case, the second slide piece 107 pulls back the second movable pin 110 radially inward due to elastic recovery of the bit body 104a. In other words, as a result of the above-descried operation, the position of the radial outward end of the tip 104c of the second intermediate finishing bit 104 moves radially inward.

[0062] As shown in FIG 5, the enlarged-diameter taper groove 204 is formed in a position shifted in phase by 90° in the circumferential direction from the first and second reduced-diameter taper grooves 202, 203 on the rear surface side of the key groove 201 so as to have a predetermined length in the axial direction from the lower end of the draw bar 36 and so that the groove bottom of the enlarged-diameter taper groove 204 increases in diameter in the downward direction in FIG. 5. The third slide piece 108 is slidably inserted into the enlarged-diameter taper groove 204. The depth of the enlarged-diameter taper groove 204 is almost equal to the radial thickness of the sliding portion of the third slide piece 108 at the lower end side of the draw bar 36 and is slightly larger than the radial thickness of the sliding portion of the third slide piece 108 at the upper end side of the draw bar 36. In other words, the third slide piece 108 moves in the radial direction of the holder body 101 by following the axial movement of the draw bar 36 relative to the holder body 101.

[0063] More specifically, in a case where the draw bar 36 moves from bottom to top with respect to the holder body 101, the third slide piece 108 moves radially outward with respect to the holder body 101. In this case, the third slide piece 108 pushes the third movable pin 111 radially outward. As a result, the bit body 105a of the precision finishing bit 105 is deflected radially outward. In other words, as a result of the above-descried operation, the position of the radial outward end of the tip 105c of the precision finishing bit 105 moves radially outward.

[0064] In a case where the draw bar 36 moves from top to bottom with respect to the holder body 101, the third slide piece 108 moves radially inward with respect to the holder body 101. In this case, the third slide piece 108 pulls back the third movable pin 111 radially inward due to elastic recovery of the bit body 105a. In other words, as a result of the above-descried operation, the position of the radial outward end of the tip 105c of the precision finishing bit 105 moves radially inward. [0065] As shown in FIGS. 1 to 3, the rotation motor 38 is fixed to the front side

(as shown in FIG 1) of the main shaft head body 31 in the vicinity of the upper ends of the draw bars 36, 37. The rotation motor 38 serves to rotate the first and second main shafts 32, 33 simultaneously. The rotation transmission mechanism 39 is driven, for example, by gear drive or belt drive. The rotation transmission mechanism 39 transmits the rotation drive force of the rotation motor 38 to the first and second main shafts 32, 33.

[0066] The first and second push-out motors 40, 41 are fixed directly above the first and second main shafts 32, 33 at the upper end of the main shaft head body 31.

These first and second push-out motors 40, 41 serve to move the first and second draw bars 36, 37 in the axial direction with respect to the first and second main shafts 32, 33. In other words, the first and second push-out motors 40, 41 serve to position the tips 103c, 104c, 105c in the radial direction.

[0067] The first and second push-out mechanisms 42, 43 serve to convert the rotational drive forces of the first and second push-out motors 40, 41 into the axial movement of the draw bars 36, 37. More specifically, the first (second) push-out mechanism 42, 43 is configured by a ball screw 301, a ball screw nut 302, and a linking member 303. The ball screw 301 is linked to the first (second) push-out motor 40 (41) and is provided at the main shaft head body 31 coaxially with the first (second) draw bar 36 (37). The ball screw nut 302 is screwed on the ball screw 301 and fixed to the linking member 303. The linking member 303 is linked to the ball screw nut 302 and draw bar 36 (37). A rolling bearing is interposed between the linking member 303 and the draw bar 36 (37). In other words, where the first (second) push-out motor 40 (41) rotates, the ball screw 301 rotates and the ball screw nut 302 moves in the axial direction with respect to the main shaft head body 31. Because of this movement of the ball screw nut 302, the linking member 303 moves in the axial direction with respect to the main shaft bead body 31. In this case, because the linking member 303 is engaged with the draw bar 36 (37) in the axial direction, the draw bar 36 (37) moves in the axial direction with respect to the main shaft head body 31 by following the axial movement of the linking member 303 with respect to the main shaft head body 31. [0068] (Detailed Configuration of Control Device 80) The detailed configuration of the control device 80 will be described below with reference to FIG 7. FIG 7 is a block diagram of the control device 80. As shown in FIG 7, the control device 80 is configured of a command unit 81, a machining control unit 82, a measurement control unit 83, and a correction unit 84. [0069] The command unit 81 outputs commands relating to machining and measuring the cylinder block W, which is the work, on the basis of a predetermined program. The machining control unit 82 controls various motors necessary for machining on the basis of the command outputted from the command unit 81 and a correction command outputted from the correction unit 84. More specifically, the machining control unit 82 controls the motor 22 for Z-axis feed that drives the main shaft head 30, rotation motor 38, first and second push-out motors 40, 41, and a motor for X-axis feed of the jig table 50.

[0070] The measurement control unit 83 controls various motors for measuring the cylinder block W, which is the work, after machining on the basis of the command outputted from the command unit 81. More specifically, the measuring command unit 83 controls the motor for Z-axis feed that drives the measuring device 60 and a motor for X-axis drive of the measuring table 70. Furthermore, the measurement control unit 83 also actuates the measurement operation of the measuring device 60. [0071] The correction unit 84 inputs the measurement results obtained by the measuring device 60. The correction unit 84 outputs a correction command to the machining control unit 82 on the basis of the measurement results. The correction command, as referred to herein, is a command that causes the correction of the radial position of the first and second intermediate finishing bits 103, 104 and the radial position of the precision finishing bit 105. More specifically, in a case where the difference between a reference value and an inner diameter of a bore portion of the machined cylinder block W that has been measured with the measuring device 60 becomes equal to or greater than a predetermined first threshold (for example, 2 μm), when the subsequent precision fishing machining of the cylinder block W is conducted, the radial position of the precision finishing bit 105 is corrected so that the difference between this radial position and the reference value becomes zero. After the correction of the radial position of the precision finishing bit 105 has been repeated several times, the correction of the radial positions of the intermediate finishing bits 103, 104 is conducted. [0072] (Method for Hole Machining with the Boring Apparatus) A hole machining method that uses the above-described boring apparatus will be described below with reference to FIGS. 8 to 10. FIG. 8 is a flowchart illustrating the hole machining method using the above-described boring apparatus. FIG 9 is a process diagram illustrating the machining processing performed by the machining control unit 82. FIG 10 is a graph showing the relationship between the accumulated value of corrected amount and number of precision finish machining correction cycles, this graph illustrating the correction processing performed by the correction unit 84 constituting the control device 80.

[0073] As shown in FIG 8, the cylinder block W, which is the work, that has been carried in to the boring apparatus is subjected to machining processing (machining process: Sl). As shown in FIG. 9, in the machining processing, the jig table 50 at which the cylinder block W is clamped is positioned in the predetermined position. In this case, the draw bars 36, 37 are moved axially down (SIl). Following this operation, the intermediate finishing bits 103, 104 move radially outward and the precision finishing bit 105 moves radially inward. In this case, the tip with the largest radial outward protrusion is the tip 102c of the rough machining bit 102, the tips with the second largest radial outward protrusion are the tips 103c, 104c of the intermediate finishing bits 103, 104, and the tip that is positioned in the radially inwardmost position is the tip 105c of the precision finishing bit 105. In a case where a correction command with respect to the intermediate finishing bits 103, 104 is outputted by the below-described correction processing, the draw bars 36, 37 are positioned in the axial positions corresponding to the correction amount. In other words, the position of the radial outward ends of the tips 103c, 104c of the intermediate finishing bits 103, 104 are corrected. [0074] Then, the tool holders 34, 35 are lowered by fast feed, while being rotated about the axis thereof, and brought close to the work W (S 12). The tool holders 34, 35 are inserted into bore portions of the first cylinder and third cylinder, respectively, of the cylinder block W (S13). In this case, the tip 102c of the rough machining bit 102 is positioned slightly lower in the axial direction than the tips 103c, 104c of the intermediate finishing bits 103, 104. As a result, the bore portions into which the tool holders 34, 35 are inserted are subjected to rough machining with the tip 102c of the rough machining bit 102 and immediately thereafter to intermediate finish machining with the tips 103c, 104c of the intermediate finish machining bits 103, 104.

[0075] Then, after all the tips 102c to 105c have protruded out of the lower end openings of the bore portions, the draw bars 36, 37 are pulled up (S14). Following this operation, the intermediate finish machining bits 103, 104 move radially inward and the precision finishing bit 105 moves radially outward. In this case, the tip with the largest radial outward protrusion is the tip 105c of the precision finishing bit 105, the tip with the second largest radial outward protrusion is the tip 102c of the rough machining bit 102, and the tips that are positioned in the radially inwardmost positions are the tips 103c, 104c of the intermediate finishing bits 103, 104. In a case where a correction command with respect to the precision finishing bit 105 is outputted by the below-described correction processing, the draw bars 36, 37 are positioned in the axial positions corresponding to the correction amount. In other words, the position of the radial outward ends of the tip 105c of the precision finishing bit 105 is corrected.

[0076] The tool holders 34, 35 that have been inserted into the bore portions of the first cylinder and third cylinder of the cylinder block W are then pulled up (S 15). In this case, the precision finish machining of the bore portion is conducted by the tip 105c of the precision finishing bit 105 that has the largest radial outward protrusion. After all the tips 102c to 105c have appeared from the upper end openings of the bore portions, the tool holders 34, 35 are raised by rapid return, while maintaining the state of rotation about the axis, and withdrawn from the work W (S16).

[0077] The draw bars 36, 37 are then pushed down (S17). Following this operation, the intermediate finish bits 103, 104 move radially outward and the precision finish bit 105 moves radially inward. In this case, the positioning of the draw bars 36, 37 is also conducted by taking into account the correction amount of the intermediate finishing bits 103, 104 by the correction processing. At the same time, the jig table 50 is moved so that the second cylinder and fourth cylinder of the cylinder block W be positioned directly below the tool holders 34, 35 (S17).

[0078] Then, the tool holders 34, 35 are lowered by fast feed and brought close to the work W (S18). The tool holders 34, 35 are inserted into bore portions of the second cylinder and fourth cylinder, respectively, of the cylinder block W (S 19). The processing is conducted in the same manner as in step S13 described above. In other words, the bore portions of the second cylinder and fourth cylinder are subjected to rough machining with the tip 102c of the rough machining bit 102 and immediately thereafter to intermediate finish machining with the tips 103c, 104c of the intermediate finish machining bits 103, 104.

[0079] Then, after all the tips 102c to 105c have protruded out of the lower end openings of the bore portions, the draw bars 36, 37 are pulled up (S20). Following this operation, the intermediate finish machining bits 103, 104 move radially inward and the precision finishing bit 105 moves radially outward. In this case, the positioning of the draw bars 36, 37 is also conducted by taking into account the correction amount of the precision finishing bit 105 by the correction processing.

[0080] The tool holders 34, 35 that have been inserted into the bore portions of the second cylinder and fourth cylinder of the cylinder block W are then pulled up (S21). In this case, the precision finish machining of the bore portion is conducted by the tip 105c of the precision finishing bit 105 that has the largest radial outward protrusion. After all the tips 102c to 105c have appeared from the upper end openings of the bore portions, the tool holders 34, 35 are raised by rapid return, while maintaining the state of rotation about the axis, and withdrawn from the work W (S22). The cylinder block W that has been thus machined is carried out to the measurement table 70 of the measuring device 60, and the cylinder block W that has been carried in is subjected to machining processing.

[0081] Referring again to FIG 8, once the machining processing of the cylinder block W is completed, the measurement processing is then conducted with the measuring device 60 (S2). The measuring device 60 measures the inner diameter of the bore portions of the second cylinder and fourth cylinder of the cylinder block W before the precision finish machining.

[0082] Once the measurement processing is completed, the correction processing is conducted in the subsequent machining of the cylinder block W (S3). More specifically, the radial positions of bites 103 to 105 of the first tool holder 34 are corrected based on the measurement results relating to the bore portion of the second cylinder, and the radial positions of bites 103 to 105 of the second tool holder 35 are corrected based on the measurement results relating to the bore portion of the fourth cylinder. Each correction processing will be described below in greater detail with reference to FIG 10. The explanation will be conducted under an assumption that the predetermined first threshold for correcting the precision finishing bit 105 is 2 μm . [0083] Let us assume that the difference between the measurement result obtained with the measuring device 60 and the reference value is 2 μm in a state after the machining of a plurality of cylinder blocks W has been completed. In this case, because the difference is equal to or greater than the predetermined first threshold (2 μm), when the precision finish machining is conducted (S15 and S21 in FIG. 9), the radial position of the precision finishing bit 105 is moved radially outward through a distance of 2 μm (first cycle of precision finishing correction). In the second cycle of precision finishing, the radial position of the precision finishing bit 105 is moved radially outward by 3 μm, and in the third cycle of precision finishing, the radial position of the precision finishing bit 105 is moved radially outward by 3 μm. The accumulated value of the correction amount of the precision finishing bit 105 at this point in time is 8 μm.

[0084] As for the correction of the intermediate finish machining bits 103, 104, in a case where the accumulated value of the correction amount of the precision finishing bit 105 is equal to or greater than the predetermined second threshold (for example, 10 μm), when the next intermediate finish machining of the cylinder block W is conducted (S13 and S19 in FIG. 9), the correction is made by 5 μm so as to bring the radial position of the intermediate finish machining bits 103, 104 close to the reference value. However, the correction of the radial position of the intermediate finish machining bits 103, 104 is necessarily such as obtain a positive difference with the reference value, rather than to bring the difference with the reference value to zero. In a case where the accumulated value of the correction amount of the precision finishing bit 105 is equal to or greater than 10 μm, the correction of the intermediate finish machining bits 103, 104 is conducted so as to bring them closer to the reference value by 5 μm.

[0085] In the third cycle of finish machining correction, because the accumulated value of the correction amount of the precision finishing bit 105 has not yet reached 10 μm, no correction of the intermediate finish machining bits 103, 104 is conducted at this point in time. Then, in the fourth correction cycle of the precision finishing bit 105, the correction amount of the precision finishing bit 105 is made 3 μm, thereby bringing the accumulated value of the correction amount to 11 μm. In other words, the accumulated value of the correction amount of the precision finishing bit 105 becomes equal to or greater than 10 μm. In this case, when next intermediate finish machining is conducted, the radial position of the intermediate finish machining bits 103, 104 is moved radially outward through 5 μm. [0086] In some cases, the correction of the precision finishing bit 105 becomes negative, that is, such as demonstrated in the fifth cycle of correction. The effect of measurement spread can be considered as a possible reason therefor. Because the correction of the intermediate finish machining bits 103, 104 is based on the accumulated value of the correction amount of the precision finishing bit 105, the negative correction is counted as negative.

[0087] By such a fine correction of the precision finishing bit 105, it is possible to obtain a machining accuracy after precision finish machining that is closer to the reference value even when the tip 105c of the precision finishing bit 105 is worn out. Meanwhile, the frequency of correction of the intermediate finish machining bits 103, 104 is lower than that of the precision finishing bit 105. Further, the correction value of the intermediate finish machining bits 103, 104 is less than the difference between the measurement value after the precision finish machining and the reference value.

[0088] This is because the correction of the intermediate finish machining bits 103, 104 is conducted based on the accumulated value of the correction amount of the precision finishing bit 105, rather than based on the results of measurements conducted on the work W after the intermediate finish machining. In other words, the correction of the intermediate finish machining bits 103, 104 is conducted after indirectly estimating the correction amount of the intermediate finish machining bits 103, 104. Because of such an indirect correction amount, the correction of the intermediate finish machining bits 103, 104 cannot be considered as very accurate.

[0089] Further, in order to improve the machining accuracy after precision finishing, the diameter of machining with the intermediate finish machining bits 103, 104 has to be made not to exceed the diameter of machining with the precision finishing bit 105. Accordingly, the correction of the intermediate finish machining bits 103, 104 is conducted as described above.

[0090] With the boring apparatus of the above-described embodiment, the following effects are demonstrated. The bits of two kinds, namely, the intermediate finish machining bits 103, 104 and precision finishing bit 105 are configured to be movable in the radial direction with respect to the holder body 101. Therefore, in a case where the tips 103c, 104c of the intermediate finish machining bits 103, 104 are worn out, the radial position of the intermediate finish machining bits 103, 104 is corrected, and when the tip 105c of the precision finishing bit 105 is worn out, the radial position of the precision finishing bit 105 can be corrected. As a result, the variation in allowance determined by the tip 105c of the precision finishing bit 105 in the precision finish machining process can be inhibited with respect to that in the conventional configuration. As a result, the service life of the tip 105c of the precision finishing bit 105 can be improved.

[0091] Further, the variation of allowance determined by the tips 103c, 104c of the intermediate finish machining bits 103, 104 between the works W can be inhibited, and the variation of allowance determined by the tip 105c of the precision finishing bit 105 between the works W can be also inhibited. Therefore, the tips 103c, 104c of the intermediate finish machining bits 103, 104 and the tip 105c of the precision finishing bit 105 can be set so as to have almost identical service lives. In other words, the replacement of the tips 103c, 104c of the intermediate finish machining bits 103, 104 and the replacement of the tip 105c of the precision finishing bit 105 can be conducted simultaneously and therefore the equipment stoppage time can be shortened.

[0092] Furthermore, the measurements of the work W are conducted only after the precision finish machining that is the final process, and the radial positions of the intermediate finish machining bits 103, 104 and precision finishing bit 105 are corrected based on the measurement results. In other words, the radial positions of the intermediate finish machining bits 103, 104 and precision finishing bit 105 are corrected by one measurement cycle. As a result, it is not necessary to increase the number of cycles with respect to that of the conventional configuration. Therefore, the increase in cycle time can be inhibited.

[0093] In this configuration, the correction of the intermediate finish machining bits 103, 104 in the intermediate finish machining process is conducted based on the measurement results obtained after the precision finish machining process. However, because the wear amount of the tips 103c, 104c of the intermediate finish machining bits

103, 104 is almost proportional to the wear amount of the tip 105c of the precision finishing bit 105, where the wear amount of the tip 105c of the precision finishing bit 105 can be measured, it is possible to estimate with a certain accuracy the wear amount of the tips 103c, 104c of the intermediate finish machining bits 103, 104. Therefore, the radial position of the intermediate finish machining bits 103, 104 is corrected based on the measurements conducted after the precision finish machining process, thereby making it possible to obtain sufficient accuracy.

[0094] The accumulated value of correction amount of the radial position of the intermediate finish machining bits 103, 104 is determined based on the accumulated value of correction amount of the radial position of the precision finishing bit 105. As a result, a sufficiently adequate value can be obtained for the accumulated value of correction amount of the radial position of the intermediate finish machining bits 103,

104. As described above, the wear amount of the tips 103c, 104c of the intermediate finish machining bits 103, 104 and the wear amount of the tip 105c of the precision finishing bit 105 are almost proportional to each other. Further, the wear amount of the tip 105c of the precision finishing bit 105 essentially corresponds to the accumulated value of correction amount of the radial position of the precision finishing bit 105. In other words, the accumulated value of correction amount of the radial position of the intermediate finish machining bits 103, 104 is determined by referring to the accumulated value of correction amount of the radial position of the precision finishing bit 105. Therefore, the accumulated value of correction amount of the radial position of the intermediate finish machining bits 103, 104 can be determined with sufficient accuracy, without directly measuring the wear amount of the tips 103c, 104c of the intermediate finish machining bits 103, 104. [0095] Further, the accumulated value of correction amount of the radial position of the intermediate finish machining bits 103, 104 is set less than the accumulated value of correction amount of the radial position of the precision finishing bit 105. As a result, the tips 103c, 104c of the intermediate finish machining bits 103, 104 can be prevented from machining in excess of the machining diameter that has to be machined by the tip 105c of the precision finishing bit 105. Therefore, the production of defective works W can be prevented.

[0096] <Second Embodiment A boring apparatus of the second embodiment will be explained below. In the boring apparatus of the first embodiment, the rough machining bit 102 is fixed to the holder body 101. In the second embodiment, the rough machining bit 102 is configured to be capable of moving in the radial direction with respect to the holder body 101, similarly to the intermediate finishing bit 103.

[0097] In this case, the rough machining bit 102 of the first embodiment, the configuration of the holder body 101 in the vicinity of the rough machining bit 102, and the configuration of the draw bars 36, 37 in the vicinity of the rough machining bit 102 are changed to configurations corresponding to the intermediate finishing bit 103 of the first embodiment. In other words, where the draw bars 36, 37 are pulled up from the upper side to the lower side, the rough machining bit 102 moves radially outward similarly to the intermediate finishing bits 103, 104. Conversely, where the draw bars 36, 37 are pulled down from the lower side to the upper side, the rough machining bit 102 moves radially inward similarly to the intermediate finishing bits 103, 104. As a result, the service life of the tips 103c, 104c of the intermediate finishing bits 103, 104 can be increased.

[0098] <Third Embodiment In the above-described boring apparatus of the first embodiment, the bore portions of the second cylinder and fourth cylinder of the cylinder block W are measured with the measuring device 60 by moving the measurement table 70 in the X-axis direction. The correction unit 84 corrects each bit 103 to 105 of the first tool holder 34 on the basis of measurement result for the bore portion of the second cylinder that follows the site that has been machined with the first tool holder 34. The correction unit 84 corrects each bit 103 to 105 of the second tool holder 35 on the basis of measurement results for the bore portion of the fourth cylinder that follows the site that has been machined with the second tool holder 35.

[0099] By contrast, in the third embodiment, the measuring device 60 may measure the inner diameter of only the second cylinder of the cylinder block W, and the correction unit 84 may correct the bits 103 to 105 of the first and second tool holders 34, 35 on the basis of the obtained measurement results. In this case, it is not necessary to move the position of the cylinder block W in the X-axis direction with respect to the measuring device 60. Therefore, the measurement table 70 becomes unnecessary. As a result, the apparatus structure can be simplified. In addition, because the time for moving the measurement table 70 is unnecessary, the cycle time can be shortened. Measuring the sites machined by bits of respective tool holders and conducting respective correction make it possible to conduct machining with higher precision, as in the above-described first embodiment.

Claims

1. A boring apparatus in which a hole in a work is machined by feeding a tool holder provided with a bit in an axial direction relative to the work, while rotating the tool holder about an axis, comprising: a holder body; a first bit that is provided at an outer circumferential surface of the holder body movably in a radial direction of the holder body with respect to the holder body and that machines a hole in the work in a first machining process; a second bit that is provided at the outer circumferential surface of the holder body movably in the radial direction of the holder body with respect to the holder body and that machines a hole in the work in a second machining process that is performed after the first machining process; a measuring unit that measures the hole in the work after the second machining process; and a correction unit that corrects a radial position of the first bit in the first machining process and a radial position of the second bit in the second machining process on the basis of a measurement result in the measuring unit when machining a hole in a next work.

2. The boring apparatus according to claim 1, wherein the correction unit determines an accumulated value of correction amount of the radial position of the first bit on the basis of an accumulated value of correction amount of the radial position of the second bit.

3. The boring apparatus according to claim 2, wherein the correction unit sets the accumulated value of correction amount of the radial position of the first bit lower than the accumulated value of correction amount of the radial position of the second bit.

4. The boring apparatus according to any one of claims 1 to 3, wherein a machining process for a hole in the work includes a precision finish machining process of precision finish machining the hole in the work and an intermediate finish machining process performed immediately before the precision finish machining process, and wherein the first machining process is the intermediate finish machining process, and the second machining process is the precision finish machining process.

5. The boring apparatus according to any one of claims 1 to 3, wherein a machining process of a hole in the work includes a precision finish machining process of precision finish machining the hole in the work, an intermediate finish machining process performed immediately before the precision finish machining process, and a rough machining process performed immediately before the intermediate finish machining process, wherein the first machining process is the intermediate finish machining process and the rough machining process, the second machining process is the precision finish machining process, and the first bit includes an intermediate finishing bit that machines the hole in the work in the intermediate finish machining process and a rough machining bit that machines the hole in the work in the rough machining process.

6. A hole machining method in which a hole in a work is machined by using a tool holder in which a bit is provided at an outer circumferential surface of a holder body and feeding the tool holder in an axial direction relative to the work, while rotating the tool holder about an axis, comprising: a first machining process of machining the hole in the work in use of a first bit that is provided at the outer circumferential surface of the holder body movably in a radial direction of the holder body with respect to the holder body; a second machining process of machining the hole in the work after the first machining process in use of a second bit that is provided at the outer circumferential surface of the holder body movably in the radial direction of the holder body with respect to the holder body; and a measuring process of measuring the hole in the work after the second machining process, wherein the first machining process includes a process of correcting a radial position of the first bit on the basis of a measurement result and machining a hole in a work when the next hole is machined in the work, and the second machining process includes a process of correcting a radial position of the second bit on the basis of a measurement result and machining the hole in the work when a next hole is machined in the work.