Abstract:
The invention relates to a tool holding assembly with an adjustment means capable of causing the tool holding end of the assembly to bend in relation to the opposite attachment end of the assembly. This is accomplished by providing first and second contact surfaces longitudinally spaced from each other along the assembly, and providing an adjustment means having at least one adjustment screw, wherein by adjusting the screw, pressure can be simultaneously applied by the adjustment means onto the first and second contact surfaces, such that an axis of one end of the assembly can be displaced by bending in relation to the other.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a tool holder which is used with being attached to a rotary part of a working machine, and more particularly to a tool holder which is used with attaching the basal side of a shaft of the tool holder to a rotary part of a working machine, which has a holding portion for holding a tool such as a cutting tool, in the tip end side of the shaft, and which can correct runout of a tip end portion of the tool such as a cutting tool attached to the holding portion. 
     BACKGROUND OF THE INVENTION 
     When a hole is to be opened in a workpiece or the inner face of a hole is to be ground by using a working machine or the like, a tool such as a cutting tool is attached to the tip end side of a tool holder mounted on a rotary part of the working machine. The tool in the tip end side of the tool holder is rotated at a high speed by rotating the rotary part of the working machine, whereby desired working can be performed on the workpiece for a short time period. 
     As described above, the tool holder is used in a high speed state. When the tip end of the tool attached to the tip end of the tool holder deflects even at a small degree as a result of the high speed rotation, there arises a problem in that the dimensional accuracy of the hole is lowered or the inner peripheral face of the hole is roughened to disable the hole from being accurately worked. 
     In order to solve the problem, conventionally, means for correcting vibrations of a shaft of a tool holder is disposed on the tool holder (see Japanese published unexamined Patent application No. 11-104931). 
     FIG. 12 shows the conventional art example. With reference to the figure, the conventional art example will be described. In a tool holder  1 , the basal side  2  of a shaft  3  is formed as a shank portion  2  which is tapered as well known in the art, in order to fit the basal side into a tapered hole  1   b  of the main shaft  1   a  of a working machine which is not shown and has a well-known configuration. A holding portion  4  for a tool  4   a  is disposed in the tip end side of the shaft  3 . (Usually, in order to fasteningly fix the tool  4   a  an example of which is a cutting tool such as a drill, a chuck  4  having a clamping hole which is expandable and contractable is disposed as the holding portion  4 . In a well-known configuration, the cutting tool  4   a  may be attached directly integrally to the shaft  3  in the position of the chuck  4 .) A large-diameter flange  5  which is used in the case where a grasping operation is to be performed by a grasping portion of a robot as well known in the art is formed on the outer periphery of the shaft  3  and on the side of the shank portion  2 . The reference numeral  5   a denotes a groove for engagement. 
     In the tool holder  1  shown in FIG. 12, plural radial holes  6  are threadedly formed in a bottom of the groove  5   a of the flange  5 , and a male screw  7  which is a screw member serving as a mass is screwed into each of the holes  6 . A hexagonal hole  8  for a hexagonal wrench is formed in the head of each of the male screws  7 . Each male screw  7  is adjusted so as to advance or retract in the corresponding hole  6  by using a wrench, so that, when the tool holder  1  is rotated at a high speed, the positional change in a radial direction appears as a change of the mass, whereby vibrations of the shaft  3  of the tool holder  1  can be corrected. 
     Even when vibrations of the shaft are corrected in a portion of the shaft  3  of the tool holder  1  the shaft center may sometimes deflect in a position closer to the tool attaching portion  4  in the tip end. Therefore, male screws  9  which similarly serve as a mass are attached to a tip end portion  3   a  so as to be advanceable and retractable, so that vibrations of the shaft in the tip end portion  3   a  can be corrected by advancing or retracting each of the male screws  9 . 
     In the conventional art described above, however, the means for correcting vibrations of a shaft can correct vibrations of the shaft, only in a place where the correcting means is disposed and in the vicinity of the place. 
     In the case of a long tool such as a drill  4   a  which protrudes by a large distance from the tool attaching portion  4 , the portion where correction of runout of the shaft center with respect to the rotation center is required at the highest degree is a blade edge  4   b  which is in the tip end of the drill. In the conventional art described above, however, it is impossible to correct runout of the shaft center with respect to the rotation center in a portion which largely protrudes from the tool attaching portion  4 , such as the blade edge  4   b  in the tip end of the drill. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a tool holder having runout correcting means for displacing an axis C of a tip end side of a shaft by a simple operation of rotating a screw member for pressurizing a part of the shaft, whereby a center of a tip end of a tool attached to a protruding tip end portion is made coincident with the rotation center Ca of the tool holder. 
     It is another object of the invention to provide a tool holder in which a screw member for displacing the axis C of the tip end side of a shaft is disposed in at least three or more uniform positions in the periphery of the main shaft of the tool holder, whereby, even when the shaft center of a cutting tool serving as a tool deflects in any direction, the tip end of the cutting tool can be moved to the rotation center of the tool holder irrespective of the direction. 
     It is a further object of the invention to provide a tool holder in which, even in a case where a screw member is disposed in one place, the position of the tip end of the cutting tool can be freely changed in either of leftward and rightward directions in accordance with the direction of rotation of the screw member. 
     It is a still further object of the invention to provide a runout correcting tool which is requested only to be attached to the periphery of a shaft of a tool holder that is already completed, whereby the tool holder can be changed to a tool holder having runout correcting means for displacing the axis C of the tip end side of the shaft to make the center of the tip end of a tool attached to a holding portion with largely protruding therefrom, coincident with the rotation center Ca of the tool holder. 
     Other objects and advantages will easily become apparent from the accompanying drawings and the following description relating to the drawings. 
     The invention has the following feature. A tool is attached to a holding portion at a tip end portion of a shaft. In the case where a drilling or cutting work is to be conducted, even when the position of the tip end of the tool is laterally shifted to deviate from the rotation center of a tool holder, a screw member is rotated to generate a force of displacing the axis of the tip end side of the shaft, between two contact portions of the tool holder, whereby the position of a blade edge of the tool can be easily moved toward the rotation center of the tool holder so as to coincide therewith. According to this configuration, it is possible to attain an effect that, in the case where a drilling or cutting work is to be conducted by rotating the tool holder at a high speed, precision working can be accurately performed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section view of a tool holder having runout correcting means of an embodiment of the invention; 
     “FIG.  2 (A) is a section view taken along the line II—II of FIG. 1, and shows an example in which a screw member is disposed in four places; 
     FIG.  2 (B) is a section view taken along the line II—II of FIG. 1, and shows an example in which a screw member is disposed in three places; 
     FIG.  2 (C) is a section view taken along the line II—II of FIG. 1, and shows an example in 
     FIG. 3 is an enlarged view of the runout correcting means shown in FIG. 1; 
     FIG. 4 is a partially cutaway view showing a state where a cutting edge of a tool deflects; 
     FIG. 5 is a partially cutaway view showing a state where the runout of the cutting edge is corrected; 
     FIG. 6 is a partial section view showing runout correcting means of another embodiment; which a screw member is disposed in six places;” 
     “FIGS.  7 (A) and  7 (B) are partial section views of a tool holder having runout correcting means of a further embodiment;” 
     “FIGS.  8 (A) and  8 (B) are partial section views of a tool holder having runout correcting means of a still further embodiment;” 
     “FIGS.  9 (A) and  9 (B) are partial section views of a tool holder having runout correcting means of a still further embodiment;” 
     “FIGS.  10 (A) and  10 (B) are partial section views of a tool holder having runout correcting means of a still further embodiment;” 
     “FIGS.  11 (A) and  11 (B) are partial section views of a tool holder having runout correcting means of a still further embodiment; 
     FIG. 12 is a view illustrating a runout correcting tool of the conventional art. 
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a section view of a tool holder  10  which is a first embodiment of the invention, and (A) of FIG. 2 is a section view taken along the line II—lI of FIG.  1 . In FIG. 2, (B) and (C) are views showing examples in which the number of screw members  24  is changed. FIG. 3 is an enlarged view of the runout correcting means shown in FIG.  1 . 
     Although a configurations of components denoted by reference numerals  10  to  18  in FIGS. 1 to  5  are well known in the same manner as a tool holder  1  shown in FIG. 12, the configurations will be described below. As well known in the art, the tool holder  10  comprises a shaft  13  having a hollow portion  13   a  serving as a main unit. A flange  11  is disposed on an outer periphery of the shaft  13 . A tapered shank portion  12  which protrudes in a rearward direction or toward a basal portion is disposed in back of the flange  11 . The portion  12  serves as an attaching portion which is to be attached to a rotary part of a working machine. 
     In a front position with respect to the flange  11 , a diameter of the shaft  13  is very smaller than that of the flange  11  (occasionally, the shaft may be larger in diameter than the flange  11 ). A step  11   b  is formed in front of the flange  11 . As well known in the art, the shaft  13  is formed into a slender hollow cylindrical shape (occasionally, the shaft may be solid). A holding portion  14  for holding a tool  17  is disposed in a tip end side of the shaft. 
     The holding portion  14  is a portion which is well known as a chuck, and as well known fasteningly fixes a basal portion of a drill  17  which is an example of a cutting tool inserted into a center hole  14   a  of the holding portion  14 , by using a collet chuck  15   a , a clamp ring  15 , etc. 
     In the holding portion  14 , the thickness as a whole is not even because of the thickness of the shaft in the holding portion  14 , and production errors of the collet chuck (contract ring)  15   a , the clamp ring  15 , etc. As a result, a position of a tool tip end  18  is laterally shifted with respect to a rotation center Ca of the tool holder  10  to deviate from the rotation center Ca of the tool holder. When the drill  17  is rotated at a high speed under such a state, the blade edge  18  at the tip end of the drill  17  deflects. This state of runout is such a state that the blade edge  18  at the tip end of the drill  17  deflects in both a direction of an arrow S in FIG. 1 and a direction of depth in FIG. 1 when the blade edge  18  is rotated around the rotation center Ca centering around the center Ca. In such runout state, precision working becomes disabled. 
     In the embodiment, therefore, a runout correcting tool  20  is detachably disposed on the outer peripheral face of the shaft  13 . The runout correcting tool  20  has a main unit which is made of a metal material having substantial thickness and weight, and which is configured as a ring-like member (annular member)  21  so as to be fittable onto the outer periphery of the shaft  13 . 
     A screw member  24  is screwed advanceably and retractably with a tapped hole  23  which is formed from an outer periphery of the annular member  21  to an inner periphery. The screw member  24  configured by a male screw is passed by the whole length through the tapped hole  23  via a well-known operation groove disposed in a head  24   b . Even when the tapped hole  23  with which the screw member  24  is screwed is formed only in one place of the periphery of the annular member  21 , the tip end  18  of the tool  17  can be adjusted with respect to runout. However, it is preferable that such a tapped hole is formed in three or more places in a circumferential direction of the periphery of the shaft (see (B) and (C) of FIG.  2 ). When these holes are respectively formed in uniform positions, adjustment of “runout” of the center of the tip end  18  of the tool can be performed very easily and rapidly. 
     An abutting portion  28  which is to abut against a first contact portion  26  disposed in the outer periphery of the shaft  13  is formed on one side face of the annular member  21 . The first contact portion  26  is formed in the step  11   b . Namely, the one side face  28  of the annular member  21  is caused to abut against the front wall face  26  of the flange. 
     A tip end  29  of the screw member  24  can abut against a second contact portion  27  which is disposed in a position separated in the longitudinal direction from the first contact portion  26 . The second contact portion  27  is configured by cuttingly forming an annular U-groove  22  in the outer peripheral face of the shaft  13 . 
     The two contact portions ( 26  and  27 ) are pressurized by rotating the screw member  24  to generate a force F of displacing the axis C of the tip end side of the shaft  13 , between the portions. 
     When the tip end portion  29  of the screw member is in press contact with the shaft  13 , a reaction force of the shaft  13  causes the end face  28  of the annular member  21  and the side end face  26  of the flange  11  to be in press contact with each other, so that the annular member  21  and the shaft  13  are firmly fixed to each other. In the embodiment, as shown in FIG. 3, when the flange side end face  28  of the annular member  21  abuts against the end face  26  of the flange  11 , i.e., in the press contact state in which the annular member  21  cannot be moved toward the flange  11 , the through hole  23  is positioned so that its center is slightly deviated from a center  22   a  of the U-groove  22  toward the holding portion  14 . 
     When the screw member  24  is screwed and the tip end portion  29  of the screw member enters the U-groove  22 , therefore, the screw tip end portion  29  abuts against only the inclined face (second contact portion)  27  of the U-groove  22  on the side of the holding portion  14 , and does not abut against an inclined face on the side of the flange  11 . As a result, when the screw member  24  is screwed into the tapped hole  23  to perform a fastening operation, the screwing force (in FIG. 3, the force directed toward the shaft center C of the tool holder  10 ) is caused by the function of the inclined face  27  to have a force in the direction indicated by F in FIG. 3, i.e., a component of force which bends the portion of the shaft  13  on the side of the holding portion  14  in the direction of the arrow Fa. 
     The reference numeral  30  denotes an annular cover which is placed on the outer periphery of the annular member in order to prevent the screw member  24  from jumping out. An end portion of the cover is detachably fixed to the shaft  13 . The reference numeral  31  denotes an operation hole which has a diameter smaller than the screw head  24   b , and which is positioned in the outer peripheral side of the screw head  24   b.    
     In the case where the drill  17  is attached to the thus configured tool holder  10  and then rotated, when the position of the blade edge  18  of the drill deviates from the rotation center axis Ca as shown in FIG. 4, the center portion of the tip end  18  of the tool  17  is made coincident with the rotation center Ca of the tool holder. In this case, as shown FIGS. 3 and 5, the shaft  13  is bent by adjusting the fastening force (the press contacting force on the inclined face  27 ) of each of the three (or more) screw members  24  shown in FIG. 2, to displace the axis C in the direction of the arrow Fa, thereby making the position of the blade edge  18  coincident with the rotation center axis Ca. 
     In the tool holder of the conventional art shown in FIG. 12, even when a screw  7  of the correcting tool is fastened, only the mass which is immediately below the screw can be changed. Therefore, it is difficult to correct runout of the shaft center which occurs on the side of the drill with respect to a correcting place. By contrast, in the embodiment, the fastening force exerted on the screw  24  causes the shaft  13  to be bent, so that the force can be applied toward the holding portion  14  as the force F shown in FIG.  3 . Therefore, runout between the rotation center of the tip end  18  of the tool and the rotation center Ca of the tool holder  10  can be easily corrected. 
     As described above, the runout correcting tool  20  of the embodiment can correct runout which occurs on the side of the tool  17  with respect to the correcting tool  20 . This is realized by the function of the inclined face  27 . When the inclination angle of the inclined face  27  is changed, the direction of the force F can be directed more closely to the drill. However, the necessary force (the force of fastening the screw  24 ) must be increased in accordance with the change. 
     FIG. 6 shows a tool holder  10  of a second embodiment of the invention in which runout of the blade edge  18  at the tip end of the drill can be corrected at a higher degree. The embodiment is different from the first embodiment in that the attaching position of the runout correcting tool  20  is set to be closer to the drill  17 . When the runout correcting tool  20  is closer to the drill as described above, it is not required to reduce the inclination angle of the inclined face  27 , and runout of the tip end  18  of the drill can be corrected by a smaller fastening force. 
     In the above description, the runout correcting means of the tool holder  10  shown in FIGS. 1 to  6  corrects “runout” of the tool holder  10  after the runout correcting tool  20  which is configured so as to be attachable and detachable with respect to the tool holder  10  is attached to the tool holder  10 . As described above, the runout correcting means  20  may be an independent component which is detachably configured. Alternatively, the means may be configured by previously attaching the annular member  21  to the outer peripheral face of the shaft in a step of producing the tool holder  10 . 
     In FIG. 6, components which seem to be functionally identical or equivalent to those of FIGS. 1 to  5  described above are denoted by the same reference numerals as those used in FIGS. 1 to  5 , and letter “e” is affixed to the reference numerals in order to distinguish the numerals from those used in the figures. Therefore, the configuration of the components can be easily realized, and duplicated description is omitted. (Also in the configuration associated to partial views of FIG.  7  and the subsequent figures, components of the tool holder  10  to which the runout correcting tool  20  is attached are denoted by the same reference numerals as those of the previous figures, letter f or g is affixed, and duplicated description is omitted.) 
     In the above-described embodiment of FIGS. 1 to  6 , one inclined face of the U-groove  22  is used as the second contact portion of the runout correcting tool  20 . It is requested only that an engagement structure for receiving the second abutting portion  29  exists correspondingly and scatteringly in the circumferential direction of the shaft  13 . The second contact portion is not required to be formed as a U-groove. In FIG. 3, the screw member  24  is opposed to the second contact portion  27 . Alternatively, the U-groove  22  may be formed in the first contact portion  26 , and the screw member  24  may be configured so as to be advanceable and retractable with respect to the U-groove. In the alternative also, the force F shown in FIG. 3 can be similarly generated. 
     Among the embodiments described above, the embodiment mode relating to the runout correcting means  20  may be configured as shown in FIGS. 7,  8 ,  9 ,  10 , and  11 . The runout correcting means  20  shown in the figures are embodiment modes which are configured in a partly different manner from the runout correcting means  20  shown in FIG.  3 . 
     The object of the runout correcting means shown in FIG. 7 is to show a possibility that runout correcting means is disposed in an arbitrary intermediate position in the longitudinal direction of the shaft so as to enable the axis of the tip end side of the shaft  13  to be displaced. 
     Referring to FIG. 7,  33  denotes a trough-shaped recess portion which is disposed in an arbitrary intermediate position in a longitudinal direction  38  of the outer periphery of a shaft  13   f . The recess is opened in front and rear portions. In the recess, inclined faces for configuring first and second contact portions  26   f  and  27   f  are formed in front and rear positions which are separated from each other in the shaft longitudinal direction  38 , respectively. A pressing member  34  made of steel is disposed in the recess  33  so as to be advanceable and retractable toward the trough bottom. First and second abutting portions  28   f  and  29   f  which are correspondingly inclined so as to be in press contact with the inclined portions of the first and second contact portions are disposed in front and rear sides of the pressing member  34 . A female screw forming portion  36  for advanceably and retractably holding a screw member  24   f  is fixedly placed around the shaft. The female screw forming portions  36  may be arranged fixedly and independently around the shaft, at a number which is equal to the numbers of the recesses  33  and the corresponding screw members  24   f . Alternatively, as illustrated, an annular member  21   f  which is integrally formed may be placed around the shaft, and a desired number of female screw forming portions  36  may be disposed in the member. Alternatively, in the same manner as the U-groove  22  of FIG. 1, the trough-shaped recess portion  33  may be formed by cutting the outer peripheral face of the shaft  13  continuously and annularly. In the case where the trough-shaped recess portion  33  is formed by cutting the outer peripheral face of the shaft  13  continuously and annularly, when the annular member  21   f  is movable in the shaft circumferential direction, the position of the female screw forming portion  36  in the circumferential direction can be changed so that the displacement direction of the axis Cf of a tip end side of the shaft  13   f  can be arbitrarily changed. Therefore, the number of the screw members  24   f  can be reduced. A recess  34   a  is formed in the pressing member  34 , and a tip end  35  of the screw member  24   f  is loosely fitted into the recess. 
     In the runout correcting means  20  of FIG. 7, when the screw member  24   f  is operated to advance the tip end  35  toward the shaft center, the pressing faces  28   f  and  29   f  on the both sides of the pressing member  34  press the inclined faces of the first and second contact portions  26   f  and  27   f  so that a distance between the inclined faces is lengthened, and, in the same manner as the case shown in FIG. 3, the force Ff of displacing the axis Cf of the tip end side of the shaft  13   f  is generated. 
     The object of provision of a runout correcting means shown in FIG. 8 is to show a possibility that the means can be easily attached to a conventional tool holder such as shown in FIG.  12  and facilitate displacement of the axis of the tip end side of the shaft. 
     Referring to FIG. 8,  11   bg  denotes a step which is usually formed in an arbitrary intermediate position in a longitudinal direction  38  and in an outer periphery of a shaft  13   g , and which, in positions separated from each other in the longitudinal direction  38 , forms a rising wall face  26   g  and a shaft outer peripheral face  27   g  that is perpendicular to the wall face. The faces constitute first and second contact portions  26   g  and  27   g , respectively. A pressing member  40  made of steel is disposed in the step  11   bg  so as to be advanceable and retractable toward a space in a corner  42 . First and second abutting portions  28   g  and  29   g  which are formed as inclined abutting faces so as to be in press contact with the first and second contact portions  26   g  and  27   g  are disposed in front and rear positions of the pressing member  40 . 
     A tapped hole  23   g  is formed in the corner  42  of the shaft  13   g . A screw member  24   g  which is passed through a hole  43  of the pressing member  40  is screwed advanceably and retractably with the tapped hole. In the runout correcting means  20   g  of FIG. 8, when the screw member  24   g  is operated to advance the pressing member  40  toward the corner  42 , the abutting faces  28   g  and  29   g  of the pressing member  40  press the abutting faces of the first and second contact portions  26   g  and  27   g  so that a distance between the abutting faces of the first and second contact portions  26   g  and  27   g  is lengthened, and, in the same manner as the case shown in FIG. 7, the force of displacing the axis Cg of the tip end side of the shaft  13   g  is generated. 
     The object of provision of a runout correcting means shown in FIGS. 9 and 10 is to provide means for enabling the screwing force of a screw member to act very effectively on displacement of an axis of a tip end side of a shaft. 
     Referring to the figures, a rising wall face  26   h  or  26   i  constituting a first contact portion  26   h  or  26   i  uses a step formed in front of a flange  11   h  or  11   i  (or an annular member which is fixed to the shaft and separately formed). An annular member  21   h  or  21   i  which is formed fixedly with the shaft  13   h  or  13   i  by an integral member (or fixedly formed in a fittable manner by a separate member) is disposed in a position of the shaft which is separated in a longitudinal direction  38 . In the annular member, a second contact portion  27   h  or  27   i  is configured by a female screw portion  23   h  or  23   i . A screw member  24   h  or  24   i  is screwed into the female screw portion  23   h  or  23   i . A tip end of the screw member is formed as a first abutting portion  28   h  or  28   i , and a basal portion of a thread portion of the screw member is formed as a second abutting portion  29   h  or  29   i.    
     According to this configuration, when the screw member  24   h  or  24   i  is operated, the gap between the rising wall face  26   h  or  26   i  and the member  21   h  or  21   i  having the female screw portion  23   h  or  23   i  is widened to generate a force of displacing the axis Ch or Ci of the tip end side of the shaft  13   h  or  13   i.    
     The axial direction of the screw member  24   h  or  24   i  in FIG. 9 or  10  may be se to be in parallel with the axis of the shaft  13   h  as shown in FIG. 9, or may be inclined or provided with an angle  47  as shown in FIG.  10 . Namely, the axial direction may be arbitrarily set so as to facilitate the operation of the member. 
     The object of provision of a runout correcting means shown in FIG. 11 is to generate a force which causes an axis of a tip end side of a shaft to be directed rightward or leftward or in different orientations depending on the direction of the operation of rotating a screw member. 
     Referring to FIG. 11, a left-hand female screw (or right-hand female screw)  23   j  is formed in a rising wall face  26   j  serving as a first contact portion  26   j . In a screw holding member  50 , a right-hand (or left-hand) female screw  23   j  which is opposite in direction to the screw of the first contact portion  26   j  is disposed in a position which is separated in a longitudinal direction  38  of the shaft  13   j , as a second contact portion  27   j . The screw holding member  50  may have an independent configuration such as a steel nut. In first and second abutting portions  28   j  and  29   j , a left-hand screw  28   j  and a right-hand screw  29   j  are formed in the both end sides of one screw member  24   j , and the screws are screwed with left- and right-hand female screws formed in the first and second contact portions  26   j  and  27   j , respectively. The contact portions  26   j  and  27   j  are made closer to or separated from each other by leftward or rightward rotating the one screw member  24   j , so that an axis Cj of the tip end side of the shaft  13   j  can be directed rightward or leftward or in different orientations. 
     The embodiment is assembled in the following manner. The both sides of the screw member  24   j  are screwed into the corresponding female screws, respectively. Under this state, the nut  50  which remains to be in a rotatable state is fixed to an outer periphery of the shaft  13   j  by any means such as welding  51 . The reference numeral  52  denotes a groove for fitting, and  53  denotes a rectangular member for operating the screw. 
     FIGS. 7,  8 ,  9 ,  10 , and  11  show the examples in which four sets of the runout correcting means  20  are arranged in the periphery of the shaft  13 . In the same manner as the first embodiment, the number of the means can be, for example, any number of 1 to 6, or arbitrarily set. 
     With respect to the runout correcting means  20  shown in the figures, the operation conducted on one set of the means has been described. When two, three, or more sets of the runout correcting means  20  which are arranged in the periphery of the shaft are alternatingly operated, the shaft center C can be bent and displaced in an arbitrary direction. For example, the position of the blade edge  18  of the drill  17  may be made closer to the rotation center Ca of the tool holder  10  so as to coincide therewith. Alternatively, an operation may be conducted so that the position of the blade edge  18  of the drill  17  is made remoter from the rotation center Ca to increase the cutting radius. 
     In each of FIGS. 7,  8 ,  9 ,  10 , and  11  showing the specific configurations, (A) is a section view of the tool holder and taken at a position corresponding to FIG. 3, and (B) is a section view taken at the position of the runout correcting means  20  shown in (A). 
     As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.