Patent Publication Number: US-2022226909-A1

Title: Cutting tool for boring, machining apparatus for boring and method for boring

Description:
TECHNICAL FIELD 
     The present disclosure relates to a cutting tool for boring that is attached to a machining apparatus and used for cutting a hole on a large-diameter side of a stepped hole of a workpiece, and also relates to a machining apparatus for boring and a method for boring that use the cutting tool. 
     BACKGROUND 
     Various kinds of machining apparatuses for cutting of differential cases and differential carriers are already known. For example, Patent Literature 1 proposes a machining apparatus for a differential case, the machining apparatus including: a shuttle unit which is rotatable and is slidable in the front-rear direction; an inner surface machining cutting tool changing apparatus that slides in the vertical direction; and a pair of left and right machining units arranged opposing each other so as to sandwich a workpiece; the machining apparatus enabling machining of different areas of a differential case without impairing the versatility and while also having a simple structure. 
     By using such a machining apparatus for a differential case, boring of a side gear hole provided in the differential case is enabled. A side gear hole is provided by expanding the diameter of an end portion on the inner side of a through hole. A tool having a cutting edge protruding laterally from a shaft part is attached to the machining unit of the machining apparatus, and after the tool is inserted into the through hole, by causing the shuttle unit to slide in the front-rear direction, the cutting edge of the tool comes in contact with the inner circumferential surface of the side gear hole, and the side gear hole can thus be machined using the tool. 
     In a case where side gear holes are composed of a pair of left-and-right side gear holes, the respective side gear holes of the pair of left-and-right side gear holes can be machined using respective tools attached to the respective machining units of the pair of left and right machining units. In this case, because each of the tools rotates in a cantilevered state, there is a concern that the machining accuracy may decrease due to chattering of the tool. In this regard, in an inner surface machining apparatus disclosed in Patent Literature 2, a center shaft is brought into contact with a cantilever-supported long cutting tool so that the long cutting tool enters a state as if both ends of the long cutting tool are held and supported, thus attempting to stabilize the posture of the long cutting tool and thereby avoid as much as possible the occurrence of vibrations or deviations in the position of the long cutting tool and thus increase the accuracy of cutting an inner circumferential surface of a cylindrical portion. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-open No. 2014-195851 
         Patent Literature 2: Japanese Patent Application Laid-open No. 2007-30101 
       
    
     SUMMARY 
     However, in the inner surface machining apparatus disclosed in Patent Literature 2, a first saddle for driving the long cutting tool is only provided on one side of the workpiece, and the center shaft only has a role of supporting the long cutting tool, and does not have a cutting function. Further, in Patent Literature 2, although it is described that inner circumferential surface machining of both left and right cylindrical portions can be completed at one time by movement of the long cutting tool in one direction, in such a case it is necessary to make the length of the long cutting tool longer, and a long cutting tool having a longer size is disadvantageous with respect to preventing vibration and preventing deviations in the position of the long cutting tool. 
     The present disclosure has been made with a view to solving the conventional problems described above, and an object of the present disclosure is to provide a cutting tool for boring, a machining apparatus for boring and a method for boring which are advantageous for reducing a cutting time while also preventing a decrease in machining accuracy caused by chatter of a tool. 
     To achieve the above object, a cutting tool for boring of the present disclosure is a cutting tool for boring that is attached to a machining apparatus and used for cutting of a hole on a large-diameter side of a stepped hole of a workpiece, the machining apparatus including a pair of left and right machining units arranged opposing each other so as to sandwich a workpiece, the cutting tool for boring being provided in a set of two which are attached to the pair of left and right machining units respectively, each of the cutting tools for boring in the set of two including: a shaft part; a cutting blade laterally protruding from the respective shaft parts; and an abutment surface that is formed at a tip of the respective shaft parts and that is for causing the tips of the respective shaft parts to butt against each other, wherein each of the cutting tools for boring performs the cutting while the two cutting tools for boring rotate integrally around a center line of the shaft part in a coupled state in which the tips of the respective cutting tools for boring are butted together. 
     According to this configuration, since the set of two cutting tools for boring can perform cutting of an inner circumferential surface of a hole while rotating around the center line in a state in which the cutting tools for boring are both held by the left and right machining units, chattering is prevented and the cutting accuracy increases. More specifically, in addition to an increase in accuracy with respect to the cylindricality, roundness and concentricity of the hole, the accuracy of the surface roughness of the inner circumferential surface of the hole also increases. 
     Further, since the respective cutting tools for boring of the set of two have a cutting blade on the respective shaft parts, when cutting two holes, a dedicated cutting tool is used to cut each hole, and hence the length of the shaft parts can be kept to a moderate length, and this fact is also advantageous for preventing chattering. In addition, it is also possible to perform cutting of two holes having different diameters by changing the cutting tool once. 
     In the cutting tool for boring of the present disclosure, preferably, among the cutting tools for boring of the set of two, one cutting tool for boring is provided with an extension part extending from the abutment surface, and the other cutting tool for boring is provided with a hole into which the extension part enters. According to this configuration, the set of two cutting tools can be prevented from slipping during rotation. 
     Further, preferably a length of the extension part is a length that, while the one cutting tool for boring and the other cutting tool for boring are being moved in opposite directions to each other with respect to a center line direction of the shaft part, is able to maintain a state in which the extension part has entered the hole, when machining of a hole of the workpiece is completed. According to this configuration, it is possible to machine two holes on the left and right at the same time, and thus the cutting time can be shortened. 
     A machining apparatus of the present disclosure is a machining apparatus that uses the respective cutting tools for boring, and includes a pair of left and right machining units arranged opposing each other so as to sandwich a workpiece. A method for boring of the present disclosure is a method for boring using the respective cutting tools for boring, wherein: in a state in which the cutting tools for boring of the set of two are arranged so that the abutment surfaces of the respective cutting tools for boring in the set of two are butted together, and a rear end side of each of the cutting tools for boring is supported by the machining apparatus, while causing the cutting tools for boring of the set of two to rotate around the center line and to move in the center line direction, an inner circumferential surface of a hole of the workpiece is cut with the cutting blade. 
     According to the machining apparatus for a differential case and the machining method for a differential case of the present disclosure, since the cutting tool for machining a differential case of the present disclosure is used, in addition to an increase in the cutting accuracy, the length of the shaft part of the cutting tool can be kept to a moderate length, and it is also possible to perform cutting of two holes having different diameters by changing the cutting tool once. 
     The advantageous effects of the present disclosure are as described above and may be summarized by saying that since the set of two cutting tools for boring can perform cutting of an inner circumferential surface of a hole while rotating around the center line in a state in which the cutting tools for boring are both held by the left and right machining units, chattering is prevented and the cutting accuracy increases, and since the respective cutting tools for boring have a cutting blade on the respective shaft parts, when cutting two holes, a dedicated cutting tool is used to cut each hole, and hence the length of the shaft parts can be kept to a moderate length, and in addition it is also possible to perform cutting of two holes having different diameters by changing the cutting tool once. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a workpiece that is an object of machining by a machining apparatus according to one embodiment of the present disclosure; 
         FIG. 2  is a longitudinal sectional view of the workpiece illustrated in  FIG. 1 ; 
         FIG. 3  is a front view of the machining apparatus according to one embodiment of the present disclosure; 
         FIG. 4  is a perspective view illustrating a state immediately before boring of a workpiece by a cutting tool for boring according to one embodiment of the present disclosure; 
         FIG. 5  is a view illustrating a state in which initial setting of the position of a workpiece has been completed in one embodiment of the present disclosure; 
         FIG. 6  is a view illustrating a state in which a pair of cutting tools are passing through a differential bearing seat surface and a differential bearing hole in one embodiment of the present disclosure; 
         FIG. 7  is a view illustrating a state in which horizontal movement of a pair of cutting tools has been completed in one embodiment of the present disclosure; 
         FIG. 8  is a view illustrating a state in which the workpiece has been moved downward by an amount Ad from the state illustrated in  FIG. 7 ; 
         FIG. 9  is a view illustrating a state in which a differential bearing hole is being cut with a cutting blade in one embodiment of the present disclosure; 
         FIG. 10  is a perspective view of the vicinity of a workpiece in a state during cutting of a differential bearing hole in one embodiment of the present disclosure; 
         FIG. 11  is a view illustrating a state in which cutting of a differential bearing hole on a left side has been completed in one embodiment of the present disclosure; 
         FIG. 12  is a view illustrating a state in which a differential bearing hole on a right side is being cut with a cutting blade in one embodiment of the present disclosure; 
         FIG. 13  is a view illustrating a state in which cutting of the bearing hole on the right side has been completed in one embodiment of the present disclosure; 
         FIG. 14  is a view illustrating a coupled state of a pair of cutting tools when new cutting tools are used according to one embodiment of the present disclosure; 
         FIG. 15  is a view illustrating a state in which the cutting tools have moved from the state illustrated in  FIG. 14 , and left and right differential bearing holes are being cut with the cutting blades; 
         FIG. 16  is a view illustrating a state in which the cutting tools have moved from the state illustrated in  FIG. 15 , and cutting of the left and right differential bearing holes has been completed; 
         FIG. 17  is a view illustrating a state before cutting of a pinion hole and an oil seal hole in one embodiment of the present disclosure; 
         FIG. 18  is a view illustrating another example of a state before cutting of a pinion hole and an oil seal hole in one embodiment of the present disclosure; 
         FIG. 19  is a view illustrating various embodiments of a tip portion of a cutting tool; and 
         FIG. 20  is a longitudinal sectional view of one example of a differential case. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereunder, one embodiment of the present disclosure will be described with reference to the drawings. First, a workpiece  10  that is an object of machining of a machining apparatus  1  (see  FIG. 3 ) according to one embodiment of the present disclosure will be described with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a perspective view of the workpiece  10 , and  FIG. 2  is a longitudinal sectional view of the workpiece  10  illustrated in  FIG. 1 . The workpiece  10  is a differential carrier. The differential carrier houses a drive pinion gear and a ring gear which are final gears, in addition to a differential case which has a built-in differential transmission mechanism. 
     In  FIG. 1 , a differential bearing seat surface  12  and a differential bearing hole  13  are formed in each of left and right legs  11 . As illustrated in  FIG. 2 , a stepped hole is formed by a small-diameter hole forming the differential bearing seat surface  12 , and a large-diameter hole forming the differential bearing hole  13 . Further, in a cylindrical portion of the differential carrier  10 , an oil seal hole  14 , a pinion hole  15 , a release hole  16 , a release hole  17  and a pinion hole  18  are formed in that order from the upper side. 
     Hereunder, the machining apparatus  1  according to one embodiment of the present disclosure will be described. First, the configuration of the machining apparatus  1  will be schematically described with reference to  FIG. 3 .  FIG. 3  is a front view of the machining apparatus  1  according to one embodiment of the present disclosure. In  FIG. 3 , a shuttle unit  3 , machining units  4 , and an ATC (automatic tool changer)  5  are mounted on a base  2 . 
     The shuttle unit  3  holds the workpiece  10 , and can rotate the workpiece  10 . The shuttle unit  3  includes an elevating body  30 , and is movable integrally with the elevating body  30  in a Y-direction (front-rear direction). The elevating body  30  is movable in a Z-direction (vertical direction) along a guide  33  by rotation of a ball screw  32  that is caused by a motor  31 . The elevating body  30  includes a clamper  34  that holds the workpiece  10 , and a rotary mount  35  that rotates the clamper  34 . 
     The machining units  4  are a left and right pair and are arranged opposing each other so as to allow the workpiece  10  that was moved downward from the position illustrated in  FIG. 3  to be sandwiched therebetween. A tool  20  is attached to a tip of each of the machining units  4 . In the present embodiment, tools  20  can be selectively used in accordance with an area to be machined, and the tool  20  attached to the respective machining units  4  is automatically replaced with a required one of the tools  20  by the ATC  5 . 
     The ATC  5  includes a rotating disk  70 , and the plurality of tools  20  are removably attached to the rotating disk  70 . The elevating body  71  is guided by a guide shaft  72 , allowing the ATC  5  to vertically move. When replacing the tool  20 , the elevating body  71  descends and the tool  20  attached to the tip of the respective machining units  4  is replaced with the tool  20  that is attached to the rotating disk  70  of the ATC  5 . 
     The machining units  4  each include a housing  21 , a motor for tool drive  22 , and a motor for sliding  23 . A driving force of the motor for tool drive  22  is transferred to a driving force transfer mechanism (not illustrated), causing the tool  20  to rotate. A driving force of the motor for sliding  23  is transferred to a ball screw mechanism (not illustrated). This causes the respective machining units  4  to slide and perform a reciprocating motion in the X-axis direction (transverse direction). More specifically, a slider  24  that is integral with the respective machining units  4  slides along a guide rail  25 . 
     Hereunder, a cutting tool for boring is described with reference to  FIG. 4 .  FIG. 4  illustrates a state immediately before boring of the workpiece  10  by a cutting tool for boring (hereinafter, referred to simply as “cutting tool”)  40 . Although the cutting tool  40  is a tool included among the tools  20  that are attached to the tip of the respective machining units  4  in  FIG. 3 , for convenience of description the cutting tool is denoted by reference numeral  40  to distinguish the cutting tool  40  from the tools  20 . 
     The cutting tool  40  is provided in a set of two, and for convenience of description one of the cutting tool  40  will be referred to as cutting tool  40   a  and the other will be referred to cutting tool  40   b . The cutting tool  40   a  and the cutting tool  40   b  each include a shaft part  41 , a cutting blade  42  laterally protruding from the shaft part  41 , and an abutment surface  43  that is formed at a tip of the shaft part  41  and that is for causing the tips of the shaft parts to butt against each other. An extension part  44  is integrated with the abutment surface  43  of the cutting tool  40   a , and a hole  45  having an opening at the position of the abutment surface  43  is formed in the abutment surface  43  of the cutting tool  40   b.    
     As will be described later in detail, during cutting, the extension part  44  enters into the hole  45  and the abutment surfaces  43  butt against each other, and a state is entered in which the cutting tool  40   a  and the cutting tool  40   b  are coupled and are both held by the housings  21  of the left and right machining units  4  (see  FIG. 10 ). In  FIG. 4 , the extension part  44  is a flat shape, and when the extension part  44  has entered into the hole  45 , slipping of the rotating cutting tool  40   a  and cutting tool  40   b  can be prevented. The extension part  44  may be a columnar shape, and as described later using  FIGS. 19( a ) and ( b ) , the extension part  44  may be omitted, and even in the case of these configurations, slip prevention can be achieved by synchronization of the rotational torques and positions of the cutting tool  40   a  and the cutting tool  40   b.    
     Hereunder, the process of machining the workpiece  10  is described in detail with reference to  FIG. 5  to  FIG. 13 .  FIG. 5  illustrates a state in which initial setting of the position of the workpiece  10  has been completed. The workpiece  10  is illustrated in a state as viewed from above, and for convenience a cross-sectional state thereof is illustrated, with the oil seal hole  14  side being the inner side of the machining apparatus  1  (see  FIG. 3 ), and the side of the left and right legs  11  being the operator side (the same also applies with respect to  FIG. 6  to  FIG. 9 , and  FIG. 11  to  FIG. 16 ). As a result of lowering the elevating body  30  of the shuttle unit  3 , the workpiece  10  that was above the machining units  4  in the state illustrated in  FIG. 3  has descended as far as an initial setting position at which the differential bearing seat surface  12  of the workpiece  10  faces the abutment surface  43  of each of the cutting tool  40   a  and the cutting tool  40   b.    
     The initial setting position is the position of the workpiece  10  in a height direction in which the cutting tool  40   a  and the cutting tool  40   b  can pass through the respective differential bearing seat surfaces  12  when the cutting tool  40   a  and the cutting tool  40   b  advance as a result of horizontal movement of the respective machining units  4  (arrow b). A height B including the cutting blade  42  of the cutting tool  40   a  and the cutting tool  40   b  is less than a diameter A of the differential bearing seat surface  12 . Therefore, by setting the position in the height direction and front-rear direction (arrow a) of the workpiece  10  at a predetermined position, it is possible for the cutting tool  40   a  and the cutting tool  40   b  to pass through the respective differential bearing seat surfaces  12 . 
       FIG. 6  a view illustrating a state in which the cutting tool  40   a  and the cutting tool  40   b  are passing through the respective differential bearing seat surfaces  12  and differential bearing holes  13 . In the state illustrated in  FIG. 6 , since the workpiece  10  is maintained in the initial setting position thereof, it is possible for the cutting tool  40   a  and the cutting tool  40   b  to pass through the respective differential bearing seat surfaces  12 , and to also pass through the respective differential bearing holes  13  which have a larger diameter than the differential bearing seat surfaces  12 . 
       FIG. 7  illustrates a state in which horizontal movement of the cutting tool  40   a  and the cutting tool  40   b  has been completed. In the state in  FIG. 7 , the abutment surfaces  43  of the cutting tool  40   a  and the cutting tool  40   b  are butted together, and the extension part  44  has entered into the hole  45 . Further, a center line  26  of the respective differential bearing holes  13  is located further to the rear side by an amount Ad relative to a center line  27  of the shaft part  41  of each of the cutting tool  40   a  and the cutting tool  40   b , and thus the center line  26  and the center line  27  do not coincide with each other. In this case, if the workpiece  10  is caused to move forward (arrow a) by the amount of Ad from the state in  FIG. 7 , the center line  26  and the center line  27  will coincide with each other. 
       FIG. 8  illustrates a state in which the workpiece  10  has moved forward by the amount of Ad from the state in  FIG. 7 . In this state, the center line  27  of the shaft part  41  of each of the cutting tool  40   a  and the cutting tool  40   b  coincides with the center line  26  of the respective differential bearing holes  13 . In this state, when the cutting tool  40   a  and the cutting tool  40   b  are rotated around the center line  27 , the trajectory of the tip edge of the cutting blade  42  is a circle with a radius C/2 centered on the center line of the differential bearing holes  13 . Therefore, in a state in which the cutting blade  42  is butted against the differential bearing hole  13 , if the cutting tool  40   a  and the cutting tool  40   b  are rotated, the inner circumferential surface of the differential bearing hole  13  can be cut to a diameter C. 
     When the cutting tool  40   a  and the cutting tool  40   b  are moved in a direction (arrow c) such that each cutting blade  42  moves toward the differential bearing hole  13  on the left side from the state in  FIG. 8 , the cutting blade  42  butts against the differential bearing hole  13 , and cutting of the differential bearing hole  13  starts.  FIG. 9  is a view illustrating a state in which the differential bearing hole  13  is being cut with the cutting blade  42 . In this state, cutting of approximately half of the differential bearing hole  13  in the depth direction thereof has been completed. 
       FIG. 10  is a perspective view of the vicinity of the workpiece  10  in a state during cutting of the differential bearing hole  13 . In the state illustrated in  FIG. 10 , the extension part  44  has entered into the hole  45 , and the abutment surfaces  43  butt against each other, and the cutting tool  40   a  and the cutting tool  40   b  are coupled and are both held by the housings  21  of the left and right machining units  4 . In this state in which the cutting tools  40   a  and  40   b  are both held, when the cutting tool  40   a  and the cutting tool  40   b  are rotated around the center line, chattering is prevented and the accuracy of cutting the differential bearing holes  13  improves. 
     More specifically, in addition to an increase in accuracy with respect to the cylindricality, roundness and concentricity of the differential bearing hole  13 , the accuracy of the surface roughness of the inner circumferential surface of the differential bearing hole  13  also increases. In particular, in  FIG. 10 , since the pair of differential bearing holes  13  are being cut, because the accuracy with respect to the concentricity of each of the differential bearing holes  13  increases, the central axes of the respective differential bearing holes  13  will also coincide with high accuracy. 
     As described above, by preventing chattering, the accuracy of cutting the differential bearing holes  13  also increase. In order to perform more stable cutting, it suffices to add a synchronization circuit to a control circuit of the machining units  4  to synchronize the rotational torques and positions of the cutting tool  40   a  and the cutting tool  40   b.    
       FIG. 11  a view illustrating a state in which cutting of the differential bearing hole  13  on the left side has been completed. In the state illustrated in  FIG. 11 , while the cutting tool  40   a  and the cutting tool  40   b  rotate around the center line, the cutting blade  42  moves as far as the end portion of the differential bearing hole  13  on the left side to thereby complete cutting of the differential bearing hole  13  on the left side. Thereafter, the cutting tool  40   a  and the cutting tool  40   b  are moved in a direction (arrow d) such that each cutting blade  42  moves toward the differential bearing hole  13  on the right side from the state in  FIG. 11 , to thereby transition to cutting of the differential bearing hole  13  on the right side. 
     Cutting of the differential bearing hole  13  on the right side is performed in the same manner as the cutting of the differential bearing hole  13  on the left side except for the movement direction of the cutting tool  40   a  and the cutting tool  40   b .  FIG. 12  is a view illustrating a state in which the differential bearing hole  13  on the right side is being cut with the cutting blade  42 . In this state, cutting of approximately half of the differential bearing hole  13  on the right side in the depth direction thereof has been completed.  FIG. 13  is a view illustrating a state in which cutting of the differential bearing hole  13  on the right side has been completed. In the state illustrated in  FIG. 13 , the cutting blade  42  has moved as far as the end portion of the differential bearing hole  13  on the right side, and cutting of the differential bearing hole  13  on the right side is completed. 
     Thereafter, if the cutting tool  40   a  and the cutting tool  40   b  and the workpiece  10  are moved in the opposite directions to the directions in the process up to this point, the cutting tool  40   a  and the cutting tool  40   b  can be taken out from the workpiece  10 . That is, when the cutting tool  40   a  and the cutting tool  40   b  are moved to the left side (arrow e) from the state illustrated in  FIG. 13 , the state illustrated in  FIG. 8  is returned to, and when the workpiece  10  is retracted by the amount of A d from this state, the state illustrated in  FIG. 7  is returned to, and if the cutting tool  40   a  and the cutting tool  40   b  are caused to move in opposite directions from this state so that the cutting tool  40   a  and the cutting tool  40   b  are separated from each other, the state illustrated in  FIG. 5  in which the cutting tool  40   a  and the cutting tool  40   b  are on the outside of the workpiece  10  is returned to via the state illustrated in  FIG. 6 . 
     The process of cutting each of the differential bearing holes  13  may be divided into the two processes, namely, rough machining and finish machining, and if it is assumed that the machining by the cutting tool  40   a  and the cutting tool  40   b  is rough machining, finish machining of the left and right differential bearing holes  13  can be performed by exchanging the cutting tool  40   a  and the cutting tool  40   b  for cutting tools for finish machining and repeating the aforementioned series of processes. 
     Although the cutting process described above using  FIG. 5  to  FIG. 13  is not a process which machines the left and right differential bearing holes  13  at the same time, in  FIG. 4 , by using cutting tools in which the length of the extension part  44  is increased and the length of the hole  45  is also increased to correspond to the increased length of the extension part  44 , it is possible to simultaneously machine the left and right differential bearing holes  13 , and the cutting time can thus be shortened.  FIG. 14  is a view illustrating a coupled state of a cutting tool  50   a  and a cutting tool  50   b  when new cutting tools  50   a  and  50   b  are used.  FIG. 14  is a view corresponding to  FIG. 8  that illustrates a coupled state of the cutting tool  40   a  and the cutting tool  40   b , in which the center line  27  of the respective shaft parts  41  of the cutting tool  50   a  and the cutting tool  50   b  already coincides with the center line  26  of each of the differential bearing holes  13 . The process up to this state is the same as in the case of the cutting tool  40   a  and the cutting tool  40   b.    
     In  FIG. 14 , with the exception of the fact that the length of an extension part  46  and of a hole  47  is long, the cutting tool  50   a  and the cutting tool  50   b  have the same configuration as the cutting tool  40   a  and the cutting tool  40   b , and components which are the same in the cutting tools  40   a  and  40   b  and cutting tools  50   a  and  50   b  are denoted by the same reference numerals, and a description of such components is omitted hereunder. The length of the extension part  46  is a length such that, while one cutting tool for boring  50   a  and the other cutting tool for boring  50   b  are being moved in opposite directions to each other with respect to the direction of the center line  27  of the shaft parts  41 , a state in which the extension part  46  has entered the hole  47  can be maintained when machining of the differential bearing holes  13  of the workpiece  10  is completed. 
     When the cutting tool  50   a  and the cutting tool  50   b  are moved in the direction in which the cutting tool  50   a  and the cutting tool  50   b  are separated from the state illustrated in  FIG. 14  (arrows f and g), the cutting blade  42  of the cutting tool  50   a  moves toward the differential bearing hole  13  on the left side, and the cutting blade  42  of the cutting tool  50   b  moves toward the differential bearing hole  13  on the right side. When movement of the cutting tool  50   a  and the cutting tool  50   b  in opposite directions to each other while rotating around the center line progresses, the differential bearing hole  13  on the left side is cut by the cutting blade  42  of the cutting tool  50   a , and the differential bearing hole  13  on the right side is cut by the cutting blade  42  of the cutting tool  50   b.    
       FIG. 15  is a view illustrating a state in which the left and right differential bearing holes  13  are being cut by the cutting blades  42 . As illustrated in  FIG. 15 , because the respective cutting blades  42  of the cutting tool  50   a  and the cutting tool  50   b  come in contact with the respective differential bearing holes  13 , the left and right differential bearing holes  13  can be machined simultaneously by the cutting tool  50   a  and the cutting tool  50   b . In the state illustrated in  FIG. 15 , cutting of approximately half of the left and right differential bearing holes  13  in the depth direction thereof has been completed. 
     During the period in which cutting of the left and right differential bearing holes  13  is being performed, since the extension part  46  is inside the hole  47 , the state is one in which the cutting tool  50   a  and the cutting tool  50   b  are coupled through the extension part  46  and the hole  47 , and are both held by the respective housings  21  (see  FIG. 10 ) of the left and right machining units  4 . Therefore, similarly to the case of the cutting tool  40   a  and the cutting tool  40   b , chattering during rotation is prevented, and the accuracy of cutting the differential bearing holes  13  is increased. 
     Subsequently, when movement of the cutting tool  50   a  and the cutting tool  50   b  in opposite directions to each other progresses, cutting of the left and right differential bearing holes  13  also progresses.  FIG. 16  is a view illustrating a state in which cutting of the left and right differential bearing holes  13  has been completed. In the state illustrated in  FIG. 16 , the tip edges of the respective cutting blades  42  have moved as far as the end portions of the left and right differential bearing holes  13 , and cutting of the left and right differential bearing holes  13  has been completed. Thereafter, the process of taking out the cutting tool  50   a  and the cutting tool  50   b  from the workpiece  10  is the same as in the case of the cutting tool  40   a  and the cutting tool  40   b.    
     Although in the foregoing embodiment the object of machining by the cutting tool  40  and the like is the differential bearing hole  13  of the workpiece  10 , the cutting tool according to the present disclosure is a cutting tool that prevents chattering and increases the accuracy of cutting of an inner circumferential surface of a hole, and an object of machining by the cutting tool is not limited to the differential bearing hole  13 .  FIG. 17  illustrates a state before a cutting tool  60  that is constituted by a pair composed of a cutting tool  60   a  and a cutting tool  60   b  cuts the pinion hole  18  and the oil seal hole  14 . 
     From the state in which cutting of the pair of differential bearing holes  13  has been completed that is illustrated in  FIG. 13 , if the cutting tool  40   a  and the cutting tool  40   b  are taken out from the workpiece  10 , the state will return to the state illustrated in  FIG. 5 . If the cutting tool  40   a  and the cutting tool  40   b  retracted from this state, and the rotary mount  35  (see  FIG. 3 ) is rotated to cause the workpiece  10  to rotate by 90 degrees (arrow h), a vertical axis  36  of the workpiece  10  will be parallel with the X-axis (see  FIG. 3 ).  FIG. 17  illustrates a state after replacing the cutting tool  40  with the cutting tool  60 , in which abutment surfaces  65  of the cutting tool  60   a  and the cutting tool  60   b  butt against each other, and an extension part  66  of the cutting tool  60   b  has entered inside a hole  67  of the cutting tool  60   a.    
     The cutting tool  60   a  is for machining the pinion hole  18 , and includes a shaft part  61  having an abutment surface  65  at the tip thereof, and a cutting blade  62  which is attached to the shaft part  61  at a position at which the cutting blade  62  can perform machining of the pinion hole  18 . The cutting tool  60   b  is for machining the oil seal hole  14 , and includes a shaft part  63  having an abutment surface  65  at the tip thereof, and a cutting blade  64  which is attached to the shaft part  63  at a position at which the cutting blade  64  can perform machining of the oil seal hole  14 . From the state illustrated in  FIG. 17 , when the cutting tools  60  are moved in a direction (arrow i) in which the cutting blade  62  moves toward the inside of the pinion hole  18  while rotating the cutting tools  60  around the center line, the inner circumferential surface of the pinion hole  18  is cut by the cutting blade  62 . After performing cutting of the pinion hole  18 , when the cutting tools  60  are moved in a direction (arrow j) in which the cutting blade  64  moves toward the inside of the oil seal hole  14 , the inner circumferential surface of the oil seal hole  14  is cut by the cutting blade  64 . 
       FIG. 18  illustrates a state before the pinion hole  18  and the oil seal hole  14  are cut with cutting tools that are different from the cutting tools illustrated in  FIG. 17 . A cutting tool  70   a  is for machining the pinion hole  18 , and includes a shaft part  71  having an abutment surface  75  at the tip thereof, and a cutting blade  72  that is attached to the shaft part  71  at a position at which the cutting blade  72  can perform machining of the pinion hole  18 . The cutting tool  70   b  is for machining the oil seal hole  14 , and includes a shaft part  73  having an abutment surface  75  at the tip thereof, and a cutting blade  74  which is attached to the shaft part  73  at a position at which the cutting blade  74  can perform machining of the oil seal hole  14 . The cutting tool  70   a  and the cutting tool  70   b  have the same configuration as the cutting tool  60   a  and the cutting tool  60   b , except that the length of an extension part  76  and a hole  77  is longer than the length of the extension part and hole of the cutting tools  60   a  and  60   b.    
     When the cutting tool  70   a  is moved in a direction (arrow i) in which the cutting blade  72  moves toward the inside of the pinion hole  18  while rotating the cutting tool  70   a  around the central axis, the inner circumferential surface of the pinion hole  18  is cut by the cutting blade  72 . On the other hand, when the cutting tool  70   b  is moved in a direction (arrow j) in which the cutting blade  74  moves toward the inside of the oil seal hole  14  while rotating the cutting tool  70   b  around the central axis, the inner circumferential surface of the oil seal hole  14  is cut by the cutting blade  74 . 
     In the case of using the cutting tool  70  as illustrated in  FIG. 18 , it is possible to perform machining of the pinion hole  18  and the oil seal hole  14  simultaneously, and the work time can be shortened. Further, during the period in which cutting of the pinion hole  18  and the oil seal hole  14  is being performed, since the extension part  76  is inside the hole  77 , the state is one in which the cutting tool  70   a  and the cutting tool  70   b  are coupled through the extension part  76  and the hole  77 , and are both held by the respective housings  21  (see  FIG. 10 ) of the left and right machining units  4 . Therefore, in the case of using the cutting tools  70  also, chattering during rotation is prevented, and the accuracy of cutting the pinion hole  18  and the oil seal hole  14  is increased. 
     Since the series of cutting processes for cutting the differential bearing holes  13 , and the pinion hole  18  and the oil seal hole  14  described above can be performed continuously in a state in which the workpiece  10  is held by the clamper  34  (see  FIG. 3 ) and without remounting the workpiece  10 , the perpendicularity between the central axes of the differential bearing holes  13  and the pinion hole  18  and the oil seal hole  14  is also highly accurate. Further, it is possible to perform cutting of the pinion hole  18  and the oil seal hole  14  that are two holes with different diameters by changing the cutting tool once. 
     Although in the embodiments described above, with regard to each of the cutting tool  40 , the cutting tool  50  and the like, an extension part is provided on one of the abutment surfaces  43 , a configuration may also be adopted in which an extension part is not provided, or in which the shape of the extension part is appropriately changed. Hereunder, various embodiments of a tip portion of a cutting tool are described with reference to  FIG. 19 . In a cutting tool  80  illustrated in  FIG. 19( a ) , with respect to each of one cutting tool  80   a  and the other cutting tool  80   b , an extension part is not provided on the abutment surface  43 . In this configuration, in order to prevent slipping when the abutment surfaces  43  are butted together, it is desirable to polish the abutment surfaces  43  to improve the accuracy of the flatness and perpendicularity and reduce the surface roughness. 
     In a cutting tool  81  illustrated in  FIG. 19( b ) , with respect to each of one cutting tool  81   a  and the other cutting tool  81   b , the diameter of the abutment surface  43  is enlarged so as to prevent slipping when the abutment surfaces  43  are butted together. In a cutting tool  82  illustrated in  FIG. 19( c ) , an extension part  84  is provided in a cutting tool  82   b , and a groove  85  for the extension part  84  to enter is provided in a cutting tool  82   a . Whilst the extension part  84  is laterally long and not longitudinally long as in the case of the extension part  44  illustrated in  FIG. 4 , with this configuration also it is possible to prevent slipping when the abutment surfaces  43  are butted together. 
     Although in the embodiments described above, an object of machining of the cutting tool  40  and the like is a hole formed in a differential carrier, as described above, the cutting tool according to the present disclosure is a tool that prevents chattering and increases the accuracy of cutting of an inner circumferential surface of a hole, and the object of machining of the cutting tool is not limited to a hole formed in a differential carrier. The object of machining of the cutting tool  40  and the like may be, for example, a side gear hole of a differential case. 
       FIG. 20  is a longitudinal sectional view of one example of a differential case. In the figure, shaft holes  91  are formed in the side surface of a central main body portion, a pair of stepped holes formed of a small-diameter hole forming an axle hole  92  and a large-diameter hole forming a side gear hole  93  are formed on both sides of the main body portion. The configuration of the stepped holes is the same as the configuration of the pair of stepped holes formed by the small-diameter hole forming the differential bearing seat surface  12  and the large-diameter hole forming the differential bearing hole  13  illustrated in  FIG. 2 . Therefore, even if the differential carrier  10  is replaced with the differential case  90 , the processes illustrated in  FIG. 5  to  FIG. 13  can be performed. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  machining apparatus 
               3  shuttle unit 
               4  machining unit 
               10  workpiece (differential carrier) 
               12  differential bearing seat surface 
               13  differential bearing hole 
               40 ,  50 ,  60 ,  70 ,  80 ,  81 ,  82  set of two cutting tools 
               40   a ,  40   b ,  50   a ,  50   b ,  60   a ,  60   b ,  70   a ,  70   b ,  80   a ,  80   b ,  81   a,    
               81   b ,  82   a ,  82   b  cutting tool 
               41 ,  61 ,  63 ,  71 ,  73  shaft part 
               42 ,  62 ,  64 ,  72 ,  74  cutting blade 
               43 ,  65 ,  75  abutment surface 
               44 ,  46 ,  66 ,  76  extension part 
               45 ,  47 ,  67 ,  77  hole 
               90  workpiece (differential case) 
               92  axle hole 
               93  side gear hole