Patent Publication Number: US-2023158579-A1

Title: Retaining member and cutting tool

Description:
TECHNICAL FIELD 
     The present disclosure relates to a retaining member and a cutting tool. 
     BACKGROUND ART 
     WO 2019/021605 (PTL 1) describes a cutting tool holder that supports a cutting insert. The cutting tool holder has a retaining member for positioning the cutting insert in a holder main body and fixing the cutting insert thereto. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: WO 2019/021605 
       
    
     SUMMARY OF INVENTION 
     A retaining member according to the present disclosure is a retaining member for fixing a cutting insert to a holder, and includes a first main body portion, a second main body portion, and a third main body portion. In the first main body portion, a coolant inflow opening is formed. In the second main body portion, a coolant ejection opening is formed. The third main body portion is located between the first main body portion and the second main body portion. A first flow path is formed in the first main body portion so as to be contiguous to the coolant inflow opening and extend along a first direction. A second flow path is formed in the second main body portion so as to be contiguous to the coolant ejection opening and extend along a second direction inclined with respect to the first direction. A boundary flow path is formed in the third main body portion so as to be contiguous to each of the first flow path and the second flow path. The boundary flow path is constituted of a pair of boundary wall surfaces facing each other in a first cross section intersecting each of the first flow path, the boundary flow path, and the second flow path. An inclination of a tangent to at least one of the pair of boundary wall surfaces is continuously changed. In a second cross section perpendicular to a direction in which the coolant flows, a lateral width of the first flow path is larger than a longitudinal width of the first flow path and a lateral width of the second flow path is larger than a longitudinal width of the second flow path when a longitudinal direction represents a direction in which the pair of boundary wall surfaces face each other and a lateral direction represents a direction perpendicular to the longitudinal direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic perspective view showing a configuration of a cutting tool according to a first embodiment. 
         FIG.  2    is a schematic perspective view showing a configuration of a holder of the cutting tool according to the first embodiment. 
         FIG.  3    is a schematic perspective view showing a configuration of a retaining member according to the first embodiment. 
         FIG.  4    is a schematic plan view showing the configuration of the retaining member according to the first embodiment. 
         FIG.  5    is a schematic cross sectional view showing a first cross section of the retaining member according to the first embodiment. 
         FIG.  6    is a schematic cross sectional view showing a configuration of a first main body portion in a second cross section perpendicular to a first direction. 
         FIG.  7    is a schematic cross sectional view showing a configuration of a second main body portion in the second cross section perpendicular to a second direction. 
         FIG.  8    is a schematic cross sectional view showing a third cross section of the retaining member according to the first embodiment. 
         FIG.  9    is a schematic perspective view showing a configuration of a cutting tool according to a second embodiment. 
         FIG.  10    is a schematic perspective view showing a configuration of a holder of the cutting tool according to the second embodiment. 
         FIG.  11    is a schematic perspective view showing a configuration of a retaining member according to the second embodiment. 
         FIG.  12    is a schematic plan view showing the configuration of the retaining member according to the second embodiment. 
         FIG.  13    is a schematic cross sectional view showing a first cross section of the retaining member according to the second embodiment. 
         FIG.  14    is a schematic cross sectional view showing a third cross section of the retaining member according to the second embodiment. 
         FIG.  15    is a schematic cross sectional view showing a first cross section of a retaining member according to a third embodiment. 
         FIG.  16    is a schematic cross sectional view showing a configuration of a retaining member according to a sample 1. 
     
    
    
     DETAILED DESCRIPTION 
     Problem to be Solved by the Present Disclosure 
     It is an object of the present disclosure to provide a retaining member and a cutting tool to reduce energy loss of a fluid. 
     Advantageous Effect of the Present Disclosure 
     According to the present disclosure, there can be provided a retaining member and a cutting tool to reduce energy loss of a fluid. 
     DESCRIPTION OF EMBODIMENTS 
     First, embodiments of the present disclosure are listed and described. 
     (1) A retaining member  100  according to the present disclosure is a retaining member  100  for fixing a cutting insert  70  to a holder  50 , and includes a first main body portion  1 , a second main body portion  2 , and a third main body portion  3 . In first main body portion  1 , a coolant inflow opening  5  is formed. In second main body portion  2 , a coolant ejection opening  6  is formed. Third main body portion  3  is located between first main body portion  1  and second main body portion  2 . A first flow path  10  is formed in first main body portion  1  so as to be contiguous to coolant inflow opening  5  and extend along a first direction D 1 . A second flow path  20  is formed in second main body portion  2  so as to be contiguous to coolant ejection opening  6  and extend along a second direction D 2  inclined with respect to first direction D 1 . A boundary flow path  30  is formed in third main body portion  3  so as to be contiguous to each of first flow path  10  and second flow path  20 . Boundary flow path  30  is constituted of a pair of boundary wall surfaces  33  facing each other in a first cross section S 1  intersecting each of first flow path  10 , boundary flow path  30 , and second flow path  20 . An inclination of a tangent to at least one of the pair of boundary wall surfaces  33  is continuously changed. In a second cross section S 2  perpendicular to a direction in which the coolant flows, a lateral width of first flow path  10  is larger than a longitudinal width of first flow path  10  and a lateral width of second flow path  20  is larger than a longitudinal width of second flow path  20  when a longitudinal direction represents a direction in which the pair of boundary wall surfaces  33  face each other and a lateral direction represents a direction perpendicular to the longitudinal direction. 
     (2) According to retaining member  100  according to (1), first flow path  10  may have a narrowed portion  41  in which the lateral width of first flow path  10  is monotonously decreased in a direction toward second flow path  20 . 
     (3) According to retaining member  100  according to (2), in a third cross section S 3  parallel to each of first direction D 1  and the lateral direction, narrowed portion  41  may be constituted of a first side surface  11  and a second side surface  12  facing each other. First side surface  11  may be in a form of a straight line and second side surface  12  may be in a form of a curve. 
     (4) According to retaining member  100  according to (2), in a third cross section S 3  parallel to each of first direction D 1  and the lateral direction, narrowed portion  41  may be constituted of a first side surface  11  and a second side surface  12  facing each other. Each of first side surface  11  and second side surface  12  may be in a form of a straight line. Each of first side surface  11  and second side surface  12  may be inclined with respect to first direction D 1 . 
     (5) Retaining member  100  according to any one of (1) to (4) may further include a cylindrical portion  4  protruding from second main body portion  2 . In first cross section S 1 , an ejection surface in which coolant ejection opening  6  is formed may be parallel to a central axis A of cylindrical portion  4 . 
     (6) According to retaining member  100  according to any one of (1) to (5), in first cross section S 1 , second flow path  20  may be constituted of a third side surface  23  and a fourth side surface  24  facing each other. Third side surface  23  may have a first wall surface  21  separated by more than or equal to 1 mm from coolant ejection opening  6 . Fourth side surface  24  may have a second wall surface  22  separated by more than or equal to 1 mm from coolant ejection opening  6 . First wall surface  21  may be parallel to second wall surface  22 . 
     (7) According to retaining member  100  according to (1), first flow path  10  may have a narrowed portion  41  in which the lateral width of first flow path  10  is monotonously decreased in a direction toward second flow path  20 . Retaining member  100  may further include a cylindrical portion  4  protruding from second main body portion  2 . In first cross section S 1 , an ejection surface in which coolant ejection opening  6  is formed may be parallel to a central axis A of cylindrical portion  4 . In first cross section S 1 , second flow path  20  may be constituted of a third side surface  23  and a fourth side surface  24  facing each other. Third side surface  23  may have a first wall surface  21  separated by more than or equal to 1 mm from coolant ejection opening  6 . Fourth side surface  24  may have a second wall surface  22  separated by more than or equal to 1 mm from coolant ejection opening  6 . First wall surface  21  may be parallel to second wall surface  22 . 
     (8) A cutting tool  200  according to the present disclosure includes: retaining member  100  according to any one of (1) to (7); and holder  50  on which retaining member  100  is disposed. 
     (9) Cutting tool  200  according to (8) may further include cutting insert  70  in contact with retaining member  100 . 
     Details of Embodiments of the Present Disclosure 
     Next, details of the embodiments of the present disclosure will be described with reference to figures. In the below-described figures, the same or corresponding portions are denoted by the same reference characters and will not be described repeatedly. 
     First Embodiment 
     First, a configuration of a cutting tool  200  according to a first embodiment of the present disclosure will be described. 
       FIG.  1    is a schematic perspective view showing the configuration of cutting tool  200  according to the first embodiment. As shown in  FIG.  1   , cutting tool  200  according to the first embodiment mainly includes a holder  50 , a cutting insert  70 , a retaining member  100 , an underlying plate  80 , and a fastening member  60 . Holder  50  has an insert holding portion  52  and a supporting portion  51 . Insert holding portion  52  is contiguous to supporting portion  51 . Cutting insert  70 , underlying plate  80 , retaining member  100 , and fastening member  60  are disposed at insert holding portion  52 . Supporting portion  51  is attached to, for example, a machine tool. 
     Cutting insert  70  has a top surface  71 , an outer peripheral surface  72 , and a cutting edge  73 . At least a portion of top surface  71  functions as a rake face. At least a portion of outer peripheral surface  72  functions as a flank face. A ridgeline between top surface  71  and outer peripheral surface  72  forms cutting edge  73 . When viewed in a direction perpendicular to top surface  71 , the outer shape of top surface  71  is substantially a shape of parallelogram. A hole portion  74  is formed in top surface  71 . Cutting insert  70  is in contact with retaining member  100 . Retaining member  100  covers a portion of top surface  71  of cutting insert  70 . Retaining member  100  fixes cutting insert  70  to holder  50 . Retaining member  100  positions cutting insert  70 . Retaining member  100  is fixed to holder  50  by fastening member  60 . Cutting insert  70  is in contact with underlying plate  80 . Underlying plate  80  is located between cutting insert  70  and holder  50 . Underlying plate  80  is in contact with holder  50 . 
     Retaining member  100  is provided with a coolant ejection opening  6 . Coolant ejection opening  6  faces a corner portion of cutting edge  73  of cutting insert  70 . Insert holding portion  52  of holder  50  is provided with a coolant sending-out opening  91 . Coolant sending-out opening  91  faces the corner portion of cutting edge  73  of cutting insert  70 . Coolant sent out from coolant ejection opening  6  is sent out from the top surface  71  side (upper side) of cutting insert  70  toward cutting edge  73 . The coolant sent out from coolant sending-out opening  91  is sent out from the outer peripheral surface  72  side (lower side) of cutting insert  70  toward cutting edge  73 . 
       FIG.  2    is a schematic perspective view showing a configuration of holder  50  of cutting tool  200  according to the first embodiment. As shown in  FIG.  2   , insert holding portion  52  of holder  50  of cutting tool  200  according to the first embodiment has an upper surface  54 , a first end surface  53 , and a second end surface  55 . Upper surface  54  is contiguous to each of first end surface  53  and second end surface  55 . First end surface  53  is contiguous to second end surface  55 . A protrusion  56  is provided at a boundary between first end surface  53  and second end surface  55 . Protrusion  56  is provided with coolant sending-out opening  91 . 
     Insert holding portion  52  is provided with a second recess  92 , a third recess  93 , and a fourth recess  94 . Cutting insert  70  and underlying plate  80  are disposed in second recess  92 . Second recess  92  is exposed at each of first end surface  53 , second end surface  55 , and upper surface  54 . Third recess  93  is provided in upper surface  54 . A portion of fastening member  60  is disposed in third recess  93 . Fourth recess  94  is provided in upper surface  54 . A portion of retaining member  100  is disposed in fourth recess  94 . 
     Next, a configuration of retaining member  100  according to the first embodiment of the present disclosure will be described.  FIG.  3    is a schematic perspective view showing the configuration of retaining member  100  according to the first embodiment.  FIG.  4    is a schematic plan view showing the configuration of retaining member  100  according to the first embodiment. 
     As shown in  FIGS.  3  and  4   , retaining member  100  according to the first embodiment mainly includes a first main body portion  1 , a second main body portion  2 , a third main body portion  3 , a cylindrical portion  4 , and a bottom surface  9 . A coolant inflow opening  5  is formed in first main body portion  1 . Coolant inflow opening  5  is a portion via which coolant is introduced into retaining member  100 . The coolant is introduced from fourth recess  94  of holder  50  into retaining member  100 . Coolant ejection opening  6  is formed in second main body portion  2 . Coolant ejection opening  6  is a portion via which the coolant is sent out from retaining member  100 . Third main body portion  3  is located between first main body portion  1  and second main body portion  2 . 
     Retaining member  100  is provided with a through hole  7 . Fastening member  60  is inserted into through hole  7 . Retaining member  100  is fixed to holder  50  by fastening member  60 . Bottom surface  9  is disposed to face top surface  71  of cutting insert  70 . Bottom surface  9  is provided with a first recess  8 . First main body portion  1  is provided to protrude from bottom surface  9 . First main body portion  1  is a tubular member. An annular groove  95  is provided in the outer peripheral surface of first main body portion  1 . Cylindrical portion  4  protrudes from second main body portion  2 . Cylindrical portion  4  extends in a direction substantially perpendicular to bottom surface  9 . Cylindrical portion  4  is disposed in hole portion  74  provided in top surface  71  of cutting insert  70 . 
     As shown in  FIG.  4   , a first flow path  10  is formed in first main body portion  1 . A second flow path  20  is formed in second main body portion  2 . A boundary flow path  30  is formed in third main body portion  3 . Boundary flow path  30  is contiguous to each of first flow path  10  and second flow path  20 . The coolant having entered first flow path  10  flows into second flow path  20  via boundary flow path  30 . Coolant ejection opening  6  is provided in an ejection surface  27  of second main body portion  2 . 
       FIG.  5    is a schematic cross sectional view showing a first cross section S 1  of retaining member  100  according to the first embodiment. First cross section S 1  intersects each of first flow path  10 , boundary flow path  30 , and second flow path  20 . As shown in  FIG.  5   , first flow path  10  is contiguous to coolant inflow opening  5 . First flow path  10  extends along a first direction D 1 . Second flow path  20  is contiguous to coolant ejection opening  6 . Second flow path  20  extends along a second direction D 2 . Second direction D 2  is inclined with respect to first direction D 1 . An angle between a straight line along first direction D 1  and a straight line along second direction D 2  may be more than or equal to 60° and less than or equal to 120°, for example. 
     As shown in  FIG.  5   , boundary flow path  30  is constituted of a pair of boundary wall surfaces  33  facing each other in first cross section S 1 . The pair of boundary wall surfaces  33  have a first boundary wall surface  31  and a second boundary wall surface  32 . Second boundary wall surface  32  may be located between first boundary wall surface  31  and bottom surface  9 . First boundary wall surface  31  is smoothly curved. From another viewpoint, it can be said that the inclination of a tangent L to first boundary wall surface  31  is continuously changed. First boundary wall surface  31  is curved to protrude on the outer side. Second boundary wall surface  32  may be bent sharply. From another viewpoint, it can be said that the inclination of a tangent to first boundary wall surface  31  may be discontinuously changed. 
     In first cross section S 1 , second flow path  20  is constituted of a third side surface  23  and a fourth side surface  24 . Third side surface  23  and fourth side surface  24  face each other. Third side surface  23  has a first wall surface  21  and a fifth wall surface  25 . Fifth wall surface  25  is a region of third side surface  23  extending to coolant ejection opening  6  from a position separated by 1 mm from coolant ejection opening  6 . First wall surface  21  is a region of third side surface  23  separated by more than or equal to 1 mm from coolant ejection opening  6 . First wall surface  21  is contiguous to fifth wall surface  25 . A distance B from coolant ejection opening  6  to a boundary between first wall surface  21  and fifth wall surface  25  is 1 mm. A step may be provided in fifth wall surface  25 . 
     Fourth side surface  24  has a second wall surface  22  and a sixth wall surface  26 . Sixth wall surface  26  is a region of fourth side surface  24  extending to coolant ejection opening  6  from a position separated by 1 mm from coolant ejection opening  6 . Second wall surface  22  is a region of fourth side surface  24  separated by more than or equal to 1 mm from coolant ejection opening  6 . Second wall surface  22  is contiguous to sixth wall surface  26 . A distance B from coolant ejection opening  6  to a boundary between second wall surface  22  and sixth wall surface  26  is 1 mm. A step may be provided in sixth wall surface  26 . First wall surface  21  may be parallel to second wall surface  22 . 
     As shown in  FIG.  5   , in first cross section S 1 , ejection surface  27  in which coolant ejection opening  6  is formed may be contiguous to cylindrical portion  4 . Ejection surface  27  may be parallel to central axis A of cylindrical portion  4 . Ejection surface  27  may be provided along the outer peripheral surface of cylindrical portion  4 . Central axis A of cylindrical portion  4  may intersect second direction D 2 . The coolant is sent out from coolant ejection opening  6  at a divergence angle θ. 
     Third side surface  23  is contiguous to first boundary wall surface  31 . Fourth side surface  24  is contiguous to second boundary wall surface  32 . In first cross section S 1 , first flow path  10  is constituted of a seventh side surface  17  and an eighth side surface  18  facing each other. Seventh side surface  17  is contiguous to first boundary wall surface  31 . Eighth side surface  18  is contiguous to second boundary wall surface  32 . In first cross section S 1 , seventh side surface  17  is inclined with respect to first wall surface  21 . In first cross section S 1 , eighth side surface  18  is inclined with respect to second wall surface  22 . 
       FIG.  6    is a schematic cross sectional view showing a configuration of first main body portion  1  in a second cross section S 2  perpendicular to first direction D 1 . The cross section shown in  FIG.  6    corresponds to a cross section taken along a line VI-VI in  FIG.  5   . As shown in  FIG.  6   , in second cross section S 2  perpendicular to the direction (first direction D 1 ) in which coolant flows in first flow path  10 , first flow path  10  formed in first main body portion  1  has an elongated shape. In second cross section S 2 , the lateral width (first width W 1 ) of first flow path  10  is larger than the longitudinal width (second width W 2 ) of first flow path  10 . In second cross section S 2  of first main body portion  1 , a longitudinal direction represents a direction in which the pair of boundary wall surfaces  33  face each other. In first flow path  10 , the direction in which the pair of boundary wall surfaces  33  face each other refers to a direction parallel to a direction from seventh side surface  17  toward eighth side surface  18 . In second cross section S 2  of first main body portion  1 , a lateral direction represents a direction perpendicular to the longitudinal direction. 
     First width W 1  may be 1.5 times or more and 4 times or less as large as second width W 2 , for example. The lower limit of first width W 1  is not particularly limited, but may be, for example, 1.7 times or more or twice or more as large as second width W 2 . The upper limit of first width W 1  is not particularly limited, but may be, for example, 3.8 times or less or 3.5 times or less as large as second width W 2 . 
       FIG.  7    is a schematic cross sectional view showing a configuration of second main body portion  2  in second cross section S 2  perpendicular to second direction D 2 . The cross section shown in  FIG.  7    corresponds to a cross section taken along a line VII-VII in  FIG.  5   . As shown in  FIG.  7   , in second cross section S 2  perpendicular to the direction (second direction D 2 ) in which coolant flows in second flow path  20 , second flow path  20  formed in second main body portion  2  has an elongated shape. In second cross section S 2 , the lateral width (third width W 3 ) of second flow path  20  is larger than the longitudinal width (fourth width W 4 ) of second flow path  20 . In second cross section S 2  of second main body portion  2 , the longitudinal direction represents the direction in which the pair of boundary wall surfaces  33  face each other. In second flow path  20 , the direction in which the pair of boundary wall surfaces  33  face each other refers to a direction parallel to a direction from third side surface  23  toward fourth side surface  24 . In second cross section S 2  of second main body portion  2 , the lateral direction represents the direction perpendicular to the longitudinal direction. Third width W 3  may be smaller than first width W 1 . Fourth width W 4  may be smaller than second width W 2 . 
     Third width W 3  may be 1.5 times or more and 4.0 times or less as large as fourth width W 4 , for example. The upper limit of third width W 3  is not particularly limited, but may be 3.0 times or less or 2.5 times or less as large as fourth width W 4 , for example. 
       FIG.  8    is a schematic cross sectional view showing a third cross section S 3  of retaining member  100  according to the first embodiment. The cross section shown in  FIG.  8    corresponds to a cross section taken along a line VIII-VIII in  FIG.  5   . Third cross section S 3  is parallel to each of first direction D 1  and the lateral direction of first flow path  10 . As shown in  FIG.  8   , first flow path  10  has a narrowed portion  41  and a constant width portion  42 . In narrowed portion  41 , the lateral width of first flow path  10  is monotonously decreased in a direction toward second flow path  20 . In third cross section S 3 , narrowed portion  41  is constituted of a first side surface  11  and a second side surface  12 . First side surface  11  and second side surface  12  face each other. First side surface  11  may be in the form of a straight line and second side surface  12  may be in the form of a curve. In constant width portion  42 , the lateral width of first flow path  10  is hardly changed in the direction toward second flow path  20 . In third cross section S 3 , constant width portion  42  is constituted of a fifth side surface  15  and a sixth side surface  16 . Fifth side surface  15  and sixth side surface  16  face each other. Each of fifth side surface  15  and sixth side surface  16  is in the form of a straight line. 
     As shown in  FIG.  8   , constant width portion  42  may be surrounded by annular groove  95 . Fifth side surface  15  is contiguous to first side surface  11 . Sixth side surface  16  is contiguous to second side surface  12 . In third cross section S 3 , the lateral width (seventh width W 7 ) of coolant inflow opening  5  may be larger than the lateral width (sixth width W 6 ) of constant width portion  42 . In third cross section S 3 , the lateral width (sixth width W 6 ) of constant width portion  42  may be larger than the lateral width (fifth width W 5 ) of narrowed portion  41 . In third cross section S 3 , fifth width W 5  becomes smaller as a distance from coolant inflow opening  5  is increased. 
     Second Embodiment 
     Next, a configuration of a cutting tool  200  according to a second embodiment of the present disclosure will be described. Cutting tool  200  according to the second embodiment is different from cutting tool  200  according to the first embodiment mainly in that the length of second main body portion  2  is long, and the other configurations of cutting tool  200  according to the second embodiment are the same as those of cutting tool  200  according to the first embodiment. Hereinafter, the configuration different from that of cutting tool  200  according to the first embodiment will be mainly described. 
       FIG.  9    is a schematic perspective view showing the configuration of cutting tool  200  according to the second embodiment. As shown in  FIG.  9   , cutting tool  200  according to the first embodiment mainly includes holder  50 , cutting insert  70 , retaining member  100 , underlying plate  80 , and fastening member  60 . Cutting insert  70  has top surface  71 , outer peripheral surface  72 , and cutting edge  73 . When viewed in the direction perpendicular to top surface  71 , the outer shape of top surface  71  is substantially a shape of rhombus. An angle of top surface  71  at a corner portion of cutting edge  73  of cutting tool  200  according to the second embodiment is smaller than the angle of top surface  71  at the corner portion of cutting edge  73  of cutting tool  200  according to the first embodiment. 
       FIG.  10    is a schematic perspective view showing a configuration of holder  50  of cutting tool  200  according to the second embodiment. As shown in  FIG.  10   , insert holding portion  52  of holder  50  of cutting tool  200  according to the second embodiment has upper surface  54 , first end surface  53 , and second end surface  55 . Upper surface  54  is contiguous to each of first end surface  53  and second end surface  55 . First end surface  53  is contiguous to second end surface  55 . Insert holding portion  52  is provided with second recess  92 , third recess  93 , and fourth recess  94 . Cutting insert  70  and underlying plate  80  are disposed in second recess  92 . Second recess  92  is exposed at each of first end surface  53 , second end surface  55 , and upper surface  54 . Third recess  93  is provided in upper surface  54 . A portion of fastening member  60  is disposed in third recess  93 . Second recess  92  and third recess  93  may be contiguous to each other. 
       FIG.  11    is a schematic perspective view showing a configuration of retaining member  100  according to the second embodiment.  FIG.  12    is a schematic plan view showing the configuration of retaining member  100  according to the second embodiment. As shown in  FIGS.  11  and  12   , retaining member  100  according to the second embodiment mainly includes first main body portion  1 , second main body portion  2 , third main body portion  3 , cylindrical portion  4 , and bottom surface  9 . In first main body portion  1 , coolant inflow opening  5  is formed. In second main body portion  2 , coolant ejection opening  6  is formed. Third main body portion  3  is located between first main body portion  1  and second main body portion  2 . 
       FIG.  13    is a schematic cross sectional view showing first cross section S 1  of retaining member  100  according to the second embodiment. First cross section S 1  intersects each of first flow path  10 , boundary flow path  30 , and second flow path  20 . As shown in  FIG.  13   , first flow path  10  is contiguous to coolant inflow opening  5 . First flow path  10  extends along first direction D 1 . Second flow path  20  is contiguous to coolant ejection opening  6 . Second flow path  20  extends along second direction D 2 . Second direction D 2  is inclined with respect to first direction D 1 . The length of second main body portion  2  of retaining member  100  according to the second embodiment is longer than the length of second main body portion  2  of retaining member  100  according to the first embodiment. The length of second flow path  20  of retaining member  100  according to the second embodiment is longer than the length of second flow path  20  of retaining member  100  according to the first embodiment. 
       FIG.  14    is a schematic cross sectional view showing third cross section S 3  of retaining member  100  according to the second embodiment. The cross section shown in  FIG.  14    corresponds to a cross section taken along a line XIV-XIV in  FIG.  13   . Third cross section S 3  is parallel to each of first direction D 1  and the lateral direction of first flow path  10 . As shown in  FIG.  14   , first flow path  10  has narrowed portion  41  and constant width portion  42 . In narrowed portion  41 , the lateral width of first flow path  10  is monotonously decreased in the direction toward second flow path  20 . In third cross section S 3 , narrowed portion  41  is constituted of first side surface  11  and second side surface  12 . First side surface  11  and second side surface  12  face each other. Each of first side surface  11  and second side surface  12  is in the form of a straight line. Each of first side surface  11  and second side surface  12  may be inclined with respect to first direction D 1 . In third cross section S 3 , the direction of inclination of first side surface  11  with respect to first direction D 1  may be opposite to the direction of inclination of second side surface  12  with respect to first direction D 1 . 
     Third Embodiment 
     Next, a configuration of a cutting tool  200  according to a third embodiment of the present disclosure will be described. Cutting tool  200  according to the third embodiment is different from cutting tool  200  according to the second embodiment mainly in that each of first boundary wall surface  31  and second boundary wall surface  32  is smoothly curved, and the other configurations of cutting tool  200  according to the third embodiment are the same as those of cutting tool  200  according to the second embodiment. Hereinafter, the configuration different from that of cutting tool  200  according to the second embodiment will be mainly described. 
       FIG.  15    is a schematic cross sectional view showing first cross section S 1  of retaining member  100  according to the third embodiment. First cross section S 1  intersects each of first flow path  10 , boundary flow path  30 , and second flow path  20 . As shown in  FIG.  15   , first flow path  10  is contiguous to coolant inflow opening  5 . First flow path  10  extends along first direction D 1 . Second flow path  20  is contiguous to coolant ejection opening  6 . Second flow path  20  extends along second direction D 2 . Second direction D 2  is inclined with respect to first direction D 1 . 
     As shown in  FIG.  15   , in first cross section S 1 , the pair of boundary wall surfaces  33  have first boundary wall surface  31  and second boundary wall surface  32 . Second boundary wall surface  32  may be located between first boundary wall surface  31  and bottom surface  9 . According to retaining member  100  of the third embodiment, an inclination of a tangent to at least one of the pair of boundary wall surfaces  33  is continuously changed. Specifically, each of first boundary wall surface  31  and second boundary wall surface  32  is smoothly curved. From another viewpoint, it can be said that each of the inclination of the tangent to first boundary wall surface  31  and the inclination of the tangent to second boundary wall surface  32  is continuously changed. First boundary wall surface  31  is curved to protrude on the outer side. Second boundary wall surface  32  is curved to protrude on the inner side. 
     In the above description, it has been illustrated that each of the inclination of the tangent to first boundary wall surface  31  and the inclination of the tangent to second boundary wall surface  32  is continuously changed; however, the configuration of retaining member  100  according to the present disclosure is not limited to this. In retaining member  100  according to the present disclosure, the inclination of the tangent to first boundary wall surface  31  may be continuously changed and the inclination of the tangent to second boundary wall surface  32  may be discontinuously changed, or the inclination of the tangent to first boundary wall surface  31  may be discontinuously changed and the inclination of the tangent to second boundary wall surface  32  may be continuously changed. 
     Next, functions and effects of retaining member  100  and cutting tool  200  according to each of the above-described embodiments will be described. 
     Retaining member  100  for fixing cutting insert  70  to holder  50  includes first main body portion  1 , second main body portion  2 , and third main body portion  3 . In first main body portion  1 , coolant inflow opening  5  is formed. In second main body portion  2 , coolant ejection opening  6  is formed. Third main body portion  3  is located between first main body portion  1  and second main body portion  2 . First flow path  10  is formed in first main body portion  1 . Second flow path  20  is formed in second main body portion  2 . Boundary flow path  30  is formed in third main body portion  3  so as to be contiguous to each of first flow path  10  and second flow path  20 . 
     When the shape of a flow path is complicated, energy loss of a fluid flowing in the flow path becomes large. In particular, when a wall surface of the flow path has an angular portion, the energy loss of the fluid is large at the angular portion. As a result, the rate of the fluid ejected from coolant ejection opening  6  becomes low. 
     According to retaining member  100  and cutting tool  200  according to each of the above-described embodiments, boundary flow path  30  is constituted of the pair of boundary wall surfaces  33  facing each other in first cross section S 1  intersecting each of first flow path  10 , boundary flow path  30 , and second flow path  20 . The inclination of the tangent to at least one of the pair of boundary wall surfaces  33  is continuously changed. That is, at least one boundary wall surface of the pair of boundary wall surfaces  33  is smoothly curved. Therefore, with retaining member  100  and cutting tool  200  according to each of the above-described embodiments, the energy loss of the fluid can be reduced as compared with a case where both the boundary wall surfaces of the pair of boundary wall surfaces  33  are not smoothly curved. 
     According to retaining member  100  and cutting tool  200  according to each of the above-described embodiments, the lateral width of first flow path  10  is larger than the longitudinal width of first flow path  10 , and the lateral width of second flow path  20  is larger than the longitudinal width of second flow path  20 . Thus, coolant spread in the lateral direction can be ejected from coolant ejection opening  6 . 
     According to retaining member  100  and cutting tool  200  according to each of the above-described embodiments, first flow path  10  may have narrowed portion  41  in which the lateral width of first flow path  10  is monotonously decreased in the direction toward second flow path  20 . Under the same energy of the fluid introduced into the flow path, the rate of the fluid in a region of the flow path having a smaller cross sectional area is higher than the rate of the fluid in a region of the flow path having a larger cross sectional area. Since first flow path  10  has narrowed portion  41  in which the lateral width of first flow path  10  is monotonously decreased in the direction toward second flow path  20 , the rate of the fluid can be increased in the direction from first flow path  10  toward second flow path  20  while reducing the energy loss of the fluid. 
     According to retaining member  100  and cutting tool  200  according to each of the above-described embodiments, retaining member  100  may further include cylindrical portion  4  protruding from second main body portion  2 . In first cross section S 1 , ejection surface  27  in which coolant ejection opening  6  is formed may be parallel to central axis A of cylindrical portion  4 . In the case where ejection surface  27  in which coolant ejection opening  6  is formed is parallel to central axis A of cylindrical portion  4 , the divergence angle of the coolant ejected from coolant ejection opening  6  can be made smaller than that in the case where ejection surface  27  in which coolant ejection opening  6  is formed is inclined with respect to central axis A of cylindrical portion  4 . 
     According to retaining member  100  and cutting tool  200  according to each of the above-described embodiments, second flow path  20  may be constituted of third side surface  23  and fourth side surface  24  facing each other in first cross section S 1 . Third side surface  23  may have first wall surface  21  separated by more than or equal to 1 mm from coolant ejection opening  6 . Fourth side surface  24  may have second wall surface  22  separated by more than or equal to 1 mm from coolant ejection opening  6 . First wall surface  21  may be parallel to second wall surface  22 . In the case where first wall surface  21  is parallel to second wall surface  22 , the fluid is facilitated to flow along the direction parallel to each of first wall surface  21  and second wall surface  22  as compared with a case where first wall surface  21  is inclined with respect to second wall surface  22 . Therefore, the divergence angle of the coolant ejected from coolant ejection opening  6  can be made small. Thus, the coolant can be discharged intensively onto cutting edge  73  of cutting insert  70 . 
     EXAMPLES 
     (Preparation of Samples) 
     First, a retaining member  100  according to a sample 1 and a retaining member  100  according to a sample 2 were prepared. Retaining member  100  according to sample 1 serves as a comparative example.  FIG.  16    is a schematic cross sectional view showing a configuration of retaining member  100  according to sample 1. As shown in  FIG.  16   , according to retaining member  100  of sample 1, boundary flow path  30  is constituted of first boundary wall surface  31  and second boundary wall surface  32  in first cross section S 1  intersecting each of first flow path  10 , boundary flow path  30 , and second flow path  20 . Each of first boundary wall surface  31  and second boundary wall surface  32  is angular. That is, the inclination of the tangent to each of first boundary wall surface  31  and second boundary wall surface  32  is discontinuously changed. 
     Retaining member  100  according to sample 2 serves as an example of the present disclosure.  FIG.  15    shows a configuration of retaining member  100  according to sample 2. According to retaining member  100  of sample 2, boundary flow path  30  is constituted of first boundary wall surface  31  and second boundary wall surface  32  in first cross section S 1  intersecting each of first flow path  10 , boundary flow path  30 , and second flow path  20 . Each of first boundary wall surface  31  and second boundary wall surface  32  is smoothly curved. That is, the inclination of the tangent to each of first boundary wall surface  31  and second boundary wall surface  32  is continuously changed. 
     (Evaluation Conditions) 
     Next, coolant was introduced into coolant inflow opening  5  of each of retaining member  100  according to sample 1 and retaining member  100  according to sample 2, and divergence angle θ of the coolant ejected from coolant ejection opening  6  was measured Divergence angle θ was measured by photographing a diverged state of the coolant in a direction perpendicular to first cross section S 1 . A value of the half of divergence angle θ of the coolant was defined as a half apical angle. Assuming that the divergence of the coolant is in an ideal form of cone, the solid angle of the coolant was calculated based on the half apical angle of the coolant. 
     (Evaluation Results) 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Sample  
                 Half Apical Angle 
                 Solid Angle 
               
               
                 Number 
                 (°) 
                 (Steradian) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Sample 1 
                 10 
                 0.03 
               
               
                 Sample 2 
                 2 
                 0.01 
               
               
                   
               
            
           
         
       
     
     Table 1 shows the half apical angle and solid angle of the coolant ejected from coolant ejection opening  6  of each of retaining member  100  according to sample 1 and retaining member  100  according to sample 2. As shown in Table 1, the half apical angle and solid angle of the coolant ejected from coolant ejection opening  6  of retaining member  100  according to sample 1 were 10° and 0.03 steradian, respectively. On the other hand, the half apical angle and solid angle of the coolant ejected from coolant ejection opening  6  of retaining member  100  according to sample 2 were 2′ and 0.01 steradian, respectively. It was confirmed that with retaining member  100  according to sample 2, the divergence angle of the coolant can be made smaller as compared with retaining member  100  according to sample 1. 
     The embodiments and examples disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the embodiments and examples described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       1 : first main body portion;  2 : second main body portion;  3 : third main body portion,  4 : cylindrical portion;  5 : coolant inflow opening;  6 : coolant ejection opening;  7 : through hole;  8 : first recess;  9 : bottom surface;  10 : first flow path;  11 : first side surface;  12 : second side surface;  15 : fifth side surface;  16 : sixth side surface;  17 : seventh side surface;  18 : eighth side surface;  20 : second flow path;  21 : first wall surface;  22 : second wall surface;  23 : third side surface;  24 : fourth side surface;  25 : fifth wall surface;  26 : sixth wall surface;  27 : ejection surface;  30 : boundary flow path;  31 : first boundary wall surface,  32 : second boundary wall surface;  33 : boundary wall surface;  41 : narrowed portion;  42 : constant width portion;  50 : holder;  51 : supporting portion;  52 : insert holding portion;  53 : first end surface;  54 : upper surface;  55 : second end surface;  56 : protrusion;  60 : fastening member;  70 : cutting insert;  71 : top surface;  72 : outer peripheral surface;  73 : cutting edge;  74 : hole portion;  80 : underlying plate;  91 : coolant sending-out opening;  92 : second recess;  93 : third recess;  94 : fourth recess;  95 : annular groove;  100 : retaining member.  200 : cutting tool; A: central axis; B: distance; D 1 : first direction; D 2 : second direction; L: tangent; S 1 : first cross section; S 2 : second cross section; S 3 : third cross section; W 1 : first width; W 2 : second width; W 3 : third width; W 4 : fourth width; W 5 : fifth width; W 6 : sixth width; W 7 : seventh width; θ: divergence angle.