Patent Publication Number: US-2023143821-A1

Title: Machining tool

Description:
TECHNICAL FIELD 
     The present invention relates to a machining tool that cuts an inner periphery of an opening of a workpiece to form a plurality of machined surfaces. 
     BACKGROUND ART 
     As an example, WO 2020/017644 A1 discloses a machining tool that cuts an inner periphery of an opening of a workpiece to form a plurality of machined surfaces. With this type of machining tool, cutting is performed by bringing a plurality of cutting tools into contact with machining target portions of the workpiece at different timings. In this way, a cutting tool forming a machined surface required to be formed with higher precision than machined surfaces with large machining allowances is prevented from becoming more prone to wear than the other cutting tools, for example. 
     Specifically, the machining tool includes a tool body that is rotatable about an axial line. On the tip side of the tool body, a fixed cutting tool is fixed, and a cartridge is provided so as to be movable along this axial line. A movable cutting tool is attached to this cartridge. Furthermore, the cartridge can move by having a cam, which is provided inside the tool body, abut against the cartridge to rotate. The cam can rotate by having an abutment portion of a rod, which moves back and forth along the axial line inside the tool body, abut against an abutment position of the cam in response to the back and forth movement of the rod. 
     When using the machining tool to cut a machining target portion of the workpiece, the fixed cutting tool is brought into contact with the machining target portion while the tool body is rotationally driven. At this time, the relative position of the cartridge with respect to the tool body is adjusted such that the movable cutting tool is distanced from the machining target portion. By performing the cutting in this manner, a prescribed machined surface, among a plurality of machined surfaces, is formed at the machining target portion. Next, by adjusting the relative position of the cartridge with respect to the tool body, the movable cutting tool, instead of the fixed cutting tool, is brought into contact with the machining target portion to perform cutting. In this way, the remaining machined surfaces are formed at the machining target portion. 
     SUMMARY OF THE INVENTION 
     In the machining tool described above, the tool body is formed from: a cutting tool holding portion, which is arranged at the tip side of the machining tool and holds a cutting tool; and a cam housing portion, which is fixed in an attachable/detachable manner to a base side of the cutting tool holding portion and houses the cam therein. Due to this, it is possible to detach the cutting tool holding portion from the cam housing portion and replace a worn-down cutting tool held by the cutting tool holding portion, for example. 
     With this machining tool, when attaching or detaching the cam housing portion and the cutting tool holding portion to or from each other or when adjusting the position of the cutting tool relative to the cutting tool holding portion, for example, there are cases where the abutment portion of the rod is distanced from the abutment position of the cam. In such a case, since the cam is rotatable, the abutment position of the cam might become skewed from an orientation facing the abutment portion. When the abutment portion of the rod is again brought near the cam in this state, there is a concern that the abutment portion will abut against a location other than the abutment position of the cam. When the abutment portion abuts against a location other than the abutment position, the load placed on the cam and the rod is increased, and therefore such a situation is preferably avoided. 
     The present invention has been devised in relation to this type of technology, and has the object of providing a machining tool that can avoid a situation where an abutment portion of a rod abuts against a location other than an abutment position of a cam. 
     A first aspect of the present invention is a machining tool that cuts an inner periphery of a workpiece including an opening to form a plurality of machined surfaces, the machining tool including: a tool body that includes a cam housing portion and a cutting tool holding portion fixed in an attachable/detachable manner to a tip side of the cam housing portion, and is rotationally driven around an axial line; a plurality of cutting tools held by the cutting tool holding portion and configured to rotate along with the tool body; a cartridge which is provided to the cutting tool holding portion in a manner to be movable back and forth, and to which at least one of the plurality of cutting tools is attached; a cam housed inside the cam housing portion and configured to move the cartridge back and forth by abutting against the cartridge and rotating; a rod configured to move back and forth along the axial line inside the tool body; an abutment portion provided on the rod and configured to cause the cam to rotate by abutting against an abutment position of the cam according to back and forth movement of the rod; and a restricting mechanism configured to adjust a phase of the cam in a state where the abutment portion is separated from the abutment position, in a manner so that the phase of the cam becomes a reference position where the abutment position faces the abutment portion. 
     This machining tool includes the restricting mechanism that is capable of adjusting the phase of the cam such that the phase of the cam becomes the reference position where the abutment position of the cam separated from the abutment portion of the rod faces the abutment portion. Therefore, it is possible to avoid a situation where the abutment portion of the rod abuts against a location other than the abutment position of the cam. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic explanatory diagram of a tip surface of a machining tool according to an embodiment of the present invention; 
         FIG.  2    is cross-sectional view of main portions taken along the line II-II in  FIG.  1    in a state where a cartridge is moved backward; 
         FIG.  3    is a cross-sectional view of main portions in a state where the cartridge of the machining tool of  FIG.  2    is moved forward; 
         FIG.  4    is a cross-sectional view of main portions for describing a state where a rod of the machining tool of  FIG.  2    is moved backward and a cutting tool holding portion and a cam housing portion are separated; 
         FIG.  5 A  is an enlarged cross-sectional side view of main portions of the machining tool for describing a restricting mechanism when a cam is at a reference position; 
         FIG.  5 B  is a cross-sectional view taken along the line VB-VB of  FIG.  5 A ; 
         FIG.  6 A  is an enlarged cross-sectional side view of main portions of the machining tool for describing the restricting mechanism when the cam is not at the reference position; 
         FIG.  6 B  is a cross-sectional view taken along the line VIB-VIB of  FIG.  6 A ; 
         FIG.  7    is a schematic partial cross-sectional view of a valve seat material and a cylinder head body before cutting; 
         FIG.  8    is a schematic partial dross-sectional view of a cylinder head after cutting of the valve seat material of  FIG.  7   ; 
         FIG.  9    is a schematic explanatory diagram for describing a state of machining a relief surface in the valve seat material of  FIG.  7    using a first cutting tool and a second cutting tool; 
         FIG.  10    is a schematic explanatory diagram for describing a state of machining a valve seat surface in the valve seat material of  FIG.  9    using a third cutting tool; 
         FIG.  11    is a cross-sectional view of main portions orthogonal to an axial line of the machining tool for describing a restricting mechanism according to a modification; 
         FIG.  12 A  is an enlarged cross-sectional side view of main portions of the machining tool for describing the restricting mechanism of  FIG.  11    when the cam is at the reference position; and 
         FIG.  12 B  is an enlarged cross-sectional side view of main portions of the machining tool for describing the restricting mechanism of  FIG.  11    when the cam is not at the reference position. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Preferred embodiments of a machining tool according to the present invention will be presented and described below with reference to the accompanying drawings. In the drawings below, constituent components that have the same or similar functions and effects may be given the same reference numerals, and redundant descriptions thereof may be omitted. 
     The following describes an example in which a machining tool  10  according to the present embodiment shown in  FIGS.  1  to  6 B  is applied, and a valve seat material  12  shown in  FIG.  7    is used as a workpiece, to cut an inner periphery of an opening  12   a  of the valve seat material  12 , thereby forming a first relief surface  14 , a valve seat surface  16 , and a second relief surface  18  shown in  FIG.  8    as a plurality of machined surfaces. The first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  are inclined surfaces having different inclination angles from each other relative to an axial direction of the opening  12   a . As shown in  FIG.  7   , the valve seat material  12  is press-fitted or joined to a cylinder head body  20 , and a valve seat  24  of a cylinder head  22  shown in  FIG.  8    is formed by performing the cutting as described above. 
     However, the target on which cutting can be performed by applying the machining tool  10  is not limited to the valve seat material  12 . Furthermore, the plurality of machined surfaces that can be formed by the machining tool  10  are not limited to the first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  described above. The machining tool  10  can be favorably applied in a case where cutting is performed on the inner periphery of an opening of a workpiece to form a plurality of machined surfaces. Examples of such a case include a case where a machined surface formed by rough boring and a machined surface formed by fine boring are each formed in an inner periphery of an opening of a workpiece, a case where a machined surface formed by boring and a machined surface formed by chamfering are each formed in the inner periphery of the opening of the workpiece, and the like. 
     First, a simple description of the cylinder head  22  is provided, while referencing  FIG.  8   . As an example, the cylinder head  22  includes the annular valve seat  24 , which is made of a sintered body of an iron-based material such as steel, and the cylinder head body  20 , which is made of an aluminum-based material such as pure aluminum or an aluminum alloy. The valve seat  24  may further include a material with high electrical conductivity such as a copper-based material. 
     A port  26  is formed in the cylinder head body  20 . The port  26  opens toward a combustion chamber (not shown). In the present embodiment, the annular valve seat  24  is inserted into an opening edge portion on the combustion chamber side of the port  26 , thereby fitting the valve seat  24  into this opening edge portion. 
     On the inner periphery of the valve seat  24 , the first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  having different surface directions from each other are arranged in the stated order from one end side (arrow X1 side) to the other end side (arrow X2 side) in the axial direction of the valve seat  24 . The first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  are each inclined in a direction in which the diameter of the opening is widened toward the combustion chamber side (arrow X1 side). As an example of the inclination angle that each of these surfaces forms with respect to the axial direction of the valve seat  24 , the first relief surface  14  forms an angle of 60°, the valve seat surface  16  forms an angle of 45°, and the second relief surface  18  forms an angle of 30°, but these angles are not particularly limited. 
     The port  26  can be opened and closed by seating or separating a valve (not shown) on or from the valve seat surface  16 , among the inner peripheral surfaces of the valve seat  24 . Therefore, in order for the valve and the valve seat surface  16  to contact each other without a gap to make the cylinder head  22  high-quality, it is necessary to perform high-precision machining particularly on the valve seat surface  16  among the inner peripheral surfaces of the valve seat  24 , with regard to the roundness, surface roughness, and the like. 
     Next, a description is provided of the valve seat material  12  before the cutting is performed to form the valve seat  24 , in other words, the valve seat material  12  in which the first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  are not yet formed, while referencing  FIG.  7   . The valve seat material  12  has an annular shape and is press-fitted into the cylinder head body  20 . The inner periphery of the valve seat material  12  has, for example, an orthogonal end surface  28  arranged at one end side (arrow X1 side) in the axial direction, an axial surface  30  arranged at the other end side in the axial direction, and a tapered surface  32  arranged between the orthogonal end surface  28  and the axial surface  30 . The orthogonal end surface  28  is orthogonal to the axial direction. The axial surface  30  is formed flush with the inner peripheral surface of the port  26 . The tapered surface  32  has a tapered shape inclined in a direction in which the diameter of the opening is widened toward the one end side (arrow X1 side) in the axial direction. 
     Next, a description is provided of the machining tool  10 , while referencing  FIGS.  1  to  6 B . The machining tool  10  cuts the inner periphery of the opening  12   a  of the valve seat material  12  shown in  FIG.  7   , to form the first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  shown in  FIG.  8   . In the present embodiment, the machining tool  10  is a composite machining tool capable of both cutting the valve seat material  12  and reaming a valve guide hole (not shown). The valve guide hole is provided in the cylinder head body  20 , and allows a shaft portion (not shown) of the valve to be inserted therethrough. 
     Specifically, as shown in  FIGS.  2  and  3   , the machining tool  10  includes a tool body  40 , a first cutting tool  42 , a second cutting tool  44  ( FIG.  1   ), a third cutting tool  46 , a cartridge  48 , a cam  50 , a cartridge biasing member  52 , a rod  54 , a restricting mechanism  56  shown in  FIGS.  5 A to  6 B , and a reamer  58 . 
     As shown in  FIGS.  2  and  3   , the tool body  40  is formed by a cutting tool holding portion  40   a  and a cam housing portion  40   b , which are fixed to each other in an attachable/detachable manner. The cutting tool holding portion  40   a  and the cam housing portion  40   b  are arranged in the stated order from the tip side (arrow Y1 side) to the base side (arrow Y2 side) of the tool body  40 . The cutting tool holding portion  40   a  has a substantially cylindrical shape with steps causing the diameter at the tip side to become less than the diameter at the base side. Furthermore, a large-diameter portion  40   a L is provided at the base side of the cutting tool holding portion  40   a . The base side of the cutting tool holding portion  40   a  and the tip side of the cam housing portion  40   b  are fixed in an attachable/detachable manner by, for example, bolting a bolt  41  to the outer peripheral side of this large-diameter portion  40   a L. 
     The base side of the cam housing portion  40   b  is fixed to a rotating spindle of a rotating drive mechanism included in a machine tool (not shown), for example. Due to this, the tool body  40  is rotationally driven about an axial line “a”. Furthermore, the tool body  40  is driven back and forth along the axial line “a” by a tool body drive mechanism included in the machine tool. 
     The first cutting tool  42  has a cutting edge  42   a  for machining the first relief surface  14  ( FIG.  8   ), and is mounted in an attachable/detachable manner to the cutting tool holding portion  40   a  of the tool body  40  via a shank  42   b . Specifically, as shown in  FIG.  1   , by gripping the shank  42   b  between an inner wall  60   a  of a housing groove  60  provided along the axial line “a” in the cutting tool holding portion  40   a  and a tightening member  62  provided in front of the inner wall  60   a  in the housing groove  60  in the rotational direction of the cutting tool holding portion  40   a , the first cutting tool  42  is directly fixed to the cutting tool holding portion  40   a.    
     By screwing together a screw  64  and a screw hole provided extending in the radial direction of the cutting tool holding portion  40   a , the tightening member  62  presses the shank  42   b  from the front in the rotational direction of the cutting tool holding portion  40   a  toward the inner wall  60   a  of the housing groove  60 , thereby making it possible to apply a tightening force. Furthermore, by loosening the engagement between the screw hole and the screw  64  with the tightening member  62 , it is possible to relax the grip on the shank  42   b , thereby making it possible to remove and insert the shank  42   b  between the inner wall  60   a  of the housing groove  60  and the tightening member  62 . 
     The second cutting tool  44  has a cutting edge  44   a  (see  FIG.  9   ) for machining the second relief surface  18  (see  FIG.  8   ), and is mounted in an attachable/detachable manner to the cutting tool holding portion  40   a  via a shank  44   b . In the same manner as the first cutting tool  42 , the second cutting tool  44  is directly fixed to the cutting tool holding portion  40   a  via the tightening member  62 . 
     The third cutting tool  46  has a cutting edge  46   a  for machining the valve seat surface  16  (see  FIG.  8   ), and is mounted in an attachable/detachable manner to the cartridge  48  via a shank  46   b . Specifically, by gripping the shank  46   b  between an inner wall  66   a  of a housing groove  66  provided along the axial line “a” in the cartridge  48  and the tightening member  62  provided in front of the inner wall  66   a  in the rotational direction, the third cutting tool  46  is fixed to the cartridge  48 . 
     The cartridge  48  is mounted to the cutting tool holding portion  40   a  in a manner to be movable back and forth along the axial line “a”, and is rotationally driven together with the cutting tool holding portion  40   a . Below, a direction toward the tip of the tool body  40  (arrow Y1 side) is the forward direction of the cartridge  48 , and a direction toward the base of the tool body  40  (arrow Y2 side) is the backward direction of the cartridge  48 . 
     As shown in  FIGS.  2  and  3   , the cartridge  48  is formed by integrating a cartridge body  68  and a push rod  70 . The cartridge body  68  is arranged to be slidable inside a groove  72  formed along the axial line “a” in the cutting tool holding portion  40   a . Furthermore, the cartridge body  68  includes a body  74  and a extending portion  76  that extends from a portion of the body  74  on the axial line “a” side (the center side of the cutting tool holding portion  40   a  in the radial direction) toward the tip side (arrow Y1 side). The housing groove  66 , which houses the shank  46   b  of the third cutting tool  46 , is provided in the entire extending portion  76  along the axial direction and in a portion of the body  74  on the tip side. 
     As shown in  FIG.  1   , when viewed from the tip side of the axial line “a”, the body  74  has a substantially rectangular shape, and a corner portion thereof that is on the front side in the rotational direction and on the outer side in the radial direction of the cutting tool holding portion  40   a  is cut away to provide a pressure receiving surface  74   a . In the body  74  and the extending portion  76 , end surfaces  78  on the backward side in the rotational direction are formed flush with each other, and end surfaces  80  on the axial line “a” side are formed flush with each other. These end surfaces  78  and  80  are orthogonal to each other. The pressure receiving surface  74   a  is inclined relative to both the end surfaces  78  and  80 . 
     Inside the groove  72 , a first inner wall surface  72   a  of the groove  72  abuts against the end surface  78  of the cartridge body  68 , and a second inner wall surface  72   b  of the groove  72  abuts against the end surface  80 . The first inner wall surface  72   a  extends along the end surface  78  and the second inner wall surface  72   b  extends along the end surface  80 , and therefore the first inner wall surface  72   a  and the second inner wall surface  72   b  are orthogonal to each other. 
     A leaf spring  82 , which is mounted on the cutting tool holding portion  40   a  by bolting or the like, abuts against the pressure receiving surface  74   a  from the front in the rotational direction. Due to this leaf spring  82 , the cartridge body  68  is pressed in a direction inclined relative to both the first inner wall surface  72   a  and the second inner wall surface  72   b  of the groove  72 , from the front in the rotational direction of the cartridge body  68 . 
     As shown in  FIGS.  2  and  3   , a fitting hole  84  is provided along the axial line “a” at the base side of the body  74 , and a small-diameter portion  70   a  at the tip side of the push rod  70  is fitted into this fitting hole  84 . A shaft portion  70   b , which has a larger diameter than the small-diameter portion  70   a , is provided at the base side of the small-diameter portion  70   a  of the push rod  70 , and a flange portion  70   c  is provided at the base side of the shaft portion  70   b . A through-hole  86  is provided along the axial line “a” in a part of the large-diameter portion  40   a L of the cutting tool holding portion  40   a  facing the groove  72 . By inserting the shaft portion  70   b  of the push rod  70  through this through-hole  86  in a manner to be movable back and forth, the majority of the push rod  70 , excluding the tip side thereof, is inserted into a push rod chamber  88  formed by the cutting tool holding portion  40   a  and the cam housing portion  40   b.    
     The inner diameter of the through-hole  86  is slightly greater than the outer diameter of the shaft portion  70   b  of the push rod  70 , and less than the inner diameter of the push rod chamber  88 . Therefore, a stepped surface  90  is formed between the push rod chamber  88  and the through-hole  86 . 
     The flange portion  70   c  of the push rod  70  has a diameter that is slightly smaller than the inner diameter of the push rod chamber  88 , and can slide inside the push rod chamber  88 . The cartridge biasing member  52 , which is made of an elastic body such as a spring, for example, is provided between the flange portion  70   c  and the stepped surface  90 . The cartridge biasing member  52  provides an elastic bias in a direction causing the flange portion  70   c  and the stepped surface  90  to move away from each other, in other words, a direction causing the cartridge  48  to move backward. 
     The cam housing portion  40   b  is provided with: a cam chamber  92  in communication with the base side of the push rod chamber  88 ; and a rod chamber  94  in communication with the axial line “a” side of the cam chamber  92  (the center side of the cam housing portion  40   b  in the radial direction). The cam  50 , which abuts against a base surface  70   d  of the push rod  70  (base surface of the cartridge  48 ), is provided in the cam chamber  92  in a manner to be rotatable via a cam shaft  96 . 
     The cam  50  abuts against the base surface  70   d  of the push rod  70  to move the cartridge  48  back and forth. Specifically, the cam  50  has a substantially circular shape and includes a cutout surface  50   a  obtained by cutting away a portion of an arc, a recessed portion  50   b  that is depressed toward the center of the circle, and an arc-shaped surface  50   c  provided between the cutout surface  50   a  and the recessed portion  50   b . An abutment portion  54   a  of the rod  54  can be inserted into the recessed portion  50   b . The abutment portion  54   a  protrudes with an annular shape from the outer peripheral surface of the rod  54  provided to be movable back and forth along the axial line “a” inside the rod chamber  94 . An abutment position  51 , which can abut against the abutment portion  54   a  inserted into the recessed portion  50   b , is provided on the inner surface of the recessed portion  50   b.    
     By moving the rod  54  back and forth while inserting the abutment portion  54   a  into the recessed portion  50   b , the cam  50  is rotationally driven with the center of the circle (arc) as the rotational center. Specifically, the direction of the rotational axis of the cam  50  passing through the rotational center described above intersects the axial line “a”, and is preferably orthogonal to the axial line “a”. Furthermore, by moving the rod  54  forward while bringing the abutment portion  54   a  into abutment against the abutment position  51 , the cam  50  rotates such that the arc-shaped surface  50   c  abuts against the base surface  70   d  of the cartridge  48  (push rod  70 ), as shown in  FIG.  3   . 
     On the other hand, by moving the rod  54  backward while bringing the abutment portion  54   a  into abutment against the abutment position  51 , the cam  50  rotates such that the cutout surface  50   a  abuts against the base surface  70   d  of the cartridge  48 , as shown in  FIG.  2   . In this way, it is possible to selectively bring the arc-shaped surface  50   c  and the cutout surface  50   a  into abutment against the base surface  70   d  of the cartridge  48 . 
     A radius r of the arc-shaped surface  50   c  is greater than a length L of a perpendicular line from the center of the circle to the cutout surface  50   a . Therefore, as shown in  FIG.  3   , by bringing the arc-shaped surface  50   c  into abutment against the base surface  70   d , it is possible for the cartridge  48  to move forward against the elastic force of the cartridge biasing member  52  by an amount corresponding to a difference between the radius r and the length L of the perpendicular line, compared to a case where the cutout surface  50   a  abuts against the base surface  70   d . In this way, in a state where the cartridge  48  has moved forward, the cutting edge  46   a  of the third cutting tool  46  is arranged on the farther toward the tip side of the tool body  40  than the cutting edges  42   a  and  44   a  of the first cutting tool  42  and the second cutting tool  44 , for example. As a result, it is possible to perform cutting with only the third cutting tool  46  contacting the inner periphery of the valve seat material  12 . 
     On the other hand, as shown in  FIG.  2   , by bringing the cutout surface  50   a  into abutment against the base surface  70   d  of the cartridge  48 , it is possible for the cartridge  48  to move backward under the effect of the elastic bias of the cartridge biasing member  52  by an amount corresponding to a difference between the radius r and the length L of the perpendicular line, compared to a case where the arc-shaped surface  50   c  abuts against the base surface  70   d . In this way, in a state where the cartridge  48  has moved backward, the cutting edges  42   a  and  44   a  of the first cutting tool  42  and the second cutting tool  44  are arranged farther toward the tip side of the tool body  40  than the cutting edge  46   a  of the third cutting tool  46 , for example. As a result, it is possible to perform cutting with only the first cutting tool  42  and the second cutting tool  44  contacting the inner periphery of the valve seat material  12 . 
     The rod  54  can be moved back and forth along the axial line “a” by an actuator (not shown) provided on the base side of the rod  54 . The actuator includes fluid pressure actuators such as hydraulic cylinders. Furthermore, an internal rod flow path (not shown) for supplying coolant to the tip side of the tool body  40  is provided along the axial line “a” inside the rod  54 . 
     A reamer hole  98 , through which the reamer  58  fixed in an attachable/detachable manner to the tip side of the rod  54  is inserted, is in communication with the tool body  40  farther on the tip side than the rod chamber  94 . The reamer  58  can move back and forth inside the reamer hole  98  in accordance with the back and forth movement of the rod  54 , and protrudes from the tip of the cutting tool holding portion  40   a.    
     With this machining tool  10 , as shown in  FIG.  4   , by releasing the bolting caused by the bolt  41 , the tool body  40  can be separated into the cutting tool holding portion  40   a  and the cam housing portion  40   b . In this case, the push rod chamber  88  is separated into a portion formed in the cutting tool holding portion  40   a  and a portion formed in the cam housing portion  40   b . Furthermore, the push rod  70  and the cartridge biasing member  52  fixed to the cutting tool holding portion  40   a  are removed from the portion of the push rod chamber  88  formed in the cam housing portion  40   b , and are separated from the cam housing portion  40   b.    
     The restricting mechanism  56  of  FIGS.  5 A to  6 B  is configured in a manner to be capable of adjusting the phase of the cam  50  to a reference position, in a state where the rod  54  moves backward and the abutment portion  54   a  of the rod  54  is separated from the abutment position  51  of the cam  50 . The reference position is where the abutment position  51  of the cam  50  faces the abutment portion  54   a  of the rod  54  that has moved back, and in the present embodiment is where the opening side of the recessed portion  50   b  of the cam  50  faces the abutment portion  54   a  of the rod  54  that has moved back. It should be noted that, as shown in  FIG.  2   , the cam  50  is at the reference position when the cutout surface  50   a  of the cam  50  abuts against the base surface  70   d  of the push rod  70 . 
     Specifically, as shown in  FIGS.  5 A to  6 B , the restricting mechanism  56  is formed from the cam shaft  96 , an engaging portion  100 , an insertion hole  102 , and an insertion member  104 . The cam shaft  96  extends along the direction of the rotational axis of the cam  50  (arrow Z1 and arrow Z2 directions), and is fixed to the cam  50  in a manner to be rotatable along with the cam  50 . One end portion of the cam shaft  96  in the extension direction (end portion on the arrow Z1 side) is rotatably fixed to the inner wall of the cam chamber  92 . The other end portion of the cam shaft  96  in the extension direction (end portion on the arrow Z2 side) is provided with the engaging portion  100 . The engaging portion  100  includes a concave portion  106 , which is depressed from the other end portion toward the one end portion of the cam shaft  96 , and an insertion groove  108 , which is formed by cutting away, in the radial direction of the cam shaft  96 , a portion of a peripheral wall  106   a  forming the concave portion  106 . 
     The insertion hole  102  is formed in a wall portion of the cam housing portion  40   b  ( FIGS.  5 B and  6 B ), and enables communication between the engaging portion  100  and the outside of the cam housing portion  40   b . Furthermore, the insertion hole  102  enables the extraction of the insertion member  104  inserted through the insertion hole  102 , only when the cam  50  is at the reference position. Specifically, the insertion hole  102  includes an axial hole  110 , a circumferential groove  112  ( FIGS.  5 A and  6 A ), and a keyhole portion  114 . The axial hole  110  extends along the direction of the rotational axis of the cam  50 . The circumferential groove  112  extends in a direction intersecting the axial hole  110 , inside the wall portion of the cam housing portion  40   b . The keyhole portion  114  enables communication, in the direction of the rotational axis of the cam  50 , between only a portion of the circumferential groove  112  and the outside of the cam housing portion  40   b.    
     The insertion member  104  can rotate along with the cam  50 , by being inserted through the insertion hole  102  from outside the cam housing portion  40   b  to engage with the engaging portion  100 . Specifically, the insertion member  104  has a shaft portion  116 , a gripping portion  118  ( FIGS.  5 B and  6 B ), and a restricting portion  120 . 
     One end side of the shaft portion  116  in the axial direction engages with the engaging portion  100  by being inserted into the concave portion  106  through the axial hole  110 . The gripping portion  118  is provided at the other end side of the shaft portion  116  in the axial direction, and can be gripped by an operator when the operator inserts the insertion member  104  through the insertion hole  102  or rotates the insertion member  104 . 
     The restricting portion  120  is a protruding piece that protrudes from the shaft portion  116  in the radial direction of the shaft portion  116 . When inserting the shaft portion  116  through the axial hole  110 , the restricting portion  120  is inserted into the keyhole portion  114 . Furthermore, when inserting the shaft portion  116  into the concave portion  106 , the restricting portion  120  is inserted into the insertion groove  108  of the engaging portion  100 . 
     In other words, as shown in  FIGS.  5 A and  5 B , when the keyhole portion  114  and the insertion groove  108  overlap according to the phase of the cam shaft  96  relative to the insertion hole  102 , the shaft portion  116  can be inserted through the axial hole  110  to engage with the engaging portion  100 , while the restricting portion  120  is inserted into the keyhole portion  114  and the insertion groove  108 . The positional relationship among the keyhole portion  114 , the insertion groove  108 , and the recessed portion  50   b  (abutment position  51 ) of the cam  50  is set such that the cam  50  is at the reference position when the keyhole portion  114  and the insertion groove  108  overlap in this manner. 
     By causing the restricting portion  120  inserted into the insertion groove  108  to abut against the inner wall of the insertion groove  108 , the rotational force of the insertion member  104  can be transmitted to the cam shaft  96 . Therefore, as an example, by gripping the gripping portion  118  and rotating the insertion member  104  in a state where the shaft portion  116  is engaged with the engaging portion  100 , it is possible to rotate the cam  50  via the restricting portion  120  and the cam shaft  96 . When rotating the insertion member  104  in this manner, the restricting portion  120  can move inside the circumferential groove  112  in accordance with the rotation of the cam  50 , as shown in  FIGS.  6 A and  6 B . 
     The restricting portion  120  inside the circumferential groove  112  can move to the outside of the cam housing portion  40   b  from the circumferential groove  112  only through the keyhole portion  114 . Therefore, as shown in  FIGS.  5 A and  5 B , the insertion member  104  can be extracted from the insertion hole  102  only when the restricting portion  120  in the insertion groove  108  is at a position where it overlaps the keyhole portion  114 , that is, only when the cam  50  is at the reference position. In other words, as shown in  FIGS.  6 A and  6 B , the insertion member  104  is restricted from being extracted from the insertion hole  102  when the restricting portion  120  in the insertion groove  108  is not at a position where it overlaps the keyhole portion  114 , that is, when the cam  50  is not at the reference position. 
     The following describes the main operation of the machining tool  10 . As shown in  FIG.  2   , by moving the rod  54  backward under the effect of the actuator described above, the cutout surface  50   a  of the cam  50  is brought into abutment against the base surface  70   d  of the cartridge  48 . Due to this, the cartridge  48  moves backward, and therefore the cutting edges  42   a  and  44   a  of the first cutting tool  42  and second cutting tool  44  attached directly to the tool body  40  can be moved farther forward than the cutting edge  46   a  of the third cutting tool  46  attached to the cartridge  48 . 
     In this state, while rotationally driving the tool body  40 , the tool body  40  is inserted into the opening  12   a  of the valve seat material  12  of  FIG.  7    from the one end side toward the other end side in the axial direction. Due to this, first, reaming of the valve guide hole is performed by the reamer  58  provided on the tip of the tool body  40 . 
     Next, as shown in  FIG.  9   , the respective cutting edges  42   a  and  44   a  of the first cutting tool  42  and second cutting tool  44  are brought into contact with the valve seat material  12 , and cutting is performed to form the first relief surface  14  and the second relief surface  18  shown in  FIG.  8   . In this cutting, the valve seat material  12  is cut from the orthogonal end surface  28  side and the tapered surface  32  side by the first cutting tool  42  and the second cutting tool  44 . 
     After the cutting has been performed to a point where a portion of the valve seat material  12  corresponding to a machining allowance S 1  shown by the two-dot chain line in  FIG.  9    has been cut away, the tool body  40  is moved backward slightly toward the one end side (arrow X1 side) in the axial direction, thereby separating the valve seat material  12  from the first cutting tool  42  and the second cutting tool  44 . As a result, a first cut surface  14   a  with the same inclination angle as the first relief surface  14 , and a second cut surface  18   a  with the same inclination angle as the second relief surface  18 , are formed in the valve seat material  12 . 
     Next, by moving the rod  54  forward under the effect of the actuator described above, the arc-shaped surface  50   c  of the cam  50  is brought into abutment against the base surface  70   d  of the push rod  70 . Due to this, the cartridge  48  moves backward, and therefore the cutting edge  46   a  of the third cutting tool  46  attached to the cartridge  48  can be moved farther forward than the cutting edges  42   a  and  44   a  of the first cutting tool  42  and the second cutting tool  44  directly attached to the tool body  40 . 
     In this state, while rotationally driving the tool body  40 , the tool body  40  is moved forward toward the other end side (arrow X2 side) in the axial direction, and as shown in  FIG.  10   , the cutting edge  46   a  of the third cutting tool  46  is brought into contact with the valve seat material  12  from which the machining allowance S 1  has been removed, and cutting is performed to form the valve seat surface  16  between the first cut surface  14   a  and the second cut surface  18   a . After the cutting has been performed to a point where a portion of the valve seat material  12  corresponding to a machining allowance S 2  shown by the two-dot chain line in  FIG.  10    has been cut away, the tool body  40  is moved backward toward the one end side in the axial direction. As a result, the valve seat  24  is obtained, in which the first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  are formed in the stated order from the one end side in the axial direction in the inner periphery of the opening. 
     Due to the cutting performed as described above, wear of the first cutting tool  42 , the second cutting tool  44 , and the third cutting tool  46  (also referred to below collectively as “cutting tools”) might occur. In such a case, as an example, the machining tool  10  is removed from a machining center (not shown) while the cutting tool holding portion  40   a  and the cam housing portion  40   b  remained fixed, and replacement or the like of the worn cutting tool is performed. 
     In the machining tool  10 , as described above, the cutting tool holding portion  40   a  and the cam housing portion  40   b  are fixed in an attachable/detachable manner. Therefore, in a state where the cutting tool holding portion  40   a  holding the cutting tool has been removed from the cam housing portion  40   b , replacement or the like of the worn cutting tool may be performed. 
     In order to perform the machining of the first relief surface  14 , the valve seat surface  16 , and the second relief surface  18  with high precision by the machining tool  10 , it is necessary for the fixed positions of the cutting edges  42   a ,  44   a , and  46   a  of the cutting tools relative to the cutting tool holding portion  40   a  to be set with high precision. Therefore, in particular, after a cutting tool has been replaced in the manner described above, it is preferable to perform an adjustment of the fixed positions of the cutting edges  42   a ,  44   a , and  46   a  of the cutting tools relative to the cutting tool holding portion  40   a  (also referred to below simply as a “fixed position adjustment”) using a tool setter (not shown) or the like, for example. 
     The following describes a method for performing the fixed position adjustment in a state where the cutting tool holding portion  40   a  is attached to the cam housing portion  40   b . When performing the fixed position adjustment, a state is brought about in which the rod  54  is moved backward such that the abutment portion  54   a  is separated from the abutment position  51  of the cam  50 . Due to this, the cam  50  is not in contact with the abutment portion  54   a  of the rod  54 . In other words, the cam  50  is released from a state of having its rotation restricted by the contact with the abutment portion  54   a . In the present embodiment, the state where the rod  54  has been moved backward is realized by removing the machining tool  10  from the machining center (not shown). 
     Before or after the rod  54  is moved backward in the manner described above and when the cam  50  is at the reference position (when the insertion groove  108  and the keyhole portion  114  are at the position where they overlap) as shown in  FIGS.  5 A and  5 B , the insertion member  104  is inserted through the insertion hole  102  of the cam housing portion  40   b  to engage with the engaging portion  100 . 
     Next, as shown in  FIGS.  6 A and  6 B , in a state where the shaft portion  116  is engaged with the engaging portion  100 , the gripping portion  118  is gripped to rotate the insertion member  104 . Thus, the arc-shaped surface  50   c  of the cam  50  of  FIG.  3    is brought into abutment against the base surface  70   d  of the push rod  70  (the abutment portion  54   a  is separated from the abutment position  51 ). In this manner, in a state where the cartridge  48  has moved forward, in other words, in a state where the cam  50  has rotated from the reference position, the fixed position adjustment of the third cutting tool  46  attached to the cartridge  48  is performed. 
     Next, as shown in  FIGS.  5 A and  5 B , by gripping the gripping portion  118  and rotating the insertion member  104 , the cutout surface  50   a  of the cam  50  is brought into abutment against the base surface  70   d  of the push rod  70 . Due to this, the cam  50  is positioned at the reference position and the restricting portion  120  in the insertion groove  108  overlaps the keyhole portion  114 , and therefore the insertion member  104  can be extracted from the insertion hole  102 . 
     Before or after the insertion member  104  is extracted from the insertion hole  102 , the fixed position adjustments of the first cutting tool  42  and the second cutting tool  44  held by the cutting tool holding portion  40   a  may be performed as needed. Furthermore, the fixed position adjustments for the first cutting tool  42 , the second cutting tool  44 , and the third cutting tool  46  may be performed in any order. 
     After the insertion member  104  is extracted from the insertion hole  102 , the machining tool  10  is mounted on the machining center (not shown) with the cutting tool holding portion  40   a  and the cam housing portion  40   b  in a fixed state, and the rod  54  is moved forward by the actuator as shown in  FIG.  2   . Due to this, the abutment portion  54   a  of the rod  54  can abut against the abutment position  51  of the cam  50 . By performing the fixed position adjustment as described above, it is possible to perform the cutting described above with the machining tool  10  in which the cutting edges  42   a ,  44   a , and  46   a  of the respective cutting tools are arranged with high precision at the prescribed positions in the cutting tool holding portion  40   a.    
     Based on the above, the machining tool  10  according to the present embodiment includes the restricting mechanism  56  capable of adjusting the phase of the cam  50  such that the phase of the cam  50  becomes the reference position where the abutment position  51  of the cam  50  separated from the abutment portion  54   a  of the rod  54  faces the abutment portion  54   a . As a result, it is possible to easily and efficiently avoid a situation where the abutment portion  54   a  of the rod  54  abuts against a location other than the abutment position  51  of the cam  50 . 
     In the machining tool  10  according to the present embodiment described above, the restricting mechanism  56  includes: the cam shaft  96  that extends along the direction of the rotational axis of the cam  50  and is fixed to the cam  50  in a manner to be rotatable along with the cam  50 ; the engaging portion  100  provided on the cam shaft  96 ; the insertion hole  102  that is formed in the cam housing portion  40   b  and enables communication between the outside of the cam housing portion  40   b  and the engaging portion  100 ; and the insertion member  104  that can rotate along with the cam  50  by being inserted through the insertion hole  102  from outside the cam housing portion  40   b  to engage with the engaging portion  100 , wherein the insertion hole  102  enables the insertion member  104  inserted through the insertion hole  102  to be extracted only when the cam  50  is at the reference position. 
     Furthermore, in the machining tool  10  according to the present embodiment described above, the insertion hole  102  includes: the axial hole  110  that extends along the direction of the rotational axis of the cam  50 ; the circumferential groove  112  that extends inside the cam housing portion  40   b  in a direction intersecting the axial hole  110 ; and the keyhole portion  114  enabling communication, in the direction of the rotational axis of the cam  50 , between only a portion of the circumferential groove  112  and the outside of the cam housing portion  40   b . The insertion member  104  includes the shaft portion  116  that is inserted through the axial hole  110  to engage with the engaging portion  100 , and the restricting portion  120  that protrudes from the shaft portion  116 . The restricting portion  120  is capable of moving inside the circumferential groove  112  in accordance with the rotation of the cam  50  in a state where the shaft portion  116  is engaged with the engaging portion  100 , and is capable of moving to the outside of the circumferential groove  112  through the keyhole portion  114  only when the cam  50  is at the reference position. 
     In the above cases, after the fixed position adjustment of a cutting tool is finished as described above, in order to extract the insertion member  104  from the insertion hole  102 , it is necessary to rotate the insertion member  104  such that the phase of the cam  50  becomes the reference position. In other words, while the insertion member  104  is inserted through the insertion hole  102  and cannot be extracted, it can be determined that the phase of the cam  50  is different from the reference position. Therefore, by performing the simple operation of extracting the insertion member  104  from the insertion hole  102  before moving the rod  54  forward to bring the abutment portion  54   a  toward the cam  50 , it is possible to easily and efficiently avoid a situation where the abutment portion  54   a  of the rod  54  abuts against a location other than the abutment position  51  of the cam  50 . The timing at which the cam  50  is rotated using the insertion member  104  is not limited to when the fixed position adjustment is performed, and can be any timing as needed. 
     In the machining tool  10  according to the present embodiment described above, the cam  50  includes the arc-shaped surface  50   c  following an arc-shaped outer periphery thereof, and the cutout surface  50   a  obtained by cutting away a portion of the arc, and by rotating with the center of the arc as the rotational center, the cam  50  can selectively bring the arc-shaped surface  50   c  and the cutout surface  50   a  into abutment against the base surface  70   d  of the cartridge  48 . The cartridge  48  moves forward when the arc-shaped surface  50   c  abuts against the base surface  70   d , and the cartridge  48  moves backward when the cutout surface  50   a  abuts against the base surface  70   d.    
     The machining tool  10  according to the present embodiment described above further includes the cartridge biasing member  52  that elastically biases the cartridge  48  in the backward direction. The cartridge  48  is moved forward against the elastic force of the cartridge biasing member  52  due to the arc-shaped surface  50   c  abutting against the base surface  70   d , and the cartridge  48  is moved backward under the effect of the elastic bias of the cartridge biasing member  52  due to the cutout surface  50   a  abutting against the base surface  70   d.    
     In these cases, as described above, the cartridge  48  can be moved back and forth by an amount corresponding to a difference between the radius r of the arc-shaped surface  50   c  and the length L of the perpendicular line from the center of the arc to the cutout surface  50   a . In other words, by bringing the arc-shaped surface  50   c  that follows the outer periphery of the arc into abutment against the base surface  70   d  of the cartridge  48 , the cartridge  48  can be moved farther forward by the above difference than when the cutout surface  50   a  is brought into abutment against the base surface  70   d.    
     At this time, since the cam  50  rotates with the center of the arc as the rotational center, the forward movement amount of the cartridge  48  can be made equal no matter what portion of the arc-shaped surface  50   c  is abutted against by the base surface  70   d . Accordingly, it is possible to easily maintain the positioning precision of the cutting tools via the cartridge  48  without the need for a high-precision adjustment of the rotational amount of the cam  50 . Furthermore, since the cutting with the cutting tool attached to the cartridge  48  is performed in a state where the arc-shaped surface  50   c  with an arc shape following the outer periphery of the arc abuts against the base surface  70   d  of the cartridge  48 , it is possible to improve the durability of the machining tool  10  with respect to the machining reaction force. 
     In the machining tool  10  according to the present embodiment described above, the abutment portion  54   a  protrudes from the outer peripheral surface of the rod  54 ; the abutment position  51  is provided on the inner surface of the recessed portion  50   b  into which the abutment portion  54   a  is insertable; the arc-shaped surface  50   c  is arranged between the recessed portion  50   b  and the cutout surface  50   a  in the circumferential direction of the arc; and by moving forward toward the tip side of the tool body  40  while the abutment portion  54   a  is brought into abutment against the abutment position  51 , the rod  54  causes the cam  50  to rotate such that the arc-shaped surface  50   c  abuts against the base surface  70   d , and by moving backward toward the base side of the tool body  40  while the abutment portion  54   a  is brought into abutment against the abutment position  51 , the rod  54  causes the cam  50  to rotate such that the cutout surface  50   a  abuts against the base surface  70   d.    
     As described above, the forward movement amount of the cartridge  48  can be made equal no matter what portion of the arc-shaped surface  50   c  is abutted against by the base surface  70   d , and so it is possible to easily maintain the positioning precision of the cutting tools via the cartridge  48  with a simple configuration without the need for a high-precision adjustment of the back and forth movement amount of the rod  54 . 
     The present invention is not limited to the above-described embodiment, and it goes without saying that various modifications could be adopted therein without departing from the essence and gist of the present invention. 
     For example, the machining tool  10  may include a restricting mechanism  130  shown in  FIGS.  11  to  12 B , instead of the restricting mechanism  56  shown in  FIGS.  5 A to  6 B . The restricting mechanism  130  includes a cam shaft  132 , a cam biasing member  134 , and a stopper  136 . Each constituent component of the restricting mechanism  130  is installed in an insertion hole  138  formed in the cam housing portion  40   b  of  FIG.  11    along the direction of the rotational axis of the cam  50 . 
     As shown in  FIG.  11   , the cam shaft  132  extends along the direction of the rotational axis of the cam  50  (arrow Z1 and Z2 directions), and is fixed to the cam  50  in a manner to be rotatable along with the cam  50 . One end portion of the cam shaft  132  in the extension direction (end portion on the arrow Z1 side) is rotatably fixed to the inner wall of the cam chamber  92 . The other end portion of the cam shaft  132  in the extension direction (end portion on the arrow Z2 side) is provided with a flange portion  140 . 
     As shown in  FIGS.  12 A and  12 B , the flange portion  140  is provided with a cutout portion  142 , which is obtained by cutting away a portion of the flange portion  140  in the circumferential direction thereof toward the center of the flange portion  140  in the radial direction. Furthermore, as shown in  FIG.  11   , an end of the flange portion  140  on the other end side in the direction of the rotational axis of the cam  50  (arrow Z2 side) is covered by a cover member  144 . In  FIGS.  12 A and  12 B , the cover member  144  and the cam biasing member  134  are omitted from the drawings. 
     As shown in  FIG.  11   , the cam biasing member  134  is formed from a torsion coil spring or the like, for example, and elastically biases the cam  50  in the rotational direction via the cam shaft  132 . The stopper  136  restricts the cam  50  that is elastically biased by the cam biasing member  134  via the cam shaft  132  from rotating beyond the reference position. In the present embodiment, the stopper  136  is formed from a pin that extends along the direction of the rotational axis of the cam  50 . 
     One end of the stopper  136  (end portion on the arrow Z1 side) is fixed to a wall portion forming the insertion hole  138  of the cam housing portion  40   b . The other end of the stopper  136  is disposed inside the cutout portion  142  of the flange portion  140 . As shown in  FIG.  12 A , by abutting against an inner wall  142   a  on one end side of the cutout portion  142  in the circumferential direction, the stopper  136  can restrict the cam  50  from rotating beyond the reference position in the direction of the elastic bias of the cam biasing member  134 . In the present embodiment, when the cam  50  is at the reference position, the stopper  136  and the inner wall  142   a  of the cutout portion  142  are arranged slightly separated from each other. 
     Furthermore, the length of the cutout portion  142  in the circumferential direction is set such that, even in a case where, as shown in  FIG.  12 B , the cam  50  is oriented such that the arc-shaped surface  50   c  thereof abuts against the base surface  70   d  of the cartridge  48  of  FIG.  3   , for example, the stopper  136  does not abut against an inner wall surface  142   b  on the other end side of the cutout portion  142  in the circumferential direction. Furthermore, the elastic bias force of the cam biasing member  134  is set to a magnitude that does not impede the back and forth movement of the rod  54  whose abutment portion  54   a  shown in  FIG.  2    abuts against the abutment position  51 . Therefore, even when the stopper  136  and the cam biasing member  134  are provided as described above, restriction of the rotation of the cam  50  for moving the cartridge  48  back and forth is avoided. 
     The restricting mechanism  130  such as described above can be favorably applied to a machining tool  10  in which, for example, the cam housing portion  40   b  forms one spindle of a multi-spindle machining head (gang head) having a plurality of machining spindles (none of which are shown in the drawings). With this type of machining tool  10 , when a cutting tool held by the cutting tool holding portion  40   a  is worn down or the like, the cutting tool holding portion  40   a  is removed from the cam housing portion  40   b  in a state where the rod  54  has moved backward, as shown in  FIG.  4   . 
     As a result, the cam  50  contacts neither the cartridge  48  nor the abutment portion  54   a  of the rod  54 , and is therefore released from a state of having its rotation restricted by the contact with the cartridge  48  or the abutment portion  54   a . In such a case, the cam  50  is kept in a state of being approximately at the reference position by the elastic bias force from the cam biasing member  134  via the cam shaft  132 , and the restriction applied from the stopper  136 . 
     Accordingly, when, after replacement or the like of the worn cutting tool has been performed, the cutting tool holding portion  40   a  is again attached to the cam housing portion  40   b  and the abutment portion  54   a  of the rod  54  is brought near the cam  50 , the abutment position  51  of the cam  50  is in a state of facing the abutment portion  54   a  of the rod  54 . As a result, it is possible to easily and efficiently avoid a situation where the abutment portion  54   a  of the rod  54  abuts against a location other than the abutment position  51  of the cam  50 . 
     In the embodiment described above, the plurality of cutting tools included in the machining tool  10  are three cutting tools, namely, the first cutting tool  42 , the second cutting tool  44 , and the third cutting tool  46 , but the number of cutting tools is not particularly limited to this. The number of cutting tools may be set according to the number of inclined surfaces to be formed on the inner periphery of the opening, and may be two or may be four or more. 
     LIST OF REFERENCE NUMERALS 
     
         
           10 : machining tool 
           40 : tool body 
           40   a : cutting tool holding portion 
           40   b : cam housing portion 
           48 : cartridge 
           50 : cam 
           50   a : cutout surface 
           50   b : recessed portion 
           50   c : arc-shaped surface 
           51 : abutment position 
           54 : rod 
           54   a : abutment portion 
           56 ,  130 : restricting mechanism 
           96 ,  132 : cam shaft 
           100 : engaging portion 
           102 : insertion hole 
           104 : insertion member 
           106 : concave portion 
           106   a : peripheral wall 
           108 : insertion groove 
           110 : axial hole 
           112 : circumferential groove 
           114 : keyhole portion 
           116 : shaft portion 
           120 : restricting portion 
           134 : cam biasing member 
           136 : stopper 
           138 : insertion hole 
           140 : flange portion 
           142 : cutout portion 
           142   a : inner wall