Patent Publication Number: US-2023134477-A1

Title: Core piece, stator core, stator, and rotary electric machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/760,775, filed on Mar. 16, 2022, which is based on PCT/JP2021/014025, filed on Mar. 31, 2021, which claims priority to Japanese Patent Application No. 2020-082831, filed on May 8, 2020, the entire contents of each are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a core piece, a stator core, a stator, and a rotary electric machine. 
     BACKGROUND ART 
     PTL 1 discloses a stator core for an axial gap type motor. The stator core includes a yoke part, teeth, and a flange part. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laying-Open No. 2009-44829 
       
    
     SUMMARY OF INVENTION 
     A core piece according to the present disclosure is a core piece that is circularly arranged to construct a stator core of an axial gap type rotary electric machine, and includes a first member in a column form extending in an axial direction of the stator core; a second member in a plate form disposed on a first end side of the axial direction in the first member; and a third member in a plate form disposed on a second end side of the axial direction in the first member, the first member has a peripheral surface connecting with the second member and the third member, the second member has a protruding portion projecting outwardly from the peripheral surface of the first member, the third member has a protruding portion projecting outwardly from the peripheral surface of the first member, and the first member, the second member, and the third member are configured by an integrally molded green compact. 
     A stator core according to the present disclosure is a stator core of an axial gap type rotary electric machine, and has a plurality of core pieces that are circularly arranged, and each of the plurality of core pieces is the core piece according to the present disclosure. 
     A stator according to the present disclosure is a stator of an axial gap type rotary electric machine, and includes the stator core according to the present disclosure, and a coil arranged on each of the first members in the stator core. 
     A rotary electric machine according to the present disclosure is an axial gap type rotary electric machine including a rotor and a stator, the rotor and the stator being arranged to face with each other in an axial direction, and the stator is the stator according to the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view showing an outline of a core piece according to a first embodiment. 
         FIG.  2    is a top view showing an outline of a core piece according to the first embodiment. 
         FIG.  3    is a view of the core piece according to the first embodiment seen from the side of an inner peripheral surface. 
         FIG.  4    is a IV-IV sectional view of the core piece shown in  FIG.  3   . 
         FIG.  5    is a V-V sectional view of the core piece shown in  FIG.  3   . 
         FIG.  6    is a VI-VI sectional view of the core piece shown in  FIG.  3   . 
         FIG.  7    is a top view showing an opening edge of a die of a mold for producing the core piece according to the first embodiment. 
         FIG.  8    is a sectional view showing an outline of a mold for producing a first member in the core piece according to the first embodiment. 
         FIG.  9    is a sectional view showing an outline of a mold for producing a second member in the core piece according to the first embodiment. 
         FIG.  10    is a sectional view showing an outline of a mold for producing a third member in the core piece according to the first embodiment. 
         FIG.  11    is a perspective view showing an outline of a stator core according to a second embodiment. 
         FIG.  12    is a perspective view showing an outline of a stator according to a third embodiment. 
         FIG.  13    is a sectional view showing an outline of a rotary electric machine according to a fourth embodiment. 
         FIG.  14    is a sectional view showing an outline of a rotary electric machine according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Problem to be Solved by the Present Disclosure 
     Improvement in productivity of a stator core is desired. It is expected that productivity of a stator core is improved by integrally molding a yoke part, teeth and a flange part. An optimum production method of integrally molding a yoke part, teeth and a flange part, however, has not been studied well. 
     One object of the present disclosure is to provide a core piece that is excellent in productivity. Another object of the present disclosure is to provide a stator core, a stator, and a rotary electric machine that are excellent in productivity. 
     Advantageous Effect of the Present Disclosure 
     A core piece, a stator core, stator, and a rotary electric machine according to the present disclosure are excellent in productivity. 
     DESCRIPTION OF EMBODIMENTS 
     Frist, description is made by listing aspects of the present disclosure. 
     (1) A core piece according to one aspect of the present disclosure is a core piece that is circularly arranged to construct a stator core of an axial gap type rotary electric machine, and includes a first member in a column form extending in an axial direction of the stator core; a second member in a plate form disposed on a first end side of the axial direction in the first member; and a third member in a plate form disposed on a second end side of the axial direction in the first member, the first member has a peripheral surface connecting with the second member and the third member, the second member has a protruding portion projecting outwardly from the peripheral surface of the first member, the third member has a protruding portion projecting outwardly from the peripheral surface of the first member, and the first member, the second member, and the third member are configured by an integrally molded green compact. 
     The core piece according to one aspect of the present disclosure is excellent in productivity. 
     A conventional core piece is configured by combining a green compact in which a first member and a second member are integrally molded, and a third member configured separately from the green compact. Alternatively, a conventional core piece is configured by combining a green compact in which ae first member and a third member are integrally molded, and a second member configured separately from the green compact. That is, a conventional core piece needs to be configured by manufacturing at least two members and combining the members. Therefore, a large number of steps and a long production time are required to produce a conventional core piece. Also, at least two molds are required to produce a conventional core piece. 
     In contrast, the core piece according to one aspect of the present disclosure need not combine a plurality of members since the core piece is configured by the green compact in which the first member, the second member, and the third member are integrally molded. Therefore, the core piece according to one aspect of the present disclosure can be produced with less steps and in a shorter time as compared with a conventional core piece. Also, the core piece according to one aspect of the present disclosure can be produced with one mold since the core piece is configured by the green compact in which the first member, the second member, and the third member are integrally molded. Therefore, it is possible to reduce the cost required for production and maintenance of the mold, and it is possible to produce the core piece according to one aspect of the present disclosure at a low cost. 
     A green compact of a conventional core piece is produced by pressure molding raw material powder filled in the mold hole of the die of the mold by means of the upper punch and the lower punch. The pressing direction is the direction along the axial direction of the stator core in the core piece, namely, the direction parallel with the first member and the second member. Of the green compact, the surface on the first end side and the surface on the second end side of the axial direction of the stator core are formed by the upper end face of the lower punch and the lower end face of the upper punch. Of the green compact, the surface on a first direction side and the surface on a second direction side of a circumferential direction of the stator core are formed by the inner peripheral surface of the lower punch. Of the green compact, the surface on an outer peripheral side and the surface on an inner peripheral side of the stator core are formed by the inner peripheral surface of the mold hole of the die. That is, the first end surface of the axial direction of the first member, and the first end surface of the axial direction of the second member are formed by the upper end face of the lower punch. The second end surface of the axial direction of the second member is formed by the lower end face of the upper punch. The surface on the first direction side and the surface on the second direction side of the circumferential direction of the first member are formed by the inner peripheral surface of the lower punch. The surface on the outer peripheral side and the surface on the inner peripheral side of the first member and the second member are formed by the inner peripheral surface of the mold hole of the die. The direction of removing the green compact from the mold is the same direction as the pressing direction, and is the direction along the axial direction of the stator core in the core piece, namely, the direction parallel with the first member and the second member. 
     The core piece according to one aspect of the present disclosure cannot be produced by the conventional production method along the same direction as the pressing direction and the removing direction of the core piece. This is because each of the second member and the third member has a protruding portion, and the protruding portions are caught by the inner peripheral surface of the mold hole of the die, and the core piece cannot come out of the mold. 
     Although the details are described later, the core piece according to one aspect of the present disclosure can be produced by setting the pressing direction and the removing direction along the radial direction of the stator core. Of the core piece, the surface on the outer peripheral side and the surface on the inner peripheral side of the stator core are formed by the lower end face of the upper punch and the upper end face of the lower punch. Of the core piece, the surface on the first direction side and the surface on the second direction side of the circumferential direction of the stator core, and the surface on the first end side and the surface on the second end side of the axial direction of the stator core are formed by the inner peripheral surface of the mold hole of the die. In this case, even though each of the second member and the third member has a protruding portion, the protruding portions are not caught by the inner peripheral surface of the mold hole of the die. Therefore, the core piece can be removed from the mold. 
     (2) In one form of the core piece, each of the first member, the second member, and the third member has: an outer peripheral surface located on an outer peripheral side of the stator core; an inner peripheral surface located on an inner peripheral side of the stator core; a first lateral surface located on a first direction side of the circumferential direction of the stator core, and connecting with the outer peripheral surface and the inner peripheral surface; and a second lateral surface located on a second direction side of the circumferential direction of the stator core, and connecting with the outer peripheral surface and the inner peripheral surface, and in each of the first member, the second member, and the third member, a length between the first lateral surface and the second lateral surface in the outer peripheral surface is longer than a length between the first lateral surface and the second lateral surface in the inner peripheral surface, and each of the first lateral surface and the second lateral surface in each of the first member, the second member, and the third member has: a first parallel face connecting with the outer peripheral surface; a second parallel face connecting with the inner peripheral surface; and a first inclined face connecting with the first parallel face and the second parallel face, and in each of the first member, the second member, and the third member, the first parallel face of the first lateral surface and the first parallel face of the second lateral surface are parallel with each other; the second parallel face of the first lateral surface and the second parallel face of the second lateral surface are parallel with each other; and the first parallel face of the first lateral surface and the second parallel face of the first lateral surface are parallel with each other. 
     The core piece has high relative density. The reason is as follows. The first parallel face and the second parallel face can be formed by a straight part along the pressing direction of the upper punch and the lower punch in the mold hole of the die of the mold as will be specifically described later. Therefore, pressure can be sufficiently applied to the raw material powder that forms the core piece. 
     The core piece is excellent in productivity. The reason is as follows. The first inclined face can be formed by a taper part crossing the pressing direction of the upper punch and the lower punch in the mold hole of the die of the mold as will be specifically described later. Since the mold hole of the die has the straight part, the upper punch and the lower punch are prevented from coming into contact with the inner peripheral surface of the taper part. Therefore, the service life of the mold is extended, and the number of core pieces that can be produced with one mold increases. 
     (3) In one form of the core piece of the above (2), in each of the first member, the second member, and the third member, an angle formed by an extended plane of the first parallel face and the first inclined face in the first lateral surface is greater than or equal to 5° and less than or equal to 20°, and an angle formed by an extended plane of the first parallel face and the first inclined face in the second lateral surface is greater than or equal to 5° and less than or equal to 20°. 
     The core piece facilitates winding of wire on the peripheral surface of the first member, and is easy to construct a stator since the angle formed in the first member satisfies the above range. The core piece is easy to arrange circularly, and easy to construct a stator core since the angle formed in the second member satisfies the above range. The core piece can suppress variation in density in the core piece since the angle formed in the third member satisfies the above range. 
     (4) In one form of the core piece of the above (2) or the above (3), each of the protruding portion of the second member and the protruding portion of the third member has: a first protruding part projecting on the first direction side of the circumferential direction; and a second protruding part projecting on the second direction side of the circumferential direction, a protruding amount of the first protruding part in the second member is larger than a protruding amount of the first protruding part in the third member, a protruding amount of the second protruding part in the second member is larger than a protruding amount of the second protruding part in the third member, the first inclined face of the first protruding part in the second member has a projecting part projecting outwardly from a first imaginary plane, the first inclined face of the second protruding part in the second member has a projecting part projecting outwardly from a second imaginary plane, the first imaginary plane is a plane connecting a connection part between the first parallel face and the first inclined face, and a connection part between the second parallel face and the inner peripheral surface in the first lateral surface of the first protruding part in the second member, and the second imaginary plane is a plane connecting a connection part between the first parallel face and the first inclined face, and a connection part between the second parallel face and the inner peripheral surface in the second lateral surface of the second protruding part in the second member. 
     The core piece is easy to construct a stator core having a large magnetic path area. The reason is as follows. 
     The stator core is configured by circularly arranging plural core pieces. As the stator core, the one configured by combining a first core piece and a second core piece adjoining in the circumferential direction such that they are in contact with each other is known. 
     For example, the core piece in which each of the first lateral surface of the first protruding part and the second lateral surface of the second protruding part has a first parallel face, a second parallel face, and a first inclined face, and the first inclined face does not have the projecting part is as follows. In circularly arranging the core pieces, if one attempts to bring the first lateral surface of the first protruding part in the second member of the first core piece, and the second lateral surface of the second protruding part in the second member of the second core piece into contact with each other, a first corner part of the first core piece and a second core part of the second core piece come into contact with each other. The first corner part is a corner part between the first lateral surface of the first protruding part and the inner peripheral surface in the second member. The second corner part is a corner part between the second lateral surface of the second protruding part and the inner peripheral surface in the second member. Therefore, it is impossible to sufficiently bring the first lateral surface of the first protruding part in the second member of the first core piece, and the second lateral surface of the second protruding part in the second member of the second core piece into contact with each other. 
     In contrast, in the core piece, each of the first lateral surface of the first protruding part and the second lateral surface of the second protruding part has a first parallel face, a second parallel face, and a first inclined face, and the first inclined face has a part projecting outwardly from each of the first imaginary plane and the second imaginary plane. In circularly arranging the core piece, even if the first lateral surface of the first protruding part in the second member of the first core piece, and the second lateral surface of the second protruding part in the second member of the second core piece are brought into contact with each other, it is possible to prevent the first corner part of the first core piece and the second core part of the second core piece from coming into contact with each other. Therefore, it is possible to sufficiently bring the first lateral surface of the first protruding part in the second member of the first core piece, and the second lateral surface of the second protruding part in the second member of the second core piece into contact with each other. 
     (5) In one form of the core piece of the above (4), the first lateral surface of the first protruding part in the second member has one selected from the group consisting of at least one of a recess or a protrusion, a step, and a second inclined face, and the second lateral surface of the second protruding part in the second member has one selected from the group consisting of at least one of a protrusion corresponding to the recess of the first lateral surface or a recess corresponding to the protrusion of the first lateral surface, a step corresponding to the step of the first lateral surface, and a second inclined face corresponding to the second inclined face of the first lateral surface. 
     The core piece is easy to construct a stator core having a large magnetic path area. The reason is as follows. The first core piece and the second core piece adjoining in the circumferential direction of the stator core can be fitted with each other by means of the steps or by means of the recess and the protrusion, or can be brought into contact with each other by means of the second inclined faces. Therefore, it is possible to sufficiently bring the first core piece and the second core piece into contact with each other, and thus it is possible to increase the contact area between the first core piece and the second core piece. 
     (6) In one form of the core piece of the above (4) or the above (5), the third member has a first end surface located on an opposite side of the side facing with the second member, and the first end surface is formed to have a convex shape toward the opposite side. 
     The core piece can construct a rotary electric machine generating little noises and vibrations. The reason is as follows. In a rotary electric machine, a stator and a rotor are arranged to face with each other. A stator is configured by arranging a coil on each first member of a stator core. The stator core is configured by circularly arranging plural core pieces. Since the first end surface of the core piece is formed to have a convex shape, abrupt change in the magnetic flux of the magnet of the rotor received by the core piece is easily suppressed. Since abrupt change in the magnetic flux is easily suppressed, cogging torque is easily reduced. Since the cogging torque is small, noises and vibrations are difficult to increase. 
     (7) In one form of any one of the core pieces of the above (2) to the above (6), the outer peripheral surface of each of the first member, the second member, and the third member has a curved surface that is convex toward the outer peripheral side, and the inner peripheral surface of each of the first member, the second member, and the third member has a curved surface that is convex toward the inner peripheral side. 
     The core piece can suppress variation in density in the core piece. 
     (8) In one form of any one of the core pieces of the above (2) to the above (7), a first joint between the protruding portion of the second member and the peripheral surface of the first member, and a second joint between the protruding portion of the third member and the peripheral surface of the first member are rounded. 
     The core piece is difficult to be damaged starting from the joints since the first joint and the second joint are rounded. 
     (9) In one form of the core piece of the above (8), the first joint and the second joint have bend radii of greater than or equal to 0.2 mm and less than or equal to 4.0 mm. 
     The core piece can reduce the load on the mold at the time of producing the core piece since the bend radii of the first joint and the second joint are greater than or equal to 0.2 mm. The core piece facilitates winding of the coil at the time of constructing the stator, and facilitates increase in the number of windings of the coil since the bend radii of the first joint and the second joint are less than or equal to 4.0 mm. 
     (10) In one form of any one of the core pieces of the above (2) to the above (9), each of the second member and the third member has a first end surface located on an opposite side of the side where the members face with each other, and in each of the second member and the third member, a corner part between the outer peripheral surface and the first end surface, and a corner part between the inner peripheral surface and the first end surface are chamfered. 
     The core piece is difficult to be damaged in the corner parts since the corner parts are chamfered. 
     (11) In one form of any one of the core pieces of the above (2) to the above (10), a total area of the outer peripheral surface in each of the first member, the second member, and the third member is greater than 1 time and less than or equal to 4 times a total area of the inner peripheral surface in each of the first member, the second member, and the third member. 
     The core piece is easy to arrange circularly and easy to construct a stator core since the total area of the outer peripheral surface is more than 1 time the total area of the inner peripheral surface. The core piece is easy to produce since the total area of the outer peripheral surface is less than or equal to 4 times the total area of the inner peripheral surface. Since the proportion of the total area of the inner peripheral surface is relatively large, the area pushing out the core piece by the lower punch is large at the time of removing the core piece from the mold. Therefore, it is easy to prevent a damage of the core piece at the time of removing the core piece from the mold. 
     (12) In one form of any one of the core pieces of the above (2) to the above (11), among sites obtained by dividing the core piece into three parts with an imaginary plane along the second parallel face of the first lateral surface, and with an imaginary plane along the second parallel face of the second lateral surface, difference in relative density between a first site on the first direction side of the circumferential direction and a second site on the second direction side of the circumferential direction, and a third site between the first site and the second site is less than or equal to 5.0%. 
     The core piece has substantially uniform physical characteristics such as magnetic characteristics inside the core piece since difference in relative density is small. 
     (13) In one form of the core piece, among the first member, the second member, and the third member, difference in relative density between a member having the largest relative density and a member having the smallest relative density is less than or equal to 5%. 
     The core piece has substantially uniform physical characteristics such as magnetic characteristics inside the core piece since difference in relative density is small. 
     (14) In one form of the core piece, the green compact has a relative density of greater than or equal to 85%. 
     The core piece can construct an axial gap type rotary electric machine that is excellent in magnetic characteristics such as saturation flux density since the core piece has a relative density of greater than or equal to 85% and thus has high density. In addition, the core piece is excellent in mechanical characteristics such as strength. 
     (15) In one form of the core piece, the green compact is configured by an aggregate of a plurality of coated soft magnetic particles having insulating coating on surfaces of soft magnetic particles, and the soft magnetic particles are iron-based particles formed of at least one metal selected from the group consisting of pure iron, an Fe—Si-based alloy, an Fe—Al-based alloy, and an Fe—Si—Al-based alloy. 
     The core piece has high density, and is excellent in accuracy of dimension. This is because soft magnetic particles are easy to deform at the time of molding the green compact since the above materials are relatively soft. 
     (16) A stator core according to one aspect of the present disclosure is a stator core of an axial gap type rotary electric machine, and has a plurality of core pieces that are circularly arranged, and each of the plurality of core pieces is any one of the core pieces of the above (1) to the above (15). 
     The stator core is excellent in productivity since each of the plurality of core pieces that are circularly arranged is the core piece that is excellent in productivity. 
     (17) In one form of the stator core, variation in length between a surface on the first end side and a surface on the second end side in the axial direction in each of the plurality of core pieces is less than or equal to 0.1 mm. 
     In the stator core, the variation in length is very small. Therefore, the stator core can construct a rotary electric machine generating little noises and vibrations. The reason is as follows. In a rotary electric machine, as described above, a stator that is configured by arranging a coil on the first member of each core piece of the stator core, and a rotor are arranged to face with each other. Since the variation in length in the stator core is small, the variation in the interval between the stator and the rotor is small. Since the variation in the interval is small, the torque ripple is small. Since the torque ripple is small, noises and vibrations are difficult to increase. 
     (18) A stator according to one aspect of the present disclosure is a stator of an axial gap type rotary electric machine, and includes the stator core according to the above (16) or the above (17), and a coil arranged on each of the first members in the stator core. 
     The stator is excellent in productivity since the stator includes a stator core that is excellent in productivity. 
     (19) A rotary electric machine according to one aspect of the present disclosure is an axial gap type rotary electric machine including a rotor and a stator, the rotor and the stator being arranged to face with each other in an axial direction, and the stator is the stator according to the above (18). 
     The rotary electric machine is excellent in productivity since the rotary electric machine includes the stator that is excellent in productivity. 
     Details of Embodiment of the Present Disclosure 
     The details of embodiments of the present disclosure are described below by referring to drawings. In drawings, the same sign indicates the object having the same name. 
     First Embodiment 
     [Core] 
     With reference to  FIG.  1    to  FIG.  6   , a core piece  1  according to a first embodiment is described. As will be described later by referring to  FIG.  11   , core pieces  1  of the present embodiment are circularly arranged to construct a stator core  7 . Although the details are described later, stator core  7  is used in an axial gap type rotary electric machine  9  as will be described later by referring to  FIGS.  13  and  14   . As shown in  FIG.  1   , core piece  1  of the present embodiment includes a first member  10  in a column form, a second member  20  in a plate form, and a third member  30  in a plate form. First member  10  extends in the axial direction of stator core  7 . Second member  20  is disposed on a first end side of the axial direction in first member  10 . Third member  30  is disposed on a second end side of the axial direction in first member  10 . One feature of core piece  1  of the present embodiment lies in that second member  20  and third member  30  respectively have specific protruding portions  21 ,  31 , and first member  10 , second member  20 , and third member  30  are configured by an integrally-molded green compact. The term “integrally-molded” means that first member  10 , second member  20 , and third member  30  are integrally formed by molding without mechanical connection using a screw or the like, or adhesion using an adhesive or the like. The details are described as follows. 
     The direction along the radial direction of stator core  7  in core piece  1  is defined as an X-axial direction. 
     The direction along the axial direction of stator core  7  in core piece  1  is defined as a Z-axial direction. 
     The direction perpendicular to both the X-axial direction and the Z-axial direction of core piece  1  is defined as a Y-axial direction. 
     Of the X-axial direction, an inner peripheral side of stator core  7  in core piece  1  is defined as an X1 direction, and an outer peripheral side of stator core  7  is defined as an X2 direction. 
     Of the Z-axial direction, the side of the second member  20  with respect to first member  10  in core piece  1  is defined as a Z1 direction, and the side of the third member  30  with respect to first member  10  is defined as a Z2 direction. 
     The Z1 direction is the first end side of first member  10 . 
     The Z2 direction is the second end side of first member  10 . 
     Of the Y-axial direction, the first direction side of stator core  7  in core piece  1  is defined as a Y1 direction, and the second direction side of stator core  7  is defined as a Y2 direction. 
     [First Member] 
     First member  10  is a columnar member extending in the Z-axial direction. First member  10  constitutes teeth both in the case that core pieces  1  construct stator core  7  of double stator single rotor configuration in axial gap type rotary electric machine  9  and in the case that core pieces  1  construct stator core  7  of single stator double rotor configuration in axial gap type rotary electric machine  9 . Axial gap type rotary electric machine  9  of double stator single rotor configuration is assembled such that one rotor  90  is sandwiched between two stators  8  as shown in  FIG.  13   . Axial gap type rotary electric machine  9  of single stator double rotor configuration is assembled such that one stator  8  is sandwiched between two rotors  90  as shown in  FIG.  14   . Hereinafter, for convenience of description, a double stator single rotor is sometimes referred to as DS/SR, and a single stator double rotor is sometimes referred to as SS/DR. 
     Examples of the form of first member  10  include a prism and a cylinder. Examples of the prism include a quadratic prism having a quadrangular section shape in the plane perpendicular to the Z-axial direction. Examples of the quadratic prisms include a trapezoidal prism having a trapezoidal section shape. The section may be uniform in the Z-axial direction. The term “trapezoidal” implicates not only geometrically trapezoidal shapes but also those substantially regarded as trapezoids as is the shape having a rounded corner part shown in the present example. The term “trapezoidal” implicates trapezoids having legs with different lengths, such as a right trapezoid, as well as trapezoids having legs of the same length, such as an isosceles trapezoid. This point also applies to later-described second member  20 , and third member  30 . 
     First member  10  in the present embodiment has a trapezoidal prism form in which the section shape is trapezoidal as shown in  FIGS.  1  and  4   . In the section shape, the length of the X2 direction side is long, and the length of the X1 direction side is short. The section shape of first member  10  is uniform in the Z-axial direction. When the first member  10  has a trapezoidal prism form, it is easy to ensure a large section area. It is also easy to reduce the dead space of core piece  1 , and it is easy to construct stator  8  having a high space factor. 
     As shown in  FIGS.  1  and  3   , first member  10  has a peripheral surface  11  connecting with second member  20  and third member  30 . Peripheral surface  11  of first member  10  shown in  FIGS.  1  and  3    has an outer peripheral surface  12 , an inner peripheral surface  13 , a first lateral surface  14   a , and a second lateral surface  14   b  as shown in  FIG.  4   . Outer peripheral surface  12  is located on the side of the X2 direction. Inner peripheral surface  13  is located on the side of the X1 direction. First lateral surface  14   a  and second lateral surface  14   b  are located on the sides leaving from each other in the circumferential direction of stator core  7  in core piece  1 . In other words, first lateral surface  14   a  is located on the first direction side of the circumferential direction of stator core  7  in core piece  1 . Second lateral surface  14   b  is located on the second direction side of the circumferential direction of stator core  7  in core piece  1 . The positional relationships among outer peripheral surface  12 , inner peripheral surface  13 , first lateral surface  14   a , and second lateral surface  14   b  also apply to later-described second member  20 , and third member  30 . 
     Outer peripheral surface  12  connects with the outer peripheral edge of first lateral surface  14   a  and the outer peripheral edge of second lateral surface  14   b . Inner peripheral surface  13  connects with the inner peripheral edge of first lateral surface  14   a  and the inner peripheral edge of second lateral surface  14   b . In other words, first lateral surface  14   a  and second lateral surface  14   b  connect with outer peripheral surface  12  and inner peripheral surface  13 . 
     The length between first lateral surface  14   a  and second lateral surface  14   b  in outer peripheral surface  12  is longer than the length between first lateral surface  14   a  and second lateral surface  14   b  in inner peripheral surface  13 . Outer peripheral surface  12  has a curved surface that is convex toward the X2 direction side in the present embodiment. Outer peripheral surface  12  may be configured by a flat surface. Inner peripheral surface  13  has a curved surface that is convex toward the X1 direction side in the present embodiment. Inner peripheral surface  13  may have a curved surface that is convex toward the X2 direction side, or may be configured by a flat surface. Bend radii of outer peripheral surface  12  and inner peripheral surface  13  may be the same or different from each other. 
     Each of first lateral surface  14   a  and second lateral surface  14   b  has a first parallel face  141 , a second parallel face  142 , and a first inclined face  143 . First parallel faces  141  of first lateral surface  14   a  and second lateral surface  14   b  are parallel with each other. Second parallel faces  142  of first lateral surface  14   a  and second lateral surface  14   b  are parallel with each other. First parallel face  141  of first lateral surface  14   a  and second parallel face  142  of first lateral surface  14   a  are parallel with each other. First parallel face  141  and second parallel face  142  are surfaces that are parallel with the X-axial direction in core piece  1 . The X-axial direction means the direction along the straight line that passes the center of stator core  7  and divides core piece  1  equally in the circumferential direction of stator core  7 . First parallel face  141  connects with outer peripheral surface  12 . Second parallel face  142  connects with inner peripheral surface  13 . First inclined face  143  connects with first parallel face  141  and second parallel face  142 . 
     It is preferred that the length along the X-axial direction of first parallel face  141  and second parallel face  142  is, for example, greater than or equal to 0.3 mm and less than or equal to 25 mm, although it depends on the size of core piece  1  or the like. When the length is greater than or equal to the above lower limit, it is possible to prevent a damage of a mold  5  accompanying the contact between a lower punch  55  and a die  50  as will be described later with reference to  FIG.  7   . Although a method for producing core piece  1  will be described later, it is possible to sufficiently apply a pressure on raw material powder forming core piece  1  when the length is greater than or equal to the above lower limit. When the length is less than or equal to the above upper limit, it is possible to increase the section area of first member  10 , and thus it is possible to improve the torque and prevent iron loss in axial gap type rotary electric machine  9 . The length along the X-axial direction of first parallel face  141  and second parallel face  142  is more preferably greater than or equal to 0.4 mm and less than or equal to 20 mm, and especially preferably greater than or equal to 0.5 mm and less than or equal to 15 mm. The aforementioned preferred ranges of the length along the X-axial direction of first parallel face  141  and second parallel face  142  in each of first lateral surface  14   a  and second lateral surface  14   b  of first member  10  also apply to a first parallel face  241  and a second parallel face  242  in each of a first lateral surface  24   a  and a second lateral surface  24   b  of second member  20 , and a first parallel face  341  and a second parallel face  342  in each of a first lateral surface  34   a  and a second lateral surface  34   b  of third member  30  as described later. 
     As shown in  FIG.  4   , it is preferred that a first inclination angle θ 11  and a second inclination angle θ 12  of first inclined face  143  are, for example, greater than or equal to 5° and less than or equal to 20°. When first inclination angle θ 11  and second inclination angle θ 12  are greater than or equal to 5° and less than or equal to 20°, it is easy to wind a later-described coil on peripheral surface  11  of first member  10 , and thus it is easy to construct stator core  7  shown in  FIG.  11   . First inclination angle θ 11  and second inclination angle θ 12  are further preferably greater than or equal to 5.5° and less than or equal to 18°, and especially preferably greater than or equal to 6° and less than or equal to 16°. First inclination angle θ 11  and second inclination angle θ 12  are preferably the same angle, but may be different from each other. First inclination angle θ 1  means the angle formed by an extended plane E 11  of first parallel face  141  and first inclined face  143  in first lateral surface  14   a . Second inclination angle θ 12  means the angle formed by an extended plane E 12  of first parallel face  141  and first inclined face  143  in second lateral surface  14   b.    
     [Second Member] 
     As shown in  FIGS.  1  and  3   , second member  20  is a plate-like member disposed on the first end side in the Z-axial direction of first member  10 . Second member  20  constitutes a yoke when core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration. Second member  20  constitutes a flange part when core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration. 
     Second member  20  in the present embodiment has a trapezoidal plate form. In the trapezoidal plate form, the section shape of the plane perpendicular to the Z-axial direction of second member  20  is trapezoidal. The section may be uniform in the Z-axial direction. The form of second member  20  may be a rectangular plate when core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration. 
     As shown in  FIG.  1    to  FIG.  3   , second member  20  has protruding portion  21 . Protruding portion  21  projects outwardly from peripheral surface  11  of first member  10 . Protruding portion  21  may protrude outwardly from peripheral surface  11  of first member  10  in part of peripheral surface  11  of first member  10 , or may protrude outwardly from peripheral surface  11  of first member  10  over the entire periphery in the circumferential direction of first member  10 . Protruding portion  21  has a first protruding part  211  and a second protruding part  212  in the present embodiment. First protruding part  211  projects on the second direction side of the circumferential direction of stator core  7 . Second protruding part  212  projects on the second direction side of the circumferential direction of stator core  7 . Protruding portion  21  may have a part projecting on the side of the X1 direction and a part projecting on the side of the X2 direction rather than having first protruding part  211  and second protruding part  212 . Protruding portion  21  may have a part projecting on the side of the X1 direction and a part projecting on the side of the X2 direction in addition to first protruding part  211  and second protruding part  212 . In this case, protruding portion  21  is circularly disposed. 
     The protruding length of first protruding part  211  and second protruding part  212  of second member  20  is longer than the protruding length of first protruding part  211  and second protruding part  212  of later-described third member  30  when core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration. The protruding length of first protruding part  211  and second protruding part  212  of second member  20  may be the same as the protruding length of first protruding part  211  and second protruding part  212  of third member  30  when core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration. The protruding length means the length along the direction perpendicular to peripheral surface  11  of first member  10 . When peripheral surface  11  has a curved surface, the protruding length means the length along the normal direction of the curved surface. 
     As shown in  FIGS.  3  and  5   , second member  20  has an outer peripheral surface  22 , an inner peripheral surface  23 , first lateral surface  24   a , second lateral surface  24   b , a first end surface  26 , and a second end surface  27 . The positional relationships among outer peripheral surface  22 , inner peripheral surface  23 , first lateral surface  24   a , and second lateral surface  24   b  are the same with to the positional relationships of respective surfaces in first member  10  as described above. First end surface  26  and second end surface  27  are located to face with each other. First end surface  26  is located on the side of the Z1 direction. First end surface  26  is located on the side of the Z1 direction of second member  20 . Second end surface  27  is located on the side of the Z2 direction. Second end surface  27  is located on the side of first member  10  in second member  20 . The positional relationship between first end surface  26  and second end surface  27  also applies to later-described third member  30 . 
     Outer peripheral surface  22  connects with the outer peripheral edge of first lateral surface  24   a , the outer peripheral edge of second lateral surface  24   b , the outer peripheral edge of first end surface  26 , and the outer peripheral edge of second end surface  27 . Outer peripheral surface  22  of second member  20  connects with outer peripheral surface  12  of first member  10 . Inner peripheral surface  23  connects with the inner peripheral edge of first lateral surface  24   a , the inner peripheral edge of second lateral surface  24   b , the inner peripheral edge of first end surface  26 , and the inner peripheral edge of second end surface  27 . Inner peripheral surface  23  of second member  20  connects with inner peripheral surface  13  of first member  10 . First lateral surface  24   a  and second lateral surface  24   b  connect with outer peripheral surface  22  and inner peripheral surface  23 . First end surface  26  connects with outer peripheral surface  22 , first lateral surface  24   a , second lateral surface  24   b , and inner peripheral surface  23 . Second end surface  27  connects with outer peripheral surface  22 , first lateral surface  24   a , second lateral surface  24   b , inner peripheral surface  23 , and peripheral surface  11  of first member  10 . 
     The length between first lateral surface  24   a  and second lateral surface  24   b  in outer peripheral surface  22  is longer than the length between first lateral surface  24   a  and second lateral surface  24   b  in inner peripheral surface  23 . The length between first lateral surface  24   a  and second lateral surface  24   b  in outer peripheral surface  22  of second member  20  is longer than the length between first lateral surface  14   a  and second lateral surface  14   b  in outer peripheral surface  11  of first member  10 . The length between first lateral surface  24   a  and second lateral surface  24   b  in inner peripheral surface  23  in second member  20  is the same as the length between first lateral surface  14   a  and second lateral surface  14   b  in inner peripheral surface  13  in first member  10 . 
     Outer peripheral surface  22  has a curved surface that is convex toward the X2 direction side in the present embodiment. Outer peripheral surface  22  may be configured by a flat surface. Inner peripheral surface  23  has a curved surface that is convex toward the X1 direction side in the present embodiment. Inner peripheral surface  23  may have a curved surface that is convex toward the X2 direction side, or may be configured by a flat surface. Bend radii of outer peripheral surface  22  and inner peripheral surface  23  may be the same or different from each other. 
     Each of first lateral surface  24   a  and second lateral surface  24   b  has first parallel face  241 , second parallel face  242 , and a first inclined face  243 . First parallel faces  241  of first lateral surface  24   a  and second lateral surface  24   b  are parallel with each other. Second parallel faces  242  of first lateral surface  24   a  and second lateral surface  24   b  are parallel with each other. First parallel face  241  of first lateral surface  24   a  and second parallel face  242  of first lateral surface  24   a  are parallel with each other. First parallel face  241  and second parallel face  242  are planes that are parallel with the X-axial direction of core piece  1 . First parallel face  241  connects with outer peripheral surface  22 . Second parallel face  242  connects with inner peripheral surface  23 . First inclined face  243  connects with first parallel face  241  and second parallel face  242 . 
     As shown in  FIG.  5   , it is preferred that a first inclination angle θ 21  and a second inclination angle θ 22  of first inclined face  243  are, for example, greater than or equal to 5° and less than or equal to 20°. When first inclination angle θ 21  and second inclination angle θ 22  are greater than or equal to 5° and less than or equal to 20°, it is easy to arrange core pieces  1  circularly, and it is easy to construct stator core  7 . First inclination angle θ 21  and second inclination angle θ 22  are further preferably greater than or equal to 5.5° and less than or equal to 18°, and especially preferably greater than or equal to 6° and less than or equal to 16°. First inclination angle θ 21  and second inclination angle θ 22  are preferably the same angle, but may be different from each other. First inclination angle θ 21  means the angle formed by an extended plane E 21  of first parallel face  241  and first inclined face  243  in first lateral surface  24   a . Second inclination angle θ 22  means the angle formed by an extended plane E 22  of first parallel face  241  and first inclined face  243  in second lateral surface  24   b.    
     When core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration, regarding first core piece  1  and second core piece  1  adjoining in the circumferential direction of stator core  7 , first lateral surface  24   a  in second member  20  of first core piece  1 , and second lateral surface  24   b  in second member  20  of second core piece  1  are in contact with each other. In this case, it is preferred that first inclined face  243  in first lateral surface  24   a  has a projecting part  244  projecting outwardly from a first imaginary plane V 21 . It is preferred that first inclined face  243  in second lateral surface  24   b  has projecting part  244  projecting outwardly from a second imaginary plane V 22 . 
     First imaginary plane V 21  is a plane that connects a first connection part and a second connection part in first lateral surface  24   a  of first protruding part  211 . The first connection part in first lateral surface  24   a  is a part where first parallel face  241  and first inclined face  243  of first lateral surface  24   a  are connected. The second connection part in first lateral surface  24   a  is a part where second parallel face  242  of first lateral surface  24   a  and inner peripheral surface  23  are connected. Second imaginary plane V 22  is a plane that connects a first connection part and a second connection part in second lateral surface  24   b  of second protruding part  212 . The first connection part in second lateral surface  24   b  is a part where first parallel face  241  and first inclined face  243  of second lateral surface  24   b  are connected. The second connection part in second lateral surface  24   b  is a part where second parallel face  242  of second lateral surface  24   b  and inner peripheral surface  23  are connected. First imaginary plane V 21  and second imaginary plane V 22  are indicated by two-dot chain lines extending diagonally with respect to the paper surface in  FIG.  5   . 
     Since first inclined face  243  in each of first lateral surface  24   a  and second lateral surface  24   b  has projecting part  244 , the magnetic path area of stator core  7  tends to increase. The reason is as follows. For example, a core piece in which each of first lateral surface  24   a  and second lateral surface  24   b  has first parallel face  241 , second parallel face  242 , and first inclined face  243 , and first inclined face  243  does not have projecting part  244  is as follows. In circularly arranging such core pieces, if one attempts to bring first lateral surface  24   a  of a first core piece and second lateral surface  24   b  of a second core piece adjoining in the circumferential direction of stator core  7  into contact with each other, a first corner part of the first core piece and a second core part of the second core piece come into contact with each other. The first corner part is a corner part between first lateral surface  24   a  and inner peripheral surface  23 . The second corner part is a corner part between second lateral surface  24   b  and inner peripheral surface  23 . Therefore, it is impossible to sufficiently bring first lateral surface  24   a  of the first core piece  1  and second lateral surface  24   b  of the second core piece into contact with each other. In other words, the contact area between first lateral surface  24   a  of the first core piece  1  and second lateral surface  24   b  of the second core piece decreases. 
     In contrast to this, in core piece  1 , first lateral surface  24   a  has first parallel face  241 , second parallel face  242 , and first inclined face  243 , and first inclined face  243  has projecting part  244  projecting outwardly from first imaginary plane V 21 . In addition, in core piece  1 , second lateral surface  24   b  has first parallel face  241 , second parallel face  242 , and first inclined face  243 , and first inclined face  243  has projecting part  244  projecting outwardly from second imaginary plane V 22 . In circularly arranging core pieces  1 , if one brings first lateral surface  24   a  of first core piece  1  and second lateral surface  24   b  of second core piece  1  into contact with each other, the first corner part of first core piece  1  and the second core part of second core piece  1  can be prevented from coming into contact with each other. Therefore, it is possible to sufficiently bring first lateral surface  24   a  of first core piece  1  and second lateral surface  24   b  of second core piece  1  into contact with each other. In other words, the contact area between first lateral surface  24   a  of first core piece  1  and second lateral surface  24   b  of second core piece  1  increases. 
     When core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration, regarding first core piece  1  and second core piece  1  adjoining in the circumferential direction of stator core  7 , first lateral surface  24   a  in second member  20  of first core piece  1 , and second lateral surface  24   b  in second member  20  of second core piece  1  are in contact with each other, as described above. In this case, it is preferred that first lateral surface  24   a  and second lateral surface  24   b  of core piece  1  respectively have steps  240  that can be fitted with each other as shown in  FIG.  3   . As result, magnetic path area of stator core  7  tends to increase. First core piece  1  and second core piece  1  adjoining in the circumferential direction of stator core  7  are fitted with each other by means of step  240  of first lateral surface  24   a  of first protruding part  211  in second member  20  of first core piece  1  and step  240  of second lateral surface  24   b  of second protruding part  212  in second member  20  of second core piece  1 . Therefore, it is possible to sufficiently bring first core piece  1  and second core piece  1  into contact with each other, and it is possible to increase the contact area between core pieces  1  adjoining in the circumferential direction of stator core  7 . Step  240  of first lateral surface  24   a  is disposed on the side of first end surface  26 . Step  240  of first lateral surface  24   a  is configured to leave first lateral surface  14   a  of first member  10  as step  240  approaches second end surface  27  from first end surface  26 . On the other hand, step  240  of second lateral surface  24   b  is disposed on the side of second end surface  27 . Step  240  of second lateral surface  24   b  is configured to leave second lateral surface  14   b  of first member  10  as step  240  approaches first end surface  26  from second end surface  27 . 
     Although omitted in drawings, first lateral surface  24   a  of core piece  1  may have at least one of a recess or a protrusion rather than a step. Second lateral surface  24   b  may have at least one of a protrusion corresponding to the recess of first lateral surface  24   a  or a recess corresponding to the protrusion of first lateral surface  24   a . That is, each of first lateral surface  24   a  and second lateral surface  24   b  may have a recess and a protrusion. Either one lateral surface of first lateral surface  24   a  or second lateral surface  24   b  may have only a recess, and the other lateral surface may have only a protrusion. The number and the shape of recesses and protrusions are not particularly limited. 
     Although omitted in drawings, first lateral surface  24   a  and second lateral surface  24   b  of core piece  1  may respectively have second inclined faces that are in contact with each other, rather than steps or recesses and protrusions. For example, the second inclined face of first lateral surface  24   a  may be inclined outwardly as it approaches second end surface  27  from first end surface  26 . And, the second inclined face of second lateral surface  24   b  may be inclined outwardly as it approaches first end surface  26  from second end surface  27 . 
     When core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration, core pieces  1  are circularly arranged such that they are out of contact with each other. In this case, each of first lateral surface  24   a  and second lateral surface  24   b  need not have all of steps  240 , recesses, protrusions, and second inclined faces that can be fitted with each other. 
     The corner part between first lateral surface  24   a  and first end surface  26 , and the corner part between first lateral surface  24   a  and second end surface  27  are rounded. The corner part between second lateral surface  24   b  and first end surface  26 , and the corner part between second lateral surface  24   b  and second end surface  27  are rounded. 
     When core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration, first end surface  26  may be configured by a flat surface. When core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration, first end surface  26  may be configured by a flat surface, or may be configured in a convex shape toward the Z1 direction side. Such core pieces  1  can construct axial gap type rotary electric machine  9  generating little noises and vibrations. The reason is as follows. In axial gap type rotary electric machine  9  of SS/DR configuration, as shown in  FIG.  14   , stator  8  and rotor  90  are arranged to face with each other. As shown in  FIG.  12   , stator  8  includes stator core  7  and a coil  80 . As shown in  FIGS.  11  and  12   , stator core  7  is configured by circularly arranging plural core pieces  1 . As shown in  FIG.  12   , coil  80  is arranged on first member  10  of each core piece  1 . When first end surface  26  of second member  20  of core piece  1  is formed to have a convex shape, abrupt change in the magnetic flux of a magnet  95  of rotor  90  received by core piece  1  is easily suppressed in axial gap type rotary electric machine  9  shown in  FIG.  14   . Therefore, cogging torque is easily reduced. Since the cogging torque is small, noises and vibrations are difficult to increase. 
     It is preferred that a corner part  28  between first end surface  26  and inner peripheral surface  23 , and the corner part between first end surface  26  and outer peripheral surface  22  are chamfered. These corner parts are difficult to be damaged since they are chamfered. The chamfering may be C chamfering or R chamfering. 
     [Third Member] 
     As shown in  FIGS.  1  and  3   , third member  30  is a plate-like member disposed on the second end side in the Z-axial direction of first member  10 . Third member  30  constitutes a flange part both in the case that core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration, and in the case that core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration. 
     Third member  30  in the present embodiment has a trapezoidal plate form. In the trapezoidal plate form, the section shape of the plane perpendicular to the Z-axial direction of third member  30  is trapezoidal. The section may be uniform in the Z-axial direction. The form of third member  30  may be a rectangular plate. For example, in core piece  1 , first member  10  may be a trapezoidal prism, and at least one of second member  20  or third member  30  may be a rectangular plate. 
     As shown in  FIG.  1    to  FIG.  3   , third member  30  has protruding portion  31 . Protruding portion  31  projects outwardly from peripheral surface  11  of first member  10 . Protruding portion  31  may protrude outwardly from peripheral surface  11  of first member  10  in part of peripheral surface  11  of first member  10 , or may protrude outwardly from peripheral surface  11  of first member  10  over the entire periphery in the circumferential direction of first member  10 . Protruding portion  31  has a first protruding part  311  and a second protruding part  312  in the present embodiment. First protruding part  311  projects on the first direction side of the circumferential direction of stator core  7 . Second protruding part  312  projects on the second direction side of the circumferential direction of stator core  7 . Protruding portion  31  may have at least one of a part projecting on the side of the X1 direction or a part projecting on the side of the X2 direction rather than having first protruding part  311  and second protruding part  312 . Protruding portion  31  may have a part projecting on the side of the X1 direction and a part projecting on the side of the X2 direction in addition to first protruding part  311  and second protruding part  312 . In this case, protruding portion  31  is circularly disposed. 
     The protruding length of first protruding part  311  and second protruding part  312  of third member  30  is shorter than the protruding length of first protruding part  311  and second protruding part  312  of second member  20  when core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration, as described above. The protruding length of first protruding part  311  and second protruding part  312  of third member  30  may be the same as the protruding length of first protruding part  211  and second protruding part  212  of second member  20  when core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration, as described above. 
     As shown in  FIG.  6   , third member  30  has an outer peripheral surface  32 , an inner peripheral surface  33 , first lateral surface  34   a , a second lateral surface  34   b , a first end surface  36 , and a second end surface  37 . The positional relationships among outer peripheral surface  32 , inner peripheral surface  33 , first lateral surface  34   a , and second lateral surface  34   b  are the same with the positional relationships of respective surfaces in first member  10  as described above. The positional relationship between first end surface  36  and second end surface  37  is the same with the positional relationship of respective surfaces in second member  20  as described above. 
     Outer peripheral surface  32  connects with the outer peripheral edge of first lateral surface  34   a , the outer peripheral edge of second lateral surface  34   b , the outer peripheral edge of first end surface  36 , and the outer peripheral edge of second end surface  37 . Outer peripheral surface  32  of third member  30  connects with outer peripheral surface  12  of first member  10 . Inner peripheral surface  33  connects with the inner peripheral edge of first lateral surface  34   a , the inner peripheral edge of second lateral surface  34   b , the inner peripheral edge of first end surface  36 , and the inner peripheral edge side of second end surface  37 . Inner peripheral surface  33  of third member  30  connects with inner peripheral surface  13  of first member  10 . First lateral surface  34   a  and second lateral surface  34   b  connect with outer peripheral surface  32  and inner peripheral surface  33 . First end surface  36  connects with outer peripheral surface  32 , first lateral surface  34   a , second lateral surface  34   b , and inner peripheral surface  33 . Second end surface  37  connects with outer peripheral surface  32 , first lateral surface  34   a , second lateral surface  34   b , inner peripheral surface  33 , and peripheral surface  11  of first member  10 . 
     The length between first lateral surface  34   a  and second lateral surface  34   b  in outer peripheral surface  32  is longer than the length between first lateral surface  34   a  and second lateral surface  34   b  in inner peripheral surface  33 . The length between first lateral surface  34   a  and second lateral surface  34   b  in outer peripheral surface  32  of third member  30  is longer than the length between first lateral surface  14   a  and second lateral surface  14   b  in outer peripheral surface  12  of first member  10 . The length between first lateral surface  34   a  and second lateral surface  34   b  in outer peripheral surface  32  of third member  30  is shorter than the length between first lateral surface  24   a  and second lateral surface  24   b  in outer peripheral surface  22  of second member  20 . The length between first lateral surface  34   a  and second lateral surface  34   b  in inner peripheral surface  33  of third member  30  is the same as the length between first lateral surface  14   a  and second lateral surface  14   b  in inner peripheral surface  13  of first member  10 . In other words, the length between first lateral surface  14   a  and second lateral surface  14   b  in inner peripheral surface  13  of first member  10 , the length between first lateral surface  24   a  and second lateral surface  24   b  in inner peripheral surface  23  of second member  20 , and the length between first lateral surface  34   a  and second lateral surface  34   b  in inner peripheral surface  33  of third member  30  are mutually the same. 
     Outer peripheral surface  32  has a curved surface that is convex toward the X2 direction side in the present embodiment. Outer peripheral surface  32  may be configured by a flat surface. Inner peripheral surface  33  has a curved surface that is convex toward the X1 direction side in the present embodiment. Inner peripheral surface  33  may have a curved surface that is convex toward the X2 direction side, or may be configured by a flat surface. Bend radii of outer peripheral surface  32  and inner peripheral surface  33  may be the same or different from each other. 
     Bend radii of at least two outer peripheral surfaces among outer peripheral surface  12 , outer peripheral surface  22 , and outer peripheral surface  32  may be the same. Of course, all bend radii of outer peripheral surface  12 , outer peripheral surface  22 , and outer peripheral surface  32  may be the same. All bend radii of outer peripheral surface  12 , outer peripheral surface  22 , and outer peripheral surface  32  may be different from one another. Bend radii of at least two inner peripheral surfaces among inner peripheral surface  13 , inner peripheral surface  23 , and inner peripheral surface  33  may be the same. Of course, all bend radii of inner peripheral surface  13 , inner peripheral surface  23 , and inner peripheral surface  33  may be the same. All bend radii of inner peripheral surface  13 , inner peripheral surface  23 , and inner peripheral surface  33  may be different from one another. 
     Each of first lateral surface  34   a  and second lateral surface  34   b  has first parallel face  341 , second parallel face  342 , and a first inclined face  343 . First parallel faces  341  of first lateral surface  34   a  and second lateral surface  34   b  are parallel with each other. Second parallel faces  342  of first lateral surface  34   a  and second lateral surface  34   b  are parallel with each other. First parallel face  341  of first lateral surface  34   a  and second parallel face  342  of first lateral surface  34   a  are parallel with each other. First parallel face  341  and second parallel face  342  are planes that are parallel with the X-axial direction of core piece  1 . First parallel face  341  connects with outer peripheral surface  32 . Second parallel face  342  connects with inner peripheral surface  33 . First inclined face  343  connects with first parallel face  341  and second parallel face  342 . 
     As shown in  FIG.  6   , it is preferred that a first inclination angle θ 31  and a second inclination angle θ 32  of first inclined face  343  are, for example, greater than or equal to 5° and less than or equal to 20°. When first inclination angle θ 31  and second inclination angle θ 32  are greater than or equal to 5° and less than or equal to 20°, variation in density of core pieces  1  can be reduced. First inclination angle θ 31  and second inclination angle θ 32  are further preferably greater than or equal to 5.5° and less than or equal to 18°, and especially preferably greater than or equal to 6° and less than or equal to 16°. First inclination angle θ 31  and second inclination angle θ 32  are preferably the same angle, but may be different from each other. First inclination angle θ 31  means the angle formed by an extended plane E 31  of first parallel face  341  and first inclined face  343  in first lateral surface  34   a . Second inclination angle θ 32  means the angle formed by an extended plane E 32  of first parallel face  341  and first inclined face  343  in second lateral surface  34   b.    
     Among first inclination angle θ 11 , first inclination angle θ 21 , and first inclination angle θ 31 , at least two inclination angles may be the same. Among second inclination angle θ 12 , second inclination angle θ 22 , and second inclination angle θ 32 , at least two inclination angles may be the same. Of course, all of first inclination angle θ 11 , first inclination angle θ 21 , and first inclination angle θ 31  may be the same. All of second inclination angle θ 12 , second inclination angle θ 22 , second inclination angle θ 32  may be the same. All of first inclination angle θ 11 , first inclination angle  021 , and first inclination angle θ 31  may be different from one another. All of second inclination angle θ 12 , second inclination angle θ 22 , and second inclination angle θ 32  may be different from one another. 
     The corner part between first lateral surface  34   a  and first end surface  36 , and the corner part between first lateral surface  34   a  and second end surface  37  are rounded. The corner part between second lateral surface  34   b  and first end surface  36 , and the corner part between second lateral surface  34   b  and second end surface  37  are rounded. 
     First end surface  36  may be configured by a flat surface as indicated by a solid line in  FIG.  3   , or may be formed to have a convex shape toward the Z2 direction side as indicated by a two-dot chain line in  FIG.  3    both in the case that core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of DS/SR configuration, and in the case that core pieces  1  construct stator core  7  provided in axial gap type rotary electric machine  9  of SS/DR configuration. When first end surface  36  is formed to have a convex shape, it is possible to construct axial gap type rotary electric machine  9  generating little noises and vibrations. The reason is as follows. As shown in  FIG.  13  or  14   , in axial gap type rotary electric machine  9 , stator  8  and rotor  90  are arranged to face with each other. As shown in  FIG.  12   , stator  8  includes stator core  7  and a coil  80 . As shown in  FIGS.  11  and  12   , stator core  7  is configured by circularly arranging plural core pieces  1 . As shown in  FIG.  12   , coil  80  is arranged on first member  10  of each core piece  1 . By forming first end surface  36  of third member  30  of core piece  1  to have a convex shape as indicated by a two-dot line in  FIG.  3   , abrupt change in the magnetic flux of magnet  95  of rotor  90  received by core piece  1  is easily suppressed in axial gap type rotary electric machine  9  shown in  FIGS.  13  and  14   . Therefore, cogging torque is easily reduced. Since the cogging torque is small, noises and vibrations are difficult to increase. 
     It is preferred that a corner part  38  between first end surface  36  and inner peripheral surface  33 , and the corner part between first end surface  36  and outer peripheral surface  32  are chamfered. These corner parts are difficult to be damaged since they are chamfered. The chamfering may be C chamfering or R chamfering. 
     A first joint between protruding portion  21  of second member  20  and peripheral surface  11  of first member  10 , and a second joint between protruding portion  31  of third member  30  and peripheral surface  11  of first member  10  are rounded as shown in  FIG.  3   . In the present embodiment, the first joint has a joint between first protruding part  211  of second member  20  and peripheral surface  11  of first member  10 , and a joint between second protruding part  212  of second member  20  and peripheral surface  11  of first member  10 . These joints are rounded. The second joint has a joint between first protruding part  311  of third member  30  and peripheral surface  11  of first member  10 , and a joint between second protruding part  312  of third member  30  and peripheral surface  11  of first member  10 . These joints are rounded. Since each joint has a rounded shape, core piece  1  is difficult to be damaged starting from such a joint. 
     It is preferred that the bend radius of the first joint and the bend radius of the second joint are greater than or equal to 0.2 mm and less than or equal to 4.0 mm. Since the bend radii of the first joint and the second joint are greater than or equal to 0.2 mm, the load on the mold at the time of producing core piece  1  is small. Since the bend radii of the first joint and the second joint are less than or equal to 4.0 mm, it is easy to wind coil  80  at the time of constructing stator  8  described later by referring to  FIG.  12   , and thus it is easy to increase the number of windings of the coil  80 . The bend radius of the first joint and the bend radius of the second joint are more preferably greater than or equal to 0.3 mm and less than or equal to 3.0 mm, and especially preferably greater than or equal to 0.5 mm and less than or equal to 2.0 mm. The bend radius of the first joint and the bend radius of the second joint may be the same or different from each other. 
     [Area Ratio] 
     The total area of outer peripheral surface  12 ,  22 ,  32  in each of first member  10 , second member  20 , and third member  30  is preferably more than 1 time and less than or equal to 4 times the total area of inner peripheral surface  13 ,  23 ,  33  in each of first member  10 , second member  20 , and third member  30 . Core piece  1  in which the total area of outer peripheral surface  12 ,  22 ,  32  is more than 1 time the total area of inner peripheral surface  13 ,  23 ,  33  is easy to arrange circularly, and easy to construct stator core  7 . Core piece  1  in which the total area of outer peripheral surface  12 ,  22 ,  32  is less than or equal to 4 times the total area of inner peripheral surface  13 ,  23 ,  33  is easy to produce. Since the proportion of the total area of inner peripheral surface  13 ,  23 ,  33  is relatively large, the area pushing out by lower punch  55  is large at the time of removing core piece  1  from mold  5 . Therefore, it is easy to prevent a damage of core piece  1  at the time of removing core piece  1  from mold  5 . The total area of outer peripheral surface  12 ,  22 ,  32  is further preferably greater than or equal to 1.2 times and less than or equal to 3.8 times, especially preferably greater than or equal to 1.5 times and less than or equal to 3.5 times the total area of inner peripheral surface  13 ,  23 ,  33 . 
     [Materials] 
     The green compact has a plurality of magnetic particles. The green compact is configured by an aggregate of soft magnetic particles. The green compact is produced by compression molding soft magnetic powder having a plurality of soft magnetic particles. Soft magnetic particles have a plurality of ironic particles composed of pure iron or an iron-based alloy. Pure iron refers to those having a purity of Fe (iron) of greater than or equal to 99% by mass. An iron-based alloy contains at least one element of Si (silicon) or Al (aluminum), and the remainder is composed of Fe and inevitable impurities. Examples of an iron-based alloy include at least one selected from the group consisting of an Fe—Si alloy, an Fe—Al alloy, and an Fe—Si—Al alloy. Examples of the Fe—Si alloy include silicon steel. Examples of the Fe—Si—Al alloy include sendust. Since the above materials are relatively soft, soft magnetic particles are easy to deform at the time of molding the green compact. Therefore, core piece  1  has high density and is excellent in accuracy of dimension. It is preferred that the green compact is configured by an aggregate of a plurality of coated soft magnetic particles having an insulating coating on the surface of soft magnetic particles. That is, it is preferred that the green compact is produced by compression molding coated soft magnetic powder having a plurality of coated soft magnetic particles. Formation of the insulating coating makes it easy to ensure the electric insulation between particles by means of the insulating coating. Therefore, it is possible to reduce the core loss of the green compact caused by the eddy current loss. The soft magnetic particles are as described above. Examples of the insulating coating include phosphate coating and silica coating. 
     [Relative Density] 
     The relative density of the green compact is preferably greater than or equal to 85%. The green compact having a relative density of greater than or equal to 85% is excellent in magnetic characteristics such as saturation magnetic flux density and in mechanical characteristics such as strength. The relative density of the green compact is further preferably greater than or equal to 90%, and especially preferably greater than or equal to 93%. The relative density of the green compact may be less than 100%. “Relative density” refers to the percentage (%) of the green compact relative to the true density of the soft magnetic particles constituting the green compact. 
     [Difference in Relative Density] 
     Among a first site, a second site, and a third site in core piece  1 , difference in relative density between the first site and the second site, and the third site is preferably less than or equal to 5.0%. Since core piece  1  has small difference in relative density, physical characteristics such as magnetic characteristics inside core piece  1  are substantially uniform. It is preferred that the difference in relative density between the first site and the second site, and the third site is as small as possible. The difference in relative density between the first site and the second site, and the third site is further preferably less than or equal to 4.0%, and especially preferably less than or equal to 3.0%. Here, as shown in  FIG.  2   , among the sites obtained by dividing core piece  1  into three parts with an imaginary plane Va along the second parallel face of first lateral surface and with an imaginary plane Vb along the second parallel face of the second lateral surface, the site of the first direction side of the circumferential direction is defined as the first site, the site of the second direction side of the circumferential direction is defined as the second site, and the site between the first site and the second site is defined as the third site. 
     It is preferred that the difference in relative density between the member having the largest relative density and the member having the smallest relative density among first member  10 , second member  20 , and third member  30  is less than or equal to 5.0%. Since the difference in relative density is small in core piece  1 , physical characteristics such as magnetic characteristics inside core piece  1  are substantially uniform. It is preferred that the difference in relative density between the member having the largest relative density and the member having the smallest relative density is as small as possible. The difference in relative density between the member having the largest relative density and the member having the smallest relative density is more preferably less than or equal to 4.0%, and especially preferably less than or equal to 3.0%. 
     It is preferred that the difference in relative density between the first site and the second site, and the third site is less than or equal to 5.0% and the difference in relative density between the member having the largest relative density and the member having the smallest relative density is less than or equal to 5.0%. 
     [Production Method] 
     Core piece  1  according to the first embodiment can be produced by a method for producing a core piece including a filling step and a molding step. In the filling step, the cavity of the mold  5  is filled with raw material powder. In the molding step, the raw material powder in the cavity is compression molded. First, by referring to  FIG.  7    to  FIG.  10   , mold  5  is described and then each step is described. 
     [Mold] 
     Mold  5  includes die  50 , an upper punch  54  and lower punch  55 . The cavity to be filled with raw material powder is formed by die  50  and lower punch  55 . 
     (Die) 
     Die  50  has a mold hole  50   h . Mold hole  50   h  is arranged such that upper punch  54  and lower punch  55  face with each other. The inner peripheral shape of mold hole  50   h  corresponds to the shape of core piece  1 . Upper punch  54  can be driven independently in the vertical direction relative to die  50 . Lower punch  55  can be driven independently in the vertical direction relative to die  50 . 
     Mold hole  50   h  has a first hole portion  51  shown in  FIGS.  7  and  8   , a second hole portion  52  shown in  FIGS.  7  and  9   , and a third hole portion  53  shown in  FIGS.  7  and  10   .  FIG.  7    shows an opening edge on the side of upper punch  54  in mold hole  50   h  of die  50 . In  FIG.  7   , die  50  is hatched for convenience of description.  FIG.  8    to  FIG.  10    are sectional views showing the state that the raw material powder filled in the cavity are pressure molded by means of upper punch  54  and lower punch  55 . The cutting position of the sectional view of  FIG.  8    corresponds to the position indicated by the VIII-VIII cutting line of  FIG.  7   . The cutting position of the sectional view of  FIG.  9    corresponds to the position indicated by the IX-IX cutting line of  FIG.  7   . The cutting position of the sectional view of  FIG.  10    corresponds to the position indicated by the X-X cutting line of  FIG.  7   . 
     First hole portion  51  has an inner peripheral surface that forms first lateral surface  14   a  and second lateral surface  14   b  of first member  10 . Second hole portion  52  has an inner peripheral surface that forms first lateral surface  24   a , second lateral surface  24   b , first end surface  26 , and second end surface  27  of second member  20 . Third hole portion  53  has an inner peripheral surface that forms first lateral surface  34   a , second lateral surface  34   b , first end surface  36 , and second end surface  37  of third member  30 . First hole portion  51 , second hole portion  52 , and third hole portion  53  are formed in a series in the direction orthogonal to the direction in which upper punch  54  and lower punch  55  face with each other. To be more specific, second hole portion  52  communicates on the first end side in the orthogonal direction of first hole portion  51 . Also, third hole portion  53  communicates on the second end side in the orthogonal direction of first hole portion  51 . 
     First hole portion  51  includes a first straight part  511 , a second straight part  512 , and a taper part  513 . First straight part  511 , taper part  513 , and second straight part  512  are formed sequentially in a series from the side of upper punch  54  toward the side of lower punch  55 . Similarly, second hole portion  52  includes a first straight part  521 , a second straight part  522 , and a taper part  523 . First straight part  521 , taper part  523 , and second straight part  522  are formed sequentially in a series from the side of upper punch  54  toward the side of lower punch  55 . Similarly, third hole portion  53  includes a first straight part  531 , a second straight part  532 , and a taper part  533 . First straight part  531 , taper part  533 , and second straight part  532  are formed sequentially in a series from the side of upper punch  54  toward the side of lower punch  55 . First straight part  511 ,  521 ,  531  forms a part on the side of outer peripheral surface of core piece  1 . Second straight part  512 ,  522 ,  532  forms a part on the inner peripheral surface side of core piece  1 . Taper part  513 ,  523 ,  533  forms a part between the outer peripheral surface side and the inner peripheral surface side of core piece  1 . 
     (Upper Punch) 
     Upper punch  54  has a first upper punch part  541  shown in  FIG.  8   , a second upper punch part  542  shown in  FIG.  9   , and a third upper punch part  543  shown in  FIG.  10   . First upper punch part  541  has a first lower end face  541   e . First lower end face  541   e  forms outer peripheral surface  12  of first member  10 . Second upper punch part  542  has a second lower end face  542   e . Second lower end face  542   e  forms outer peripheral surface  22  of second member  20 . Third upper punch part  543  has a third lower end face  543   e . Third lower end face  543   e  forms outer peripheral surface  32  of third member  30 . First upper punch part  541 , second upper punch part  542 , and third upper punch part  543  may be formed in a series, or may be formed independently from each other such that they can ascend and descend independently. When first upper punch part  541 , second upper punch part  542 , and third upper punch part  543  are formed in a series, first lower end face  541   e , second lower end face  542   e , and third lower end face  543   e  are formed in a series. The shape of first lower end face  541   e  corresponds to the shape of outer peripheral surface  12  of first member  10 . The shape of second lower end face  542   e  corresponds to the shape of outer peripheral surface  22  of second member  20 . The shape of third lower end face  543   e  corresponds to the shape of outer peripheral surface  32  of third member  30 . 
     (Lower Punch) 
     Lower punch  55  has a first lower punch part  551  shown in  FIG.  8   , a second lower punch part  552  shown in  FIG.  9   , and a third lower punch part  553  shown in FIG.  10 . First lower punch part  551  has a first upper end face  551   e . First upper end face  551   e  forms inner peripheral surface  13  of first member  10 . Second lower punch part  552  has a second upper end face  552   e . Second upper end face  552   e  forms inner peripheral surface  23  of second member  20 . Third lower punch part  553  has a third upper end face  553   e . Third upper end face  553   e  forms inner peripheral surface  33  of third member  30 . First lower punch part  551 , second lower punch part  552 , and third lower punch part  553  may be formed in a series, or may be formed independently from each other such that they can ascend and descend independently. When first lower punch part  551 , second lower punch part  552 , and third lower punch part  553  are formed in a series, first upper end face  551   e , second upper end face  552   e , and third upper end face  553   e  are formed in a series. The shape of first upper end face  551   e  corresponds to the shape of inner peripheral surface  13  of first member  10 . The shape of second upper end face  552   e  corresponds to the shape of inner peripheral surface  23  of second member  20 . The shape of third upper end face  553   e  corresponds to the shape of inner peripheral surface  33  of third member  30 . 
     [Filling Step] 
     The cavity formed by die  50  and lower punch  55  is filled with raw material powder. As the raw material powder, soft magnetic powder or coated soft magnetic powder as described above can be used. The raw material powder may contain a binder and a lubricant in addition to the soft magnetic powder or the coated soft magnetic powder. A lubricant may be applied on the inner peripheral surface of mold hole  50   h  of die  50 . 
     [Molding Step] 
     The raw material powder in the cavity is compression molded by means of upper punch  54  and lower punch  55 . The direction of compressing the raw material powder is the direction along the radial direction of stator core  7 . The higher the pressure of compression molding, the higher the relative density of produced core piece  1 . The pressure may be, for example, greater than or equal to 700 MPa, and further may be greater than or equal to 980 MPa. 
     [Other Steps] 
     After the molding step, a heat treatment may be conducted as necessary. For example, by removing strains by a heat treatment, it is possible to produce core piece  1  of low loss. For example, by a heat treatment, a binder or a lubricant may be removed. When the raw material powder contains the above-described coated soft magnetic particles, it is preferred that the heat treatment temperature is less than or equal to the decomposition temperature of the insulating coating. 
     [Operation and Effect] 
     Core piece  1  of the present embodiment is excellent in productivity since it is configured by a green compact in which first member  10 , second member  20 , and third member  30  are integrally molded. 
     Second Embodiment 
     [Stator Core] 
     With reference to  FIG.  11   , stator core  7  according to a second embodiment is described. Stator core  7  of the present embodiment has plural core pieces  1  that are circularly arranged. Each of plural core pieces  1  is core piece  1  according to the first embodiment. Plural core pieces  1  are circularly combined such that, regarding core pieces  1  adjoining in the circumferential direction, step  240  of first lateral surface  24   a  in second member  20  of first core piece  1  and step  240  of second lateral surface  24   b  in second member  20  of second core piece  1  fit with each other. Stator core  7  is used in axial gap type rotary electric machine  9  of DS/SR configuration shown in  FIG.  13   . 
     It is preferred that variation in length between the surface on the first end side and the surface on the second end side in the Z-axial direction in each of plural core pieces  1  is less than or equal to 0.1 mm. The length between the surface on the first end side and the surface on the second end side in the Z-axial direction means the maximum length of lengths between first end surface  26  of second member  20  and first end surface  36  of third member  30 . 
     If the variation in length between first end surface  26  of second member  20  and first end surface  36  of third member  30  in each of plural core pieces  1  is less than or equal to 0.1 mm, the variation in length is very small. Therefore, stator core  7  can construct axial gap type rotary electric machine  9  generating little noises and vibrations. The reason is as follows. Axial gap type rotary electric machine  9  is arranged such that stator  8  and rotor  90  face with each other as shown in  FIG.  13   . Since the variation in length in stator core  7  is small, the variation in the interval between stator  8  and rotor  90  is small. Since the variation in the interval is small, the torque ripple is small. Since the torque ripple is small, noises and vibrations are difficult to increase. The variation in length is determined in the following manner. In each core piece  1 , the length from first end surface  26  of second member  20  to first end surface  36  of third member  30  is measured. This length is the maximum length along the Z-axial direction of core piece  1 . Difference between the maximum value and the minimum value of the length in each of plural core pieces  1  is calculated. The difference is defined as the aforementioned variation in length. The variation in length between first end surface  26  of second member  20  and first end surface  36  of third member  30  in each of plural core pieces  1  is further preferably less than or equal to 0.05 mm, and especially preferably less than or equal to 0.01 mm. 
     [Operation and Effect] 
     Stator core  7  of the present embodiment is excellent in productivity since plural core pieces  1  constituting stator core  7  are configured by core piece  1  of the first embodiment that is excellent in productivity. 
     Third Embodiment 
     [Stator] 
     With reference to  FIG.  12   , stator  8  according to a third embodiment is described. Stator  8  of the present embodiment includes stator core  7  and coil  80 . As stator core  7 , stator core  7  according to the second embodiment can be used. Coil  80  is wound around first member  10  in each core piece  1  of stator core  7 . Stator  8  is used in axial gap type rotary electric machine  9  of DS/SR configuration shown in  FIG.  13   . 
     Each coil  80  includes a cylindrical part formed by winding wire. As the winding wire, coated round wire is used. Coated round wire includes a conductor of round wire, and an edge coating provided on the outer periphery of the conductor. In  FIG.  12   , only the cylindrical part of each coil  80  is shown in a simplified form, and illustration of both end parts of the winding wire is omitted. Stator core  7  can be manufactured by winding wire outside first member  10  of each core piece  1 . 
     [Operation and Effect] 
     Stator  8  according to the third embodiment is excellent in productivity since it includes stator core  7  of the second embodiment that is excellent in productivity. 
     Fourth Embodiment 
     [Rotary Electric Machine] 
     With reference to  FIG.  13   , rotary electric machine  9  according to a fourth embodiment is described.  FIG.  13    is a sectional view cut on a plane that is parallel with a rotary shaft  91  of rotary electric machine  9 , and passes the center of the circumferential direction of core piece  1 . This point also applies to  FIG.  14    referred to in a later-described fifth embodiment. Rotary electric machine  9  of the present embodiment is an axial gap type rotary electric machine. Rotary electric machine  9  of the present embodiment is of DS/SR configuration including one rotor  90  and two stators  8 . In other words, in rotary electric machine  9 , rotor  90  and stator  8  are arranged to face with each other in the axial direction. Rotor  90  and stator  8  are assembled such that one rotor  90  is sandwiched between two stators  8 . As each stator  8 , stator  8  according to the third embodiment described above may be used. Rotary electric machine  9  can be utilized in a motor or a generator. Rotary electric machine  9  includes a casing  92 . 
     Casing  92  has a cylindrical interior space that accommodates stator  8  and rotor  90 . Casing  92  includes a cylindrical part  921  and two plates  922 . Cylindrical part  921  surrounds the outer peripheries of stator  8  and rotor  90 . Plates  922  are respectively arranged on both ends of cylindrical part  921 . Two plates  922  are fixed to both end surfaces of cylindrical part  921  in such a manner that plates  922  sandwich stator  8  and rotor  90  from both sides of the axial direction. Both plates  922  have a through hole in the center part. The through hole is provided with a bearing  93 . Rotary shaft  91  penetrates the through hole with the bearing  93  interposed therebetween. Rotary shaft  91  penetrates inside of casing  92 . 
     Rotor  90  includes magnet  95  and a rotor body. Rotor  90  is a flat plate-like member in the present embodiment. The number of magnet  95  may be plural as in the present embodiment, or only one magnet  95  may be provided unlike the present embodiment. When the number of magnet  95  is plural, a specific number of magnet  95  may be the same with the number of core piece  1 . Plural magnets  95  are arranged at regular intervals in the circumferential direction of the rotor body. In the present embodiment, each magnet  95  is in the form of a flat plate having a surface shape corresponding the surface shape of first end surface  36  of third member  30  in each core piece  1 . Each magnet  95  may be in the form of a convex lens having a convex surface toward the side of each stator  8 . When the number of magnet  95  is one, magnet  95  has a circular shape. In one magnet  95 , the S pole and the N pole are alternately arranged in the circumferential direction. The rotor body supports plural magnets  95 . The rotor body is a circular member. The rotor body is rotatably supported by rotary shaft  91 . Magnets  95  are arranged at regular intervals in the circumferential direction of the rotor body. Magnets  95  are polarized in the axial direction of rotary shaft  91 . Magnetization directions of magnets  95  adjoining in the circumferential direction of the rotor body are opposite to each other. Rotor  90  rotates by repetition of attraction and repulsion between magnet  95  and each core piece  1  by a rotating magnetic field generated in stator  8 . 
     Stator  8  is arranged such that first end surface  36  of third member  30  in each core piece  1  faces with magnet  95  of rotor  90 . As rotor  90  rotates, first end surface  36  of third member  30  in each core piece  1  receives a magnetic flux from rotating magnet  95 . As shown in  FIG.  3   , when first end surface  36  of third member  30  in each core piece  1  is formed to have a convex shape as described above, noises and vibrations of rotary electric machine  9  can be reduced. The reason is as follows. Since first end surface  36  of third member  30  in each core piece  1  is formed to have a convex shape, abrupt change in the magnetic flux of magnet  95  of rotor  90  received by core piece  1  is easily suppressed. Therefore, cogging torque is easily reduced. Since the cogging torque is small, noises and vibrations are difficult to increase. 
     [Operation and Effect] 
     Rotary electric machine  9  according to the fourth embodiment is excellent in productivity since it includes stator  8  of the third embodiment that is excellent in productivity. 
     Fifth Embodiment 
     [Rotary Electric Machine] 
     With reference to  FIG.  14   , rotary electric machine  9  according to a fifth embodiment is described. Rotary electric machine  9  of the present embodiment is an axial gap type rotary electric machine. Rotary electric machine  9  of the present embodiment is different from rotary electric machine  9  of the fourth embodiment mainly in that the rotary electric machine is of SS/DR configuration including two rotors  90  and one stator  8 . In other words, in rotary electric machine  9 , rotor  90  and stator  8  are arranged to face with each other in the axial direction. Rotor  90  and stator  8  are assembled such that one stator  8  is sandwiched between two rotors  90 . The following description focuses on a point of difference from the fourth embodiment. Description of the same configuration as that in the fourth embodiment is omitted. 
     Each rotor  90  includes the rotor body, plural magnets  95 , and a back yoke  98 . The rotor body and plural magnets  95  are as described in the fourth embodiment. Back yoke  98  is disposed between rotor  90  and plate  922 . Back yoke  98  is a flat plate member. Back yoke  98  is configured by the same green compact as that of core piece  1  described above, or a laminated steel plate. 
     Stator  8  includes plural core pieces  1  that are circularly arranged, coil  80  wound on first member  10  of each core piece  1 , and a supporting member that retains plural core pieces  1 . Illustration of the supporting member is omitted. Core pieces  1  are the same in configuration of second member  20  and third member  30 . That is, core pieces  1  are the same in the protruding amount of first protruding part  211  and second protruding part  212  in second member  20 , and in the protruding amount of first protruding part  311  and second protruding part  312  in third member  30 . Also, first lateral surface  24   a  of first protruding part  211  and second lateral surface  24   b  of second protruding part  212  in second member  20  are not provided with a step as described above. Coil  80  is as described in the third embodiment. A holder retains plural core pieces  1  such that intervals between core pieces  1  are regular intervals. This holder prevents core pieces  1  adjoining in the circumferential direction from coming into contact with each other. 
     [Operation and Effect] 
     Rotary electric machine  9  according to the fifth embodiment is excellent in productivity since it includes stator  8  that is excellent in productivity likewise rotary electric machine  9  according to the fourth embodiment. 
     The scope of the present invention is indicated by the appended claims rather than by the foregoing illustration, and is intended to include all modifications within the equivalent meaning and scope of the claims. For example, the rotary electric machine may be the one that includes one rotor and one stator. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : Core piece 
               10 : First member 
               11 : Peripheral surface;  12 : Outer peripheral surface;  13 : Inner peripheral surface 
               14   a : First lateral surface;  14   b : Second lateral surface 
               141 : First parallel face;  142 : Second parallel face;  143 : First inclined face 
               20 : Second member 
               21 : Protruding portion;  211 : First protruding part;  212 : Second protruding part 
               22 : Outer peripheral surface;  23 : Inner peripheral surface;  24   a : First lateral surface;  24   b : Second lateral surface 
               240 : Step 
               241 : First parallel face;  242 : Second parallel face;  243 : First inclined face 
               244 : Projecting part 
               26 : First end surface;  27 : Second end surface;  28 : Corner part 
               30 : Third member 
               31 : Protruding portion;  311 : First protruding part;  312 : Second protruding part 
               32 : Outer peripheral surface;  33 : Inner peripheral surface 
               34   a : First lateral surface;  34   b : Second lateral surface 
               341 : First parallel face;  342 : Second parallel face;  343 : First inclined face 
               36 : First end surface;  37 : Second end surface;  38 : Corner part 
               5 : Mold 
               50 : Die;  50   h : Mold hole 
               51 : First hole portion 
               511 : First straight part;  512 : Second straight part;  513 : Taper part 
               52 : Second hole portion 
               521 : First straight part;  522 : Second straight part;  523 : Taper part 
               53 : Third hole portion 
               531 : First straight part;  532 : Second straight part;  533 : Taper part 
               54 : Upper punch 
               541 : First upper punch part;  541   e : First lower end face 
               542 : Second upper punch part;  542   e : Second lower end face 
               543 : Third upper punch part;  543   e : Third lower end face 
               55 : Lower punch 
               551 : First lower punch part;  551   e : First upper end face 
               552 : Second lower punch part;  552   e : Second upper end face 
               553 : Third lower punch part;  553   e : Third upper end face 
               7 : Stator core;  8 : Stator;  80 : Coil 
               9 : Rotary electric machine 
               90 : Rotor;  91 : Rotary shaft;  92 : Casing 
               921 : Cylindrical part;  922 : Plate 
               93 : Bearing;  95 : Magnet;  98 : Back yoke 
             E 11 , E 12 , E 21 , E 22 , E 31 , E 32 : Extended plane 
             Va, Vb: Imaginary plane; V 21 : First imaginary plane; V 22 : Second imaginary plane 
             θ 11 , θ 21 , θ 31 : First inclination angle 
             θ 12 , θ 22 , θ 32 : Second inclination angle