JP2010252500A - Method of manufacturing yoke - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a yoke which removes compression stress remaining in the yoke by drawing and prevents roundness from being deteriorated even if trimmed. <P>SOLUTION: A material 20 formed in a circle with a prescribed diameter d0 is drawn at a prescribed drawing rate and a first intermediate article 21 in a bottomed cylinder shape whose inner diameter is larger than an inner diameter D of the yoke, which is the last diameter, is formed in a first drawing process. Then, in a second drawing process, the first intermediate article 21 is drawn much more, and a second intermediate article 31 in the bottomed cylinder shape, whose inner diameter is smaller than the inner diameter D of the yoke, which is the last diameter, is formed. In an inner diameter expansion process, a shaping punch is inserted into the second intermediate article 31 formed in the bottomed cylinder shape whose inner diameter is smaller than the inner diameter D of the yoke, which is the last diameter, by double drawing, and plastic deformation is performed so that the inner diameter d2 of the second intermediate article 31 is expanded to the inner diameter D being the last diameter. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a bottomed cylindrical yoke used in a rotating electrical machine such as an electric motor or a generator.

  As a yoke of an electric motor used as a drive source for a power slide door device, a power window door device or the like, a bottomed cylindrical yoke is often used. In such a yoke, a flange portion extending radially outward is integrally formed at the opening end, and the yoke is attached to a member such as a gear case at the opening end by the flange portion. Also, a bottomed cylindrical bearing holding portion is integrally formed at the bottom of the yoke, and one end of the armature shaft of the armature housed inside the yoke is rotatably supported by the bearing held by the bearing holding portion. It has become so.

  Such a bottomed cylindrical yoke is formed, for example, by drawing (deep drawing) a steel plate such as a blank formed in a disk shape as disclosed in Patent Document 1. In this case, the drawing process is performed a plurality of times, and the inner diameter of the bottomed cylindrical intermediate product formed in each drawing process is gradually reduced to a predetermined final diameter. In addition, after the drawing process, the pressing portion formed at the opening end is trimmed (trimmed) to form a flange portion having a predetermined shape that matches the shape of the gear case.

JP-A-9-331657

  In a yoke formed into a bottomed cylindrical shape through a plurality of drawing processes, compressive stress due to the drawing process remains in the cylindrical part and the flange part. Since the yoke immediately after drawing is closed in the circumferential direction, the component acting in the circumferential direction around the axis of this residual stress works uniformly over the entire circumference, but does not affect the roundness of the yoke. When the flange portion or the cylindrical portion is trimmed after the drawing, the residual stress is partially released and the roundness of the yoke is lowered. When the roundness of the yoke is lowered, there arises a problem that the accuracy of attaching the magnet to the inner peripheral surface of the yoke and the accuracy of attaching the yoke to the gear case are lowered.

  An object of the present invention is to provide a yoke in which the compressive stress remaining in the yoke is removed by drawing and the roundness does not decrease even when trimmed.

  The yoke manufacturing method according to the present invention is a yoke manufacturing method that is formed into a bottomed cylindrical shape and used for a rotating electrical machine, and is a first bottomed cylindrical shape having a larger inner diameter than a final diameter by drawing a steel plate. A first drawing step for forming an intermediate product, a second drawing step for drawing the first intermediate product to form a bottomed cylindrical second intermediate product having an inner diameter smaller than a final diameter, and the second intermediate product. An inside diameter expanding step of inserting a tool into the product and enlarging the inside diameter of the second intermediate product to the final diameter with the tool.

  In the yoke manufacturing method of the present invention, after the second drawing step and before the inner diameter expanding step, a shaping step is performed in which a flange portion is formed at the open end and a bearing holding portion is formed at the bottom portion. It is characterized by.

  The yoke manufacturing method of the present invention is characterized in that after the inner diameter expanding step, a shaping step is performed in which a flange portion is formed at the open end and a bearing holding portion is formed at the bottom.

  The yoke manufacturing method of the present invention is characterized in that only the inner diameter of the magnet mounting portion within a predetermined range from the open end of the second intermediate product is enlarged by the inner diameter enlargement step.

  According to the present invention, a bottomed cylindrical intermediate product having an inner diameter smaller than the final diameter is formed by the first drawing process and the second drawing process, and the inner diameter of the intermediate product is expanded to the final diameter by the inner diameter enlargement process. Since the yoke having a predetermined inner diameter is formed, the residual compressive stress generated in the drawing process can be offset by the tensile force generated in the inner diameter expanding process. As a result, it is possible to improve the roundness of the yoke by reducing the influence of the release of the residual compressive stress that occurs when the cylindrical portion, flange portion, and the like of the yoke are trimmed. Further, since it is possible to prevent the roundness from being lowered due to the trim processing, it is possible to increase the degree of freedom in designing the shape of the flange portion and the like without considering the change in the roundness due to the trim processing.

  Further, since only the magnet mounting portion within a predetermined range from the opening end of the second intermediate product is enlarged by the inner diameter enlarging process, the step portion formed on the inner peripheral surface of the yoke can be used for magnet positioning. .

It is a perspective view of the yoke manufactured by the manufacturing method of the yoke which is one embodiment of the present invention. It is a cross-sectional view of the yoke shown in FIG. (A)-(e) is explanatory drawing which shows the manufacture procedure of the yoke shown in FIG. 1, respectively. It is sectional drawing of the processing apparatus used for an internal diameter expansion process. (A)-(c) is explanatory drawing which shows the processing state of the processing apparatus shown in FIG. 4, respectively. (A) is a front view of the yoke before the flange portion is trimmed, and (b) is a front view of the yoke after the flange portion is trimmed. It is a transverse cross section of the yoke manufactured by the manufacturing method of the yoke which is other embodiments of the present invention. (A)-(e) is explanatory drawing which shows the manufacture procedure of the yoke shown in FIG. 7, respectively.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  A yoke 11 shown in FIG. 1 is used for an electric motor (not shown) as a rotating electrical machine used as a drive source of a power slide door device mounted on a vehicle.

  As shown in FIGS. 1 and 2, the yoke 11 is formed in a bottomed cylindrical shape having a cylindrical portion 12 and a bottom portion 13. The cylindrical portion 12 has a structure in which a large-diameter cylindrical portion 12a extending over a predetermined range from the opening end in the axial direction and a small-diameter cylindrical portion 12b having an inner diameter smaller than the large-diameter cylindrical portion 12a are arranged in the axial direction. The inner diameter of the yoke 11 on the open end side, that is, the inner diameter of the large-diameter cylindrical portion 12a is D. The inside of the large diameter cylindrical portion 12a is a magnet mounting portion 12c, and a step portion 12d is formed between the large diameter cylindrical portion 12a and the small diameter cylindrical portion 12b. A pair of segment-shaped magnets 14a and 14b inserted into the inside of the yoke 11 from the open ends are abutted against the stepped portion 12d at the axial end portions and positioned in the axial direction of the large-diameter cylindrical portion 12a. It is fixed to the inner peripheral surface, that is, the magnet mounting part 12c by means such as adhesion. A bottomed cylindrical bearing holding portion 15 is integrally formed on the bottom portion 13, and a bearing (not shown) is attached to the bearing holding portion 15. An armature (not shown) housed inside the yoke 11 is supported by a bearing mounted on a bearing holding portion at one end of the rotating shaft, and is rotatably supported inside the yoke 11. A flange portion 16 is integrally formed at the opening end of the cylindrical portion 12. The flange portion 16 is formed in a predetermined shape corresponding to the shape of a gear case (not shown) of the power slide door device, and the yoke 11 has a bolt or the like (not shown) in a state where the opening end and the flange portion 16 are abutted against the gear case. Is fixed to the gear case.

  Next, the manufacturing method of this yoke 11 is demonstrated.

  The yoke 11 is manufactured by a yoke manufacturing method according to an embodiment of the present invention. This yoke manufacturing method is performed by the first drawing step, the second drawing step, the shaping step, the inner diameter expanding step, and the trim step.

  In this manufacturing method, as a material 20 of the yoke 11, as shown in a cross section in FIG. 3A, a blank material, that is, a circular steel plate formed by stamping a steel plate into a circle having a predetermined diameter d0 by pressing. Is used.

  In addition, as this raw material 20, you may use not only the blank material as shown to Fig.3 (a) but what formed the steel plate circularly by other processing methods, such as a laser cutting process. The shape of the material 20 is not limited to a circle having a diameter d0, and may be any other shape as long as it can be drawn, such as a square shape having a side length d0.

  When the material 20 is prepared, first, a first drawing process is performed.

  In the first drawing step, the material 20 is drawn at a predetermined drawing rate by a pressing device (not shown) to form a first intermediate product 21 shown in FIG. The first intermediate product 21 has a bottomed cylindrical shape including a cylindrical portion 22 and a bottom portion 23. The inner diameter d1 of the cylindrical portion 22 is larger than the inner diameter D of the yoke 11, which is the final diameter. That is, the drawing ratio in the first drawing step is set so that the inner diameter d1 of the first intermediate product 21 is larger than the inner diameter D of the yoke, which is the final diameter.

  When the first intermediate product 21 is formed by the first drawing step, the second drawing step is then performed.

  In the second drawing step, the first intermediate product 21 is further drawn to form a second intermediate product 31 shown in FIG. In the second drawing step, the first intermediate product 21 is further thinned and deeply drawn. Accordingly, the second intermediate product 31 is formed in a bottomed cylindrical shape that is longer than the first intermediate product 21 including the cylindrical portion 32 and the bottom portion 33. The inner diameter d2 of the cylindrical portion 32 of the second intermediate product 31 is smaller than the inner diameter d1 of the first intermediate product 21 and the inner diameter D of the yoke 11 which is the final diameter. That is, the drawing ratio in the second drawing step is set so that the inner diameter d2 of the second intermediate product 31 is smaller than the inner diameter D of the yoke 11 which is the final diameter. Thus, in this yoke manufacturing method, the inner diameter d2 of the second intermediate product 31 forming the yoke 11 is once made smaller than the inner diameter D of the yoke 11, which is the final diameter.

  When the second intermediate product 31 is formed by the second drawing process, a shaping process is performed next.

  In this shaping step, as shown in FIG. 3D, the bottom 33 of the second intermediate product 31 is shaped by a predetermined drawing process or the like, and the bearing holding part 35 is formed on the bottom 33. Further, the pressing portion at the time of drawing at the opening end portion of the second intermediate product 31 is shaped so as to be perpendicular to the axial direction, and the flange portion 36 is formed at the opening end.

  In this shaping step, the inner diameter of the second intermediate product 31 remains d2 and does not change.

  When the bearing holding portion 35 and the flange portion 36 are formed by the shaping process and the entire shape of the second intermediate product 31 is adjusted, the inner diameter expanding step is performed next.

  In this inner diameter expanding step, the cylindrical portion 32 of the second intermediate product 31 in which the bearing holding portion 35 and the flange portion 36 are formed has the same inner diameter d2 as the inner diameter D of the yoke 11 which is the final diameter. Enlarged by drawing.

  FIG. 4 is a cross-sectional view of a drawing apparatus used in the inner diameter expansion process.

  The drawing device 41 includes a base 42 and a head 43 each formed in a plate shape. The base 42 and the head 43 are fixed to a pair of guide posts 44 extending in the vertical direction, and are arranged at predetermined intervals in the vertical direction. The second intermediate product 31 to be processed is the base 42 is arranged. A shaping die 45 is provided between the base 42 and the head 43. The shaping die 45 is supported by the guide post 44 so as to be movable in the vertical direction between the base 42 and the head 43, and can be moved in the vertical direction by being driven by a driving device (not shown). ing. In the shaft center of the shaping die 45, a shaping large-diameter hole 45a that opens toward the base 42 and a holding small-diameter hole 45b that extends toward the head 43 are coaxially connected in the axial direction. Yes. The inside diameter of the shaping large-diameter hole 45a is set larger than the inside diameter of the holding small-diameter hole 45b, and the boundary between the shaping large-diameter hole 45a and the holding small-diameter hole 45b is inclined with respect to the axial direction. The taper surfaces 45c are connected.

  A holder 46 is fixed to the lower surface of the head 43. The holder 46 is formed in a cylindrical shape slightly smaller in diameter than the holding small-diameter hole 45b, and is slidably inserted into the holding small-diameter hole 45b. In addition, a recess 46a is provided in the axial center of the end face in the axial direction facing the base 42 side of the holder 46. The second intermediate product 31 arranged on the base 42 has a bearing holding portion 35 whose recess 46a In the state of being inserted into the bottom portion 33, the bottom portion 33 is held on the axial end surface of the holder 46.

  A shaping punch 47 as a tool is disposed below the base 42. The shaping punch 47 has a punch main body 47a formed in a columnar shape with steel or the like, and the outer diameter of the punch main body 47a is set to be the same as the final diameter, that is, the inner diameter D of the yoke 11. A cylindrical protrusion 47b having an outer diameter set to be the same as the inner diameter d2 of the second intermediate product 31 is formed coaxially and integrally with the punch main body 47a at the tip of the punch main body 47a of the shaping punch 47. The boundary between the punch body 47a and the protrusion 47b is a curved surface 47c. A through hole 42a is provided in the axial center of the base 42, and the shaping punch 47 is driven by a cylinder device (not shown) so as to be able to move upward from the through hole 42a to the base 42. Yes.

  Next, a processing procedure of the inner diameter expanding process performed by the drawing device 41 will be described with reference to FIG.

  First, as shown in FIG. 5A, an inner diameter d2 is formed at a predetermined position on the base 42 through the first drawing process and the second drawing process so that the inner diameter d2 is smaller than the inner diameter D of the yoke 11, which is the final diameter. The bottom cylindrical second intermediate product 31 is arranged facing downward so that the opening end is aligned with the through hole 42 a of the base 42. At this time, the second intermediate product 31 is in a state where the bottom 33 abuts against the axial end surface of the holder 46 and the bearing holding portion 35 is inserted into the recess 46 a and supported by the holder 46.

  Next, from this state, the shaping die 45 is driven downward in the figure by a driving device (not shown). When the shaping die 45 is driven downward, the flange portion 36 is sandwiched between the lower surface of the shaping die 45 and the upper surface of the base 42 as shown in FIG. Retained. Further, when the shaping die 45 moves relative to the holder 46, the outer peripheral portion of the cylindrical portion 32 in a predetermined range from the bottom 33 side of the second intermediate product 31 is the inner peripheral surface of the holding small-diameter hole 45b of the shaping die 45. Retained. In this state, the outer peripheral surface in a predetermined range from the opening end of the cylindrical portion 32 of the second intermediate product 31 is opposed to the inner peripheral surface of the shaping large-diameter hole 45a with a predetermined gap. .

  Next, from this state, the shaping punch 47 is driven upward, that is, toward the inside of the second intermediate product 31 by a cylinder device (not shown). Then, the shaping punch 47 protrudes above the base 42 from the through hole 42a of the base 42 and is inserted into the cylindrical portion 32 of the second intermediate product 31 as shown in FIG. At this time, the punch body 47a of the shaping punch 47 protrudes from the opening end of the cylindrical portion 32 of the second intermediate product 31 to a predetermined range, that is, a position corresponding to the shaping large-diameter hole 45a of the shaping die 45, and the protruding portion 47b It protrudes to a portion corresponding to the holding small diameter hole 45b.

  Here, since the outer diameter of the punch body 47a of the shaping punch 47 is formed to be the same as the inner diameter D of the yoke 11, that is, larger than the inner diameter d2 of the second intermediate product 31, the punch body 47a of the shaping punch 47 is formed. Is inserted into the inside, a portion within a predetermined range from the opening end of the cylindrical portion 32 of the second intermediate product 31, that is, a portion whose outer peripheral surface faces the inner peripheral surface of the shaping large-diameter hole 45 a is the punch of the shaping punch 47. The main body 47a is spread outward in the radial direction, and its outer peripheral surface comes into contact with the inner peripheral surface of the shaping large-diameter hole 45a. Thereby, a portion within a predetermined range from the opening end of the cylindrical portion 32 of the second intermediate product 31, that is, a portion corresponding to the magnet mounting portion 12c is sandwiched and narrowed by the shaping punch 47 and the shaping large-diameter hole 45a, It is plastically deformed in the direction in which the inner diameter is expanded and expanded to the final diameter, that is, the same diameter as the inner diameter D of the yoke 11.

  In addition, the part by which the internal diameter of the 2nd intermediate product 31 was expanded becomes the large diameter cylindrical part 12a of the yoke 11, and the inner side becomes the magnet mounting part 12c.

  On the other hand, a portion within a predetermined range from the bottom 33 side of the cylindrical portion 32 of the second intermediate product 31, that is, a portion where the outer peripheral surface of the cylindrical portion 32 is held by the inner peripheral surface of the holding small-diameter hole 45b is formed inside the shaping punch. 47 projecting portions 47b are inserted and held between the inner peripheral surface of the holding small-diameter hole 45b and the outer peripheral surface of the projecting portion 47b. Thereby, the part of the predetermined range from the bottom 33 of the cylindrical part 32 of the second intermediate product 31 does not change in inner diameter from the inner diameter d2 of the second intermediate product 31 and has a smaller diameter than the large cylindrical part 12a of the yoke 11. It becomes the small diameter cylindrical part 12b. Further, a part of the cylindrical portion 32 of the second intermediate product 31 is sandwiched between the tapered surface 45 c of the shaping die 45 and the curved surface 47 c of the shaping punch 47, so that the cylindrical portion 12 of the yoke 11 has an axial direction. Accordingly, a step portion 12d inclined in a taper shape is formed.

  Thus, in the yoke manufacturing method of the present invention, the bottomed cylindrical second intermediate product 31 having an inner diameter smaller than the inner diameter D of the yoke 11 which is the final diameter is formed by the first drawing step and the second drawing step. The yoke 11 having a predetermined inner diameter D is formed by expanding the inner diameter d2 of the second intermediate product 31 to the same inner diameter D as the final diameter by an inner diameter expanding step. As a result, the circumferential compressive stress remaining in the second intermediate product 31 by the drawing process performed in the first drawing process and the second drawing process is offset by the circumferential tensile force generated in the inner diameter expanding process. Can be reduced.

  In this embodiment, the diameter d0 of the material 20 is 202 mm, the inner diameter d1 of the first intermediate product 21 is 108 mm, the inner diameter d2 of the second intermediate product 31 is 81 mm, and the inner diameter D of the yoke 11 is the final diameter D = 82. The aperture ratio in the first aperture process is 0.53, the aperture ratio in the second aperture process is 0.75, and the aperture ratio (enlargement ratio) in the inner diameter expansion process is 1.02.

  After the inner diameter expanding step, a trim step for forming the flange portion 36 of the second intermediate product 31 on the flange portion 16 having a predetermined shape is performed.

  In the trimming process, as shown in FIG. 6 (a), the circular flange portion 16 matched to the material 20 is predetermined by pressing using a blank die or the like as shown in FIG. 6 (b). Punched into the shape of

  At this time, in the yoke 11 manufactured by the manufacturing method of the present invention, the compressive residual stress in the circumferential direction generated in the second intermediate product 31 by the drawing process performed in the first drawing process and the second drawing process is By being subjected to plastic deformation so that the inner diameter is enlarged by the inner diameter expanding step, it is offset and reduced by the tensile force in the circumferential direction due to the plastic deformation. Therefore, even if the flange portion 16 is trimmed into a non-uniform shape in the circumferential direction, the influence of the compressive residual stress on the roundness is reduced, and the roundness of the cylindrical portion 12 of the yoke 11 is reduced. Can be improved.

  Thus, in the yoke manufacturing method of the present invention, the bottomed cylindrical second intermediate product 31 having an inner diameter smaller than the inner diameter D of the yoke 11 which is the final diameter is formed by the first drawing step and the second drawing step. The yoke 11 having a predetermined inner diameter D is formed by expanding the inner diameter d2 of the second intermediate product 31 to the final diameter by the inner diameter expanding step. Therefore, the cylindrical portion 12 and the flange portion 16 of the yoke 11 are trimmed by canceling the residual compressive stress in the circumferential direction generated by the drawing process in each drawing process by the tensile force generated in the circumferential direction in the inner diameter expanding process. The influence on the roundness due to the release of the residual compressive stress that occurs sometimes can be reduced. Thereby, the compressive stress remaining in the yoke 11 is removed by the drawing process in each drawing process, and the yoke 11 in which the roundness does not decrease even when trimmed can be provided.

  Further, since it is possible to prevent the roundness from being lowered due to the trim processing, it is possible to increase the degree of freedom in designing the shape of the flange portion 16 and the like without considering the change in the roundness due to the trim processing.

  Further, in the yoke manufacturing method of the present invention, only the inner diameter of the portion corresponding to the magnet mounting portion 12c within a predetermined range from the opening end of the second intermediate product 31 is enlarged by the inner diameter enlargement process. A step portion 12d is formed on the peripheral surface. The stepped portion 12d is used for axial positioning of the magnets 14a and 14b in the yoke 11, thereby increasing the positioning accuracy of the magnets 14a and 14b without using other members or jigs. it can.

  FIG. 7 is a cross-sectional view of a yoke manufactured by a yoke manufacturing method according to another embodiment of the present invention, and FIGS. 8A to 8E respectively show the manufacturing procedure of the yoke shown in FIG. It is explanatory drawing.

  In the yoke manufacturing method shown in FIG. 3, a stepped shape having a large diameter cylindrical portion 12a and a small diameter cylindrical portion 12b is performed by performing a shaping step after the second drawing step and before the inner diameter expanding step. The yoke 11 is formed.

  On the other hand, in the yoke manufacturing method shown in FIG. 8, the inner diameter expanding step is performed after the second drawing step and before the shaping step. That is, the bearing holding portion 15 is not provided in the inner diameter expanding step, and the yoke 11 whose overall shape is not shaped is formed, and then the shaping step is performed. In this embodiment, in the inner diameter expanding step, the entire inner diameter of the cylindrical portion 32 of the second intermediate product 31 is increased. After the inner diameter expanding step, the yoke 11 is formed in the shaping step. The inner diameter of s does not change. Thus, as shown in FIG. 7, the yoke 11 is formed in a bottomed cylindrical shape including the cylindrical portion 12 having a uniform inner diameter without the stepped portion 12d as shown in FIG. As described above, the inner diameter enlargement step may be performed at any timing as long as it is after the first drawing step and the second drawing step regardless of the shaping step.

  In FIGS. 7 and 8, members corresponding to those described above are denoted by the same reference numerals.

  It goes without saying that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention. For example, in the embodiment described above, the shaping process and the trimming process are performed in addition to the first drawing process, the second drawing process, and the inner diameter expanding process. It may be performed arbitrarily, and may be omitted if unnecessary.

  Moreover, in the said embodiment, although the raw material 20 is processed to the 2nd intermediate product 31 by the 2nd drawing process of a 1st drawing process and a 2nd drawing process, it is not restricted to this, The 1st drawing process Alternatively, another drawing process may be added before or between the first drawing process and the second drawing process, and the second intermediate product 31 may be formed through two or more drawing processes.

  Furthermore, in the said embodiment, although this invention was applied to the yoke 11 of the electric motor used as a drive source of a power slide door apparatus, not only this but electric motors, generators, etc. which are used for other uses You may make it apply this invention to the yoke of another rotary electric machine.

  Furthermore, in the above-described embodiment, trimming is performed only on the flange portion. However, the present invention is not limited to this, and processing such as providing a notch extending in the axial direction in a part of the cylindrical portion 12 of the yoke 11, etc. The cylindrical portion 12 may be processed.

11 Yoke 12 Cylindrical portion 12a Large-diameter cylindrical portion 12b Small-diameter cylindrical portion 12c Magnet mounting portion 12d Step portion 13 Bottom portion 14a, 14b Magnet 15 Bearing holding portion 16 Flange portion 20 Material (steel plate)
21 First intermediate product 22 Cylindrical portion 23 Bottom portion 31 Second intermediate product 32 Cylindrical portion 33 Bottom portion 35 Bearing holding portion 36 Flange portion 41 Drawing device 42 Base 42a Through hole 43 Head 44 Guide post 45 Shaping die 45a Large diameter hole for shaping 45b Small diameter hole 45c for holding Tapered surface 46 Holder 46a Concave portion 47 Shaping punch (tool)
47a Punch body 47b Protruding part 47c Curved surface D Inner diameter d0 Diameter d1, d2 Inner diameter

Claims (4)

A method of manufacturing a yoke that is formed into a bottomed cylindrical shape and is used in a rotating electric machine,
A first drawing step of drawing a steel plate to form a bottomed cylindrical first intermediate product having a larger inner diameter than the final diameter;
A second drawing step of drawing the first intermediate product to form a bottomed cylindrical second intermediate product having an inner diameter smaller than the final diameter;
A method of manufacturing a yoke, comprising: inserting a tool into the second intermediate product, and expanding the inner diameter of the second intermediate product to the final diameter with the tool.   2. The yoke manufacturing method according to claim 1, wherein a shaping step of forming a flange portion at an opening end and forming a bearing holding portion at a bottom portion is performed after the second drawing step and before the inner diameter expanding step. A method for manufacturing a yoke.   2. The method for manufacturing a yoke according to claim 1, wherein after the inner diameter expanding step, a shaping step is performed in which a flange portion is formed at the open end and a bearing holding portion is formed at the bottom portion.   4. The method for manufacturing a yoke according to claim 1, wherein only the inner diameter of a magnet mounting portion within a predetermined range from the opening end of the second intermediate product is enlarged by the inner diameter enlarging step. Yoke manufacturing method.

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