Abstract:
Provided is a method for making a double-walled yoke that allows an inner yoke ( 4 ) and outer yoke ( 3 ) to be integrally joined to each other both easily and securely. Such a yoke provides an adequate cross sectional area for a magnetic path of the yoke while minimizing the weight of the yoke. A tubular member ( 14 ) destined to be form the inner yoke and a cup-shaped member ( 13 ) destined to form the outer yoke are both fitted on a punch ( 7 ) and a drawing process is executed by using an appropriate drawing die ( 8 ). The outer yoke may have a small wall thickness as long as it is capable of retaining its shape during use or supporting a bearing for the motor. The inner yoke is required to be relatively small in size as long it is capable of forming a satisfactory magnetic path for the motor. A satisfactory magnetic path can be obtained if the yoke is provided with an adequate wall thickness so as to provide an adequate cross sectional area for the magnetic path.

Description:
TECHNICAL FIELD 
   The present invention relates to a double-walled yoke including a cup-shaped outer yoke and a tubular inner yoke nested in and secured to the outer yoke in a coaxial relationship and a method of making such a double-walled yoke. 
   BACKGROUND OF THE INVENTION 
   A conventional motor casing typically comprises an end plate supporting a brush holder and a cup-shaped yoke connected to the end plate. The cup-shaped yoke is typically formed by a deep drawing process.  FIG. 14  shows such an example. A radial outer flange  31   a  is formed at an open end of a cup-shaped yoke  31 . The flange  31   a  is formed at the same time as forming the yoke  31  by a deep drawing process. 
   The circumferential wall  31   b  of the yoke  31  that supports permanent magnets  32  is required to have a certain thickness so that a magnetic path having an adequate cross sectional area may be achieved. Therefore, the material for forming the yoke  31  is required to consist of a plate member having a corresponding thickness. When such material is subjected to a deep drawing process, the bottom portion that defines a bearing support portion  31   c  is stretched, and is given with a reduced wall thickness upon completion of the deep drawing process. However, the radial flange  31   a  would not be stretched to such an extent during the deep drawing process, and is given with a wall thickness d similar to that of the circumferential wall upon completion of the deep drawing process. Therefore, the thickness of the radial flange  31   a  is greater than desired, and this adds to the weight of the yoke without providing any benefits. 
   However, there is an ever growing demand to reduce the weight of a motor, and it is desired to minimize the weight of a yoke without unduly complicating the manufacturing process. 
   Japanese patent laid-open publication 11-089123 discloses a double-walled yoke that combines a tubular inner yoke for supporting permanent magnets and a cup-shaped deep-drawn outer yoke having a relatively small thickness. Thereby, only the part of the yoke that defines the magnetic path for the permanent magnets is given with an increased thickness while the remaining part of the yoke is given with a reduced thickness. Therefore, the weight of the yoke can be minimized without compromising the magnetic performance of the yoke. 
   According to this previously proposed yoke, the inner yoke and outer yoke are integrally combined apparently by press fitting the inner yoke into the outer yoke. However, a press fitting process may involve seizure and/or deformation of the inner yoke unless the inner and outer yokes are prepared at a very high precision, and a considerable amount of effort and cost are required to carry out the press fitting process for a yoke of this type in a successful manner. 
   BRIEF SUMMARY OF THE INVENTION 
   In view of such problems of the prior art, a primary object of the present invention is to provide a method for making a double-walled yoke that allows an inner yoke and outer yoke to be integrally joined to each other both easily and securely. 
   A second object of the present invention is to provide a method of making a double-walled yoke at a high precision. 
   A third object of the present invention is to provide a double-walled yoke which provides an adequate cross sectional area for a magnetic path of the yoke while minimizing the weight of the yoke. 
   A fourth object of the present invention is to provide a double-walled yoke which joins an inner and outer yoke in a highly secure manner and can be manufactured at low cost. 
   According to the present invention, such objects can be accomplished by providing a method for making a double-walled yoke including a cup-shaped outer yoke and a tubular inner yoke nested in and secured to the outer yoke in a coaxial relationship, comprising: preparing a cup-shaped member destined to form the outer yoke and having a diameter greater than that of the outer yoke; preparing a tubular member destined to form the inner yoke; preparing a cylindrical punch having an outer profile for defining an inner profile of the double-walled yoke; fitting the tubular member on the punch; fitting the cup-shaped member on the punch over the tubular member; preparing a drawing die having an inner profile for defining an outer profile of the double-walled yoke; and fitting the drawing die on the cup-shaped member and drawing the outer yoke onto the tubular member so that the tubular member is fixedly secured to the inner surface of the cup-shaped member. 
   The outer yoke may have a small wall thickness as long as it is capable of retaining its shape during use or supporting a bearing for the motor. The inner yoke is required to be relatively small in size as long it is capable of forming a satisfactory magnetic path for the motor. A satisfactory magnetic path can be obtained if the yoke is provided with an adequate wall thickness so as to provide an adequate cross sectional area for the magnetic path. 
   The punch may be provided with an annular shoulder to engage a lower axial edge of the tubular member so that the tubular member may be accurately positioned in the axial direction relative to the cup-shaped member in a highly simple manner. In such a case, if the cup-shaped member is provided with an annular shoulder that abuts an upper axial edge of the tubular member when the cup-shaped member is fitted on the punch over the tubular member, the tubular member is axially compressed between the annular shoulder of the cup-shaped member and annular should surface of the punch while the outer circumferential surface of the tubular member is constricted by the drawing die during the drawing process so that the tubular member can be firmly secured to the inner circumferential surface of the cup-shaped member by the drawing process. 
   The annular shoulder may be defined by a tapered surface so that the lower axial edge of the tubular member may flare out and dig into the cup-shaped member and the secure attachment between the two members may be effected by the drawing process. If the lower axial edge of the tubular member is already flared outwardly before the drawing process, a similar result can be achieved without regard to if the annular shoulder surface is tapered or not. 
   Typically, the cup-shaped member is provided with a radial outer flange at an open end thereof, and the punch is provided with a base block defining a flat upper surface that abuts the radial outer flange of the cup-shaped member when the cup-shaped member is fitted on the punch over the tubular member. Thereby, the axial positioning of the cup-shaped member can be achieved in a highly simple manner. 
   To ensure a secure engagement between the tubular member and cup-shaped member against a relatively rotation in the circumferential direction, an outer circumferential surface of the punch may be provided with at least one axial ridge or an inner circumferential surface of the drawing die is provided with at least one axial ridge. Alternatively, for the same purpose, the punch and drawing die may be configured in such a manner that the tubular member and cup-shaped member are drawn so as to have a cross section that deviates from a true circle. Thereby, the relative rotation between the tubular member and cup-shaped member can be effectively prevented. 
   Prior to the drawing process, lubricating oil is applied to the tubular member and/or cup-shaped member so that the drawing process may be executed without any undesired seizure and/or deformation of the tubular member and/or cup-shaped member. However, the lubricating oil could be trapped between the punch and tubular member, and it could cause deformation in the shape of the tubular member. Such a problem can be avoided if the lubricating oil can be released from the gap between the punch and tubular member during the drawing process. To promote the release of the lubricating oil during a drawing process, an axial lower edge of the tubular member may be provided with at least one notch. 
   The tubular member may be provided with an axial slit extending over an entire axial length thereof so that the cost for preparing the tubular member may be minimized. The slit would not cause any undue increase in the magnetic resistance of the yoke if the permanent magnets are secured to the tubular member (inner yoke) so that the slit is located between the opposing edges of the adjacent permanent magnets. 
   According to a certain aspect of the present invention, the present invention provides a double-walled motor yoke including a cup-shaped outer yoke and a tubular inner yoke nested in and secured to the outer yoke in a coaxial relationship, characterized by that the motor yoke is formed by preparing a cup-shaped member destined to form the outer yoke and having a diameter slightly greater than that of the final outer yoke and a tubular member destined to form the inner yoke, and drawing the cup-shaped member onto the tubular member so as to form the inner yoke and outer yoke that are integrally joined to each other. 
   Thereby, the inner yoke and outer yoke can be joined integrally to each other without requiring any special arrangements, and are not required to be prepared at a very high precision as opposed to the conventional press fitting process. 
   To restrict a relative axial movement of the inner yoke toward the closed end of the outer yoke, the outer yoke may be provided with an annular shoulder in a part thereof adjacent to a closed end thereof, the annular should of the outer yoke engaging a corresponding axial end of the inner yoke. To restrict a relative axial movement of the inner yoke toward the open end of the outer yoke, an axial end of the inner yoke corresponding to an open end of the outer yoke may be configured so as to at least partly dig into an inner circumferential surface of the outer yoke. 
   The inner yoke and outer yoke may be engaged by each other by a projection that is formed in one of the yokes and digs into the other so as to secure the inner yoke to the outer yoke against a relative circumferential movement. 
   An axial end of the inner yoke corresponding to an open end of the outer yoke may be provided with at least one notch so that lubricating oil that may be present on the surface of the tubular member and/or cup-shaped member may be allowed to escape during the drawing process. If lubricating oil is trapped in a gap that may exist between different layers of the material or between the die assembly and the material, it could cause undesired deformation of the finished outer yoke. 
   The tubular member destined to form the inner yoke may be provided with a slit extending axial over an entire length thereof so that the tubular member may be economically prepared by rolling a strip of steel plate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Now the present invention is described in the following with reference to the appended drawings, in which: 
       FIG. 1  is a sectional view of a motor unit embodying the present invention; 
       FIG. 2  is an exploded sectional view showing the cup-shaped member and tubular member in relation with the drawing die and punch; 
       FIG. 3   a  is a longitudinal sectional view of the punch on which the tubular member is fitted; 
       FIG. 3   b  is a longitudinal sectional view of the punch on which the cup-shaped member is fitted over the tubular member; 
       FIG. 4  is a longitudinal sectional view of the punch and drawing die during the drawing process; 
       FIG. 5   a  is a view similar to  FIG. 3   a  showing a punch of a modified embodiment of the present invention; 
       FIG. 5   b  is a view similar to  FIG. 4  showing the punch of the modified embodiment and drawing die during the drawing process; 
       FIG. 6  is an exploded sectional view showing a tubular member of a modified embodiment in relation with the punch; 
       FIG. 7   a  is a side view or an alternate embodiment of the punch partly in section; 
       FIG. 7   b  is a cross sectional view taken along line VIIb-VIIb of  FIG. 7   a;    
       FIG. 8  is a cross section view of an assembly including permanent magnets, inner yoke and outer yoke that is formed by using the punch illustrated in  FIGS. 7   a  and  7   b;    
       FIG. 9   a  is a longitudinal sectional view of a drawing die of an alternate embodiment; 
       FIG. 9   b  is a cross section view of an assembly including permanent magnets, inner yoke and outer yoke that is formed by using the drawing die illustrated in  FIG. 9   a;    
       FIG. 10  is a cross sectional view of a punch of yet another embodiment of the present invention along with the tubular member and cup-shaped member; 
       FIG. 11  is a perspective view of a tubular member of an alternate embodiment of the present invention; 
       FIG. 12  is an enlarged fragmentary sectional view of the punch and drawing die illustrating the mode of operation of the tubular member shown in  FIG. 11 ; 
       FIG. 13  is an enlarged fragmentary sectional view of a part at which the inner yoke and outer yoke are joined to each other as a result of the drawing process by using the tubular member illustrated in  FIG. 11 ; and 
       FIG. 14  is a longitudinal sectional view of a conventional motor unit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a sectional view of a motor unit embodying the present invention. The illustrated motor unit may be applied to an automotive wiper unit among other possibilities, and integrally incorporates an electric motor  1  and a reduction gear unit  2 . The motor  1  comprises a rotor  1   a  and a casing  1   b  that surrounds the rotor  1   a  and serves as a yoke  1   b . This yoke  1   b  is provided with a double-walled structure that includes a cylindrical cup-shaped outer yoke  3  and a tubular inner yoke  4  provided along the inner circumferential surface of the outer yoke  3 . To the inner circumferential surface of the inner yoke  4  are attached a pair of arcuate permanent magnets  9  in a symmetrical arrangement. 
   The reduction gear unit  2  comprises a casing  2   a  that is provided with an open end configured to be connected to the open end of the outer yoke  3 , and this casing  2   a  includes a cylindrical portion  2   b  that accommodates brushes  1   b  in sliding engagement with a part of the rotor  1   a  of the motor  1 . The brushes  1   b  are secured to the inner surface of the cylindrical portion  2   b  via a mounting portion not shown in the drawing. The open end of the reduction gear unit casing  2   a  is provided with outwardly directed radial flange  2   c , and the open end of the outer yoke  3  is also provided with a similar corresponding radial outer flange  3   a . The two radial flanges  2   c  and  3   a  abut each other and joined to each other by using threaded bolts so that the outer yoke  3  and the casing  2   a  may be securely joined to each other. 
   The rotor  1   a  is supported by a motor shaft  1   c  which is rotatably supported by a first bearing  5   a  provided in a bottom portion of the outer yoke  3  and a second bearing  5   b  provided in a partition wall separating the cylindrical portion  2   b  of the casing  2   a  from the main part of the casing  2   a  accommodating a gear mechanism. An axial end of the motor shaft  1   c  adjacent to the reduction gear unit  2  is formed with a worm that projects into the chamber defined in the casing  2   a  and meshes with a pair of input gears  6   a  each consisting of a worm wheel and forming a part of the gear mechanism. The gear mechanism additionally comprises an output gear  6   b  having a relatively large diameter that meshes with pinions coaxially secured to the input gears  6   a , and the output gear  6   b  is provided with an output shaft  6   b  extending out of the casing  2   a . The outer end of the output shaft  6   b  is connected to a wiper arm via a link mechanism not shown in the drawing. The casing  2   a  is provided with an open side which is closed by a cover not shown in the drawing. 
   The process of forming the inner and outer yokes  3  and  4  of the illustrated motor unit is described in the following. In the following process, the inner and outer yokes  3  and  4  are formed separately, and are joined later. First of all, a cup-shaped member  13  that is to be formed into the outer yoke  3  as shown in  FIG. 2  is prepared by deep drawing a circular piece of sheet metal. The cup-shaped member  13  comprises cylindrical drum portion  3   b  and a bottom portion that includes a bearing support portion  3   c  for supporting the first bearing  5   a . The bottom portion of the cup-shaped member  13  is given with a progressively reduced diameter from the drum portion  3   b  to the bearing support portion  3   c  by forming a pair of reduced diameter portions and intervening tapered portions. The radial outer flange  3   a  is formed around the edge of the open end of the drum portion  3   b  of the cup-shaped member  13 . The cup-shaped member  13  is similar in shape to the finished outer yoke  3  but is somewhat larger in size. 
   Then, a cylindrical or tubular member  14  that is to be formed into the inner yoke  4  is prepared. The tubular member  14  may consist of a seamless tubular member formed by deep drawing a circular piece of sheet metal or may be formed by rolling a strip of sheet metal. In the latter case, the tubular member  13  is provided with a slit  4   a  extending over an entire length thereof in an axial direction as illustrated in  FIG. 2 . The tubular member  14  is similar in shape to the finished inner yoke  4  but is somewhat larger in size. 
   The cup-shaped member  13  and tubular member  14  are placed in a nested relationship in a die assembly including a punch  7  and a drawing die  8  as illustrated in  FIG. 2 . The punch  7  comprises a cylindrical portion  7   a  that has a diameter D 1  to define the inner circumference of the inner yoke  4  and a stepped free end  7   b  configured to form the inner surface of the bottom portion of the outer yoke  3  that includes the reduced diameter portions, intervening tapered portions and bearing support portion  3   c  as mentioned earlier. The drawing die  8  comprises a straight bore portion  8   a  that has a diameter D 2  (=D 1 +2t 1 +2t 2 ; where t 1  and t 2  are the wall thicknesses of the inner and outer yokes  3  and  4 , respectively) to define the outer circumference of the outer yoke  3  and a stepped bottom portion  8   b  that defines the bottom portion of the outer yoke  3  in cooperation with the stepped free end  7   b  of the punch  7 . 
   The drawing process that integrally combines the inner yoke  4  with the outer yoke  3  is now described in the following. First of all, the cylindrical or tubular member  14  is fitted on the cylindrical portion  7   a  of the punch  7  as illustrated in  FIG. 3   a . The punch  7  is provided with a base block  7   g  having a flat upper surface  7   d  and an enlarged diameter portion  7   c  of a certain height provided between the base block  7   g  and the cylindrical portion  7   a  of the punch  7 , and the enlarged diameter portion  7   c  has a diameter which is greater than that of the rest of the cylindrical portion  7   a  but smaller than the inner diameter of the drum portion  3   b  of the finished outer yoke  3  to define an annular shoulder surface that engages the lower edge of the cylindrical or tubular member  14 . Preferably, the outer diameter of the enlarged diameter portion  7   c  is greater than the outer diameter of the cylindrical portion  7   a  by twice the wall thickness of the tubular member  14  so that the axial end of the tubular member  14  fitted on the punch  7  is securely engaged by the annular shoulder defined by the enlarged diameter portion  7   c . This annular shoulder surface supports the tubular member  14  against the pressure applied by the drawing die  8  during the drawing process, and thereby determines the axial position of the tubular member  14 . The upper surface  7   d  of the base block  7   g  supports the radial outer flange  3   a  during the drawing process, and thereby determines the axial position of the cup-shaped member  13 . In other words, the annular shoulder surface of the enlarged diameter portion  7   c  and the upper surface  7   d  of the base block  7   g  jointly determine the relative position between the cup-shaped member  13  and tubular member  14  during the drawing process 
   The inner diameter D 3  of the tubular member  14  may be slightly greater than the outer diameter of the cylindrical portion  7   a , but should be small enough so that the lower edge of the tubular member  14  may be engaged by the annular shoulder surface of the enlarged diameter portion  7   c . Because it suffices if the lower edge of the tubular member  14  is engaged by the annular shoulder surface of the enlarged diameter portion  7   c , the cross section of the tubular member  14  may also be slightly warped, elliptic or otherwise deviated from a true circle, and is therefore not required to be highly precise in shape or dimension. Therefore, the cost for preparing the tubular member  14  can be minimized as opposed to the prior art that requires a high precision in the dimension and shape of the cross section of the inner yoke for the required press fitting process. 
   Then, as illustrated in  FIG. 3   b , the cup-shaped member  13  is fitted on the punch  7  over the tubular member  14 . The radial outer flange  3   b  is placed on the upper surface  7   d  of the base block  7   g  as mentioned earlier. The inner diameter D 4  of the cylindrical drum portion  3   b  of the cup-shaped member  13  is selected in such a manner that there is a small gap between the cylindrical portion  7   a  of the punch  7  and the inner surface of the cup-shaped member  13  when it is fitted on the punch  7 . A similar gap may exist between the bottom portion of the cup-shaped member  13  and stepped free end  7   b  of the punch  7 . Because the shape of the cup-shaped member  13  is not required to be highly precise, the manufacturing cost can be minimized. When a press fitting process is used, at least the drum portion  3   b  is required to be prepared at a fairly high precision, and this increases the manufacturing cost. On the other hand, according to the illustrated embodiment, because the final shape of the outer yoke  3  is formed by a drawing process, the cup-shaped member  13  is not required to be prepared at a high precision in the shape and dimensions. 
   The drawing process is executed by lowering the drawing die  8  relative to the punch  7  and effecting a cooperation between the drawing die  8  and punch  7  as illustrated in  FIG. 4 . Thereby, the cup-shaped member  13  is wrapped tightly around the tubular member  14  so that the cup-shaped member  13  is shaped into its final shape and integrally joined with the tubular member  14 . The radial outer flange  3   a  of the cup-shaped member  13  is somewhat larger than the radial outer flange  3   a  of the finished outer yoke  3 , and is therefore cut into the final size following the drawing process. 
   Thus, as a result of the drawing process, a double-walled yoke  1   b  joining the outer yoke  3  and inner yoke  4  in a coaxial relationship is produced. Thereby, the part of the yoke  1   b  that supports the permanent magnets  9  is given with an adequate wall thickness to provide a necessary magnetic path without unnecessarily increasing the wall thickness of the remaining part of the yoke  1   b . In particular, in the case of a yoke provided with a radial outer flange such as that  3   a  of the illustrated embodiment, if the yoke is formed simply drawing a plate member having a necessary thickness, the radial outer flange  3   a  would be given with a thickness as large as that of the drum portion  3   b . However, in the illustrated embodiment, the outer yoke  3  can be made of a relatively thin plate member, and the radial outer flange  3   a  does not need to be any thicker than the remaining part of the outer yoke  3 . Also, the final drawing step is executed by fitting the cup-shaped member  13  and tubular member  14  formed nearly to their final forms onto the punch  7 , there would be no risk of seizure or deformation of the inner yoke  4  as opposed to the conventional press fitting process. 
   The inner yoke  4  is provided with a longitudinal slit  4   a , but it would not adversely affect the magnetic efficiency of the motor  1  particularly if the permanent magnets  9  are placed in a such a manner that the slit  4   a  is located in a gap between opposing side edges of the permanent magnets  9 . It is also possible to prepare the tubular member  14  in such a manner that the axial slit  4   a  is almost invisible in the finally formed inner yoke  4 . 
   The enlarged diameter portion  7   c  of the punch  7  defines the annular shoulder surface to limit the axial (thrust) movement of the inner yoke  4  during the drawing process as mentioned earlier. The drawing process firmly joins the outer yoke  3  and inner yoke  4  to each other, but there are also other means for ensuring a firm attachment between them as described hereinafter. 
   The cup-shaped member  13  of the illustrated embodiment is formed with a shoulder portion  3   d  between the bottom portion and drum portion  3   b  ( FIG. 3   b ). When the cup-shaped member  13  is drawn into the final shape of the outer yoke  3 , the shoulder portion  3   d  engages the corresponding (upper) axial edge of the tubular member  14 . Therefore, during the drawing process, the tubular member  14  is axially compressed firmly between the shoulder portion  3   c  and annular shoulder surface of the enlarged diameter portion of the punch  7  while the outer circumference is constricted by the straight bore portion  8   a  of the drawing die  8  as illustrated in  FIG. 4 . As a result, the inner yoke  4  becomes firmly attached to the outer yoke  3  once the drawing process is completed. 
   The axial position of the shoulder portion  3   d  is determined such that it substantially coincides with the upper edge of the tubular member  14  when the tubular member  14  is supported by the annular shoulder surface of the enlarged diameter portion  7   c  of the punch  7  and the cup-shaped member  13  is fitted on the punch  7  over the tubular member  14  until the radial outer flange  3   a  abuts the flat upper surface  7   d  of the base block  7   g.    
   Optionally, the annular shoulder surface defined by the enlarged diameter portion  7   c  of the punch  7  may consist of a tapered surface  7   e  as illustrated in  FIG. 5   a . In this case, during the drawing process, the tapered annular shoulder surface  7   e  causes the corresponding axial edge of the inner yoke  4  to flare out as illustrated in  FIG. 5   b . The flared axial end of the inner yoke  4  at the same time digs into the outer yoke  3 , and this firmly joins the inner yoke  4  with the outer yoke  3  against any relative axial movement. 
   In the embodiment illustrated in  FIG. 6 , the corresponding lower axial edge of the tubular member  14  is formed as a flared portion  4   b  from the beginning. In this case, the annular shoulder surface defined by the enlarged diameter portion  7   c  of the punch  7  may be provided with a flat surface perpendicular to the axial line or alternatively a tapered surface as was the case in the previous embodiment. In either case, the flared portion  4   b  digs into the outer yoke  3  similarly to the previous embodiment, and this also firmly joins the inner yoke  4  with the outer yoke  3  against any relative axial movement. 
   The drawing processes of the foregoing embodiments are capable of joining the inner yoke  4  with the outer yoke  3  against a relative circumferential movement as well as against a relative axial movement. However, to positively prevent any relative circumferential rotation between the inner yoke  4  and outer yoke  3 , the outer circumferential surface of the cylindrical portion  7   a  of the punch  7  may be provided with a pair of axial ridges  7   f  at a diagonally opposed positions as illustrated in  FIGS. 7   a  and  7   b . The lower end of each axial ridge  7   f  may merge with the enlarged diameter portion  7   c  of the punch  7 . For the ease of fitting the tubular member  4  on the punch  7 , the inner diameter of the tubular member  14  may be slightly larger than the general profile of the cylindrical portion  7   a  including the axial ridges  7   f , and the enlarged diameter portion  7   c  may be slightly greater than the general profile of the cylindrical portion  7   a . However, owing to the presence of the axial slit  4   a , even when the general profile of the cylindrical portion  7   a  including the axial ridges  7   f  is equal to or greater than the inner diameter of the tubular member  14 , the tubular member  14  may be fitted on the punch  7  by spreading the tubular member  14  wider. 
   When a drawing process is executed by using this punch  7 , as shown in  FIG. 8 , the tubular member  14  is pushed out by the axial ridges  7   f , and is formed with corresponding axial ridges  4   c  which in turn dig into the inner circumferential surface of the outer yoke  3  as shown in  FIG. 8  so that the inner yoke  4  can be firmly secured to the outer yoke  3  against a relative rotation between them. 
   In the embodiment illustrated in  FIG. 9   a , axial ridges  8   c  are formed on the inner circumferential surface of the drawing die  8  instead on the outer circumferential surface of the punch  7 . When a drawing process is executed by using this drawing die  8 , the outer yoke  3  is pushed inward by the axial ridges  8   c  and is thereby formed with corresponding axial ridges  3   e  which in turn dig into the outer circumferential surface of the inner yoke  4  so that the inner yoke  4  can be firmly secured to the outer yoke  3  against a relative rotation between them. 
   The axial ridges formed in the punch and drawing die in the preceding embodiments can be freely modified without departing from the spirit of the present invention. Each axial ridge may have a smaller length or smaller height as required. A reduced height or reduced length means a reduced resistance during the drawing process. The number of axial ridges are not limited to two, but may be one, three or more. 
   There are other possible means for preventing the relative rotation between the inner yoke  4  and outer yoke  3 . In the embodiment illustrated in  FIG. 10 , the cylindrical part  17   a  of the punch  17  is configured in such a manner that the part of the tubular member  14  carrying the permanent magnets  17  is truly circular (having a radius R) in shape but other part of the tubular member  14  bulges slightly outward from the true circle (having a radius R) defining the part carrying the permanent magnets  17 . The corresponding straight bore portion  8   a  of the drawing die  8  is configured to conform to the profile of the punch  17  although it is not shown in the drawings. Typically, the inner profile of the straight bore portion  8   a  may be greater than the outer profile of the punch  17  by the combined thicknesses of the inner and outer yokes  4  and  3 . 
   When a drawing process is executed by using the punch  17  and drawing die  8  described above, the inner yoke  4  is given with a pair of radially outward bulging portions  4   d  and the outer yoke  3  is given with a corresponding pair of radially outward bulging portions  3   f  by conforming to the outer profile of the punch  17 , and the engagement between the mutually engaging bulging portions  4   d  and  3   f  effectively prevents a relative rotation between the inner and outer yokes  4  and  3 . 
   Lubricating oil is present between the cylindrical portion  7   a  ( 17   a ) of the punch  7  ( 17 ) and the tubular member  14  during the drawing process in any of the foregoing embodiments. If such lubricating oil remains trapped in any part of the yoke  1   b  upon completion of the drawing process, it could adversely affect the final configuration of the inner circumferential surface of the inner yoke  4 . According to the present invention, the lower axial edge of the tubular member  14  may be optionally provided with a plurality of notches  14   a  at a regular interval as illustrated in  FIG. 11 . The tubular member  14  may be formed by stamping a continuous strip of sheet metal into a plurality of short strips of sheet metal and rolling each short strip of sheet metal. Such notches  14   a  can be formed at the time of stamping the sheet metal without substantially increasing the manufacturing cost. 
   Referring to  FIG. 12 , during the drawing process using such a tubular member  14 , as the drawing die  8  moves downward in the direction indicated by arrow A, the cup-shaped member  13  and tubular member  14  are constricted radially inward so that the lubricating oil that may be present between the punch  7  and cylindrical member  14  is squeezed downward as indicated by arrow B. The axial lower end of the tubular member  14  may abut the annular shoulder surface of the enlarged diameter portion  7   c  of the punch  7  and may block the release of the lubricating oil if no countermeasure is taken, but in the illustrated embodiment, the lubricating oil is allowed to be released from the notches  14   a  as indicated by arrow C. Thereby, no lubricating oil is trapped between the punch  7  and the tubular member  14  so that the inner circumferential surface of the inner yoke  4  can be formed in a highly accurate manner without being affected by the presence of lubricating oil. 
   The illustrated tubular member  14  was provided with four of such notches  14   a , but this number is quite arbitrary. If the tubular member  14  is provided with an axial slit  4   a , such a slit may help the release of lubricating oil and it may be enough to form a single notch  14   a  at a diametrically opposite position to the axial slit  4   a.    
   When a drawing process is executed by using a tubular member  14  that is formed with such notches  14   a , the material of the outer yoke  3  may be squeezed into the recesses defined by the notches  14   a . The resulting engagement between the inner yoke  4  and outer yoke  3  is effective in preventing not only the relative rotation but also the relative axial movement between the inner yoke  4  and outer yoke  3 . According to such an embodiment, the manufacturing arrangement can be simplified and manufacturing cost can be minimized by eliminating the need for ridges  7   f  and  8   c  and other features to be provided to the punch  7  and drawing die  8 . 
   Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. 
   The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application are incorporated in this application by reference.