Patent Publication Number: US-2021178712-A1

Title: Method for manufacturing worm wheel

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a worm wheel. 
     BACKGROUND ART 
     An electric power steering device is known which transmits a rotational force of an electric motor to a rack shaft via a worm shaft and a worm wheel. The worm wheel includes a plurality of teeth to be meshed with a tooth of the worm shaft. Each of the teeth of the worm wheel has a first tooth surface to be pushed by the tooth of the worm shaft when the worm shaft rotates in a predetermined direction and a second tooth surface to be pushed by the tooth of the worm shaft when the worm shaft rotates in a reverse direction. 
     JP2013-160334A discloses a worm wheel in which parts of first tooth surfaces and parts of second tooth surfaces are in the form of concave curved surfaces. Specifically, a substantially half area of the tooth surface is formed as a concave curved surface part and the other area is formed as a helical surface part. The concave curved surface part is formed so as to be more away from a tooth trace from a center toward an end of the tooth, and the helical surface part is formed in parallel to the tooth trace. 
     Further, JP2013-160334A discloses a method for manufacturing a worm wheel using a molding mold dividable into first molds and second molds. The first mold has a first molding surface for molding a first tooth surface and the second mold has a second molding surface for molding a second tooth surface. The first and second molds are so combined with each other that the first molding surfaces and the second molding surfaces face each other while being spaced apart and first dividing surfaces of the first molds opposite to the first molding surfaces are in contact with second dividing surfaces of the second molds opposite to the second molding surfaces. Cavities for molding teeth of the worm wheel are defined by the first and second molding surfaces. The molded worm wheel is removed from the first and second molds by separating the first and second molds from each other along a tooth trace direction. 
     SUMMARY OF INVENTION 
     If a worm wheel is meshed with a worm shaft, contact surface pressures are produced on the teeth of the worm wheel. These contact surface pressures are reduced by forming curved surface parts of first and second tooth surfaces into shapes corresponding to a tooth of the worm shaft and increasing contact areas of the tooth of the worm shaft and the teeth of the worm wheel. To further reduce the contact surface pressures, it is desired to make the curved surface parts larger. 
     However, in the method disclosed in JP2013-160334A, the first and second molds are separated from each other in the tooth trace direction in removing the molded worm wheel from the first and second molds. To separate the first and second molds from each other in the tooth trace direction, the first and second dividing surfaces need to be formed in parallel to the tooth trace direction. Thus, the curved surface parts of the first and second tooth surfaces cannot be made larger. 
     If the curved surface parts of the first and second tooth surfaces are made larger, the first molding surface of the first mold and the second molding surface of the second mold are not only formed so as to be more away from the tooth trace from the center toward one end of the tooth, but also formed so as to be more away from the tooth trace from the center toward the other end of the tooth. Since the first and second dividing surfaces are formed along the tooth trace, central parts of the first and second molds are thicker than both end parts. Thus, the first and second molds cannot be withdrawn from the molded worm wheel. 
     As just described, the curved surface parts of the first and second tooth surfaces cannot be made larger by the method disclosed in JP 2013-160334A. 
     The present invention aims to make curved surface parts of tooth surfaces of a worm wheel larger while forming the curved surface parts into shapes corresponding to a tooth of a worm shaft. 
     According to one aspect of the present invention, a method for manufacturing a worm wheel including a plurality of wheel teeth to be meshed with a shaft tooth of a worm shaft includes a step of combining first molds for molding first tooth surfaces of the wheel teeth to be pushed by the shaft tooth when the worm shaft rotates in a predetermined direction and second molds for molding second tooth surfaces of the wheel teeth to be pushed by the shaft tooth when the worm shaft rotates in a reverse direction to form cavities defined by the first molds and the second molds; a step of injecting molten material into the cavities to mold the wheel teeth; and a step of separating the first molds and the second molds from each other in a direction oblique to tooth traces of the wheel teeth. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a worm wheel manufactured by a manufacturing method according to an embodiment of the present invention, showing a state where the worm wheel is meshed with a worm shaft; 
         FIG. 2  is a perspective view showing a part of the worm wheel in  FIG. 1 ; 
         FIG. 3  is a partial development view of a pitch cylindrical surface of the worm wheel in  FIG. 2 ; 
         FIG. 4  is a view for explaining a manufacturing method according to the embodiment of the present invention, showing a state where first molds and second molds are combined; 
         FIG. 5  is a view for explaining the manufacturing method according to the embodiment of the present invention, showing a state where molten resin is injected in cavities; 
         FIG. 6  is a view for explaining the manufacturing method according to the embodiment of the present invention, showing a state where the first and second molds are separated from each other; 
         FIG. 7  is a view for explaining a manufacturing method according to a comparative example; and 
         FIG. 8  is a view for explaining a manufacturing method according to a modification of the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a method for manufacturing a worm wheel  10  according to an embodiment of the present invention is described with reference to the drawings. 
     As shown in  FIG. 1 , the worm wheel  10  is used in a worm speed reducer  100 . In the worm speed reducer  100 , the worm wheel  10  is meshed with a worm shaft  20 . The worm shaft  20  is coupled to a rotary shaft (not shown) of a motor, and the worm wheel  10  is coupled, for example, to a pinion (not shown). A rotational force output from the motor is transmitted to a rack shaft for turning wheels through the worm wheel  10  and the pinion and assists the steering of the wheels by a driver. 
     When the worm shaft  20  rotates according to the rotation of the rotary shaft of the motor, the worm wheel  10  rotates. At this time, the rotation of the worm shaft  20  is decelerated and transmitted to the worm wheel  10 . The pinion rotates according to the rotation of the worm wheel  10 . In this way, the worm speed reducer  100  decelerates and transmits the rotation of the motor to the pinion via the worm shaft  20  and the worm wheel  10 . 
     The worm shaft  20  includes a cylindrical shaft body  21  and a shaft tooth  22  spirally formed on an outer periphery of the shaft body  21 . The worm wheel  10  includes a ring-shaped wheel body  11  and a plurality of wheel teeth  12  projecting from an outer periphery of the wheel body  11 . A pitch of the wheel teeth  12  is substantially equal to a pitch of the shaft tooth  22  and the wheel teeth  12  are meshed with the shaft tooth  22 . 
     The wheel body  11  and the wheel teeth  12  are integrally molded from a resin material. A sleeve  17  made of metal is provided on an inner periphery of the wheel body  11 . 
     The wheel teeth  12  are formed on an outer periphery of the worm wheel  10 . Further, the wheel teeth  12  have first and second tooth surfaces  13 ,  14  intersecting with a circumferential direction of the worm wheel  10 . The second tooth surface  14  is located on a side of the wheel teeth  12  opposite to the first tooth surface  13 . 
     When the worm shaft  20  rotates in a predetermined direction (direction D 1  shown in  FIG. 1 ), the first tooth surfaces  13  of the wheel teeth  12  are pushed by the shaft tooth  22 . As a result, the worm wheel  10  rotates in a direction D 3  shown in  FIG. 1 . When the worm shaft  20  rotates in a reverse direction (direction D 2  shown in  FIG. 1 ), the second tooth surfaces  14  of the wheel teeth  12  are pushed by the shaft tooth.  22 . As a result, the worm wheel  10  rotates in a direction D 4  shown in  FIG. 1 . 
       FIG. 2  is a perspective view showing a part of the worm wheel  10 .  FIG. 3  is a partial development view of a pitch cylindrical surface of the worm wheel  10 . 
     As shown in  FIGS. 2 and 3 , the wheel teeth  12  further have first and second end surfaces  15 ,  16  perpendicular to a direction along a rotation axis R of the worm wheel  10  (hereinafter, merely referred to a “rotation axis direction”). The second end surface  16  is located on a side of the wheel teeth  12  opposite to the first end surface  15 . 
     As shown in  FIG. 3 , the first tooth surface  13  has a flat surface part  13   a  formed into a flat surface from the first end surface  15  along the rotation axis R of the worm wheel  10  and a curved surface part  13   b  formed into a curved surface from the flat surface part  13   a  to the second end surface  16 . The curved surface part  13   b  is formed by being curved in the direction D 4  from the flat surface part  13   a  in correspondence with the shaft tooth  22  (see  FIG. 1 ) of the worm shaft  20 . 
     Similarly to the first tooth surface  13 , the second tooth surface  14  has a flat surface part  14   a  and a curved surface part  14   b.  The flat surface part  14   a  is formed into a flat surface from the second end surface  16  along the rotation axis R of the worm wheel  10 . The curved surface part  14   b  is formed into a curved surface shaped in correspondence with the shaft tooth  22  of the worm shaft  20 . 
     Since the curved surface part  13   b  is formed on the first tooth surface  13  and the curved surface part  14   b  is formed on the second tooth surface  14 , a tooth trace direction of the wheel teeth  12  is inclined with respect to the rotation axis direction. 
     Here, “tooth trace” means a virtual line passing through a center of the first end surface  15  and a center of the second end surface  16  and is represented by a dashed-dotted line denoted by reference sign “T” in  FIG. 3 . “Tooth trace direction” means a direction along the tooth trace T. 
     Since the first tooth surface  13  has the curved surface part  13   b,  a contact area of the first tooth. surface  13  and the shaft tooth  22  when the worm shaft  20  rotates in the direction D 1  can be increased as compared to the case where the entire first tooth surface  13  is formed only by the flat surface part  13   a.  Similarly, since the second tooth surface  14  has the curved surface part  14   b,  a contact area of the second tooth surface  14  and the shaft tooth  22  when the worm shaft  20  rotates in the direction D 2  can be increased as compared to the case where the entire second tooth surface  14  is formed only by the flat surface part  14   a.    
     By increasing the contact areas of the first and second tooth surfaces  3 ,  14  and the shaft tooth  22 , contact surface pressures produced on the first and second tooth surfaces  13 ,  14  can be reduced. Thus, the durability of the wheel teeth  12  can be improved. 
     Next, a mold  30  for molding the wheel teeth  12  of the worm wheel  10  is described with reference to  FIGS. 4 to 6 . The mold  30  is formed so as to be dividable into a first mold block  40  and a second mold block  50 . 
     The first mold block  40  includes a plurality of first molds  43  for molding the first tooth surfaces  13  (see  FIG. 3 ) of the wheel teeth  12 . The first mold  43  projects from a body part  41  of the first mold block  40 . A flat surface part  44   a  and a curved surface part  44   b  are formed on a molding surface  44  of the first mold  43  in correspondence with the first tooth surface  13 . That is, the flat surface part  44   a  is formed into a flat surface along the rotation axis R and the curved surface part  44   b  is formed by being curved in the direction D 4  from the flat surface part  44   a.    
     The second mold block  50  includes a plurality of second molds  53  for molding the second tooth surfaces  14  (see  FIG. 3 ) of the wheel teeth  12 . The second mold  53  projects from a body part  51  of the second mold block  50 . A flat surface part  54   a  and a curved surface part  54   b  are formed on a molding surface  54  of the second mold  53  in correspondence with the second tooth surface  14 . 
     With the first and second mold blocks  40 ,  50  combined, the molding surfaces  44  and the molding surfaces  54  face each other while being spaced apart. Tips of the first molds  43  are in contact with a side surface  52  of the body part  51  of the second mold block  50 . Tips of the second molds  53  are in contact with a side surface  42  of the body part  41  of the first mold block  40 . 
     A cavity  31  for molding the wheel teeth  12  (see  FIG. 3 ) is defined by the molding surfaces  44 ,  54  and the side surfaces  42 ,  52 . That is, the side surface  42  of the body part  41  molds the first end surface  15  of the wheel teeth  12  and the side surface  52  of the body part  51  molds the second end surface  16  of the wheel teeth  12 . 
     Further, with the first and second mold blocks  40 ,  50  combined, surfaces  45  of the first molds  43  opposite to the molding surfaces  44  and surfaces  55  of the second molds  53  opposite to the molding surfaces  54  are in contact with each other. That is, the mold  30  is dividable with the surfaces  45 ,  55  as boundaries. The “surface  45 ” and the “surface  55 ” are also respectively referred to as a “dividing surface  45 ” and a “dividing surface  55 ”. 
     Next, a method for manufacturing the worm wheel  10 , more specifically a method for molding the wheel teeth  12 , is described. 
     First, as shown in  FIG. 4 , the first and second mold blocks  40 ,  50  are combined to form the cavities  31 . Subsequently, as shown in  FIG. 5 , molten resin is injected into the cavities  31  and cooled. The resin is cured into a shape corresponding to the shape of the cavities  31  to mold the wheel teeth  12  made of resin. 
     Subsequently, as shown in  FIG. 6 , the first and second mold blocks  40 ,  50  are separated from each other in the rotation axis direction. As a result, the molded worm wheel  10  is removed from the first and second mold blocks  40 ,  50 . 
     Since the first and second mold blocks  40 ,  50  are separated from each other in the rotation axis direction in the present embodiment, the dividing surfaces  45  of the first mold block  40  and the dividing surfaces  55  of the second mold block  50  are formed along the rotation axis R of the worm wheel  10 . 
     The curved surface part  44   b  of the first mold  43  is formed by being curved in the direction D 4  from the flat surface part  44   a  along the rotation axis R as described above. Since the dividing surface  45  of the first mold  43  is formed along the rotation axis R, the curved surface part  44   b  is formed by being curved so as to approach the dividing surface  45  from the flat surface part  44   a  toward the tip of the first mold  43 . Thus, the first mold  43  is formed so as to become narrower from a base end (end part of the first mold  43  in a separation direction of the first mold  43 ) toward the tip. Thus, the first molds  43  can be withdrawn from the molded worm wheel  10 . 
     Similarly, the curved surface part  54   b  of the second mold  53  is formed by being curved in the direction D 3  from the flat surface part  54   a  along the rotation axis R. Since the dividing surface  55  of the second mold  53  is formed along the rotation axis R, the curved surface part  54   b  is formed by being curved so as to approach the dividing surface  55  from the flat surface part  54   a  toward the tip of the second mold  53 . Thus, the second mold  53  is formed so as to become narrower from a base end (end part of the second mold  53  in a separation direction of the second mold  53 ) toward the tip. Thus, the second molds  53  can be withdrawn from the molded worm wheel  10 . 
     Here, a worm wheel manufacturing method according to a comparative example is described with reference to  FIG. 7 . Also in the comparative example, a first mold block  140  includes a plurality of first molds  143  each having a molding surface  144  and a dividing surface  145 , and a second mold block  150  includes a plurality of second molds  153  each having a molding surface  154  and a dividing surface  155 . A flat surface part  144   a  and a curved surface part  144   b  are formed on the molding surface  144 , and a flat surface part  154   a  and a curved surface part  154   b  are formed on the molding surface  154 . 
     In the comparative example, the first and second mold blocks  140 ,  150  are separated from each other in a tooth trace direction of wheel teeth  112 . In this case, the dividing surface  145  of the first mold  143  is formed along a tooth trace T. Thus, the flat surface part  144   a  of the first mold  143  is formed along the tooth trace T, and the curved surface part  144   b  of the first mold  143  is formed by being curved in the direction D 4  from the flat surface part  144   a.    
     If the flat surface part  144   a  is formed along the rotation axis R as shown by a chain double-dashed line in  FIG. 7  in the comparative example, the flat surface part  144   a  extends more away from the dividing surface  145  from a base end toward the curved surface part  144   b.  Further, if the curved surface part  144   b  is formed by being curved from the flat surface part  144   a,  a part of the curved surface part  144   b  is curved so as to be more away from the dividing surface  145  from the flat surface part  144   a  toward a center of the first mold  143 . Thus, the first mold  143  is formed so as to become thicker from the base end toward the center. Therefore, the first molds  143  cannot be withdrawn from the molded worm wheel. 
     As just described, in the comparative example, the curved surface part  144   b  is formed by being curved from the flat surface part  144   a  since the flat surface part  144   a  is formed along the tooth trace T. That is, in the comparative example, the flat surface part  144   a  cannot be formed obliquely to the tooth trace T, and the curved surface part  144   b  cannot be formed so as to approach the tooth trace T from the flat surface part  144   a  toward the center of the first mold  143 . 
     Since a first tooth surface  113  of the wheel teeth  112  is molded into a shape corresponding to the shape of the molding surface  144  of the first mold  143 , a flat surface part  113   a  of the wheel teeth  112  cannot be formed obliquely to the tooth trace T. Further, a curved surface part  113   b  of the wheel teeth  112  cannot be curved so as to approach the tooth trace T from the flat surface part  113   a  toward a center of the wheel teeth  112 . Thus, the curved surface part  113   b  of the wheel teeth  112  cannot be made larger while being formed into a shape corresponding to the shaft tooth  22  (see  FIG. 1 ) of the worm shaft  20 . 
     Similarly, in the comparative example, the dividing surface  155  of the second mold  153  needs to be formed along the tooth trace T. Thus, a flat surface part  114   a  of the wheel teeth  112  cannot be formed obliquely to the tooth trace T and a curved surface part  114   b  cannot be curved so as to approach the tooth trace T from the flat surface part  114   a  toward the center of the wheel teeth  112 . Thus, the curved surface part  114   b  of the wheel teeth  112  cannot be made larger while being formed into a shape corresponding to the shaft tooth  22  (see  FIG. 1 ) of the worm shaft  20 . 
     Since the first mold block  40  is separated from the second mold block  50  in the rotation axis direction as shown in  FIG. 6  in the present embodiment, the dividing surface  45  of the first mold  43  is formed along the rotation axis R. Thus, the flat surface part  44   a  of the first mold  43  formed along the rotation axis R extends along the dividing surface  45 . Further, the curved surface part  44   b  formed by being curved from the flat surface part  44   a  is curved so as to approach the dividing surface  45  from the flat surface part  44   a  toward the tip of the first mold  43 . Thus, the first mold  43  is formed so as to become narrower from the base end toward the tip. Therefore, the first molds  43  can be withdrawn from the molded worm wheel  10 . 
     As just described, in the present embodiment, the flat surface part  44   a  can be formed along the rotation axis R and the curved surface part  44   b  can be formed by being curved from the flat surface part  44   a.  That is, the flat surface part  44   a  can be formed obliquely to the tooth trace T, and the curved surface part  44   b  can be formed so as to approach the tooth trace T from the flat surface part  44   a  toward the center of the first mold  43 . 
     Since the first tooth surface  13  of the wheel teeth  12  is formed into a shape corresponding to the shape of the molding surface  44  of the first mold  43 , the flat surface part  13   a  of the wheel teeth  12  can be formed obliquely to the tooth trace T. Further, the curved surface part  13   b  of the wheel teeth  12  can be curved so as to approach the tooth trace T from the flat surface part  13   a  toward the center of the wheel teeth  12 . Thus, the curved surface part  13   b  of the wheel teeth  12  can be made larger while being formed into a shape corresponding to the shaft tooth  22  (see  FIG. 1 ) of the worm shaft  20 . 
     Similarly, the dividing surface  55  of the second mold  53  is formed along the rotation axis R. Thus, the flat surface part  14   a  of the wheel teeth  12  can be formed obliquely to the tooth trace T, and the curved surface part  14   b  can be curved so as to approach the tooth trace T from the flat surface part  14   a  toward the center of the wheel teeth  12 . Thus, the curved surface part  14   b  of the wheel teeth  12  can be made larger while being formed into a shape corresponding to the shaft tooth  22  (see  FIG. 1 ) of the worm shaft  20 . 
     Further, since the first mold  43  is separated from the second mold  53  in the rotation axis direction in the present embodiment, the dividing surface  45  of the first mold  43  is formed along the rotation axis R. Thus, the molding surface  44  can be formed by being curved more away from the tooth trace T from the center toward the base end until a tangent to the molding surface  44  becomes parallel to the rotation axis R. Similarly, the molding surface  54  can be formed by being curved more away from the tooth trace T from the center toward the base end until a tangent to the molding surface  54  becomes parallel to the rotation axis R. Thus, the curved surface parts  13   b,    14   b  of the wheel teeth  12  can be made larger while being formed into shapes corresponding to the shaft tooth  22 . 
     Although the flat surface part  13   a  and the curved surface part  13   b  are formed on the first tooth surface  13  in the present embodiment, the entire first tooth surface  13  may be formed into a curved surface. Similarly, the entire second tooth surface  14  may be formed into a curved surface. That is, the entire first tooth surface  13  may be formed into a curved surface using the first mold  43  and the entire second tooth surface  14  may be formed into a curved surface using the second mold  53 . 
     Since the entire first tooth surface  13  may be formed into a curved surface using the first mold  43  and the entire second tooth surface  14  may be formed into a curved surface using the second mold  53 , the wheel teeth  12  formed by molding need not be processed in forming the entire first and second tooth surfaces  13 ,  14  into curved surfaces. Thus, the curved surface parts  13   b,    14   b  of the first and second tooth surfaces  13 ,  14  can be made larger with fewer man-hours. 
     The separation of the first and second mold blocks  40 ,  50  is not limited to the one along the rotation axis direction, and the first and second mold blocks  40 ,  50  may be separated in a direction oblique to the tooth traces T of the wheel teeth  12 . Also in this case, the dividing surfaces  45 ,  55  of the first and second mold blocks  40 ,  50  are formed obliquely to the tooth traces T. Thus, the molding surfaces  44 ,  54  of the first and second molds  43 ,  53  can be formed by being curved more away from the tooth traces T from the centers toward the base ends. Therefore, the curved surface parts  13   b,    14   b  of the wheel teeth  12  can be made larger while being formed into shapes corresponding to the shaft tooth  22 . 
     The separation direction of the first and second mold blocks  40 ,  50  is preferably inclined toward the rotation axis R with respect to the tooth traces T. In this case, the dividing surfaces  45 ,  55  are inclined toward the rotation axis R with respect to the tooth traces T. Thus, the molding surfaces  44 ,  54  of the first and second molds  43 ,  53  can be formed by being curved more away from the tooth traces T from the centers toward the vicinities of the base ends. Therefore, the curved surface parts  13   b,    14   b  can be made larger while being formed into shapes corresponding to the shaft tooth  22 . 
       FIG. 8  is a view for explaining a manufacturing method according to a modification of the present embodiment, showing wheel teeth  212  manufactured by this manufacturing method and a mold  230  used in this manufacturing method. Cavities  231  for molding the wheel teeth  212  (see  FIG. 3 ) are defined by a first mold block  240  and a second mold block  250 . 
     Dividing surfaces  245  of the first mold block  240  are inclined from tooth traces T toward the rotation axis R and beyond the rotation axis R. Similarly, dividing surfaces  255  of the second mold block  250  are inclined from the tooth traces T toward the rotation axis R and beyond the rotation axis R. In the mold  230 , the first and second mold blocks  240 ,  250  are separated along the dividing surfaces  245 ,  255 , i.e. in a direction inclined from the tooth traces T toward the rotation axis R and beyond the rotation axis R after the wheel teeth  212  are molded. 
     In the manufacturing method according to the modification, molding surfaces  244 ,  254  on base ends of first and second molds  243 ,  253  can be formed into curved surfaces until the molding surfaces  244 ,  245  become parallel to the dividing surfaces  245 ,  255 . Thus, curved surface parts  213   b,    214   b  of first and second tooth surfaces  213 ,  214  of the wheel teeth  212  can be enlarged, and a contact area of the wheel teeth  212  and the shaft tooth  22  (see  FIG. 1 ) can be made larger. 
     The configuration, functions and effects of the embodiment of the present invention are summarized below. 
     The present embodiment relates to the method for manufacturing the worm wheel  10  including the plurality of wheel teeth  12 ,  212  to be meshed with the shaft tooth  22  of the worm shaft  20 . The method for manufacturing the worm wheel  10  includes a step of combining the first molds  43 ,  243  for molding the first tooth surfaces  13 ,  213  of the wheel teeth  12 ,  212  to be pushed by the shaft tooth  22  when the worm shaft  20  rotates in the direction D 1  and the second molds  53 ,  253  for molding the second tooth surfaces  14 ,  214  of the wheel teeth  12 ,  212  to be pushed by the shaft tooth  22  when the worm shaft  20  rotates in the direction D 2  to form the cavities  31 ,  231  defined by the first molds  43 ,  243  and the second molds  53 ,  354 , a step of injecting the molten resin into the cavities  31 ,  231  to mold the wheel teeth  12 ,  212 , and a step of separating the first molds  43 ,  243  and the second molds  53 ,  253  from each other in. the direction. oblique to the tooth traces T of the wheel teeth  12 ,  212 . 
     Since the first molds  43 ,  243  and the second molds  53 ,  253  are separated from each other in the direction oblique to the tooth traces T in this configuration, the dividing surfaces  45 ,  245 ,  55 ,  255  of the first molds  43 ,  243  and the second molds  53 ,  253  are formed obliquely to the tooth traces T. Thus, the molding surfaces  44 ,  244 ,  54 ,  254  of the first molds  43 ,  243  and the second molds  53 ,  253  can be formed by being curved more away from the tooth traces T from the centers toward the base ends. Therefore, the curved surface parts  13   b,    213   b,    14   b,    214   b  of the wheel teeth  12 ,  212  can be made larger while being formed into shapes corresponding to the shaft tooth  22 . 
     Further, in the method for manufacturing the worm wheel  10 , the first molds  43 ,  243  and the second molds  53 ,  253  are separated from each other in the direction inclined toward the rotation axis R with respect to the tooth traces T of the wheel teeth  12 ,  212 . 
     Since the first molds  43 ,  243  and the second molds  53 ,  253  are separated from each other in the direction inclined toward the rotation axis R with respect to the tooth traces T of the wheel teeth  12 ,  212  in this configuration, the dividing surfaces  45 ,  245 ,  55 ,  255  of the first molds  43 ,  243  and the second molds  53 ,  253  are formed by being inclined toward the rotation axis R with respect to the tooth traces T. Thus, the molding surfaces  44 ,  244 ,  54 ,  254  of the first molds  43 ,  243  and the second molds  53 ,  253  can be formed by being curved more away from the tooth traces T from the centers toward the vicinities of the base ends. Therefore, the curved surface parts  13   b,    213   b,    14   b,    214   b  of the wheel teeth  12 ,  212  can be made larger while being formed into shapes corresponding to the shaft tooth  22 . 
     Further, in the method for manufacturing the worm wheel  10 , the first molds  43  and the second molds  53  are separated from each other in the direction along the rotation axis R of the worm wheel  10 . 
     Since the first molds  43  and the second molds  53  are separated from each other in the rotation axis direction, the dividing surfaces  45 ,  55  of the first molds  43  and the second molds  53  are formed along the rotation axis R. Thus, the molding surfaces  44 ,  54  can be formed by being curved more away from the tooth traces T from the centers toward the base ends until the tangents to the molding surfaces  44 ,  54  become parallel to the rotation axis R. Therefore, the curved surface parts  13   b,    14   b  of the wheel teeth  12  can be made larger while being formed into shapes corresponding to the shaft tooth  22 . 
     Further, in the method for manufacturing the worm wheel  10 , the entire first tooth surfaces  13 ,  213  are formed into curved surfaces using the first molds  43 ,  243  and the entire second tooth surfaces  14 ,  214  are formed into curved surfaces using the second molds  53 ,  253 . 
     Since the entire first tooth surfaces  13 ,  213  are formed into curved surfaces using the first molds  43 ,  243  and the entire second tooth surfaces  14 ,  214  are formed into curved surfaces using the second molds  53 ,  253 , the wheel teeth  12 ,  212  formed by molding need not be processed in forming the entire first and second tooth surfaces  13 ,  213 ,  14 ,  214  into curved surfaces. Therefore, the curved surface parts  13   b,    213   b,    14   b,    214   b  of the wheel teeth  12 ,  212  can be made larger with fewer man-hours. 
     The embodiments of the present invention described above are merely illustration of some application examples of the present invention and not of the nature to limit the technical scope of the present invention to the specific constructions of the above embodiments. 
     For example, in the above embodiment, the wheel teeth  12 ,  212  are made of resin. Instead of resin, the wheel teeth  12 ,  212  may be formed from a meltable material. 
     The present application claims a priority based on Japanese Patent Application No. 2016-63851 filed with the Japan Patent Office on Mar. 28, 2016, all the contents of which are hereby incorporated by reference.