Patent Publication Number: US-2019168351-A1

Title: Method for manufacturing ring and ring polishing apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2017/035127, filed Sep. 28, 2017, claiming priority based on Japanese Patent Application No. 2016-193487, filed Sep. 30, 2016. 
    
    
     TECHNICAL FIELD 
     Aspects of the present disclosure relate to methods for manufacturing a ring and ring polishing apparatuses. 
     BACKGROUND ART 
     Conventionally, methods using barrel polishing are known as methods for processing the lateral ends of a ring in a belt-type continuously variable transmission. In barrel polishing, auxiliary materials such as abrasive are expensive. Moreover, since the inner and outer peripheral surfaces of a ring are also polished, fine scratches are made on the surfaces, which may adversely affect fatigue strength. An alternative method to barrel polishing has been proposed in which a ring wound around a tension roll and a return roll and under tension is rotated in the circumferential direction, and end face processing rolls (grinding wheels), each having in its peripheral surface a groove with a radius of curvature R satisfying the relationship R≥t/2 for the thickness t of the ring, are pressed against both lateral ends of the ring to process the lateral ends of the ring into a round shape (see, e.g., Patent Document 1). 
     Patent Document 1: Japanese Patent Application Publication No. 2002-248522 (JP 2002-248522 A) 
     SUMMARY 
     In the technique of Patent Document 1, sufficient processing accuracy can be achieved if there is no variation in thickness of rings to be processed. However, there is actually a variation in thickness of the rings. Accordingly, if the grooves in the end face processing rolls (grinding wheels) have a fixed shape (radius of curvature), processing accuracy is reduced. For example, edges remain on the end faces of the ring. 
     It is an aspect of the present disclosure to achieve accurate processing of lateral ends of a ring regardless of a variation in thickness of rings. 
     It is an aspect of the present disclosure to achieve accurate processing of lateral ends of a ring regardless of a variation in thickness of rings. 
     A method for manufacturing a ring according to the present disclosure is a method for manufacturing an endless metal ring for a continuously variable transmission using a transmission belt that is formed by binding a plurality of elements with the ring. In the method, after a tubular body formed by butt welding ends of a strip is cut into a plurality of ring bodies, the ring body being under tension is rotated in a circumferential direction, and a grinding wheel is pressed against a lateral end of the ring body from an outer peripheral side toward an inner peripheral side in a thickness direction of the ring body to remove an edge portion which is on the outer peripheral side of the lateral end of the ring body and has been formed by the cutting, and a grinding wheel is pressed against the lateral end of the ring body from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring body to remove an edge portion which is on the inner peripheral side of the lateral end of the ring body and has been formed by the cutting. 
     In the method for manufacturing a ring according to the present disclosure, after a tubular body formed by butt welding ends of a strip is cut into a plurality of ring bodies, the ring body being under tension is rotated in the circumferential direction, and the grinding wheel is pressed against a lateral end of the ring body from the outer peripheral side toward the inner peripheral side to remove an edge portion which is on the outer peripheral side of the lateral end of the ring body and has been formed by the cutting, and the grinding wheel is pressed against the lateral end of the ring body from the inner peripheral side toward the outer peripheral side to remove an edge portion which is on the inner peripheral side of the lateral end of the ring body and has been formed by the cutting. The outer peripheral side and the inner peripheral side of the lateral end of the ring body are thus separately processed (edge removal). Accordingly, processing accuracy of the lateral end of the ring body can be improved even if there is a variation in thickness of the ring bodies. 
     A first ring polishing apparatus according to the present disclosure is a ring polishing apparatus for polishing into a convex arc shape (round shape) a lateral end of an endless metal ring for a continuously variable transmission using a transmission belt that is formed by binding elements with the ring. The apparatus includes: a rotating unit that has a first roller and a second roller and that rotates the ring wound around the first roller and the second roller and under tension in a circumferential direction; a first polishing unit that has a concave arc-shaped first polishing portion and that presses the first polishing portion against the lateral end of the ring from an outer peripheral side toward an inner peripheral side in a thickness direction of the ring to polish the outer peripheral side of the lateral end of the ring into a round shape; and a second polishing unit that has a concave arc-shaped second polishing portion and that presses the second polishing portion against the lateral end of the ring from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring to polish the inner peripheral side of the lateral end of the ring into a round shape. 
     In the first ring polishing apparatus according to the present disclosure, a polishing unit for polishing a lateral end of the ring into a round shape is divided into the first polishing unit having the concave arc-shaped first polishing portion and the second polishing unit having the concave arc-shaped second polishing portion. The first polishing portion is pressed against the lateral end of the ring from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring to polish the outer peripheral side of the lateral end of the ring into a round shape, and the second polishing portion is pressed against the lateral end of the ring from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring to polish the inner peripheral side of the lateral end of the ring into a round shape. Since the outer peripheral side and the inner peripheral side of the lateral end of the ring are thus separately polished, processing accuracy of the lateral end of the ring can be improved even if there is a variation in thickness of the rings. 
     A second ring polishing apparatus according to the present disclosure is a ring polishing apparatus for polishing into a round shape a lateral end of an endless metal ring for a continuously variable transmission using a transmission belt that is formed by binding elements with the ring. The apparatus includes: a rotating unit that has a first roller and a second roller and that rotates the ring wound around the first roller and the second roller and under tension in a circumferential direction; and a polishing unit that has a concave arc-shaped first polishing portion and a concave arc-shaped second polishing portion which are formed next to each other in a thickness direction of the ring so that the ring is located between the first polishing portion and the second polishing portion, and that presses the first polishing portion against the lateral end of the ring from an outer peripheral side toward an inner peripheral side in the thickness direction of the ring to polish the outer peripheral side of the lateral end of the ring into a round shape, and presses the second polishing portion against the lateral end of the ring from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring to polish the inner peripheral side of the lateral end of the ring into a round shape. 
     In the second ring polishing apparatus according to the present disclosure, a polishing unit for polishing a lateral end of the ring into a round shape is the polishing unit having the concave arc-shaped first polishing portion and the concave arc-shaped second polishing portion which are formed next to each other in the thickness direction of the ring so that the ring set on the rotating unit is located between the first polishing portion and the second polishing portion. The first polishing portion is pressed against the lateral end of the ring from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring to polish the outer peripheral side of the lateral end of the ring into a round shape, and the second polishing portion is pressed against the lateral end of the ring from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring to polish the inner peripheral side of the lateral end of the ring into a round shape. The outer peripheral side and the inner peripheral side of the lateral end of the ring are thus separately polished. Accordingly, processing accuracy of the lateral end of the ring can be improved even if there is a variation in thickness of the rings. Moreover, since the outer peripheral side and the inner peripheral side of the lateral end of the ring can be separately polished by the single polishing unit, processing accuracy of the ring can be improved by using a simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram schematically showing the configuration of a continuously variable transmission  1 . 
         FIG. 2  is a configuration diagram schematically showing the configuration of a transmission belt  10 . 
         FIGS. 3A to 3M  are illustrations showing an example of a manufacturing process of a ring. 
         FIG. 4  is a configuration diagram schematically showing the configuration of a polishing apparatus  30 . 
         FIGS. 5A to 5C  are illustrations showing how the outer peripheral side of an end of a ring body  23  is rounded by using an outer peripheral-side polishing roll  41 . 
         FIGS. 6A to 6C  are illustrations showing how the inner peripheral side of the end of the ring body  23  is rounded by using an inner peripheral-side polishing roll  46 . 
         FIGS. 7A to 7D  are illustrations showing how a lateral end of the ring body  23  is rounded by using a polishing roll  41 B of a comparative example. 
         FIG. 8  is a configuration diagram schematically showing the configuration of a polishing apparatus  130  of another embodiment. 
         FIGS. 9A to 9C  are illustrations showing how a lateral end of a ring body  23  is rounded by using a polishing roll  141 . 
     
    
    
     PREFERRED EMBODIMENTS 
     Modes for carrying out the various aspects of the present disclosure will be described with reference to the accompanying drawings. 
       FIG. 1  is a configuration diagram schematically showing the configuration of a continuously variable transmission  1 . The continuously variable transmission  1  is mounted on a vehicle including a power source such as an engine, and as shown in the figure, includes a primary shaft  2  serving as a drive-side rotary shaft, a primary pulley  3  provided on the primary shaft  2 , a secondary shaft  4  disposed parallel to the primary shaft  2  and serving as a driven-side rotary shaft, a secondary pulley  5  provided on the secondary shaft  4 , and a transmission belt  10  wound around a pulley groove (V-groove) of the primary pulley  3  and a pulley groove (V-groove) of the secondary pulley  5 . The continuously variable transmission  1  changes the groove widths of the primary pulley  3  and the secondary pulley  5  and thus steplessly shifts power of the primary pulley  3  to transmit the shifted power to the secondary pulley  5 . 
       FIG. 2  is a configuration diagram schematically showing the configuration of the transmission belt  10 . As shown in  FIG. 2 , the transmission belt  10  includes a multiple (e.g., several hundreds) elements  11  and a laminated ring  12  and is formed by binding the multiple elements  11  into a ring shape with the laminated ring  12 . For example, the elements  11  are parts punched out from a steel sheet by pressing. The elements  11  are subjected to a clamping force from the pulley (the primary pulley  3 , the secondary pulley  5 ) at their right and left side surfaces, and those elements  11  which are located ahead in the traveling direction of the belt are pushed out by the friction force in a direction tangential to the pulley, whereby power is transmitted. 
     The laminated ring  12  is formed by laminating a plurality of endless metal rings  20  (single rings) having slightly different circumferences from each other in the radial direction. The laminated ring  12  is manufactured by a manufacturing process illustrated in  FIGS. 3A to 3M . 
     The manufacturing process of the ring  20  is comprised of (A) a strip cutting process (see  FIG. 3A ), (B) a bending process (see  FIG. 3B ), (C) a pre-welding cleaning process (see  FIG. 3C ), (D) a welding process (see  FIG. 3D ), (E) a first solution treatment process (annealing process, see  FIG. 3E ), (F) a ring cutting process (see  FIG. 3F ), (G) a rounding process (see  FIG. 3G ), (H) a pre-rolling cleaning process (see  FIG. 3H ), (I) a rolling process (see  FIG. 3I ), (J) a post-rolling cleaning process (see  FIG. 3J ), (K) a second solution treatment process (see  FIG. 3K ), (L) a circumference adjusting process (see  FIG. 3L ), and (M) an aging and nitriding process (see  FIG. 3M ). 
     The strip cutting process (A) is a process of cutting a strip steel (maraging steel) having a predetermined thickness (e.g., 0.4 to 0.5 mm) and wound around a drum in the lateral direction into strips  21  with a predetermined size. The strip cutting process can be performed by using a cutter cutting machine having a cutter edge, a laser cutting machine, etc. The bending process (B) is a process of forming a tubular body  22  by bending the strip  21  into a tubular shape such that the ends of the strip  21  abut on each other. The bending process can be performed by using a roll or a die. 
     The pre-welding cleaning process (C) is a process of degreasing and cleaning the tubular body  22  before welding the abutting portions of the tubular body  22 . The pre-welding cleaning process can be performed by, e.g., shower cleaning, ultrasonic cleaning, etc. The welding process (D) is a process of performing butt welding, namely welding the abutting portions of the tubular body  22 . The welding process can be performed by, e.g., laser welding, plasma welding, etc. The first solution treatment process (annealing process) (E) is a process that is performed in order to level hardness distribution around the weld, which has been changed by the welding process, to improve ductility. 
     The ring cutting process (F) is a process of cutting the tubular body  22  into a plurality of ring bodies  23  with a predetermined width, and this process can be performed by using a cutter cutting machine, a laser cutting machine, etc. The rounding process (G) is a process of rounding lateral ends of the ring body  23  by polishing with a polishing roll, and this process is performed by using a polishing apparatus  30  illustrated in  FIG. 4 . As shown in  FIG. 4 , the polishing apparatus  30  includes: a rotating unit  31  having a drive roller  31   a  and a driven roller  31   b  around which the ring body  23  is wound; an inner peripheral-side backup roller  33  that supports the ring body  23  from the inner peripheral side; an outer peripheral-side polishing unit  40  that faces the inner peripheral-side backup roller  33  and polishes the outer peripheral side of a lateral end (the outer peripheral side of an end) of the ring body  23  into a round shape; an outer peripheral-side backup roller  34  that supports the ring body  23  from the outer peripheral side; and an inner peripheral-side polishing unit  45  that faces the outer peripheral-side backup roller  34  and polishes the inner peripheral side of the lateral end (the inner peripheral side of the end) of the ring body  23  into a round shape. The drive roller  31   a  is pressed against the inner peripheral surface of the ring body  23  in a direction away from the driven roller  31   b . The drive roller  31   a  is driven to rotate, whereby the rotating unit  31  can rotate (turn) the ring body  23  under tension in the circumferential direction. The rotating unit  31  may include a separate tension roller that tensions the ring body  23 . The outer peripheral-side polishing unit  40  includes: an outer peripheral-side polishing roll  41  having an axis of rotation extending in a direction parallel to the thickness direction of the ring body  23  set on the rotating unit  31 ; and a rotating and moving unit  43  capable of rotating and moving the outer peripheral-side polishing roll  41  in the rotational direction of the outer peripheral-side polishing roll  41 , the axial direction of the axis of rotation of the outer peripheral-side polishing roll  41  (forward and backward in the figure), and a direction perpendicular to this axial direction and parallel to the lateral direction of the ring body  23  (upward and downward in the figure). The inner peripheral-side polishing unit  45  includes an outer peripheral-side polishing roll  46  and a rotating and rotating unit  48  which are similar to those of the outer peripheral-side polishing unit  40 . 
       FIGS. 5A to 5C  are illustrations showing how the outer peripheral side of an end of the ring body  23  is rounded by using the outer peripheral-side polishing roll  41 .  FIGS. 6A to 6C  are illustrations showing how the inner peripheral side of the end of the ring body  23  is rounded by using the inner peripheral-side polishing roll  46 . In  FIGS. 5A to 5C  and  FIGS. 6A to 6C , L-shaped grooves  42 ,  47  of the outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46  are shown exaggerated for convenience of description. The outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46  are columnar members and have the L-shaped grooves  42 ,  47  formed in the circumferential direction in the distal end portions of their outer peripheral surfaces along the entire circumference. The bottoms of the L-shaped grooves  42 ,  47  are formed by linear portions  42 b,  47 b extending straight in the axial direction of the outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46 , and corners of the bottoms of the L-shaped grooves  42 ,  47  are formed by concave arc-shaped portions  42   a ,  47   a  having an arc angle θ of about 90 degrees and a radius of curvature r that is about ½ of the thickness t of the ring body  23 . The L-shaped grooves  42 ,  47  have an abrasive grain layer comprised of abrasive grains bonded together. The ring body  23  is rounded by using the outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46  as follows. First, the outer peripheral-side polishing roll  41  is moved in the lateral direction of the ring body  23  (downward in the figure) while being rotated, and the inner peripheral-side polishing roll  47  is moved in the lateral direction of the ring body  23  (downward in the figure) while being rotated. The linear portions  42   b ,  47   b  are thus pressed against the end face of one lateral end of the ring body  23  (see  FIGS. 5A, 6A ). In the case where a laser cutting machine is used in the ring cutting process (F) described above, each laser cut portion (lateral end) of the ring body  23  has a heat-affected zone that has increased hardness due to structural changes caused by heat. In this case, the heat-affected zone of the ring body  23  can be removed by pressing the linear portions  42   b ,  47   b  of the outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46  for the ring body  23  against the end face of the lateral end of the ring body  23  in the rounding process (R). The outer peripheral-side polishing roll  41  is then moved from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring body  23  (to the left in the figure) while being rotated, so that the concave arc-shaped portion  42   a  is pressed against the outer peripheral side of the one lateral end (the outer peripheral side of the end) of the ring body  23 . The outer peripheral side of the end of the ring body  23  is thus formed into a round shape (see  FIGS. 5B, 5C ). At the same time, the inner peripheral-side polishing roll  46  is moved from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring body  23  (to the right in the figure) while being rotated, so that the concave arc-shaped portion  47   a  is pressed against the inner peripheral side of the one lateral end (the inner peripheral side of the end) of the ring body  23 . The inner peripheral side of the end of the ring body  23  is thus formed into a round shape (see  FIGS. 6B, 6C ). The outer peripheral side and the inner peripheral side of the end of the ring body  23  need not necessarily be processed at the same time, but may be processed at different timings. That is, the inner peripheral side of the end of the ring body  23  may be processed after the outer peripheral side of the end is processed, or the outer peripheral side of the end of the ring body  23  may be processed after the inner peripheral side of the end is processed. The rotational direction of the outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46  at the contact point with the ring body  23  may be either the same as or opposite to the turning direction of the ring body  23 . After the one lateral end of the ring body  23  is thus polished, the ring body  23  is removed from the rotating unit  31  and is mounted again on the rotating unit  31  with its one lateral end and the other lateral end switched, and the other lateral end of the ring body  23  is similarly polished. The heat-affected zones are thus removed from both lateral ends of the ring body  23 , and both lateral ends of the ring body  23  are formed into a semicircular shape with almost no edge. The pair of the outer peripheral-side polishing unit  40  and the inner peripheral-side polishing unit  45  may be provided for each of the one lateral end and the other lateral end of the ring body  23  so that each of the one lateral end and the other lateral end of the ring body  23  is polished by a corresponding pair of the outer peripheral-side polishing unit  40  and the inner peripheral-side polishing unit  45 . 
       FIGS. 7A to 7D  are illustrations showing how a lateral end of the ring body  23  is rounded by using a polishing roll  41 B of a comparative example. The polishing roll  41 B of the comparative example has a semicircular groove  42 B formed in the middle of its outer peripheral surface in the axial direction along the entire circumference, and the semicircular groove  42 B has an arc angle θ of about 180 degrees and a radius of curvature r that is about ½ of the thickness t of the ring body  23 . In the comparative example, the polishing roll  41 B is moved in the lateral direction of the ring body  23  while being rotated, so that the groove  42 B of the polishing roll  41 B is pressed against the end face of the lateral end of the ring body  23 . The lateral end of the ring body  23  is thus polished and formed into a round shape. In the case where the ring body  23  is rounded by using the polishing roll  41 B of this modification, the lateral end of the ring body  23  can be formed into a semicircular shape with almost no edge if the thickness t of the ring body  23  satisfies the relationship t=2·r for the radius of curvature r of the groove  42 B. However, there is actually a variation in thickness of the ring bodies  23 . If the thickness t of the ring body  23  is t&lt;2·r, edges may remain in both ends in the thickness direction of the end face of the ring body  23  (see  FIGS. 7A, 7B ). If the thickness t of the ring body  23  is t&gt;2·r, both ends of the end face of the ring body  23  in the thickness direction may be polished by the outer peripheral surface of the polishing roll  41 B other than the groove  42 B, which may cause undercuts (see  FIGS. 7C, 7D ). In the present embodiment, the polishing roll is divided into the outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46 , and the outer peripheral side and the inner peripheral side of each lateral end of the ring body  23  are polished into a round shape by the outer peripheral-side polishing roll  41  and the inner peripheral-side polishing roll  46 . This allows both lateral ends of the ring body  23  to be formed into a semicircular shape with almost no edge regardless of the variation in thickness of the ring bodies  23 . 
     The pre-rolling cleaning process (H) is a process of, before rolling the ring body  23 , removing polishing debris etc. that has stuck to the ring body  23  in the rounding process. The rolling process (I) is a process of rolling the ring body  23  to a required thickness with a rolling roller to produce a ring body  24 . As described above, in the case where a laser cutting machine is used in the ring cutting process (F), the heat-affected zones with high hardness are removed from the ring body  23  by the rounding process (G) after the ring cutting process (F). The ring body  23  can therefore be rolled to a desired thickness by the rolling process (I) without breakage. The post-rolling cleaning process (J) is a process of removing rolling oil etc. that has stuck to the ring body  24  by the rolling. The second solution treatment process (K) is a process of heating the ring body  24  produced by the rolling to recrystallize a metallic structure transformed by the rolling. 
     The circumference adjusting process (L) is a process of finely adjusting the circumferences of a plurality of the ring bodies  24  produced by the rolling so that the ring bodies  24  can be laminated in the radial direction. The aging and nitriding process (M) is a process of aging the ring bodies  24  with the adjusted circumferences and then nitriding the ring bodies  24  to strengthen the surfaces of the ring bodies  24 . 
     According to the embodiment described above, the ring body  23  under tension is rotated in the circumferential direction, and the concave arc-shaped portion  42   a  is pressed against a lateral end of the ring body  23  from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring body  23 , whereby the outer peripheral side of the lateral end of the ring body  23  is polished into a round shape. The concave arc-shaped portion  47   a  is also pressed against the lateral end of the ring body  23  from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring body  23 , whereby the inner peripheral side of the lateral end of the ring body  23  is polished into a round shape. Since the outer peripheral side and the inner peripheral side of the lateral end of the ring body  23  are thus separately polished, processing accuracy of the lateral end of the ring body  23  can further be improved even if there is a variation in thickness of the ring bodies  23 . 
     In the above embodiment, the polishing roll for the rounding process (G) is divided into the outer peripheral-side polishing roll  41  that polishes the outer peripheral side of an end of the ring body  23  and the inner peripheral-side polishing roll  46  that polishes the inner peripheral side of the end of the ring body  23 . However, the outer peripheral side and the inner peripheral side of the end of the ring body  23  may be rounded by using an integrated polishing roll  141 .  FIG. 8  is a configuration diagram schematically showing the configuration of a polishing apparatus  130  of another embodiment. As shown in the figure, the polishing apparatus  130  of the another embodiment includes: a rotating unit  31  having a drive roller  31   a  and a driven roller  31   b ; a polishing unit  140  that rounds the outer peripheral side and the inner peripheral side of an end of the ring body  23 ; inner peripheral-side backup rollers  133   a ,  133   b  that are disposed in front of and behind the polishing unit  140  in a direction in which the ring body  23  is fed and support the ring body  23  from the inner peripheral side; and outer peripheral-side backup rollers  134   a ,  134   b  that are disposed in front of and behind the polishing unit  140  in the direction in which the ring body  23  is fed and support the ring body  23  from the outer peripheral side. The polishing unit  140  includes: a polishing roll  141  having an axis of rotation extending in a direction parallel to the thickness direction of the ring body  23  set on the rotating unit  31 ; and a rotating and moving unit  143  capable of rotating and moving the polishing roll  141  in the rotational direction of the polishing roll  141 , the axial direction of the axis of rotation of the polishing roll  141  (forward and backward in the figure), and a direction perpendicular to this axial direction and parallel to the lateral direction of the ring body  23  (upward and downward in the figure). 
       FIGS. 9A to 9C  are illustrations showing how a lateral end of the ring body  23  is rounded by using the polishing roll  141 . In  FIG. 9 , a groove  142  of the polishing roll  141  is shown exaggerated for convenience of description. The polishing roll  141  is a columnar member and has the groove  142  formed in the circumferential direction in the middle of its outer peripheral surface in the axial direction along the entire circumference. The bottom of the groove  142  is formed by a linear portion  142   b  extending straight in the axial direction of the roll and both corners of the bottom of the groove  142  are formed by concave arc-shaped portions  142   a ,  142   c  having an arc angle θ of about 90 degrees and a radius of curvature r that is about ½ of the thickness t of the ring body  23 . The groove  142  has an abrasive grain layer comprised of abrasive grains bonded together. The ring body  23  is rounded by using the polishing roll  141  as follows. First, the polishing roll  141  is moved in the lateral direction of the ring body  23  (downward in the figure) while being rotated, so that the linear portion  142   b  is pressed against the end face of one lateral end of the ring body  23  (see  FIG. 9A ). The polishing roll  141  is then moved from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring body  23  (to the left in the figure) while being rotated, so that the concave arc-shaped portion  142   a  is pressed against the outer peripheral side of the one lateral end (the outer peripheral side of the end) of the ring body  23 . The outer peripheral side of the end of the ring body  23  is thus formed into a round shape (see  FIG. 9B ). The polishing roll  141  is then moved from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring body  23  (to the right in the figure) while being rotated, so that the concave arc-shaped portion  142   c  is pressed against the inner peripheral side of the lateral end (the inner peripheral side of the end) of the ring body  23 . The inner peripheral side of the end of the ring body  23  is thus formed into a round shape (see  FIG. 9C ). After the one lateral end of the ring body  23  is thus polished, the ring body  23  is removed from the rotating unit  31  and is mounted again on the rotating unit  31  with its one lateral end and the other lateral end switched, and the other lateral end of the ring body  23  is similarly polished. Heat-affected zones are thus removed from both lateral ends of the ring body  23 , and both lateral ends of the ring body  23  are formed into a semicircular shape with almost no edge. The polishing unit  140  may be provided for each of the one lateral end and the other lateral end of the ring body  23  so that each of the one lateral end and the other lateral end of the ring body  23  is polished by a corresponding one of the polishing units  140 . In this another embodiment, the inner peripheral side of the end of the ring body  23  is processed after the outer peripheral side of the end is processed. However, the outer peripheral side of the end of the ring body  23  may be processed after the inner peripheral side of the end is processed. The outer peripheral side and the inner peripheral side of the end of the ring body  23  may be alternately processed little by little by repeatedly reciprocating the polishing roll  141  in the axial direction. 
     As described above, a method for manufacturing a ring according to the present disclosure is a method for manufacturing an endless metal ring ( 23 ) for a continuously variable transmission ( 1 ) using a transmission belt ( 10 ) that is formed by binding a plurality of elements ( 11 ) with the ring ( 12 ,  20 ,  23 ). In the method, after a tubular body ( 22 ) formed by butt welding ends of a strip is cut into a plurality of ring bodies ( 23 ), the ring body ( 23 ) being under tension is rotated in a circumferential direction, and a grinding wheel ( 42   a ,  142   a ) is pressed against a lateral end of the ring body ( 23 ) from an outer peripheral side toward an inner peripheral side in a thickness direction of the ring body ( 23 ) to remove an edge portion which is on the outer peripheral side of the lateral end of the ring body ( 23 ) and has been formed by the cutting, and the grinding wheel ( 47   a ,  142   b ) is pressed against the lateral end of the ring body ( 23 ) from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring body ( 23 ) to remove an edge portion which is on the inner peripheral side of the lateral end of the ring body ( 23 ) and has been formed by the cutting. 
     In the method for manufacturing a ring according to the present disclosure, after a tubular body ( 22 ) formed by butt welding ends of a strip is cut into a plurality of ring bodies ( 23 ), the ring body ( 23 ) being under tension is rotated in the circumferential direction, and the grinding wheel ( 42   a ,  142   a ) is pressed against a lateral end of the ring body ( 23 ) from the outer peripheral side toward the inner peripheral side to remove an edge portion which is on the outer peripheral side of the lateral end of the ring body ( 23 ) and has been formed by the cutting, and the grinding wheel ( 47   a ,  142   b ) is pressed against the lateral end of the ring body ( 23 ) from the inner peripheral side toward the outer peripheral side to remove an edge portion which is on the inner peripheral side of the lateral end of the ring body ( 23 ) and has been formed by the cutting. Accordingly, as compared to the case where a semicircular groove is pressed against a lateral end of a ring body in a lateral direction of the ring body to process edge portions, processing accuracy of the lateral end of the ring body ( 23 ) can further be improved even if there is a variation in thickness of the ring bodies. 
     In the method for manufacturing a ring according to the present disclosure, a part of the grinding wheel ( 42   a ,  142   a ,  47   a ,  142   b ) which removes the edge portion may have a concave arc shape so as to polish the outer peripheral side or the inner peripheral side of the lateral end of the ring body ( 23 ) into a round shape. This allows the edge portion of the ring body which has been formed by the cutting to be processed into a smooth round shape. 
     A first ring polishing apparatus according to the present disclosure is a ring ( 23 ) polishing apparatus ( 30 ) for polishing into a round shape a lateral end of an endless metal ring ( 23 ) for a continuously variable transmission ( 1 ) using a transmission belt ( 10 ) that is formed by binding elements ( 11 ) with the ring ( 12 ,  20 ,  23 ). The apparatus includes: a rotating unit ( 31 ) that has a first roller ( 31   a ) and a second roller ( 31   b ) and that rotates the ring ( 23 ) wound around the first roller ( 31   a ) and the second roller ( 31   b ) and under tension in a circumferential direction; a first polishing unit ( 40 ) that has a concave arc-shaped first polishing portion ( 42   a ) and that presses the first polishing portion ( 42   a ) against the lateral end of the ring ( 23 ) from an outer peripheral side toward an inner peripheral side in a thickness direction of the ring ( 23 ) to polish the outer peripheral side of the lateral end of the ring ( 23 ) into a round shape; and a second polishing unit ( 45 ) that has a concave arc-shaped second polishing portion ( 47   a ) and that presses the second polishing portion ( 47   a ) against the lateral end of the ring ( 23 ) from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring ( 23 ) to polish the inner peripheral side of the lateral end of the ring ( 23 ) into a round shape. 
     In the first ring polishing apparatus according to the present disclosure, a polishing unit for polishing a lateral end of the ring ( 23 ) into a round shape is divided into the first polishing unit ( 40 ) having the concave arc-shaped first polishing portion ( 42   a ) and the second polishing unit ( 45 ) having the concave arc-shaped second polishing portion ( 47   a ). The first polishing portion ( 42   a ) is pressed against the lateral end of the ring ( 23 ) from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring ( 23 ) to polish the outer peripheral side of the lateral end of the ring ( 23 ) into a round shape, and the second polishing portion ( 47   a ) is pressed against the lateral end of the ring ( 23 ) from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring ( 23 ) to polish the inner peripheral side of the lateral end of the ring ( 23 ) into a round shape. The outer peripheral side and the inner peripheral side of the lateral end of the ring ( 23 ) are thus separately polished. Accordingly, processing accuracy of the lateral end of the ring ( 23 ) can further be improved even if there is a variation in thickness of the rings ( 23 ). 
     In the first ring polishing apparatus according to the present disclosure, the first polishing unit ( 40 ) may include: a first polishing roll ( 41 ) having an L-shaped groove ( 42 ) formed in its outer peripheral surface along entire circumference and having an axis of rotation parallel to the thickness direction of the ring ( 23 ) set on the rotating unit ( 31 ), the L-shaped groove ( 42 ) having the concave arc-shaped first polishing portion ( 42   a ) in a corner of a bottom of the groove; and a first rotating and moving unit ( 43 ) that, while rotating the first polishing roll ( 41 ), moves the first polishing roll ( 41 ) in an axial direction so that the first polishing portion ( 42   a ) is pressed against the lateral end of the ring ( 23 ) from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring ( 23 ), and the second polishing unit ( 45 ) may include: a second polishing roll ( 46 ) having an L-shaped groove ( 47 ) formed in its outer peripheral surface along entire circumference and having an axis of rotation parallel to the thickness direction of the ring ( 23 ) set on the rotating unit ( 31 ), the L-shaped groove ( 47 ) having the concave arc-shaped second polishing portion ( 47   a ) in a corner of a bottom of the groove; and a second rotating and moving unit ( 48 ) that, while rotating the second polishing roll ( 46 ), moves the second polishing roll ( 46 ) in an axial direction so that the second polishing portion ( 47   a ) is pressed against the lateral end of the ring ( 23 ) from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring ( 23 ). The outer peripheral side and the inner peripheral side of the lateral end of the ring ( 23 ) can thus be processed at the same time by moving the first polishing roll ( 41 ) in the axial direction by the first rotating and moving unit ( 43 ) and moving the second polishing roll ( 46 ) in the axial direction by the second rotating and moving unit ( 48 ). 
     A second ring polishing apparatus according to the present disclosure is a ring ( 23 ) polishing apparatus ( 130 ) for polishing into a round shape a lateral end of an endless metal ring ( 23 ) for a continuously variable transmission ( 1 ) using a transmission belt ( 10 ) that is formed by binding elements ( 11 ) with the ring ( 12 ,  20 ,  23 ). The apparatus includes: a rotating unit ( 31 ) that has a first roller ( 31   a ) and a second roller ( 31   b ) and that rotates the ring ( 23 ) wound around the first roller ( 31   a ) and the second roller ( 31   b ) and under tension in a circumferential direction; and a polishing unit ( 140 ) that has a concave arc-shaped first polishing portion ( 142   a ) and a concave arc-shaped second polishing portion ( 142   c ) which are formed next to each other in a thickness direction of the ring ( 23 ) so that the ring ( 23 ) set on the rotating unit ( 31 ) is located between the first polishing portion ( 142   a ) and the second polishing portion ( 142   c ), and that presses the first polishing portion ( 142   a ) against the lateral end of the ring ( 23 ) from an outer peripheral side toward an inner peripheral side in the thickness direction of the ring ( 23 ) to polish the outer peripheral side of the lateral end of the ring ( 23 ) into a round shape, and presses the second polishing portion ( 142   c ) against the lateral end of the ring ( 23 ) from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring ( 23 ) to polish the inner peripheral side of the lateral end of the ring ( 23 ) into a round shape. 
     In the second ring polishing apparatus according to the present disclosure, a polishing unit for polishing a lateral end of the ring ( 23 ) into a round shape is the polishing unit ( 140 ) having the concave arc-shaped first polishing portion ( 142   a ) and the concave arc-shaped second polishing portion ( 142   c ) which are formed next to each other in the thickness direction of the ring ( 23 ) so that the ring ( 23 ) set on the rotating unit ( 31 ) is located between the first polishing portion ( 142   a ) and the second polishing portion ( 142   c ). The first polishing portion ( 142   a ) is pressed against the lateral end of the ring ( 23 ) from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring ( 23 ) to polish the outer peripheral side of the lateral end of the ring ( 23 ) into a round shape, and the second polishing portion ( 142   c ) is pressed against the lateral end of the ring ( 23 ) from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring ( 23 ) to polish the inner peripheral side of the lateral end of the ring ( 23 ) into a round shape. The outer peripheral side and the inner peripheral side of the lateral end of the ring ( 23 ) are thus separately polished. Accordingly, processing accuracy of the lateral end of the ring ( 23 ) can further be improved even if there is a variation in thickness of the rings ( 23 ). Moreover, since the outer peripheral side and the inner peripheral side of the lateral end of the ring ( 23 ) can be separately polished by the single polishing unit ( 140 ), processing accuracy of the ring ( 23 ) can be improved by using a simple configuration. 
     In the second ring polishing apparatus according to the present disclosure, the polishing unit ( 140 ) may include: a polishing roll ( 141 ) having a groove ( 142 ) formed in its outer peripheral surface along entire circumference and having an axis of rotation parallel to the thickness direction of the ring ( 23 ), the groove ( 142 ) including the concave arc-shaped first polishing portion ( 142   a ) and the concave arc-shaped second polishing portion ( 142   c ) which are formed in both corners of a bottom of the groove and a linear portion ( 142   b ) serving as the bottom and connecting arc ends of the first polishing portion ( 142   a ) and the second polishing portion ( 142   c ); and a rotating and moving unit ( 143 ) that, while rotating the polishing roll ( 141 ), moves the polishing roll ( 141 ) toward one side in an axial direction so that the first polishing portion ( 142   a ) is pressed against the lateral end of the ring ( 23 ) from the outer peripheral side toward the inner peripheral side in the thickness direction of the ring ( 23 ), and moves the polishing roll ( 142 ) toward the other side in the axial direction so that the second polishing portion ( 142   c ) is pressed against the lateral end of the ring ( 23 ) from the inner peripheral side toward the outer peripheral side in the thickness direction of the ring ( 23 ). The outer peripheral side and the inner peripheral side of the lateral end of the ring ( 23 ) can thus be separately processed by the single polishing unit ( 140 ) by reciprocating the polishing roll ( 141 ) in the axial direction. 
     Although the embodiments of the invention of the present disclosure are described above, it is to be understood that the invention of the present disclosure is not limited in any way to the above embodiments and may be carried out in various forms without departing from the spirit and scope of the invention of the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     The invention of the present disclosure can be utilized in manufacturing industries of transmission belts for use in continuously variable transmissions.