Patent Publication Number: US-9889532-B2

Title: Double disc surface grinding machine and grinding method

Description:
TECHNICAL FIELD 
     The present invention relates to double disc surface grinding machines and grinding methods, and more specifically, to a double disc surface grinding machine and a grinding method for grinding two surfaces of middle- and large-sized annular or generally annular works such as piston rings for large marine vessel engines, inner and outer rings of cross roller bearings for rotary tables in machine tools, and inner and outer rings of large bearings in wind-driven power generators. 
     BACKGROUND ART 
     Double disc surface grinding machines are conventionally employed to grind works. For example, Patent Literature 1 discloses a double disc surface grinding machine, in which a work has a center hole, where an in-hole driven roller is disposed, and a drive roller and an outer circumferential driven roller are disposed on an outer circumference of the work, whereby the work is held sandwiched by the rollers while being rotatably supported. With this arrangement, a rotating upper wheel and a rotating lower wheel are fed to the work from above and below, and both surfaces of the work is ground simulataneously. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A 2002-96262 
     SUMMARY OF INVENTION 
     Technical Problem 
     When this grinding apparatus simultaneously grinds both surfaces of a large object, such as a piston ring, or an inner or an outer ring of a bearing which has a diameter of approximately 500 mm through 1200 mm, the work moves spontaneously with respect to the in-hole driven roller, the drive roller and the outer circumferential driven roller in a direction in which the pair of grinding wheels are opposed to each other (i.e., in an axial thickness direction of the work). In other words, the work is sandwiched and is supported rotatably by the rollers, and because of this the work moves with respect to each roller, and as a result, the work also moves spontaneously in the direction in which the pair of grinding wheels are opposed to each other during the grinding operation. Consequently therefore, if the work has a warp which may look a mountain-shape or an s-shape as shown in  FIG. 16( a ) and ( b ) , on its surfaces to be ground when the work is sandwiched by the pair of grinding wheels, the warp will cause the portion which is not sandwiched by the grinding wheels to vibrate, flail or violently swing depending upon the extent of the warp, with the sandwiching grinding wheels working as a fulcrum point as shown in  FIG. 17 , as the work is rotated. This interferes with smooth rotation of the work on the roller, making the work snaking or vibrating during the rotation, leading to a problem that a good level of grinding accuracy cannot be achieved. The work&#39;s flailing movement when the work is sandwiched between the two grinding wheels becomes greater if the amount of warp is greater. In other words, grinding accuracy decreases further if the work has a greater warp. Also, the work&#39;s distortion/warp is greater if the work&#39;s diameter is larger and the work&#39;s wall thickness (radial thickness) is thinner. Likewise, the work is less rigid if the work&#39;s axial thickness is smaller. These mean that so called thin-walled works, such as bearing outer rings and piston rings are more difficult to grind. 
     When grinding a large, thin-walled work with the grinding apparatus according to Patent Literature 1, all of the three rollers, i.e., the in-hole driven roller, the drive roller and the outer circumferential driven roller are substantially aligned on a single straight line. In other words, work supporting points are eccentrically-located, and it is not possible to hold the work stably. 
     Therefore, a primary object of the present invention is to provide a double disc surface grinding machine capable of holding various works stably and grinding them at a high grinding accuracy, and a grinding method therefor. 
     Solution to Problem 
     According to an aspect of the present invention, there is provided a double disc surface grinding machine for grinding two main surfaces of an annular work. The machine includes a pair of rotary grinding wheels opposed to and spaced from each other in a first direction; a rotation shaft extending in the first direction; a work holding section for holding an inner circumferential surface of the work at a plurality of locations; a position adjustment section connecting the rotation shaft and the work holding section to each other for adjusting a position of the work holding section radially of the rotation shaft; a rotation-driving section for integrally rotating the rotation shaft, the position adjustment section, the work holding section and the work around the rotation shaft while the inner circumferential surface of the work is held by the work holding section; and a grinding wheel feeding section for feeding at least one of the grinding wheels to the work so as to sandwich part of the rotating work between the pair of grinding wheels threreby grinding two main surfaces of the work. 
     According to another aspect of the present invention, there is provided a method of grinding two main surfaces of an annular work with a pair of rotating grinding wheels opposed to each other in a first direction at a space therebetween. The method includes a holding step of holding an inner circumferential surface of the work at a plurality of locations with a work holding section; a rotating step of rotating the work holding section and the work integrally with each other around a rotation shaft which extends in the first direction; and a grinding wheel infeeding step of sandwiching part of the rotating work between the pair of grinding wheels and advancing at least one of the grinding wheels to grind two main surfaces of the work. 
     According to the invention described above, the inner circumferential surface of the work is held by the work holding section so that the work will not move with respect to the work holding section (in other words, the work will move integrally with the work holding section) during the grinding. Therefore, it is possible to decrease movement of the work with respect to the work holding section in the direction in which the pair of grinding wheels are opposed to each other, during the grinding. Also, the work holding section does not hold any of the two main surfaces (surfaces to be ground) of the work. Therefore, even if there is a warp in the work&#39;s surfaces which are to be ground, it is possible to decrease flailing movement of the work during grinding. Further, it is possible to adjust the position of the work holding section in the radial direction of the rotation shaft. This allows to move the work holding section until it makes contact with the inner circumferential surface of the work at a plurality of locations, regardless of the diameter or shape of the inner circumferential surface of the work, and therefore to reliably hold the inner circumferential surface of the work with the work holding section. As a result, it is possible to stably hold the work of a various kinds and achieve good grinding accuracy. 
     Preferably, the work holding section includes a plurality of holding members extending radially as viewed from the rotation shaft; and each of the holding members is movable radially inward and outward of the rotation shaft, and is contactable to the inner circumferential surface of the work. In this case, since a plurality of holding members which extend radially as viewed from the rotation shaft hold the inner circumferential surface of the work, only a small holding force (a pushing force exerted from the holding member onto the inner circumferential surface of the work) is sufficient at each location of contact on the inner circumferential surface of the work. Also, the holding members may simply be advanced or retracted radially of the rotation shaft to establish easily contact of the holding members with the inner circumferential surface of the work. 
     Further preferably, the position adjustment section is arranged to allow individual position adjustment of each of the holding members. In this case, position adjustment with respect to the inner circumferential surface of the work can be made individually for each of the holding members. Therefore, it is possible to hold the inner circumferential surface of the work suitably regardless of the kind of the work. 
     Further, preferably, the position adjustment section is arranged to allow simultaneous position adjustment of the plurality of holding members. In this case, it is possible to perform position adjustment of a plurality of the holding members in a single operation. Therefore, it is possible to perform a task of holding the inner circumferential surface of the work with the plurality of holding members, within a short time. 
     Preferably, the plurality of holding members include at least a first holding member, a second holding member and a third holding member, which satisfy the following condition that: in the work&#39;s inner circumferential surface, with a first location which is defined as a location contacted by the first holding member, a second location which is defined as a location contacted by the second holding member, and a third location which is defined as a location contacted by the third holding member, the second location and the third location are on an opposite side from the side where the first location is, with respect to the rotation shaft; whereas the second location and the third location are on mutually opposite sides from each other with respect to a straight line which passes through the first location and the center of the rotation shaft. In this case, the first holding member, the second holding member and the third holding member are pressed onto the inner circumferential surface of the work. This makes it possible to hold the work by pressing it with a sufficient amount of force with the first holding member, the second holding member and the third holding member. With respect to the rotation shaft as a reference, the second location and the third location are on an opposite side from the first location. In this case, a force applied from the first holding member to the work and forces applied from the second holding member and the third holding member respectively to the work at least include components acting in directions away from each other. Further, with respect to the straight line which passes through the first location and the center of the rotation shaft, the second location and the third location are on mutually opposite sides from each other. In this case, a force applied from the second holding member to the work and a force applied from the third holding member to the work at least include components acting in directions away from each other. As a result of these, it is possible to hold the work more stably. 
     Further preferably, the rotation shaft and the work are concentric with each other. In this case, it is possible to perform position adjustment of the work holding section with the position adjustment section, and holding of the inner circumferential surface of the work more smoothly. 
     It should be noted here that in the present invention, the term two main surfaces of the work means a pair of surfaces which connect to the inner circumferential surface of the work. For example, if the work is circular-annular, the two main surfaces of the work refer to a pair of circular-annular surfaces (in other words, two surfaces of the work other than the outer circumferential surface and the inner circumferential surface). 
     The above-described object and other objects, characteristics, aspects and advantages of the present invention will become clearer from the following detailed description of embodiments of the present invention to be made with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a double disc surface grinding machine according to an embodiment of the present invention.  FIG. 1( a )  is a front view,  FIG. 1( b )  is a plan view and  FIG. 1( c )  is a side view. 
         FIG. 2  is an illustrative side view which shows a primary portion of the double disc surface grinding machine in  FIG. 1 . 
         FIG. 3  is an illustrative sectional view which shows a primary portion of the double disc surface grinding machine in  FIG. 1 . 
         FIG. 4  is a flowchart which shows an example of operation of the double disc surface grinding machine in  FIG. 1 . 
         FIG. 5  shows a case where an oval-annular work is held by the double disc surface grinding machine in  FIG. 1 . 
         FIG. 6  is an illustrative side view which shows a primary portion of a double disc surface grinding machine according to another embodiment of the present invention. 
         FIG. 7  is an enlarged view which shows a vicinity of a work holding section and a position adjustment section of the double disc surface grinding machine in  FIG. 6 . 
         FIG. 8  is a an enlarged view which shows a vicinity of a work holding section and a position adjustment section of a double disc surface grinding machine according to still another embodiment of the present invention (with an oval-annular work held thereby). 
         FIG. 9  is an enlarged view which shows a vicinity of the work holding section and the position adjustment section of the double disc surface grinding machine in  FIG. 8  (with an prismatic-annular work held thereby). 
         FIG. 10  is an illustrative sectional view which shows a primary portion of a double disc surface grinding machine according to still another embodiment of the present invention. 
         FIG. 11  is an illustrative side view which shows a primary portion of the double disc surface grinding machine in  FIG. 10 . 
         FIG. 12  is an enlarged view which shows a vicinity of a work holding section and a position adjustment section of a double disc surface grinding machine according to still another embodiment of the present invention. 
         FIG. 13  is an illustrative sectional view taken in line A-A in  FIG. 12 . 
         FIG. 14  is an enlarged view which shows a vicinity of a work holding section and a position adjustment section of a double disc surface grinding machine according to still another embodiment of the present invention. 
         FIG. 15  is an illustrative sectional view taken in line B-B in  FIG. 14 . 
         FIG. 16( a )  shows a work having a mountain-shaped warp.  FIG. 16( b )  shows a work having an s-shaped warp. 
         FIG. 17  is an illustrative drawing which shows a flailing movement of a work according to a conventional art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. Referring to  FIG. 1( a ) through ( c ) , a double disc surface grinding machine  10  is a horizontal double disc surface grinding machine, and includes a bed column  12 . The bed column  12  has a front surface, at a center region thereof, there is formed an opening  14  extending in an up-down direction. The opening  14  allows a work W to be brought in and out of the bed column  12 . Inside the bed column  12 , a pair of grinding wheels  16   a,    16   b  for grinding the work W is opposed coaxially to each other with a gap therebetween in an arrow H direction (in a horizontal direction in the present embodiment). In the present embodiment, the arrow H direction represents the first direction, and each of the grinding wheels  16   a,    16   b  is circular-annular in a side view. Also, in the present embodiment, the work W is circular-annular in a side view, and the work W has an inner circumferential surface which has a circular section. 
     The pair of grinding wheels  16   a,    16   b  are supported by the grinding wheel shafts  18   a,    18   b.  The grinding wheel shafts  18   a,    18   b  are supported by the grinding wheel shaft units  20   a,    20   b  rotatably and movably in a horizontal direction, and are driven by the drive motors  24   a,    24   b  via belts  22   a,    22   b.  Therefore, rotational driving forces from the drive motors  24   a,    24   b  are transmitted via the belts  22   a ,  22   b,  to the grinding wheel shafts  18   a,    18   b,  whereby the grinding wheels  16   a,    16   b  are rotated. 
     The grinding wheel shafts  18   a,    18   b  are each movable horizontally by grinding wheel feeding sections  26   a,    26   b . As the grinding wheel shafts  18   a,    18   b  are moved horizontally by the grinding wheel feeding sections  26   a,    26   b,  the pair of grinding wheels  16   a,    16   b  are moved horizontally. The bed column  12  has an upper surface, at a center region of which there is provided a lid  28  which is openable/closable. 
     The bed column  12  has a front surface, where a front bed  30  is disposed. The front bed  30  supports a transport section  32  and a rotation-driving section  34 . 
     The transport section  32  includes a pair of guide rails  36   a,    36   b,  a work table  38 , a table mover screw  40  and a drive motor  42 . The pair of guide rails  36   a,    36   b  is formed to extend in a fore-aft direction on the front bed  30 . In other words, the pair of guide rails  36   a,    36   b  are disposed to cross at right angles with the grinding wheel shafts  18   a ,  18   b.  The work table  38  is disposed slidably on the pair of guide rails  36   a,    36   b.  The table mover screw  40  is connected to a lower surface of the work table  38 . The drive motor  42  is connected to the table mover screw  40 . When the drive motor  42  is driven, the table mover screw  40  is rotated, whereby the work table  38  is slid on the guide rails  36   a ,  36   b,  moving closer to or away from the bed column  12 . The arrangement makes the work W movable to and away from the pair of grinding wheels  16   a,    16   b.  The rotation-driving section  34  is disposed on the work table  38 , and includes a drive motor  44 . At an end region of the rotation-driving section  34 , a rotation shaft  46  extends in the arrow H direction to rotate the work W. The rotation-driving section  34  is driven by the drive motor  44 , and rotates the rotation shaft  46  to rotate the work W. 
     Referring to  FIG. 2  and  FIG. 3 , the rotation shaft  46  includes a cylindrical section  47  extending in the arrow H direction and a flange-like disc section  48  formed at an end region of the cylindrical section  47 . A generally cylindrical screw ring  49  is provided at a center region in the disc section  48  of the rotation shaft  46 . The screw ring  49  includes a cylindrical section  50  and a flange section  52  formed at an end region of the cylindrical section  50 . The cylindrical section  50  has its side surface formed with a plurality (twelve, in the present embodiment) of screw holes  54  substantially equidistantly in a circumferential direction. 
     The screw ring  49  is attached to a base flange  56 . The base flange  56  is substantially hollow and disc-like. The base flange  56  has a circular hollow portion  58  to which the screw ring  49  is fitted and a plurality (twelve, in the present embodiment) of guide grooves  60  each formed to correspond to (in line with) one of the screw holes  54  in the screw ring  49 . Each of the guide grooves  60  extends radially (in a radial direction) of the base flange  56 . The plurality of guide grooves  60  are disposed substantially equidistantly in a circumferential direction of the base flange  56 . In other words, the base flange  56  has a plurality (twelve, in the present embodiment) of protrusions  61  formed substantially equidistantly in the circumferential direction of the base flange  56 , and the guide grooves  60  are formed between mutually adjacent ones of the protrusions  61 . While the cylindrical section  50  of the screw ring  49  is fitted to the hollow portion  58 , the flange section  52  of the screw ring  49  is attached to the base flange  56  with fasteners  62 . Then, the base flange  56 , to which the screw ring  49  is attached, is fixed to a shaft end region of the rotation shaft  46  (to the disc section  48 ) with a plurality (four, in the present embodiment) of fasteners  64 . In this state, the rotation shaft  46 , the screw ring  49  and the base flange  56  are coaxial with each other and rotatable simultaneously. A work holding section  65  is disposed in the base flange  56 . The work holding section  65  includes a plurality (twelve, in the present embodiment) of strip-like holding members  66 . Each holding member  66  is fitted into the guide groove  60  of the base flange  56  so that it can slide accurately, and is disposed radially (in a radial direction) of the base flange  56 . The plurality of holding members  66  are disposed substantially equidistantly in a circumferential direction of the base flange  56 . Also, in order to prevent the holding members  66  from being ground by the grinding wheels  16   a,    16   b,  each holding member  66  has a thickness which is thinner than an axial thickness T of the work W. The holding member  66  on the guide groove  60  is sandwiched by the base flange  56  and a guide plate  68 . The guide plate  68  is attached to the base flange  56  with fasteners  70 . The guide plate  68  as described above guides the holding member  66  to slide smoothly, without rattling or moving out of the guide groove  60 . Adjustment screws  72  are threaded into the screw holes  54  in the screw ring  49 . The adjustment screw  72  is disposed in line with the holding member  66  so that a head portion of the adjustment screw  72  can push an end of the holding member  66 . By rotating (clockwise or counterclockwise) the adjustment screws  72  with a jig such as a torque wrench, the adjustment screws  72  and the holding members  66  can be moved radially outward/inward of the rotation shaft  46  with respect to an inner circumferential surface of the work W. The adjustment screws  72  are driven to move in the circumferential direction thereby pushing the holding members  66  outward until tip portions of the holding members  66  make pressing contact onto the inner circumferential surface of the work W. By doing so, the inner circumferential surface of the work W receives an appropriate amount of radially outward pressure evenly from a plurality (twelve, in the present embodiment) of the holding members  66 , and the work W is held by the plurality of holding members  66  in an outwardly expanding (pushed from inside out) fashion. Above the base flange  56 , a guiding plate  74  is provided to assist positioning of the work W when the work W is set onto the holding members  66 . The guiding plate  74  is movable in an axial direction of the rotation shaft  46 , and is moved back to a predetermined position by an unillustrated advancement/retraction drive motor when the setting of the work W onto the holding members  66  is completed. 
     In the present embodiment, as shown in  FIG. 2 , twelve holding members  66  are disposed substantially equidistantly (at an angular interval of approximately 30 degrees) in a circumferential direction. Therefore, it is clear that the work holding section  65  (a plurality of the holding members  66 ) includes the first holding member, the second holding member and the third holding member which satisfy the condition stated as “In the work&#39;s inner circumferential surface, with the first location which is defined as a location contacted by the first holding member, the second location which is defined as a location contacted by the second holding member, and the third location which is defined as a location contacted by the third holding member, the second location and the third location are on an opposite side from the side where the first location is, with respect to the rotation shaft; and the second location and the third location are on mutually opposite sides from each other with respect to a straight line which passes through the first location and the center of the rotation shaft”. Referring to  FIG. 2 , using the clock reading system, the holding member  66  which points the “12 o&#39;clock” position will be called the first holding member  66   x;  the holding member  66  which points the “4 o&#39;clock” position will be called the second holding member  66   y;  the holding member  66  which points the “8 o&#39;clock” position will be called the third holding member  66   z,  for example. Then a location where the first holding member  66   x  makes contact with the work W is the first location x; a location where the second holding member  66   y  makes contact with the work W is the second location y; and a location where the third holding member  66   z  makes contact with the work W is the third location z. In this case, with respect to the rotation shaft  46 , the second location y and the third location z are on an opposite side from the first location x; whereas with respect to a straight line P which passes through the first location x and the center C of the rotation shaft  46 , the second location y and the third location z are on mutually opposite sides from each other. In the present embodiment, the position adjustment section  76  includes the screw ring  49 , the base flange  56  and the adjustment screws  72 . It is possible with the position adjustment section  76 , to connect the rotation shaft  46  and the work holding section  65  and to adjust the position of the work holding section  65  radially of the rotation shaft  46 . 
     Next, description will be made for a method of setting the work holding section  65  (a plurality of the holding members  66 ) to the work W. 
     First, a set of four holding members  66  which are in a crisscross relationship in the twelve holding members  66  are set to an extending length which is shorter than the inner radius of the work W by approximately 0.5 mm. All of the other holding members  66  are set to a shorter extending length than these four. Next, one surface of the work W is pressed onto the guiding plate  74 , to position the work W. Next, the four holding members  66  in the cross pattern are divided into two sets, with each set consisting of two holding members  66  which are in a rightly opposed relationship to each other. In each set, the two holding members  66  are alternately pushed radially outward. In this process, the adjustment screw  72  is turned with a torque wrench for example, whereby the holding member  66  is pushed. Then, once all of the four holding members  66  make contact with the inner circumferential surface of the work W, the holding members  66  are alternately torqued in each set, to a predetermined torque. Likewise, the remaining holding members  66  are divided into sets of two rightly opposed holding members  66 , and in each set the holding members  66  are alternately torqued to the predetermined torque. Following the process described above, the inner circumferential surface of the work W is held by the work holding section  65  (a plurality of the holding members  66 ). 
     Next, a primary operation of the double disc surface grinding machine  10  will be described with reference to  FIG. 4 . 
     First, the work W is held with the work holding section  65  as described above (Step S 1 ). Next, the work table  38  is moved forward by the transport section  32 , toward the bed column  12  to move the work W, which is held by the work holding section  65 , to a grinding position. In other words, the work W is sent to between the pair of grinding wheels  16   a,    16   b  (Step S 3 ). 
     Next, the rotation-driving section  34  rotates the work W (Step S 5 ). Specifically, a driving force from the drive motor  44  rotates the rotation shaft  46  integrally with the position adjustment section  76 , the work holding section  65  and the work W, around the rotation shaft  46  in a direction indicated by an arrow R 1  in  FIG. 2 . 
     Next, the drive motors  24   a,    24   b  rotate the grinding wheels  16   a,    16   b  respectively, in a direction indicated by an arrow R 2  in  FIG. 2 . Simultaneously therewith, the grinding wheel feeding section  26   a  moves the left (see  FIG. 1 ) grinding wheel  16   a  forward, toward the work W. As the grinding wheel  16   a  makes contact with the work W, an unillustrated sensor detects a position where the contact was established (Step S 7 ). Thereafter, the grinding wheel  16   a  is returned to its grinding start position (Step S 9 ). 
     Next, the grinding wheel feeding section  26   b  moves the right (see  FIG. 1 ) grinding wheel  16   b  forward, toward the work W. As the grinding wheel  16   b  makes contact with the work W, an unillustrated sensor detects a position where the contact was established (Step S 11 ). Thereafter, the grinding wheel  16   b  is returned to its grinding start position (Step S 13 ). 
     Next, the left and right grinding wheels  16   a,    16   b  are advanced to the positions of contact (Step S 15 ), then a coarse grinding infeed is performed with the grinding wheels  16   a,    16   b  (Step S 17 ), and further, a fine grinding infeed is performed with the grinding wheels  16   a,    16   b  (Step S 19 ). Although the pair of grinding wheels  16   a,    16   b  sandwich only part of the work W at any moment, the work W is rotating and therefore all parts of the surfaces of the work W which must be ground make passes between the pair of grinding wheels  16   a,    16   b,  allowing both main surfaces of the work W to be ground simultaneously. Then, after a spark out (Step S 21 ), the grinding wheels  16   a,    16   b  are returned to their original positions (Step S 23 ). Then, rotation of the work W is stopped (Step S 25 ), and the work table  38  recedes from the bed column  12  to the location where the work W is removed/mounted (Step  27 ). 
     A cycle of Step S 1  through Step S 27  is repeated to sequentially grind a plurality of the works W. 
     Hereinafter, functions and advantages of the double disc surface grinding machine  10  will be explained. 
     According to the double disc surface grinding machine  10 , the inner circumferential surface of the work W is held by the work holding section  65  so that the work W will not move with respect to the work holding section  65  (in other words, the work W will move integrally with the work holding section  65 ) during the grinding. Therefore, it is possible to decrease movement of the work W with respect to the work holding section  65  in the direction in which the pair of grinding wheels  16   a,    16   b  are opposed to each other, during the grinding. The work holding section  65  does not hold two main surfaces (surfaces to be ground) of the work W. Therefore, even if the work W has warps in its surfaces to be ground, it is possible to decrease flailing movement of the work W during grinding. In other words, regardless of whether or not the work W has a warp, distortion, etc. in its surfaces to be ground, it is possible: to hold the inner circumferential surface of the work W with the work holding section  65  so that the rotation shaft  46 , the position adjustment section  76 , the work holding section  65  and the work W will rotate integrally with each other; to grind the work W based on the rotation shaft  46 ; and to reduce flailing movement of the work W during grinding. 
     It is possible to adjust the position of the work holding section  65  in the radial direction of the rotation shaft  46 . This allows to move the work holding section  65  until it makes contact with the inner circumferential surface of the work W at a plurality of locations, regardless of the diameter or shape of the inner circumferential surface of the work W, and therefore to reliably hold the inner circumferential surface of the work W with the work holding section  65 . It is possible to stably hold the work W of a various kinds. As a result, it is possible to use grinding wheels  16   a,    16   b  of a small diameter when grinding the work W regardless of the diameter, warp, etc. of the work W; to ensure appropriate parallelism and flatness regarding the surfaces of the work W to be ground; to obtain good grinding accuracy; and to provide accuracy as reference surfaces used in a later process of machining on the inner and outer diameters. 
     A plurality of the holding members  66  which extend radially as viewed from the rotation shaft  46  hold the inner circumferential surface of the work W. Therefore, at each location of contact on the inner circumferential surface of the work W, only a small holding force (a pushing force exerted from the holding member  66  onto the inner circumferential surface of the work W) is sufficient. The holding members  66  may simply be advanced or retracted radially of the rotation shaft  46 , to establish easily contact of the holding members  66  with the inner circumferential surface of the work W. 
     Position adjustment with respect to the inner circumferential surface of the work W can be made individually for each of the holding members  66 . Therefore, it is possible to hold the inner circumferential surface of the work W suitably regardless of the kind of the work W. In other words, the inner circumferential surface of the work W need not necessarily be a true circle. It is possible to handle annular works having various shapes other than circler, such as oval, rectangular, hexagonal or rice-ball shaped. For example, it is possible to handle an oval-annular work W 1  as shown in  FIG. 5 . 
     Referring to  FIG. 2 , as the first holding member  66   x , the second holding member  66   y  and the third holding member  66   z  are pressed onto the inner circumferential surface of the work W, the first holding member  66   x,  the second holding member  66   y  and the third holding member  66   z  press thereby hold the work W with sufficient amount of forces. With respect to the rotation shaft  46  as a reference, the second location y and the third location z are on an opposite side from the first location x. In this case, a force applied from the first holding member  66   x  to the work W and forces applied from the second holding member  66   y  and the third holding member  66   z  respectively to the work W at least include components acting in directions away from each other. Further, with respect to the straight line P which passes through the first location x and the center C of the rotation shaft  46 , the second location y and the third location z are on opposite sides from each other. In this case, the force applied from the second holding member  66   y  to the work W and the force applied from the third holding member  66   z  to the work W at least have components acting in directions away from each other. As a result of these, it is possible to hold the work W more stably. 
     Since the rotation shaft  46  and the work W are concentric with each other, it is possible to perform position adjustment of the work holding section  65  with the position adjustment section  76  and holding on the inner circumferential surface of the work W more smoothly. 
     Since the work W can be ground from a state of material to a finished size within a single grinding cycle, productivity is improved, and since the grinding wheels  16   a ,  16   b  of a narrow effective grinding width can be used, it is possible to decrease cost on the grinding wheels. Further, it becomes possible to make the double disc surface grinding machine  10  compact. 
     In the embodiment described above, individual adjustment screws  72  are used to move corresponding ones of the holding members  66  radially outward to hold the inner circumferential surface of the work W. However, as in an embodiment shown in  FIG. 6 , advancing/retracting movement of all of the holding members  66  may be performed by a single operation by employing an automatic chucking mechanism. 
     Referring to  FIG. 6  and  FIG. 7 , in the present embodiment, a cam shaft  78  is inserted through the center of the rotation shaft  46   a,  coaxially therewith. In other words, the rotation shaft  46   a  includes a cylindrical section  47   a  extending in the arrow H direction (see  FIG. 1 ) and a flange-like, hollow disc section  48   a  formed at an end region of the cylindrical section  47   a.  The cam shaft  78  is inserted through the cylindrical section  47   a  and the hollow disc section  48   a.  The cam shaft  78  is rotatable with respect to the rotation shaft  46   a,  and is connected to an unillustrated cam rotary-drive actuator. At an end portion of the cam shaft  78 , a disc cam  80  is fixed with a plurality (four, in the present embodiment) of fasteners  82 . The disc cam  80  includes an annular cam groove  84 . The cam groove  84  has the same number of ridges  84   a  and valleys  84   b  as the plurality (twelve, in the present embodiment) of holding members  66   a  included in work holding section  65   a.  Also, in the present embodiment, there is provided a locking mechanism (not illustrated) for fixing the cam shaft  78  to the rotation shaft  46   a  (to make the rotation shaft  46   a  un-rotatable with respect to the cam shaft  78 ). Therefore, it is possible to fix the cam shaft  78  to the rotation shaft  46   a  with the locking mechanism, under a state where rollers  92 , which will be described later, are on the ridges  84   a  or on the valleys  84   b  of the cam groove  84 . With this arrangement, by driving the cam rotary-drive actuator while the locking mechanism fixes the cam shaft  78  to the rotation shaft  46   a,  the cam shaft  78  and the rotation shaft  46   a  rotate integrally with each other. Each of the holding members  66   a  is formed with a spring hole  86 . In the spring hole  86 , a spring  88  and a plunger  90  are inserted. The plunger  90  has an end, to which a roller  92  is attached. The roller  92  is fitted into the cam groove  84 . The base flange  56   a  is formed substantially hollow and disc-like. An inner diameter of the base flange  56   a  is substantially equal to an outer diameter of the disc cam  80 . The base flange  56   a  is fitted around the disc cam  80 , and is fitted to the hollow disc section  48   a  of the rotation shaft  46   a  with a plurality (four, in the present embodiment) of fasteners  64 . In the present embodiment, the position adjustment section  76   a  includes the base flange  56   a,  the cam shaft  78 , the cam rotary-drive actuator, the disc cam  80 , the fasteners  82 , the springs  88 , the plungers  90  and the rollers  92 . Other arrangements are the same as the embodiment in  FIG. 1 , so repetitive description will not be given here. 
     In the present embodiment, the work W is set, and then the cam shaft  78  is rotated leftward (counterclockwise (in an arrow R 3  direction) in  FIG. 7 ) to cause the disc cam  80  to push up the rollers  92 , to press the plungers  90 . This causes the holding members  66   a  to be pressed out via the spring  88  which is adjusted to exert an appropriate clamping force, and the inner circumferential surface of the work W is held by all of the holding members  66   a.  In this way, it becomes possible to hold the inner circumferential surface of the work W with all of the holding members  66   a,  by rotating a single cam shaft  78 . Then, the unillustrated locking mechanism is used to lock the holding members  66   a  at their work W holding positions, so that the holding members  66   a  can continue to hold the work W with a constant force during grinding. 
     According to this embodiment, the position adjustment section  76   a  is arranged to be capable of making a simultaneous position adjustment of a plurality of the holding members  66   a.  Therefore, the arrangement allows position adjustment of a plurality of the holding members  66   a  in a single operation. It is possible to perform a task of holding the inner circumferential surface of the work W with a plurality of the holding members  66   a  within a short time. 
       FIG. 8  shows an embodiment as another arrangement for causing all of the holding members  66   b  to be advanced or retracted with a single operation. In the present embodiment, an oval work W 1  is held by the work holding section  65   b.  A major axis radius of the work WI is longer than a radius of a true circle by a length S. 
     In the present embodiment, there is provided a plurality (twelve, in the present embodiment) of air cylinders  94  at a base flange  56   b,  together with a plurality (twelve, in the present embodiment) of holding members  66   b  which are included in the work holding section  65   b.  Each of the air cylinders  94  has a piston rod  96 , to which a corresponding one of the holding members  66   b  is connected. Each air cylinder  94  is connected to an air supply joint  100  via a corresponding pipe  98 . The air supply joint  100  is inserted through the center of the rotation shaft  46   b  coaxially therewith, and is rotatable integrally with the rotation shaft  46   b.  In other words, the rotation shaft  46   b  includes a cylindrical section  47   b  extending in the arrow H direction (see  FIG. 1 ) and a flange-like, hollow disc section  48   b  formed at an end region of the cylindrical section  47   b.  The air supply joint  100  is inserted through the cylindrical section  47   b  and the hollow disc section  48   b,  and fixed to the rotation shaft  46   b.  The air supply joint  100  has an end portion protruding out of an end portion of the rotation shaft  46   b,  and the plurality of the pipes  98  are radially connected to an outer side surface of the end portion of the air supply joint  100 . An unillustrated air supply source supplies air through the air supply joint  100  and each of the pipes  98 , to each air cylinder  94 , then all of the holding members  66   b  move outward to press an inner circumferential surface of the work W 1  thereby holding the work W 1 . The base flange  56   b  is formed substantially hollow and disc-like. An inner diameter of the base flange  56   b  is substantially equal to an outer diameter of the air supply joint  100 . The base flange  56   b  is fitted around the air supply joint  100 , and is fixed to the hollow disc section  48   b  of the rotation shaft  46   b  with a plurality (four, in the present embodiment) of fasteners  64 . In the present embodiment, the position adjustment section  76   b  includes the base flange  56   b,  the air cylinders  94 , the pipes  98 , the air supply joint  100  and the air supply source. Other arrangements are the same as the embodiment in  FIG. 1 , so repetitive description will not be given here. 
     According to this embodiment, it is possible to drive each of the holding members  66   b  individually with a single operation. With the use of air cylinders  94 , it is possible to cause all of the holding members  66   b  to press and hold the inner circumferential surface of any work, which has a large inner diametrical difference (e.g., between the major and minor axes) or of a work of an irregular shape, with a uniform force regardless of the extended length (amount of stroke) of the individual holding members  66   b.  The air cylinders may be replaced with hydraulic cylinders. The same applies to embodiments shown in  FIG. 12  and  FIG. 14 . 
     In the embodiments described above, the holding members may be varied in their length and shape of tips, in accordance with the work to be machined. Then it becomes possible to handle works more flexibly, not only circularly annular works such as inner and outer rings of bearings, inner and outer rings of cross roller bearings, and piston rings but also works of any loop shapes such as triangular, rectangular, etc. For example, the length of each holding member  66   b  in the embodiment shown in  FIG. 8  may be changed as shown in  FIG. 9 . Then, it becomes possible to easily hold a work W 2  of a prismatic (hexagonal) ring shape as shown in  FIG. 9 . 
       FIG. 10  and  FIG. 11  show still another embodiment with an arrangement for individual position adjustment of each holding section. 
     Referring to  FIG. 10  and  FIG. 11 , in the present embodiment, the work holding section  65  and the position adjustment section  76  of the double disc surface grinding machine  10  shown in  FIG. 1  are replaced with a work holding section  65   c  and a position adjustment section  76   c.    
     The position adjustment section  76   c  includes a screw ring  49   c,  a base flange  56   c  and a holding plate  102 . 
     The screw ring  49   c  is generally cylindrical, and is made the same as the screw ring  49  except that the screw ring  49   c  does not have the screw holes  54 . In other words, the screw ring  49   c  is at a center region in the disc section  48  of the rotation shaft  46 , and includes a cylindrical section  50   c  and a flange section  52  formed at an end region of the cylindrical section  50   c.  The screw ring  49   c  is attached to the base flange  56   c.    
     The base flange  56   c  is made the same as the base flange  56  except that the base flange  56   c  does not have the guide grooves  60  and the protrusions  61 . In other words, the base flange  56   c  is substantially hollow and disc-like, and has a circular hollow portion  58  into which the screw ring  49   c  is inserted. While the cylindrical section  50   c  of the screw ring  49   c  is fitted to the hollow portion  58 , the flange section  52  of the screw ring  49   c  is attached to the base flange  56   c  with fasteners  62 . Then, the base flange  56   c , to which the screw ring  49   c  is attached, is fixed to an end portion (the disc section  48 ) of the rotation shaft  46  with a plurality (four, in the present embodiment) of fasteners  64 . The holding plate  102  is attached to the base flange  56   c.    
     The holding plate  102  is hollow and disc-like, has a circular hollow region  104  through which the screw ring  49   c  is inserted, and a plurality (twelve, in the present embodiment) of substantially rectangular through-holes  106 . The plurality of through-holes  106  are formed substantially equidistantly in a circumferential direction of the holding plate  102 . The holding plate  102  has a thickness which is slightly smaller than an axial thickness T of the work W so as to give the holding plate  102  increased bending rigidity without a risk that the holding plate  102  will be ground by the grinding wheels  16   a,    16   b.  Also, the holding plate  102  has an outer diameter which is slightly smaller than an inner diameter of the work W. The outer diameter of the holding plate  102  is designed to make a gap G of, e.g., approximately 5 mm, between the holding plate  102  and the work W. The holding plate  102  is fixed to the base flange  56   c  with a plurality (six, in the present embodiment) of fixing bolts  108 . Therefore, the rotation shaft  46 , the screw ring  49   c,  the base flange  56   c,  and the holding plate  102  are coaxial with each other and simultaneously rotatable. In each of the through-holes  106  in the holding plate  102 , there is formed a screw hole  110  extending radially (in radial directions) of the holding plate  102 , penetrating to reach an outer circumferential surface of the holding plate  102 . The screw holes  110  are provided to penetrate an appropriate thickness position (substantial center region of the thickness, in the present embodiment) of the holding plate  102 . 
     The work holding section  65   c  includes a plurality (twelve, in the present embodiment) of bolt-like holding members  66   c.  Each of the holding members  66   c  is threaded into a corresponding one of the screw holes  110 , radially (in a radial direction) of the holding plate  102 . The plurality of the holding members  66   c  are disposed substantially equidistantly in a circumferential direction of the holding plate  102 . By rotating the holding members  66   c  (in a clockwise or counterclockwise direction), it is possible to move closer to and away from the holding members  66   c  with respect to the inner circumferential surface of the work W, radially of the rotation shaft  46 . In other words, it is possible to advance or retract the holding members  66   c  with respect to the screw holes  110  thereby adjusting the position of the holding members  66   c  by way of interaction between male threads in the holding members  66   c  and female threads in the screw holes  110 . The holding members  66   c  are thus rotated in the circumferential direction to move outward until tip portions of the holding members  66   c  press onto the inner circumferential surface of the work W. By doing so, the inner circumferential surface of the work W receives an appropriate amount of radially outward pressure evenly from a plurality (twelve, in the present embodiment) of the holding members  66   c,  and the work W is held by the plurality of holding members  66   c,  in an outwardly expanding (pushed from inside out) fashion. The holding members  66   c  are fixed with locknuts  112 . Other arrangements are the same as the embodiment in  FIG. 1 , so repetitive description will not be given here. 
     According to this embodiment, the holding plate  102  which holds the holding members  66   c  is disc-like, and is formed as a single-piece without being divided in a circumferential direction. This allows the holding plate  102  to have a strong bending rigidity. Also, the embodiment allows the holding plate  102  to be increased in size until its outer diameter is close to the inner diameter of the work W. This allows the holding plate  102  to have a further increased bending rigidity. Further, a portion of each holding member  66   c  to be extended beyond the outer circumferential surface of the holding plate  102  is short, being equal to the dimension of the gap G. Thus, the holding members  66   c  resist bending forces and can become substantially integral with the holding plate  102 . Therefore, each holding member  66   c  has substantially the same level of rigidity as the holding plate  102 , i.e., it is possible to increase rigidity. This embodiment is advantageous particularly when the work W to be ground has a small thickness. 
     The work may be automatically clamped and held according to an embodiment shown in  FIG. 12  and  FIG. 13 . 
     Referring to  FIG. 12  and  FIG. 13 , in the present embodiment, the work holding section  65   b  and the position adjustment section  76   b  in the embodiment shown in  FIG. 8  are replaced with a work holding section  65   d  and a position adjustment section  76   d.    
     The position adjustment section  76   d  includes a base flange  56   d,  air cylinders  94 , pipes  98 , an air supply joint  100 , a holding plate  102   a,  and an air supply source. 
     The base flange  56   d  is the same as the base flange  56   b  except that the base flange  56   d  does not have the guide grooves  60  and the protrusions  61 . In other words, the base flange  56   d  is formed substantially hollow and disc-like. An inner diameter of the base flange  56   d  is substantially equal to an outer diameter of the air supply joint  100 . The base flange  56   d  is fitted around the air supply joint  100 , and is fixed to the hollow disc section  48   b  (see  FIG. 8 ) of the rotation shaft  46   b  with a plurality (four, in the present embodiment) of fasteners  64  (see  FIG. 8 ). It should be noted here that although the embodiment in  FIG. 12  has the hollow disc section  48   b  and the fasteners  64 ,  FIG. 12  does not show the hollow disc section  48   b  and the fasteners  64  to avoid complication in the drawing. The same applies to  FIG. 14  (which will be described later). 
     The holding plate  102   a  is hollow and disc-like. In order to give a high level of bending rigidity to the holding plate  102   a  and in order to prevent the holding plate  102   a  from being ground by the grinding wheels  16   a,    16   b,  the holding plate  102   a  has a slightly smaller thickness than an axial thickness T (see  FIG. 10 ) of the work W. Also, the holding plate  102   a  has an outer diameter which is slightly smaller than an inner diameter of the work W. The outer diameter of the holding plate  102   a  is designed to make a gap Ga of, e.g., approximately 5 mm, between the holding plate  102   a  and the work W. The holding plate  102   a  is fixed to an outer circumferential edge region of the base flange  56   d  with a plurality (six, in the present embodiment) of fixing bolts  108   a.  The holding plate  102   a  has a plurality (twelve, in the present embodiment) of through-holes  110   a . The plurality of through-holes  110   a  are formed substantially equidistantly in a circumferential direction of the holding plate  102   a,  penetrating from an inner circumferential surface of the holding plate  102   a  to an outer circumferential surface thereof, radially (in radial directions). The through-holes  110   a  are provided to penetrate an appropriate thickness position (substantial center region of the thickness) of the holding plate  102   a.    
     The work holding section  65   d  includes a plurality (twelve, in the present embodiment) of rod-like holding members  66   d.  The plurality of holding members  66   d  are disposed substantially equidistantly in a circumferential direction of the holding plate  102   a.  Each of the holding members  66   d  is inserted through a corresponding one of the through-holes  110   a  to extend radially (in a radial direction) of the holding plate  102   a  and to move inward and outward smoothly with respect to the inner circumferential surface of the work W radially of the rotation shaft  46   b . Each holding member  66   d  is connected to a piston rod  96  of a corresponding one of cylinders  94 , via a connection joint  114 . Thus, as an unillustrated air supply source supplies air through the air supply joint  100  and each of the pipes  98 , to each of the air cylinders  94  which is disposed in the base flange  56   d,  all of the holding members  66   d  move outward to press an inner circumferential surface of the work W thereby holding the work W. After clamping the work W, the holding members  66   d  are fixed with locking screws  116  threaded into the holding plate  102   a,  integrally with the holding plate  102   a  (see  FIG. 13 ). Therefore, each holding member  66   d  has substantially the same level of rigidity as the disc-like holding plate  102   a,  i.e., it is possible to increase rigidity. Other arrangements are the same as the embodiment in  FIG. 8 , so repetitive description will not be given here. 
     Further, the work may be automatically clamped and held according to an embodiment shown in  FIG. 14  and  FIG. 15 . 
     Referring to  FIG. 14  and  FIG. 15 , in the present embodiment, the work holding section  65   d  and the holding plate  102   a  in the embodiment shown in  FIG. 12  are replaced with a work holding section  65   e  and a holding plate  102   b.    
     The holding plate  102   b  is hollow and disc-like, and has a thick portion  118  in its outer circumferential region. Therefore, the holding plate  102   b  is formed to have an L-shaped section in its outer circumferential region. For increased bending rigidity of the holding plate  102   b,  the thick portion  118  of the holding plate  102   b  may be made greater than an axial thickness T of the work W. Also, the holding plate  102   b  has an outer diameter which is slightly smaller than an inner diameter of the work W. The holding plate  102   b  is fixed to an outer circumferential edge region of the base flange  56   d  with a plurality (six, in the present embodiment) of fixing bolts  108   a.  The thick portion  118  of the holding plate  102   b  has a plurality (twelve, in the present embodiment) of through-holes  110   b.  The plurality of through-holes  110   b  are formed substantially equidistantly in a circumferential direction of the holding plate  102   b  (thick portion  118 ), penetrating from an inner circumferential surface of the thick portion  118  to an outer circumferential surface thereof, radially (in radial directions). 
     The work holding section  65   e  includes a plurality (twelve, in the present embodiment) of rod-like holding members  66   e.  The plurality of holding members  66   e  are disposed substantially equidistantly in a circumferential direction of the holding plate  102   b.  Each of the holding members  66   e  is inserted through a corresponding one of the through-holes  110   b  to extend in a radial direction (radially) of the holding plate  102   b  and to move inward and outward smoothly with respect to the inner circumferential surface of the work W radially of the rotation shaft  46   b . Each of the holding members  66   e  has a tip portion  120 , which has a slightly smaller diameter than a width T of the work W so that the tip portion  120  can be brought between the grinding wheels  16   a,    16   b  without making contact with the grinding wheels  16   a,    16   b  while holding the work W. By positioning an outer circumferential surface of the holding plate  102   b  closely to the grinding wheels  16   a,    16   b,  it becomes possible to shorten a length of the tip portion  120 . Part of the holding member  66   e  other than the tip portion  120  is outside of the grinding wheels  16   a,    16   b  and therefore can be made thicker to give increased rigidity to the holding members  66   e.  Each holding member  66   e  is connected to a piston rod  96  of a corresponding one of cylinders  94 , via a connection joint  114 . After clamping the work W, the holding members  66   e  are fixed with locking screws  116   a  threaded into the holding plate  102   b,  and becomes integral with the holding plate  102   b.  Therefore, each holding member  66   e  has substantially the same level of rigidity as the disc-like holding plate  102   b,  i.e., it is possible to increase rigidity. Other arrangements are the same as the embodiment in  FIG. 12 , so repetitive description will not be given here. 
     According to this embodiment, there is no need for inserting the holding plate  102   b  between the grinding wheels  16   a,    16   b,  so it is possible to increase the thickness of the holding plate  102   b  without being limited by the thickness of the work W, and therefore to further increase the rigidity of the holding plate  102   b.  Also, by shortening the length of the tip portion  120  in the holding member  66   e , it becomes possible to further increase rigidity which is given by an integral structure of the holding plate  102   b  and the holding members  66   e.  Especially, this can be achieved more effectively by increasing a distance between the center C 1  of the rotation shaft  46   b  which rotates the work W and the center of the grinding wheels  16   a,    16   b  and decreasing a distance L (see  FIG. 14 ) by which the work W is brought between the grinding wheels  16   a,    16   b,  because these allow the tip portion  120  to be made shorter. 
     The present invention is not limited to cases where both in the pair of grinding wheels are advanced to the work, but is also applicable to cases where only one in the pair of grinding wheels is advanced to the work when the work is sandwiched by the pair of grinding wheels for grinding both main surfaces of the work. 
     In the embodiments described above, the present invention was applied to horizontal double disc surface grinding machines. However, the present invention is also applicable to vertical double disc surface grinding machines in which grinding wheel shaft is made vertical. 
     Also, the present invention is applicable not only to infeed grinding but also to so called creep feed grinding in which a rotating work is fed from outside of the grinding wheels toward the grinding wheels, and also to traverse creep feed grinding in which creep grinding is repeated. 
     The present invention being thus far described in terms of preferred embodiments, it is obvious that these may be varied in many ways within the scope and the spirit of the present invention. The scope of the present invention is only limited by the accompanied claims. 
     REFERENCE SIGNS LIST 
     
         
           10  Double disc surface grinding machine 
           16   a,    16   b  Grinding wheels 
           26   a,    26   b  Grinding wheel feeding sections 
           34  Rotation-driving section 
           46 ,  46   a,    46   b  Rotation shafts 
           65 ,  65   a,    65   b,    65   c,    65   d,    65   e  Work holding sections 
           66 ,  66   a,    66   b,    66   c,    66   d,    66   e  Holding members 
           66   x  First holding member 
           66   y  Second holding member 
           66   z  Third holding member 
           76 ,  76   a,    76   b,    76   c,    76   d  Position adjustment sections 
         C, C 1  Rotation shaft centers 
         P Straight line which passes through the first location 
         and the center of the rotation shaft 
         W, W 1 , W 2  Works 
         x First location 
         y Second location 
         z Third location