Patent Publication Number: US-6336855-B1

Title: Grindstone for ELID grinding and apparatus for ELID surface grinding

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ELID grinding wheel for performing electrolytic dressing while grinding a workpiece, and to an ELID surface grinding apparatus using the grinding wheel. 
     2. Description of Related Art 
     In order to grind a high-precision magnetic disk substrate, mainly a plane lapping machine has heretofore been used. This plane lapping machine is the lapping machine of a simultaneous double-sided lapping system. In the machine, gears are formed in the periphery of a fixture to which a workpiece is attached, the fixture meshes with the middle and peripheral gears to rotate itself and the machine, a pressure by an upper cylinder is applied, a loose abrasive material is interposed between a tool called a lapping plate and the workpiece, and lapping is performed by the relative movement of the lapping plate and the workpiece. 
     Although the above-described conventional lapping apparatus (e.g., the plane lapping machine) has a characteristic that a high machining accuracy can be obtained by relatively simple equipment, but contrarily has the following problems. 
     (1) Since the losse abrasive is used in the lapping, the lapping is very low ({fraction (1/10)} or less of the grinding removal rate), and the lapping requires much time. Therefore, large-sized equipment is usually used, and a plurality of magnetic disk substrates are simultaneously lapped, but the lapping time per disk is long. 
     (2) The lapping plate as a standard is preparaed beforehand with a high accuracy, and the workpiece is grinded in conformity with the standard. Therefore, when the accuracy of the lapping plate itself is deteriorated by wear, the re-sharpening is necessary again. In this case, “mutual lapping” which is a process for grinding the upper and lower lapping plates with each other is usually performed. Although irregularities are flattened by this means, but squareness cannot be secured with respect to a rotating shaft, and the parallelism of the worked workpiece is deteriorated. Specifically, in a constant-pressure lapping system, truing (mutual lapping) can be performed on the machine. However, even when the flatness of a grinding wheel (lapping plate) is enhanced, the parallelism between a workpiece holding jig and the grinding wheel cannot be adjusted, and the parallelism of both workpiece surfaces (thickness precision) cannot be enhanced by this adjustment. 
     Furthermore, in recent years, the breakage of a hard disk, and particularly the breakage of a motor have frequently occurred. One of the causes for this lies in the unbalance of the thickness of the magnetic disk substrate. Specifically, there is a problem that the eccentric force generated by the unbalance of the magnetic disk substrate shortens the motor bearing life, and the motor breaks in a short time. Therefore, in order to enhance the reliability of the hard disk, it has strongly been desired that the parallelism (thickness precision) of the magnetic disk substrate be enhanced more than before. However, if this tries to be achieved by the above-described conventional lapping machine, the lapping time is further lengthened, which is impractical and raises a problem. 
     In order to solve various above-described problems, the present inventors, et al. have devised “Mirror Working Apparatus and Method of Magnetic Disk Substrate” (Japanese Patent Application No. 136198/1998, not laid open) and filed an application. In this invention, as schematically shown in FIG. 1, the apparatus comprises: a metal bond grinding wheel  2  which rotates centering on a vertical shaft center Z 1  and includes a horizontal working surface  2   a ; workpiece holding rotation means  4  which includes a horizontal support surface 4 a  disposed opposite to the working surface and rotates centering on a vertical shaft center Z 2 ; voltage applying means  6  which uses the metal bond grinding wheel as an anode, and an electrode  6   a  disposed opposite to the working surface of the metal bond grinding wheel in a non-contact manner as a cathode, and applies a pulse voltage between both electrodes; and electrolytic fluid supply means  8  for supplying a electrolytic fluid to the working surface of the metal bond grinding wheel, and the workpiece holding rotation means  4  brings a disc-shaped magnetic disk substrate  1  or a disc-shaped truing grinding wheel in close contact with the support surface to hold and rotate the substrate or the grinding wheel, and is constituted to be horizontally and vertically movable. The method by this apparatus comprises: (A) plane-working the working surface  2   a  of the metal bond grinding wheel on the machine; subsequently electrolytic dressing the metal bond grinding wheel; and simultaneously performing (B) grind-working of the support surface  4   a  of the workpiece holding rotation means and (C) grind-working of the magnetic disk substrate  1  attached to the workpiece holding rotation means alternately on the machine. Additionally, in FIG. 1, numerals  3 ,  5  denote drive apparatus. 
     According to this apparatus and method, as compared with the conventional lapping apparatus, the removal rate of the magnetic disk substrate can largely be enhanced, and both surface parallelism (thickness precision) and surface roughness of workpiece can be enhanced more than before. 
     In the conventional art, the grinding wheel  2  shown in FIG. 1 is integrally molded and the surface  2   a  is entirely formed into a flat surface without any recess. However, when the grinding wheel  2  is enlarged in size with the enlargement of the workpiece  1  (magnetic disk substrate), in order to manufacture the large-sized grinding wheel with a high accuracy by press molding and sintering, there are many limitations in respect of equipment and there is a problem that manufacture cost is excessively increased. 
     Moreover, since the surface of the grinding wheel is a flat surface without any recess, an electrolytic fluid is not easily supplied to a gap with the workpiece, and there is a problem that chips and grinding wheel pieces generated by grinding are not easily removed from the grinding wheel surface. Therefore, scratches are generated on the workpiece by the chips or grinding wheel pieces generated during grinding, the supply of the electrolytic fluid partially becomes insufficient and the electrolytic dressing becomes insufficient, and other problems occur. 
     Furthermore, when the supply amount of the electrolytic fluid is excessively increased to prevent the scratches, the workpiece floats up from the grinding wheel by the pressure of the electrolytic fluid so that the grinding cannot be performed, that is, a so-called hydroplaning phenomenon easily occurs. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed to solve the above-described problems. Specifically, an object of the present invention is to provide an ELID conductive grinding wheel in which size is easily enlarged, chips and grinding wheel pieces can smoothly be removed, insufficient grind fluid does not easily occur, and hydroplaning phenomenon does not easily occur and to provide an ELID surface grinding apparatus which uses this grinding wheel. 
     According to the present invention, there is provided an ELID grinding wheel which is a conductive grinding wheel rotating centering on a vertical shaft center and including a horizontal working surface, the grinding wheel comprises a conductive base member of an integrally molded flat plate, and a plurality of fan-shaped segments detachably attached to the base member to entirely constitute the working surface in an annular shape, each segment comprises a base metal directly attached to the base member and a grinding wheel part formed on the surface of the base metal, the grinding wheel part comprises a recessed groove for partitioning the working surface into a plurality of working areas and an electrolytic fluid supply hole for directly supplying an electrolytic fluid to the respective working areas, so that the electrolytic fluid is supplied to each electrolytic supply hole through the base member. 
     According to the constitution of the present invention, since the conductive grinding wheel comprises the conductive base member of the integrally molded flat plate and a plurality of fan-shaped segments detachably attached to the base member to entirely constitute the annular working surface, the fan-shaped segment is reduced in size, and a large-sized grinding wheel can easily be manufactured using conventional molding equipment and sintering equipment. 
     Moreover, since the grinding wheel parts of the respective segments include a plurality of working areas partitioned by the recessed grooves, and the electrolytic fluid is directly supplied from the electrolytic supply holes disposed in the respective working areas, the electrolytic fluid flows in a radial one direction from the electrolytic fluid supply hole to the recessed groove surrounding the hole, and the chips and grinding wheel pieces can smoothly be removed with this flow. Furthermore, since it is easy to supply an optimum amount of electrolytic fluid to the respective working areas, the insufficient grind fluid does not easily occur, and the hydroplaning phenomenon by the excessive fluid does not easily occur. 
     According to a preferred embodiment of the present invention, the respective working areas have substantially the same area, the respective electrolytic fluid supply holes have substantially the same sectional area and the same number of holes are disposed in the respective working areas. According to this constitution, only by supplying substantially the same amount of electrolytic fluid to the respective working areas, the insufficient or excessive part of the grinding fluid can be prevented from being generated over the entire working surface. 
     Moreover, the base metal is provided with an electrolytic fluid supply port connected to each electrolytic fluid supply hole. Since the electrolytic fluid supply port is disposed, the electrolytic fluid can easily be supplied to each electrolytic fluid supply hole through the base member. 
     Furthermore, the recessed groove includes a boundary groove extending in the radial direction of the grinding wheel along a boundary line with the adjacent segment, a middle groove disposed between the boundary grooves and extending in the radial direction of the grinding wheel, and an intersecting groove extending to intersect the boundary groove and/or the middle groove. According to this constitution, the chips and grinding wheel pieces having reached the intersecting groove can smoothly be discharged to the outside of the grinding wheel through the boundary groove and/or the middle groove by the action of a centrifugal force. 
     Moreover, according to the present invention, there is provided an ELID surface grinding apparatus comprising an electrolytic fluid supply apparatus for distributing/supplying a conductive electrolytic fluid to a plurality of electrolytic fluid supply ports of the above-described ELID grinding wheel. 
     According to the constitution of the present invention, the conductive electrolytic fluid can be distributed/supplied to the plurality of electrolytic fluid supply ports of each ELID grinding wheel from the electrolytic fluid supply apparatus. 
     According to the preferred embodiment of the present invention, the electrolytic fluid supply apparatus includes an electrolytic fluid chamber disposed in the upper part of the grinding wheel, and a plurality of pipelines for independently connecting the chamber to the respective electrolytic fluid supply ports. According to this constitution, the electrolytic fluid pooled in the electrolytic fluid chamber can be distributed/supplied to the respective electrolytic fluid supply ports by its own weight, and the constitution is suitable especially for the upper grinding wheel. 
     Moreover, the electrolytic fluid supply apparatus may be constituted of a rotary connector disposed on the rotation centerline of the grinding wheel, and a plurality of pipelines for independently connecting the rotary connector to the respective electrolytic fluid supply ports. According to this constitution, even in the lower grinding wheel, the electrolytic fluid can smoothly be supplied to the respective electrolytic fluid supply ports from the rotary connector. 
     Other objects and advantageous characteristics of the present invention will be apparent from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illistration of a mirror finishing apparatus according to the prior application which has not been laid open. 
     FIG. 2 is an entire constitution diagram of an ELID surface grinding apparatus which uses an ELID grinding wheel of the present invention. 
     FIG. 3 is a partial view in I-I′ line of FIG.  2 . 
     FIG.  4 (II) is a sectional view in II-II′ line of FIG. 3, and FIG.  4 (III) is a sectional view in III-III′ line of FIG.  3 . 
     FIG. 5 is a constitution diagram of a segment  12  in a case where a circular arc center angle is 60 degrees. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinafter with reference to the drawings. Additionally in the respective drawings, common members are denoted with the same reference numerals and redundant description is omitted. 
     The present inventors et al. have developed “Electrolytic Dressing Method and Apparatus for Conductive Grinding wheel” which is referred to as an electrolytic inprocess dressing grinding method (ELID grinding method) (Japanese Patent Publication No. 075823/1994). In the method and apparatus, a satisfactory mirror surface and smooth flat surface can be obtained with a high efficiency and high speed by applying a voltage to a conductive grinding wheel, and electrolytic dressing the conductive grinding wheel. 
     In the ELID grinding method, since the clogging of the grinding wheel is not caused by the electrolytic dressing, with fine abrasive grains a remarkably superior working surface like the mirror surface can be obtained by a grinding work, and the mirror surface can be worked at a speed several ten times that of lapping with a loose abrasive material. The present invention has been achieved by further developing the ELID grinding method with a higher precision. 
     FIG. 2 is an entire constitution diagram of an ELID surface grinding apparatus using an ELID grinding wheel of the present invention. This ELID surface grinding apparatus is constituted of voltage applying means (not shown), an electrolytic fluid supply apparatus  18 , upper and lower ELID grinding wheels  10 , and workpiece holding rotation means  24 . 
     The upper and lower ELID grinding wheels  10  are conductive grinding wheels, include working surfaces  10   a  disposed opposite to each other, and are rotated/driven centering on vertical shaft centers Z 3 , Z 4  by independent drive apparatus (not shown). The shaft centers Z 3 , Z 4  are constituted parallel to each other with a high precision. Moreover, either one of the upper and lower grinding wheels can move horizontally and vertically in addition to the rotation/driving. According to this constitution, by relatively horizontally moving the upper and lower grinding wheels  10 , and allowing the grinding wheels to closely abut on each other on the machine, the respective working surfaces  10   a  can horizontally be plane-worked. 
     In this example, the workpiece holding rotation means  24  includes a sun gear  24   a  which are independently rotated/driven centering on the center shaft Z 4  of the lower grinding wheel  10 , a planet gear  24   b  which meshes with the sun gear  24   a , and a ring gear  24   c  which meshes with the outer periphery of the planet gear  24   b . The planet gear  24   a  is provided with a through hole in which the workpiece  1  (e.g., magnetic disk substrate) is loosely engaged in a position deviating from the rotation center. Moreover, this planet gear  24   a  is constituted to be always positioned in the middle position between the upper and lower grinding wheels  10 . 
     According to this constitution, by rotating the planet gear  24   b  in an appropriate angle range, the workpiece  1  is held between the upper and lower grinding wheels  10 , rotated centering on the vertical axis and can horizontally be rocked. Specifically, in this example, the planet gear  24   b  functions as a carrier for holding/rocking the workpiece  1 . Additionally, the carrier  24   b  is thicker than an electrode (not shown). 
     The voltage applying means (not shown) can set the upper and lower grinding wheels  10  to be plus (+). Moreover, the carrier  24   b  can be set simultaneously with the electrode or switched to be minus (−) via an electric supply wire connected to the carrier  24   b  of the workpiece holding rotation means  24 . 
     The electrolytic fluid supply apparatus  18  positioned above the ELID surface grinding apparatus of FIG. 2 is constituted of an electrolytic fluid chamber  19   a  disposed above the upper grinding wheel  10 , and a plurality of pipelines  19   b  for independently connecting the chamber  19   a  to an electrolytic fluid supply port  16  disposed in the grinding wheel  10 . According to this constitution, the electrolytic fluid stored in the electrolytic fluid chamber  19   a  can be distributed/supplied to the respective electrolytic fluid supply ports  16  (described later) by its own weight, and the constitution is suitable particularly for the upper grinding wheel. 
     Moreover, the electrolytic fluid supply apparatus  18  positioned below the ELID surface grinding apparatus of FIG. 2 is constituted of a rotary connector  20   a  disposed on the rotation center line of the lower grinding wheel  10 , and a plurality of pipelines  20   b  for independently connecting the rotary connector  20   a  to the electrolytic fluid supply port  16  disposed in the grinding wheel  10 . According to this constitution, even in the lower grinding wheel, the electrolytic fluid can smoothly be supplied to the respective electrolytic fluid supply ports  16  from the rotary connector  20   a.    
     Therefore, according to the constitution, the conductive electrolytic fluid can be distributed/supplied to the electrolytic fluid supply port  16  of each ELID grinding wheel  10  by the electrolytic fluid supply apparatus  18 . Additionally, the electrolytic fluid supply apparatus  18  using the rotary connector  20   a  is not limited to the lower grinding wheel, and may be used on the upper grinding wheel. 
     FIG. 3 is a partial view in I-I′ line of FIG.  2 . Moreover, FIG.  4 (II) is a sectional view in II-II′ line of FIG. 3, and FIG.  4 (III) is a sectional view in III-III′ line. 
     As shown in FIG. 2, the ELID grinding wheel  10  of the present invention is constituted of a conductive base member  11  formed of an integrally molded flat plate, and a plurality of fan-shaped segments  12  detachably attached to the base member  11  with a bolt, and the like. 
     Moreover, as shown in FIG. 3, the plurality of fan-shaped segments  12  are adjacent to one another to entirely constitute the annular working surface  10   a . Furthermore, as shown in FIGS.  4 (II) (III), each segment  12  is constituted of a base metal  12   a  directly attached to the base member  11  of FIG. 2 and a grinding wheel part  12   b  formed on the surface of the base metal. 
     The surface of the segment  12 , that is, the grinding wheel part  12   b  includes a recessed groove  14  (grooving) for partitioning the working surface into a plurality of working areas  13 , and an electrolytic fluid supply port  15  for directly supplying the electrolytic fluid to the respective working areas  13 . 
     The recessed groove  14  is constituted of a boundary groove  14   a  extending in the radial direction of the grinding wheel along a boundary line with the adjacent segment, a middle groove  14   b  disposed between the boundary grooves  14   a  and extending in the radial direction of the grinding wheel, and an intersecting groove  14   c  extending to intersect the boundary groove  14   a  and/or the middle groove  14   b . The size of each recessed groove (grooving) is set to an appropriate size for passing the chips, for example, a depth of around 3 mm and width of around 4 mm. Moreover, the boundary groove  14   a  is constituted to be narrow (e.g., 2 mm) so that the same grooving is disposed in a gap with the adjacent segment. According to this constitution, the chips and grinding wheel pieces having reached the intersecting groove  14 C can smoothly be discharged to the outside of the grinding wheel through the boundary groove and/or the middle groove by the action of a centrifugal force. Additionally, in this example, the intersecting groove  14   c  is obliquely disposed to form the flow of the chips by the action of the centrifugal force, but this constitution is not indispensable, and the groove may be disposed in an arbitrary direction, for example, radial direction. 
     Moreover, in this example, the respective working areas  13  have substantially the same area. Furthermore, the respective electrolytic fluid supply holes  15  have substantially the same sectional area and the same number of holes (one hole in this example) are disposed in the respective working areas  13 . According to this constitution, only by supplying substantially the same amount of electrolytic fluid to the respective working areas, the insufficient or excessive part of the electrolytic fluid can be prevented from being generated over the entire working surface  10   a.    
     As shown in FIG.  4 (II), the base metal  12   a  is provided with the electrolytic fluid supply ports  16  connected to the respective electrolytic fluid supply holes  15 . Additionally, in this example, the electrolytic fluid supply hole  15  and electrolytic fluid supply port  16  are constituted of the common through hole, but the single electrolytic fluid supply port  16  may be disposed, and an inner manifold may be disposed for connection to the plurality of electrolytic fluid supply holes  15  inside the base member  11 . Moreover, a channel connected to the electrolytic fluid supply port  16  is disposed in the base member  11  of FIG. 2, and the electrolytic fluid is supplied to the respective electrolytic fluid supply holes  15  through the base member  11 . 
     Moreover, as shown in FIG.  4 (III), an attachment hole  17  is disposed in a part of the segment in which the head of a bolt is set to be lower than the grooving bottom, and the respective segments  12  are detachably attached to the base member  11  by this attachment hole  17 . Furthermore, the relative position can accurately be determined by a positioning tool (not shown), for example, a knock pin. 
     Assuming that the center angle of the circular arc of the segment  12  shown in FIG. 5 is 60 degrees and a segment area is 258 cm 2 , the segment  12  is constituted of six pieces considering from the grinding wheel total area. When the grinding wheel total area is changed, the division number results in Table 1. In examples 1 to 3, the division number varies in a range of 2 to 8. In this case, the center angle is in a range of 180 degrees to 45 degrees. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Divi- 
                   
                   
                   
                 Grinding 
               
               
                 Exam- 
                   
                 sion 
                   
                 Inner 
                 Outer 
                 wheel 
               
               
                 ple 
                 Segment 
                 No. 
                 Angle 
                 Dia. 
                 Dia. 
                 Total Area 
               
               
                   
               
             
            
               
                 Exam- 
                 258 cm 2   
                 2 
                 180° 
                 162 mm 
                  275 mm 
                 387.6 cm 2   
               
               
                 ple 1 
               
               
                 Exam- 
                 same 
                 6 
                  60° 
                 324 
                  550 
                 1550.6 
               
               
                 ple 2 
               
               
                 Exam- 
                 same 
                 8 
                  45° 
                 648 
                 1100 
                 1975.2 
               
               
                 ple 3 
               
               
                   
               
            
           
         
       
     
     According to the above-described constitution of the present invention in the φ3.5 in. magnetic disk, since the conductive grinding wheel  10  is constituted of the conductive base member  11  formed of the integrally molded flat plate, and a plurality of fan-shaped segments  12  detachably attached to the base member to entirely constitute the annular working surface, the fan-shaped segment  12  is reduced in size, and the conventional molding equipment and sintering equipment can be used to easily manufacture the large-sized grinding wheel. 
     Moreover, since each segment grinding wheel part  12   b  is constituted of the plurality of working areas  13  partitioned by the recessed groove  14  (grooving), and the electrolytic fluid is directly supplied from the electrolytic fluid supply holes  15  disposed in the respective working areas, the electrolytic fluid flows in one radial direction from the electrolytic fluid supply hole  15  to the recessed groove  14  surrounding this hole, and the chips and grinding wheel pieces can smoothly be removed with the flow. Moreover, since the optimum amount of electrolytic fluid can easily be supplied to the respective working areas, the grinding fluid insufficient part is not easily generated, and the hydroplaning phenomenon by the excessive part is not easily generated. 
     [Example] 
     In order to confirm the effect of the above-described recessed groove  14  (grooving), three types of grinding wheels provided with no grooving, ten grooves and 20 grooves were tested. As a result, for the surface roughness measured value (Ra measured value: unit of angstrom), when there was no grooving, the value was 1298 at maximum, 14 at minimum, and 127 on average. On the other hand, for ten grooves the value indicated 258 at maximum, 14 at minimum, and 31 on average, for 20 grooves the value indicated 28 at maximum, 12 at minimum, and 18 on average, and a superior surface roughness was obtained. 
     Particularly, for Ra maximum value, the grinding wheel provided with no grooving indicated Ra 1298, but the grinding wheel provided with ten grooves indicated Ra 258, the grinding wheel provided with 20 grooves indicated Ra 28, and the value was remarkably improved. Therefore, it is seen that according to the present invention, with the flow of the electrolytic fluid the chips and grinding wheel pieces are smoothly removed, the scratches are prevented, the insufficiency or excess of the grinding fluid is prevented, and a satisfactory ELID grinding is performed. 
     As described above, for the ELID grinding wheel of the present invention and the ELID surface grinding apparatus using this grinding wheel, the size enlargement is facilitated, the chips and grinding wheel pieces can smoothly be removed, the grinding fluid insufficient part is not easily generated, the hydroplaning phenomenon does not easily occur, and other superior effects are provided. 
     Additionally, the present invention is not limited to the above-described embodiment, and can variously be modified within the scope of the present invention. For example, the working of both surfaces of the workpiece has been described above in the embodiment, but the present invention is not limited to this, and can similarly be applied to single surface grinding.