Patent Publication Number: US-6341999-B1

Title: Glass substrate chamfering method and apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a chamfering method and apparatus for processing the edges of a glass substrate for a hard disk. 
     2. Prior Art 
     A hard disk is a storage device of a computer; the surface of a thin doughnut-like circular disk is coated with a magnetic substance in which data are stored. Aluminum has been used conventionally as the material of this circular disk (hereafter referred to as “hard disk substrate”). However, recently, chemically reinforced glass or crystalline glass have been used as the substrate of a hard disk (referred to as “glass substrate”). 
     FIGS. 1A and 1B schematically show a glass substrate; FIG. 1A is a general view, and FIG. 1B shows a sectional view of the outer and inner peripheries. For a glass substrate with a nominal size of 2.5 inches, for example, outer and inner diameters are 2.5 and 1 inches (about 63 mm and about 25 mm), respectively, and the thickness is about 1.6 mm. The outer and inner peripheries (also referred to as “edge portions”) are composed of end surfaces (with a width of 0.6 mm) at outer and inner peripheries, and a pair of oblique surfaces sandwiching the peripheries (with an angle of about 45°). 
     The aforementioned glass substrate is subjected to high-speed revolutions of several tens thousand revolution per minutes to shorten the access time of the hard disk. Consequently, the entire body thereof must be processed precisely, and a high dynamic balance must be achieved. Both surfaces of the glass substrate are finished to a mirror surface of an Rz of 1 μm or less, so that a magnetic substance can be coated at a high density to maximize memory capacity. 
     In addition, edges (including end surfaces and oblique surfaces) should be finished to mirror surface quality with such high accuracy and quality as required by both surfaces of the glass substrate. The reason edge portions are finished to mirror surface quality is explained below. 
     (1) There are microscopic cracks on a rough pear-like surface. As a result, a glass reinforcing treatment may result in unevenness, or cracks may grow due to high-speed revolutions during operation, eventually causing breakage of the substrate. 
     (2) Contaminants may remain in microscopic recesses of a crack, so cleaning may be incomplete, and it may be a source of contamination in a subsequent process. 
     Edge portions of the above-mentioned glass substrate were, in the prior art, finished to mirror surface quality first by using a formed plated grindstone,which is highly precise, and then by buffing to mirror surface quality. 
     However, compared to an aluminum substrate, a glass substrate provides poor machinability, so one problem was that machining efficiency, when using a plated grindstone, was low. More explicitly, although rather coarse grinding particles of #500 (grain diameter of about 30 μm) to #600 (grain diameter of about 25 μm) are used, grinding particles still peel considerably; therefore, the plated grindstone must be replaced frequently, so the continuous processing period with the machine is short and operating ratio is low because the grindstone must be replaced. In addition, since the processed surface is in a pear-like state with a rough surface, buff-polishing must be carried out for a long time (about one hour) as a subsequent process to the machining with the plated grindstone. Consequently, overall productivity is low, which is a practical problem. 
     SUMMARY OF THE INVENTION 
     The present invention aims to solve these problems. That is, an object of the present invention is to provide a chamfering method and apparatus for the glass substrate of a hard disk, with which edge portions of the substrate can be processed with high accuracy, quality, and efficiency, thereby greatly reducing the need for a subsequent process, such as buff-polishing, or even eliminating it. The present invention provides, as a method of processing end surfaces of outer and inner peripheries of a doughnut-like glass substrate ( 1 ) with a circular hole ( 1   a ) at the center thereof and chamfering oblique surfaces sandwiching the end surfaces, a glass substrate chamfering method using a metal-bonded outer-surface grindstone ( 12 ) for simultaneously processing the end surface and oblique surfaces of the outer periphery, and a metal-bonded inner-surface grindstone ( 14 ) for simultaneously processing the end surface and oblique surfaces of the inner periphery, wherein by means of the outer-surface grindstone and the inner-surface grindstone, end surfaces and oblique surfaces of outer and inner peripheries of the glass substrate are ground at the same time, and during a grinding process, the outer-surface grindstone is dressed electrolytically to sharpen the grinding particles, and during a non-processing period to replace the glass substrate, the inner-surface grindstone is dressed electrolytically. 
     The present invention also provides a chamfering apparatus for processing end surfaces of inner and outer peripheries of the doughnut-like glass substrate ( 1 ) having a circular hole ( 1   a ) at the center thereof and chamfering oblique surfaces sandwiching the end surfaces, composed of the metal-bonded outer-surface grindstone ( 12 ) for processing the end surface and oblique surfaces of the outer periphery, the metal-bonded inner-surface grindstone ( 14 ) for simultaneously processing the end surface and oblique surfaces of the inner periphery, the outer-surface electrode ( 18 ) for electrolytically dressing the outer-surface grindstone during the grinding process, the inner-surface electrode ( 20 ) for electrolytically dressing the inner-surface grindstone during the non-processing period, a grinding liquid feeder ( 22 ) for supplying a conductive grinding liquid between the outer-surface grindstone and the outer-surface electrode and between the inner-surface grindstone and the inner-surface electrode, and a voltage application means ( 24 ) for applying an electrolytically dressing voltage across the outer-surface grindstone and the outer-surface electrode and across the inner-surface grindstone and the inner-surface electrode, wherein the outer-surface grindstone is dressed electrolytically when the outer-surface grindstone is grinding the end surface and oblique surfaces of the outer periphery of the glass substrate, and during a non-processing period when the glass substrate is being replaced, the inner-surface grindstone is dressed electrolytically. 
     According to the method and the apparatus of the present invention, as described above, end surfaces and oblique surfaces of outer and inner peripheries of the glass substrate ( 1 ) can be ground simultaneously using the outer-surface grindstone ( 12 ) and the inner-surface grindstone ( 14 ). In addition, during this grinding period, the outer-surface grindstone is dressed electrolytically (also known as “in-process dressing”) using the voltage application means ( 24 ) while supplying a conductive grinding liquid between the grindstone and the outer-surface surface electrode ( 18 ), thereby even when fine grinding particles are used, the outer-surface grindstone can be sharpened, therefore, while maintaining a high efficiency, the workpiece can be processed with a high accuracy and quality. Furthermore, the conductive grinding liquid is supplied between the inner-surface grindstone and the inner-surface electrode ( 20 ) when the glass substrate is replaced during a non-work period, the grindstone is dressed electrolytically (also known as “interval dressing”) by the same voltage application means. Therefore, even when similar fine grinding particles are used, the inner-surface grindstone can be sharpened while maintaining high efficiency and can process a workpiece precisely with high quality. 
     In addition, because the outer-surface grindstone undergoes in-process dressing during grinding, and the inner-surface grindstone undergoes interval dressing during a non-processing period, the load of the voltage application means ( 24 ) can be averaged, and compared to an alternate method where both grindstones are dressed at the same time, the power supply facility can be made more compact. 
     According to a preferred embodiment of the present invention, an electrode ( 16 ) for discharge truing is installed in place of the above-mentioned glass substrate ( 1 ), the aforementioned outer-surface grindstone ( 12 ) and inner-surface grindstone ( 14 ) are positioned at a grinding position, the aforementioned electrode ( 16 ) and grindstones ( 12 ,  14 ) are charged positive and negative, respectively, and both grindstones are formed by discharge truing. 
     Using the above method, even if the outer-surface grindstone ( 12 ) or inner-surface grindstone ( 14 ) deteriorates in terms of shape accuracy after long-term use, an error in the installation of the grindstones caused by removing and remounting the grindstones can be avoided. In addition, the workpiece can be formed with a high accuracy because the grindstones can be subjected to truing by discharge while being mounted. Moreover, the power supply for electrolytic dressing (also referred to as the “voltage application means”) can be used for discharge truing simply by changing polarity, so the power supply need not have a large supply capacity. 
     Also, the above-mentioned metal-bonded outer-surface grindstone ( 12 ) and metal-bonded inner-surface grindstone ( 14 ) have circular cylindrical outer surfaces and frustum-type grooves ( 12   a ,  14   a ) that can contact end surfaces and oblique surfaces of the circular cylindrical surfaces. 
     In this configuration, edge portions of the glass substrate can be processed precisely with high quality simply by making frustum-type grooves ( 12   a ,  14   a ) of the formed grindstones come into contact with edge portions (also referred to as the “outer periphery” and “inner periphery”) of the glass substrate ( 1 ). 
     The aforementioned metal-bonded outer-surface grindstone ( 12 ) and/or metal-bonded inner-surface grindstone ( 14 ) are provided with a plurality of frustum-type grooves ( 12   a ,  14   a ) with separating intervals around the same axis. With this configuration, if the accuracy of the shape of a groove deviates from the specified value due to wear caused by processing, the grindstone can be shifted axially and the grooves can be used sequentially, thereby the process can be carried out without a stopping, and continuous operation can be greatly extended. 
     In addition, the above-mentioned metal-bonded outer-surface grindstone ( 12 ) and/or metal-bonded inner-surface grindstone ( 14 ) are provided with frustum-type grooves ( 12   a ,  14   a ) for rough processing and finish processing, with intervals around the same axis. 
     Due to this configuration wherein the grindstones are provided with frustum-type grooves for rough processing and finish processing respectively, after the workpiece is processed roughly by rough-processing frustum-type grooves ( 12   a ,  14   a ), the grindstone is shifted in the axial direction, and finish processing frustum-type grooves ( 12   a ,  14   a ) on the same grindstone can be used for finish processing. Therefore, compared to the prior art method in which the worn grindstone is replaced with another fresh grindstone, down-time can be greatly reduced, while avoiding the deterioration of processing accuracy caused by decentering (also known as “axial deviation”) of the center of rotation. 
     Moreover, another preferred embodiment of the present invention is provided with a substrate drive device ( 32 ) that drives the glass substrate ( 1 ) to rotate around the axial center thereof, an outer-surface grindstone drive device ( 34 ) that drives the metal-bonded outer-surface grindstone ( 12 ) around the axial center thereof, and an inner-surface grindstone drive device ( 36 ) that drives the metal-bonded inner-surface grindstone ( 14 ) around the axial center thereof; the substrate drive device, outer-surface grindstone drive device and/or inner-surface grindstone drive device are configured to be movable between the processing position of the glass substrate and a non-processing position where the outer-surface grindstone and the inner-surface grindstone are kept away from the glass substrate while the glass substrate is replaced. 
     In this configuration, when the metal-bonded outer-surface grindstone ( 12 ) and the metal-bonded inner-surface grindstone ( 14 ) are positioned at the processing location, the outer-surface grindstone is dressed electrolytically (the process of in-process dressing), while grinding the workpiece, and when both grindstones are positioned at a non-processing location, the glass substrate can be replaced while the inner-surface grindstone is being dressed electrolytically (the process of interval dressing). 
     The aforementioned substrate drive device ( 32 ) is provided with a vacuum suction head ( 33   a ) to suck the glass substrate ( 1 ) and a supporting head ( 33   b ) that supports the glass substrate between the vacuum suction head. This configuration allows the vulnerable glass substrate ( 1 ) to be driven and rotated precisely, while being protected when it is sandwiched by the vacuum suction head ( 33   a ) and the supporting head ( 33   b ). 
     In another configuration, a detachable discharge truing electrode ( 16 ) is sandwiched between the aforementioned vacuum suction head ( 33   a ) and the supporting head ( 33   b ), and electrode ( 16 ) being provided with an outer periphery and inner periphery that match the glass substrate ( 1 ). In this configuration, even if the outer-surface grindstone ( 12 ) or the inner-surface grindstone ( 14 ) deteriorates in terms of shape accuracy after a long-term operation, the grindstone can continue high-accuracy forming by being discharge trued, without the need to remove the grindstone and remount it to the machine, simply by setting the discharge truing electrode ( 16 ) in place of the glass substrate ( 1 ). 
     Other objects and advantages of the present invention are revealed according to the following description referring to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are schematic views of a glass substrate. 
     FIG. 2 is an isometric view of a conventional electrolytic interval-dressing grinding device used in the prior art method. 
     FIG. 3 is a plan view showing the general configuration of the chamfering apparatus used in the method according to the present invention. 
     FIG. 4 is an enlarged view of A part in FIG.  3 . 
     FIG. 5 is an enlarged view of B part in FIG.  3 . 
     FIG. 6 is an enlarged view of a head portion for discharge truing. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described below referring to the drawings. The same components in each drawing are identified with the same numbers, and no duplicate description is given. 
     The electrolytic in-process dressing (ELID) grinding method developed as a means of grinding an ultra-precise mirror surface with high efficiency, which was considered impossible using conventional grinding technologies, is disclosed. According to this ELID grinding method, conductive joined parts of a metal-bonded grindstone are dissolved by electrolytic dressing to sharpen the grindstone while it is grinding. According to this grinding method, even a cemented carbide material can be efficiently processed to a mirror surface using a metal-bonded grindstone with fine grinding particles: therefore, the efficiency and the precision of processing can be improved. 
     The inventors of the present invention have applied for a Japanese patent, “Electrolytic Interval Dressing Method for Grinding” (unexamined Japanese patent application No. 115867/1992). A sketch of this prior art device and method is shown in FIG. 2; an electrode  5  is provided with a gap in which workpiece  1  is ground, a conductive grindstone  4  with a voltage is driven and reciprocated between the workpiece  1  and the electrode  5 , a conductive grinding liquid is interposed between the conductive grindstone  4  and the electrode  5 , and electrolytic dressing and grinding is carried out alternately. 
     The present invention combines the aforementioned in-process dressing (ELID method) and interval dressing, and adds various novel devices for chamfering a glass substrate. 
     The chamfering apparatus according to the present invention is used to process end surfaces of outer and inner peripheries of a doughnut-like glass substrate  1 , as shown in FIGS. 1A and 1B, with a circular hole  1   a  at the center thereof, and processing and chamfering edges of oblique surfaces flanked on the end surfaces. 
     In FIG. 3, this chamfering apparatus  10  is provided with a metal-bonded outer-surface grindstone  12 , a metal-bonded inner-surface grindstone  14 , an outer-surface electrode  18 , an inner-surface electrode  20 , a grinding liquid feeder  22 , and a voltage application means  24 . 
     The metal-bonded outer-surface grindstone  12  and the metal-bonded inner-surface grindstone  14  are composed of grinding particles (for instance, diamond or CBN) and a conductive joining part (referred to as the “metal-bonded part”) that fixes the grinding particles. The metal-bonded part is a metal such as cast steel or bronze, molten and solidified. 
     As shown in FIGS. 3 to  5 , the outer peripheries of metal-bonded outer-surface grindstone  12  and metal-bonded inner-surface grindstone  14  are circular and cylindrical, and are provided with frustum-like grooves  12   a  and  14   a  respectively that can contact the end surface and oblique surfaces on the cylindrical surface. These frustum-like grooves  12   a ,  14   a  are formed for processing edges of the glass substrate  1  with great precision and high quality simply by having the grooves contact the edge portions (outer periphery and inner periphery) of the glass substrate during grinding. 
     As shown in FIGS. 4 and 5, a plurality of frustum-like grooves  12   a ,  14   a  are provided (for example, ten grooves each) on the same axis of the metal-bonded outer-surface grindstone  12  and the metal-bonded inner-surface grindstone  14  respectively at predetermined intervals. In this example, among the ten grooves, one half of the frustum-like grooves  12   a ,  14   a  are formed for performing a rough processing, and the remainder for performing a finish processing. 
     In this configuration, the metal-bonded outer-surface grindstone  12  can simultaneously process the end surface and oblique surfaces of the outer periphery of glass substrate  1 , and the metal-bonded inner-surface grindstone  14  can also simultaneously process the end surface and oblique surfaces of the inner periphery of the substrate  1 . 
     In FIG. 3, the chamfering apparatus  10  according to the present invention is further provided with substrate drive device  32 , outer-surface grindstone drive device  34 , and inner-surface grindstone drive device  36 . 
     The substrate drive device  32  is composed of a vacuum suction head  33   a  for sucking the glass substrate  1 , and a supporting head  33   b  for pinching and supporting the glass substrate  1  to the vacuum suction head  33   a . The detachable vacuum suction head  33   a  is mounted to a drive axis  37 , which is driven and rotated around the axial center of the glass substrate  1 , by screwing threads of a connecting rod  37   a . In addition, the end surface (left side in this figure) of the vacuum suction head  33   a  can vertically suck and hold the glass substrate  1  by evacuating a space at the contact surface of the glass substrate  1  via an exhaust passage provided in the vacuum suction head  33   a.    
     The supporting head  33   b  is composed of a pinching head  38   a  contacting the opposite side (left side in this figure) of the glass substrate  1  and a supporting part  38   c  for supporting the substrate via a bearing  38   b.    
     The substrate drive device  32  is configured to be capable of traveling in the left and right direction (X direction) between position W for processing the glass substrate  1  and non-processing position R (state of FIG.  3 ), where the outer-surface grindstone  12  and the inner-surface grindstone  14  are removed from the glass substrate  1  and the substrate can be replaced. 
     The above-mentioned supporting part  38   c  is powered at position W for processing the glass substrate  1 , by a powering device not illustrated at a predetermined pressure to the right in the figure, thereby it pinches and holds the glass substrate  1  at the vacuum suction head  33   a  with a satisfactory pressure. 
     Due to this configuration, the glass substrate  1  can be driven and rotated by the substrate drive device  32  around the axial center thereof, while resisting a processing load during grinding of the outer and inner peripheries of the glass substrate  1 . 
     The outer-surface grindstone drive device  34  is provided with a drive shaft  34   a  that fixes the metal-bonded outer-surface grindstone  12  to the right end in the figure. This drive shaft  34   a  is parallel to the axial center of the glass substrate  1 , and is driven and rotated by a drive device not illustrated. In addition, this outer-surface grindstone drive device  34  is arranged to be capable of freely traveling in backward and forward directions (Y direction) perpendicular to left and right directions (X direction) in the figure, by an NC control system. In this configuration, a predetermined frustum-like groove  12   a  for processing the glass substrate  1  is positioned corresponding to the glass substrate  1  located at a processing position W by moving the outer-surface grindstone  12  in the X direction from the non-processing position R shown in FIG. 3, and the outer-surface grindstone  12  is moved in the Y direction so that it contacts the outer periphery of the glass substrate  1  and the edge portion can be processed. 
     The inner-surface grindstone drive device  36  is also provided with a drive shaft  36   a  that fixes the metal-bonded inner-surface grindstone  14  to the right end in the figure, in the same way as with outer-surface grindstone drive device  34 . This drive shaft  36   a  is parallel to the axis of the glass substrate  1 , and is driven and rotated by a drive device, which is not illustrated. In addition, this inner-surface grindstone drive device  36  is also capable of freely traveling in backward and forward directions (Y direction) perpendicular to left and right directions (X direction) in the figure by the NC control system. In this configuration, a predetermined frustum-like groove  12   a  that processes the glass substrate  1  by moving the inner surface grindstone  14  in the X direction is positioned according to the glass substrate  1 , and further the inner-surface grindstone  12  is moved in the Y direction so that it contacts the inner periphery of the glass substrate  1  and can process the edge portion. 
     The outer-surface electrode  18  is mounted in the main body  34   b  of the outer-surface grindstone drive device  34  via an insulation member (not illustrated), and can travel together with the outer-surface grindstone drive device  34 . This outer-surface electrode  18  is also provided with an opposite surface  18   a  at a predetermined spacing from a plurality of frustum-like grooves  12   a  of the metal bond outer-surface grindstone  12 . 
     The inner-surface electrode  20  can travel in backward and forward directions (Y direction) from the non-processing position R shown in FIG. 3 to a position close to the metal-bonded inner-surface grindstone  14 . This inner-surface electrode  20  is also provided with an opposing surface  20   a  with a predetermined spacing from a plurality of frustum-like grooves  14   a  of the metal-bonded inner-surface grindstone  14 . 
     The grinding liquid feeder  22  supplies a conductive grinding liquid between the outer-surface grindstone  12  and the outer-surface electrode  18  and between the inner-surface grindstone  14  and the inner-surface electrode  20 . More explicitly, the grinding liquid feeder  22  is provided with a nozzle  22   a  fixed to the main body  34   b  of the outer-surface grindstone drive device  34 , and can supply the conductive grinding liquid between the outer-surface grindstone  12  and the outer-surface electrode  18 , irrespective of where the outer-surface grindstone drive device  34  is located. 
     Also, the grinding liquid feeder  22  is provided with another nozzle  22   a  that travels together with the inner-surface electrode  20 , and can supply conductive grinding liquid between the inner-surface grindstone  14  and the inner-surface electrode  20  at a location where the inner-surface electrode  20  comes close to the inner-surface grindstone  14 . 
     A voltage application means  24  is composed of a power source  24   a , a brush  24   b  that contacts the rotating axes of grindstones  12 ,  14 , and a power line  24   c  that connects brush  24   b  and electrodes  18 ,  20  electrically to the power source  24   a , and applies positive and negative voltages to grindstones  12 ,  14  and electrodes  18 ,  20 , respectively. The power source  24   a  should be a constant-current-type ELID power supply that can provide DC voltage pulses. 
     According to the method of the present invention using the aforementioned apparatus, end surfaces and oblique surfaces of the outer and inner peripheries of the glass substrate  1  are ground simultaneously by means of the outer-surface grindstone  12  and the inner-surface grindstone  14 . In addition, the outer-surface grindstone  12  is dressed electrolytically and sharpened during grinding. Furthermore, during a non-processing period when the glass substrate  1  has completed processing and is being replaced, the inner-surface grindstone  14  is dressed electrolytically to sharpen it. 
     According to the method and the apparatus of the present invention described above, end surfaces and oblique surfaces of outer and inner peripheries of the glass substrate  1  can be ground simultaneously using the outer-surface grindstone  12  and the inner-surface grindstone  14 . In addition, a conductive grinding liquid is supplied during this grinding between the outer-surface grindstone  12  and the outer-surface electrode  18 , and the grindstone is in-process dressed (ELID grinding) using the voltage application means  24 . Therefore, even when fine grinding particles, for instance, #1200 (grain diameter of about 15 μm) to #2000 (grain diameter of about 8 μm) are used, the glass substrate can be processed highly efficiently with high quality and high precision because of the periodic sharpening of the outer-surface grindstone. Moreover, the inner-surface grindstone is dressed electrolytically by the same voltage application means by supplying conductive grinding liquid between the grindstone and the inner-surface electrode  20  during a non-processing period when the glass substrate  1  is replaced (the interval dressing), so even if similar fine grinding particles to those above are used, the inner-surface grindstone can be sharpened continually and can process the workpiece with high precision and high quality while maintaining a high work efficiency. 
     In addition, because the outer-surface grindstone  12  undergoes in-process dressing during processing and the inner-surface grindstone  14  undergoes interval dressing when processing is stopped, power loads of the voltage application means  24  can be averaged, and the power source can be made more compact than the case in which both grindstones are dressed electrolytically at the same time. 
     In addition, since outer peripheries of the outer-surface grindstone  12  and the inner-surface grindstone  14  are cylindrical surfaces provided with frustum-like grooves  12   a ,  14   a  respectively that can contact end surfaces and oblique surfaces, the edge portions of the glass substrate can be processed by grinding with high precision and highquality simply by contacting the frustum-like grooves  12   a ,  14   a  with respective edge portions of the glass substrate  1  (outer and inner peripheries). 
     In this configuration, the plurality of frustum-like grooves  12   a ,  14   a  are arranged with spacings on the same axis of the outer-surface grindstone  12  and the inner-surface grindstone  14 , respectively; therefore, the grinding process does not cease, but can be carried out continuously simply by shifting the grooves in the axial direction as soon as the accuracy of shape of the groove no longer satisfies a predetermined value due to wear during processing. This shifting of the grooves permits an extended period of continuous operation. 
     When frustum-like grooves  12   a ,  14   a  are configured separately for rough and finish processing with intervals on the same axis of the outer-surface grindstone  12  and the inner-surface grindstone  14 , the workpiece can first be roughly processed using the frustum-like grooves  12   a ,  14   a  that are used for rough processing and then the workpiece can be shifted axially and the step of finish processing performed using the frustum-like grooves  12   a ,  14   a  of the same grindstone that are used for finish processing; therefore, compared to prior art processing methods where the worn grindstone is replaced with another fresh grindstone, the method of the present invention reduces down time significantly while avoiding deterioration of processing accuracy due to decentering of the rotation axis (also known as the “shifting of rotating center”). 
     FIG. 6 is an enlarged view of the head part during discharge truing. In FIG. 6, the chamfering apparatus  10  according to the present invention is provided with a vacuum suction head  33   a  and a detachable discharge truing electrode  16  sandwiched between the vacuum suction head  33   a  and a supporting head  33   b . The outer and inner peripheries of this electrode  16  are identical in shape to the outer and inner peripheries of the glass substrate  1 . 
     In addition, the above-mentioned voltage application means  24  can apply positive and negative voltages to the discharge truing electrode  16  and the grindstones  12 ,  14 , respectively by switching the connection. 
     According to the method of the present invention, using the aforementioned apparatus, the discharge truing electrode  16  is installed between the vacuum suction head  33   a  and the supporting head  33   b  in place of the glass substrate  1 , and the outer-surface grindstone  12  and the inner-surface grindstone  14  are positioned at a grinding location and both grindstones are formed by discharge truing. A conductive mist is also used during this discharge truing, thereby efficient and high-accuracy discharge forming is enabled. 
     Using this method, even if the shape accuracy of the outer-surface grindstone  12  or the inner-surface grindstone  14  adversely changes because of long-term operation, the deteriorated grindstone can be trued by discharge and formed highly precisely without the need to remove the grindstone from the system; therefore, installation errors caused by removing or reinstalling the grindstone can be avoided. In addition, the power source for electrolytic dressing (the voltage application means) can also be used for discharge truing simply by changing polarity, so the power facility need not be large. 
     According to the present invention described above, because in-process dressing (ELID grinding) and interval dressing are used jointly, even if grinding particles as fine as #1200 (grain diameter of about 15 μm) to #2000 (grain diameter of about 8 μm) are used, grindstones are sharpened as required at any time and can process the workpiece highly accurately and with high quality while maintaining a high processing efficiency. Because the grindstones undergo in-process dressing during processing and interval dressing when processing is stopped, work loads of the voltage application means  24  can also be averaged and the power facility can be made more compact. Furthermore, since a plurality of frustum-like grooves  12   a ,  14   a  are arranged at intervals on the coaxis of the grindstones  12 ,  14  respectively, any working groove for which the accuracy of shape can no longer satisfy a predetermined accuracy due to processing wear can be shifted axially, and the grooves can be used sequentially; therefore, the process of grinding need not be interrupted, but can be carried out continuously, so continuous operating time can be greatly extended. In addition, frustum-like grooves for rough and finish processing respectively can be provided on the same axis, thus after a rough grinding process is completed, the grindstone can be shifted axially and can finish grinding the workpiece during a finish grinding process, thereby down-time is reduced dramatically and the degradation of processing accuracy caused by deviation of the center can be avoided. Alternatively, a discharge truing electrode  16  can be installed in place of the glass substrate  1  and can true both grindstones by discharge, so that any installation error due to removing or reinstalling grindstones can be avoided. Instead, high-accuracy forming can be maintained by discharge truing with the grindstones in place. 
     Accordingly, the method and the apparatus for chamfering a glass substrate disclosed by the description of the present invention provides superior results over the prior art methods including the benefits of the processing of edge portions of a glass substrate for a hard disk with precision, improved efficiency, and with high quality to such an extent that the need for a subsequent process, such as buff-polishing, can be reduced or even eliminated. 
     Although the present invention has been described referring to several preferred embodiments, the scope of rights encompassed by the present invention should not be interpreted to be limited only to these embodiments. To the contrary, the scope of rights encompassed by the present invention shall include all improvements, corrections, and equivalent entities covered by the scope of the attached claims.