Conventionally, disk materials have been widely used as substrates for information recording media such as hard disks for personal computers and audio media. In recent years, there has been an ever-increasing demand for glass plates capable of recording information with high density. Therefore, attention has been focused on the development of improved machining processes for the glass plates.

When a glass plate is machined into a disk substrate for an information recording medium, the glass plate is punched out by a glass cutter to form a recording disk raw plate. The recording disk raw plate, as punched out, has sharp edges. The sharp corners of the edge are cut off with a chamfering process. The new chamfered edge is then finished.

Conventionally, the chamfering process is conducted by grinding and shaping the edge of the recording disk raw plate with a diamond wheel to form a trapezoidal configuration on the edge. A plurality of recording disk raw plates are superposed and fixed, and the edges of the plurality are polished with a brush while supplying cerium oxide slurry to the edges.

However, a number of pits are formed in the edges by the aggressive chamfering processes with the diamond wheel. The polishing of the edges with the brush and slurry fails to completely remove the pits. Consequently, fine particles derived from glass plate shavings and polishing slurry enter the remaining pits. The fine particles then may contaminate later processes by dislodging from the pits.

Additionally, this conventional method generates a large amount of waste-water containing cerium oxide, which is a soil pollutant, resulting in high costs for waste- water treatment.

Japanese Patent Kokai Publication No. 01-005759 (1989) discloses a method for grinding the edge of a glass hard disk raw plate by using a grinding wheel.

However, this method needs to use the grinding wheel together with elastic members, resulting in a complex grinding operation. Moreover, the complex device results in high costs.

Japanese Patent No. 2000042889A, which was filed by the same applicant of the present application, discloses a method for grinding the edge of a recording disk raw plate, which can solve the above-mentioned conventional problems. In this method, the edge of a disk raw plate, as punched out, is polished by using a resin bonded grinding wheel composed of a comparatively soft bonding agent. As a result, it is possible to reduce the size of the pits remaining on the face of the edge.

However, in recent years, higher recording density has been demanded in the recording media. As a result, the pits on the edge of the substrate causing contamination on the substrate surface must be virtually eliminated.

Moreover, the specification of Japanese Patent No. 2000042889A, discloses a method of independently grinding the edge from the upper face, the right slanting face and the left slanting face of the trapezoidal configuration by using three grinding wheels in an attempt to effectively eliminate the pits from the edge of a disk raw plate.

For this reason, a complicated operation is required in which the three grinding wheels need to be precisely positioned and removably attached to three driving shafts. This operation tends to cause offsets in the mounting positions, resulting in deviations in the quality of the finished substrate. Additionally, the grinding wheel disclosed in the method disclosed in Japanese Patent No. 2000042889A has a short service life.

The present invention has been devised to provide a method of finishing the edge of a recording disk raw plate which can virtually eliminate pits on the edge of a recording medium substrate and which is easily operated without causing any deviations in the quality of the finished recording medium.

The present invention is to provide a process for mirror-finishing the edge of a recording disk raw plate comprising: (a) grinding the edge of a recording disk raw plate with a diamond wheel to shape the edge into a trapezoidal configuration ; (b) grinding the edge of the recording disk raw plate with a resin bonded grinding wheel which contains abrasive grains having a particle size of between mesh sizes #220 to #1200 ; and (c) grinding the edge of the recording disk raw plate with a resin bonded

grinding wheel which contains abrasive grains having a particle size of between mesh sizes #5000 to #20000. Thus, it becomes possible to finish the edge of a recording disk raw plate while virtually eliminating the pits on the edge of the recording medium substrate. Additionally, the process is easily operated without causing any deviations in the quality of the finished recording medium.

In the present invention, the recording disk raw plate refers to a disk material used for a disk substrate of an information recording medium on and from which electronic information is written and read. For example, the term recording disk raw plate includes a hard-disk use glass substrate and a silicon wafer substrate.

In certain embodiments, the glass used as a recording disk raw plate may be either amorphous glass or crystalline glass. In such hard-disk use glass embodiments, the thickness of the disk raw plate is set to 0.4 to 1.4 mm, more preferably, 0.6 to 0.8 mm.

When a glass plate is machined into a recording disk raw plate for an information recording medium, the glass plate is punched out into a round shape by a glass cutter. Then, the edge of the recording disk raw plate, thus punched out, is ground by a diamond wheel to shape the edge into a trapezoidal configuration so as to cut off corners of the edge thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view that shows the shape of the edge of a recording disk raw plate that is ground and shaped by using a diamond wheel.

FIG. 2 is a schematic cross-sectional view that shows the shape of a groove that is formed in the peripheral face of a grinding wheel used in the present invention.

FIG. 3 is a perspective view that shows one example of a combined wheel used in the present invention.

FIG. 4 is a cross-sectional view that shows a process in which a dressing process is carried out on a grinding wheel used in the present invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a schematic cross-sectional view that shows the shape of the edge 100 of a recording disk raw plate that has been ground and shaped by a diamond wheel. In certain embodiments, the thickness of the disk raw plate is 0.6 mm, the

upper side 102 of the trapezoidal configuration has a length of approximately 300 pLm, and the angle of inclination A on both sides 104 is approximately 45°.

Upon grinding and shaping a fragile material such as glass by using a diamond wheel, pits are formed all through the ground surface. The pits formed typically have a diameter of 20 to 50 um.

The pits tend to fill with foreign matter such as the shavings produced by the grinding process. This foreign matter may dislodge during later processes and result in contamination on the substrate surface. In recent years, higher recording density has been desired in the recording media, and it has been demanded that contamination on the substrate surface be minimized. Therefore, minimization of the pits on the edge of the substrate is desirable.

In the present invention, the pits are minimized or eliminated by mirror- finishing the edge of the disk raw plate that has been ground and shaped into a trapezoidal configuration. The method includes at least two grinding processes as described below. The grinding process of the present invention requires a supply of water that is added so as to prevent scattering of glass powder generated during the grinding process. In other words, it is not necessary to provide a grinding assistant such as cerium oxide slurry. Therefore, no waste water containing a soil pollutant is generated during the grinding process, making it possible to cut costs for the corresponding treatment.

In the first grinding process, the grinding process is carried by using a resin bonded grinding wheel (hereinafter, referred to as"first grinding wheel") which contains abrasive rains having a particle size of between mesh sizes #220 and #1200 (JISR6001, published in 1987), preferably between mesh sizes #320 and #800, and more preferably between mesh sizes #400 and #600.

The particle size greater than a #220 mesh size sometimes results in new pits on the edge of the glass plate during the grinding process. The particle size finer than a #1200 mesh size may result in a long polish time to eliminate the pits. Examples of the material of abrasive grains include SiC, A1203, CeO2, and other known and normally used polishing materials.

The first grinding wheel preferably has a shore D hardness of not less than 80,

more preferably, between 85 and 95. The shore hardness D less than 80 makes the grinding face too erodable, resulting in a short service life of the grinding wheel. The density of the first grinding wheel is preferably set in the range between 1.6 and 2.5 g/cm3. The density less than 1.6 g/cm3 makes the grinding face too erodable, resulting in a short service life of the grinding wheel. The density exceeding 2.5 g/cm3 may form additional pits on the edge of the glass plate during the grinding process. A preferred resin for the resin grinding wheel is polyurethane. A preferred polyurethane includes that described in Japanese Patent Kokai Publication No. 294336/1990 (U. S.

Patent No. 4,933,373), which discloses cross-linked polyurethane that has a glass transition temperature of approximately 10 °C and a glass transition temperature range of approximately higher than 70 °C.

In addition to the above-mentioned setting of the particle size and material of the abrasive grains, the first grinding wheel is preferably manufactured by using a method disclosed in Japanese Patent Kokai Publication No. 294336/1990 (U. S. Patent No. 4,933,373). Such a grinding wheel is generally known as a foamed elastic grinding material, and examples thereof include such wheels as the DLO WHEEL commercially available from Sumitomo 3M Ltd.

The first grinding wheel may be provided with a groove on its peripheral surface corresponding to the edge shape of the disk raw plate. For example, in an embodiment where the edge of the recording disk raw plate is ground by a diamond wheel to shape the edge into a trapezoidal configuration, a groove having a reversed trapezoidal configuration, elongated in a direction perpendicular to the shaft, may exist on the first grinding wheel's peripheral surface. FIG. 2 is a schematic cross- sectional view that shows the shape of the groove 200 formed in the peripheral surface of the grinding wheel used in the method of the present invention. In this case, the upper face and right and left slanting faces of the edge of the disk raw plate are simultaneously ground by using a single grinding wheel. One groove 200 or a plurality of grooves may be formed. The edges of a plurality of disk raw plates, for example superposed and fixed together, may be simultaneously ground by using a grinding wheel with a plurality of grooves.

In certain embodiments, the disk raw plate may include an inner

circumferential portion defining a center hole. The first grinding wheel may be formed into a cylinder shape. In this case, the grinding wheel can be used to finishing the inner circumferential portion of the disk raw plate.

The disk raw plate is then subjected to a grinding process. The grinding wheel and a disk raw plate are respectively counter-rotated and a load is applied to the edges thereof so as to allow them to come into contact with each other. The grinding conditions are properly adjusted depending on the level of the finishing required. In certain embodiments, the peripheral surface velocity of the grinding wheel is set in the range of approximately 1000 to 3000 m/min, the peripheral surface velocity of the disk raw plate is set in the range of 20 to 500 m/min., the load applied to the edges is set in the range of approximately 0.2 to 5 kg, and the grinding time is in the range of 5 to 60 seconds, more preferably, 10 to 30 seconds.

The second grinding process is carried out by using a resin bonded grinding wheel (hereinafter, referred to as"second grinding wheel") containing abrasive grains having a particle size of between mesh sizes #5000 and #20000, preferably between #6000 and #15000, more preferably between #8000 and #10000.

The abrasive grains having a particle size greater than #5000 tends to cause an insufficient pit removing process. The abrasive grains having a particle size finer than #20000 makes the amount of polishing too small, taking a long time in eliminating pits. The density of the second grinding wheel is preferably set in the range of 1.6 to 2.5 g/cm3. The density less than 1.6 g/cm3 makes the grinding face too erodable, resulting in a short service life of the grinding wheel. The density exceeding 2.5 g/cm3 may form additional pits on the edge of the glass plate during the grinding process.

Except for the particle size of the abrasive grains, the second grinding wheel has the same structure as the first grinding wheel. Therefore, the second grinding wheel can be manufactured in the same manner as the first grinding wheel.

The grinding method of a disk raw plate is the same as that of the first grinding process. The grinding conditions are properly adjusted depending on the level of the finishing required. In certain embodiments, the peripheral surface velocity of the grinding wheel is set in the range of approximately 100 to 3000 m/min, the

peripheral velocity of the disk raw plate is set in the range of 20 to 500 m/min., the load applied to the edges is set in the range of approximately 0.2 to 5 kg, and the grinding time is in the range of 5 to 60 seconds, more preferably, 10 to 30 seconds.

An additional embodiment may include three grinding processes. In this case, in the first grinding process, grinding is carried out by using a resin bonded grinding wheel containing abrasive grains having a particle size of between mesh sizes #120 and #220, in the second grinding process, grinding is carried out by using a resin bonded grinding wheel containing abrasive grains having a particle size of between mesh sizes #220 and #1000, more preferably between mesh sizes #400 and #600. The third grinding process is carried out by using a resin bonded grinding wheel containing abrasive grains having a particle size of between mesh sizes #1000 and #20000, more preferably between mesh sizes #8000 and #10000.

The grinding wheels used in the grinding processes of the present invention may be used as an independent device in each of the grinding processes. Moreover, the first grinding wheel and the second grinding wheel may be joined to form a combined grinding wheel, and this may be used in the respective grinding processes.

FIG. 3 shows a perspective view that shows one example of the combined grinding wheel to be used in the method of the present invention. In FIG. 3, a first grinding wheel 301 and a second grinding wheel 302 are laminated and joined to each other coaxially.

In this case, moving from the first grinding process to the second process requires only moving the disk raw plate along the shaft direction. Therefore, it is possible to eliminate the time consuming tasks of exchanging the wheels to move from the first grinding process to the second grinding process.

Moreover, the coaxially laminated and joined structure makes it possible to minimize the grinding face of the first grinding wheel 301 and the second grinding wheel 302, and consequently to further uniformly finish of the edge of the substrate.

In general, attaching or detaching the grinding wheels may cause an error in the mounting position. For this reason, each time the grinding wheel is attached or detached in a grinding process for the edge of a disk raw plate, offsets occur in the circularity and concentricity of the disk raw plate. This may result in deviations in the

dimensional precision and quality in the recording medium substrate. However, in the present invention, the combined grinding wheel requires neither attaching nor detaching when shifting from the first grinding process to the second grinding process, so no deviation occurs in the quality of the finished recording medium substrate.

The first grinding wheel 301 and the second grinding wheel 302, may be bonded together with a double sided tape, a bonding agent, or fastened with bolts.

The width 11 of the first grinding wheel 301 and the width 12 of the second grinding wheel 302 are not necessarily the same, and may be appropriately changed. In the case when one grinding wheel is more erodable, it is preferable to increase the thickness of the more erodable grinding wheel. Thus, the service lives of the two wheels are made coincident with each other. More preferably, the widths of the respective grinding wheels are changed in proportion to the amounts of erosion of the wheels as they are used on the same object for the same period of time.

The combined grinding wheel may be provided with a groove on its peripheral surface corresponding to the edge shape of the disk raw plate. (Not shown) For example, in an embodiment where the edge of the recording disk raw plate is ground by a diamond wheel to shape the edge into a trapezoidal configuration, a groove having an inverted trapezoidal configuration, elongated in a direction perpendicular to the shaft, may exist on the combined grinding wheel's peripheral surface, as shown in FIG. 2 (not shown in FIG. 3). One groove or a plurality of grooves may be formed. In the case when there is a difference in the abrasive property of the respective grinding wheels constituting a combined grinding wheel, it is preferable to increase the number of grooves in the more erodable grinding wheel in the same manner as the width of the grinding wheel.

In the case when a plurality of disk raw plates are simultaneously ground, the gap between the grooves is preferably set to be constant all over the entire combined grinding wheel. In this case, the respective grinding wheels constituting the combined grinding wheel have the numbers of grooves in proportion to their respective widths.

A structure in which a plurality of the first grinding wheels are coaxially laminated and a structure in which a plurality of the second grinding wheels are coaxially laminated may be further laminated and combined coaxially to form another

combined grinding wheel. Moreover, in order to deal with the grinding operation consisting of the three processes, three kinds of grinding wheels may be laminated and combined coaxially to form a combined wheel.

In general, a grinding wheel needs to be subjected to a dressing process so as to recover its grinding strength after a predetermined service time. For example, in the case of a grinding wheel having a groove in its peripheral portion with a shape corresponding to the shape of an object to be ground, the dressing process is generally carried out by first removing the grinding wheel from the driving shaft of a grinding device, and attaching it to a driving device. Next, a dresser is pressed onto the peripheral surface of the grinding wheel so as to grind it to a flat face. Thereafter, another dresser having the same shape as the object to be ground is pressed onto the peripheral surface of the grinding wheel so as to form a groove corresponding to the shape of the object to be ground.

In the present invention, the grinding wheel may have a groove on its peripheral portion corresponding to the shape of the edge of a disk raw plate.

FIG. 4 shows a cross-sectional view that shows the method for carrying out the dressing process on a grinding wheel used in the method of the present invention.

FIG. 4 (a) shows a cross-sectional shape of a peripheral face of a grinding wheel at the initial stage of a grinding process. A disk raw plate 401 is ground by a groove 402 having an inverted trapezoidal configuration.

FIG. 4 (b) shows a cross-sectional shape of the peripheral face of the grinding wheel that has been used for a predetermined time. The groove 402 becomes deeper due to abrasion, and a protrusion 403 between grooves is formed. Since the protrusion 403 between grooves wears the surface 404 of the disk raw plate out, it is necessary to remove this through a dressing process.

FIG. 4 (c) schematically shows a dressing method. This dressing method features that a dresser 405, which has the same outer diameter and the same edge shape as a disk raw plate and also has a thickness m that is not less than the groove pitch n of the grinding wheel, is used.

The application of the dresser having the above-mentioned specific dimensions makes it possible to carry out a dressing process on the grinding wheel

without the need for preliminarily carrying out a grinding process to make the grinding face of the grinding wheel flat.

In this dressing method, the operation is simplified since it is not necessary to remove the grinding wheel from the driving shaft of the grinding device. Moreover, since the outer diameter of the dresser is the same as the disk raw plate, the dressing operation is carried out in the same manner as the grinding operation with the exception that the disk raw plate is changed to a dresser.

An explanation will be further given of the present invention by means of examples. The following examples are not intended to limit the present invention.

Example 1 A glass disk, measured 2 mm in thickness, 63.5 mm in diameter and 20 mm in hole diameter, was prepared as a recording disk raw plate. The edge of this disk raw plate was ground and shaped to a trapezoidal configuration as illustrated in FIG. 1 by using a #500 diamond grinding wheel ("MED 500"made by Mitsubishi Material k. k. diameter 63.5 mm, hole diameter 20 mm). The length of the upper side 102 (as shown in Fig. 1) of the trapezoidal configuration was approximately 400 Rm, and the angle of inclination 0 on both of the ends was approximately 45°. On the upper face of the edge of the disk raw plate and the right and left slanting faces, pits were formed all over the faces.

A #600 aluminum oxide grinding wheel ("DLO WHEEL"commercially available from Sumitomo 3M Ltd. diameter 160 mm, density 1.8 g/cm3, shore D hardness 90) and a #10000 cerium oxide grinding wheel ("DLO WHEEL" commercially available from Sumitomo 3M Ltd. diameter 160 mm, density 2.0 g/cm3, shore D hardness 95) were prepared. A groove having a shape corresponding to the shape of the edge of the disk raw plate was formed on each of the grinding wheels by using a dresser.

First, the edge of the disk raw plate was ground by using the #600 aluminum grinding wheel. The following grinding method was used. The #600 aluminum grinding wheel and the disk raw plate were respectively counter rotated, and a load was applied so as to allow them to contact each other. The grinding conditions were: a

peripheral velocity of 2000 m/min. of the grinding wheel, a peripheral velocity of 46 R. P. M of the disk raw plate, a load of 2 to 5 Kg, and a grinding time of 10 sec.

Next, the #600 aluminum grinding wheel was replaced by the #10000 cerium oxide grinding wheel, and the same method was carried out so as to grind the edge of the disk raw plate. The grinding conditions were: a peripheral velocity of 2000 m/min. of the grinding wheel, a peripheral velocity of 46 R. P. M of the disk raw plate, a load of 2 to 5 Kg, and a grinding time of 10 sec.

After the grinding process, the ground face was visually observed under a laser microscope, and a pit removing rate was calculated. Moreover, an outline measuring device (made by"Mitsutoyo"k. k.) was used to measure the curvature of the angle of the end face. Table 1 shows the results of the tests.

Here, in the present example, since, upon making a shift from the first grinding process to the second grinding process, the grinding wheels were exchanged on the grinding device, an error occurred in the mounting position of the grinding wheel, and the peripheral face of the disk raw plate shown a of FIG. 1 was not made completely flat.

Example 2 The edge of the recording disk raw plate was ground and shaped into a trapezoidal configuration in the same manner as Example 1.

A #600 aluminum oxide grinding wheel ("DLO WHEEL"commercially available from Sumitomo 3M Ltd. diameter 160 mm, density 1.8 g/cm3, shore D hardness 90) and a #10000 cerium oxide grinding wheel ("DLO WHEEL" commercially available from Sumitomo 3M Ltd. diameter 160 mm, density 2.0 g/cm3, shore D hardness 95) were prepared. These two members were bonded to each other with their axes being coincident with each other by using a bonding agent ("Scotch- Weld DP-420"made by Sumitomo 3M Ltd.). A groove having a shape corresponding to the shape of the edge of the disk raw plate was formed on each of the grinding wheels by using a dresser.

First, the portion of the #600 aluminum oxide grinding wheel was used so as to grind the edge of the disk raw plate. In the grinding method, grinding wheel and

the disk raw plate were respectively counter rotated, and a load was applied so as to allow them to contact each other. The grinding conditions were: a peripheral velocity of 2000 m/min. of the grinding wheel, a peripheral velocity of 46 R. P. M of the disk raw plate, a load of 2 to 5 Kg, and a grinding time of 10 sec.

Next, the disk raw plate was shifted toward the axial direction, and the portion of the #10000 cerium oxide grinding wheel was used so as to grind the edge of the disk raw plate. The grinding conditions were: a peripheral velocity of 2000 m/min. of the wheel, a load of 2 to 5 Kg, a peripheral velocity of 46 R. P. M of the disk raw plate, and a grinding time of 20 sec.

The pit removing rate of the ground face and the curvature of the angle of the end face were calculated in the same manner as Example 1. The results are shown in Table 1.

In Example 2, no exchanging process of grinding wheels was made upon making a shift from the first grinding process to the second grinding process, no error occurred in the mounting position of the grinding wheel. Therefore, the upper side 102 of the disk raw plate (shown in FIG. 1) was made completely flat.

Comparative Example 1 The edge of the same recording disk raw plate as used in Example 1 was ground and shaped into a trapezoidal configuration in the same manner as Example 1.

A plurality of these were superposed and fixed to a rotary shaft.

The edge of the disk raw plate was ground with grinding brush. The grinding method was as follows: The grinding brush and the disk raw plate were respectively rotated reversely, and while supplying a water slurry containing 10 to 20 % of cerium oxide as a grinding assistant at a rate of 10 liter/min, the edge of the disk raw plate and the brush were allowed to contact each other. The grinding conditions were: a peripheral velocity of 1000 m/min. of the grinding brush, a peripheral velocity of 46 R. P. M of the disk raw plate and periods of grinding time of 60 seconds and 3600 seconds.

The pit removing rate of the ground face and the curvature of the angle of the end face were calculated in the same manner as Example 1. The results are shown in

Table 1.

In the conventional brush grinding method, since a plurality of the disk raw plates were superposed, the brush was not sufficiently applied to portions at which the disks are closely adjacent with each other, resulting in an insufficient pit removing rate. In contrast, at the tip of the edge of the disk raw plate, since abrasion by the brush developed rapidly, the curvature of the edge became too great after the grinding process for a long time.

Comparative Example 2 Example 1 in the specification of Japanese Patent No. 2000042889A.

The edge of the same recording disk raw plate as used in Example 1 was ground and shaped into a trapezoidal configuration in the same manner as Example 1.

A #220 aluminum oxide grinding wheel ("DLO WHEEL"commercially available from Sumitomo 3M Ltd., diameter 160 mm, density 1.0 g/cm, shore D hardness 35) was prepared. A groove having a shape corresponding to the shape of the edge of the disk raw plate was formed on this grinding wheel by using a dresser.

The edge of the disk raw plate was ground by using this grinding wheel. The following grinding method was used. The grinding wheel and the disk raw plate were respectively counter rotated, and a load was applied so as to allow them to contact each other. The grinding conditions were: a peripheral velocity of 2000 m/min. of the grinding wheel, a peripheral velocity of 46 R. P. M of the disk raw plate, a load of 2 to 5 Kg, and a grinding time of 10 sec.

The pit removing rate of the ground face and the curvature of the angle of the end face were calculated in the same manner as Example 1. The results are shown in Table 1.

Comparative Example 3 Example 2 in the specification of Japanese Patent No. 2000042889A.

The edge of the same recording disk raw plate as used in Example 1 was ground and shaped into a trapezoidal configuration in the same manner as Example 1.

Three #220 aluminum oxide grinding wheels ("DLO WHEEL"commercially

available from Sumitomo 3M Ltd., diameter 50 mm, density 1.5 g/cm3, shore D hardness 35) were prepared. The three grinding wheels were set to a grinding device (a hard disk end-face grinding device made by Shonan Engineering k. k.) capable of simultaneously grinding the upper face and the right and left slanting faces of the edge of the disk raw plate, and a grinding process was carried out under the conditions of a peripheral velocity of 500 mm/sec. and a grinding time of 30 seconds.

The pit removing rate of the ground face and the curvature of the angle of the end face were calculated in the same manner as Example 1. The results are shown in Table 1.

Table1 Exa ple I Exam le 2 Comparative Comparative Comparative First Second First Second Example 1 Example 2 Example 3 Hardness (Shore D) 90 95 90 95-35 35 Grinding Density (/cm) 1.8 2.0 1.8 2.0-1.0 1.0 Wheel Abrasive grain Al203 CeO2 Al203 CeO2 A 1203 Al203 Particle size (mesh) 600 10000 600 10000-220 220 Peripheral velocity 2000 2000 2000 2000 1000 2000 500 Grinding(m/min.) Conditions Load (kg) 2 to 5 2 to 5 2 to 5 2 to 5 2 to 5 0.2 to 2 Time (sec.) 10 10 10 20 60 3600 10 30 Removing rate of more than more than more than more 5 95 80 95 Results of large pits (%) a 95 95 95 than 95 Grinding Removing rate of more than more than more than more 0 90 0 0 small pits (%) b 10 80 10 than 95 Curvature (m) <10 <10 <10 <10 <50 < 100 <35 <20 Kemoving rate of large pits having a diameter of not less than 10 um<BR> b Removing rate of small pits having a diameter of not more than 10 J. m<BR> [0079] The polishing method of the edge of a recording disk raw plate of the present invention makes it possible to virtually eliminate pits in the edge of a recording medium substrate, to provide an easy operation, and also to prevent deviations from occurring in the quality of the finished recording medium substrate.