U.S. patent application number 10/215893 was filed with the patent office on 2003-05-01 for clamping jig for glass substrate, buffer sheet, method for processing glass substrate, and glass substrate.
This patent application is currently assigned to Nippon Sheet Glass Co., Ltd.. Invention is credited to Matsuno, Kensuke, Okuhata, Koji, Watanabe, Takeo, Yoshikawa, Takamasa.
Application Number | 20030082999 10/215893 |
Document ID | / |
Family ID | 26621913 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030082999 |
Kind Code |
A1 |
Yoshikawa, Takamasa ; et
al. |
May 1, 2003 |
Clamping jig for glass substrate, buffer sheet, method for
processing glass substrate, and glass substrate
Abstract
The present invention provides a method which can simultaneously
grind inner peripheral edges and outer peripheral edges of a
multiplicity of glass substrates used for magnetic storage media or
the like. In order to chamfer the outer edges of the glass
substrates g, a multiplicity of annular glass substrates g are
firstly stacked with buffer sheets 5 interposed between the glass
substrates to make a glass substrate block G. Then, a first plates
1, 1 of the above described clamping jig are applied to both sides
of the glass substrate block G, and a first fastening tool 3 is
inserted through center holes of the first plates 1, 1 and the
glass substrate block G. Next, the glass substrate block G is
clamped from an inside thereof for chamfering the outer edges with
a grindstone 6.
Inventors: |
Yoshikawa, Takamasa;
(Osaka-shi, JP) ; Okuhata, Koji; (Osaka-shi,
JP) ; Matsuno, Kensuke; (Osaka-shi, JP) ;
Watanabe, Takeo; (Osaka-shi, JP) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
Nippon Sheet Glass Co.,
Ltd.
Osaka-shi
JP
|
Family ID: |
26621913 |
Appl. No.: |
10/215893 |
Filed: |
August 8, 2002 |
Current U.S.
Class: |
451/41 |
Current CPC
Class: |
B24B 9/065 20130101;
B24B 41/06 20130101 |
Class at
Publication: |
451/41 |
International
Class: |
B24B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 10, 2001 |
JP |
2001-273581 |
Oct 15, 2001 |
JP |
2001-317136 |
Claims
What is claimed is:
1. A clamping jig for holding a multiplicity of annular glass
substrates with the annular glass substrates being stacked,
comprising: a first pair of plates, each plate of which is pressed
against each side of a glass substrate block fabricated by stacking
the multiplicity of glass substrates and each plate of which is
provided with a fastening tool insertion hole at a center of the
each plate; a first fastening tool which is inserted through an
inner hole of said glass substrate block, end portions of the first
fastening tool being attached to the fastening tool insertion holes
of said first plates; a second pair of plates, each plate of which
is disposed outside said first plate and each plate of which has an
inner diameter smaller than an outer diameter of the first plate;
and a second fastening tool for fastening the second pair of plates
at a portion outside said glass substrate block.
2. A buffer sheet which is interposed between annular glass
substrates, wherein the buffer sheet is made of a flexible material
whose Rockwell hardness is 40 or less.
3. A buffer sheet which is interposed between annular glass
substrates, wherein the buffer sheet comprises a two-layered
structure including a flexible layer and a hard layer.
4. A buffer sheet which is interposed between annular glass
substrates, wherein the buffer sheet comprises a three-layered
structure in which a hard layer is sandwiched between flexible
layers.
5. The buffer sheet according to claim 3 or 4, wherein said hard
layer has a compressive elasticity modulus of 100 MPa or more.
6. The buffer sheet according to claims 2 to 5, wherein a thickness
of the buffer sheet is 0.2 mm or less.
7. The buffer sheet according to claims 2 to 6, wherein the buffer
sheet is shaped such that an inner diameter of the buffer sheet is
larger than an inner diameter of the glass substrate and an outer
diameter of the buffer sheet is smaller than an outer diameter of
the glass substrate.
8. A method for processing a glass substrate, wherein a
multiplicity of annular glass substrates are stacked alternately
with the buffer sheets according to claims 2 to 7, said glass
substrates are clamped at an outer or inner peripheral side of the
glass substrates, and said outer or inner peripheral side which is
not clamped is ground.
9. A method for processing a glass substrate, wherein a stack of a
multiplicity of annular glass substrates is clamped at an outer
side or an inner side thereof while buffer sheets are respectively
interposed between the annular glass substrates such that the
buffer sheets do not protrude from inner peripheral edges and outer
peripheral edges of the glass substrates, the outer peripheral
edges of the multiplicity of glass substrates are simultaneously
ground with the stack being clamped at the inner side thereof, the
inner peripheral edges of the multiplicity of glass substrates are
simultaneously ground with the stack being clamped at the outer
side thereof, and further, the clamping situation of the glass
substrates is maintained even at a time of switching between an
operation for grinding said inner peripheral edges and an operation
for grinding said outer peripheral edges.
10. The method for processing a glass substrate according to claim
9, wherein the grinding of said outer peripheral edges and said
inner peripheral edges is chamfering which is conducted by allowing
a grindstone to come into rotationally contact with the outer
peripheral edges and the inner peripheral edges, the grindstone
being made by bonding diamond abrasive grains to an alloy.
11. The method for processing a glass substrate according to claim
9, wherein the grinding of said outer peripheral edges and said
inner peripheral edges is polishing which is conducted by allowing
a polishing brush or a polishing pad to come into rotationally
contact with the outer peripheral edges and the inner peripheral
edges while supplying a polishing agent thereto.
12. The method for processing a glass substrate, wherein the outer
peripheral edges and the inner peripheral edges are polished by
allowing the polishing brush or the polishing pad to come into
rotationally contact with the outer peripheral edges and the inner
peripheral edges while supplying the polishing agent thereto, with
the clamping situation of the glass substrates being continuously
maintained without releasing respective glass substrates from the
clamping situation, after the grinding according to claim 9.
13. The method for processing a glass substrate according to claims
9 to 12, wherein said buffer sheet to be used is a buffer sheet
defined in claims 2 to 7.
14. The method for processing a glass substrate according to claims
9 to 12, wherein said buffer sheet to be used has a convex portion
at an inner periphery or an outer periphery of the buffer sheet,
the buffer sheet and the glass substrate are stacked such that the
convex portion reaches to the inner peripheral edge or the outer
peripheral edge of the glass substrate, and are subjected to the
grinding, and a portion of the inner periphery or the outer
periphery of the glass substrate, corresponding to said convex
portion, is left as an insufficiently ground portion.
15. A glass substrate, wherein the glass substrate is obtained by
the processing methods according to claims 8 to 12.
16. A glass substrate, wherein the glass substrate is obtained by
the method for processing the glass substrate according to claims
13.
17. A glass substrate, wherein the glass substrate is obtained by
the method for processing the glass substrate according to claim 14
and wherein a mark is formed at an inner peripheral edge and/or an
outer peripheral edge of the glass substrate by insufficiently
grinding the glass substrate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a clamping jig for holding
a multiplicity of annular glass substrates with the annular glass
substrates being stacked, a buffer sheet to be interposed between
the glass substrates, a method for grinding (chamfering or
polishing) inner peripheral edges and outer peripheral edges of the
multiplicity of annular glass substrates, and a glass substrate
obtained by this method.
[0003] 2. Description of the Related Art
[0004] Glass is superior to aluminum in shock resistance, rigidity,
hardness, and strength, so that the glass has been increasingly
used as a magnetic storage medium which is built into a hard disc
drive or the like.
[0005] In particular, a configuration of a personal computer has
been gradually changed from a desktop type to a notebook type or a
mobile type recently, thus the demand for glass substrates which
are excellent in flatness and densification has been increased
instead of aluminum substrates.
[0006] In order to obtain a glass substrate to be used for the
magnetic storage medium, a disc-like (a doughnut-like) glass
substrate is cut out from an untreated glass plate, and then, as
also disclosed in Japanese Patent Laid-Open No. 10-154321 for
example, peripheral edges (an inner peripheral edge and an outer
peripheral edge) of this disc-like glass substrate are subjected to
lapping which is conducted by means of a diamond grindstone and to
polishing which is conducted by means of a cerium oxide suspension,
and subsequently, two recording sides (main surfaces) of this glass
substrate are subjected to lapping by means of alumina abrasive
grains and to polishing by means of a cerium oxide suspension.
[0007] The above described method is ineffective because the glass
substrates should be processed every single substrate.
Consequently, a method in which a multiplicity of glass substrates
are simultaneously polished has been disclosed in Japanese Patent
Laid-Open No. 11-221742. According to this disclosure, a
multiplicity of glass substrates are stacked as they are and
clamped to make them into a block, then a polishing brush or a
polishing pad is allowed to become contact with an inner hole of
this block, and subsequently, the inner hole, in other words, inner
peripheral edges of the multiplicity of glass substrates are
simultaneously polished while supplying a polishing agent
thereto.
[0008] However, the prior art as described above has problems as
follows.
[0009] Firstly, although the inner peripheral edges of the
multiplicity of glass substrates can be simultaneously polished,
the outer peripheral edges have to be polished by other steps
completely different from the polishing steps which have been
conducted for the inner peripheral edges, because the outer
peripheral edges of the glass substrates are still remained within
a substrate case. That is, the multiplicity of glass substrates are
removed from the substrate case, and then the outer peripheral
edges are polished every single substrate, or the multiplicity of
glass substrates are newly held by another damper to simultaneously
polish the outer peripheral edges. However, the processing becomes
inefficient when the polishing is conducted every single substrate,
on the other hand, the use of another damper not only complicate
the polishing steps but also causes misalignment of centers of the
glass substrates at a time of newly holding the glass substrates by
the another clamper, so that the processing precision
deteriorates.
[0010] Secondly, cracks are easily produced on the recording
surfaces since the glass substrates are directly contacted with
each other. That is, a great amount of cullets (glass powder) are
produced in a working field of the polishing, so that the cullets
may be interposed between the glass substrates, and consequently, a
pressure applied by the clamp to the glass substrates causes deep
cracks (25 .mu.m or more) on the surfaces. In this way, the
presence of the cracks on the recording surface of the glass
substrate has a disadvantage for forming a uniform magnetic film,
so that the polishing has to be continued until the cracks
disappear. The polishing time becomes longer when a polishing
margin becomes thicker, and the glass substrate itself also
requires to be manufactured in consideration of the polishing
margin, so that the glass substrate inevitably becomes thicker.
SUMMARY OF THE INVENTION
[0011] In order to solve the above described problems, a clamping
jig according to the present invention comprises: a first pair of
plates, each plate of which is pressed against each side of a glass
substrate block fabricated by stacking the multiplicity of glass
substrates and each plate of which is provided with a fastening
tool insertion hole at a center of the each plate; a first
fastening tool which is inserted through an inner hole of the above
described glass substrate block, end portions of the first
fastening tool being attached to the fastening tool insertion holes
of the above described first plates; a second pair of plates, each
plate of which is disposed outside the above described first plate
and each plate of which has an inner diameter smaller than an outer
diameter of the first plate; and a second fastening tool for
fastening the second pair of plates at a portion outside the above
described glass substrate block.
[0012] According to this clamping jig, inner peripheral edges and
outer peripheral edges can be continuously subjected to lapping and
polishing, without setting free the clamping situation of the glass
substrate block which is fabricated by stacking the multiplicity of
glass substrates.
[0013] And in a method for processing a glass substrate according
to the present invention, a stack of a multiplicity of annular
glass substrates is clamped at an outer side or an inner side
thereof while buffer sheets are respectively interposed between the
annular glass substrates such that the buffer sheets do not
protrude from inner peripheral edges and outer peripheral edges of
the glass substrates, the outer peripheral edges of the
multiplicity of glass substrates are simultaneously ground with the
stack being clamped at the inner side thereof, the inner peripheral
edges of the multiplicity of glass substrates are simultaneously
ground with the stack being clamped at the outer side thereof, and
further, the clamping situation of the glass substrates is
maintained even at a time of switching between an operation for
grinding the above described inner peripheral edges and an
operation for grinding the above described outer peripheral
edges.
[0014] In the above described method, the grinding may be started
from either of the outer peripheral edges and the inner peripheral
edges. The grinding includes chamfering, rough grinding (lapping),
and polishing.
[0015] According to the present invention, the inner peripheral
edges and the outer peripheral edges of the multiplicity of glass
substrates can be continuously ground, and further, the polishing
can be subsequently performed after the completion of the
lapping.
[0016] In order to perform chamfering as one of the grinding
operations, a grindstone in which diamond abrasive grains are
bonded to a back metal for example is allowed to come into
rotationally contact with the glass substrate. In addition, in
order to perform lapping or polishing, a polishing brush or a
polishing pad is allowed to come into rotationally contact with the
glass substrate while supplying a polishing agent thereto, for
example.
[0017] As above described buffer sheet, a resin sheet having a
thickness of 0.2 mm or less whose inner diameter is larger than
that of the glass substrate and whose outer diameter is smaller
than that of the glass substrate is preferably used.
[0018] Setting the inner and outer diameters within a range
described above is required to prevent poor lapping or poor
polishing, and setting the thickness at 0.2 mm or less provides an
RC (Radial Curvature) of 40 nm or less.
[0019] Further, as above described buffer sheet, it is appropriate
to use a sheet which is made of a flexible material having a
Rockwell hardness of 40 or less, a sheet comprising a two-layered
structure including a flexible layer and a hard layer (for example,
its compressive elasticity modulus is 100 MPa or more and
preferably 1000 MPa or more), a sheet comprising three-layered
structure in which a hard layer is sandwiched between flexible
layers, or the like.
[0020] A relation between the Rockwell hardness and the compressive
elasticity modulus is shown in FIG. 14. It is apparent from this
figure that the materials for the flexible layers include PO
(polyolefin), PU (polyurethane), PE (polystyrene), and S (suede)
and that the materials for the hard layers include PP
(polypropylene), PVC (polyvinyl chloride), PET (polyethylene
terephthalate), and PS (polyester).
[0021] In addition, the above described buffer sheet to be used has
a convex portion at an inner periphery or an outer periphery of the
buffer sheet; the buffer sheet and the glass substrate are stacked
such that the convex portion reaches to the inner peripheral edge
or the outer peripheral edge of the glass substrate, and are
subjected to the grinding; and a portion of the inner periphery or
the outer periphery of the glass substrate, corresponding to the
above described convex portion, is left as an insufficiently ground
portion. In this manner, markings for the quality control can be
made on a surface of the glass substrate with simultaneously
performing the grinding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a block diagram showing a fabrication sequence of
a glass substrate for a magnetic storage medium;
[0023] FIG. 2 is a perspective view of a clamping jig;
[0024] FIG. 3 is a side view of a glass substrate block in which a
multiplicity of glass substrates are stacked;
[0025] FIG. 4 is a top view showing a situation in which a buffer
sheet is overlaid on a glass substrate;
[0026] FIG. 5(a) and (b) are cross sectional views showing other
examples of the buffer sheet;
[0027] FIG. 6 is a graph showing a relation between an RC (Radial
Curvature) and a buffer sheet thickness;
[0028] FIG. 7 is a conceptual view of the RC (Radial
Curvature);
[0029] FIG. 8 is a side view showing a situation in which outer
peripheral edges of the glass substrates are chamfered;
[0030] FIG. 9 is a side view showing a situation in which inner
peripheral edges of the glass substrates are chamfered;
[0031] FIG. 10 is a side view showing a situation in which outer
edges of the glass substrate are polished;
[0032] FIG. 11 is a side view showing a situation in which inner
edges of the glass substrates are polished;
[0033] FIG. 12(a) is a top view of a buffer sheet having convex
portions at its outer periphery;
[0034] FIG. 12(b) is a top view of a buffer sheet having convex
portions at its inner periphery;
[0035] FIG. 13 shows a glass substrate manufactured by the use of
the buffer sheet having the convex portions; and
[0036] FIG. 14 shows a relation between a compressive elasticity
modulus and a Rockwell hardness.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] Embodiments of the present invention will now be described
with reference to accompanying drawings. FIG. 1 is a block diagram
showing a fabrication sequence of a glass substrate used for a
magnetic storage medium. Firstly, an untreated glass plate is made
into an annular (disc-like) glass substrate, then an outer edge and
an inner edge of this glass substrate are subjected to chamfering
and polishing, before performing lapping and polishing of a
recording surface of this glass substrate. Subsequently, after
performing scrub washing, the surface is allowed to be chemically
strengthened before washing the substrate, then inspection is
carried out to make a final product. Among the steps described
above, the present invention is applied to the chamfering of the
glass substrate and to the grinding of the edges.
[0038] FIG. 2 is a perspective view of a clamping jig used for
practicing the present invention. The clamping jig is comprised of
a first pair of plates 1, 1, a second plates 2, 2, a first
fastening tool 3 inserted between the first pair of plates 1, 1,
and a second fastening tool 4 inserted between the second pair of
plates 2, 2.
[0039] The first plate 1 is an annular plate having an outer
diameter smaller than that of a glass substrate g and an inner
diameter generally identical with that of the glass substrate g,
while the second plate 2 is an annular plate 2 having an outer
diameter larger than that of the glass substrate g and an inner
diameter smaller than the outer diameter of the first plate 1. Each
of the first and second fastening tools 3 and 4 comprises a
combination of a bolt and nuts, and three second fastening tools 4
are equally spaced apart in a circumferential direction of the
second plate 2.
[0040] When the outer edge of the glass substrate g is ground by
means of the above described clamping jig, the first plates 1,1 and
the first fastening tool 3 are used as will be described below in
detail with reference to FIG. 8. On the other hand, when the inner
edge of the glass substrate g is ground, the first plates 1,1, the
second plates 2, 2, and the second fastening tool 4 are used as
will be described below in detail with reference to FIG. 9.
[0041] FIG. 3 shows a glass substrate block G in which a
multiplicity of glass substrates g are stacked, and in the present
invention, this glass substrate block G is subjected to chamfering,
lapping, and polishing for example. The glass substrate block G has
buffer sheets 5 each of which is interposed between the glass
substrates g.
[0042] If foreign substances such as cullets are present between
the glass substrates g, cracks may be produced on the recording
surface when the glass substrates g are directly stacked to each
other. Therefore, the buffer sheet 5 is interposed between the
glass substrates 9 to allow the foreign substances such as cullets
to be embedded within the buffer sheet in order to prevent the
occurrence of the cracking. For this purpose, the buffer sheet 5 is
required to be made of a more flexible material than the glass
substrate g. However, since the glass substrates g are liable to
become misaligned during the polishing operation if the buffer
sheets are too flexible, a preferable Rockwell hardness of the
buffer sheet 5 is set at 5 or more.
[0043] In addition, if the buffer sheet 5 is protruded from the
inner edge or from the outer edge of the glass substrate at the
time of grinding thereof, the portion of the glass substrate
covered with the buffer sheet can not be ground. Therefore, an
inner diameter of the buffer sheet 5 is made larger than that of
the glass substrate g by 0.01 mm to 5.0 mm for example and is also
made smaller than an outer diameter of the glass substrate g by
0.03 mm to 10.0 mm for example, as shown in FIG. 4.
[0044] In order to prevent the buffer sheet 5 from protruding from
an outer periphery of the glass substrate g even when the buffer
sheets 5 are interposed between the glass substrates g with the
buffer sheets 5 misaligned with each other, a difference between an
outer diameter of the glass substrate g and an outer diameter of
the buffer sheet 5 is preferably set at a certain value which is at
least two times larger than a difference between an inner diameter
of the glass substrate g and an inner diameter of the buffer sheet
5.
[0045] FIG. 5(a) and FIG. 5(b) are cross sectional views showing
another embodiments of a buffer sheet. As a buffer sheet 5, a
two-layered structure including a flexible layer 51 and a hard
layer 52 can be used as shown in FIG. 5(a), or a three-layered
structure having a flexible layer 51 sandwiched between hard layers
52 as shown in FIG. 5(b) can also be used. In addition, a material
which has a compressive elasticity modulus of 100 MPa, and
preferably of 1000 MPa, is used as the hard layer 52.
[0046] A relation between the compressive elasticity modulus and
the Rockwell hardness is as shown in FIG. 14, and among the
materials having their compressive elasticity modulus of 100 MPa or
more are PP (polypropylene), PVC (polyvinyl chloride), PET
(polyethylene terephthalate), and PS (polyester).
[0047] The buffer sheet of the present invention including the hard
layer and the flexible layer comprises one of the sheets described
below or a laminate thereof. That is, a relation between
compressive elasticity modulus Y (MPa) and a Rockwell hardness R of
each layer to be used for the buffer sheet is defined so as to be
included within a region between two dashed lines in FIG. 14, the
two dashed lines respectively representing two formulae, log
Y=0.022R+0.78 and log Y=0.022R-0.42.
[0048] FIG. 6 is a graph showing a relation between the RC (Radial
Curvature) and the buffer sheet thickness. In this case, the RC is
indicated by a maximum distance which is measured between points A
and B of a sagging portion of the glass substrate g, the sagging
portion being created radially between a point at a predetermined
distance R1 from a center of the disc-like glass substrate g and a
point at a distance R2 from the center of the disc-like glass
substrate g substantially corresponding to an end of the recording
surface, as shown in FIG. 7. Generally, the RC requires to be 42
.mu.m or less. To meet this requirement, it can be seen from FIG. 6
that the thickness of the buffer sheet 5 should be 0.2 mm or
less.
[0049] Next, a procedure for carrying out the chamfering of the
outer edge and the inner edge of the glass substrate g will be
described with reference to FIG. 8 and FIG. 9.
[0050] Firstly, a multiplicity of annular glass substrates g are
stacked alternately with buffer sheets 5 to form a glass substrate
block G. Then, the first plates 1, 1 of the above described
clamping jig are applied to both sides of the glass substrate block
G, and the first fastening tool 3 is inserted through center holes
of the first plates 1, 1 and the glass substrate block G to clamp
the glass substrate block G from an inside of the block, then the
outer edges are subjected to the chamfering by means of a
grindstone 6.
[0051] The grindstone 6 has diamond abrasive grains bonded to a
back metal, and an outer periphery of the grindstone 6 has a rugged
shape adapted to an intended chamfering shape. Then, in order to
chamfer the inner edges of the glass substrates g, the glass
substrate block G is allowed to be rotated while simultaneously
rotating the grindstone 6.
[0052] After simultaneously subjecting the outer edges of the
multiplicity of glass substrates g to the chamfering process as
described above, the second plates 2, 2 of the clamping jig are
applied to the outside of the first plates 1, 1, while keeping this
clamping situation without setting free the glass substrate block
G. Further, three second fastening tools 4 are inserted between the
second pates 2, 2, and the glass substrate block G is clamped from
an outside of the block. Then, the first fastening tool 3 is
removed and another grindstone 7 is inserted through the center
hole of the glass substrate block G as shown in FIG. 9 to
simultaneously chamfer the inner edges of the multiplicity of glass
substrates g.
[0053] After this chamfering, as shown in FIG. 10 and FIG. 11, a
polishing brush or a polishing pad 8, 9 such as nylon is used to
polish the chamfered outer edges and inner edges of the glass
substrate block g without setting free the block G, while supplying
thereto a polishing agent in which cerium oxide particulates are
suspended.
[0054] The outer edges of the glass substrates g can be firstly
processed before processing the inner edges thereof as described in
this example, and vice versa.
[0055] FIG. 12 shows another example of a buffer sheet, a buffer
sheet 5 shown in FIG. 12(a) having convex portions 5a at an outer
periphery thereof and a buffer sheet 5 shown in FIG. 12(b) having
convex portions 5b at an inner periphery thereof. The buffer sheet
5 having convex portions 5a, 5b at the outer periphery or the inner
periphery thereof as described above is interposed between the
glass substrates g such that the convex portions 5a, 5b reach to
the outer edges or inner edges of the glass substrates g, then
lapping or polishing is performed with the buffer sheet kept at
this position. Consequently, portions of the glass substrate g
covered with the convex portions 5a, 5b will not be sufficiently
lapped or polished, so that marks m can be positively produced at
the outer edge or the inner edge of the glass substrate g as shown
in FIG. 13.
[0056] The above described marks m can be used for the quality
control, that is, the marks can be used for checking a product
number, a production plant, and a date and time of the production,
for example, for every lot. However, the quality of the glass
substrates may be affected, if such portions which have not been
sufficiently polished exist excessively. Therefore, the sizes and
the numbers of the convex portions 5a, 5b are limited within a
range which dose not affect the quality.
[0057] Next, results of the experiments conducted by the use of
various buffer sheets which are different in their materials and
their laminar structures are shown in the following table. The
experiments were conducted under the conditions as follows.
[0058] Chamfering of the outer peripheral edges and chamfering the
inner peripheral edges were respectively performed by using ten
grooves type of grindstones having an outer diameter of 80 mm .phi.
and an inner diameter of 22 mm .phi. respectively. In this case,
ten sheets of glasses which were stacked by the clamping jig were
simultaneously processed by the ten grooves type of grindstone,
each groove having a predetermined cross sectional shape. As for
each of the ten grooves of the grindstone, a #500 diamond
grindstone was fixed to a back metal. In this case, various buffer
sheets having different materials were interposed between the glass
plates. Then, polishing was performed with a nylon brush roll,
while supplying thereto a polishing agent in which cerium oxide
particles were suspended.
[0059] After the polishing, the glasses were removed from the
clamping jig to be released from the stacked arrangement. Then,
main surfaces (recording surfaces) of the glass plate were polished
by 25 .mu.m for each surface using a polishing pad and a polishing
agent including cerium oxide grinding particles whose mean particle
sizes are 0.5 to 1.7 .mu.m, and were subsequently washed and
dried.
[0060] Next, the glass plate was subjected to a chemically
strengthening treatment which was performed by ion exchange which
uses mixed molten salt of potassium nitrate and sodium nitrate, and
the treated glass plate was washed and dried again. After this
treatment, the inspection was conducted for cracks on the main
surfaces of the glass plate. Alternatively, processing (grinding)
of the outer peripheral edges and the inner peripheral edges of the
glass substrates using the diamond grindstone may be performed
every sheet of glasses, and then the multiplicity of the ground
glass substrates may be stacked by the clamping jig to be
collectively subjected to the polishing such that the inner
peripheral edges and the outer peripheral edges of the glass
substrates may be simultaneously polished by a polishing pad or a
polishing brush while supplying a cerium oxide polishing agent
thereto.
1 TABLE 1 Effects Com- Workability at a time of press- Yield after
Radial buffer sheet treatment ive Laminar the curvature At a At a
elas- struc- inspection after time of time of ture ti- val- for
cracks edge setting a removing a ues buffer Thick- Rockwell city
are thickness on main polish- sheet on buffer sheet Material ness
(mm) hardness modulus in surfaces ing (nm) a glass from a .mu.ms.)
surface glass Example 1 Flexible Polyurethane 0.275 10 40 Three
100(PU) + 94% 178 4 1 surface (PU) MPa layers 75 (PET) + 100(PU)
Example 2 Polyurethane 0.2 10 10 Single 200 100% 45 1 1 (PU) MPa
layer (PU) Example 3 Polyolefin 0.15 5 3 Single 150 92% 8 3 3 (PO)
MPa layer (PO) Example 4 Polyolefin 0.1 5 3 Single 100 96% 4 2 3
(PO) MPa layer Example 5 Polyurethane 0.19 10 40 Three 90(PU) + 35
98% 11 5 5 (Satin- MPa layers (PET) + like) (PU) 50(PU) Example 6
Polyethylene 0.1 40 20 Single 100 92% 5 5 5 (PE) MPa layer (PE)
Example 7 Hard- Polyurethane 0.175 10 40 Two 100(PU) + 86% 20 4 2
flexible (PU) MPa layers 75(PET) surface Comparative Hard Polyester
0.1 140 1400 Single 100 58% 7 4 4 Example 1 surface (PS) MPa layer
(Polyester) Comparative Polypropylene 0.3 110 1300 Single 300 78%
187 4 4 Example 2 (PP) MPa layer (PP) Comparative Hard polyvinyl
0.15 120 1500 Single 150 50% 10 4 4 Example 3 chloride (PVC) MPa
layer (PVC) Workability evaluation 1: Wrinkles easily occur on a
sheet surface, and the setting of the sheet requires a certain
technique. It is extremely difficult to remove the sheet from the
glass. 2: Workability between 1 and 3 3: The setting can be
performed at a predetermined position. The sheet can be removed
from the glass in a short time 4: Workability between 3 and 5 5:
The setting can be easily performed within a predetermined
position. The sheet can be easily removed.
[0061] In the above described table, each of Examples 1 to 6 used a
buffer sheet comprising a single layer whose Rockwell hardness was
40 MPa or less, and the occurrence of cracks on a main surface of
the glass was prevented. That is, in a visual inspection which was
carried out by illuminating the glass surface with a lamp of 500 W,
an yield after passing the inspection provided that no cracks were
found on the glass surface was as high as 92% or more. Among these
Examples, Example 1 showed that a total thickness of the buffer
sheet was 0.275 mm which was the largest value among these examples
and also showed that the radial curvature was as large as 178 nm.
In contrast to Example 1, thickness of each buffer sheets used for
Examples 2 to 7 was 0.2 mm or less, so that each of their radial
curvatures was as small as 45 nm or less.
[0062] In each of Comparative Examples 1 to 3 where single layered
buffer sheet was used, it was found that cracks occurred on a main
surface of the glass substrate and its yield after the inspection
for cracks was as low as 78% or less. In Comparative Example 2
where a buffer sheet whose thickness was the largest among three
Comparative Examples was used, the radial curvature exhibited a
large value, so that it was found that this buffer sheet was not
suitable for manufacturing a substrate used for a magnetic storage
medium.
[0063] In addition, evaluation points as for the workability of the
buffer sheets at the time of setting the buffer sheets on the glass
surfaces and at the time of removing the buffer sheets from the
wetting glass surfaces may vary depending on the thickness of the
buffer sheets, adhesion properties to a surface of the glass
substrate under the wet condition caused by the material to be
used, and the nerve, for example.
[0064] According to the present invention as described above, a
multiplicity of glass substrates can be simultaneously treated at a
time of lapping or polishing inner peripheral edges and outer
peripheral edges of the glass substrates to be used as magnetic
storage media, and therefore, the cost-reduction can be
substantially carried out.
[0065] In addition, a clamping situation of the multiplicity of
glass substrates can be kept as it is without setting free the
clamping situation when the outer peripheral edges are subjected to
lapping or polishing after continuously performing lapping or
polishing of the inner peripheral edges thereof, so that the
misalignment of the glass substrates will not occur at the time of
newly holding the glass substrates by the another clamping jig.
[0066] In addition, a buffer sheet is interposed between the glass
substrates at a time of stacking the multiplicity of glass
substrates. Therefore, even if foreign substances such as cullets
are sandwiched between the glass substrates, such foreign
substances will be embedded into the buffer sheets and cracks will
not be produced on a surface of the recording surface.
[0067] According to the buffer sheet of the present invention which
is used at a time of stacking a multiplicity of glass substrates,
its Rockwell hardness exhibits a predetermined value, so that the
occurrence of cracks on a main surface (a recording surface) of the
glass substrate is prevented. In addition, a buffer sheet having a
multi-layered structure including a flexible layer prevents the
glass substrate from becoming misaligned in its lateral direction,
so that the chamfering and the polishing of the edges can be
performed completely roundly.
[0068] In addition, convex portions are provided for a certain
portion of the buffer sheet, and lapping or polishing is performed
such that the convex portions reach to the inner peripheral edge or
the outer peripheral edge of the glass substrate, so that marks
used for the quality control can be easily produced on the glass
substrate.
* * * * *