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BACKGROUND OF THE INVENTION
This invention relates to an abrader for use in the mirror polishing of
glass articles such as optical glasses, to a method for shaping the
grinding surface of said abrader and to a method for effecting the mirror
polishing of glass by use of said abrader.
Optical glasses with mirror surfaces such as lenses for use in cameras,
spectacles and microscopes, prisms and filters are today manufactured by a
series of operations comprising roughing, fine lapping (smoothing) and
mirror polishing operations.
The roughing operation constitutes a step wherein the glass surface is
shaped with a grindstone or the glass surface is roughed up by moving a
lapping board on the glass surface while keeping a granular lapping powder
and a lapping liquid supplied to the interface therebetween.
The fine polishing operation is a step in which the glass surface is
treated by interposing between the lapping board and the glass surface a
lapping powder having a fine particle diameter of the order of #2000 to
#500 Japanese Industrial Standard (average diameter 8 .mu.m to 34 .mu.m)
which is finer than that of the lapping powder used in the roughing
operation.
In a modified form, this fine polishing operation may be carried out by
directly grinding the glass surface with diamond pellets formed by
blending diamond powder and a binder.
The mirror polishing operation is for finishing the glass surface to
perfect mirror smoothness by grinding the glass surface with the lapping
board while continuously supplying a suspension of cerium oxide, zirconium
oxide or red iron oxide to the interface between the lapping board and the
glass article.
As the lapping board for use in the operations described above, there is
used the type of lapping board which is formed by attaching a sheet of
pitch, wax, woolen cloth or polyurethane resin to the surface of a disc
made of cast iron.
According to the generally accepted theory, while the shaping of the glass
surface in the roughing and fine polishing operations is mainly
accomplished by causing fine glass fracture in the surface region, the
shaping of the glass surface to the perfect mirror smoothness in the
mirror polishing operation is attained by the fine cutting of the glass
surface owing to the interaction among the glass surface, the lapping
board and the lapping powder such as cerium oxide, zirconium oxide or red
ion oxide, coupled with the elimination of bulges and dents on the surface
due to heat flow in the glass surface, chemical reaction, etc.
For the mirror polishing of glass surface to be effected advantageously by
means of the lapping board, it is imperative that the following
requirements be fulfilled:
(1) The glass surface should be pressed with a uniform magnitude of
pressure by the lapping board.
(2) The lapping powder should be uniformly distributed throughout the
entire interface between the glass article and the disc abrader.
(3) The particle size of the lapping powder and the concentration of
lapping powder in the suspension should both be proper for the operation.
It is no easy matter, however, to fulfill all these requirements. Further,
since this mirror polishing operation necessitates use of suspension, the
work is messy. As the lapping board is operated by an abrading machine,
the suspension is likely to cause difficulties in the maintenance of the
entire abrasion system.
What is more, it frequently happens in this operation that fine particles
of the lapping powder from the suspension collect on the surface of a
glass article under treatment and continue to cling thereto even after
termination of said mirror polishing operation. For removal of the
clinging fine particles, ultrasonic washing or even manual work involving
use of a sharp blade is often resorted to.
The suspension is used repeatedly in this operation. As the number of
repetitions increases, the lapping powder in the suspension undergoes
gradual size reduction through friction and consequently the abrading
capacity thereof dwindles by degrees. Eventually, it becomes necessary to
start using a fresh lapping powder, in which case the new lapping powder
tends to inflict scratches on the glass surface. In fact, rejects occur
mostly because of such defects.
The inventors experimentally manufactured an abrader by blending a lapping
powder with a polyvinyl acetal resin and molding the resultant blend and a
lapping plate incorporating diamond dust and tried them in actual
operations. However, both proved to be deficient in abrading capacity,
durability, polishing accuracy, etc. and therefore unacceptable for
practical use.
U.S. Pat. No. 3,915,671 which involves as one of the inventors thereof the
inventor of the present invention covers a method for the manufacture of a
porous, resin-bonded grinding tool. The grinding tool manufactured by this
method comprises a cured unsaturated polyester resin and an abrasive
material. Examples of the abrasive material usable in the manufacture of
said grinding tool include fused alumina, silicon carbide, diamond dust,
emery, garnet and glass powder.
With the grinding tool which contains such abrasive material, mirror
polishing of the grade proper to optical glasses cannot be accomplished.
The present invention also embraces a method for shaping the grinding
surface of an abrader.
This method is intended for uniformizing the pressure with which the glass
surface being abraded and the lapping board are kept in contact with each
other and also for ensuring uniform distribution of the suspension which
is supplied continuously to the interface during the polishing operation,
whereby the accuracy of polishing will be heightened.
Where a pitch plate is used as the lapping board, for example, the shaping
of the grinding surface is effected by employing a method which takes
advantage of the thermoplasticity of pitch and which comprises pressing
the pitch plate against the surface of the standard plate in water heated
to 40.degree. to 70.degree. C and allowing the plate in that state to cool
off gradually. In the case of a lapping board in which a sheet of
polyurethane is attached to the disc surface, the shaping of the grinding
surface is effected by a method which comprises fastening this sheet by
the medium of an adhesive agent to the surface of the disc while keeping
the sheet pressed against the surface of the standard pate and thereafter
grinding the surface of the attached sheet and the surface of the standard
plate against each other. Since this sheet of polyurethane is very
resistant to wear, it may be necessary on some occasions to continue said
mutual grinding for a long time (on the order of several hours). The
operation of surface shaping the lapping board prior to the polishing of
the glass surface consumes a fairly long time no matter which method is
employed. And this operation must be repeated at frequent intervals,
because the grinding surface of the abrader is gradualy deformed as the
polishing is continued.
Such being the case, need has long been felt for simplifying the method of
surface shaping and reducing the time required for surface shaping.
The primary object of this invention is to provide an abrader for use in
the mirror polishing of glass, which abrader permits the mirror polishing
operation to be carried out with ease and convenience and produces a
polished surface of high accuracy.
Another object of this invention is to provide a method for shaping the
grinding surface of said abrader for use in the mirror polishing of glass.
Still another object of this invention is to provide a method for effecting
the mirror polishing of glass by use of said abrader.
SUMMARY OF THE INVENTION
To accomplish the objects described above according to the present
invention, there is provided an abrader for mirror polishing of glass,
which abrader comprises a porous cured unsaturated polyester resin and a
metal oxide distributed uniformly throughout said resin, said metal oxide
being one member selected from the group consisting of zirconium oxide,
cerium oxide and red iron oxide. In the abrader, said metal oxide selected
from among zirconium oxide, cerium oxide and red iron oxide is contained
in an amount corresponding to 40 to 90% by weight. While this abrader is
effectively used in the form of a unitary molded piece, it may otherwise
be advantageously used in the form wherein a multiplicity of molded unit
pieces are mounted on a unitary circular disc.
That surface of the abrader which is brought into direct contact with the
glass surface, namely the grinding surface of the abrader, is required to
have a definite shape. An abrader must be given a grinding surface of a
definite shape by grinding the given abrader and a disc of the standard
surfacial shape against each other under continuous supply of a suspension
containing fine particles of a hardness of 2 to 6 on the Old-Mohs' scale
of hardness. To accomplish the mirror polishing of glass surface by use of
said abrader, the polishing must be carried out while water or a cutting
oil is fed continuously to the interface of grinding.
In the polishing operation performed by the method described above, the
progress of polishing gradually slows down as the striations formed by
fine grit on the glass surface begin to vanish. Even after total
disappearance of these striations, the mirror polishing operation must be
continued in the event that the glass surface has not yet been polished to
the degree of mirror smoothness aimed at. In this case, discontinuation of
the supply of water or cutting oil to the interface of grinding serves the
purpose of speeding up the progress of polishing and consequently
permitting the glass surface to be finished with an increased degree of
mirror smoothness.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents a typical abrader of this invention, made of a unitary
molded piece and used for the mirror polishing of glass surfaces.
FIG. 2 illustrates an abrader of this invention, made of a unitary molded
piece, having a concave edge and used for the mirror polishing of glass
surfaces.
FIG. 3 illustrates an abrader of this invention, made of a unitary molded
piece, having a convex edge and used for mirror polishing of glass
surfaces.
FIG. 4 illustrates an abrader of this invention, made of a unitary molded
piece having a bulbous edge and used for mirror polishing of glass
surfaces.
FIG. 5 represents a typical abrader of this invention for use in mirror
polishing of glass surfaces, which comprises a circular disc and a
plurality of molded unit pieces mounted on said circular disc.
FIG. 6 illustrates an abrader of this invention for use in mirror polishing
of glass surfaces, wherein the circular disc has an inwardly curved
surface of a suitable radius of curvature.
FIG. 7 illustrates an abrader of this invention for use in mirror polishing
of glass surfaces, wherein the circular disc has an outwardly curved
surface of a suitable radius of curvature.
FIG. 8 illustrates an abrader of this invention for use in mirror polishing
of glass surfaces, wherein the forward end of a cylinder containing at the
center thereof an inwardly curved surface serves as a circular disc.
FIG. 9 illustrates an abrader of this invention for use in mirror polishing
of glass surfaces, wherein the circular disc is modified to an annular
shape.
FIG. 10 is a graph showing the relation between the size of molded unit
pieces and the thickness of polishing as determined of an abrader of this
invention having a plurality of molded unit pieces mounted on a circular
disc.
FIG. 11 is a graph showing the relation between the number of molded unit
pieces and the thickness of polishing as determined of an abrader of this
invention having a plurality of molded unit pieces mounted on a circular
disc.
FIG. 12 is a photomicrograph of the surface of a lens taken after the fine
polishing performed as indicated in Example 1.
FIG. 13 is a photomicrograph of the surface of a lens taken after the
grinding performed by the conventional technique as described in Example
1.
FIG. 14 is a photomicrograph of the surface of a lens taken after the
grinding performed by the method of this invention as indicated in Example
1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The abrader for use in the mirror polishing of glass surface according to
the present invention will be described. This abrader comprises a porous,
cured unsaturated polyester resin and at least one metal oxide selected
from the group consisting of zirconium oxide, cerium oxide and red iron
oxide.
The method by which this abrader is manufactured is as follows:
The unsaturated polyester resin is obtained by first preparing an
unsaturated polyester through the reaction of an unsaturated dibasic acid
such as maleic acid or fumaric acid with a dihydric alcohol such as
ethylene glycol or diethylene glycol and subsequently dissolving said
unsaturated polyester in a vinyl type monomer such as styrene vinyl
acetate or methyl methacrylate. The unsaturated polyester resin thus
produced is generally a viscous oily liquid and is insoluble in water.
Then, this unsaturated polyester resin and water are combined to produce a
water-in-oil emulsion. In the preparation of this emulsion, the mixing
ratio by weight of the resin to water is in the range of from 1/0.5 to
1/3, preferably from 1/1.5 to 1/2.5. Then, at least one member selected
from the group consisting of zirconium oxide, cerium oxide and red iron
oxide is dispersed and suspended in the emulsion. The amount of such metal
oxide to be added to the emulsion is such that the ratio by weight of the
metal oxide to the emulsion falls in the range of from 0.3/1 to 4/1,
preferably from 0.5/1 to 2/1.
The emulsion now containing the metal oxide is subsequently added with a
known curing agent such as methylethyl ketone peroxide, for example, and
poured in a mold of a desired shape and left to stand therein at
temperatures ranging from normal room temperature to 130.degree. C,
preferably from 60.degree. to 120.degree. C. As the result of this
standing, the charge in the mold is cured and partially or wholly
dehydrated, affording an abrader of said desired shape.
The abrader thus obtained contains pores 0.1 to 50 .mu. in diameter, with
the porosity falling in the range of from 20 to 70% (the porosity
calculated on the assumption that the abrader has been dehydrated to
100%). The amount of the metal oxide present in the abrader is in the
range of from 40 to 90% by weight based on the weight of the abrader. If
the content of the metal oxide exceeds the upper limit of 90%, then the
abrader wears off quickly and tends to sustain scratches. If it fails to
reach the lower limit of 40%, then the abrader is deficient in grinding
capacity.
The abrader can effectively be used in the form of a unitary molded pieice.
It may otherwise be used similarly effectively in the form wherein a
plurality of molded unit pieces are spaced regularly on a circular disc.
The form in which the abrader of this invention should be used is
determined by the shape and dimensions of the product to be obtained by
the grinding, the kind of glass of which the glass article under treatment
is made and the intended use of the product.
Preferred embodiments of the abrader of this invention for use in the
mirror polishing of glass surfaces are illustrated in FIGS. 1-9.
In the drawings, 1 denotes an abrader of this invention which consists
solely of one unitary molded piece.
In FIG. 1(A), the abrader is in the shape of a tall cylinder. In FIG. 1(B),
it is in the shape of a short cylinder. In each of the abraders, the end 2
at which the abrader comes into contact with the article subjected to
grinding has a flat surface.
FIGS. 2, 3 and 4 show other examples of cylindrical abraders of which the
ends 2a, 2b and 2c have concave, convex and bulbous surfaces.
The abraders of the shapes described above are porous, so that the liquid
to be used in the polishing operations described afterward will be
occluded in the pores thus distributed therein. During the actual
polishing operations, therefore, the liquid thus occluded in the pores is
constantly released to produce a cooling effect and thereby preclude
seizure. Consequently, the grinding can be continued with the grinding
surface of the abrader kept in constant contact with the glass surface
without causing any clogging in the interface. Since the abrader of this
invention has proper elasticity, it maintains contact advantageously with
the glass surface. Because of its shock-absorbing property, the abrader
inflicts no scratches on the glass surface. Thus, this abrader enables the
glass article to be finished with mirror surface of high accuracy.
This abrader may be provided in the end face thereof with grooves incised
in the form of a network. The grooves serve as a kind of reservoir for the
liquid during the polishing operations, with the result that the abrader's
ability to prevent seizure will be further enhanced.
So far as the size of the glass article subjected to grinding is within
certain limits, the abrader made of one molded unitary piece and adapted
for the glass article will suffice for the purpose.
Now, other preferred embodiments of the abrader of this invention for the
mirror polishing of glass surface will be described.
FIG. 5 illustrates an abrader of the present invention which is formed by
having a plurality of molded unit pieces 1 disposed at proper intervals
from one another on the surface of a circular disc 3, with the bases of
said pieces attached immovably to said surface.
Said disc may be made of a metallic substance such as cast iron or of a
synthetic resin, for example. To the surface of this disc, the bases of
said unit pieces are to be immovably fastened by the medium of an adhesive
agent, for example.
FIG. 6 illustrates another abrader of the present invention which is formed
by having a plurality of molded unit pieces 1 disposed at proper intervals
from one another on the concave surface of a disc 3a, said concave surface
having a proper radius of curvature and the bases of said pieces immovably
fastened to said surface.
FIG. 7 illustrates an abrader identical in construction with that of FIG.
6, except that the disc has a convex surface 3b instead of the concave
surface.
FIG. 8 illustrates still another abrader of this invention which is formed
by having a plurality of molded unit pieces 1 disposed at proper intervals
in the pattern of a ring along the edge portion of the centrally concave
end surface of a cylinder 3c, with the bases of said unit pieces immovably
attached to said end surface.
FIG. 9 illustrates an abrader which is formed by having a plurality of
molded unit pieces 1 disposed at proper intervals in the pattern of a
plurality of rings along the annular surface of a disc 3d, with the bases
of said unit pieces immovably attached to said surface.
The size and the shape of each abrader and the size and the number of
molded unit pieces immovably attached to the surface of the disc are
suitably selected with due consideration paid to the nature of the glass
article subjected to grinding.
The abrader thus formed by having molded unit pieces immovably attached to
the surface of the disc gives good performance in the grinding operations
while minimizing the total area of contact with the glass surface.
If the grinding is carried out at a high rate of speed, fine particles
generated such as by abrasion are partly driven into the pores contained
in the unit pieces and partly discharged through the voids intervening
between the unit pieces and, therefore, are not suffered to inflict
scratches on the glass surface, enabling the desired mirror polishing to
be finished easily and rapidly and with high accuracy.
Now, the method which is also embraced by this invention and which is
directed to shaping the grinding surface of the abrader will be described.
To be effectively used, the abrader is required to have its grinding
surface shaped so as to conform to the surface which the finished glass
article is expected to possess.
The present invention accomplishes this shaping of the grinding surface of
the abrader by a method which comprises grinding the surface of the
abrader and the surface of the standard disc against each other while a
suspension of fine particles having a hardness lower than that of glass is
being continuously fed to the interface of grinding.
Since the hardness of glass generally ranges from 4 to 7 on the Old Mohs'
scale of hardness, said fine particles in the suspension are desired to
have a hardness approximately in the range of from 2 to 6. Examples of
substances which satisfy this requirement include borax, gypsum, calcium
carbonate, cryolite, zinc white, barium sulfate and aluminum hydroxide
prepared in the form of fine particles. The choice from among these
substances, of course, depends on the kind of glass subjected to grinding.
Use of a suspension containing fine particles of a substance having a
hardness higher than that of glass must be avoided, for such fine
particles tend to pierce and lodge on the surface of the molded unit
pieces and may result in scratches being inflicted on the glass surface in
the course of grinding.
The shaping of the grinding surface of the abrader can be accomplished in
an extremely short period of time by mutually grinding the abrader and the
surface of the standard disc as described above while continuously feeding
to the interface of grinding the suspension of fine particles have a
hardness lower than that of glass. This is because the unit pieces which
make up the abrader are made of a porous substance containing fine pores
measuring 0.1 to 50 .mu. in diameter and the shells enclosing the
individual pores have a very thin wall thickness such that even the fine
particles in the suspension having a hardness lower than that of glass
will readily crush said shells, enabling the grinding surface of the
abrader to be shaped in a short period of time.
The method of this invention by which the glass surface is given desired
mirror polishing by use of the abrader of this invention will be
described.
When the glass surface is subjected to mirror finishing by the conventional
method, namely the method which involves use of a lapping plate and an
abrasive slurry, there is a possibility that the glass surface will
sustain scratches. The method of mirror polishing according to this
invention is characterized by having the glass surface ground with the
abrader of this invention while continuously feeding to the interface of
grinding either water or a cutting oil. Since the grinding surface of the
abrader perfectly fits the glass surface, the mirror polishing is capable
of finishing the glass surface with mirror smoothness of high accuracy
without inflicting any scratches on the glass surface.
Specifically in this mirror polishing method, as the grinding surface at
the end of the abrader rubs the glass surface, the particles of zirconium
oxide, cerium oxide or red iron oxide which are exposed on said grinding
surface cut the surface layer of the glass and eventually effect the
desired mirror polishing. Said mirror polishing proceeds quite smoothly
and finishes the glass surface with highly accurate mirror smoothness
because the cured unsaturated polyester resin which is the other main
component of the abrader possesses a shock-absorbing property. The
finished glass surface, accordingly, shows substantially no uneven
polishing.
As the mirror polishing of the glass surface advances, the abrader itself
undergoes abrasion and throws off fine particles. The fine particles thus
generated from the abrader find their way into the pores in the abrader
and, therefore, are prevented from causing clogging of the interface of
grinding. As a result, the particles of zirconium oxide, cerium oxide or
red iron oxide are constantly exposed on the surface, so that the mirror
polishing of glass surface is allowed to proceed smoothly without any
hindrance.
Even if any of the unit pieces of the abrader happens to contain coarse
grains of zirconium oxide, cerium oxide or red iron oxide, such coarse
grains are embedded in the resin and only limited parts of such coarse
grains come into contact with the glass surface. Thus, there is no
possibility that pressure will be concentratedly exerted on particular
coarse grains of said metal oxide to inflict scratches on the glass
surface.
In the case of the abrader of a type having a plurality of molded unit
pieces disposed on the disc, the fine particles generated from the
grinding surface thereof find their way into the voids intervening between
said unit pieces and they are also washed out of the abrader by virtue of
these voids. This abrader, accordingly, is completely free of causes which
might inflict scratches on the glass surface.
If the mirror polishing by the method of this invention is carried out in
the total absence of supply of water or cutting oil, then the ground
surface of the glass will sustain stains from fusion caused by frictional
heat. Water or the cutting oil is highly effective in cooling the
interface of grinding and has a low degree of viscosity. In the presence
thereof, the mirror polishing proceeds quite smoothly. Cutting oil is
readily available as, of course, is water.
The method of the present invention is simple compared to the conventional
methods, capable of finishing the glass surface with mirror smoothness of
high accuracy and advantageous in that it contributes to improvement of
the work environment, permits simplification of process control and
promotes simplification of the work of cleansing the glass which has
undergone mirror polishing.
Generally, the glass surface can be finished with mirror smoothness of high
accuracy by the method described above. If the mirror finish obtained by
this method proves to be insufficient, then the glass surface must be
further subjected to mirror polishing by the method to be described below.
The mirror polishing performed by the method described above proceeds
rapidly so far as the glass surface being ground is coarse. As the mirror
polishing is continued until the striations inflicted on the glass surface
by friction substantially vanish, the glass surface resulting from the
mirror polishing usually acquires the mirror smoothness aimed at.
Continued mirror polishing may on some occasions be found necessary where
the ground glass surface is still deficient in Newtonia accuracy (accuracy
determined by examination of interference fringes) or said surface is
found to retain detectable striations or other marks.
Generally in this case, the mirror polishing has already been continued
until disappearance of the striations inflicted by the abrader particles.
Moreover, if the mirror polishing is continued while under continued
supply of water or a cutting oil to the interface of grinding, the rate at
which the polishing proceeds is remarkably lowered. The reason why the
polishing slows down is that the striations have ceased to exist, the
glass surface being polished has come near the smoothness of mirror
surface and the presence of water or cutting oil has substantially
eliminated frictional resistance to the interface between the abrader and
the glass surface.
At this stage, the continued mirror polishing begins to proceed at a
greatly increased rate when the supply of water or cutting oil to the
interface of grinding is stopped. Stopping the supply of water or cutting
oil results in a gradual increase in said frictional resistance. The water
or cutting oil which has been occluded in the continuous fine pores
distributed in the abrader exudes to the surface so that the minimum
amount of necessary lubricating liquid will be constantly present in the
interface between the glass surface and the abrader. Thus, the speed of
polishing is substantially increased, the polishing time required for
finishing the glass surface with acceptable mirror smoothness is shortened
and the exudation of the occluded water or cutting oil which lasts for a
fairly long period of time serves the purpose of enhancing the Newtonian
accuracy and eliminating surface scratches. These functions constitute a
characteristic feature which has never been attained by any of the
conventional glass polishing methods.
Typical data indicating the relation between the size and number of molded
unit pieces and the grinding thickness as determined for abraders of this
invention each having a plurality of molded unit pieces disposed on a disc
are shown in FIG. 10 and FIG. 11.
Unit pieces in the shape of short cylinders were molded in varying sizes
with a mixture consisting of a given amount of porous unsaturated
polyester resin and an amount of cerium oxide of a volume twice as large.
A number of unit pieces of each size was immovably attached to the surface
of a flat disc 100 mm in diameter of cast iron at virtually the same
intervals from one another to produce an abrader. The abrader had its
grinding surface shaped as required. A lens 34 mm in surface diameter and
7 mm in thickness made of BK-7 (borosilicate glass called "borosilicate
crown") was subjected to polishing with the abrader under the following
conditions.
______________________________________
Lens grinding machine
Oscar grinding machine (re-
volution number of lower axis -
100 rpm) with a 200-W motor
Stroke 55 mm .times. 180 strokes/minute
Flow volume of water
200 ml/minute
Pressure of grinding
235 g/cm.sup.2 (unit area of lens
surface)
______________________________________
The data obtained in this polishing operation are given in FIG. 10. In the
graph, the vertical axis is graduated for the grinding thickness of the
lens and the horizontal axis is graduated for the length of polishing
time. In the graph, the curve A represents the data obtained for the
abrader using unit pieces 13 mm in diameter and 4 mm in height (the
combined area of the grinding surface of unit pieces was 51% of the total
area of the disc), the curve B represents the data obtained for the
abrader using the unit pieces 6 mm in diameter and 4 mm in height (the
combined area of the grinding surface of unit pieces was 49% of the total
area of the disc) and the curve C represents the data obtained for the
abrader using the unit pieces 25 mm in diameter and 4 mm in height (the
combined area of the grinding surface of unit pieces was 50% of the total
area of the disc) respectively.
Unit pieces in the fixed shape of short cylinders 13 mm in diameter and 4
mm in height were molded with the same mixture as described above.
Abraders were produced by having varying numbers of these unit pieces
immovably fastened to the surface of discs. The lens of the same
description was polished with each of the abraders. The data are shown in
FIG. 11 in the form of the relation between the polishing thickness of the
lens and the length of polishing time. In the graph, the vertical axis and
the horizontal axis are graduated similarly to those of FIG. 10. The curve
D represents the data obtained of the abrader using a total of 30 unit
pieces and curve E those of the abrader using a total of 40 unit pieces
respectively.
It is clear from these graphs that the amount of polishing varies with the
diameter of the molded unit pieces and the total area of the grinding
surface of unit pieces disposed on the disc. The data, therefore, suggest
that the conditions of the abrader should be decided with due
consideration paid to the material of the glass being polished and the
intended use of the finished glass.
The present invention will be described hereinbelow with reference to
working examples.
EXAMPLE 1
An abrader of this invention as illustrated in FIG. 5 was obtained by
causing a total of 30 molded unit pieces 13 mm in diameter and 4 mm in
height to be disposed immovably on the surface of a disc 100 mm in
diameter (with the combined area of the grinding surface of unit pieces
amounting to about 50% of the total area of said disc). This abrader was
used to give mirror polishing to the surface of a glass made of BK-7 which
had undergone a fine polishing treatment with brown alumina (A) #1200
(having an average particle diameter of 13 mm as measured by the method of
JIS), with water supplied to the interface of grinding at 200 cc per
minute and pressure applied to the disc at 235 g/cm.sup.2.
FIG. 12 is a photograph of the surface of said lens taken after said fine
polishing treatment through an electron microscope at 4,000 magnifications
(by replica method).
FIG. 14 is a photograph of the surface of the lens taken through an
electron micrograph at 20,000 magnifications to show the outcome of the
mirror polishing carried out by the method of this invention by use of
said abrader of this invention on the surface of lens resulting from the
fine polishing treatment.
COMPARATIVE EXAMPLE 1
Entirely the same lens surface as obtained by the fine polishing treatment
in Example 1 was polished with a lapping board under a pressure of 115
g/cm.sup.2 under continued supply to the interface of grinding at 15
cc/minute of a lapping agent in the form of a water slurry containing
cerium oxide at a concentration of 15%. The results are shown in FIG. 13.
Superiority of the lens surfaces obtained by the mirror polishing treatment
of the present invention to those obtained by the conventional method is
evident from comparison of the data of FIGS. 12, 13 and 14.
EXAMPLE 2
A water-in-oil emulsion was obtained by agitating 100 parts (by weight; the
same hereinafter) of an unsaturated polyester resin having a styrene
content of 30%, 2 parts of cobalt naphthenate as the curing accelerator
and 3 parts of triethanolamine as the emulsifier in a household blender
wwhile under continued gradual dropwise addition thereto of a total of 150
parts of water. This emulsion was gently agitated with 200 parts of cerium
oxide and the resultant mixture was further agitated with 2 parts of a
curing accelerator (methylethylketone). The mixture thus obtained was cast
in a cylindrical mold 13 mm in diameter and left to cure at normal room
temperature for 12 hours. The molded piece was released from the mold,
dried at 80.degree. C for 12 hours to be cured and dehydrated and cut into
unit pieces 4 mm in height. On the surface of a flat disc of cast iron 100
mm in diameter, 30 unit pieces of the shape of short cylinders (containing
continuous pores about 5 .mu. in diameter) were immovably attached at one
end by the medium of an epoxy adhesive agent at substantially equal
intervals to produce an abrader. A quick-drying ink was applied to the
surface of each unit piece.
The abrader was set in position as the lower disc in an Oscar polishing
machine and rotated at 100 r.p.m. As the upper disc, a standard disc
having a flat surface was set in position and caused to reciprocate while
sliding on the lower disc under a load of 1 kg, with a slurry containing
30% of borax (having a hardness of 2 to 2.5) continuously poured to the
interface of grinding for 30 seconds. The two surfaces were then ground
against each other under an increased load of 3 kg and continued pouring
of the slurry for 30 seconds. Then, the abrader was removed from the
machine and washed with water. The ink applied to the surface of the
pieces was found to have completely vanished, indicating that the shaping
of the grinding surface of the abrader was accomplished in an extremely
short period of time.
EXAMPLE 3
The procedure of Example 2 was repeated, except the molded unit pieces of
the shape of short columns measured 6 mm in diameter and 4 mm in height.
To the surface of a disc 35 mm in diameter containing in said surface a
concave 20 mm in radius of curvature, 25 molded unit pieces were immovably
attached at substantially equal intervals to produce an abrader. A
quick-drying ink was applied to the surface of each unit piece.
The abrader thus obtained was set in position as the lower disc in the
Oscar polishing machine. As the upper disc, a standard disc 20 mm in
diameter containing a convex 17 mm in radius of curvature was set in
position. The two discs thus held in position were ground against each
other under a load of 1 kg for five minutes, with a slurry containing 50%
of barium sulfate continuously poured to the interface of grinding. A the
end of the mutual grinding, the abrader was released from the machine and
washed with water. The quick drying ink applied to the surface of the unit
pieces was found to have completely vanished, indicating that the shaping
of the grinding surface of abrader was accomplished advantageously in a
very short period of time.
EXAMPLE 4
A mixture consisting of a given amount of porous unsaturated polyester
resin and an amount twice as large in volume of a cerium oxide powder 1
.mu. in particle diameter was molded in the shape of a thin disc (100 mm
in diameter and 4 mm in length) and was immovably fastened at one surface
by the medium of an epoxy adhesive agent to the surface of a flat-surfaced
disc of cast iron. After the fastening, the exposed surface of the molded
disc was scraped to perfect smoothness by a precision lathe. On the flat
surface of the disc, straight grooves about 1 mm in breadth were incised
at fixed intervals of 5 mm in the pattern of a checkerboard. The abrader
thus produced was operated to give mirror polishing to the lens surface
under the following conditions.
______________________________________
Lens polishing machine
Oscar's machine (lower shaft
revolution number 100 r.p.m.),
provided with a 200-W motor
Lens One flat-surfaced lens 34 mm in diameter and
7 mm in thickness, made of BK-7 (borosilicate
crown), which had undergone a fine polishing
treatment using a brown electro-fused alumina
abrader having an average particle diameter of
13 .mu.m (JIS #1200).
Stroke 55 mm .times. 180 strokes/minute
Flow volume of water
200 ml/minute
Pressure 200 g/cm.sup.2 (unit area of lens surface)
______________________________________
After one hour of this mirror polishing, the lens was measured for
thickness by a micrometer to find the loss of thickness due to the
polishing. The loss was found to be 12 .mu.. When the polished surface of
the lens was elaborately examined with the aid of a light-concentration
lamp, absolutely no scratch was detected. When the polished surface of the
lens was photographed by the replica method through an electron microscope
at 20,000 magnifications, it was found to be a mirror surface of extremely
high degree of smoothness. After the lens had undergone a total of 10
cycles of such mirror polishing, no infliction of scratches was detected
on the surface.
COMPARATIVE EXAMPLE 2
The polishing described in Example 4 above was repeated under the same
conditions, except a slurry containing 15% of cerium oxide was fed at a
flow volume of 15 ml/minute. After one hour of this polishing, the amount
of lapping on the lens was found to be 15 .mu.. Scratches were found on
two of a total of ten lenses used.
EXAMPLE 5
With the mixture of the composition of Example 4, unit pieces of the shape
of discs 13 mm in diameter and 4 mm in height were molded instead of the
molded unitary piece of Example 4. On the surface of a disc of cast iron
100 mm in diameter, 30 unit pieces were immovably fastened at
substantially equal intervals. The abrader thus obtained was ground to
have its grinding surface shaped as required. With this abrader, the same
lens (SK-7 heavy crown) was subjected to mirror polishing under the same
conditions as those of Example 4, except the pressure applied was 250
g/cm.sup.2 and a commercially available water-soluble cutting oil diluted
with water to 20 times the original volume was poured continuously to the
interface of grinding. After 10 minutes of this polishing, the thickness
of lens lost by abrasion was 8 .mu. and the striations remained to a
slight extent. After 20 minutes of the polishing, the thickness of lens
lost totalled 10 .mu. and the striations remained to a very slight extent.
After 30 minutes of the polishing, the thickness of lens lost increased to
12 .mu., the striations were completely absent and no scratch was
detected. After one hour of the polishing, the thickness of lens lost
reached 13 .mu. and the glass surface had the appearance of a mirror
surface, with no infliction of scratch detectable.
EXAMPLE 6
Unit pieces were molded in the shape of short cylinders 19 mm in diameter
and 6 mm in height with a mixture consisting of a given amount of porous
unsaturated polyester resin and an amount twice as large in volume of
zirconium oxide. On the surface of a flat-surfaced disc of cast iron 100
mm in diameter, 13 such unit pieces were immovably fastened at
substantially equal intervals. The abrader thus produced was ground to
have its grinding surface shaped as required. By repeating the procedure
of Example 4, the lens of the foregoing description was given mirror
polishing by use of the abrader. After two hours of the polishing, the
thickness of lens lost by the abrasion was 12 .mu. and the glass surface
had the appearance of a mirror surface and was found to sustain absolutely
no scratch.
COMPARATIVE EXAMPLE 3
The procedure of Example 4 was repeated to give mirror polishing to the
lens of the same description, except that a sheet of polyurethane 1 mm in
thickness was set in position as the lower disc, the pressure applied was
100 g/cm.sup.2, and a slurry containing 15% of cerium oxide was fed at a
flow volume of 15 ml/minute. In this case, the slurry was not used
cyclically but a fresh supply of slurry was fed continuously throughout
the entire period of polishing.
After one hour of polishing, the thickness of lens lost by the abrasion
totaled 15 .mu | | |
