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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a grinding wheel for mirror finishing, and
especially, to a grindstone containing fine abrasives for finishing
optical glass, semiconductor substrates or the like.
2. Prior Art
Processes for machining a surface of work to a mirror finish generally
include honing, super-finishing, lapping, grinding with coated abrasives,
polishing, buffing and so on. Honing and super-finishing use a grinding
wheel similar to a normal grinding wheel, but are superior in working
efficiency. The reason is as follows: Sharp grinding is always conducted
due to fracture of abrasive grain which comes from the high speed of
vibration of the grinding wheel and the frequent changes of the directions
of these vibrations. Also, the efficiency of the honing operation depends
upon the performance of sharp grinding due to fracture of abrasive grain.
Lapping is a finishing process for the surface of the work to be machined
as a cutting tool, which is used for highly accurate dimension and fine
surface. The grinding process with the coated abrasive aims only for
finishing the surface of the work to be machined without questioning
accuracy of dimensions.
As mentioned above, on the machining in which high accuracy of finishing is
required, for example, surface finishing for lenses of cameras,
microscopes, etc., lapping is the process of choice. However, this process
has its drawback in taking much time in lapping. The finishing process by
grinding wheel is superior in operationability, but its abrasive
performance depends on filling ratio of abrasive grain. Accordingly, it
has been required to raise the filling ratio of abrasives grain so as to
improve the abrasive performance. However there is a practical limit in
obtaining the filling ratio of abrasive grains due to the problem of
bonding force between the abrasive grains to each other. And therefore,
the filling ratio of abrasive grains has been 90% at the most, in
practice, thus there is a limit in abrasive performance.
SUMMARY OF THE INVENTION
The present invention provides a grinding wheel in which the filling ratio
of abrasives grain is raised by the improvement of a bonding agent which
is able to obtain high accurate mirror finishing in a short period of
time.
A object of the present invention is to provide a grinding wheel which
improves the abrasive performance so that machining accuracy and the
function of the coolant is strengthened, and long span of the life can be
enjoyed.
A further object of the present invention is to provide a grinding wheel
which can obtain excellent mechanical strength and abrasive performance by
raising the filling ratio of abrasives grain to more than 90%.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a grinding wheel characterized in that in
the structure of a bonding agent which bonds abrasive grains is the
reaction product of a resin and a solution of cashew nut shell liquid or
its derivative and which may also contain a surface active agent.
Cashew nut shell liquid used in the present invention is a liquid similar
to rhus lacquers (Japanese lacquers) contained in the outer shell of a nut
of cashew which is obtained from a kind of tropical plant.
Its main ingredient comprises a mixture of Cardanol
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Cashew nut shell liquid or its derivative is reacted with a resin of
synthetic resin-bonding agent to prepare a cashew modified resin reaction
product. The synthetic resin-bonding agent containing this reaction
product is mixed with abrasive grains and, optionally, a small amount of
surface active agent. The mixture is heated and pressurized according to
the normal method to manufacture a formed product, thus obtaining the
grinding wheel of the present invention.
As the resin, i.e., bonding agent which is reacted with cashew nut shell
liquid or its derivative, thermosetting resins such as phenol resins,
epoxy resins, unsaturated polyester resins, etc. and thermoplastic resin
such as polyamide resin, polyolefin resin, vinyl chloride resin, etc. can
be used. Abrasive grains which can be used in the present invention do not
receive any limitation at all. Depending on the purpose, abrasive grains,
such as metallic oxide having high melting points such as cerium oxide,
magnesium oxide, chromium oxide; abrasives grain such as cubic crystalline
boron nitride; diamond, alumina, sapphire, silicon carbide, boron carbide,
silicon nitride, etc. can be used.
In the present invention, the surface active agent is mainly used to give
wettability to the surface of an abrasive grinding wheel which receives a
supply of grinding fluids at the time of the grinding operation. The
cashew-modified resin prepared by reacting cashew nut shell liquid or its
derivative with resins of a synthetic resin bonding agent repels water.
However, by adding a small amount of surface active agent to the structure
of an abrasive grinding wheel, the interfacial tension between solids and
liquids is reduced; and by emulsifying, wettable and permeable actions
contained therein, liquid film is not discontinued between the abrasive
grinding wheel and the work to be finished, thereby hightening griding and
abrading operationality and cleaning force as well as preventing
attritious wear of the abrasive grains in a short time.
The surface active agents used here are not restricted to any one. For
example, sodium alkylsulfonic acid, sodium salt of higher alcohol
sulfo-acid ester, and others may be used. The surface-active agent is
added in amounts within the range in which the mechanical strength of an
abrasive grinding wheel is not damaged.
The present inventors have found that an abrasive grinding wheel having a
bonding agent containing cashew modified resin improves its abrasive
performance in relation to an increase in the amount of cashew nut shell
liquid (amount of cashew modification), and they have recognized that it
is possible to increase the filling ratio of abrasives grain to 98% in
case the amount of the bonding agent is constant. In comparison with the
conventional method which does not employ a cashew nut liquid grinding
wheel, grinding required about 45 minutes to obtain a finished surface of
Rmax (.ANG.) 600 in surface roughness of a finished surface from
crystalized glass (Rmax 1000 .ANG.) of surface roughness. A finished
surface of 100 .ANG. can be obtained in about three minutes in case of an
abrasive grinding wheel in which 2.4% of cashew nut shell liquid is
contained, under the same condition.
EXAMPLES
Examples of the present invention are shown below.
A grinding wheel is formed under the following conditions by bonding powder
of cerium oxide (2 .mu..sup..phi.) with a bonding agent which is a
reaction product of phenol resin containing phenol novolac and a reaction
product with cashew nut shell liquid (cashew modified phenol).
Forming Condition:
1. Mixing ratio between abrasive grains and a bonding agent
Powder of cerium oxide . . . 96 wt%
Bonding agent containing cashew modified phenol resin . . . 4 wt%
2. Additive ratio (weight %) of cashew nut shell liquid against phenol
novolac
(phenol novolac: cashew nut shell liquid)
100:0 (Comparative Example)
90:10 (Example 1)
80:20 (Example 2)
60:40 (Example 3)
40:60 (Example 4)
3. Form-machining
Press forming
Forming temperature: 150.degree. C.
Pressure: 200 kg/cm.sup.2
Forming time: 10 minutes.
4. Shape and Dimension of the formed product
150 mm.sup..phi. .times.10 mm
On the surface to be grinded was formed V-shaped grooves each being 3 mm in
depth at the intervals of 8m/m.
By using the formed products prepared by respective Examples 1 to 4 and the
Comparative Example, the grinding operation was carried out under the
following conditions and the ground products were subjected to the
property tests.
Test Conditions
(1) Grinding machine: Oscar-type machine
(2) Number of rotations of grinding wheel per minute: 500 rpm
(3) Load: 1 kg/cm.sup.2
(4) Tommy amplitude: 15 mm.times.122 s.p.m.
(5) Material to be ground: Crystalized glass (surface roughness:Rmax 1,000
.ANG.)
(6) Concentration of slurry: Water: 100 gr/5 liters
The result of the tests are shown in Tabe 1.
TABLE 1
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Comparative
Example
Example 1
Example 2
Example 3
Example 4
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Filling ratio of cashew*
0 0.4 0.8 1.6 2.4
in grindwheel (%)
Hardness of grindwheel
Very hard
hard normal soft Very soft
Time required for grinding
45 20 10 4 3
mirror (min.)
Surface roughness of
600 300 200 150 100
finished surface (Rmax .ANG.)
V-grooving machinability
cracked
little cracked
not so much
not cracked
not cracked
cracked
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Note
*Cashew nut shell liquid
According to Table 1, within 4 wt% of the bonding agent, grinding
performance was raised as the additive amount of a solution of cashew nut
shell liquid (amount of cashew modification) increased. In the Comparative
Example, it took 45 minutes to obtain a finished face of Rmax (.ANG.) 600
in surface roughness of the finished face. In Example 1, on the other
hand, it took only 3 minutes to obtain the finished face of 100 .ANG..
Nevertheless, the amount of cashew nuts shell liquid should not increase
limitlessly because there is naturally a limit in obtaining increased
bonding strength from a bonding agent. Accordingly, it can be said that it
is desirable to increase the additive amount of cashew nut shell liquid or
a derivative thereof as much as possible within the range as mentioned
above.
Furthermore, in proportion to cashew nuts shell liquid increased, the
occurence of "Crack" at the time of machining V-shaped grooves has
decreased. "Crack" has not occurred at all in the Examples 4 and 5 due to
the long-chained structure possessed by cashew nut shell liquid.
The ratio of abrasives grain could be increased to 98% using a bonding
agent containing cashew nut shell liquid.
According to the present invention, as described above, it is clear that
the performance of an grinding wheel can be greatly increased by raising
the filling ratio of abrasives grain more than 96% which is greater than
the filling ratio of the conventional method.
As described above, the present invention has its effects that in order to
give surface-wettability at the time of operation by adding further
surface active agent to a bonding agent containing a reaction product of a
resin and a solution of cashew nut shell liquid or its derivative, the
abrasive performance is further improved, and in addition, various
properties contained in the surface active agent are effectively made the
best use for abrasive performance, whereby presenting a grinding wheel
which can raise machining accuracy of the work to be finished and highten
the washability of the surface of the work, and also has long life span.
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