U.S. patent application number 09/764095 was filed with the patent office on 2001-08-30 for vitrified grindstone having pores partially filled with resin, and method of manufacturing the same.
This patent application is currently assigned to NORITAKE CO., LIMITED. Invention is credited to Ito, Kenji.
Application Number | 20010018324 09/764095 |
Document ID | / |
Family ID | 18544281 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010018324 |
Kind Code |
A1 |
Ito, Kenji |
August 30, 2001 |
Vitrified grindstone having pores partially filled with resin, and
method of manufacturing the same
Abstract
A vitrified grindstone having a vitrified abrasive structure
which has pores and which includes abrasive grains and an inorganic
bonding agent that holds the abrasive grains together, wherein
10-95% of a total volume of the pores is filled with a cured resin.
The vitrified abrasive structure may further include an aggregate
such that the abrasive grains and the aggregate are held together
by the inorganic bonding agent.
Inventors: |
Ito, Kenji; (Kaizu-gun,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
277 S. WASHINGTON STREET, SUITE 500
ALEXANDRIA
VA
22314
US
|
Assignee: |
NORITAKE CO., LIMITED
|
Family ID: |
18544281 |
Appl. No.: |
09/764095 |
Filed: |
January 19, 2001 |
Current U.S.
Class: |
451/542 ;
451/541 |
Current CPC
Class: |
B24B 5/02 20130101; B24D
3/18 20130101; B24D 18/0027 20130101; B24D 3/348 20130101 |
Class at
Publication: |
451/542 ;
451/541 |
International
Class: |
B24D 007/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2000 |
JP |
2000-017267 |
Claims
What is claimed is:
1. A vitrified grindstone having a vitrified abrasive structure
which has pores and which includes abrasive grains and an inorganic
bonding agent that holds said abrasive grains together, wherein
10-95% of a total volume of said pores is filled with a cured
resin.
2. A vitrified grindstone according to claim 1, wherein said
vitrified abrasive structure further includes an aggregate such
that said abrasive grains and said aggregate are held together by
said inorganic bonding agent.
3. A vitrified grindstone according to claim 1, wherein 40-90% of
the total volume of said pores is filled with said cured resin.
4. A vitrified grindstone according to claim 1, wherein said cured
resin consists of at least one thermosetting synthetic resin which
is selected from a phenol resin and an epoxy resin.
5. A vitrified grindstone according to claim 1, wherein said
abrasive grains consist of diamond abrasive grains.
6. A vitrified grindstone according to claim 1, wherein said
abrasive grains consist of CBN abrasive grains.
7. A vitrified grindstone according to claim 1, wherein said
abrasive grains in said vitrified abrasive structure have a
concentration of 10-230.
8. A vitrified grindstone according to claim 1, wherein said
abrasive grains have an average grain size of 20-220 .mu.m.
9. A vitrified grindstone according to claim 1, wherein said
vitrified abrasive structure has a porosity of 20-75% by volume,
before said vitrified abrasive structure is impregnated with said
resin.
10. A vitrified grindstone according to claim 1, wherein said
inorganic bonding agent includes a crystallized glass in which
willemite precipitates.
11. A vitrified grindstone according to claim 1, wherein said
vitrified abrasive structure includes 15-35% by volume of said
inorganic bonding agent.
12. A vitrified grindstone according to claim 1, wherein said
vitrified abrasive structure further includes a pore forming
agent.
13. A method of manufacturing a vitrified grindstone having a
vitrified abrasive structure which includes abrasive grains and an
inorganic bonding agent that holds said abrasive grains together,
said method comprising: a step of preparing a mixture of a liquid
resin and a liquid diluent which is other than said liquid resin
and which dilutes said liquid resin, such that said mixture
includes 10-95% by volume of said liquid resin; a step of
impregnating said vitrified abrasive structure with said mixture;
and a step of curing said liquid resin contained in said mixture
with which said vitrified abrasive structure is impregnated.
14. A method according to claim 13, wherein said liquid diluent
consists of a volatile liquid.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates in general to a vitrified
grindstone which has a vitrified abrasive structure impregnated
with a cured resin, and a method of manufacturing such a vitrified
grindstone.
[0003] 2. Discussion of the Related Art
[0004] There is known a vitrified grindstone having a vitrified
abrasive structure, in which abrasive grains and an optionally used
aggregate are held together by a glassy inorganic vitrified bonding
agent, and in which a multiplicity of pores or voids are formed
between the abrasive grains. Owing to excellent properties of the
vitrified grindstone, such as a relatively high degree of strength
with which the bonding agent holds the abrasive grains together,
and a relatively easy dressing operation, the vitrified grindstone
is widely used for precision grinding operations. In recent years,
there is a need for a vitrified grindstone capable of performing a
grinding operation with improved efficiency, to meet a demand for
shortening of the required grinding time.
[0005] On the other hand, there has been proposed for practical use
a vitrified grindstone wherein the proportion or content of the
inorganic bonding agent is made relatively high for increasing a
grade of the vitrified grindstone and also a degree of strength
with which the inorganic bonding agent holds the abrasive grains
together. This vitrified grindstone has a relative high hardness,
and can meet, to some extent, the above-indicated need for
improving the grinding efficiency. However, this vitrified
grindstone is not completely satisfactory. Namely, the use of the
inorganic bonding agent in a relatively large proportion assures an
increase in the strength of the vitrified grindstone, which
contributes to an improvement in the grinding efficiency, but
considerably reduces the porosity of the vitrified abrasive
structure and results in difficult or insufficient fracturing and
removal of the abrasive grains, leading to relatively easy glazing
or clogging of the surface of the vitrified grindstone, relatively
easy chipping of the abrasive structure, relatively difficult
dressing operation of the vitrified grindstone, and other drawbacks
encountered during use of the vitrified grindstone as a grinding
tool. In addition, the use of the inorganic bonding agent in the
relatively large proportion is likely to cause various drawbacks
during the manufacture of the vitrified grindstone, such as
cracking or deformation of the grindstone and insufficient removal
or burn-out of the primary binder of the inorganic bonding agent,
in the firing process. The insufficient removal of the binder may
cause the manufactured vitrified grindstone to have some amount of
residual carbon.
[0006] Such a vitrified grindstone with reduced porosity may be
manufactured by hot-pressing of the materials of the grindstone,
and may be used for performing a highly efficient grinding
operation. However, this manner of manufacturing the vitrified
grindstone requires special manufacturing equipment which usually
suffers from low manufacturing efficiency, leading to a relatively
high cost of manufacture of the vitrified grindstone, and a
considerable limitation in the range of size of the vitrified
grindstone that can be manufactured.
SUMMARY OF THE INVENTION
[0007] It is therefore a first object of the present invention to
provide a vitrified grindstone which has a vitrified abrasive
structure impregnated with a resin, and which is less likely to
suffer from the above-indicated drawbacks relating to its use and
manufacture and assures a high degree of grinding efficiency.
[0008] It is a second object of the present invention to provide a
method of manufacturing such a vitrified grindstone.
[0009] The first object may be achieved according to a first aspect
of this invention, which provides a vitrified grindstone having a
vitrified abrasive structure which has pores and which includes
abrasive grains and an inorganic bonding agent that holds the
abrasive grains together, wherein 10-95% of a total volume of the
pores is filled with a cured resin.
[0010] In the vitrified abrasive structure of the present vitrified
grindstone, a suitable volumetric percentage of the total volume of
the pores or voids is filled with the cured resin, so as to prevent
filling of the pores with metal particles which are removed from
the workpiece during a grinding operation using the present
vitrified grindstone and which would otherwise be fused in the
pores, causing clogging or glazing on the grinding surface of the
vitrified grindstone. It is also noted that since the resin filling
the pores is softer than the abrasive grains, the surface of the
vitrified grindstone is comparatively recessed at local spots
corresponding to the resin-filled pores, during the grinding
operation on the workpiece, so that the abrasive grains adjacent to
the surface of the abrasive structure gradually fracture or break
down and are removed, making it possible to prevent an excessive
rise of the temperature on the workpiece surface due to an
excessive amount of heat of friction which would be generated
between the workpiece surface and the abrasive grains that remain
dull, and also prevent chipping of the vitrified grindstone while
permitting easy dressing of the vitrified grindstone. The abrasive
grains which are only loosely held together by the inorganic
bonding agent can be tightly held together with an additional
bonding force provided by the cured resin, so that the cured resin
functions to avoid early removal of those abrasive grains, assuring
a high grinding ratio.
[0011] It is further appreciated that the arrangement in which
10-95% of the total volume of the pores is filled with the cured
resin is effective to prevent the hardness of the grindstone from
being excessively hardened by the impregnation of the vitrified
abrasive structure with the cured resin, while assuring a high
grinding ratio. Since an excessive increase in the hardness of the
vitrified grindstone is prevented, the present vitrified grindstone
can be easily dressed as needed, making it possible to prevent a
reduction in the service life of the vitrified grindstone. If the
volumetric percentage of the pores that is filled with the cured
resin is smaller than 10%, the effect of the impregnation of the
abrasive structure with the cured resin cannot be expected. If the
volumetric percentage of the pores that is filled with the cured
resin is larger than 95%, the vitrified grindstone is excessively
hardened, making it difficult to dress the vitrified
grindstone.
[0012] It is to be understood that the vitrified abrasive structure
of the vitrified grindstone may further include an aggregate such
that the abrasive grains and the aggregate are held together by the
inorganic bonding agent.
[0013] According to a first preferred form of the first aspect of
the invention, 40-90% of the total volume of the pores is filled
with the cured resin. In the vitrified grindstone of this first
preferred form, an excessive increase in the hardness of the
vitrified grindstone is more reliably prevented owing to the
arrangement in which at least 10% of the total volume of the pores
remains unfilled with the cured resin. Further, since at least 40%
of the total volume of the pores is filled with the cured resin, it
is possible to more reliably prevent the conventionally encountered
drawbacks such as easy filling of the pores with metal particles,
easy glazing or clogging of the surface of the vitrified
grindstone, easy chipping of the abrasive structure, and easy
removable of the abrasive grains. Thus, the present vitrified
grindstone can be easily dressed without having to increase a load
applied to the vitrified grindstone, leading to a further increased
grinding ratio and providing a further improved surface smoothness
of the workpiece.
[0014] According to a second preferred form of the first aspect of
the invention, the cured resin consists of at least one
thermosetting synthetic resin which is selected from a phenol resin
and an epoxy resin, so that the vitrified grindstone has a higher
degree of hardness than where the resin consists of a thermoplastic
resin.
[0015] According to a third preferred form of the first aspect of
the invention, the abrasive grains includes super abrasive grains
(considerably fine abrasive grains) consisting of diamond abrasive
grains, CBN abrasive grains, or mixture of diamond and CBN abrasive
grains. The super abrasive grains preferably have Knoop hardness of
at least 3000. It is preferable that the super abrasive grains have
an average particle size of 20-220 .mu.m. The particle sizes of 20
.mu.m and 220 .mu.m correspond to 800 and 60 meshes, respectively.
Preferably, the super abrasive grains in the vitrified abrasive
structure has a concentration of larger than 10 and smaller than
230, more preferably, a concentration ranging from 20 to 200.
[0016] According to a fourth preferred form of the first aspect of
the invention, the vitrified abrasive structure has a porosity of
20-75% by volume, more preferably, 30-65% by volume, before the
vitrified abrasive structure is impregnated with the resin.
[0017] According to a fifth preferred form of the first aspect of
the invention, the inorganic bonding agent consists of a
borosilicate glass or a crystallized glass which is suitable for
holding super abrasive grains together. The crystallized glass may
be, for example, a glass in which willemite precipitates. The
inorganic bonding agent preferably has a thermal expansion
coefficient ranging from .alpha.-(2.times.10.sup.-6) to
.alpha.+(2.times.10.sup.-6) [1/K] (where .alpha. represents a
thermal expansion coefficient of the super abrasive grains), so
that the super abrasive grains can be tightly bonded together by
the inorganic bonding agent.
[0018] According to a sixth preferred form of the first aspect of
the invention, the vitrified abrasive structure includes 15-35% by
volume of the inorganic bonding agent, so that the porosity of the
vitrified abrasive structure is held in the volumetric range as
described above, without deteriorating the holding strength with
which the bonding agent holds the abrasive grains together. The
vitrified abrasive structure may include, as an aggregate, a pore
forming agent such as an inorganic balooning agent or other
inorganic hollow substance.
[0019] The second object may be achieved according to a second
aspect of this invention, which provides a method of manufacturing
a vitrified grindstone having a vitrified abrasive structure which
includes abrasive grains and an inorganic bonding agent that holds
the abrasive grains together. This method comprises (a) a step of
preparing a mixture of a liquid resin and a liquid diluent which is
other than the liquid resin and which dilutes the liquid resin,
such that the mixture includes 10-95% by volume of the liquid
resin; (b) a step of impregnating the vitrified abrasive structure
with the mixture; and (c) a step of curing the liquid resin
contained in the mixture with which the vitrified abrasive
structure is impregnated.
[0020] According to this method, the vitrified abrasive structure
is impregnated with the mixture of the resin and the liquid diluent
in the impregnating step, and the resin contained in the mixture is
then cured or hardened in the curing step, for thereby making it
possible to obtain the vitrified grindstone having the vitrified
abrasive structure whose pores are partially filled with the cured
resin, wherein a volume ratio of the cured resin to the pores is
determined depending upon a kind of the liquid diluent and a mixing
ratio of the liquid resin to the liquid diluent. Since the mixture
includes 10-95% by volume of the liquid resin, 10-95% of a total
volume of the pores is filled with a cured resin, as a result of
the impregnation of the vitrified abrasive structure with the
mixture. Thus, a suitable percentage of the total volume of the
pores or voids of the vitrified abrasive structure is filled with
the cured resin, so as to prevent filling of the pores with metal
particles which are removed from the workpiece during a grinding
operation using the present vitrified grindstone and which would
otherwise be fused in the pores, causing clogging or glazing on the
grinding surface of the vitrified grindstone.
[0021] It is also noted that since the resin filling the pores is
softer than the abrasive grains, the surface of the vitrified
grindstone is comparatively recessed at local spots corresponding
to the resin-filled pores, during the grinding operation on the
workpiece, so that the abrasive grains adjacent to the surface of
the abrasive structure gradually fracture or break down and are
removed, making it possible to prevent an excessive rise of the
temperature on the workpiece surface due to an excessive amount of
heat of friction which would be generated between the workpiece
surface and the abrasive grains that remain dull, and also prevent
chipping of the vitrified grindstone while permitting easy dressing
of the vitrified grindstone. The abrasive grains which are only
loosely held together by the inorganic bonding agent can be tightly
held together with an additional bonding s force provided by the
cured resin, so that the cured resin functions to avoid early
removal of those abrasive grains, assuring a high grinding
ratio.
[0022] It is further appreciated that the present method merely
requires, in addition to the steps performed in the conventional
method of manufacturing the vitrified grindstone, the impregnating
step and the curing step, and does not require conventionally used
special equipment such as hot pressing equipment, which usually
suffers from low manufacturing efficiency and a limitation in the
range of size of the vitrified grindstone that can be manufactured.
Still further, the volume ratio of the cured resin to the pores,
which ratio is held in a range of 10-95%, is effective to prevent
the hardness of the grindstone from being excessively hardened due
to the impregnation of the vitrified abrasive structure with the
cured resin, while assuring a high grinding ratio. Since an
excessive increase in the hardness of the vitrified grindstone is
prevented, the vitrified grindstone can be easily dressed as
needed, making it possible to prevent a reduction in the service
life of the vitrified grindstone. If the above-described volume
ratio is smaller than 10%, the effect of the impregnation of the
abrasive structure with the cured resin cannot be expected. If the
volume ratio is larger than 95%, the vitrified grindstone is
excessively hardened, making it difficult to dress the vitrified
grindstone.
[0023] Since the hardness of the vitrified grindstone manufactured
according to this method is determined depending upon the
volumetric percentage of the pores filled with the cured resin, it
is possible to manufacture vitrified grindstones having respective
grades different from each other, by impregnating vitrified
abrasive structures identical in grade with each other, with
respective mixtures having different mixing ratios of the liquid
resin to the liquid diluent. Further, since the pores of the
vitrified abrasive structures are not fully filled with the resin,
the hardness of the vitrified grindstone can be changed even after
the manufacture of the grindstone has been completed, for example,
by additionally impregnating the grindstone with a suitable amount
of the liquid resin. It is accordingly unnecessary to manufacture,
as end products, vitrified grindstones having different grades for
respective different applications, and it is possible to suitably
change the grade of each of vitrified grindstones which have been
prepared in the same manner to have the same grade.
[0024] Preferably, the mixture prepared in the mixture preparing
step includes 40-90% by volume of the liquid resin, so that 40-90%
of the total volume of the pores is filled with the cured resin.
Since at least 10% of the total volume of the pores remains
unfilled with the cured resin, an excessive increase in the
hardness of the vitrified grindstone is more reliably prevented.
Since at least 40% of the total volume of the pores is filled with
the cured resin, it is possible to more reliably prevent the
conventionally encountered drawbacks such as easy filling of the
pores with metal particles, easy glazing or clogging of the surface
of the vitrified grindstone, easy chipping of the abrasive
structure, and easy removable of the abrasive grains. Thus, the
vitrified grindstone can be easily dressed without having to
increase a load applied to the vitrified grindstone, leading to a
further increased grinding ratio and providing a further improved
surface smoothness of the workpiece.
[0025] Preferably, the liquid diluent is a volatile liquid, so that
the liquid diluent contained in the mixture is rapidly volatilized
before the curing of the resin in the curing step. The vitrified
abrasive structure of the vitrified grindstone is effectively
reinforced by the impregnation of the vitrified abrasive structure
with the resin, since the resin is cured more efficiently than
where the liquid diluent is volatized during the curing of the
resin or after the curing of the resin.
[0026] More preferably, the liquid diluent is volatilized at a
temperature of 40-100.degree. C., so that the liquid diluent in the
mixture is not substantially volatilized at a normal temperature,
and is rapidly volatilized at a relatively low temperature in the
curing step before the resin is cured. Since the liquid diluent is
not substantially volatilized at a normal temperature, each step of
the method is easily implemented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The above and other objects, features, advantages and
technical and industrial significance of this invention will be
better understood by reading the following detailed description of
the presently preferred embodiment of the invention, when
considered in connection with the accompanying drawings, in
which:
[0028] FIG. 1 is a perspective view showing an abrasive segment as
a vitrified grindstone according to one embodiment of this
invention, which grindstone has a vitrified abrasive structure
reinforced by impregnation thereof with a resin;
[0029] FIG. 2 is a perspective view of a grinding wheel whose
radially outer portion consists of the abrasive segments of FIG. 1,
which are arranged in the circumferential direction; and
[0030] FIG. 3 is a view illustrating a process of manufacturing the
abrasive segment of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] FIG. 1 shows an abrasive segment 10 as a vitrified
grindstone constructed according to one embodiment of this
invention, which grindstone has a vitrified abrasive structure
reinforced by impregnation thereof with a resin. The abrasive
segment 10 consists of a plate member which is curved so as to have
a generally arcuate shape. FIG. 2 shows a grinding wheel 18
consisting of an inner core 12, and a circular array of abrasive
segments 10 which are bonded to the outer circumferential surface
of the inner core 12 with a suitable bonding adhesive such as an
epoxy resin adhesive such that there does not exist any gap or
clearance between the adjacent abrasive segments 10. The inner core
12 is formed of a metallic material, a fiber-reinforced synthetic
resin or a vitrified grindstone. The inner core 12 has in its
center a mounting through-hole 19, into which a rotary shaft of a
grinding machine is to be fitted.
[0032] Each abrasive segment 10 consists of a radially outer layer
14 having an outer grinding surface, and a radially inner layer 16
which is formed integrally with the outer layer 14 and which
functions as a base support layer for mechanically supporting the
outer layer 14. Each of the outer and inner layers 14, 16 consists
of abrasive grains and an inorganic bonding agent or a vitrified
bond by which the abrasive grains are held together. Generally, the
outer and inner layers 14, 16 use the same kind of inorganic
bonding agent, but use different kinds of abrasive grains. For
instance, the outer layer 14 includes super abrasive grains, such
as cubic-crystal boron nitride (CBN) or diamond abrasive grains,
which have a Knoop hardness value of at least 3000, while the inner
layer 16 includes ordinary ceramic abrasive grains such as fused
alumina grains, silicon carbide grains or mullite grains, which are
relatively inexpensive. Preferably, the super abrasive grains used
for the outer layer 14 have an average particle size within a range
of 0.5-250 .mu.m. The lower and upper limits of 0.5 .mu.m and 250
.mu.m respectively correspond to 32000 and 60 meshes.
[0033] Each of the abrasive segments 10 used for the grinding wheel
18 may be manufactured by a method illustrated in the block diagram
of FIG. 3. Initially, a mixture preparing step 20 is implemented to
prepare mixtures or compositions for the radially outer and inner
layers 14, 16 of the abrasive segment 10. Each of the mixtures for
the outer and inner layers 14, 16 includes abrasive grains, a
glassy inorganic bonding agent, a caking additive such as dextrin,
and optionally suitable additives such as an organic substance and
an inorganic balooning agent or other pore forming agent. Desired
proportions of these materials are weighed and well mixed under
stirring to prepare the mixture for each of the outer and inner
layers 14, 16. In the present embodiment, the inorganic bonding
agent consists of a borosilicate glass, or a crystallized glass in
which willemite precipitates. The inorganic bonding agent
preferably has a thermal expansion coefficient ranging from
.alpha.-(2.times.10.sup.-6) to .alpha.+(2.times.10.sup.-6) [1/K]
(where a represents a thermal expansion coefficient of the super
abrasive grains), at a temperature ranging from a room temperature
to 500.degree. C. The proportion of the inorganic bonding agent is
determined such that the fired vitrified abrasive structure of the
abrasive segment 10 includes 15-35% by volume of the inorganic
bonding agent. For example, the mixture for the outer layer 14 is
prepared by mixing 18.0 parts by volume of a vitrified boding agent
and 10.0 parts by volume of a caking additive into 50.0 parts by
volume of CBN abrasive grains (type-I #80/#100 available form GE
company), while the mixture for the inner layer 16 is prepared by
mixing 18.0 parts by volume of a vitrified bonding agent and 10.0
parts by volume of a caking additive into 50.0 parts by volume of
silicon carbide abrasive grains (#80).
[0034] The mixture preparing step 20 is followed by a forming step
22 in which the prepared mixtures for the outer and inner layers
14, 16 are introduced into a suitable metal mold whose cavity has a
shape following the abrasive segment 10 to be manufactured. The
mixtures in the mold are compressed or compacted by a press, to
form an unfired or green precursor for the abrasive segment 10.
Then, a firing step 24 is implemented to fire or burn the unfired
precursor at a temperature of about 900.degree. C., so as to obtain
the fired abrasive segment 10 having, for example, a length of 40
mm, a width of 10.4 mm and a thickness of 7.4 mm. In this firing
step 24, the caking additive included in the compositions is burnt
out, while the inorganic bonding agent is fused to bond the
abrasive grains together, so that the abrasive segment 10 has a
porous vitrified abrasive structure having a network of continuous
pores or voids, wherein the abrasive grains are held together by
the inorganic bonding agent. In this vitrified abrasive structure,
the abrasive grains have a concentration of 10-230, preferably,
20-200. The vitrified abrasive structure has a porosity of 20-75%
by volume, preferably, 30-65% by volume.
[0035] In the meantime, a resin preparing step 26 is implemented to
prepare a mixture by mixing a liquid phenol resin into a liquid
diluent which is other than the liquid phenol resin and which does
not impede curing of the resin. The liquid diluent preferably
consists of a solvent, which is not easily volatilized at a normal
temperature but is easily volatilized in a dried ambient when the
resin is cured, and which has a relatively low boiling point, for
example, about 200.degree. C. or lower. In this respect, furfuryl
alcohol having a boiling point of about 170.degree. C. is
preferably used as the liquid diluent to be mixed into the liquid
phenol resin. 5-95 parts by volume of the furfuryl alcohol is mixed
into predetermined parts by volume of the liquid phenol resin in
the form of, for example, PR-9400 (available from Sumitomo Durez
Company, Ltd), such that the sum of the volumes of the furfuryl
alcohol and the liquid phenol resin corresponds to 100 parts by
volume. The furfuryl alcohol and the liquid phenol resin are
stirred for about three minutes, for thereby obtaining the mixture
having a relatively low viscosity.
[0036] In the following impregnating step 28, the fired vitrified
abrasive structure of the abrasive segment 10 which has the network
of the multiple continuous pores is impregnated with the mixture
obtained in the resin preparing step 26, so that the liquid phenol
resin (thermosetting synthetic resin) and the liquid diluent
(furfuryl alcohol) contained in the mixture are dispersed evenly
into the entirety of the porous vitrified abrasive structure. This
impregnating step 28 may be performed by: pouring the mixture in
the fluid or liquid state in a suitable container made of stainless
such that the mass of the fluid or liquid mixture in the container
has a depth of about 10 mm; immersing the entirety of the abrasive
segment 10 (which has been formed and fired in the forming and
firing steps 22, 24) in the mass of the mixture; and evacuating the
abrasive segment 10 so that the network of the continuous pores is
filled with the mixture whose major component is the phenol resin.
However, only a selected portion of the abrasive segment 10 may be
immersed in the mass of the fluid or liquid mixture.
[0037] The impregnating step 28 is followed by a curing step 30 of
holding the resin-impregnated abrasive segment 10 in a suitable
drier, at a suitable temperature for a suitable length of time, for
instance, at 18.degree. C. for two hours. This curing step 30 may
be implemented after the liquid mixture which sticks to the surface
of the abrasive segment 10 without permeating into the abrasive
segment 10, is removed by a suitable cloth. With implementation of
the curing step 30, the phenol resin which is contained in the
mixture filling the pores is cured, while the liquid diluent
(furfuryl alcohol) contained in the mixture is rapidly volatilized
as the temperature rises to a volatilization point of the furfuryl
alcohol at the initial stage of the process of the curing of the
resin. As a result of the volatilization or elimination of the
furfuryl alcohol, only the cured phenol resin remains in the pores
of the vitrified abrasive structure, so that the pores of the
vitrified abrasive structure are partially filled with the cured
phenol resin, wherein a volume ratio of the cured phenol resin to
the pores is held in a range of 10-95% which corresponds to a
volume ratio of the phenol resin to the mixture.
[0038] The abrasive segments 10, each of which is manufactured
according to the method as described above, are bonded to the outer
circumferential surface of the inner core 12 having an outside
diameter of .phi.366 mm, with an epoxy resin adhesive, so that the
grinding wheel 18 as shown in FIG. 2 is formed.
[0039] In the vitrified abrasive structure of each of the abrasive
segments 10 of the grinding wheel 18, a suitable percentage of the
total volume of the pores or voids is filled with the cured phenol
resin, so as to prevent filling of the pores with metal particles
which are removed from the workpiece during a grinding operation
using the grinding wheel 18 and which would otherwise be fused in
the pores, causing clogging or glazing on the grinding surface of
each abrasive segment 10. It is also noted that since the phenol
resin filling the pores is softer than the abrasive grains, the
surface of the abrasive segment 10 is comparatively recessed at
local spots corresponding to the resin-filled pores, during the
grinding operation on the workpiece, so that the abrasive grains
adjacent to the surface of the abrasive segment 10 gradually
fracture or break down and are removed, making it possible to
prevent an excessive rise of the temperature on the workpiece
surface due to an excessive amount of heat of friction which would
be generated between the workpiece surface and the abrasive grains
that remain dull, and also prevent chipping of the abrasive segment
10 while permitting easy dressing of the vitrified grindstone. The
abrasive grains which are only loosely held together by the
inorganic bonding agent can be tightly held together with an
additional bonding force provided by the cured resin, so that the
cured resin functions to avoid early removal of those abrasive
grains, assuring a high grinding ratio.
[0040] It is further appreciated that the above-described volume
ratio of the cured phenol resin to the pores, which ratio is held
in a range of 10-95%, is effective to prevent the hardness of the
abrasive segment 10 from being excessively hardened by the
impregnation of the vitrified abrasive structure with the cured
resin, while assuring a high grinding ratio. Since an excessive
increase in the hardness of the vitrified grindstone is prevented,
the present vitrified grindstone can be easily dressed as needed,
making it possible to prevent a reduction in the service life of
the vitrified grindstone.
[0041] In the manufacture of the abrasive segment 10 according to
the above-described method, the vitrified abrasive structure is
impregnated with the mixture of the phenol resin and the furfuryl
alcohol in the impregnating step 28, and the phenol resin contained
in the mixture is then cured or hardened in the curing step 30, for
thereby making it possible to obtain the resin-impregnated abrasive
segment 10 having the vitrified abrasive structure whose pores are
partially filled with the cured phenol resin, wherein the volume
ratio of the cured phenol resin to the pores is determined
depending upon the mixing ratio of the phenol resin to the furfuryl
alcohol. Since the mixture includes 10-95% by volume of the phenol
resin, the vitrified abrasive structure is impregnated with the
mixture such that the volume ratio of the cured phenol resin to the
pores is held in a range of 10-95%.
[0042] It is further appreciated that the above-described method
merely requires, in addition to the steps performed in the
conventional method of manufacturing the vitrified grindstone, the
impregnating step 28 and the curing step 30, and does not require
conventionally used special equipment such as hot pressing
equipment, which usually suffers from low manufacturing efficiency
and a limitation in the range of size of the abrasive segment that
can be manufactured.
[0043] Since the hardness of the resin-impregnated abrasive segment
10 manufactured according to the above-described method is
determined depending upon the volumetric percentage of the pores
filled with the cured phenol resin, it is possible to manufacture
abrasive segments 10 having respective grades different from each
other, by impregnating vitrified abrasive structures which are
obtained at the firing step 24 and which have respective grades
identical with each other, with respective mixtures having
different mixing ratios of the phenol resin to the furfuryl
alcohol. In addition, since the pores of the vitrified abrasive
structures are not fully filled with the resin, the hardness of the
abrasive segment 10 can be changed even after the manufacture of
the abrasive segment 10 has been completed, for example, by
additionally impregnating the abrasive segment 10 with a suitable
amount of the liquid resin. It is accordingly unnecessary to
manufacture, as end products, abrasive segments 10 having different
grades for respective different applications, and it is possible to
suitably change the grade of each of abrasive segments 10 which
have been prepared in the same manner to have the same grade.
[0044] Experiments were conducted to clarify a relationship between
the grinding performance of the grinding wheel 18 using the
abrasive segments 10 and the volumetric percentage of the pores of
the vitrified abrasive structure filled with the cured resin.
EXAMPLES
[0045] The inner core portion 12 of the grinding wheel 18
manufactured in this experiment is a steel disk having the center
mounting hole 19, while the radially outer and inner layers 14, 16
of each of the abrasive segments 10 of the wheel 18 have the
following compositions:
[0046] Outer Layer 14
1 CBN grains (Type-I #80/#100 50.0 (parts by volume) available from
GE company) Vitrified bonding agent 18.0 (parts by volume) Caking
additive 10.0 (parts by volume)
[0047] Inner Layer 16
2 Silicon carbide grains (#80) 50.0 (parts by volume) Vitrified
bond agent 18.0 (parts by volume) Caking additive 10.0 (parts by
volume)
[0048] An unfired or green abrasive structure formed of the
above-indicated compositions were fired at 900.degree. C. for five
hours, to prepare each abrasive segment 10 having a length of 40 mm
(as measured in the circumferential direction of the grinding wheel
18), a width of 10.4 mm (corresponding to the thickness or axial
dimension of the grinding wheel 18) and a thickness of 7.4 mm. The
outer layer 14 had a thickness of 3.8 mm while the inner layer 16
had a thickness of 3.6 mm. In the meantime, five different mixtures
Nos. 1-5 as indicated in Table 1 were prepared by mixing the phenol
resin (PR-9400 available from Sumitomo Durez Company, Ltd) and the
furfuryl alcohol, with proportions thereof as indicated in Table 1
(in which the phenol resin and the furfuryl alcohol are simply
referred to as "Resin" and "Diluent", respectively). Each of the
mixtures Nos. 1-5 was stirred for three minutes, and then the
abrasive segment 10 was impregnated with each of the mixtures Nos.
1-5 according to the method as described above, whereby vitrified
grindstones of Examples 1-5 as indicated in Table 2 were
obtained.
[0049] Table 2 indicates compositions of the vitrified grindstones
of Examples 1-5 and Comparative Examples 1 and 2. The vitrified
grindstone of Comparative Example 1 was obtained by impregnating
the abrasive segment with the phenol resin, in place of the mixture
of the phenol resin and the furfuryl alcohol. The vitrified
grindstone of Comparative Example 2 corresponded to a conventional
vitrified grindstone which is not impregnated with the phenol
resin. It is noted that "Filling ratio" in Table 2 represents a
percentage of the total volume of the pores or voids which was
replaced by the resin, i.e., a percentage of the total volume of
the pores which was filled with the resin, as a result of the
impregnation of the vitrified abrasive structure with the resin. It
is noted that the indicated volume of each component of the
compositions is with respect to the total volume of the vitrified
grindstone. It is also noted that the volume of the diluent is
indicated in parentheses, since the diluent was eliminated and did
not remain in the grindstone after the curing of the resin.
3 TABLE 1 Resin (vol. %) Diluent (vol. %) Mixture No.1 95.0 5.0
Mixture No.2 90.0 10.0 Mixture No.3 70.0 30.0 Mixture No.4 40.0
60.0 Mixture No.5 10.0 90.0
[0050]
4 TABLE 2 CBN Inorganic abrasive boding Filling grains agent Resin
Diluent Pores ratio Comparative 50.0 18.0 32.0 0.0 0.0 100 Example
1 Example 1 50.0 18.0 30.4 (1.6) 1.6 95 Example 2 50.0 18.0 28.8
(3.2) 3.2 90 Example 3 50.0 18.0 22.4 (9.6) 9.6 70 Example 4 50.0
18.0 12.8 (19.2) 19.2 40 Example 5 50.0 18.0 3.2 (28.8) 28.8 10
Comparative 50.0 18.0 0.0 0.0 32.0 0 Example 2
[0051] Using the vitrified grindstones of Examples 1-5 and
Comparative Examples 1 and 2, grinding operations were successively
performed on workpieces on a cylindrical grinding machine, in the
following conditions:
[0052] [Conditions]
[0053] Dimensions of grindstone (grinding wheel): 380 mm (outside
diameter).times.10 mm (thickness).times.80 mm (inside diameter)
Workpiece: Cylindrical workpiece made of SCM435 having dimensions
of 60 mm (outside diameter).times.5 mm (width) Type of grinding
operation: Plunge grinding to reduce diameter of workpiece from 60
mm to 37 mm, using a grinding fluid (emulsion water-soluble
grinding liquid)
[0054] Peripheral speed of grindstone (grinding wheel): 160 m/sec
Grinding efficiency: 70 mm.sup.3/mm
[0055] Results of the grinding operations by the vitrified
grindstones of Examples 1-5 and Comparative Examples 1 and 2 are
indicated in Table 3, wherein "Surface roughness" means the surface
roughness of the tenth workpiece, "Consumed electric power" means
the amount of electric power consumed when the tenth workpiece was
machined, and "Filling ratio" represents the percentage of the
total volume of the pores or voids which was filled with the
resin.
5 TABLE 3 Burning Con- Fill- Resistance Grind- on Surface sumed ing
to ing finished roughness electric ratio dressing ratio surfaced Rz
power Comparative 100 0.50(kW) 2300 NO 2.5 (.mu.m) 2.0(kW) Example
1 Example 1 95 0.38(kW) 3600 NO 2.5 (.mu.m) 1.8(kW) Example 2 90
0.38(kW) 3700 NO 2.3 (.mu.m) 1.7(kW) Example 3 70 0.35(kW) 4000 NO
2.3 (.mu.m) 1.6(kW) Example 4 40 0.34(kW) 3700 NO 2.4 (.mu.m)
1.6(kW) Example 5 10 0.32(kW) 3300 NO 2.9 (.mu.m) 1.6(kW)
Comparative 0 0.30(kW) 1200 YES 3.2 (.mu.m) 1.7(kW) Example 2
[0056] As is apparent from Table 3, the grinding ratio in the 15
grinding operations with the vitrified grindstones of Examples 1-5
was about 2.7-3.3 times as high as the grinding ratio in the
grinding operation with the conventional vitrified grindstone of
Comparative Example 2, which was not impregnated with the resin.
Further, each of the vitrified grindstones of Examples 1-5 provided
the workpiece with an excellent smoothness on the finished surface
without burning or glazing on the surface. The resistance to
dressing operation of each of the grindstones of Examples 1-5 was
larger than the resistance to dressing operation of the grindstone
of Comparative Example 2, by an amount of as small as 0.02-0.08 kW.
The electric power consumed in the grinding operations with the
grindstone of Examples 1-5 was almost equal to that consumed in the
grinding operation with the grindstone of Comparative Example 2. As
compared with the grindstone of Comparative Example 1 in which 100%
of the total volume of the pores or voids was filled with the
resin, is the dressing resistance and the consumed electric power
of the grindstone of Examples 1-5 were considerably lower than
those of the grindstone of Comparative Example 1. The grinding
ratio of the grindstones of Examples 1-5 was about 1.4-1.7 times as
high as the grinding ratio of the grinding stone of Comparative
Example 1. The surface roughness provided by the grindstones of
Examples 1-5 was almost equal to that provided by the grindstone of
Comparative Example 1. As a whole, the grindstones of Examples 2-4
whose fill ratio was 40-90% gave better results than the
grindstones of Comparative Examples 1 and 2, except that the
dressing resistance applied to the grindstones of Examples 2-4 was
slightly larger than that applied to the conventional grindstone of
Comparative Example 2.
[0057] The grindstone of Comparative Example 1, in which the
filling ratio was excessively large and the abrasive grains were
completely surrounded by the resin, had an increased hardness
thereof, and required a comparatively large force to be applied to
its grinding surface in the dressing operation so as to crack the
inorganic bonding agent for removing dull abrasive grains from the
grinding surface. The increased hardness and dressing resistance
results in a considerably difficult dressing operation of the
grindstone of Comparative Example 1, in which a comparatively large
amount of stock has to be removed from the grindstone so as to
sufficiently roughen the grinding surface of the grindstone;
otherwise it would be difficult to restore sharpness of the
abrasive grains, making difficult to continue the grinding
operation with the grindstone. Such a comparatively large amount of
stock to be removed in the dressing operation undesirably reduces
the grinding ratio and shortens the service life of the grindstone.
After the difficult dressing operation, the grindstone of
Comparative Example 1 necessitates an additional operation for
removing the resin between the abrasive grains from the grinding
surface, by using a so-called "soft" grindstone, so that cutting
edges of the abrasive grains are exposed, namely, so that the
grinding surface is assuredly roughened. This additional operation
is somewhat cumbersome and causes a risk of removals of the
abrasive grains whose sharpness has been restored in the dressing
operation.
[0058] In contrast to such drawbacks of the grindstone of
Comparative Example 1, each of the grindstones of Examples 1-5, in
which the filling ratio was held in a suitable range and the resin
served to reinforce the abrasive structure, did not require a large
force to be applied to its grinding force in the dressing
operation, in which sharpness of the abrasive grains can be easily
restored without having to remove a large amount of stock from the
grindstone, thereby advantageously increasing the grinding ratio
and lengthening the service life of the grindstone. Further, each
of the grindstones of Examples 1-5 does not necessitate the
above-described additional operation for removing the resin from
the grinding surface, so that the grinding operation can be
restarted with the grindstone after the easy dressing
operation.
[0059] Each of the grindstones of Examples 1-5 did not suffer from
large wear thereof and fusion of metal particles in the pores,
leading to an effective use of the grindstone which was equipped
with the expensive super abrasive grains. On the other hand, the
grindstone of Comparative Example 1 whose filling ratio was 100%
suffered from fusion of metal particles in the pores, although did
not suffer from burning on the surface of the workpiece ground by
this grindstone. The grindstone of Comparative Example 2 whose
filling ratio was 0%, suffered from considerable fusion of metal
particles in the pores and even burning on the ground surface of
the workpiece. It is considered that the fusion of metal particles
in the grindstone of Comparative Example 1 was caused by an
excessive rise of temperature due to substantial absence of the
pores in the grinding surface, which pores would have served as
chip pockets for accommodating therein the grinding liquid. It is
also considered that the considerable fusion of metal particles in
the grindstone of Comparative Example 2 was caused by a reduced
number of the abrasive grains dedicated for the grinding operation,
due to absence of the resin which would have served for holding the
abrasive grains and thereby preventing easy removal of the abrasive
grains from the grinding surface.
[0060] While the presently preferred embodiment of the present
invention have been described above with a certain degree of
particularity, by reference to the accompanying drawings, it is to
be understood that the invention is not limited to the details of
the illustrated embodiment, but may be otherwise embodied.
[0061] In the above-illustrated embodiment, the phenol resin is
used as the thermosetting synthetic resin with which the vitrified
abrasive structure is impregnated. However, the phenol resin may be
replaced by one- or two-liquid epoxy resin, or even by a
thermoplastic synthetic resin such as urethane resin and polyvinyl
alcohol. The two-liquid epoxy resin is prepared, for example, by
mixing fluid or liquid hardening agent including polyamide resin,
into fluid or liquid primary agent of epoxy resin such as bisphenol
A.
[0062] In the above-illustrated embodiment, the furfuryl alcohol
having a high degree of volatility is used as the liquid diluent
which serves to dilute the resin. However, the furfuryl alcohol may
be replaced by other alcohol solvent (such as ethanol and
methanol), water, thinner, or any other liquid which is capable of
diluting the resin and does not impede the curing of the resin. The
diluent liquid does not necessarily have to be a liquid capable of
dissolving the resin, but may be a liquid capable of suspending the
resin.
[0063] It is preferable that the liquid diluent has a high degree
of volatility and does not remain in the abrasive structure.
However, the liquid diluent may be a liquid having a low degree of
volatility, as long as the liquid does not impede the curing of the
resin even if the liquid remains in the abrasive structure. For
instance, while the resin filling 10-95% of the total volumes of
the pores of the vitrified abrasive structure is being cured, the
liquid diluent may remain in the rest of the total volumes of the
pores which is not filled with the resin. That is, the liquid
diluent may be any kind of liquid, as long as the liquid is capable
of being eliminated as a result of volatilization thereof, or
capable of contracting so as to be volumetrically reduced, for
thereby forming pores or voids in the abrasive structure before the
curing of the resin is completed.
[0064] In the above-illustrated embodiment, the volumetric ratio of
the resin to the mixture (of the resin and the liquid diluent)
corresponds to the volumetric ratio of the pores filled with the
resin. However, these ratios do not always correspond to each
other, for example, due to possible contraction and gasification of
the resin and the liquid diluent. Where these ratios do not
correspond to each other, it is desirable to determine the
volumetric ratio of the resin to the mixture such that the
volumetric ratio of the pores filled with the resin ranges from
10-95 vol. %.
[0065] While the mixture is prepared by mixing the phenol resin
into the furfuryl alcohol in the above-illustrated embodiment, it
is possible to use a resin solution in which these components are
mixed with suitable proportions thereof.
[0066] In the above-illustrated embodiment, the grindstone takes
the form of the abrasive segment 10 which consists of the radially
outer layer 14 assigned to perform a grinding operation and the
radially inner layer 16 backing up the outer layer 14. However, the
abrasive segment may consist of a single layer formed of the same
composition as used for the outer layer 14. Further, the grindstone
may take the form of an elongated horning bar or a super-finishing
block.
[0067] While the radially outer layer 14 of the abrasive segment 10
includes super abrasive grains such as cubic-crystal boron nitride
(CBN) or diamond abrasive grains in the above-illustrated
embodiment, the outer layer 14 may include alundum grains (fused
alumina abrasive grains), carborundum grains (silicon carbide
abrasive grains) or other ordinary abrasive grains, in place of the
super abrasive grains.
[0068] While the grinding wheel 18 shown in FIG. 2 has a circular
array of the abrasive segments 10, the grinding wheel may use a
single integral annular abrasive solid mass.
[0069] It is to be understood that the invention may be embodied
with various other changes, modifications and improvements, which
may occur to those skilled in the art, without departing from the
spirit and scope of the invention defined in the following
claims.
* * * * *