Grinding media and a production method thereof

A new grinding medium consisting essentially of a synthetic resin matrix in which small-sized foam, coarse powdery abrasives and fine powdery abrasives are dispersed is provided. A powder having a low electric resistance is added to the medium to form relatively large-sized foam around the powder. The medium is preferably produced by softening and blocking a synthetic resin powder by high-frequency dielectric heating to form a matrix of the synthetic resin and dispersing small-sized foam, coarse powdery abrasives, fine powdery abrasives and powder having a low electic resistance in the synthetic resin matrix wherein relatively large-sized foam is formed around the powder. The medium has a high ability of abrasion and the surface of the medium is smoothly renewed during the grinding process so that the medium has an excellent grinding effectiveness.

FIELD OF THE INVENTION 
The present invention relates to new grinding media which are used in the 
barrel grinding of metal parts and the like. More particularly, the 
present invention relates to a new grinding medium consisting essentially 
of a synthetic resin matrix in which small-sized foam, coarse powdery 
abrasives, and fine powdery abrasives are dispersed and a production 
method thereof comprising softening and blocking a synthetic resin powder 
by high-frequency dielectric heating to form said synthetic resin matrix 
and dispersing said minute amount of foam, said coarse powdery abrasives, 
said fine powdery abrasives and powder having a low electric resistance in 
said synthetic resin matrix to form large-sized foam around said powders 
having a low electric resistance. 
DESCRIPTION OF THE PRIOR ART 
A barrel grinding method is a grinding method wherein workpieces are ground 
by a medium in a barrel which is rotated or swung. 
Hitherto, a medium consisting of a synthetic resin matrix in which 
small-sized foam and powdery abrasives are dispersed has been provided 
(Tokkai Sho 60-242960). 
The surfaces of workpieces are ground by powdery abrasives exposed on the 
surface of the medium when the workpieces are stirred with the medium in a 
barrel and the medium then scours the surfaces of the workpieces. 
When the medium scours the surfaces of the workpieces, the media may be 
elastically deformed since the medium includes small-sized foam and is 
pressed by the workpieces while it is stirred along with the workpieces. 
As a result, the contacting effectiveness between the medium and the 
workpieces is increased by this elastic deformation of the medium. 
Further, the surface of the medium is being abraded through the grinding 
process and the powdery abrasives within the medium are successively 
exposed so that the grinding effect of the medium is renewed. This renewal 
of the surface of the medium is called the "dressing effect". 
To increase the grinding effectiveness of the medium, it is desirable to 
use powdery abrasives having a large particle size. Nevertheless, in a 
case where powdery abrasives having a large particle size are mixed in the 
medium, the dispersal density of the powdery abrasives may gradually 
decrease as the particle size of the powdery abrasives gradually increases 
if the added amount of the powdery abrasives is fixed. This low dispersal 
density of the powdery abrasives may cause unevenness of the powdery 
abrasives in the medium. 
Accordingly, it is feared that the surface of a medium having no or less 
powdery abrasives is not well renewed during the grinding process. Said 
surface of such a medium having no or less powdery abrasives has a large 
number of small holes originating from the minute amount of foam and 
traces of the powdery abrasives peeled from the surface of the medium and 
the powder from the ground workpieces and grinding oil may become clogged 
in said small holes to obstruct the renewal of the surface of the medium 
so that the dressing effectiveness is decreased and as a result, the 
grinding effectiveness of the medium may be decreased. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a new 
grinding medium in which the renewal of the surface of said medium is 
smoothly performed to increase the dressing effectiveness. 
A further object of the present invention is to provide a new grinding 
medium in which less powder from the ground workpieces clogs the surface 
thereof. 
Still a further object of the present invention is to provide a new 
grinding medium having an excellent grinding effectiveness. 
Still a further object of the present invention is to provide a suitable 
production method of said grinding medium. 
Briefly, said objects of the present invention can be attained by a new 
grinding medium consisting essentially of a synthetic resin matrix in 
which small-sized foam, coarse powdery abrasives and fine powdery 
abrasives are dispersed and a production method of said grinding medium 
comprising softening and blocking a synthetic resin powder by 
high-frequency dielectric heating to form a matrix of said synthetic resin 
and dispersing small-sized foam, coarse powdery abrasive, fine powdery 
abrasive and a powder having a low electric resistance in said synthetic 
resin matrix wherein relatively large-sized foam is formed around said 
powder.

medium No. 1 
.smallcircle.--.smallcircle. medium No. 2 
X--X medium No. 3 
DETAILED DESCRIPTION OF THE INVENTION 
Synthetic resin 
In the present invention, a synthetic resin is used as a matrix for the 
grinding medium. Said synthetic resin is a thermosetting synthetic resin 
such as phenol resin, urea resin, melamine resin, epoxy resin, urethane 
resin and the like or a thermoplastic synthetic resin such as cellulose 
triacetate resin, polyvinyl chloride resin, polycarbonate resin, methacryl 
resin, polystyrol resin, polyamido resin (nylon 6, nylon 12 or the like), 
acrylonitrile-butadiene-styrene resin and the like. 
Powdery abrasives 
In the present invention, powdery abrasives are dispersed in the matrix of 
said synthetic resin. Said powdery abrasives may be ceramic powder which 
has been traditionally used. Said ceramic powder is such as almina powder, 
silica powder, zirconia powder, titania powder, boron nitride powder, 
silicon nitride powder and the like. 
In the present invention, the coarse powdery abrasives and the fine powdery 
abrasives are dispersed in the matrix of said synthetic resin and the 
particle size of said coarse powdery abrasives is generally larger than 
#220 and the particle size of said fine powdery abrasives is generally 
less than #240. 
Powder having a low electric resistance 
In the present invention, a relatively conductive powder having a low 
electric resistance is dispersed in the matrix of said synthetic resin. 
The volume resistivity of said powder is generally less than 10.sup.6 
(.OMEGA..multidot.cm) and said powder is such as silicon carbide powder, 
metal powder, carbon powder and the like. The particle size of said powder 
is desirably close to the particle size of said powdery abrasives 
dispersed in the matrix of said synthetic resin. 
Formulation 
In said grinding medium of the present invention, the weight ratio of the 
matrix of said synthetic resin and said powdery abrasives is generally 
40:60 to 10:90 and the weight ratio of said coarse powdery abrasives and 
said fine powdery abrasives is generally 30:70 to 70:30, desirably 40:60 
to 60:40. In a case where said powder having a low electric resistance is 
added, said powder is generally added in an amount less than 10% by 
weight, desirably 5% by weight of the matrix of said synthetic resin. 
Production method 
Said grinding medium of the present invention may be produced by methods 
such as a method comprising the dispersal of powdery abrasives in a 
softened thermoplastic synthetic resin and molding said mixture by 
injection molding, extruding molding, casting molding or the like, or a 
method comprising mixing powdery abrasives in a powdery thermoplastic 
synthetic resin, packing said powdery mixture in a mold and heating said 
powdery mixture to soften and block said powdery thermoplastic synthetic 
resin, or the like. 
In the former method, a chemical blowing agent should be added in said 
mixture or said mixture should be mechanically agitated to make 
small-sized foam in the resulting medium while in the later method, said 
softenend powdery thermoplastic synthetic resins stick together to form 
small-sized foam between said powdery thermoplastic synthetic resin. 
In the later method, said powdery mixture in a mold is heated by electric 
heating or high-frequency electric heating. In high-frequency electric 
heating, it is desirable to select a thermoplastic synthetic resin as a 
dielectric substance having a power factor dielectrics of more than 0.02 
in the range of a using frequency between 10 c/s to 10.sup.6 c/s to obtain 
a high effectiveness of the high frequency electric heating. When said 
powder having a low electric resistance is dispersed in the matrix of said 
themoplastic synthetic resin, said powder is selectively heated by 
high-frequency induction heating and said thermoplastic synthetic resin 
around said powder is selectively heated by said powder to form relatively 
large-sized foam around said powder. 
Said relatively large-sized foam promotes the abrasion of the surface of 
said medium to increase the dressing effectiveness. Nevertheless, when the 
added amount is beyond 10% by weight of the matrix of said thermoplastic 
synthetic resin, it is feared that high-frequency heating cannot be 
performed by electric discharge. 
In the present invention, a thermosetting synthetic resin is also used to 
form the matrix. In this case, generally, said powdery abrasives are mixed 
in an uncured thermosetting synthetic resin, a monomer of thermosetting 
synthetic; resin, an oligomer of thermosetting synthetic resin, a 
prepolymer of thermosetting synthetic resin and said mixture is foamed by 
a chemical blowing agent or by mechanical agitation and molded by cast 
molding to produce said grinding medium. 
It is desirable to prevent the formation of a skin layer on the surface of 
said medium so as to expose said powdery abrasives on the surface of said 
medium. To prevent the formation of said skin layer on said surface of 
said medium, it is necessary to control the heating condition, generally 
the heating time, or remove said skin layer from said surface of said 
medium by abrasion, a solvent, or the like. 
In accordance with to the present invention, the abrasion effectiveness is 
elevated by said coarse powdery abrasives in said medium, and the spaces 
between said coarse powdery abrasives are filled with said fine powdery 
abrasives to prevent sedimentation and separation of said powdery 
abrasives in said medium to acquire the uniform dispersion of said powdery 
abrasives in said medium, and further, said fine powdery abrasives are 
easily peeled from the surface of said medium so that said surface of said 
medium is smoothly abraded and renewed to maintain an excellent dressing 
effectiveness. 
Further, in a case where said powder having a low electric resistance is 
used, when the mixture of the thermoplastic synthetic resin and said 
powdery abrasives and said powder are heated by high-frequency dielectric 
heating to soften and block said thermoplastic synthetic resin, relatively 
large-sized foam is formed around said powder and as a result, the 
dressing effectiveness is elevated. 
When said high-frequency dielectric heating is applied to heat said 
mixture, the heating time in the mold process is shortened. 
EXAMPLE 1 
A mixture of nylon 6 powder and alumina powder (25:75 weight ratio) was 
prepared. Said alumina powder as a powdery abrasive consisted of 45 parts 
by weight of coarse alumina powder (average particle size being #150) and 
55 parts by weight of fine alumina powder (average particle size being 
#600). 
As shown in FIG. 1, said mixture (103) was poured into each molding hole 
(102) of a mold panel (101) and said mold panel (101) was put between 
electrodes to heat said mixture in each molding hole (102) by 
high-frequency dielectric heating and said mixture was softened and 
blocked. 
Said mold panel (101) was made of a dielectric such as synthethic resin and 
the desirable material for said mold panel (101) was such as 
polyvinylfluoride, silicone rubber and the like. 
The heating conditions of said high-frequency dielectric heating were 
voltage: 300 to 1000 volts, frequency: 10 to 10.sup.6 c/s and ordinary 
heating time: 0.5 to 3 minutes. As above described, since said heating 
time was very short in the case of high-frequency dielectric heating, the 
synthetic resin powder incompletely melted so that small-sized foam was 
formed to disperse in the resulting medium and a skin layer was not formed 
on the surface of the resulting medium. After heating, the mixture (103) 
in the molding holes (102) of said mold panel (101) were lightly pressed 
by a press mold to adjust the shape of the medium, and after cooling the 
resulting medium No. 1 having a diameter of 8 mm was removed from the 
molding holes (102) of said mold panel (101). 
The structure of said medium No. 1 is shown in FIG. 2. In the Figure, (103) 
is the matrix of the synthetic resin, (104) is the coarse powdery 
abrasives, (105) is the fine powdery abrasives, and (106) is the foam. 
EXAMPLE 2 
A mixture of nylon 6 powder, almina powder, and silicon carbide powder 
(25:70:5 weight ratio) was prepared. The average particle size of said 
silicon carbide powder as a powder having a low electric resistance was 
#600 and said almina powder as a powdery abrasive consisted of 45 parts by 
weight of coarse almina powder (average particle size being #150) and 55 
parts by weight of fine almina powder (average particle size being #600). 
Media No. 2 having a diameter of 8 mm was produced from said mixture by the 
same method as in EXAMPLE 1. 
The structure of said medium No. 2 is shown in FIG. 3. In the Figure, (203) 
is the matrix of the synthetic resin, (204) is the coarse powdery 
abrasives, (205) is the fine powdery abrasives, (206) is the foam, (207) 
is the silicon powder, and (208) is the relatively large-sized foam formed 
around said silicon carbide powder (207). 
COMISON 1 
Medium No. 3 was prepared by using a mixture of nylon 6 and coarse powdery 
abrasives (25:75 weight ratio) and said coarse powdery abrasives had a 
particle size of #150. The production method of said medium NO. 3 was the 
same as in 
EXAMPLES 1 and 2. 
TESTS 
Surface roughness Rmax, average surface roughness Ra, medium abrasion 
easiness (dressing ability) and work grinding tests on medium No. 1, No. 2 
and No. 3 were carried out. 
The resulting surface roughness Rmax and Ra are shown in Table 1. 
TABLE 1 
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Media R.sub.max (.mu.m) 
R.sub.a (.mu.m) 
______________________________________ 
No. 1 6.2 0.92 
No. 2 7.5 0.88 
No. 3 7.2 0.90 
______________________________________ 
Referring now to Table 1, medium No. 1 and No. 2 in which both coarse 
powdery abrasives and fine powdery abrasives were used, and medium No. 3 
in which only coarse powdery abrasives were used have almost the same 
surface roughness respectively, and it is confirmed that said surface 
roughness is not so much effected by the addition of fine powdery 
abrasives. 
To determine the dressing ability of each medium, each said medium was put 
in a barrel rotating at a speed of 420 rpm to determine the relation 
between the abrading amount of each medium and the treatment time. This is 
shown in FIG. 4. 
Referring now to FIG. 4, the dressing ability of medium No. 1 and No. 2 in 
which both coarse powdery abrasives and fine powdery abrasives were used 
are larger than the dressing ability of medium No. 3 in which only coarse 
powdery abrasives were used, and it is remarkable that medium No. 2 in 
which powder having a low electric resistance was used had an especially 
large dressing ability. 
For the workpiece grinding tests, each one of the said media was put in a 
barrel rotating at a speed of 420 rpm together with a workpiece (SUS 304 
panel 50.times.20.times.1.2 mm) to determine the relation between the 
grinding amount of the workpiece and the treatment time. This relation is 
shown in FIG. 5. 
Referring now to FIG. 5, the workpiece grinding abilities of media No. 1 
and No. 2 in which both coarse powdery abrasives and fine powdery 
abrasives were used are larger than the workpiece grinding ability of 
medium No. 3 in which only coarse powdery abrasives were used, and it is 
remarkable that medium No. 2 in which powder having a low electric 
resistance was used has an especially large workpiece grinding ability.