Note: Descriptions are shown in the official language in which they were submitted.
132~92
COMROSIlE GRINDI~G hHE~L
RICHARD H. SIO~I
lB Avery Heights Drive
Holàen, Massachusetts 01520
FIELD OF THE INVENTION
The in~ention relates to grinding wheels havins a
non-abrasive core supporting an abrasive containing rim, in
particular, a super abr~sive material in which the rim
5 abrasive section contains active filler material and silver
powder.
B~CKGROU~D OF THE INVENTION
Diamond grit or CBN grit containing grinding
whePls are customarily made with an abrasive rim section and
10 a supportin~ core. Because of the need for strength and
thermal conductivity, the core is desireably an organic
polymer filled with a metal powder. Aluminum is a
particularly desireable filler metal for the core because of :
its relatively low density, high conducti~ity, and low co~t.
U.S. Patent 3,863,233 teaches that such cores may
employ aluminum, copper, or a mixture of aluminum and copper
as a filler. In addition, graphite has been employed to
facilitate dressing away of the core material near the
abr~sive rim in cup-type wheels. Such wheels may employ the
~0 graphite additive only in the portion of the core adjacent `
to the abrasive rim. ` `
With the advent of the use of cubic boron nitride
abrasive grits, the use of active fillers in the abrasive
rim section became commercially used to improve the grinding
25 of ferrous and other metals. Such fillers, which improve `
the grindability of the metal workpiece may also be used in
cases where diamond is the abrasive, or where a blend of
diamond and CBN is used. Secondary, non premiumJ abrasives
may also be used. The diamond and CB~ are preferably metal
30 clad. Nickel is a common cladding. ~
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For optimum performance in the grinding of tool
steels, the incl~sion of s~lver pow~er in the abrasive
section, tooether with the active fill~r has been found
particu'arly advantageous. ~To reduce the power consumption
5 it is also desire3ble to include a dry film lubricant filler
in the abrasive rim. Such fillers include ~eflon* graphite,
hexagonal boron nitride, and molybàenum disulphiàe. British
published Application 2136011-A corresponding to West German
Application 3404851-A, published July 16, 1984, àiscloses
10 resin bonded ~rinding wheel rims containing CB~, active
filler, dry film lubricant, and silver powder.
Although such wheels, when the rim is mounted on
the core containing aluminum powder as a filler, have shown
high efficiency in the grinding of difficult-to-grind tool
15 steels, they have suffered from ins~ability when stored in a
humid atmcsphere, In such wheels the damage occurs at the
interface between the abrasive rim and the aluminum bearing
core, and in severe cases causes the rim to crumble apart.
DESCRIPTION O~ THE INVENTION
~0 Further investigation of the degradation effect in
a humid atmosphere led to the discovery that the problem was
caused by an electrolytic action between t~e aluminum in the
core ~nd the silver in the rim, in the presence of
electrolytes such as the halogen salts used as active
25 fillers. In accordance with the ~nown standard oxidation
reduction potentials of silver salts and aluminum it is
postulated that silver ions (Ag+) from the æurface of the
si}ver particles migrate toward the core section due to the
electrolytic effect of the couple between Ag+ and aluminum
30 metal. Since removal of the silver from the rim would
reduce the grinding efficiency of the abrasive, and
substitution of some other metal for aluminum would increase
the cost, density, (resulting in balance problems) and/or
thermal conductivity of the core, -he solution to the
35 problem was not apparent.
The effective solution, the subject of this
invention, was found to be a substitution of copper for the
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*Trademar~
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13244~2
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~luminum in the core 2t those portions of the core
immediately aàjacent the abrasive containins section,
while retaining aluminum filler in the major portion of
the core.
5 BRIEF DESCRIP~10~ OF THE DRAh'lN~
In the drawing, Figure 1 shows a straight wheel
with a composite core 10 of aluminum filled resin, with a
non-aluminum section 11 adjacent to the abrasive rim 12, an~
a mounting hole 13.
~igure 2 shows a cup-type wheel with an aluminum
filled ~orë 20, a non-aluminum filled portion 21, an
abrasive rim 22, and a mounting hole 23.
DETAILED DESCRIPTION O~ THE INVENTION
In the manufacture of cup-type wheels which were
15 tested for reaction to a humid environment, reported below,
first the core was formed in a mold oriented such that the
portion cup shaped core which will be ad3acent to the CBN
section is in the bottom. Sufficient copper filled resin
powder to build the section 21 in Figure 2 is placed in the
20 mold, hand packed, and leveled, The aluminum powder
containing resin is then placed on top, packed and leveled,
The entire assembly is then pressed and the core removed ` `
from the mold. The lower portion of the core is then shaped
on a lathe to provide a recess into which the drive rim
25 section fits in a subsequent molding operation,
Various methods of constructing a composite wheel
of this type, and various core compositions containing
aluminum powder are discussed in U.S. Patent 3,868,233.
Typical molding conditions are 5 tons per sgure
inch pressure, a temperature of 160C, and a molding time of
20 minutes. The final cure can be carried out in an air
atmosphere oven for 24 hours at 175C. As is known in the
art, the time and temperature of cure is varied to control
35 the grade of harndess of the wheel depending upon the
specific application.
'A, .
24~2
A~rasi~e rimC were fa~ricateà containing CB~
(cubic boron nitride~ a~rasive grits, nickel clad, 25% ~y
volume (based on the CB~ volume), and mounted on cores of
different composition. A sui~able formulation for the bond
and fillers for the rim is as follows:
Material Volume
KAlF4 active filler (powder) 25
Silver powder 20
MoS2 po~der ~dry film lubricant) 10
Phenolic resin (novolac powder
containing 2 vol ~ Cao based on
total bond volume) 45
The mix for the ri~ sections was prepared by
wetting the CBN abrasive grit material with furfural and
15 blending it with the mixture of bond and fillers. The mix
was cold pressed to a shape in the recess in the cup, and
the whole assembly cured by beat.
Nheels, for test purposes, were made having
unitary aluminum filled cores, and some having composite
20 cores, as in Figures 1 and 2. Wheels havinq composi~e
cores, in which the section adjacent the rim contained
copper or no metallic filler survived a period of 8 days
exposure to hot humid air (85 to 90S relative humidity at
120C), while the wheel with all-a}uminum filled cores,
25 suffered catastrophic degradation of the rim sections.
Other metals than copper, having a lower EMF
difference than that between aluminum and silver may be
used, such as nickel, iron, or cobalt. Copper is preferred :-
as less active and of higher thermal conductivity. The core
30 material may also contain graphite and other fillers. A
suitable core composition is taught in U.S. Patent
3,868,233.
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