Method of making a resin-metal composite grinding wheel

A metal-resin composite material consisting of a continuous metal matrix and a continuous resin matrix fabricated by hot-pressing a mixture of precursors, the precursors for the metal matrix including an elemental metal having a melting point below 450.degree. C, the metal matrix including an intermetallic compound or alloy having a melting point above 500.degree. C, and the continuous resin matrix being fabricable at a temperature above 250.degree. C. Such composite materials have particular utility as a bonding matrix for premium abrasives such as diamond and boron nitride, to form grinding tools.

FIELD OF THE INVENTION 
This invention relates to a resin-metal composite material for use in 
fabricating articles for applications where heat stability, heat 
conductivity, strength, and frictional properties are important. The 
invention also relates to grinding wheels formed by bonding premium 
abrasives with the described resin-metal composite material to provide 
good wear resistance and abrasive retention. 
BACKGROUND OF THE INVENTION 
British Pat. No. 1,279,413, published June 28, 1972, discloses a process 
for making abrasive tools, such as grinding wheels, wherein diamond 
abrasive grits are uniformly dispersed throughout a porous metal matrix, 
which matrix is then impregnated with a liquid resin, either a 
thermosetting pre-polymer, or a molten thermoplastic. The liquid resin 
fills all accessible pores in the metal matrix and is then cured or cooled 
to a solid condition. Such construction is intended to retain the 
advantages of the strength and heat conductivity of a metal bond, with the 
controlled wear properties of a resin bond, particularly in the dry 
grinding of cemented carbide tools. 
U.S. Pat. No. 2,258,774 to Kuzmick, discloses forming diamond wheels by 
mixing the abrasive with a low melting metal powder composition and a 
powdered pre-polymer of a thermosetting resin, and molding tools by the 
application of pressure and heat to the mixture contained in a mold of the 
desired shape. The metal is selected to have a melting point between 
55.degree. C. and 327.degree. C., said to be equal to or lower than the 
temperature developed in the wheel during the grinding operations. As a 
result, the metal melts during grinding so as to provide a lubricating 
action. 
Although related in structure to the composite matrices of the British 
patent and of Kuzmick, the composite material of this invention is 
intended for different grinding applications than either prior art 
reference and thus differs materially in its physical properties and 
composition. In particular, it is designed for the wet grinding of 
cemented carbide although it also gives improved results in dry grinding. 
Sears U.S. Pat. No. 3,523,773 discloses a composite glass-resin bond. 
SUMMARY OF THE INVENTION 
Applicant has discovered that grinding wheels which are particularly 
effective for the wet grinding of cemented carbide tools can be made by 
employing diamond grit bonded in a composite matrix of resin and metal so 
fabricated that all of the powder particles (both resin and metal) have 
been coalesced into solid continuous, or essentially continuous, phases. 
Neither the resin nor the metal then is a "filler" in the other, in the 
sense of a particulate powder, the powders having lost their identity as 
such in the application of heat and pressure. 
In order to achieve the above-described result, it has been found necessary 
to employ, as the metal part of the system, a combination of metal powders 
which are fabricable below 450.degree. C., but which, after fabrication, 
result in an intermetallic compound or an alloy which melts above 
500.degree. C. The limit of 450.degree. C. is established by the recently 
available high temperature resins, none of which are sufficiently heat 
stable to be fabricated at temperatures above 450.degree. C. It will be 
possible to increase this limit as resins of increasingly greater thermal 
stability are developed. 
Of the possible metal systems, many are eliminated because of expense, 
toxicity, or chemical instability. The preferred systems require the 
presence of at least one elemental metal powder in the mix to be 
fabricated, selected from the group consisting of tin, bismuth, and 
indium. Tin will form intermetallic compounds (melting above 500.degree. 
C.) with silver, cobalt, copper, iron, manganese, nickel, tantalum, and 
titanium. Bismuth will form suitable intermetallics with manganese, 
nickel, and titanium, and indium will form suitable intermetallics with 
silver, cpper, manganese and nickel. Bonds may also be formed by 
combinations of the above systems, the only requirement being that the mix 
to be fabricated include a metal powder which will melt during fabrication 
and react with other metal present to form a metal phase having a melting 
point above 500.degree. C. 
Under processing conditions employed in this invention, it has been found 
that the elemental metals do not completely react with each other. Thus, 
when a copper-tin system is employed, the resulting product will include 
elemental tin, elemental copper, and the intermetallic Cu.sub.3 Sn and a 
lesser amount of other Cu-Sn intermetallics. The metal matrix in such a 
case is thus composed of three individual phases which form a single 
mechanically interconnecting or continuous matrix. A preferred embodiment 
of the invention employs resin bond type copper clad diamond, with a 
mixture of copper and tin powders as the precursor of the metal matrix. In 
this preferred embodiment, a portion of the elemental tin reacts with the 
copper cladding of the diamond to make the cladding a mechanically 
continuous part of the metal matrix of the bonding matrix. In cases where 
borazon (cubic boron nitride) is employed as the abrasive, in coated form, 
a nickel coated abrasive grain is preferred. 
The resin phase of the matrix may be any bonding resin which is infusible 
in its final form. Thus the precursor for the resin phase of the bond may 
be a thermosetting pre-polymer such as a "B" stage phenolic powder, or may 
be a coalescible powder of an infusible polymer such as a polyimide as 
taught in U.S. Pat. No 3,523,773. By infusible, we mean a resin which does 
not melt upon heating to 250.degree. C. This term thus includes 
thermosetting resins and high temperature essentially noncross-linked 
polymers such as the polyimides and polyphenylene sulfides, which can be 
molded by application of heat and pressure to the resin in a powdered 
form. 
The bond of this invention may also include conventional finely divided 
particulate fillers heretofore employed in grinding wheels such as 
aluminum oxide, silicon carbide, and boron carbide as abrasive fillers, 
MoS.sub.2, polytetrafluoroethylene, graphite, hexagonal boron nitride, as 
lubricating fillers, and metal fillers that do not melt or coalesce to 
become part of the continuous metal matrix. Among these fillers, silicon 
carbide and graphite are preferred. 
The operative ratio of metal to resin, by volume, for improved results 
against a standard commercial phenolic bonded diamond wheel of the same 
diamond content, is from 5/95 to 95/5, the preferred range is from 15/85 
to 85/15, and the optimum range is from 55/45 to 75/25. The abrasive 
content can be as high as 65 volume percent, the preferred range is from 5 
to 50% by volume, the optimum is from 10 to 30% by volume. 
The metal powder, for forming the metal matrix of the bond, may contain 
from 10 to 80%, by weight of the low melting metal, preferably from 30 to 
50% by weight, of the metal powders. 
It is conventional in the art of making premium abrasive grinding wheels to 
fabricate wheels in which only the outer rim is fabricated with included 
premium abrasive grains. The core of the wheel to which the abrasive rim 
is attached can be prepared of the same or similar composition, exclusive 
of the premium abrasive (diamond or cubic boron nitride), as the abrasive 
rim, so as to match thermal expansion with, and enhance adhesion to, the 
grinding section. Silicon carbide may be substituted for diamond or boron 
nitride in the core material. For thin wheels (less than 3/32 inch or 2.5 
mm) steel cores are preferred. Thick wheels (over 1/4 inch) may be 
cemented to filled phenolic cores. 
When the work to be ground are tools of high speed steels or tool steels, 
cubic boron nitride abrasive is employed instead of diamond. In such 
cases, the preferred abrasive is cubic boron nitride having a nickel 
coating. For grinding T15 steel, phenol-formaldehyde resin is preferred 
over a polyimide, while for grinding 52100 steel, an infusible polyimide 
is preferred. 
Whether diamond or boron nitride is the abrasive, the particular field of 
use of the wheels of this invention is in the shaping and sharpening by 
wet grinding of tools such as drills, rotating burrs and indexable inserts 
.

EXAMPLE OF PREFERRED EMBODIMENTS 
Metal powder, resin powder and diamond of the following kinds and amounts 
were homogeneously mixed: 
______________________________________ 
Wt. (gm) 
Vol. % 
______________________________________ 
Toray KC 5000 polyimide resin powder 
1.65 18.4 
available from Toray Industries 
Inc., Tokyo, Japan 
Copper Powder 16.81 27.4 
Tin Powder 13.81 27.4 
Diamond, 140/170 mesh, copper 
4.54 18.7 
clad, resin bond type 
Copper (as Coat on diamond) 
4.54 8.1 
______________________________________ 
The mixture was placed in a ring mold and molded at 5 tons per square inch 
to a temperature of 350.degree. C. (20 minutes to heat to 350.degree., 
then cooled to 100.degree. C. and removed from mold). The ring was 
cemented to an aluminum filled phenolic resin core with epoxy cement to 
produce a 5 inch diameter, 3/16 inch thick grinding wheel with a 11/4 inch 
center hole. Grinding tests against a standard commercial phenolic bonded 
wheel, containing silicon carbide filler, and the same amount of diamond 
as the test wheel, showed an increase of better than 100% and up to 298% 
in efficiency in wet grinding cemented tungsten carbide. The test employed 
a surface grinder to grind a 22.64 square inch surface of Kennametal K3H 
cemented tungsten carbide; the conditions were: 
Wheel speed: 4100-5300 surface feet per minute 
Table traverse: 50 feet per minute 
Unit cross-feed: 50 mils (.050 inches) per pass 
Downfeed: 1 mil per pass for a total of 30 passes 
Coolant: Standard commercial coolant diluted 40 to 1 with water (Norton 
Wheelmate 203) 
In this application when reference is made to "abrasive boron nitride" we 
mean to refer to boron nitride in one of the crystal forms in which it is 
harder than aluminum oxide. One such form is cubic boron nitride, the 
other is the hexagonal (wurtzite structure) form. The other hexagonal 
form, analagous to graphite, is soft and not considered to be an abrasive.