US 2216728 A
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Description (OCR text may contain errors)
Oct. 8, 1940. R. c. BENNER Er Al. 2,216,728
ABRASIVE ARTICLE AND METHOD OFMAKING THE SAME Original Filed Deo. 31, 1935 PREBONDED AGGREGATES A a s fr( N lNTERAGC-:REGATE BOND INVENTORS.
RAYMOND C. BENNER.
Patented Oct. 8,
UNITED. STATES PATENT Acarica ABRASIVE ARTICLE AND LIETHOD F MAK- ING THE SAME Application December 31, masa-1an No. 57,060 Renewed June 30, 1939 4 Claims.
y This invention relates to the production of bonded abrasive articles wherein the abrasive granules are held firmly in a setting of .binder whereby they continue to cut until worn away.
5 This is in contradistinction to the action of ordinary abrasive articles wherein it is intended that when an abrasive granule is worn to dullness the increased friction causes the granule to fracture, exposing a fresh cutting edge, or to break out of the bond structure entirely, making way for bringing into action an unworn granule. In further contrast to ordinary abrasive articles, those with which the present invention is concerned are extremely dense with vsubstantially all the space between the abrasive granules lled with binder and require periodical sharpening to bring y fresh cutting edges into play, whereas ordinary abrasive articles are somewhat porous to facilitate -the action described above.
An extremely dense structure is made necessary by the character of thework to ybe done, or by the nature of the abrasive material used. In cutting certain extremely hard materials arelatively porous bond structure would be so weak as to let loose the abrasive granules at the viirst attempt to cut the material thereby making the cutting operation ineflicient with respect to the work. 4In the use of certain rather expensive abrasives, such as diamonds and boron carbide, this premature breaking out of the granules before they have been completely used up would render the process ineiiicient with respect to the abrasive, regardless of the work.
It has been suggested to bond abrasive granules into dense cutting tools .by means of metals, for example by mixing them with molten metal such as zinc or by mixing the abrasive specifically diamonds, with finely divided electrolytic iron particles and molding the mixture under o great pressure to form a dense homogeneous article.
Such articles have a number of disadvantages however. As stated above they must be dressed or sharpened periodically as the granules wear to remove Ibond from around the granules to enable them to penetrate the work, and provide space for accumulation of material cut from the work. This is dimcult to do without unduly loosening the granules from their foundation, especially in the case of finely divided abrasive material because of the small spaces between adjacent grains and the relative softness of the bonds used. For this reason it is practically impossible to use satisfactorily in this way extremely fine abrasive ma- Il terial, such as the diamond dust produced in crushing and otherwise preparing larger granules for use. Furthermore there are certain disadvantages in the use of dense metal .bonds with respect to the action of the wheel as a whole. More accurately it might be saidA that there are 5 certain valuable characteristics of an abrasive article bonded with other materials, such as resins,
or rubber which are not present in a metal article. 'I'hese advantages are dimcult to deiine but no less present and may be said to give a grind- [lo ing wheel better grinding characteristics.
It is an object of our invention therefore to provide a method of making abrasive articles, of the dense type described above, by which the cutting properties of the article can .be controlled i5 closely within a wide range of variation; to provide a method wherein a binder for holding the abrasive grains in their setting and a binder for providing the wheel structure are selected as hav--` ing the properties best suited to the purpose lfor zo which they are intended; to provide a method wherein abrasive grains are formedv into aggregates by means of a binder having the properties of thoroughly wetting the grains when liquid or plastic and forming a rela-tively hard brittle bindg5 er for the grains when set, the aggregates being then bonded into a dense body by a binder which is liquid or plastic at a temperature which does not injure the rst binder whereby it flows, with or without pressure, into all spaces between the oy aggregates and all spaces communicating therewith within the aggregates. It is a further object to provide a method of making abrasive articles wherein the cutting qualities of the finished article can be selectively determined by varying u the properties of the binders used with respect to the grain used and with respect to each other. It is a further object to provide a structure which combines the advantages of the use of small abrasive granules with the cutting power and clearance spaces characteristic of abrasive structures utilizing large abrasive granules. It is also an object of the invention to provide a means ofv utilizing abrasive dusts which could otherwise not be utilized in a bonded condition, in the produca tion of abrasive articles whose properties approach 'those of articles formed of larger abrasive granules.
The aggregates may be formed in a manner to make them somewhat porous whereby the binder 5g used to form the abrasive article penetrates the pores of the aggregates to assist in holding the aggregates in place, or the aggregates maybemade to have a dense structure with substantially no voids therein. 55
The binder used to form the aggregates may be any one suitable for use in binding abrasive grains including those already used in the production of abrasive articles. These include inor- 5 ganic binders such as metals, clays and glasses, and organic binders such as resins (thermoplastic and heat hardenable) rubber, and cellulose derivatives. 'I'he method of making the aggregates will depend on the nature of the binder used and will l0 be described by reference to specific examples below. The binder used may be any one which can be put in a liquid or plastic form, as in solution or in the molten state, whereby it will penetrate and illl all spaces in a mass of aggregates, for
example, a solution of a reslnous binder, or a mixture of a liquid reslnous binder and a nely divided solid reslnous binder, or a molten thermoplastic binder such as certain resins and metals. Of course the binder selected should be one which U can be used at a temperature which will not injure the binder in the aggregates.
The invention will be described with reference to the use of specific binders for the grain and for the aggregates. It is to be understood how- Il ever that the invention is not limited thereby but includes the use of any combination of binders to selectively control the cutting action oi' the resulting abrasive product. Furthermore, although the invention specifically provides a n means of using nes from valuable abrasive materials it is evident that the principles of the invention apply to use of other, larger, or less expensive abrasive materials.
, A type of abrasive article which can be made u in accordance with the applicants invention is illustrated in the accompanying drawing. This drawing shows a fragmentary section of an abrasive wheel, the section being taken in a plane at right angles to the axis of the wheel.
"0' Prebonded aggregates are indicated on an enlarged scale, each aggregate containing a plurality of abrasive particles (such as diamond dust) which have been strongly bonded under heat and pressure. The aggregates are formed by crushing the bonded mass after it has cooled. The aggregates are thereafter united by means of a dense bond which makes intimate contact with the irregular surfaces of the aggregates.
ln the drawing abrasive particles 2 are shown in clusters, each cluster containing abrasive particles which have been prebonded with a binder 3 before the breaking up of the prebonded mass into aggregates. The binder 3 may be composed (as mentioned above in some of the examples) Il of metal or glass. After the prebonded mass has been broken up into aggregates the latter are formed into an abrasive article with the aid of an interaggregate binder, such for example as a phenol condensation product resin, and the 00 whole article is heated in this case under pressure to form a hard dense bond.
Example iy I 70 diamonds are mixed with metal which is in nely divided form. The amount of bond material used is that needed substantially to fill the volumebetween the abrasive particles. Simple calculations based upon the speciiic gravity, appar- 75 ent density, grit size, etc. ofthe materials to be used permit the calculation of this amount with reasonable accuracy. The mixture is pressed and subjected to temperatures ranging from approximately 700 C.'to 1500 C. depending on the metal used. Diamonds are subject to oxidation 5 at temperatures above 700 C. It is desirable .therefore that the heating should be performed in a non-oxidizing atmosphere such as may be produced by deposition of ilnely divided carbon on the mix or by the use of an atmosphere of l0 hydrogen. A hydrogen atmosphere facilitates the wetting of the diamond particles by the metal. It is desirable also that the heating should be rapid to minimize the action of any residual oxygen in the mix on the diamonds. Il The mix can also be subjected to jarring while the metal is in a liquid or softened condition in order to bring the metal into closer contact with the abrasive particles, so that the attractive forces between the metal and the abrasive par- I ticles may be more fully utilized and so that a dense mass of metal and included abrasive may be obtained. After the mixture has cooled the mass is broken up by any convenient method common to the art of crushing into aggregates il each of which has a plurality of abrasive particles.
Another. hard abrasive that can be thus incorporated with metal is boron carbide. Boron carbide can be alloyed, or incorporated into intla mate mixture, with a number of metals, and metal-bonded aggregates can be prepared by either of the following processes.
Example II u Examples III A mixture of boron carbide and a metal is, heated until the entire mass becomes fluid. On 50 cooling the mass the boron carbide crystallizes to give fairly well developed crystals embedded in a metal matrix. 'I'he temperature required to produce fluidity of the entire mass is usually about 2000 C. or higher, depending on the metal used and on the proportions of metal and boron carbide. Examples of metals which can be used singly or in combination in this manner with boron carbide are copper, nickel, cobalt, iron, etc. 'Ihe cooled mass is broken up into aggregates containing boron carbide crystals and interconnecting metal.
In the production of metal bonded aggregates, alloying agents may be incorporated into the metal to produce a satisfactory degree of brittleness for crushing purposes and to give the proper grinding action. 'I'hese addition agents are well known in the metallurgical art and need not be described in detail. As specinc examples, copper can be embrittled by the addition of tungsten, aluminum or tin, the amount added being dependent upon the properties desiredin the bonded aggregate. Thisalso constitutes a mechanism for controlling the slow but necessary I breakdown of the metal bonded aggregate duru ing use; this in turn makes possible considerable control of the cutting properties of the abrasive article because the brittleness or' toughness of the bond and abrasive may be adjusted-to suit the character of the cutting action desired. Copper may also be embrittled by dissolving therein boron carbide as disclosed in Example III. Dense masses of abrasive particlesand an interconnecting medium can be made by mixing diamond particles with powdered glass and melting the glass in contact with the diamond particles in a reducing atmosphere. One glass which has been found suitable forthis purpose is made by fusing, crushing and screening the following ingredients:
' Parts by weight 20 Boi-ax"- i Flint--- 48 Soda ash 12 Zinc oxide 20 A small amount of dextrin as a temporary binder may be used when compacting a mixture of glass and diamonds. For ring, an oxidizing atmosphere is maintained at 600 C. until the dextrin has burned away, then the article may be transferred to a graphite container and quickly heated for a short time at 900 C. in a reducing atmosphere followed by any convenient cooling schedule.
After a dense mass containing abrasive particles distributed throughout has been prepared by one of the methods described above, it is broken up into much smaller aggregates in each of which a number of abrasive particles are held in a comparatively non-porous lump or aggregate. These lumps or aggregates are then admixed with a bonding material such as a heat hardenable resin; e. g. a phenolic condensationv product resin in the A or B stage. In view of the comparatively non-porous character of the aggregates they can be molded with such a resin bond (and inert filler if desired) into a compact body having few pore spaces. The tendency is for the resinous bond to ilow into and illl the pores of the abrasive aggregates. p The molded body is cured at high pressures and at temperatures which produce a curing of the bond, and the body may be subjected to a baking process to further cure the bond.
It is possible to use a wide ra-nge in proportion and variety of abrasive aggregates, bonds and fillers for making abrasive articles. One mix which may beused comprises: y
Per cent by weight 40 mesh and finer aggregate clusters of glass bonded diamonds; the diamonds used comprising a mixtureof grit sizes passing an mesh screen and being retained on a 200 mesh screen 10 Fused alumina particles passing a 325 mesh screen 22 B stage phenol-formaldehyde type resin 12 Previously cured and crushed phenol-formaldehyde type resin, screened to pass a mesh screen 56 and baking the mold and contents for 16 hours at 350.F.; upon cooling and dissembly of the mold, the abrasive wheel is ready for use.
Instead of a-bonding material composed of a heat hardenable resin, a thermoplastic resin can used. An example of such a resin is a polymerized resin tha-t can be made from a vinyl acetate base. .'Ihe vinyl acetate is polymerized by means of light or heat until the viscosity of a molar solution of the resin in benzol is about 15 centipoises. About seventy percent of the acetate ygroups are replaced by acetaldehyde. The resultant resin is thermoplastic and is well adapted formolding in combination with fillers to form strong structures of low porosity. It is also possible to use mixtures of reversible and irreversible types of resin to bring about desired elasticity,
toughness and resilience of the bond.
lent than the glass. In the process of bonding, y the aggregates are surrounded by molten metal under pressure to make a dense structure. Aside from the aggregates and binders therefor, described above, aggregates may be made using other binders, such as rubber, resin solutions, and clay.
In making resin bonded aggregates with norma-lly solid heat hardenable resin 220 grit abrasive grain is mixed with a solution of A stage phenol formaldehyde resin in acetone in a proportion to supply 2% of resin based on the abrasive grain. The mixture is then dried in the" oven for 6 to 10 hours at 150 F. and the temperature is then raised to 250 F. and held at that figure for one half hour. The resulting mass is then cooled, crushed, and screened to produce a fraction of aggregates having the desired size.
Rubber bonded aggregates are made from the following ingredients:
, Percent Smoked sheet 8 Sulfur 4 Abrasive grain- 88 ing 5% clay bona with 95% abrasive grain with suitable additions of liquid to give the mixture the proper consistency. The mass is then baked to vitrify the bond at 1250 C. After cooling sui'- ilciently the vitrified mass is broken up to produce aggregates each containing a plurality of abrasive grains.
I'he aggregates thus made may be rebonded into dense structures by means of any binder which is plastic at a temperature which will not injure the aggregates, to flow into all spaces between the aggregates and spaces communicating `therewith within the aggregates, e. g. thermoplastic resins, low melting metals such as copper and zinc, and mixtures of liquid resin and solid resin.
' Articles made according to this invention may be somewhat smooth-surfaced when first made. They may be made ready for use by subjecting the working surfaces to a gentle disking action by oscillating the article slowly over a surface plate charged with iine, granular loose abrasive which cuts away the bond between the aggregates due to its relative softness and also small portions of bond from the abrasive'clusters, thus leaving protruding cutting edges. Thlsdisking process, or its equivalent, may be resorted to periodically during the use of the article to remove any detritus lodged in its working surface or to bring new cutting edges into use.
It is an important advantage of the invention that a structure having relatively tine abrasive granules may lbe provided with relatively large clearance spaces between aggregates. Relatively large non-abrasive spaces on the working face oi the abrasive article are illled with the binder for the aggregates, which spaces may be made concave or hollow by removal of the binder therefrom by a proper dressing action to provide clearance spaces for the abrasive granules and space for the accumulation of material removed from the work. If the binder fuorV the aggregates is made relatively softer than the -binder for the grains, the dressing Vaction will tend to remove more material from between the aggregates than in the aggregates, thus providing the proper setting for the aggregates.
Abrasive wheels made by bonding abrasive aggregates of the type described above and in the manner described have many advantages. The abrasive particles are surrounded for the most part by material that adheres more rmly to the abrasive granules than most resinous bonds. Again the abrasive aggregates have a comparatively large size and can be made with roughened surfaces so that the resinous bond used in uniting the aggregates can hold the aggregates strongly. Moreover the use of diilerent bonding materials in combination with closely compacted abrasive aggregates makes it possible to obtain an exceptionally wide range of cutting characteristics.
'Ihe present invention makes it possible to use more elciently abrasive dusts such as diamondfines, since the aggregates give some of the effects of larger abrasive particles such as are commonly selected for use in the manufacture of bonded diamond articles. 'I'here is always an excess oi' ne grit sized material produced in crushing diamonds to obtain the larger sizes used i'or diamond abrasive wheels. etc. Insuilicient market exists to absorb the quantities of fines produced and the material is therefore rela tively inexpensive. This invention makes it possible to utilize the fines -to do a large portion of the work oi coarser, more expensive grits. Similar remarks apply to the utilization oi the ilnerv particles obtained by crushing other rare and costly abrasive materials.
l.l The steps in the method oi making an abrasive article which comprise mixing diamond particles with powdered glass and a carbonaceous binder, heating up the mixture in an oxidizing atmosphere at temperatures below the melting point of the glass until the camonaceous binder is substantially dissipated, then heating the mixture in a reducing atmosphere until the glass is melted around the diamond particles, cooling the mass and crushing the cooled mass to form aggregates consisting oi' diamond particles and glass, and bonding the aggregates with a reactive resin under heat and pressure.
2. The steps in the method of making an abrasive article which comprise mixing diamond particles with powdered boro-silicate' glass and a carbonaceous binder, heating up the mixture in an oxidizing atmosphere at about 600 C. until the carbonaceous binder is substantially dissipated, then heating the mixture for a short time to about 900 C. to melt the glass around the diamond particles, crushing the cooled mass to form aggregates consisting of diamond particles and glass, and bonding the aggregates with a reu active resin under heat and pressure. 3. An abrasive article comprising aggregates composed of glass-bonded diamond particles and an interaggregate bond composed oi' synthetic resin cured in intimate contact with said aggregates.
4. An abrasive article comprising aggregates composed of diamond particles bonded with a borosilicate glass and an interaggregate bond composed of a synthetic resin and a hard filler which have been compressed into intimate con- `dl tact with said aggregates under pressure and heat.
RAYMOND C. BENNER. ALBERT L. BALL.