Grinding Tool Having A Rigid And Dimensionally Stable Resin Binder

Abstract

A grinding tool useful for rapid grinding of a precision cut is described which comprises a high concentration of abrasive grains uniformly dispersed in a non-brittle resin binder body which is rigid and dimensionally stable under the required grinding pressure.

Parent Case Text

This application is a continuation-in-part of my co-pending applications Ser. No. 813,377, filed May 15, 1959, entitled "Abrading Tool;" and Ser. No. 829,665, filed July 27, 1959, entitled "Abrading Tools," the latter being a continuation of the former. This application is also a continuation-in-part of my co-pending applications Ser. No. 854,468, filed Nov. 20, 1959, entitled "Method of Making Articles from Foamed Elastomeric Material"; Ser. No. 12,303, filed Mar. 2, 1960, entitled "Composition and Method for Making Grinding Wheels and the Like;" and Ser. No. 15,135, filed Mar. 15, 1960, entitled "Method of and Apparatus for Centrifugal Molding", all of which are now abandoned.

Description

The invention relates as indicated to abrading tools, and more particularly to rotary abrading tools of the nature of grinding wheels.

A grinding wheel, in contrast to a polishing or finishing wheel, is capable of making a cut of substantial depth in a work-piece which may be of cast iron or steel, for example, and the characteristics of prior grinding wheels are well known and described, for example, in "The Grinding Wheel" by Kenneth B. Lewis, published 1959, by The Grinding Wheel Institute, Cleveland, Ohio. Ordinarily, such grinding wheels have comprised a mass of densely compacted discrete abrasive grains bonded together by a molded and fired ceramic material or a resin bonding agent. Such wheels have been notoriously difficult of manufacture, requiring very careful placement of the granular abrasive in a mold and usually rather lengthy baking or curing periods. Many of them are quite fragile or brittle and easily fracture if carelessly handled. They also require frequent dressing to ensure maintenance of the desired tool face profile to obtain a uniform cut. Those wheels which have been hard enough to be capable of a fast or deep cutting action such as is needed for abrasive machining, requiring imposition of high unit pressures, have not also been capable of simultaneously producing a surface finish of the quality desired and frequently cause metallurgical damage to the work. Consequently, in very many cases a preliminary rough grinding step has required a subsequent finishing operation and indeed a conventional cutting tool such as a milling cutter may first be employed followed by a rough grinding step and then a finishing step.

Polishing tools such as polishing pads and wheels have also been known in which polishing materials have been incorporated in a body of readily yielding elastomeric material such as natural and artificial rubber and various synthetic resins. While suitable for use in cleaning or polishing operations, such articles have lacked entirely the dimensional stability and rigidity necessary in a grinding wheel which must remove stock accurately in amounts generally measured in thousandths of an inch and which must do so in a precise path or line of cut to achieve the desired dimension and geometry of the work.

In contrast to the tools generally described above, I have found that by the proper placement and concentration of abrasive grains or granules in a selected resin or plastic binder I have been able to produce an improved abrasive tool, and particularly a grinding wheel, wherein the granular abrasive material is disposed within the binder matrix or body in such manner as to achieve a tool which is essentially rigid considered as a whole, but wherein the individual abrasive grains exposed at the working face of the tool are slightly spaced apart and capable of individual micro-movement or adjustment relative to each other without being dislodged from their sockets in the binder, the binder being capable of a limited amount of local elastic deformation when exceptionally high pressures are imposed on the individual exposed grains. In consequence, a grinding tool in accordance with this invention may be fed rapidly into the work to produce a relatively deep cut without producing either excessive scoring of the work surface or prematurely dislodging excessively protruding grains in the tool face which would result in rapid break-down of the tool profile. Any such excessively protruding grains are forced inwardly of the tool face under the operating pressure imposed thereon until substantially all of the grains exposed in the working face of the tool bear against and act upon the work; this being achieved, however, without appreciable distortion of the tool considered as a whole, so that a dimensionally true cut is produced.

It is accordingly an important object of my invention to provide a novel abrading tool capable of fast cutting action but which nevertheless will neither itself break down prematurely or produce excessive scoring of the work, but instead will produce an exceptionally good finish for such a rapid cut.

Another object is to provide an abrading tool capable of fast cutting action to produce a true dimensionally accurate grinding cut without, however, also producing rapid erosion of the tool edges (which would necessitate frequent dressing and machine down time) or causing metallurgical damage to the work.

Still another object is to provide an abrading tool, and especially a grinding tool, having abrasive grains uniformly slightly spaced apart in an essentially rigid non-brittle resin to afford a large number of cutting points exposed at the working face of the tool which are slightly individually adjustable under working pressures imposed thereon due to stubborn elastic yielding action of the resin bond without affecting the essentially rigid character of the tool body.

A further object is to provide such tool having a multitude of very small closed cells in the resin bond thus spacing the individual grains slightly apart and incapable of absorbing water or other liquid to avoid imbalance of the tool in wet grinding, for example.

A still further object is to provide an abrading tool such as a grinding wheel which is not fragile and will not fracture easily when dropped or otherwise maltreated.

Another object is to provide a method of manufacturing such abrading tool, and especially a grinding wheel, which is rapid and inexpensive in that uniformly reproducible results are obtainable without appreciable production of rejects, utilizing centrifugal force preliminarily to distribute the granular abrasive material in a body of liquid resin, with the viscosity of the binder resin thereupon being increased and a foaming action produced properly to space the individual grains in the now viscous binder following cessation of effective centrifuging, such binder then being gelled or set to the desired rigid but non-brittle condition capable of local stubborn elastic yielding action when excessive pressure is imposed upon an individual abrasive granule exposed at the working face of the tool.

While a variety of resin or plastic bonding agents are suitable for employment in accordance with my invention and are commercially available, I particularly prefer certain selected polyurethane compositions which, when properly employed, evidence the desired physical characteristics mentioned above. Such polyurethane compositions are capable of foaming with the assistance of the small amount of moisture normally present and the various other resins may be caused to foam through the provision of appropriate well-known foaming agents activated by means of heat, catalysts and the like.

To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

In said annexed drawing:

FIG. 1 is a diagrammatic elevation partly in cross-section of a circular mold mounted upon a turntable or centrifuge and adapted for the production of a rotary abrading wheel in accordance with the invention;

FIG. 2 is a view similar to FIG. 1 but illustrating a further stage in the process;

FIG. 3 is a vertical section illustrating a subsequent stage in the process;

FIG. 4 is a vertical section through the closed mold in the blowing and gelling or setting stage;

FIG. 5 is a view of a typical abrading wheel produced in accordance with this invention;

FIG. 6 is a fragmentary transverse section on an enlarged scale taken on the line 6--6 on FIG. 5;

FIG. 7 is a magnified diagrammatic view indicating the disposition of the abrasive grains and binder resin during initial centrifuging;

FIG. 8 is a magnified view similar to FIG. 7 but indicating the relationship and form of such grains and binder subsequent to blowing and setting of the binder;

FIG. 9 is a magnified view on the same scale as FIG. 8 showing the cells formed in the radially inner non-abrasive portion of the wheel;

FIG. 10 is a magnified diagrammatic view in cross-section showing only the abrasive grains protruding at the working face of the tool; and

FIG. 11 is a magnfied diagrammatic view indicating the manner in which the disposition of such grains is modified by the working pressure imposed thereon during performance of a grinding operation.

The preferred method of making the improved tools of the present invention will first be generally described, reference being had to the foregoing figures of the drawing. In such description, the matrix or binder material will generally be referred to as a resin, or more specifically as a polyurethane composition. It will be understood, however, that various resins or plastic compositions, and especially thermosetting plastic compositions may be utilized; for example: the reaction products of a member selected from the group consisting of an aromatic polyether and an aromatic polyester with a polyisocyanate, certain epoxy resin compositions, certain phenolic resin compositions, and certain silicone resins. In general, for grinding wheels and the like in accordance with the present invention which will produce deep accurate cuts at high feed pressure, a cross-linked polymer or thermosetting resin is preferred which will produce a rigid infusible dimensionally stable foam. The abrasive material, certain filler materials, and more detailed descriptions of the composition of the substantially rigid dimensionally stable infusible cellular foamed body which constitutes the preferred matrix or binding material will then be successively described.

METHOD OF MANUFACTURE

Utilizing the preferred polyurethane resin composition as the binder material, the abrasive material may be incorporated in the unreacted or partially reacted polyurethane constituent mixture and the reaction then completed in an appropriate mold to form the desired abrading tool such as a grinding wheel. A blowing or foaming agent will ordinarily be incorporated in the mixture substantially simultaneously with the incorporation of the abrasive therein, and the blowing operation which occurs prior to final setting of the resin assists in accomplishing uniformly spaced distribution of the abrasive grains through the resin body and in maintaining such grains in suspension prior to solidification. Furthermore, the prompt setting of the polyurethane likewise militates against settling of the grains after these have thus become properly located.

In the production of many types of abrading tools, and especially grinding wheels for example, it is preferred to employ a method of the type diagrammatically illustrated in FIGS. 1-4 inclusive of the drawing. An annular mold 1 is shown having its base 2 inset in a turntable 3 adapted to be rotated about its vertical axis by worm gear unit 4 driven by electric motor 5. The mold is provided with a removable cover plate 6 having a central opening through which protrudes an axial stud 7 having a threaded reduced outer end portion 8. In the initial step of the operation, a measured quantity of the fluid or unset resin such as the mixture of polyurethane reactant constituents may be discharged from upper reservoir 9 into the central opening 10 of the mold cover plate 6 and the turntable 3 is revolved at a speed sufficient to cause the resin to flow radially outwardly and accumulate in the radially outer portion of the mold as shown at 11. In this manner, approximately the radially outer half of the mold may be filled.

Now referring to FIG. 2 which illustrates a subsequent operation at the same station, a measured quantity of granular abrasive may next be discharged from hopper 12 into the rotating mold which will be rotated at a sufficiently high speed to cause such abrasive to flow radially outwardly under the influence of centrifugal force into the previously deposited resin through which it migrates toward the radially outer periphery or circumferential portion of the mold cavity, accumulating in uniform manner in the radially outer circumferential region 13 in concentrated contacting or substantially contacting relationship with a relatively small quantity of the resin filling the interstices between the abrasive grains.

In the next stage of the operation, at the same station, additional resin may be discharged into the central region of the mold as shown in FIG. 3, but still ordinarily preferably not entirely filling the mold cavity. The mold is thus now partially filled with the resin-abrasive mixture with the abrasive grains uniformly circumferentially distributed in a radially outer annular region of the mold and with the remainder of the resin body being substantially abrasive free. The resin constituents have been sufficiently liquid to permit such centrifuging but as they continue to react, the viscosity rapidly increases. Furthermore, the binder now tends to foam, but such tendency may be largely inhibited during these preliminary stages of the manufacturing process due to the centrifugal force imposed on the rapidly rotating mass. When effective centrifuging is now ceased, ordinarily by completely stopping rotation of the turntable, a foaming of the resin body takes place in the now quite viscous resin, producing relatively small, ordinarily closed cells in the interstices between the abrasive grains to an extent sufficient to space the latter apart preferably not more than one grain diameter, with the entire resin-abrasive body accordingly expanding radially inwardly of the mold which is centrally vented as previously indicated. A multitude of small cells are simultaneously generated in the radially inner non-abrasive portion of the resin body, such cells ordinarily being smaller than those formed between the abrasive grains in the outer portion of the mold and having thinner walls or webs therebetween than the webs between the cells formed in such outer portion in which the abrasive grains are embedded.

An annular tapered plug 14 may be employed to close the mold as shown in FIG. 4, secured by an outer washer 15 and nut 16 threaded on stud 8. The resin binder within the mold is now permitted to set or cure, as may be required, to essentially rigid but non-brittle condition. In the case of the preferred polyurethane resin, the reaction of the constituent ingredients comprising the same is permitted to go to completion while left in the mold for from about 2 to 7 hours (depending on the thickness of the abrasive article to be produced) and heated during this period to approximately 200.degree. F.

As above indicated, it is ordinarily very much preferred to include in the resin a small amount of an appropriate blowing agent effective to produce a very large number of minute cells or voids throughout the body of the finished article. Depending upon the particular blowing agent employed, as discussed more in detail below, it may be desirable to heat the material while enclosed within the mold and prior to setting of the resin to activate such agent. The mold will ordinarily have been removed from the turntable at this stage since it is usually not desired to centrifuge the same during performance of the blowing operation. Such minute cells or voids in the outer abrasive region of the tool assist in permitting micro-deformation at locally overstressed points of the working face in use, such response to high unit pressures being at least somewhat due to partial collapse of free volume therein. Furthermore, the interstices between the abrasive grains are relatively open so that the grains exposed at the working face of the tool instead of being substantially solidly embedded in individual sockets have their cutting edges much more fully exposed for active work than has been the case in prior practice. When polyurethane is employed as the bonding agent, such preferred resin evidences an unexpectedly strong bond to the abrasive grains effective to secure the latter even under severe working conditions and despite the fact that the bonding resin may contact only certain portions of the individual abrasive grains rather than substantially completely solidly embedding the latter. Excessive wear and damage to the abrading tool are likewise somewhat minimized by the inner resin or plastic central portion of the tool which serves to support the relatively more rigid outer abrasive portion (which itself is capable of local micro-deflection) in a manner to absorb violent shock and stresses which may be encountered when the latter engages the work. In some cases, however, it is preferred to remove the inner non-abrasive portion of the wheel and quite frequently at least the extreme inner portion may thus be removed to adapt the wheel to various sizes of arbors or the like.

As shown in FIGS. 5 and 6, a rotary abrading wheel produced in accordance with the invention may ordinarily be of the usual cylindrical form and provided with a central arbor hole 17 formed by stud 7 within the mold or drilled to a larger diameter as may be desired. Various metal hubs and the like may be placed within the mold and thereby included as a part of the finished article. As shown in FIGS. 5 and 6, the wheel will comprise a radially outer circumferential portion 18 ordinarily of substantial width having a large number of abrasive grains 19 slightly spaced apart out of contact with each other by the cellular resin, and an inner non-abrasive portion 20 of the resin body having a multitude of small cells therein. For purposes of clarity, such cells in both the radially inner and outer portions of the wheel are not shown in these figures, but their relative size and disposition are indicated in certain other figures of the drawing. The side or end faces 21 and 22 of the wheel will ordinarily have an integral substantially imperforate resin skin formed thereon in the molding operation, and this skin adds appreciably to the strength of the wheel. If desired, however, thin annular face plates of sheet metal, strong paper, cardboard or plastic may be molded and bonded to the respective end faces of the wheel.

Instead of separately introducing into the mold the binder resin and the discrete abrasive elements, these components may themselves be intermixed in advance so as to be simultaneously supplied to the mold in the same manner as above described. While it is desirable that the abrasive granules be easily wetted by the liquid resin composition and that the components be well and uniformly intermixed to the extent feasible, the uniform disposition of the abrasive grains in the final product is not dependent thereon but is obtained by means of the centrifuging and foaming steps above described. The amount of resin-abrasive mixture delivered to the mold, while only partially filling the same, will ordinarily be selected to be sufficient completely to fill the mold upon expansion of the resin-abrasive body as a result of the blowing operation. The blowing operation takes place at atmospheric pressure (the mold being centrally vented), and a small amount of flash may be produced which can easily be trimmed from the finished article. In some cases, when an especially strong blowing action is obtained, centrifuging may be continued, although usually at reduced speed, during the blowing operation in order to obtain the desired uniform spacing of the individual grains while at the same time limiting such spacing to not more than approximately one grain diameter. It will be seen that there are accordingly a number of factors which may be utilized in regulating the manufacture of the new abrasive tool including the speed of centrifuging, the viscosity of the resin binder and the activity of the blowing agent, for example.

Now referring more particularly to FIGS. 7, 8 and 9 of the drawing, FIG. 7 illustrates in much magnified, somewhat diagrammatic form the placement of the abrasive grains 19 in the outer circumferential portion of the mold as a result of the centrifuging operation, such grains being concentrated into subtantially contacting relationship with the interstices therebetween filled with the binder resin 23, although as above indicated some degree of foaming may already be taking place. Upon further foaming of the binder resin, ordinarily after cessation of effective centrifuging and after such resin has substantially increased in viscosity, the abrasive grains 19 are moved apart thereby as generally indicated in FIG. 8, the individual grains being uniformly spaced apart preferably not more than about one grain diameter, on the average. The resin is then set in this condition and, while forming a rather complex structure as viewed under the microscope, may nevertheless properly be described as comprising relatively thin coatings on the grain surfaces bonded to and holding the latter and interconnected by heavy web portions 24 having cells or voids therebetween, and sometimes also apparently smaller bubbles or cells formed within such web portions themselves. Such webs are of considerably greater dimensional thickness than webs which are produced when the same resin is caused to foam freely under atmospheric pressure without employment of the centrifuging step. Such heavy webs are of considerable advantage in the finished article since they play a large part in limiting the stubborn local deformation of the resin body to the required micro amounts needed to permit readjustment of an individual protruding grain and absorb the excess energy of the force imposed thereon when such protruding grain is brought into engagement with the work surface under grinding pressure.

Using either of the centrifuging methods above described, a uniform circumferential region of densely concentrated abrasive material is soon produced which may be observed through a transparent cover plate, and as the blowing operation proceeds this annular circumferential abrasive region may be seen to widen appreciably in a radially inward direction due to radially inward displacement of the abrasive granules resulting from the formation of gas pockets or cells therebetween. The width of such region or band of abrasive concentration may in some cases even be doubled due to such action of the blowing compound although in many cases the effect will be very considerably less. Of course, the radially inner non-abrasive portion 20 of the resin or plastic body will likewise simultaneously expand radially inwardly due both to such expansion of the outer circumferential abrasive bearing portion and also the formation of cells or bubbles 25 within the portion 20 itself. As indicated in FIG. 9, such cells or bubbles 25 are normally both smaller and have thinner webs separating the same than is the case with the cells which assist in spacing the abrasive grains 19 (FIG. 8). It will be noted from the foregoing that centrifugal force is employed initially to distribute the abrasive grains in an annular region in uniform (substantially contacting) relation to each other with liquid resin in the interstices therebetween, the excess resin being separated from the grains in a radially inward direction by direct displacement. The resin is not set, however, until after the grains have been moved slightly out of contact with each other by means of the foaming operation. When the entire mass is expanded by means of the blowing operation, it is interesting and important to note that no differential effect or result is obtained in the abrasive annulus. In other words, a radial cross-section through such abrasive annulus shows the abrasive grains to be uniformly distributed and spaced not only circumferentially of the tool but also radially of such abrasive annular portion, the consequence being that the grinding characteristics of the tool in use are not appreciably altered as the diameter of the tool is reduced through repeated dressings. It is desirable that the resin be at least preliminarily gelled or set as promptly as possible after the abrasive grains have thus been properly uniformly slightly spaced, such prompt gelation cooperating with the cells in the resin body to prevent settling of the abrasive content under influence of gravity following cessation of centrifuging. The grinding tool produced in this manner will have a maximum number of cutting points exposed at the working face of the tool consistent with the requirements that the abrasive grains be slightly uniformly spaced for individual micro-adjustment and absorption of stress. Such uniform spacing of the grains is obtained in accordance with the invention inasmuch as the foaming may continue at atmospheric pressure until the resin gels sufficiently to inhibit and finally stop the foaming. At this point, the forces producing the foaming and the forces resisting the foaming, i.e., the gelation or increased viscosity of the resin and, optionally, continued rotation of the centrifuge, will be in balance and further foaming will cease.

One of the most difficult problems encountered in the manufacture of conventional grinding wheels is the obtaining of uniformity from wheel to wheel within specific grades, such wheels being manufactured with dry granular materials and the compacting of such a mass into a particular shape and size being hard to reproduce. Because of the unique manufacturing methods above described and the materials employed, exceptional uniformity of wheel quality is an inherent feature of the present invention, and consequently less frequent adjustment is required to maintain close tolerances in production operations.

ABRASIVE MATERIAL

The type, grit size and amount of abrasive may be varied to produce a wide variety of useful products. All commonly available abrasive grains are adapted to be employed in the articles and manufacturing methods of this invention. In each particular case, however, it is necessary to take into consideration the size, shape, purity and other aspects of the materials employed to ensure obtaining the desired dense concentration of the abrasive grains in the working portion of the abrading tool, the exact amount of the abrasive grain to be employed being directly determined by the bulk density (grams/unit volume obtained by free fall) for the specific grade, size, shape, etc., of abrasive being employed. The term "bulk" or "pack" density of abrasive grains is well known and understood in the art, and figures are available for all of the common abrasive grains. The term is defined by The Grinding Institute as weight in air of a given volume of the permeable material (including both permeable and impermeable voids normal to the material) expressed in grams per cubic centimeter. For the purposes of the present invention, I very much prefer that grinding tools in accordance therewith have an abrasive content of a density equal to from about 75 percent to about 100 percent of the bulk or pack density of the particular abrasive employed. The abrasive grains should constitute from about 30 percent to about 45 percent of the abrasive-resin body, by volume, with the resin correspondingly normally constituting from abut 45 percent to about 30 percent of the tool body by volume.

Any suitable abrasive material may be utilized such as silicon carbide, aluminum oxide, emery, garnet, talc, pumice, and lime silicon dioxide, depending upon the abrading action and the resultant surface finish desired. While grit sizes of from 600 to 10 mesh may be utilized, the ordinary range will be from about 320 to about 24 mesh and most frequently from 60 to 36 mesh.

Such abrasive grains should have reasonably close size control so that they will not centrifuge differentially through the liquid media (i.e., the finer grains predominately towards the center of the wheel and the coarse grains toward the outer periphery of the wheel). For example, it may be possible to centrifuge a blend of 46, 54 and 60 grit fairly uniformly but not a blend of 36 and 100 grit. Likewise, if two different types of abrasives are employed as a blend, the true densities of each must be carefully considered so that the mass of the particle remains approximately the same. Thus, boron carbide having a specific gravity of 2.51 will centrifuge quite differently from aluminum oxide with a specific gravity of 3.95 even though they might both be classified as 60 grit.

The wetting ability and purity of the grain is important. The wetting properties of the abrasive material determine the speed of blending the grain with the reactive resin mixture. Acid or basic impurities on the surface of the abrasive may tend to catalyze the resin reaction. Moreover, the abrasive grain must have friability, shape, and hardness properties which are compatible with the bond-filler media in which it is embedded.

FILLER MATERIAL

A filler such as mica (325 mesh), graphite powder (325 mesh), iron pyrites (approximately 200 mesh), silicon carbide and aluminum oxide of flour fineness, etc., may be incorporated in the resin-abrasive mix. Such filler should be so finely ground that it may be uniformly dispersed throughout the grinding wheel even when present in small quantities and should be of such specific gravity and fineness that it will essentially not centrifuge to the outer portion of the wheel through the media of the reacting resin foam mix and abrasive slurry at the centrifuge speeds involved. That is, there should be no great difference in filler concentration in relation to the foam resin at the hub and at the outer periphery of the new abrasive wheel.

The filler may desirably in certain cases help remove some of the heat of reaction by absorbing heat from the reacting mass and such filler will ordinarily assist in reducing stresses within the grinding wheel which may be present in an unfilled plastic system. The cellular structure of the cured system helps to alleviate this problem but at areas of differential coefficient of linear expansion, such as at the interface of the abrasive annulus-plastic hub portion of the grinding wheel, a filler may be useful to reduce stresses which might lead to cracking of the wheel. The filler then adds to the desirable grinding qualities of the wheel, and fillers such as graphite or mica may impart lubricating qualities to the wheels. One such as sulfur, iron pyrite, or cryolite may impart cooling qualities during grinding because they decompose or boil at or below normal grinding temperatures. Fillers such as fine silicon carbide or aluminum oxide flour may also impart additional abrasive action to the wheel and may be desirable for fine finish work.

MATRIX OR BINDING MATERIAL

A satisfactory polyurethane composition for producing a substantially rigid, dimensionally stable, infusible, cellular foamed body such as characterizes the improved grinding wheel of the present invention which will make a precise grinding cut at substantial high pressure may be made by using a polyester in which one of the components, usually the hydroxyl bearing group is trifunctional or higher. The polyester, with free hydroxyl groups present, can be subsequently cross-linked with a diisocyanate to form the finished polyurethane. Some examples of desirable polyester compositions are the following formulations.

______________________________________ Formula No. 1 Formula No. 2 moles moles glycerol 4.0 trimethylol propane 4.0 adipic acid 2.5 adipic acid 2.5 phthalic anhydride 0.5 phthalic anhydride 0.5 Formula No. 3 Formula No. 4 moles moles glycerol 2.0 trimethylol propane 3.0 pentaerythritol 0.5 phthalic anhydride 2.0 phthalic anhydride 1.0 sebacic acid 3.0 Formula No. 5 moles trimethylol propane 4 adipic acid 1 phthalic anhydride 1/2 dimer acids 1/2 ______________________________________

Wheels may thus be produced with the above formulations which comprise a substantially rigid, dimensionally stable, non-brittle, infusible, cellular foamed body made by reacting a material selected from the group or class consisting of aromatic polyesters and polyethers with a polyisocyanate to produce such rigid polyurethane. The reaction products may be termed resins selected from the group consisting of aromatic polyester polyurethanes and aromatic polyether polyurethanes.

A formulation such as that below may also be employed utilizing aliphatic polyesters and polyethers in the reaction with the polyisocyanate also to produce a cross-linked rigid thermosetting type foam:

Formula No. 6 ______________________________________ trimethylol propane 3-9/16 moles dimer acids 1/16 mole oxalic acid 21/2 moles ______________________________________

Cross-linking can be obtained in a polyurethane system if one of the components of the polyester, usually the hydroxyl bearing group is trifunctional or higher. The polyester with free hydroxyl groups present can be subsequently cross-linked with a diisocyanate to form the finished polyurethane. Also, such cross-linking in a polyurethane system can be obtained if the system is a polyether, the polyether, however, being in the form of a triol or higher to cross-link with the diisocyanate; the shorter the distance between the cross-linking hydroxyl positions, the more rigid the structure obtained. Also, such cross-linking can be obtained if a linear polyester based on glycols or a polyether system containing diols is cross-linked by using a triisocyanate (i.e., triphenylmethane triisocyanate). However, in commercial practice, the first two methods of cross-linking a polyurethane system are desirably employed to obtain such rigid foams.

Polyesters such as the above used in polyurethane formulations should have an acid number of from less than one to forty and have the following ratio range of the hydroxyl to the carboxyl groups in the resin reactants: From four hydroxyl (OH) to one carboxyl (COOH); to one hydroxyl (OH) to one carboxyl (COOH). The preferred ratios are from three hydroxyl (OH) to one carboxyl (COOH); to 1-1/2 hydroxy (OH) to one carboxyl. The excess of hydroxyl groups ensure the subsequent reaction with polyisocyanate to form polyurethanes.

The dimer acids or dimerized fatty acids included in certain of the above examples of alkyd resins are dimeric polymers of unsaturated fatty acids such as: dimerized linolenic or linoleic acids. These dimer acids may be prepared by heating the methyl esters of polyunsaturated acids such as linoleic or linolenic acids at high temperatures. This is represented diagrammatically by a Diels-Alder reaction to form the dilinoleic acid (dibasic unsaturated acid) as follows: ##SPC1##

A suitable one-shot polyurethane may be made by blending one of the above polyesters with the theoretical amount of slight excess of polyisocyanate, preferably toluene diisocyanate (either 2,4 or 2,6 toluene diisocynate or mixtures thereof), to react with the excess hydroxyl groups present in the polyester. A possible reaction is shown in FIG. No. 1, using toluene diisocyanate and a polyester as indicated in formula 1.

The polyisocyanate employed in preparing the reactant foaming compositions may be used either with or without one or more thermoplastic polymeric resin additives, the latter serving to stabilize the foam during the reaction. Ethyl cellulose has been a particularly effective additive in this respect and the preferred range of addition would be from 0 to 8 parts of ethyl cellulose per 100 parts of toluene diisocyanate, by weight.

Heat resistance of the above formulations may be improved by adding polymethylol phenyls in the reactant compositions or mixtures for producing cellular plastics.

The preferred method of producing foam in the above systems is to incorporate from 01. percent to 3.0 percent H.sub.2 O by weight into the alkyd resin. The water may be incorporated as liquid water; however, other means may be employed, such as one or more metallic salt hydrates. Wetting agents such as glycerol monoricinoleate may also be incorporated to aid in the uniform dispersion of the water into the alkyd resin. The reaction between the polyisocyanate and water forms an intermediate product, carbamic acid, which decomposes to give a primary amine and carbon dioxide gas, the blowing agent.

A typical system would be a polyester of the type indicated in formula 1, with a hydroxyl number of 450 - 470, a water content of 0.1 to 1.0 percent and a viscosity of 120,000 to 160,000 CPS at 70.degree. F. The polyester may have additives such as 2, 4, 6 trimethylol allyloxy benzene included to improve heat resistance. This composes one component of the one-shot system. The other component is composed of a diisocyanate, preferably toluene diisocyanate (either 2,4 or 2,6 toluene diisocyanate or combinations thereof) which can be blended with from 0 to 8 parts by weight of ethyl cellulose per 100 parts toluene diisocyanate. In machine mixing, the polyester, or resin component is normally heated to a suitable temperature, from 100.degree. to 150.degree. F., so that the resin can be pumped and dispensed more readily. The toluene diisocyanate component may have an initial viscosity of from 1 to 5,000 CPS at 70.degree. F. depending upon the amount and type of ethyl cellulose present. A filler such as mica above noted then may be incorporated.

The ingredients may usually be mixed at room temperature although they may, if desired, be preheated to reduce viscosity and increase the rate of reaction. They may be mixed for about 1 minute and then poured into the spinning mold, the latter operation requiring about 30 seconds and the centrifuging about 45 to 240 seconds. The duration of centrifuging at speed depends upon several factors. Such centrifuging must be sufficiently long and the speed sufficiently high to cause the abrasive grains to be uniformly concentrated in the outer periphery of the wheel. The duration of centrifuging must also be sufficiently long to allow polymer and viscosity build-up so that the abrasive annulus will not slump seriously due to gravity when effective centrifuging ceases. However, the centrifuging should be stopped while foaming action is still sufficient to separate the abrasive grains and to cause the reacting, foaming resin to travel radially inwardly and complete the dimensions of the mold. The speed of centrifuging may vary from several hundred r.p.m. to several thousand r.p.m. depending primarily upon the diameter of the wheel produced. The turntable or spinning mold may then be stopped and the foaming operation proceeds for aproximately 10 minutes to fill the central portion of the mold and to widen the outer circumferential abrasive region radially inwardly through uniform spreading of the abrasive elements slightly apart. Another 10 minutes may be required for initial setting, and then 20 minutes or more for final setting.

Foam may, of course, be generated in known manner in various types of resins by whipping or beating, or by inclusion of soluble granules which are subsequently dissolved out, or by introduction of gases under pressure. The term "foam" as herein employed is intended to include cellular structures without regard to the particular manner in which such cells may be formed.

To manufacture a preferred abrading wheel, 162 grams of an alkyd resin, such as given in formula 1, is mixed with 138 grams of toluene -2,4 diisocyanate for 1 minute. An abrasive material such as 330 grams of 36 grit fused aluminum oxide may be mixed into the above alkyd-diisocyanate mixture. The foregoing mixture is immediately placed into the aforedescribed mold and then rotated at about 3,000 r.p.m. for 1 minute. The mold is then placed into an oven at approximately 250.degree. F. for 2 hours. The mold may then be removed from the oven and cooled before the finished, foamed abrading wheel is removed. The finished wheel should weigh 520 grams. The difference in the original weight of materials and the article weight is accounted for by the cling in the mixing cup. The cling material and the material in the mold each contain the same proportion of grit and plastic as the mix.

Satisfactory wheels may also be made by varying the foregoing procedure; for example, the abrasive material may be premixed into the alkyd resin or diisocyanate portion of the mixture. The alkyd resin may be varied as to the nature of its chemical components as given, for example, in formulas 2, 3, 4 and 6. Abrasive type and grit size may also be varied to produce the desired type of abrading action in the finished wheel. The toluene diisocyanate portion of the foregoing mixture may be varied by using mixtures of toluene -2,4 diisocyanate with toluene -2,6 diisocyanate.

Another formulation for making, for example, 7 inches O.D. by 1/2 inch wheels using a resin sold by Nopco Chemical Company under the trade name "Lockfoam" would be as follows:

162 grams A-625-R "Lockfoam" resin manufactured by

The Nopco Chemical Company

138 grams A-625-C foaming agent also manufactured

by The Nopco Chemical Company

330 grams Abrasive grit as the aforementioned

aluminum oxide, for example

The foregoing material may be mixed in the order given above, at a temperature of 70.degree. F. to start. The resin and foaming agent may be mixed for 45 seconds and then the abrasive grit is added and mixed for an additional 45 seconds. This mixture may then be placed in a mold having a volume larger than that of the mixture, such mold being made so that it is open to the atmospheric pressure. The mold is then rotated to centrifuge the contents for 45 seconds at about 2,800 r.p.m. While still in the mold, the article is cured at about 200.degree. F. for 1-3/4 hours, after which it is cooled to room temperature before the mold is opened. The weight of the final wheel is 520 grams.

WHEEL COMPOSITION AND DENSITY

The composition of the abrasive annulus 13 can be obtained by burn-out tests utilizing certain procedures to obtain the composition breakdown. The density and volume of the grinding wheel can be accurately determined by means of ASTM Test D792-50 (Specific gravity by water displacement) if the grinding wheel is essentially closed cell and does not absorb water rapidly, this latter feature being important in grinding wheels which are often run with coolants. If fluids are absorbed by the wheel, the latter may tend to become out of balance. In such tests, sections of wheels of known weight and volume are placed in a crucible and fired for at least 1 hour in an oven maintained at 1,300.degree. F. During this period, all organic bond material is driven off as volatile matter. Also, if a filler is present which either boils or decomposes below the oven temperature, it will be driven off with the bond material. If not, the filler will remain in the crucible with the abrasive which is unchanged at this temperature. Examples of fillers that are not affected at this temperature are mica and graphite. Since these are present as very fine particles, they can be separated from the heavier or larger abrasive particles by washing in a manner similar to that employed in ore separating processes. An example of a filler which is volatilized in the oven at such temperature is sulfur which boils at 832.degree. F. This must be subsequently separated from the bond by chemical analysis, using a fresh sample from the same grinding wheel. By weighing the remaining contents after burn-out, with an analytical balance and also after wash-out and drying, a very accurate determination of composition by weight can be made with excellent duplication.

However, weight composition does not reveal the complete story of the grinding wheel. Volume composition, which includes air or gas space, sometimes referred to as porosity in the grinding industry, is a very significant factor. To obtain volumetric composition, the densities of the abrasive, bond and filler can be ascertained. As an example:

Aluminum oxide abrasive density 3.95 gm/cc. Polyurethane bond density 1.20 gm/cc. Mica density 2.84 gm/cc.

The aluminum oxide abrasive density can vary from 3.90 to 3.97 gm/cc., depending on friability, but 3.95 gm/cc. is a good over-all average. The 1.20 gm/cc. density for the non-foamed polyurethane bond system can be obtained from suppliers of the resins. This density may, however, vary somewhat for other rigid polyurethanes employed, but such variance is not particularly significant. Since the total volume and the volume occupied by the constituents of the grinding wheel can be ascertained, the volume occupied by the air space can be determined by the difference. The over-all density is determined as above set forth and the abrasive density can be determined as simply the over-all density times the weight percent of abrasive. By using the volumetric composition, different abrasives such as silicon carbide or diamond; or different bonds, such as epoxy and phenolic; or different fillers, such as sulfur or cryolite, find utility in the wheel of the present invention. The weight make-up may change quite considerably with such different components, but the volumetric make-up will remain approximately the same. The weight make-up is important when related to a specific system; i.e., aluminum oxide, polyurethane bond, mica filler; and is also included to help define the product of the present invention.

The following examples 1-5 indicate the results of this analysis: ##SPC2##

In the examples 1-5 above, the resin system used is a polyester-based one-shot polyurethane such as that previously set forth. The filler (325 mesh white waterground mica) was preblended with the toluene diisocyanate component of the system so that the foaming resin system contained filler as it was dispersed from the mixing machine. The foam system in each case is a normal 25 pound per cubic foot free foam density rigid polyurethane. The resin is preheated to 130.degree. F. The toluene diisocyanate component was thinned to obtain a toluene isocyanate with 2 parts ethyl cellulose per 100 parts T.D.I. The filler was then preblended into the above solution and the blend dispensed at a temperature of approximately 80.degree. F. The mix of mica-filled foam reactants was dispensed into a cup and the abrasive added to the top and blended into a uniform mixture by mixing with a loop-type mixer for 15 seconds. The uniform blend was then dispensed into a steel mold of dimensions 7 inches O.D. .times. 11/4 inches I.D. .times. 1/2 inch thick maintained at a temperature of 150.degree. F. The top was placed on the mold secured with a nut and centrifuging was started 75 seconds after the foam shot was taken. The mold was rotated at 2,200 r.p.m. for 45 seconds, then allowed to coast down to rest (about 20 seconds). The mold was then removed from the centrifuge and placed into an oven for three hours at 200.degree. F. The wheels, as indicated, were subjected to burn-out tests by splitting the abrasive annulus on the parting line 26 in FIg. 5 and then running the burn-out tests separately on each portion.

The above fixe examples are indicative of the compositions obtained in a seven inch diameter grinding wheel. Larger wheels must generally be made by more rapid means because much larger amounts of material must be handled in the same period of time. In example 6, set forth below, a 24 inches O.D. .times. 3/4 inch thick wheel was employed, such wheel being made by blending abrasive with the reactant mix in two separate cups and filling by pouring into a rotating mold, stopping and inserting the vented core into the filled mold before going into high speed centrifuging. The results of burn-out tests at various positions in the abrasive indicate some differences in composition as the radial distance from the outer edge changes. However, it is to be noted that at identical radial distances (such as work encounters when acted upon by the grinding wheel since the grinding wheel is rotated on the same axis as that on which it was initially produced), the wheel has excellent uniformity.

Example 6 __________________________________________________________________________ (24" O.D. .times. 3/4" thick) __________________________________________________________________________ Radial distance from outer edge 0.375 1.13 1.75 2.25 3.13 (in.) Over-all density 2.48 2.45 2.37 2.30 2.23 (gm/cc) Abrasive density 1.83 1.81 1.74 1.68 1.61 (gm/cc) __________________________________________________________________________ Abrasive 73.8 74.0 73.3 72.8 72.1 Composition Filler (325 3.5 3.0 3.0 3.0 3.0 mesh mica) Weight % Bond-rigid 22.7 23.0 23.7 24.2 24.8 polyurethane __________________________________________________________________________ Abrasive 46.9 46.4 44.6 43.0 41.3 Composition Filler (325 3.0 2.6 2.5 2.4 2.4 mesh mica) Volume Bond-rigid 47.0 46.9 46.8 46.4 46.2 polyurethane Air 3.1 4.1 6.1 8.2 10.1 __________________________________________________________________________

In the above example, 1,916 grams of the rigid polyurethane reactants were mixed with 259 grams of 325 mesh mica and with 3,325 grams of the abrasive and were agitated with a loop-type mixing blade until the materials were well blended (approximately 15 seconds). The ingredients were then poured into a mold rotating at approximately 550 r.p.m. In approximately 75 seconds from the start of the first shot of the foam before the ingredients had been poured into the mold, the centrifuging was stopped and the mold plug secured. Even with the plug in place, there is adequate central venting of the mold, however. The centrifuge was then spun at 800 r.p.m. for approximately 90 seconds. The mold was then removed from the centrifuge and cured in an oven for 5 hours at 200.degree. F.

In example 7 set forth below, a 20 inches O.D. .times. 2 inches thick wheel was produced by mixing all ingredients in the mold. A specially designed core containing mixing blades may be located below the abrasive and foam feeding stations to blend the mixture with no external hand mixing and acts as the center vented core after the mix is achieved. The compositions at different radial positions roughly coincide with example 6 as indicated:

Example 7 ______________________________________ (20" O.D. .times. 2" thick) ______________________________________ Radial distance from 0.375 1.13 1.75 outer edge (in.) Over-all density (gm/cc) 2.51 2.49 2.41 Abrasive density (gm/cc) 1.87 1.86 1.78 ______________________________________ Abrasive 74.4 74.5 74.3 Composition Filler (325 3.2 3.3 2.9 mesh mica) Weight % Bond (rigid 22.4 22.2 22.8 polyurethane) ______________________________________ Abrasive 47.9 47.6 45.7 Composition Filler (325 2.8 2.9 2.4 mesh mica) Volume Bond (rigid 47.2 46.1 45.6 polyurethane) Air 2.1 3.4 6.3 ______________________________________

In the above example, 6,147 grams of the rigid polyurethane reactants were mixed with 829 grams of 325 mesh white waterground mica along with 10,708 grams of 60 grit size abrasive and dispensed into a mold rotating at 700 r.p.m. The mixing was accomplished by four blades rotating at the same speed as the mold plus an air nozzle to keep the material from building up on the sides of the mixing chamber. The mold was then stopped, cored and then rotated at 1,060 r.p.m. for 120 seconds. The wheel was then cured for seven hours at 200.degree. F. with the final weight of the wheel being 17,450 grams.

Another excellent polyurethane system can be obtained using polyether as a base. Examples of usable polyethers include reactive polyglycols. The preferred type of polyether is one which is trifunctional or higher (i.e., triols, pentols, hexols). A good system for use with grinding wheels of the present invention is called a quasi-system and is shown as follows as weight percent.

______________________________________ Part A Part B ______________________________________ polyether 20% polyether 99.3% toluene diisocyanate 78% water 0.1% catalyst 0.2% catalyst 0.6% emulsifier 1.8% ______________________________________

Parts A and B can be blended in a mixing machine in the ratio of 4 parts A to 3 parts B.

For the preferred grinding wheel of the present invention which will make a very precise cut at predetermined high pressure, it is important to select a rigid, infusible, dimensionally stable, cross-link resin for the foam. Cross-linking of this type can be obtained in a polyurethane system if:

1. One of the components of the polyester, usually the hydroxyl bearing group, is trifunctional or higher. The polyester, with free hydroxyl groups present, can be subsequently cross-linked with a diisocyanate to form the finished polyurethane.

2. If the system is a polyether, the polyether must be in the form of a triol or higher to cross-link with a diisocyanate. The shorter the distance between the cross-linking hydroxyl positions, the more rigid the structure.

3. A linear polyester based on glycols or a polyether system containing diols could possibly be cross-linked by using a triisocyanate (i.e., triphenylmethane triisocyanate) but in commercial practice systems 1 and 2 are usually used to obtain rigid foams.

As previously stated, other resins than the polyurethanes mentioned are suitable for use with the grinding wheels of the present invention. One example thereof is an epoxy resin mixture which will produce a foam system, such as the following:

Amount Purpose (grams) Ingredient ______________________________________ Resin 147.7 EPON 828 - epoxy resin (Shell Chemical) Resin 31.3 EPON 1004 - epoxy resin (Shell Chemical) Filler 26.0 Mica - 325 mesh white waterground Diluent - heat absorber 6.0 Toluene - technical grade Curing agent 11.0 Diethylene triamine Wetting agent 4 drops Tween 20 - polyoxethylene (20) sorbitan monolaurate Blowing agent 0.5 Ammonium carbonate - powdered, purified Abrasive 334.0 Aluminum oxide - 60 grit ______________________________________

PROCEDURE

1. Preblend 147.7 grams EPON 828, 31.3 grams EPON 1004, 26 grams mica, 4 drops Tween 20 and 6 grams xylene and heat to 170.degree. F.

2. Add 11 grams diethylene triamine and blend with a high speed loop-type mixer for 10 seconds.

3. Add 334 grams aluminum oxide, 60 grit, at 170.degree. F. and blend for additional 10 seconds.

4. Add 0.5 grams powdered ammonium carbonate and blend additional 15 seconds until finely dispersed.

5. Add above mixture to mold of dimensions 7 inches O.D. .times. 11/4 inches I.D. .times. 0.500 inch thick (305.3 cc.), close mold and centrifuge at 2200 r.p.m. for 60 seconds.

6. Allow mold to coast to a stop (20 seconds), then remove mold and place in oven to cure for three hours at 200.degree. F.

7. Final Weight -- 520 grams.

EPON resin 828 has an epoxide equivalent of 175-210, molecular weight of 350-400. EPON resin 1004 has an epoxide equivalent of 870-1,025 and a molecular weight of 1400. Other epoxy resins such as EPON 834 (epoxide equivalent 225-290, molecular weight 450) and EPON 1001 (epoxide equivalent 450-525, molecular weight 900-1000) may be used. Diethylene triamine is the curing agent, but curing agents such as metaphenylene diamine may be included to improve strength, heat and chemical resistance of the bond. Mica is the filler, but other fillers previously described may be employed. Toluene is the solvent-diluent used to modify and control the foaming process by absorbing excessive heat of reaction. Tween 20 is used as a wetting agent to provide a fine and uniform dispersion of gas bubbles. Ammonium carbonate is the blowing agent, but various other blowing agents can be incorporated, such as Celogen (P, P.sup.1 oxybis (benzenesulfonyl hydrazide)), nitroso compounds, azo compounds, hydrazides, etc. Thus, the resin-catalyst wetting agent-blowing agent system may be varied to obtain a proper cross-linked system for abrasive wheels.

Phenolic foams have also been found suitable for production of grinding wheels in accordance with the present invention.

______________________________________ Part A (parts by weight) Part B (parts by weight) ______________________________________ BRLA 2761 80 Water (as ice) 50 BRLA 2760 20 Sulfuric acid 68.degree. Baume 50 Isopropyl ether 6.6 Phosphoric acid 85% 7 Tween No. 40 1 ______________________________________

Part A is prepared as follows: The Tween 40 is dispersed in the isopropyl ether and then, with continuous stirring, this blend is mixed into the blend of BRLA 2761 and BRLA 2760. Part B is prepared by adding the sulfuric acid very slowly to ice. When this addition is complete and well stirred, the phosphoric acid may be added and mixed well. The ratio of components is from 8 parts of Part B, to 92 parts of Part A, to 16 parts of Part B, to 84 parts of Part A.

BRLA 2761 and BRLA 2760 are liquid phenolic resins produced by the Union Carbide Plastics Company. Each is composed of an incompletely condensed resin made by the interaction of phenol and formaldehyde. The water miscibility of BRLA 2761 is 205 percent, that is, 2.05 parts by volume of water mixed with 1 part by volume of BRLA 2761 will still form a solution. Above this point, additional water will cause an emulsion to be formed. The water miscibility of BRLA 2760 is 195 percent. When these resins are mixed with catalyst and blowing agent, an exothermic reaction of further condensation causes liberation of the gas by the blowing agent. The increasing viscosity of the mix prevents escape of this gas and hence the mass expands until the resin sets up. Tween 40 (polyethylene (20) sorbitan mono palmitate) is a wetting agent which helps to provide a fine and uniform dispersion of gas bubbles.

WHEEL CHARACTERISTICS

It has long been recognized that most production grinding operations are necessarily a compromise between such essential factors as amount of metal being removed, quality of finish, size control and heat damage. The wheel of the present invention substantially eliminates this element of compromise, permitting significant increases in feed and speed of grinding without sacrificing quality of finish or other important performance characteristics. Because of the wheel's greater tensile and impact strength, as compared to more conventional wheels, it can make faster and deeper cuts and accordingly can obtain greater stock removal per time unit while still maintaining excellent surface finish, accuracy and cool operating characteristics. Moreover, the wheel of the present invention can be operated at extremely high grinding speeds. The wheel can operate at feeds and speeds beyond anything heretofore thought possible with conventional grinding tools. For example, while conventional wheels must be confined to the 6,500 to 9,500 SFPM range, wheels of the present invention can be operated at speeds as high as 13,000 SFPM (surface feet per minute).

Production tests as well as field tests have indicated that the wheel of the present invention has remarkably superior form holding qualities. For example, when considering a slot cut in a solid steel plate, the slot cut by the wheel of the present invention has substantially square corners while a slot made by a conventional vitrified wheel, for example, will have rounded corners. Thus, the wheel of the present invention has remarkable corner holding ability even under severe operating conditions.

Accordingly, the wheel of the present invention is believed to be the safest grinding wheel ever employed. This is evidenced by its ability to withstand higher speeds, operate with heavier cuts, grind cooler, resist coolant saturation, and resist fracture or breakage even under severe operating conditions. In grinding wheel tests conducted by an independent testing laboratory, the wheel of the present invention surpassed all standard vitrified or resinoid grinding wheels of leading manufacturers. For example, in a grinding face impact test involving dropping heavy metal objects onto the faces of grinding wheels in operation, the wheel of the present invention suffered no damage while standard vitrified or resinoid wheels were either severely damaged or no longer usable. In breaking speed tests, the wheel of the present invention surpassed all standard vitrified wheels of leading manufacturers as indicated below:

The present invention 250% Standard vitrified wheel 91% Standard vitrified wheel 100% Standard vitrified wheel 95%

Although the grinding wheel of the present invention obtains a faster rate of metal removal, it produces a fine surface finish of low RMS. The wheel resists loading so well that rough and finish cuts can often be combined, making it possible in many cases to eliminate preceding or subsequent operations. The wheel also has remarkably unusual stability whereby uniform finish can be held from part to part for longer periods than with conventional types of grinding wheels. Accurate balance and uniform quality are also features of the wheel of the present invention. Because of the production techniques, every wheel is produced dimensionally accurate and in true balance. Thus, regardless of size, precision made wheels do not require balancing at installation and set-ups can be made on grinding machines in much less time. Moreover, exceptional uniformity of wheel quality is an inherent feature. Due to such uniform physical characteristics, less frequent adjustments are required to maintain close tolerances on production operations. Because of the unusual qualities of the wheel, it does not break down or load as rapidly as conventional wheels. This minimizes the number of dressings required on production runs and results in greatly improved efficiency. For example, a conventional wheel may require dressing approximately once every half hour whereas the wheel of the present invention may be dressed at the start of each shift as a precautionary measure only. Needless to say, dressing of a wheel is a time-consuming and expensive operation.

In the aforementioned grinding edge impact test, grinding wheels were mounted to the holding flanges and rotated at approximately 6,500 SFPM (1800 r.p.m.) while the plane of the wheel was in a vertical direction. A chrome steel ball of 2 inches in diameter and weighing 1.18 pounds was dropped from different heights to produce impacts ranging from 1.18 to 10.62 foot pounds on the center of the grinding edge or face of the wheels. Upon completion of those impacts, the wheels were inspected for damage. Out of the eight wheels tested, which were in accordance with the present invention, only in two cases and at the highest impact figure did a small crack result, all of the others showing no damage or merely a small mark. This is in sharp contradistinction with respect to the same test conducted on conventional vitrified, resinoid or rubber based wheels wherein all were either damaged or no longer usable as the result of such impact loads.

Thus, despite the above-described cellular construction of the new grinding wheel, by employment of properly oriented abrasive grain, of appropriate resinous ingredients, and of thermosetting plastics and particularly polyesters of the type described reacted with isocyanates or diisocyanates to produce substantially rigid, dimensionally stable, infusible cross-linked polyurethane, a grinding wheel is obtained having an extraordinarily strong rigid work-engaging cutting face considered as a whole, whereby precision grinding operations can be performed at much accelerated rate. Remarkably enough, despite the high cutting rate which may accordingly be obtained, the tool is not nearly so subject to wear as conventional grinding wheels, particularly at the edges or shoulders and need not be dressed so frequently to maintain a precision cutting face. At the same time, scoring of the work does not occur despite the exceptional depth of cut that may be taken and a much improved surface finish is produced on the work.

It will readily be recognized that this combination of properties would appear to be incompatible since a high rate of tool feed and an exceptional depth of cut obviously impose severe working stresses on the tool, and individual upstanding abrasive grains at the tool face would normally either be broken out of the face or would produce corresponding deep score marks in the work surface.

The difference between a conventional grinding wheel and the present grinding wheel lies largely in the fact that the new structure is capable of handling a substantially higher stress load, as can be calculated from the higher bursting speed, especially under conditions where shock loading is expected. The specific physical difference is that the abrasive grains are not touching and do, in fact, find support in a substantially rigid but non-brittle plastic matrix. This kind of matrix, especially selected from plastics in the thermosetting category, is able to provide individual micro-movement of the individual grains, which action is the stress relieving or stress absorbing means. This stress absorbing deformation occurs on a local basis without involving change in dimension of the whole body and without permanent relocation of the individual abrasive grains. The physical effect is to prevent the concentration of stress from occurring as long as possible and to keep a crack from starting and propagating.

This feature is partially illustrated in very diagrammatic fashion in FIGS. 10 and 11 of the drawing, the former indicating a grinding tool profile in which the exposed abrasive grains are embedded and accordingly protrude to varying degrees at the working face. When such tool is caused to make a grinding cut in the work as shown in FIG. 11, however, the impact shock is absorbed and the excessively protruding grains are locally individually readjusted in position so that the working pressure is supported across the entire working face of the tool, this being accomplished without substantial yielding of the tool body as a whole. Such stress absorbing deformation occurs on a local basis only without involving a change in dimension of the whole body and without permanent relocation of the individual abrasive grains. The physical effect is to prevent a concentration of stress from occurring as long as possible and to keep a crack from starting and propagating. This has never before been accomplished in a tool of this type which is dimensionally stable and has a high abrasive grain density or concentration with a grain weight per cubic centimeter of about 1.8 grams depending upon the exact chemical content and grain size and shape within the specific wheel.

It will accordingly be seen that in such rigid resin grinding wheel, brittleness is avoided through the mechanism of absorbing the load applied to any single abrasive grain by distributing it over its relatively large local area of the holding matrix, stubborn deformation of which can absorb energy without passing such load along to adjacent grains. On the contrary, loads applied to relatively large areas of the wheel face will not change the dimensional stability of the face and, of course, the tool as a whole in any amount different from that of conventional grinding wheels.

Extremely light grinding can be performed even when the wheel is pressed against the work at pressures as low as 100 lbs./sq. in., using a flat steel surface 1/2 inch square as the test work-piece, and light grinding can be performed at 200 lbs./sq. in. At 1,000 lbs./sq. in., which is considered heavy pressure for conventional wheels, the wheel of this invention performs excellently as described, and in many cases the wheel of this invention may be employed at working pressures of as high as 2,000 lbs./sq. in. where conventional wheels would fail. It will be appreciated that in these tests the entire surface of the test work-piece does not ordinarily engage the working face of the wheel and the figures given are computed from the area of contact actually observed.

The explanation of this unique capacity of the wheel is believed clear and is found in the stress absorption obtained through such stubborn micro-deformation permitted on an individual and local basis by the combination of a substantially rigid but non-brittle thermosetting resin bond and the concentrated yet uniformly spaced apart abrasive grains. The working face of the tool which constitutes abrasive grains mounted in a rigid non-brittle resin in relatively large volumes, but in such a manner as to minimize unit stress thereon, provides a structure which will permit such independent, stubborn, local deformation in amounts sufficient to absorb stress of any highly stressed grains. Such novel structure is markedly different from conventional vitrified or resinoid grinding wheels wherein brittle expulsion of the grains is easily observed. Moreover, such structure is not even in the same class with soft rubber or highly flexible plastic wheels where deformation of large areas of the wheels is equally observable. Such latter wheels, of course, cannot make a precise cut at high feed or pressure and are suitable only for polishing or finishing purposes.

When compared with the best of the currently available conventional grinding wheels, a grinding tool of the present invention using a dimensionally stable, only locally yielding polyurethane body, as hereinbefore described, produces unexpected performance results. The following is a comparison table between a superior conventional commercially available grinding wheel and a similar wheel but made in accordance with the invention:

Conventional Wheel Polyurethane Wheel Best Grade of This Invention __________________________________________________________________________ Total weight 700 grams 525 grams Bursting speed 10,500-12,500 r.p.m. Over 14,500 r.p.m. Note: Top speed of test- ing machine is 14,500 r.p.m. Wheel wear rate .013" wheel break- .0085 wheel break- on cold rolled down per .050" total down per .050" total steel feed made at .002" feed made at .002" per pass per pass Stock removal .037" stock removal .042" stock removal on C.R.S. depth per .050" down depth per .050" down feed feed Surface finish using .002" down 45 microinches 35 microinches feed per pass Maximum depth feed advance .003" .005" assuming equal wheel wear Static axial 440 inch pounds In excess of 880 breaking load inch pounds Static axial deflection .004" .019" under load of 10.7 pounds Maximum static deflection before .200" In excess of .5625" wheel breakage Total depth of cutting between .020" .030" dressings, using .005" down feed Shape holding characteristics Shows substantial Substantially as for a slot (for break-down originally formed a corner) after .050" down feed Work-piece Heat build-up Little or no temperature grind- quickly and easily temperature increase ing without discernible coolant Ratio of amount of metal removed 1.0 1.7 for equal wheel diameter loss __________________________________________________________________________

In addition to the foregoing performance characteristics, the improved wheel construction is distinguished by the feature that its working face is fixed, even though the cutting points presented by the individual grains are capable of stubborn temporary micro-deflection. It thus becomes possible to use the wheel for precision, dimensional cutting work.

One interesting phenomenon which may be observed when the unique wheel of the present invention is formed without the abrasive grains added is that the multitude of small or minute gas bubbles that initially form as the result of the foaming action of the resin material are caused to coalesce in the outer portion of the body with a corresponding increase in size of the individual bubbles or resultant cells while still maintaining the closed character of the latter and uniform distribution thereof throughout the area in question. Coincidentally, the web structure between the bubbles is caused dimensionally to thicken in the area in question. The resulting wheel-like article will accordingly then comprise a central portion, in which the resultant cells remain numerous and minute in size, and an outer peripheral or rim portion the radial depth of which may be varied as desired, in which the resultant cells are of increased size and the intervening web structure is dimensionally thickened. Such peripheral or rim portion will furthermore be of greater density than such central portion, e.g., the weight of the former per cubic centimeter may be two or three times that of the latter, where foamable polyurethane is the resin used.

TOOL FORMS

While the tool form in which the present invention will probably find its greatest utility is that of a grinding wheel, above described, it will be appreciated that other tool forms utilizing the improvements of the present invention may also be employed. For example, the resin may be extruded or molded in the form of an abrasive containing cylindrical member or stick in which, if desired, the abrasive may be concentrated and yet uniformly spaced adjacent the outer periphery thereof by centrifuging about its longitudinal axis. The tool may comprise a metal cup provided with a co-axial stem adapted to be chucked in a drill press or the like. A cylindrical abrading element of the general character of such stick may be secured within the cup, or alternatively a cylindrical abrading element having a conical tip portion may be similarly mounted. The cylindrical abrading elements may be formed with the abrasive concentrated and yet uniformly spaced in their working end portions by centrifuging the molds in which they are produced. Other tool forms such as blocks, belts, toothed or slotted wheels, may also employ the features of the improvements of the present invention.

Abrasive discs may likewise be produced in accordance with this invention comprising the usual circular base plate to which the disc is adhered by a suitable adhesive in well-known manner. Such disc may be in the form of a truncated cone formed of foamed polyurethane resin containing granular abrasive therewithin preferably concentrated in the region of the flat circular working face of the tool. A co-axial stud or stem is provided on the plate for chucking in an appropriate rotary power tool.

it will be obvious that by employing a mold of different contour and varying dimensional relationships of parts, not only may such improvements of the present invention be employed for making grinding wheels, but abrasive tools of any of the several types currently employed. For example, wheels having special curved faces may be employed for various applications. Also, a cup, cylinder or cone may be employed either by centrifuging the components in a mold of corresponding shape or by cutting sections from a portion of a wheel-like article made as hereinbefore described.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

Claims

I, therefore, particularly point out and distinctly claim as my invention:

1. A grinding tool comprising an essentially rigid, dimensionally stable body of cellular polyurethane resin binder selected from the group consisting of aromatic polyether polyurethanes and aromatic polyester polyurethanes, and granular abrasive dispersed uniformly and embedded therein, the grains comprising such abrasive being spaced only slightly apart and said resin being capable of slight local stubborn resilient yielding action to an extent allowing corresponding slight local individual movement of said grains exposed at the tool face relative to other adjacent grains in such face, but said body as a whole being sufficiently rigid to support the grinding face of said tool to make a grinding cut of precise predetermined depth under operating pressures in use, such slight local yielding action of said resin at such grinding face being sufficient to ensure readjustment of the positions of individual grains protruding excessively from such face under such operating pressures to bring such protruding grains into substantially the same plane as the other adjacent grains in the portion of the tool face under pressure contact with the work, whereby the work load is sustained by substantially all said grains across such face, simultaneously to permit the making of a deep precision cut in the work by such face while nevertheless producing a relatively smooth finish thereon and without premature dislodgment of such excessively protruding grains from such face.

2. The grinding tool of claim 1, wherein said grains are thus spaced apart approximately one grain diameter, on the average.

3. The grinding tool of claim 1, wherein said tool is in the form of a grinding wheel.

4. The grinding tool of claim 1, wherein said tool is in the form of a grinding wheel, and said grains are spaced apart approximately one grain diameter, on the average.

5. A grinding tool adapted for rapid grinding of a deep predetermined precision cut in a steel part, said tool comprising

a non-brittle resin binder body which is rigid and dimensionally stable under the required grinding pressures thereby to ensure such precision,

and a high concentration of abrasive grains uniformly incorporated and embedded in said binder,

substantially all said grains nevertheless being only very slightly spaced apart in said body

and the latter being just sufficiently locally yieldable under such grinding pressures at the working face of said tool to afford individual relative micro-adjustment of said spaced grains exposed thereat for energy absorption and stress distribution, when such grains are subjected to heavy impact forces when engaging the work without appreciable change in the conformation of such working face.

6. The grinding tool of claim 5 wherein said resin is selected from the class consisting of polyurethane resin, epoxy resin, phenolic resin, and silicone resin.

7. The grinding tool of claim 5 wherein said resin is a foamed resin selected from the class consisting of polyurethane resin, epoxy resin, phenolic resin, and silicone resin.

8. The grinding tool of claim 5 wherein said resin is a closed cell foamed resin selected from the group consisting of aromatic polyether polyurethanes and aromatic polyester polyurethanes.

9. The grinding tool of claim 5 wherein the abrasive content of a unit volume of said tool by weight is equal to at least approximately 75 percent of the pack density of the particular abrasive grains employed.

10. The grinding tool of claim 5 wherein the abrasive content of a unit volume of said tool by weight is equal to from about 75 percent to about 100 percent of the pack density of the particular abrasive grains employed, and said body is thus rigid and dimensionally stable under grinding pressures on the order of at least 1,000 pounds per square inch.

11. The grinding tool of claim 5 wherein the abrasive grains constitute from about 30 percent to about 48 percent of the abrasive-resin body by volume.

12. The grinding tool of claim 5 wherein said resin is foamed resin selected from the class consisting of polyurethane resin, epoxy resin, phenolic resin, and silicone resin, and the abrasive content of a unit volume of said tool by weight is equal to at least approximately 75 percent of the pack density of the particular abrasive grains employed.

13. A grinding tool adapted for rapid grinding of a deep predetermined precision cut in a steel part, said tool comprising

a non-brittle resin binder body which is rigid and dimensionally stable under the required griding pressures thereby to ensure such precision,

and a high concentration of abrasive grains uniformly incorporated and embedded in said binder,

the abrasive density of said tool in grams per cubic centimeter being at least 1.28,

substantially all said grains nevertheless being spaced apart in said body just short of contact with each other,

and the interposed resin of said body being just sufficiently yieldable to afford individual micro-adjustment of said grains exposed at the working face of said tool when said tool is subjected to grinding pressure in use on the order of at least 1,000 pounds per square inch, thereby to achieve energy absorption and stress distribution when said exposed grains are subjected to heavy impact forces when engaging the work, without, however, appreciable change in the dimensions of said body or the conformation of such working face.