US2537541A - Grinding wheel construction - Google Patents

Description

1951 L. H. MlLLlGAN ET AL 2,537,541

GRINDING WHEEL CONSTRUCTION Filed July 19, 1945 2 Sheets-Sheet l PIGl I iv

Lou/51.1. H. M/LL/GAN 20552.7 H. LaMamzp um m 17 Jan. 9, 1951 H. MILLIGAN ET AL 5 5 GRINDING WHEEL CONSTRUCTION Filed July 19, 1945 2 Sheets-Sheet 2 loc- Lam/ELL h- MILL/6AM I ,Qoeser H-LoMsmzD Patentecl Jan. 9, i951 GRINDING WHEEL GONSTRUCTION Loweii H. Milligan and Robert H. Lombard, Worcester, Mass., assignors to Norton Company, Worcester, Mass, a corporation of Massachusetts Application July 19, 1945, Serial No. 605,97

9 Claims.

This invention relates to built-up grinding wheels and to a method of constructing them to overcome certain difficulties heretofore encountered in interrelating the rigid, inflexible, bonded abrasive element and a rigid element which is to reinforce it or support it. 1

One of the objects of this invention is to provide a method and means for dependably securing together the relatively rigid or inflexible, bonded abrasive element, such as one in which the abrasive grains are bonded by a vitrified bond, and a companion element which is relatively rigid or inflexible and which may serve as the support for the abrasive element or as a reinforcement for the rigid abrasive element. Frequently the supporting or reinforcing element is of metal, and frequently it serves principaliy or solely as a means for mounting the abrasive element, as, for example, upon a shaft, for rotation of the grinding wheel, and, particularly in such cases, the thermal coefficients of expansion are suihciently different so that the mere cementing together of the two is insufficient to cope with strains and stresses set up in the junction when changes in temperature take place, as during the use of the grinding wheel. And such cementing cannot invariably be relied upon to cope with strains and stresses setup in the junction independently of temperature change, such as, for

example, by centrifugal forces tending to rupture the grinding wheel or tending to displace the relatively heavy and rigid abrasive part. Various expedients have heretofore been resorted to in the endeavor to overcome such difficulties or deficiencies, but some of them are cumbersome and others require complicated procedures, and none of them overcomes the problems posed by, or the deficiencies and weaknesses in, the continuous face-to-face junction produced by known rigid cementing methods. One of the dominant aims of this invention is to provide a mechanical junction or union between such rigid elements of a grinding wheel that will overcome the difficulties and deficiencies such as those above mentioned and to provide a practical and economical method of achieving the same. Other objects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the features of construction, combination of elements, arrangements of parts, and in the several steps and relation and order of each of the same to one or more of. the others, all as will be illustratively described herein, and the scope of the ers, of suitable thickness, throughout which are application of which will be indicated in the following claims.

In the accompanying drawings, in which are shown by way of illustration several illustrative embodiments of the mechanical features of our invention, v

Fig. l is a vertical central sectional view through a composite or built-up, rigid abrading tool in the form of a grinding wheel;

Fig. 2 is a fragmentary vertical sectional view, on a greatly enlarged scale, as seen along the line 2-2 of Fig. 1, showing certain features of one embodiment of our invention;

Figs. 3 and 4 are transverse sectional views as seen along'the lines 33 and 4A, respectively, of Fig. 2, being in effect greatly enlarged and somewhat exaggerated fragmentary transverse. sections like the section of Fig. 1; l

Fig. B is a-transverse sectional view as seen along the line 3'---3 of Fig. 2, showing, in enlarged and somewhatv exaggerated form, certain possible structural relationships of certain of the parts;

Fig. 5 is a fragmentary vertical sectional view, on a greatly enlarged .scale, as seen along the line 2-2 of .Fig. '1,.showing certain features ofanother embodiment of 'ourrxinvention;,, i ;.r

Fig. 6 is a transverse sectional view as seen along the line B-6 of Fig. 5, being in eifecta;

- greatly enlarged. and somewhat exaggerated 'vention.

Similar reference characters refer to similar parts throughout the several views of the drawings.

Our invention is best illustratedv with respect to the construction of a so-called cup wheel comprising a rigid or relatively inflexible abrasive annulus or ring In, which may be assumed to comprise a body of ceramic or vitrified material by and throughout which the abrasive grains are bonded, or, in the case of diamond abrasive grains, may comprise one or more external laydistributed and bonded the diamond abrasive grain, such as the layer Hi and a body portion Hl united or integrally formed therewith, as by utilizing a ceramic or vitrified bonding material throughout both portions; in the latter case the body portion 20 can include any suitable granular body material (abrasive or non-abrasive) bonded by the same or compatible vitrified bond as is used in the part Ill In any such case, and for convenience, we term the member 10 as the rigid or inflexible abrasive part; it and a companion member I I, also of rigid or inflexible material, are to be secured together, and, for purposes of illustration, the member Il may be of metal, such as steel or cast iron, being provided with any suitable means, which may take any one of a wide variety of known forms to mount or secure it to an appropriate part of the grind.- ing mechanism, such as a shaft, collar, hub, or

' the like.

In somewhat exaggerated form, there is indicated at !2 in Fig. l the means forming the junction between the rigid parts [I] and II in one embodiment, these are shown in larger scale in Fig. 3 and in another embodiment, in Fig. 6. They comprise a large number of substantially uniformly distributed, rigid and strong, individual masses which we term columns or posts, being indicated at H in Fig. 3 and also in Fig. 3 and at I3 in Fig. 6, and made of a hardened inflexible material which is preferably of a character such that it can be applied, preferably in the above-mentioned form of the columns or posts, in plastic state between the adjacent faces of the rigid parts it) and II and thereafter hardened, cured, or matured in situ, the material having in plastic form appropriate qualities such as adhesiveness to join or bond itself, at the respective ends of the posts, to the adjacent faces of the two rigid parts l and II.

The joining parts l3 and I?) are preferably of substantially uniform effective cross-section; and in the form of Fig. 3 they are preferably connected as is better seen in Fig. 2 by laterally extending webs M which may or may not, but preferably do, extend between the two faces of the rigid parts Ill and H, and which in either case can form laterally extending cross-braces joining adjacent masses or columns [3. the-cross-connecting webs [4 do not engage with the faces of the rigid parts H] and I I, they would have the general conformation, and relationship to the columns or posts, as is indicated in Fig. 3. They are made of the same material and when hardened they have qualities generally similar to those of the individual columns or posts H). Tog'ether the individual massesor columns l3 and their laterally extending cross-connecting parts [4 have, in the cross-section as seen in Fig. 2, a grid-like conformation,-which, as later pointed out, may be of any desired pattern, preferably of substantially uniform geometric design or configuration, illustratively, as indicated in Fig. 2, comprising a succession'of substantially equal hollow squares with a column [3 at each apex thereof; within each of the squares is a hollow space or cell l5 (see also Figs. 3 and 4) and these are therefore substantially uniformly distributed, representing voids, as better appears in Figs. 3 and 4, so that the connecting and bridging material between the rigid. parts l0 and ii is discontinuous, as seen along the line 2-2 of Fig. 1, and the cross-sectional area of the material joining' the parts Hi and H is thereby materially less than the cross-sectional area of either of the Where adjacent faces throughout which it is desired to secure the two parts together. Where the crossconnecting webs do not engage the adjacent faces of the rigid parts It and II, as shown in Fig. 3 these spaces or cel.s 15 are connected with each other, to greater or less extent, by way of the gaps between the webs l4 and the respective faces of the rigid parts Iii and H; these gaps are clearly shown in Fig. 3

The material of which the bridging and securing means I2 is made is preferably a resinous or synthetic resin composition having the characteristics above mentioned, being cured to hardened relatively rigid or inflexible condition. As

' better appears in Figs. 3 and 4, the parts 13 and their connected cross-bracing parts Hi are preferably of increasing cross-sectional area in both directions away from a central vertical plane which is indicated at AA in Figs. 3 and 4, being in that respect outwardly divergent in cross-section, and thus the parts l3 and the lateral crossconnecting parts 54 (where the latter engage the rigid parts H] and H) present to the faces of the rigid parts H3 and Ii greater areas for strong bonding or adhesion to the faces of the parts than is its minimum cross-sectional area. In fact, the entire areas of adjacent faces of the rigid parts ii) and H may in this manner he utilized, while retaining the voids or cells I5, so as to achieve maximum area of junction of the securing means SE to the adjacent faces.

The rigid abrasive annulus l6 and the rigid part H are thus rigidly secured together, and when forces are exerted, usually forces due to differentials in the thermal coeflicients of expansion of the rigid parts Iii and ii, relative movement, vertically as viewed in Figs. 1, 3, and 4, tends to take place between the parts :8 and ii, subjecting the individual rigid connecting masses to shear. Were the two rigid parts simply cemented together with a material that is hard and rigid upon setting or curing, the magnitude and directions of such shear are such that the rigid abrasive annulus cracks, usually in radial directions, with the cracking commencing at the smallest radius of the rigid abrasive annulus; breakage of this character usually results when the rigid abrasive annulus i0 is put into tension, that is, subjected to forces tending to increase its radii, and they can be brought about when the rigid part i i has a higher temperature coerlicient of expansion that the rigid part I8, so that the part ll increases in radius faster than does the vitrified bonded annulus ill. Or the forces of shear, were the rigid parts ii! and H to be rigidly secured together by a hard material, can simply cause failure of the hard cemented junction by actual shear thereof in whole or in part.

Such destructive results as these are avoided according to our invention, for, when forces arise tending to subject, or actually subject, the connecting means l2 to shear, the individual bridging parts i3 thereof, laterally unsupported in the regions of the voids or cells l5, can actually partake of bending and, though rigid and hard, are strained or deformed in a manner analogous, for example, to the deformation that a rigid, structural steel truss or cantilever undergoes when subjected to load. As a result, the strain set up by stresses in the one rigid part is not transmitted Wholly to the other rigid part because of the deformation or strain, in shear, of the numerous hard or rigid and small crosssec tioned bridging columns [3 and their cross-connections 14. For example, should the rigid part l l in Figs. 1, 3, and 4, upon rise in temperature, expand radially outwardly faster than does the rigid abrasive annulus H), the hard or rigid parts I3 and it are subjected to vertical shear, as along the vertical plane AA of Figs. 3 and 4, and they individually deform or bend somewhat, aided by their minimum central cross-section, and thus the radial outward movement of the expanding rigid part I l is not, or cannot be, completely enforced upon the rigid abrasive part the flexing of the hardened, rigid parts l3 and Id of Figs. 3 and 4 thus permits sufficient relative movement between the two rigid parts It! and H and protects the connecting means 12 as a whole against actual cleavage under shear. The construction, moreover, has good strength under the pressures of actual grinding operations, even if those pressures are directed radially or have radial components; and, where grinding pressures are exerted in directions parallel to the lengths of the individual columns or posts, as for example when exerted in directions toward the right as viewed in Figs. 1, 3, and 4, the junction-forming means 52, that is, the many individual posts or columns, has good compressive strength, the hard or rigid material of each of the many bridging increments taking the load without failing.

The connecting pillars or posts I3 are numerous, as above noted, and preferably uniformly distributed; for example, there may be several hundred voids or cells per square inch of cross-sectional area along the line 2-2 of Fig. They may, for example, be arranged in some suitable uniform geometric design, such as a checker-board design, with one pillar or post for each individual square which may be dimensioned to have, say 14 squares per linear inch; in such case there would be about 196 connecting elements 53 per square inch. According to our invention'we provide an inexpensive and economical way of achieving a construction of the above-described character and to facilitate the provision of a large number of individually small and substantially uniformly distributed individual connecting or bridging parts. Illustratively,

when we employ a curable or settable synthetic resinous cement, such as phenol-aldehyderes inous cement, we use the cement in fluid form of suitable viscosity and prefer initially to coat each of the faces, indicated at 0 and l respectively, of the rigid parts l0 and H, with one or more coats of. the cement, and then we provide a suitable means which can at once function as a carrier and distributor of such cement or subsequent applications of cement for the purpose of building up the numerous increments of uniformly distributed connectingbridges or parts I3 and i4 and to serve also as a spacing means between the two rigid parts is and H in order thereby to give the parts 13 appropriate length in a horizontal direction as viewed in Figs. 1, 3, and 6. Such a means, in the form of Figs. 2, 3, and 4, conveniently comprises strands of any suitable material, preferably arranged in any suitable way, as by weaving, to provide interstices therebetween and of suitable size. For example, the strands may be interwoven to form an open mesh fabric, with the interstices or apertures substantially square, and in Fig. 2 are indicated several horizontal strands H and several vertical strands V so arranged; thus, the material may be made up to have, for example, 14 meshes or squares per linear inch, in which case the material would have 196 squares or in- 513, wetting it, or it merges 6 terstices per square inch of area. The spacep carrier then has the resinous cement applied to its strands, and this may be achieved in any convenient manner.

For example, the open mesh structure may have the liquid form of cement painted onto it, or it may be clipped in the liquid cement, or the latter may be sprayed onto the strands, the cement 'if necessary being preferably given a viscosity appropriate to the character of the material of which the strands are made. For example, if the strands are made of textile fibers, it is desirable to impregnate them as well as coat thestrands, and in such case higher fluidity may be more suitable than would be the case if the strands were of a material that need not be im-- pregnated or that is impervious. Synthetic as well as natural organic fiber may be used, and also glass fiber; solid, and hence impervious, strands are also usable, and these may be of any convenient or suitable material. Strands or stranded materials, if pervious, may be and preferably are first treated in any suitable way so that they will not have a wicking or capillary action upon liquids such as water, and this may be done by impregnating them in any suitable way; illustratively they may be impregnated by suitably thin solutions of the same cementing material that is to be used in making up the connecting posts or pillars or in preliminarily coating the faces of the rigid parts ill and ll.

The open mesh spacer-carrier structure, coated heavily enough with the liquid or plastic cement which preferably does not fill the interstices thereinand which, if desired, may be partially or completely matured thereon, is now laid or spread out against one of the rigid surfaces to be connected together, for example, against the face ll of the rigid part H, which, as above noted, is preferably first coated with cement. As it is laid in place, and with the face already coated, the liquid or semi-liquid cement carried by the strands coalesces with the coating on the rigid surface or wets the latter if it has not previously been coated, spreading out materially beyond the dimension of the strands themselves, and, should one or more interstices of the spacer-carrier structure have been filled with liquid cement, its contents are drawn to the rigid surface and spread out or intermingled with other regions of cement on the surface.

Then the companion rigid part, such as the member if! with its rigid face i preferably coated, is placed on top of the assemblage of the rigid part if and the spacer-carrier, whence cement from the strands attaches itself to the face with the cement coating previously applied to the face it, and thus the general configuration of the voids or cells I5 is brought into being, the strands V H of the 'spacer-carrier preventing where they cross each other at the .apexes of the squares, such close approximation of the two rigid parts it and I! as would obliterate the voids and as would make the cement continuous or uninterrupted by voids throughout its entire extent between the two rigid faces W and il. This action of the crossing portions of the strands V and H, in lim iting the approach of parts it and ii, is indicated in Figure 3 where the cross-sections of the crossing portions of the strands are shown more or less of oval shape due to the compressive forces exerted thereon, necessarily at the apexes,

of the square meshes of the spacer-carrier structure where the thickness of the latter is other- 7. Wise greatest because of the crossing of the strands at these apexes; in contrast, the lengths of these strands intervening the apexes are substantially uncompressed as is indicated by the circular cross-sections of the strands as pertrayed in Figure 4. The assemblage is then preferably subjected to some pressure, illustratively, on the order of a few pounds per square inch, insufficient to destroy the cells or voids I5, the application of the pressure being primarily to insure the making of good contact between he now plastic or semi-fluid columns or posts 13 (and where desired, also the cross-connectors i4) and the rigid faces iii and i I the action insuring enlargement of the areas of contact, accompanied also by elongation in the vertical plane AA of Figs. 3 and 4 ofthe cells l5 which are prevented from being filled by cement or from being obliterated by the air that is trapped in them when the companion rigid part, such as the abrasive annulus lb, was placed in position in course of assembly. The moderate pressing together of the assemblage lessens the horizontal dimension of the cells l5, as viewed in Figs. 3 and e, and the resultant compression of the entrapped air has the effect of enlarging the vertical dimension thereof, as viewed in Figs. 3 and 4, thus, also, contracting the cement of the columns l3 and parts it and sustaining the still plastic or semi-fluid material thereof against collapse, aided by the strands of the spacer-carrier structure which also functions to limit the approach between the two rigid parts i El and l i. It is such actions as these that give the individual columns or supports 13 and cross-connecting parts is cross-sections generally like those indicated in Figs. 3 and 4, and they may be aided by the facility with which the cement wets the rigid surfaces iii and l i, particularly if the latter are preliminarily coated with cement.

The cement is now allowed to harden and any suitable treatment, depending upon the character of the cement, is here applicable. Synthetic resin cements are available that cure at ordinary room temperature; others are available that require some heat treatment, and in such cases the assemblage subjected to appropriate heat treatment to harden the cement. During the cure or setting, the moderate pressure above mentioned is maintained, and a simple way of doing that is simply to put a weight upon the last of the several parts to be assembled. As the cement sets, the strands H and V become solidly encased or embedded therein and in the final structure they need not take part in actions that ensue when the completed rigid grinding structure is subjected to forces like those earlier above described.

Or we may employ cements that undergo shrinkage during cure or utilize the factor of shrinkage which various usable cements are known to have. In such case we apply the ce ment in liquid form to the open-mesh spacercarrier element above described in such manner as to be certain that the interstices are well filled with the liquid or plastic cement; it may be assembled to the rigid parts ill and H, preferably with the faces thereof initially coated, all in the illustrative manner above described, and the assemblage then subjected to some pressure, which may again be on the order of several pounds per square inch, in order to bring the two rigid parts toward each other to the desired extent, all as limited by the action of the strands as above described.

Curing or setting may then be proceeded with and as the cement cures or sets it shrinks, shrink ing onto or about the strands V and H, and in so doing creating the voids or cells 15 by the coaction of the strands V and H with the shrink-- ing cement; by reference to Fig. 2 it will be seen that the strands V and H extend around individual areas or increments of the rigid surfaces to be joined together, these areas being of square shape in the illustration. As the cement shrinks and undergoes movement as the result of the shrinkage, the strands impede such movement while the cement within the areas is unimpeded, and hence it is cement within those areas that partakes of movement outwardly away from the center of each area that forms the voids or cells. In other words, those portions of the cement that are in these areas and hence not impeded from partaking of movement are shrunk or moved into or toward those portions of the cement that are restricted against movement by the strands V and H.

In Figs. 5, 6, '7, and 8 we have shown another embodiment of certain structural features of our invention and of our method, and reference may first be made to Fig. '7 in which a fragment of one of the rigid parts such as the part I l is shown in cross-section; to its face II we apply a layer L of the cement in suitable plastic or semi-liquid form preferably after havin first applied and set a coating of cement to the face H as is preferably done in carrying out the methods above described in connection with Figs. 2, 3, and 4. With the part II positioned so that the face H and hence the layer L are horizontal, we now distribute, in any suitable manner and as nearly uniformly as possible, a suitable number of pellets or granules G which are preferably uniformly dimensioned or of substantially the same shape and size, made of a material having such physical characteristics that the pellets or granules employed have preferably little or no resistance to shear. For example, the material may be cork or rubber so that the pellets or granules do not transmit substantial shear stresses; such materials can be comminuted or broken up in any suitable or known way, and they are sized by screening so as to provide substantial equality of size or dimensions. A wide range of materials is available for making these pellets. By way of further example, the pellets may comprise vegetable matter in the form of certain seeds. For example, poppy seeds which are generally spherical, more or less uniformly sized, and capable of size selection by screening; individually they appear to have some rigidity, but structurally they have relatively small resistance to shear; Seeds, if employed, are preferably first thoroughly dried and, according to their nature, may be impregnated if desired.

The pellets G are spread onto the relatively thick layer L of cement which preferably is in a form having appropriate liquidity so that it will wet the pellets, being thereby drawn toward or onto the pellets from the areas or regions intervening the pellets, thinning itself out throughout such regions somewhat in the manner indicated in exaggerated form in Fig. 7. The pellets may be distributed in appropriate spacings, for example, 8 or 10 or so per linear inch or about or so per square inch. The companion rigid part Ill (see Fig. 8) next has its face Hi supplied with a layer L of the cement, preferably after having applied and set thereon a thin coating of the cement, and with the layer L thereof facing downwardly, the rigid part it may now be brought down onto the above-described assemblage of the rigid part i l, its cement layer L and the pellets G held in and on the cement layer L Conveniently the layer L is of smaller mass or thickness than the layer L and those portions of the pellets G that are exposed upwardly are engaged by the cement of layer L and in effect are embedded therein.

Suitable pressure, on the order of several pounds per square inch, may now be applied to bring the two rigid parts l0, ll together to the desired extent, effectively limited by the uniform vertical dimensions of the pelletsG as viewed in Figs. '7 and 8. Curing or setting may now be proceeded with. v

If the cement in layers L and L is applied in insufficient quantity to fill all of the space about the pellets G and inbetween the two rigid parts and Il, voids I are again formed inasmuch as the cement of one layer can join itself to the cement of the other layer only through the medium of the pellets G, the surfaces of which the cement envelops. The pellets G, in this mode of carrying on this embodiment of our invention, prevent such a close approach of the two rigid faces 16 and H as would permit the cement of one layer joining with the cement of the other layer throughout their entire areas.

On the other hand, we may employ, for the layers L and L of Figs. '7 and 8, cements having the shrinkage characteristics above mentioned and applythem in greater quantity than just described, namely. a quantity sufiicient to fill all of the space between the two rigid faces lll and li as the volume of that space is determined by the action of the pellets G in limiting the approach of the two rigid faces H3 and H toward each other. When cure or setting proceeds, the cement shrinks and againthose portions that are unimpeded shrink or move to those portions, namely the portions of the cement, about the 5 functions as above described. It is economical? to manufacture from the viewpoints of both structural elements and procedural steps employed. The multitude of relatively small cross-v sectional areas of junction between the two rigid parts are easily and withfacility achieved, par-- ticularly when there is employed the unitary stranded spacer-carrier structure like that of Figs. 2, 3, and 4, the strands of which, accord sembled of the multitudinously distributed increments of cement, 'care being taken that theamount of cement in fluid form be appropriately proportioned, as will now be clear, to the sizes pellets G that are restricted or held from migration by the pellets themselves.

In either case the structure, when cure or setting is completed, appears in general as isshown in Figs. 5 and 6 wherein the individual pillars or columns are indicated by the reference character I3 each column having within it and encompassing a pellet G which is solidly encased and embedded in the cement. The columns or supports it will be seen to be individual or uniformly distributed; they are rigid and strong, and. in their action absence of resiliency therein is not detrimental. formly sized pellets, they are of substantially uniform effective crossse'ction so that each does the same amount of work as any other. Furthermore, due to such actions as those above described, the pellets are effective to give the joining posts or columns 13 cross-sectional areas as viewed in Fig. 6 which are progressively increasing in both directions away from a central Vertical'plane indicated at AA in Fig. 6; they are thus outwardly divergent in cross-section, and each part presents tothe faces of the rigid parts I!) and I! greater areas for strong bonding or' adhesion than is its minimum cross-sectional area. In that manner also the entire areas of the adjacent faces NB and H of the rigid parts may be utilized for achieving maximum area of junction of the securing means iii to the adjacent faces. H

" Elhe'resultant structure of either Fig. 2 or Fig.

By using substantially uniof strands and the sizes of the meshes or interstices in the carrier struc ure.

four inches, a suitable and illustrative spacercarrier to employ is mosquito netting of about 16 mesh.v The thickness of the strands, the ma-" terial thereof, and the size of the interstices of the spacer-carrier may bewidely varied accordof the pellets may be widely varied according to circumstances, as will now alsobeunderstood It will also be understood that thoughthe rigid abrasive part I0 is above described as vitrifiedbonded, our invention is not limited tovitrifiedbonded abrasive structures, but' 'that the rigid abrasive structure may embody any other suit able or known bonding medium or bond structure; also the description of the rigid'part I as: of metal is not to be interpreted as a limitation,- since that part may be of any other suitable ma terial, for example, powdered aluminum bonded by resin or a resinoid. In any such cases', 'where forces or stresses of the kind earlier above 'me'ntioned are brought into play, due principally 'to differences in thermal coefficients of expansion," or to centrifugal action," their detrimental effects may be reliably overcome according to our-in vention. 3 5

Because of the unique actions of the individ-E ual increments of bridging connectionfprovided by parts like the parts l3 and M when sub-- jected to shear, we areenabled, if desired, to carry out our invention in a manner to give theserigid bridging increments agreater range 0? action. Thiswe may do by using a cement in terial that requires application of heat to'cu're and harden it so that the heat treatment asifi an oven, of the assembly raises the temperature of the rigid parts I!) and l I to expand them to the different extents according to their respective temperature coeffi-cients of expansionklri such case, if the member H of Fig. 1 is the one that has the higher temperature coefficient, thj bridging increments is or 13 and the like are} set and hardened to rigid condition to secure the two rigid parts Ill and II together in their differentially expanded condition. Upon coolingoff, the twoparts contractto respectively diff ferent dimensions corresponding to normal -or room temperature, and the many bridging or securing increments are thereby "strained to-put the rigid abrasive part H! in compression,- the forces of compression acting radially inwardly in-a direction to tend to reduce the radii of-the part lllglthe companion rigid part I i would-Basa 'r'es'ult; be in tension. When the resultantfg-rind- For small-diamx-t: etered grinding wheels, on the order of three or ing" wheel is then put into use, it rises in temperature, due to the heat generated under the abrading action of the rigid abrasive part H, and the latter expands, as does also the rigid part H, as temperature equilibrium throughout the entire structure is reached, and hence the initial strain to which the securing columns or posts are subjected becom'es" relieved, in whole or inpart, so that the operation of the grinding wheel can continue with shear stresses, due to different temperature coefficients of the parts it and I I. in whole or in part'eliminated from the connecting columns or increments. The latter can thus better cope with the stresses otherwise imposed by the rotation of the grinding wheel and its grinding operation."

. A preferred manner in carrying out this aspect of our invention is to utilize a cementing material that cures at a temperature above room temperature but substantially at or below the intended operating temperature of the abrasive structure; for the latter, the upper limit is usually around 100 0., the boiling point of water, which is frequently employed as a coolant in grinding operations. We prefer to employ a cement that cures at a temperature intermediate ofthe operating temperature of the grinding wheel and room temperature, those being the upper and lower temperature extremes to-which the abrasive structure is subjected in ordinary use. preferred and illustrative curing. temperature is in the neighborhood of 65 C. and various syn thetic resin cements curable at this temperature as well as cements curable within the preferred range above mentioned are available.

With a cement that hardens or matures at this intermediate temperature, the connecting posts or columns It or iii become set when the two rigid parts, whatever their differences in thermal coefilcients, are dimensionally less changed relation to each other than if the temperature of maturing or curing were outside of the normal or ordinary range of change of temperature of the ultimate abrasive structure; upon. cooling down to room temperature, comparatively small relative dimensional change of the rigid parts l and H takes place, subjecting the connecting posts or columns to a correspondingly small strain or bending in one direction. At around room temperature, therefore, the connecting posts are normally under some strain, a strain which becomes less and less as the grind ing structure warms up when it is put into operation. When the abrasive structure-reaches the temperature at which the material of the connecting columns was matured, for example, a temperature of 65 0., the columns are free from strain caused by difierentials in. thermal coefiicients, and as the. temperature of the abrasive structure continues to rise to, for eX- ample, 100 0., to operating temperature, the columns are strained in opposite directions but to a far lesser extent than would be the case if the entire rise in temperature had beenedective to cause strain in the same direction. As a result it is possible to cut the range of change of strain in the connecting columns down to about 50%.

As earlier above noted, however, so-called air drying cements, namely, compositions that cure at ordinary room temperature, may be employed in carrying out ourinvention.

{The junction I2 ofFig. 1, being made up of a large number of columns or posts-isee Figs. 3, 4, new ha a le ser capacity; -.seadu ve f.

heat from the abrasive part It to the rigid sup porting or backing part H than it would have if its resinous composition were solid throughout the entire junction; the cross-sectional area' of the junction (see Figs. 2 and 5) is materially less than its cross-section would be were it to be solid throughout, and hence conduction of heat does not so readily take place. This lower capacity for heat transmission'is enhanced also by the voids or hollow spaces 1 5 in Fig. 2 and by the connected spaces I5 of Fig. 5, and may also be aided by the material of which the parts H and V of Fig. 2 or the parts G of Figs. 5 and 6 are made, if such material is selected to have a heat conductivity less than that of the cement composition employed. Hence a substantial temperature difierential is maintainable between the part 19 and the part i i, so that the part H, if it is the part of higher. thermal coeflicient of expansion, partakes of diminished increase in its dimensions, and the connecting posts or columns of the junction l2 are subjected to a correspondingly less strain or deflection. Where the junction. i2 is of the kind described in connection with Figs. 5 and 6, the spaces between the connecting columns 13 are all connected and open to the atmosphere, bothat the inner and at the outer circumferences of the junction l2 (see Fig. 1), and thus heat-withdrawing movement or circulation of air from the atmosphere can take place therethrough, and, when embodied in a grinding wheel form as shown in Fig. I centrifugal forces aid in discharging such heat-withdrawing air from the outer periphery of the junction 12, fresh air as a result being drawn radially inwardly of the junction l2 at its inner circumference. Such air cooling throughout the junction further lowers the rate of possible heat transmission from the abrasive part i 9 to the companion part II, thus enhancing the above-described advantageous result. If a liquid coolant is employed during the grinding operation, an analogous action takes place in forcing the liquid coolant centrifugally through the connected spaces in the junction, often-times materially aided by the above-described movement of air therethrough where the liquid coolant is concentrated principally at the point or line of grinding contact of the abrasive part with the Work-piece being ground. Strain of the junction,' due to thermally responsive dimensional changes in the joined-together parts, can thus be greatly reduced. Generally similar coactions and advantages are achievable where the junction [2 is of the kind described in connection with Fig. 3 the spaces or cells I5 being interconnected and open to the atmosphere, both at the inner and outer circumference of the junction l2 (see Fig. l), the interconnection of the spaces 15 being effected, as above described, by the gaps (see Fig. 3 between the cross-connecting webs M and the respective faces of the two rigid parts I!) and II.

By providing a series of successive heat-barrier junctions in the path of flow of heat from the abrasive surface to the part II, We are enabled to provide an abrasive structure of improved action and of certain desirable unique coactions amongst its several parts, as is illustratively shown in Fig. 9, in which the abrasive part 1 0 may comprise a structure having a vitrifled-bonded diamond abrasive layer W and a vitrified-bonded body portion "l such as de scribed above in connection with Fig. 1, and a rigid part II, such as that above described; in-

terposed between the two parts l and H of Fig. 9, however, are several heat-barrier junctions l2, illustratively three in number, alternated with one or more, illustratively two, rigid parts P and P preferably constructed and composed in the same manner as the part It", and hence being also vitrified-bonded, conveniently and illustratively comprising suitable granular body material such as fused alumina, vitreous silica, feldspar, clay, or the like, a suitable bond of a Suitable glass, the parts l0 and P and P being preferably of similar composition and having the same thermal coefficient of expansion. The succession of parts, as indicated in Fig. 12, are socured together by junctions l2 constructed preferably in the manner and with structural features illustratively above described, and the intermediate parts such as parts P and P may be employed in any number and in any desired axial dimension or thickness. For cup wheels of substantial or large depth or axial dimension, the interposed parts may be relatively thick axially and may be made up of uniform dimensions to thus lend themselves to facilitate of building up a cup Wheel of any desired axial depth. Where the cup wheel is relatively shallow as in Fig. l, the part I!) may be made up in smaller axial thickness and likewise the interposed part or parts such as parts P and P in order, within the shallow cup depth, to get the desired number of heat'barrier junctions l2.

Each of the succession of junctions l2, functioning thermally as above described, can thus be effective to maintain a substantial tempera t re differential between the parts which it secures together, so that the usually substantial rise in temperature of the abrasive part Hi, from room temperature to operating or grinding tem perature, is step by step precluded from being communicated to the mounting or supporting part it which, if made of metal or other mate rial having a markedly different thermal coefficient of expansion, is thus precluded'from partaking of substantial dimensional changes and hence from subjecting its immediate junction !2 to substantial strain. Though the overall temperature differential, that is, between the part It] and the part H of Fig. 9, may thus be substan tial, the succession of interposed heat-barrier junctions has the effect of subdividing that overall temperature differential into a succession of fractional temperature differentials between the successive rigid parts, so that their respective junctions are subjected to but fractional strains due to their relative dimensional changes. These actions, moreover, are of amplifying or multiplying effect in the direction from the part of high est temperature to the part of lowest temperature, since the rate of heat flow through any one junction is a function of the difference between the absolute temperatures of the two parts joined together, and in each case that difference is but a fraction of the difference in temperature between the parts it and l I. By such cumulative coaction of parts such as those just described, we are enabled with safety and efficiency "and economy to cope with large temperature dife 'ferentials between the parts it and H of substantially different thermal coefficients of expansion, illustratively with the part H having the higher coefficient of the two.

Though in the illustrative embodiments above described we have illustrated our invention with lrlc'spectto a grinding wheel,..the invention .is to be interpretedas embodying, or as applicable to,

any other abrasive articles, such as an abrasive stick, wheel segment, or the like, for which it is desired to achieve the many advantages of the invention, and in the claims the term grinding wheel, unless otherwise qualified, is therefore to be interpreted to include any such abrasive article.

It will thus be seen that there has been provided in this invention a construction and method of achieving the same in which the various objects hereinbefore noted, together with many thoroughly practical advantages, are successfully achieved. The junction or joining means effected according to our invention, though comprising individual resinous cement parts, can be made to have advantageously a low modulus of elasticity, being as a joint less rigid than the rigid abrasive member It that is oined to another rigid part;

havinga low modulus of elasticity, it can deflect under stress, yet the iniividual hard and relatively rigid connecting columns under compression do not permit of relative out-of-truth shift of one of the parts relative to the other as has been known to happen, under some grinding conditions, where the two parts are secured together 'by resilient cement as has heretofore been used in the endeavor to overcome the difijcu ties posed by the different thermal coefiicients of expansion.

With our invention we are enabled to employ cements which mature to a hard and relatively rigid condition and to cause them to function, under the peculiarly difficult and changing conditions wrought by the substantial and otherwise uncontrollable stresses and strains brought into being by centrifugal forces and by thermal changes, not only dependably to secure the two rigid parts together but also to lessen, or avoid, risk of failure, as by rupture in shear as could happen according to prior attempts to utilize such cements.

It will thus be seen that there has been provided by this invention an article and a method in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the mechanical features of the above invention and as the art herein described might be varied in various parts, all without departing from the scope of theinvention, it is to be understood that all matter hereinabove set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. A grinding Wheel comprising a rigid abrasive part and a rigid supporting part adapted to be rotated, said two'parts having different thermal coefficients of expansion and therefore responding with different dimensional changes. to changes in temperature throughout the range from normal or room temperature to grinding wheel operating temperature, said two rigid parts having adjacent spaced surfaces throughout which they are secured together, and means securing said two rigid parts together and permitting relative dimensional changes to take place therebetween in response to thermal changes and to centrifugal forces, said means comprising an initially substantially plastic cementitio s material maturable at a temperature intermediate of the aforesaid iimiting temperatures of said temperature range, said cementitious material being matured at said intermediate temperature, and with both of said rigid parts raised to said intermediate temperature, to a sub stantialiy rigid condition in the form of a D 1- rality of distributed and spaced compression-resistant masses of said material matured to said rigid condition and bridged between and joined at their respective ends to said spaced adjacent surfaces of said two parts whereby said rigid masses, upon the latter and said joined-together two parts reaching normal or room temperature, are individually strained in shear in one direction and, upon subsequent rise in temperature to grinding wheel operating temperature, are individually strained in shear in opposite direction.

2. An abrasive article comprising a rigid abrasive part and a rigid supporting part, said two parts having different thermal coemcients of expansion and therefore responding with different dimensional changes to changes in temperature throughout the range from normal or room temperature to operating temperature, said two rigid parts having spaced faces throughout which they are secured together, and means securing said two rigid parts together comprising at least one interposed rigid part to provide a pluralit of pairs of adjacent surfaces throughout which the k parts are to be secured together and a securing means between and joining together each pair of adjacent surfaces of said plurality of parts, the securing means being made of a matured cementitious material and having relatively low heat conductivity and thereby interposing a plu-, rality of barriers between said rigid abrasive part and said rigid companion part and each resistive of the transfer of heat therethrough and thereby oppose lessening of the temperature differential between said last two mentioned parts.

3. A cup type of grinding wheel comprising a plurality of bonded annuli adapted to be superimposed in a series one upon another in axial direction, the bonded annulus at one end of the series comprising abrasive grains and presenting an abrasive face for grinding and a rigid supporting part adjacent the annulus at the other end of the series, said supporting part and said end abrasive annulus having difierent thermal coefficients of expansion, and means forming junctions between the adjacent faces of said annull and between the adjacent faces of said supporting part and of the bonded annulus at the other end of said series, said junction-forming means comprising matured cementitious material, and means giving the junction-forming means a lesser heat conductivity than the heat conductivity of the matured cementitious material per se, thereby to interpose between the abrasive annulus and the supporting part a plurality of serially related barriers resistant to the flow of heat.

4. A cup type of grinding wheel comprising a plurality of bonded annuli adapted to be superimposed in a series one upon another in axial direction, the bonded annulus at one end of the series comprising abrasive grains and presenting an abrasive face for grind ng and a rigid supporting part adjacent the annulus at the other end of the series, said supporting part and said end abrasive annulus having different thermal coefiicients of expansion, and means forming junctions between the adjacent faces of said annull and between the adjacent faces of said supporting part and of the bonded annulus at the other end of said series, said junction-forming means comprising an initially substantially plastic cementitious material maturable ata temperature intermediate in the range from normal or room temperature to grindingwheel operating temperature, said cementitious material being matured at said intermediate temperature and with all of said annuli and said supporting part raised to said intermediate temperature to a substantially rigid condition in the form of a plurality of distributed and spaced relatively rigid securing elements of said material matured to said substantially rigid condition and cementitiously joined at their respective ends to the respective adjacent surfaces whereby said securing elements, upon the latter and said joined-together annuli and supporting part reaching normal or room temperature, are individually strained in shear in one direction.

5. The steps in a method of making a grinding wheel that comprises a rigid abrasive part and a rigid companion part that has a different thermal coefficient of expansion than said rigid abrasive part with at least one rigid part interposed therebetween, said steps comprising arranging said parts serially and interposing between adjacent surfaces of successive parts a combined carrier and spacer means comprising an open-work fabric of which the strands carry cementitious material in insufiicient quantity to completely fill, upon setting, the interstices between said strands, and treating the assemblage to set the cementitious material and thereby join successive parts together, whereby the said cementitious material has distributed substantially uniformly therethrough a plurality of hollow spaces to lessen the effective heat conductivity of the resultant junction below that which it would have were it of solid cementitious material, thereby to interpose a series of barriers resistive to flow of heat between the endmost members of said serially-arranged parts.

6. The steps in a method of making a grinding wheel that comprises a rigid abrasive part and a rigid companion part that has a different thermal coefficient of expansion than said rigid abrasive part, said steps comprising placing between and in contact with the adjacent spaced surfaces of the two parts a plurality of substantially uniformly distributed relatively small masses of an unmatured cementitious material that is maturable at a temperature intermediate of roomtemperature and grinding wheel operating temperature, holding the two parts in spaced relation, maturing the cementitious material of the plurality of masses by heat-treating the assemblage at said intermediate temperature to thereby dimensionally change said rigid parts to respective values corresponding to temperature increase thereof to said intermediate temperature and to mature said masses in the form of a plurality of distributed and spaced securing elements bridged between and joined at their respective ends to the adjacent surfaces of said rigid parts, and then cooling the assemblage to room temperature whereby said securing ele ments are individually strained in shear in one direction by the resultant relative dimensional changes in said two rigid parts.

7. The steps in a method of making a grind: ing wheel. that comprises a rigid abrasive'part and a rigid companion part that has a different thermal coeflicient of expansion than said rigid abrasive part, said steps comprising placing between adjacent surfaces of said parts an unmatured cementitious material maturable to role-'- tivelyrigid condition at a temperature inter,- mediate of room temperature and grinding whee] operating temperature, heating the resultant as semblage to said intermediate temperature to thereby effect corresponding relative dimensional change between said two rigid parts and to mature said cementitious material at said intermediate temperature and rigidly join together said two parts of the grinding wheel while dimensionally changed due to temperature increase to said intermediate temperature, and cooling the resultant assemblage to room temperature whereby the junction formed by the matured cementitious material is strained in shear in one direction and upon subsequent rise in temperature,

to grinding wheel operating temperature is strained in shear in opposite direction.

8. A grinding wheel comprising a rigid abrasive part and a rigid companion part adapted to be rotated, said two parts having different thermal coeificients of expansion and having adjacent spaced surfaces throughout which they are to be secured together, and means securing said two rigid parts together and permitting relative dimensional changes to take place therebetween, said means comprising an initially substantially plastic cementitious material matured to substantially rigid condition in the form of a multiplicity of diminutive individually small-crosssectioned and substantially uniformly distributed hardened and rigid compression resistant elements bridged between said space-d adjacent surfaces of said two rigid parts and with their axes substantially at right angles to said adjacent surfaces, each element being bonded at its respective ends to said two surfaces, said cementitious material having embodied therein smalldimensioned spacer elements comprising a pluthat take place between said two rigid parts in the general directions of their adjacent surfaces into a multiplicity of strain effects respectively at said small-cross-sections thereof of a magnitude insufficient to cause rupture'in shear.

9. A grinding wheel comprising a rigid abrasive part and a rigid companion part adapted to be rotated, said two parts having different thermal coefficients of expansion and having adtributed hardened and rigid compression resistant elements bridged between said spaced adjacent surfaces of said two rigid parts and with their axes substantially at right angles to said adjacent surfaces, each element being bonded at itsrespective ends to said two surfaces, said cementitious material having embodied therein small-dimensioned spacer elements comprising a plurality of individual pellet-like members substantially uniformly dimensioned to effect substantially uniform spacing between said two spaced surfaces of said two rigid parts and substantially uniformly distributed between the lat ter, said pellet-like elements being individually enveloped by and encased within the matured cementitious material, said small-dimensioned spacer elements serving, while the cementitious material is in initial plastic condition, to space said two rigid parts from one another and substantially to determine the axial length of said compression resistant elements in the direction from one of said spaced surfaces to the other, said multiplicity of small-cross-sectioned hard and rigid elements converting stresses caused by relative dimensional changes that take place between said two rigid parts in the general directions of their adjacent surfaces into a multiplicity of strain effects respectively at smallcross-sections thereof of a magnitude insufficient to cause rupture in shear.

LOWELL H. MILLIGAN,

\ ROBERT H. LOMBARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,767,821 Thompson June 24, 1930 1,970,835 Benner Aug. 21, 1934 2,048,905 Webster July 28, 1936 2,065,941 Lane Dec. 29, 1936 2,065,942 Lane Dec. 29, 1936 2,070,764 Webster Feb. 16, 1937 2,353,864 Wooddell July 18, 1944 2,379,544 Scutt July 3, 1945 2,479,078 Milligan et a1. Aug. 16, 1949

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