US3427759A - Prestressed grinding wheel - Google Patents

Description

Feb. 18, 1969 s. s. KlsTLER ETALjv PRESTRESSED GRINDING WHEEL Sheet 2 of2 Filed Aug. 25, 1965 United States Patent O 3,427,759 PRESTRESSED GRINDING WHEEL Samuel S. Kistler, Salt Lake City, Utah, and Charles V. Rue, Tiliin, Ohio, assignors, by mesne assignments, to International Telephone and Telegraph Corporation Filed Aug. 25, 1965, Ser. No. 482,431 U.S. Cl. 51-206 4 Claims Int. Cl. B24d 5 00, 7 00 ABSTRACT OF THE DISCLOSURE The present invention broadly relates to abrasive articles, and more particularly, to abrasive grinding wheels such as snagging wheels incorporating therein a reinforcing network enabling the grinding wheel to be rotated at substantially higher peripheral speeds without fear of fracture or disintegration of the abrasive wheel structure. More specifically, the present invention is directed to a novel reinforced grinding wheel incorporating a reinforcing network embedded and interlocked in the abrasive matrix which is prestressed in a manner so as to apply a residual compressive stress in a tangential direction which is effective to neutralize or offset the subsequent tensile stresses imposed on` the abrasive matrix during rotation of the wheel enabling thereby the attainment off-substantially higher peripheral speeds than with similar grinding wheels heretofore know The development of various reinforcing materials foruse in grinding wheels has permitted a continuing increase in safe operating speeds which hitherto had been regarded as far beyond practical limits. The impetus for higher operating speeds of grinding wheels is principally dictated by considerations of increased economy `and increased elliciency obtained in metal removal operations. For example, in the connection with resinoid bonded grinding wheels of the types employed in snagging steel billets,v

past experience has shown that at grinding speeds up to about 16,000 surface feet per minute (s.f.p.m.) in both factory operation and in controlled laboratory tests, the cost of steel billet snagging is materially reduced by an increase in wheel speed. This reduction in cost is mainly in labor and overhead due to the fact that metal removal per minute increases steadily with increases in speed and with no evidence of an upper limit. In View of the foregoing, maximum peripheral speeds of 9,500 s.f.p.m., which formerly were considered the upper practical limit, have now been displaced by speeds of l12,500 s.f.p.m., and in some limited instances speeds of 16,000 s.f.p.m. are in commercial use.

The upper practical grinding speed limitation is dictated by the safety hazard that is presented to operating personnel and the equipment itself as a result of the fracture and high speed disintegration or bursting of a grinding wheel operating at high speeds. The failure of grinding wheels is generally a progressive deterioration first evidenced by the development of a plurality of radial cracks through the abrasive matrix which extend outwardly toward the periphery of the wheel resulting in a substantial reduction in the strength and in the integrity of the abrasive matrix. For this reason it has heretofore been customary to discard large heavy duty grinding wheels 3,427,759 Patented Feb. 18, `1969 such as snagging wheels, for example, as soon as the development of such radial cracks were observed in spite of the fact that a grinding wheel in such condition may nevertheless provide many additional days of satisfactory operation. 'In spite of such precautions, however, grinding wheels do break occasionally sometimes resulting in serious injury to the operation as well as frequently causing serious physical damage to the attendant grinding equipment.

Another problem which still further aggravates this situation is the pretesting to which vabrasive grinding wheels are subjected by ethical manufacturers before shipment t-o a purchaser. In order to assure a safe operating margin, ethical manufacturers conventionally test grinding wheels prior to shipment at peripheral speeds about 50 percent higher than the intended commercial operating speed. It is not unusual under such testing to cause cracks or fissures to form in the abrasive wheel matrix as the result of the excessively high speeds employed wherein the differences in the modulus of elasticity of the reinforcing networks employed and the modulus of elasticity of the abrasive matrix results in the imposition of stresses in the abrasive matrix beyond their rupture points. Conventionally, the reinforcing network is adequate to prevent the wheel from bursting but the presence of such cracks or fissures in the abrasive matrix necessitates a rejection of the wheel for commercial use which constitutes a rather costly loss of valuable materials and manpower.

It is, accordingly, a principal object of the present invention to provide an improved reinforced grinding wheel which overcomes the problems and disadvantages associated with grinding wheels of similar types heretofore known.

Another object of the present invention is to provide av novel'method for making prestressed reinforced grinding wheels which are substantially superior to grinding wheels of similar type heretofore known.

A further object of the present invention is to provide a novel, prestressed reinforced grinding wheel incorporating a prestressed reinforcing network embedded therein and interlocked with the abrasive matrix and prestressed in a manner so as to apply a residual compressive stress in a tangential direction upon the abrasive matrix which is effective to neutralize or offset at least a. portion of the stresses imposed on the abrasive matrix during operation of the wheel.

A still further object of the present invention is to provide a prestressed reinforced -grinding wheel which is of superior strength enabling operation thereof at substantially higher operating speeds than heretofore considered practical thereby enabling substantial increases in the economy and efficiency of commercial metal removal operations.

Yet still another object of the present invention is to provide a method for making prestressed reinforced grinding wheels which assu-res the attainment of a controlled residual stress in the resultant abrasive matrix and which further assures consecutive manufacture of high quality grinding wheels which can be safely operated at higher speeds than those heretofore employed Without fear of rupture or disintegration thereof.

The foregoing and other objects and advantages of the present invention are -achieved by forming a grinding wheel comprising a plurality of abrasive grains and a bonding agent and embedding in the abrasive matrix during the molding or shaping thereof, a reinforcing network which is placed under tension during the setting of the abrasive matrix such that upon release of the tension on the network, a residual compressive stress in a tangential direction is imposed on the abrasive matrix comprising the wheel. The reinforcing network is further characterized as preferably incorporating surface irregularities therein effecting an interlocking thereof with the abrasive matrix forming the body of the wheel. The prestressing of the abrasive matrix by the reinforcing network embedded therein is controlled so as to impose a residual compressive stress below the compressive strength of the abrasive matrix.

Other objects and advantages of the present invention will become apparent upon .a reading of the following description, taken in conjunction with the accompanying drawings, wherein:

FIGURE l is a perspective view of a typical grinding wheel to which the present invention is applicable;

FIGURE 2 is a pl-an view partly in section of a grinding wheel formed with an arbor through the center thereof and incorporating a reinforcing network in accordance with one of the embodiments of the present invention;

FIGURE 3 is a magnified transverse sectional view through the reinforcing ring embedded in the grinding wheel shown in FIGURE 2 and taken along the line 3 3 thereof;

FIGURE 4 is a side elevational view partly in section of a mold employed for forming the grinding wheel shown in FIGURE 2 and the means for effecting a tensioning of the reinforcing ring embedded therein;

FIGURE 5 is a plan view of an alternative satisfactory mold and reinforcing network for forming grinding wheels in accordance with the practice of the present invention;

FIGURE 6 is a magnified fragmentary transverse sectional view through the mold ring shown in FIGURE 5 and taken along the lines 6--6 thereof;

FIGURE 7 is a fragmentary magnified side elevational view of the mold ring shown in FIGURE 5 as viewed in the direction of the arrow indicated at 7;

FIGURE 8 is a plan view of still another alternative satisfactory construction of a mold ring including means for tensioning a polygonal shaped reinforcing element during the manufacture of the prestressed grinding wheel comprising the present invention;

FIGURE 9 is a plan view of still another satisfactory mold and radial reinforcing network which is particularly applicable for the manufacture of arborless-type grinding wheels;

FIGURE 10 is a side elevational view of the mold shown in FIGURE 9, and

FIGURES 11 through 14 are fragmentary perspective views of typical configurations of reinforcing elements incorporating surface irregularities along the lengths thereof for attaining an interlock with the abrasive matrix comprising the body of the wheel.

Referring now in detail to the drawings, a typical grinding wheel of the type to which the present invention is applicable is illustrated in FIGURE 1 and comprises a cylindrically shaped abrasive body conventionally com prising abrasive grains tenaciously bonded into a matrix by a suitable bonding agent. The -grinding wheel 20 is suitably supported for rotation by means of a pair of flanges 22 which are secured to a drive shaft 24 for rotatably supporting and driving the grinding wheel in a manner well known in the art. Grinding wheels may be of the arborless type in which the clamping fianges 22 are formed with a plurality of circumferentially spaced apertures through which bolts extend through correspondingly located apertures through the abrasive body for securely clamping the abrasive body 20 therebetween. Alternatively, the grinding wheel may be formed with a suitable arbor or hole through the center thereof for mounting the wheel on a suitable hub or between clamping plates as may be desired.

The advantages achieved in accordance with the prestressed reinforcing network embedded in the grinding wheel are obtained regardless of the particular type of abrasive matrix of which the body `of the wheel is composed. The abrasive grains employed in the abrasive matrix may consist of any of the types well known in the art including silicon carbide, boron carbide, tantalum carbide, tungsten carbide or other hard metal carbides; alumina, diamond grains, glass, quartz, garnet, etc. The bonding agent employed for tenaciously bonding the abrasive grains into a hard Awear resistant matrix may be of the vitrified, resinoid, shellac, rubber, silicate, magnesium, oxychloride type or metallic type. Conventionally, the bonding agent employed in the manufacture of heavy duty grinding wheels of the type employed for snagging steel billets consists of suitable thermosetting resins or mixtures thereof including phenol aldehyde resins, cresol aldehyde resins, resorcinol aldehyde resins, urea aldehyde resins, melamine formaldehyde resins, furfuryl alcohol resins, epoxy resins, polyurethane resins, polyamide resins and the like. Of the foregoing bonding agents, the condensation product of phenol itself with formaldehyde is the most common and preferred bonding material.

The particular technique employed for forming the abrasive matrix is also not critical in attaining the benefits of the present invention. Accordingly, the abrasive matrix may be formed by conventional hot or cold pressing techniques in which the abrasive grains are preliminarily coated with a suitable bonding agent such as, for example, a phenol aldehyde A-stage resin either in a powdered or a liquid form and thereafter are placed around the reinforcing network and are compacted under pressure generally of several tons per square inch. In the cold pressing technique, the compacted mass is subsequently heated to an elevated temperature effecting a curing or setting of the bonding agent whereas in the hot pressing technique, the

coated abrasive grains are hot compacted effecting concurrent curing thereof which may be followed by a supplementary final curing operation. Alternatively, the abrasive matrix can be formed by casting or molding the abrasive ,grains and a liquid bonding agent in a suitable mold preferably employing a differential pressure to the uncured mass to assure a substantially complete penetration of the liquid bonding agent in and around the individual abrasive grains. Of the various techniques employed for forming the abrasive matrix, the so-called displacement technique is preferred to which reference is made to United States Patent No. 2,860,961 assigned to the same assignee as the present invention for additional details. Suffice it to say, the displacement method possesses the advantage of avoiding excessive pressures as required in the hot and cold pressing techniques avoiding thereby the possibility of damage or deformation of the individual abrasive grains and reinforcing elements embedded therein. The bonding agent is subsequently cured at an elevated temperature forming a high strength matrix tenaciously interlocked with the reinforcing network of the types herein subsequently described.

In the manufacture of heavy duty type grinding wheels of the type satisfactory for snagging steel billets, the bonding agent and abrasive grains are conventionally employed in proportions such that the resultant abrasive matrix comprises from about 40 percent up to about 64 percent by volume of abrasive grains with the balance thereof consisting of the bonding agent, reinforcing element or elements, filler materials, pores and plasticizers, and the like. Any one of a variety of filler materials of the types well known in the art may be incorporated in the abrasive matrix and may include, for example, powdered cryolite, metallic sulfides and other filler materials which are either inert or which improve the cutting efficiency of the resultant abrasive grinding wheel. It will be understood by those skilled in the art that the particular composition of the bonding agent, the composition or type of abrasive grains employed, as well as the inclusion or omission of various filler materials of the types well known in the art, are not critical in order to obtain the benefits of the present invention.

In accordance with the practice of the present invention, a prestressing of the grinding wheel matrix is achieved either by the application of a tension at a central region or adjacent to the arbor through the grinding wheel or by the application of a compressive stress around the periphery of the wheel, or by combinations thereof. In either case, the resultant prestressing imposes a residual compressive stress in a tangential direction on the grinding wheel matrix. An exemplary reinforcing network and manner for effecting a prestressing of a grinding wheel will now be described with particular reference to FIGURES 2-4. As shown in these gures, a grinding wheel indicated at 26 formed with a hole or arbor 28 through the center thereof, is reinforced by a reinforcing network 30, comprising a composite ring 32, the inner surface of which defines the arbor 28. A plurality of radially extending reinforcing elements 34 are interlocked with the ring 32 and extend therefrom to a point adjacent to the periphery of the wheel. The composite ring 32, as may be best seen in FIGURE 3, comprises a plurality of annular discs 36 which are formed with a plurality of circumferentially spaced and aligned apertures or bores 38, through each of which a bolt 40 extends preferably having its shank end disposed in threaded engagement in the upper disc 36. The upper surfaces of each of the annular discs 36, with the exception of the upper one thereof, are recessed such as by milling indicated at `42 to provide a clearance space for the insertion of the reinforcing elements 34 which are Wrapped around and are interlocked with the bolts 40. In accordance with the construction illustrated in FIG- URES 2 and 3, the reinforcing elements 34 are preferably in the form of a hairpin of an elongated U-shape wherein the leg portions thereof are crimped or otherwise provided with surface irregularities to facilitate an interlock with the abrasive matrix while the bight portion of the reinforcing elements are smooth so as to loop around and engage the bolts 40.

As a typical example, in the manufacture of a heavy duty type snagging wheel having an outside diameter of 24 inches, an arbor of a diameter of 12 inches and wherein the wheel is 3 inches thick, a reinforcing network designed to provide safe operation at 16,000 s.f.p.m. may conveniently comprise a composite ring 32 containing eleven annular discs 36, each of which is 0.282 inch thick and of which ten are milled to provide recesses 42 for receiving and interlocking a hard drawn oil tempered steel wire of 0.054 inch diameter in the form of crimped hairpins comprising the reinforcing elements 34. A total of fifty bolts, 40 of a nominal diameter of Vlg-inch are spaced equally around the composite ring and are disposed inwardly 1i-inch from the periphery of the discs. The ends of the hairpin shaped reinforcing elements 34 terminate at a point adjacent to the periphery of the wheel such as, for example, about 1/z-inch therefrom.

In accordance with this construction, a total of 1000 radially extending elements comprised of 500 hairpinshaped reinforcing elements are positioned substantially uniformly throughout the abrasive wheel matrix. Exposure of the ends of the reinforcing elements at the periphery of the wheel during use thereof does not detract from the grinding efficiency of the abrasive wheel. It has been found that wires of a circular cross-section ranging from about 0.030 inch to about 0.130 inch or wires of an irregular or regular cross sectional shape of equivalent cross sectional area have only a negligible effect on the grinding efficiency of the wheel upon becoming exposed at the grinding face thereof. Reinforcing elements of a diameter of less than about 0.030 inch or regularly or irregularly shaped elements of equivalent cross sectional area have generally been found to possess insufiicient load carrying capacity necessitating the inclusion of an extensive number thereof to provide the requisite t-otal reinforcement necessary. On the other hand, the use of round wires of a nominal diameter of 0.130 inch or regularly or irregularly shaped wires of equivalent cross sectional area have been found in some instances and under certain grinding conditions to cause interference in the grinding action and,

accordingly, it is for this reason that the nominal diameter of circular metallic wires is preferably held to a size of about 0.130 inch or less.

It will also be understood that the reinforcing elements 34 may be comprised of any high strength material which can be mechanically anchored and interlocked in the grinding wheel matrix and strongly attached to the central reinforcing ring in lieu of the hard drawn tempered steel wires previously described. Alternative high strength materials which can be satisfactorily employed as reinforcing elements of a prestressed reinforcing network include ber glass cords or cables comprising a plurality of interwoven or twisted rovings, each of which in turn is comprised of a plurality of continuous ine sized fiber glass laments. The liber glass cord or cable is preferably twisted or braided so as to provide surface irregularities therealong increasing the mechanical engagement and anchorage of the element with the abrasive in which it is embedded. It is'also contemplated that the fiber glass cord or cable can be interwoven with high strength metallic wires or reinforcing elements providing therewith a composite material. The metallic reinforcing elements similarly may be braided or may be in the form of perforated or crimped ribbons to increase the mechanical interlock thereof in the abrasive matrix. For this purpose the reinforcing elements, in addition to high strength steel and fiber glass, may be composed of such materials as bronze, beryllium, copper, nickel, molybdenum, as well as plastic materials such as nylon and Daeron, for example. High strength steel wires, however, constitute a preferred material Vin consideration of their high tensile strength, high modulus of elasticity and comparatively low cost.

' In the manufacture of' a heavy duty type grinding wheel convenient for the snagging of steel billets employing the reinforcing network 30, as shown in FIGURES 2-4, a convenient means for effecting a prestressing of the networ-k is by the employment of a tapered plug 44 which is pressed into the interior of the composite ring 32 (the taper of which is exaggerated for the purposes of clarity as illustrated in FIGURE 4), which similarly is provided l \with a correspondingly contoured taper along the inner surfaces of the individual annular discs 36. The inner Surface of the ring and the peripheral surface of the plug are preferably coated with an extreme pressure lubricant such as molybdenum or tungsten disulfide grease, a suspension of Teflon powder or the like. The composite ring 32 which is held in tension as the result of the stretching thereof by the tapered plug 44 and including the radially extending hairpin shaped reinforcing elements 34 may be suitablyv inserted in a mold as shown in FIGURE 4 for the formation of an abrasive grinding wheel employing the displacement method in accordance with the technique as more fully described in the aforementioned United States Patent No. 2,860,961. The speci-fic molding operation as illustrated in FIGURE 4 employs a mold assembly cornprising a base plate 46 mounted on a member 49 defining vacuum chamber 48 connected by means of a tube 50 to a suitable vacuum source for withdrawing and at least partially evacuating the interior of the chamber 48. The base plate 46 is formed with a plurality of apertures 51 therethrough disposed in communication with the chamber 48 over which an annular retainer ring 52 is placed which is of a fairly coarse wire construction and on top of which a barrier ring 54 is placed which is pervious to air but substantially impervious to or will resist penetration by the liquid bonding material. A circular split band outer mold member 56 is mounted on the base plate 46 concentrically to the composite ring 32 defining the periphery of the resulting grinding wheel.

As shown in FIGURE 4, after the reinforcing network is placed in the mold, the annular recess defined between the inner surface of the mold band 56y and the periphery of the composite ring 32 is filled with abrasive grains 58 preferably by tapping or vibrating the mold to cause substantially complete filling thereof. Thereafter, a preliminarily mixed bonding material is applied directly over the top of the abrasive layer and vacuum is applied to the chamber 48 effecting a downward movement and a substantially complete filling of the voids between the individual abrasive grains. Thereafter, the mold assembly is heated to effect a curing of the resinous bonding agent after which the mold assembly is allowed to cool followed thereafter by a removal of the tapered plug 44. As a result of the removal of the tapered plug, the composite ring 32 tends to shrink to its original diameter imposing a corresponding radial inward tensile force on each of the individual reinforcing elements 34 and imposing thereby a corresponding residual compressive stress on the abrasive matrix.

It is also contemplated in accordance with the practice of the present invention to impose a residual prestress on the abrasive wheel matrix by employing reinforcing network of a cuspate geometrical pattern of the general types as described in United States Patent No. 3,123,948 which is assigned to the same assignee as the present invention. A convenient manner for achieving this is illustrated by the mold assembly and reinforcing network illustrated in FIGURES -7. As shown in FIGURE 5, a mold ring 60 is provided comprising two semi-circular elements which are hingedly connected at one of their ends by means of a pivot pin 62 and formed with chamfered end surfaces 64 at the opposite ends thereof. The inner surface of the mold ring 60` is formed with pairs of inwardly projecting ears 66 through which a suitable pin or bolt 68 extends serving as a plurality of circumferentially spaced anchor points for a reinforcing network which may comprise, for example, a continuous high strength reinforcing element indicated at 70.

The reinforcing element 70, as best seen in FIGURE 5, is woven around the pins or bolts 68 in a manner so as to form a series of chords extending between angularly spaced points adjacent to the periphery of the wheel. The number of ears 66 and pins 68 provided along the inner surface of the mold ring 60, and the particular weaving pattern employed, can be varied so as to provide the desirate cuspate geometrical pattern which may be comprised of a plurality of plies or layers to provide the requisite reinforcement.

A tensioning of the reinforcing element 70 is achieved, for example, by pressing a suitable wedge 72 between the chamfered end surfaces 64 of the mold ring, effecting an increase in the diameter thereof with a corresponding stressing or tensioning of the reinforcing elements. The resultant mold ring incorporating the tensioned reinforcing network therein, can be employed directly for forming a grinding wheel either by the cold pressing, hot pressing, casting or displacement techniques in a manner well known in the art. In each case, however, the mold ring is retained in the expanded condition retaining the reinforcing element 70 under tension until such time that the abrasive matrix has solidified or cured after which the wedge 72 is removed followed by a removal of the pins or bolts 68 enabling the mold ring to be removed from the pheripheral surface of the wheel. In order to facilitate a removal of the mold ring 60 and the pins 68 at the completion of the curing of the abrasive matrix, the surfaces thereof may be preliminarily coated with a release compound such as, for example, a coating of a polytetrauoroethylene plastic preventing substantial adherence of the bonding agent thereto.

It will be apparent, in accordance with the mold assembly illustrated in FIGURES 5-7, that the peripheral surface of the grinding wheel will incorporate a series of interruptions corresponding to the locations of the ears 66 and pins 68 on the mold ring. These interruptions, which represent only a very small fraction of the grinding face area of the grinding wheel, will not normally interfere with the use of the grinding wheel for rough snagging of steel billets. It is contemplated, however, that such surface interruptions can subsequently be filled with an additional uncured abrasive material and bonding agent and a recuring of the wheel forming an integral smooth grinding surface. Alternatively, such surface interruptions can be filled with a mixture of abrasive grains and a cold setting bonding agent such as an epoxy bonding material.

The embedment of a prestressed reinforcing network having a cuspate geometrical pattern can alternatively be achieved in accordance (with the mold assembly illustrated in FIGURE 8. As shown in FIGURE 8, a mold ring 74 is provided with a plurality of circumferentially spaced, radially extending tapped bores in which threaded bolts 76 are threadably disposed with their shank ends positioned inwardly of the inner surface of the mold ring 74. The shank end portions of the bolts 76 are formed with suitable swivel fittings through which a reinforcing element 78 is threaded forming a reinforcing network of polygonal shape comprising a series of chords. A preselected tensioning of the reinforcing element 78 is achieved by turning each of the bolts 76 outwardly effecting thereby a preselected tensioning of the reinforcing element 78. The resultant mold ring 74 can thereafter be employed for the hot or cold pressing or molding of an abrasive matrix therein after which the bolts 76 are severed or otherwise removed imposing a residuary compressive stress on the abrasive matrix. The inner surface of a mold ring 74 and the swivel ends of the bolt 76 disposed in contact with the bonding agent, may similarly be coated with a suitable release agent facilitating a disengagement thereof at the completion of the curing of the abrasive matrix.

The reinforcing networks, as illustrated in FIGURES 5 and 8, comprising a cuspate geometrical pattern comprising a plurality of chord elements extending between circumferentially spaced points adjacent to the periphery of the grinding wheel may be comprised of a reinforcing element of high tensile strength of the same types as described in connection with the hairpin reinforcing elements 34 in connection with FIGURES 1-4. The surfaces of the reinforcing elements are preferably formed with a plurality of irregularities therealong to facilitate an anchoring or interlocking thereof with the abrasive matrix such that upon a release of the tensioning stress of the mold ring, a corresponding compressive stress is imposed on the abrasive wheel matrix.

The same general technique as described in connection with FIGURES 5-8 can similarly be applied for making arborless type grinding wheels such as shown in FIGURE 9. In accordance with the mold designs shown in FIG- URES 5-8, an arborless type as well as an arbor type wheel can be formed whereupon a suitable cylindrical insert is positioned concentrically within the mold ring and around which the abrasive matrix is formed. These mold ring devices are equally applicable for the formation of arborless type grinding wheels in which a plurality of circumferentially spaced apertures are provided through which fastening bolts extend for clamping the wheel to suitable clamping flanges. As shown in FIGURES 9 and l0, a mold ring 86 is provided comprising two halves having the adjacent ends pivotally connected by means of a pin 88 and the opposite adjacent ends thereof formed with a tapered end sulface indicated at 90 for removably receiving a tapered wedge 92 for effecting an expansion of the mold ring and a tensioning of the reinforcing elements 94 connected thereto. The mold ring 86 is formed with a plurality of small sized circumferentially spaced apertures 96 therethrough, through which a high strength reinforcing element is interlaced in any desired pattern such as illustrated in FIGURE 9. After the reinforcing network has been developed, a prestressing thereof to the desired tension is achieved by insertion of the `wedge 92 between the tapered end surfaces 90 effecting an expansion of the diameter of the mold ring 86. During the formation of the abrasive matrix, a series of plugs indicated at 98, which preferably are provided with a suitable release coating can be inserted in the mold to define the subsequent bolt holes through the matrix for mounting the finished grinding Iwheel. At the completion of the molding operation and a substantially complete curing or setting of the a'brasive matrix, the wedge 92 is removed and the reinforcing elements 94 can simply be cut or severed to enable a removal of the mold ring from the periphery of the wheel.

It is also contemplated as shown in FIGURE 9 that some of the reinforcing elements can be looped around and connected to a reinforcing ring indicated at 100 whereupon a tensioning of the reinforcing elements 94 effects a corresponding stretching and tensioning of the reinforcing ring 100. This latter technique comprises an alternative satisfactory method in lieu of the method described in connection with FIGURES 2-4 for effecting a stretching or tensioning of the composite ring 32. It will be understood however that satisfactory arborless type grinding wheels can be made in accordance 'with the technique shown in FIGURES 9 and l0 without the use of the reinforcing ring 100.

In the various reinforcing networks illustrated in the drawings, it is preferred in order to impart an optimum prestressing to the abrasive matrix to form the individual reinforcing elements in a manner so as to effect an anchoring or mechanical interlock thereof with the abrasive matrix effecting thereby a transfer of at least a portion of the prestress to the matrix. FIGURES 1l-14 illustrate various typical means whereby an interlock of the reinforcing elements with the abrasive matrix is enhanced in view of the physical irregularities along the length of the reinforcing elements. FIGURE ll is illustrative of a reinforcing element 102 having a. crimped construction forming a plurality of engaging surfaces along the lengththereof. FIGURE 12 is illustrative of a braided reinforcing element 104 comprising interwoven strands of a high strength or a mixture of high strength reinforcing -ilaments. FIGURE 13 is illustrative of a twisted reinforcing element 106 which may additionally include knots or other surface irregularities to enhance its mechanical anchoring to the abrasive matrix. A reinforcing element 108 is illustrated in FIGURE 14 incorporating a series of knobs of an enlarged diameter disposed at intervals therealong Iwhich similarly serves'the purpose of effecting a mechanical interlock with the abrasive matrix. It will be understood that alternative satisfactory configurations of the reinforcing elements including perforated ribbons, for example, to achieve a mechanical interlock with the abrasive matrix can be satisfactorily employed in accordance with the practice of the present invention.

The magnitude of the prestress imposed on the abrasive matrix by the reinforcing networks illustrated in the drawings and as hereinbefore described can be varied consistent with the physical strength properties of the abrasive matrix itself, the intended operating speed of the abrasive wheel, the size of the abrasive wheel and the relative strength of the reinforcing elements of which the reinforcing network is comprised. -For any given grinding wheel composition, configuration and operation, the appropriate degree of prestressing can be mathematically approximated to provide optimum and safe operation at the intended operating speed. The stresses imposed on a grinding wheel matrix during rotation thereof are composite stresses including tensile stresses in a radial direction as well as tensile stresses acting in a tangential direction at any given point of a wheel a the result of the hoop stress created by the centrifugal forces acting on the abrasive matrix. The principal cause of failure of a grinding wheel is due to the tangential stress imposed on the matrix and the stresses extending in a radial direction are of secondary importance in connection with grinding wheel failure. The mathematical formula for computing the tangential stress on a grinding wheel matrix as the result of the stresses induced by rotation thereof can be approximated by the following equation:

Employing the foregoing equation, the computation of the tangential stresses in a typical hea-vy duty snagging wheel having an outside diameter of 2A inches, a hole or arbor through the Wheel of 'l2 inches in diameter and a thickness of 3 inches rotating at 16,000 s.f.p.m. is listed in Table l, column A.

TABLE 1 [Tangential Stress DistributlonGlrilndlng Wheel 24' O.D. x12' LD. x 3

Radius, inches A B C D 2, 450 -3, 350 900 2, 170 2, 180 2, 620 -440 2, 050 l, 800 2, -370 1, 900 1, 600 1, 870 -270 1, 740 1, 400 l, 650 -250 1, 550 l, 220 1, 480 -260 1, 32() l, 060 -1, 350 -290 1, 090

.A =Taugential stress at 16,000 s.f.p.m.; B=Prestressed at 2,000 p.s.. n'ii. at LD.; C=Net stress at 16,000 s.f.p.m.; D=Net stress at 24,00 In the foregoing computation, Poissons ratio is assumed to be 0.3. As will be noted in column A of Table l, the tangential `stress adjacent to the hole or arbor is greater than at the periphery of the wheel which indicates that failure of the abrasive matrix starts at the hole considerably before the mean stress reaches the rupture strength of the grinding wheel matrix. Accordingly, when a crack starts at the hole it propagates rapidly through the wheel toward the periphery thereof as a result of the stress concentration formed around the end of the crack. Accordingly, grinding wheel failure occurs at a substantially lower speed under normal operation than it would if the tangential stress Iwere uniform throughout the abrasive matrix.

In accordance with the practice of the present invention by incorporating a reinforcement network in the wheel structure wherein the abrasive matrix and the rein-y forcing network are held together by cohesion, adhesion and mechanical anchorage such that the wheel behaves as an elastic body, a prestressing of the abrasive matrix can be achieved imposing a residual compressive stress in a tangential direction which neutralizes at least a portion of the subsequent tangential stresses imposed thereon during high speed rotation of the wheel. For example, by applying a uniform radial tension of 2000 p.s.i. to the inside surface of the same grinding wheel employed for the computation of thel stress pattern, as indicated in column A of Table 1, a recomputation of the residual stress pattern is obtained as enumerated in column B of Table l. In this case, it will be apparent that the residual compressive stresses in a tangential direction are greatest adjacent to the arbor or hole through the grinding wheel and diminish toward the periphery thereof.

Since the stress systems imposed on elastic bodies are algebraically additive, the resultant tangential stresses imposed on the prestressed grinding wheel rotating at 16,000 s.f.p.m. constitutes the algebraic sum of the values as listed in columns A and B with a resultant operating stress as indicated in column C. As will be noted in column C of Table l, the residual compressive stress imposed on the abrasive matrix overcomes all of the tangential tensile stresses such that the matrix remains under compression even when rotated at 16,000 s.f.p.m.

Column D of Table l represents the stress pattern developed in the same grinding wheel which has been preliminarily prestressed at a totative speed of 24,000 s.f.p.m. In this case, it will be noted that the residual compressive stress in a tangential direction is overcome by the tensile stress in a tangential direction due to the centrifugal force but the magnitude of the resultant tangential tensile stress is less than the tangential tensile stress of the same wheel in a non-prestressed condition as represented by the data set forth in column A, rotating at a substantially lower operative speed. The benefits derived in accordance with the practice of the present invention are readily apparent from the foregoing mathematical analysis, and it will be appreciated tht the magnitude of the residary prestress imposed on the abrasive matrix is controlled so as not to exceed the ultimate compressive strength of the abrasive matrix or otherwise incur injury or damage thereto.

In order to further illustrate the improved reinforced grinding wheel comprising the present invention, the beneiits derived from the use of a reinforcing network 30 of the type illustrated in FIGURES 2-4 will now be described. On the assumption that there is no clearance between the bolts 40 and the bores 38 in which they are retained or between the hairpin shaped reinforcing elements 34 and the surfaces of the bolts and that the grindof the hairpins and that the hairpins themselves are not elastically stretched, the tension applied to each square inch of the inner surface of a grinding wheel can be theoretically computed by the following Lame-Clapeyron equation:

Assigning a modulus of elasticity to steel as 3 l06 p.s.i., its Poisson ratio 0.25 and the thickness of the ring in a radial direction as one inch, a diametral stretch of the ring in a radial direction of 4.2 mills will provide a tension on the wheel of 2000 p.s.i. or a stretch of 4.8 mills will provide a tension of 4000 p.s.i. This latter stretch gives a maximum tension of the ring of approximately 25,000 p.s.i., which is well within the elastic limit of mild steel such that the composite reinforcing ring can be made of inexpensive and easily fabricated steel.

In actual practice, some clearance or looseness in the reinforcing assembly and the reinforcing matrix will be present whereupon the prestressing of the composite ring will be increased to a value beyond that calculated to compensate for the resultant residual stress desired. On the other hand, it is not necessary to compensate for elastic stretch in the hairpins or elastic compression of the wheel since as the centrifugal force increases in response to an increasing rotative speed of the wheel, the calculated values of tension will be achieved when the expansion or stretch of the wheel under rotational stress equals the stretch introduced at the time of manufacture.

From a practical standpoint, the imposition of a 2000 p.s.i. or a 4000 p.s.i. tensile stress at the hole of the grinding wheel as hereinbefore described, can be translated by calculation to reveal that if this tensile prestress were all that holds the rotating wheel together (that is, if the wheel were cut in two across a diameter and the two halves were held together solely by tension in the hole), the 2000 p.s.i. tension at the hole would hold the wheel together up to about 15,000 s.f.p.m., and the 4000 p.s.i. tension at the hole would hold the same cracked wheel together up to about 24,000 s.f.p.m. The inclusion, accordingly, of the reinforcing network 30 as shown in FIGURES 2-4, comprising 500 hairpin-shaped reinforcing elements of a hard drawn tempered steel wire 0.054 inch in diameter, is more than adequate to maintain a tensile stress of 4000 p.s.i. Accordingly, by a study of the amount of prestressing in relationship to the breaking speed of a grinding wheel, a

maximum strength of the grinding wheel can be achieved which will exceed substantially a bursting speed for a unreinforced Wheel of about 24,000 s.f.p.m. Theoretically, the foregoing reinforcing network would provide a breaking speed of as high as 34,000 s.f.p.m.

The optimum amount of prestressing to provide a maximum breaking speed will vary, as hereinbefore described, upon the elastic modulus of the abrasive matrix. The presence of the reinforcing networks or other reinforcing elements embedded within the abrasive matrix cause variations in the modulus thereof and, accordingly, while the foregoing mathematical equations provide an approximation of the stresses and bursting speeds of grinding wheels, the optimum degree of prestressing is best determined by experimentation.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulll the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. A reinforced grinding wheel comprising an abrasive body of a circular cross section adapted to be mounted for rotation about an axis passing through substantially the center thereof, said abrasive body comprising an abrasive matrix including a plurality of abrasive grains tenaciously bonded by a bonding agent and a reinforcing network embedded in and interlocked with said abrasive matrix, said reinforcing network comprising a ring disposed concentrically of the axis of rotation of said wheel and a plurality of reinforcing elements interlocked with said ring and extending substantially radially therefrom to the periphery of said wheel, said ring and said reinforcing elements of said network prestressed in tension for imposing a residual compressive stress in a tangential direction on said abrasive matrix of a magnitude below the compressive strength of said matrix.

2. A reinforced grinding wheel comprising an abrasive body of a circular cross section adapted to be mounted for rotation about an axis passing through substantially the center thereof, said abrasive body comprising an abrasive matrix including a plurality of abrasive grains tenaciously bonded by a bonding agent and a reinforcing network embedded in and interlocked with said abrasive matrix, said reinforcing network comprising a ring disposed concentrically about the axis of rotation of said 4wheel and a plurality of high strength steel wire reinforcing elements of a diameter of from about 0.030 inch to about 0.130 inch diameter interlocked with said ring and extending substantially radially therefrom to the periphery of said wheel, said reinforcing elements formed with surface irregularities along the length thereof for mechanically engaging said abrasive matrix, said ring and said reinforcing elements of said network prestressed in tension for imposing a residual compressive stress in a tangential direction on said abrasive matrix of a magnitude below the compressive strength of said matrix.

3. A reinforced grinding wheel comprising an abrasive body of a circular cross section adapted to be mounted for rotation about an axis passing through substantially the center thereof, said abrasive body comprising an abrasive matrix including a plurality of abrasive grains tenaciously bonded by a bonding agent and a reinforcing network embedded in and interlocked with said abrasive matrix, said reinforcing network comprising a plurality of diametrically extending reinforcing elements of high tensile strength extending between opposed peripheral surfaces of said wheel incorporating surface irregularities along the length thereof for facilitating a mechanical interlock with said abrasive matrix, said reinforcing elements prestressed in tension for imposing a residual compressive stress in a tangential direction on said abrasive matrix of a magnitude below the compressive strength of said matrix.

4. A reinforced grinding wheel comprising an abrasive body of a circular cross section adapted to be mounted for rotation about an axis passing through substantially the center thereof, said abrasive body comprising an abrasive matrix including a plurality of ab-rasive ygrains tenaciously bonded by a bonding agent and a reinforcing network embedded in and interlocked with said abrasive matrix, said reinforcing network comprising a plurality of diametrically extending high tensile strength metallic wires disposed at substantially equal angular increments through said matrix and extending between opposed peripheral surfaces of said wheel, said wires formed with surface irregularities along the length thereof for enhancing the mechanical interlocking thereof with s-aid matrix, said metallic reinforcing elements of a diameter ranging from about 0.030 inch to about 0.130 inch and prestressed in tension for imposing a residual compressive stress in a tangential direction on said abrasive matrix of a magnitude below the compressive strength of said matrix.

References Cited UNITED STATES PATENTS Hart 51-206 Spohn 51-206 Fowler 51-206 Bennett 52-223 Hurst 51-206 Warnken 1836 X Kistler.

Fischer 51-206 Fischer 51-206 Francis 52-223 15 LESTER M. SWINGL-E, Primary Examiner.

D. G. KELLY, Assistant Examiner.

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