US2173462A - Grinding wheel - Google Patents

Description

Sept. 19, 1939.. H. w. WAGNER E, AL 2,113,462

GRINDING WHEEL Original Filed NOV. 4, i935 lEpim f6 b1/newton HERBERT W. WAGNER KENNETH l Wmv-00MB :BY @@UTQ flltofwwn Patented sept. 19, 1939 UNITED STATES PATENT OFFICE Y` GRINDING WHEEL sachusetts Original application November 4, 1935, Serial No. 48,181. Divided and this application April 1, 1938, Serial No. 199,534

6 Claims.

This invention relates to grinding wheels, and particularly to grinding wheels made of abrasive grains bonded together by vitrified ceramic materials, rubber, shellac, resin, the articial resinoids, etc.

When a grinding wheel of a given mass is rotated, each element of the wheel has a certain centrifugal force which acts outwardly. This force depends upon the speed at which the wheell is rotating, the distance of the element from the center of the disk and the mass per unit of volume of the wheel. This centrifugal force sets up stresses in therotating wheel and so causes strain and deformation which tend to break the wheel when it is rotated at a certain maximum or breaking speed. Similarly, the act of grinding on the periphery of the wheel causes frictional heat to be generated, and this results in a temperature gradient existing between the periphery of the wheel and the center which also sets up disruptive stresses within the wheel. Hence, both the centrifugal force and the frictional heat combine to cause wheel breakage. Because of these two factors, each type of grinding wheel is limited in its use to a certain standard wheel speed which should not be exceeded because of danger to the operator, although the efficiency of a grinding operation is increased by raising the wheel speed. I

'Ihe primary object of this invention is to increase the standard grinding speed for a given type of wheel by strengthening the wheel so that it will have the same factor of safety at the higher speeds, and to provide a grinding wheel structure which may be rotated rapidly and subjected to a high frictional heat during a grinding operation without danger of breakage of the wheel. Other objects will be apparent in the following disclosure.

The tenslonal stresses within a -grindingrwheel due to the centrifugal force and to frictiial heat are both maximum at the center of the wheel. The standard grinding wheel has a hole at its center for mounting the same on an arbor or spindle.l The principal stresses within the wheel are in' two directions,l radial and tangential; but at the edge of the hole of a rotating Wheel, the radial stress is zero. Hence, the maximum stress occurring at the edge of the hole is tangential in direction. When this tangential tensional stress equals the tensile strength of the material, then a radial crack starts at the edge of the hole and grows until the periphery of the wheel is reached. This happens whether the stress be due to centrifugal force or to heat applied at the periphery of the Wheel, and the stresses due to these two forces combine by simple addition to cause wheel breakage.

This invention contemplates fortifying a grinding wheel by omitting the central hole 5 thereof and by providing bonded abrasive material, in that position of maximum stress, which has a higher intrinsic strengthy than has the outer zone.

Referring to. the drawing; which illustrates various embodiments of this invention;

Fig, 1 is a plan view, partly broken away, of an imperforate grinding wheel having two annular zones, the outer one being suitable for grinding purposes and the inner zone constructed for strengthening the wheel;

Fig. 2 is a fragmentary plan view of a Wheel having small mounting holes arranged and located remote from the irnperforate center of the wheel;

Fig. 3 is a similar fragmentary view showing a two zone wheel provided with mounting holes;

Fig. 4 is a fragmentary sectional view showing how an imperforate wheel may be mounted by cernenting it to a supporting member;

Fig. 5 is a sectional fragmentary view showing how a supplemental bond may be applied to a wheel of uniform structure so as to form a two zone wheel as shown in Fig. 3; and

Fig. 6 shows hows a vitriiied wheel may be 30 strengthened during the ring operation.

As shown in Fig. 1, a grinding wheel may be made as a two' zone structure in which the outer zone I0 is adapted for the normal grinding operations and the inner zone I2 serves to strength- $6 en the grinding wheel. This wheel may be provided with small mounting holes I4 (Figs. 2 and 3) which are arranged concentric to the axis but remote from the center of the wheel and are of such sizesand locations that the wheel 40 is not materially weakened in its structure by the presence thereof. Bolts are adapted to pass through these holes and secure the wheel on a suitable flat faced backing plate. Such a wheel may, however, be mounted in other ways, such l, as being made entirely imperforate and cemented to a backing plate. I

The structure of the grinding wheel may be made in accordance with the standard methods of manufacture, except as regards leaving out the central hole of the wheel and fortifying the central zone. The wheel comprises abrasive grains, such as crystalline alumina, silicon carbide, boron. carbide or diamond of suitable grit sizes, which are cemented or bonded together by standard bonds, such as vitrifled ceramic materials, vulcanized rubber, heat set resinoid,.shellac, resin, sodium silicate, etc. The invention is particularly applicable to thevltried wheels because of their being relatively fragile although of great commercial value. The vitriflable ma-l terials comprise ball clay, slip clay, feldsp'ar, kaolin, flint and various other materials compounded in suitable proportions which are intermixed with the abrasive grains in a plastic condition for molding the wheel shape. The molded wheel is red in a ceramic kiln at a suitable ltemperature to mature the bond to a glassy or porcelanic condition. Rubber and resinoid wheels are heated at lower temperatures to vulcanize the rubber or to convert the plastic resinoid to a hard infusible body. Various well lknown procedures are adopted for the purpose of setting the particular bond chosen. One primary d'fference over the prior art lies in so shaping the wheels as to omit from such wheels the central spindle hole. If outer mounting holes I4 are provided, these may be made either in the plastic moldable mass prior to heat setting the bond or by drilling the holes in the finall wheel, structure after the bond has been matured or heat set. Various expedients may be adopted for this purpose. By such methods, we provide a wheel having no central` hole i. e., having a continuous imperforate structure throughout the central zone of high stress, without regard to the natural porosity of the wheel.

We may further strengthen such a wheel by impregnating the pores in the central portion I2 (Fig. l) of a standard one zone wheel of uniform structure with a resinoid or other matekor frictional heat than will a wheel made wholly of the outer zone composition and structure. In such a wheel, the moduli of elasticity of the two zones may be substantially equal and the materials are so chosen as to quantity and kind that the drying and ring shrinkages do not cause detrimental strains or cracks at the junction between the two zones.

Such a wheel may comprise an outer zone made of vitrified ceramic bonding material and suitable abrasive grains cemented together thereby and an inner zone likewise made of abrasive or other granular material cemented together by vitrifled ceramic material, but in which the grains are much smaller or the bond is present in a larger amount in the inner than in the outer zone, so that the central zone is the stronger of the two. The outer zone -may be made, for example, of crystalline alumina vabrasive grains of 16 grit size bonded by means of 2% ounces of a standard ceramic bond per pound of the abrasive. The central zone may be made of similar abrasive grains of v36 grit size, and 31/4 ounces of the grain bond per pound of abrasive may be employed. When these two mixtures have been molded together into the annular shapes illustrated and fired in a ceramic As a further example, the outer zone may be' made of crystalline alumina abrasive of between 30 and 36 grit size, and 3% ounces of bond to the pound of abrasive may be employed. In

-the inner zone, the abrasive may be made up of a mixture of one half of 60 grit, one quarter.

of 'l0 grit and vone quarter of 80 grit size, and the ceramic bond may be used in the proportion of 31/2 ounces per pound of abrasive.

The inner zone will be of such size radially that it will receive a considerable proportion of the stress within the wheel, and preferably extend to such distance from the center that the mounting holes I4 may be located as above defined within this inner zone. That is, the mounting holes of the wheel may be located at approximately 40% of the distance outwardly from the center, as indicated in Figs. 2 and 3, and the material of this inner zone will extend suciently far beyond these holes so that the localized zones of stress around each of the mounting holes will be ,located substantially wholly within this inner zone material. It is satisfactory to locate these holes at least 1/ inch inside of the periphery of the inner zone. Thus, we may produce a wheel which is far stronger than either thel one zone no-hole wheel or the two zone wheel having a central mounting hole, whereby the wheel will stand a very high rate of rotation as well as a high temperature gradient from the periphery to the center of the wheel. In order to show the superiority of this two zone no-hole wheel over the standard wheel, two

wheels were made as follows: A two zone wheel was made with the outer zone of crystalline alumina abrasive of 36 grit size and bonded to what is known as grade J on the Norton scale of hardness. This outer zone employed 3A of an ounce of standard ceramic bond per pound of abrasive. The inner zone without a central hole was made of 60 grit size of the same abrasive and with the same bond in the proportions of one ounce of bond per pound of abrasive so that the grade of the central zone was grade K. In that wheel the inner zone had a radius of 41/2 inches and the outer periphery of the wheel a-radius of ten inches. Four 1/2-inch holes were located four inches from the center and inch bolts employed in mounting the wheel. For comparison, a single zone wheel with a 3-inch central hole was made to grade J of 36 grit size so as'to correspond with the structure of the outer zone of the first wheel. This single zone wheel broke at the speed of v11,890 surface feet per minute, while the two zone wheel broke at 'a speed of 18,850 surface feet per minute or approximately 58% higher breaking speed. An average increase ,of 40 to 50% in this breaking speed is thus easily obtained. I

As shown in Fig. 4, the no-hole wheel may be mounted on a metal or other rigid plate I8 by means of a layer of vulcanized hard or soft rubber 20. This plate I8 is adapted to be suitably fastened as by means of bolts 22' to a hub 24 constructed for mounting on the end of a wheel spindle of a standard grinding machine. The plastic rubber may be placed between the side of the completed grinding wheel and the iron plate and then subjected to heat and pressure to cause the rubber to stick to the iron plate and to permeate the surface pores of 'the wheel where it is vulcanized in position. Various expedients may be adopted for the purpose of increasing the cohesion of the rubber cement with the wheel face and the metal plate. Likewise, a resinoid cement may be adopted for this purpose, such as the unconverted Bakelite phenol aldehyde condensation product which is applied in a plastic condition between the mounting plate and wheel and is set to a hard Yinfusible condition by means of heat, as is well understood in the art.

m Another way of strengthening the no-hole Wheel involves impregnating the central portion of the wheel with a strengthening bond or cement. A porous wheel of uniform structure which has no central hole may be made of desired grade and structure in accordance with the prior art methods but may be provided with mounting holes as indicated above. The central zone of this wheel is then impregnated with a strengthening agent which is capable of i'llling the pores of the wheel suiiciently and of adhering to the surfaces thereof so as to materially aid the wheel in withstanding the stresses of the grinding, but the outer zone is left uniilled. Of the various materials which may be employed for that purpose, vulcanizable rubber and a plastic unconverted resinoid of a type of the Bakelite phenolic condensation product are preferred, although'other materials such as rosin, sodium silicate, and other well known cementitious materials or grinding wheel bonds may be. employed for the purpose. The wheel may be impregnated with this material by any suitable procedure. For example, as shown in Fig. 5, the wheel may have annular plates 30 and a ring 22 mounted so as to enclose the outer grinding zone which is not to be impregnated to any material extent. Then the strengthening material may be flowed or forced into the pores of the wheel in the central zone thereof. For instance, melted rosin or a liquid resinoid or sodium silicate may be poured onto the two wheel sides and allowed to permeate the pores by the natural capillary action. A vacuum mayI be applied to the wheel face to cause the material to permeate the same. If rubber is used, a layer of the plastic rubber compounded with sulfur in the right proportions may be forced under pressure and heat into the wheel faces and thus caused to permeate the same. The rubber may also be employed as a solution or in any other suitable form for the purpose. After the strengthening material has lled the pores of the central zone sumciently, it is caused to set to a hard condition, as by vulcanizing the rubber compound or converting the resinoid to the infusible condition by the application of sufficient heat for the purpose. The wheel may be preheated prior to the introduction of the lling material in order to cause it to flow more readily, or one may preheat only n the inner zone by means of a iiame or by heating the central zone within an oven while the outer zone is suitably insulated by thermal insulation. .Other expedients may be adopted for the purpose. By this means, the' vulcanized rubber or the resinoid or other medium employed is 'caused to grip the pore surfaces and to be sufficiently integral therewith so that it aids the wheel materially in resisting 'the stress set up by centrifugal force and the heat of grinding. 'I'hus the wheel is strengthened in thatimperforate continuous zone where the maximum stresses are present. 'I'his internal strengthening zone may be sufciently Wide so that it will receive the major -portion of thev stress, but the rosin, etc. should preferably not extend materially into the zone used for grinding. The impregnatedzone may extend throughout approximately 40% of the radial distance within the wheel, as indicated in Fig. 1.

Likewise, a vitrified grinding wheel made of abrasive grains cemented together by vitried' ceramic bonds may be so made as to have no hole there supported while red within a ceramic kiln.

The wheel 34 may be covered with refractory clay 36 or other suitable heat insulating material in the manner shown in Fig. 6 so that the clay is increasingly deeper going from the center of a wheel outwardly towards its periphery. Thus the wheel will radiate heat more quickly at the central zone during the cooling operation than it will on the outer portion and the center will set first from a plastic to a solid condition and thereafter the still plastic outer zone will shrink as it sets and so impose a compressional strain upon the central zone of the wheel. 'Ihis strained condition within the lwheel material tends to resist the stresses set up by rotation and heat during use of the wheel.

The grinding wheel may be so shaped and mounted that it will grind on its peripheral face or on a at side face. The wheel may therefore be shaped as a diskor in any other form which is foud best suited for a given grinding operation. The principles of this invention apply irrespective of the shape, kind, type, size or other physical characteristics of the wheel as well as to the various wheel structures made by using the different type of abrasive and of bond in any suitable proportions and structural arrangement.

'Ihis case is a division of our co-pending, application Serial Number 48,181 filed November 4, 1935. A

We claim:

1. A composite grinding wheel comprising a body of bonded granularabrasive material having an outer annular zone of required grinding characteristics and an inner zone integral therewith which is continuous and imperforate at the wheel center', said inner zone comprising abrasive material and a bond which has a greater strength than has the outer zone and being of such size that the wheel may be rotated safely at a rate in excess of the maximum safe speed for a wheel made solely of the outer zone material and structure! 2. A composite grinding wheel comprising an outer annular zone of bonded granular abrasive material of required grinding characteristics, and

an inner strengthening zone of bonded granular material of predetermined different characteristicsintegrally united with the outer zone and which is continuous andimperforate at the wheel center in the zone of maximum stress, the inner zone containing sufficient granular material of a finer grit size than has the average grain in the outer zone and the bond and granular material being of such character and proportions relative to those of the outer zone that the inner zone has a higher strength than has the outer zone and such a rupturaldeformation and modulus of elas- `:f5

ticity that the composite wheel will withstand greater stress due to the rotation or peripheral heat than will a. wheel made wholly of the outer zone composition and structure.

3. A grinding Wheel according to claim 1 in which the bond and the abrasive material in the inner zone are so constituted and proportioned that the ruptural deformation and modulus of elasticity are such that the wheel will withstand a greater stress due to rotation or peripheral heat than will a wheel made solely of the outer zone `structure and material.

4. A grinding wheel comprising abrasive grains united by a bond into a required grinding structure and which is continuous and imperforate throughout the central high stress zone, said wheel being impregnated with a strengthening material within the-central zone of the wheel.

5. A grinding wheel of the type covered by claim 1 in which the central zone comprises bonded abrasive material united as a porous structure which is impregnated with a heat set strengthening material.

6. A grinding wheel comprising abrasive grains united by a bond as a flat sided disk of the required grinding structure having an outer zone providing a peripheral grinding face and an inner strengthening zone integral with the outer zone which is continuous and imperforate at the Wheel center and has a stronger structure and so is more resistant than is the outer grinding zone to the stresses of centrifugal force and frictional heat at the periphery of the Wheel, a supporting plate and a cementitious medium uniting one face of the plate integrally to one side of the wheel solely within the inner zone of high strength.

. HERBERT W. WAGNER.

KENNETH F. WHITCOMB.

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