TECHNICAL FIELD

The present disclosure relates to depressed center grinding wheels.

BACKGROUND

Depressed center grinding wheels are often used in combination with a handheld portable grinder held by an operator, either at an angle of 90 degrees (e.g., when used as a cut of wheel) or at an angle of up to about 30 degrees (e.g., when used for grinding welding beads, flash, gate, and risers off of castings), more typically about 15 degrees, relative to the surface of the workpiece being abraded. Depressed center wheels may also be referred to in the abrasive art as raised hub wheels or by their shape designation of "Type" (e.g., Types 27, 28, and 29), with Type 27 being the most popular.

The depressed center design allows a flange/lock nut to recess within the wheel so that it can be used for various grinding and cutting applications. Bonded abrasive articles have abrasive particles bonded together by a bonding medium. The bonding medium is typically an organic resin, but may also be an inorganic material such as a ceramic or glass (i.e., vitreous bonds). Examples of bonded abrasive articles include stones, hones, and abrasive wheels such as, for example, grinding wheels and cut-off wheels.

Grinding wheels are of various shapes may be, for example, driven by a stationary-mounted motor such as, for example, a bench grinder, or attached and driven by a hand-operated portable grinder.

Hand-operated portable grinders are typically held at a slight angle relative to the workpiece surface, and may be used to grind, for example, welding beads, flash, gates, and risers off castings.

SUMMARY

In recent years there have efforts to include shaped abrasive particles in various grinding wheels

(e.g., depressed center grinding wheels); however, the relatively high cost of such abrasive particles remains an obstacle to their widespread acceptance in the industry. It would be desirable to have alternative constructions that can reduce cost of grinding wheels by reducing the amount of shaped abrasive particles while achieving comparable abrading performance.

Advantageously, the present disclosure solves this technical problem in the case of depressed center grinding wheels by providing a depressed center grinding wheel comprising an abrasive disc having a working surface and a back surface opposite the working surface, wherein the working surface has a depressed center portion, and wherein the abrasive disc comprises:

a working layer comprising first abrasive particles retained in a first binder material, the first abrasive particles comprising first shaped abrasive particles, wherein the first shaped abrasive particles comprise from 40 to 100 weight percent of the first abrasive particles; a back layer comprising second abrasive particles retained in a second binder material comprising first crushed abrasive particles and essentially free of shaped abrasive particles;

an intermediate layer disposed between the working layer and the back layer, the intermediate layer comprising third abrasive particles retained in a third binder material, the intermediate layer comprising second shaped abrasive particles and second crushed abrasive particles, wherein the second shaped abrasive particles comprise 25 to 75 weight percent of the second abrasive particles;

a first reinforcing scrim sandwiched between the back layer and the intermediate layer; and a second reinforcing scrim adjacent one of:

the back layer opposite the intermediate layer;

the intermediate layer opposite the back layer; or

the working layer opposite the intermediate layer.

Depressed center grinding wheels according to the present disclosure are useful; for example, for abrading a surface of a workpiece.

Accordingly, in another aspect, the present disclosure provides a method of abrading a workpiece, the method comprising contacting a workpiece with the working surface of a depressed center grinding wheel according to the present disclosure and moving the working surface relative to the workpiece to abrade the workpiece.

As used herein, the term "nominal" means: of, being, or relating to a designated or theoretical size and/or shape that may vary somewhat from the actual (e.g., within a manufacturing process tolerance).

As used herein, the term "shaped abrasive particle" refers to an abrasive particle (e.g., a ceramic abrasive particle) with at least a portion of the abrasive particle having a nominal predetermined shape corresponding to a mold cavity used to form a precursor shaped abrasive particle, which is then calcined and sintered to form the shaped abrasive particle. Shaped abrasive particle as used herein excludes abrasive particles shaped solely by a mechanical crushing process.

As used herein, the term "crushed abrasive particle" refers to an abrasive particle shaped solely by a mechanical crushing process.

As used herein, the term "essentially free of means containing less than 5 weight percent of (preferably less than 1 weight percent of, or even free of).

Features and advantages of the present disclosure will be further understood upon consideration of the detailed description as well as the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic side view of an exemplary depressed center grinding wheel 100 according to the present disclosure.

Figs. 2A-2F are schematic cross-sectional representations showing various exemplary configurations of scrim placement in exemplary depressed center grinding wheels lOOa-lOOf.

Fig. 3 is a schematic perspective view of exemplary shaped abrasive particle 300. Fig. 4 is a schematic side view showing a depressed center grinding wheel 100 abrading a workpiece 400 according to the present disclosure.

Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. The figures may not be drawn to scale.

DETAILED DESCRIPTION

Referring now to Fig. 1, depressed center grinding wheel 100 comprises an abrasive disc 1 10 having a working surface 122 and a back surface 142 opposite the working surface 122. Working surface 1 12 has a depressed center portion 1 14. Abrasive disc 1 10 comprises working layer 120, intermediate layer 130, and back layer 140.

Working layer 120 comprises first abrasive particles 124 retained in a first binder material 126. The first abrasive particles 124 comprise first shaped abrasive particles 125. The first shaped abrasive particles 125 comprise from 40 to 100 weight percent of the first abrasive particles 124. The first abrasive particles 124 may also comprise crushed abrasive particles, if desired, in amounts of up to about 60 percent by weight (e.g., 5 to 60 percent by weight, 20 to 60 percent by weight, or 40 to 60 percent by weight), based on the total weight of the first abrasive particles. The first shaped abrasive particles 125 may all be of the same size and shape or they may be a mixture of various shaped abrasive particles with different sizes, shapes, and/or compositions. In some preferred embodiments, the first abrasive particles are all shaped abrasive particles having the same nominal size and shape. The first shaped abrasive particles 125 may all be of the same size and shape or they may be a mixture of various shaped abrasive particles with different sizes and/or shapes. Likewise, any optional crushed abrasive particles included in the first abrasive particles may have any size distribution and/or compositional distribution.

Back layer 140 comprises second abrasive particles 144 retained in a second binder material 146 comprises first crushed abrasive particles 148, and is essentially free of shaped abrasive particles (e.g., first shaped abrasive particles, second shaped abrasive particles, or other shaped abrasive particles).

The second abrasive particles 144 comprise at least 95 percent by weight of first crushed abrasive particles, preferably at least 99 percent by weight, and more preferably about 100 percent by weight of first crushed abrasive particles, based on the total weight of second abrasive particles. Accordingly, the back layer is essentially free of shaped abrasive particles. The second abrasive particles 144 may be of any size. The second abrasive particles 144 may have any size distribution and/or compositional distribution.

Intermediate layer 130 is disposed between the working layer 120 and the back layer 140. The intermediate layer 130 comprises third abrasive particles 134 retained in a third binder material 136. The intermediate layer 130 comprises second shaped abrasive particles 135 and second crushed abrasive particles 138. The second shaped abrasive particles 135 comprise 25 to 75 weight percent (e.g., 30 to 60 weight percent, or 40 to 60 percent) of the second abrasive particles. The second shaped abrasive particles 135 may all be of the same size and shape or they may be a mixture of various shaped abrasive particles with different sizes, shapes, and/or compositions. In some preferred embodiments, the second abrasive particles include second shaped abrasive particles having the same nominal size and shape. The second shaped abrasive particles 135 may all be of the same size and shape or they may be a mixture of various shaped abrasive particles with different sizes and/or shapes. Likewise, the second crushed abrasive particles may have any size distribution and/or compositional distribution.

Preferably, the first and second shaped abrasive particles are the same (i.e., same compositional, shape, and size distribution); however, they may be different in other embodiments, if desired. Likewise, the first and second crushed abrasive particles are preferably the same (i.e., same compositional and size distribution); however, they may be different in other embodiments, if desired.

While shaped abrasive particles in Fig. 1 are shown as vertically aligned triangles, this is for illustration purposes only, and the shaped abrasive particles may have any orientation (e.g., randomly aligned or aligned parallel to the backing).

Referring again to Fig. 1, depressed center grinding wheels include at least two reinforcing scrims deployed at various locations throughout the grinding wheel. First reinforcing scrim 150 is sandwiched between back layer 140 and intermediate layer 130. Second reinforcing scrim 152 (not shown) is positioned adjacent to one of: a) back layer 140 opposite intermediate layer 130; b) intermediate layer 130 opposite back layer 140; or c) working layer 120 opposite intermediate layer 130.

Optional centrally disposed arbor hole 170 extends through abrasive disc 1 10. Optional attachment member 175 is centrally disposed, and optionally secured by nut 180, to back surface 142 of abrasive disc 1 10, although this is not a requirement.

In some embodiments, the second reinforcing scrim 152 is sandwiched between the working layer 120 and the intermediate layer 130. Examples include the embodiments shown in Fig. 2B (shown as 152b), Fig. 2D (shown as 152d), and Fig. 2F (shown as 152f). In some of these embodiments, a third reinforcing scrim 156 is bonded to the working layer 1 10 opposite the intermediate layer (e.g., see 156b in Fig. 2B).

In some embodiments, second reinforcing scrim 152 is secured to the back layer opposite the intermediate layer. Examples are shown in Fig. 2A (shown as 152a), Fig. 2C (shown as 152c), and Fig. 2E (shown as 152e). In some of these embodiments, third reinforcing scrim 156 is sandwiched between the intermediate layer and the working layer (e.g., shown as 156e in Fig. 2E). In some of these embodiments, an optional fourth reinforcing scrim 169 is bonded to working layer 120 opposite intermediate layer 130 (e.g., shown as 169e in Fig. 2E).

In Fig. 2C, optional third scrim 156c is secured to working layer 120 opposite the intermediate layer 130. Likewise, in Fig. 2F, optional third scrim 156f is secured to working layer 120 opposite the intermediate layer 130. Depressed center grinding wheels according to the present disclosure are generally made by compression molding, injection molding, transfer molding, or the like. The molding can be done either by hot or cold pressing or any suitable manner known to those skilled in the art. During the

manufacturing, the individual components (e.g., working layer, intermediate layer, back layer, and scrims) are typically layered up into a green body that is then subjected to curing conditions. The green body typically contains one or more binder material precursors, either liquid organic, powdered inorganic, powdered organic, or a combination of thereof, mixed with abrasive particles (i.e., shaped abrasive particles and crushed abrasive particles selected and positioned as described herein), and reinforcing scrims (positioned at desired locations in the wheel). In some instances, a liquid medium (either resin or a solvent) is first applied to the abrasive particles to wet their outer surface, and then the wetted particles are mixed with a powdered medium.

The various binder materials in the working layer, intermediate layer, and back layer (which may be the same or different, preferably the same) typically comprise a glassy inorganic material (e.g., as in the case of vitrified abrasive wheels), metal, or an organic resin (e.g., as in the case of resin-depressed center grinding wheels) .

Glassy vitreous binders may be made from a mixture of different metal oxides. Examples of these metal oxide vitreous binders include silica, alumina, calcia, iron oxide, titania, magnesia, sodium oxide, potassium oxide, lithium oxide, manganese oxide, boron oxide, phosphorous oxide, and the like. Specific examples of vitreous binders based upon weight include, for example, 47.61 percent Si0 ₂ , 16.65 percent A1 ₂ 0 ₃ , 0.38 percent Fe ₂ O3, 0.35 percent Ti0 ₂ , 1.58 percent CaO, 0.10 percent MgO, 9,63 percent Na ₂ 0, 2.86 percent K ₂ 0, 1.77 percent Li ₂ 0, 19.03 percent B ₂ C>3, 0.02 percent Mn0 ₂ , and 0.22 percent P ₂ C>5 ; and 63 percent Si0 ₂ , 12 percent A1 ₂ C>3, 1.2 percent CaO, 6.3 percent Na ₂ 0, 7.5 percent

K ₂ 0, and 10 percent B ₂ C>3. During manufacture of a vitreous bonded depressed center grinding wheel, vitreous binder in powder form, may be mixed with a temporary binder, typically an organic temporary binder. The vitrified binders may also be formed from a frit, for example anywhere from about one to 100 percent frit, but generally 20 to 100 percent frit. Some examples of common materials used in frit binders include feldspar, borax, quartz, soda ash, zinc oxide, whiting, antimony trioxide, titanium dioxide, sodium silicofluoride, flint, cryolite, boric acid, and combinations thereof. These materials are usually mixed together as powders, fired to fuse the mixture and then the fused mixture is cooled. The cooled mixture is crushed and screened to a very fine powder to then be used as a frit vitreous binder precursor. The temperature at which the frit vitreous binder precursor is matured to form a vitreous binder is dependent upon its chemistry, but typically ranges from about 600° C to about 1800° C, although this is not a requirement.

Examples of metal binders include tin, copper, aluminum, nickel, and combinations thereof. Metal binder materials can be formed by sintering metal powders, optionally containing a temporary organic binder material that burns off during sintering. Organic binder materials are typically included in an amount of from 5 to 30 percent, more typically 10 to 25, and more typically 15 to 24 percent by weight, based of the total weight of the depressed center grinding wheel. Phenolic resin is the most commonly used organic binder material, and may be used in both the powder form and liquid state. Although phenolic resins are widely used, it is within the scope of this disclosure to use other organic binder materials including, for example, epoxy resins, urea-formaldehyde resins, rubbers, shellacs, and acrylic binders. The organic binder material may also be modified with other binder materials to improve or alter the properties of the binder material.

Useful phenolic resins include novolac and resole phenolic resins. Novolac phenolic resins are characterized by being acid-catalyzed and having a ratio of formaldehyde to phenol of less than one, typically between 0.5: 1 and 0.8: 1. Resole phenolic resins are characterized by being alkaline catalyzed and having a ratio of formaldehyde to phenol of greater than or equal to one, typically from 1 : 1 to 3 : 1. Novolac and resole phenolic resins may be chemically modified (e.g., by reaction with epoxy

compounds), or they may be unmodified. Exemplary acidic catalysts suitable for curing phenolic resins include sulfuric, hydrochloric, phosphoric, oxalic, and p-toluenesulfonic acids. Alkaline catalysts suitable for curing phenolic resins include sodium hydroxide, barium hydroxide, potassium hydroxide, calcium hydroxide, organic amines, or sodium carbonate.

Phenolic resins are well-known and readily available from commercial sources. Examples of commercially available novolac resins include DUREZ 1364, a two-step, powdered phenolic resin (marketed by Durez Corporation of Addison, Texas under the trade designation VARCUM (e.g., 29302), or HEXION AD5534 RESIN (marketed by Hexion Specialty Chemicals, Inc. of Louisville, Kentucky). Examples of commercially available resole phenolic resins useful in practice of the present disclosure include those marketed by Durez Corporation under the trade designation VARCUM (e.g., 29217, 29306, 29318, 29338, 29353); those marketed by Ashland Chemical Co. of Bartow, Florida under the trade designation AEROFENE (e.g., AEROFENE 295); and those marketed by Kangnam Chemical Company Ltd. of Seoul, South Korea under the trade designation "PHENOLITE" (e.g., PHENOLITE TD-2207).

Curing temperatures of organic binder material precursors will generally vary with the material chosen and wheel design. Selection of suitable conditions is within the capability of one of ordinary skill in the art. Exemplary conditions for a phenolic binder may include an applied pressure of about 20 tons per 4 inches diameter (224 kg/cm^) at room temperature followed by heating at temperatures up to about 185 °C (degrees Celsius) for sufficient time to cure the organic binder material precursor.

In some embodiments, the depressed center grinding wheels include from about 10 to 60 percent by weight of abrasive particles; typically 30 to 60 percent by weight, and more typically 40 to 60 percent by weight, based on the total weight of the binder material(s) and abrasive particles.

Shaped abrasive particles composed of crystallites of alpha alumina, magnesium alumina spinel, and a rare earth hexagonal aluminate may be prepared using sol-gel precursor alpha alumina particles according to methods described in, for example, U.S. Patent No. 5,213,591 (Celikkaya et al.) and U.S. Publ. Patent Appl. Nos. 2009/0165394 Al (Culler et al.) and 2009/0169816 Al (Erickson et al.). In some embodiments, alpha-alumina-based shaped abrasive particles can be made according to a multistep process. Briefly, the method comprises the steps of making either a seeded or non-seeded sol- gel alpha alumina precursor dispersion that can be converted into alpha alumina; filling one or more mold cavities having the desired outer shape of the shaped abrasive particle with the sol-gel, drying the sol-gel to form precursor shaped abrasive particles; removing the precursor shaped abrasive particles from the mold cavities; calcining the precursor shaped abrasive particles to form calcined, precursor shaped abrasive particles, and then sintering the calcined, precursor shaped abrasive particles to form shaped abrasive particles. The process will now be described in greater detail.

The first process step involves providing either a seeded or non-seeded dispersion of an alpha alumina precursor that can be converted into alpha alumina. The alpha alumina precursor dispersion often comprises a liquid that is a volatile component. In one embodiment, the volatile component is water. The dispersion should comprise a sufficient amount of liquid for the viscosity of the dispersion to be sufficiently low to enable filling mold cavities and replicating the mold surfaces, but not so much liquid as to cause subsequent removal of the liquid from the mold cavity to be prohibitively expensive. In one embodiment, the alpha alumina precursor dispersion comprises from 2 percent to 90 percent by weight of the particles that can be converted into alpha alumina, such as particles of aluminum oxide monohydrate (boehmite), and at least 10 percent by weight, or from 50 percent to 70 percent, or 50 percent to 60 percent, by weight of the volatile component such as water. Conversely, the alpha alumina precursor dispersion in some embodiments contains from 30 percent to 50 percent, or 40 percent to 50 percent, by weight solids.

Aluminum oxide hydrates other than boehmite can also be used. Boehmite can be prepared by known techniques or can be obtained commercially. Examples of commercially available boehmite include products having the trade designations "DISPERAL", and "DISPAL", both available from Sasol North America, Inc. of Houston, Texas, or "HiQ-40" available from BASF Corporation of Florham Park, New Jersey. These aluminum oxide monohydrates are relatively pure; that is, they include relatively little, if any, hydrate phases other than monohydrates, and have a high surface area.

The physical properties of the resulting shaped abrasive particles will generally depend upon the type of material used in the alpha alumina precursor dispersion. In one embodiment, the alpha alumina precursor dispersion is in a gel state. As used herein, a "gel" is a three dimensional network of solids dispersed in a liquid.

The alpha alumina precursor dispersion may contain a modifying additive or precursor of a modifying additive. The modifying additive can function to enhance some desirable property of the abrasive particles or increase the effectiveness of the subsequent sintering step. Modifying additives or precursors of modifying additives can be in the form of soluble salts, typically water soluble salts. They typically consist of a metal-containing compound and can be a precursor of oxide of magnesium, zinc, iron, silicon, cobalt, nickel, zirconium, hafnium, chromium, yttrium, praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium, titanium, and mixtures thereof. The particular concentrations of these additives that can be present in the alpha alumina precursor dispersion can be varied based on skill in the art.

Typically, the introduction of a modifying additive or precursor of a modifying additive will cause the alpha alumina precursor dispersion to gel. The alpha alumina precursor dispersion can also be induced to gel by application of heat over a period of time. The alpha alumina precursor dispersion can also contain a nucleating agent (seeding) to enhance the transformation of hydrated or calcined aluminum oxide to alpha alumina. Nucleating agents suitable for this disclosure include fine particles of alpha alumina, alpha ferric oxide or its precursor, titanium oxides and titanates, chrome oxides, or any other material that will nucleate the transformation. The amount of nucleating agent, if used, should be sufficient to effect the transformation of alpha alumina. Nucleating such alpha alumina precursor dispersions is disclosed in U.S. Patent No. 4,744,802 (Schwabel).

A peptizing agent can be added to the alpha alumina precursor dispersion to produce a more stable hydrosol or colloidal alpha alumina precursor dispersion. Suitable peptizing agents are monoprotic acids or acid compounds such as acetic acid, hydrochloric acid, formic acid, and nitric acid. Multiprotic acids can also be used but they can rapidly gel the alpha alumina precursor dispersion, making it difficult to handle or to introduce additional components thereto. Some commercial sources of boehmite contain an acid titer (such as absorbed formic or nitric acid) that will assist in forming a stable alpha alumina precursor dispersion.

The alpha alumina precursor dispersion can be formed by any suitable means, such as, for example, by simply mixing aluminum oxide monohydrate with water containing a peptizing agent or by forming an aluminum oxide monohydrate slurry to which the peptizing agent is added.

Defoamers or other suitable chemicals can be added to reduce the tendency to form bubbles or entrain air while mixing. Additional chemicals such as wetting agents, alcohols, or coupling agents can be added if desired. The alpha alumina abrasive particles may contain silica and iron oxide as disclosed in U.S. Patent No. 5,645,619 (Erickson et al.). The alpha alumina abrasive particles may contain zirconia as disclosed in U.S. Patent No. 5,551,963 (Larmie). Alternatively, the alpha alumina abrasive particles can have a microstructure or additives as disclosed in U.S. Patent No. 6,277,161 (Castro).

The second process step involves providing a mold having at least one mold cavity, and preferably a plurality of cavities. The mold can have a generally planar bottom surface and a plurality of mold cavities. The plurality of cavities can be formed in a production tool. The production tool can be a belt, a sheet, a continuous web, a coating roll such as a rotogravure roll, a sleeve mounted on a coating roll, or die. In one embodiment, the production tool comprises polymeric material. Examples of suitable polymeric materials include thermoplastics such as polyesters, polycarbonates, poly(ether sulfone), poly(methyl methacrylate), polyurethanes, poly(vinyl chloride), polyolefin, polystyrene, polypropylene, polyethylene or combinations thereof, or thermosetting materials. In one embodiment, the entire tooling is made from a polymeric or thermoplastic material. In another embodiment, the surfaces of the tooling in contact with the sol-gel while drying, such as the surfaces of the plurality of cavities, comprises polymeric or thermoplastic materials and other portions of the tooling can be made from other materials. A suitable polymeric coating may be applied to a metal tooling to change its surface tension properties by way of example.

A polymeric or thermoplastic tool can be replicated off a metal master tool. The master tool will have the inverse pattern desired for the production tool. The master tool can be made in the same manner as the production tool. In one embodiment, the master tool is made out of metal, e.g., nickel and is diamond turned. The polymeric sheet material can be heated along with the master tool such that the polymeric material is embossed with the master tool pattern by pressing the two together. A polymeric or thermoplastic material can also be extruded or cast onto the master tool and then pressed. The thermoplastic material is cooled to solidify and produce the production tool. If a thermoplastic production tool is utilized, then care should be taken not to generate excessive heat that may distort the thermoplastic production tool limiting its life. More information concerning the design and fabrication of production tooling or master tools can be found in U.S. Patent Nos. 5, 152,917 (Pieper et al.); 5,435,816 (Spurgeon et al.); 5,672,097 (Hoopman et al.); 5,946,991 (Hoopman et al.); 5,975,987 (Hoopman et al.); and 6, 129,540 (Hoopman et al . ) .

Access to cavities can be from an opening in the top surface or bottom surface of the mold. In some instances, the cavities can extend for the entire thickness of the mold. Alternatively, the cavities can extend only for a portion of the thickness of the mold. In one embodiment, the top surface is substantially parallel to bottom surface of the mold with the cavities having a substantially uniform depth. At least one side of the mold, that is, the side in which the cavities are formed, can remain exposed to the surrounding atmosphere during the step in which the volatile component is removed.

The cavities have a specified three-dimensional shape to make the shaped abrasive particles. The depth dimension is equal to the perpendicular distance from the top surface to the lowermost point on the bottom surface. The depth of a given cavity can be uniform or can vary along its length and/or width. The cavities of a given mold can be of the same shape or of different shapes.

The third process step involves filling the cavities in the mold with the alpha alumina precursor dispersion (e.g., by a conventional technique). In some embodiments, a knife roll coater or vacuum slot die coater can be used. A mold release can be used to aid in removing the particles from the mold if desired. Typical mold release agents include oils such as peanut oil or mineral oil, fish oil, silicones, polytetrafluoroethylene, zinc stearate, and graphite. In general, mold release agent such as peanut oil, in a liquid, such as water or alcohol, is applied to the surfaces of the production tooling in contact with the sol- gel such that between about 0.1 mg/in^ (0.02 mg/cm^) to about 3.0 mg/in^ 0.46 mg/cm^), or between about 0.1 mg/in^ (0.02 mg/cm^) to about 5.0 mg/in^ (0.78 mg/cm^) of the mold release agent is present per unit area of the mold when a mold release is desired. In some embodiments, the top surface of the mold is coated with the alpha alumina precursor dispersion. The alpha alumina precursor dispersion can be pumped onto the top surface. Next, a scraper or leveler bar can be used to force the alpha alumina precursor dispersion fully into the cavity of the mold. The remaining portion of the alpha alumina precursor dispersion that does not enter cavity can be removed from top surface of the mold and recycled. In some embodiments, a small portion of the alpha alumina precursor dispersion can remain on the top surface and in other embodiments the top surface is substantially free of the dispersion. The pressure applied by the scraper or leveler bar is typically less than 100 psi (0.7 MPa), less than 50 psi (0.3 MPa), or even less than 10 psi (69 kPa). In some embodiments, no exposed surface of the alpha alumina precursor dispersion extends substantially beyond the top surface to ensure uniformity in thickness of the resulting shaped abrasive particles.

The fourth process step involves removing the volatile component to dry the dispersion.

Desirably, the volatile component is removed by fast evaporation rates. In some embodiments, removal of the volatile component by evaporation occurs at temperatures above the boiling point of the volatile component. An upper limit to the drying temperature often depends on the material the mold is made from. For polypropylene tooling the temperature should be less than the melting point of the plastic. In one embodiment, for a water dispersion of between about 40 to 50 percent solids and a polypropylene mold, the drying temperatures can be between about 90 °C to about 165 °C, or between about 105 °C to about 150 °C, or between about 105 °C to about 120 °C. Higher temperatures can lead to improved production speeds but can also lead to degradation of the polypropylene tooling limiting its useful life as a mold.

The fifth process step involves removing resultant precursor shaped abrasive particles with from the mold cavities. The precursor shaped abrasive particles can be removed from the cavities by using the following processes alone or in combination on the mold: gravity, vibration, ultrasonic vibration, vacuum, or pressurized air to remove the particles from the mold cavities.

The precursor abrasive particles can be further dried outside of the mold. If the alpha alumina precursor dispersion is dried to the desired level in the mold, this additional drying step is not necessary. However, in some instances it may be economical to employ this additional drying step to minimize the time that the alpha alumina precursor dispersion resides in the mold. Typically, the precursor shaped abrasive particles will be dried from 10 to 480 minutes, or from 120 to 400 minutes, at a temperature from 50 °C to 160 °C, or at 120 °C to 150 °C.

The sixth process step involves calcining the precursor shaped abrasive particles. During calcining, essentially all the volatile material is removed, and the various components that were present in the alpha alumina precursor dispersion are transformed into metal oxides. The precursor shaped abrasive particles are generally heated to a temperature from 400 °C to 800 °C, and maintained within this temperature range until the free water and over 90 percent by weight of any bound volatile material are removed. In an optional step, it may be desired to introduce the modifying additive by an impregnation process. A water-soluble salt can be introduced by impregnation into the pores of the calcined, precursor shaped abrasive particles. Then the precursor shaped abrasive particles are pre-fired again. This option is further described in U.S. Patent No. 5, 164,348 (Wood). The seventh process step involves sintering the calcined, precursor shaped abrasive particles to form alpha alumina particles. Prior to sintering, the calcined, precursor shaped abrasive particles are not completely densified and thus lack the desired hardness to be used as shaped abrasive particles. Sintering takes place by heating the calcined, precursor shaped abrasive particles to a temperature of from 1,000 °C to 1,650 °C and maintaining them within this temperature range until substantially all of the alpha alumina monohydrate (or equivalent) is converted to alpha alumina and the porosity is reduced to less than 15 percent by volume. The length of time to which the calcined, precursor shaped abrasive particles must be exposed to the sintering temperature to achieve this level of conversion depends upon various factors but usually from five seconds to 48 hours is typical.

In another embodiment, the duration for the sintering step ranges from one minute to 90 minutes.

After sintering, the shaped abrasive particles can have a Vickers hardness of 10 GPa, 16 GPa, 18 GPa, 20 GPa, or greater.

Other steps can be used to modify the described process such as, for example, rapidly heating the material from the calcining temperature to the sintering temperature, centrifuging the alpha alumina precursor dispersion to remove sludge and/or waste. Moreover, the process can be modified by combining two or more of the process steps if desired. Conventional process steps that can be used to modify the process of this disclosure are more fully described in U.S. Patent No. 4,314,827 (Leitheiser).

Shaped abrasive particles used in the present disclosure may comprise plates, rods, or a combination thereof, for example. In preferred embodiments, the shaped abrasive particles have shapes that can be characterized as thin bodies having triangular, rectangular (including square), or other geometric shapes with sharp points. Such shaped abrasive particles have a front face and a back face, both of which faces have substantially the same geometric shape. The faces are separated by the thickness of the particle. The ratio of the length of the shortest facial dimension of an abrasive particle to its thickness is at least 1 to 1, preferably at least 2 to 1, more preferably at least 5 to 1, and most preferably at least 6 to 1.

Preferred shaped abrasive particles are shaped as rectangular (including square), or triangular plates, preferably having a sloping sidewall; for example, triangular particles having a sloping sidewall as described in U.S. Patent No. 8,142,531 (Adefris et al.). Fig. 3 shows an exemplary such shaped abrasive particle 300 having the shape of a truncated triangular pyramid.

Further details concerning methods for making shaped abrasive particles are described in U.S.

Patent Nos. 8,764,865 (Adefris et al.), 8, 142,532 (Adefris et al.), 8,123,828 (Adefris et al.), 8,142,891 (Culler et al.), 5,366,523 (Rowenhorst et al.), and 5,204,916 (Berg et al), and in U.S. Publ. Patent Appln. No. 2009/0165394 Al (Culler et al.) and 2013/0040537 Al (Erickson et al.).

The shaped abrasive particles used in the present disclosure can typically be made using tools (i.e., molds) cut using diamond tooling, which provides higher feature definition than other fabrication alternatives such as, for example, stamping or punching. Typically, the cavities in the tool surface have planar faces that meet along sharp edges, and form the sides and top of a truncated pyramid. The resultant shaped abrasive particles have a respective nominal average shape that corresponds to the shape of cavities (e.g., truncated pyramid) in the tool surface; however, variations (e.g., random variations) from the nominal average shape may occur during manufacture, and shaped abrasive particles exhibiting such variations are included within the definition of shaped abrasive particles as used herein.

The shaped abrasive particles are typically selected to have a length in a range of from 0.001 mm to 26 mm, more typically 0.1 mm to 10 mm, and more typically 0.5 mm to 5 mm, although other lengths may also be used. In some embodiments, the length may be expressed as a fraction of the thickness of the depressed center grinding wheel in which it is contained. For example, the shaped abrasive particle may have a length greater than half the thickness of the depressed center grinding wheel. In some

embodiments, the length may be greater than the thickness of the depressed center grinding wheel.

The shaped abrasive particles are typically selected to have a width in a range of from 0.001 mm to 26 mm, more typically 0.1 mm to 10 mm, and more typically 0.5 mm to 5 mm, although other lengths may also be used.

The shaped abrasive particles are typically selected to have a thickness in a range of from 0.005 mm to 1.6 mm, more typically, from 0.2 to 1.2 mm.

In some embodiments, the shaped abrasive particles may have an aspect ratio (length to thickness) of at least 2, 3, 4, 5, 6, or more.

The first abrasive particles (i.e., in the working layer) may contain solely of first shaped abrasive particles, or first shaped abrasive particles in combination with an amount of third crushed abrasive particles. Likewise, the second abrasive particles (i.e., in the intermediate layer) may contain solely of second shaped abrasive particles, or second shaped abrasive particles in combination with an amount of second crushed abrasive particles. In any event, the ratio of the weight percent of the first shaped abrasive particles in the first abrasive particles to the weight percent of the second shaped abrasive particles in the second abrasive particles is from 40:60 to 60:40, preferably from 45 :55 to 55 :45.

The first and/or second abrasive particles may comprise more than one size or shape of shaped abrasive particles, although a single size and shape is typically preferred. The first and second abrasive particles may be the same or different, preferably the same, with regard to shape, size, and/or composition.

Surface coatings on the shaped abrasive particles may be used to improve the adhesion between the shaped abrasive particles and a binder material in abrasive articles, or can be used to aid in electrostatic deposition of the shaped abrasive particles. In one embodiment, surface coatings as described in U.S. Patent No. 5,352,254 (Celikkaya) in an amount of 0.1 to 2 percent surface coating to shaped abrasive particle weight may be used. Such surface coatings are described in U.S. Patent Nos.

5,213,591 (Celikkaya et al.); 5,01 1,508 (Wald et al.); 1,910,444 (Nicholson); 3,041, 156 (Rowse et al.); 5,009,675 (Kunz et al.); 5,085,671 (Martin et al.); 4,997,461 (Markhoff-Matheny et al.); and 5,042,991

(Kunz et al.). Additionally, the surface coating may prevent the shaped abrasive particle from capping.

Capping is the term to describe the phenomenon where metal particles from the workpiece being abraded become welded to the tops of the shaped abrasive particles. Surface coatings to perform the above functions are known to those of skill in the art.

Useful crushed abrasive particles include, for example, crushed particles of fused aluminum oxide, heat treated aluminum oxide, white fused aluminum oxide, ceramic aluminum oxide materials such as those commercially available under the trade designation 3M CERAMIC ABRASIVE GRAIN from 3M Company of St. Paul, Minnesota, black silicon carbide, green silicon carbide, titanium diboride, boron carbide, tungsten carbide, titanium carbide, diamond, cubic boron nitride, garnet, fused alumina zirconia, sol-gel derived abrasive particles, iron oxide, chromia, ceria, zirconia, titania, silicates, tin oxide, silica (such as quartz, glass beads, glass bubbles and glass fibers) silicates (such as talc, clays (e.g., montmorillonite), feldspar, mica, calcium silicate, calcium metasilicate, sodium aluminosilicate, sodium silicate), flint, and emery. Examples of sol-gel derived abrasive particles can be found in U.S. Patent Nos. 4,314,827 (Leitheiser et al.), 4,623,364 (Cottringer et al.); 4,744,802 (Schwabel), 4,770,671 (Monroe et al.); and 4,881,951 (Monroe et al.). It is also contemplated that the abrasive particles could comprise abrasive agglomerates such, for example, as those described in U.S. Patent Nos. 4,652,275 (Bloecher et al.) or 4,799,939 (Bloecher et al.).

Typically, conventional crushed abrasive particles are independently sized according to an abrasives industry recognized specified nominal grade. Exemplary abrasive industry recognized grading standards include those promulgated by ANSI (American National Standards Institute), FEPA

(Federation of European Producers of Abrasives), and JIS (Japanese Industrial Standard). Such industry accepted grading standards include, for example: ANSI 4, ANSI 6, ANSI 8, ANSI 16, ANSI 24, ANSI 30, ANSI 36, ANSI 40, ANSI 50, ANSI 60, ANSI 80, ANSI 100, ANSI 120, ANSI 150, ANSI 180, ANSI 220, ANSI 240, ANSI 280, ANSI 320, ANSI 360, ANSI 400, and ANSI 600; FEPA P8, FEPA P12, FEPA P16, FEPA P24, FEPA P30, FEPA P36, FEPA P40, FEPA P50, FEPA P60, FEPA P80, FEPA P100, FEPA P120, FEPA P150, FEPA P180, FEPA P220, FEPA P320, FEPA P400, FEPA P500, FEPA P600, FEPA P800, FEPA P1000, FEPA P1200; FEPA F8, FEPA F12, FEPA F16, and FEPA F24;.and

JIS 8, JIS 12, JIS 16, JIS 24, JIS 36, JIS 46, JIS 54, JIS 60, JIS 80, JIS 100, JIS 150, JIS 180, JIS 220, JIS 240, JIS 280, JIS 320, JIS 360, JIS 400, JIS 400, JIS 600, JIS 800, JIS 1000, JIS 1500, JIS 2500, JIS 4000, JIS 6000, JIS 8000, and JIS 10,000. More typically, the crushed aluminum oxide particles and the non-seeded sol-gel derived alumina-based abrasive particles are independently sized to ANSI 60 and 80, or FEPA F36, F46, F54 and F60 or FEPA P60 and P80 grading standards.

Alternatively, shaped abrasive particles can be graded to a nominal screened grade using U.S.A. Standard Test Sieves conforming to ASTM E-l 1 "Standard Specification for Wire Cloth and Sieves for Testing Purposes". ASTM E-l 1 prescribes the requirements for the design and construction of testing sieves using a medium of woven wire cloth mounted in a frame for the classification of materials according to a designated particle size. A typical designation may be represented as -18+20 meaning that the shaped abrasive particles pass through a test sieve meeting ASTM E-l 1 specifications for the number

18 sieve and are retained on a test sieve meeting ASTM E-l 1 specifications for the number 20 sieve. In one embodiment, the shaped abrasive particles have a particle size such that most of the particles pass through an 18 mesh test sieve and can be retained on a 20, 25, 30, 35, 40, 45, or 50 mesh test sieve. In various embodiments, the shaped abrasive particles can have a nominal screened grade

comprising: -18+20, -20/+25, -25+30,

-30+35, -35+40, -40+45, -45+50, -50+60, -60+70, -70/+80, -80+100, -100+120,

-120+140, -140+170, -170+200, -200+230, -230+270, -270+325, -325+400, -400+450,

-450+500, or -500+635. Alternatively, a custom mesh size could be used such as -90+100.

In some embodiments, some or all of the abrasive particles (shaped and/or crushed) are treated with a coupling agent (e.g., an organosilane coupling agent) to enhance adhesion of the abrasive particles to the binder. Coupling agents are well-known to those of skill in the abrasive arts. Examples of coupling agents include trialkoxysilanes (e.g., gamma-aminopropyltriethoxysilane), titanates, and zirconates. The abrasive particles may be treated before combining them with the binder material, or they may be surface treated in situ by including a coupling agent to the binder material.

In some embodiments, depressed center grinding wheels according to the present disclosure contain additional grinding aids such as, for example, polytetrafluoroethylene particles, cryolite, sodium chloride, FeS2 (iron disulfide), or KBF4; typically in amounts of from 1 to 25 percent by weight, more typically 10 to 20 percent by weight, subject to weight range requirements of the other constituents being met. Grinding aids are added to improve the cutting characteristics of the cut-off wheel, generally by reducing the temperature of the cutting interface. The grinding aid may be in the form of single particles or an agglomerate of grinding aid particles. Examples of precisely shaped grinding aid particles are taught in U.S. Patent Publ. No. 2002/0026752 Al (Culler et al.).

In some embodiments, the organic binder materials may contain plasticizer such as, for example, that available as SANTICIZER 154 PLASTICIZER from UNIVAR USA, Inc. of Chicago, Illinois.

Depressed center grinding wheels according to the present disclosure may contain additional components such as, for example, filler particles, subject to weight range requirements of the other constituents being met. Filler particles may be added to occupy space and/or provide porosity. Porosity enables the depressed center grinding wheel to shed used or worn abrasive particles to expose new or fresh abrasive particles.

Depressed center grinding wheels according to the present disclosure have any range of porosity; for example, from about 1 percent to 50 percent, typically 1 percent to 40 percent by volume. Examples of fillers include bubbles and beads (e.g., glass, ceramic (alumina), clay, polymeric, metal), cork, gypsum, marble, limestone, flint, silica, aluminum silicate, and combinations thereof.

Depressed center grinding wheels according to the present disclosure are useful, for example, as

Type 27 (e.g., as in American National Standards Institute standard ANSI B7.1-2000 (2000) in section 1.4.14) depressed-center grinding wheels.

Depressed center grinding wheels according to the present disclosure are typically 0.80 millimeter (mm) to 16 mm in thickness, more typically 1 mm to 8 mm, and typically have a diameter between 2.5 cm and 100 cm (40 inches), more typically between about 7 cm and 13 cm, although other dimensions may also be used (e.g., wheels as large as 100 cm in diameter are known). An optional center hole may be used to attaching the depressed center grinding wheel to a power driven tool. If present, the center hole is typically 0.5 cm to 2.5 cm in diameter, although other sizes may be used. The optional center hole may be reinforced; for example, by a metal flange. Alternatively, a mechanical fastener may be axially secured to one surface of the cut-off wheel. Examples include threaded posts, threaded nuts, Tinnerman nuts, and bayonet mount posts.

As discussed previously, depressed center grinding wheels according to the present disclosure include at least two scrims that reinforce the depressed center grinding wheel. Examples of scrims include woven or knitted cloth, mesh, and screens. The scrim may comprise glass fibers (e.g., fiberglass), organic fibers such as polyamide, polyester, or polyimide. The scrim may comprise an open mesh selected from the group consisting of woven, nonwoven, or knitted fiber mesh; synthetic fiber mesh; natural fiber mesh; metal fiber mesh; molded thermoplastic polymer mesh; molded thermoset polymer mesh; perforated sheet materials; slit and stretched sheet materials; and combinations thereof. The scrim need not be woven in a uniform pattern but may also include a nonwoven random pattern. Thus, the openings may either be in a pattern or randomly spaced. The scrim network openings may be rectangular or they may have other shapes including a diamond shape, a triangular shape, an octagonal shape or a combination of shapes.

In some instances, it may be desirable to include reinforcing staple fibers within the bonding medium, so that the fibers are homogeneously dispersed throughout the grinding wheel.

Depressed center grinding wheels according to the present disclosure are useful, for example, for abrading a workpiece. During use, the depressed center grinding wheel can be used dry or wet. During wet grinding, the wheel is used in conjunction with water, oil-based lubricants, or water-based lubricants. Depressed center grinding wheels according to the present disclosure may be particularly useful on various workpiece materials such as, for example, carbon steel sheet or bar stock and more exotic metals (e.g., stainless steel or titanium), or on softer more ferrous metals (e.g., mild steel, low alloy steels, or cast irons).

Depressed center grinding wheels according to the present disclosure are useful for grinding a workpiece at an acute angle with the workpiece. Such an abrading process is shown in Fig. 4, wherein depressed center grinding wheel 100 abrades workpiece 400. During grinding, the working, intermediate, and back layers experience wear and participate in the abrading of the workpiece.

SELECT EMBODIMENTS OF THE PRESENT DISCLOSURE

In a first embodiment, the present disclosure provides a depressed center grinding wheel comprising an abrasive disc having a working surface and a back surface opposite the working surface, wherein the working surface has a depressed center portion, and wherein the abrasive disc comprises: a working layer comprising first abrasive particles retained in a first binder material, the first abrasive particles comprising first shaped abrasive particles, wherein the first shaped abrasive particles comprise from 40 to 100 weight percent of the first abrasive particles;

a back layer comprising second abrasive particles retained in a second binder material comprising first crushed abrasive particles and essentially free of shaped abrasive particles;

an intermediate layer disposed between the working layer and the back layer, the intermediate layer comprising third abrasive particles retained in a third binder material, the intermediate layer comprising second shaped abrasive particles and second crushed abrasive particles, wherein the second shaped abrasive particles comprise 25 to 75 weight percent of the second abrasive particles;

a first reinforcing scrim sandwiched between the back layer and the intermediate layer; and a second reinforcing scrim adjacent one of:

the back layer opposite the intermediate layer;

the intermediate layer opposite the back layer; or

the working layer opposite the intermediate layer.

In a second embodiment, the present disclosure provides a depressed center grinding wheel according to the first embodiment, wherein the second reinforcing scrim is sandwiched between the working layer and the intermediate layer.

In a third embodiment, the present disclosure provides a depressed center grinding wheel according to the second embodiment, further comprising a third reinforcing scrim bonded to the working layer opposite the intermediate layer.

In a fourth embodiment, the present disclosure provides a depressed center grinding wheel according to the first embodiment, wherein the second reinforcing scrim is secured to the back layer opposite the intermediate layer.

In a fifth embodiment, the present disclosure provides a depressed center grinding wheel according to the fourth embodiment, further comprising a third reinforcing scrim sandwiched between the intermediate layer and the working layer.

In a sixth embodiment, the present disclosure provides a depressed center grinding wheel according to the fourth or fifth embodiment, further comprising a fourth reinforcing scrim bonded to the working layer opposite the intermediate layer.

In a seventh embodiment, the present disclosure provides a depressed center grinding wheel according to any one of the first to sixth embodiments, wherein the first shaped abrasive particles comprise triangular shaped abrasive particles.

In an eighth embodiment, the present disclosure provides a depressed center grinding wheel according to any one of the first to seventh embodiments, wherein the second shaped abrasive particles comprise triangular shaped abrasive particles.

In a ninth embodiment, the present disclosure provides a depressed center grinding wheel according to any one of the first to eighth embodiments, wherein the ratio of the weight percent of the first shaped abrasive particles in the first abrasive particles to the weight percent of the second shaped abrasive particles in the second abrasive particles is from 40:60 to 60:40.

In a tenth embodiment, the present disclosure provides a depressed center grinding wheel according to any one of the first to eighth embodiments, wherein the ratio of the weight percent of the first shaped abrasive particles in the first abrasive particles to the weight percent of the second shaped abrasive particles in the second abrasive particles is from 45:55 to 55:45.

In an eleventh embodiment, the present disclosure provides a depressed center grinding wheel according to any one of the first to tenth embodiments, wherein the first and second shaped abrasive particles comprise alpha alumina.

In a twelfth embodiment, the present disclosure provides a depressed center grinding wheel according to any one of the first to eleventh embodiments, further comprising a centrally disposed arbor hole extending through the abrasive disc.

In a thirteenth embodiment, the present disclosure provides a depressed center grinding wheel according to any one of the first to twelfth embodiments, further comprising an attachment member centrally disposed on the back surface of the abrasive disc.

Objects and advantages of this disclosure are further illustrated by the following non-limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure. EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

The following abbreviations are used for materials in the examples.

TABLE OF ABBREVIATIONS

PR1 liquid phenolic resin, obtained under trade designation "PREFERE

825136G1" from Dynea Oy Corporation, Helsinki, Finland.

PR2 phenolic resin powder (a solid phenolic resin), obtained under

trade designation "VARCUM 29302" from Durez Corporation, Dallas, Texas.

CRY Sodium hexafluoroaluminate, obtained under trade designation

"CRYOLITE" from Freebee, Ullerslev,

Denmark.

SCRIM 1 fiberglass mesh having the trade designation "STYLE 4400" from

Industrial Polymer and Chemicals, Inc., Shrewsbury,

Massachusetts.

SCRIM2 fiberglass mesh having the trade designation "STYLE 184" from

Industrial Polymer and Chemicals, Inc..

HC5 0.7 micron aluminum trihydroxide particles available as HYDRAL

COAT 7 from Almatis, Inc., Leetsdale, Pennsylvania.

PREPARATION OFAP2

Shaped abrasive particles were prepared according to the disclosure of U.S. Pat. No. 8, 142,531 (Adefris et al.). The shaped abrasive particles were prepared by molding alumina sol gel in equilateral triangle-shaped polypropylene mold cavities of 0.028 inch (0.71 millimeter) depth and 0.11 inch (0.28 millimeter) on each side. The draft angle a between the sidewall and bottom of the mold was 98 degrees. After drying and firing, the shaped particles were calcined at approximately 650 °C, and then saturated with a magnesium nitrate solution (10.5 percent by weight as magnesium oxide, and having 0.02 percent by weight of HC5 dispersed therein). Excess nitrate solution was removed, and the saturated shaped particles were allowed to dry after which the particles were again calcined at 650 °C and sintered at approximately 1400 °C resulting in shaped ceramic abrasive particles. Both the calcining and sintering were accomplished using rotary tube kilns.

GRINDING TEST

Abrasive wheels were tested by grinding a rectangular mild steel bar (0.25 inch (0.6 cm) x 18 inches (45.7 cm) x 3 inches (7.6 cm)) over a 0.25 inch (0.6 cm) x 18 inches (45.7 cm) area of the surface while mounted on a 12000 rpm air driven grinder that oscillated back and forth (one cycle = 18 inches (45.7 cm) each way for a total of 36 inches (91 cm)) for ten one-minute cycles. The applied load was the grinder weight of 9 pounds (4.1 kg) and the abrasive wheel was held at an angle of 15 degrees relative to the surface (i.e., 0 degrees). The steel bar was weighed before and after each cycle, and the weight loss (i.e., cut) was recorded. The steel bar was traversed 16 times from end to end per cycle. Weight loss from the grinding disc (i.e., disc wear) was recorded after each 10-cycle test.

EXAMPLE 1 and COMPARATIVE EXAMPLES A- B

Mixes were prepared according to the amounts and components listed in Table 1. Mix 1, Mix 2 and Mix 4 were prepared by combining the indicated components using a paddle-type mixer (obtained as "CUISINAPvT SM-70" from Conair Corporation, East Windsor, New Jersey, operated at speed 1) for 10 minutes. Mix 3 was prepared by combining Mix 1 and Mix 2 using a paddle-type mixer for 10 minutes. Mix 5 was prepared by combining Mix 4 and Mix 2 using a paddle-type mixer for 10 minutes. Mix 6 was prepared by combining 50% Mix 1 and 50% Mix 4 using a paddle-type mixer for 10 minutes.

TABLE 1

EXAMPLE 1

A Type 27 depressed-center composite grinding wheel was prepared as follows. A 4.5-inch (11.4 centimeters) diameter disc of SCRIM1 was placed into a 4.5-inch (11.4 centimeters) diameter cavity die. Mix 3 (50 grams) was spread out evenly. A second 4-inch (10.2 centimeters) diameter of SCRIM2 was placed on top of Mix 3. Mix 6 (50 grams) of was spread out evenly and a third 4-inch (10.2 centimeters) diameter of SCRIM2 was placed on top of Mix 6. Then Mix 5 (50 grams) of was spread out evenly. The filled cavity mold was then pressed at a pressure of 40 tons/38 square inches (14.5 megapascals).

The resulting wheel was removed from the cavity mold and placed on a spindle between depressed center aluminum plates in order to be pressed into a Type 27 depressed-center grinding wheel. The wheel was compressed at 5 ton/38 square inches (1.8 megapascals) to shape the disc. The wheel was then placed in an oven to cure for 7 hours at 79 °C, 3 hours at 107 °C, 18 hours at 185 °C, and a temperature ramp-down over 4 hours to 27 °C. The dimensions of the final grinding wheel were 180 millimeter diameter x 7 millimeter thickness. The center hole was 7/8 inch (2.2 centimeters) in diameter. The resultant depressed-center composite grinding wheel was configured such that a layer of Mix 5 was the working layer. COMPARATIVE EXAMPLE A

Comparative Example A was a Type 27 depressed-center grinding wheel prepared according to the procedure of Example 1, except that Mix 5 was used instead of Mix 6 in middle layer (so that Mix 5 was used in both middle and top layers). The resultant depressed-center composite grinding wheel was configured such that a layer of Mix 5 was the working layer.

COMPARATIVE EXAMPLE B

Comparative Example B was a Type 27 depressed-center grinding wheel prepared according to the procedure of Example 1, except that Mix 6 was used instead of Mix 5 in top layer (so that Mix 6 was used in both middle and top layers). The resultant depressed-center composite grinding wheel was configured such that a layer of Mix 6 was the working layer.

Grinding test results of Example 1 and Comparative Examples A and B are reported in Table 2

(below).

TABLE 2

EXAMPLES 2-7 and COMPARATIVE EXAMPLES C- E

Mixes were prepared according to the amounts and components listed in Table 3. Mix 7, Mix 8 and Mix 10 were prepared by combining the indicated components using a paddle-type mixer

(CUISINART SM-70 from Conair Corporation, East Windsor, New Jersey, operated at speed 1) for 10 minutes. Mix 9 was prepared by combining Mix 7 and Mix 8 using a paddle-type mixer for 10 minutes. Mix 11 was prepared by combining Mix 10 and Mix 8 using a paddle-type mixer for 10 minutes. Mix 12 was prepared by combining 25% Mix 9 and 75% Mix 11 using a paddle-type mixer for 10 minutes. Mix 13 was prepared by combining 50% Mix 9 and 50% Mix 11 using a paddle-type mixer for 10 minutes. Mix 14 was prepared by combining 75% Mix 9 and 25% Mix 1 1 using a paddle-type mixer for 10 minutes.

TABLE 3

A Type 27 depressed-center composite grinding wheel was prepared for each Example in

Examples 2-7 and Comparative Examples C-E as follows. Mixes used in each Example as bottom, middle and top layers and their amounts are reported in Table 4. A 4.5-inch (11.4-cm) diameter disc of SCRIMl was placed into a 4.5-inch (11.4-cm) diameter cavity die. Bottom layer mix was spread out evenly. A second 4.0-inch (10.2-cm) diameter of SCRIM2 was placed on top of bottom layer mix. The middle layer mix was spread out evenly and then top layer mix was spread out evenly. A third 3-inch (7.6-cm) diameter of SCRIM2 was placed on top of top layer mix. The filled cavity mold was then pressed at a pressure of 40 tons/38 square inches (14.5 mPa).

The resulting wheel was removed from the cavity mold and placed on a spindle between depressed center aluminum plates in order to be pressed into a Type 27 depressed-center grinding wheel. The wheel was compressed at 5 ton/38 square inches (1.8 mPa) to shape the disc. The wheel was then placed in an oven to cure for 7 hours at 79 °C, 3 hours at 107 °C, 18 hours at 185 °C, and a temperature ramp-down over 4 hours to 27 °C. The dimensions of the final grinding wheel were 180 millimeter diameter x 7 millimeter thickness. The center hole was 7/8 inch (2.2 cm) in diameter. The resultant depressed-center composite grinding wheel was configured such that a top layer was the working layer.

Grinding test results of Example 2-7 and Comparative Examples C-E are reported in Table 4

(below).

TABLE 4

Various modifications and alterations of this disclosure may be made by those skilled in the without departing from the scope and spirit of this disclosure, and it should be understood that this disclosure is not to be unduly limited to the illustrative embodiments set forth herein.