US3476537A - Abrasive composition with limestone as the porosity-inducing agent - Google Patents

Description

United States Patent OfiFice 3,476,537 Patented Nov. 4, 1969 US. Cl. 51-296 4 Claims ABSTRACT OF THE DISCLOSURE The abrasive composition contains granular abrasive material, granular limestone, a granular filler and a binder. The abrasive and limestone grains are of the same order in size, While the filler grains are much smaller size. The ratio by volume of abrasive and limestone in the composition approximates less than 50% :10%.

The present invention relates to improvements in an abrasive composition intended for use, most commonly, in rotary grinding or finishing wheels of a wide variety of descriptions. However, and as will be readily appreciated by those skilled in the art, the formulation or composition of ingredients of which such wheels are fabricated is also well adapted for use in abrading or finishing units of a non-rotary type, for example in blocks, belts, sheet-s and the like.

More particularly, the invention relates to an abrasive wheel composition of this sort which has provision for inducing porosity therein in a novel, economical and efficient manner, with the result of greatly improving the performance of the wheel or other unit in a considerable number of respects. These may be in regard, for example, to the length of life of the unit; to its efficiency and/r speed in removing metal or other material from a workpiece, without burning or chipping the material; to its lowness of cost as compared with conventional, generally similar abrasive compositions; to the diminished operator effort involved in the use of the unit, etc. For simplicity, reference will be hereinafter made only to the use of the improved composition in rotary abrasive wheels, on the understanding that it has the other fields of application mentioned above.

As is commonly understood, the grade or hardness of an abrasive grinding or finishing wheel is dependent upon three physical factors, all expressed in percentage per cubic inch. These are: its porosity or presence of voids, its total volumetric content of abrasive grains, and its total volumetric content of non-abrasive bonding or other agents. Theoretically, the most efficient grinding wheel for any given operation would be the hardest possible wheel, as determined by the three above factors.

As is well known, grinding wheels remove metal or other material by a process of abrasive breakdown; that is, the abrasive grains must fracture or break down, so as to present continually renewed, sharp cutting points to the material being ground. When a grain does not break down fast enough, it tends to become fiat-surfaced, and it generates heat which is transferred to the material being ground, as Well as to the wheel.

Such a wheel is then considered to be hard; and this has come about as the result of one or both of two conhow easily a grain is crushed.

In the past, and at present, attempts have been made to alleviate the drawback of excessive grain toughness by mechanically introducing holes, grooves or slots in the grinding surface of the wheel. This of course calls for specialized wheel mold parts for each wheel specification as to type and size, with resultant great increase in the cost of wheel production. It requires the stocking of a vast number of individually expensive mold parts, a reduction of production time due to mold changeovers, etc.

As another alternative, efforts have been made to reduce wheel toughness by non-mechanically imparting voids or porosity in the molding procedure, for example, by introducing aluminum oxide bubbles in the abrasive composition. Due to the vastly different specific gravity of the aluminum oxide bubble, it tends to float to the top of the mix, resulting in a non-uniform product.

It is therefore a general object of the present invention to provide an abrasive block composition for use in grinding and finishing wheels or other abrasive-type material removing devices, which composition has porosity intentionally introduced therein in a novel and improved manner, thus vastly to improve the wheels performance in respect to the various considerations of length of life, grinding capacity and efficiency, and the like, mentioned above.

More specifically, porosity is induced by the addition to the abrasive composition or formulation of a random distribution of a granular agent approximating the abrasive grains in grit size, and softer than the abrasive grains. In this connection it is to be understood that the addi tion in question is by no means a filler, or equivalent of a filler, on any basis, i.e., its intended function, its particle size or its identity. It is to be emphasized that the purpose and action of the porosity inducing agent is to replace an approximately equal number of the abrasive grains of a harder nature. Thus, the agent stabilizes the abrasive mix in curing, then wears, decrepitates, decomposes or dissolves during the grinding process in a way to enable the abrasive grains to serve more efficiently individually as cutters.

In further accordance with the invention, the porosity inducing agent referred to above is preferably one which is widely available at very low cost, as compared with that of the abrasive grain material. I find that a limestone of onetype or another, for example, calcspar (calcium carbonate) or dolomite (calcium-magnesium carbonate), is ideally suitable from the standpoints of commercial availability and low cost, wide selection of grain size, etc. It is to be clearly understood that the limestone (CaCO intended for use in the improved composition is to be sharply distinguished from the lime (CaO) conventionally used in grinding Wheels and the like. Other possible selections for the porosity inducing agent may be made from natural or activated bauxite, and minerals such as olivine, gypsum, chromite, coquimbite, pyrolusite, molybdenite, galena, halite, and the like, as well as a variety of manufactured products for a similar purpose.

It will be noted that the materials referred to above vary quite widely in hardness on the Mob and Knopp scales, i.e., from Moh Nos. 1-3 to as high as 6 or 7. Accordingly, the selection will be governed to a considerable extent by the selection of the particular granular abrasive which is used, as well as on the basis of the grinding operation involved, in point of kind and amount of material to be removed, desired speed, finish, life, etc.

The effect of the porosity introducing agents referred to above, in a random distribution in the abrasive mass of cutting grains, binder, filler, etc., is to offer continuing relief for a considerable quantity of the abrasive grains, as the porosity inducing agent mechanically wears away, decrepitates, decomposes or dissolves under the effects inherent in the grinding operation.

As for the granular abrasive component of the wheel, it may be in one of various types commonly employed, typical of which are hereinafter mentioned in tabulations of volume structure data taken from certain comparative tests.

Another object of the invention is to provide abrasive material formulations incorporating one or another type of porostiy inducing agent, in which the performance qualities, capacity and life of a wheel fabricated therefrom may be readily and extensively controlled by simply changing from one porosity agent to another, by mixing different agents, by altering the total percent content of the agent in the mixture, by varying its granular size, etc.

A further object of the invention is to provide a porosity-increased composition which is capable of being employed in grinding wheels or the like for a large number of different types of operation. Examples are face grinding, foundry snagging, roll, centerless or off-hand grinding, segmental disc wheel grinding, cylindrical grinding, internal grinding, ceramic tile grinding and the like; and in which many gradations of rough or finished surface are required.

For example, the improved wheel of the invention has been found very effective in the end grinding of coiled torsion springs ranging widely in diameter; and this type of operation will prove useful in explaining my theory in regard to the improved performance of the wheel.

Thus, a conventional grinding wheel will display, in microscopic cross section, a more or less uniform dis tribution of abrasive grains, resinous binder and natural voids, in what may roughly be considered a checkerboard distribution, for the purpose of discussion. Assume that in the improved wheel of the invention, a corresponding cross section will exhibit a generally, but not exactly, similar distribution of abrasive grains, binder and natural voids or porosity plus, in addition, a random placement of porosity inducing granules.

It will therefore be seen that in at least some partial cross sectional area, smaller or larger, of the improved wheel composition there will be a distribution of abrasive grains, binder, agent and natural voids which is the same as that of the conventional wheel section, lacking entirely the porosity inducing grains contemplated by the invention. In another portion, or the remainder, of the cross sectional area, the porosity inducing granules will be found in their random distribution.

It follows, naturally, that the two wheels under consideration will have substantially identical cutting characteristics, being of equal hardness, at their areas of corresponding distribution of abrasive grains, binder and natural voids. By the same token, the wheel of the invention is softer in the area of non-corresponding distribution. The advantage is that the porosity imparting granules, having first mechanically stabilized the wheel composition in the molding and curing thereof, will in turn gradually wear out, decrepitate, decompose or dissolve in the cutting operation. Accordingly, a certain percentage of the abrasive grains, deprived of lateral support, will more readily chip or break down progressively in a desirable manner. The abrasive grains are made more eflicient as cutting tools, in the accepted sense referred to above, and last longer. The improved wheel will remove more than, or at least as much metal as, the conventional wheel, and usually without burning.

Now, in reference to the instanced grinding of coiled torsion springs, it is obvious that a coil with a small wire diameter will be subject to a relatively high unit pressure at its relatively low total area of engagement or contact sweep by the wheel. On the other hand, a spring of considerably larger wire diameter will be subjected to a lower unit pressure against the wheel, but by way of compensation is operated on with greater cutting efiiciency due to the increased presence of the porosity inducingagent in a larger area of sweep by the wheel, i.e., an area in which more of the porosity inducing granules or particles are present. The result is a uniformly good and efficient grinding of the smaller and larger size springs. Essentially the effect is the same as in the case in which porosity is incorporated in the wheel by the production molding of slots, recesses, etc. therein a mechanical way, but in accordance with the invention, the full and much less costly equivalent of such holes or recesses is bad in a vastly different way.

The tabulations in terms of volume structure which follow compare various formulations of the improved wheel of the invention with conventional grinding wheels, and are based on data as to comparative performance on several different types of grinding operation, also on different types of work. The wheels of the respective sets under comparison were intentionally and carefully fabricated to have as much in common as was practically possible, for example, in regard to percentage by volume formulation.

The powdered and liquid binder and filler components were of well known and widely available types. Actually, a powdered two-stage thermosetting phenolic resin of the phenol formaldehyde type, known as Varcum 3030 and commonly used to bond grinding wheel abrasive grains, was the dry binder in each instance. Other thermosetting resins or the like would be equivalents, as well as vitreous-type binders of one sort or another such as are commonly used in the industry. A one-step water soluble phenolic resin, known as Varcum 8121, was used as a wetting agent or liquid resin, and would have its equivalent in furfural, furfural alcohol, etc. Cryolite, a double fluoride of sodium and aluminum, also referred to as ice stone was the filler, one commonly employed in the grinding wheel industry. Equivalents might be lime, calcium oxide, potassium sulfate, iron sulfide, wollastonite, fine emery, etc.

It is here well to emphasize that such fillers are commonly used in the industry in #200 and finer mesh sizes, (although they have been used in sizes as great as #46 and #60 mesh). The fillers contemplated by me are of much finer nature than the grit sizes contemplated for the important porosity inducing agent of the improved composition, which range in the neighborhood of #10 through #24. The function of that agent is in no sense merely the function of a filler.

Limestone, as used for the porosity inducing agent of the improved composition is equally sharply distinguishable from lime, in the contemplation of the invention.

As for the abrasive grain composition of the wheels under comparison, the aluminum oxides were, generally speaking, fused bauxite, with additions of zirconia and/or titania in some instances for additional toughness. Black silicon car-bide was employed in some instances, also for a desired degree of toughness. Finally all of the wheels in question, improved and conventional, had their ingredients mixed and baked in an industrially well-known procedure, identical in the case of each of the respective comparative formulations.

CONVENTIONAL WHEEL A IMPROVED WHEEL B These two wheels, both in a 30" outer diameter, a 12 inch inner or hole diameter, and a ,4 inch axial length, were employed in a horizontal spindle disc operation, using grinding fluid, on coiled springs. The composition of the respective wheels was as follows:

Conventional wheel A Volume structure: Percent Aluminum oxide#20 grit S2 Limestone None Powdered resin 12 Liquid resin 4 Porosity 32 Density, .079549 #/in.

Improved wheel B Volume structure: Percent Aluminum oxide-#20 grit (23% decrease) 40 Limestone-# 14 and #16 grit Powdered resin 18 Liquid resin 6 Porosity 26 Density, .077547 #/in.

Conventional wheel A removed 530 pounds of metal per set of two wheels, with frequent bad burning of the workpieces. Improved wheel B, on the other hand, removed 655 pounds per two wheel set with no burning of the material. This is a 22.6% improvement in output capacity, without need for regrinding of burned pieces and possible scrap loss.

CONVENTIONAL WHEEL C- IMPROVED WHEEL D These wheels were performance-compared at 9500 s.f.p.m. in a swing frame foundry grinding operatlon on annealed malleable iron. The composition of these two wheels was as follows:

Conventional wheel C Volume structure: Percent Aluminum oxide (40% zirconia)--#14, #16

Density, .099882 #/in.

Wheel C ground 19,800 pounds of malleable castings, ranging in weight from 60 to 200 pounds, with riser or gate dimensions to be removed which ranged from one cubic inch to five cubic inches. As operated on the same machines by the same operators, improved wheel D ground 54,000 pounds of castings, representing a produc tion improvement factor of about 172%.

CONVENTIONAL WHEEL EIMPROVED WHEEL F These wheels, in dimensions of six inch CD. by five/ eighth inch ID. by one inch thickness, were used at 9500 s.f.p.m. in the offhand portable grinding of stainless steel. The respective compositions of the wheels in question was as follows:

Conventional wheel E Density, .084831 #/in.

Conclusions in a comparison operation of this type are based primarily on the opinion of the operator or operators,'rather' than upon production records kept at the plant, as in the case of the first two comparative tabulations discused above, which records determine the policy of the plant in purchasing grinding wheels.

On the basis of operator opinion (also referred to commonly in decisions regarding reordering) conventional wheel E was rejected for the reason that it required more operator effort and occasionally burned workpieces. On the other hand, the improved wheel F cut at a much more rapid rate than wheel E, requiring less operator effort to perform the operation. Moreover, wheel F was a very free cutting one, while having a longer life of operation than the conventional wheel in the same operation.

CONVENTIONAL WHEELS G AND H-- IMPROVED WHEEL I These three wheels, all dimensioned. 30" CD. by 6'' ID. by 3" thickness, were disc wheels run at 7000 s.f.p.m. in the grinding of vitrified and glazed tile products for decorative or conventional purposes. Conventional wheel G was an entirely standard one, which had in the past done an apparently very satisfactory job. Wheel H was also of generally conventional composition, differing slightly from wheel G in regard to percentage porosity and resin content. However, in these porosity and resin factors, wheel H was identical to improved wheel I. It is to be noted that the abrasive material employed in wheel I was of somewhat finer mesh or grit size than in the case of wheels G and H, with what might be expected to result in a lessened material removing capacity. The respective compositions were as follows:

Conventional wheel G Density, .067462 #/in.

Improved wheel I Volume structure: Percent Black silicon carbide#30 grit (19.3% decrease) 42 Limestone-#20 & #24 grit 10 Powdered resin 12 Liquid resin 4 Porosity 32 Density, .065612 #/in.

CONVENTIONAL WHEEL J--IMPROVED WHEEL K The wheels compared were Type 11 cup wheels (6/4% by 2 /2" by /6"11). The operation was portable oif-hand grinding at 9500 s.f.p.m. of Super Stone Metal, a combination of manganese, aluminum, nickel and bronze. The compositions of wheels I and K are as follows:

Conventional wheel J Volume structure: Percent Black silicon carbide #12, #14 and #16 grit--- 52 Limestone Powdered resin 15 Liquid resin Porosity 28 Density, .069252 #/in.

Improved wheel K Volume structure: Percent Black silicon carbide#12, #14 and #16 grit (14.5% decrease) 46 Limestone# grit 10 Powdered resin 14 Liquid resin 4 Porosity 26 Density, .071145 #/in.

The operators opinion as to the cutting etliciency and overall life of the wheel, i.e., the time required to wear a wheel down to a no longer useful size, is the performance standard on this type of grinding job.

Conventional wheel I was satisfactory (performancewise) from the operators standpoint. However, it had a useful life of approximately 1 hour and 45 minutes. Improved Wheel K was equally satisfactory in performance to operator, but it lasted with etficiency for approximately 2 hours and 30 minutes, a life increase of 45 minutes. Thus, by the replacement of six parts of abrasive material by limestone, efiiciency was increased by approximately 43% in improved wheel K.

One of the important conclusions to be drawn from the foregoing tabulations is that in all of the five instances, the improved wheel enables a very significant diminution in the amount of expensive abrasive or cutting agent which was employed, in the neighborhood of about 14%24%.

Equally significant are the improvements in output capacity; the diminution of operator effort; the minimizing of burned workpieces, requiring reworking with the possibility of not meeting tolerance; the longer effective life, etc.

It is to be noted that the granular size of the limestone porosity inducing material is approximately of the same order as that of the granular abrasive material,

i.e., in a range of #10 mesh-#30 mesh, actually #10 mesh-#24 mesh for the limetsone. 'In other words, the porosity inducing granules, as random distributed in the abrasive composition as a whole in an approximate ratio by volume of less than 50% abrasive grains to 10% limestone grains, are of suflicient bulk to sustain the granular abrasive during mold-ing and baking or curing, yet will readily and gradually disappear as wheel wear proceeds, enabling the abrasive grains to present newly restored and sharp cutting points. The size differential relationship is of an entirely different order than that existing in regard to the abrasive grains and filter material, the latter of which is often of flour-like proportion, in a size of as little as #400 mesh.

Such fillers are commonly selected and used on the basis of one chemical characteristic or another, whereas this is not a factor in the selection of the porosity affording agent of the improved composition. In fact, in addition to other attractive features of granular limestone, as discussed above, one of its desirable aspects in the light of the present invention is that it is substantially free of chemical impurities, so that its performance of its intended function is entirely predictable.

Actually, the porosity inducing agents contemplated by the invention lose their porosity inducing characteristic and capability in the finer granular sizes.

As indicated above, the hardness of the granular limestone and other optional porosity imparting agents may range considerably, i.e., from 1 to 6 or 7 on the Mob scale. The abrasive material will normally have a Mob number of 9 or 9.5, although hardnesses of as little as 6 or 7 have been employed. Accordingly, it is not possible to express any particular range of hardness numbers for the porosity agents, its selection depending, as it does, on the selection of the abrasive agent, and this in turn depending upon a large number of considerations, such as the material of the workpiece, the fineness of finish desired, etc.

What I claim as my invention is:

1. A grinding member which, in final set condition ready for use, includes an abrasive composition comprising solid granular abrasive material bonded in combination with solid granular porosity inducing material, the porosity inducing material being essentially limestone, being of a hardness substantially less than that of the abrasive material, a granular filler material, and a thermosetting resin binder material, the grains or granules of the respective abrasive and porosity inducing materials being of approximately the same order in size, and much greater in size than those of the granular filler material.

2. A grinding member which, in final set condition ready for use, includes an abrasive composition comprisin-g solid granular abrasive material bonded in combination with solid granular porosity inducing material, the porosity inducing material being essentially limestone, being of a hardness substantially less than that of the abrasive material, a granular filler material, and a thermosetting resin binder material, the grains or granules of the respective abrasive and porosity inducing materials being of approximately the same order in size, and much greater in size than those of the granular filler material, the grains or granules of the abrasive material and porosity inducing material being in the approximate range of #10 mesh-#30 mesh in size.

3. A grinding member which, in final set condition ready for use, includes an abrasive composition comprising solid granular abrasive material bonded in combination with solid granular porosity inducing material, the porosity inducing material being essentially limest ne, being of a hardness substantially less than that of the abrasive material, a granular filler material, and a thermosetting resin binder material, the grains or granules of the respective abrasive and porosity inducing materials being of approximately the same order in size, and much greater in size than those of the granular filler material, the limestone being of a granule size in the range of approximately #10 mesh#24 mesh.

4. A grinding member which, in final set condition ready for use, includes an abrasive composition comprising solid granular abrasive material bonded in combination with solid granular porosity inducing material, the porosity inducing material being essentially limestone, being of a hardness substantially less than that of the abrasive material, a granular filler material, and a thermosetting resin binder material, the grains or granules of the respective abrasive and porosity inducing materials being of approximately the same order in size, and much greater in size than those of the granular filler material, the abrasive and porosity inducing materials being in an References Cited UNITED STATES PATENTS Halstead 264'12 Taylor 106-422 Kistler 51298 Kistler 51-298 Wooddell et al 51298 Robie 51-298 Cofran 51296 DONALD J. ARNOLD, Primary Examiner approximate ratio of less than 50%:10% by volume in 15 51 298, 308

the composition as a Whole.

US. Cl. X.R.