US2960392A - Metal bonded grinding wheel - Google Patents

Description

METAL BONDED i' l- DING WHEEL N Drawing. Filed Nov. 5, 1958, Ser. N0.771,956

Claims. (Cl. 51308) The invention relates to metal bonded grinding wheels, especially to diamond grinding wheels. This application is a continuation in part of my copending application Serial No. 671,411, filed July 12, 1957, and now abandoned, which was a continuation in part of my copending application Serial No. 632,008, filed January 2., 1957, now abandoned.

One object of the invention is to provide a grinding wheel of the type indicated which will last longer and cut just as well or better. Another object of the invention is to provide a grinding wheel of the type indicated which will cut better and last just as long or longer. Another object of the invention is to increase the strength of metal bonded wheels without sacrificing the self-dressing action which keeps the wheels sharp. Another object is to provide a superior diamond grinding wheel of the metal bonded type.

Another object of the invention is to provide a superior wheel for use in electrolytic work eroding and abrading machines. Another object is to provide'an electrically conducting wheel. Another object is to produce a wheel which has improved self-dressing characteristics during grinding. Another object is to provide abrasive articles such as homes, lens generators, segments or other shapes with the improved bond herein described.

Other objects will be in part obvious or in part pointed I out hereinafter.

The'principal application of my invention is for the manufacture of diamond grinding wheels of the metal bonded type. The invention is also applicable for the manufacture of wheels for electrolytic work eroding and abrading machines. These machines are relatively new in a commercial sense, but have their inception in a atent to Charles Francis Jenkins No. 1,017,671 of 1912.

As conducive to a better understanding of the invention, in prior diamond tools, the diamond particles are held in place by a matrix of metal formed by compressing a mixture of metal powder and diamond particles and sintering the metal powder by raising it to an elevated temperature for a suitable period of time, either with or without simultaneously applying pressure. The metal particles undergo a difiusion and plastic flow effect aided by surface tension to produce a more 'or less solid metallic matrix, the strength of which is dependent on the degree of diifusion between adjoining particles. This process is well known to the art. i

A disadvantage of diamond tools formed by the use of metal powders is that the densification obtained is such that when grinding hard, brittle materials such as cemented tungsten carbide, there is not sufiicient dress- .ing action, that is, erosion of metal from around the diamonds, to keep the diamonds exposed. Thus such a. conventional tool tends to become glazed and'ceases to cut effectiyely. In order to obtain a :satisfactory dressing action, it has been necessary in the past to remeasurements of about 0.1 mil to about 5 mils.

Patented Nov. 15, 1960 sort to the use 'of weaker, more friable bonds, which permit the diamond to be dislodged easily upon dulling and expose fresh diamond without making full use of all exposed diamond. This dressing action results in rapid wear and consequently high wheel costs.

If a harder, more dense bonding matrix is used, it is the practice sometimes to admix a quantity of friable, non-metallic material in powder form, in order to aid in the dressing action. This practice has limited success owing to the relatively large bond areas between individual friable particles which still can cause smearing and loading. If sufficient friable powder is added to insure freedom of cut, the entire bond structure is weakened to the point where wheel life is sacrificed. The purpose of this invention is to overcome the previous disadvantages of diamond wheels for grinding hard, brittle materials and to permit free cutting action with little or no sacrifice in life.

Through the use of metal fibers rather than metal powders conventionally used in the art, advantage is taken of the strong interlocking tendency of the metal fibers and the tendency of the fibers to align themseives perpendicularly to the direction of pressing. The desirable orientation which occurs during pressing is peculiar to systems in which particles are not equidimensional. The alignment of the fibers during pressing is in accordance with the principles of Le Chatelier. Since the direction of pressing is from the side in the case of a peripheral Wheel with an abrasive section around the circumference, the fibers which form the bond tend to become perpendicular to the direction of pressing and parallel to the planes of the wheel faces. Also the fibers in the sintered wheel are most of them parallel to the nearest tangents to the wheel. Through the use of glass fibers or other non-metallic fibers of a brittle nature a maximum dressing effect can be obtained without seriously weakening the matrix. The same volume of non-metallic material, when reduced to powder, presents more weakening discontinuities in a given matrix. However, for some purposes I may depend on the interlocking metal fibers for producing the necessary strength for handling, and I may introduce glass in the mixture as a powdered contituent.

The theory of the dressing action can best be exemplified in a peripheral-type diamond wheel which is conventionally pressed from the sides. The aligning tendency of the fibers causes them to be oriented in a direction parallel to the direction of cutting so that there is a rapid and effective erosion of parallel paths in short lengths throughout the wheel surface. The matrix, having similar directional tendencies, has a maximum resistance to tearing out of the diamond yet is frequently interrupted by short, parallel glass areas which effectively minimize smearing and loading, but not all of the advantages of my wheel are restricted to the presence of the orientation of the fibers.

In a grinding test using a 6" diameter by /s" thick peripheral diamond wheel with a metal fiber matrix and glass fiber filler, the face of the wheel remained open andwell dressed after prolonged constant pressure grindmg of cemented tungsten carbide, Whereas wheels made conventionally, using metal powder matrices, exhibited smearing and loading. The wear of the fiber-bonded diamond wheel was of the same order of magnitude as the conventional wheels.

EXAMPLE I I took Brillo 0000 size steel wool which has an average diameter computed from maximum and minimum I also took Pyrex glass wool produced by Corning Glass Comeeann" ranging from about 2 to 1 to about 80 to 1.

pany, of average diameter similarly measured from about 0.3 mil to about 5 mils. For comminuting the fibers I used a Waring Blendor which is quite satisfactory for the purpose. This blender is used in factories and also in houshold kitchens and is well known. It is disclosed in US. Patent No. 2,109,501.

Approximately 50 grams of the steel wool were first cut with shears into convenient tufts and gradually fed into the blender in which was contained about 800 cubic centimeters of water having 1% of a 40% water solution of sodium nitrite rust inhibitor. The downward current created by the blender impeller carried the tufts toward the impeller. The blender cut the steel wool strands in about 20 minutes into short fibers with an average length to diameter ratio of about 20 to 1 but I have found that the fiber lengths can be controlled pretty well by the length of the cutting time. By using shorter times I produced longer fibers such as about 125 to 1 in ten minutes. Washing of the metal fibers was accomplished by decanting away the solution, adding water, then stirring and decanting again. This decanting procedure was repeated five times and then three decantations were made with methyl alcohol to remove the last of the water. Drying was accomplished at 60 C. for a number of hours, and the product was stored in air-tight glass containers.

The glass wool was comminuted in exactly the same way. About 25 grams of glass wool were added gradual- 1y to the container of the blender with about 800 cubic centimeters of water without any sodium nitrite or anything else. Ten minutes in the blender produced glass fibers with a length to diameter ratio from about 2 to 1 to about 60 to 1. These fibers showed some tendency to cluster into aggregates which could however easily be separated in the subsequent screening and mixing operations. The water was decanted away from the glass fibers which were then dried a number of hours at 60 C. and stored in glass containers. It was estimated that 80% ofthe fibers had a ratio of length to diameter of more than to 1 and were from 0.010 inch to 0.100 inch long, with 50% at least 0.020 inch long, and of average diameter from three-tenths of a mil to 2 mils.

For making the bond I took 14.57 grams of the metal fiber and 2.57 grams of the glass fiber. Mixing of these two kinds of dry fibers was accomplished by repeated screening through a silk screen with twenty-three openings per inch or an average hole size of .0376 inch. Firm pressure was used in screening the fibers by hand using rubber gloves for protection from the fibers. Four repeated screenings through the silk screen resulted in a uniform, easily handled mixture which showed no tendency to separate. Diamond abrasive was added to the steel and glass fiber mixture and uniformly distributed by mechanical stirring with a spatula. Diamond size was 100 grit as defined by the Bureau of Standards, Abrasive Grain Standards, 25% by volume. Distribution of the diamond was improved by the addition of 1 /2% (0.256 gram) by weight of powdered Acrowax C, which is ethylene diadipimide, to the fiber mixture before the screening operation. The Acrowax provided an adhesive film on the fibers which helped to retain the diamond in the mixture after the abrasive was added, and prevented its separation during subsequent loading into a mold. The Acrowax is expelled by volatilization below 300 C. in the early stages of the sintering cycle.

A non-abrasive center preform was made by pressing 363 grams of electrolytic iron powder, minus 325 mesh, manufactured by the Plastic Metals Division of National Radiator Company. The powder was loaded into a steel mold with an inside diameter of 5.775 inches, and pressed to a thickness of .202 inch, said thickness being greater than the finished thickness of the wheel to facilitate bonding of the abrasive section to the preform. The outer band of the mold was then removed and a second band,

conditions to use diamond grinding wheels.

. grain was tested.

with an inside diameter of 6.025 inches was positioned around the partially pressed preform. The fiber and diamond mixture containing 17.14 grams of fiber mixture and 4.2 grams of diamond was then charged into the space between the periphery of the preform and the inside diameter of the steel mold, and a pressure plate positioned on top of the wheel. The mold was subjected to a pressure of 1300 tons, using a hydraulic press, and the band was removed while maintaining pressure on the pressure plates to prevent cracking of the molded wheel.

After the wheel was removed from the mold, it was placed on a refractory batt and sintered in a nitrogen atmosphere for one hour at a temperature of 1040 C. The finished diamond wheel had a diameter of 6.020 inches, with a 43 diamond abrasive section. The thickness of the wheel was 0.132 inch before the truing and finishing operations. For purpose of complying with the patent statute, this example is chosen as the best mode of my invent-ion. This grinding Wheel was used to grind cemented tungsten carbide and proved exceptionally resistant to smearing and loading commonly occuring in other diamond wheels manufactured by previous techniques.

EXAMPLE II I make up a wheel as in Example 1, except that for the abrasive I use fused aluminum oxide. For an alumina abrasive periphery type wheel for use in electrolytic grinding and similar to the wheel in Example I in many respects, I would use #60 grit size of #44 Alundum which is a strong aluminum oxide abrasive. I would make the wheel with a larger volume of Alundum abrasive than I used of diamond and 50 volume percent of Alundmn with 50 volume percent of the same mixture of metal and glass fibers of Example I would be suitable.

The mixing, pressing and firing procedure would be similar to Example I. The electrolytic iron powder center would have a high electrical conductivity which would be advantageous for electrolytic grinding. This kind of a wheel is useful in electrolytic work eroding and abrading machines in which machines it is not necessary under all Since the electrolytic action takes off a great deal of the stock when grinding cemented carbides, the softer and cheaper aluminum oxide can be used.

An experiment was made showing that glass powder can be used in place of glass fibers in mixture with the metal fibers. The bond alone in the absence of abrasive I prepared two different mixes which were the same except for the use of glass in the form of fibers in one and in the form of powder in the other. The fibers used have been described in Example I. The glass powder was made by crushing a Pyrex glass beaker and then pulverizing it in a small rotary steel plate mill. The portion used was approximately #180 grain size which was obtained by using the portion that passed through a 16X silk screen (158 meshes per inch) and remained on an 18X silk screen (183 meshes per inch), although much broader grain size limits are feasible, such as from about mesh down to the finest sizes readily obtainable.

The mixings were made with 10.1 grams of steel fibers and 1.8 grams of glass fibers or 1.8 grams of glass powder. The steel mold employed was rectangular in shape with a cavity to press bars 1.25" long x 0.50" wide. Rectangular test bars of 0.25" thickness were pressed at 50,- .000 pounds total pressure corresponding to 40 tons per ulu's of rupture of the product at room temperature was rim:-

determined by cross-bending on a one inch span. Results were:

Modulus of rupture (pounds per square inch) Metal Fibers Metal Fibers Plus Glass Plus Glass Fibers Powder Thus it is seen that the strengths of the two products were substantially the same. They had an identical appearance. They also looked alike under a metallurgical microscope. Metal fibers could be identified in both samples and these were seen to be sintered together at various places along their length. For further examination, the structure of the pieces was loosened by deform ing plastically as a result of hammering in a steel mortar, and then such hammered pieces of the same sample were rubbed together to produce a powder for examination under a petrographic microscope after immersing in a liquid of 1.59 refractive index.

In the product made with glass fibers some rod-shaped fragments that corresponded dimensionally to the original glass fibers could be identified and these were seen to be sometimes fused together in spots and sometimes attached to aggregates of angular shaped pieces of glass. A considerable amount of the glass fibers, evidently merged together by fusion, lost their fiber identity. No rod-shaped pieces could be seen in the product made with glass powder. All the glass in this latter sample was in the form of irregular particles which were either isolated or fused together.

From these data I conclude that the major factor conveying unique properties on my product is the metal fibers in combination with glass and, while glass fibers may have some desirable properties, glass powder may also be used. Other ceramic constituents equivalent to glass may be used in the raw batch mixture as an addition to or in place of glass.

The diameter range of commonly available metal fibers is from about 0.1 mil to about 5 mils average diameter. Metal whiskers of very small diameter such as about 0.01 mil have been produced and they may be used in my process, raw batch and product. With long fibers, the maximum length to diameter ratio for 1 mil fibers is about 125 to 1 and for fibers of about 0.1 mil in diameter, the ratio becomes about 1300 to 1. The crosssectional shape may vary from circular to flattened forms and the ratios for maximum to minimum diameters on the same fiber may be from about 1 to 1 up to 5 to 1. The surface character of fibers may vary from smooth to irregular.

Although difiiculties of blending the fiber and handling fiber in wheel manufacture impose a practical fiber length limit of about /s", for a felting process of manufacture still longer fibers may be used. In a felting process, a mixture of metal fibers, glass powder and/or fiber, abrasive grain and Water would be mixed with a suspending agent such as ammonium alginate and agitated to main tain uniform suspension while simultaneously removing the liquid phase by gravity or pressure or vacuum. The cake could then be further compacted if necessary and fiinally sintered to develop the product.

My product is a metal fiber-bond abrasive product with glass or other ceramic material in a secondary-bonding role. The metal fibers constitute a continuous matrix of electrically conducting material. As a result of heat treatment, the metal fibers are sintered together at points of contact. A certain amount of shrinkage may occur in the firing operation. The metal fibers constitute the primary continuous phase. As a matter of fact, experiments have shown that bonds with even lower ratios of metal to glass than the minimum shown in Table I, such as 3 volume parts of metal fibers to 7 volume parts of 3 glass, have high electrical conductivity. Along with the fibers, powdered metal up to amounts equal to that of the fibers may be employed to contribute additional strength, because metal powders have very high surface areas and consequently sinter more readily.

The glass which is put into the mixture adheres and ooalesces and may devitrify or crystallize to a greater or lesser extent during the firing operation. The word glass in the specification and claims is intended to mean the glass that is put into the product and the ceramic phase derived from said glass which is found in the product. TABLE I.SOME STRUCTURAL RELATIONSHIPS TOR DIFFERENT WHEELS MADE IN ACCORDANCE WITH MY INVENTION [The tabulated numbers are the volume percents for the different column headings] Total Total Glass Total Item No. Abrasive Metal (Ceramic Bond Pores constituent) If it is desired to convert the above volume percentages of solid constituents to corresponding parts by Weight this is very simply done by multiplying them by their respective specific gravities.

When the volume percentage of abrasive is above 25 vol. percent as is commonly the case with grinding wheels made with abrasives that are cheaper than diamonds, the total bond contents of necessity are in a lower range. Ordinarily the volume percent of glass is no greater than the volume percent of metal fibers, but when the glass constituent is put into the wheel in fiber form, a higher volume percent of glass can be used since some of the glass fibers will functionally replace metal fibers in molding and handling the wheel.

My invention constitutes an abrasive product, usually a wheel having diamond abrasive grain, but alternatively having other kinds of abrasive, and fused alumina is the illustrative abrasive in Example II. The percentage of the abrasive is not critical and is determined by factors already known in the art. At least half of the metal content is metal fibers. There may or may not be some porosity to the wheel, but volume percentages are stated on the total volume of the wheel. With simultaneous application of heat and pressure (hot pressing) the porosity can be reduced to a negligible level, less than 1%.

The volume percentages of those products for which diamond is the abrasive in the above are calculated on the basis that the abrasive will usually be from about 5 volume percent to about 25 volume percent, and both metal fibers and glass fibers are used. When the volume percentage of the abrasive is as low as 5 volume percent the metal fibers can vary between 47 volume percent and 90 volume percent when there are no pores, and between 37 volume percent and 70 volume percent when 20 percent of pores are present. When the volume percentage of abrasive is 25 the metal fibers can vary from 37 volume percent to 70 volume percent when there are no pores, and between 27 volume percent and 50 volume percent when 20 percent of pores are present. When the abrasive is 5 volume percent the glass fibers can vary from 47 volume percent to 5 volume percent and when the abrasive is 25 volume percent the glass fibers can vary from 37 volume percent to 5 volume percent. These usual diamond abrasive contents are shown in Table I together with higher abrasive contents which may be used. Non-diamond abrasives are more commonly employed in higher volume percents such as 50 and 60 which are likewise illustrated in the table but diamond abrasive can be employed in high volume percents and nondiamond abrasives can be employed in low volume percents, when desired.

If the volume percentage of glass fibers is low and the volume percentage of metal fibers is high, the resultant abrasive body will have maximum strength and a minimum self-dressing capacity. Conversely if the volume percentage of glass fibers is high and the volume percentage of metal fibers is low the abrasive body will have a minimum of strength and a maximum self-dressing capacity. Therefore in the range stated the degree of selfdressing capacity can be varied at will to give the desired grinding action.

Having specified diamonds and aluminum oxide abrasive which is also known as fused alumina abrasive in the examples, I want to point out that other abrasives can be used. Silicon carbide is widely used as an abrasive and has been much used for grinding cemented carbide. Recently fused zirconia has been used as an abrasive and can be used in my invention. In the broader aspects my invention is not limited to any particular abrasive.

In electrolytic grinding, the grinding wheel must be conductive. But it has already been pointed out in a number of patents that the ordinary metal bond is too conductive. With the use of the ordinary metal bonded wheel in an electrolytic operation, arcing frequently develops which mars the work piece. Many circuits and arrangements have already been devised to reduce this arcing. Many of the circuits require expensive electronic equipment.

Due to the incorporation of ceramic materials, wheels according to my invention have a higher resistivity (lower conductivity) than ordinary metal bonded grinding wheels. Silicon carbide is conductive while diamonds and aluminum oxide are, for all practical purposes, completely nonconductive. This gives an advantage to the use of aluminum oxide abrasive over silicon carbide abrasive. However conversely silicon carbide cuts the hard cemented carbide better than does aluminum oxide, silicon carbide being harder than aluminum oxide. With a strong electrolytic attack on the work piece which can be achieved without spoiling the work piece if the conductivity of the wheel is low, about all that the abrasive has to do is to clear away the material weakened by electrolytic attack.

In such cases the aluminum oxide abrasive may be preferred as arcing is less apt to occur with a lowered wheel conductivity. An outstanding advantage according to the invention is the low conductivity of the bond itself due to the presence of non-conducting ceramic material in the wheel and the fiber-like structure of the metal that is employed. In many cases for electrolytic grinding a low percentage of metal fibers will be preferred.

It will thus be seen that there has been provided by this invention a metal bonded grinding wheel in which the various objects hereinbefore set forth together with many thoroughly practical advantages are successfully achieved. As various possible embodiments may be made of the above invention and as many changes might be made in the embodiment above set forth, it will be understood that all matter hereinbefore set forth is to be interpreted as illustrative and not in a limiting sense.

having a length to diameter ratio of from 2 to 1 to 125 to I claim:

1. Abrasive wheel comprising abrasive grain bonded with from 37% to 90% by volume of sintered metal fibers having a length to diameter ratio of from 2 to 1 to 125 to 1, of which have a ratio of more than 10 to 1 and are from .002 inch to .125 inch long and which fibers have an average diameter of from one tenth of a mil to five mils, containing from 5% to 47% by volume of glass, and the glass of said bond including portions in which glass fiber residue can be identified.

2. Abrasive Wheel according to claim 1 in which the metal fibers are steel fibers.

3. Process for the manufacture of abrasive wheels comprising providing abrasive grains from 5% to 60%, metal fibers of average diameter from 0.01 mil to 5 mils and of length to diameter ratio between 2 to 1 and 1300 to 1 from 15% to and glass in physical form selected from the group consisting of glass fibers 80% of which have a length to diameter ratio of more than 10 to 1 and the average diameters of which are from one third of a mil to two mils and glass powder from mesh size to the finest powders, from 5% to 47%, mixing the abrasive grains, the metal fibers and the glass, charging the mixture into a mold, compacting the mixture and heating the product at sintering temperature for the metal to aggregate the glass and to sinter the metal fibers together at points of contact, thus forming a metal bonding phase with glass in secondary bonding role, all percentages being by weight.

4. .Process according to claim 3 in which the metal fibers have an average diameter in the range of from 5 mils to 0.1 mil.

5. Raw batch for the manufacture of abrasive wheels by compacting and sintering consisting of abrasive grains from 5% to 60%, metal fibers of average diameter from 0.01 mil to 5 mils and of length to diameter ratio between 2 to 1 and 1300 to 1 from 15% to 90% and glass in physical form selected from the group consisting of glass fibers 80% of which have a length to diameter ratio of more than 10 to 1 and the average diameters of which are from one third of 21 mil to two mils and glass powder from 100 mesh size to the finest powders, from 5% to 47%, all percentages being by Weight.

6. Raw batch according to claim 5 in which the glass is the fibers defined.

7. Raw batch according to claim 6 in which the average diameter of the metal fibers is from 0.1 mil to 5 mils.

8. Raw batch according to claim 5 in which the average diameter of the metal fibers is from 0.1 mil to 5 mils.

9. Abrasive wheel comprising abrasive grain bonded with from 37% to 90% by volume of sintered metal fibers l, 80% of which have a ratio of more than 10 to 1 and are from .002 inch to .125 inch long and which fibers have an average diameter of from one tenth of a mil to five mils, and containing from 5% to 47% by volume of glass.

l0. Abrasive wheel according to claim 9 in which the metal fibers are steel fibers.

-' References Cited in the file of this patent UNITED STATES PATENTS 226,066 Hart Mar. 30, 1880 1,918,242 Benner et al July 18, 1933 2,162,387 Radabaugh June 13, 1939 2,232,389 Jurkat Feb. 18, 1941 LA. a.

Claims (1)

1. ABRASIVE WHEEL COMPRISING ABRASIVE GRAIN BONDED WITH FROM 37% TO 90% BY VOLUME OF SINTERED METAL FIBERS HAVING A LENGTH TO DIAMETER RATIO OF FROM 2 TO 1 TO 125 TO 1, 80% OF WHICH HAVE A RATIO OF MORE THAN 10 TO 1 AND ARE FROM .002 INCH TO .125 INCH LONG AND WHICH FIBERS HAVE AN AVERAGE DIAMETER OF FROM ONE TENTH OF A MIL TO FIVE MILS, CONTAINING FROM 5% TO 47% BY VOLUME OF GLASS, AND THE GLASS OF SAID BOND INCLUDING PORTIONS IN WHICH GLASS FIBER RESIDUE CAN BE IDENTIFIED.