US2476699A - Diamond abrasive - Google Patents

Description

July 19, 1949. R. F. CLlNE DIAMOND ABRASIVE Filed Nov. 6, 1945 25 I I i ROBE/er E Q WE Patented July 19, 1949 assignor to Norton a corporation of Company, Worcester, Mass.,

Application November 6, 1945, Serial No. 620,982

1 Claim.

3 The invention relates to diamond abrasives. One object of the invention is to provide a diamond grinding wheel bonded with a hard, heat-resistant bond. Another object of the invention is to produce a sintered metal type bond of great hardness. Another object of the invention is to provide a method for the manufacture of diamond abrasives yielding a hard metallic bond under such conditions that the diamonds are neither oxidized nor graphitized nor otherwise impaired.

Another object of the invention is to provide for diamonds a bond of the metallic class which is harder acting than the non-ferrous metal bonds and also harder than most sintered steel bonds now known. Another object of the invention is to provide a diamond abrasive useful in new types of grinding operations. Another object of the invention is to provide a grinding wheel having one or more of the above characteristics and which is nevertheless free-cutting.

Another object of the invention is to produce a diamond grinding wheel especially for cutting or grinding of hard, brittle substances. Another object of the invention is to provide a hard and somewhat friable bond which will allow dull diamond grains readily to break away from the wheel to permit new sharp edges of diamond to be presented to the working face. ject of the invention is to provide a whole range of bonds varying in hardness from about Rockwell A 85 or higher down to the hardness of bronze and varying from quite friable or brittle to fairly ductile.

Another object of the invention is to provide a well bonded diamond grinding wheel. Another object of the invention is to provide a combination of elements which, in powder form, will quickly sinter to produce a uniform composition to bond diamond abrasive grains. Other objects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the features of construction, combinations of elements, and in the several steps and relation and order of each of said steps to one or more of the others thereof, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claim.

In the accomp nyi drawings illustrating two Another obof many possible embodiments of the mechanical features of this invention,

Figure 1 is an axial sectional view of a graphic mold assembly and induction heating apparatus;

Figure 2 is an end view of a diamond wheel made in the mold of Figure 1;

Figure 3 is an axial sectional view taken on the line 3-3 of Figure 2;

Figure 4 is an axial sectional view of another graphite mold assembly for the production of a hollow cylindrical wheel including a non-abrasive portion;

Figure 5 is an end view of the wheel made in the mold of Figure 4; and

Figure 6 is an axial sectional view, taken on the line $6 of Figure 5.

For the grinding of difierent materials and under difierent conditions and to produce specific results, many different kinds of abrasives and bonds have been used. Diamond abrasive bonded with synthetic resin fulfills particular needs while vitrified bonded diamond wheels are preferred for certain operations. Where hard bonded diamond wheels have been wanted, resort has usually been had to metal bonds. Sintered copper-tin bond has been used with great success to bond diamonds making, relatively speaking, a hard bonded diamond wheel. Sintered steel has been used to some extent to provide a still harder acting bond for diamonds. I have discovered a composition and a method for the production of diamond abrasives with a still harder bond which ical composition:

Per cent B. 17.32 C 21.62 F 0.03 Undetermined L03 Percen e MIMI-l8 by w tgg 1 Finer than 1 I provide a quantity of iron powder. The iron powder that was actually used would pass through a 325 mesh screen and analysis indicated it to be more than 98% iron. It was obtained fromacommercial source on the open market.

I made a mixture of these powders to give a volume percentage of 23 B40 to 7'7 iron. Actually, to provide this percentage ratio, I took 17.8 grams of B40 and 182.2 grams of iron. The two powders were mixed together in a standard type of pan mill for a period of about 18 hours to insure uniform mixing. A weight of 15.7 grams of this dry mixture was then taken as bond for the wheel. To this was added 2.77 grams of diamond grainof 100 grit size to produce a resulting wheel containing 25 volume per cent of diamond. This small amount of mixture which totalled 18.47 grams was mixed with a spatula by hand to reasonable uniformity and then loaded into a graphite mold.

Referring now to Figure 1, the mold consisted of an outer hollow cylinder I of graphite with a bore fitted with hollow cylindrical graphite plungers 2 and 3 which would just slide in the bore and themselves having a 3/4" bore in which a cylindrical graphite core rod 4 fitted. The wheel mixture was placed in the annular zone 5 between the core rod 4 and the outer cylinder I. The plungers 2 and 3 moved from both ends to compress the mixture in the annular zone 5. The mixture, which was dry. was-compacted in the mold by a moderate cold pressing operation under a. pressure slightly less than 1000 pounds tothe square inch. I

The mold was then placed between graphite rods 6 and I and inside of a high frequency induction furnace coil 8 havinga refractory lining. While heating the mold it was subjected to a pressure of 2260 pounds-per square inch by a press ram l0 acting against therod 6, the rod 1 being supported by the press platen II. The

- heatingand' pressing were continued until the required contraction shown by a press gage had been obtained toyield the desired length of wheel. The moldassembly was then removed from the furnace and allowed to cool slowly in diatomaceous earth powder. 1'

It is usually desirable to make a diamond grinding wheel integral with a back or center of nonabrasive properties so that all orsubstantially all of the diamond material may-be used in grinding, leaving little or none in the discarded stub. Figure 4 illustratesa mold. to make such a wheel. An outer cylinder ll of graphite is fitfed with hollow cylindrical graphite plungers i8 and I4 which will just slide inthe bore of the cylinder l2 and in the bores or the plungers l3 and H is a hollow core It. also of graphite. The

annular zone ii of this mold is partly filled with a mixture such as already described, and partly with such mixture without the diamonds, i. e. with iron and boron carbide. The two mixtures are cold pressed in the mold as already described and then the mold is placed between pressure elements and in a high frequency induction furnace of a size to receive it and the mixtures are compacted under heat and pressure of the order of and as already described. A feature of the mold of Figure 4 is that-the core I5 is hollow and thinwalled so that the piece can c001 from the inside as well as from the outside to prevent fracture of the product due to cooling stresses. This is important for relatively large diameter wheels.

The grinding wheel 20 made in the mold of Figure l is shown in Figures 2 and 3. When the cold mold was broken apart and the wheel 20 cleaned of graphite, it was mounted on a. mandrel and found to rotate with remarkable truth. While rotating it was dressed with fine boron carbide grain of about 200 grit size to cause the diamond grains to project above the surface of the bonding material.

This wheel 20 had a good distribution of diamond grains around its periphery and across its ends. The diamonds themselves were undamaged by the heat treatment due to the short time during which they were above 1000 C. (about five minutes). The actual temperature on the outside of the mold was 1085 C. The diamonds were well stuck orattached in the wheel, indicating that good'bonding had been achieved. I

Grinding tests were made with the wheel 20 in comparison with 'a standard metal bonded diamond wheel containing the same concentration of diamonds, the bond being 18.5% tin and 81.5% copper (by weight), and also in comparison with a standard resinoid (phenolic resin) bonded diamond wheel, also having the same concentration of diamonds. The grinding machine was a Heald 72a internal grinder, wheel speed 25,000 R. P. M. The following tables give the grinding results for particular materials.-

Material on Diameter 7 Mill Tenn:

5 Diameter Of the invention Btandard'Metal bonded when Wear on Diameter Mils Wm] Diameter Mils 0f the invention Standard Besinoid Bonded Removed on MaterlalR-e movedo'n Tents III Grinding Sondersons cartridge die steel Material Re- Wheel Wear Wheel on Diameter gm Mill Mile

! the invention 0.1 12 0 Standard Metal Bonded. 0. 7 12. 0

example of a relatively thin zone of abrasivebearing bond molded integrally with another zone having the same alloy composition but no abrasive. A wheel such as the wheel 2| or any other integral shape can be mounted on a steel shank or any other suitable mounting. It can be brazed to a shank by a subsequent heating operation and the bond will not be deleteriously affected as long as the brazing temperature is less than 100 0.

The above chemical analysis should not be considered as a limiting case since, in the commercial preparation of boron carbide, the boron con-, tent may, if desired, be raised to 95% with the balance mainly carbon. The presence of the carbon was considered to be of doubtful value. Since 17.8 grams of boron carbide plus 182.2 grams of iron ads up to 200 grams of mixture, the proportion is 8.9% of boron carbide to 91.1% of iron by weight. Since there was but 21.62% of carbon in the boron carbide, there was only 1.94% of carbon in the mixture.

In an attempt to find out the true nature of the'bondingmaterial of the invention, I prepared a sample of 25% by volume of boron carbide of the foregoing analysis and 75% by volume of Fe (this differs only slightly from the proportions used in making the wheels described) This mixture was heated under pressure as already set forth and the compacted sample was then powdered and subjected to X-ray analysis. Then another mixture was made of 40% B40 and 60% Fe by volume, likewise heated under pressure. powdered and subjected to X-ray analysis. The first of these will be called sample No. 1 and the second will be called sample No. 2. Excellent powder photographs were obtained from both samples. The following conclusions are believed to be correct:

(a) The principal phase present in both samples No. 1 and No. 2 was FeaB, sometimes referred to as F6432, which contained little or nothing in solid solution.

(0) Sample No. 1 contained little or no B40 or Fe.

(0) Sample No. 2 seemed to contain a small amount of each of B40 and Fe.

(:2) No indication of FeB or of 13 could be found in either of sample No. 1 or sample No. 2.

(e) Graphite was indicated by the X-ray photographs to a slight extent in sample No, 2. From all considerations graphite should be present in both of the samples and to a greater extent in sample No. 2 than in sampleNo. 1.

(I) From all considerations there is doubt as to the presence of FeaC in either sample. The X-ray photographs failed to indicate any. Furthermore, the sintering time was too short to make the formation of FeaC likely.

It is, therefore, considered that the principal sintering reaction may be indicated by the for- This would indicate that to combine all of the iron and boron and provided the boron carbide is represented by the formula B40, 11% B40 to 89% Fe by weight should beused. But, as indicated, an excess of boron over that required by the formula 1340 may be present (in solid solution) in the boron carbide powders. In such cases more iron may be provided to combine all of the boron to FezB.

In order that the various weight and volume percentages already mentioned may be seen at a glance, the following table is provided:

TABLE IV Volume per cent B10 Weight per cent 10.0 3.87 23.0 Formula from 8.9 which were prepared wheels of the two examples given 25.0 Sample No. l 9.67 28.4 Rgti) fol 8F 11.0 4F B o l e -r e: 30.0 11.6 40.0 Samplg No. 2 15.5

c 90.0 06.13 77.0 Formula from 91.1 which were prepared wheels oi the two examples given 75.0 Sample No. 1 90.3 71.6 Ratio ior 89.0

B 8Fe 4F02B C 70.0 88.4 60.0 Sample No. 2 84.0

The composition of sample No. 1 appears to yield the highest hardness values, indicated by tests to be in the range of Rockwell A scale 80 to 85. This is very hard. Rockwell A 87 is about 600 times as hard as plate glass in wear resist-. ance. Commercial cemented tungsten carbide has a. hardness range on the Rockwell A scale of around 86 to 91. It may be desirable in some cases to provide a bond which, while much harder than the widely used copper-tin bonds, isyet not so hard as the above. In general, thehardness and brittleness of the bond can be reduced by decreasing the proportin of boron carbide,

For example, with 10% by volume of B40 and 90% by volume of Fe (see Table IV the resulting metallic material becomes more ductile and actually can be deformed without snap fracturing; This is doubtless due to a. substantial proportion of iron as ferrltein the composition. Assuming all of the boron combined with iron to form FezB, this composition is by weight:

Per cent Free iron 71.86 Combined iron 24.27 Combined boron-l-carbon 3.87

Total 100.00

While boron carbide, B40, is of great hardness,

and is in fact considered next to the diamond itself in hardness, it has been noted that 90.3% iron, the remainder 1840, produced the hardest bond. This is doubtless because the particular time-temperature factor for sintering does not cause the boron carbide grains readfly to bond .shows, upon metaliographic inspection, discrete crystals of 1346. These tend to weaken the structure somewhat. Yet for certain grinding results it 'is desirable to have a friable bond. 80, therefore, I may use as much as 40% 1340 by volme (see Table IV) or, stated in another way,

- as little as 84.58% by weight of iron (sample No.

2). The low limit of iron is, therefore, stated,

in an even percentage, as 84% by weight.

Instead of sintering the mixture of boron carhide and iron under combined heat and pressure,

the grinding wheel or other abrasive body may first be cold pressed under a relatively high pressure and subsequently sintered without pressure. To substantiate this, a test bar was made by pressing a mixture of 23 volume per cent of 34C and 77 volume percent of iron under a pressure of 10,000 pounds per square inch without any heat. later this test bar was heated in an induction furnace in a vacuum for fourhours at a temperature of 1000 C. This process made a strongbar and accordingly is deemed to be useful for the manufacture of grinding wheels containing diamonds. From certain tests made by my associates it is clearthat the diamonds will not be detrimentally graphitized by such heat treatmentin the presence of the range of compositions according to the present invention, provided carbon ispresent even in combined form to the extent of at least 3% by weight of the bond, as will usually be the case. In either method of preparing the product the bond is a silvery metallic product having a density of from 6.0 to 7.0. Accordingly if for manufacturing reasons it is desired to do the heating in a furnace which cannot be associated with pressure apparatus, cold pressing and subsequent sintering may be re- Y sorted to.

While the utility of the invention is now known in connection with diamond abrasives, it is contemplated that the bonding ingredientsmay be used to bond other hard materials such as aluminum oxide, and especially silicon carbide, not

' material and I wish to claim the same separately.

It will thus be seen that there has been provided .by this invention an article and a method in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As various possible embodiments might be made of the mechanical features the above invention and as the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

An abrasive composition comprising diamond grains bonded with a sintered mixture containing from 25% to Fe'zB by weight and from 91% to 97.8% iron by weight, including both free and combined iron.

ROBERT F. CLINE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 952,290 Whitney Mar. 15, 1910 1,562,043 Pacz Nov. 17, 1925 2,046,912 Kormann et al.' July 7, 1936 2,076,952 Krathy Apr. 13, 1937 2,358,459 Kelleher Sept. 19, 1944 FOREIGN PATENTS Number Country Date 491,659 Great Britain Sept. 6, 1938 OTHER REFERENCES Mellor, Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 5.

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