The Abrasive Handbook

Grinding plays a most important part in present-day industry for without it the cost; of many, everyday commodities would be prohibitive. For example, without the modern grinding machines and wheels it would be impossible to manufacture low priced cars. Without the help of abrasive engineers such vehicles would cost several thousand dollars each. The abrasive, industry has developed rapidly during the past quarter of a century and during this period there have been, evolved many interesting and economical ways of applying grinding processes.

Data pertaining to abrasives is not always obtained readily, although the splendid work done by the publishers of Abrasive Industry has aided materially in spreading abrasive knowledge. The data contained in this handbook represent a painstaking collection covering a period of many years.

As a reference work for works managers, chief engineers, designers, abrasive engineers, grinding-room foremen, and grinding machine operators, it is the hope of the editor that this book soon will be recognized as a standard covering the best current practice. This book points out how many classes of work can be ground advantageously and it answers practically most questions which confront the users of abrasive materials or grinding machines. It stands by itself as being the only work of its kind ever published.

Abrasive materials are classed into two groups, natural and manufactured. The natural abrasives are emery, corundum, quartz, flint, garnet, diamond, tripoli, diatomaceous earth, sandstone, pumice, and natural sharpening stones. The manufactured abrasives are carbide of silicon, aluminum oxide, glass, and the metallic abrasives such as steel wool and steel shot and grit. Following is an alphabetical list of the abrasives described in this section:

Glass has been utilized as an abrasive for a number of purposes. Many years ago, before the advent of manufactured abrasives, glass was considered a good medium for grinding locomotive throttle valves in their seats, for grinding brass valves for various purposes, etc. The glass was powdered very fine and used with water or oil.

In England abrasive glass is used in considerable quantities for coating abrasive paper and cloth, taking the place of the flint used in the United States. As the British supply is obtained from old liquor bottles, it is cheap and abundant. In the United States, however, glass coated paper has never been a serious competitor with flint and garnet. This is due probably to the superior cutting qualities of these natural materials and to large sources of the raw product. However, under certain conditions glass paper has been known to give good results. It also is used to quite an extent in Canada.


ALUMINUM OXIDE

This material, which is also termed manufactured alumina, is an electric furnace product made by fusing materials high in alumina, such as bauxite. It has also been made from emery, resulting in the so-called German iron-free corundum. Manufactured alumina also is made from corundum. This product is used extensively for making cool cutting wheels for specific grinding operations. Bauxite is used principally for making aluminum oxide as it can be mined cheaply, while its alumina content is high.

Bauxite is a clay like material, taking its name from the village of Lee Beaux in Southern France where it first was observed. It never is found in a crystallized state. Its color runs from light yellow to deep red. Its constituents are aluminum oxide, iron oxide, silica .and titanic acid. It is thought to be a decomposition of an igneous rock. It is mined in open cuts, calcined to remove the excess of moisture and shipped to abrasive plants in carload lots.

In the manufacture of aluminum oxide at the plant of the Carborundum Co., the following process is followed: The furnace is a simple affair consisting of an outer shell which rests on a base, while two electrodes supply the current. While the shell is cooled by water circulation, it does not contain a refractory lining as the charge forms this itself. The furnace is mounted on wheels so that it can be moved from under the electrodes after the burning operation is completed.

In making up the charge, the bottom of the furnace first is lined with a carbon and tar mixture. Then a layer of bauxite is introduced and the electrodes lowered to rest on it A path of graphite then is laid between the electrodes. This forms a good passage for the current, but as soon as the bauxite is melted it forms its own conductor. The current is turned on and the charge brought to a molten state. The current is alternating, 6000 amperes at 100 volts.

After the first layer is melted another layer is put in place and the electrodes raised. This process is continued for about 36 hours at which time the furnace is full. During the melting process the oxide of iron and silica in the raw material unite and form ferrosilicon so that the abrasive is practically pure. After the furnace has cooled off the outer shell is removed and the ingot taken out. It is broken up under a skull cracker and next passed through an ore crusher. Next it goes through a magnetic separator. Subsequent operations consist of feeding the material through a roller crusher after which it is graded into various sizes.

Aluminum oxide varies in color from a light purple to a dark brown. A special variety is almost white in color. Aluminum oxide is used for making grinding wheels for finishing materials of high tensile strength. It is used in grain form for setting up polishing wheels, coating abrasive paper and cloth, and to a certain extent for finishing stone, glass, etc.

The ordinary variety of aluminum oxide is used for general steel grinding on both rough and precision work, while a refined variety is used largely for grinding alloy steels, for cutter sharpening, etc.

Aluminum oxide first was made in 1837 by M. A. A. Gaudin who was performing experiments to produce artificial rubies. Other experimenters took up the work from time to time laying the foundation for the latter day investigators who produced the material on a commercial basis. Perfection of this abrasive is due to American genius chiefly to that of Charles B. Jacobs and Frank J. Tone.


CARBIDE OF SILICON

Carbide of silicon is a manufactured abrasive, a chemical composition of two elements, carbon and silica. It is made in an electric furnace of the resistance type, the ingredients forming the charge being coke, salt, sawdust and sand. Coke supplies the element of carbon and sand that of silica. The salt brings about certain reactions in the manufacturing process, while the sawdust makes the mass porous so that the gas generated finds a ready means of escape.

Ordinary carbide of silicon furnaces are approximately 50 feet long, 10 feet wide and five feet high. The walls and sides are brick, while the top is open. The electric terminals run through the end walls. They are carbon rods three inches in diameter arranged in bundles of 60. The spaces between the rods are packed with graphite. The outer ends are capped with copper to form electrical connections.

The furnace charge is made up of 34 parts coke, 54 parts sand, 10 parts sawdust and 2 parts salt. These materials are mixed by mechanical means and usually they are brought to the furnace by a conveyor. In charging, enough of the mixture is placed in position to fill the furnace up to the level of the electrodes. A trench is dug between the electrodes and filled with granulated coke. This allows a free passage of current when the burning operation is started. The furnace is then filled completely and the current turned on. This current is alternating, 190 volts and 6000 amperes. The resistance lowers as the furnace charge heats and after four hours of operation it remains constant at 125 volts, 6000 amperes. The sawdust burns out first and then carbon monoxide is given off, which burns with a yellow flame. As the action goes on the mass shrinks which necessitates the adding of new material. About 86 hours are consumed in the burning operation. Then the furnace is permitted to cool for a day after which it is broken open. The top crust is practically unaltered and under this is found a layer of amorphous carbide of silicon. Under this is the pure crystallized material of commerce. At the center is found a mixture of carbide of silicon and graphite. The heat necessary is estimated at 7500 degrees Fahr., which is sufficient to turn the core into practically pure graphite.

The outer layer of material, that is the amorphous carbide of silicon, for many years was thought to have no commercial value. Later it was discovered that this material has high refractory properties so that today it forms a valuable by product. Carbide of silicon is of various colors from green to gray. The color is not an index of its value. Carbide of silicon has a well defined crystalline structure and it is exceedingly hard and sharp. It is nearly if not quite as hard as a diamond. Carbide of silicon has never been found naturally, the reason probably being that the heat to form it is so near that necessary to form graphite that the latter material only was formed when the earth cooled off.

Carbide of silicon is used for the manufacture of grinding wheels, being especially valuable for grinding cast iron and brass. It also is an economical abrasive for grinding stone, glass, etc. In grain and powder form it is used for stone finishing, glass grinding, etc. Coated on paper and cloth it is used for a diversity of purposes such as leather finishing, paint rubbing, etc.

Carbide of silicon was discovered by Edward G. Acheson in 1891 while he was experimenting with a small electric furnace. This material first was used for polishing precious stones, at which time it sold for $880 a pound.


CORUNDUM

This mineral is an impure form of the ruby, a composition of alumina and oxygen, with impurities such as silica, ferric oxides, etc., and combined water. Some 60 years ago it was considered a comparatively rare mineral but now it is known to exist in several parts of this country. Large deposits have been worked (and worked out) in Hastings county, Canada. During the World war, immense reserves of this mineral were opened in the Transvaal district of South Africa.

Corundum in reality is a pure form of emery as its chief constituent is alumina. Its crystals present no true cleavage, but parting planes are present. If these are too numerous, however, so as to be present in the individual grains of which a grinding wheel is composed, such a corundum is low in efficiency. An ideal corundum for abrasive wheel manufacture is one wherein the dull grains will break with an irregular to a conchoidal fracture. The hardness of corundum varies from 8.8 to 9. It is found in three forms called boulder, crystal and grain corundum.

At the present time the principal deposits in the United States worked for commercial purposes are in North Carolina near Franklin. This property is known as the Corundum Hill Mine and is controlled by the Hampden Corundum Wheel Co. This material is found in small crystals running through decomposed rock. As it is cleaned readily, it forms a valuable source of abrasive supply. In the United States corundum also has been found in Maine, Massachusetts, Connecticut, New York, Pennsylvania, Delaware, Virginia, South Carolina, Tennessee, Georgia, Colorado, Montana, California and Idaho. In Canada the chief deposits were found in Renfrew county, Ontario. As previously stated these deposits have been worked out. However, operations now are being carried on in working over an enormous pile of tailings, the residue left by cleaning operations of other days, so that a source of supply of this material still is available and may continue for some years to come. In speaking of corundum in general it can be stated that it crystallizes in the rhombohedral division of the hexagonal system and under the head of crystal corundum is included all the crystal varieties of corundum which occur in block corundum or in sand and gravel.

The corundum deposits are found about 400 feet from the company's mill. They occur in reef as well as in eluvial form. The reefs are opened up through a series of surface workings consisting of irregular pits, while trenches and short cross cuts expose a solid reef to a depth of 60 feet. The ore forms vertical bodies up to 12 feet thick. It is composed of coarse, white plumasite (feldspar corundum rock) carrying from 15 to 60 per cent corundum. The remainder is almost entirely feldspar with a small amount of black mica, magnetite, etc. The water supply comes from a driven well located 500 feet from the mill. This well furnishes not less than 180,000 gallons per day which is ample for all milling operations. A 54 horse power gas engine generates power for pumping water, running the mill and working the dynamo for the magnetic separator and also for electric illumination. The milling operations include crushing, concentrating and grinding.

The ore arrives at the mill as coarse, gravelly material mixed with earthly debris, but including larger blocks. These are reduced to about half the size of a man's fist in a 12-inch Robey stone crusher. The ore then is crushed by five stamps, each weighing 1500 pounds, in a continuous supply of water. The product passes out through a screen with eight meshes to the linear inch to a Frenier pump.


CROCUS COMPOSITION

A material composed of crocus mixed with a suitable binder and molded in cake form. It is used for bringing up the high luster seen on iron and steel parts such as cutlery. This material also is used on tin and similar soft materials.


DIAMONDS

Diamonds are of two general types, the white or gem variety and the black or carbonado stones. Bort diamonds as used for grinding wheel truing are in reality imperfect gem stones which because of flaws, construction characteristics, etc., cannot be cut into gems.

Diamonds are found in India, South America, South Africa, New South Wales, Borneo, British Guiana and in the United States. They are found in the craters of extinct volcanoes and also in river beds or in localities where once rivers existed. Such stones have been washed down from mountain ranges for it is generally conceded that diamonds were formed in volcanoes only. Why this is so is not clear, but it is the consensus of opinion among geologists who have made a deep study of the, subject. Diamonds run in color from pure white to dark brown. The specific gravity of the diamond is 3.5, while its hardness is 10 on Moh's scale. It is conceded to be the hardest substance known, but this fact sometimes is disputed. Diamonds crystalize in the cubic system, generally as octahedrons.

The majority of diamonds are found in South Africa the more important mines being the Premier, Kimberley and De Beers. The output of diamonds is controlled by a British syndicate which puts only so many stones on the market annually.

In the South African mines, the stones are found in what is technically termed blue ground. Formerly this material was spread out above ground for weathering before the diamonds were sorted out, but of late years a more direct method has been employed. In general about four tons of blue ground must be mined to produce a carat weight of diamonds. The workings extend underground for a great distance, often to a depth of 3000 feet or more.

The blue ground is washed to separate worthless material from the gems and a further concentration follows on pulsating tables that are greased with vaseline. Sorting follows and it is here that the gem stones are recovered from the bort. The percentage of bort may run anywhere from 10 to 70, depending on the workings. The process followed in working river workings, as they are termed, is simple. The soil is sifted, the diamonds sorted out and later classified. Diamonds have been found in the United States in Arkansas. These stones are small but of great purity. These workings, however, do not institute an important factor in the diamond market.


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