A complete treatise on the manufacture and practical use of abrasives, abrasive wheels and grinding operations including natural and artificial abrasives, production and preparation of abrasives, grits, grades and bonds, sharpening and grinding stones and wheels, testing wheels for efficiency, truing, rebushing and installing wheels, safety devices, and dust-collecting systems, complete exposition on surface, external and internal grinding and comprehensive data covering the physical and chemical nature of abrasives in general.

The art of finishing metals by abrasion is one of the oldest mechanical practices in existence, dating from the time prehistoric man discovered that he could fashion his wood and bone implements by rubbing them on rocks of a gritty nature.

The grindstone is, without doubt, the oldest form of grinding wheel known. With the early development of the mechanical arts, it was discovered that a sandstone cut in circular shape and mounted upon a revolving shaft, showed higher efficiency than the side of a rock for sharpening and shaping various implements. It is definitely known that grindstones, rotated by power, were used in the manufacture of armor as early as the year 1570. It is also known that the emery deposits of the Grecian Archipelago were known to the ancients and the value of this abrasive recognized, as many writers of early days referred to emery under various names. In considering some of the mechanical achievements of the handicraftsmen who worked with metals centuries before the Christian era, it is hard to conceive how they attained so high a degree of perfection without the use of an alumina abrasive for tool-sharpening purposes.

While the practice of fashioning tools and implements by abrasion is in all probability as old as civilization itself, modern grinding, as we accept this term, is a comparatively recent development. About half a century ago, the individual workman made his own grinding wheels of glue and emery.

The first attempt at precision grinding consisted of finishing the chilled iron calender rolls used in the paper-making industry. Owing to the hard nature of the material in question, it was a long and tedious process to turn these rolls accurately.

The development of the sewing-machine industry in the New England States gave impetus to the development of the grinding-wheel business. As a matter of fact, the first attempts at cylindrical grinding, aside from roll grinding, consisted of finishing parts of the Wilcox & Gibbs sewing machine. The work was done by the Brown & Sharpe Mfg. Co.

With the advent of the automobile industry, over twenty years ago, the grinding-wheel business received a fresh impetus as a rapid means was in demand for the accurate finishing of parts.

Today, the modern grinding wheel is among the most useful of modern shop accessories. Without it, it would be impossible to maintain the present-day standard of rapid production. In practically every line of metal working, the grinding wheel plays an important part, its usefulness ranging all the way from the rough grinding of castings and forgings to the finishing of accurate surfaces, both plane and cylindrical.

In presenting this work, the writer has taken great precaution to make sure that every statement is authentic. Aside from knowledge gained through many years as a journeyman machinist, later supplemented with several years experience as a grinding-wheel salesman, many months were spent in collecting data, verifying statements and consulting reliable authorities, both in this country and abroad.

Natural abrasives are being found in many parts of the world. In a broad sense, the list includes all minerals capable of abrasive action, but from a commercial point of view, the principal natural abrasives are sandstone, emery, corundum and garnet. The diamond is, of course, a natural abrasive; indeed it is the hardest of all, but it is needless to state that its rarity excludes it from the list of commercial abrasive materials.


SANDSTONE

The first abrasive to be used in the form of a wheel was in all probability sandstone. The use of a revolving stone for sharpening purposes is so old that the beginning is lost in antiquity. It seems reasonable to believe, however, that the artificers of early civilization borrowed the idea of a revolving sharpening stone from the crude mills used many centuries ago for the grinding of grain.

Sandstone is a very curious mineral, indeed, as it consists of uniform grains of sand (generally quartz with a small percentage of feldspar and mica) firmly cemented together with silica. Some varieties of sandstone, the Craigleith stones used in the cut-glass industry for instance, are practically pure silica, this material often running as high as 98 per cent. Sandstone is found in many parts of the world and in this country the most extensive deposits that are worked for the production of grindstones are in Ohio and Michigan. The Gray Canyon quarry at Amherst, Ohio, is classed as the largest quarry in the world. Sand- stones are of various colors, these being derived from impurities that penetrated the mass during the formative stage. Pure siliceous stones are white, or pale yellow in cases where small quantities of iron oxide are present. A red tinge is generally due to hematite, yellow to limonite, green to glauconite, gray to clay and shale, and black, as observed in black Graileith stones for example, to manganese dioxide.

The average layman is of the opinion that all grindstones are alike, but this supposition is erroneous for, in forming the sandstone of which the grindstones of commerce are made, it would appear that Nature anticipated the wants of man by providing not only several grits to choose from, but several grades as well. To insure an ample supply of grits and grades, grindstone manufacturers generally control holdings in various localities.

Before the advent of the grinding wheel, sandstone was the only abrasive to be used in the form of a wheel. Its use was, of course, limited, as practically the only grinding done in the early manufacturing days consisted of tool sharpening. Grindstones are used at present in large quantities for sharpening edge tools, cutlery, etc., often in preference to modern abrasive wheels. Many reasons for this practice are explained later, under the heading, Grindstones Vs. Grinding Wheels.


EMERY

The specific gravity of emery varies in different specimens from 3.7 to 4.3 and the percentage of alumina oxide from 30 to 70. The abrasive power, sometimes called the effective hardness, is not proportional to the amount of alumina contained, being influenced to a great extent by the proportions of other component parts in the form of impurities such as silica, lime magnesia, etc., and the structure of the grain itself. For the purpose of grinding-wheel manufacture, the value of emery as an abrasive agent is determined by the amount of alumina oxide present and the toughness of the grain itself.

In common with other natural products, emery ore varies in a number of characteristics. This is true not only of specimens from different mines, but of the product of one mine as well. To market a high-grade emery, it is necessary to pay especial attention to the selection and grading of the crude ore, to which end the use of the microscope cannot be recommended too strongly, as by this means more can be learned of the quality of the emery than by resorting to any other method, aside from the actual working test of the finished product.

As emery may have a high percentage of alumina and at the same time the ore may be so constituted that, after crushing, the grains will seem to possess no cutting points. Such an emery does not make a very efficient grinding wheel.

Some specimens of emery crush up into the proper kind of grains, as far as cutting points are concerned, but at the same time nothing but fine grains are produced. Again it is sometimes noticed that small flakes of mica are scattered through the ore, which is sure to cause trouble in the vitrifying process if the emery is used in the manufacture of vitrified grinding wheels. It is readily seen that an efficient abrasive cannot be made of an emery ore selected at random. It is of the utmost importance to know the nature of the grain to adapt the same for a given abrasive purpose.

At one time, all grinding wheels were made of emery. Of late years, however, corundum and the different artificial abrasives are used. At the same time, notwithstanding the efficiency of modern abrasives, the more ancient emery wheel still has its field of usefulness, and, strange as it may seem, on some classes of grinding, steel castings and heavy malleables, for instance, emery wheels continue to show the highest efficiency. This statement is not made thoughtlessly, but is the result of several years of observation spent in representing grinding-wheel manufacturers.

Emery wheels, or emery stones, as they are termed in this case, are also largely used for hulling oats and rice, taking the place of the bed and runner natural stones as used in the ordinary buhr mill. To whom the credit belongs for introducing the above stones, is not known for a certainty, but experience has proven that the emery stone compares favorably with the natural stones heretofore used for this purpose.

Coarse emery is used in the form of bricks for rubbing stone and various metals while the finer grades of emery are made into sharpening stones, scythe stones, etc. Of late years, however, artificial sharpening stones made of electro-thermic abrasives (Carborundum, for example) have largely replaced those made of emery. For a few specific purposes, emery stones are still in use owing to the high finish they produce.


CORUNDUM

The name corundum was originally applied to the ruby and sapphire of India, the word being derived from the Sanskrit (kurminda) and literally means ruby. The variety of corundum we are dealing with is called, by the older mineralogists, impure corundum and comprises the numerous varieties that are not transparent or perfect enough to be used as gem stones.

Of the origin of corundum but little is known for a certainty, as a study of his mineral calls for exhaustive research work on the part of the geologist, who is sometimes reticent when it conies to establishing hard and fast rules. Prof. J. H. Pratt, who has made a deep study of the occurrence of corundum, in this country and elsewhere, considers that
the corundum of North Carolina segregated from a molten magma, the separation taking place at an early period of consolidation.  

Canadian corundum has of recent years been regarded as an essential rock constituent. Regarding the corundum of Ontario, H. E. T. Haultain states that the corundum-bearing rocks are not dykes, that they are not eruptive, and that there is no sign of separation from magma.

Corundum possesses no true cleavage, but parting planes are usually present along which the crystal fractures. How- ever, if these planes are so numerous as to be present in the small grains of abrasive material that constitute a grinding wheel, a low abrasive efficiency will be the result because the grains will readily fracture, thus breaking away before becoming dull and useless.

An ideal corundum for an abrasive wheel is one wherein the grains are free from parting lines, thus they will, on becoming dull, break with the irregular to conchoidal fracture, which is a characteristic of corundum. As a matter of fact, all varieties of corundum have comparatively the same degree of hardness, that is, from 8.8 to 9., but some varieties are much higher in abrasive efficiency than others. This is due to the fact that the parting planes are sometimes too numerous as previously stated. This accounts for the fact that some makes of corundum wheels are superior to others. The only practical test for the abrasive efficiency of a doubtful corundum is to make some sample wheels of it and have them tested on actual work under every-day working conditions.


DIAMOND

Diamonds are divided into three groups, the transparent and practically flawless varieties used as gem stones and imperfect stones called bort diamonds. There is also a black diamond often called carbonado. The diamond has a specific gravity of 3.50 and is the hardest substance known, being placed at 10 on the mineralogist's scale. The diamond crystallizes in the cubic system, generally taking the form of an octahedron.

The fracture of the diamond is conchoidal and the crystals invariably cleave along planes parallel to the octahedral faces. Diamond cutters avail themselves of this characteristic when reducing the stone to the best shape for cutting. Of late years, however, a sawing process has been developed which is said to be superior to the older method of cleaving by means of a sharp blow.

The diamond is found in India, South America, South Africa, New South Wales, Borneo, and British Guiana. At the present time, most of the diamond industry centers in South Africa, the mines in this locality having been worked since 1870. Previous to this time, diamonds were found in alluvial deposits and in conglomerates, but in the South African mines, the most famous of which are the Kimberley and the De Beers mines, the diamonds are found imbedded in a kind of blue clay in what are termed "pipes." These are supposed to be filled-up craters of long-extinct volcanoes.

Of the origin of the diamond but little is known, although many eminent geologists have advanced well-grounded theories concerning its formation, but, as the original condition of the carbon, of which the diamond is composed, remains a question, the genesis of the diamond is still unsolved.

The De Beers Company mine many hundred thousand dollars' worth of diamonds weekly and it is needless to state that operations are conducted on a large scale, under the supervision of the most able mining engineers available. To get at the diamond-bearing blue clay, a shaft is sunk several hundred feet into the earth just outside the pipe, tunnels from this shaft running into the diamond-bearing deposits. This material is hoisted to the ground above, where it is spread out in large fields to allow the sun and rain to crumble it to the extent of being easily washed. This weathering process is materially aided by going over the deposits occasionally with steam-plows.

The disintegrated soil is next washed in shallow cylindrical troughs wherein the diamonds are swept to the rim by means of revolving toothed arms, the lighter material escaping at the center. The findings are now concentrated to separate the diamonds from hard foreign substances and then a further separation is effected by passing the concentrates over a greased surface. For some unaccountable reason, the greased surface holds the diamonds while the other worthless materials escape.

It takes, on an average, four tons of blue ground to yield one carat weight of diamond, and as the De Beers mines often yield from three to four pounds of diamonds a day, it is seen that an immense amount of blue ground has to be worked.

The next step, and a very interesting one, is to sort out the diamonds both for color and purity. The color runs from clear white to black, a pale yellow being the most common color. The only difference between a gem stone and a bort diamond is that the former is practically flaw- less and of good color, while the latter contains black specks and other flaws, has no brilliancy and possesses an irregular fracture.

It is needless to state that the diamond sorters are expert at their work and they never let a gem stone pass for a bort. As a matter of fact, dealers in bort stones look in vain for a gem stone that might have missed the eye of the inspector, but there is no record of their efforts being rewarded.

Aside from truing grinding wheels, bort diamonds are used for many other purposes. In powdered form, they are used for diamond cutting, this process being introduced by L. von Berquen in the year 1476, for cutting and drilling very hard substances, for certain kinds of delicate lapping and grinding in watch factories and occasionally for very minute turning operations in the watch or jeweler's lathe.


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