The manufacture and uses of abrasive materials

The industrial importance of abrasives is seldom realized, even by those who are concerned in their production. Yet there is scarcely any manufactured article of commerce which does not come in contact with some form of abrasive during one or more stages of manufacture. Such staple articles of food as wheat and oats are ground to flour or meal by means of abrasive stones or crushing rolls, many metallic articles are now "ground to size" instead of being cut or turned as was formerly the case, their rough portions are made smooth or a cutting edge is produced on them by the application of a suitable abrasive "stone," and a fine polish may be imparted to glass, metal, wood, leather, ivory and numerous other materials by means of abrasive powders.

In the engineering industries in particular, recent years have seen the development of much that is entirely new and have also witnessed great extensions in the previous practice of grinding. Much of this development is a direct result of the vast growth of motor work, which offers for solution many problems peculiarly its own, but a large proportion has arisen in consequence of new views relating to the economies of grinding in the wider field of general machine shop practice.

It is hoped that the present volume will prove of value to many who are called upon to deal with grinding problems without any very extended or specialized experience in the art. No one can become an expert grinder by reading a book or articles in the technical press, but if the underlying principles of grinding are carefully considered they will frequently assist in solving many of the problems associated with abrasives and with grinding. In solving the more difficult problems, the assistance of a grinding specialist or other expert should be sought and welcomed, as it is easy to incur serious losses through "experimenting" without sufficient knowledge.

An abrasive may be defined as a material or article employed for removing undesirable projections from other articles and, eventually, for imparting a smooth or even a polished surface to the latter. The action is of an erosive nature, the abrasive material gradually "wearing away" or "grinding off" the material to which it is applied. When the abrasive is in the form of relatively large, sharp pieces the material is removed rapidly by a cutting action, but for polishing when much smaller particles are used the object is not to remove the material rapidly but to take such extremely shallow cuts that the material shows no signs of scratches. When abrasives are used in the form of hones, oilstones and whetstones, consisting of very small particles, their action is necessarily slow. Coarser particles would remove too much metal at a time and would prevent a sufficiently sharp cutting edge being produced.


Abrasive wheels are also employed as cutting agents when their employment is cheaper or more

Nature of Abrasive Action - The extent to which one material will wear away or cut away another depends on the hardness of the abrasive, the sharpness of its cutting edges, the pressure at which it is applied, the resistance of the material to be removed and the freedom with which the abraded particles fall away after they have been separated by the action of the abrasive.

The ductile metals yield in a very peculiar manner to abrasives, the grains rolling over one another until each finds its place between some of the remainder. Thus, the raw edges of such a metal as aluminum appear like stiff dough when the metal is prodded with a sharp needle point during microscopical inspection.

Moreover, the cohesion of the various particles of metal to one another is unequal, and during abrasion the looser ones, especially those of a crystalline character, tumble out and leave tiny cavities in their places, thus causing irregularities in their vicinity.

The metallic grains which are larger than those of the abrasive are scored and grooved by the frictional contact, cut up into smaller dimensions, and may become more closely welded together.

The fracturing of the grains of the abrasive during use is of more importance than is generally tupposed. Corundum, carborundum, and similar substances, being very hard, do not readily split; but when they do so, they form sharp cutting edges and enable the abrasion to be continued until the binding particles are reached. The bond must be of such a nature that it breaks off exposing fresh grains of abrasive. Sand (which is somewhat rounded, with a few points and edges) and glass (which is jagged) leave conchoidal surfaces when their grains break; but although they are service- able in some ways, the particles are so liable to crumble completely that they are useless for grinding metals.

When grinding hard substances, the sharp ends of the abrasive particles sometimes get worn instead of broken. In some cases this may be beneficial, but it usually entails a loss of gripping and abrasive power.

In grinding and smoothing metals, the abrasion is at first due to the cutting action of the most projecting points of the grains of the abrasive. As these are removed by the friction, a larger number of less-effective prominences are opposed to the metal. Their value is lowered, because the particles of metal which are rubbed off fall between the fixed grains, and are compacted so tightly together as to render the surface of the abrasive smooth and slippery, or " glazed," unless means are found to remove the incorporated dust.

The heat generated on account of friction is often very considerable, but as its area of distribution is circumscribed, it often passes unnoticed. Naturally, when either water or other lubricant is present it tends to prevent too rapid a rise in temperature, but lubricants may introduce undesirable chemical reactions which prevent their being used in some cases.

In dry grinding, the rise in temperature occasionally melts numerous particles of metal, which do not necessarily become removed, but are pressed more firmly into the surrounding interstices, and thereby produce a smoother surface, which is retained when the substance is thoroughly cooled again. If, however, the fusion goes beyond the useful limit the harder grains between the softer ones fall out and leave the metal jagged in parts. This is especially the case with iron and steel. Ductile metals do not disclose this defect so conspicuously as brittle ones.


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