The object of this book has been to embody in concise form the main principles of workshop precision grinding, and in the pages following will be found the results of many years' experience in the design and construction of grinding machines and close observation of their practical utility.

The subject matter is the basis of a series of lectures on grinding machines given by the author at the Royal Technical College, Salford. The author hopes, however, that the book will prove helpful, not only to the technical student, but also to the fully trained engineer, because precision grinding plays such an important part in all engineering work that every engineer should be aware of its possibilities. It is hoped the book will not merely be perused and put aside, but that it will be studied carefully and a desire thus be stimulated for closer acquaintance with actual grinding practice.

In the space available it has not been possible to give complete details of machines, and the reader who desires fuller information on these points is referred to Grinding Machinery by J. J. Guest, and Precision Grinding Machines by the author.

The forerunner of the modern grinding machine was the grindstone, which has been in use from time immemorial and still finds useful applications. Between the modern grinding machine and the grindstone there is, however, nothing in common. The art of "precision" grinding has made great advances since the beginning of the twentieth century, largely owing to the demands of the automobile manufacturer, and precision grinding machines are capable of dealing with materials and obtaining a degree of accuracy quite beyond the powers of a grindstone. Mechanically, the grinding machine is a machine tool of the highest quality and there is an enormous difference between the grindstone a solid block of soft natural stone and the modern abrasive wheel which is made artificially from particles of extremely hard material. When the wheel is properly chosen and used these particles actually cut the work and do not "grind" or "abrade" it in the ordinary sense of these words.

Grinding is a Cutting Process. The operation of grinding, in which metal or other substance is removed by contact with a rapidly revolving grinding wheel, is an actual cutting process. The cutting tools are hard, sharp particles of abrasive extending from the working face of the wheels. When these small, sharp tools harder than any substance they are called upon to cut are moved at high speed into contact with the material to be ground, each particle cuts its own minute chip from the work. The modern grinding wheel, properly selected and used in the modern grinding machine, is just as surely a milling cutter as if it were made of steel. Under the microscope the material removed is seen to resemble the chips from other machine tools, being, for instance, very similar to those produced by a milling cutter or lathe tool.

Abrasives - These are of two kinds, natural and artificial. The natural abrasives emery and corundum are both mineral substances, similar in composition, except that emery is not as pure as corundum but contains a large percentage of iron, which is undesirable in a grinding wheel as it has no abrasive qualities.

Initially the principal abrasive available was emery, that most in repute, as being the purest, coming from Naxos. Deposits of nearly pure corundum have since been found, and this is the natural material now most in use.

The best grade of corundum is found in Canada, and has a higher percentage of aluminum oxide than has emery. Under the pressure of grinding the grains of corundum fracture, thus presenting new cutting edges or points to the work. Since both emery and corundum are natural products, they cannot be obtained free from all impurities. Artificial abrasives have, to a very large extent, superseded natural abrasives in the manufacture of grinding wheels, because improvements in the design and construction of grinding machines created the demand for better and more reliable grinding wheels which could be used on them. The excellence of the grinding wheel as obtainable today is undoubtedly due to the requirements of the grinding machine.

The artificial abrasives used are aluminous abrasives and carbide of silicon. Both of these are products of intense heat in an electric furnace. Aluminous abrasives are made from bauxite, a hydrate of alumina, which is found at Baux, France, and in several parts of the southern United States; and silicon carbide is made from a mixture of coke, sand, salt and sawdust.

Abrasives Used for Various Materials. The extreme hardness and brittleness of carbide of silicon is an essential element in the successful grinding of metals of low tensile strength, such as cast iron, brass, bronze, and copper. It is also used for grinding granite, pearl, earthenware, firebrick, glazed sanitary ware, wood, cork, leather, and a variety of other articles. Wheels made from this abrasive by different makers bear various names including Crystolon, Carborundum, Carbolite, etc.

An abrasive material perfectly adapted to the grinding of materials of high tensile strength, such as steel and malleable iron, differs in essential characteristics from one suitable for the grinding of cast iron and brass. For materials of high tensile strength an abrasive must be hard and sharp and possess greater toughness than is required for the successful grinding of weaker materials.

Such extreme hardness and sharpness as is found in carbide of silicon is not essential or even desirable in an abrasive for the grinding of steel and malleable iron. The aluminous abrasives are therefore used for these metals, under trade names of Alundum, Aloxite, etc.

For the grinding of steel, then, an abrasive must possess the following properties: (1) A degree of hardness which will permit it to penetrate easily into the material to be ground. (2) An irregular crystallization producing the property of sharpness. (3) A degree of toughness which will permit the crystals of the grain to stand up and not break down or fracture too rapidly under the strain placed upon them by the high resistance of a tough material during a grinding operation.

Such wheels cut rapidly and freely, but at such a rate and in such a way that excessive heat is not generated. They are used for all classes of steel grinding, varying from fine precision and tool-room work to snagging of heavy castings.


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