The subject dealt with here is that of grinding as employed in engineering machine shops, and is one which is, for several reasons, of continually increasing importance to manufacturers.

The book has been written in response to the frequently expressed wishes of engineers, works managers, and machine operators, that I should give them detailed information, often of a character beyond that which could easily be dealt with in conversation or by letter. As such readers are familiar with ordinary workshop practice and tool details, these matters are as a rule only referred to briefly; but the nature of many of the inquiries addressed to me for advice - not on grinding only, but on many other questions as to plant and methods of production - has led me to the conclusion that the subject of grinding could not be adequately presented without some brief treatment of various topics connected with it.

The book has been planned so that the whole subject is presented as systematically as possible, and so as to lay bares the reasons underlying the various matters. Upon knowledge of these depends a sound judgment as to what is suitable plant, of the possibilities of the process, and concerning the best mode of using the machinery? The Table of Contents fully indicates the arrangement of the book.

I have treated the vexed subject of work speeds first from the point of view of the best modem practice, and then introduced my theory of grinding. This was first published in a paper before the British Association for the Advancement of Science in September 1914. It dispels the current belief in a standard work speed, but offers in return an explanation of the phenomena encountered, and supplies the best methods of meeting the various difficulties.

The machines illustrated have been carefully chosen with regard to the ends in view, and in the selection a preference has always been given to those of the pioneer firms; machines of my own design have, however, only been introduced as illustrating special points, e.g. the automatic steady, which could not otherwise be shown. Machines have throughout been treated from the. Point of view of the grinding process, and not as interesting examples of mechanism, so that unless the detail is directly connected with grinding the description is as brief as is compatible with lucidity.

The various problems presented by different classes of work have been treated in the same manner. The reader will detect these various phenomena under other guises and, understanding the nature of the case, will treat it appropriately.

The feeds used and the errors involved in grinding are so small that in a number of the illustrations some of the dimensions are very much exaggerated for the sake of clearness; in all cases, however, the fact is quite evident, and can lead to no misapprehension.

Where the best method of presenting any matter has involved the use of equations, the results have been also given in ordinary language so as to be available to any reader.

Where my opinion is expressed, particularly if opposed to current belief, I have written in the first person.

Grinding - In modem machine-shop practice the term grinding has now acquired a definite meaning, and is confined to the shaping of material by means of rotating abrasive wheels of practically rigid substance. The shaping may be done by hand, as in sharpening a lathe tool on a grindstone, or may be a mechanically guided operation, as in the truing-up of a hardened steel mandrill in a grinding machine, but owing to the importance of the latter work the term grinding or more definitely precision grinding as an operation, is practically confined to it. As opposed to turned or milled work the quality of ground work which first makes itself appreciated is fineness of surface; this, however, is surpassed by that of polished work, which does not possess the first characteristic of ground work - namely an accuracy considerably surpassing that of work produced by ordinary cutting tools.

Polishing and Lapping. - Polishing consists in removing the small inequalities of surface by rubbing the work with soft material charged with abrasive powder. By using successively finer powders the material is removed by smaller and smaller amounts with a corresponding improvement in the quality of the surface. To do this work rapidly the soft material is made into bobs or belts and run at a very high rate of speed, but as this soft material follows the larger irregularities of the surface of the work, the result is that accuracy is not a feature of the process.

The accuracy given to certain work by machine grinding can be improved by lapping (Chapter XII), which consists in making a lap (or piece of metal, or other material softer than the work) to envelop the work, charging it with abrasive, and working the two together until a better fit is obtained. This is a slow process, and demands much care. It consequently is only used in those operations for which the accuracy of form or the quality of surface given by grinding is insufficient.

Both grinding and lapping are really cutting processes when closely looked into, and in the heavier kinds of grinding chips which can be handled are produced.

Although the use of grinding or abrasive processes is of primeval antiquity, and grinding machines have long been in use, it is only of recent years that machine grinding has become one of the recognized shop operations. At first applied to the manufacture of gauges, hardened steel spindles, and to the cutters and mandrills of the shop, now all the more accurate parts of engines, motor cars, machine tools, sewing machines, and machinery in general are ground, and the use of the process is extending to pieces in which the precision is not of such importance.

A number of causes have combined to effect this rapidly, and a review of these will assist in the formation of a broad judgment of the possibilities of the process, of its nature, and of its limitations.

Mechanically guided Grinding - It is not so long ago that turning was a mechanically guided operation only in so far that the work was carried between centres, as the slide rest dates back only to Maudslay; yet now the art of turning metal by hand exists as a commercial process in very few trades – such as axle box making - and almost all manufacturing machines using steel tools have them mechanically guided.

That this substitution of mechanically guided for hand guided tools has taken place is, by the nature of commercial progress, due to the fact that it results in a cheaper product. And this economy is due to the comparatively unskilled labour which can be employed, and to the opportunity it offers of speeding-up the process. After mechanically guided tools became usual, the appreciation of accuracy became more possible, and so a way was opened for the employment of grinding as a productive method. The replacement of a steel tool by a grinding wheel was first adopted to deal with the problem of hardened work - the correction of distortion due to the process of hardening; in France grinding machines are still termed Machines Rectifier. In its early days the process, although it gave more accurate results than turning, and produced a superior surface, was so tedious that it was con- fined to those cases where the requirements warranted the expense, e.g. the spindles of machine tools. Single point diamond tools were sometimes used on very small hardened work, but the expense and difficulties encountered were great.

Modern Manufacturing - Following the development of the steam engine and machine tools, with the resulting spread of the facilities for making machine parts, came the development of modem mass production, involving the use of special small tools of advanced accuracy. This called again on the use of mechanically controlled grinding for the purpose of finishing and sharpening these tools, And for the production of the gauges simultaneously required. The old process of making a reamer, for example, necessitated several careful annealing and operations removing little metal, and a then- satisfactory reamer was an inferior and expensive tool compared with those finished by the grinding operations of to-day.

These demands resulted in the production of machines - the universal grinding machine and the cutter grinder - for the manufacture and sharpening of the special tools, and the production of gauges and other hardened parts. The capability of dealing with hard steel (due to the very much harder nature of the abrasive particles of the wheel and their freedom from heat effects), and the adaptability to precision work (due to the sharp edges of the particles taking a very fine chip with little normal force), are the properties which render the grinding process especially suitable to the requirements of tool manufacture. From this footing in the tool-room, the process of grinding has extended, aided by improvements in both wheels and machines, until today mechanically guided grinding machines have a place in all manufacturing shops where accurate work is required, and not on hard steel parts alone, but on many classes of material. This could not be so unless the grinding machine produced work of certain required accuracy or of other desired qualities, at a cost unmistakably less than it can be produced at by other processes.

It would be premature to discuss here the question of the advisability of adopting the process and installing the machinery in any special case; knowledge to this end is to be gathered throughout the book, and the matter is again referred to in the conclusion, after the nature of the process, the trend of modem development, and the reaction of this art upon other manufacturing methods have been considered.

For reasons which can be easily understood, the process of grinding is more accurate than that of turning, and less accurate than that of lapping under proper conditions, and the surface produced corresponds fairly with the accuracy. When using these three processes within limits of accuracy easily attained by them, the cost is generally least in the roughest process - that is, with the single point cutter - and is greatest with lapping; but as any particular limit of accuracy is made finer the cost of finishing by the rougher processes increases very rapidly. Hence if we are fixed to certain limits of accuracy it will prove to be cheaper to finish by grinding than by turning, or by lapping than by grinding, according partly to what these limits are, and partly to the character and condition of the machines and appliances available. Speaking generally, therefore, work necessarily of a very high degree of accuracy should be first turned, then ground, and finally lapped.

Accuracy is compulsory. - The limits of accuracy required are therefore of primary importance in determining what processes should be used in the production of a part of a machine, and whether grinding is desirable or necessary.

In order to be satisfactory, to run and wear well, machinery demands in its construction certain accuracies, due to properties inherent in the nature of the materials employed, the use to which they are put, the oil to be used in the bearings, &c. These are the primary factors which enforce limits upon the dimensions of machine parts. These limits may be very liberal, and attainable by mere careful casting or forging, or they may be very narrow, and require very accurate workmanship to meet them. Of the former, many loom and agricultural machine parts are illustrative: and as an example of the latter we may take the case of a forced fit, where a cylindrical piece is forced by a press into a hole slightly smaller in diameter than itself - say a wheel and axle which rotate together. Consider this case more closely.

Supposing that the female part or hole is made first, it is necessary that the plug should be made a certain amount larger in diameter than the hole, else it will be loose, or at any rate insufficiently tight when forced in: on the other hand, it must not exceed it by a certain (other) amount (dependent on the external and internal diameters of the female piece and the material of the parts), else it will be impossible to force the plug into the hole, or damage will result in doing so. These considerations determine certain dimensions between which the diameter of the plug must lie; the stresses in the parts of the forced fit, and the force necessary to press the parts together, or to turn the plug inside the hole, will depend upon the particular diameters to which the parts are formed, but their amounts must be within a satisfactory range. The margin of diameter or the limits are therefore determined for any particular case by the elastic properties of the material used.

Beyond this, the quality of the surfaces, and the nearness of the material surface to the ideal geometrical surface (if one may express it so), affect the problem; more so, however, in cases of running fits where there is wear than in the forced fit example which we have taken. In all cases, however, a certain maximum and minimum difference of size is entailed by the physical properties of the materials.

Limits, Tolerances, and Allowances - When the problem is extended from the production of a single shaft for one particular hole to the case of manufacturing these parts in quantity, the matter becomes a little more complicated, as the holes will vary in diameter amongst themselves in whatever way they are produced, and it is very desirable that manufactured parts should be interchangeable.

Our physically controlled limit of difference of fit has then to be divided into two parts applicable to the two parts of the fit. Prom the point of view of manufacturing one particular fit, the division of the allowable margin (between the male and female parts of a fit) should be such as to make the cost of manufacturing the particular parts of that fit in quantity a minimum; but for manufacturing reasons the margin is divided according to broader considerations, with the result that it has become possible to make limit gauges for the different classes of fit commercial articles.

It should be observed that owing to the difficulty of working close to the actual size of a gauge, parts - whether male or female - made to limit gauges come regularly well within the limits, so that when a physically determined margin is divided between two sets of limits, a little may safely be added to its amount.

We have considered the hole to have been produced first; if a shaft had been required to be a running fit in the hole it would have been made to a somewhat smaller diameter than a shaft to be a press fit, so that it would rotate easily and provide room for oil. In quantity manufacturing the holes would all be made the same size within the small limit allowed, but the shafts would have different diameters (each with its limits) according as they were to be press or running fits. It would, however, be a matter of indifference whether the holes or shafts were actually made first.

The sizes in this case are said to be on the hole as a basis, and there is one set of limit gauges for the holes for whatever purpose they are intended; the variations for the various types of fit being made on the shaft, there being different sets of limit gauges for the shaft according to the purpose of the fit.


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