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rundum, rouge, crocus, tripoli, putty powder, crushed steel, diamond dust or bort, oilstone. The great variations in the hardness and degree of finish necessary in work are sufficient to account for so many different kinds of substances being in use. Although several sorts of abrasives may be employed indifferently for certain work, there are instances where but one composition is adapted to a particular operation. Varying grades of the same material are also essential for different purposes, or for roughing and finishing processes, this being a question of coarseness or fineness of the abrading agent. The great demand for abrasives has resulted in increased supplies, and in the development and production of new materials, as well as in improvements in the applications of the existing varieties. Machinery for their employment has also been immensely developed, and the forms in which abrasives are shaped specially for use in these machines are innumerable.

Abrasives take either what we may term natural forms, i.e., powders, or blocks, or they are made up into artificial shapes, to suit the purpose of their application to the work. In the former case, of which sand and natural grindstones are typical examples, the powder is used in either its loose dry state, or in a paste with liquids, and the solid stone is used in the form of blocks, or it is shaped into discs and rotated on an axle. These two instances were the first ones of the application of abrasives, but they have since been considerably affected, and in some cases ousted by other materials formed into special shapes for particular work, of which emery is the principal example.

The primitive method of abrading-using a loose powder-is practised to a large extent, and all abrasives are brought into use by this mode of working. The substances depend for their successful action upon the constant changing of position which they undergo, being rolled or rubbed about incessantly, so that fresh abrading points are exposed. There is a limit to the abrading capacity, however, of the materials, so that after a certain amount of work has been done, they begin to lose their efficiency and get dull. This is noticed in such operations as glass grinding, where sand is employed between the glass and a rubber or

disc of iron; the sand has to be constantly replenished, as the used grains begin to dull and cease effective action. In this particular respect abrasives vary widely, since some kinds will do much more work than others; emery, corundum, and carborundum, for instance, are superior to sand in their endurance.

The grading of the powders for this form of abrasion produces a wide range of coarse and fine qualities suited to all classes of work. The varying grades of emery are an example of this, ranging from "corn" to "flour" sizes. For those finishes which the finest flour emery is not delicate enough to produce, other materials are employed-rouge, crocus, &c. ; though these are not of so much value in engineers' work as in that of gold and silver-smiths, and various fine metal trades. The choice of an abrasive material cannot always depend upon its rapid action, since a very free and fast working material may produce too much scratching to be desirable, and therefore a more slowly operating substance must be chosen, even at the expense of time. Here, however, the value of grading comes in, since a surface can be roughed down with a rapidly working abrasive, and then finished with a smoother quality of the same or of another material. The hardness or softness also of an abrasive have much to do with the surface produced, apart from the fineness of grain. A very hard material may produce scratches, though it is finely divided, while a coarser grade of another substance may work satisfactorily.

The advent of the practice of forming abrasives into solid shapes or attaching them to the surfaces of moving bodies has resulted in a considerable extension of the use of abrasive materials. In the form of wheels, of which emery discs, and cylinders are typical, they perform all kinds of intricate operations, and other plainer ones with rapidity and accuracy. This is due directly to the device of artificially shaping the wheels into forms suitable for the application of the abrasive to the work, frequently to enable processes to be performed which could not be done at all with loose powders or pastes. The production of an efficient bond or cementing material has enabled manufacturers to mould emery and other substances into in

numerable shapes, some of them of delicate outlines, in which a natural grindstone would be too fragile. It is also possible to make wheels in numerous grades suited to the particular requirements of various metals and finishes necessary. See Grinding Wheels.

The action of an artificial wheel while at work bears similarities to that of a powder in motion between surfaces-the particles of emery abrading the work being loosened and detached, so exposing fresh grains to operate. In the case of a revolving wheel these particles become scattered, instead of remaining between the wheel and work, so that a really more efficient and cleaner cut is attained. The loss of grains must not, however, be rapid, or the wheel will become worn away too quickly. On the other hand, retention of the particles for too long a period results in non-efficiency, because the front surface becoming dulled, ceases to operate, and the fresh grains below not being exposed, are able to do nothing. The happy mean in abrasion must therefore be struck to get the best results out of wheels, and special grades used for special work, and for different metals, hard, and soft. "Glazing," which occurs when a wheel is run too slowly, means the filling up of the pores of the wheel with the metal being ground, choking the action and stopping further abrasion from taking place.

Many of the finer grades of abrasives, chiefly for polishing, are employed when spread over revolving bobs or wheels of wood, covered with leather, &c., and on travelling belts, by which awkward shapes are tackled. All these various aspects of abrasive materials will be fully discussed under the respective headings of the substances; and their composition, manufacture, and specific uses will be described together with the machines in which they are employed. Reference may also be made to the general remarks on Abrasive Processes.

Abrasive Processes.-The property which certain substances possess of abrading or rubbing away softer materials is turned to valuable account in engineering, and other trades. Both the very roughest and the very finest classes of mechanical operations are alike effected by abrasion, from the fettling of castings to the finishing of high-class gauges, and lens-grinding,

and polishing, work done to extremely minute fractions of the inch. The process of abrasion has peculiarities which are not possessed by true cutting tools. The extreme hardness of the abrading agent is the principal factor which enables many abrading operations to be performed, work which could not be done at all in some instances with steel cutting tools. This applies to all hardened steel pieces, and to tempered tools which can only be reduced or sharpened by abrasion. Their degree of hardness is of little account, since there is no hardened steel which cannot be easily attacked by the various abrading materials in use.

The sharpening of cutting tools was obviously one of the first human applications of abrasion, and it still constitutes the most important operation. But another function which has immensely grown during recent years is the practice of finishing pieces of work either roughly or to the most precise dimensions at present attainable. The development of this class of abrasion or grinding has opened up a new field in mechanical operations, especially in connection with hardened pieces. The advantages of hardening wearing parts have always been apparent, but the distortion which follows the hardening process imposes limits to the extent of the operation, unless some means of rectifying the effects of such distortion is avail able. With the advent of emery and other wheels, the truing up of hardened pieces has become a comparatively simple job, and the effect upon engineers' and other work has been far reaching.

But the most remarkable feature of the process of abrasion lies in the minute amounts of material which can be removed at a time. The case differs essentially from that of cutting tools. These must penetrate a surface to an appreciable extent before they will cut, so that very fine amounts of material cannot be taken off (except by a scraping action, which is another thing). The very finest results in finishing are therefore found to be effected by abrasion. The faintest contact, with friction of the abrading agent and the work, suffices to produce some amount of reduction, however small. What this means is that any number of separate rubbings or grindings can be made

in succession until the work has been brought to its desired state. The degree of pressure exercised during contact has a considerable effect upon the quantity of material removed, and this is a matter of sensitiveness that depends on the skill of the operator in the higher classes of abrasion. In some of this fine reduction and polishing, no machinery has yet taken the place of the human hand as a controlling agent, lensgrinding for instance, although the comparatively rough preparatory processes may be done by machine.

The highly finished surfaces

necessary in certain classes of work are only possible by the use of an abrasive which combines innumerable grains constantly on the move, so that no one particle can make any decided effect, or in other words, produce scratching.

Although the sharpening of tools, as previously remarked, has long been performed by abrasion, the introduction of emery and other wheels has induced a great change in the methods of dealing with certain classes of cutting tools. Those of circular shape, as reamers, boring tools, and milling cutters, were formerly softened each time it was necessary to sharpen them, and the teeth then filed, after which re-hardening and tempering was done. The many disadvantages and the waste of time involved in this method are obviated now that emery wheels can be applied to the grinding of the cutting teeth, using a special form of machine for controlling the operation. Great accuracy can be attained thus, with consequently increased economy of production. Flat-edged tools are also ground truly and economically by machine.

Although abrasion by the help of machinery has taken such an important place in workshop practice, there is nevertheless a considerable amount of work which is done by hand methods, quite apart from the precision finishing just now mentioned. The most elementary form of abrasion, that of rubbing two faces together with an abrading material between them, in the form of a powder or paste, is followed extensively. This method is employed largely for grinding in valves of various kinds, which must make a steam, or air, or gas-tight fit. A fine quality of material, emery or other kind, must

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be used in order to get smooth surfaces. valves are rubbed or rotated, applying the abrading agent at intervals until perfect contact is obtained, usually over narrow surfaces. This mode of procedure, though seemingly rather crude, is the best method of giving the final fit to certain kinds of valves, especially those for gas and petrol engines, and as they wear, or become pitted during service, truing up is done on them in the same manner.

Other kinds of work are also fitted mutually by abrasion, but the method is limited to small areas, because of the inaccuracies which are likely to result if large surfaces are worked in this way. An objection to the process is that the particles of abrasive material tend to bury themselves in the work, and in soft metals that does occur. It is therefore not desirable to grind revolving shafts into soft metal bearings, because the latter retain some of the emery, &c., and continue to abrade while in service. But hard steel spindles can be ground into hard steel bearings or bushes (as in the case of lathe mandrels), and satisfactory results produced. The same applies to pins and pivots in joints of all kinds, notably those of link motions and other engine parts. Here, however, abrasion in another form is rendering mutual grinding together of less use than formerly. The circular spindles are ground instead, separately on special machines with wheels, and the holes are lapped or ground out independently to suit. An equally good fit is obtained, with more economy of time, and less trouble than by mutual abrasion with a loosely applied material.

This aspect of abrasion is one of the most striking features of the newer practice as contrasted with the older. Numerous types of machines have been, and are constantly being devised to employ abrading agents for various services, and while many machines are only for rough work, there are some which perform very precise operations. Grinding within part of an inch can be done on the best machines, employing the means of setting provided, and finer limits can be worked to as a matter of skill on the workman's part. An interesting point about this is that the sparks arising from the contact of wheel and work while revolving are a surer guide to the fact that abrasion is

occurring than measurement is, since a faint contact and consequently abrasion may occur, the result of which cannot be measured sensibly, although the workman may be sure that reduction, however minute, is going on.

This process, therefore, once spoken of with some little contempt, as being a rather clumsy and rough operation, now takes front rank in machine shop work from the point of view of accuracy, and the precision machines which it has developed excel in fine construction those used for other work, as turning, planing, &c. Further reference will be found to these machines under their headings, in connection with the various kinds of grinding.

Previous to the introduction of such precision machines, circular work was done by the use of laps of lead, charged with abrading material, as an alternative to mutual grinding together. Accurate guidance of these, in the sense of machine work, is not possible, but good results can nevertheless be secured, and the method is employed considerably. But, like the mutual grinding, it has been greatly affected by the more economical and certain methods of the precision machines, using wheels.

Leaving this aspect of the question, we may note the extensive use of abrasives for the rougher classes of operations. There are many kinds of jobs for which cutting tools cannot be successfully employed in the reduction of the surfaces. Chilled rolls, chilled wheels, and other pieces are cases in point, but more especially we may mention that large variety of work on which only a small amount of material has to be removed, often chiefly for good appearance. steel tool is not well adapted to taking off a thin cut from the skin of a casting or forging, the hard scale of which quickly ruins the cutting edge.

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Here the abrasive wheel comes in, for it is admirably suited for the dressing-off of scale, mixed with sand, and is little affected by either. For trimming and polishing all kinds of pieces, therefore, the process of abrasion is followed extensively. Stove parts are typical examples of the use of grinding wheels, sometimes no cutting tools whatever being used on them, drills excepted. The cast iron used in stovefounding is frequently of a hard nature, which cannot be successfully and rapidly attacked by

steel tools, especially on shallow surface cuts, but abrasion solves the difficulty, and either plane joints may be trued up, or edges and faces trimmed and polished for good appearance. The edges of armour plates are corrected by grinding, after hardening of the faces has been done. These remarks apply also to large quantities of other metal work, where either hardness interferes with tooling, or cheapness and rapidity of trimming is essential. The rapidity of action of a grinding wheel suitable for the work is very great, quicker in many instances than a steel tool could cut off shavings. Much of the work that was once left black is now finished neatly and cheaply by grinding, and of course new classes of machinery are always being introduced, giving impetus to the practice of abrasion. A considerable, amount of hand filing has been dispensed with, in favour of the more rapid grinding, especially in the rougher classes of work, from the fettling of castings to the jointing of edges and parts for agricultural machinery, steel work, wagon work, hardware, &c.

Another immense field which abrasives have all to themselves is that of polishing. The finer grades of emery, corundum, and the crocus, and allied substances, are used on buffs, mops, and on belts, and all kinds of articles are finished rapidly, with any degree of polish desired. Much of this work has been done in the past more laboriously with emery cloth manipulated by the hands. Practically any work which can be performed by hand is also capable of execution by machine. For the more awkward jobs, the travelling belts, charged with abrasive material, allow of the most awkward outlines and corners being got at.

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Abrasive agents are formed into many special shapes, besides wheels, for the purpose of application, from the common emery cloth to the emery files and rubs, used in the hand. only is metal work abraded, but wood, granite, marble, glass, pottery, pearl, horn, leather, indiarubber, &c., are operated on by suitable materials.

The operation of abrasion involves the use of liquids in certain cases. Two purposes are served, either that of cooling, or of enabling the materials to work smoothly. Abrasion produces heat much more quickly than pure cutting

with steel tools, and the temperature of the abrasive and work rises rapidly. This is objectionable for two reasons. One is that excessive heating of the pieces will result in distortion, a thing that would ruin the majority of machine pieces; the other is that tempered tools would be softened by the heating due to the friction of abrasion. Cold water is therefore flooded over work, and the abrasive agent in cases where it is necessary, and the heat is thus kept down. Moreover, where a fine finish is required, dry abrasion is often unsuitable, so a paste is prepared, using water or oil, the latter especially, with the result that the action of the abrading material is facilitated, and yet softened and toned down, so that a fine smooth finish is imparted, free from the scratches which would be produced by dry particles rubbing on the work. Oilstones are perhaps the most familiar examples of the employment of oil. These cannot be worked dry, or abrasion will not take place to any extent, neither will the surface produced be good. But directly oil is applied, the tool being treated commences to cling" to the stone, and a sensible amount of steel is rubbed off, constituting the sharpening.

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A peculiar and modern mode of application of abrasives is that of Sand-blasting. Here the material is blown in a spray very forcibly on to the work, every portion being exposed to the action of the blast, which is naturally of a penetrating character. Many operations are now done in this manner which were once impossible, such as file sharpening, and other processes are done with greater facility, cleaning of metal work being one of the principal.

Detailed descriptions of the various abrasive processes will be found under their proper headings in this work. A few general remarks on the materials used are placed under Abrasive Materials.

Absolute Accuracy. This term, and its equivalent, "perfect accuracy," has no meaning in workshop practice, notwithstanding that it is employed loosely. No competent mechanician would guarantee the absolute accuracy of any piece of work, neither is there any instrument in existence which would be capable of checking or measuring without error. Instead, therefore, of using the old phrase, “perfectly

true," and its equivalent, error is admitted, and its amount stated. This is really the basis of the numerous limit gauges that are so prominent a feature of present-day manufacture, and which are of recent growth. If, for in

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stance, an error of 1 of an inch is considered permissible in a given class of product, as not being detrimental to its proper fitting, or to the operation of the mechanism of which it forms a part, then a limit gauge is made to embody that amount of error, and if the piece of work passes that gauge it is accurate enough for its purpose.

But though admitting the impossibility of securing absolute accuracy in mechanical production, nearer approximations to that ideal are being constantly made. This is only practicable by the gradual elimination of hand work, and the substitution of the work of some special machines, or machine adjuncts for it, the latter in the form of jigs.

The automatic screw machines have been very influential in the production of approximately accurate and uniform results. This is not so much due to these machines themselves as to the adjuncts which are fitted to them in the form of box tools. The latter in many cases fulfil the function of gauges as well as cutters, either by their hollow form in some cases, or by the insertion of steady pieces in opposition to the cutters. But the grinding machine is fast becoming the principal instrument by means of which these results are achieved, since it is possible to grind not only with extreme pre-cision, but also to predetermined dimensions within highly refined micrometric limits, a triumph which cannot (some few special cases excepted, as scraping) be achieved by hand work, and then only at a greatly enhanced cost. If a gauge has to be corrected finely it is ground, and if a screw requires correction it must be ground; so must spindles, bushes, cutters, and much besides. See Grinding Machines, &c.

Three things render perfect accuracy impossible of attainment, or of retention, if obtained by chance. These are changes due to temperature, the flexure of bodies, and the internal changes which take place even in the most homogeneous materials. In doing the finest work, uniform temperature in the workroom has to be maintained. Even the approach of a person

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