1902 Encyclopedia > Machine Tools

Machine Tools

MACHINE TOOLS. The very small degree of anti-quity to which machine tools can lay claim appears forcibly in the sparse records of the state of the mechanical crafts a century ago. A few tools of a rude kind, such as tilt-hammers, and a few special ones which aimed at accuracy, but were of very limited application, such as " mills " for boring cannon, or " engines " for cutting the teeth of clock wheels, were almost their only representatives. Machine tools of the modern type indeed would not then have been likely to have found much favour even if they had been invented, owing to the difficulty of providing sufficient power for driving them, except in the comparatively few positions where water-power was available. The transmis-sion of power was unthought of, except for the very limited distances which were possible with the ill-fitted " gudgeons " and " lanterns and trundles " of the old mill-wrights.

The steam-engine, however, changed all this. On the one hand the hitherto unheard-of accuracy of fit required by its working parts created a demand for tools of increased power and precision, and on the other it rendered the use of such tools possible in almost any situation. Thus, acting and reacting on each other, machine tools and steam-engines have grown side by side, till our workshops have become peopled with a race of giants, capable of uncomplainingly performing tasks altogether beyond the powers of the easily wearied hands which have brought them into existence. But the first steps in the process were costly and difficult to a degree which it is not now easy to realize. James Watt, for instance, in 1769, was fain to be content with a cylinder for his " fire-engine " of which, though it was but 18 inches in the bore, the diameter in one place exceeded that at another by about | ('375) of an inch; its piston was not unnaturally leaky, though he packed it with " paper, cork, putty, pasteboard, and old hat." In the bore of a cylinder of 120 inches there would not now be admitted an error of of an inch, and the leakage past the piston is practically nil. Even this must by no means be taken to represent the extreme limit attainable in respect of size and accuracy.

Machine tools present so many points of difference that no classification of them will be attempted beyond a broad division into general and special tools,—those included under the latter head being such as are intended to perform repeatedly one single operation (or one of a small number of varieties of that operation), or are mainly employed in particular processes or manufactures.

As an instance of special tools working successively in a series,-—which is a frequent arrangement with special tools,—the block machinery, for making ships' blocks at Portsmouth dockyard, may be mentioned. Erected in 1807 by Mr IT. Maudslay from designs by Mr (afterwards Sir Marc) Brunel, on the recommendation of Sir S. Bentham, it enabled ten men to do in a superior manner work which previously required one hundred and ten, and effected in the annual expenditure of the nation a saving of about £24,000. Into the particulars of the beautifully arranged sawing, mortising, shaping, and other machines by which this was accomplished we cannot enter, but they are of great interest, not only from their intrinsic merits, but also as being, if not the very first, certainly superior to any which had previously been used. Our limited space, however, will be more profitably devoted to giving a few examples of the general tools used in engineers' work-shops.

The Steam-Hammer, which in some respects may be regarded as the most important of machine tools, has already been noticed (see HAMMER, vol. xi. p. 425). Second only to
it in importance, and long anterior to it in date, stands the lathe. At what exact point of its development from the simple foot lathe it first became entitled to rank as a machine tool we will not stop to inquire, for the origin of this, as of most of the mechanical legacies which have been handed down to us by successive inventors and improvers, is involved in much obscurity. But as far as tools laying any claim to precision are concerned it appears certainly to have been the first to come into existence. On the Continent, mechanism to be used in conjunction with it for oval turning, and for producing mouldings oblique to the axis of the work, had been devised as early as 1569, in which year one Jacques Besson published drawings of two lathes so arranged. Whether much additional beauty was obtained by thus departing from the circular sections producible with the simple lathe, and converting them into distorted ones, such as that sketched in fig. 1(reduced from Besson), may perhaps be questioned, but the taste for this "swash" work, as it is called, ere long extended also to England. Moxon, the first English writer on the subject, gives a drawing of a very similar lathe, and he men-tions the name of an established London maker whose oval engines and swash engines, and all other engines, were " excellently well made," so they were apparently in some demand at the time of his writing (1680).

FlG- 1.—Swash Work,

Screw cutting in the lathe was another problem—and a more worthy one—which occupied the attention of inventors at the same early period. A curious but mechanically very imperfect arrangement for accomplish-ing it (with which, however, threads either right or left handed could be cut on tapered and oval as well as on cylindrical work) is given in another of Besson's engrav-ings. In this the tool is entirely supported and its movements are controlled by the machine instead of being held in the hand,—an arrangement of which the great advantage appears to have been but tardily appreciated, though it contains the germ of the principle which, applied first in the slide-rest of the lathe, and subsequently in machine tools of almost every type, has enabled tasks of constantly increasing severity to be successfully dealt with.

Nearly two centuries seem to have elapsed before what we now know as the slide-rest became a recognized adjunct to the turning lathe, though in the meantime arrangements had been devised for controlling the motion of the tool by attaching it to some portion of the mechanism in some special cases,—as in that of two curious lathes for turning hyperbolic, spherical, or plane mirrors for optical purposes, of which engravings were published at Rome in 1648. Its first definite appearance in print occurs in the great French Encyclopedie, published in 1772. Detail drawings of an admirable slide-rest are given in one, and evidence of its being then in regular use occurs in several of the very interesting engravings of that ponderous work, which gives so clear an insight into the methods then employed in France in the various crafts. The description, however, by no means settles the question of its origin.

It is pretty certain that the slide-rest was reinvented in England by the ingenious Henry Maudslay, when he was employed in Mr Bramah's workshop in London, where "Maudslay's go-cart" (as it was at one time derisively called) was first set to work in 1794. That he had not previously seen the drawings just mentioned cannot of course be proved, but the high price at which the Encyclopedic was published makes it very probable that no copy of it had at that time come under the notice of a hard-working English mechanic. The intrinsic differences of the two slide-rests tend towards a similar conclusion. Who-ever may have been its first inventor, the slide-rest has cer-tainly proved itself to be the most invaluable of all the additions made to the turning lathe. Its indispensability to the modern power-lathe will be readily appreciated from the following examples.

An engraving of a simple slide-rest for use with a foot lathe has already been given (see LATHE), and its effect in reducing the labour of the turner was then pointed out. The self-acting slide-rest (fig. 2) carries this reduction still farther; and, by deriving from the lathe itself the small " feed " movement ne-cessary for bringing the tool to bear on succes-sive portions of the work, it dispenses wholly with the need for physical exertion on the part of the work-man, and does not even demand his con-tinuous supervision. One result of this is that the slide lathe (for so complete is the union between the slide-rest and the lathe that they must now be re-garded as one machine) affords a complete solution of the screw-cutting problem, since, by varying the extent to which the rest traverses the lathe bed during each revolu-tion of the mandrel, a screw thread of any desired pitch can be cut with a single tool.

In fig. 3, which shows a self-acting screw-cutting lathe with double-geared headstock, of a type now well estab-lished, the arrangements for obtaining and varying this traversing motion may be observed. A steel leading screw

FIG. 3.—Self-Acting Screw-Cutting Lathe.

runs along the front of the lathe bed, and with it the slide-rest can be connected at pleasure. Two or more change wheels, properly proportioned as to the number of their teeth, connect the head of the screw with the hinder end of the mandrel.

Although a leading screw is not the only nor in all cases the best mode of rendering a lathe self-acting, ordinary screw-cutting lathes are very largely used for other pur-poses than that implied by their name. The advantage of perfect regularity in the feed is very great even for plain turning, and this can only be secured when it is inde-pendent of human vigilance. The feed in a direction transverse to the bed is also very commonly rendered automatic by means into which we cannot here enter, lathes so provided being distinguished as self-acting stir-facing lathes. In this case, however, the varying diameter of the successive cuts introduces serious objections to a uniform rate of feed. These were remedied as long ago as 1827 by that excellent mechanician Joseph Clement, —who was one of the greatest improvers of the power lathe; but his arrangement has never come into general use.

To enable a comparatively small lathe to be used for surfacing work of larger diameter than it Would naturally admit, a portion of the bed is frequently made removable so as to leave a " gap " close to the fixed headstock. An 8-inch gap lathe, for instance, such as fig. 3, can thus admit an article of 26 inches diameter instead of 16 inches only.

Break lathes, such as fig. 4, carry the same principle still farther, so that they can take in work of considerable length as well as of large diameter,—the treble-geared headstock and all other parts being in their case made of sufficient strength to bear the heavy strains which result from the increased size and weight of the work, a quality

FIG. 4.—Self-Acting Break Lathe.

hi which gap lathes are not unfrequently deficient. Lathes of this kind Were made by Mr (now Sir Joseph) Whitworth as long ago as 1840, and the type is still the accepted one for general heavy turning. The face plates on which large work is chucked in these lathes are sometimes as much as 15 feet in diameter.

Face lathes, of which the main duty is surfacing articles whereof the diameter is great but the length small, are very similar to the foregoing minus the entire right-hand portion of the bed and all that it carries. They have occasionally been made for work of Very large diameter,— such as turning the roller paths of 40 feet railway turn-tables,—though it is now found preferable to turn such things in a horizontal position, in lathes of which the mandrels are vertical.

But the point to which the growth of power-lathes has now attained will be best illustrated by the following interesting particulars of two which have been quite recently designed and made in the Boyal Gun Factories at Woolwich. Each of these can take in a piece of work having a maximum diameter of 12 feet and a total length of 36 feet,—which represents a truly appalling weight of metal to have to deal with,—their main dimensions, &c, being

Height of centre of mandrel above the bed 6 ft.
Total length of bed 60 „
Length of fixed headstock 12 ,,
Diameter of front bearing of mandrel in do. ... 18 in.
Length of do. do. .... 36,r
Length of leading screw over all 52 ft. 3 in.
Diameter of do. do 7 in.
Weight of fixed headstock, about 55 toffs
Do. movable do., about 18 ,,
Do. slide-rest and saddle, about 154 ,,
Total weight, nearly 300 ,,

In lathes of this enormous size—as in all machine tools \>{ the heaviest class—great weight and a proper disposal of it on a thoroughly secure foundation are necessary for obtaining the rigidity which is a first essential to success. When, however, this and all other conditions have been fulfilled, and the tool and the speed have been suitably adjusted, the operation of paring off great shavings from the revolving mass becomes one of such apparent facility that it is almost difficult for a stranger to believe that it is not lead or even some yet softer substance, rather than-wrought irou or steel, which is under treatment.

It has been found that in heavy turning the best results are obtained by taking deep cuts at a low rate of speed, fast driving bringing no corresponding increase in the amount of work got through. Various other means have therefore been devised for accelerating operations. Each of the Woolwich lathes just mentioned is furnished with two slide-rests, so that two independent cuts can be taken: at once at different parts of the work. The duplex system effects the same thing in a different way, two slide-rests (one in front and the other at the back of the lathe) being mounted on one saddle and adjusted simultaneously by a single right and left handed screw,—a plan which has the advantage of subjecting the work to two opposite strains which either wholly or partially balance each other.- In some instances both the above advantages are combined by using two duplex rests at different parts of the bed. A quick hand traverse is another time-saving arrangement, now common to almost all screw cutting lathes. It enables the slide-rest to be run quickly back from the end of one cut to the starting point of the next. In turning up a number of similar articles upon each of which several different tools have to be used in succession, the time which would be lost in changing the tools is sometimes saved by employing a capstan rest, in which the whole series of tools is so fixed once for all that each in turn can be brought to bear upon the work without further adjust-ment.

FIG. 5.—Slide-Rest Tools.

Three examples of turning tools are given in fig. 5,- the middle one being an ordinary hook tool, suited for outside work on wrought iron or steal, and the one above it a left-hand tool which can be used also for inside. Their cut-ting edges are of course forged and ground straighter or more pointed or otherwise varied ac-cording to circumstances, and for cast iron or brass the angle of the edge is made much less acute,- as in the lowest of the three in the engraving. The size of the steel from which they are made also varies, 2 inches square being by My means exceptionally large, so that the weight of it uselessly employed in the shanks is very considerable, and altogether disproportionate to that required for the cutting edges. The plan of fixing a short steel cutter in an iron tool-holder, suggested many years ago by Mr Babbage (which has already been men-tioned in connexion with foot lathes), has, however, not found the favour which at first sight might have been expected for it, in spite of the saving which it effects in this respect.

For chasing long or coarse-threaded screws the above-mentioned screw-cutting lathes leave little or nothing to be desired. But for producing the large number of screwed bolts, studs,. &c, now required in mechanical workshops more rapid methods must be had recourse to, and special machines for forging, turning, screwing, and finishing them have accordingly come into common use. Of these one example only can be given—the screwing machine, fig. 6—with which the threads of bolts or nuts are cut to the "standard pitch" whieh now (happily) is almost universally accepted. Immense loss and inconvenience were formerly caused by the absence of uniformity in this respect, but, thanks to the persevering manner in which the efforts of Maudslay and Clement to put an end to this evil have been followed up by Whitworth, it has now almost ceased to exist, and any bolt or nut can be sub-stituted for any other of a like size, however different the processes by which the two have been manufactured. The machine (fig. G) is in fact a lathe with a few special features, such as the hollow man-drel, whichenables it to operate upon a barof any length. Dies mounted on a modified form of slide-rest cut the thread to the full depth at a single traverse, and a simple arrangement enables nuts to be tapped with equal facility. In some other varieties of screwing machines, more particularly those in-tended for hand power only, the outward resemblance to the turning lathe is less apparent, but if their action is looked into it will be found that in them as in almost all machine tools it is the principle of the slide which is mainly conducive to their success.

Second only to the lathe in its importance stands the planing machine. Just as the slide lathe renders it easy to turn a cylindrical surface true from end to end, a task which before its introduction had been one of extreme difficulty, even for the most highly skilled workman, so the planing machine supersedes, by a method giving vastly superior results, the difficult and costly process of hand chipping and filing, by which flat surfaces of metal were formerly produced. Although it is a comparatively modem invention, its real origin is obscure. No drawings or description of any planing machine at all resembling those now in use were published in England previously to those of one made by Clement in 1825, which appeared in the Transactions of the Society of Arts. With this beauti-ful machine, which was of considerable size, being capable of admitting articles measuring as much as 6 feet in height or width, he obtained results which would satisfy all ordinary requirements at the present day.


process of wood planing, but is analogous to that by which the successive cuts of a narrow tool produce a cylindrical surface in a slide lathe. A traversing table carries the


The ordinary self-acting planing machine is shown in fig. 7. Its action bears no resemblance to the familiar work and forces it against the tool, which is stationary while making its cut, but between the cuts has a slight " feed" motion along its horizontal slide. Perfectly parallel cuts are thus taken from every portion of the work in succession, the result being a surface, not indeed per-fectly smooth and free from scores, but (what is generally far more important) possessing a general flatness and free-dom from twist which can be obtained only with a great expenditure of time and trouble by hand labour. The extent to which machinery has cheapened work of this kind will be appreciated from the fact that in 1826 the cost of rendering a square foot of surface true by hand chip-ping and filing was 12s., whereas in 1856 it could be done in the planing machine at a cost of less than one penny.

Planing machines, equally with lathes, are required not only to give good results but to give them quickly. Pro-vision is therefore made for regulating the traverse of the table to suit the length of the cut, and for utilizing or accelerating its return journeys. The former is sometimes done by fixing the tool in a revolving tool-holder or "jim crow," so that its face can be always turned towards its cut, and for accomplishing the latter there are various arrangements which give a " quick return " to the table. It is also a common practice to use two tools at once, as in turning. It will be observed that the size of the work which can be treated in a planing machine, such as fig. 7, is strictly limited by the clear width between the standards, and the height of the horizontal slide above the table wheii at its highest point. Although these dimensions are very considerable in the larger sizes, which can occasionally take in articles over 9 feet in width and height and 50 feet in length, yet it is sometimes desirable to be able to exceed them, and in these large machines the weight of the table and the power consumed in driving and reversing it become a serious consideration. It is therefore mechani-cally preferable to keep the work at rest when it is large or heavy, and to give all the requisite movements to the tool. This view is now gradually gaining favour, and the makers of some recent machines have adopted a form of construction entirely different from the above, which has the advantage of enabling cuts either horizontal or vertical to be taken from any piece of work which can be secured to the base-plate, so that its full size is almost immaterial.

- Vertical Drilling Machine.

In the first case the tool is than distinguished as a "pillar " and in the second as a "radial" drilling machine. Either of these methods enables the drill to be brought to bear exactly upon the desired spot

An ordinary vertical drilling machine is represented in fig. 8, one of comparatively small size and single-geared having been chosen rather than a larger example with greater complication. When once properly started, this machine is self-acting, but for each hole the work has to be adjusted by hand so as to bring the required portion exactly under the drill spindle, and the small size of the table prevents its being at any great distance from the edge. These objections are remedied in larger machines, either by making the table cap-able of horizontal adjustment,—a good way of doing this being to pivot a circular table at the end of an arm which can revolve round the main standard of the machine,—or by mounting the drill spindle on a radial arm, and enabling its distance from the standard to be varied.
(within certain limits as to distance from the edge, &c), the first by adjusting the work below the drill, the second by adjusting the drill over the work. A wall drill dis-penses with a table altogether, and gives great facilities for operating on large pieces of work, especially if the means of adjustment is secured by the radial arm just men-tioned. Multiple drilling machines, with which a series of holes can be drilled at once, are serviceable tools for some purposes, mainly on account of the sav-ing of time which they effect. Three drills are shown in fig. 9, the first the old, bad, but not yet quite superseded pattern, which is incapable of making a straight or clean hole of any consider-able depth, and which loses its ori-ginal diameter both in wear and in sharpening ; the second the twist drill, which compares favourably with it in every one of these respects ; and the third a pin drill, for enlarging a hole already existing.

Boring machines deal chiefly with apertures of large diameter, for which great straightness and accuracy are required, such as the cylinders of pumps, steam-engines, &c, or the bores of guns. The latter object brought them very early into existence, as already mentioned, and the general principle upon which the rude machinery of more than a century ago bored out the old cast iron mortars is still used for the powerful weapons of our own day. It consists in the employment of a boring bar formed by mounting a series of cutters (or a combination of guides and cutters) round the periphery of a cylindrical " head " somewhat less in diameter than the required bore. Fig. 10 will render evident the great similarity which exists between the oldest and the most re-cent gun-boring heads, the one be-ing taken from the Encyclopédie already referred to, and the other from a draw-ing of a boring-bar used for a similar purpose at Woolwich, The head may be either a fixture at the end of its bar, in which case it forms a kind of drill with several cutting edges, or it may be so arranged as to traverse the bar to a small extent at each revolution,—a plan which is generally preferred for all open-ended cylinders, &c, and which admits of the work being kept stationary throughout the operation. The bar when in use is mounted either vertically or hori-zontally, according to circumstances, in a lathe or boring machine. The excellent results obtainable in this manner will be appreciated from the fact that with the gun-boring machinery at Woolwich a hole 10 inches in diameter and 10 feet deep can be bored in solid steel at a single operation, and holes have been carried to a depth of 2-1 feet with a variation of less than Ti^- of an inch in the diameter. The accuracy of modern machine work indeed not unfrequently brings into prominence sources of error which were previously unsuspected. The boring of large cast iron cylinders affords an instance of this, for it has been found that, however true the boring tool may be, the distortion of the cylinder itself, through being laid on its side, is sufficient to mar the results obtained with it ; consequently it has been found necessary always to bore a large cylinder in the vertical position which it will occupy when in use.

In the construction of modern machinery, &c, it is often neces-sary to depart from the simple geometric forms to the production of which the tools which have thus far occupied our attention are mainly adapted. Wo will now glance at some of the labour-saving contrivances applicable to other cases.

FIG. 11. —Slot Drilling.

The slot-drilling machine effects (by a method said to have been first used about the year 1848) the conversion of the circular cavity producible with an ordinary drilling machine into an elongated " slot " or slit. The extent of the elongation can be varied by in-creasing or diminishing the re-ciprocating movement of the slide which carries the rotating drill. An example of it is given in fig. 11, and the cutting end of a roughing drill is shown to an enlarged, scale. Where smooth-ness of the sunken surface is required this is followed by a rose or some other finishing tool.
The slotting machine (fig. 12) also cuts grooves and slots, but in an entirely different manner. Those who are acquainted with the wood mortising machine, from which the idea of this tool was derived by Roberts of Manchester, will at once understand its principle, and will appreciate the good service which can be rendered by this powerful paring tool. A large proportion of the shaping, &c, required in heavy work is now done in these machines, which
are sometimes of great size and power. The table on which the work is placedis pivoted and mounted on a compound slide, and a self-acting horizontal transverse or circular movement can thus be given to it.

FIG. 12.—Slotting Machine.

For work of moderate size shaping machines, which are more of recent introduction than either slotting or planing machines, both of which they resemble in their action, are in some respects more convenient. The slide which carries the tool is in their case hori-zontal, and its short but variable strokes are in a direction trans-verse to the bed, along which it can travel, just as a slide-rest travels along a lathe bed. Curved surfaces, either convex or concave, as well as flat ones, can generally be worked up automatically in these machines, one of which is shown in fig. 13, but their details \ and arrangements vary consider-ably. For operating upon small surfaces, especially those of com-plicated outline, the plan of employing a revolving cutter, re-sembling a circular file, is now gaining favour. It is interesting to note that this is but a return to a system which is stated to have been devised by Dr Hooke in 1664, and which was certainly used in some of the early '' engines " for cutting the teeth of wheels. One such cutter or milling tool is shown in fig. 14. Others are of a plain cylindrical form, or are varied in outline to any extent to suit the particular purpose for whtch they are intended, amongst which purposes may be mentioned that of cutting the teeth of ot'jer milling tools. "When mounted on a compound slide and used in a milling machine, a tool of this kind is a labour-saving contrivance of a very efficient kind, and it should be observed that it may in ome cases be employed for finishing metal surfaces possessing a couble curvature, to which none of the foregoing planing or shaping machines could be applied.

Profiling or 'Age-milling machines are a still more recent appli-cation of the milling-tool system. They enable the curved or com-plicated outlir e of a previously prepared templet to be reproduced with certainty any number of times in succession. They are in fact copying machines, acting in a similar manner to Jordan's carving machine or Illanchard's copying lathe, in both of which the form of the copy ;s derived from the original pattern by causing this pattern to control the movements of the revolving tool.

Another class of machine tools, which has sprung up of late years and is rapidly extending, is that of emery grinders. One thing which has given much impetus to these is the now not unfrequent necessity for turning or shaping steel in a more or less hard con-dition, for doing which these and natural grinding-stones are the only substances practically available on a large scale, while the rapid wear of the latter unfits them for many of the purposes to which the artificial preparations of emery can be applied with great advantage. Accordingly emery wheels are now mounted for use in a great many diil'erent ways,—either on slide-rests as turning tools, in . emery planers and emery shaping machines (such as fig. 15), and various others in which they take the place of steel cutters, or as tool grinders either general or special, in which the rival material, so far from supplanting steel, does much to-wards increasing its efficiency, by enabling the process of grinding to be applied to many cutting tools which could previously be sharpened only with much greater labour and cost by other methods. Saws, grooved rimers and screw taps, and twist drills are familiar instances of this appli-cation. A high rate of speed is essential for obtaining the full effect of an emery wheel, half a mile a minute being by no means an unusual or excessive rate of travel for its cutting surface. A con-siderable amount of heat is consequently developed at the point of contact with the work, and the composition of the wheel must be such that it can endure this without injury. Some which could not fulfil this requirement have long been used by native workmen in India, but others w hich could fulfil it were patented in England in 1842, though for years after this they were but little known or used.

FlG- 15.-Emery Shaping Machine.

Punching and shearing machinery holds the same isolated position amongst machine tools that punches and shears occupy amongst cutting tools used by hand—if indeed either the one or the other can be regarded as cutting tools at all. Yet, for performing rapidly and in many cases without any waste of material, shears can often claim superiority to any other means available for ac-complishing the same ends. The diagram (fig. 16) shows the old arrangement known as cropping shears, still in use at many iron-works, where early appliances seem to enjoy a remarkable vitality. An ex-ample of a self-contained shearing and punching machine is given in fig. 17. The apparent ease with which machines of this kind, acting wdth a slow quiet stroke, shear or perforate plates of iron, even when of considerable thickness, gives an altogether false impression of the amount of power which the operation requires. _ Arrangements for obviating the difficulty of placing the work exactly in the correct position for each one of a series of holes to be punched in it were devised by Maudslay ; his plan, which is the one now usually adopted, being to place a traversing table in front of the machine, from some part of which it is moved to a distance depending on the '' pitch " of the holes after each stroke of the punch. Another system, by which the holes could be arranged in any required pattern, was subsequently invented by Roberts.

The above examples of workshop tools have been confined to those to which the requisite power is transmitted from an inde-pendent steam-engine or some other prime mover —the usual mode of transmission being by lines of shafting carrying pulleys or drums. Belts pass from these to similar pulleys, which may be observed on many of the machines in the engravings. But this is not the invariable method. The prime mover may itself form part of the machine, as it does in the case of a steam-hammer. Or steam may be dispensed with and water confined under a high pressure substi-tuted,—which constitutes the hydraulic system of distribution, now largely applied to the working of cranes and many other purposes, and to some extent also to machine tools. Punches and shears lend themselves readily to this system on account of their slow move-ments ; so, too, do riveting machines. The distribution of power by hydraulic means, and also by compressed air, was patented by Mr firamah in 1796. Another formidable rival to steam also has now sprung up in the shape of electricity, and the results from it which are promised to us—and which indeed seem likely to be ob-tained—will go far towards revolutionizing all our present ideas as to the difficulty of transmitting power to a distance, and will work a complete transformation in the aspect of the machine tools of the future.

FIG. 17.—Punching and Shearing Machine.

One other class of machines must be mentioned before conclud-ing, viz., measuring machines. The greatly increased accuracy of modern work has rendered necessary the recording of very minute dimensions, such as are quite beyond the measuring powers of ordi-nary rules and callipers. Difference engines, i.e., machines which can measure minute differences between two articles—such as a standard gauge and an intended copy of it—have thus found a place in engineers' works. To their arrangement and manufacture, as well as to that of standard measuring bars and gauges, Sir J. Whitworth has paid great attention, and he has achieved such success that in his workshop measuring machines a difference of _______ an inch is readily appreciable. At the standards de-partment of the Board of Trade there is one of these machines, used for the verification of standard gauges, which reads to the ______ an inch ; and with his most sensitive machines—which, however, require great care and special precautions in their use— the ______ an inch can be detected.

The social influence of machine tools we cannot discuss, though it is a subject upon wdiich the diffusion of correct ideas is greatly needed. The days of mill-burning and implement-breaking mobs indeed are past, but the effect of the introduction of the machinery of which these tools are the parents is one which is still much misunderstood. More particularly is this the case amongst the hand-working classes in England, who see clearly the local and temporary hardships which its introduction occasionally causes, but are blind to the greatly preponderating advantages which they reap from it in an especial degree. (C. P. B. S.)

The above article was written by C. P. B. Shelley, Lecturer on Machinery, Kings College, London.

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