Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/172

Rh 154 MACHINE TOOLS 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 sur 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 &quot;gap&quot; 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 Fro. 4. Self-Acting Break Lathe. in 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 taming 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 Royal 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, &amp;lt;fec., 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,, Length of leading screw over all 52 ft. 3 in. Diameter of do. do 7 in. Weight of fixed headstock, about 55 tons Do. movable do., about 18 ,, Do. slide-rest and saddle, about 15^,, Total weight, nearly 300~ ,, In lathes of this enormous size as in all machine tools of 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 iron 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. 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 steel, 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 no 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 screiving machine, fig. 6 with which the threads of bolts or nuts are cut to the &quot;standard pitch&quot; which now (happily) is almost Fiu. 5. Slide-Rest Tools.