LATHE. In its simplest forma form which is still employed by the natives of Indiathe lathe consists of two upright posts each carrying a fixed pin or dead centre, between which the work in hand is caused to revolve by an assistant pulling alternately the two ends of a cord passed round it. A tool held firmly on a bar which forms a "rest" then attacks in succession the projecting parts, and in this way the entire surface is brought to an equal distance from the central axis; in other words, the cross section becomes everywhere circular.
Fig. 1 shows a "dead-centre lathe " of the kind used in Europe during the 18th century, in which the centres are
FIG. 1.Dead-centre Lathe.
carried by "puppets" or "poppets" which can be adjusted to suit the length of the work, the turner giving the rotation by means of the treadle and spring-lath attached to the ceiling. This lath, having immortalized itself by giving its name to the "lathe," has now almost entirely disappeared, the waste of time in its up-ward stroke (during which the work revolves in the wrong direction) being a fatal objection to its use in an age in which economy in that respect is of such importance. Dead-centre lathes them-selves are now almost things of the past, though within their own limits,which are of course con-fined to such articles as are turned on the outside only, and can be supported at the ends (such as fig. 2)they offer a steadiness of support and a freedom of rotation which others seldom equal and never surpass. The system, however, still sur-vives in the small lathes or " throws" used by Fig. 2. watch and clock makers; and for their purposes it is not likely to be soon superseded.
The lathe seems to have but tardily developed into the "foot-lathe," the application to it of a fly-wheel worked by a crank and treadle having been exceptional rather than usual even in the early part of the present century, though a separate fly-wheel turned by an assistant had long previously been employed, and must have rendered possible the turning of heavy work which could not have been attempted without it. The naves of cart wheels were doubtless a case in point, and for these many other purposes detached fly-wheels still render good service where steam or other motive power is not available.
The early attempts at modifying the dead-centre lathe so that articles such as fig. 3 could be turned " en Fair," or without the support of a " back-centre," cannot have been very encouraging. The introduction of a spindle or mandrel carrying a pulley for the lathe band and screwed at one
end so that the work could be attached to it was a tolerably obvious mode of effecting it, a "headstock" resembling fig. 4 being the result. But the discarding of the dead-centre point and the substitution of a front bearinga step which was essential to setting free the end of the mandrel, and so enabling it to carry the workmust have been accompanied by a loss of power and an amount of unsteadiness which quite account for the tenacity with which the simple pole-lathe and the very similar "spring-bow lathe" survived, and make it im-probable that the mandrel was at first ever used in cases for which the older form was admissible. For even if it had been possible with the then existing means to render a mandrel sufficiently true, and to obtain an accurate fit between it and the bearing in which it revolved, wrong ideas prevailed as to the best form to be given to it,the question indeed having only become a settled one within the memory of persons now living, after various unsatis-factory patterns had been tried and discarded. It is a matter of great importance, since the proper performance of a lathe is mainly dependent on the mandrel's maintaining a thoroughly good fit.
The types of modern lathes are as various as are the occupations of those who use them. The mechanic, the soft-wood turner, and the amateur, for instance, differ so greatly in their requirements that a lathe which would be well suited to the one would be very ill adapted, even if not wholly useless, to the other. Thus the professional turner of soft wood, with a lathe of which the frame and even the fly-wheel are of timber (its value in shillings being not very different from the price of an amateur's lathe in pounds) will use a high rate of speed and sharp tools and
FIG. 5.Mechanic's Lathe.
light cuts, and so obtain results with which the owner of an elaborate instrument cannot at all compete. A modern mechanic's lathe on the other hand, such as fig. 5, has very different demands made upon it. For this the greatest possible steadiness in all the working parts is the main desideratum, and it is of great advantage to have the means of obtaining a slow speed, so as to be able to take the heaviest cuts which its strength and the power avail-able warrant. Timber has accordingly given place either to cast iron or gun-metal or steel in almost every part of a lathe of this class, the resulting increase of weight and firmness enabling the hand turner successfully to operate on small sizes of wrought iron or even steel, notwithstanding that in driving the fly-wheel his force can be applied only during a portion of each revolution.
In turning hard materials such as these it is of primary importance that the tool should be held more rigidly than it can with the hand when no support is available except that of a narrow T-headed rest. The difficulty of doing this was to some extent got over formerly by employing "heel tools," which transferred most of the strain directly to a flat-topped rest and made correspondingly reduced demands upon the arm of the turner; but it was never completely overcome till the introduction of the "slide-rest" placed the movement of the tool under complete control, and grasped it in a hand that never tires. Fig. 6 shows a
slide-rest such as would be used with the lathe in the previous engraving, for which purpose simplicity of construction and steadiness in all its parts are the points chiefly aimed at. Slide-rests designed for amateurs' use are sometimes very different from this in respect of conv plication and the number of different movements of which they are capable, but each increase in the number of parts intervening between the lathe-bed and the tool is a source of possible unsteadiness which should not be introduced without reason.
Foremost amongst the more complicated lathes both in utility and in the date of their introduction stand " screw-cutting lathes," in which a regular spiral can be traced upon the work by self-acting means. The traversing mandrel, in which this end was formerly attained by giving a longitu-dinal motion to the mandrel and the work attached to it, and keeping the tool stationary, is now but little used, the modern plan of causing the slide-rest to travel along the bed automatically being more convenient in most instances. It involves, however, an amount of gearing almost inadmissible in a foot-lathe, and it is for those driven by steam-power that it is chiefly employed. These, being machine tools, do not come within our present subject. It should be mentioned that screws can be cut in foot-lathes by hand-chasing tools without any special arrangement, and they are done in this way to a great extent by telescope makers and others with beautiful regularity.
" Chucks"a term which embraces most of the contriv-ances by which the turner establishes connexion between his work and the mandrelhave been made to contribute in various ways to the production of abnormal forms. The oval chuck is used (as its name implies)-for giving an elliptical path to the work in lieu of a circular one. The eccentric chuck .enables any point or any series of points in succession to be brought into a line with the axis of the mandrel. With the former chuck, therefore, a fixed tool can trace an ellipse on the face of the work, and with the latter a series of intersecting or adjacent circles can be de-scribed by it. In this way a great variety of intricate
"engine-turned" patterns can be produced in the lathe, some idea of which may be gathered from the compara-tively simple one shown in fig. 7. To the complicated apparatus known as the geometric chuck neither straight lines nor irregular curves are impossible. The " rose-engine" is a very old device for producing a somewhat similar kind of ornament, such as fig. 8, by giving a chattering motion to the mandrel, which is specially mounted on a vibrating frame for that purpose. The wavy lines on the backs of watches are engraved in this
FIG. 7.An Engine-turned FIG. 8.Rose-engine
way, the curvature of the case not preventing the use of the rose-engine, as it would that of the eccentric chuck. But it is probable that these methods of producing face-work ornament will gradually disappear, and that all who still have leisure for doing them will prefer to use elliptic and eccentric aud rose cutters fixed in the slide-rest and driven independently of the mandrel by overhead motion. With these similar results can be obtained, and the tool only instead of the entire mass of the work has to follow the desired curve.
Sketches of a few characteristic turning tools are given in fig. 9 : A, a chisel, and B, a gouge, for soft wood ; C, a heel tool for wrought iron; D, E, the enlarged ends of a pair of chasing tools for cutting outside and inside screw-threads; and F, G, two slide-rest tools. Of these last F is forged from a square bar of steel (an operation which must be repeated from time to time as
FIG. 9.Turning Tools.
the edge gets worn away), and G has an iron shank made once for
all, from which the steel-cutting portion can be removed for the
purpose of sharpening or renewal. The saving of tool-steel thus
effected is of course no great consideration in the case of these
small tools, but it is very considerable in the large sizes used with
the power lathes of the present day. Examples of these will be found
under the heading MACHINE TOOLS (q.v.). (C. P. B. S.)