Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/342

324 LATHE

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 bearing – a step which was essential to setting free the end of the mandrel, and so enabling it to carry the work – must 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 improbable 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 unsatisfactory 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.

Fig. 4. – Headstock.

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 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 available 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.

Fig. 5. – Mechanic's Lathe.

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 complication 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.

Fig. 6. – Slide-Rest.

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 longitudinal 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 contrivances by which the turner establishes connexion between his work and the mandrel – have 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 described by it. In this way a great variety of intricate