The apparatus now referred to is that invented by Mr. Healey of Dublin, an amateur; * it is universal, or capable within certain limits of cutting all kinds of screws, either right or left handed, and is represented in plan in tig. 596, in which C is the chuck which carries the work to be screwed, and t is the tool which list upon r r the lathe-rest, that is placed at right angles to the bearer, and is always free to move in its socket s, as on a center became the binding screw is either loosened or removed.
* Thiout's Traite d'Horlogerie, Mechanique et Pratique, etc., 4to, Paris, 1741, vol. i, page 69, plate 27. The name of the inventor is not given.
† Three are described in Thiout's Treatise: namely, in plates 25, 26, and 27, the first by Regnaudde Chaalon. Other examples will be found in Rees's Cyclopedia, Article Fusee, Plates Horology, 36 and 37.
On the outside of the chuck C is cut a coarse guide screw, which we will suppose to be right-handed. The nut n n, which fits the screw of the chuck, is extended into a long arm, and the latter communicates with the lathe-rest by the connecting rod c c. As the lathe revolves backwards and forwards the arm n (which is retained horizontally by a guide pin g), traverses to and fro as regards the chuck and work, and causes the lathe-rest r r', to oscillate in its socket s. The distance s t being halfsr', a right hand screw of half the coarseness of the guide will be cut; or the tool being nearer to, and on the other side of, the center s, as in the dotted position t', a finer and left hand screw will be cut.
The rod c c may be attached indifferently to any part of n n, but the smallest change of the relation of s t to s r', would mar the correspondence of screws cut at different periods, and therefore t and r should be united by a swivel joint capable of being fixed at any part of the lathe r r', it r r' , which is omitted in Mr. Healey's perspective drawing of the apparatus.
This is one of the least perfect of the modes of originating screws, it should therefore be only applied to such as are very short; as owing to the variation in the angular relation of the parts, the motion given to the tool is not strictly constant or equable; when in the midway position, the several parts should lie exactly at right angles to each other, in order, as far as possible, to avoid the error. The inequality of the screw is imperceptible in the short fusee, and it would be there harmless even if more considerable; but a perfect equality of coarseness or of angle, is imperative in longer screws, and those to be fitted one to the other, a condition uncalled for in the fusee, which has only to carry a chain.
Fig. 596, varley's screw-cutting lathe.
* First described in Tilloch's Philosophical Mag. for 1804, Vol. six., pp. 172 - 175.
The apparatus invented by the late Mr. S. Varley, and represented in plan in figs. 597 and 598, although it does not present the universality of the last, is quite correct in its action and far more available; it is evidently a combination of the fixed mandrel, and the old screw-mandrel, fig. 592, p. 613. Four different threads are cut on the tube which surrounds the mandrel, and the connection between the guide screw and the work, is by the long bar b b, which carries at the one end a piece g filed to correspond with the thread, and at the other, a socket in which is fixed a screw tool t, corresponding with the guide at the time employed.
The lathe revolves with continuous motion; and the long bar or rod being held by the two hands in the position shown, the guide g, and the tool t, are traversed simultaneously to the left by the screw guide; and when the tool meets the shoulder of the work, both hands are suddenly withdrawn, and the bar is shifted to the right for a repetition of the cut, and so on until the completion of the screw. The guide g, is supported upon the horizontal plate p, which is parallel with the mandrel, and the tool t, lies upon the lathe rest r.
Beneath the tool is a screw which rubs against the lathe rest r, and serves as a stop, this makes the screw cylindrical or conical, according as the rest is placed parallel or oblique. For the internal screw, the tool is placed parallel with the bar, as in fig. 598; and the check screw is applied on the side towards the center, against a short bar, parallel with the axis of the lathe.
As in the screw-mandrel lathes, the screws become exact copies of the screw-guides, and to a certain extent the mechanism fulfils the office of the slide-rest; but at the same time, more trouble is required for the adjustment of the apparatus. In general the guide-rod must be supposed to act somewhat as an imcumbrance to the free use of the tool, which is applied in a less favourable manner, when the screw is small compared with the exterior diameter of the work, as it must then project considerably from the bar: so that on the whole the traversing mandrel is a far more available and convenient arrangement.*
None of the machines which have been hitherto described, are proper for cutting the accurate screws, of considerable length or of great diameter, required in the ordinary works of the engineer; but these are admirably produced by the screw-cutting lathes, in which the traverse of the tool is effected by a long guide-screw, connected with the mandrel that carries the work, by a system of change wheels, after the manner employed a century back, as in fig. 595. The accuracy of the result now depends almost entirely upon the perfection of the guide-screw, and which we will suppose to possess very exactly 2, 4, 5, 6, or some whole number of threads in every inch, although we shall for the present pass by the methods employed in producing the original guide screw, which thus serves for the reproduction of those made through its agency.