This apparatus, already slightly referred to in the chapter on screw cutting tools in the second volume, connects the revolutions of the mandrel with those of the slide rest screw, by a train of change wheels. It is adapted to plain foot lathes having either back center or traversing mandrels, and affords the screw cutting powers of the ordinary slide or screw-cutting lathe, together with some others. It is perhaps the most convenient arrangement for cutting screws and spirals, not exceeding 8 or 9 inches in length, which length may be said to comprise the whole of the screws required in works executed in the foot lathe. The slide lathe is essential for cutting long screws, and for the execution of large and long plain turning. On the other hand the slide rest apparatus, precisely the same in principle, may be fairly considered to have certain special advantages; in being more compact, less costly, far less laborious in its application, and in permitting the production of the spiral upon surfaces, cones and curved forms. The same apparatus is employed for cutting metal screws with the slide rest fig. 146, and for spirals and ornamental twists in wood and ivory, with the slide rest for ornamental turning. For these reasons therefore, it is very generally adopted in foot lathes by the professional mechanician and by the amateur, and to the latter, it is perhaps the most generally useful addition that can be made to the powers of the plain lathe.
The principle upon which screws of various pitches are produced, as enlarged or reduced copies of a guide screw, by aid of a train of wheels, and also some of the forms taken by the apparatus, have been given in the second volume. It may however be convenient to the reader, to succinctly recal some few points, so far as may appear necessary, in describing the construction of the spiral apparatus and its combinations for different screw threads. The manipulation of the apparatus, will be found in the chapter on screw cutting.
Precisely the same in system and derived from the slide lathe, at first, the details of construction of the spiral apparatus nearly followed the necessary arrangement of the details of the former. The first wheel of the train revolving with the mandrel, was attached to its back end, fig. 163; the last turning with the screw, was carried by a long metal rod, one extremity of which, was connected by a square or fork to the end of the slide rest screw, and the other, supported on a rectangular bracket attached to the bearers behind the lathe head. The intermediate wheels revolved upon arbors, carried by a radial arm circulating around the mandrel, and fixed in position at the back of the lathe head, by a bolt and nut passing through the hole at other times occupied by the pivot of the conducting apparatus of the screw guides.
The apparatus was usually without other means of obtaining multiplex threads, than that of raising the radial arm, to place the wheels out of gear with that on the slide rest screw; and then, shifting the mandrel or the slide rest screw round through the partial revolution required. This deficiency was sometimes removed, with more or less success, by various forms of shifting carrier plates, carrying the first wheel on the back end of the mandrel, or by shifting the work round within the chuck. The original form of spiral apparatus however proved inconvenient in use; among other reasons, from the necessity of every time, removing the conducting apparatus of the screw guides for its attachment; and also, from having to shift the lathe head forward upon the bearers to make place for the bracket, by which the pulley no longer stood over the fly wheel. A more serious objection, lay in the use of the rod carrying the last wheel of the train; for unless the rod was adjusted exactly axially with the screw of the slide rest, it interfered with the truth of the thread in every revolution or coil of the screw. The apparatus also, was not suitable to lathes having back centers.
The original form has been replaced by that indicated by fig. 164; in which the last wheel of the train revolves upon the end of the slide rest screw; the radial arm is attached to the face of the lathe head, and the first wheel, is carried by a chuck on the nose of the mandrel; the chuck being also arranged for cutting screws of multiplex threads. The modern apparatus being more compact, and free from the objections to which its predecessor was open.
The mechanical details, figs. 165 to 171, comprise the spiral chuck, carrying the work and the first wheel of the train. The radial arm with arbors for the intermediate wheels, with various blanks, nuts and washers, to enable the wheels with either the large or small apertures to be carried by the arbors. A socket, arranged in the same manner, to carry the wheels on the screw, and a series of about 15 change wheels, usually of brass, with from 15 to 144 engine cut teeth. The larger wheels, are bored out with plain holes, to fit upon the back of the chuck at a, fig. 166, to which they are fixed by a screw ring and washer; the smaller, fit only upon the arbors, and upon the socket on the end of the slide rest screw.
The front of the chuck, carries a rachet wheel of 96 teeth with a detent, by which the work is moved round and refixed, one half, third, quarter, or less part of a revolution, in cutting double, triple, or other multiplex threads, without interference with the train of wheels. The rachet wheel is provided in front with a screw, a copy of the nose of the mandrel, to carry the work, which may be placed in any of the ordinary fixing chucks; its opposite end being supported by the point of the popit head. The radial arm fig. 165, is bored out with a hole rather larger than the back of the spiral chuck, and around this hole, on the under surface, not seen in the figure, it has a projecting ring, which fits within a circular recess sunk in the face of the lathe head. The arm moves partially around the mandrel as a center, and is fixed to the lathe head by a screw bolt passing through a circular mortise. The arbors for the intermediate wheels can be placed at any required distance from the mandrel, and are carried on the front side of the radial arm in a long rectangular mortise, in which they are fixed by screw caps behind. Figs. 167 and 168 give the details of the arbors, their shafts revolve with the wheels, fitting the holes in the smaller and carrying the larger, by blanks or central filling pieces, fig. 169; by which means any single wheel may be employed, or any pair of wheels can be placed together on the same arbor, with either the larger or the smaller of the two, next to the face of the radial arm.