The smaller and most simple application of the system of change wheels for producing screws, is shown in fig. 599. The work is attached to the mandrel of the lathe by means of a chuck to which is also affixed a toothed wheel marked M. therefore the mandrel, the wheel, and the work partake of one motion in common: the tool is carried by the slide-rest, the principal slide of which is placed parallel with the axis of the lathe as in turning a cylinder, and upon the end of the screw near the mandrel, is attached a tooth wheel S, which is made to engage in M, the wheel carried by the mandrel.
As the wheels arc supposed to contain the same number of teeth, they will revolve in equal times, or make continually turn for turn; and therefore in each revolution of the mandrel and work, the tool will be shifted in a right line, a quantity equal to one thread of the guide-screw, and so with every coil throughout its extent of motion. Consequently, the motion of the two axes being always equal and continuous, the screw upon the work will become an exact copy of the guide-screw contained in the slide-rest, that is, as regards the interval between its several threads, its total length, and its general perfection.
* The details of this apparatus will be found in the description of the same by Mr. Cornelias Varley, the nephew of the inventor, in the Trans. Soc of Arts, vol. xliii., p. 90, 1825.
But the arrows in M and S, denote that adjoining wheels always travel in opposite directions; when therefore the mandrel and slide-rest are connected by only one pair of wheels, as in fig. 599, the direction of the copy screw is the reverse of that of the guide. The right-hand screw being far more generally required in mechanism, when the combination is limited to its most simple form, of two wheels only, it is requisite to make the slide-rest screw left-handed, in order that the one pair of wheels may produce right-hand threads.
But a right-hand slide-rest screw may be employed to produce at pleasure both right and left hand copies, by the introduction of either one or two wheels, between the exterior wheels M and S, fig. 559. Thus, one intermediate axis, to be called I, would produce a right-hand thread: two intermediate axes, I I, would produce a left-hand thread, and so on alternately; and this mode, in addition, allows the wheels M and S to be placed at any distance asunder that circumstances may require.
In making double thread screws the one thread is first cut, the wheels are then removed out of contact, and the mandrel is moved exactly half a turn before their replacement, the second thread is then made. In treble threads the mandrel is twice disengaged, and moved one-third of a turn each time, and so on.
When intermediate wheels are employed, it becomes necessary to build up from the bearers some description of pedestal, or from the lathe-head some kind of bracket, which may serve to carry axes or sockets upon which the intermediate wheels revolve. These parts have received a great variety of modifications, three of which arc introduced in the diagrams 600 to 602; the wheels supposed to be upon the mandrel, are situated on the dot line M M, and those upon the slide-rest on the line S S.
The rectangular bracket in fig. 600, has two straight mortises; by the one it is bolted to the bearers of the lathe, and by the other it carries a pair of wheels, whose pivots are in a short piece, which may be fixed at any height or angle in the mortise, so that one or both wheels, I I, may be used according to circumstances. In fig. 601, the intermediate wheel, or wheels, are carried by a radial arm, which circulates around the mandrel, and is fixed to the lathe head by a bolt passed through the circular mortise. In fig. 601, a similar radial arm is adjustible around the axis of the slide-rest screw, in the fixed bracket.
Sometimes the wheel supposed to be attached to the slide-rest, is carried by the pedestal or arm, fixed to the bed or headstock of the lathe; in order that a shaft or spindle may proceed from the wheel S, and be coupled to the end of the slide-rest screw, by a hollow square or other form of socket, so as to enable the rest to be placed at any part of the length of the bearer, and permit a screw to be cut upon the end of a long rod.*
* The shaft sometimes terminates at each end in universal joints, in order to accommodate any trifling want of parallelism in the parts, if however the shaft be placed only a few degrees oblique, the motion transmitted ceases to be uniform, or it is accelerated and retarded in every revolution, which is fatal in screw cutting.
This change in the position of the slide-rest, is also needful in cutting a screw, which exceeds the length the rest can traverse, as such long screws may then be made at two or more distinct operations; before commencing the second trip the tool is adjusted to drop very accurately into the termination of that portion of the screw cut in the first trip, which requires very great care, in order that no falsity of measurement may be discernible at the parts where the separate courses of the tool have met. This method of proceeding, has however from necessity, been followed in producing some of the earliest of the long regulating screws, which have served for the production of others by a method much less liable to accident, namely, when the cut is made uninterruptedly throughout the extent of the work.
In the larger application of the system of change wheels, the entire bed of the lathe is converted into a long slide-rest, the tool carriage with its subsidiary slides for adjusting the position of the tool, then traverses directly upon the bed; this mode has given rise to the name "traversing or slide-lathe," a machine which has received, and continues to receive, a variety of forms in the hands of different engineers. It would be tedious and unnecessary to attempt the notice of their different constructions, which necessarily much resemble each other; more especially as the principles and motives, which induce the several constructions and practices, rather than the precise details of apparatus, are here under consideration.