The first remarkable deviation from the usual form is in the shaft or axis of the dividing plate. See [fig. 1] and [2] of the [Plates 15] and [16]. The dividing plate a b, is concentric with, and fixed to an axis A B made as perfectly cylindrical as possible, so as both to slide and turn in the bars C D, and E F composing the frame. These bars are bushed, to fit the axis A B, either with a contracting ring of brass, as usual in some mathematical instruments; or with type metal, cast around the axis into rough holes in those bars:—which metal, closing upon the axis makes a good centre; and will last a long time. My Engine is made in this manner; and has been renewed in this part only twice in several years. This frame C D E F of the Engine, is strongly connected with the feet G G H H, by means of the nuts E F in the plan: and by these feet it is fixed to its bench or table, as will be seen in [Plate 16].
[Figure 2] of the present [Plate], represents the plan of the Machine, but turned upside down; so that the feet G H screwed under the lower plate E F, are wholly visible. In this figure, also, is shewn at c d, the edges (without the bottom) of the horizontal slide which carries the stand for the cutter frame represented in [fig. 4]. This stand is indicated by the dotted lines of this [figure 2], as situated under the arm D of the bar C D; but it is better shewn in [fig. 5], where e f marks the slide in which the cutter frame ([fig. 4]) moves up and down, by means of the screw and handle e f. In general I avoid dwelling much on these smaller parts, because they exist, probably in a more perfect state, in most other machines. In this [fig. 5], g h shews the screw that moves this stand nearer to, or further from the axis A B of the Engine, according to the diameter of the wheels: which is also a common process in Machines of this kind, on which therefore much need not be said. But a somewhat greater importance attaches to the cutter frame represented in the [4th. figure]: which is a kind of small lathe whose spindle n o, carries the cutter n, outside the frame, for the purpose of changing the former without displacing the latter. The cutter (of any proper section) is placed in or near that line which is a continuation of the centre of the fixing screw o p. It is in that line for wheels whose teeth can be finished with once cutting: but near it for those whose teeth must be cut at twice. In this same [figure], i k represent the ends of the standards that form the vertical slide e f of [fig. 5]; and the separate figure p q, shews the back of the cutter frame l m, the flat part of which, p, presses correctly on these uprights i k, and thus fixes this instrument at any desired height, and to any given angle with the perpendicular: the use of which arrangement we shall soon have occasion to exemplify.
