Fig. 311 represents a modification of the eccentric chuck, when the
latter is used as a fixture, to present to revolving cutters and
drills the different parts of the work which is to be operated on. The
eccentric chuck is commonly made to slide in one direction only, and
the traverse is limited. In the present case there is ample traverse
in both directions, the slide being arranged to descend to the lathe
bed, and upwards to an equal degree. A is a cross section of this
chuck, the length of which is 7-1/2 inches. It is very strongly made
in brass, and is altogether much more substantial than the eccentric
chuck. B and C are the guide bars, between which works the sliding
part. The nut of the leading screw is below this as usual. The tangent
wheel has 120 teeth, and is thus divided: 0--12--24, &c. The tangent
screw head is divided thus: 0--1--2--3--4--5, with half divisions,
marked but not numbered. This rectilineal chuck is most commonly used
in a vertical position, but may be otherwise placed. In using it for
any work likely to bring a strain upon it, the ordinary spring index
attached to the lathe for use with the face plate, should not be
entirely relied on to keep it in position. It is safer to make use in
addition of the segment engine stops or other available contrivance.
The special function of the chuck is the production of straight lines
on the face of work forming stars or radial flutes, which can be
worked with a drill. Fluting is also readily done by its aid, with the
addition of the vertical eccentric, or dome chuck, already described.
Its use is, however, by no means confined to ornamental work--small
tenons, mortises, and even dovetails are producible by it; and in
fitting together the various parts of temples, shrines, and similar
complicated specimens, its uses will be innumerable; and here may be
noted the extension of the lathe and its apparatus to work apparently
in no way suited to it. It has now become more of a universal shaping
machine than it used to be, owing to the great accuracy of the work
done by it, and the variety of fittings that can be added to it. In a
later page will be found a drawing and description of a new device of
the kind--a planing machine, devised by an ingenious and first-class
maker, Munro, of Lambeth, and patented by him. Mention is made of it
in this place, because the rectilineal chuck is in some degree capable
of similar work. The slide moved up and down by a screw the handle
of which is of extra length to allow the vertical traverse, is also
capable of being moved by a cam-eccentric chain or rack and cogged
wheel, so that by pulling down a handle the slide may be made to slide
up and down more rapidly than by the screw motion; any piece of wood
of rectangular or other figure may thus be planed on the face, by
being fixed on the rectilineal chuck, and acted upon by a fixed tool
in the slide rest, the latter affording the horizontal traverse of
the tool across the face of the work, the former the perpendicular
movement of the material. If a slide rest is thus arranged in
combination with the chuck in question, and the lathe bed is imagined
to be set up on end with the chuck downwards and horizontal, the
whole will become, in fact, a precise counterpart of those planing
machines, the bed of which traverses to and fro, with the work under
a fixed tool. Munro's arrangement is, of course, of far more extended
application, and more suited for metal work; but for lighter and more
delicate operations of a similar kind the rectilinear chuck and slide
rest will be found very serviceable. It is with such an adaptation of
this chuck as has been alluded to, namely, a quick speed movement of
the slide by a lever handle, that the rays are drawn so exquisitely
fine and close upon the faces of many gold dial plates of watches,
the handle being arrested by a stop at any given point, so that these
rays shall not transgress their appointed limits. It will be hardly
necessary to allude to those other applications of this apparatus, or
other particulars in which it is identical with the eccentric chuck,
as the description already given of the latter applies to both alike,
the extra traverse of the present in both directions being its chief
distinguishing feature.
[Illustration: FIG. 311.]
[Illustration: FIG. 312.]