Many turners take special interest in the production of objects in the
lathe, that at first sight appear impossible to be produced solely
by its means. Inasmuch as such works manifest the skill and patience
of the artificer, they will always meet with appreciation; and,
although otherwise useless, they serve as elegant objects of vertu,
and are well worthy a place among the rare ornaments of the drawing
rooms. When first the Chinese balls, consisting of a set of hollow
spheres one within the other, all exquisitely carved, were brought
to England, it was believed they were made in hemispherical pieces,
united round the equatorial line with some kind of cement, the joint
being carefully concealed. I am not sure that they are made in a lathe
in China; but, at all events, they are so made in England, and our
home productions almost rival those of that strange yet clever nation.
I say almost, because the carving in ivory done by the Chinese is in
some respects unequalled, nor do I suppose that work requiring in many
instances years of patient industry could be made to repay the cost
of manufacture in England. No sooner were these curiosities in vogue
here than all kinds of similar impossibilities were manufactured.
Stars with from three to a dozen rays made their appearance, enclosed
sometimes in similar sets of hollow spheres--the rays projecting
beyond the limits of the outer shell--others were wondrously enclosed
in cases with flat sides, cubes, pyramids, six, eight, twelve-sided
hollow cases, all turned fairly in the lathe, were produced with
similar contents, so that the apple in King George's dumpling became a
very secondary wonder. The starry inmates were evidently too large for
the houses; yet there they were--legs and arms, of course, sticking
out through doors and windows, simply because there was no room for
them inside. We will penetrate the mystery, commencing with a single
hollow ball containing a star of six rays, the bases of the latter
standing on a central cube.
In the first place a perfect sphere is required, and consequently the
slide rest and template, or spherical rest, must come into requisition
unless the turner can produce a ball by hand tools alone. Let this
sphere, or rather its boundary line, be drawn on paper of full size
with the compasses, Fig. 262, A, B, C, D. Draw the diameters A, D,
C, B, at right angles to each other. This will give you five points,
which on the sphere itself (on which these lines will have to be
drawn, including also another, answering to A, B, C, D) are centres of
six openings, here represented by the circles, through which the tools
have to be introduced to hollow out the sphere and form the star. The
points of the latter will be in the centres of these openings. Draw in
addition the plan of the central cube, and one ray of the proposed
star; next draw an inner circle, here dotted to mark the thickness of
the outer envelope. The object of this drawing is to enable you to
make a set of curved tools, one of which is shown black at E, and a
set are marked on a plate of steel, from which they must be cut out.
A close inspection of the figure will show that if ball, Fig. 262,
were turning on the point A, A D being its axis of revolution, tools
of the given section introduced at D would cut away the material
round the point or ray, leaving the latter standing;[21] and this
operation repeated at the five remaining openings would entirely free
the central cube with its rays according to the proposed design. The
tools have to be introduced in order, beginning with the smallest; and
although the above remarks will make clear the principle, there are
several points to be attended to in practice, and some few accessories
are required which will now be explained. It is evident that for
every different sized sphere fresh sets of tools will be requisite,
which will also vary in pattern according to the intended form of
the central base on which the rays stand; a cube or flat-sided solid
requiring one tool at least, with a rectilineal edge; spherical or
other solids demanding others whose ends are of different section.
Hence, in all cases, full-sized plans of the proposed work must be
drawn, and special tools designed therefrom.
[21] There is an error in the position of this tool, which, thus
placed, would not leave the point of the star. Fig. 270 will explain
the method better.
[Illustration: FIG. 262.]
[Illustration: FIG. 269A.]
[Illustration: FIG. 269B.]
Fig. 269A is introduced to show more clearly the result of
the application of the first set of tools, or rather of the first
application of the set, as the latter are used throughout.
The blackened part will be entirely cut away in this operation, the
shaded part meeting it will be removed when the tools are transferred
to the adjacent opening, the cuts meeting those first made. Hence
the tools need only reach from _a_ to _b_, and can be more easily
introduced than if the curved part were longer. Gauges, Fig.
269B, A, must likewise be made of thin brass or tin, that the
progress of the work may be examined, and each opening in the sphere
should likewise be measured with a gauge, or with compasses fixed to
one width by an adjusting screw.
The proper chuck for this work is the capped ball chuck already
described, by loosening the cap of which any one of the six openings
may be brought under the action of the tool, these openings being, in
fact, bored out simultaneously with the formation of the star. After
the first point or ray of the star has been completed, the ball may
be reversed and the opposite ray formed. These are now to be secured
by plugs, which are to be turned conical, to fit the opening of the
ball at one end, and of a length to rest upon the central cube at the
other, being also bored out to fit over the rays, which they should
embrace closely at top and bottom, even if not at the other points
of its length. (Fig. 269C, A and B.) This is to be repeated
as each ray is formed, so that the central star may be held in place
until the work is finished, when the plugs are removed, and the star
will be entirely detached. The above-named tools being straight on the
right hand side of the shank will not form a finished _conical_ point
or ray. Hence it is recommended to file away that side, so that when
flat upon the rest, the back of the tool may be an exact counterpart
of a ray, Fig. 270, A. There is, however, no absolute necessity for
this, as the star point can be first made blunt, with perpendicular
sides, which can then be neatly finished by a separate tool made for
the purpose, and kept up to a very keen edge. The first and smallest
of the set of tools here shown, is the one with which the flat sides
of the cube are formed, and it must be bevelled from underneath, so as
to present a cutting edge on the end. The curved tools should cut on
the end and both sides of the crook.
[Illustration: FIG. 269CC.]
[Illustration: FIG. 270.]
[Illustration: FIG. 270A.]
[Illustration: FIG. 271.]
It is quite possible to make the above in mahogany, but a closer
grained wood is much to be preferred, as the tools used--which are
held flat upon the rest--are rather scraping than cutting, and
mahogany, and fibrous woods in general, cannot be thus worked neatly.
Boxwood is, in every respect, the best material to begin upon, ivory
and blackwood being reserved until the eye and hand have become
accustomed to such work. The whole operation requires great care, and
is rather tedious, but the result ought to be a sufficient reward.
The external surface may, of course, be ornamented with the usual
apparatus, but the star should be left clear and sharp. The edges of
the openings should have a light beading, cut with a bead tool, Fig.
271, A and B.