There is no class of work on the whole more interesting than that
executed by the aid of the spiral chuck, especially with an addition
to be described here. This apparatus has grown, almost as a matter
of course, from the adaptation to the ordinary lathe of the system
of change wheels for the production of screws of various pitches. A
spiral is, in fact, a screw with very extended pitch, the threads
either closely enwrapping a cylinder which forms the core or body of
the screw, or being entirely separate and independent of such core,
the latter being by far the most light and elegant. The chuck here
described is used with the same arm or bracket as has been already
spoken of, standing out from the poppet to carry change wheels, and
is itself adjustable to suit different diameters of the same. In
the sectional view of this chuck given here, A, H, is the body with
internal screw as B, to fit the mandrel. The cog wheel of the chuck
which gears into the first on the movable arm or standard, is cast
with a large central hole to allow it to be stopped on at D, where it
is retained by a nut C. This permits a change of such wheel for one of
different size or for the apparatus to be presently described.
Thus far the chuck is only applicable to the production of screws
or spirals with a single thread. F, is a dividing plate with racked
edge acted on by the tangent-screw E, and carrying the screw, G, a
counterpart of that upon the mandrel. This plate carries 96 divisions
or teeth. The latter may be used with a spring click if preferred; but
the racked edge gives perhaps the more delicate power of adjustment.
The spiral chuck constructed in this way is capable of producing
any required number of screw threads or spirals, solid or detached,
and of any ordinary pitch. It is, however, chiefly intended for the
production of spirals or twists for articles of _virtu_. The method
of proceeding has already been described in a previous page. It is
one rather of care than skill, as the lathe apparatus ensures the
correct movement of the tool where the shape of the latter determines
the form of thread, angular, round, or moulded at pleasure. A few
of the tools required are shown in the drawing. For finishing the
rounded threads, Nos. 3 and 4 may be used, which are similar to those
required for turning ivory rings, the one completing half the thread,
the other applied in the opposite direction, meeting the cut of the
first and finishing the operation. As it is necessary to get round to
the back of the threads in this case, no inner mandrel can be used to
support the work, and, therefore, great care and delicate handling
are necessary to prevent breaking the twists. The stops should also
be used upon the bed of the slide-rest, to limit the traverse of the
tool and prevent it from striking the shoulder, and destroying any
bead or other moulding formed there. This is more specially needed,
when there are two or more such twists rising from the same base (that
is when there are two or more threads to the screw). The additional
apparatus now to be described, adds considerably to the powers of the
spiral chuck. It is called the reciprocating apparatus, and its effect
is, to cause a to and fro movement of the work, at the same time that
the motion of the tool is continued in a horizontal direction. Fig.
320 shows the simplest of the effects thus produced. The screw is
commenced and carried to any desired distance on the cylinder. The
action and horizontal traverse of the tool is continued, but that of
the cylinder reversed, and the cut is thus carried upwards. The tool
may be a revolving cutter, the action of which, being continuous and
in the same direction, would seem preferable, as the greater the
speed with which the tool attacks the material the better is generally
the result in work of this kind. A fixed tool, moreover, must have a
central edge chamfered above and below, and there is also a tendency
with any such fixed tool to unscrew the chuck, as the resistance
occurs in that direction in the upward cut.
[Illustration: FIG. 320, 321, 322, 323, 324, 325.]
The details of the arrangement are as follows:--The several parts
being drawn of full size, Fig. 321 A is an eccentric capable of
slight adjustment by the use of either hole, one being further from
the centre than the other. Fig. 322 gives another view of this
eccentric, which is precisely similar to that used in model engines.
It is turned as a circular plate of gun-metal, with one flange. The
plate being five-tenths of an inch thick, a second plate, forming
another flange, is attached by four small screws, after the ring of
the eccentric is in place. This ring is of iron, or, still better,
of steel, and is made in one piece, with the arm B, which is six
inches long; the main part of it is flat, but it is rounded towards
the end, and turned at E, after which it is again flattened to work
against the arm D, or still better forked to embrace the latter. It
will be seen that D is also a flat plate, with a turned ring similar
to the first, but without the enclosed eccentric. In this is drilled
a series of ten holes, into any of which a pin can be fitted, so as
to unite the arms B and D, the pin becoming a hinge or centre of
oscillation. The circular ring of the part D fits on the part D, D
of the chuck, on which it can be secured by the ferrule C, the arm
D being then in a vertical position. The holes in the eccentric can
(either of them) be fitted over and secured to the end of the leading
screw of the slide-rest. The handle being then placed on the other
end of the screw and turned by the hand, the eccentric will cause the
arm D to oscillate to and fro through the medium of the connecting
rod B, thereby giving to the chuck, and to the work attached to it,
a similar to and fro movement. The extent of this movement depends
upon the length of the lever D brought into action. With the pin in
the holes 1, 2, 3, the oscillation will be inconsiderable, but with
the pin in either hole numbered 8, 9, 10, it will be much increased.
In the first, therefore, a short wave, Fig. 320, will result; in the
second case these will be more like Fig. 320 B. This apparatus will
completely alter the character of a spiral, which, if cut through a
hollow cylinder (as in the case of _detached_ twists) becomes a zigzag
of curved sides curious enough to behold. The apparatus, it must
be understood, is worked entirely by the handle of the slide-rest,
the lathe-cord being thrown off unless the latter is carried to the
overhead instead, to put in action revolving cutters. The reciprocal
action is, in fact, a self-acting segment engine.
It has been already stated that for the production of spiral work
revolving cutters are preferable to fixed ones, unless, indeed, it is
required to finish up a perfectly round thread, when Figs. 3 and 4
of the tools drawn are required. Revolving cutters must be placed in
the frame of the universal cutter and set to the rake of the thread.
Drills may be used for the reciprocating movement, as they make very
clean work, and the rake need not with these be attended to. In face
work drills are specially to be used to produce patterns like Fig.
325, and others derived from this simple one. An additional apparatus,
represented in Fig. 323, is required for the latter process, to enable
the rest to be turned in its socket so as to face the work, and
notwithstanding the alteration of its position still to keep up the
gearing of the wheels. A rest socket to be made and mounted as usual
is fitted with a stem surmounted by an accurately drilled boss A,
through which passes a spindle fitted with the wheel C, to gear with
that on the arm carrying the change-wheels, and which may be changed
for one of larger or smaller size. This is for cutting such work as
Fig. 324, representing, of course, only a single spiral, very open and
of itself of no beauty, but which by intersection of other spirals
can be converted into a pattern of great elegance. When it is desired
to produce the waved spiral the eccentric is fixed to the rod instead
of the wheel C, and the work proceeds the same as when a cylindrical
surface is to be worked. In the Fig. shown the division plate of this
chuck is of course used.
[Illustration: 1.]
[Illustration: 2.]
[Illustration: 3.]
[Illustration: 4.]
[Illustration: 5.]
[Illustration: 6.]
It is evident that the variations producible by working intersecting
spirals and waved lines are very numerous, and these may be
additionally varied by the combination of eccentric and spiral
movements.
The following six patterns are re-engraved from Valicourt's Hand
Book, which is almost identical with that of Bergeron. They were not
engraved in time to be inserted under the head of eccentric chuck
work. The notation and description annexed is from a manuscript book
kindly lent to the writer by an amateur. "_Bed-plate_," 4 deg., must
be taken to mean that the work is shifted (on one of the beds of the
chuck) four of the marked divisions for eccentricity, counting from
the axial line of the mandrel.
"_Slide rest_, 4 deg.," means that the cutting tool is moved four
corresponding divisions in its bed for radius of the circle to be cut;
and so throughout.
"4 *," means four times repeated.
The cutting tools, unless otherwise expressed, are double angled, and
"25 cutting tool," means 25 deg. of cutting edge.