One of the most useful tools for ornamentation, especially of plain
surfaces, such as the top of a box cover, is the eccentric cutter,
Fig. 283. The shank, A, lies in the receptacle holder of the slide
rest, and is drilled throughout to receive a steel spindle, carrying
at one end a double pulley, B, to receive the cord from the overhead
motion, and at the other frame, E, with its leading screw, of which
the movable milled and graduated head is seen at H. This frame has
one surface, level with the centre of the main spindle, which is cut
away as shown, and, consequently, as the point of the tool is on its
flat side, which latter rests upon the frame (the bevel being below),
this point can, by the tangent screw, be brought into a line with the
centre of the main spindle, so that when the cord from the overhead
is passed round B, the spindle revolves with great rapidity, and the
point of the tool, K, in the position described, makes a simple dot.
By turning round the milled screw head, H, either by the thumb and
finger or by a small winch handle, fitted on the square part beyond
the head of the screw, the tool holder, D (which is in one piece with
the nut of the leading screw), is made to traverse the frame, and the
tool will cut a circle small or large according to the eccentricity
thus given to it. In Fig. 285 D, is the tool holder on the front of
the frame; C, the end of the spindle; L, a bell-shaped washer, which
is acted on by the small square-headed screw, drawing D towards the
frame and clamping the tool. The whole is in the figure of full size.
The tool holder is in one piece with the nut, through which passes
the leading screw, and which is continued as a screw for the action
of the bell-shaped washer and tightening nut; hence it is necessary
to allow a degree of play between the nut and leading screw, to
prevent bending the latter when clamping the tool. This is effected
by filing off the threads in the nut at the top and bottom, to render
the whole slightly oval. The remaining threads suffice for the action
of the leading screw: a very slight degree of play in the required
direction will be found sufficient. The powers of the eccentric
cutter frame will be found sufficiently extensive to make it a most
serviceable, perhaps necessary, piece of lathe apparatus. If it cannot
be said absolutely to supply the place of the eccentric chuck, it has
nevertheless the advantage of great lightness of construction, lowness
of cost, and ease of manipulation. The weight of the eccentric chuck,
whether single or double, as of all chucks in which sliding plates
are used, is a sad drawback to their value--a drawback unfortunately
beyond remedy, and specially felt when the slides are drawn out to
a great degree of eccentricity. Combined together, these two form a
_compound_ eccentric chuck, and in this way are capable of nearly
everything in the way of eccentric ornamentation. Where the _chuck_
is not to be had, it is by all means advisable to procure the cutting
frame, for which the writer confesses a great partiality. It appears,
indeed, to him a far more rational proceeding, as it is also now of
extensive application, to act upon fixed work by revolving or moving
tools, instead of proceeding in the contrary way; and all these little
tools used with the overhead apparatus are so lightly and elegantly
constructed, and so well adapted for the parts they have to perform,
that the originator of them (_native talent devised them_), deserves
to be well and lastingly remembered; instead of which it is doubtful
whether his name is even known. (_Sic transit_ is a quotation too
stale for this work.)
[Illustration: FIG. 283.]
[Illustration: FIG. 285.]
[Illustration: FIG. 285B]
To cut circles deeper in one part than another--the shell pattern,
for instance--with this tool, it is not necessary to alter the level
of the sole of the rest, as it is when the eccentric chuck is used
with a fixed tool, as it suffices to set the rest itself at an angle,
by moving it round in the socket, so that the revolving tool should
touch the face of the work sooner at one point than at an opposite
one. In the same way the work may be considerably undercut on one
side of the circles, by giving the angular set to the rest, and
placing a tool in the holder, with a point of the form shown at 285
B. There is nothing prettier than this undercut work when well and
sharply done, for which purpose the tool should not only be rendered
keen on the hone, but burnished and polished on the brass and iron
slabs already described.
The following remarks on the work of this cutter frame on flat
surfaces only, will be useful to the reader in designing and working
out the various combinations of circles, intersecting or otherwise,
which it is calculated to produce. On a surface represented by 286 A,
the line of circles, _a, a_, is on a diameter, and, supposing them to
be described by the eccentric cutter (or by a simple double-pointed
drill), their centres are obtained by means of the leading screw of
the slide rest, moved the requisite number of turns between each cut,
while the work is retained in a fixed position on the mandrel. But
if the line of circles is on such a line as _b b_, above or below
the centre, and consequently not on a diameter, it is plain that no
movement of the slide rest or cutter, or both, can avail to place
them in position, except with great difficulty and tedious working
with the division plate of the lathe and the screw of the slide rest.
Hence the eccentric chuck must be brought into play, and being fixed
with its slide in a vertical position, the screw is turned and the
work is lowered thereby until the line _b, b_, is on a level with the
point of the tool. The eccentric cutter or double drill will then
suffice to work the row of circles. When the centres of the circles
are themselves on parts of the circumferences of other circles, the
division plate of the lathe or of the eccentric chuck will be called
into requisition, according as these circles are concentric with
the mandrel or otherwise. In Fig. 286B, the curved lines are parts
of circles of equal size with that representing the surface of the
work, and their centres lie on one and the same diameter, viz., at
opposite extremities of the line, _a, b_. Being thus eccentric to
the work, the division plate of the chuck is used to arrange the
intersecting circles of the pattern--its slide having been first
drawn down, until the centre of the arc to be worked with circles
is brought opposite to the tool. The work will be in position when,
on turning the mandrel slowly, the cutting point of the tool passes
across its centre. The division of the original circle is in this
instance into four parts, two of which are thirds, and two sixths of
its circumference. The arcs of circles are also lines equal to thirds
of the circumference of the work. It is well to remember this division
of a circle by other equal circles described round it from points
on its circumference, these circles passing through the centre. The
original circle will in this way be divided, as shown at C, into six
equal parts. To produce it with the aid of the eccentric cutter is
easy. Set the tool of the cutter first to the centre of the work, so
that on revolving it will make a simple dot. This should always be
done, whatever pattern is subsequently to be cut. Fix the index of
the division plate of the lathe at 360. Move the screw of the slide
rest until the point of the cutter, on being advanced, rests on the
circumference of the circle previously cut upon the work, or on the
circumference of the work itself, if the divisions are to reach the
edge. Screw back the _tool_ (not the rest) until its point reaches
the centre of the work and cause it to revolve so as to cut one arc.
Move the lathe pulley forward to 60° and cut a second arc, and so on,
advancing 60° each time, and the figure will be cut. This division
of the circle will form the groundwork of many handsome patterns.
When the arcs thus formed are intended merely to be the lines of
centres, and not themselves to form integral parts of the pattern,
they should, nevertheless, be marked with a pencil in the tool holder,
if possible, as there will be less liability to error in working
the proposed pattern. In the present advanced stage of the art of
turning, mere surface work done by the eccentric cutter is rather
apt to be despised, owing to the extended powers of Ibbetson's or
Plant's geometric chuck; but, valuable as the two latter are, they are
necessarily so costly that few can obtain them, whereas the little
cutter frame is comparatively cheap, and it is really capable of very
exquisite work in skilful hands.
[Illustration: FIG. 286A]
[Illustration: FIG. 286B]
[Illustration: FIG. 286C]
SEGMENT ENGINE. Fig. 287.
In very many cases of ornamentation it is required that the mandrel,
instead of making an entire revolution, should stop at a given point
in both directions, so that, for instance, the turner should be able
to move it 60, 80, or 100 divisions to and fro, with the certainty
of its not advancing beyond that distance. This is effected by the
racked and divided brass wheel B, fixed on the mandrel against the
small end of the pulley. This wheel is sufficiently thick to allow of
racking part of its edge to be acted on when necessary by the tangent
screw, and leaving the other part for divisions, which are generally
seventy-two in number, and marked in figures at every sixth division.
On the other side of the plate are a number of holes drilled through
its whole thickness to receive stop pins, Fig. 289, P, which are sawn
through as shown, that they may spring, and fit the holes tightly.
There are seventy-two holes corresponding with the divisions. These
pins are about 3/16ths of an inch diameter, generally with flattened
heads, and a hole through them to receive a pin to aid in removing
them. The holes are sometimes made in the edge, instead of the side
of the segment plate, but the latter is the best position. At Fig.
288, T, is seen the interior part of the poppet, with a piece of brass
let in, and fixed securely, in which are inserted two screws, against
which the segment stops abut, and prevents further rotation of the
pulley. Side by side with this latter piece is placed the frame which
carries the tangent screw. It is shown at Fig. 289. This frame is not
fixed to the base of the poppet, but pivotted at _e_, between two
short standards screwed into the poppet for that purpose. When not in
use, the whole frame, therefore, drops down towards the front, but
it can be raised by the small cam, K, Fig. 289, so as to gear with
the worm wheel. In many cases the latter is not used, but the pulley
turned by hand. The screw, however, gives a steadier and more easily
regulated movement, essential in delicate operations, and sometimes
convenient, even when the stops do not require to be inserted. The
use of the cam, acting on the frame which carries the tangent screw,
is now generally followed in the eccentric and oval chucks, and also
in the dome chuck. It enables the workman, by throwing out of gear
this part, to turn the worm wheel with the fingers, to set it at the
required number on the division plate, a slow process when effected by
the screw.
[Illustration: FIG. 287.]
[Illustration: FIG. 289.]
[Illustration: FIG. 288.]