We must now recur to the sphere of which we have already spoken. The
method previously given for producing it is not sufficiently accurate,
although a very close approximation can thus be made to the perfect
figure. It is probably impossible without special apparatus, rendering
the tool independent of the hand, to turn out an absolutely correct
sphere--indeed, it is a sufficiently delicate operation, even with the
following or similar apparatus. For ordinary purposes, indeed, where
the object is simply to produce a croquet ball, a spherical box, or a
globe to be afterwards covered with paper, or any such work, the plan
already given will generally suffice, and, indeed, is very extensively
used. Some practised workmen, too, will, without even the aid of ruled
lines, turn out spheres of average excellence by the eye alone, aided
by a template. When, however, it is proposed to hollow out a sphere
so as to leave a mere shell of 1/8 in. or less, and perhaps include a
number of such shells one within the other, and a star in the centre
of all, it evidently becomes necessary to work with greater accuracy,
and still more so with respect to billiard balls, in which even the
slight variation caused by increased temperature will seriously affect
the result of the most skilful play, and cause the very best players
to fail. The principle of the spherical rest is displayed by the
diagram, Fig. 210.
[Illustration: FIG. 210.]
[Illustration: FIG. 211.]
A is the chuck carrying the ball to be turned, of which C is the
centre. In a right line with the latter, and below it, is a pin fixed
to a block between the bearers of the lathe, and on this the arm, D,
turns. The latter carries a tool-holder in which a pointed tool, E, is
fixed. The point of this tool will evidently move in a circle, when
the arm is moved by means of the handle, D; and, as the centre of the
circle is exactly under that of the proposed sphere, the latter will
be correctly shaped when the lathe is put in motion. Fig. 211 gives
another view of the tool-holder. It is essential that the point of the
tool should be in a line with the centre of the lathe mandrel, so that
it shall act on a diametrical plane as it is carried round the work.
Such is the principle upon which a practically useful tool for turning
spheres has to be arranged.
The faults in the above simple machine are many. In the first place
no provision is here made for the advance of the tool towards the
work. In the second place the requisite firmness and stability cannot
be obtained by merely causing the bar to revolve upon a centre-pin;
and thirdly, as the tool post is fixed to the horizontal bar, the
diameter of the ball must be limited. In point of fact, therefore, the
above arrangement would not answer, and it is only described in order
to illustrate the principle of all inventions for the production of
spheres in the lathe. To give steadiness of action the pin forming
the centre of motion is connected with a circular metal plate truly
turned, upon which a second similar plate works, and to the latter is
attached the tool-holding apparatus. It is difficult to make choice
of a circular or spherical rest so as to give it precedence, since
most of the patterns ordinarily made are good. To obtain the requisite
movement is, indeed, by no means a matter of difficulty; and one or
two adjustments in respect of the height and radius of the tool being
provided, a very simple apparatus will answer the purpose. To commence
with Bergeron's, which, though venerable, is by no means inefficient.
This is represented in Figs. 212 and 213. A, B, is the base, the
tenon, B, accurately fitted to slide between the bearers of the lathe,
the whole being held down as usual by the bolt and nut, _c_. The top
part of the base is surmounted by the accurately faced plate 214, on
which a side sectional view is given in Fig. 215. This is fastened
to the base plate (which, in Bergeron's description, is of wood) by
four countersunk screws. It is turned with a recess, so that the
outer part stands up in the form of a rim, and from its centre rises
a conical pin, _b_, the upper part of which is first octagonal, and
then rounded and tapped. It is this strong pin which forms the centre
of motion, and it must stand with its axial line precisely in the
centre of the lathe bed, so that if the plate were slipped close to
the poppet head this line would bisect the nose of the mandrel. This
is essential in all patterns of spherical slide rest. Upon the lower
circular plate rests that represented in Fig. 216, A and B, the latter
being the sectional representation. This plate is drilled in the lathe
with a central hole, the lower part conical to fit the pin in the
base plate, the upper part countersunk as in the figure, to receive
the octagonal part of the pin and the nut. The projections _a_, and
_b_, in B, represent the projecting rim, _a_, in the Fig. 216, A, and
this is made to fit very nicely within the rim of the lower plate,
while the adjacent part, _c_, rests upon the rim itself. The accuracy
of these bearing surfaces is of the utmost importance, It is evident
that this arrangement is calculated to give great stability during
the revolution of the upper part of the rest, which is fixed securely
to the plate last named. This plate has a hollowed edge cut with a set
of fine teeth to be acted on by the tangent, screw D, Figs. 212, 213,
and shown in Fig. 217 on a larger scale. The bearings of this screw
are attached to the base plate, and the screw is prevented from moving
endwise by collars as usual.
[Illustration: FIG. 212.]
[Illustration: FIG. 213.]
[Illustration: FIG. 214.]
[Illustration: FIG. 215.]
[Illustration: FIG. 216.]
[Illustration: FIG. 217.]
[Illustration: FIG. 218.]
[Illustration: FIG. 219.]
[Illustration: FIG. 220.]
On the upper surface of the top plate are fixed parallel bars
chamfered beneath like those of a slide rest, and between these the
tool holder slides, the advance of the latter being effected by a
screw precisely as in the slide rest already described. In the plan of
this instrument, as seen from above, Fig. 218, A, B, are the bars. The
tool is seen in position at C, the tangent at D. A scale is attached
to the sliding part of the tool holder for determining the size of
the sphere. The tool is again seen in position in 219, and detached
in 220. In Bergeron's description of the above spherical slide rest
the method of using the apparatus is thus described:--"Commence by
placing in a chuck a cylinder of some sound wood, and reduce it to
a convenient diameter, which should be a little greater than that
of the proposed ball. With the gouge work down this, and give it
roughly the form of a ball attached to a cylindrical base." This base
serves to sustain the ball during the operation, and the form of
an inclined plane is to be given to it where it is attached to the
ball, as seen in the drawing, to facilitate the passage of the tool.
After this preliminary work, place the instrument on the lathe bed,
and cause the tool holder to advance by means of a screw (the one
attached to the lower slide, not the tangent screw) until, reckoning
from 0in., the starting point, the index attached to the sliding
part has travelled over the graduated divisions of the scale, so as
to denote the size of the ball in its present rough state. Then
slide the instrument along the lathe bed, until the tool, accurately
adjusted as to height, just touches the ball at the quarter circle.
This will be better understood by the diagram Fig. 221, in which A,
B, are the lathe bearers; C, the tool in position; D, the ball; E,
the chuck or the base of the cylinder. Having previously determined
the size of the finished ball, the work may now be carefully begun by
clamping the rest underneath the bed, and making use of the tangent
screw. Little by little the work is to be reduced, taking care not
to cut too near the base on which the ball is yet carried. This
base is to be cut away little by little as the tool comes round,
and at the last cut the ball will drop off finished, and it is not
to be further touched with sand paper or other material. Hence, as
it approaches the finish, the tool must be delicately and steadily
made to traverse, so as to leave a finished surface as it advances.
Bergeron adds certain precautions as follows:--"If the material is
very rough from the gouge, so that at any point the tool is likely to
meet with such resistance as would endanger the work, such part may be
pared down again by using the machine as a common rest for gouge or
chisel; for the apparatus once arranged should not be altered nor the
fixed tool shifted until the work is done. The tool throughout is to
be advanced very gently forward at each turn by means of the screw,
which causes the parallel movement, and the tool is to be accurately
adjusted so as to be exactly in a line with the centre of the mandrel;
it is also to be very keenly sharpened, and even polished." The first
impression given by an inspection of the above figures is that the
ball would be liable to drop off before it could be fairly severed by
the tool. The writer determined to test this objection personally.
He selected a piece of sycamore, which is very fit for the purpose,
and useful as a kind of medium between hard and soft woods. A ball
was turned by hand from this material having a diameter of about two
inches. The neck by which it was held was retained of the size of a
cedar pencil during the final shaping of the rest of the ball. It
was of course not thus reduced until the ball was nearly spherical.
Gradually the neck was cut away until it was perhaps as small as
the lead of a pencil, yet still the ball retained its position, and
the final stroke cut it off truly and tolerably cleanly, E, 212 and
213. It is really astonishing how small a portion of sound wood will
retain a ball so turned, but the lathe should not be allowed to stop,
else the tendency to hang down or sag would overcome the sustaining
power of the fibres. Bergeron states, indeed, plainly, that the
process answers satisfactorily, and as a man of large experience his
opinion is certainly reliable. Nevertheless, since sand papering or
after process is to be eschewed, it does appear to the writer that
the final cut would leave a minute portion untouched requiring to
be afterwards removed. Not having one of the rests in question the
writer's opinion is to be taken _quantum valeat_. The spherical rest
more commonly used is that represented in the "Handbook of Turning,"
since it is necessary not only that the machine should be efficient
for turning a sphere, but likewise applicable to the ornamentation
of the same by the revolving cutter and other apparatus used in such
processes. This and some other forms will presently be described, and
also an ingenious adaptation of the ordinary slide rest by means of
guide pieces or templets to work of this character. A very good form
of chuck for holding spheres during the operation of hollowing them
out or forming stars or cubes within them will, however, be first
introduced here as described by Bergeron. It will be at once seen how
simply and efficiently a ball can be thus held during such processes.
[Illustration: FIG. 221.]
Fig. 222 represents a chuck for holding balls, A being a sectional
view, B an elevation. In the first, _b, b_, represents the body of the
chuck, made as usual to screw on the mandrel. At _a_, the chuck is
formed with a shoulder like an ordinary box. This part of the chuck is
to be hollowed out to fit the ball on which it is intended to operate.
On the side of the chuck at _b_, is to be cut a screw of medium pitch.
Mounting another piece of wood in the lathe a kind of cover, _c, c_,
is now to be made to fit over the body of the chuck like the cover of
a box, but hollowed out to the curvature of the ball. A ring of brass
or wood of section D being screwed on the inside to the pitch of the
male screw on the outside of the chuck will hold the three separate
parts of the apparatus firmly together. Let them be thus arranged and
finished as one piece on the mandrel. Afterwards drill a hole in the
centre of the part which forms the cover, and enlarge it so that its
diameter shall slightly exceed that of the openings necessary to be
made in the ball for the purpose of hollowing out and forming within
it stars, or lesser spheres, box, cube, or other design, at pleasure.
In this chuck the ball will be held not only centrally but securely.
It is of course necessary to have a chuck specially made for each
different sized ball, but when it is considered that such things as
these are merely turned as curiosities and to prove the capabilities
of the workman, it is not probable that more than two or three sizes
of chuck will be needed, and the difficulty of making them is not
great nor the necessary expenditure of time, either. They should be
made entirely of sound boxwood, and so arranged as to the size of the
respective parts that when the ball is inserted and the cover placed
on, the latter shall not quite reach the shoulder on the base of the
chuck. In the spherical rest of more modern times the principle of
that already described is almost necessarily retained. It is figured
in 223. The sole A is formed like that of an ordinary hand rest, so
that it can be advanced across the lathe bed, and secured by the nut
and screw underneath as usual. From this rises a central circular
plate, which need not be more than a quarter of an inch thick, but
turned truly flat, that it may be parallel with the surface of the
lathe bed. From this rises the conical central pin upon which works
the plate B, the edge of which is racked to be moved by the tangent
screw C. Across this circular plate is securely fixed the chamfered
frame D surmounted by the part E which carries the socket and tool
receptacle, the details of which will be entered into when describing
the rest for ornamental work.[15]
[15] The drawing shows a band at X encompassing the screw. This is
an error, as the whole upper part, including this screw, is made to
revolve by means of the tangent screw.
[Illustration: FIG. 222.]
[Illustration: FIG. 223.]
The method of turning from a pattern acting on the tool has been
alluded to. In some cases a similar method is pursued, in which the
hand supplies the place of automatic machinery, an instance of this
is the application of pattern plates or templets to the small slide
rest now to be described, by which not only parallel or spherical
work may be done, but the elevations and hollows in moulded work may
be followed without difficulty. The pattern plates can be made by
the amateur or workman so that not only is no extra cost incurred,
but any desired form can be given to the work and as many duplicates
made as may be requisite. The simple slide rest itself is represented
in Fig. 224. The lower iron or steel frame is rectangular, chamfered
underneath, and is cast with a projecting part B underneath to the
socket of the hand rest. There is, therefore, no sole or saddle
required, and the height is adjustable at pleasure to suit 3, 4 or
5-inch centres. C is a plate of brass, cast with one V-piece similar
to D, the second E being removable at pleasure, or both V-pieces or
guide bars may be attached by screws. They are attached by two or
three screws passing through oval holes in the plate and tapped into
the bar, so that a little play to or fro is allowed, by which they
can be adjusted to grasp with more or less friction the iron bevelled
frame. They are kept up to their places by a pair of large headed
screws tapped into the edge of the brass plate and marked _e, e_, in
the drawing of the rest. On the top of the brass plate is fixed in
a like manner another pair of chamfered bars similar to those of a
slide rest for metal, but in the ornamental turning slide rests, where
lightness and accuracy are more needed than strength, the parts are
proportionally smaller. The frame, for example, may be six inches
or six and a half inches in length by two in width, the brass plate
two inches square, or perhaps two and a half by two, the longest
measurement in the direction of the upper guide bars, between which
the tool receptacle will slide. This will be quite large enough for
a five-inch lathe, although the measurement may be more or less if
desired. If taken as above, the iron frame may be half an inch deep
and the face of each side of similar size, leaving one inch between,
in which the screw will lie. 224 B shows the under side of the frame
with a cross piece to which the part, B 224, is attached. The guide
bars for the tool receptacle may be similarly small and light. If
the plate is 1/4 in. thick and two in width the bars may be 3/8 in.
stuff before being bevelled, the attaching screws can then be 1/3 in.
or 3/16 in. in the shank, the heads being large and flat, and not
countersunk. It is probable that many amateurs would like to attempt
such a rest themselves, hence we have given the above details. We
must however, warn them that as the above is for ornamental turning,
where accuracy is of the utmost importance, great care must be
exercised in the work, and the various parts must be fitted to the
utmost perfection. The upper face of the lower frame must be quite
level, and the sides quite parallel, and the upper or cross slide must
be fitted precisely at right angles to it. Above all, the screw by
which the upper part is advanced in either direction requires great
precision. It should be made with a fine thread deeply cut, and must
fit its nut under the brass plate, D, without shake. The nut itself
should be long, and must be carefully bored and tapped; it should
be also sawn through its length underneath, to give it a spring, so
that it may grasp the screw tightly yet easily, and this will also
compensate for wear. This will be understood when it is stated that
the milled head by which the screw can be turned is graduated round
its circumference, as is also the face of the bed or lower frame of
the rest. Hence to give the head a turn, or half, or a quarter turn,
must draw the sliding plate exactly the same distance to or fro, at
whatever part of the frame it may be at the time. This necessitates
all the threads being precisely alike. [We say precisely with a
certain mental reservation, for, strictly speaking, perfection is
hardly possible, and the skill and science brought to bear upon screw
cutting when perfect work has been necessary, as for astronomical
instruments, would hardly be credited, so extremely difficult is this
part of the mechanical art.] It would be better for the amateur to
get the screw and nut cut by Holtzapffel, Munro, or other first-class
maker, who has the requisite means and can command the highest skill.
Between the upper guide bars are fitted various tool receptacles. The
only one which need as yet be spoken of is that which holds the little
inch-long tools for ordinary work or for use with the eccentric chuck.
This consists of a brass plate bevelled to fit the chamfered bars, on
the end of which is a small piece of steel with a rectangular hole and
set screw, as seen in the drawing marked X. This plate has a tailpiece
of rather thicker metal, through which pass two set screws, which
regulate the depth of the cut to be made, their points bearing against
the end of the brass plate on which this tool receptacle works. No
screw is attached to move this upper slide, but a wooden handle
projects from it at right angles to be moved by hand, or a lever Y is
made use of. This has two projecting pins, the front one taking one
of the holes, 1, 2, 3 of the slide, the other one of a set of similar
holes in the top of the chamfered bars. By this the slide can be
advanced with ease and great steadiness. We now come to the pattern
plates or templets F, alluded to. These are formed of sheet iron about
1-20 in. to 1-16 in. thick, and must be long enough to reach from one
end to the other of the iron frame A, underneath which at each end are
two holes _a, a_., Fig. 224 B, to receive screws by which the templets
are attached by means of the slots, _b, b_. The outer edge, _a_, of
templet is the pattern to be copied. The dotted lines in Fig. 224 show
a plate in position. The tailpiece of the tool holder to be used has
a projecting stud, or is made with a screw with or without the roller
seen at H, and this is kept in contact with the templet by the hand
or by a spring, so that when the slide traverses the lower frame the
tool-holder necessarily follows all the curves of the pattern plate.
An inspection of K will make it evident that the projecting screw must
be so regulated as to length as to allow the tool-holder to go to the
extremes of the projections and hollows. It must therefore in the
pattern shown be at least as long as the line _a, b_, Fig. K, and a
tool-holder of sufficient length must for similar reasons be selected,
or the tailpiece might touch the fixed plate on which it works before
the guide pin had penetrated the deepest hollow of the pattern. With
these precautions nothing can exceed the ease with which this clever
adaptation of the slide rest is made and used, the greatest advantage
being that the workman can make his own templets to any curve or
series of curves that may be desired.
[Illustration: FIG. 224.]
[Illustration: FIG. 224B.]
At a later page is described a slide rest with the arrangements of
the tool-holder somewhat improved, and calculated for the reception
of larger tools and apparatus. The little rest here described is,
however, very light and useful.