The first of the chucks comprised under the above head is the oval or
elliptical chuck, and it is introduced first in order because it is
not essentially a machine for ornamental turning, as are the eccentric
and others of this class. There are many plain works required of
elliptical section, as bradawl and other tool handles, for which a
very simple arrangement is required.
The principle of the oval chuck is as follows:--There is an
arrangement of slide, by which as the piece revolves it is drawn
gradually further from the tool during half a revolution, and in
a similar manner caused to approach it during the remaining half
revolution, each point in the circumference alternately partaking of
such movement; the whole of these points together, which, of course,
form the circumference, will become an ellipse. Let D, Fig. 228, be
the centre of the mandrel, A, B the direction of the slide moving
up and down in a right line, and carrying the work upon a screw
in the centre of it. C, E become centres, and may be taken as the
extremes, for as the work revolves a succession of centres are formed
and instantaneously changed. The figure produced will be the oval
shown. To render this, however, clearer, Fig. 229 may be taken, which
represents the chuck in its most simple form with separate details of
the parts which compose it. A is the chuck with central slider and
chamfered bars, as described in speaking of the slide and rest and
previous apparatus; B is the slide detached; D, front view of the
same. The short arms _a, b_, pass through slots in the back plate as
seen at C, which shows this plate with slide removed. Through these
short arms pass a pair of adjusting screws; or still better _a_ and
_b_ are themselves cross arms or pallets extending the width of the
plate as seen next drawing, and in the chuck of Muir which follows.
They are merely flat plates of steel embracing the guide ring, so that
some point in their inner surface may rest against it during every
part of the revolution of the chuck.
[Illustration: FIG. 228.]
[Illustration: FIG. 229.]
The guide ring here alluded to is shown at E, and also at G, fixed in
its place upon the poppet. It is in the form of a raised ring with
arms, B, C., which are turned at right angles near their ends, and
through which pass adjusting screws with conical points. This plate
is flat at the back and bears against the face of the poppet, the
mandrel nose falling into the central opening E. It is kept in place
by the points of the screws falling into conical holes at the sides
of the poppet head. At F is a side sectional view of this plate,
with its raised and accurately turned ring, H, and at G is seen the
poppet with the plate attached, the left arm being dotted to show
the position of the adjusting screw. It is this ring and plate which
regulate the movement of the slider, and, with it, of the work, the
latter being attached to the screw in the centre of the sliding plate,
which screw is a counterpart of that upon the nose of the mandrel.
Suppose the chuck A screwed to the mandrel, and the ring accurately
concentric with the mandrel, in which position, the pallets must touch
at two opposite points. In the best chucks there is an adjusting screw
to each, by which the contact can be regulated. In this position
any object of a circular form can be turned, for the slider remains
in one position, and its screw, upon which the work is fixed, is a
continuation of the mandrel. But if now the adjusting screws of the
part E are turned, the one being loosened and the other tightened,
the guide ring will no longer be concentric with the mandrel, and,
as the screws of the slider bear upon it, the slider will during
its revolution be moved to and fro to a distance regulated by the
eccentricity of the guide ring. The combination of this circular
motion of the chuck and rectilineal movement of the slider will
produce an ellipse, and a stationary tool applied to the work will cut
it, into that form.
The above simple arrangement of oval chuck suffices only for plain
work. The only figures that can be described by its means, upon the
cover of a box, for instance, being a series of ellipses of which
the longest diameters fall in the same right line, and of which the
centres are coincident with the axis of the mandrel, as Fig. 229.
[Illustration: FIG. 229.]
Even these, however, cannot be done without some compensating
arrangement, as the minor axis does not diminish in length at the same
rate as the major--hence the ellipses get narrower and narrower until
the central one becomes a mere right line. This is referred to again
in the ornamental section of this work.
[Illustration: FIG. 231.]
[Illustration: FIG. 232.]
[Illustration: FIG. 233.]
[Illustration: FIG. 234.]
[Illustration]
Combinations like 231, in which the ellipses intersect, cannot be
so obtained. Hence the oval chuck is provided with a wheel, either
racked to work by a tangent screw or fitted with a spring catch, by
which it becomes a dividing plate. This wheel revolves on a central
pin[18] fixed to the middle of the sliding plate, and carries a screw
of the same pitch as that upon the mandrel to which other chucks can
be attached. By this means the axial lines of the ellipse can be
varied in direction. This addition is shown in Fig. 232, which is
a section, and 233, which is a front view. In the former, A is the
wheel, which, as previously explained, should be so arranged as to
contain a number of cogs divisible by as many figures as possible; 96
is such a number, being divisible by 2, 3, 4, 6, 8, 12; 72 is also
a good number, as it will divide by 2, 3, 4, 6, 8, 9. If the edge
is racked and moved by a tangent-screw with divided head a greater
range can be taken and finer work done. In this case the face of the
wheel can be marked with divisions, and a fine steel pointer, as
shown at F, added to count by. The pin B, which is firmly attached to
the centre of the slider plate, must be strong, and the lower part
at least should be conical. It is drilled and tapped at the smallest
end, and when the circular plate with its screw is slipped upon it, a
screw, E, the head of which is countersunk into the face of the large
screw, retains it in place. The slide, C, has a recess turned to fit
the wheel plate, and the latter is cut as shown at X, which ensures a
more accurate bearing than if it was left flat on the lower surface.
In making this chuck certain precautions are necessary. In the first
place, the guide ring fixed to the poppet must be exactly concentric
with the mandrel when in its central position; and when it is drawn by
the adjusting screws to the right or left the central line must remain
parallel with the surface of the lathe bed. To ensure this centrality
it is necessary to turn its outer surface when it is in position on
the lathe head. So at least says Bergeron; and it is perhaps the best
method whereby to ensure the accuracy that is required.[19] For this
purpose Bergeron directs the use of a cutter similar to Fig. 234
attached to the mandrel as a chuck, the edge which is on the inside of
the bent part at _a_ acting on the exterior of the ring as the mandrel
revolves. The screws allow the tool to be advanced closer to the ring
as the work proceeds, while they secure it at any desired point. Such
a contrivance as this, used merely as a finishing tool to correct any
slight error, is no doubt sufficiently satisfactory. The various parts
of this and other compound chucks should be first turned separately
to near the required size, and accurately finished when in their
respective places upon the chuck. Any parts which present a difficulty
from the impossibility of retaining them in place while operated upon,
may be soldered with tinman's solder, and thus turned, after which
the application of moderate heat will detach them, and the fluid
solder can at the same time be wiped off with a pledget of tow or
cotton waste. As many of our readers may wish to make such apparatus
as the above, it may be desirable to add a few directions for the
preparation of chamfered edges such as those of the slide and guide
bars, the latter of which should be of iron or steel. Let the slide,
for instance, be cast as a rectangular plate and the two flat surfaces
be roughly levelled with a file. One of these must now be made
perfectly true, either by mounting it with solder upon the face plate
of the lathe, and levelling it with the aid of the slide rest, which
is perhaps the safest plan, or by careful working with the file, using
a straight-edge in all directions, and finishing by careful grinding
upon a flat stone slab with water, or on a wooden grinder charged
with emery and oil. After one side is finished, the opposite face may
be similarly treated; but for this the plate may be secured to the
finished surface of the lower plate of the chuck itself, and turned
with a tool fixed in the slide rest. The edges must now be filed truly
at right angles to the sides, care being taken to keep the long sides
of the plate parallel. (The short sides or ends will be rounded by
being turned true with the edge of the chuck.) The work must now be
tested with the straight-edge and small steel square, and any error
carefully corrected. Of course, if the reader is the happy possessor
of a planing machine, all these operations will be facilitated and
accuracy more likely to be ensured. It may here be mentioned that,
to supply the want of such planing machine (a want often felt by
amateurs who have not mastered the use of file and scraper), Monro, of
Gibson-street, has cleverly added a planing apparatus to the ordinary
foot lathe, rendering the latter tool complete for all purposes of
amateur engineering.
[18] This pin should have been shown of a conical not cylindrical
form, and much stronger in proportion.
[19] This part is always so turned by the best makers.
This handy apparatus will be found on a later page fully described and
illustrated by a photograph of the machine. The writer has seen it and
used it, and can testify to its satisfactory working, as a lathe thus
fitted does not run heavier or require greater exertion than when used
for ordinary turning.
The next step will be the chamfering of the edges of the plate. Let
235 represent the plate in its present condition, with rectangular
edges. To produce a chamfer of 45°, draw a line, _a, b_, at a distance
from the edge equal to the thickness of the piece. If a smaller angle
is desired, the line must be drawn further back. An angle of 30° to
35°, is, in the writer's opinion, better than one of 45°, as the
chamfered bars will then have a wider bearing on the upper surface of
the plate, tending to hold it more securely down upon the lower part
of the chuck. Nothing remains but to file carefully down from the
line thus drawn to the lower edge, by no means a difficult operation
if care is exercised not to obliterate the mark, or to trespass the
least beyond the assigned limit. A template, cut like Fig. 236, of the
desired angle, will be a gauge for the edges of the plate, as well as
for those of the chamfered bars, and will serve to make assurance
doubly sure. The arms which stand out at the back of the slider to
embrace the guide ring are not fastened to the plate immovably, but
with power of adjustment. A pair of short slots are made in the
slider, into which a square projection from the arms fits, and the
whole is clamped by a screw, as shown in 237, A, B, and C.
[Illustration: FIG. 235.]
[Illustration: FIG. 236.]
[Illustration: FIG. 237.]
A more accurate method is shown in the Ornamental Section
for finer adjustment than can be secured in this way, but for a home
made chuck the above will suffice and is the easiest plan to carry
into effect. To use this chuck, the guide is first arranged, so that
its ring is concentric with the mandrel. A mark is generally made
upon it, and also upon the lathe-head, by which this position can be
readily insured. The chuck is then screwed upon the mandrel, and the
arms adjusted, as just described, so as to embrace accurately, but
not too tightly, the guide ring. They are then, once for all, fixed
in that position by the screws alluded to. A few drops of oil are
necessary to lubricate their inner surface and the exterior of the
guide, and the latter being withdrawn by its adjusting screws to the
desired eccentricity, the work may be proceeded with. A rough piece
of wood, however, should always be first turned to a cylindrical
form, as an oval chuck being an expensive article is to be carefully
preserved, and not exposed to the shocks inseparable from the process
of roughing down the work. Moreover, there should always be one or
two screws passing through the slider into the back plate, to take
away the strain from the chamfered bars, which can be removed when
the slider is to be brought into action. Two precautions are here
laid down respecting oval turning, which, in all probability, a
tyro would not suspect to be necessary until taught by failure. In
the first place, at whatever point of the circumference the tool is
held, at that point it must remain, or rather, it must remain in the
same horizontal line, being neither raised nor depressed. Hence, for
all work where accuracy is needed, oval turning should be done with
the slide rest. In the second place, when it is desired to place a
succession of ellipses one within the other on the face of the work,
like Fig. 229, it will not be sufficient to place the tool nearer to
the centre for each ring, but the eccentricity of the guide ring must
be reduced at the same time; otherwise, when the middle is reached, a
straight line will be the result, instead of the proposed ellipse, as
already stated. The lathe should not be driven at a very high speed,
and the moving parts should be lubricated from time to time. There are
other ways of compensating the error produced by the oval chuck, or
elliptic cutting frame, which however are so entirely connected with
ornamental turning that they are reserved to be introduced into that
section. A contrivance for turning ovals invented, and communicated to
the _English Mechanic_ by a Suffolk amateur, deserves a place here. It
is thus described by the inventor:--