A compound spherical slide-rest, for ornamental turning lathes, which
has been patented by Mr. Hoblyn, Rickling-green, Essex, is capable
of turning and ornamenting accurately spheres and any segments of
circles convex or concave, either on the surface or cylinder; also,
it is stated that by the addition of templates any other curves,
not segments of circles, can be turned out, in addition to the work
performed by an ordinary compound slide-rest. In our illustrations,
Fig. 1 is a side view, and Fig. 2, an elevation of the rest. A is
the lower, and B the top plate, C the carriage, and the saddle as
usual. The lower plate, of iron, is planed on bottom and sides,
and has a longitudinal bevelled slot in the bottom to receive the
fastening bolt, so that the plate can be tightened up at any point
on the lathe bed. At one end of this bottom plate is a piece raised
the whole of its breadth, accurately turned and faced, with a stout
turned pin, _b_, for a pivot in the centre. A wheel, _c_, is placed
around this raised piece, having any specified number of cogs, and on
its top any specified number of small holes drilled to receive two
pins as segment stops. The top plate is slightly narrower than the
bottom one, and has on its underside a corresponding surface to that
raised on the plate A, turned and faced with countersunk hole in the
centre to fit on the pivot _b_, on which it is tightened by a screw
and washer. The top of B is placed to receive two parallel bars of
brass, bevelled on the inner edge, so as to form sliding bars for the
carriage C, one bar being a fixture, the other capable of adjustment
by tightening screws. The sides are also accurately planed, and on
one a tangent screw, _e_, is so fixed as to be put in and out of gear
with the brass wheel, _c_, for the purpose of driving the top plate
round the pivot _b_. In the raised surface of B, is a countersunk
slot forming a quarter circle, with screw passing through it into the
bottom plate; by this means the top plate can be firmly fixed at any
angle to the bottom plate for the purpose of slide-rest turning. A
small pointer, _f_, works clear of the brass wheel in the capacity
of segment stop--two small pins being placed in the specified number
of holes in _c_. On the opposite side of the screw, _e_, on the top
surface, a screw, _g_, works in two standards screwed into the plate
through slots in the bar, which works through a traveller, _h_, firmly
attached to the carriage, C, and therefore capable of driving the
carriage in a rectilinear direction--for slide-rest purposes. This
carriage, C, consists of an iron plate _i_, with standard _k_, top
plate _l_, with parallel brass bars to receive tool receptacle _m_,
the bottom plate and standard being in one piece with the plate,
planed side and bottom, so as to slide truly between the parallel bar
on B. The standard has its surface accurately turned with a pin in
centre to form pivot. The top plate is planed true, bottom, sides,
and top, and has a countersunk hole in the exact centre, so as to
work on the pin, and tightens up with a large nut. In the plate is a
countersunk slot forming a quarter-circle, with screw passing through
into the standard, to set the tool receptacle either parallel with
B for spherical turning, at right angles to B for slide turning, or
at any angle for thick and thin cuts, and similar patterns when used
either as a spherical or slide rest. The parallel bars are to be
similar to those on B, to enable the tool receptacle to be advanced or
retired by screw or lever. This part of the machine can be adjusted
for height of centre thus:--The standard _k_ to be a hollow cylinder
with fixing screw on one side; the plate _i_ to be made with turned
pin on bottom, to fit into standard; elevating screw; saddle as usual,
but with the addition of a small hole drilled in one of the sides, and
a corresponding one in the side of the lower plate A, so placed that
when a pin is fixed into these holes the pivot is exactly central to
the lathe centre.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
The way of using the spherical rest is thus described by Mr.
Hoblyn:--"When altogether, and the point of tool adjusted by means of
a square exactly over centre of pivot, it is evident, if the top plate
moves round, the point of the tool will still be in the same place;
but if we retire the point one half-inch, on moving the plate right
round it would describe an inch circle, so that if the centre of pivot
be exactly under the centre of the lathe, and we move the rest the
half-circle, it will cut a perfect ball (or any part of one, if the
cut be less than half a circle) of such size as the distance doubled
of point of tool from centre of pivot. Therefore, by adjustment of
the lower plate on the lathe bed according to the size of material
(this, of course, does not allude to the act of turning a ball, when
the centre of pivot must be exactly under centre of lathe), adjustment
of carriage from centre according to the size of circle required, and
adjustment of tool for depth of cut, we are enabled to turn any convex
curve. To turn the concaves, instead of retiring from centre of pivot,
it is only requisite to advance the point beyond the centre to the
distance required, when the same rules apply as to the convex curves.
In turning the concaves, however, it is necessary to turn the plates
half-round, so that A and B are in the same straight line, instead of
over each other. This enables you to work on any sized piece of wood,
the object of the longitudinal slot in the lower plate being to enable
you to adjust your previously arranged curve to any sized wood or
ivory that your lathe will take, less 2 in., the height of machine.
The desired curve having been turned, take out the chisel, and place
in the receptacle any ornamenting instrument, drill, eccentric, &c.;
advance the point till it touches the work, then set your screw for
depth of cut, and work according to fancy. Whatever it is it must be
mathematically correct on the curve, as you have not altered it in
any way. You then remove your rest to cut the next desired curve, and
proceed as before. The best way to execute a piece of work is first to
make a sectional pattern of your design, either by drawing or cutting
it out with scissors from a double piece of paper, when, of course,
both sides come the same, the line where the paper is doubled being
the line of centre of lathe. You can then, with a pair of compasses,
ascertain with accuracy the necessary size of circle, and the position
of centre of pivot to procure the desired curves. You may also produce
a very good effect by a combination of two different curves in this
way:--Let your ornamental instrument be the universal cutter; working
horizontally follow up your curve with it, but, instead of cutting
right out on your curve, let the instrument finish of itself in wood
previously turned so as to fit its curve, when you get the lesser and
greater curves following in unbroken succession. With the eccentric
chuck numberless effects of the most curious description may be
produced, even work supposed to be only possible with a rose engine.
We believe the tool is to be seen at the shop of Mr. Evans, 104,
Wardour-street.
To turn a sphere by means of a template attached to the slide rest as
described, the following adjustment of the rest and mode of proceeding
should be followed. A, B, Fig. 225, is the chuck containing the
material, H, to be worked into a sphere. Upon this the length, or
diameter (equal to that of the template, as will be explained), is
to be marked at K, K, which being divided equally by the line L, L,
will give the dimensions of the ball as if it were about to be turned
by hand. The corners are to be turned off with the gouge, as far as
shown, K being equal to _e_ L, and K _f_ equal to K _e_. The outline
of the ball will with these measurements not be touched. The angles
left may also carefully be removed, but (as shown by the figure) this
operation must be conducted with great care. A template must now
be made containing a full semicircle, _every part_ of which can be
traversed by the stud or screw upon the under part of the slide, or
the ball will not be severed by the final cut. It is evident that the
traverse of the slide during the operation will be the full radius
of the ball, and in this, and indeed all cases of deep recesses, and
greatly projecting mouldings, the ordinary tool-holder with tailpiece
had better be removed, and replaced with a slide like M, having a pin
straight through it to rest in contact with the template. This will
preclude the necessity for the long stud or screw spoken of before
as necessary when the slide with the tailpiece is used, but the
tool cannot be advanced independently of the template as when the
other form is used. Fix the rest so that when the top slide is at its
central position the tool may stand as in the sketch exactly upon the
central line of the ball. Take care that thus placed the tangent-pin
of the slide is on the central mark of the template. The long frame
of the rest must likewise be parallel with the bed of the lathe,
keeping the top slide pressed against the template with the left hand,
while the top part traverses the frame under the action of the screw
moved by the right hand, the ball will be correctly cut.[16] One or
two cautions must, however, be given here, to ensure a satisfactory
result. In the first place the cylinder from which the ball is to
be made must be exactly of the diameter of the semicircle on the
template. H, H is the cylinder to be turned to a sphere, G, G, B shews
the position of the tool at starting, the dot on A, the templet, the
tangent pin of the slide, Fig. 226. As the work proceeds the tool will
take the several positions shown, the dotted lines, D, being equal and
parallel. The tool will thus repeat the form of the template. Let the
latter remain as before, but let a smaller cylinder be inserted in
the lathe, or, which is the same thing, let the tool be now lengthened
so as to start at C on the inner dotted line. When the pin, F, has
reached K, which should be the axis of the ball, the tool will be
at M, quite out of cut. Fig. 227 represents three forms of tool in
contact with the ball at two points. The first two will evidently be
out of cut at the axial line, as the side of these bevels will then
touch the piece to be turned. C is a form that will remain in contact
from the diameter to the axial line. The left side of the edge is
slightly overhanging the side line of the tool, D.
[16] See Appendix.
[Illustration: FIG. 225.]
[Illustration: FIG. 226.]
[Illustration: FIG. 227.]
When the part shown has been cut this tool must be removed and a
similar tool bevelled in the reverse direction, adjusted by reference
to the central line of the ball as before.[17] It is recommended to
roughly shape the work with the gouge, and partially to cut it off
with the parting tool so as to relieve the tool as much as possible,
and when the last finishing cut is to be taken a freshly sharpened
tool is to be made use of. It is evident that in the above and
similar work the rest may be placed across the end of the cylinder if
preferred to turn the _right_ hand hemisphere, but it would have to
be moved for the second half, which should be avoided, if possible.
The advantage which the circular rest has over the above is due to
the fact that the tool and rest once in position, neither has to be
readjusted until the work is complete. The slide rest and semicircular
template forms, however, if judiciously used, a very serviceable
substitute and makes very satisfactory work. Whether or no the reader
has a complete rest for spherical work, he should decidedly provide
templates to use as above. They are not only useful for turning or
ornamenting spheres, but any forms whatever that may be desired, and
they possess this special advantage, that when a dome or other pattern
has been thus turned with a plain tool the same template used with
revolving cutters will enable the work to be ornamented with perfect
ease, doing away in a great measure with the need of a dome chuck.
Suppose, again, that a number of pieces are desired precisely similar,
as a set of pawns for a set of chessmen, a sectional drawing made and
transferred to a piece of sheet-iron, and the latter cut to form a
pattern plate, will enable the most unskilful to work satisfactorily.
Nothing more need be said of the uses of templates, and for the
present the subject will be dismissed, though it may possibly be
referred to again in a future page.
[17] Holtzapffel uses a tool, the plan of which is semicircular, like
a small round tool, cutting on front and two side edges; the tool is
very narrow and bevelled below.