Among the various uses to which a lathe may be put, wheel-cutting
is one of the most important, so many pieces of mechanism requiring
cogged wheels of various pitches and forms of tooth. By the aid of
the slide rest such an apparatus as figured may be readily arranged,
and the work rapidly and accurately performed. The guide by which the
cogs and spaces are determined is the division plate already alluded
to, and which is not visible in the present drawing, but the index of
which is shown at G. C is the cutter frame with a side pulley (one on
each side) to conduct the catgut band from the revolving spindle to
the overhead motion on to the flywheel. The spindle carries a pulley
for the cord and a cutter wheel, I, to which an exceedingly rapid
evolution is given. There are many patterns of these wheels, the edges
of which, cut into teeth like a fine saw or file, are the exact form
of the spaces required between the cogs; hence, some are rectangular,
some have a triangular section. The thicker the wheel to be cut the
larger should be the cutter, so that the bottom of the cut may be
virtually level. In Fig. 181 another form of cutter is shown, which
if put into sufficiently rapid motion answers as well, if not better,
than the wheel-shaped cutters. It is a simple short bar of hard steel,
with the edges bevelled in alternate directions, fitted into a slot
in the spindle and held by a wedge or screw. The shape of the end is
as before, a cross section of the space between two teeth. The cutter
frame is here arranged to fit into the ordinary tool-holder of the
slide rest, but the form may, of course, be varied at pleasure. It
will be noticed that the slide rest, as delineated here, is different
to that of which details have been given. It is made without the sole,
and fits into the socket of an ordinary rest. Thus it can be turned
on a centre, and becomes, to all intents and purposes, a compound
slide rest. It is on this plan the small ornamental turning rest of
lighter construction is made. The spindle fitting the socket projects
from the centre of the lowest frame, and is cast in one piece with it.
If the apparatus is compactly and strongly made, it becomes a very
serviceable form, and is much used for the small lathes in sea-going
steamers. The sides are made very short, so that the extent of
traverse is small. We may here mention an addition to the rest socket,
enabling the workman to raise this kind, or that used with drills and
cutters, which is simple and convenient. Inside the iron socket a few
turns of a screw are cut, and a second socket of brass with an outside
thread is made to fit into it Figs. 182 A and B, the latter being a
section. The edge of the inside socket is sometimes milled round, to
facilitate holding it by the thumb and finger. In this way the height
of the slide rest, or tee of the common rest, is adjustable to a great
nicety.
[Illustration: FIG. 180.]
[Illustration: FIG. 181.]
[Illustration: FIG. 182.]
When a wheel is to be cut of large size, or of substance exceeding
that of clock-wheel work, the above method is not suitable. The wheel
is then generally laid flat, and the cogs are shaped by a slotting
machine, the chisel of which has a vertical motion. The lathe is
then no longer used; as a separate machine is more convenient and
economical.
A most serviceable addition to a lathe, especially an amateur's foot
lathe, is the circular saw, with guides for cutting parallel, taper,
or mitred work. Great rapidity of work is thus combined with perfect
accuracy. A five-inch lathe will of course take a saw nearly ten
inches diameter, but it is not advisable to attach one of quite this
size, for the larger the saw the greater is the leverage against which
the turner has to contend, and the friction caused by a deep cut in
stuff of two inches diameter is quite sufficient to make the labour
considerable. When such work is necessary, it must be very gently
brought to bear upon the saw, and the flywheel of the lathe should
be heavy. The cord should also pass from the latter to the slowest
division of the pulley. If the workman, amateur or professional,
desires a lesson in practical mechanics, he has nothing to do but turn
a piece of ash six inches in diameter, with the lathe-cord extending
from the flywheel to the smallest part of the pulley, the diameter of
which is about half that of the object to be turned. This will teach
him what hard work is. Then let him try the job with the cord, from
the smallest part of the flywheel rim, to the largest diameter of
pulley. The change to a slower motion and greater power will not be
disagreeable. It must be remembered that a circular saw of six inches
diameter will not penetrate three-inch stuff, owing to the boss or
nut by which it is attached to its spindle. The above size will not
make good work of stuff exceeding two inches in thickness, and even
less thickness would be preferable. As to the size of saw, indeed,
that is most suitable to a five or six-inch foot lathe, much depends
upon the proposed work, and still more upon the weight and size of
the flywheel. As a general rule it is better to err upon the size
of smallness. The service to which this tool is commonly put is but
light; sawing narrow strips of mahogany, such as used at the angles
of bird-cages, cutting strips or segments of ivory (for which let the
saw be kept wet) sawing out mitred or dovetailed joints, and similar
work is within the compass of a five or six inch saw, and it is better
not to exceed this. The teeth should be tolerably fine for hardwood
and ivory, and coarser for deal and soft woods. Smaller saws of hard
steel, and made of thick plate, are used for metal.
The method of mounting saws of small size, such as are suited to be
worked by the treadle of a foot-lathe, is shown in Fig. 183. E is
a steel spindle, of which the diameter equals that of the central
hole in the saw B. At F, about the middle of the spindle, is a fixed
flange, at the base of which is a short feather or inlaid key, X 184,
which fits the small slot seen in the centre of the saw and reaches
also within a similar slot in the movable flange, G, but it must not
be so long as to come through to the back of the latter. This flange
and nut H having been removed, the saw is slid upon the spindle till
it rests against the fixed flange; the movable one is now to be
brought against it and clamped by the nut. The spindle is sometimes
made with a square end to fit the square hole chuck, and centred at
the other, or it is drilled at both ends, so as to be driven by the
carrier or driver chuck, C, D. It must be so placed as to run towards
the operator. The above arrangement must now be made complete by
the addition of a platform, B, on which to lay the work that is to
be sawn, and on which some contrivance can be adjusted to guide the
passage of the saw through the same, so as to cut the work in parallel
pieces, or at any desired angle, such as would be necessary for mitred
joints. For the general uses of the amateur a mahogany or hardwood
platform is as good as any, and such as is delineated in vol. ii.
of Holtzapffel's valuable work is perhaps the best arrangement. The
saw table rests on the opposite ends of a kind of open box, which
is represented without the two sides, although they may be added if
desired, and the whole when removed from the lathe would then form a
case for the saws, or serve other similar purpose. The platform is
hinged, so as to overlap, as seen in the Fig. 184A, and there
is in the middle of it a slit cut by the saw itself, which, when it
is mounted on its centres will be in the position shown. If sides are
added a notch must be made in the upper edge of both for the passage
of the spindle. The fillet B fits between the bearers of the lathe,
securing the parallelism of the whole. When made with four sides the
box must first be placed on the lathe bed and loosely held by the bolt
beneath. The saw is then mounted, and the box adjusted to its place
and fixed. The cover is then (if for the first time) brought carefully
down upon the saw, and the lathe being put in motion the slit is made,
and its position will be truly at right angles with the spindle and
the lathe bed. Of course, in future operations the platform is lowered
over the saw before the holding down bolt is permanently screwed
up. The sides of the saw-kerf may be edged with brass if preferred,
but on the whole the plan is not to be recommended, for if, as will
occasionally happen, the saw should get slightly out of truth, or
vibrate a little when in use, the teeth will come into contact with
the metal and be blunted or broken. If the saw-kerf by constant wear
should widen too much, the whole platform is renewable at little
cost, or a new piece can be let in, and a fresh saw-kerf made. There
are several guides for parallel work. The one shown in Fig. 185 is
precisely such as is to be seen in the ordinary parallel ruler--A is
the back bar screwed to the platform at the right-hand edge; B the
guide or fence which, when the connecting links C, C are perpendicular
to A, should touch the saw; D, D are arcs of circles of which E, E
are the centres. They may be arcs of brass pivoted to the links and
passing through a slot in the bar, A, A, or may themselves be cut as
mortises in the platform, in which fit a pair of bolts with thumb-nuts
passing through the links, by which to clamp the fence in any desired
position. This form of parallel guide is not very substantial, and is
not correct in practice unless the pins are very nicely fitted, and
the links precisely of a length. The second figure shows the sectional
form of the fence G, the links being represented at H, and the fixed
bar at K. The following is a more solid and unyielding guide, and
much to be preferred. Holtzapffel attributes it to Professor Willis.
It is merely a modification of the T-square as used with the ordinary
drawing board with an arrangement for fixing it in the required
position. The present arrangement differs from that in Holtzapffel's
work in the manner of fixing it and the addition of a second T-piece
on the side next the workman. In Fig. 186 A is the upright part of
the fence, B the bottom or sole, to which is attached at each end
the T or cross pieces, E, E, which slide along the straight edges of
the platform and secure the constant parallelism of the fence to the
surface of the saw; C is the groove or slot, in which a screw, D,
traverses, and the fence is thus fixed by a turn of the thumb-nut,
D. This fence can by no possibility get out of truth; it is easily
removed by taking out the single screw, and it is far more simple and
more easily made than the one previously described. The nearest edge
of the platform may be marked in inches and eighths, and the fence
can then be instantaneously adjusted for sawing pieces of any desired
width. It is not always, however, that straight, rectangular, or
parallel strips are required, and an additional arrangement is needed
to form a guide for sawing angular pieces. Now it is not sufficient to
lay the guide fence at a given angle, for if the latter were arranged
for that adjustment by taking off the tees and causing it to turn
upon the screw which secures it in place, and a piece of board were
placed against it to be sawn, the latter would press against the saw
sideways as seen in Fig. 187. The guide fence for angles must itself
therefore travel in a line parallel to the saw and carry with it the
piece to be cut. The simplest and usual arrangement is that given
in Holtzapffel's work. A dovetailed groove in the platform running
in a direction parallel with the saw carries a sector attached to a
bar which fits the groove, and this bar is free to move forward or
backward without lateral movement. The piece to be sawn is thus rested
against the fence forming the straight face of the sector, and the
whole is moved forward together against the edge of the saw. Fig. 188
explains this. It will be seen that several grooves are made side
by side, all of which fit the slide alike, and by moving the latter
into either of these a lateral adjustment is effected to suit pieces
of different widths. The sliding strip should be made of hard wood
and nicely fitted, and may be lubricated with soap, or polished with
black lead, either of which will cause it to slide with diminished
friction. Fig. 189 shows a somewhat different arrangement, by which
more lateral adjustment may be given. The sector is replaced by a
T-square, the blade of which has a slot through which a screw passes
into the sliding bar. A second jointed rod is added, passing through
a staple in the slide, and by a screw in the latter the T is fixed as
required. The staple must turn on a centre to accommodate itself to
all positions of the T-square, and a second eye may be placed on the
opposite arm to allow the guide rod to be removed to that side, which
is sometimes more convenient. The sector also may be made adjustable
as in Fig. 190, and clamped by a simple screw in the slot. Either of
the above methods will allow sufficient range for small work, such as
is likely to be the object of amateurs or those who add small saws to
the foot lathe. The guide for angular pieces may indeed be in many
cases dispensed with by making use of patterns of wood chiefly in
the form of triangles; these sliding against the parallel guide, and
carrying the work with them, will answer well in a number of cases
where other provision for such work has not been made in arranging
the saw table. It must be remembered, however, that in this case the
length of the piece (supposing it to be the side of a picture frame
to be mitred) is limited to the space between the fence and the saw,
which in the more perfect arrangement is an evil plainly avoided.
It may sometimes be necessary to cut pieces with various angles not
in the same plane. By using the hinged platform, and adding a screw
attached to the front of the box, and standing perpendicularly, the
front of the table can be raised to a given angle, but those who are
likely to enter extensively into the cutting mathematical figures
are referred to vol. ii. of Holtzapffel's work, where the subject is
fully explained and illustrated. The following remarks upon the proper
speed of saws and sizes of teeth are copied from that work, and may
therefore be relied on:--
"The harder the wood the smaller and more upright should be the
teeth, and the less the velocity of the saw.
"In cutting with the grain the teeth should be coarse and
inclined, so as rather to remove shreds than sawdust.
"In cutting across the grain the teeth should be finer and more
upright, and the velocity greater, so that each fibre may be cut
by the passage of some few of the consecutive teeth rather than
be torn asunder by the passage of one tooth only.
"For gummy or resinous woods and ivory, the saw teeth must
be keen, and the speed comparatively slow, to avoid the
dust becoming adhesive (by reason of the heat engendered by
friction), and thus sticking to, and impeding the action of the
saw."
[Illustration: FIG. 183.]
[Illustration: FIG. 184.]
[Illustration: FIG. 184.]
[Illustration: FIG. 185.]
[Illustration: FIG. 184A.]
[Illustration: FIG. 186.]
[Illustration: FIG. 187.]
[Illustration: FIG. 188.]
[Illustration: FIG. 189.]
[Illustration: FIG. 190.]
By raising the platform so as only to expose a small portion of the
saw, it is easy to cut rebates, grooves for tongueing, and other work
of a similar kind. The above arrangement of hinged table facilitates
this application of the saw.