The slide rest previously described, although applicable to the
purposes of ornamental turning, has one disadvantage. It is necessary
that the various pieces of apparatus to be used with it should have a
foundation plate with chamfered edges to fit accurately between the
guide bars. This is often inconvenient, and adds to the difficulty of
making, and consequently to the cost of such pieces. In addition to
this drawback, it may happen that one of these fittings by being more
frequently used becomes more worn than another, so that the guide bars
require constant re-adjustment, and their accuracy and parallelism
become impaired. To obviate these and similar inconveniences the
slide rest is now commonly made like Fig. 277, and a tool receptacle,
Fig. 278, is fitted to slide between M, M, and is so arranged as to
hold securely the universal cutters and other apparatus required
for ornamentation or for plain turning. These are all made with a
rectangular bar fitting the longitudinal channel in the middle of the
receptacle, and are secured by the following simple contrivance. It
will be seen by the drawing that the central channel is widened at
A, A, and that a groove or saw-cut B runs along the inside from end to
end. This groove is continued in a similar manner on the side next
to the reader. Fig. 280 represents an ordinary tool holder, with a
rectangular shank A, and clamping screw B, by which the tool _c_ is
secured. The part A is laid in the central channel, and a small piece
of metal shaped like Fig. 279 is inserted in one of the open spaces,
A, A of the receptacle and slid along with its lower flange in the
saw-cut until clear of the enlarged part of the channel. It is thus
retained, and the clamping screw which passes through its centre is
brought to bear upon the piece to be fixed, which is thereby securely
held in its required position. Two of these holdfasts are generally
used at the same time. If the main bar of the tool holder is not
quite thick enough to be clamped, then it is only necessary to lay a
small plate below it. By the above simple means, the necessity for
fitting each individual piece of apparatus to work upon the chamfered
guides is done away. In order to ensure the position of the sole of
the rest at right angles to the lathe bed a kind of saddle, A, Fig.
281, is used. This is of cast iron or brass, accurately planed on
the upper surface, and has a projection fitting between the bearers
of the lathe. The usual holding down-bolt passes through the hole in
the centre, securing the saddle and the rest at the same time. The
usual arrangement of a kind of double socket, the inner one rising at
pleasure by being tapped into the outer, has already been described,
and serves for accurate adjustment of the height of the rest. It is
convenient, in addition, to have a stop or set screw under the bed
of the rest, and a similar one on the top of the socket, so placed
that when the frame is swung round it shall stop precisely at right
angles to its former position. Thus, if the tool is first required
to be used upon the side, and then upon the face of the object to
be turned, these two positions are obtained at once, and can, if
necessary, be alternated without any re-adjustment of the moving
parts of the rest by the aid of the set square. The receptacle-holder
is generally advanced by the hand lever, Fig. 279, one pin of which
fits into the hole in the guide-bar as seen in the drawing, while
the other falls into a short slot _e_, made in the upper surface of
the receptacle, or of the piece of apparatus to be used in it. Of
course, this arrangement may be reversed, one or both pins being
fixed to the rest and its receptacle slide, and the holes made in
the lever. Sometimes, however, a slower and more regular movement is
required than it is possible to give in this way, and the lever is
replaced by the leading screw C, D, Fig. 278, the head of which is
removable, and can be replaced by a small winch handle. This screw
is tapped into the lug cast upon the receptacle, and its point is of
the form shown. The latter fits into a hole in the pillar A, 278, and
is retained by a pin, which falls into the groove, D, Fig. 278, and
prevents the screw from advancing or receding without carrying the
sliding plate with it. The pin being removed, the screw will no longer
act in this way, and the slide may be moved by the lever instead.
The other screw, E, F, of fine pitch, serves to regulate the advance
of the receptacle, and consequently the depth of cut of the tool--a
round head with divisions on its edge is attached to one end, which
abuts against the pillar B, Fig. 277, which latter has a mark on its
top to act as an index. Thus the advance of the tool can be regulated
to a great nicety, and successive predetermined and different depths
may be reached and repeated at pleasure, as is sometimes necessary.
C, C, Fig. 277, is one of a pair of stops which can be fixed by their
screws at any two points of the bed of the slide rest. These serve
to regulate the distance which the top slide and tool holder are
intended to traverse, as in drilling a number of flutes of equal
length, and many similar works. They are usually made of gun-metal,
the screws of iron or steel, or of a metal called homogeneous, which
may be described as between the two, and, being pleasant to work, is
worthy of notice. It is absolutely necessary that the slide rest for
ornamentation should be made with the greatest nicety. The slides must
work equally smoothly from end to end of their traverse. The pitch of
the screws must be not only fine, but even and regular, and the screw
itself of precisely the same diameter from end to end, else it will
work loosely through its nut in one place, and jamb in another. It is
extremely pleasant to feel the exquisite smoothness and oiliness, for
no other word will express it, of the movements of sliding parts in
the workmanship of Munro or Holtzapffel, especially if compared with
inferior work. _Good_ amateur's work indeed is often far superior to
that which is sometimes advertised, and perhaps a few hints may not
be out of place here, relative to the construction of this necessary
addition to the lathe.
[Illustration: FIG. 277.]
[Illustration: FIG. 279.]
[Illustration: FIG. 280.]
[Illustration: FIG. 278.]
[Illustration: FIG. 281.]
[Illustration: FIG. 282.]
First of all, the frame of the rest must be accurately at right angles
to the spindle, which fits into the socket. These should, therefore,
be turned together, supposing the amateur not to have a planing
machine. The whole may be mounted as Fig. 282, where A represents the
carrier plate or chuck; B, the driver, the tail of which should be as
long or nearly so as the frame from _c_ to D; F is the side tool to
be fixed in the slide rest for metal. The effect of this arrangement
is to plane the face of the slide with transverse strokes instead of
lengthwise. It may be afterwards finished and polished with oilstone
powder on a flat slab of planed iron. When the face is finished,
the whole must be reversed, the pin of the carrier plate will bear
against the frame, which thus acts as a driver, and the spindle must
be turned. In this way accuracy is ensured if the slide rest used is
carefully set. The chamfered sides of the slides are difficult to work
with the file, but may be so done with care, and with a template of
the desired bevel as a guide. The great secret is to take plenty of
time, not to press too much upon the file nor to move it too quickly
over the surface; fine even strokes, especially towards the finish,
must be given, and a final polish with oilstone powder and oil used
on a piece of a stick. In turning the screw a back stay must be fixed
opposite to the tool in the slide rest to insure the contact of the
cutting edge without bending the work.
Presuming that the screw will be cut with stock and dies, it may be
stated as a caution that the latter must not be tightened except at
the commencement of cutting the thread deeper. The return of the tool
by a backward motion (or unscrewing), should not be used as a cutting
action, and therefore, should be carried on with the dies in the same
position which they had during their descent.
At the beginning, therefore, of each downward movement the dies must
be tightened and oiled, and they must not again be touched till the
bottom of the screw has been reached, and the upward movement also has
been completed, so that they have arrived again at the starting point.
If tightened at any other time the screw will be either conical or of
a wavy section, either of which forms would be fatal to its use. The
castings for such a rest should be of malleable iron, if possible, as
being much more easy to work; the guide bars may be of gun metal, as
also the chamfered bars, which work on the main frame. This will give
a more finished appearance, and will on the whole be more durable and
satisfactory.