jaws of a face-plate, on the head-stock spindle, the loose
poppet being omitted.
These, however, are broad types only, since proportions
of length to diameter differ, and with them lathe designs
are modified whenever there is a sufficient amount of work
of one class to justify the laying down of a special machine
or machines to deal with it. Then further, we have duplicate
designs, in which, for example, provision is made in
one lathe for turning two or three long shafts simultaneously,
or for turning and boring two wheels or tires at
once. Further, the position of the axis of a face lathe
need not be horizontal, as is necessary when the turning
of long pieces has to be done between centres. There are
obvious advantages in arranging it vertically, the principal
being that castings and forgings can be more easily
set and secured to a horizontal chuck than to one the face
of which lies vertically. The chuck is also better supported,
and higher rates of tuming are practicable. In
recent years these vertical lathes or vertical turning and
boring mills (fig. 30) have been greatly increasing in numbers;
they also occur in several designs to suit either
general or special duties, some of them being used for
boring only, as ehucktng lathes. Some are o immense
size, capable of boring the field magnets of electric
generators 40 ft. in diameter.
Standard Lathes.-But for doing what is termed
the general work of the engineer's turnery, the standard
lathes (fig. 29) predominate, i.e. self-acting, sliding
and surfacing lathes with headstock, loose poppet and
slide-rest, centres, face plates and chucks, and an equipment
by which long pieces are turned, either between
centres or on the face chucks, and bored. One of
the greatest objections to the employment of these
standard types of lathes for indiscriminate dut is due
to the limited height of the centres or axis of the headstock,
above the face of the bed. This is met generally
by providing a gap or deep recess in the bed next
the fast headstock, deep enough to take face work of
large diameter. The device is very old and very common,
but when the volume of work warrants the employment
of separate lathes for face-work and for that done
between centres it is better to have them.
Screw-cutting.-A most important section of the work
of the engineer's turnery is that of cutting screws (see
SCREW). This has resulted in differentiation fully as
great as that existing between centres and face-work.
The slide-rest was designed with this object, though
it is also used for plain turning. The standard “ self acting
sliding, surfacing and screw-cutting lathe " is
essentially the standard turning lathe, with the addition
of the screw-cutting mechanism. This includes a
master screw-the lead or guide screw, which is
gripped with a clasp nut, fastened to the travelling
carriage of the slide-rest. The lead-screw is connected
to the headstock spindle by change wheels, which are
the variables through which the relative rates of movement
of the spindle and the lead-screw, and therefore
of the screw-cutting tool, held and traversed in the
slide-rest, are effected. By this beautiful piece of
mechanism a guide screw, the pitch of which is permanent,
is made to cut screw-threads of an almost
infinite number of possible pitches, both in whole and
fractional numbers, by virtue of rearrangements of
the variables, the change wheel s. The objection to
this method is that the trains of change wheels have
to be recalculated and rearranged as often as a screw
of a different pitch has to be cut, an operation which
takes some little time; To avoid this, the nest or
cluster system of gears has been largely adopted, its
most successful embodiment being in the Hendey-Norton
lathe. Here all the change wheels are arranged
in a series permanently on one shaft underneath the
headstock, and any one of them is put into engagement
b a sliding inion operated by the simple movement
of, a lever. 'ighus the lead-screw is driven at different
rates without removing any wheel from its spindle.
This has been extensively applied to both small and
large lathes. But a moment's thought will show that
even this device is too cumbrous when large numbers of
small screws are required. There is, for example, little
in common between the screw, say of 5 or 6 ft. in
length, for a massive penstock or valve, and é-in. bolts,
or the small screws required in thousands for electrical
fittings. Clearly while the self-acting screw-cutting
lathe is the best possible machine to use for the first,
it is unsuitable for the last. So here at once, from the
point of view of screw cutting only, an important divergence
takes place, and one which has ultimately led
to very high specialization.
Small Screws.—When small screws and bolts are cut in
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(Webster & Bennett, Ltd. Coventry.) Lathe. ap Fig. 29.-8-in. Centre G
Driving Back-gears. Change wheels. Swing plate Hand leve and surf
abinets. COHCS.