better to have a stream
of oil constantly playing upon the tool-point. This constant flow may be
obtained by mounting a can having a spout leading to the tool, on a
bracket at the rear of the carriage.

[Illustration: Fig. 7. Cutting Thread by using Compound Rest]

[Illustration: Fig. 8. (A) V-thread. (B) U. S. Standard Thread. (C)
Square Thread. (D) Left-hand Thread. (E) Double Square Thread. (F)
Triple Square Thread]

=Threads Commonly Used.=--Three forms of threads or screws which are in
common use are shown in Fig. 8; these are the V-thread (_A_), the U. S.
standard (_B_), and the square thread (_C_). The shapes of these threads
are shown by the sectioned parts. The V-thread has straight sides which
incline at an angle of 60 degrees with each other and at the same angle
with the axis of the screw. The U. S. standard thread is similar to the
V-thread except that the top of the thread and bottom of the groove is
left flat, as shown, and the width of these flats is made equal to 1/8
of the pitch. The square thread is square in section, the width _a_,
depth _b_ and space _c_ being all equal. All of these threads are
right-hand, which means that the grooves wind around to the right so
that a nut will have to be turned toward the right to enter it on the
thread. A left-hand thread winds in the other direction, as shown at
_D_, and a nut is screwed on by turning it to the left.

=Multiple Threads.=--Threads, in addition to being right-and
left-handed, are single, as at _A_, _B_, _C_ and _D_, double, as at _E_,
and triple, as at _F_, and for certain purposes quadruple threads or
those of a higher multiple are employed. A double thread is different
from a single thread in that it has two grooves, starting diametrically
opposite, whereas a triple thread has three grooves cut as shown at _F_.
The object of these multiple threads is to obtain an increase in lead
without weakening the screw. For example, the threads shown at _C_ and
_E_ have the same pitch _p_ but the lead _l_ of the double-threaded
screw is twice that of the one with a single thread so that a nut would
advance twice as far in one revolution, which is often a very desirable
feature. To obtain the same lead with a single thread, the pitch would
have to be double, thus giving a much coarser thread, which would weaken
the screw, unless its diameter were increased. (The lead is the distance
_l_ that one thread advances in a single turn, or the distance that a
n