plained
in Chapter IV, which contains descriptions of different thread forms and
methods of cutting them. Internal thread tools are shown at _K_ and _L_
for cutting U. S. standard and square threads in holes. It will be seen
that these tools are somewhat like boring tools excepting the ends which
are shaped to correspond with the thread which they are intended to cut.

[Illustration: Fig. 3. Turning Tool with Inserted Cutter]

A tool for turning brass is shown at _M_. Brass tools intended for
general work are drawn out quite thin and they are given a narrow
rounded point. The top of the brass tool is usually ground flat or
without slope as otherwise it tends to gouge into the work, especially
if the latter is at all flexible. The end of a brass tool is sometimes
ground with a straight cutting edge for turning large rigid work, such
as brass pump linings, etc., so that a coarse feed can be used without
leaving a rough surface. The tools at _N_ and _O_ are for boring or
finishing drilled or cored holes. Two sizes are shown, which are
intended for small and large holes, respectively.

The different tools referred to in the foregoing might be called the
standard types because they are the ones generally used, and as Fig. 2
indicates, they make it possible to turn an almost endless variety of
forms. Occasionally some special form of tool is needed for doing odd
jobs, having, perhaps, an end bent differently or a cutting edge shaped
to some particular form. Tools of the latter type, which are known as
"form tools," are sometimes used for finishing surfaces that are either
convex, concave, or irregular in shape. The cutting edges of these tools
are carefully filed or ground to the required shape, and the form given
the tool is reproduced in the part turned. Ornamental or other irregular
surfaces can be finished very neatly by the use of such tools. It is
very difficult, of course, to turn convex or concave surfaces with a
regular tool; in fact, it would not be possible to form a true spherical
surface, for instance, without special equipment, because the tool could
not be moved along a true curve by simply using the longitudinal and
cross feeds. Form tools should be sharpened by grinding entirely on the
top surface, as any grinding on the end or flank would alter the shape
of the tool.

[Illustration: Fig. 4. Heavy Inserted-cutter Turning Tool]

=Tool-holders with Inserted Cutters.=--All of the tools shown in Fig. 1
are f