inciples
governing their shape which apply generally; so in what follows we shall
not attempt to explain in detail just what the form of each tool used on
the lathe should be, as it is more important to understand how the
cutting action of the tool and its efficiency is affected when it is
improperly ground. When the principle is understood, the grinding of
tools of various types and shapes is comparatively easy.

[Illustration: Fig. 11. Plan View of Lathe Turning and Threading Tools]

=Shape or Contour of Cutting Edge.=--In the first place we shall
consider the shape or contour of the cutting edge of the tool as viewed
from the top, and then take up the question of clearance and slope, the
different elements being considered separately to avoid confusion. The
contour of the cutting edge depends primarily upon the purpose for which
the tool is intended. For example, the tool _A_, in Fig. 11, where a
plan view of a number of different lathe tools is shown, has a very
different shape from that of, say, tool _D_, as the first tool is used
for rough turning, while tool _D_ is intended for cutting grooves or
severing a turned part. Similarly, tool _E_ is V-shaped because it is
used for cutting V-threads. Tools _A_, _B_ and _C_, however, are regular
turning tools; that is, they are all intended for turning plain
cylindrical surfaces, but the contour of the cutting edges varies
considerably, as shown. In this case it is the characteristics of the
work and the cut that are the factors which determine the shape. To
illustrate, tool _A_ is of a shape suitable for rough-turning large and
rigid work, while tool _B_ is adapted for smaller and more flexible
parts. The first tool is well shaped for roughing because experiments
have shown that a cutting edge of a large radius is capable of higher
cutting speed than could be used with a tool like _B_, which has a
smaller point. This increase in the cutting speed is due to the fact
that the tool _A_ removes a thinner chip for a given feed than tool _B_;
therefore, the speed may be increased without injuring the cutting edge
to the same extent. If, however, tool _A_ were to be used for turning a
long and flexible part, chattering might result; consequently, a tool
_B_ having a point with a smaller radius would be preferable, if not
absolutely necessary.

The character of the work also affects the shape of tools. The tool
shown at _C_ is used for taking light finishing cuts with a wide f