ope and Clearance change as Tool is raised or lowered]

=Angle of Tool-point and Amount of Top Slope.=--The lip angle or the
angle of keenness [delta] (Fig. 10) is another important consideration
in connection with tool grinding, for it is upon this angle that the
efficiency of the tool largely depends. By referring to the illustration
it will be seen that this angle is governed by the clearance and the
slope [beta], and as the clearance remains practically the same, it is
the slope which is varied to meet different conditions. Now, the amount
of slope a tool should have depends on the work for which it is
intended. If, for example, a turning tool is to be used for roughing
medium or soft steel, it should have a back slope of about eight degrees
and a side slope ranging from fourteen to twenty degrees, while a tool
for cutting very hard steel should have a back slope of about five
degrees and a side slope of nine degrees.

[Illustration: Fig. 13. (A) Blunt Tool for Turning Hard Steel. (B)
Tool-point Ground to give Keenness]

The reason for decreasing the slope and thus increasing the lip angle
for harder metals is to give the necessary increased strength to the
cutting edge to prevent it from crumbling under the pressure of the cut.
The tool illustrated at _A_, Fig. 13, is much stronger than it would be
if ground as shown at _B_, as the former is more blunt. If a tool ground
as at _A_, however, were used for cutting very soft steel, there would
be a greater chip pressure on the top and, consequently, a greater
resistance to cutting, than if a keener tool had been employed;
furthermore the cutting speed would have to be lower, which is of even
greater importance than the chip pressure; therefore, the lip angle, as
a general rule, should be as small as possible without weakening the
tool so that it cannot do the required work. In order to secure a strong
and well-supported cutting edge, tools used for turning very hard metal,
such as chilled rolls, etc., are ground with practically no slope and
with very little clearance. Brass tools, while given considerable
clearance, as previously stated, are ground flat on top or without
slope; this is not done, however, to give strength to the cutting edge,
but rather to prevent the tool from gouging into the work, which it is
likely to do if the part being turned is at all flexible and the tool
has top slope.

Experiments conducted by Mr. F. W. Taylor to determine the most
effic