Machinery Repairman

Figure 5-13.—Surface finish vs nose radius chart.
NOSE RADIUS.—Selecting the nose radius can be
important because of tool strength, surface finish, or
perhaps the need to form a fillet or radius on the work.
To determine the nose radius according to strength
requirements, use the chart shown in figure 5-13.
Consider that the feed rate, depth of cut, and workpiece
condition determine strength requirements.
Large radii are strongest and can produce the best
finishes, but they also can cause chatter between tool
and workpiece. For example, the dashed line on the
chart shows that a 1/8-inch radius would be required
Figure 5-14.—Indexable inserts.
5-11
Figure 5-15.—Side view of rake angles.
for turning at a feed rate of 0.015 inch to obtain a
100 microinch finish. A 1/4-inch radius would be
required with a 0.020-inch feed rate.
INSERT SHAPES.—Indexable inserts (fig. 5-14)
are clamped in toolholders of various designs. Each
of these inserts has several cutting edges. After you
have used all of the edges, discard the insert.
Round inserts have the greatest strength and, like
large radius inserts, they offer higher feed rates with
equal finishes.
Square inserts have lower strength and fewer
possible cutting edges than round tools, but they are
much stronger than triangular inserts
Triangular inserts have the greatest versatility.
For example, you can use them in combination
turning and facing operations where round or square
inserts are often not adaptable to such combinations.
Because the included angle between cutting edges is
less than 90°, you also can use triangular inserts for
tracing operations. The main disadvantages are their
reduced strength and fewer cutting edges per insert.
RAKE ANGLE.—When selecting rake angles
(fig. 5-15), you need to consider the machining
conditions. Use negative rake where there is maximum
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