Page 664 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 23.5
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too large an angle can cause chipping or E,
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breaking of the cutting edge. In addition to 2° E E
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conventional point drills, several other O) g 'E
drill-point geometries have been developed 9 E Y; ‘g g .2
to improve drill performance and increase 'E 8 53 8 8 gg 8 High-pressure
the penetration rate. Special grinding tech- coolant
niques and equipment are used to produce
these geometries.
Other Types of Drills. Several types of
drills are shown in Fig. 23.20. A step drill
produces holes with two or more different
diameters. A core drill is used to make an
existing hole larger. Counterboring and
FIGURE 23.20 Various types of drills and drilling and reaming operations.
countersinking drills produce depressions
on the surface to accommodate the heads
of screws and bolts below the workpiece surface. A center drill is short and is used
to produce a hole at the end of a piece of stock, so that it may be mounted between
centers of the headstock and the tailstock of a lathe (Fig. 23.2). A spot drill is used
to spot (to start) a hole at the desired location on a surface.
Spade drills (Fig. 23.21a) have removable tips or bits and are used to produce
large-diameter and deep holes. These drills have the advantages of higher stiffness
(because of the absence of flutes in the body of the drill), ease of grinding the cutting
edges, and lower cost. A similar drill is the straight-flute drill (Fig. 23.21b).
Solid carbide and carbide-tipped drills (Figs. 23.21c and d) are available for
drilling hard materials (such as cast irons), high-temperature metals, abrasive mate-
rials (such as concrete and brick-masonry drills), and composite materials with
abrasive fiber reinforcements (such as glass and graphite).
Camei
Gun Drilling. Developed originally for drilling gun barrels, gun drilling is used for
drilling deep holes and requires a special drill (Fig. 23.22). The depth-to-diameter

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FIGURE 23.2I Various types of drills. (a) Spade drill; (b) Straight-flute drill; (c) Drill with
indexable carbide inserts; (d) drill with brazed-carbide tip.

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