Countersinks (Figure 6-21) are special angled cutters used to countersink holes for flathead screws so they are flush with the surface when mounted. The most common countersinks are cone shaped with angles of 82°. Cone angles of 60°, 90°,100°,110°, and 120° are for special needs.
Counterbores (Figure 6-21) are special cutters that use a pilot to guide the cutting action to enlarge a portion of a hole. Common uses are for enlarging a hole to make a bolt head fit flush with the surface.
COMBINED COUNTERSINK AND CENTER
This special drilling tool (Figure 6-21) is used to start holes accurately. These tools are mainly used to center drill and countersink the end of round stock in a lathe machine.
Reamers (Figure 6-21) are cutting tools that are used to enlarge a drilled hole by a few thousandths of an inch for a precise fit.
Boring tools (Figure 6-21) are not usually considered with drilling, but they can be used to bore a hole using the power-feed drilling machines. These tools consist of an arbor with a tool bit attached that cuts a preset sized hole according to the distance that the tool bit protrudes from the arbor.
FIELD EXPEDIENT CUTTERS
Under battlefield conditions, the exact tools may not be available for each job. Simple flat drills can be made quickly from a high-speed steel lathe tool bit or a drill blank. If a grinder is available, then a crude drill can be ground that has a point and two flat edges, which could produce a hole if enough pressure is applied and the workpiece is machinable.
Figure 6-21. Other types of cutters.
TAP AND DIE WORK
Hand tapping and hand die work can be done on a drilling machine. The drill chuck is used to align the tap or die.
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