Tc 9-524 fundamentals of machine tools

4-6

---

TC 9-524
SPECIAL DRILLS
Special drills are needed for some applications that a normal
high-speed industrial operations. Other types of special drills
general purpose drill cannot accomplish quickly or accurately.
are: left hand drill, Silver and Deming, spotting, slow spiral,
Special drills can be twist drill type, straight fluted type, or
fast spiral, half round, die, flat, and core drills. The general
special fluted. Special drills can be known by the job that they
purpose high-speed drill, which is the common twist drill used
are designed for, such as aircraft length drills, which have an
for most field and maintenance shops, can be reground and
extended shank. Special drills are usually used in.
adapted for most special drilling needs.
SHARPENING TWIST DRILLS
Twist drills become dull and must be resharpened. The
preferred method of resharpening a twist drill is with the drill
grinding machine, but this machine is not always available in
field and maintenance units, so the offhand method of drill
sharpening must be used (Figure 4-10). The off hand method
requires that the operator have a knowledge of the drilling
geometry (Figure 4-11) and how to change drill angles as
needed for any drilling job (see Table 4-2 in Appendix A).
Tools needed are a utility or bench grinder with a dressed
wheel and a drill point gage (Figure 4-12) or protractor head
on the combination square. The drill point gage is set at 59°
and adjusted along the steel rule to fit the drill to be
sharpened. The cutting lips must be of the same angle, the lip
clearance angle must be within a specific degree range, and
the cutting lips must be of an equal length. There are several
basic characteristics that all twist drills must have to cut
properly. The following will cover those characteristics.