Twist drill bits quoted from Machinery's Handbook 27th Edition
Twist drill bits are rotary end-cutting tools having one or more cutting lips and one or more
straight or helical flutes for the passage of chips and cutting fluids. Twist drills are made
with straight or tapered shanks, but most have straight shanks. All but the smaller sizes are
ground with “back taper,” reducing the diameter from the point toward the shank, to prevent
binding in the hole when the drill is worn.
Straight Shank Drill bits: Straight shank drills have cylindrical shanks which may be of the
same or of a different diameter than the body diameter of the drill and may be made with or
without driving flats, tang, or grooves.
Taper Shank Drill bits: Taper shank drills are preferable to the straight shank type for drilling
medium and large size holes. The taper on the shank conforms to one of the tapers in the
American Standard (Morse) Series.