Originally Posted by Machinery's Handbook-26th Edition
Chamfer Length: Three types of hand taps, used both for hand and machine tapping, are
available, and they are distinguished from each other by the length of chamfer. Taper taps
have a chamfer angle that reduces the height about 8–10 teeth; plug taps have a chamfer
angle with 3–5 threads reduced in height; and bottoming taps have a chamfer angle with 11⁄2
threads reduced in height. Since the teeth that are reduced in height do practically all the
cutting, the chip load or chip thickness per tooth will be least for a taper tap, greater for a
plug tap, and greatest for a bottoming tap.
For most through hole tapping applications it is necessary to use only a plug type tap,
which is also most suitable for blind holes where the tap drill hole is deeper than the
required thread. If the tap must bottom in a blind hole, the hole is usually threaded first with
a plug tap and then finished with a bottoming tap to catch the last threads in the bottom of
the hole. Taper taps are used on materials where the chip load per tooth must be kept to a
minimum. However, taper taps should not be used on materials that have a strong tendency
to work harden, such as the austenitic stainless steels.