Section 13. qxd

13-62
MACHINING PROCESSES AND MACHINE TOOLS
Fig. 13.4.16
(
a
) Plain milling cutter teeth; (
b
) face milling cutter.
THREADING
Threads may be formed on the outside or inside of a cylinder or cone
(1) with single-point threading tools (see Fig. 13.4.1), (2) with thread-
ing chasers, (3) with taps, (4) with dies, (5) by thread milling, (6) by
thread rolling, and (7) by grinding. There are numerous types of taps,
such as hand, machine screw, pipe, and combined pipe tap and drill.
Small taps usually have no radial relief. They may be made in two,
three, or four flutes. Large taps may have still more flutes.
The 
feed
of a tap depends upon the lead of the screw thread. The
cutting speed
depends upon numerous factors: Hard tough materials,
great length of hole, taper taps, and full-depth thread reduce the speed;
long chamfer, fine pitches, and a cutting fluid applied in quantity
increase the speed. Taps are cut or formed by grinding. The ground-
thread taps may operate at much higher speeds than the cut taps. Speeds
may range from 3 ft/min (1 m/min) for high-strength steels to 150
ft/min (45 m/min) for aluminum and magnesium alloys. Common high-
speed steel tool materials for taps are M1, M7, and M10.
Threading dies,
used to produce external threads, may be solid,
adjustable, spring-adjustable, or self-opening die heads. Replacement
chasers are used in die heads and may be of the fixed or self-opening
type. These chasers may be of the radial type, hobbed or milled; of the
tangenital type; or of the circular type. Emulsions and oils are satisfac-
tory for most threading operations.
For thread rolling, see Sec. 13.2.
MILLING
Milling
is one of the most versatile machining processes and is capable
of producing a variety of shapes involving flat surfaces, slots, and con-
tours (Fig. 13.4.15).