Page 374 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

3 Chapter 14 Metal-Forging Processes and Equipment

with the part in the forefront. This part is made of a titanium alloy and weighs ap-
proximately 135 0 kg.

Mechanical Presses. These presses are basically of either the crank or the eccentric
type (Fig. 14.17a). The speed varies from a maximum at the center of the stroke to
zero at the bottom of the stroke; thus, mechanical presses are stroke limited. The
energy in a mechanical press is generated by a large flywheel powered by an electric
motor. A clutch engages the flywheel to an eccentric shaft. A connecting rod trans-
lates the rotary motion into a reciprocating linear motion. A knuckle-/oint mechan-
ical press is shown in Fig. 14.17b. Because of the linkage design, very high forces can
be applied in this type of press (see also Fig. 11.20).
The force available in a mechanical press depends on the stroke position and be-
comes extremely high at the end of the stroke. Thus, proper setup is essential to avoid
breaking the dies or equipment components. Mechanical presses have high production
rates, are easier to automate, and require less operator skill than do other types of ma-
chines. Press capacities generally range from 2.7 to 107 MN. Mechanical presses are
preferred for forging parts with high precision.

Screw Presses. These presses (Fig. 14.17c) derive their energy from a flywheel;
hence, they are energy limited. The forging load is transmitted through a large verti-
cal screw, and the ram comes to a stop when the flywheel energy is dissipated. If the
dies do not close at the end of the cycle, the operation is repeated until the forging is
completed. Screw presses are used for various open-die and closed-die forging oper-
ations. They are suitable particularly for small production quantities, especially thin
parts with high precision, such as turbine blades. Press capacities range from 1.4 to
280 MN.

Hammers. Hammers derive their energy from the potential energy of the ram,
which is converted into kinetic energy; hence, they are energy limited. Unlike hy-
draulic presses, hammers (as the name implies) operate at high speeds, and the re-
sulting low forming time minimizes the cooling of a hot forging. Low cooling rates
then allow the forging of complex shapes, particularly those with thin and deep re-
cesses. To complete the forging, several successive blows usually are made in the
same die. Hammers are available in a variety of designs and are the most versatile
and the least expensive type of forging equipment.

Drop Hammers. In power drop hammers, the ram’s downstroke is accelerated by
steam, air, or hydraulic pressure at about 750 kPa. Ram weights range from 225 to
22,500 kg, with energy capacities reaching 1150 k]. In the operation of graz/ity drop
hammers (a process called drop forging), the energy is derived from the free-falling
ram. The available energy of a drop hammer is the product of the ram’s weight and
the height of its drop. Ram weights range from 180 to 4500 kg, with energy capac-
ities ranging up to 120 k].

Counterblow Hammers. These hammers have two rams that simultaneously
approach each other horizontally or vertically to forge the part. As in open-die forg-
ing operations, the part may be rotated between blows for proper shaping of the
workpiece during forging. Counterblow hammers operate at high speeds and trans-
mit less vibration to their bases. Capacities range up to 1200 k].
High-energy-rate Forging Machines. In these machines, the ram is accelerated
rapidly by inert gas at high pressure and the part is forged in one blow at a very high
speed. Although there are several types of these machines, various problems associ-
ated with their operation and maintenance, as well as die breakage and safety con-
siderations, have greatly limited their use in industry.