Press Tools
3.
Presses and Equipment for Sheet Metal
dies
POWER PRESS TYPES
The types
of power presses available for metal-cutting and forming operations are varied,
the selection depending upon the type of operation. Not all types of presses
will be described because of space limitations. The basic types of presses and
press mechanisms will be described to give the beginner the necessary background
for designing press tooling.
Presses
are classified by (1) type of frame, (2) source of power, (3) method of
actuation of slides, (4) number of slides incorporated, and (5) intended use.
Most presses are not classified by only category one but several. For example, a
straight-side press may be mechanically or hydraulically driven and may be
either single or double acting.
3.1.1 Classification by frame type: The frame
of a press is fabricated by casting or by welding heavy steel plates. Cast
frames are quite stable and rigid but expensive. Cast frame construction also
has the advantage of placing a mass of material where it is needed most. Welded frames are generally less
expensive and are more resistant to shock loading because of the greater
toughness of steel plate.
The
general classification by frame includes the gap frame and the straight
side. The gap frame is cut back
below the ram to form the shape of a letter C. This allows feeding a strip from
the side. Some gap-frame presses have an open back to permit strip feeding from
front to back or ejection of finished parts out the back. Gap-frame presses are
manufactured with solid frames fixed in a vertical or inclined position. Others
are manufactured with a separate frame mounted in a base, which allows the frame
to be inclined at an angle in three different positions.
The reason for inclining the press is to allow parts to fall through the open back by gravity. The three-position inclinable press is frequently referred to as an open-back inclinable (OBI) press (see Fig. 3-1). Solid gap-frame presses are obtainable in higher tonnages than inclinable ones because of the rigid base and solid construction.
The OBI
press is the most common press in use today. It ranges from a small 1-ton bench
press to floor presses rated up to 150 tons. Its main use is for blanking and
piercing operations on relatively small work pieces, although bending, forming,
and drawing operations can also be done.
Fig, 3-2
shows the major components of an OBI press, as follows:
1)
A rectangular bed, the
stationary and usually horizontal part of the press, serving as a table to which
a holster plate is mounted.
2)
A bolster plate,
consisting of a flat steel plate from 50 mm. to 125 mm. thick, secured to the
press for locating and supporting the die assembly.
3)
The ram, sometimes
called the slide, which reciprocates within the press frame and to which the
punch or upper-die assembly is fastened.
4)
A knockout, consisting
of a crossbar through a slot in the ram that contacts a pin in the die to eject
the work piece.
5)
The flywheel, which
absorbs energy from the motor continuously and delivers its stored energy to the
work piece intermittently, making it possible to use a smaller motor.
6)
The pitman, consisting
of a connecting rod to convey motion and pressure from the main shaft or
eccentric to the press slide.
