Metal-stamping presses can be classified according to drive
mechanism—mechanical, hydraulic, servo--and press-frame
construction—gap-frame, c-frame, straightside. These classifications,
detailed here, impart certain important characteristics to the press.
Also described are stamping processes—blanking, piercing, notching,
bending, drawing, coining.
Presses function as the signature pieces of shopfloor equipment in a
stamping operation. The types of metal-stamping presses depend on the
nature of the stamping work. Stamping presses function by providing
energy to force a ram downward, providing force for the stamping dies
and tooling. Attached to the press ram is an upper die. The ram
descends toward the lower die. Located between these die halves is the
part material. As the die halves meet, a part is cut, shaped or
otherwise worked within the tooling. The ram then ascends, a part is
removed or the part material is indexed, and the stamping cycle
repeats. Sounds simple. But generating the force necessary to stamp
metal parts, especially given newer, higher-strength steels, demands
attention to press design and method of press motion. Designs and drive
mechanisms impart characteristics to stamping-press operations that
affect the ability to form parts. With stampers demanding flexibility
in these expensive pieces of capital equipment, press manufactures have
responded, and press technology has evolved to serve stampers’ diverse
needs.
Stamping presses generally conform to two basic designs. Gap-frame
presses, also referred to as c-frame presses because of their shape,
are connected from bottom to top at one location, behind the work area.
These presses feature lower capacities and typically perform as
stand-alone machines, often manually fed. Straightside presses are
supported on each side of their rectangular footprint, and given their
robust construction they are less susceptible to deflection arising
from off-center loading (a condition where stresses drive the ram out
of its normal, parallel condition) than gap-frame presses. Due to their
beefed-up framework, straightside presses offer high capacities and
often operate in press lines, either teamed with other straightside
presses or outfitted with ancillary equipment.
Hydraulic presses
Hydraulic presses depend upon the pressurization of hydraulic fluid
in cylinders to provide force to the ram. These presses, by controlling
the pressure, allow force control, and full force if need be,
throughout the motion of the ram as the part is formed. This amount of
vertical motion is referred to as the stroke length. The notion of
force control throughout the stroke is important. In certain
applications, such as working with difficult-to-form material, or when
performing drawing operations, force control throughout the stroke, and
the ability to provide full power throughout the stroke, is needed for
proper forming. Force control and full-force capability, along with
relatively simple maintenance and lower energy costs, are major reasons
why stampers may consider choosing a hydraulic press over its
mechanical kin.
At one time, hydraulic presses were seen as maintenance-intensive,
given the occurrences of leaking seals and hoses attributed to the
fluid-handling nature of the machines. In recent years, better
hydraulic-system designs and improved seals and connectors have all but
eliminated such problems.
Applications for hydraulic presses include deep drawing,
high-tonnage blanking at lower speeds, and short job runs, since press
speed is less critical given smaller part volumes. Hydraulic presses
are available in capacities from 20 to 10,000 tons, with work strokes
from 0.4 to 32 in.
Mechanical presses
Drive methods of mechanical presses differ from those of their
hydraulic counterparts. In most mechanical stamping presses, flywheels,
driven by motors, store energy that is then transferred to ram motion.
Because flywheels expend energy with each downstroke of the ram, they
slow down. That energy must be restored, and is, by the motor in time
for the next stamping-press cycle to begin. Mechanical presses operate
at much faster speeds—well above 1,000 strokes/min.--than hydraulic
presses, but work strokes are shorter, due to the fact that full force
develops in a mechanical press near the bottom position of the press
stroke. With their high-speed capability, mechanical presses get the
call for many high-volume metal-stamping jobs where parts are flat or
at least somewhat shallow. Mechanical presses fare well in stamping
flat or low-depth parts. That is because mechanical presses can only
provide full forming force in a smaller stroke range than hydraulic
presses. Typical applications for mechanical presses include high-speed
blanking, precision flat-part production and shallow drawing.
Mechanical press capacities range from 20 to about 6,500 tons with strokes from 0.2 to 20 in.
Servo presses
Recent years have brought new technology in the form of
servo-driven presses, or servo presses, to metal stampers. Servo
presses, though technically classified as mechanical presses, employ
servo drives to provide power, negating the need for flywheels.
Advantages of servo presses include the ability to control the stamping
press’ stroke length and velocity. Another plus: Servo presses allow
for dwell time at the bottom of a press stroke, where forming work
occurs. This is ideal when material must be given time to flow or
stretch into a part shape. Features such as these bring benefits of
mechanical and hydraulic presses into a single machine, providing
flexibility to the stamper.
Types of metal-stamping operations
Given the proper tooling, metal-stamping presses can perform a multitude of part-shaping operations.
- Blanking: Cutting flat sheetmetal into a defined size and shape. Typically performed in one hit of the press, the result may be a finished part or a blank destined for further forming or processing into its final shape.
- Piercing: Similar to blanking, the pierced piece instead is scrapped, with the surrounding material as the part.
- Notching: Similar to piercing, but here material is removed from the edges of the workpiece.
- Bending: Sometimes referred to as forming, tooling bends workpiece material into various angles.
- Drawing: The press essentially stretches sheetmetal to a depth.
- Coining: The die forms an imprint on the workpiece.
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