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Press feed options
for stampers:
Examining the choices
in roll and gripper feeds
The two
basic press feed types commonly in use today are roll feeds and gripper
feeds. Roll feeds, as the name implies, use rollers to move the material
into the tool, while gripper feeds employ gripping clamps and a linear
motion to move the strip. Both types of press feeds can be powered by
the press, by a self-contained drive system (such as air- or hydraulic-powered),
or by a servo-drive system.
Roll
Feeds
Press-driven.
Feeds that are driven by the press, such as rack-and-pinion or cam feeds,
always are synchronized with the press rotation. They begin their motion
at some predetermined point in the press cycle and finish at another predetermined
point, regardless of press speed or die engagement. Although the index
speed must increase or decrease to keep pace with the press, the feed
can draw as much power as it needs from the press to accomplish this,
within the limitations of the mechanical coupling to the press.
This synchronization
feature makes press-driven feeds suitable for high-speed indexing, feeding
in-die transfers, or for use with unloaders and other applications that
require feed motion to be tied to press rotation to avoid a collision.
When the press stops, the feed motion stops as well.
Conversely,
because most press-driven feeds lack timing adjustment, the feed motion
for all dies cannot begin until after a point in the stroke at which the
deepest draw die disengages.
Inherent
in the design of press-driven feeds is smooth motion, called an S-curve
move profile, in which the rate of acceleration varies throughout the
index. These feeds make gradual transitions in velocity, with high acceleration
and deceleration in the interim. Eliminating sharp velocity transitions,
called jerk points, helps these feeds make high-speed indexes with good
accuracy.
Most press-driven
feeds are limited in feed length and feed length adjustment. They cannot
perform inching and use no controls interface. Most require that gear
sets, rollers, or mechanical linkages be changed or adjusted to modify
feed length.
Also, since
these feeds are coupled directly to the press rotation, they lack the
ability to jog the strip for threading. Without electrical controls, mechanical
feeds cannot accept setup information from or provide feedback to press
control or automation systems.
Self-powered.
Self-powered units begin their motion in response to a signal from the
press but have a finite, minimum amount of time in which they are capable
of indexing based on the amount of power available and the load that is
encountered.
Thus, the
point at which the feed finishes varies with the press speed. The faster
the press runs, the later in the stroke a self-powered unit will finish.
These feeds always finish the feed motion once it has been started, regardless
of what the press does.
Self-powered
feeds operate independently of the press and can be adjusted for each
application to begin feeding as soon as the die opens. Most go from a
stationary condition directly into a fixed rate of acceleration, resulting
in jerk points at the beginning, middle, and end of each move.
Servo-driven.
Servo-driven roll feeds use a closed-loop positioning drive, usually a
servo but sometimes a stepper, to control the index position of the feed
rolls.
These feeds
operate at high speeds and take up minimal space. They have unlimited
feed length capability and use a microprocessor-based control that provides
programmable move patterns, self-diagnostics, autocorrection, and the
ability to communicate with automation.
Servo-driven
roll feeds are available in a variety of configurations, including conventional
two- and four-roll units, feeder-straighteners, unwinder-feeder-straighteners,
and zig-zag units. Feed control packages range from single setup controls
with thumbwheels or keypads to systems that allow programming of multiaxis
move patterns. Some can control auxiliary functions and devices and offer
varying levels of memory and communications capability.
Most servo
feeds use a trapezoidal move profile, which has four distinct jerk points.
Some also are available with controls that can execute S-curve move profiles.
Some systems are electronically synchronized with press rotation. These
units require a controls package and a feedback device, either a resolver
or encoder, that is attached to the press crank to track press rotation.
Their top speeds are limited by the available drive power.
Servo-drive
technology has matured over the past few decades to the point that these
drives are more reliable and less expensive than they were in the past.
However, a considerable amount of technical expertise may be required
to troubleshoot them.
Gripper
Feeds
As mentioned
previously, gripper feeds employ a linear motion to move the strip. They
are available in a variety of sizes, from compact press-mounted units
to large cabinet-mounted models, which include pull-through straighteners.
Gripper
feeds are limited to a specific maximum feed length based on the model
selected, so the longest feed length requirement must be anticipated at
the time of purchase. After that, each additional increment of length
costs more, and the longer feed length capability means the machine itself
must become longer, thus requiring more floor space.
The tendency
is to buy the shortest machine that will fill the need. If the machine
must run a feed length that is longer than it was designed for, it must
perform multiple cycles on each press stroke, commonly referred to as
multistroking. This capability requires an optional controls package,
and the press typically is operated in the single-cycle mode because of
the time required for the return stroke. Gripper feeds use a pair of clamps:
the retainer, which remains stationary, and the gripper, which moves during
the feed and return strokes. During the feed stroke, the retainer releases
the strip as the gripper holds and moves it forward through the top half
of the press cycle while the tool is open.
On the
return stroke, the gripper releases the strip, and the retainer holds
it while the gripper retracts from the press through the bottom half of
the press cycle while the tool is closed. Since the return and feed strokes
take about the same amount of time, gripper feeds are limited to a 180-degree
feed window at the maximum operating speed.
The gripper's
pulling force can be provided by air- or hydraulic-powered cylinders,
a hydraulic motor, a servomotor, or the press. The gripper usually is
supported by guide bars or rails and is driven by cylinder rods, chains
and sprockets or ball screws, or a mechanical linkage to the press.
Self-powered.
With self-powered (air- or hydraulic-powered) units, feed length is adjustable.
The gripper moves between an adjustable stop and a stationary stop, and
a cushion softens the blow at the end of each stroke. Feed length adjustment
may require the use of tools and can involve some trial and error.
Air-powered
gripper feeds generally are low in cost and commonly are used in conjunction
with pull-through straighteners for applications requiring low to moderate
speeds and limited feed lengths.
The cost
of the compressed air used by these feeds and the loss of air through
leaks, pressure drops, and contamination can offset the savings realized
from the feed's lower initial cost. Air-powered gripper feeds have many
moving parts and wear components, which can add to maintenance costs.
Servo-driven.
Servo-powered gripper feeds use a closed-loop servo drive coupled to a
ball screw and nut to position the gripper, and they don't use stops or
cushions. Feed setup information is programmed into the control unit via
an operator interface, or it can be serially downloaded from another device.
The control unit then commands the servo drive to position the gripper
accordingly. These feeds also can interface with press automation systems.
A variety
of methods are used to unwind and feed coiled stock; only the most common
have been covered here. Careful thought must be given to the equipment
that is selected to avoid limiting the productivity of the system as a
whole.
Jim
Ward is National Sales Manager of Coe Press Equipment Corporation, 40549
Brentwood Drive, Sterling Heights, MI 48310, phone 586-979-4400, fax
586-979-2970.
About The Company
Coe Press
Equipment Corporation, with headquarters in Sterling Heights, Michigan,
is a leading producer of pressroom feed equipment including servo roll
feeds, power straighteners, coil reels and cradles, and complete coil
processing systems. They also design and engineer fully-integrated coil
processing systems for metal stamping and processing operations.
For more
information, contact Jim Ward at Coe Press Equipment Corporation, 40549
Brentwood, Sterling Heights, MI 48310, (586) 979-4800, fax (586) 979-2970.
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