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Getting
it straight:
Understanding coil
straightening in
stamping operations
Before coiled
material can pass through a die to produce an acceptable part, it must
be straightened. Coil straightening is accomplished by bending a strip
of material around sets of rollers that alternately stretch and compress
the upper and lower surfaces so that the material's yield point is exceeded.
Surfaces then are the same length after springback, which results in flat
material. Straightening requirements vary depending on material defects,
die design, and part requirements.
Two
Ways to Get Straight
Modern
straightening equipment falls into two categories: straighteners and precision
levelers. Straighteners, or flatteners, are used more in the stamping
process. These machines use between five and 11 rolls, depending on the
material's width, thickness, strength, and type. Their large-diameter
rollers are widely spaced, which removes coil set rather than levels material.
Straighteners
are available in powered or pull-through versions. In powered units, an
electric or hydraulic motor moves the material to the next operation.
In pull-through units, the feed equipment pulls the material through the
straightener, which is nonpowered.
The second
classification of straightening devices is precision levelers, which,
like straighteners, remove coil set but also can remove other material
imperfections, such as camber, wavy edges, center buckles, and trapped
stresses. Levelers are distinguished by closely spaced, small-diameter
rollers with backups.
Normally,
a precision leveler has more rollers than a straightener. Rollers have
the ability to flex and work the material harder. Unlike straighteners,
precision levelers are always powered because of the amount of work they
must do to the material.
The
Constant Problem of Inconstant Coil Set
Understanding
basic straightening principles is necessary to obtain consistent results
in the setup and operation of straightening equipment. In theory, three
staggered rolls should be enough to straighten most materials. This approach
will work if the amount of coil set in the material is constant throughout
the coil.
Depending
on material composition (thickness and yield strength), the amount of
coil set can dramatically increase as the coil is depleted. In most cases,
coil set is induced in the material during a previous process, such as
slitting, edge conditioning, or finishing.
The outer
wraps of the coil are placed under the tension and compression required
to bend the material around the coil's OD, usually from 54 to 72 inches.
The coil's inner wraps are placed under the tension and compression required
to bend the material around the ID of the coil, usually from 16 to 24
in.
The range
between the inside and outside diameters of a coil can result in a dramatic
change in the amount of coil set. Using only three staggered rollers,
the operator has to adjust the machine constantly to obtain an acceptable
level of flatness.
Power straighteners
are built with multiple work rolls to deal with varying coil set. As more
work rolls are used in a straightener, the amount of coil set that can
be removed becomes greater.
Straightening
Through Thick, Thin
Thick materials
require large-diameter rollers. The center distance spacing of these rollers
can be relatively large and still do an effective job of back-bending
the material. Thick materials normally require fewer straightener rollers.
As the material thickness increases, the roller diameters and support
journal diameters also must increase. The work rolls must be able to withstand
the forces required to back-bend the material without excessive deflection
across their width.
Conversely,
thin materials require small-diameter rollers. The rollers' center distance
spacing should be shorter to stretch and compress the material. Thin materials
normally require more rollers to remove varying amounts of coil set in
the material.
A consideration
also should be given to the support journal diameters of work rolls on
light-gauge applications. As the material and machine widths increase,
the tendency for small-diameter rollers to flex and deflect also increases.
Balancing
Flexibility With Productivity
Stampers
struggle to increase volume, quality, and production efficiency while
retaining a high level of flexibility. Most companies don't have the luxury
of building their capital equipment to meet the needs of a single product
line. Therefore, flexibility and capacity should be maximized to meet
the demands of changing product lines, customers, and markets.
When choosing
a straightener, stampers must think about the trade-offs between producing
a single product versus producing a range of products. These options should
be explored at the onset of a search for straightening equipment. Some
fundamental decisions must be made concerning the level of the straightener's
flexibility and the breadth of the demands it must meet.
When determining
the level of flexibility needed for the range of materials a straightener
will process, stampers must evaluate the maximum material width and the
machine along with the range of material thicknesses that will be run.
As the width of a straightener increases, its ability to process a material
with a given thickness and width is compromised.
The tendency
for work rolls and end journals to deflect becomes greater as the machine
width increases. For example, a 12-in.-wide straightener with 3-in.-diameter
work rolls located on 5-in. centers can process 6-in.-wide, 0.187-in.-thick
material. The same straightener configuration but 36 in. wide cannot effectively
process the same material because of potential roll deflection. Excessive
roll deflection may result in a loss of contact surface area, decreased
straightening efficiency, material slippage through the straightener,
and broken work rolls.
Because
of variations in material type, thickness, and width, a single straightener
can't meet the demands of every application. Therefore, during the equipment
specification process, all variables associated with the straightening
process, such as yield strength and surface finish, should be considered.
Stampers
using nonpowered pull-off coil reels, which rely on a straightener's horsepower
to pull the material off of the coil and through the straightener, should
consider ID, OD, coil width, and maximum coil weight variables. However,
for all straightening applications, the maximum line speed must be defined.
Stampers
evaluating straightening equipment may find that the more detail they
can offer in defining coil variables for a specific application, the easier
it will be to work with a machine supplier to select the best straightening
equipment.
Bruce
Grant is research and development manager for Coe Press Equipment, 40549
Brentwood, Sterling Heights, MI 48310, 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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