The Electromagnetic Approach to Forming
One of the problems associated with producing aluminum body panels as compared
with making the same old typical steel sheet is that there is a tendency for
the aluminum to tear or wrinkle during forming operations. Consequently, those
panels that are produced out of aluminum are often comparatively soft in shape,
avoiding the sharp edges that can be stamped into steel as a matter of course.
Electromagnetism may be the means by which this limitation is overcome. At
least the work being performed at Ohio State University by Glenn S. Daehn, a
professor of materials science and engineering, and his colleagues have been
working on the past few years on methods to use electromagnetism for forming
aluminum. Although the process itself is fairly remarkable, its name is, well,
rather pedestrian: “electromagnetically assisted stamping” (EMAS).
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Prof. Glenn S. Daehn and his colleagues at Ohio State are applying electromagnetism to forming. The results include shapes that ordinarily cause tearing or wrinkling when stamping alone is used. The potential is there for greater deployment of aluminum body panels. (Photo: Ohio State University)
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Although you might immediately think, “Wait a minute, aluminum isn’t
magnetic,” Daehn answers that the real issue here is that of a material
being electrically conductive. Which aluminum is. So, too, are materials including
high-strength steel, which is a material that can also be difficult to form.
In the setup for forming, there is an actuator (e.g., a coil of wire) through
which a large pulsed current is passed. “The changing magnetic field created
by the transient current induces eddy currents in any conductor nearby,”
Daehn explains, adding, “These currents have their own magnetic fields.
There is a mutual magnetic repulsion between the two sets of currents. This
can cause a very rapid motion of the metal.” And if the motion is down
into a mold, such as a sharp create or character line across a body panel, then
the result is an aluminum panel that otherwise probably couldn’t be produced.
In tests at Ohio State, aluminum sheet that could ordinarily be stretched no
more than 30% of its length was stretched 100% without tearing. Which means
that complex shapes can be produced in aluminum through EMAS.
And it can be done with comparatively simpler tooling than would be necessary
with the conventional stamping approach, which could necessitate the use of
multiple die sets in order to attain the required form (a form which isn’t
as complex as that which can be attained through EMAS).
EMAS is actually something of a hybrid process, inasmuch as the conventional
stamping press and tooling are still part of it. Inserts are integrated into
the tooling. According to Daehn, design work remains to be done on the configuration
of the setup as well as on the strategy used to deploy the electromagnetic force
(e.g., there could be a big push of electromagnetism when the ram is at the
bottom of the stroke, or there could be a series of small impulses, which is
a process that’s called “bump forming,” which Daehn and OSU post-doc
researcher Vincent J. Vohnout developed with Ishikawajima-Harima Heavy Industries
of Japan). Still, he suggests that the actuators can be easily protected and
so it shouldn’t be a troublesome issue. He is particularly bullish on the
bump forming approach, about which he notes, “I think lots can be done
with bump forming with very little technology development.”
While it might seem that something formed more slowly might resist tearing,
Daehn acknowledges that while it isn’t particularly intuitive, there are
solid reasons why faster is better. For one thing, when the pulse occurs and
launches the metal in a uniform way, the only way for a tear to occur is if
there is a local change in velocity. “Inertia resists this—at high
speed, things want to keep moving in the direction in which they were launched.
“Second, when a sheet of metal strikes a die surface at high speed, large
compressive stresses are developed at impact. The forming process resembles
forging more than sheet forming at impact.” Once again, tearing is typically
circumvented. “Third,” Daehn concludes, “when we use electromagnetic
forming with stamping we can alter the strain distribution in a part very significantly.
We can also exploit this to move strain away from regions where a part is likely
to tear.”
One of the other beneficial aspects that they’ve discovered with relation
to EMAS is that they can do it without utilizing lubricant, so there is an environmental
gain.
Although this is something that is going on in a lab at Ohio State, Daehn says
that if there was a manufacturing company that was interested in aggressively
pursing EMAS, he thinks that it could become commercially viable in about 12
months.
Done In One With Innovative Fineblanking Process
The closer you can get to a finished part in one process, the better off you
are from a variety of points of view, ranging from inventory and logistics to
capital equipment and manpower requirements. Consequently, for those who make
parts with sheet metal, the Forming, Fineblanking and Stamping (FFS) technology
developed by Feintool System Parts ought to be of particular interest.
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The steps involved in making a seat belt housing in a single tooling setup. (There
are, of course, multiple stages within the tool.)
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Consider: FFS is high-volume, precision-part production that is, in effect,
from coil-to-component in a single press. The Feintool Systems Parts facility
in Nashville, TN, for example, has a unique 500-ton hydraulic press that is
fitted with multiple tooling modules fed by a CNC part transfer system with
the result that all operations, including in-tool deburring, are conducted in
a single pass. This setup combines both stamping modules and fine-blanking modules
to get the job done—and this single-pass job includes operations that can
eliminate the need for secondary operations including bending, milling, grinding,
broaching, and drilling.
The types of parts that it can handle include racks, clutch plates, and seat
recliner hardware, just to name a few. It can handle ferrous and nonferrous
materials up to 5.0-mm thick. The dimensional accuracy that can be attained
is +/-0.025 mm, and flatness is held to 0.025 mm/25 mm for parts sized from
150- to 200-mm square. (Fineblanking, if you’re not familiar with it, is
a cold extrusion process, not a stamping process per se: the material isn’t
ripped.)
An example of the operations-consolidation capability of FFS is an HSLA U-shaped
housing for a seat belt retractor. It had been fineblanked, belt sanded, washed
and dried, offset bended, and U-bended. Apparently, this multiple-process, multiple-handling
process took so long that sometimes there was actually part corrosion before
the part was completed.
With the FFS approach and a redesign of some aspects of the part, however,
these individual steps are replaced by a clever arrangement of modular tools
that complete 15 parts per minute. The tooling includes a deburring operation
in which burrs are actually pushed back into the component so that a clean edge
is left on the part. While there was originally a need for a semi-piece operation
followed by the spin riveting of a reinforcing washer on the side of the housing,
the revised design calls for a coining operation that produces a locking profile
that performs like a washer without the additional part.
One operation that had been tricky was creating the U-bends because there was
a tendency for the width of the bends to be inconsistent, which meant rework.
Thanks, in part, to CNC control of the transfer in the tooling, the U-bend outside
width is held to 60 mm +/-0.1 mm.
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A TMK press from Schuler. Because this is a modular approach to press build, equipment can be quickly tailored to application needs.
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There are two 37-mm holes on the part (used for seatbelt spool retention) that
are held to +/-0.2 mm. There are also 5-mm holes on the part that are used to
fasten the housing’s cover. In the previous design, these were straight-through
holes. A washer was used around the holes to help position the pushpins for
assembly. In the new design, the holes are formed with a lead-in chamfer to
facilitate pushpin insertion and a counterbore formed inside the hole that creates
a shoulder to hold the pins in place. The shoulder position tolerance is +/-0.04
mm.
The Modular Approach
While Schuler Inc. (Canton, MI) has long been known for the custom presses and
press systems that it engineers and builds for the OEMs, it hasn’t always
had product suitable to the requirements of the supplier community. This is
something that it is remedying with what it is calling its ProfiLine segment,
which includes a new lineup of presses designated TMK. TMK signifies Transfer/Mechanical/Knuckle
joint drive. The TMK presses are available with capacities from 200 to 800 tons
and are offered with firm or adjustable stroke lengths of 40 to 250 mm. The
bed widths range from 1,500 to 3,660 mm.
Fundamentally, this is a modular approach to press and press system building.
The machines can be used as transfer, drawing or blanking presses; they can
perform operations including cutting, drawing, embossing, and punching. There
is an assortment of peripheral devices that are specifically engineered to be
utilized with the TMK presses, including coil feeding lines, die change carriages,
die-change frames, NC-transfer devices, and other units.
The modified knuckle-joint system that’s employed can be adjusted for
the appropriate stroke height and rake for each die. The reason why this is
called a “modified” system is because although the slide moves downward
in a way similar to a conventional knuckle-joint press, when it gets near bottom
dead center (BDC), the slide speed diminishes so that the impact speed is lower
than would be the case with a crank drive. The speed can be adjusted depending
on the type of forming process that’s being performed (e.g., it would be
different, say, for drawing and embossing). The return from BDC is performed
quickly to help assure that cycle time is optimized.
Because the TMK lineup is based on preexisting modules, there are a number
of benefits, including the fact that these are proven components. What’s
more, standardization permits prices that are competitive, and it also means
that delivery times are significantly faster than they are for customized systems.
Forming for the SSR
One of the clear, key characteristics of the soon-to-be-arriving 2003 Chevrolet
SSR are the fenders and rear quarter panels. Whereas those sorts of structures
were common back in the late 1940s and early ‘50s, the era of trucks that
are echoed in the SSR’s design, nowadays, maximum draws on vehicles tend
to be nothing more than character lines, creases, in effect. As David F. Bjerke,
lead body integration engineer for the SSR admitted, “The challenge was
whether the fenders and rear quarter panels could be made with that much draw.”
“That much” is on the order of 10 in. for the front fenders and 18
in. for the rear quarters. Back in the day, that much draw would be accommodated
by multiple dies and multiple people. But today, the goal is to minimize both
the number of hits and the manpower to get the job done. Another challenge for
an ’03 product as compared with a ’53 truck is that the tolerances
required (e.g., 0.003 in.) are a whole lot stricter than they once were (e.g.,
0.010 in.).
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How do they create those deep drawn fenders?
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A solution to the SSR situation was devised by engineers at Fuji Dietec Corp.
(Troy, MI). As Werner Speidel, Fuji project manager puts it, “We developed
a die and employed a stamping process—an inverted toggle draw—that
is really a marriage between the old and the new: the original toggle draw stamping
process and the more recent stretch draw concept.” In toggle draw, the
sheet is placed over a cavity, a toggle-driven upper binder ring comes down,
and a punch presses the metal into the cavity. In stretch draw, the punch is
stationary and a retaining ring forces the sheet over a punch, thereby stretching
it.
To create the SSR panels, a two-step inverted toggle draw process is used:
sheet is located over a punch and the binder ring presses it down to form the
basic shape, then a secondary punch descends and completes the forming operation.
Forming Fabric
Not all presses process metal. Some—like the “Large Area Edge Turning
Press” model LF 1626 from Freeman Co. (Erlanger, KY)—actually form
such things as cloth, leather, vinyl, and carpeting for applications on visors
and seats. The pneumatically actuated press is capable of handling tools that
measure 16 x 26 in., and it quickly processes parts in a fraction of the time
that is normally required by manual operations—and at a fraction of the
cost of systems that cost upwards of $90,000.
Consider, for example, an automotive visor. It is a two-piece part (excluding
the mounting hardware). There is a chipboard base and the covering fabric. In
process, adhesive is applied to the edge of the chipboard. The fabric is in
the female cavity of the tooling, and then the chipboard is inserted. When actuated,
the press folds the edges of the material around the part.
According to Greg DeFisher, president and COO of Freeman, the press can reduce
production time by as much as 70% compared to manual methods. “Companies
can quickly recoup the cost of the capital equipment through the efficiencies
they gain,” he says. The press costs around $20,000 (depending on options,
of course) and the tooling can cost from $16,000 to $25,000. (Tools can be swapped
in and out of the press in about 10 minutes to handle different parts.)
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This press forms fabric—for sun visors and other interior trim components.
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They’re developing three-dimensional dies that permit both covering and
die cutting of pieces onto forms. One application for this would be putting
map pockets onto the backs of car seats.
While the company is known by some automotive suppliers for its cutting presses
and hot-melt adhesive applicators, this edge turning press is a comparatively
new undertaking for the company.
(One interesting non-automotive application: book covers for Bibles.)
Model It First
Given the amount of time required to produce even prototype tools for stamping,
the need for simulation of the process is absolutely essential for those who
are interested in proceeding in something less than weeks or months. One product
that can help engineers quickly get the job done—one based on work that’s
been going on for the last 10 years with many major automakers (from Audi to
Renault, with plenty in between)—is pam-stamp 2g from ESI Group (in North
America: Shelby Twsp., MI). This is a collection of modules that provide the
means to go all the way through the process, from initial simulation to process
validation. The modules are:
- pam-diemaker. This imports CAD geometry and then can be used to generate
the necessary binder surface and die addendum geometries, based on the formability
of the part. It also facilitates multi-part grouping.
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The front fender of the Renault Kangoo undergoing pam-stamp 2g analysis.
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- pam-quickstamp. This is all about formability evaluation. This not
only takes into account the physical attributes of the forming operation (the
plasticity of the material, the blank holder pressure, etc.), but also such
things as accuracy and time. It is a quick 3D evaluation tool.
- pam-autostamp. Validation of the process is conducted with this module.
It permits the user to perform virtual try-out of the forming process, so a
determination can be made of the performance of the dies, both in terms of behavior
and output. There is implicit solver technology included that provides springback
predictions.
The overall architecture of pam-stamp 2g is such that there is data sharing
between each of the modules.
Move It, Don’t Lose It
When you’re looking at a 78-in. to 84-in. pitch between from six to eight
dies in a transfer press that’s being used to produce large body panels,
you’re also often looking at transfer automation that employs heavy finger
tooling rails for the simple reason that you’re dealing with wobbly parts
and the need for rigidity. Apparently, you’re also looking at the potential
for there to be some reliability and maintainability problems because the heavy
mass of the rails means that this is a high-inertia operation, which can result
in damage to the tooling, dies, or press should something go awry in part transfer.
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If you look closely to either side of the picture, you can see the finger tooling rails that are attached to the FLEX 5000 permanent transfer rails.
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By way of providing an alternative to this situation, Atlas Technologies (Fenton,
MI) has developed a low-inertia approach to part transfer, one that employs
aluminum rails and multiple points of strategically placed support for the rails.
What’s more, Atlas, for its servo-driven, tri-axis FLEX 5000 in-press transfer
system, has developed telescoping rails. During die change, after the transfer
rails are brought through the front of the press with the dies, the ends of
the finger tooling rails, which reach through the press during stamping operation
to load the blanks to the first station and unload parts to the exit conveyor,
are automatically extended.
