Forming: a mini-compendium of developments. (Equipment & Applications).
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).
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 Ca 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.
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.
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.).
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.
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.)
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.
* 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.
Hove 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.
By way of providing an alternative to this situation, Atlas Technologies (Fenton, Ml) 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.
You may want to learn more about these developments. So, you can get information From the Following companies by visiting these links:
* Feintool: www.feintool.com
* Schuler: www.schulergroup.com
* Freeman: www.freeman-die.com
* ESI Group: www.esi-group.com
* Fuiji Dietec: www.Fujidietec.com
* Atlas Technologies: www.atlastechnologies.com/
You can contact Prof. Daehn at Ohio State via email at firstname.lastname@example.org
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|Title Annotation:||aluminum forming for auto body panels using electromagnetism|
|Comment:||Forming: a mini-compendium of developments. (Equipment & Applications).(aluminum forming for auto body panels using electromagnetism)|
|Author:||Vasilash, Gary S.|
|Publication:||Automotive Design & Production|
|Date:||Jan 1, 2003|
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