Pressworking: Cross-Bar Press Considerations
Handling parts that are hard to handle—like large body panels— parts that have a physical propensity to flex, can be tricky, says Andreas Sambel, general manager, Mechanical Press Sales, Schuler Inc. (Dearborn, MI). One of the approaches that the people at Schuler recommend for processing body panel parts is to use a cross-bar transfer press. This press type has an individual slide for each forming station rather than, say, a tri-axis transfer press that employs a common slide for multiple stations. (It should be noted that Schuler makes those presses, too, so it’s not like they have anything against them. Rather, it’s that when it comes to big panels, cross-bar transfer is an advisable approach. But if it is something like producing frame parts, then the tri-axis approach will probably be recommended.) Cross bars are fitted across the width of the press; each cross bar is equipped with an arrangement of suction cups that attach to the panels for transfer. The slides are individually adjustable, which is beneficial for tryouts and setups. The configuration provides good accessibility to dies, clamps, and other components. Between the forming stations there are universal stations (see “Trick moves,”).
Schuler produces these presses to size (e.g., 52,000 kN with a 2,000-mm feed pitch; 68,000 kN with a 2,600-mm feed pitch), depending on the application. (One thing about these big-capacity units: although there may be body sides produced, there are many other body panels that aren’t quite as large. So it should be kept in mind that double-attached parts can be readily run.)
Here are some other aspects worth considering. . .
A considerable factor
A goal of stamping operations is to have high output rates. There are several reasons:
1. Few body panels mean few cars
2. Press lines are extraordinarily expensive pieces of machinery, and if they aren’t running, they are pieces of sculpture that are more expensive than most things found in Sotheby’s
3. Press lines are ahead of all of the other operations in an assembly plant (e.g., body build, trim, and final), so if there aren’t sufficient stampings, then all of that becomes idle (or at least not running at the appropriate rate).
Consequently, there is a goal to move panels as expeditiously as possible through the press operations. Everyone talks about quick die change. Here we’re talking about moving metal. And realize that the vacuum that may be used to pickup parts and transfer them between presses only has so much gripping force. So one thing to keep in mind when calculating the output of a press line is the speed at which the material can be safely transferred; the number of strokes per minute isn’t the only factor.
There are situations when it is necessary to have a die tilted by x°—maybe 30—in order to, say, produce a sharp character line on a body panel. (Even vehicles that aren’t part of the Ford New Edge Design approach have these features.) One of the things that Schuler offers on its multislide cross bar transfer press is a series of universal stations (place one between each press; so a five-press line gets four universal stations) that actually provides the tilt to load the part in the proper orientation into a draw station in order to get the edge. A universal station can provide seven-axis of controlled motion. One advantage of having the manipulation of the material done by the universal station is that it simplifies die design; there doesn’t necessarily have to be the cam-actuation within the die that might otherwise provide the orientation. And what is important to note is that the universal station approach facilitates shortening the feed pitch. Less pitch means more production. (There is a slight lengthening of the total line, however, due to the use of universal stations between the presses.)
About the number of stations
Some of you might have read that preceding paragraph and noted the words “a five-press line.” And then you said something like: “Hey, the goal is to reduce the number of presses in a line. Five! Four are better!” Which could be true. . .but there is another factor that needs to be taken into account: the dies. Fewer stations mean more complex dies. More complex dies mean difficulties when it comes to (1) developing the dies; (2) building the dies; (3) setting up the dies; (4) maintaining the dies. So what may seem like a virtue (i.e., shorter press lines) may actually be a practical pain in the posterior for die/manufacturing/production engineers alike. You might want to recalculate.
There is sometimes a need to lighten up a vehicle. Which can lead to the implementation of aluminum panels. What is the consequence on pressworking equipment? Not much, Schuler’s Andreas Sambel says. Mainly, it is an issue of changing the destacking system: magnetic pickups can be used for the steel; aluminum requires suction cups. Other than that, the status is pretty much quo.
The Systems Approach
Atlas Technologies (Fenton, MI) is probably best known for the die carts that the company has been providing to industry for a number of years. The die carts and associated racks (for handling dies) make it possible to do such things as to change over a five-press tandem line in 103 seconds. Even stamping plants that are known for quick die change capability don’t often make the switch that expeditiously—not by several minutes. Company president Mike Austin explains that Atlas has been working on a number of press-related fronts, from innovative destacking equipment for blanks to some advanced projects with the Auto Body Consortium (Ann Arbor, MI), such as developing “smart dies” (Austin says that these dies have adjustable settings for such parameters as pressure, pad depth, draw bead and that adjustments to the dies can be made based on the conditions of materials both going into and out of the press).
One area that Atlas engineers are concentrating on is developing standard stamping cells for small to medium sized (800- to 3,500-ton) press applications. “We’re looking to simplify systems and to improve performance through the building of equipment in a more standardized manner and then integrating the control systems,” he says. According to Austin, the general modus operandi for getting a press line today is for the customer to specify the size of press and some of the auxiliary functions that need to be performed. Generally, press companies receive the RFQ. Then the response tends to be a custom system, with slight modifications to meet the customer’s specific demand.
Here’s the kick of his idea: “If the customer can configure a cell out of standard components and still achieve over 90% of what he’s looking for, then it is possible to improve performance, reduce cost, reduce lead-time, and create more standardization.”
He admits that there is an understandable reticence on behalf of people from getting “shoe-horned into a standard solution.” But he points out that just as there has been increased standardization in metalcutting, there is now the potential to do the same in metal forming.
Austin then cites the familiar desktop computer analogy, of being able to get a computer configured with the precise specs made up of components that are standard and available from a number of vendors. In the case of the press cell, it is a matter of pre-engineered modules for the front of line equipment, press, end of line equipment, automated storage and material system, and controls.
Among the benefits he ticks off are:
•Faster equipment delivery (e.g., reduced engineering is required)
•Flexible production capacity
•The ability to make fast upgrades or modifications (in approximately two weeks)
•Common spare parts
Atlas isn’t undertaking this initiative on its own. They are working with a number of suppliers—both domestic and off-shore—who have capabilities in controls, coil handling, scrap conveying, and equipment installation.
Tandems Keep Going
“Tandem lines are not new to the auto industry,” observes Dennis Boerger, product manager, AIDA-Dayton Technologies Corp. (Dayton, OH). And he acknowledges that there is a trend, in cases where the components produced are large body panels, toward transfer presses, away from tandem lines. But he points out that there are still notable advantages to using tandem equipment. In the case of AIDA and equipment for producing, as he describes it, “large outer skin components,” he points to the SMX series of double crank straight-side presses, stamping machines that range in capacity from 300 to 2,000 metric tons. Bed sizes vary, as well (e.g., 8 ft. x 5 ft.; 20 ft. x 10 ft.).
One of the benefits of this equipment is, Boerger says, the fact that there is a patented link motion provided, a motion that can be modified to meet the forming requirements of the station; combined with ancillary automation: “It pretty much provides whatever slide motion that is best suited for producing a high-quality part in any operation.” He notes, “Most presses in tandem lines provide straight eccentric motions.” So, he explains, with the SMX there is the means by which the press stroke can be slowed down during its working portion so that there is more time—on the order of 30 to 40%—with the die engaged with the material. A key benefit of this approach is that the longer hold pressure means that the steel has an opportunity to flow plastically and as a result there tends to be less springback. Because cycle time is certainly an issue, there is a fast return of the ram to the top for the next stroke.
“One of the big areas of change in tandem lines today versus 20 years ago,” Boerger notes, “is in automation and control systems.” In terms of automation, in addition to the dedicated between press automation, there are also robotic handling systems being used to load and unload presses. AIDA recently introduced a PLC-based control to facilitate setup and operation, as well as to provide enhanced monitoring capability.
“I think the flexibility of tandem lines is a key advantage,” Boerger says. He cites the example of part changes. In the case of a large transfer press, he explains, it might be necessary to work on the entire die in order to make a small change to a component. But with the tandem approach, there are smaller, individual dies, which are easier to work on. He admits, however, that although there is the benefit of flexibility, you do “sacrifice some output.”
Still, the trade-off can be advantageous.
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