Maybe you should feed it both
December 11, 2007
Stampers today need to process more grades and types of material than ever before, yet are under relentless pressure to reduce costs. They increasingly rely on systems integrators and equipment suppliers to design and install versatile stamping lines with quick-change capabilities. A notable case was that of a truck frame and chassis components manufacturer. It worked with vendors to develop a line that processes material up to 0.280 in. thick and 72 in. wide, in a range of yield strengths, in three forms – coil, blanks, or sheet.
As U.S. auto manufacturers continue to strive for global competitiveness, the Big Three search relentlessly for ways to reduce costs. They look for any and all methods, both internal and external, to cut expenses. Is this a short-term requirement that will ease as profitability improves and stabilizes? In a word, no.
As the cost-reduction pressures increase, the deck remains stacked against the suppliers. All too commonplace is the mandate that the next order of 50,000 widgets will be awarded only if the piece price drops 4 percent. At the same time, raw material prices, the labor rate, and healthcare costs continue to rise.
So how are the successful coil processors and stampers able to meet these challenges? The most prominent requirements today are (1) as much flexibility as possible and (2) as little changeover time as possible.
Stampers with large transfer systems traditionally bring raw material into the plant in sheet or blank form and use tailored processes to accommodate blank-fed operations. The plant layout, tooling, transfer systems, and loading/unloading equipment are set up specifically for sheet- or blank-fed operations.
Today, more stampers are interested in running coil on the same line. They want to run in strict transfer mode for blank destacking systems; in coil-fed transfer mode in which the blank is parted from the coil in the initial stations of the die and then moved through the remainder of the tooling stations with the transfer system; or in coil-fed progressive-die mode, in which the coil processing system maintains control of the material location as it travels through the tooling.
As with any major capital project, planning is crucial. Often many good intentions without sufficient forethought result in production pitfalls that could have been avoided. Hurrying through a project will allow you to start running parts sooner, but taking short cuts means you'll have to live with the consequences for years to come. Simply put, it's a "pay now or pay later" kind of thing, and retrofitting to make corrections is always more costly than taking sufficient time to address known issues in the first place.
Five main planning steps are:
While most coil processing systems operate in similar fashion, be aware of these considerations when developing a large transfer/destack system.
Feed Location. In many cases, the feed system must be located away from the press bed/bolster to align with either the transfer pickup location or the destacker. In a conventional press feeding system, the feed is mounted to the press. However, this may not be possible in your application. You might have to explore other options, such as floor-mounted stanchions.
Tail-Out. When the feed is positioned away from the press, material control is lost when the tail of the coil leaves the feed rolls. While this is not a problem on thin-gauge product when the distance is relatively short, it is serious when the material is 0.250 inch thick and 72 in. wide and the distance is 10 feet. Pulling the tail through the die area with a chain and tow motor may be one solution, but it creates a logistical bottleneck. Other solutions, such as a bolster-mounted tail-out feed, or using the transfer to move the tail through the tooling stations, may be better.
Coil Weight. An issue that frequently is overlooked is coil weight. Many stampers match coil weight to crane capacity. Other factors (limitations or restrictions) play a role. There is no sense in specifying 60,000-pound coils when your supplier charges a premium price for that size, or if road restrictions require you to get an overload permit to transport every coil to your plant.
Production Speed. In many cases, stampers arbitrarily set a higher feed speed requirement than necessary to make sure that the coil-feeding process is not the speed-limiting factor. However, in more cases than not, this can add an unnecessary expense for production rates that the line will never reach. There's no sense in having a high part-per-minute throughput rate on the feed line when the part removal process at the die exit can't keep up, or long feed lengths necessitate a looping pit that can't be installed because of facility limitations.
Electrical Interface. In any coil-fed operation, an electrical interface between the feed system and the press ensures proper timing between the feed movement and the press stroke. Usually the press control provides a start signal to the feed; in most cases, the feed sends a completion signal to the press. However, with transfer and destack systems, additional electrical lockouts are required so the feed system cannot be energized when operating in destack mode, or vice versa.
Many of these factors converged in a recent stamping line project. A Tier 1 automotive stamping supplier involved in manufacturing truck frame and chassis components was interested in increasing its production capabilities in wide, heavy, high-yield-strength material primarily for blank transfer and, to a lesser extent, coil processing. It would require large tonnage for blank transfer (its current application) and coil feeding (a future requirement). The company runs material from 0.062 in. to 0.280 in. thick; from 18 in. to 72 in. wide; with yield strength up to 80,000 pounds per square inch (PSI). It determined the optimal material size to be 0.280 in. thick and 72 in. wide, and optimal coil weight to be 40,000 lbs.
The press supplier accepted responsibility for the entire project, and subsequently coordinated with a second party to provide the coil processing system. The press supplier worked with the stamper and the other suppliers to coordinate the mechanical design, integration, and assembly.
Because the stamper required destack capabilities with minimal downtime, the system included two powered destack carts on an in-floor track to allow lateral shuttling from the stack-loading station to the press to a position under the transfer bars. In this configuration, one cart is loaded while the other is in operation, so the only lag time is the time it takes to shuttle the newly loaded cart into position.
To allow coil-fed operation, the press manufacturer provided details of the destack cart track design to the coil system supplier. A mating track allows the feed to be shuttled out of position in the opposite direction of destack cart travel. In addition to the conventional interface requirements, such as press cycle signals (feed initiate and feed complete), electrical interlocks shut down the press and coil processing system anytime an operator opens an access door.
Because the feed system can traverse, its human-machine interface (HMI) is located near the press control pendant. This convenient location saves the operator countless steps.
The system comprises several key pieces of equipment designed for the two main criteria—flexibility and quick-change:
Rail-mounted feed unit, destacker, and transfer system. The feed control is located adjacent the press column to allow ease of access for the operator. When feeding coil, the feed unit is located directly under the destack system to keep it as close as possible to the tooling. When running in destack mode, the feed unit moves out on rails to allow the destack carts to come into position.
Four-roll servo feed. The system includes a four-roll servo feed with four 4-in.-dia., matte chrome-finished feed rolls, designed to prevent slippage and marking. Five sets of backups located across the width of the rolls prevent roll deflection and provide the required roll pressure to process the heavy, high-yield-strength materials.
Adjustable edge guides allow the operator to control material position within the feed system from a single point of adjustment. Anti-backup rolls prevent material from backing into the looping pit in a case of a power outage or air pressure loss.
Threading tables. The system has a looping pit for long-feed-length jobs. Threading tables span the looping pit—double-sided tables which, when elevated, cover the looping pit and allow hands-free jogging of the material from the straightener into the feed.
Heavy-duty stock straightener. This straightener, designed for pull-off operation, uses four 6-in.-dia., matte chrome pinch rolls to flatten incoming material. The rolls are hydraulically opened for thread-up, and when closed they allow the straightener to pull material from a nonpowered stock reel. Adjustable edge guides allow the operator to control material position within the straightener from a single point of adjustment.
A regenerative drive system with ultrasonic loop control maintains adequate slack in the looping area. It also provides constant power to the rolls, even when the loop is full, to provide constant control of the material. The operator uses the HMI to adjust the roll settings to adapt to the material's thickness.
Hold-down Peeler system. A hydraulically raised and lowered peeler table, mounted to the entry end of the straightener, helps guide material from the reel at the appropriate angle for various coil diameters.
A motor drive to the rider roll helps direct the outer wrap of material toward the straightener when the reel is inched.
hydraulically operated coil end debender system opens wide enough to accept the material and holds it in place while the bender leaves bend the material up or down (operator selectable) and prepares it for threading. This unit also can be used on the tail end of the coil.
Powered thread-up pinch rolls are diamond-knurled for maximum material traction and open wide to allow easy material passage when threading and during run mode.
Solenoid-operated valves with push-button controls move all threading features to a neutral position in preparation for automatic operation. An HMI prompts the operator for threading procedures and provides the maintenance schedule and service points for the entire system. It also provides in-depth diagnostic fault messages with recommended remedies. The touchscreen also allows the operator to program in the material thickness, and with this information the control automatically adjusts the powered straightener head to the proper position to remove coil set.
Nonpowered stock reel. A nonpowered stock reel with a 40,000-lb. capacity supports the coils. The reel has an ID range of 19 in. to 24.5 in. and an OD capacity of 72 in. It also has a variable-tension brake system. As the coil depletes, an ultrasonic sensor sees a reduction in coil OD and adjusts the brake automatically. A secondary floor-mounted hold-down arm with powered rider roll provides for safe processing of high-yield-strength materials.
Coil-loading car. A traveling coil-loading car stages the coil and positions it on the mandrel. The car uses the same track system as the stock reel and is hydraulically powered and controlled with a remote jog operator pendant. The coil car has a 24-in. lift capacity and antitip arms to accommodate a range of coil widths and ODs. It has an automatic down-and-out feature to prevent the car from being left under the stock reel inadvertently. When the autoready function is energized, the car automatically drops and clears the stock reel, moving to a neutral position in preparation for automatic operation.
The focus on having flexibility and quick-change characteristics—the versatility to process a variety of material thicknesses and strengths, whether coil or sheet, and the capability to reconfigure the line quickly—didn't mean that safety wasn't a critical consideration.
In keeping with standard practices, the line has safety fences that protect operators and safety interlocks that interrupt the line's operation when operators open the access doors. The line was designed for hands-free threading, which reduces the operators' needs to open access doors and work close to the line. However, when manual threading is necessary, an access door adjacent the feed and press entrance allows manual intervention in jog/single-stroke-mode only.
In addition to protecting the operators, some of the interlocks protect the equipment from damage. A sliding gate in the perimeter fence allows the feed to traverse out of the way for destacking, and an electrical interlock prevents unsafe operation. Furthermore, interlocks prevent coil system operation when in destack mode, and prevent destack cart motion when feeding coil.