The ins and outs of magnetic die clamping
Magnetic clamping of dies adds flexibility to stamping operations
For the right application, magnetic die clamping can add significant flexibility to a stamping operation, eliminate physical clamps, and simplify maintenance.
As automotive OEMs have continued their struggle, so have their suppliers. Sometimes stampers haven't had enough work to keep their doors open. But the work they did have hasn't gone away, and this is where Eagle Wings Industries, a Rantoul, Ill., stamper, steps in. Unfortunately, all that takeover work can add up to some significant retrofit costs, fitting another company's die onto an Eagle Wings press. One technology, though, has helped matters. At the company's Olney, Ill., subsidiary, Richland Manufacturing, a new 330-ton, straight-side SEYI press doesn't hold a die assembly in place with mechanical or hydraulic clamps.
Instead, it uses electro-permanent magnets.
Electro-Permanent Magnet Basics
The key word is permanent, according to Paul Van Every, operations manager at Sterling Heights, Mich.-based Tecnomagnete Inc. Electro-permanent magnets aren't electromagnets, nor are they conventional permanent magnets. The electro-permanent magnet "uses electricity to energize and to de-energize," he said. "The rest of the time, it's not drawing any current," but instead is emitting magnetism just as a permanent magnet does: constantly and without electricity (see Figure 1).
It's this characteristic, he added, that makes it practical and safe in industrial applications, die clamping included. The technology involves bolted, 2-in.-thick magnetic plates, which can either use a press's existing slide and bolster plates or be drilled and tapped in place. Magnetics eliminate moving parts: no mechanical clamping, and no leaks or other maintenance that comes with hydraulic die clamping systems. Because there are no physical clamps, dies don't need to line up with clamps attached to T-slots, and the magnetic force can span the entire face of the die, not just around the perimeter, so die flexing is less of an issue. With the die at rest, an operator turns a key and pushes a button to demagnetize the system and release the die.
Machine shops have used magnetic workholding devices for years. By the 1980s, a few metal stampers in Europe were trying out the new magnet technology on their presses, and about six years ago, magnetic die clamping started showing up in North American stamping shops. Some early adopters initially had safety concerns, for understandable reasons. A die is incredibly heavy and, if unsecured, incredibly destructive. To see the die move up and down with no physical clamp can be a bit unnerving.
As Van Every explained, a permanent magnet cannot hold halfway; the holding force is either there (current systems are capable of up to 14 tons per square foot), or it's not. If the power goes off, the state of the permanent magnet doesn't change, so the die stays locked in the press. The system draws electricity only to magnetize and demagnetize. Proximity switches on the slide and bolster prevent magnetization if anything is between the die and magnetic plate, fingers included. If the proximity switches sense more than 0.007 in. of space, the magnet doesn't energize at all.
Electro-permanent magnets are hybrids. Permanent magnets use materials that hold their magnetism, as the name implies, continuously. Electromagnets use electricity to create magnetism by energizing the iron in ferrous material. Send electricity through a wire wrapped around a nail, and you get an electromagnet. As the electricity stops, so does the magnetism. As Van Every explained, "For an electro-permanent magnet, pull the nail out of that coil of wire, and put in a piece of aluminum-nickel-cobalt," a material that exhibits high force and low coercivity, meaning it can become magnetized and demagnetized very easily.
Aluminum-nickel-cobalt, or AlNiCo, has its origins as a secret wartime material used to keep deck guns aimed at the sky even if an enemy shell knocked out a ship's power. By the 1950s the British and U.S. governments allowed the makers of AlNiCo to start selling it commercially. In the 1960s the material was sold as part of workholding devices for grinding and machining, but back then the material had some huge drawbacks. To increase the holding force, engineers simply added more AlNiCo—to the point where "they magnetized the whole darn part, and even the whole machine," Van Every explained.
The real breakthrough came when developers found a way to temper this magnetic force with another element. The element used today, neodymium, has extremely high coercivity, meaning it can't be magnetized and demagnetized easily at all. But when AlNiCo influences the neodymium, less magnetic material can exert a much stronger magnetic field in a smaller area (see sidebar, How Electro-Permanent Magnets Work). According to sources, today these kinds of electro-permanent magnets can hold 230 pounds per square inch.
Like their permanent magnet counterparts, electro-permanent magnets have live north and south poles, and magnetism takes the path of least resistance, from pole to pole. The magnetic plates that hold dies never really turn "off." The electricity just reverses the poles, which change the position of the magnetic field. When "energized," the magnetic field extends only inches beyond the magnetic plate. When turned off, the poles change such that the magnetic field stays entirely in the plate itself (seeFigure 2).
In a conventional clamping system, the actual clamps go where they make sense to hold the range of dies on the press (see Figure 3). "If you have a die that's 6 feet wide and 4 feet deep, you don't use one 8-inch clamp in the middle," said Todd Wenzel, president of stamping integrator TCR Inc., Wisconsin Rapids, Wis. "You probably have three or four equally spaced." In magnetic clamping, it's about placing the magnetic poles, and the same thinking applies.
At Eagle Wings, for instance, "we put the magnetic poles right in line with where clamping would go," said Jim Schwartz, general manager of marketing. "For our standard hydraulic clamps, there's either a plate with slots that the clamps slide into or, if there are risers on top of the die, the risers go across in line with the clamps. So for magnetic clamping, we put the magnetic poles over where those clamps would be." Eagle Wings' press, which has a 54- by 98-in. bed, uses enough poles in its magnetic clamping setup to be able to hold the press's smallest die, one that's 24 in. square producing small, two-out brackets.
The more poles a magnetic plate has, and the closer they're placed together, the stronger the holding force, and the greater variety of dies that can be held in the press. More poles cost extra, of course, but they add flexibility.
Wenzel added that QDC plates can reduce the number of poles required, which reduces upfront cost. It's true that with enough poles a magnet clamping system can hold dies of various sizes directly, even dies on parallels. "If you use a QDC plate, you can use fewer magnetic [poles] on the press," he explained, because no matter the die, its standard QDC plate ensures "you're covering poles."
Costs and Benefits
These magnetic mechanisms aren't a panacea for every stamping situation. They can't handle forging presses very well, for instance. Magnets are available that can handle 250 degrees F, and specialty magnets are available that can endure up to 450 degrees F, but the cost for such magnets can be significant. The die space also has to be able to accommodate 2 to 4.5 in. of magnetic plate, so the system may have issues with jobs that fill up die height on a press.
"Gap and straight-side presses often have plates that can be removed and the magnet can be put in its place," Wenzel said (seeFigure 4). "Sometimes the existing clamping surface is welded in, and that can create problems. But generally there are ways to get around the [die height] issue."
Comparing costs of different technologies isn't as straightforward as it seems, said Wenzel, whose company offers the gamut of clamping systems, from manual die clamping to hydraulic and magnetic technology. Magnetic systems offer significant flexibility comparable to more expensive traveling hydraulic clamping systems. But if less flexible, standard hydraulic systems are an option, cost comparisons should take the press size into account.
"With the labor costs associated with installing the hydraulic components, it's sometimes a wash" between the cost of standard hydraulic and magnetic clamping systems, he said, particularly for smaller presses. For larger- tonnage presses, standard hydraulic clamps may cost less initially, because a large press just needs more clamps; magnetic systems require more magnetic poles, which can add significant cost. On the other hand, magnets don't require hydraulics, which may be attractive from a maintenance standpoint. As always, application needs and available funds guide the final decision.
For the operator, a magnetic clamp means he doesn't need to walk around the press and tighten manual clamps or check for leaks from hydraulic clamps. But, said Wenzel, the magnetic plates do need to be clean and free of slugs. If there is any significant space (again, more than 0.007 in.) between the magnetic plate and die assembly, the magnet won't energize. "Operators have no choice but to be more careful about cleaning the bolster and slide. They can't leave a slug on them."
Off and Running
During this harsh business climate, Schwartz said magnetic die clamping is serving Eagle Wings nicely. In recent months, for instance, the stamper has taken over work from financially troubled firms—something all too common today. These companies often don't produce many parts ahead, so to keep the supply of parts in the pipeline, Eagle must take the tooling and get the press up and running quickly.
"While it used to take us about 10 days to bring a tool in and retrofit it in our press, we can now get it up and running within one to two days," Schwartz said. For the company's 330-ton press, "there's no machining [to adapt a die to a press's existing physical clamps]. We get the tooling in, and within a few days we're off and running."
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