The science behind the servo press

Servomotor-driven mechanical press spurs a new way of thinking about forming metal

THE FABRICATOR® JANUARY 2008

January 15, 2008

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Flexibility sums up where the servo-driven mechanical press stands in its evolution. Early adopters are seeing that flexibility and asking, "What if?" What if I could control ram motion throughout the stroke and dwell for a certain period at bottom dead center (BDC)? According to sources, those "what ifs" have led to new ways of thinking about forming metal.

Komatsu H2F Series Servo Press

A Komatsu H2F series servo press line stamps perforated sheets for the backs of plasma TVs. This press is programmed to form each part with multiple strokes, eliminating the need for a traditional progressive-die setup.

In stamping, when you get right down to it, it's not about tonnage. It's about maximizing energy, or the machine's ability to deliver tonnage, where it's needed most: between the die and workpiece. And until recently the only way to increase tonnage in a mechanical press was through bigger presses with bigger motors and flywheels.

But what if a press delivered tonnage differently?

That question spurred a new wave of mechanical press designs. Press-makers removed the main motor, flywheel, and clutch, substituting it all with a servomotor that focused energy only where needed and, in effect, made the ram a controllable axis.

The flywheel-clutch mechanical press likely will remain the industry's workhorse for some time. Still, its servo-driven cousin probably won't stay a niche player forever. Toyota, for instance, has switched several lines over to high-tonnage Komatsu servo presses, producing panels for the Tundra® in San Antonio, Texas, and the RAV4® in Woodstock, Ont. According to Executive Vice President Jim Landowski of Wood Dale, Ill.-based Komatsu, Toyota plans to adopt more servo-driven mechanical presses during the next several years, with the intent to make its pressrooms more flexible.

Flexibility sums up where the servo-driven mechanical press stands in its evolution. Early adopters are seeing that flexibility and asking, "What if?" What if I could control ram motion throughout the stroke and dwell for a certain period at bottom dead center (BDC)? According to sources, those "what ifs" have led to new ways of thinking about forming metal.

"In a servo press, you always know, within a few microns, what the slide position is," said Dennis Boerger, product manager for Dayton, Ohio-based AIDA-America Corp. "That opens up a lot of possibilities."

Capabilities

As Boerger explained, the ram motion of a press can be boiled down to a physics equation: "Energy comes from the mass times velocity, or mass times rotating speed." The faster that source—be it a flywheel or servomotor—spins, the more energy it has. But a flywheel-driven press has inherent inefficiencies. Energy must be delivered from the flywheel through a clutch, down the connecting rods, which drive the ram that provides the maximum tonnage at some point above BDC. The main drive motor then has to get the flywheel back up to speed before the punch hits the material again. For this reason mechanical presses can't run too slowly because the minimized rotating speed of the flywheel won't be able to provide enough energy to produce the needed force to cut through and form metal.

"But if I replace the flywheel and clutch with a servomotor, I can deliver maximum torque at any speed," Boerger said, from next to zero to the maximum rating.

With a servomotor, "you can match the velocity and dwell and stroke [length] based on the application," Landowski said. Consider a part that requires forming through a 3-inch stroke, and say the slide on the press stroke is 7 in. "You can set the stroke length so you travel only 3 inches, allowing for a certain height to clear a flange after it's formed up So, you can come down at a fast velocity, then slow down that last quarter inch to make the form, then speed back up to a 4-inch dimension height in order to clear the flange," maintaining fast cycle times.

"As you shorten the stroke length, you can significantly increase speeds," Boerger added. A hydraulic press also can use shorter stroke lengths, but the nature of hydraulic power gives those presses some speed limitations, he said.

Servo driven mechanical press

For the servo-driven mechanical press, press-makers removed the main motor, flywheel, and clutch, substituting it all with a servomotor that focuses energy only where needed.

Also, because the servo press's slide can slow and the ram can dwell at or just above BDC, more in-die operations such as tapping can occur inside the press. The ram's die, dwelling at the bottom, actually holds the part stable, like a fixture, securing the part as the in-die operation takes place.

A servo press can perform progressive forming under one die. Landowski referred to a titanium eyeglass frame application. Titanium springback can be a bear to deal with, so the application traditionally has called for a progressive-die setup, with each hit forming it 1 in., 1.25 in., 1.5 in., and so on, perhaps through five or 10 steps. The servo press can be programmed to perform all these steps in one stroke, with the ram stopping above BDC and then slowly progressing down to form the part, moving back up, then going back down a bit farther, and so on, until the part is formed.

Because the ram speed can be controlled precisely, the amount of shear can be controlled as well. During prototype work or testing, stampers can see exactly when a fracture will start to occur in the metal, then design the process to suit. Stampers also can combine this with sensors, such as linear glass scales and other closed-loop setups, to monitor off-center loading and account for thickness variation and hard spots caused by variances of carbon in the sheet, depending on the press model and application, sources said.

A shop buys a standard mechanical press with specifications designed for what it needs to do. Not so with a servo press, said Boerger. "The servo drive doesn't care. It can give stroke lengths from 1 to 12 inches long or more. It can give full tonnage at 1 stroke per minute to maximum speed, and you can program the stroke length and the profile." Blanking work can be done one day, deep draws the next.

He added that because the ram represents a controllable axis, the speed may be controlled and reverse-tonnage effects minimized after material fracture. This means blanking operations can use a greater percentage of a press's overall tonnage rating. Using a 250-ton press, traditionally stampers would have blanked at a 125-ton maximum (half the tonnage rating). With the servomotor, a 250-ton-rated press could blank up to, say, 220 tons, depending on the application, Boerger said.

Note, however, that servo presses still cannot deliver full tonnage throughout the stroke, as hydraulic presses can. "The servo press has a tonnage rating curve like a [flywheel-clutch-driven] mechanical press," Boerger said. "A 150-ton standard mechanical press might be rated 6.5 mm above the bottom of the stroke; the higher up the stroke, the less tonnage there is available." The same rules apply for the servo press. The difference? A servo press can stop anywhere in the stroke, then descend to BDC and provide maximum tonnage. How? A servomotor, unlike a flywheel, can provide maximum torque almost immediately.

Servos: Custom or Off-the-Shelf?

During the late 1990s came a fork in servo press development. Some press suppliers decided to develop their own servomotors offering much more torque than anything commercially available. Others used off-the-shelf servos together with leverage components that, thanks to Newtonian laws, multiply torque and, hence, tonnages—up to 5,000 tons to date.

The direct-drive approach has for the most part been limited to lower tonnages, but tonnage ratings have been steadily gaining ground. Direct-drive presses are approaching capacities of 2,000 tons as a result of recent developments, including the higher torque capacities provided by 400-volt motors (double that of previous generations) and the ability to use multiple servomotors to directly drive a single ram, Boerger said. "Instead of having one high-torque servomotor, you can have a gear on the driveshaft and build a housing that holds [multiple] servomotors, all with pinions that drive off the same gear," he explained.

AIDA took the direct-drive route. The company's first motors developed in-house "had five times as much torque as the largest commercially available motor did," Boerger explained. "At the time, [one manufacturer] had one that had about 3,000 foot-pounds of torque. The one we came up with had about 15,000 foot-pounds."

Amada also took the direct-drive approach, using a high-torque, low-RPM motor specifically designed for the company's press. According to David Stone, product manager, the direct drive maximizes the energy; the ram has more energy available along a greater portion of the stroke. "The direct-drive [servo] press can provide more strokes per minute and high energy for the ability to apply force high up off the bottom of the stroke," which, he said, is advantageous for deep draws and similar work.

Amanda SDE Press

An Amada SDE press uses direct-drive servomotor technology.

So what makes these servos different from their off-the-shelf counterparts? As Stone explained, "The fundamental difference is the number of poles in the motor. A standard servomotor may have six to eight [magnetic] poles" that drive the motor rotation, while the motors used in Amada direct-drive servo presses have 24 poles. The more poles, the more torque a motor has at low speeds. This enables the press to develop full torque and energy at fewer strokes per minute.

Even so, due to the physics involved when using a crankshaft to drive the slide, full tonnage isn't available through the full stroke, as it is with a hydraulic press, although the high energy still allows many applications to be run (blanking or forming) at very slow speeds, at 1 SPM or less—something impossible with a flywheel-driven press. Nevertheless, due to stroke-length limitations of a mechanical press, a hydraulic press still may be the best option for extremely deep draws.

For its servo presses, Komatsu took the torque-multiplier route. "We use a standard, off-the-shelf motor and torque multiplier" consisting of a shaft and knuckle arrangement, Landowski said. The latest servo presses using this technology go up to 5,000 tons, he said. It's about leverage; the greater the lever effect (produced in this case through knuckles and rods), the more torque is produced. Also, servos aren't designed to take the harsh ram forces directly, so they're set apart, coupled to the ram assembly with timing belts or other coupling methods based on the press's capacity.

Taking the Tough Jobs

"I haven't had one scenario where the servo press hasn't done it better," said Tom Ward, vice president of Ward Manufacturing Co., Evanston, Ill. "Our standard way of looking at a job became very rigid. I had to run a certain job that couldn't exceed a certain tonnage. We always asked, 'How do I build the tool to withstand the shock of running a certain speed so I can make money?' With the servo press, we threw all that out the window."

In 2004 Ward replaced some 30-year-old equipment with four 250-ton AIDA gap-frame servo presses. The impetus for the purchase came from an upcoming job, but the job itself didn't necessarily require a servo press. Ward said company management looked beyond that one job. "We could have saved money and bought a standard mechanical press, but we asked ourselves, where does that leave us? Does that give us any technical advantage?"

The company saw stamping work going overseas, so to keep profitable, Ward said the company had to focus on precision, low-volume, difficult, "China-unfriendly" work. For instance, Ward took on a job that involved aluminum and a perforated sheet layered on top, designed to provide heat shielding. The inner perforated material had limited formability, tearing easily under the forceful ram of a standard mechanical press. For this application, the servo press could move down quickly, stop just before the material, then form the material extremely slowly, balancing loads and ensuring smooth material flow. "This could all be done in one press stroke," he said.

Ward added that the technology has allowed him to automate material handling. "If I dwell at any point in the stroke, I can come in with a mechanical part extractor and remove the part during that programmed dwell. And to ensure quality, I can tell the ram, 'Give me two seconds while you're at the top so I can confirm, via sensors, that the part has come out of the tool with the extractor.'

"Some jobs that would have taken me four weeks now take me four days," Ward added.

The presses also have freed up enough capacity so that the company could perhaps get rid of some of its 30-year-old behemoths, opening up much-needed floor space. "In that space we could integrate new state-of-the-art equipment, further adding to our flexibility."

AIDA ServoPro Press

A bank of AIDA ServoPro presses allows Evanston, Ill.-based Ward Manufacturing Co. to take on difficult, high-value, "China-unfriendly" work.

Designing for New Technology

For the past year PTL Manufacturing, Belleville, Ill., has used a Komatsu gap-frame servo press to perform deep-draw and forming work. "It can slow down when in the drawing area," said Daniel Stock, vice president of engineering. "When you're at bottom dead center, that's when you're doing all of your work. And for some material, we need to have that ram go at a certain [slower] speed to prevent cracking" in the forming portion of the cycle, while speeding through the rest of the stroke. The press has allowed the company to increase speed for one job from 40 SPM to 75, stamping materials like 1050 spring steel as well as higher-carbon steels like 1095.

But integration at PTL hasn't been plug-and-play. The servo press is a different animal, and with that comes a learning curve. According to Stock, PTL Manufacturing has had to relearn the stamping process. Yes, the new press allows the company to be creative, to control the ram and material flow. But with that control comes a whole new way to operate a press. "We had dies designed with the old technology in mind. If you put a die designed in the traditional way in the servo press, you sometimes may not be harnessing all the [servo press's] capabilities," Stock said.

Specifically, with a traditional die design, all of the piercing and forming operations happen at BDC. "With the servo, you have more flexibility, with more tonnage available at different locations. It allows you to start engaging your stripper at a different time," along with the piercing pilot and forming operations.

This brings up a tradeoff, Stock said. If a die design takes advantage of the added capabilities of a servo press, the die may not run on standard mechanical presses.

Servo's Status

Servo-driven mechanical presses won't replace their standard counterparts any time soon, sources said. The flywheel-driven mechanical press still can do high-volume, relatively straightforward work faster and cheaper than the servo press. And the hydraulic press still remains the only technology with the maximum tonnage available throughout its stroke, ideal for extremely deep draws. (But sources said the stroke's positioning accuracy does not match that of the servo press, which can move a ram to a certain point within a few microns.)

Boerger said he sees the servo press undergoing the same "adoption curve" as other servo technologies used on the stamping house floor, such as servo-driven coil feeds and servo transfer mechanisms on a transfer line.

"I believe the servo press is going to change the landscape of stamping," Stone added, "but it's not going to happen overnight." If a traditional mechanical press is working well for a company now, it most likely will work in the foreseeable future. Areas where Stone sees the greatest impact include complex forming and exotic-material applications in which parts can't be formed any other way.

As Ward put it, the servo press will push shops toward high-value, creative work—in other words, China-unfriendly. "For us, that's a very good place to be."



FMA Communications Inc.

Tim Heston

Senior Editor
FMA Communications Inc.
833 Featherstone Road
Rockford, IL 61107
Phone: 815-381-1314

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