Without line components having the capability to run within the parameters of unique servo press motion profiles, the high-performance, higher-cost machine becomes just another stamping press. “From a transfer-automation perspective, we will ‘supercharge’ our systems to handle servo press technology,” Stirrett says, adding that for its part, Linear employs special software in this effort.
By ‘supercharge,’ Stirrett means designing a transfer system’s drive train—increasing the sizes of drive systems and motors in the X, Y and Z axes, for example—to account for quicker speed changes and stresses that come with operating within pendulum mode.
“With pendulum motion squeezing the time available per stroke for automation,” he says, “we need the automation to run faster, as much as 100% faster than normal operation with a 360-deg. press drive-shaft rotation. We don’t want to be the bottleneck on a servo press line.”
Factors that affect part-transfer automation also affect coil feed. In a recent Tooling by Design column in MetalForming, author Pete Ulintz offered an additional explanation specific to material feed:
“Factors affecting critical angle include press speed (strokes/min.) and press stopping time. Press stopping time increases with press speed, which in turn changes the critical angle. Now the sensor must signal the press to stop earlier in the stroke, resulting in a smaller feed angle. This means that the feed cycle also must be completed sooner, requiring greater feed acceleration.
Feed-roll slippage becomes an issue with higher feed accelerations, producing a feed-length distance that measures less than the programmed feed length.”
Again, coil-feed equipment also must be supercharged to deal with servo press motion profiles.
Program With Automation in Mind
When programming slide velocity in the portion of the press stroke where automation unfolds, keep in mind details such as the angle at which automation must grab or set a part into the die, as some angles may prove difficult for part grabs and for proper setting into the die.
“Faster slide velocities through transfer automation and material feed increase the possibility of losing control of the material,” Phillips says.
And, not only must transfer automation be complete, but also must be out of the way once the critical angle is reached.
“Automation may have correctly positioned the part material into the die, but then the die smashed the transfer arms,” says Phillips. “The metal former must factor in the time needed for automation to move fully out of the way. This is a major cause of crashes. Transfer arms, robot arms … I see crashed end-of-arm tooling when metal formers don’t factor this in during changes to pendulum mode—changing the angle or speeding up the stroke.
“Instead of efforts to address all of this, sometimes metal formers unfortunately run servo presses at 360-deg. rotations, effectively operating these higher-investment machines as conventional presses,” he continues. “They are not optimizing press strokes and doing other things that make servo technology so game-changing.”
Over the years, press and automation controls have helped metal formers better position themselves for servo press success.
“Initially, standard servo press controls had difficulty with automation when running in pendulum mode,” reports Lee Ellard, North American sales manager at Stamtec Inc. “Because the drive shaft did not make a full revolution and reversed instead, the control had to be made to think that a full cycle had been completed. Controls didn’t quite have that full capability, but I think by now that has changed and controls can coordinate movement across the press line. Certainly, timing automation is tricky with some of these servo-motion profiles.”
Tips for Improved Part Forming
A major selling point of servo-driven presses involves slide-velocity-slowdown programming to prevent snapthrough shock and help protect tooling and press components—and reduce the ear-splitting sound.
“It’s not just a matter of hitting tougher material harder,” Phillips says.
Another not-so-well-known benefit of this capability offered by Phillips: an increased cut line and resultant higher cut quality.
“The controlled slowdown—while maintaining force—first compresses the part material upon die contact, followed by cutting and then material fracture, or snapthrough” says Phillips. “Increasing the cut line through controlled cut-through reduces snapthrough and provides a cut quality that can approximate that of fineblanking. This means less burring and tighter tolerances, thus metal formers may not need to grind or tumble-finish parts after forming, saving a costly and time-intensive secondary step.
“Success here,” advises Phillips, “requires increased attention to tool positioning and die timing. Improving these tool characteristics helps improve servo press forming while increasing tool performance and life.”
And, a pulse-type mode—servo press manufacturers have various names for this feature—combats springback. Here, the tool basically applies force to the part material, withdraws a bit, then descends again a bit deeper, and continues in this manner until forming completes. This reduces springback along each successive step, but tasks metal formers to ensure proper sensor operation and pulse timing to enable any automation, lubrication and other functions during pulsing steps.
Metal formers also can instantly pause a servo-driven press anywhere along the drive-shaft rotation to perform any needed automation or other tasks. While not improving productivity, this capability ensures inclusion of all needed complex-part-production parameters.
Ultimately, servo press success depends on finessing the slide velocity to eliminate wasted motion, enable automation, and ensure proper speed and force when needed, combined with attention to tooling characteristics. One more piece of advice to gain the most from servo presses: Keep up with press maintenance.
“Ensure ram parallelism and gib clearances to give tooling the chance to work at its best,” Phillips says. “Otherwise, metal formers can’t take advantage of less burring and higher part quality if tooling requires higher clearances to compensate. And, tooling may collide and wear out more quickly with an out-of-parallel press.” MF
Industry-Related Terms: Center,
Die,
Forming,
Ram,
Run,
Springback,
Stroke,
Transfer,
Forming,
Dwell Time,
Forming,
StampingView Glossary of Metalforming Terms
See also: Stamtec, Inc., Linear Automation Inc.
Technologies: Stamping Presses
Consider a servomechanical press operating in pendulum mode. This mode enables slide movement within a narrower range—say, 90 through 270 deg. rather than through a full 360-deg. rotation (see figure on following page). Reversing the direction of the slide before reaching top dead center eliminates unneeded motion and decreases stroke length, allowing for increased production. For a job that runs at 60 strokes/min., for example, the ability to reduce 0.5 sec./stroke via pendulum motion and a shorter stroke gains 30 sec./min, 30 min./hr., etc. Foregoing that means lost production time that never can be regained—a powerful argument for this technology. 
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