Page 23 - MetalForming November 2019

management, and how that can impact AHSS forming. As Patil explains, more press energy and tonnage are required in order to form a part if that same part must be produced from AHSS instead of medium- or low-carbon, or high-strength, low-alloy steel. Mechan- ical presses use flywheels to store ener- gy and then provide it during forming operations. Servo-driven presses pro- vide alternatives. Aida, for example, employs a capacitor-bank energy-man- agement system, where capacitors store energy in DC form, with capacitor sizes based on a user’s speed and stroke requirements. Here, the capacitors are charged continuously, and when the servo motors need excess energy, such
as when drawing or coining, the capac- itor banks bump up the supply. During the nonforming cycle, incoming elec- tricity charges the capacitors so that they can supply extra energy when needed.
For slower-running applications, such as those often associated with AHSS, capacitors have more time to accept a charge, enabling full charging, and pro- viding more energy, more efficiently, throughout the stroke for forming.
Quicker Advance and Return Aids Strokes/Min.
Even if more forming energy becomes available with a servo-driven press, strokes/min. and, in turn, pro- ductivity suffer due to the need for slower forming speeds when working with AHSS, right? Not so, Patil explains, thanks to the adjustable velocity along the stroke length, an important advan- tage inherent in servo-driven presses.
Suppose that part production includes a relatively deep draw of dual- phase (DP) 1400-MPa steel. Obviously, forming this material requires more energy than when working with DP 600- MPa steel, and may, for example, entail the need for six draw stations instead of three. And again, forming speed must be slowed. Whereas parts from the lower-strength material may be pro- duced at 30 strokes/ min., working with the stronger material may require speeds not surpassing 15 strokes/min.
Stampers employing a servo-driven press can run at 15 strokes/min. during forming, but during all nonforming portions of the stroke, the press runs at top speed, thus achieving an overall speed of 30 strokes/min. and keeping production on track.
Effective in Battling Springback
Springback represents a huge chal- lenge when stamping AHSS. At the EWI conference, Patil provides the example of a wing-channel project, where DP 350-MPa steel exhibits significant springback. Due to workhardening, separate restrike stations won’t work. A shop-floor servo-press solution
involved restrikes in the same cycle at bottom dead center, again, owing to the ability of servo-driven presses to control motion during the stroke. With- in that single cycle, the tool restrikes two more times at bottom dead center for about 0.2 sec. each time, and uses heat as an ally to provide improved part definition and set the wings at required 90-deg. positions.
Reverse-Tonnage Reversal
Patil sees more and more cases of reverse tonnage, or snapthrough, in his travels, especially on older presses built before the dawn of AHSS. Reverse tonnage results from the release of pressing force when the part material fractures. This energy and vibration transfers through the tooling and press, and may result in significant damage. Presses not built to deal with the stress- es and strains of forming AHSS are can- didates for succumbing to reverse ton- nage. Patil asks how a metal former without tonnage-monitoring capability knows if a press is experiencing reverse tonnage.
“When you see pieces falling off of your press, that’s a sign of reverse ton- nage,” he bluntly assesses.
Shock dampeners provide some protection on traditional mechanical presses, says Patil, who offers the vari- able-stroke-speeds benefit of servo presses as a reverse-tonnage answer. Using a pressroom example, Patil describes a blanking die placed in a standard crank-motion mechanical press. Measurement reveals 114 tons of forward press force and reverse ton- nage of 9 tons. The same die then is placed in a servomechanical press, with the ram descending rapidly prior to slow blanking. Not only does the bot- tom-stroke speed reduction lower the noise level significantly, from 101 to 75 dB, but forward force reduces to 104 tons, with reverse tonnage dropping to 2.6 tons, well within the recommend- ed reverse-tonnage limit of 20 percent. Aida-America, Patil reports, will present this demonstration of a 70-percent- plus reverse-tonnage reduction at FABTECH in Booth D46027. MF
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