Page 17 - MetalForming Magazine May 2023

Ram driven by hydraulic cylinder located above
Bolster plate does not move
Pneumatic cylinders Hydraulic cylinder Servo valves
Slide magnetostrictive linear displacement transducer (MLDT)
Die
Die cushion descends as the ram contacts the die
Cushion MLDT
Manifold with pressure transducers
Another challenge: responsiveness of the valve. Should the cushion’s hydraulic controller wait too long to begin increasing oil flow to the valve, the time lag could damage the work- piece. The hydraulic oil must be moving in time to move the cushion just as the ram readies to make contact.
Dayton Die Cushions’ solution to this challenge: a programmable motion controller that could set up a virtual motion axis to which a physical axis could be geared. ‘Gearing’ refers to the ability of the motion of a slave axis to follow a master axis, which can be another physical axis or a virtual axis that exists only within the motion controller.
“We created a virtual closure of the die-cushion control loop before the ram hits the cushion,” Pedersen says, “which no one had done before.”
With the virtual control loop closed, oil flow initiates before the die cushion contacts the ram, thus the two hydraulic systems come together smoothly, even with the ram moving quickly and with tremendous kinetic energy.
To achieve high dynamics, the team at Dayton Die Cushions selected a servo valve with high flow but very low mass. Other objectives: minimize the volume of oil flow, reduce the size of the hydraulic power unit and reduce oil conditioning, i.e., cooling, filtering, etc. Constructing this die-cushion application entirely with hydraulics would add difficulty in ensuring move- ment of enough hydraulic oil in the volume and time required, as the cushion measures 125 by 69 in. and must provide 250 tons of maximum holding force. As a result, Pedersen and his team incorporated two passive pneumatic cylinders beside the hydraulic cylinder (Fig. 2) to reduce the amount of hydraulic oil to be moved. The pneumatic cylinders pro- vide the base tonnage of the cushion, and the hydraulic cylinder provides the variable tonnage that assists in shaping the workpiece.
Fig. 2—This illustration shows layout of a die cushion in a press at Spartanburg Steel Products. A rapid development and installation process helped ensure the ability of the manufacturer to meet part-production requirements for a new automotive-model year.
platforms that reside under the die in the press, move to cushion and oppose the motion of the main press ram (Fig. 1). A die cushion typically pinches the periphery of a workpiece with var- ious pad pressures to control the rate at which the workpiece is pressed into the die by the ram. The use of die cush- ions enables manufacture of automo- tive components, such as body panels, to tight tolerances and with features such as bends that prove unattainable with press-only stamping operations.
Given such requirements, Dayton Die Cushions manufactured a die cush- ion (Fig. 2) and retrofitted it to a stamp- ing press at Spartanburg Steel Products, Spartanburg, SC. Not initially designed with a die cushion, the press was iden- tified as a candidate by plant managers, who understood that to supply more of the new-generation body panels to automotive manufacturers, they would need the capabilities and precision offered by such a device.
Cushion, Motion Control Meshed to Press
Challenges to be met during the retrofit project at Spartanburg Steel Products included fitting the new machine components within the phys-
ical constraints of its existing press. “We had to take the motion of the existing ram into account to make sure that the new cushion could work with it,” says Tom Pedersen, Dayton Die Cushions manager. “The old press had a distorted bed, and we needed to align the structure before we could add our hardware. We reconstituted the bed with adjustable guides that we aligned
with a 3D laser to within 0.001 in.” And, the new die cushion needed to contact the workpiece with the press ram already in motion and moving at a very high velocity, which in turn required the hydraulics to act before the ram contacted the die. This con- trasts with traditional press operations, where the ram hits the cushion before the cushion moves, with the contact causing hydraulic pressure to spike and generating an error in the closed- loop algorithm, which produces a response according to the algorithm. However, such a method would not work in the press application at Spar-
tanburg Steel Products.
“We couldn’t tolerate the collision,”
Pedersen explains. “Instead, we looked for a way to predict the collision and take action prior to the ram hitting the workpiece.”
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