CNC Die Cushions incorporate several different features depending upon the installation or application. These functions may include:
1.Pressure Control during Down Stroke
2.Position Control during Return Stroke
3.Hold Down
4.Pre Acceleration
5.Cushion Stroke Adjust
6.Cushion Drop
7.Teach Stroke
These cushions are more expensive. They involve linear and pressure transducers, PLCs, programming, electronic wiring, transformer, servo valves, driver cards, hydraulic power units consisting of filtering, cooling, pump / motor, motor-starters, manifolds, hoses & plumbing.
Usually they combine gas pressure with oil control. This is done to minimize the cost of the hydraulic system - its flow, cooling, pressure drop and valve size. During cushion down-stroke, gas provides the base tonnage with the hydraulic providing the adjustable tonnage. On the return stroke, gas provides the return (stripping) power and the oil provides the position control to prevent slamming.
At bottom of stroke, in hold-down mode, the hydraulic has to be able to stop the gas forces from returning the cushion. When released to position control, hydraulic pressure drop, which generates shear and heat in the oil, needs to be routed out of the cushion back to the reservoir/cooler.
Pre-acceleration is normally only used on very large cushions where the inertial mass of the cushion resists a radical change in velocity. Upon ram contact the cushion moves from still-stand to ram pick-up velocity. This causes an undesired cushion tonnage spike resulting in coining or splitting of the work piece. Pre-acceleration ahead of the ram contact minimizes this but requires a power source independent of the press that must be re-charged at the given strokes per minute. It also requires accurate timing coordinated with the cushion stroke.
Cushion stroke adjustment is done when the cushion is in its de-energized state. In smaller presses, this is done by hand. Larger cushions it is done with worm gears or by hydraulic stop. Our cushions use the hydraulic stop method.
Cushion drop is used on large "rolling bolster" type presses. The extra cushion stroke length is necessary to have the cushions position themselves LOWER than the bolster trolley. This drop length must be added to the active stroke. It is only used when changing out a die using trolley swap.
A teach stroke is used during job setup to give the electronics a chance to test tonnage and position control for proper timing. The faster these changes occur during an actual working stroke, the more difficult it is for the valve and loop closure to keep up. This may require using fewer setpoints for the curve programmed to be followed.