Page 26 - MetalForming April 2012

Piper Picked a Perfect Press
Scott Pryer, president of Pryer Tech- nology Group, adds: “Using older press technology when trying to form a dif- ficult part, metalformers can experi- ence differences in pressure from cycle to cycle, and the position of the punch can vary. The skill and experience of the machine operator needs to be very high in order to deal with the variabil- ity. What metal fabricators want is a machine to do the same thing every
time, so that operators can focus on other areas of the operation.”
For his Triform presses, Pryer opted for an RMC multiaxis programmable electro-hydraulic motion controller from Delta Computer Systems. The result is precise position control to ±0.002 in., and pressure control to one percent of full scale. On fluid-cell press- es such as that used at Piper, the Delta RMC controls the cylinder that varies
the bladder pressure. On deep-draw machines featuring hydraulic punch cylinders, the RMC controls two motion axes: the punch cylinder that pushes the tooling into the bladder (see figure), and the cylinder that varies the bladder pressure. On larger Triform models, the RMC pulls triple duty by also controlling movement of the con- tainment clamps and the upper cham- ber that closes around the bladder and workpiece.
Programs developed for the motion controller allow operators to select pro- duction recipes that contain as many as 30 different steps in a press cycle, each step corresponding to a target bladder pressure and punch position (on deep- draw presses). Besides the target values, motion instructions specify how the machine is to get to the next target. At the beginning of a machine cycle, a quick-move instruction sends the punch cylinder axis to its initial posi- tion, while the bladder is precharged to a particular pressure. Then, during forming, the punch axis is given a posi- tion command with an acceleration and deceleration parameter for each process step. Meanwhile, the RMC sets bladder pressure and coordinates it with punch-axis movement. Bladder pressure is monitored and fed back to the motion controller by a pressure transducer typically capable of reading between 1 and 10,000 psi. To ensure precise punch operation, control out- puts from the motion controller to the punch hydraulics connect to a servo- quality proportional valve.
“The trick is to avoid wrinkling the sheet as it bends around the corners of the tool,” says Harrelson. “To accom- plish this, most of the programmable motion steps in each press cycle occur within the first inch of punch travel.”
“Our customers can do more than they ever thought possible because they’re controlling to a very precise level,” adds Pryer. “In the past, depend- ing on the complexity of the forming projects, a fabricator could experience scrap rates of 10 to 25 percent. Now, with a repeatable process we see scrap rates near zero.” MF
24 MetalForming/April 2012
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