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Casting in Control
Multi-axis control within 0.050 inches permits precise part pulling
Rick DeMeis -- Design News, July 7, 2003
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Control Key: The Delta Computer motion controller matches slave cylinder position to that of the master cylinder. The PLC is the larger box in the middle of the rack. |
Entiat, WA — Entiat, WA—In order to achieve fine-grained aluminum vacuum or pressure vessels, up until now they most likely had to be hogged out from solid metal billets or forgings—an expensive process that could produce much wasted metal. Now thanks to precise motion control, engineers at US-Castings have developed a less expensive casting process to produce such high-quality vessels.
The process yields very fine-grained (i.e., small crystal substructure), and thus tougher, aluminum castings compared to sand castings—whose larger grains are prone to leak or trap gases in their crystalline structure, precluding use as pressure or vacuum containers. But because they are cast, the fine-grained cast vessels can cost up to 50% less than their machined counterparts.
Gentle giant
One key to the process is the use of metal mold liners, which allows quick cooling of the molten aluminum—cutting the growth of crystal grains in the finished casting. But one challenge US-Castings engineers had to overcome was the need for precise control over the high force (up to 24 tons) needed to remove the casting, mold liner, and plugs from the die cavity. Casting tolerances dictated precise positioning of the mold as it was opened, according to Bill Brown, US-Castings machine designer. "Finished casting tolerances can be ±30 thousandths of an inch," he notes.
Adding to the challenge of controlling the rams at the four corners of the mold were the steep walls of the mold cavity—as little as one degree over an 18-inch "deep draw" mold depth. "The hydraulics have to operate within a tolerance of 50 thousandths of an inch. If any one of the cylinders didn't, we wouldn't be able to achieve the dimensional requirements of the finished product, and we might cause damage to the machine and tooling," Brown says.
While the four hydraulic jacks move in unison, the force produced by each ram may vary greatly (with pressures varying up to 2,500 psi), because different areas of the casting may tend to bind differently with the die. The engineers chose a master-slave control architecture for this four-axis system—one cylinder is a master whose position is tracked by the other three, each applying enough force to overcome the friction between the casting and the mold.
To make it work, Brown's team looked for a motion controller tailored for multi-axis applications involving control of both ram pressure and position. As it turned out, position was the vital driving parameter with pressure being applied as needed to produce the correct displacement. "If we didn't have servo control that held position, regardless of force, the castings could hangup in certain areas during removal," notes Brown. Bensonville, IL-based
Parker Hannifin (www.parker.com), US-Castings' hydraulics supplier, recommended the RMC100 controller from Vancouver, WA-based Delta Computer Systems (www.deltacompsys.com).
The RMC100 can control up to eight axes, and has control and feedback provisions for linking slave axes to a designated master. In the US-Castings system, an MTS Temposonics (www.mts.com) magnetostrictive displacement transducer (MDT) provides position feedback from each cylinder (see figure). The Delta controller provides direct interfaces to these transducers.
Another positive for Brown was the motion programming and tuning software that came with the controller. "Programming motion sequences using the RMC's event step table is as easy as working with a spreadsheet. I had many other complex issues to deal with. It was good that the controller was easy to use," he adds.
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