When the U.S. Department of Defense asked one of its key contractors to reduce the cost of a five-part assembly on control indicators for military avionics, engineers were faced with a difficult challenge: cut the expense, but continue to meet the same strict standards for performance and aesthetics.
The unlikely goal was achieved with the help of Alloy Die Casting (ADC, Buena Park, CA), which reduced total cost of the five parts by an estimated 60%, despite low volumes of around 2,000 parts per year, without sacrificing the reliability, longevity or visual appeal of the original design.
Viasat Inc. (Carlsbad, CA) had been machining all five of the parts individually, maintaining the close tolerances and smooth finish required to pass the customer’s 100% testing requirements. Faced with the new cost-cutting request, the firm turned to ADC to see if there was a way to reach the same quality standards with greater cost efficiency.
“These indicators are used in standard military airborne avionics,” says Viasat Design Engineer Lon Plourde. “The assembly involves a heat sink, front panel, or bezel, access door and two door pivots. At first, we were concerned that die casting wouldn’t be cost effective, because of the low volumes and high quality requirements. But ADC showed us how it could be done so efficiently that our investment would pay off after just 600 parts.”
The assembly is subjected to harsh military environmental requirements, and must withstand operating temperatures of -40 C to 55 C, as well as storage temperatures from -62 C to 85 C. All parts are required to successfully resist 100% humidity, and must survive difficult shock and crash loads according to MIL-STD-5400.
The repeatability of the die casting process was an additional benefit for Viasat. “Our customer is very sensitive to the aesthetics of these parts,” says Plourde. “When you die cast, the result is fairly predictable, and if you run into a problem, it is typically corrected in the tool and it goes away.”
Casting ProcessEven though Viasat’s Carlsbad location had never specified a die cast part before, it had one advantage: instead of presenting a blueprint and requesting a quote, Plourde and his team sat down with ADC tool designers, who made suggestions for minor changes to improve manufacturability and strength, while optimizing tool performance and durability.
“ADC provided intuitively sound advice throughout the process,” Plourde adds. “They laid out a detailed timetable for each stage, and they delivered on it.
“In addition to the cost and aesthetics, one of our primary requirements was the thermal performance of the heat sink,” Plourde continues. “The design had to dissipate enough heat energy to maintain component temperatures, and we found through testing that we could achieve the thermal properties we needed using A360 aluminum.”
The heat sink is a complex design, approximately 5 inches by 3 inches with a series of 25 rectangular “windows” arranged in five columns. Cast on a Toshiba 250-ton press, the 2-inch plunger delivers a gate velocity of about 1,200 inches per second, for a 30-millisecond fill time.
“The large gates on this tool help us get a quick fill, without a lot of turbulence,” says ADC Design Engineer Gary Gray. “The relatively low inlet velocity also helps minimize tool erosion.”
Weighing in at 0.13 pound after casting, the parts require about five minutes of machining time, during which they get a number of drilled and tapped holes. The heat sink also gets nine helicoil inserts, requiring that the cast parts be kept flat within 0.01 inch to accommodate them. “We used ejector pins at virtually every intersection, all the windows and corners, to ensure that the parts came out straight, with a minimum of torque that could lead to warping,” Gray says.
The control indicator bezel is somewhat larger, as the heat sink nestles inside, measuring about 5.75 inches by 3.35 inches. Cast on the same size press as the heat sink, this panel requires a 35-millisecond fill time, yielding about 60 of the ¼-pound parts each hour.
Because the heat sink is designed to transfer thermal energy away from components inside the indicator and out through the panel, it must make complete surface contact with the bezel for maximum heat transfer. That means the same flatness tolerances have to apply.
“The bezel is the most cosmetic part of the five,” Gray observes. “It gets quite a few secondary operations, including trimming, sanding, drilling and tapping to accommodate switches, controls and gages. After hand cleaning, it also receives a chemical film coating for corrosion resistance, and then it is selectively powder coated, with key areas masked off for electrical contacts or other design considerations.”
The door is a much smaller part, measuring 2.5 inches by less than ¾ inch and having a cast weight of less than 1⁄10 of a pound. With so little metal, the fill time is very short, about 20 milliseconds even at a reduced gate velocity of 1,100 inches per second on the 135-ton press. The design was not without its challenges, however.
“This part gets a small hole at either end,” Gray says. “It is just 0.068 inch in diameter, with a tolerance of 0.005 inch. That is pretty small for casting in place with aluminum, but we did not want to machine such a small part any more than was necessary. There is also a pair of 0.025-inch holes in the door design, which were just too small to cast. Those we have to drill,” Gray adds. Like the bezel, the door gets a chemical film after machining, and then is masked and powder coated.
The two door pivots are a matching pair, originally quoted in the same aluminum alloy as the other components. But Gray and his design team found that they could achieve the fine detail required for this small part by casting it in zinc instead, taking advantage of zinc’s superior flow properties and ability to maintain close tolerances.
Because of the hole locations and small part size of less than ¾ inch by ½ inch and weighing barely 2.5 ounces, ADC casts the door pivots on a Techmire 4-slide machine. The technique can produce very thin parts, often 0.02 inch or less, and tolerances of 0.001 inch are not uncommon. The result is a tightly controlled, high-speed process that can yield thin-walled components so complex they would need to be assembled from two or more pieces if alternative casting methods were used. After casting, the door pivots are finished with chemical film, masking and powder coating.
Alloy Die also provides final assembly of the door pivots, with springs and other components in place, ready for subsequent manufacturing steps in the Viasat facility.
- Alloy Die Casting