Anritsu Co. (Morgan Hill, CA), a provider of test and measurement devices for telecommunications, optical and wireless systems, has replaced traditionally machined radio frequency electromagnetic radiation (RF) shields with precision die cast parts in its handheld cable and antenna analyzers. The move was designed to reduce the unit cost of about a dozen different shields across the Site Master Series of its analyzers.
“This family of analyzers helps field technicians perform verification and fault location testing to detect problems before they become costly, time-consuming system failures,” explains Process Engineer Sam Krull. “Their capabilities include precision return loss/voltage standing wave ratio (VSWR), cable loss and distance-to-fault (DTF) measurements. The RF shields are an important element in these designs, as they prevent crosstalk within the devices.” Site Master Cable and Antenna Analyzers are used in the installation, troubleshooting and maintenance of microwave cables and communication systems in the cellular and broadcast, and aerospace and defense industries.
“Most of these components were produced by traditional machining from plate or bar stock at first,” Krull explains. “Because we’re a low-volume, high-mix manufacturer, we sometimes don’t have the opportunity to switch to more cost-effective production techniques. But when we do see a chance to make a change, the earlier we take that step, the more we can save over hog-out machining. We’ve seen payback in these parts in as little as three months.”
Anritsu contracted Alloy Die Casting (ADC, Buena Park, CA) to develop the replacement designs cast from A380 aluminum. “These shields are fairly complex, which causes some casting firms to shy away from smaller production runs,” observed ADC Design Engineer Gary Gray. “But we’ve developed a process for designing die cast tools and manufacturing techniques for intricate, close-tolerance parts that other shops might avoid in low to medium volumes.”
According to Gray, the dimensions of these shields were part of the challenge. “These are very thin-walled designs,” he says. “Some of them are just 0.03-inch thick, with fairly complex geometry. Any time you get below 0.06 inch in an aluminum casting, you can run into complications, so it’s important to get the right balance of injection speed and venting capacity, as well as very precise temperature control.”
The shields are all cast from single-cavity tools, typically on a 250-ton press. Raw ingots are melted in a furnace at approximately 1,200 F. The small 2-inch plunger size delivers a fill time of about 25 milliseconds (0.025 seconds), yet maintains a relatively low gate velocity around 1,200 inches per second. Depending on the specific shield being cast, shot sizes range from 4 ounces to about 1 pound, and ADC usually runs at a conservative speed of 70 to 80 cycles per hour.
“We want the injection speed to be fast enough to fully atomize the material and give us a quick fill time,” Gray explains. “But we have to temper that with the understanding that higher gate velocities will erode the tool steel more quickly, and can shorten die life.”
After the cast parts are removed from the mold, finishing includes a straightening operation and as many as 19 through-holes, two of which are threaded. ADC uses a single-spindle computer numerical controlled (CNC) manufacturing center for machining, drilling and tapping, which requires approximately 6 minutes apiece.
“Drilling the holes rather than casting them in place helps us to maintain closer tolerances and minimize pin breakage,” says Gray. “If we were running large quantities at a time, we’d consider a multi-spindle drill operation, but in these volumes, drilling one at a time is still pretty efficient.” The shields are then sent out for anodizing to help protect them from corrosion.
- Alloy Die Casting, a Sanders Industries Co.