Colorado Springs, CO —Sometimes the best screws are the ones you don't use. Just ask Rick Euker, R & D mechanical designer for Agilent Technologies. "I'm biased against screws," he admits, noting that they add complexity, assembly time, and cost. So when Euker designed an enclosure for a new strain-gauge remote conditioning unit, he abolished screws in favor of integrated fastening features. "The whole unit pretty much snaps together," he says.
The result of Euker's war on screws? While he counts 40-plus loose screws in comparable rack-mounted enclosure designs, the new Agilent enclosure employs just a single screw—and that one for an electrical ground connection, not fastening. The whole remote unit, a 14-piece object lesson in parts consolidation, assembles in roughly two minutes and disassembles in about 45 seconds. Euker says that conventional enclosures he looked at had about 60 loose parts and took about 1/2-hour to put together. "And that was once you had some practice," he says.
Even without screws, the resulting box is plenty rugged. As a remote unit which sees use out in the field, the enclosure needs to withstand a battery of environmental, vibration, and impact tests. "We have a test manual as thick as a phone book," Euker says, noting that the screwless enclosure components come together with a sufficient retention force to survive a 32-inch drop test and a 6-G slam test.
While notable for its elimination of loose fasteners, the housing also embodies the first commandment of design for manufacturability and assembly (DFMA): "Thou shalt consolidate parts." In fact, Euker can list several functions, including accessibility, that would likely have required separate parts in traditional designs.
Self-clinching keyhole fasteners, which install to become a permanent feature of the sheet metal, let the motherboard slip on and off the bottom cover.
Euker began his attack on screws with unit's top and bottom sheet metal covers, which come together with the help of two built-in features. For the rear of the unit—which measures 1.75×19×9 inches—Euker chose a common "poor man's hinge," or offset tabs lanced into the sheet metal. A less common snap-fit feature lets the sides of the cover grip one another. The two covers likewise attach to the front panel without screws, instead relying on a folded "lip" that engages in the extrusion that forms the unit's front panel.
The "screwless" theme continues inside the box, where two kinds of self-clinching fasteners from Penn Engineering & Manufacturing Corp. (Danboro, PA) secure the remote unit's internal components. Four PEM SNAP-TOP stand-offs attach the unit's power supply and vibration-damping rubber bump pads to the bottom cover, while eight PEM KEYHOLE fasteners let the motherboard slip on to the bottom cover.
According to Euker, self-clinching fasteners do double duty from a DFMA perspective: Unlike loose fasteners, they arrive at Agilent as an integral part of sheet metal, reducing parts count. And as their names suggest, these two fasteners allow quick assembly though their respective snapping or sliding connections.
Other internal features also helped Euker do away with screws. The motherboard, for instance, slips into a groove in the front panel extrusion. And Euker specified only communications and electrical connectors that would snap into the enclosure—such as the ac power receptacle he found by "digging deep" into his stack of component catalogs.
Multiple functions. A true DFMA effort, the enclosure not only skimps on loose fasteners but also cuts the number of components needed to perform non-fastening functions. Euker's list of functions for the box that normally would have required separate parts includes things as simple as silk-screening the wiring diagrams and instructions on the covers.
His list also includes more ingenious integration instances. To take one example, a series of bumps punched in the lower cover provide RFI shielding instead of a separate gasket. Euker explains that the bumps mate with dimples in the top cover, tightening the fit enough to offer shielding for the 30 MHz remote unit. Another example can be found in the top cover, where two more self-clinching standoffs from Penn physically back-up the remote unit's 64-pin connector in order to offset insertion forces of up to two lbs.
Even the enclosure's single screw—which the electrical designers needed for grounding purposes—performs a second task in that it adds a structural reinforcement point for the top cover. "We placed the screw where the cover was most likely to warp," he says.
Why did he do it? Euker's approach to cutting parts by consolidating functions might be torn right from the pages of a DFMA handbook, but he adds that the most important DFMA strategy may be personal. He acknowledges that the relatively low production volumes of the high-end strain gauge didn't necessarily call for a revamping of the enclosure design. In fact, a "knock-off" of conventional enclosures would have fit within Euker's $50,000 tooling budget. "There wasn't a lot of pressure to come up with a better design in this case," he says.
So why take the time to design something better? Sure, cost reduction is always attractive, and the DFMA enclosure cost less than $2,000 for tooling—not to mention lower assembly costs. Field reliability may improve too, as all the quick-releasing components make servicing a breeze.
But the real reason for design improvement, according to Euker, often comes down to a personal zeal. "Management isn't always interested in the nuts and bolts," he says. "It's up to the individual design engineer to develop a passion for building simple, low-cost things that work reliably."