These Fins are Made for Walking (Tech Trends Feature)

Designcraft brings underwater gear onto dry land with Pro/ENGINEER.

Somewhere in Lake Zurich, Illinois, approximately 30 miles away from Chicago, Casey Stahl slipped his feet into a pair of swim fins. What he was doing would have made a lot more sense if he were going scuba diving in the 250-acre lake roughly two miles west of his office. But he wasn't. He happened to be in the middle of a machine shop littered with black and yellow pieces of resin. Unperturbed by the strange looks from the machinists standing nearby, he proceeded to walk in the swim gear. And amazingly, he managed to do it with-out teetering over.

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Normally, the fins are the last items that divers put on before they go into the water and the first items they take off when they're back on shore. The oversized rubber flippers are never meant for walking — unless they're designed to be amphibious, like the Flip Fins that Stahl was taking for a stroll.

Flipping Out

Last year, Omega Aquatics approached Designcraft, a full-service product development firm, to help realize its new creation — high-performance swim fins that divers can walk in. Unlike the fins currently on the market, Omega Aquatics' model would feature a pivoted blade attached to the foot pocket by a spring. When underwater, a diver can lock the blade into place with just a few kicks. When a diver exits the water, he or she can snap the pivoted blade upright to turn the swim gear into something resembling a boot. Stahl, a designer at Designcraft, recalled how it all began.

"[Omega Aquatics] had a basic idea. It was an outline, an AutoCAD sketch," he said. "We were hired to take the design all the way to initial production. The most critical part was creating all the 3D files. It needed them for prototyping and tooling."

Immediately, Stahl and the other designers noticed a few conceptual issues with the design. For ease of use, the designers recommended repositioning the self-locking mechanism that controlled the blades. But that change meant Stahl and his colleagues could no longer reuse the original AutoCAD drawing. They had to start afresh.

Solids, Surfaces, and Tangents

Having used and taught SolidWorks and Rhino, Stahl felt he was fully equipped to handle both complex surfaces and solids. But when he took the job at Designcraft, he found himself working in Pro/ENGINEER, a program he revered but with which he was unfamiliar.

"I suppose, if I had really wanted to, they might have let me use a different [CAD package] and import it into Pro/E," he reasoned. "But you lose a lot in the translation." He knew this because he often had to clean up the clients' IGES and STEP files that contained errors: "edges that don't meet or missing surfaces; for instance, pretty disastrous stuff," he said.

In the end, Stahl decided to rely on the Pro/E ISDX (interactive surfacing design extension) module. "Pro/E is very dimension-driven," he observed, "but with the ISDX module, you're controlling the curves through their tangency. It gives the surfaces a much more organic look and a natural flow."

Something Borrowed, Something New

"There's no need to reinvent something that already exists," Stahl reasoned. Even though he and his colleagues were designing a new kind of swim fin, the shape of the standard fin needn't be recreated from scratch. "So I used a photograph of a generic fin to draw my initial curves, then proceeded to modify and customize the shape to the client's specifications," he said.

Initially, Omega Aquatics had envisioned the use of a double-torsion spring that wrapped around the front of the boot to connect the pivoted fin to the foot pocket. But Stahl and his colleagues soon realized this wasn't feasible. "It would be very difficult to assemble," Stahl explained. "And the spring got in the way of the self-locking mechanism we wanted to use." In the end, Designcraft came up with two smaller torsion springs on each side of the boot to control the fin (figure 1).

Figure 1. In the original design from the client, the pivoted blade (the fin) was attached to the foot pocket via a single spring at the center (top). But realizing the difficulty the design would create in assembling the product, Designcraft proposed an alternate spring system (bottom).

After 16 or 17 design iterations, the model went into the prototype phase. "In the tests, we found out that, if you kick hard enough, the latch would pop and become loose," Stahl recalled. "We tried snaps, tried some other features, and finally settled on a 316 stainless steel screw that would withstand saltwater corrosion."

Logo Motif

If the swim fin were constructed in flat surfaces, imprinting the client's logo might have been a fairly straightforward task, accomplished via the chamfer tool. "Well, doing that on a curved surface that went in several directions took a couple of days," Stahl recalled (figure 2). "[The fin] was a very organic surface to begin with, so putting another organic item on top was a challenge."

Figure 2. Imprinting the organic shape of the logo onto the curved surface of the blade proved to be a difficult challenge.

Because the design comprises both rigid and flexible areas, Stahl and his colleagues had to consider how the two different types of materials — polypropylene for the rigid part and thermoplastic elastomer (TPE) for the soft part — would be injected into the mold.

"We had to make sure there were channels to fill the TPE from a single [material injection] gate," said Stahl. "We had to see that the materials could flow all around inside and bond, not just chemically but mechanically. We needed to place the correct drafts, holes, and parting lines in the places where the two different materials would meet. That affected how the model was drawn." To disguise the injection gate in the final product, Designcraft designers strategically placed it at the center of the O in the Omega imprinted on the sole of the boot.

Will It Float? Will It Break?

Designcraft prototyped the boot section using a transparent material to understand how a diver's foot would fit within the swim fin. "That let us see whether we needed to pull the toe a little bit up or open up somewhere else," said Stahl.

In addition to stereolithography, the company uses high-speed computer numerical control (CNC) machining, PolyJet 3D printing, silicone molding, and urethane casting for design visualization. "It's a lot cheaper to do those [to test the design] than to fix the tooling during the molding process," Stahl said. "You also get a better idea of the physical part. It gives you a sense of scale and a tactile object," something a digital model can't provide.

The real-world tests that Stahl personally conducted included filling up his bathtub and dropping the polypropylene prototype into the water to verify its buoyancy. "It sank pretty slowly," he noted. In another instance, he took great pleasure in applying brute force to the springs to check their durability. "Once, the assembly came apart right in my face," he recalled. No CAD designer was harmed during these experiments, he was glad to report.

Organic Fashion on the Rise

Because the client, Omega Aquatic, doesn't use Pro/E, Designcraft used Adobe Acrobat files and JPEG images for reviews, annotations, and approvals. The toolmakers involved in the project used SolidWorks, allowing Designcraft to communicate with them through STEP files.

Figure 3. Designcraft used Pro/E interactive surfacing design extension to produce the 3D files required for manufacturing Omega Aquatics' Flip Fins.

"Most of our clients are in consumer electronics," said Stahl. "Some of the handheld devices could get pretty organic." The curved profiles of the camcorder, mouse, and MP3 player showcased in Designcraft's online gallery reinforce this trend. "The Flip Fin project was definitely far more advanced. Without the ISDX module, we couldn't have done it," said Stahl (figure 3).

Kenneth Wong is a former editor of Cadence magazine. As a freelance writer, he explores innovative usage of technology and its implications. E-mail him at Kenneth.Wong at cadalyst.com.