cnc Manufacturing Methods

Decavitator was built from scratch by undergraduate students at MIT (by the end of the project, some of the undergraduates had become graduate students). Decavitator is part hobby-shop model, part high-tech design and manufacturing wonder, and part last-minute hack job.

The fuselage design was copied from the Daedalus fuselage in order to take advantage of the Daedalus studies of position issues. But whereas Daedalus used a propellor in front of the rider powered by a carbon fibre drive shaft, Decavitator used a propellor behind the rider driven by a long length of chain.

Unlike the carbon fibre frame in Daedalus, the Decavitator fuselage was made by lashing thin-walled aluminum tubes together using kevlar rovings. Hard foam plugs were inserted in the ends of the tubes then the tubes were mitred on milling machines and lashed by hand. The aluminum tubes were more resilient than carbon to dings and impacts, and they provided a good "breadboard" for trying out the various boat configurations over the years.

Fuselage Fairing
The mold for Daedalus' fairing was used to lay up the Decavitator fairing. Two layers of bidirectional kevlar cloth were laid in the mold. Carbon-rohacell-carbon stiffeners were then bonded to the interior of the shell. The still somewhat flimsy shell was then mounted to the fuselage with a number of small carbon tubes and lots of tweaking.

The hulls were built using technology and techniques developed at Composite Engineering. They are modified kayaks built of a glass-Nomex-glass sandwich with plywood bulkheads embedded inside for hardpoints. Each hull is 18 ft (5.5 m) long and weighs 7 lbs (3.2 kg). For details about the hull manufacturing process, contact Ted VanDeusen at Composite Engineering in Concord, Massachusetts.

The fuselage is suspended between hulls on a carbon tube pyramid. The pyramid is a tetrahedron made of carbon fibre tubes. The tubes were made by laying up pre-preg carbon fibre around aluminum mandrels. The tubes were baked in an oven we built specially for baking tubes. One of the tubes had flattened ends, so the mandrel was etched out using hydrochloric acid. The tubes were lashed together with carbon fibre rovings.

The (two-bladed) propellor was laid up in a glass composite mold. Each blade half was laid up separately then the two halves were bonded together. Each half consists of a kevlar-rohacell-kevlar sandwich. Balsa wood and rohacell inserts were used at the root to stabilize the embedded thin-walled aluminum tube attachment point.

Wings and Control Surfaces
The wings and control surfaces were custom designed using the "xfoil" software developed by Professor Mark Drela for design low Reynolds Number airfoils.

The first surfaces were built from glass molds made from a male plug. Templates of the foil shape were used to fine-tune a glass-covered foam plug. A two-part glass mold was then made around the male plug. Once the mold was complete, carbon fibre parts were cast in the mold. The first surfaces took six months to complete (from conception to part in hand).

Molds were machined from solid blocks of aluminum then sanded and polished to achieve a very smooth surface finish. Carbon fibre was laid in each mold half and allowed to cure under a vacuum. The two halves were then sanded down to mate perfectly then bonded together.

Frustration with the slow fabrication times of composite molds led to the development of a cnc-based mold-making process. Foil shapes were downloaded to a 3-axis cnc milling machine and the cavities were milled directly out of aluminum blanks. The two-part molds were then sanded and polished by hand to get a satisfactory finish. The first parts from molds made using this process took six weeks to complete. By the end of the project, design to part-in-hand took six days.

Drive Train
Chain rings were machined on the cnc mill from 1/8 in aluminum blanks. The titanium bottom bracket was donated by Fat City Cycles. A stock Shimano crank set was drilled out to lighten it up a bit (every ounce counts!). The chain was a 1/4 inch pitch, stainless steel chain from Berg. We made it into a mobius chain (the 20 ft loop of chain contains a single twist).

The seat was made by lashing thin-walled aluminum tubes together then stretching heavy kevlar cloth between the tubes and bonding the edges of the cloth with epoxy.

Timing System
Marc Schafer designed a simple but elegant solution to the problem of setting up timing traps across a wide river. His system uses any bright light source (no laser required) and miminal electronics.