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Composite Chassis Swing Arm

Polaris Rover

Polaris (shown in the picture above) prospects for water at the lunar poles by using a drill to sample lunar soil and scientific instruments that detect water. The rover is capable of driving and avoiding obstacles autonomously including traverses into dark regions in the lunar pole’s long shadows. Polaris suspension includes raise and lower capability to vary chassis ground clearance to lower for drilling and raise for driving on rough terrain.

CAD Model of Polaris

The Polaris Rover actuates a swing arm at each corner to raise, lower, and tilt the chassis. This vastly improves the ability to drive, work, and get out of trouble. Each swing arm is cut from a rectangular composite tube described in this blog.

Swing Arm

Fabrication of the swing arm requires a high temperature cure resin, which limits the mold selection to metal or a high-temperature, high-density foam. Metal (aluminum) releases easily, creates a good surface finish, can produce many parts, but is expensive. Foam is cheap, easy to machine, but cannot withstand higher temperatures and will produce fewer parts as a result.

Foam Swing Arm Mold

Thus, a 3-part mold was designed which consists of 2 foam blocks with a metal insert in the middle (shown in the picture above). The metal insert is removed after the part cures in the oven which provides space to extract the foam blocks.

In the picture below the high-density foam for the Polaris swing arm mold is machined on a 3-axis CNC machine.

High-Density Foam Is CNC'ed for the Swing Arm Mold

The mold is sanded to remove waviness caused by the CNC bit, cleaned, and coated with two thin layers of 5-minute epoxy. Next, it is baked at 150 degrees F for an hour to fully cure the resin. Afterwards, the epoxy layer is sanded down to create a very smooth surface. The mold is cleaned again with mold cleaner or acetone and sealed with a mold sealant. Finally, multiple layers of release agent are applied to the surface of the mold to prepare for the composite layup. The release agent prevents the resin from sticking to the surface of the mold. The next step is to test the composite swing arm to determine what force must be applied in order for the part to fail.

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