Jan 28, 2015

Ripstik Chassis and Baking Aluminum

I'm rushing to get a deliverable out by Feb 2nd, so I've been lagging on documentation. But here's some of what I've been doing with the ripstik chassis:

I'm using eighth-inch 2024 aluminum plate for the body, originally going with a 2 layer shell design because I wasn't convinced that one layer would hold a person. I modeled sheet metal folding in Solidworks and waterjetted the flat pattern.

I chose 2024 instead of the more common 6061 aluminum alloy to facilitate bending. 6061 (especially the T-6 variety available to me) is extremely susceptible to cracking when bending. 2024 T-3 is doable, and is still readily machinable. T-3 and T-6 refers to the level of temper in the standard sheet. For me, lower temper is better for bending. 

Then, I put my cutouts on the bending brake and put in angles. I ended up needing a larger bending radius than I had planned for; turns out that the specific sheet I was using needs a bit more than the "thickness of part" rule of thumb.

In addition, turns out my 0.125" plate was just barely within the maximum thickness allowed in this brake. I had to throw around my body weight on the lever to get even a slight bend, and I wasn't looking forward to spending hours and hours to get the angles I wanted.

The internet tells me 2024 aluminum has an annealing temp (temperature at which it softens) of around 750°F, and a hardening temperature of around 300°F. My oven gets to 575°F+ on broil, so I stuck my plates in there for 2 hours, let cool to room temp (1 hour), and tried again.

This strategy worked reasonably well (beware of well-intentioned friends leaving your aluminum in the oven while baking cookies!) Only the narrow tube-holding plate cracked severely after the pseudo-annealing, and I was okay with cutting those off and waterjetting replacements.

Aluminum 2024 Recipe

        1. While oven is at room temperature, place aluminum on top shelves
        2. Set oven to broil
        3. Open the windows if necessary. (broiling aluminum smells much like hot glue)
        4. Wait 2 hours
        5. Turn oven off. Without removing aluminum, let rest for 1 hour
        6. Remove aluminum. May be warm to the touch.