Daily Equation #2 - Astroid x^(2/3) + y^(2/3) = a

This is Day 2 of my collection of printable equations. To see the whole collection, go to: http://www.thingiverse.com/thing:227210

Found a great site of famous & interesting equations: http://www-groups.dcs.st-and.ac.uk/~history/Curves/Curves.html. For today's equation, I picked the Astroid: x^(2/3) + y^(2/3) = a where a = r^2/3

From the site (edited & shortened for clarity):

The astroid can be formed by rolling a circle of radius r/4 on the inside of a circle of radius r. It can also