Magnetic EggBot/SphereBot Pen Arm

I've split the pen arm from [thing:844167](https://www.thingiverse.com/thing:844167) into two parts that are attached together with magnets. You should print magnetic_pen_arm_top_part2.stl once, and magnetic_pen_arm_top_part1.stl as many times as you have pens, then you can switch between pens easily and quickly. (You'll obviously need a pair of magnets for each piece that you print too.)

The magnet dimensions and other settings are parametric, check the OpenSCAD file for details. I used 8x1 mm neodymium magnets, you can find them [on Aliexpress](http://s.click.aliexpress.com/e/6a2RrVj) for instance.

pen_arm_top.stl is the original file by [jsc](https://www.thingiverse.com/jsc), magnetic_pen_arm_top_part1.stl and magnetic_pen_arm_top_part2.stl are the ones I printed.

Action video: https://www.youtube.com/watch?v=wXIDBlRRZM4