It’s not easy to print a large sphere on a Makerbot.  There are a lot of cool sphere things on Thingiverse, including some pretty sweet hollow sphere patterns.  That hollow sphere is allegedly printable on a Makerbot, but I’m not sure it’s the best option for my AUV.

I’m working on a design for a self-supporting hemisphere in Autodesk’s new 123D CAD program.

123D is a pretty good tool.  It’s kind of frustrating at times (it’s pretty slow on my laptop), but it’s still in beta.  Learning from the built-in tutorials, I managed to design a hemisphere that might turn out to be printable (I am currently 6000-odd km away from my Makerbot):

A hemisphere designed in 123D

Okay, it’s not a hemisphere, it’s a hemidoughnut.  The hole is for water to enter (from the bottom) or air to escape (from the top).  There’s a support structure inside to hold it up while it’s printing.   I tried to make the support structure flimsy so I can cut it apart to expose the inner cavity.  Here is the 2D cutout that I revolved around the Z axis:

2D cutout of the hemisphere.

There are a few things that are noteworthy in this version:

• I put little 0.1 mm slices into the walls so that the inner cavities would topologically be the outside and wouldn’t get filled in.  This is kind of like how the walls in the whistle model that comes with ReplicatorG are non-manifold and therefore don’t get filled.  I’m not sure how else to specify an empty inner volume in 123D.  Hopefully a 0.1 mm empty space will not confuse Skeinforge.
• The small solid rectangles in the struts are to give the tall, thin walls some rigidity.
• The bottom surface is a filled-in circle so as to stick well to the build platform.  The model might benefit from another horizontal surface half-way up to keep everything in line.  I’d like to make it a cross pattern instead of printing a full circle, so as to reduce waste.
• Near the top are some flat areas filling in the space between the struts and the hull.  These will be part of the final hull.  I might be able to use them as mounts for the inner components.

One good feature in 123D is that it can save to STL format.  Unfortunately when I save the 10 cm hemisphere to STL and load it into ReplicatorG, it appears very small.  Eyeballing it, it looks like in RepG it has been scaled to 0.10 the intended size.  Hopefully I can recover the original size by scaling by 10x.  I will be able to test this hypothesis with a calibration cube when I get home.