It’s pretty hard to get a watertight object out of our Makerbot Thing-O-Matic. The walls of printed objects are pretty solid, but unexpectedly porous; even a thick block printed with 100% infill will allow water to penetrate it due to errors around the edges and imperfectly fused strands of plastic. If you want to make a hollow object waterproof you’re going to have to do some post-processing.
It gets worse when the object is a curved surface, as is my AUV hull. I’ve read that objects can be made watertight by adding outer shells. That may be true for some objects, but on objects that curve along the z-axis the number of shells exposed to the surface grows as the tangent plane gets closer to parallel to the printer’s build platform—and big holes start to form.
My hull doesn’t actually have to be watertight, it’s a wet hull. But it has to be airtight in order to hold the bubble of gas that controls the robot’s buoyancy. Here’s what the airtight hull looks like:
I tried a solenoid I scavenged from underwater valves on my 3D printed valve system and it didn’t work (duh). It was just too weak. The original valve spreads the force from the high-pressure side across a larger area, so I guess the spring return can be comparatively weak. My design didn’t do that. I also didn’t cut my compression spring down very much, but I qualitatively determined that the solenoid wasn’t generating a useful amount of force by holding it on the magnetic core while turning the power on and off a bunch of times. I am a terrible engineer.
So I went back to square 2 and decided to remake the original brass valve body in lighter ABS plastic using our Makerbot 3D printer. The beta version looked like this: Continue reading →
The hull is done for now. As I mentioned before, I have some improvements in mind (mainly to get rid of the outer bolts), but it took about 34 hours to print all eight pieces and I’m not eager to do it again. If I ever get around to putting motors on this thing then I will have to re-print at least four of the semidemihemispheres, as the current ones don’t have any mounting points for motor attachments.
I printed four copies of the semidemihemisphere and refined it a bit as I went. Here’s the whole thing, including the hull that I covered in my last log entry. The only difference in the hull is I removed the top hole, as only two of the semidemihemispheres need it (as valve mounting holes).
Tweaked version of the AUV model.
I made a few interesting changes to the cutaway portion though:
I did a bit of testing on my AUV hull design, and finally got a prototype ready to go. I’ve been working on the design for several months in Autodesk 123d, which is a pretty great program if you can put up with the crashes, corrupt save files, and slowness (hey, it’s beta).
Here is my current design in full, including the support structures:
The 3D model for my spherical AUV hull.
This is one eighth of the hull; I call it a semidemihemisphere. Eight of these will make a sphere 18 cm in diameter. I split it up into eighths because I’m using a Makerbot 3D printer and its build platform is limited to a cube about 10 cm to a side. Also, this way each piece can be identical (or nearly so).
One limitation in the Makerbot is that it can’t print overhangs very well, so I had to add some support structures that can be cut away. In this entry I will ignore that and focus on the hull design:
Update: 123D beta 5 uses the proper units for exporting STL files, so the scaling operation is no longer necessary.
As I noted previously, objects in STL files created by Autodesk’s new 123D CAD program are not correctly scaled when loaded into ReplicatorG (at least not for me, I’m using the public beta from a few weeks ago). Eyeballing it, I guessed the object in RepG was about 1/10 the size defined in the 123D model.
To test this hypothesis, I designed a rectangular prism, 50 mm by 25 mm by 10 mm (yes, the temptation to go 90x40x10 was strong). Results follow:
I’m working on a prototype for the AUV hull. I plan to print a 10 cm diameter hollowish sphere and use it to develop a buoyancy system.
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.