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:

Finished (‽) AUV Hull.

So that’s kind of neat.  I did the job using the following procedure:

1. Clean the surface of each semidemihemisphere with isopropanol or something else that dries clear.
2. Assemble each hemisphere with big wads of epoxy clay filling in the gaps between the four semidemihemispheres.  Then slather the assembled hemisphere in more epoxy clay so that it’s fully covered.  Then leave it until the clay cures.
3. Sand down the epoxy clay until the plastic shows through (I used a 120-220-400-800 grit progression of sandpaper).
4. Coat with lacquer to make it shiny.  I used shellac at first, then switched to polyurethane as it was easier to use (shellac has a very short work time and also discolours in water) and food-safety wasn’t a concern.

I actually did step 2 in reverse because I’m a terrible engineer and didn’t think to seal the gaps between the semidemihemispheres with clay until after I’d done the outer surfaces.

Epoxy clay turned out to be an effective material for smoothing and sealing 3D prints.  I bought 2 ounces of the reddish-brown (“flesh” coloured) stuff on eBay and later supplemented it with a 1 lb. white batch of the unsettlingly spelled Apoxie Sculpt from Sculpture Supply Canada (n.b. what I discovered regarding the difference between Apoxie Clay and Apoxie Sculpt).  The clay doesn’t adhere well to ABS as you’re applying it, I had to keep my fingers lubricated with water so that I could press the clay into the object’s filament ridges without the clay sticking to me.  (The water also made it easier to smooth the clay out so that I could squish it into a thin layer that was quick to sand down.)  Once the clay cures it’s really fricking hard to get it off of whatever it’s on, but it sands and drills cleanly.

The clay can also be used for filling holes and errant curves (e.g. due to the bottom of a print curling up as it cools) and for making good-looking, smooth, heat-resistant surfaces (I made a totally sweet coaster).  It also takes acrylic paint a little better than ABS plastic does, which isn’t saying much.

Another miraculous substance that I’ve discovered is Plumbing Goop.  It’s clear, creates a thin layer, is very easy to apply, and sticks really well to ABS.  It doesn’t peel off like silicone rubber.  It also provides a rubber-like grip, something that bare ABS is sorely lacking.  The downsides: because it’s applied in a thin, clear layer, it’s harder to get a thorough seal with Plumbing Goop than with epoxy clay; it doesn’t make spherical prints look like Jupiter; it doesn’t smooth out the filament ridges; and it’s flexible (comparable to silicone rubber), which might not actually be a downside depending on what you’re doing.

Pictures and video follow:

A semidemihemisphere with overhang supports attached (but not the walls that the cutaway rests on).

Cutting away an overhang support.

The full AUV hull. Neat!

Here are two videos of a semidemihemisphere being printed. Part 1:

Part 2 (the cool part):

Tweaked version of the AUV model.

I made a few interesting changes to the cutaway portion though:

The cutaway is needed to hold up the hull while it prints.  Without the cutaway, the plastic, heated to its melting point by the Makerbot, will droop and hang or fall.  The cutaway gives the hull something to sit on as the plastic cools.  Here is the cutaway portion, viewed from the side:

AUV hull cutaway pieces from the side.

A – I added a bunch of little bits on the bottom layer of the model.  This is a hack to get the printer to print a larger raft.  (The raft is a thick layer of plastic that the model sits on as it prints, intended to let the model stick well to the build platform.)  I had to reduce the raft margin so that it would fit on the build platform, but with the smaller raft the pieces were susceptible to being knocked around by the toolhead as it goes.  These pieces get raft added around them, which merges with the raft of the actual model so that the real pieces get a nice stable raft.

B – This bit connects the cutaway material to the hull.  The hull is getting pretty tall at this point, so if it gets hit by the toolhead there’s a force applied that is strong enough to knock around the hull and perhaps misalign the layer.  This part (and its equivalent on the other side of the model) help hold the hull steady against the cutaway shells as they get close to being joined together.

C – This bar connects the cutaway shells together to keep them steady.  In a previous model (shown in the last log entry) I made this part too wide, and it took so long to print the horizontal bar that the hull cooled too much and split a little.  It wasn’t anything a little plastic welder couldn’t fix, but the supports don’t need to be very wide to fulfill their purpose.  After shrinking them to 1 mm wide the printer could print one bar layer with just two quick strands of plastic.

AUV hull cutaway pieces from the top.

D – This is the support for the upper tab that hangs freely over the semidemihemisphere.  You can see it from the side at the top of the cutaway side image above.  This is a pretty cool piece that’s anchored vertically on the cutaway shell, and ends up a horizontal flat surface at the upper tab bottom.  It is topped by a 0.5 mm shell that the tab sits on (see E).  I did this piece with a loft from a rectangle on the vertical plane to a rectangle on the horizontal plane.

E – The lattice pattern holds up the hull’s overhang.  The lattice rests on a 2-layer thick platform.  The bottom layer of the platform is printed over open space, and droops badly.  The second layer of the platform is printed on top of that and is pretty flat.  Then the lattice is printed on the nice flat layer, and the actual hull is printed over the lattice.  The lattice is sparse enough that it can be separated from the hull easily with a sharp utility knife (same as the shell in D).  The squares in the lattice are 5 mm to a side (I originally used a 10 mm lattice, but it was too sparse and the hull overhang drooped too much) and 0.5 mm thick, i.e. the width of one strand of plastic.  Note that the top-most (innermost) hull overhang is narrow enough that it doesn’t need to be supported by a lattice.

F – The circular patterns support the circular holes in the hull.  Without these the Makerbot would try to print a circle over the lattice, and it would just collapse into the open spaces.  The square support on the upper lattice supports the receptacle that accepts the upper assembly tab.  Without it, the printer would again try to print the square over the lattice and the first layer would fall into the holes.  With the support the first hull overhang layer is a full surface, and the receptacle square sits on that full layer instead of the lattice.

Here is an old version of the cutaway that had a bunch of problems:

An early, problematic cutaway version.

Problem 1: The bottom layer had these wedges to hold everything together and provide a solid base.  After I started using a raft these were no longer necessary; in fact, they were a detriment because they took a long time to print and the first couple hull layers cooled too much and came apart.

Problem 2: As stated before (C) the cutaway support bar is too wide and takes too long to print.

Problem 3: The lattice is radial instead of square.  The printer couldn’t print the off-angle 0.5 mm lines.  The lattice would be printed near the edge of the lattice pattern, but there wouldn’t be anything in the middle, and the hull overhang would not get its support.

Problem 4: The circle supports in the lattice are too thin (0.5 mm) and they don’t get printed properly.

Problem 5: The innermost cutaway support is thicker than the others.  Once again, there was no benefit to this and it took significantly longer to print than the thinner version in the later versions.

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:

My design requirement was simple: I needed a large, spherical hull with a hole in the top and bottom for buoyancy control.  As I said previously, in order to make it big enough I had to split the sphere into eighths.  The eight pieces needed to be bolted together securely, but I wanted to avoid having a lot of nut and bolt holes on the outside of the hull.  (You’ll see that there are several outer holes in the current design, but I have ideas to remove them all that I might implement in a future version of the hull).

The sphere is composed of an upper hemisphere and a lower hemisphere, each consisting of four more-or-less identical pieces like this:

AUV semidemihemisphere hull component.

These two hemispheres are assembled using connectors inside the hull, and then the hemispheres are bolted together from outside the hull.

The upper assembly looks like this:

AUV hull upper assembly.

The tab on the right fits into the matching receptacle on the left to connect neighbouring pieces in a hemisphere.  In my test prints of the upper part of the semidemihemisphere, two pieces snapped together quite securely.  The circular hole in the middle fits a #4 bolt that will connect the valve to the top hemisphere.  It will only be used on two of the semidemihemispheres; I’ll delete it from the model file for the other units so that I don’t have to plug it.

Just below the upper assembly you can see the inner component mounting points:

AUV hull inner component mounting points.

These can be used to mount components inside the AUV, e.g. a microcontroller or a sensor/control module board.  The slots are sized to fit a #4 nut, so that the mounting board can be bolted on easily from below, without having to hold the nut (many of the nuts on the Makerbot itself are held in this manner). A mounting board will be bolted to one mounting point on two neighbouring semidemihemispheres, which will handily hold the semidemihemispheres together.  It might be worth connecting neighbouring mounting points together even if nothing is mounted on them, just to reduce stress in the upper assembly tab.

The flat, right-angled circular protrusions inside the hull are actually an artifact of the support structure needed to print this thing on the Makerbot.  This nod to the machine’s limitations turned out to be quite useful; I’m not sure what the best solution would be if I got this manufactured commercially.

Down at the bottom of the hull you can see two more square connectors to join the lower parts of the four neighbouring units that make up a hemisphere:

AUV inner hemisphere connectors (right).

One side has a hex nut inset, again so that the nut doesn’t need to be held when tightening the bolt.  Two neighbouring semidemihemispheres can be joined at the bottom with a 1/2″ #4 bolt using these inner connectors.

Once the two hemispheres have been assembled, they are connected using the outer joins:

AUV outer hull joins.

The left connector accepts a #4 bolt, and the right connector accepts a #4 nut.  By matching one hemisphere’s bolt holes to the other’s nut holes, the two hemispheres can be joined together.  This is obviously a major flaw in the outside of the hull.  I currently plan to print plugs that can be inserted into holes to smooth out the hull surface.  This is one thing I’d like to improve in the future, using a technique similar to the protruding tab in the upper assembly.  I couldn’t use the same trick here because the bottom surface needs to be flat to sit on the Makerbot build platform while it prints, but a similar piece printed separately and glued in should be quite effective.  I decided not to do that in this version of the hull, because I wanted to get the heck on with things and it’s not that important.

One problem is that when the semidemihemisphere is printed the bottom is not flat.  It gets curled up, the bottom layer sticks to the raft interface, and so on.  I currently plan to sand the bottom layer down and replace it with a cheap-o gasket made out of silicone rubber.