Just a minor progress update. I’ve been adjusting the CAD design and working on some additional parts. A snapshot below….
I also finished grooving the prototype bulkhead ring. It wasn’t super easy, and I’ll need to experiment with feeds and speed some more to get nicer cuts. I’ve also had a plastic order come in, so I’ve done a test fit of the bulkhead into an extra piece of acrylic tube. It’s a tight, but good fit! I just need to machine the remaining bulkheads and anodize them before I start pressure testing.
I’ve also been working away on the nosecone plug — Adding layers, then machining it down to get it smooth — Once it’s in the right shape, I’ll manually sand it smooth and start the process of sealing with epoxy. This is a pretty messy operation, as the spackle turns to dust, so I typically do the machining with the vacuum hose right beside to suck it all up.
In the photo below, you’ll also notice the standoff I made to mount the toolposts higher in order to machine large diameter parts — Here I’ve got two spacers mounted to get over 6″ swing over the bed with the Sherline lathe. Since it’s a very soft material, I think it’s okay, but this would be quite non-ideal for machining anything harder. (To do the outside of the bulkhead rings, I just mount a single spacer and use the smaller toolpost. Still not as rigid as I’d like, but it’ll do for this small job. I do wish I had room for a bigger machine, though…)
While designing the thrust/steering module for the AUV, I figured I should determine how much torque the rudder will require (for motor selection) and what forces it will result (for eventual control development). The rudder itself is a fairly simple plane with an airfoil type profile, with the rotational axis a quarter of the way from the front to the rear, making it what’s known as a balanced rudder. More on this later.
I set up an Open Foam case to calculate the forces and torque, and made a script to run it through 0 to 50-degree steering in 5-degree increments. I opted to just use an isolated rudder for the initial tests, although in reality there will be interaction between the body and the rudder, especially since the rudders will be located in the region where flow is beginning to separate from the AUV hull. Note that in earlier CAD models, I had shown the rudder being only a portion of a fin — in order to simplify things, and balance the rudder, I opted to make the rudder consist of the entire fin.
The simulated results I got are shown in the chart below. I only ran simulations up to 2 m/s, as I don’t expect the AUV to go much faster than that operationally.
Although there appears to be an outlier at 30 degrees deflection (Either due to some real hydrodynamic effect or errors in the simulation), this result is actually what’s expected for a balanced (or partially balanced) rudder. If the rotational axis was at the front edge of the rudder, all the forces would be acting on one side of the rotational axis which would result in significant torque requirements to move the rudder. By locating the pivot point near the rudder foil’s center of pressure, the forces in front of and behind the rotational axis negate each other resulting in reduced torque requirements.
With the balanced rudder, the forces start off minimal while the flow is laminar. Once the rudder’s foil begins to stall, the torque will actually invert and go the other direction. To illustrate this, some renderings of the results from Open Foam are shown below. Note that the streamline colours represent particle velocity, but the colour scales are slightly different between all images.
In the first case above, with 10 degrees deflection, the flow is still smooth around the rudder’s foil. While the torque value at this point is very low (hence why it’s “balanced”), using the right-hand rule to interpret the torque around the Z-axis, it appears that the torque is actually wanting the rudder to keep deflecting!
At 20-degrees separation, per the plotted results, the rudder foil is around the stall point. At this point, the torque around the rotational axis is neutral. Any further deflection wants to push the rudder back towards the forward position.
Throwing the rudder even further, to 40-degrees, we can see that it’s now clearly stalled. You can see the turbulent flow behind the rudder. The streamlines make for a really cool graphic!
For curiosity’s sake, I ran a couple of extra simulations, varying the location of the rudder’s pivot axis 4mm forward and 4mm back from the 1/4 chord position (this turned out to be 6.6% of the mean chord). A plot of the results below. I still have some optimization to do in terms of rudder profile and mounting point, but these give me a good order of magnitude understanding of the rudder torque for initial design work.
In terms of motors to drive the rudders, I’m currently planning a geared down brushless motors, as I want to have fairly fine and smooth control over the mechanism (reduce noise, vibrations, improve fine control over hobby servos).
I’ve been bounding around design and work on some different parts of the AUV but had been meaning to get back to working on the bulkhead ring prototypes. I’ve made some tweaks to the design, mostly so that I can mount internal components directly to the inside of the ring, necessitating the creation of some flat spots on the inner surface, where I can drill and tap mounting holes. These should be fairly easy to cut out on the mill when doing the finishing drill steps.
I had some time today to sit down and make some progress on the machining and managed to get most of the lathe work done, minus the external grooves which will seal up against the body tube. For the most part, things went well, although I made a fairly stupid mistake and zeroed off the wrong side of the grooving bit when cutting the face grooves — Luckily, not all the rings will need seals and o-rings, so I can put this one into a “wet” compartment where that mistake won’t matter.
One frustrating battle I did have to fight, however, was with chatter. I had significant chatter on the external turning operation. I think I’ve tracked that down (a little too late) to one of the gibs in the slide, so will need to tweak that before spinning anything more.
One tricky part about the setup is that the part needs to be flipped part way through. To make sure that everything is lined up, I had to place a dial indicator behind the part and very carefully adjust it in the chuck to make sure that I had very little runout. In the end, I got about 2-3 thou peak-to-peak deviation at 2.7″ radius. Not too shabby, and good enough for my purposes.
All in all, however, the g-code works well, and future rings should be much faster/smoother to machine. I ended up settling on a feed speed of 145mm/min and 0.1mm depth of cut with the mill’s motor geared down (~1400rpm with carbide tooling, so a very fast surface speed). I wouldn’t push the machine any harder than this at these large diameters (~5″), especially since the chucking method isn’t super secure, and subject to going out of alignment relatively easily. I suspect I’m feeding too fast with too light of a cut, so on the next ring I’ll experiment with deeper cuts and slower feeds to see if I can improve the chatter/finish/headaches. — Disclaimer: Do not take any of this as good advice. I’m not an expert machinist! This a very non-ideal method for clamping large parts on a small lathe, and can be dangerous if not secured properly.
Next steps will be to finish this part, and work on the next batch. I’ll need about 8 with my current design, so it may take a bit of time to get them all done…
The AUV’s nosecone will be a free-flooded chamber but needs hold its form for hydrodynamic purposes. I had originally planned to make the nose cone have a spherical form at the front with a radius something smaller than the AUV’s diameter, then blending into the body. The purpose of that was to eventually be able to put a phased-array forward-looking sonar in the nose for mapping and obstacle avoidance, and the spherical form looking forward would reduce the refractive effects of the water-fiberglass-water transition.
However, given the magnitude of the scope of building an AUV and a side looking Synthetic Aperture Sonar, any kind of additional forward-looking sonar would be so far down the road that I opted for a simpler, more hydrodynamic and more aesthetically pleasing elliptical nose-cone form instead.
The first step in making the nose cone was to build a plug, to be used to make a mold. Using 0.75″ blue insulation foam, I cut out profiles using the CNC machine. The profiles were slightly oversized to compensate for any slop in the assembly and cutting, with the excess to be sanded down to the correct dimensions.
The pieces were then glued together, onto a wooden dowel which served to keep everything aligned and to give something for the lathe chuck to bit onto. To keep things tight, I used the mill itself to act as a “clamp”. The joy of working with foam is that you can get away with doing things you shouldn’t do with harder materials…
I cut out another profile, and lined the inside with sandpaper — This was used to help sand the foam to the right shape. Once in the spackle stage, the form was used to help evenly spread the spackle. It would have been better to use a more rigid material for the form, like wood, but I didn’t have any kicking around, so opted to work with what I had.
The first sanding step was to sand the foam past the target dimensions, and then build up a layer of spackling filler – The spackling will help give a smoother, stronger finish than is achievable with foam, which is essential for a good fiberglass mold, and hydrodynamic surface. I used quick-dry spackle to speed up working time, although since it off-gases quite a bit as it dries, I had to use it outside and the cold weather slowed the dry time back down to a crawl. The quick-dry spackle does shrink somewhat as it dries, causing cracking in thick layers, but that’s okay as the cracks are filled in over subsequent, finer applications.
Once a decent layer of spackle was built up over top of the foam, I used the CNC lathe to bring the plug to the correct dimensions and symmetry. To create the G-Code, I used e-Cam running in my Windows VM. Since I didn’t have to rough away all the material from a full cylinder, I created a DXF file with offset profiles from the final shape, starting at 0.25mm and building up to 1mm steps. Importing these, I created a series of “finishing” profiles, which resulted in tracing the final lines.
Normally the Sherline lathe can’t handle very large diameters, but using riser blocks and some custom made tool risers (Another thing I worked on over the break, which I’ll need to finish other AUV parts) I managed to swing the large diameter part. Luckily, the plaster is relatively soft and easy to work with. I’ve run it through the CNC program once, and then added some extra plaster to fill in some gaps where it wasn’t thick enough and will run it through again once that’s dry.
Next steps will be to seal the surface with epoxy resin, polish smooth and wax to perfection — The more time spent working on the plug, the better the end result will be. Since I don’t have a suitable indoor workshop for curing resin (fumes), I’ll need to use a resin formulated to cure at temperatures near freezing so that I can do the epoxy/fiberglass work on the patio. Winter in my neck of the woods is more akin to spring in the rest of Canada and doesn’t go below freezing very often. Unfortunately, this year winter has been a bit colder than usual, so it may be a while before it warms up enough for fiberglass work…
Otherwise, AUV design is slowly progressing. I’ve been taking some more test cuts and refining the CNC programs to generate the universal rings. I’ve been putting more thought into the electronics and control of the AUV, but will leave that as a topic for a future post.