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Month: November 2016

AUV Thrust Module Design Progress

A quick update on some design work I’ve been picking away at recently. I’ve managed to roughly place all the parts for the drivetrain in the thrust module and started work on the control fin motor/gear layout. Still all subject to change, but it looks like I’ll be able to fit everything I need in the tail cone.

Cut-away view of the work in progress AUV propulsion module.
Cut-away view of the work in progress AUV propulsion module.

Next step of this design is to figure how to seal the control surface shafts. Ideally, those will be magnetic couplers as well, but I have to analyze how much torque will be required, and how much I can transmit with a small magnetic coupler. Failing that, I’ll design it to use Ikelite glands — I have an Ikelite camera case which uses these, and the performance seems good.

I found a source for BLDC gear motors that run off 24 volts and aren’t insanely expensive, which I’m hoping to use for the control surfaces. Standard hobby servos run of much lower voltages, and I was hoping to implement my own control scheme for the motors — More on that later, as I’ve ordered one motor to test out to verify it will actually be suitable.

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A Lesson in Rigidity

After getting some metal stock this week, I was anxiously waiting for the weekend so I could start machining some prototypes for the AUV’s universal ring bulkheads.

Now, I need to throw out a disclaimer — I’m not an expert machinist by any stretch of the imagination. In fact, quite the opposite and I’m learning as I go, which is what this post is all about.

Having read through different things on feeds and speeds, I set up some programs with what I thought were somewhat aggressive cuts (for the Sherline) that I hoped the machine could handle — 1mm depth with a 3/8″ end mill into 6061 aluminum, with a feed of about 260 mm/minute. I’d read complaints about the rigidity of the 2000 series mill, but figured I’d try anyways.

The intent was to mill out the center of a 6″ diameter piece of Aluminum, so I could use the 3.1″ Sherline chuck to clamp from the inside onto the lathe for all the turning operations I’d need to do. I was hoping to salvage the inside chunk, since the stock wasn’t cheap. I planned on accomplishing this by milling in half way from one side, then flipping the part and milling in from the other side.

Now, the Sherline motor handled the first cuts like a champ. The 2000 series vertical column? Not so much. Unfortunately, as the machine went through the cuts, it put enough stress on the vertical column causing it to pivot along the vertical axis and throwing it off center. You can see the steps caused by this in the picture below, due to the column gradually pivoting at each cut, throwing off the alignment more and more. Luckily this was a roughing cut, so the the stock wasn’t lost.

Stepped profile caused by the wandering alignment of the mill's vertical column
Stepped profile caused by the wandering alignment of the mill’s vertical column

Eventually, I tightened the column well enough that it finished the last cuts without too much added deflection. Figuring I could tighten up the mill just a little bit more before flipping the part and trying the other side, I put the wrench on again, applied some torque and snapped the bolt clean off, leaving me with a Z-axis no longer attached to my mill, and the mill out of commission!

Not to be deterred, I figured I would try and see how well the the part fit in the lathe’s chuck using the pocket that was milled in one side. Not too bad, I must say:

6" diameter part on a 3,1" diameter chuck
6″ diameter part on a 3,1″ diameter chuck

I then screwed the chuck inot the lathe. I had to use two riser blocks to get clearance on the large part, causing some inappropriate cutter geometry. I needed that much offset to get the part below 6″ diameter, after which the intent is to only use a single riser block, and some special tool holders I’m planning to make in order to hold tools perpendicular to the part.

Turning large diameter parts
Turning large diameter parts

Turning the large diameter part was easier than I’d thought I used 0.2mm depth of cut and a 60 mm/min feed rate, which the machine handled well. The part stayed solidly affixed to the chuck, even when I accidentally crashed the tool into the part.

Now that I’ve gotten the stock cut such that I can mount it on the lathe, and turn it freely on the lathe, hopefully soon I’ll be able to start working it to the final shape. Unfortunately, I ran out of time to machine today and had to start cleaning up at this point.

Next steps, I’ll have to fix the mill. From what I’ve read online, the rigid column Sherlines fare much, much better than the fancy articulating column mills, and luckily the part to do the conversion is fairly inexpensive. Although perhaps the Sherline isn’t the right tool for machining large parts en-masse, for the quantities of parts I need and considering it’s what I already had available, it looks like it’ll fit my needs just fine.

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AUV Bulkhead Rings

One of the goals I’ve had for most of my AUV project was to make it as modular as possible, which would allow future upgrades and change of mission in the future — Want to take video? Swap out the sonar with a video module. Need better control in the water column for inspecting things? Add lateral thruster modules. Want to take water quality samples? Swap out the sonar module for a water quality sensor module… And the list goes on.

To achieve this, years ago I came up with an idea for a universal bulkhead design, which would allow each module to plug into any other, provided that the electrical interfaces matched through the bulkheads. The design would also allow mix-and-matching of flooded and non-flooded compartments, depending on the payload and mission configuration.

AUV Universal Bulkhead Ring with o-rings

The design is fairly simple, each bulkhead essentially consists of two identical rings with a flat plate bulkhead sandwiched in between them. The bulkheads are screwed together and O-rings keep everything sealed up nice and tight. The compartment tubes are held into place by a series of radial screws (not properly shown in the photo above) which don’t clamp down on the tube (causing stress points in the material) but rather act as pins to prevent the tube from sliding out.

One of the challenged was getting the design to play nicely with standard cast acrylic tubing, with its loose tolerances. This necessitates usage of large diameter o-rings to make up for the variance in tube diameters. One option could have been to machine the acrylic tube’s inside diameter to appropriate tolerance, but I decided that the difficulty of doing so with the tools available to me outweighed the convenience of using stock cast acrylic profiles.

 

AUV Universal Bulkhead Assembly
AUV Universal Bulkhead Assembly
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