Acrylic/aluminum hull problems

Waterproof Housing, Frames, and Buoyancy Methods.
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puzzler300
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Joined: Jan 2nd, 2013, 12:26 pm
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Acrylic/aluminum hull problems

Post by puzzler300 »

Hi all,

I've been working on building a hull for my electronics. This is my 3rd iteration, and the one I'm most confident about. I'm working on the primary housing --housing my camera, batteries, and Arduino-- and so far it looked like it was going pretty well.

It's primarily made out of a 3" (76.2mm) ID 3.5" (88.9mm) OD acrylic tube. The ends are machined aluminum with 2 3" (76.2mm) ID 3.5" (88.9mm) OD o-rings, one on each end. The white brackets on the ends serve only to hold the end caps in place by means of four threaded rods. I tested it in my bathtub just to make sure the idea wasn't fundamentally flawed and that worked really well, actually. I took a trip to the pool at my university and threw it in the deep end and I'd say it lasted 6' (1.8m) before it started leaking.

I feel pretty stumped as to where to go from here. Any input would be greatly appreciated!
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12722451_928970360483685_664056920_o.jpg
12722451_928970360483685_664056920_o.jpg (338.04 KiB) Viewed 6820 times
Bathtub test, looking good!
Bathtub test, looking good!
12788342_928970393817015_1550511540_o.jpg (90.9 KiB) Viewed 6820 times
12' (3.6m) depth test, not so good.<br /><br />(Kudos to the swim team for letting me test in their pool)
12' (3.6m) depth test, not so good.

(Kudos to the swim team for letting me test in their pool)
12768101_928970447150343_1006197478_o.jpg (99.33 KiB) Viewed 6820 times
Complete end assembly. Aluminum cap + bracket + o-ring
Complete end assembly. Aluminum cap + bracket + o-ring
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The aluminum cap
The aluminum cap
12787081_928987593815295_494889958_o.jpg (284.29 KiB) Viewed 6820 times
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puzzler300
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Re: Acrylic/aluminum hull problems

Post by puzzler300 »

I'm adding a CAD drawing that might help clarify what how the tube end works
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Jarubell
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Re: Acrylic/aluminum hull problems

Post by Jarubell »

To start, I have no experience and yet to even start building.

How straight are the cut ends? How did you prep them?
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puzzler300
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Re: Acrylic/aluminum hull problems

Post by puzzler300 »

Approximately straight? I haven't done anything to it. I bought it off McMaster Carr and just left it. Definitely has saw marks on the ends, no idea the depth, but I can feel them. I want to put it in a lathe and face it, but I also don't want to shatter it (if it isn't evident, I have no idea how to work with acrylic).

--edit--
I found this video: https://www.youtube.com/watch?v=QhAXbA2lmnE and I think I'll try the sander + buffing wheel tomorrow and test it one more time.

I'll post results
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TigerShark
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Re: Acrylic/aluminum hull problems

Post by TigerShark »

Clean those ends up and that might work. You could swirl it on fine sandpaper over a very flat surface. Also you might need more compression.
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puzzler300
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Re: Acrylic/aluminum hull problems

Post by puzzler300 »

Sounds good, I tightened it today until it sounded like the ABS was cracking, but I might try a bit more.

--edit--

I've stripped out the threads on my aluminum threaded rod, any good places to get threaded stainless rod?

Also, has anyone tried silicon grease on their o-rings to help with sealing?
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TigerShark
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Re: Acrylic/aluminum hull problems

Post by TigerShark »

You also shouldn't need to get it that tight. You definitely need to use some grease. I use vasoline. Just a little to moisten the o-ring. You have to keep the area very clean also. I think if you had a part of the end cap that extended into the tube that would work better. I have double O-rings on my setup. One with a seal inside the tube and one seals on the end of the tube. You have to think where is the o-ring going to go when water pressure is squeezing it? I think yours would likely get pushed inside with your current design if you take it too deep.

Home Depot should have threaded rod. You could also get unthreaded and use a die to thread just the ends which would be stronger. You can also order it from Mcmastercarr.com or onlinemetals.com
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puzzler300
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Re: Acrylic/aluminum hull problems

Post by puzzler300 »

I was thinking the easiest thing to do would be to re-machine the aluminum caps with a taller center piece, or deeper o-ring grooves. Is there any advantage to the double o-ring system other than redundancy?

Also, do you think plain threaded rod will work? I'm mostly worried about corrosion.
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TigerShark
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Re: Acrylic/aluminum hull problems

Post by TigerShark »

Here is how my end cap seals:
DSCF6548.jpg
DSCF6548.jpg (66.83 KiB) Viewed 6749 times
This works for me, I have never had an issue with this design.

I think your rods would be fine but I would use SS or Brass. You could thread just the ends of solid rod.
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