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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 00:57 
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Joined: 29 Aug 2012, 21:34
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Location: Palm City, FL
I wanted to start a discussion about aluminum machine screws, mainly because I'm curious about their potential in tin boat applications. I was planning out how I will mount my floor and console in the Weldbilt, and thought about using aluminum rivnuts and aluminum machine screws. On paper it sounds like a great idea, even considering how soft aluminum is. Screws would have to be very lightly torqued, but I feel like they'd be a heck of a lot stronger than the 3/16 pop rivets that held the floor in previously. Thoughts?



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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 09:08 

Joined: 05 Oct 2008, 12:14
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Location: Algonquin Il
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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 09:40 
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Location: Eastern Mass
Great question and idea! But while I can find tensile and shear loads for 3/16" aluminum rivets on-line, a quick look didn't show any values for the aluminum machine screws, as it is alloy dependent. What I do recall from my GE machinist program training is that aluminum possesses the best strength-to-weight ratio of any metal in common use. It can even be made stronger than mild steel.

You mentioned the riv-nuts, I bought some and decided against it. Here's why ...

1) Per the Machinist's Handbook (let's say these are 'best practices') one needs at least 1.5 to 2 full diameters of threads of engagement for aluminum fastener joints. With the riv-nuts (at least the one I got) one would be lucky to get one full thread engagement.

2) Riv-nuts might be good for tensile loading, but I'm more worried about shear loading in an aluminum boat, as the pieces work against each other. And if the riv-nut was really strong - it could be too strong - causing the stresses to propagate somewhere else, like where it wasn't designed to go. Pictures of the typical load stresses attached.

I tell you, after re-riveting my boat back together with the solid rivets, compressed air tool and bucking bar, I am totally impressed with the rivet process! Dare I say ... it was even fun to do 8) !

In fact, I'd opine that if riv-nuts and bolts were such a superior method, why don't we see them used for structural methods in aircraft construction? I bet they're only used there for adding support items to the fuselage that might need to be removed at some point. But another point, they are more costly (2 parts to buy per joint) and more time intensive an installation, so that's probably the primary decision point for companies.


Shear.JPG
Shear.JPG [ 15.19 KiB | Viewed 387 times ]
Tension.JPG
Tension.JPG [ 14.17 KiB | Viewed 387 times ]

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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 11:48 
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FWIW, I installed my aluminum floor with closed end blind aluminum rivets. Nice and strong. My captains seat is bolted to the floor and then through another piece of aluminum under the floor that rests on aluminum angle between the ribs. Sort of a frame to help reinforce the captains seat. This is all riveted to the ribs and angle under the floor to secure it down. No issues in the 3 years it has been like that.

I have also used rivnuts for my driving light mounts and my trolling motor mount. They are very strong and take the abuse I put on my trolling motor with ease. Only problem with them is removal. I've had to remove a few for various reasons and some of them ended up in a mess, especially the ones that lost their "grip" and started spinning.



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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 15:07 
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Joined: 29 Aug 2012, 21:34
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Location: Palm City, FL
That's some great info Dale. Thanks for sharing. I would love to use solid rivets on my boat, but the reality is, it wouldn't be practical in any of my applications, such as attaching the floor to my stringers.

My thought process was, yes I would spend more on rivnuts and aluminum screws, but the ease of removing the floor (for whatever reason) would be worth it.

I guess the only other option. Would be 1/4" blind rivets. I haven't had too good of luck with 3/16" so 1/4" would be my next guess.



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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 15:27 
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mbweimar wrote:
Would be 1/4" blind rivets. I haven't had too good of luck with 3/16" so 1/4" would be my next guess.

FWIW I have used some awesome high strength/high shear 1/4" rivets from McMaster-Carr. They might be available but other places too, but I can get you the specs if needed. Thing is, you WILL need a large A-frame type rivet gun to install them, but they would work great for that. Heck, you could even opt for a few blind rivets and a few riv-nuts if you wanted to.



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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 17:06 

Joined: 19 Feb 2013, 15:33
Posts: 371
5200 & aluminum pop rivets to aluminum angle to the floor is plenty strong for a console. 1 rivet per 2" or so



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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 20:11 

Joined: 17 Apr 2016, 19:24
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The advantage of aluminum is weight and if used with aluminum materials in a corrosive environment, less likely to dissolve from electrolysis. If you are working on a floor in an aluminum boat weight is not important. Use aluminum flat bar and sandwich the assembly with stainless bolts. It will take a long time to deteriorate and will remain strong. 5200 and rivets is a superb idea if you have maneuvering room for assembly.


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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 20:23 

Joined: 20 Apr 2015, 21:06
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Location: Secaucus, New Jersey
I have used aluminum screws in some specialized mechanical designs and trust me when I say an aluminum boat is not the place for them......

Use rivets as there is about 100 years of field testing to prove it out.



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 Post subject: Aluminum machine screws
PostPosted: 27 Jun 2016, 20:58 

Joined: 05 Oct 2008, 12:14
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Location: Algonquin Il
Have you looked at the specs for stainless steel rivets?



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 Post subject: Aluminum machine screws
PostPosted: 28 Jun 2016, 08:34 
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Joined: 29 Aug 2012, 21:34
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Location: Palm City, FL
I have not. BUT, my boat is used almost exclusively in salt water, so I try to keep the stainless hardware to a minimum where I can. I use aluminum rivets wherever possible. I live about a mile from the Atlantic Ocean, so even when it sits at the house, its subjected to salt spray.



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