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JimJarvis50

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So I have this metal rocket. The coupler is true, but the upper airframe tube is a bit out of round, and the tube wobbles more than I would like. My tried-and-true blue tape technique that works well on composites just won't work with the metal tube - the metal just chews up the tape. Is there something else that I can try?

Jim
 

dixontj93060

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What type of metal is it Jim? Aluminum? Have you tried metal tape?
 

dixontj93060

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JimJarvis50

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It's hard to say what the gap is. Seems significant from the motion, but even a few pieces of tape tighten it up. Maybe I just need to exaggerate the chamfer?

Jim
 

CORZERO

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Source a new (concentric) tube. How terribly expensive could a new aluminum tube be?
 

JimJarvis50

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Source a new (concentric) tube. How terribly expensive could a new aluminum tube be?
Cost isn't really an issue. The existing tube has been trimmed, squared, fit for the altimeter, shear pinned, and a few other things that are hard to explain. I don't want to replace it, I just want to make it fit a bit better.

Jim
 

JimJarvis50

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Gotcha. How about bore and sleeve?
I don't think that is going to happen from the perspective of time (not to mention that I have no experience doing this or have the equipment it would require). It would be interesting to determine if this would actually be feasible. I assume (demonstrating my ignorance) that the idea would be to bore out the tube and then insert an appropriately sized sleeve. Would it be possible to bore out a 6" tube of 1/8" aluminum? How would it be done?

Jim
 

mpitfield

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Jim I have nothing to add but this sounds like an interesting build and pretty serious rocket...looking forward to hearing about it.
 

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Jim I have nothing to add but this sounds like an interesting build and pretty serious rocket...looking forward to hearing about it.
Ditto. Even more interesting if it is held together with duct tape. LOL
 

CORZERO

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Correct. Bore/sleeve is often performed on internal combustion engine cylinders to increase displacement and or modify bore/stroke ratio. Probably more work than you were looking forward to but your local machinist/motor builder would either have the facilities to perform the task with relative ease or be able to refer you to one whom could. At the level you fly I would consider anything less than a proper fit to be unacceptable. Any temporary or afro-engineered modification to mitigate "wobble" would not be considered and the project halted were it my own.
 

manixFan

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Index the tube to the coupler (likely already is) and find the 'low spots' in the upper airframe. Scuff/clean the coupler and coat with a very thin layer of epoxy in the low areas. When cured sand to fit.

I've never tried this with a metal airframe but have with other slip fit joints that had excessive play where tape would not work. The hard part might be getting it thin enough depending on your tolerances. I used epoxy filled with wood flour for easy sanding but you might try phenolic micro balloons or something similar.

Sounds like a BALLS flight. I hope to see it there.


Tony
 

Kip_Daugirdas

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Shim stock with pressure sensitive adhesive?

I haven't used it for rocketry but we used some at work and it holds impressively well.
 

JimJarvis50

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Well, I over-exaggerated the chamfer and buffed it smooth. Tried metal tape. No joy. Still trying things.

Jim
 

ECayemberg

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Strategic strikes with a rubber mallet or soft blow hammer. No joke. Especially if it can be supported on the inside.

What is the wall thickness, Jim?
 

JimJarvis50

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Strategic strikes with a rubber mallet or soft blow hammer. No joke. Especially if it can be supported on the inside.

What is the wall thickness, Jim?
I had a certain motor vendor advise me to do that when I managed to drop a motor casing. It worked! In this case, I think making the tube round would just make it a little over-sized all the way around. I'd still have to tape it somehow.

I did manage to have a little luck with the metal tape. I reduced the movement by about half. One trick is to slide the airframe straight on and in alignment, and not with any side-to-side movement. A little oil on the tape helps too. I can't get any more pieces on the full length of the coupler, but I think I can add some right at the top of the tube (the last inch of travel). If this works, I think it will be OK.

Jim
 

sharkbait

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Look up DuraBond 7025 by Cotronics.

It's an aluminum repair epoxy that you can apply then hone or machine for fitment, good to 2,000 degrees.
 

3stoogesrocketry

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Jim I would suggest getting some dykem blue and paint your coupler with it . Once you slide the tube onto the coupler , it will scuff the coupler where it is making contact , so you will know where you need to build up the coupler .

Eric
 

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If time isn't a huge issue, you may be able to have a nesting piece made that is slightly larger than your coupler that is an interference fit (0.001-0.003") with the tight spots. Bash it in there to re-form the tube, then somehow bash it out.

Tim Allen's preferred method.
 

manixFan

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Look up DuraBond 7025 by Cotronics.

It's an aluminum repair epoxy that you can apply then hone or machine for fitment, good to 2,000 degrees.
Good to know about that stuff, definitely a lot better than epoxy and wood flour for this use. However doing a bit of research I found this:

"Dear George,
Thank you for your interest in our 7025 product.
The material may work, but there is one or two things you should know.
First, you should apply the material to a clean roughened surface.
Second, the material cures to be porous and weak. It will require a post cure at 250°F for 2 hours."

It is the reply from a Cotronics rep to a post on a car repair forum. So for Jim's use that may be a bit of an issue. I also could not find a source for it. But it does sound pretty promising for a variety of uses. (The actual spec says good to 1000 degrees but still plenty sufficient.)


Tony
 

JimJarvis50

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I think I might have an idea for how to do this with epoxy. Thinking out loud, I wrap the coupler tube, which is true, with some very thin mylar. Then, in the airframe, slather some epoxy around the ID, maybe starting a few inches back and repeating a couple of times to get everything smooth and to near the top of the tube. I have an extra section of the tubing (cut from the ends) that I can pilot this with.

Jim
 

sharkbait

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Good to know about that stuff, definitely a lot better than epoxy and wood flour for this use. However doing a bit of research I found this:

"Dear George,
Thank you for your interest in our 7025 product.
The material may work, but there is one or two things you should know.
First, you should apply the material to a clean roughened surface.
Second, the material cures to be porous and weak. It will require a post cure at 250°F for 2 hours."

It is the reply from a Cotronics rep to a post on a car repair forum. So for Jim's use that may be a bit of an issue. I also could not find a source for it. But it does sound pretty promising for a variety of uses. (The actual spec says good to 1000 degrees but still plenty sufficient.)


Tony
Not going to get into a debate on questioning the Rep but our guys use this stuff at work repairing the process piping at the batch plant. We also use the 7032 Stainless putty which is good to 2,000 sorry for the confusion on the temp ratings. you are correct that the 7025 is only rated to 1,000 degrees, but it does not require a post heat cure. 7025 will cure in 24 hors at room temp or you can post heat at 250 for a 2 hour cure. both are capable of being drilled, tapped, ground, or otherwise machined after cure.
 

sharkbait

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Sorry, accidently hit the post button before I was done typing.

The 7025 does have some minor inherent porosity, but it's not weak by any means, it has a compressive strength of over 4,000 psi. Unfortunately there are no distributors for 7025 & 7032, you must set up an account and purchase them directly from Cotronics.

That being said, I thought for some reason that there was a high temperature requirement involved with this repair, if not then Cotronics RK454 Aluminum repair epoxy would be a better choice. It's only rated at 500 degrees but it has a higher bond capacity and has a much higher compressive strength of 10,000 psi. We use this on our low temp applications, and the good news is that you buy RK454 directly from good ole McMaster-Carr
 

manixFan

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Not going to get into a debate on questioning the Rep but our guys use this stuff at work repairing the process piping at the batch plant. We also use the 7032 Stainless putty which is good to 2,000 sorry for the confusion on the temp ratings. you are correct that the 7025 is only rated to 1,000 degrees, but it does not require a post heat cure. 7025 will cure in 24 hors at room temp or you can post heat at 250 for a 2 hour cure. both are capable of being drilled, tapped, ground, or otherwise machined after cure.
No debate from me, I'll defer to your real world experience to something I read on the web, regardless of the source. I appreciate your follow-up and very informative reply. I much prefer actual personal experience and recommendations over something I read. If you know it works based on your experience, that's good enough for me.


Tony
 

JimJarvis50

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The first pilot test with epoxy went OK, but some improvements are needed. For the pilot, I covered the waxed coupler with thin mylar, coated about 2 inches of the mylar with epoxy and West 410 filler (with microballons I think) and then pushed on a scrap air frame section. The filler was to thicken the epoxy so that it would stay on the coupler without just running off. The air frame section was "buttered" with epoxy to help with bonding to the aluminum. It looks like the epoxy managed to get reasonable coverage on the larger gap on either side of the tube. In the areas where the gap was smaller, the coverage was just spotty.

I don't think it would be possible to put a thicker coat of epoxy on the inside of the tube and then push it onto the mylar.

The trick to this will be to figure out how to get the most coverage possible of the coupler. I don't think it will be possible to do this more than once. After the first application, the large gap is filled so repeating the process won't work. It would probably help to make the epoxy thinner than what I did and just deal with the mess. It might be possible to do one side of the tube and then the other using gravity to fill the low spots. What would be really helpful is to figure out how to inject epoxy into the coupler to fill the gap from the inside out. Ideas on logistics would be welcome.

Jim

IMG_0881.jpg
 

3stoogesrocketry

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The first pilot test with epoxy went OK, but some improvements are needed. For the pilot, I covered the waxed coupler with thin mylar, coated about 2 inches of the mylar with epoxy and West 410 filler (with microballons I think) and then pushed on a scrap air frame section. The filler was to thicken the epoxy so that it would stay on the coupler without just running off. The air frame section was "buttered" with epoxy to help with bonding to the aluminum. It looks like the epoxy managed to get reasonable coverage on the larger gap on either side of the tube. In the areas where the gap was smaller, the coverage was just spotty.

I don't think it would be possible to put a thicker coat of epoxy on the inside of the tube and then push it onto the mylar.

The trick to this will be to figure out how to get the most coverage possible of the coupler. I don't think it will be possible to do this more than once. After the first application, the large gap is filled so repeating the process won't work. It would probably help to make the epoxy thinner than what I did and just deal with the mess. It might be possible to do one side of the tube and then the other using gravity to fill the low spots. What would be really helpful is to figure out how to inject epoxy into the coupler to fill the gap from the inside out. Ideas on logistics would be welcome.

Jim
Jim , how about if you built a rig to spin your tube around 100 rpm , then poured some thin epoxy into the tube . Tge epoxy will leave a thin film and should fill in the low spots. You can then either use a hone or carfully sand the epoxy to get a perfect fit.

Eric
 
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