YouTube tutorial question... I must not understand something...

Discussion in 'Techniques' started by Pariah Zero, Dec 25, 2019.

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  1. Dec 25, 2019 #1

    Pariah Zero

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    I’ve been watching John Coker’s build tutorials on YouTube, and something is really bugging me:

    In every case, I see centering rings, fins, etc. being epoxied together, but with fillets only - there is no epoxy applied between the mating surfaces. (such as the inside/outside of the centering rings, or the roots of fins.)

    I only see epoxy fillets being made.

    I’m not going to claim extensive amount of epoxy experience, but I was taught that fillets are used to supplement the joint between the mating surfaces — and fillets aren’t supposed to be the only thing used to bond.

    So what am I missing? I get the feeling I don’t understand something properly.
     
    Last edited: Dec 25, 2019
  2. Dec 25, 2019 #2

    prfesser

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    I'm anything but expert on this, but...epoxying a 1/16" centering ring into a tube gets a line of adhesive that's 1/16" wide, or a bit more. Applying a decent fillet to each side expands that by a factor of 8 or more. Much if not most of the strength is in the fillet, not the original joint (if the surfaces are properly prepared). That being said, I apply epoxy to the two pieces, put it together, let it set, then add fillets.

    Side note: I suggest applying epoxy to both parts being joined, rather than applying to just one part and assuming it will wet the other part properly. Sometimes...it doesn't.

    Best -- Terry
     
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  3. Dec 25, 2019 #3

    kuririn

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    My 2 cents: Don't forget that epoxy will permeate and soak into porous materials. The longer the set time, the deeper the penetration. So for thin pieces like centering rings and fins it MAY not make a difference.
    Personally, I do apply epoxy to the mating surface before attaching: root chords of fins and a line on the body tube where the centering ring will go. I don't apply it to both surfaces. Rather impossible to do that with centering rings. Provides me with added insurance and peace of mind. After letting that tack bond cure I apply the fillets. So do what you feel comfortable with.
     
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  4. Dec 28, 2019 #4

    Ez2cDave

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    I tack the Centering Rings in position, using a tiny drop of CA, in a couple of spots. Then, I fillet each of them, on one side ( the same side, obviously ), with Epoxy. After the Epoxy cures, I use CA to completely bond the Centering Rings to the Motor Mount Tube. Then, I use Epoxy to complete the Filleting process on the other side of the Centering Rings.

    Dave F.
     
  5. Dec 28, 2019 #5

    richP

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    There's more than one way to skin a cat. People have different build techniques.
    The main reason why some folks don't epoxy the rings to MMT, is because they use a few drops of CA to tack the rings into position. CA will wick into any available space and will inhibit penetration of epoxy; but it does make aligning and setting a final position much easier than using epoxy for the initial bond.
    Personally, I like to use a slow-cure epoxy for initial bonding. But I am also in no rush and can wait a day to let individual components cure before moving on. There are risks- you will find occasionally that the fin/CR/bulkhead that you aligned perfectly the day before has slumped out-of-alignment before the epoxy has cured completely.
     
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  6. Dec 28, 2019 #6

    Pariah Zero

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    I’m glad to hear it’s more a personal preference/style thing than me just being ignorant about proper technique.


    Yeah, my son will only allow me a 30-60 minutes a day at most for my own free time. That’s about though time to mix & apply Epoxy, and then clamp it up. (He’s ~3 years old; Epoxy is not something I want him to play with while “helping” dad... maybe in a few more years.)

    You win again, Gravity!!!
     
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  7. Jan 3, 2020 #7

    solid_fuel

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    that sounds like you'd be good with slow cure epoxy.
     
  8. Jan 3, 2020 #8

    Pariah Zero

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    Agreed.
     
  9. Jan 17, 2020 #9

    jqavins

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    I know that with wood glue (e.g. for balsa fins on paper tubes) the fillets provide most of the strength, while butt gluing the root edge is only good enough for a temporary hold while applying the fillets. I don't see any reason this would be different for epoxy, but I won't swear it isn't. And I glue all mating surfaces, whatever glue I'm using.

    All that said, why not ask John?
     
  10. Jan 17, 2020 #10

    Pariah Zero

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    Oh,you know... I didn’t realize he was on the forum when I wrote the initial question...:confused:
     
  11. Jan 18, 2020 #11

    jlabrasca

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    Did nobody warn you about starting adhesive threads?

    Think in terms of stresses. The fillets increase the contact area for the joint, and so reduce the stress for a given force.

    For the fin-tab to motor-mount connection the contact area approaches a mathematical line (a plane surface on the bottom of the tab in contact with a circular cylinder).

    That joint is intended to keep the fin from rotating in the slot. This would be a shear stress at the mating surface, but would -- sort of -- be a compressional or tensile stress for the fillets.

    The internal fillets between the body tube and the fin tabs are intended to keep the fin from pulling out of the slot -- again this would be a shear stress exerted over a small contact area without the fillets.

    For centering rings the area of the mating surfaces is likewise very small and, for a rocket with through-wall fins, the centering rings might not be load-bearing.

    https://www.apogeerockets.com/education/downloads/Newsletter126.pdf

    I know I got at least some insight into this from correspondence with John Coker -- but i will not attribute any part of it, in case I am misremembering.

    edit: typos + autocorrect + wrong damned glasses for working at this screen
     
  12. Jan 18, 2020 #12

    Pariah Zero

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    It’s kind of hair splitting, but my question is just as valid with Glitter Glue as with whatever miracle material... and to be honest, so were all the answers.

    Thanks for the replies, faith in humanity temporarily restored.

    I’m definitely not up for debating esoterica about whatever hot snot somebody prefers. The important things are that the adhesive I chose is the best one, and as long as I never say what animal it came from, nobody will be able to contradict me.

    Alright, alright, it came from humans

    edit: spelling, and the fact I can’t blame anything on the wrong glasses.
     
    Last edited: Jan 18, 2020
  13. Jan 18, 2020 #13

    jlabrasca

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    Despite the name, the glass transition temperature of hot snot is actually very low (depends on the mfg, but figure no higher than about 110°C). You might get away with using it for forward bulkhead, but I would not use it for the motor mount or for any component that is exposed to ejection gasses.
     
  14. Jan 18, 2020 #14

    Pariah Zero

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    No question, and I agree with you.

    I mentioned hot melt adhesive and glitter glue specifically because they are less than good choices for rocketry. Sadly, the internet isn’t a great place to be facetious. Sorry for that misunderstanding.
     
  15. Jan 18, 2020 #15

    jlabrasca

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  16. Jan 18, 2020 #16

    Pariah Zero

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  17. Jan 18, 2020 #17

    jlabrasca

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    I did a Science Direct search for "mucus+glass+transition" while composing my reply in #13. I got more than 100 hits. Mucus is really interesting. The stuff that snails and slugs exude -- for example -- is a non-newtonian fluid; it is solid and strongly adhesive under tensile strain, but flows under compression.

    Adding the word "rocket" to the search significantly changed the character of the search results -- there is (apparently) a "Rocket sampling syringe" favored for collecting cervical mucus as well as a protein assay technique referred to as "Rocket immunoelectrophoresis". I didn't dig too deeply (I was briefly distracted by this title: Influence of caffeine on movement characteristics, fertilizing capacity and ability to penetrate cervical mucus of human spermatozoa) but I did not find any reports of the use of "snot" of any variety in the construction of rockets.

    Before writing up a research proposal, a more exhaustive literature search is indicated.
     
  18. Jan 18, 2020 #18

    Pariah Zero

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    I wonder if somebody has used snot as propellant...
     
  19. Jan 18, 2020 #19

    JohnCoker

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    I generally cut the CRs so they are a tight fit (just barely able to slide on). This means there is no room for epoxy between. Also, I prefer to tack the parts in place with CyA first, then do the fillets for the strong bonding. Epoxy is stronger in compression than tension so a decent fillet on either side of the CR maximizes the mechanical properties of the joint as I understand it.
     
    Last edited: Jan 18, 2020
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  20. Jan 18, 2020 #20

    manixFan

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    As John says, the fit between the centering rings and the tubes is generally fairly tight. What I do is lay down a thin fillet of regular epoxy first and wait a minute for that to soak in and 'wick' into any spaces big enough for it to flow into. Then I'll use thickened epoxy to form the actual fillet. Often times the thin epoxy will seep out the other side of the centering ring so I know it's getting between the tube and the ring.


    Tony
     
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  21. Jan 18, 2020 #21

    jlabrasca

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    Discussion of propellant formulations is restricted to the Experimental Forum.
     
  22. Jan 18, 2020 #22

    jqavins

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    Too high in water content. beyond that, see above.
     
  23. Jan 18, 2020 #23

    Pariah Zero

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    [​IMG]
    (Imagine this is me, as realization sets in...) ohh crap, that’s right!

    ... I totally forgot it’s not just about things that burn. Hopefully I won’t get banned... I realize that’s something the moderators absolutely have to take seriously... no joking intended. I don’t want anybody getting in trouble, especially me. It’d really suck to get banned the day after subscribing to the forum. And for a snot joke at that!

    Sorry!
     
    Last edited: Jan 18, 2020
  24. Jan 30, 2020 #24

    Greg Furtman

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    PZ, what I usually do is apply epoxy inside the tube all the way around just before where the centering ring will be so when I push it into position it gets to the epoxy first and as it slides across that area the center of the ring accumulates epoxy. After it sets up then I do my filleting with thickened epoxy.
     
  25. Jan 30, 2020 #25

    jqavins

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    The puts a thin layer of epoxy between the ring and the wall, which may or may not help, and surely doesn't hurt. If the fit is really snug, wouldn't it be nice to have a little lubricant to make insertion easier that, afterward, doesn't interfere with bonding and just maybe even makes the bond a little better? So I use glue there too, whether white, wood, or epoxy.
     
  26. Jan 30, 2020 #26

    Greg Furtman

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    I like epoxy for internal things because it is not "grabby". I made the mistake once of using Titebond to glue the inner tube of an avionics bay. The Titebond was so grabby that I barely got the tube in all the way. Learned my lesson. Epoxy gives me time to work & does provide a little bit of lubrication. ;)
     
  27. Jan 30, 2020 #27

    jqavins

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    White glue is also not grabby, for those situations where its strength is sufficient. I use wood glue for inserting motor mounts in LB body tubes, but you've got to 1) only pass the centering rings each through a small ring of glue, and 2) get them into position in one smooth motion; if you stop, you're stuck.
     
  28. Jan 30, 2020 #28

    mbeels

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    Yeah, according to the "Woodworker's Handbook", white (PVA) glue and wood glue have similar relative ultimate strength. I don't know if there is really a situation where wood glue is strong enough, but white glue isn't. I use white glue anywhere I need to slide something into position.
     
  29. Jan 30, 2020 #29

    jqavins

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    Is that true of the glue itself or the whole glued joint (i.e. including the glue-to-surface adhesion)?
     
  30. Jan 30, 2020 #30

    mbeels

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    That I don't know. It'd be interesting to find some test results.

    With my rockets, it seems that it is usually the material surrounding a glue joint that breaks, not the glue joint itself (white, wood, CA, or epoxy). But 80% of my rockets are LPR or MPR, and 90% of my rockets are cardboard and wood. Sometimes I've seen glue pull loose from smooth surfaces, like the glassine on tubes, or gloss finish on cereal boxes.
     

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