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Shear Pin Shear Plates in your cardboard tubes

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Banzai88

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Part 1 of 2


I’ve mentioned several times that I use .005 brass plate/shim material to make shear plates in cardboard body tubes for nose cone shear pins, and I’ve been asked to do a tutorial more than once. I'm sure that there are other ways to do it, and I'm open to critique and useful conversation!

Not being one that documents things that I do well, this is my first attempt at a tutorial thread, so I’ll do the best that I can.

Determine how many and where you want your shear pins (there are many schools of thought on that, you decide what is best for your application). In this example I’m using 3 2-56 shear pins in a 3 inch LOC tube to secure a Pinnacle nose cone equipped with a MAC nose cone tracker bay. I’ve already marked the locations, so I drill my 1/16” holes in the three locations through the body tube and the nose cone. As I drill each hole, I leave a steel pin in the hole to ensure alignment. Remove nose cone. Treat holes with thin CA to harden the cardboard. I put a drop inside and out, blow the hole clear, and allow to cure.

Take your brass shim stock, in this case I used K&S .005 brass shim stock from Hobby Lobby, $3.49. I use a quarter as a template. Draw as many circles as you need for your rocket and cut out. I use standard hobby scissors.

I make another marking template out of paper (yellow circle), and fold it in half, then half again to find the center. I poke a hole in the center and use this to mark the center line of the brass circles. Drill out the center with a 1/16” drill bit. DO NOT PRESS HARD, you want a hole without a burr on the back side. In the event that you do get a burr, just hit it with a hammer gently to flatten it out, and run the drill bit through again. I then mark the circles to correspond to the hole that they will go into and a vertical line for alignment. Here you can see 1/A, 2/B, 3/C.


Poke the drill bit through the hole that you drilled into the airframe and on the inside, hang one of your brass circles. Press it flat against the inside of the tube and mark around the circle and the center line. Label accordingly so that you don’t get them all mixed up.


This is a Dremel #503 120grit 3/8” flap brush. We’re going to use this to excavate a recess in the tube for our shim. You can use a different tool if you like, but I find that this particular one gives the fastest, best controlled results.


Before we start, measure the thickness of your tube. DON’T trust the advertising hype….measure it! In this case we get .052”. I’m happy with the dry fit of the nose cone shoulder, so a little math is in order here. I want the brass shim, which is .005 thick, about .005 recessed into the cardboard. So .005 shim + .005 recess = .010 to remove from a .052 tube, leaving a thickness of about .042. A little DEEPER isn’t so bad, but a little SHALLOW can be bad and interfere with the fit of the nose cone. GO SLOW, measure twice, cut once.


Use the flap brush to CAREFULLY excavate your recess. Go all the way out to the lines of the circle that you drew! You will see the layers of the cardboard peel away a little at a time, and the hardened center where your hole is will be a little tougher. GO SLOWLY and carefully.


Here you see the initial results, and the circle remarked. I generally do a first pass, remark, and do a second pass to refine the size and depth of the hole before a final marking.


Now it’s time to measure with our shim in place. Here you see that I have a reading of .047. Our target thickness was .042 + .005 shim = .047. OK, so I’m right where I want to be.


Time to smooth out the transition all 3 recesses and make sure that the vertical marks and labels are legible.


Pre-bend the shim to conform to the inside of the tube. It only takes just a little bit of force.


First I’m going to describe what we want, I’m going to show you what it looks like, then I’m going to show you how to do it. The pictures hop numbers because the 5 minute kicks quickly. That and having glue everywhere didn’t lend itself well to getting all the pictures that I wanted on a single hole!

We’ll be using 5 minute epoxy, so timing is important. I use BSI 5 minute, and the smallest amount that I can mix reliably is 8 grains of hardener and 8 grains of resin. YMMV based on your cups, sticks, epoxy, technique, whatever. That 16 grains of BSI is about 3X as much as I can actually use in this particular application.

Here you see the completed layup with locater pin (spare drill bit). From outside to inside it’s a layer of tube-epoxy-shim-epoxy-wax paper.


This is how I do it:
a. Measure and mix epoxy
b. Lay a thin film of mixed epoxy into the round recess inside the tube, just enough to wet the surface

c. Insert the drill bit/pin from the outside of the tube

d. Hang the proper shim onto the pin

e. Press the shim into the recess
f. Coat the top surface of the shim with a thin film of epoxy
g. Cover the whole thing with a piece of wax paper

h. Press and hold the shim in place while removing the pin
i. Clean the pin with alcohol and a paper towel
j. You’re still holding the shim in place, right? Epoxy should be getting to the gel stage now. Gently massage any extra epoxy out around the shim. Remember, we want a .005 thickness of epoxy to return the tube to its original size
k. Once the epoxy gets warm and kicks off, you can let go. If the shim pops up, or the wax paper moves, it’s not ready for you to let go yet!
l. Carefully wipe any excess epoxy off of the outside of the tube where it might have squeezed out
m. Repeat for the other holes

Once the epoxy has hardened, probably 15-30 minutes, you can cleanly remove the wax paper. If the wax paper does not remove cleanly, the epoxy hasn’t cured enough yet. Patience, Grasshopper.

(MAX PICTURE LIMIT REACHED. SEE PART 2 BELOW)
 
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mccordmw

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For cardboard, things aren't super heavy or stressed, so I don't find the need to be too complicated.

For shear plates, here are my steps:

1. Each 2-56 pin can hold 30# of separation force. For my cardboard rockets, 2 pins is generally way more than enough to prevent drag separation. No need to calculate unless you're getting into big, heavy rockets.
2. Drill 2 holes in the tube on opposite sides. No need to harden with CA. We'll have a shear plate doing the cutting, not a hardened cardboard hole.
3. Push a nylon shear pin through each hole.
4. Using tin snips, cut a 1/2" x 1/2" square from thin steel sheets (a 2'x2' sheet from Lowes is cheap) and drill a hole in the center. I also snip off the sharp corners.
5. Bend the square so it's an arc approximately like the tube. Just eyeball it.
6. Paint a thin coating of epoxy on the inside of the tube around the nylon screw.
7. Push the steel square on the nylon screw, wipe off any excess epoxy so the interior is smooth.

The hole should be small enough so the screw is on tight. That holds the steel plate firmly to the tube. Once the epoxy is cured, back out the screw. Epoxy doesn't stick to nylon. You'll also have some epoxy threads that were created. Those hold the screw in well.

I find that most cardboard tubes have enough space between the tube and coupler that there hasn't been a need to sand out a space. I have a stack of pre-cut 1/2" squares of steel sheeting (I think it's 164th inch? Can't recall), so it just takes minutes to do all of this.
 

Banzai88

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Maybe it's just the tubes and nose cones that I seem to be getting, or maybe it's the variable humidity and temperature here in Tidewater, Virginia, but almost NONE of the combinations that I have in cardboard tubes are consistent day to day, never mind loose enough that I wouldn't need to excavate for a shear plate to get proper clearance. Every time that I tried surface mount inside the tube, it ended in disappointment.
 
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Flyfalcons

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Thank you for the tutorial, Tom. I'm right at that stage with my BBIII build, and your technique seems to confirm what my plans were for my nose cone. I take it you are just using shear pins for the payload tube/nose cone joint, and not for the booster/avbay joint, and relying on friction fit on the booster/avbay?
 

Banzai88

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Thank you for the tutorial, Tom. I'm right at that stage with my BBIII build, and your technique seems to confirm what my plans were for my nose cone. I take it you are just using shear pins for the payload tube/nose cone joint, and not for the booster/avbay joint, and relying on friction fit on the booster/avbay?
You're welcome, I hope that it helps develop a practice that leads to reliability of deployments and nose retention for you. Yup, I usually just rely on friction fit for the lower joint, tuned for conditions for the day with tape.
 

dixontj93060

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I've used brass stock for years but I've never thought of putting the shim on the inside of the tube. Nice technique. Thanks for sharing.
 

mpitfield

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Great post, I have a build on the table that I will use this technique on. I agree with you in that even though it's a bit more work, I feel more comfortable putting the extra effort in upfront. The last thing I want to deal with is a tear in the airframe, which results in bodywork, filling, sanding, priming, painting, etc. That is assuming the tear didn't somehow result in a deployment issue and worst case a complete loss of the rocket.
 

Banzai88

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Part 2 of 2

This is what you’ve created. Likely, as you see here, you’ll have epoxy spread out in a larger area than you need, and probably some harder ridges. We’ll remove those in the next step.


Once the epoxy has hardened a full 24 hours, I go back with my #503 flap brush and GENTLY and CAREFULLY remove and blend the excess epoxy, leaving what you see here.


A gentle touch of some #400 sandpaper and all is right with the world.


Re-measure, you should be back to your original .052. Whatever you get, make sure that:

  1. You don’t leave any hard ridges
  2. You’re happy with the fit and slide of your nose cone

You can see here that a LITTLE more sanding is in order. It's OK if you go through the epoxy and onto the brass, just go slow and be careful.


Align the holes properly, if necessary, re-drill the 1/16” pilot hole in the tube.
Tap each of the holes 2-56. Put a screw in each as it’s finished to ensure proper alignment as you progress around the tube.


DONE!
 
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rharshberger

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Nice tutorial Tom, thank you. Putting the shim on the inside I believe is a key point as the shim and the nosecone shoulders are both rigid and the screw wont move or cock slightly before shearing, it also should shear more consistently and no potential long pieces to jam in the space between tube and NC shoulder.
 

Banzai88

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Nice tutorial Tom, thank you. Putting the shim on the inside I believe is a key point as the shim and the nosecone shoulders are both rigid and the screw wont move or cock slightly before shearing, it also should shear more consistently and no potential long pieces to jam in the space between tube and NC shoulder.
Thank you, sir. High praise for my first instructional thread, especially from someone of your skill level.

Since I started doing it this way, I haven't torn a tube again or had a pin related failure. It's such positive retention in the tube that I routinely recover all of the shear pin screw heads as well!

On another note, doing it this way also allows me to use only 1 shear pin on rockets where I used to fight the friction fit genie. Blasphemy, I know, because everyone 'knows' that one pin will cock and jam the nose cone. To that I say "NOT if it's done correctly with properly fitted shear surfaces".
 

farsidius

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Great tutorial! Thanks for posting this here. It's great to have these resource to come back to
 

Banzai88

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Just checked on this thread. Surprised that it's still here, as I lost the source pictures not long after posting this thread. Hopefully it's helped someone along the way.
 

Bat-mite

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You did a great job. Thanks. Only question I had was: what did you use to apply the epoxy into the recess? Popsicle stick, paintbrush, toothbrush?
 

Banzai88

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I just used the popsicle stick that I used to mix the epoxy. It's not really deep, nor does it require a lot of epoxy, but it's enough that you can feel the sides of the recess that you excavate for the plate. I've managed to do this a few times since this and get it just about right to where I didn't need to do any other feathering of the epoxy other than a 400 grip polish.

I've seen this done with plates in the plastic nose cones, but my reasoning for putting the plates in the cardboard tube was to reinforce the tube, since the nose cone is already hard enough. I did a few rockets with just CA strengthened holes, and noticed after a few ejections that the holes were starting to elongate, and once had a tear through. Since I've been doing it this way I haven't had a single failure to shear.
 

KevinM

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Thanks so much for posting this Tom! I've been fighting with (and losing) the "friction fit" battle myself for a while now. Have you ever tried this aproach with Estes/BMS thin-walled (0.035") tube, or do you think that would be asking for disaster?
 

gary7

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I would like to shim my polypropylene nose cone. My problem is finding what epoxy will best hold the shims in place? It seems that a friction fit of the nose cone will keep them in place until separation but then I would lose them. I may also just try simple Scotch Tape. to hold them in place.

On the other hand, I have flown poly nose cones with shear pins and without tear out or elongation of the hole in the cone although someone else has mentioned that it can happen.

Any one else have thoughts about this?

Finally, 0.005" brass is rather thin and fragile. Seems like it would fail before a 2/56 screw shears.
 
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Banzai88

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Thanks so much for posting this Tom! I've been fighting with (and losing) the "friction fit" battle myself for a while now. Have you ever tried this aproach with Estes/BMS thin-walled (0.035") tube, or do you think that would be asking for disaster?
I've never tried it with tube walls that thin. In fact, I've only ever done it with MadCow/LOC tubes. If you have a spare piece of tube, that would be the thing to do a test run on.
 

Banzai88

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I would like to shim my polypropylene nose cone. My problem is finding what epoxy will best hold the shims in place? It seems that a friction fit of the nose cone will keep them in place until separation but then I would lose them. I may also just try simple Scotch Tape. to hold them in place.

On the other hand, I have flown poly nose cones with shear pins and without tear out or elongation of the hole in the cone although someone else has mentioned that it can happen.

Any one else have thoughts about this?

Finally, 0.005" brass is rather thin and fragile. Seems like it would fail before a 2/56 screw shears.
Good luck finding something that will bond, and STAY bonded with a poly nose cone long term. I think that we're all searching for that unicorn, although I've seen people have good luck with rocketpoxy, 4500, and JB weld. YMMV.

The whole reason for doing it this way was to prevent shear screw/pin tear through or wear on the hole in the cardboard. I've had a few disappointments there, including a tear that caused a nose cone to hang up and fail to separate and allow laundry deployment. Since I've been doing it this way, not one single failure. This method provides 2 solid shear surfaces between the shim and the plastic of the nose cone shoulder, much like the two halves of a pair of scissors.

.005 brass as installed like I've demonstrated is MUCH stronger than a 2/56 shear screw. I have a few rockets that have dozens of flights on them with no failures of the shear plates or elongation of the holes (unlike CA strengthened cardboard holes).
 

KevinM

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I've never tried it with tube walls that thin. In fact, I've only ever done it with MadCow/LOC tubes. If you have a spare piece of tube, that would be the thing to do a test run on.
Test report!

I really appreciate it when data is posted in response to questions (even when they're my own) so I went and made a mock-up to try this technique out using a scrap section of 3" Estes thin-walled BT and a modified NC from a Big Daddy. I slightly varied the install technique from above in that I used a single shim which I glued in first, then drilled and tapped after the epoxy had set up, but otherwise followed the directions above.
01_setup.jpg

After everything had set up I used one of the many online BP ejection charge calculators to figure out that 0.11g was about the right amount of charge for this 3" x 3" section, so I made up a couple of charge pouches with 0.15g and 0.2g to test how this section would hold up to an aggressive charge.
02_internalView.jpg

I'm pleased to report that the body tube held up with no apparent signs of damage after firing both (oversized) charges. The only trouble I have to report is that after each firing the "head" of the screw would twist off what was left in the body tube, leaving a small threaded piece trapped in the shim that was very difficult to remove.
03_shearPlate.jpg

04_shearPinPostFiringBT.jpg

04_shearPinPostFiringNC.jpg

Based on these results I'm now planning to retrofit a shear plate like this into my DD-capable Leviathan. Thanks again to the OP for posting this!

KMc
 

Banzai88

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Awesome test rig! Since you documented it with data and pictures, it's officially "rocket science"! Glad that you found the thread helpful.
 

Nytrunner

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Based on these results I'm now planning to retrofit a shear plate like this into my DD-capable Leviathan. Thanks again to the OP for posting this!
I'll add in my experience with thin wall nose cones like the Pro series II. The plastic is so thin that it can deform and lead to pin bending and jamming after a couple firings. I've had deployment problems due to that, and although I love the brass strip and shear pin technique, I just friction fit thin nosecones like on the PSII rockets
 

KevinM

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I'll add in my experience with thin wall nose cones like the Pro series II. The plastic is so thin that it can deform and lead to pin bending and jamming after a couple firings. I've had deployment problems due to that, and although I love the brass strip and shear pin technique, I just friction fit thin nosecones like on the PSII rockets
Thanks for that bit of wisdom. I recall being concerned about that, so I made sure I found the thickest side of the NC to drill the hole through, and I also tried to make sure the NC's side wall would snug right up against the plate to minimize the opportunity to torque. I'm glad to know that my concerns were warranted, but also that my implementation may still be succeptible.

It's funny you mention friction fit, one of the reasons I was so excited when I saw this thread is specifically because I've had so much trouble with friction fitting my NC.
 

Nytrunner

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Thanks for that bit of wisdom. I recall being concerned about that, so I made sure I found the thickest side of the NC to drill the hole through, and I also tried to make sure the NC's side wall would snug right up against the plate to minimize the opportunity to torque. I'm glad to know that my concerns were warranted, but also that my implementation may still be succeptible.

It's funny you mention friction fit, one of the reasons I was so excited when I saw this thread is specifically because I've had so much trouble with friction fitting my NC.
If you scuff the interior near the whole (so it has some toothiness) maybe an epoxy application would strengthen the area?

Yeah, I dislike friction fit on principle because it's difficult to quantify, but I dislike damaged rockets more o_O
 
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