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Dlee

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I am working on my L2 rocket

I have received from McMaster Carr a bag of 100 5/24 black plastic ( nylon ) machine screws to use as shear pins.

My plan is to use 3 of them on the nosecone.

I have had 1 L3 tell me to just make the holes really really tight, and just push them through so they hold themselves in ( no cutting threads )

I have had another L3 tell me to cut threads into the plastic nosecone

My own warped logic tells me to harden the hole in the airframe and try to cut threads into that.

What do other folks do?
 

n5wd

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I have had 1 L3 tell me to just make the holes really really tight, and just push them through so they hold themselves in ( no cutting threads )...I have had another L3 tell me to cut threads into the plastic nosecone...My own warped logic tells me to harden the hole in the airframe and try to cut threads into that.
You can always install a threaded insert or a T-nut into the hole.
 

WillMarchant

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If it is a relatively soft blow-molded nosecone then my experience is that it will distort after a few flights. But you can take a strip of thin fiberglass and epoxy that onto the outside of the nosecone and another on the inside of the body tube. Or you can use CA glue to reinforce the hole in the body tube.

I usually put threads into the hole in the nosecone and don't thread the body tube. But I see a lot of people pushing the shear pin in and then they put tape over it to hold it in place.

Whatever you do make sure that you ground test!
 

delta22

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My own warped logic tells me to harden the hole in the airframe and try to cut threads into that.

What do other folks do?
What you said. Works like a charm.

- Drill out body tube shear pin locations to 1/4" and fill the holes with epoxy. Use tape and wax paper to contain epoxy to shape of tube.
- Then insert NC into body tube and drill through the center of the (hardened and sanded) epoxy spots with a bit smaller than the screw OD.
- Remove NC and drill holes to just larger than screw OD and NC is done.
If NC material is not very hard, optionally can create hardened spots using same method as for body tube.
- Tap body tube holes.
- Paint
- Fly

Done this with several rockets and never a problem with shear pin control, including L2 flight.
 

BHP

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What size screws?

I used 2/56 nylon screws from McMaster and simply drill a clearance hole through whatever the body tube material is and into whatever the NC material is. Never had a problem with them not shearing or enlarging holes in many dozens of flights simply by pushing the screws through the holes with my thumb.

I use two 2/56 screws opposite each other but I always drill 4 holes 90 degrees apart during the build stage.

It works for me....

Pete
 

Dlee

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I mis-posted the size. the screws are indeed 2/56 screws.

I appreciate all the feedback.

2 things stick out in the replies to me

Reinforce the nosecone shoulder where the hole will be with fiberglass. Those nosecones shoulders have ridges, and I will be drilling between those, I think I do have room to reinforce the shoulder with fiberglass, I can always sand it smooth, great tip!

and I really liked the idea of making threads in the airframe, it just makes sense to me, that is the part I want to hold itself in and I cant imagine that nosecone holding threads for long.

But an Epoxy plug to hold the threads.... I can do that!

Thanks guys!!!
 

BHP

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That is interesting on the 2 vs 3 or 4 shear pins. Those results seem counter-intuitive but I think Doc knows what he's doing so have no intention of arguing! :D

I'll continue using two #2 nylons since it works for me, however.

I've seen a few people tap their shear pins holes. I don't and to me it makes little sense but do what you'd like. How, though, are you gong to remove the sheared off head portion of the screw that is in a tapped hole in your nosecone shoulder? Punch it though the threads? That'll work but eventually your threads will be wrecked.

Edit: Whoops. I see now I misread your threading idea. My bad. (But I still don't think it necessary :cool:

Pete
 

Diosces

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For cordboard or plastic, repeated shearing will oval out the interior shoulder or coupler and you might not get a shear. I usually epoxy/glue a strip of thin brass (or tin) on the interior part to help shear the pin.

Here's a link with pic for an early implementation Jaguar shear using tin. Brass sheet works much better and thats all I use now.
 

Dlee

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That one was already in my head, it is why I bought 30 ejection canisters from Newtons 3rd rocketry. I plan to use up a LOT of them in ground testing.

I have already burned up 2 of them just testing the altimeters to make sure they fire them, and that properly loaded they will easily set off the BP.

I plan on doing a lot of ground testing with the shear pins as well.
 

cobra1336

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Great choice on the ejection canisters. Use them all the time.
 

Dlee

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I like the canisters, I worry that there is no form of pyrogen in them.

But I do like the canisters, a lot. Tested a couple already... work great!

I also bought screw type switches as I wanted to mount the switches on the AV bay sled and NOT the airframe, just a hole to get to the screw switches which will be mounted very close to the inside of the coupler.
 

cobra1336

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No pyrogen, just the wire. I have ignited up to 4 grams with them.
 

new2hpr

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Ditto on using the brass shimstock to make shearing edges on the nosecone holes. I thread the airframe and clearance the brass. You do NOT want the airframe cinched down against the cone. The only forces you want are the two bodies to slide and shear the screw. With the screw in tension, you have additional friction between the cone and airframe and risk binding. Also, the head and stub of screw just unscrew after the flight.

And, as always, ground test, ground test, ground test!
-Ken
 

SteelyEyed

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Speaking of shear pins... Here's a photo of one of the three brass shear plates that I added to the shoulder of the nosecone on my L3 project. I use a small sanding drum on my Dremel tool to grind out a shallow spot so that the brass plate will be flush with the surrounding surface of the nosecone shoulder. I also cut a slot in the shoulder and put a 90 degree bend in the upper end of the brass plate, rough up the back side of the plate with 80 grit sandpaper, apply JB Weld and insert the brass plate into the slot/trough and tape it down with painters tape until the JB Weld sets up. I use #2-56 nylon screws as my shear pins and a 5/64ths drill bit for the holes. If you happen to use this technique be sure and drill the holes with the nosecone installed in the airframe as it will be during flight. After you drill the first hole install a shear pin before you rotate the assembly to drill the next hole. Similarly, after you drill the second hole install another shear pin before you proceed to the next hole. That way you keep everything properly aligned. This same setup has worked flawlessly on 8 of 8 flights of my Nuclear Stonebreaker (which is also a 5.5 inch airframe rocket with DD).

 
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sandmantoy

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I use the 4-40 nylon screws but don't tap anything, I just make them snug. On a plastic nose cone I just epoxy a small piece of brass shim stock where the shear pin is going to be. Never had one pop out. Hardening cardboard is a must on it if is not glassed.
 
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Diosces

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Speaking of shear pins... Here's a photo of one of the three brass shear plates that I added to the shoulder of the nosecone on my L3 project. I use a small sanding drum on my Dremel tool to grind out a shallow spot so that the brass plate will be flush with the surrounding surface of the nosecone shoulder. I also cut a slot in the shoulder and put a 90 degree bend in the upper end of the brass plate, rough up the back side of the plate with 80 grit sandpaper, apply JB Weld and insert the brass plate into the slot/trough and tape it down with painters tape until the JB Weld sets up. I use #2-56 nylon screws as my shear pins and a 5/64ths drill bit for the holes. If you happen to use this technique be sure and drill the holes with the nosecone installed in the airframe as it will be during flight. After you drill the first hole install a shear pin before you rotate the assembly to drill the next hole. Similarly, after you drill the second hole install another shear pin before you proceed to the next hole. That way you keep everything properly aligned. This same setup has worked flawlessly on 8 of 8 flights of my Nuclear Stonebreaker (which is also a 5.5 inch airframe rocket with DD).
Thats nice and clean!
On aligning shear holes, I put a little bitty notch on the mating edges so it's easy for me to line up//
 
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