3D Printing Anyone thread AV bay into nosecone?

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Kelly

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I need a nosecone AV Bay in a 2.6" rocket. I've done this using John Coker's technique, but in a 2.6" rocket there isn't much of a hole left for the sled once you glue in a partial bulkhead to hold the nuts. I thought a threaded connection might give me more room, but I was concerned about strength. For testing, I printed a PETG ring with female threads, epoxied it into a 2" PVC pipe, and also printed a bulkhead (which could hold a sled) with male threads:

IMG_20210220_143904466.jpg

I screwed them together, and was able to hang ~300 pounds of weight on the bulkhead, without any sign of damage. Plenty strong for my use, and I only lose a few mm off the diameter. The threads have a pitch of 2mm, and there is about 10mm worth of threads here.
Something like a twist-lok might have made rotational alignment easier (for example, for ensuring that a hole in the NC shoulder will align with an internal disarming switch) but I think the threads are stronger.
Any reason not to thread an AV sled into the NC like this? My plan is to print the female ring, glue it into a 2.6"-ish fiberglass coupler that serves as the NC shoulder, and then print a sled with the male threads and bulkhead.
 

David Schwantz

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As far as the switch, would a magnetic one work? Just a swipe to turn on or off.
 

Kelly

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Mark, OZ: Thanks, those are nice looking bays. I think I'll conclude that while fully-threaded is probably stronger, I may have gotten into the realm of overkill. Maybe I'll do some testing on the twist-locks.
 

Evolvant

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I've been thinking about doing exactly this on a 2.2" project. My efforts to print threads (Male and female) using tinkercad were an epic failure though.

Curious what software was used to create the threads and if you'd share any tricks you figured out. They look great!
 

cwbullet

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Fusion 360 and TinkerCad both can make threads.
 

Kelly

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I use OpenSCAD for almost everything these days. There's a thread package available, so creating threads is as easy as calling a function and specifying the major diameter, the pitch, the length, and the thread profile (e.g., UTS or NPT).

Tips? I'm just starting with threads, so I'm no expert. I found that, for this pitch/diameter, I needed to allow about 1mm of clearance (difference between pitch diameter of the male and female parts). I do get some stringing when printing the female threads - I might need to play with head temperature a bit. The threads need to be cleaned up a little with a sharp tool - especially at the 'reversal point' ridge in the print where the print head completes putting down a circle, and turns around and goes the other way. Finally, I found that just a small amount of silicone grease on the threads makes them operate much more smoothly. I may be able to dial the clearance down a bit, in fact.
 

Scrapmaster87

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I use fusion 360. When I do threaded motor retainers I uses class 3 (loosest tolerance) threads in the built in thread tool. So far they work pretty good for 38mm, I have yet to do 29 or 54mm versions.

The twist-lost was a lot more interesting to develop as I had to design it to be printable. Once I figured it out I can scale it up or down and not have worry about it not working. I can also add more locking lugs if I need more holding strength. Since I use it on my e-bay coupler, I did add a basic labyrinth seal to keep ejection gasses out of the locking lugs. It's amazing to be able to do 3+ dual deploy flights in an afternoon with the same rocket!
 

cwbullet

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Yes, I know. I watched all the how to videos on youtube for tinkercad. Gave it a shot. Didn't even come close. Guess I just ran out of talent. Lol
Be patient, you will get it. It took me months of tinkering.
 

roytyson

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I do this with everything up to 4” rockets.
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D9E6F29C-3256-47ED-8089-F2E75E12B9A8.jpeg
EEAE6546-5B5A-4F8F-803D-DFF8080F183C.jpeg
 

Kelly

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Not speaking for Roy, but on my prints (not yet flight-tested) the threads fit close enough - and can always be made so by adjusting print parameters - that they can be screwed on tight and have little-no chance of coming out during a flight.
 

Crazyrocket

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I would think that it would be highly unlikely to unscrew (providing it is on tight) given that the nosecone itself is free to rotate with the shock cord. I have had an ebay partially unscrew due to a twisted shockcord between the fin can and the ebay/payload bay assembly. There is enough weight and resistance between the parts that can cause something to unscrew. I just do not see that happening on a nose cone at the end of a shock cord.
 

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