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Custom end caps for ally pipe?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Custom end caps for ally pipe?

Unread postAuthor: chinnerz » Thu Nov 18, 2010 3:11 am

Hey guys, it has been 1 year since i made my last rifle and with school out, i got the urge once again :) :)

I am planning on using some square ally tube for the pressure chamber for a simple "inside a T barrel sealer" i have everything else figured out.. maybe..
But i have run into a bit of a snag with this pressure chamber. i can attach it to my t, but i have no idea how to seal off the ends.

any ideas??
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Unread postAuthor: inonickname » Thu Nov 18, 2010 3:41 am

Using a square chamber isn't a very good idea. Unless it's very thick and at low pressure, you'll put far too much stress on the corners.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 4:00 am

inonickname wrote:Using a square chamber isn't a very good idea. Unless it's very thick and at low pressure, you'll put far too much stress on the corners.


As the man says, there's a very good reason why there are no sharp cornered pressure chambers.

I made one once :roll:
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Unread postAuthor: chinnerz » Thu Nov 18, 2010 4:25 am

yeah i though about this heaps, but i need a flat surface to thread T's and other thingo's into.

I would like to go ahead anyways as i'm planning on running this at <100psi~

I have filed a little notch into one of the corners so i know exactly where it will blow when it does.

This is going to be a small cal 10mm od target rifle primarily, but i am making it so that i can unscrew the barrel, trigger valves and chamber to change/replace things. I may even use it at 50psi for nerf. idk yet.

yeah i remember seeing that before jack... thats the nasty side of this hobby i guess. :(
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 4:42 am

Just to have an idea of the forces involved, imagine your chamber is 1 inch square and 12 inches long.

At just 50 psi, there is a force of 600lbs/272kg on each of the 4 sides trying to rip the thing apart. I would urge you very strongly to reconsider using this material as a chamber, unless it's exceptionally thick walled and very small cross section.
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Unread postAuthor: ThornsofTime » Thu Nov 18, 2010 4:52 am

you could always hide a round pipe inside of a square body. Get the fit rite (telescoping) and you can just tap straight thru both of them and keep your flush square corners. Much better idea than pressurizing square tube.
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Unread postAuthor: inonickname » Thu Nov 18, 2010 5:02 am

Or, have an endplug machined from square Al that you can thread the tee into. I think a round chamber is more visually appealing anyway.
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Unread postAuthor: Crna Legija » Thu Nov 18, 2010 6:46 am

i taped a 1/2''bsp hole in the top of a 1'' od steel pipe with 3mm walls and its held 600psi, tho i usually keep it at 300 and the odd 500. Its held fine for 2 months now
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Unread postAuthor: chinnerz » Thu Nov 18, 2010 7:12 am

Well even if i do use a round tube (which i will be), i still have the same problem. what to do about the ends? :S



my current situation:

i'm still at school so im in constant lack of funds.

I own a bench grinder, vice, hammer, saw, drill and dremal, other then than no metal work tools.
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Unread postAuthor: Crna Legija » Thu Nov 18, 2010 7:28 am

use epoxy, just make sure you roughed up the pipe really good use a broken drill bit and if you can put some screws through the wall of you pipe and into the plug.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 7:33 am

-_- wrote:use epoxy, just make sure you roughed up the pipe really good use a broken drill bit and if you can put some screws through the wall of you pipe and into the plug.


Unsurprisingly exactly what I was going to suggest.

As a non-JSR example, have a look at what ilovefire did with his hybrid cartridges;)
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Unread postAuthor: chinnerz » Thu Nov 18, 2010 8:15 am

hmm.. i may need a crash course in epoxy casting :/
I'll have to have a look at what is available in my local hardware store.


Say i can't find any epoxy... or the right kind of epoxy, is there any other product or something i could use?
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 8:39 am

chinnerz wrote:hmm.. i may need a crash course in epoxy casting :/


Detailed tutorials here and here ;)

Isn't ilovefire an Aussie too, maybe he can point you in the right direction when it comes to sourcing materials. There are a few ebay links in the above tutorials, you can get a reasonable amout for not a lot of $$$ online if you can't find it locally.

I know what it means to work on a student budget, that was one of the appeals of epoxy when I was a younger spudder - why buy/pay for specialised parts to be machined when you can make your own ;)

Even now that my budget has grown considerably, I would still find it hard to justify paying for something like a QEV as opposed to making my own piston valve.
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Unread postAuthor: Ragnarok » Thu Nov 18, 2010 11:07 am

I know we've mostly sorted this issue now, but for a demonstration of why "square is bad", take a look at the De Havilland DH106 Comet 1.
The stress concentrations around the corners of the square windows meant that two of them tore themselves apart in mid air only a couple of years after their introduction.

Square is not safe as a pressure vessel, even if you do think you're using low pressures. The stresses involved can be whole orders of magnitude greater than with a round pressure vessel.

I should mention that this relates to the internal shape of the vessel. If you were to bore out square bar stock to have an cylindrical inside, that would be perfectly safe, if carrying a lot of excess weight in the "corners" which would add no strength over a completely cylindrical vessel.
Machining a square inside into cylindrical stock... still got the problems of a square vessel.
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Unread postAuthor: Technician1002 » Thu Nov 18, 2010 11:10 am

To have end caps as strong as the pipe, I would recommend having the ends welded on with a TIG welder. Due to the material used, I recommend taking it to a metal shop to be done properly. It only takes a few minutes per end so shop charges should be reasonable. Look up Metal Fabricators in the local area.
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