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stock as chamber

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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stock as chamber

Unread postAuthor: cheeseboy » Sat Dec 06, 2008 10:37 pm

I have made a few PVC launchers with piston valves and such , and am confident that i can make a decent copper cannon, I was thinking of my design and had an Idea.:idea:

My Idea is to - using the machine on campus - melt tin to a stock shape using a mould and using it as a chamber, my question is whether tin can hold 300psi easily and be a safe chamber, as I am planning to make the most realistic gun possible and the stock being the chamber would work well in this way.

(sorry if there is already a topic on this, i did a quick search and didn't find anything)

edit: the metal has to be tin, as it has a low melting point and is what the machine is designed for.
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Unread postAuthor: jitup » Sat Dec 06, 2008 10:50 pm

Aluminum has a fairly low mw=elting point as well. Ask if you are able to melt aluminum, if you are go to the scrap yard, ask if they havbe any brocken aluminum engine blocks. If they have not sold the metal for scrap, I'm sure they will agree to a fair price (how ever much srap alum. is worth these days) and you will have plenty of metal. THe reason I suggest this is because my school also has one of those machines (the name escapes me at the moment) but I thought it would only melt tin until one day some asked the instructor if they turned up the heat a little if it could melt alum. the instroctor said yes. Yours might be different, but now I have my first initial cast in alum!
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Unread postAuthor: mega_swordman » Sat Dec 06, 2008 11:50 pm

My only concern with this is to make extra sure that everything is secure and there are no weak points on this. This is simply due to the fact that you have a lot of pressure right next to your face.

Otherwise, I agree with the post above. I think aluminum would be a better choice for this project.
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Unread postAuthor: Daltonultra » Sat Dec 06, 2008 11:54 pm

Aluminum is the better bet. Tin is not very strong, really. You can bend a pure tin 2cm bar with your bare hands, I did that in middle-school science class.
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 12:08 am

No, not a good idea.

If it's not cylindrical (or spherical, if you can find such a thing), don't use it as a pressure vessel, because the forces on it are unbalanced and uneven. Stocks do not have a circular cross section (the only shape where the forces are evenly distributed), so it's not safe to use them as a pressure vessel.

Not that you can't contain a proper air chamber inside a stock, or use an air chamber as a stock if done properly.
iknowmy3tables did it with Deus Ex Machina.
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Unread postAuthor: jook13 » Sun Dec 07, 2008 12:13 am

I would guess it would be fine if the exterior was shaped like a stock, but he hollowed a cavaty that was cylindrical. Of course that assumption works if the wall thicknesses are enough all around.
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 12:19 am

jook13 wrote:I would guess it would be fine if the exterior was shaped like a stock, but he hollowed a cavaty that was cylindrical.

That would be workable, but pressure vessel "cavities" that do not have a circular cross section are a big no.
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Unread postAuthor: psycix » Mon Dec 08, 2008 11:58 am

Ragnarok wrote:That would be workable, but pressure vessel "cavities" that do not have a circular cross section are a big no.

What about a pressure vessel that is has a non circular cross section, but, the center of the large surfaces are supported with internal rods to pick up the force acting on those?
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Unread postAuthor: Ragnarok » Mon Dec 08, 2008 12:22 pm

psycix wrote:What about a pressure vessel that is has a non circular cross section, but, the center of the large surfaces are supported with internal rods to pick up the force acting on those?

Still not a good idea really, because the forces are still very unfavourable. A circular cross section is naturally the shape the pressure is trying to force the vessel into so it automatically stabilises itself, and all the forces are safely balanced. Seriously - stick to round.

There's absolutely nothing to stop cheeseboy from making a stock shape to fit over a conventional cylindrical chamber so he gets the look he wants while still safe.

A very good example of this kind of thing is the Parker Hale Phoenix.
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Unread postAuthor: Hotwired » Mon Dec 08, 2008 12:26 pm

That would be fun to make.

The issue is not that you can't make a pressure vessel that isn't a cylinder with a convex or concave bottom because you can.

It's that a cylinder with a rounded bottom is the most efficient shape for it. Actually I think it's a sphere but spheres are rubbish when it comes to efficient use of space so few people use them.

Making a high pressure vessel that is cubic for example would need to be much heavier to withstand deformation such as the walls ballooning out which is vastly easier to withstand in a cylindrical shape.
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Unread postAuthor: cheeseboy » Mon Dec 08, 2008 5:43 pm

wow :shock:, thanks for all the replies guys! just got back from coast, so sorry for not replying :oops:, my Idea now is to not use the mould at all because the reason I wanted to use It was to make a pressure vessel, but now I am going to forgo that and make a fat wooden stock, drill a hole in the back and store my pressure vessel- in this case a small metal cylinder- in it. I am also considering a regulated co2 setup and switch out canisters through the back, but what I like about pneumatics is once you make them it costs basically nothing to run, unless you lose the projectile. so thanks for the replies guys this is an awesome community!
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Unread postAuthor: FishBoy » Mon Dec 08, 2008 5:58 pm

[quote="cheeseboy"]unless you lose the projectile.quote]

Which you will, no matter how hard you try not to :D
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Unread postAuthor: psycix » Tue Dec 09, 2008 4:09 pm

Some projectiles are not lost, they are simply smooched all over the place or embedded deeply in something.
You know exactly where they are, but you can't use em.
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