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Coaxial piston questions

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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Coaxial piston questions

Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 7:30 pm

After looking at my sprinkler valved cannon, i have seen how simple it is to build a coaxial cannon.

I was wondering if there were any disadvantages to this type of cannon?
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Unread postAuthor: pizlo » Wed Jul 16, 2008 9:00 pm

Its harder to make, you have to deal with th form its in, the pilot might not be able to be a blowgun.
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Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 9:55 pm

I know. I was thinking 1/2 bv? too big?

Chamber would be 1 foot of 2 inch pvc. Barrel would 1 inch. The piston is an undetermined material as of now. Whats the ideal size for the piston? Length wise i mean? And when piloted how big should the gap between the barrel and piston be?
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Unread postAuthor: frankrede » Wed Jul 16, 2008 10:02 pm

1/4"
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 17, 2008 12:24 am

This questions have been asked and answered repeatedly, take some time to use the search feature - it will answer questions you never even thought to ask ;)
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Unread postAuthor: iisthemuffin » Thu Jul 17, 2008 9:47 am

Asked or not, i cant find it. And i do not have the time to spend sifting through thousands of od posts. I jsut tried many different search titles and still did not find my answer. I respect that you guys dont want to waste your time answering questions over and over but if i cant find it what else am i to do? And i know more than well that one of you will get on the search and find exactly what i asked and tell me it took you 2 seconds. Well tell me how you find things so quickly, and ill be out of your hair.

Until then, would you mind answering my other question?

Also, i saw someone using a union (while i was searching) to attach the pilot to the back of the chamber. I assume this was so they could get to the piston if desired. Is a union ok for this purpose?
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Unread postAuthor: psycix » Thu Jul 17, 2008 12:03 pm

iisthemuffin wrote:I was wondering if there were any disadvantages to this type of cannon?
Breach-loading systems will be very complicated.

iisthemuffin wrote:1- I know. I was thinking 1/2 bv? too big?
2-Whats the ideal size for the piston?
3-Length wise i mean?
4-And when piloted how big should the gap between the barrel and piston be?

1-The bigger the pilot valve, the better.
2/3-If its too short, it may jam because it may allow it to rotate. The same length as diameter is basically a good idea. Remember to make it as light as possible.
4-Mathematically seen: minimal 1/4*Barrel Diameter, but a few bits more are recommended. Note that your piston may bounce back a little and it may not constrict flow while doing that.

Edit:
@ your last post:
It is HIGHLY recommended to make the piston serviceable. This can be done with almost anything, usually done with threaded adapters.
If your piston screws up, then you dont have to break open your gun.
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Unread postAuthor: iisthemuffin » Thu Jul 17, 2008 12:35 pm

So if i have a 1 inch barrel then the gap between the barrel and piston, when piloted, should be roughly a bit larger than 1/4 inch?

Im trying to make a mini coaxial right now. Just to try my hand at pistons before i spend some real money. Should the sealing face be soft? Like very soft rubber? Im trying to use rubber intertube and ive tried about four layers of it. My piston and barrel are both square but it just doesnt seem to want to seal. Any ideas?
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Unread postAuthor: Coodude26 » Fri Jul 18, 2008 1:47 pm

Hey psycix, how much pressure can a 4" endcap take?

And muffin, it should be somewhat firm, otherwise it may get cut up from the high pressures.
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Unread postAuthor: psycix » Fri Jul 18, 2008 2:22 pm

Just depends on what the cap is rated for.
Please note that the piston is slamming back into the bumper, and that the cap also might have to endure that aswell.
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Unread postAuthor: starman » Fri Jul 18, 2008 2:33 pm

iisthemuffin wrote: And i do not have the time to spend sifting through thousands of od posts.


Oops, that's the wrong answer there muffin. I, and many others take the time to do that very thing, why shouldn't you?
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Unread postAuthor: iisthemuffin » Fri Jul 18, 2008 4:02 pm

I never said i was exempt from that. And like i had said, i DID try searching and i looked at many things and still had many un answered questions. So i proceeded to make a thread.
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