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help needed with coaxial

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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help needed with coaxial

Unread postAuthor: Cosmic Muffin » Thu Mar 06, 2008 5:29 am

hi guys i made a coaxial from 15mm copper pipe with a 10mm copper pipe barrel i have a piston that fits snugly inside the 15mm pipe. the gun is actuated by a blowgun thats inline with the chamber so i tried to keep the pilot volume low (i think its the pilot volume). anyway i fill it from behind the piston like you should but when i try to fire it air just comes out the blowgun and i cant figure out why that is, can u help?
by the way I'm not sure if this post is in the right place so forgive me if im right. :roll:
Cheers Ant
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Re: help needed with coaxial

Unread postAuthor: jackssmirkingrevenge » Thu Mar 06, 2008 5:35 am

Cosmic Muffin wrote:...I have a piston that fits snuggly


How snuggly is snuggly? What's your piston made out of?

Pictures would be helpful ;)
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Unread postAuthor: BigGrib » Thu Mar 06, 2008 5:36 am

umm not in the right place should have been in the pnuematic cannon discussion but anyway, either equalization hole is too big or there in not one at all or something else
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: Cosmic Muffin » Thu Mar 06, 2008 6:23 am

well when i posted my other gun i couldnt get the pics onto the post so i had to get PCGUY to put them up after i sent them to him over MSN. anyway the distance between the piston and the wall of the pipe is about 0.000001 mm, put it this way it cant move (apart from sliding). the piston is made from a peice of an olde bike pump with epoxy to fill some gaps in the middle and the sealing face is rubber but i tried so many different types of rubber like inner tube, some red stuff and so on. there isnt much space between the barrel and chamber wall so i wasnt sure where to put the equilisation hole. i put groves down the side of the piston so air could go through and fill the chamber. ill try to get PCGUY to put up some of my pics but the best i can do is get them on tomorrow.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 06, 2008 6:54 am

There isnt much space between the barrel and chamber wall


That's a bit of a problem, ideally there's a significant difference between barrel outer diameter and chamber inner diameter. If it's a coaxial, it means your chamber volume is far too tiny for an effective shot.

You don't need equalisation holes unless you're using o-rings to seal your piston either.

Fix these two issues and you should be ok, no equalisation grooves and (for a 10mm barrel) a chamber that's at least 20mm internal diameter.
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Re: help needed with coaxial

Unread postAuthor: Antonio » Thu Mar 06, 2008 7:29 am

Cosmic Muffin wrote:hi guys i made a coaxial from 15mm copper pipe with a 10mm copper pipe barrel i have a piston that fits snugly inside the 15mm pipe. the gun is actuated by a blowgun thats inline with the chamber so i tried to keep the pilot volume low (i think its the pilot volume). anyway i fill it from behind the piston like you should but when i try to fire it air just comes out the blowgun and i cant figure out why that is, can u help?
by the way I'm not sure if this post is in the right place so forgive me if im right. :roll:
Cheers Ant


Sounds great what u are working on here. It would be nice to c some diagrams, animations or pics of this concept as its kinda hard to understand. I would really like to give advice or help as this setup is allot like my airguns.
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Re: help needed with coaxial

Unread postAuthor: jackssmirkingrevenge » Thu Mar 06, 2008 7:36 am

ant wrote:I would really like to give advice or help as this setup is a lot like my airguns.


I don't think so ant, your projects tend to be hammer valves while this is an exhaust valved piston launcher from what I gather.
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Re: help needed with coaxial

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Re: help needed with coaxial

Unread postAuthor: Antonio » Thu Mar 06, 2008 7:38 am

jackssmirkingrevenge wrote:
ant wrote:I would really like to give advice or help as this setup is a lot like my airguns.


I don't think so ant, your projects tend to be hammer valves while this is an exhaust valved piston launcher from what I gather.


Well atleast the materials being used are familiar, so I hope I can go to piston type of airguns once I c his work. I tried working with pistons but with no succes. I hope I can relate my stuff to this new prototype''.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 06, 2008 7:51 am

I tried working with pistons but with no success


I find that hard to understand, you've managed to make some extremely sophisticated and functional launchers using only basic tools and materials, surely a simple piston valve should be no real challenge ;)
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Unread postAuthor: Cosmic Muffin » Fri Mar 07, 2008 1:46 am

by the way the barrel/chamber is about 60cm but im not sure of the exact chamber volume[/quote]
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 07, 2008 1:54 am

Cosmic Muffin wrote:by the way the barrel/chamber is about 60cm but im not sure of the exact chamber volume


The difference between barrel outer diameter and chamber diameter is far too small, and this will remain a problem no matter how long the chamber. Either get a wider chamber or smaller barrel, otherwise you won't have much chance of success.
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Unread postAuthor: Cosmic Muffin » Fri Mar 07, 2008 2:24 am

in that case i will get a smaller barrel
any idea of what i could use and what could i use to stick it to the copper ?
cos i was thinking of just cutting out the barrel and then puting the new one thru the existing peice left in the reducer
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 07, 2008 2:41 am

Cosmic Muffin wrote:any idea of what I could use and what could I use to stick it to the copper?


Well... ;)

As to a barrel, check out your local model/hobby shop, they should stock brass tubing in convenient sizes like 6mm.
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Unread postAuthor: Antonio » Fri Mar 07, 2008 5:01 am

jackssmirkingrevenge wrote:
I tried working with pistons but with no success


I find that hard to understand, you've managed to make some extremely sophisticated and functional launchers using only basic tools and materials, surely a simple piston valve should be no real challenge ;)


Thats a good one lol. Atm I am working on a half-half piston-hammervalve system. Ill post some stuff on that. Not sure if someone already thought it out.
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Unread postAuthor: Cosmic Muffin » Fri Mar 07, 2008 9:04 pm

ok well im an Aussie so would thay still sell the pipe over here too ?
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