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coaxial air gun design will it work?

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 air gun design will it work?

Unread postAuthor: hawktalk » Sun Aug 11, 2013 7:04 am

I have been thinking about making a airgun/combustion rifle but still cant decide what to do all my attempts making a pneumatic air gun have resulted in having no stock. I did make a small butane/propane powered rifle but after one or two shots it became boring. normally resulting in me taking off the barrel and using it as a blow gun. then i saw a coaxial air gun. i still cant get my head round it properly but this is what i have got so far. any advise, links would be really helpful. not looking for something advanced or flashy but something simple. easy to make.
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Unread postAuthor: turkeypig666 » Tue Oct 01, 2013 8:28 pm

So, where exactly is your ball valve?
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Unread postAuthor: Gippeto » Tue Oct 01, 2013 9:37 pm

@ turkey...why does he need a ball valve??

Hawk, on the right track with that.:)

Piston needs to be a VERY close fit. Can be made from a piece of wood dowel, or really...anything cylinder shaped and appropriately sized.

Another good option is cast epoxy.

http://www.spudfiles.com/forums/making- ... t8919.html


There are several very good examples of coax "rifles" in the pneumatic showcase section.

A quick search for "coax" later...
http://www.spudfiles.com/forums/simple- ... 25204.html
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Unread postAuthor: Technician1002 » Tue Oct 01, 2013 11:59 pm

There is a sticky on how to make small rubber face pistons on a drill press. You can choose from several suitable materials.
Image

Making a small piston that seals well is not too hard if you have the tools.
http://www.spudfiles.com/forums/tutorial-making-a-small-o-ringed-piston-on-a-drill-press-t21740.html
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Unread postAuthor: dart guy » Wed Oct 02, 2013 3:05 pm

i use golfballs and erasers
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Re: coaxial air gun design will it work?

Unread postAuthor: hawktalk » Fri Oct 25, 2013 1:18 pm

completely forgot after a couple of weaks or so waiting for help on this subject.
thanks everyone for the replies.
ive been looking at this guys online blog for a while, ive given up on it tho because i cant find any of the components. i liked how simple he made it, the only thing i think he modified i the 3/4- 1/2 reducer i wounder if its possible with metric parts.
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Re:

Unread postAuthor: hawktalk » Fri Oct 25, 2013 1:20 pm

dart guy wrote:i use golfballs and erasers

what?how?
this better not be a stupid comment
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Re: coaxial air gun design will it work?

Unread postAuthor: dart guy » Fri Oct 25, 2013 2:41 pm

I spin a golf ball on my lathe and glue a eraser on as a sealing face,and it works!(even i am shocked)
I have a wood lathe so i am limited with what i can use.
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Re: coaxial air gun design will it work?

Unread postAuthor: hawktalk » Fri Oct 25, 2013 3:13 pm

OK at least i can believe that. isnt there rubber inside golf ball ,and in theory it should help absorb the forces upon it lol and erasers are a bit physically weak to use as after a while surly they will crumble
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Re: coaxial air gun design will it work?

Unread postAuthor: dart guy » Fri Oct 25, 2013 3:38 pm

The erasers are pretty good as long as they are not overly flexed and the golf balls insides are a plastic /rubber mix it is very resistant.
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Re: coaxial air gun design will it work?

Unread postAuthor: Gippeto » Fri Oct 25, 2013 7:32 pm

dart guy wrote:I spin a golf ball on my lathe and glue a eraser on as a sealing face,and it works!(even i am shocked)
I have a wood lathe so i am limited with what i can use.



Can one assume that pictures will be forthcoming?

I admit to skepticism. :roll:
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Re: coaxial air gun design will it work?

Unread postAuthor: dart guy » Fri Oct 25, 2013 8:36 pm

go ahead be a skeptic but this is the only part on my mini qev, but it was the only part that sealed on the whole thing :( I'll have pics in the morning
edit/ here's the pics (i can hear it now "kid your gonna blow yourself up" :D ) Image
and here is a piston influenced by airdannon Image
its under his topic because he couldnt get the piston to seal so i thought about the idea and mad something like a prototype
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little update on my coaxial gun.

Unread postAuthor: hawktalk » Mon Nov 11, 2013 4:47 pm

I started this weekend just gone making the housing and push rod guides for the trigger and I also bodged up a 1/4" bsp - 15mm bit of copper pipe so when I get the compression fitting it will fit nice and air tight.
I've never used epoxy before so it was fun to learn :D

Apart from glue it how can I make it really air tight I've had bad experience with compression fittings as they leek where the olive isn't tight enough against the fitting and locking nut when the thread has been over tightened (not by me that I suppose is what u get when you take the fittings out of the scrap pile at my old college lol)..... Any tried and tested methods plz ?
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Re: coaxial air gun design will it work?

Unread postAuthor: Ragnarok » Mon Nov 11, 2013 8:01 pm

While some people will tell you that compression fittings need goopy stuff in them to seal them, it's not necessary.
Get them finger tight, then add half a turn with a wrench (less if a really large size, more if a really small one). Tighten it a bit more if that's not proving air/water tight.

As for the earlier talk about rubber, it's a bad mistake to assume that "elastic" also means "compressible". Rubber, while very elastic, has a Poisson's ratio of 0.50 - in layman's terms, you cannot squash it into a smaller volume.
(While this sounds counter intuitive, we're used to water doing much the same - fluid, but incompressible).

So solid rubber pistons aren't actually that great an idea.
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Does that thing kinda look like a big cat to you?
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Re: coaxial air gun design will it work?

Unread postAuthor: hawktalk » Tue Nov 12, 2013 2:16 am

I used to tighten them finger tight but didn't really help, tightening them further and too far they make this horrible grinding cutting noise but I could use some sort of sealant that will break when I dismantle it for service.

Any particular type of sealant?
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