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coaxial piston gun

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 gun

Unread postAuthor: dilweed » Thu Aug 14, 2008 9:53 am

RAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARGHHHHHHHHHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!11
I hate it I HATE IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! i've spent all my money (£70-a lot for a 13 year old!) and ir leaks and doesnt work. I've spent about 8/10 ths of my stupid summer holidays on this!!!!. I have decided to just scrap the te piston and go on to a coaxial piston design. I have asked many times what measurements i need and how do I do it, but i dont get musch info, yes i have seen the design and understand the theory but i need proportions and measure ments!!! not blaming anyone just this is realy driving me nuts. really. really. really................................nuts.......? I will give you measuremens now:

CHAMBER:
-copper
-wall thickness=0.1 cm
-chamber width= 1.5 cm
-chamber length(can change)= 16 cm
-operating pressure= 50-200 psi

BARREL:
-aluminium
-wall thickness= 0.85 cm
-inner barrel width= 0.5 cm
-length= 78 cm

Projectile:
-steel
-width= 0.4 cm (ball)
-width= 0.4 cm (minie ball)
-length= 1cm (minie ball)
-kinetic energy= 4 joules

Piston:
-woden dowel
-width= 1 cm
-length= 2.5 cm
-wieght= 0. 01 kg
-sealing face (s f)= 3 layers of bike tire threaded into dowel
-sf width= 1.3 cm
-small spring to push piston
-distance from fill/ valve to piston when up against barrel= 2 cm

PLEASE HELP MEEEEEEEE, thank you.

p.s. once i am finished ill show you the design. im calling it a potato mauser 98. It looks like a real gun apart from the trigger (of course)
who likes beatoven? listen to him when you work it gets your creative juices flowing ( or sends an electric current somewhere to your left hemisphere.) Cheers!!!
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Unread postAuthor: starman » Thu Aug 14, 2008 10:14 am

Some up-to-date pictures might be helpful for us to help you.

Looks like Beethoven has made you a nervous wreck my man. Your post looks like something from a Pee Wee Herman episode... :wink:
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Unread postAuthor: trollhameran » Thu Aug 14, 2008 10:51 am

Also tell us about what isn't working on it, is it not sealing? is it not actuating? and like starman said, post some pics of what you have made so far.
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Unread postAuthor: Lentamentalisk » Thu Aug 14, 2008 11:31 am

Well for one, you piston diameter is way to small. You said that the ID of your chamber is 1.3cm (I did the math) and your piston is 1cm in diameter. that means 1.5mm on either side for the air to rush past. You want to make sure that the piston is a tight fit. It should slide easily, but not have wiggle room. Also, 3 layers of bike tire is a little much for something that small.
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Re: coaxial piston gun

Unread postAuthor: Gippeto » Thu Aug 14, 2008 1:48 pm

[quote="dilweed"]

CHAMBER:
-copper
-wall thickness=0.1 cm
-chamber width= 1.5 cm
-chamber length(can change)= 16 cm
-operating pressure= 50-200 psi

BARREL:
-aluminium
-wall thickness= 0.85 cm
-inner barrel width= 0.5 cm
-length= 78 cm


Piston:
-woden dowel
-width= 1 cm
-length= 2.5 cm
-wieght= 0. 01 kg
-sealing face (s f)= 3 layers of bike tire threaded into dowel
-sf width= 1.3 cm
-small spring to push piston
-distance from fill/ valve to piston when up against barrel= 2 cm

quote]

Take your measurements again, or at least correct them.

You have your barrel as being 22mm od. and your chamber as being 15mm id.

We can't help you if you don't provide accurate information. :?
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Unread postAuthor: dilweed » Thu Aug 14, 2008 2:03 pm

i use cm as a standerdized unit of measure not mm, i think you misunder stood, the inner barrel width is the part that lets the bullet through, the wall thickness is just how thick it is. the barrel is about 4 1/2 times longer than the chamber.
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Unread postAuthor: Lentamentalisk » Thu Aug 14, 2008 2:08 pm

give us your info as such

Outside diameter of chamber:
Inside diameter of chamber
chamber length
Outside diameter of barrel
inside diameter of barrel
barrel length
Diameter of piston
Length of piston


measuring the ID and OD is much more accurate than trying to measure the wall thickness because you have 2x the amount, so you don't have to worry about guessing between the lines so much.
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Unread postAuthor: psycix » Thu Aug 14, 2008 4:07 pm

You spent 8/10th of your of your summer holidays building this?
I guess you should've spent 7/10th of your summer holidays on research and 1/10th on building it, not reverse.
Research for months, measure twice, cut (or build) once.

I hope for you that your piston is serviceable, because you may need to open it up again. Your piston seems way not tight enough.
If your piston is so tight that you think it cannot be tighter, then you are almost there, but not completely. ;)
Also, are you filling from the chamber or from the pilot? You should fill from the pilot.

What are you using as pilot valve??

Pics would certainly help.
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Unread postAuthor: dilweed » Wed Aug 20, 2008 3:06 am

im using a 1/4 ball valve, an endcap threaded on to it, and a bike valve pushed through the hole. I did do lots of research, for about 3-4 months but my piston doesn't work!
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Unread postAuthor: CpTn_lAw » Wed Aug 20, 2008 5:12 am

There ain't much research to do on such a project, it is still relatively simple to understand. Something that might be helpful; put some heavy duty grease on the sealing face of the piston. Make sure your piston moves in the chamber with very little pressure. A well calibrated piston will move with very little pressure, and make the chamber air-tight. Your pilot has to be fast enough, but a ball valve will do about a fine as you could do.
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Unread postAuthor: ALIHISGREAT » Wed Aug 20, 2008 5:23 am

CHAMBER:
-copper
-wall thickness=0.1 cm
-chamber width= 1.5 cm
-chamber length(can change)= 16 cm
-operating pressure= 50-200 psi

BARREL:
-aluminium
-wall thickness= 0.85 cm
-inner barrel width= 0.5 cm
-length= 78 cm

Projectile:
-steel
-width= 0.4 cm (ball)
-width= 0.4 cm (minie ball)
-length= 1cm (minie ball)
-kinetic energy= 4 joules

Piston:
-woden dowel
-width= 1 cm
-length= 2.5 cm
-wieght= 0. 01 kg
-sealing face (s f)= 3 layers of bike tire threaded into dowel
-sf width= 1.3 cm
-small spring to push piston
-distance from fill/ valve to piston when up against barrel= 2 cm


i think when he says "wall thickness" he means outside diameter?

so his barrel is 8.5mm meanign that there is only 3.25 mm either side of the barrel in the chamber...

so this isn't much area for the air to push the piston back..

How tight is the piston?

if it is tight, lose the spring and try again, if not put some tape around it, loose the spring and try again.
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Unread postAuthor: psycix » Wed Aug 20, 2008 8:29 am

Dilweed, please refine the data and include some pics if you want us to help you.
We know that you tried hard, and we know that it doesnt work. But for us to see the reason, we need more information.
One or two pictures of your piston and gun, and we could say "this is the problem, and these are your options to to fix it."
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