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Aluminium keychain coax

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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Aluminium keychain coax

Unread postAuthor: turner » Thu Mar 17, 2011 4:59 pm

what do you guys think? Barrel is .18" brass and theoutside is 1/2" with .035 wall.
Image
I used apresta valve because they have no spring and are way more ergonomic. It has a cutting board piston made on a drill press and surgical tubing on the barrel to seal.
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I take it to 300 psi with my pb tanks but now it doesn't work. it hold untill 50 psi, but then blows out of the barrel. But some times I can use a bike pump, and jump on it and it holds at like 120. Does anyone have and experience with this?
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Unread postAuthor: JDP12 » Thu Mar 17, 2011 5:01 pm

Means your piston isn't sealing... Why did you post this 3 times?
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Unread postAuthor: turner » Thu Mar 17, 2011 5:07 pm

I'm sorry I'm on my phone on the middle of no where, my thumb nail didn't work either. Can any mods delete the other 2?
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Unread postAuthor: Gaderelguitarist » Thu Mar 17, 2011 7:14 pm

How was your surgical tubing connected to the barrel? Was it simply glued on or slid over the sealing face, etc.

Most people on here would have just made a solid rubber piston and lubricated the sides. Still, it looks like a quality build. Just get that piston figured out.
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Unread postAuthor: turner » Thu Mar 17, 2011 9:45 pm

I super glued it
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Unread postAuthor: Gaderelguitarist » Thu Mar 17, 2011 9:50 pm

You might want to check that it is actually still inside your gun. There are several instances of sealing faces coming off and shooting out of the barrel. This would explain why your piston isn't sealing properly and holding pressure.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 17, 2011 11:10 pm

Gaderelguitarist wrote:You might want to check that it is actually still inside your gun. There are several instances of sealing faces coming off and shooting out of the barrel.


This is certainly a possibility. Simply supergluing the tubing in place isn't really going to cut it, it needs to be supported, thisi s what I had done before I started using solid rubber pistons as suggested earlier):

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Shame because it looks good, epoxy FTW! I also like the use of the presta.
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Unread postAuthor: turner » Fri Mar 18, 2011 3:59 pm

I love how no body believes its an air gun, un till you shot it haha. I can still feel the tubing if I stick something down the barrel and it holds til 50 psi so idk.
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Unread postAuthor: saefroch » Fri Mar 18, 2011 6:46 pm

I had a similar problem with an earlier version of a piston valve that used a hot glue piston with a sealing face made from layers of bike inner tube. It would leak then pilot itself past about 150psi. Never figured out what the problem was, but it stopped when I used a more robust sealing face material.
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Unread postAuthor: turner » Fri Mar 18, 2011 6:59 pm

That would make sence, but it.doesn't hold between 50 to 120. Saturday I will be home so then I will put some oil or hacksaw it apart.
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Unread postAuthor: saefroch » Fri Mar 18, 2011 7:06 pm

It's probably leaking because of the elasticity of the surgical tubing. Is the leak coming out the barrel? Does it hold past 120psi?
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Unread postAuthor: turner » Fri Mar 18, 2011 8:16 pm

Yup when I hit 50 it bursts out of the bartel but it I get out to 120 fast I can pump it up to like 350
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Unread postAuthor: saefroch » Fri Mar 18, 2011 8:38 pm

That's really odd... I'd try replacing the sealing face with something more robust to see if that fixes it.
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 21, 2011 2:05 am

saefroch wrote:I'd try replacing the sealing face with something more robust to see if that fixes it.


... or use a sharpened tube to cut a solid rubber piston:

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Unread postAuthor: mark.f » Mon Mar 21, 2011 7:36 am

A solid rubber piston is definitely a good idea for something this small, and hardware (screws and washers) might be difficult to obtain and align in these small sizes.
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