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Pneumatic Pen Gun Help

Discuss miniature cannons built from things such as pill bottles and pen caps.
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Pneumatic Pen Gun Help

Unread postAuthor: sagthegreat » Thu Feb 16, 2012 5:25 pm

I am making a penumatic Pen Gun and need some help, No matter what I do my piston wont seal the barrel, I am open to all suggestions thanks. :P
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Unread postAuthor: jsefcik » Thu Feb 16, 2012 5:26 pm

is your piston lubed?
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 16, 2012 5:27 pm

Some photos of your setup or at the very least a detailed description of the materials you used and the general dimensions will help us help you :)
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Unread postAuthor: sagthegreat » Fri Feb 17, 2012 6:47 pm

thanks guys, i am using a hot glue stick with a bb glue to it as the piston, the barrel has a rubber ring glued onto it to make a seal, the piston is lubed but it wont seal, not even at like 40psi
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Unread postAuthor: killerbanjo » Fri Feb 17, 2012 7:22 pm

What is the rubber ring? an oring? How is it glued? I think that the problem is likely to be the ring. It doesnt hurt to take the shrader core out and pour some lube into where the piston is though.
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Unread postAuthor: sagthegreat » Fri Feb 17, 2012 7:25 pm

the rubber ring is a eraser with a hole cut into it(so ghetto), and the piston is lubed(how does the lube help it seal)
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Unread postAuthor: killerbanjo » Fri Feb 17, 2012 7:32 pm

It only really prevents micro leaks but i was thinking that your piston could be jamming and mushrooming under pressure because it is soft and therefore not sealing or moving. As for using an eraser, I would say that is probs your problem, its very soft and can tear easily. On ebay a 20cm x 20cm sheet of 3mm neoprene rubber can be had for about 3-4 quid all in, perfect for piston heads.

EDIT: The other thing is when i made mine my piston wouldnt move, this was due to a trace of epoxy gluing it to the chamber wall, i took the shrader core out and pushed on the piston with a bit of the same tubing i used for the barrel and that broke the bond...
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Unread postAuthor: sagthegreat » Fri Feb 17, 2012 8:06 pm

I know the piston is moving cuz I removed the valve to check. I'm starting a new gun. any recommendations for the barrel and piston
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Unread postAuthor: killerbanjo » Fri Feb 17, 2012 9:26 pm

If its a pen gun then i made mine from 10mm steel with 1mm walls. The barrel is 5mm with 1m walls so it fires 3mm projectiles. Because shrader valves are 8mm the 10mm OD tube works perfectly and makes it very easy to make. Because it is 1mm wall steel the gun can handle 1500psi+

For the piston hot glue will work but epoxy would be a bit bitter. Get some rubber sheet, it will last ages and you will not regret it. Use the rubber for the piston head.

Hope that helps, Ed.
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Unread postAuthor: inonickname » Sat Feb 18, 2012 3:00 am

Junky seal. Go for some rubber sheet, it needs to be harder than you'd expect. Get some fairly hard sheet say, 6mm or so, and sharpen a piece of your chamber material and use it as a punch to make the piston.
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Unread postAuthor: sagthegreat » Sat Feb 18, 2012 1:57 pm

so do i need to attach anything to the barrel as a seal, or should a rubber piston on the barrel itself be fine?

-thanks :roll:
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Unread postAuthor: killerbanjo » Sat Feb 18, 2012 5:06 pm

Rubber piston/piston head will be fine :)
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Unread postAuthor: sagthegreat » Sat Feb 18, 2012 5:09 pm

Sorry for being a noob but will epoxying the neoprene head onto the piston work? Or do I nned to screw it on
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Unread postAuthor: killerbanjo » Sat Feb 18, 2012 5:31 pm

I personally would screw it in as you could get it more precise. If you epoxy it on and get it wrong which would be easy on a small piston then its a bit of a bugger :lol:
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Unread postAuthor: Gun Freak » Sat Feb 18, 2012 8:03 pm

You can't epoxy rubber, you have to super glue it. And for such a small piston you can't really find a screw small enough because it has to be smaller than the barrel ID, so I'd just super glue it. I've done that on both of mine and it works.
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