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PenGun Problem

Discuss miniature cannons built from things such as pill bottles and pen caps.
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PenGun Problem

Unread postAuthor: iemand » Sat Jan 31, 2009 8:54 am

I've recently made a pengun, using jsr's work as inspiration.
But, there's a problem: it has absolutely no power at all, even at 145 psi

Yes, I know the chamber is probably too small, but it's not even capable of pushing a 1 gram bolt out of the barrel :cry:

I think it has something to do with the schrader valve, it might release some, if not all air when I disconnect it(the piston is sealing because I can shoot small pieces of tissue)

Any help would be appreciated
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Unread postAuthor: inonickname » Sat Jan 31, 2009 9:18 am

Without seeing the internals, it's difficult. Is it possible that you didn't have a buffer on the end of the barrel, and it cut a piece of rubber out from the seal, which gets jammed in there and limit flow? unsure..
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Unread postAuthor: iemand » Sat Jan 31, 2009 9:30 am

Nope, I've got piece of rubber to seal the barrel, and the piston isn't jammed because I can hear it moving when I turn the pengun upside down. It's also unlikely that the rubber sealing piece is cut by the barrel, because of the way I put the barrel in there.

Like this: ((o))
o is the barrel
outer () is the chamber tube
inner() is a piece of tube which fits into the chamber and around the barrel(some kind of adaptor piece you could say)
It's all lined and grinded down so its smooth, no sharp edges.
This prevents imo the rubber from getting cut.

I however have a bit large space built in into this thing(between the piston and the filling valve), which is about 0.2 inch.


But I still think the problem is that there is to much air released when I disconnect the pump from the valve, because those schrader valves have a pin sticking out which allows filling and releasing. That pin is also pressed when I attach it to my pump, and then when I disconnect it, quite some air leaks out before the valve seals again(I guess)
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 31, 2009 11:25 am

iemand wrote:But I still think the problem is that there is to much air released when I disconnect the pump from the valve, because those schrader valves have a pin sticking out which allows filling and releasing. That pin is also pressed when I attach it to my pump, and then when I disconnect it, quite some air leaks out before the valve seals again(I guess)


That's definitely a problem, have a look through the original pen gun thread for solutions. Also, you definitely need 300 psi plus pressures for decent power in such a small calibre, so think of investing in a shock pump assuming the construction can take it.
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Unread postAuthor: Sticky_Tape » Sat Jan 31, 2009 11:34 am

Maybe the shrader is cut too short and the sealing thing on the inside of the shrader valve opposite of the threads is pushing on the back of the piston seriously limiting flow? There are a whole lot of problems that could be occuring maybe you should just make a new one. It won't take too long.
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Unread postAuthor: jook13 » Sat Jan 31, 2009 12:58 pm

I bet the piston is too loose. If it isnt tight enough, too much air escapes around the piston.

If air is escaping as you disconnect but it doesnt fire, that tells me the piston isnt too effecient.
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Unread postAuthor: iemand » Sat Jan 31, 2009 4:41 pm

jook13 wrote:I bet the piston is too loose. If it isnt tight enough, too much air escapes around the piston.

If air is escaping as you disconnect but it doesnt fire, that tells me the piston isnt too effecient.


Hmm, that could be the problem, but the gap around the piston is less then 0,5mm :wink:
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Unread postAuthor: clide » Sat Jan 31, 2009 7:52 pm

Any noticeable gap is probably too much. You want your piston to fit as close as possible while still being able to move. If you have something like a 7 mm chamber and a 6.5 mm piston then that gap is equivalent to a 2.6 mm hole. That is going to cause problems if you are venting with a schrader valve.
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Unread postAuthor: psycix » Mon Feb 02, 2009 11:07 am

Maybe you could epoxy a syringe tube in there, and use the syringe piston as actual piston. This grants you a very well fitting piston.
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Unread postAuthor: iemand » Tue Feb 03, 2009 10:09 am

psycix wrote:Maybe you could epoxy a syringe tube in there, and use the syringe piston as actual piston. This grants you a very well fitting piston.


I'll give that a try, but not on this one :P Maybe for my next project... Anyway, thanks for the help :D
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Unread postAuthor: psycix » Tue Feb 03, 2009 10:52 am

So this pengun is ditched? You could still fix it.

Hmm, that could be the problem, but the gap around the piston is less then 0,5mm

For a piston of only a few mm diameter, thats ONE HELL OF A GAP
Maybe add a layer of duct tape or even better, an O-ring.
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Unread postAuthor: iemand » Tue Feb 03, 2009 12:24 pm

Well, for the fixing part, I used metal epoxy, so that's not gonna work unless I cut it open which would destroy it totally :lol: It's now somewhere hidden with the other "epic fail" projects.

btw isn't the syringe-tube-piston a bit too tight fitting?
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Unread postAuthor: psycix » Tue Feb 03, 2009 3:48 pm

The fitting is a perfect seal, though I do not now how well it seals on higher pressures.
You could poke a hole with a needle or something like that to let air trough.
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