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First coaxial gun help

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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First coaxial gun help

Unread postAuthor: sputnick » Wed Jan 28, 2009 2:24 pm

Well, Everyone here has heard it a million times before,

My piston gun will not fire, and I need peoples help.

This is my first coaxial gun, and I am very excited to get it working. It is simple enough, the chamber is 3/4 PVC (really 1 inch, I never understood that... and the barrel is 1/2 inch copper.

Now, I realize I should make a more accessible piston, that is a modification for another day. The piston is a section of 1/2 pvc, (again, really 3/4 inches, what the hell?) wrapped in electrical tape for a decent seal, and then filled with hot glue with a section of inner tube as the sealing face.

Whenever I fill it to around 100 psi, it seals up nicely, no hissing or leaks to speak of, then when I fire, Nothing. All the air comes through the blowgun.


Then I tried firing it vertically, to maybe give it some gravity, still, nothing. all the air drains through the blowgun, then after it is neural, the piston drops.

I have tried a few dry and wet lubricants (not at the same time obviously) and different pistons, I am totally stuck here guys, any help would be great.

Here is the layout of the barrel, housing and piston, as well as the firing/filing rig,
Image

Here is the layout of the stock I will be making, I have only hand tools so cutting this out will be the largest ordeal :roll: Image
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Unread postAuthor: john bunsenburner » Wed Jan 28, 2009 2:38 pm

Are you sure it has a tight fit, it sounds like the piston is just sitting in the chamber and the chamber drains through the pilot valve while you fire, o rmaybe it has a very bad seal and the air drains out through the barrel. Do you have an equalization hole or do you fill the pilot chamber and the chamber seperately.

Oh and the stock is not too hard, you need quite some time but i managed and i am by far not the best crafts man. what i dont like about woad it is unforgiving, mess up once and the stock is useless. I practised a few times on cheep scrap pieces of woad got some ok stocks but gave up on the idea becaus ei am not patient eanoth. Now i make stocks from fiber glass, resin and nylon stockings(i might make a how to some day), in my eyes it is far more simple, justyou wont get the cool woad look, good luck though.
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Unread postAuthor: sputnick » Wed Jan 28, 2009 2:41 pm

No it is a tight fit, and it is not leaking through the barrel since there is still air to blow out the blowgun, i have put my finger over the hole (no ammo of course) there is no air leaving the barrel.
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Unread postAuthor: psycix » Wed Jan 28, 2009 2:46 pm

The gap between the piston and the wall is large enough to compensate for the blowgun. A common problem.
You can:
1- Slap on an overkill pilot valve. (Big ball valve) [inefficient solution but could work]
2- Make a better piston that fits better. [some work, but best results]
3- Add a few layers of duct tape around the piston to make it fit better [cheap n quick fix, wont last for ages though]
4- Add o-rings [very hard to do. Makes an uber piston.]

Good luck!
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Unread postAuthor: jeepkahn » Wed Jan 28, 2009 3:54 pm

What is the true inner diameter of the chamber, and what is the true outer diameter of the barrel, also the true inner diameter of the barrel??? It's quite possible that with not enough differance in diameters, that there isn't enough pressure on the piston to move it... I calculate below to illustrate using made up numbers...

outer diameter of piston .75"=surface area .44"sq
outer diameter of barrel .55= surface area reduction from above .2374"sq
.44-.2374=.2026"sq of surface area where the pressure is trying to move the piston backwards.

if the inner diameter of the barrel is .51" then you have .2041"sq of surface area holding the piston forward...Which mean the piston ain't gonna move...

But, if you give me the dimensions requested, I can do the calculations exactly....

edit: I know most on here know this but for any noobs I'll post it. This is for any solid/non-toolie piston valve.

the force sealing the barrel is pressure times the surface area equal to what is exposed to atmosphere via the barrel.

the force (with pilot opened) that is trying to move the piston backwards/open is the pressure times the total surface area minus the surface area hidden from the chamber pressure by the barrel outer diameter.

if the first number is higher than the second, the piston will not move...

I think people forget this sometimes...
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Unread postAuthor: sputnick » Wed Jan 28, 2009 5:27 pm

I do not know the perfectly exact measurments, the chamber is about 2mm's short of an inch, and the outer diameter of the barrel is 1/2 of an inch.

To everyone, I will cease trying untill I get the new backend, I am sure that will make it easier for testing and everything, also, I will get a fitting where the threads are larger than the pipe ID, and I think that the reason is that the threads at the front of the gun are smaller overall than the pipe ID, so a super tight piston will not fit past, so I comprimised slighly. I will have updates likely tommorow.
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Unread postAuthor: jeepkahn » Wed Jan 28, 2009 5:52 pm

sputnick wrote:I do not know the perfectly exact measurments, the chamber is about 2mm's short of an inch, and the outer diameter of the barrel is 1/2 of an inch.

To everyone, I will cease trying untill I get the new backend, I am sure that will make it easier for testing and everything, also, I will get a fitting where the threads are larger than the pipe ID, and I think that the reason is that the threads at the front of the gun are smaller overall than the pipe ID, so a super tight piston will not fit past, so I comprimised slighly. I will have updates likely tommorow.


chamber .90" inner diameter=.636"sq
barrel .5" outer diameter= .19625"sq
differance = .43975"sq

You should be good, Look at your piston fit like everyone said...
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Unread postAuthor: maggotman » Wed Jan 28, 2009 6:26 pm

i founf with my coaxel i needed to use hiher pressures it would not fire under 70 psi
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Unread postAuthor: tghhs » Wed Jan 28, 2009 8:48 pm

yeah i made my first coax exzactly like ur's using same material and sizes. i had the same problem but i just wrapped it with some duct tape and it worked perfectly.

My problem was that i used casting epoxy for the piston which works great and quick, with no need to lube ur mold, but it's designed to pull away form it so the mold can be removed.

there was only like 1/2 mm gap with out the tape, yet i had the same problem.

with the tape on, it was sick!!!. but only for ~ 10 shots. After which i opened it up to find the tape stuck to the chamber wall.

try using a hole saw for piston, if u've already got tape on it wont last long.
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