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Barrel in Chamber, Gun #4

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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Barrel in Chamber, Gun #4

Unread postAuthor: JET47 » Tue Mar 25, 2008 8:17 pm

Ok, i got all the parts together to build the full scale gun that i was working on before, sorta gave up on the little one lol, might finish it later.

Just wanted to know if anyone sees anything obviously wrong before i glue the last few parts together. I've got a male and female threaded connection with teflon tape where the piston is, so i can open it up to fix it if i have to, 4"outter chamber, 2" inner barrel, using uncut pipe end for the sealing face of the barrel, nice thick rubber pipe cap on the piston.... looks pretty solid to me, very heavy though.

Will be in the cannons section shortly if no one sees anything wrong. Questions in the picture comments

Thanks
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IMG_0510 Medium Web view.jpg
piston, sealing face is a rubber pipe cap, with a diaphramish face.
IMG_0511 Medium Web view.jpg
piston from back, atm its hollow, should i make the back face solid?
IMG_0512 Medium Web view.jpg
back end. schrader valve is screwed directly into the pvc (used a threading tool) with teflon tape
IMG_0513 Medium Web view.jpg
chamber and barrel. only worry is that the air rushing past that support peice might break it, not sure how though lol
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Unread postAuthor: pizlo » Tue Mar 25, 2008 8:27 pm

Looking good, exept the pilot volume reminds me of my failed Coax. maybe it should be less.
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Unread postAuthor: JET47 » Tue Mar 25, 2008 8:30 pm

ya? the piston goes all the way back into the 4-2" reducing coupling. i dunno how i could make it smaller.
maybe coat the insides with hot glue hahaha
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Unread postAuthor: JET47 » Tue Mar 25, 2008 8:33 pm

and wow... is a barrel inside the chamber what defines a coaxial? ive been thinking they just have two brached fill chambers, thats what it looked like to me in clide's diagram at least. like the one on the front page that won the contest
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Unread postAuthor: pizlo » Tue Mar 25, 2008 8:34 pm

take out the y and endcap, put the scrader into the pipe.
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Unread postAuthor: elitesniper » Tue Mar 25, 2008 8:37 pm

Holy CRAP!, That is massive pilot volume, you can just get a male adapter and a reducing bushing. And yes block ff the back of your piston.
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Unread postAuthor: JET47 » Tue Mar 25, 2008 8:37 pm

haha, pretty obvious lol =P
i designed it like that originally cause i was using these really long tire replacement schrader valve from the local auto zone, and i couldn't slide em up into a hole in a pipe cause they were too long. spent the weekend stateside though and found some nice threaded ones at the lowes there lol. gunna have to buy more parts :( i think ill put the change on hold though till i know if it will work or not
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Unread postAuthor: JET47 » Tue Mar 25, 2008 8:40 pm

and i was thinking since the rubber is sorta diaphramy, keeping the inside open would help seal. but not worth it?
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Unread postAuthor: elitesniper » Tue Mar 25, 2008 8:42 pm

nope, it would increase more pilot volume and you should have a washer below your sealing face, make sure the pipe inside is nice square and smooth so it can seal.
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Unread postAuthor: pizlo » Tue Mar 25, 2008 8:44 pm

when you post, and noone else has after you use the edit button on you post to add content, don't make another post.
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Unread postAuthor: Carlman » Tue Mar 25, 2008 8:45 pm

you have a s**t loada DWV on the rear end of that, the Y, the cleanout cap, reducer and god knows the pipe may be too!

you need to fill the piston to reduce pilot volume, but thats not going to make a dent in the all ready massive one allready
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Unread postAuthor: JET47 » Tue Mar 25, 2008 8:50 pm

im not rlly sure about how to recognize DWV vs the pressure rated stuff, but i know at my home depot there are thick strong looking fittings, and weak thin ones, i made sure to pick the strong stuff for parts holding pressure, but i used sum of the weak [sewage?] pipe for like, the piston and barrel support.

thanks for all this advice guys i dont really have the experience lol
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Unread postAuthor: elitesniper » Tue Mar 25, 2008 8:54 pm

dwv, pipe has shallow socket fittings, just look for fittings that say sch 40 nsf-pw.
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Unread postAuthor: starman » Tue Mar 25, 2008 9:06 pm

JET47 wrote:and wow... is a barrel inside the chamber what defines a coaxial?
Yep, you have a coax alright. You're only going to need 1/2" of piston "throw" to get the maximum airflow into the barrel so adjust the piston length to where it falls in that range. Also, make sure the piston fits the 4" cylinder extremely close but without a full seal and without binding...this is crucial, otherwise air will just slip out around the piston and out the pilot before it does the work you really want it to, which is unseal the barrel. You should also try to seal the pilot hole with the back side of the piston upon blowback. Your ball valve pilot will be cumbersome and slow so you'll lose some performance there. A modded 1" sprinkler valve would improve performance a great deal.

I'm not a pneumatic expert but just completed one very similar to this...here http://www.spudfiles.com/forums/bofors- ... 13488.html
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Unread postAuthor: fireman565 » Tue Mar 25, 2008 9:28 pm

Try a search on "dwv AND fittings". With that search alone, you should be able to learn about dwv fittings, when you can use them and when definitely not to use them. Your statement "thanks for all this advice guys i dont really have the experience lol" means that while you're at searching, try reading for a few nights on materials to use for pneumatic guns, the theory behind how they actually work, and how pistons work. You're headed for trouble if you don't research this before pressurizing something you have put together that won't hold up. :)

Now, with that said, building a pneumatic for the first time, is good. You have just learned by earlier posts some serious mistakes you have made. So now, head back to the drawing board, and try again. :D

Back to DWV for a second... If you will look for any raised-lettering on the piece, it will tell you if its dwv or pw. Look on the edge of the piece, it will be in raised lettering and very small lettering. See the arrows in the pic you posted below.
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EDIT..... Just to give you an idea about piston valves... My first piston valve gun I built, it took two days to put together, and two weeks to get right. My second piston gun, it took me two weeks of planning and cutting and sanding, just to get everything to work out before I glued anything, another week to glue everything up, just being careful. On the first attempt, it worked perfectly. So, you can spend a day gluing everything together, and a week figuring out what's wrong, or a good week of planning and a week of slowly fitting everything together, and one minute to find out it works on the first attempt... :D
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