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My plans plus a few questions

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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My plans plus a few questions

Unread postAuthor: Sean » Mon Jul 14, 2008 1:19 pm

Well I've been wanting to build a pneumatic for a long time and finally found a forum with good information. I do have a few questions about my design though. On the bottom with the schrader valve, should i put it on the end like I have or through 2 layers of pvc on the side? I have herd of people doing it both ways and would like to know which one would be better. Another question is about the chamber/stock. In my research I've been told that 1.5 C:B would be good for pneumatics. since I have the elbows, adding extra room, how would I add that into my calculations because if I don't I'm afraid I might lose performance. Also I'm not sure how to calculate the right length of pipe to use for each gap. Thanks for the help!
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Re: My plans plus a few questions

Unread postAuthor: jackssmirkingrevenge » Mon Jul 14, 2008 1:39 pm

Sean wrote:On the bottom with the schrader valve, should i put it on the end like I have or through 2 layers of pvc on the side?


The latter option is stronger but even one layer is reasonably safe.

In my research I've been told that 1.5 C:B would be good for pneumatics. since I have the elbows, adding extra room, how would I add that into my calculations because if I don't I'm afraid I might lose performance.


Don't be too fussy, there is no "ideal" ratio for pneumatics - the more volume you have, the more power you'll have, it's very simple. What you do have is a point where no matter how much more volume you add, it won't really improve performance. For example, in your case a litre of air will give much better performance than half a litre of air, but the difference between say 50 litres and 100 litres will be negligible.

Also remember that for a given quantity of air, half the volume at double the pressure will give better performance.
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Unread postAuthor: CS » Mon Jul 14, 2008 1:47 pm

SpudFiles Wiki - Chamber to barrel ratio

SpudFiles Wiki wrote:The C:B ratio plays a much smaller role in the design of pneumatic cannons since factors like chamber charge pressure and valve performance play a greater role in determining overall performance. It is advisable to use GGDT to determine the best design for pneumatic cannons.


So with that being said the only reason C:B ratios are mentioned for pneumatics is to convey a relative ratio. You might try putting your gun design into GGDT, but I wouldn't consider the results very accurate. Especially considering the design of your chamber.

Now looking at the gun I would say that your chamber volume is not large enough - however that is just from my experiences. As well you shouldn't have problems taping through a cap like that. The threads on one of those schraders is going to fairly small. Tapping it into a rounded portion of the gun (pipe wall, etc) will not allow for all the thread to engage.

One more thing. Your show a 3/4" M adapter and 1" M adapter coming from a sprinkler valve. Your sprinkler valve is going to either have a 3/4" or 1" connection.
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Unread postAuthor: Sean » Mon Jul 14, 2008 2:27 pm

Yes I messed up on the picture but forgot to go back and edit it. Still trying to figure all this stuff out, it's more time consuming than I thought :P I'm still a little unclear, instead of threading it through one layer, are you saying I should have the schrader valve go through the side of the end cap and the pipe wall? Sorry, a little confused
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Unread postAuthor: CS » Mon Jul 14, 2008 2:29 pm

No problem. I recommend tapping it in like you have it shown in the picture. I've tapped in schraders the exact same way and not had a problem.
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Unread postAuthor: Sean » Mon Jul 14, 2008 2:44 pm

Maybe I could have a 2" diameter chamber connected to the bottom T on the stock instead of the stock having to serve as the chamber. I have herd about people blocking off the T so air doesn't get through, how would one go about doing this, not sure what it is called. Searched it but nothing came up :(
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Unread postAuthor: Pilgrimman » Mon Jul 14, 2008 3:16 pm

I wouldn't bother with blocking the stock. It'd be best to extend the barrel to ~5' , and compensate for the change in volume by adding a small 2" tank where the schrader would be and putting the schrader in the end of the tank. This way, you can attatch a barrel support to the tank so that the barrel doesn't flex when firing. Flexing can wreck your barrel, and maybe even crack the threaded part on your valve. If anything here is confusing, just ask for clarification.

Kudos for doing some research! :D
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Unread postAuthor: Sean » Mon Jul 14, 2008 3:42 pm

Hmm 5 foot barrel, that's pretty long. would there be any way I would be able to keep the barrel a little shorter than that, add the 2 inch tank, and still get good performance?
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Unread postAuthor: psycix » Mon Jul 14, 2008 4:44 pm

Sean wrote:Yes I messed up on the picture but forgot to go back and edit it. Still trying to figure all this stuff out, it's more time consuming than I thought :P I'm still a little unclear, instead of threading it through one layer, are you saying I should have the schrader valve go through the side of the end cap and the pipe wall? Sorry, a little confused


Well, you could place the schrader trough just anywhere on the gun BUT, when drilling trough just 1 layer of pipe isnt recommended. The pipe will be roughly 4mm thick over there.
However, on the spot where a fitting is glued to the pipe, youve got two layers of pvc: Double thickness, making sure the schrader is installed better.
Less chance on leaking and its just better in place.
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Unread postAuthor: pocket » Mon Jul 14, 2008 5:37 pm

it looks good for a basic pneumatic one thing i would suggest is to take out those 2 elbows that attach the stock to the sprinkler and replace them with a straight pipe it will help dish out the recoil better
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Unread postAuthor: CS » Mon Jul 14, 2008 7:11 pm

If you don't mind you may consider changing your design. Here I have the plans to a little spud sniper I drew up a while back:
Image

And if you wanna get PVC elaborate maybe something like this:
Reventon 25MM Pneumatic Sniper Rifle
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[This image is clickable]

In whatever you choose I might recommend that you post the parts list just so you don't forget something. Even now that I think I've moved out of the "noob" phase I always forget something at Home Depot. Damn memory...
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Unread postAuthor: Sean » Mon Jul 14, 2008 8:46 pm

I like the first one, now I gotta figure out the correct dimensions. I want to have a 3/4" barrel specifically for paint balls. I was kinda thinking of changing mine to be like that and have a 2 inch chamber on the bottom. I'll make sure to have a parts list :P
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