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Building my first cannon...some 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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Building my first cannon...some questions

Unread postAuthor: aj23 » Wed May 09, 2012 9:57 pm

So let me start by what I want to do:
My plan is to build a cannon with a 2 inch barrel, and a 4inch chamber. Initially, I was thinking of just building something simple and attaching them with a sprinkler valve as this is for a school project, but I'm aiming to create something superior to what everyone else is doing and well, everyone is just attaching two PVC pipes with a valve lol.

I believe using a piston valve can make the cannon perform better, correct? The only issue I see is if something goes wrong, I can't fix it easily or anything since I'll have to do a live test and hit a target with the cannon.

In regards to the piston, with a 4" chamber and a 2" barrel, what would the best way to build it be? Would it have to be 4" as well and snug with the chamber? Any suggestions on dimensions / materials? I'm unsure of how I should be making the piston...actual pics / videos would definitely help a lot too.

Also, I already have a sprinkler valve I was planning on using, I could still use that as the exhaust if I make a piston right? Or would it not make a difference at all?

Edit: Plan on making the cannon straight where the barrel goes into the chamber btw, if that makes a difference to suggestions.

Any help is appreciated.

Thanks
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Unread postAuthor: warhead052 » Wed May 09, 2012 11:23 pm

My question is, what type of class is this?!

You can make a 4" piston, I suggest taking a look at the piston valves explained visually thread, I believe its a sticky. You can use the sprinkler valve to exhaust the valve, of course, but a faster method would be using a quick exhaust valve. That should help a bit...
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Unread postAuthor: jackssmirkingrevenge » Thu May 10, 2012 12:31 am

From another thread:

If you want a high performance big bore without breaking the bank, consider something like this design:

Image

You can make the chamber and barrel out of the same type of tubing, all you need is the chamber capped on one end with a valve of some sort in order to supply the air, and on the other end you attach a union.

All you have to do is sandwich a disk of frangible material (foil etc.) and when a certain pressure is reached in the chamber, BOOM!, Robert's your mother's brother.

It is a bit of a pain in the arse having to install new burst disks for every shot but really this is the only way to get the best performance out of a pneumatic on a very low budget.

At 100-150 psi with a long enough chamber and barrel, performance should be quite impressive.
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Unread postAuthor: aj23 » Thu May 10, 2012 7:05 am

It's for a grade 11 physics ISU. And I don't want to restrict the budget too much, having to modify the cannon (even slightly) for every single shot would probably make it considered a lower end compared to the rest. If anything, I would just use a valve in between chamber and barrel but would prefer a piston.

I did read the piston valves explained visually thread and understand it mostly, but I'm unsure what type of materials the piston itself should be made of, and how should it fit. Is there nothing attaching it to the chamber, its just placed in there? And should it be a fairly tight fit where it wont slide unless pushed with force?
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Unread postAuthor: jackssmirkingrevenge » Thu May 10, 2012 7:15 am

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Unread postAuthor: mattyzip77 » Thu May 10, 2012 8:54 am

4 inch seems big, I would go with a 3 inch chamber and make a few different size barrels to shoot different projectiles. Below I have posted a youtube link for you to look at. This is one of the best bigger piston valves I have seen. Good luck!!


http://www.youtube.com/watch?v=75O71AHTIr4
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Unread postAuthor: aj23 » Thu May 10, 2012 2:52 pm

I'd like to make it 4" since I'll be putting the 2" barrel into the chamber already.

Basically, I'm aiming to make something like this - http://www.xinventions.com/main/spud/pac5.htm

Should I make the piston out of wood as in that video above, or is there a better option? And yea, how does the fit work in terms of holding the piston in place / allowing for movement?
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Unread postAuthor: mattyzip77 » Thu May 10, 2012 3:22 pm

You can make the piston out of anything, keep in mind that you want to keep it as light as you can. Do you plan to house it in a tee? I dont understand when you said you were putting the barrel into the chamber? Were you thinking of a coax design???
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Unread postAuthor: dewey-1 » Thu May 10, 2012 4:15 pm

Come on now matty! :D
Click on his link and see the coaxial launcher!
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Unread postAuthor: mattyzip77 » Thu May 10, 2012 6:51 pm

awwww shizznitt, I missed the link. I would go with the 3 inch coupling for the piston, unless you have acess to a lathe and if you do there are a million options!!
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Unread postAuthor: Crna Legija » Fri May 11, 2012 1:11 am

aj23 wrote:Should I make the piston out of wood as in that video above, or is there a better option? And yea, how does the fit work in terms of holding the piston in place / allowing for movement?


the piston isn't held in place at all it should move freely only 1/4 the diameter of the barrel, it needs to be a very close fit with a chamber near air tight.
Wood is fine the piston just make sure you seal it up with vanish.
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Unread postAuthor: aj23 » Fri May 11, 2012 6:00 pm

So it should be able to move an inch backwards / forwards with a 4in barrel, in a coaxial design cannon? Also, what stops the potato from going to far into the barrel? Since it's loaded before filling the cannon with air for safety, couldn't it get pushed down to far and have pieces go into the chamber / stick between the piston and barrel and cause issues after multiple launches?

Anyone have a good "how to" with parts list for something like this though? I understand the basics, but there might be a few small parts I'm not thinking of.

And for the piston, just simply using a 3inch piston with rubber bolted on the front would work, or would I need to do additional modifications?

Thanks
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Unread postAuthor: mattyzip77 » Fri May 11, 2012 8:09 pm

You need to do some research and take a look around in the pneumatic cannon showcase to see if you can find one there you like and get ideas from. To stop the potato from going to far in the barrel use a little common sense. like a pre measured ram rod, or a screw in the barrel to stop it from going to far maybe??? The piston is going to be harder than it looks. It may need o rings, a check valve, etc.
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Unread postAuthor: Crna Legija » Fri May 11, 2012 8:31 pm

aj23 wrote: Also, what stops the potato from going to far into the barrel? Since it's loaded before filling the cannon with air for safety, couldn't it get pushed down to far and have pieces go into the chamber / stick between the piston and barrel and cause issues after multiple launches?


use a measured stick that wont push it too far.

And for the piston, just simply using a 3inch piston with rubber bolted on the front would work, or would I need to do additional modifications?

Thanks


yeah thats all but it needs to be very close to the size of the chamber for a 4 inch id say 0.2 or less
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Unread postAuthor: aj23 » Sat May 12, 2012 9:47 am

Alright thanks for the help guys! After looking up some costs though, doesn't look like the coaxial version is my best bet if I want to build a piston valve cannon, as the 4" pipe is $50 alone for a 6 foot length and I'm trying to keep the whole thing around $50-60 lol.

I'm thinking of doing a barrel sealing piston instead, so for that I'd have a 2" T, into the barrel, with a coupler to do into a 3" chamber or something...but then could I simply by a 1.5in coupler to use as the piston with this design?

I guess I should really ask, is the piston that much better than just using a modified sprinkler valve between the barrel and the chamber? I kind of want to do the piston to make mine unique, but I don't know if I'll be capable lol.
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