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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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Unread postAuthor: Crna Legija » Sat May 12, 2012 10:22 am

well what do you need the cannon to do? if you are looking for moar powar than id say go with the burst disk jar said at the start, when they hear the difference in noise they wont think its crappy have pre cut disks and it wont take much longer then sprinkler valves.
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Unread postAuthor: aj23 » Sat May 12, 2012 10:46 am

I'm going more for precision than anything. But I'll look into that still.

Also, thinking about the coaxial one a bit more, would 3' of a 4in pipe be good enough for the chamber, or does the barrel need something a bit longer at still at least?

In the chamber construction section of the following page, what are the 3 parts in front of the piston used for? Do they attach to the barrel?

And I've never realized tubing was used, what is that for?
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Unread postAuthor: Crna Legija » Sat May 12, 2012 9:38 pm

i dont get what your on about now?
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Unread postAuthor: mattyzip77 » Sat May 12, 2012 10:02 pm

I'm lost as well and why are you asking if 3 feet of 4 inch pipe is long enough for a chamber? I'm cofused!!! If this is your first cannon, I suggest you start with a easy build. A barrel sealing piston valve inside of a coax isnt exactly a piece of cake!!
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Unread postAuthor: Crna Legija » Sat May 12, 2012 10:33 pm

lol he changed his mine its a barrel sealer not coax now
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Unread postAuthor: aj23 » Sat May 12, 2012 11:27 pm

lol I'm confusing myself, and everyone else a bit too much it seems. I'm back to the coax design, and have all the parts ready to build it other than the piston. I picked up a 3inch coupling, but it doesn't fit within the 4in barrel (using abs pipe) so it seems that idea for a piston won't work.

Should the piston be pretty snug, but movable in the barrel? What would the easiest piston be to build for a beginner cannon?

Thanks
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Unread postAuthor: Crna Legija » Sun May 13, 2012 1:30 am

Cast some hot glue or epoxy about a 1 1/2'' thick in a bit off the chamber material make sure you lube the walls, that's the easiest piston and will work fine you still need a rubber sealing face.
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Unread postAuthor: mattyzip77 » Sun May 13, 2012 6:13 am

I would cast a epoxy piston, hot glue I dont thing will last in a big 3 inch design!!
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Unread postAuthor: jakethebeast » Sun May 13, 2012 7:26 am

Sorry, don't have time to read the whole thread but...

Why not to use a hole saw to cut nice piston-blank from plywood, and sand it to close fit, and then some neoprene to seal it? :)
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Unread postAuthor: aj23 » Thu May 17, 2012 8:22 pm

Yea we'll get the piston thing figured out this weekend, but one more question, when my piston is touching the back of the chamber, how much space should be between it and the barrel? Since that's a stationary length, I don't want to mess it up.

At the back of the chamber I have a coupling and male to female adaptors to allow for a screw on back which is a reducer to lead to the back valves and everything.

The extra pieces alone are pretty long in length, so I'm thinking the barrel will have to extend alightly further than the whole regular chamber, into the additional parts a bit?

I was trying to find pictures of some opened from the back but couldn't seem to get any.
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Unread postAuthor: jackssmirkingrevenge » Thu May 17, 2012 10:05 pm

The piston should travel around 30-50% of the barrel inner diameter.
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Unread postAuthor: aj23 » Sat May 26, 2012 11:52 pm

I finished this cannon, but there was one issue. Since I did a coax design, I sanded out my 4" to 2" reducer coupling so the barrel could go straight through. I sanded it a bit too much though, and the fit was loose. When I filled it up with air, there was space between the barrel and the coupling and some leaked out.

I put epoxy around to seal the two together and hopefully plug the whole, but do you guys think that will hold the pressure?

Also, on the piston does the rubber at the front have to be neoprene? I put some cheap rubber I had on the front as I couldn't find any, it should be fine though right?
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Unread postAuthor: kydavies » Sun May 27, 2012 2:26 am

You can use a thick (at least 3mm and no it does not have to be 1 piece) piece of rubber or neoprene. I found (with my cane gun) that thin flexable pieces of rubber have a tendancy of being ripped off the piston and getting lodged half way down the barrel (such a pain to remove flexable greasy gaskets situated in the middle of a 1/2" pipe....). Good luck

PS. The epoxy will be fine if you give it time to cure.
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Unread postAuthor: mattyzip77 » Sun May 27, 2012 8:32 am

You can get neoprene discs up to 2 1/4 inch in diameter at any lowes or home depot. The epoxy should be fine, however you should have just solvent welded it like the rest of the cannon, I hope!!
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Unread postAuthor: aj23 » Sun May 27, 2012 10:52 pm

Is there any way I can get 4" neoprene from any of those big stores? I realized with the coax design, I have nothing holding the barrel in the center of the chamber properly...should I maybe put screws in to hold it down and make sure it doesn't move? I'm curious what people normally do.

But yea, for the neoprene I didn't even see anything 2" at home depot...anyone have a link to it or something? The employees were clueless as well and told me they didn't sell it after searching for like 20 minutes.

Right now I used some rubber from a mouse pad I had laying around...I'm assuming thats the cause of my leak, but could it possibly be something else?

I mean the cannon could still technically function, but once I put a potato in I'm thinking the pressure leaking through the barrel will start to push it a bit before I let anything go, or that blockage might cause some other issues.

And yea the cannon was all solvent welded...just we sanded the reducer coupling out a bit too much and I guess didn't put enough of the pvc cement to compensate, and then once it was in that stuff couldn't do anything after.
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