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first cannon ever very bad

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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first cannon ever very bad

Unread postAuthor: taytayswims » Sun May 04, 2008 4:12 pm

This is my first cannon. I made the mistake of using ABS pipe for the air chamber and an ABS reducer. I also messed up drilling out the second valve for the breech. It is my first and now i have a much better idea of what to do on my next one. Like a sprinkler valve ect.
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small5.jpg
the whole gun
small4.jpg
the drill bit i had chewed up the "breech" but it works
small6.jpg
i used epoxy and super glue

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Unread postAuthor: bigbob12345 » Sun May 04, 2008 4:14 pm

Ok if it was pvc, and pressure rated, and had a faster valve then it would be a nice cannon although a lager bore barrel could do you good.
And dont use it, just wait until you can make a better one.
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Unread postAuthor: Lentamentalisk » Sun May 04, 2008 4:17 pm

Well the ABS pieces aren't pressure rated, and the glue between the PVC and ABS will not hold any pressure what so ever. Other than that the chamber to barrel ratio is ridiculously huge, you could lengthen that barrel quite a bit.
sorry to be such a downer, but I wouldn't use it over 20psi.
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Unread postAuthor: elitesniper » Sun May 04, 2008 4:18 pm

dude scrap that, pvc does not bond with abs!
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Unread postAuthor: SpudFarm » Sun May 04, 2008 4:21 pm

he is fine at 40psi and down.. and a longer barrel will make you get more speed at those low pressures ;)

think of the thin soda bottle and the pressure it can handle..
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Unread postAuthor: Pilgrimman » Sun May 04, 2008 4:22 pm

Well, as you are no doubt aware, that cannon is dangerous. You could probably rebuild it for relatively cheap, however. Simply cut off the ball valves with about an inch of pipe left on either side, and then use couplers to attatch them to your new barrel. The chamber will have to be rebuilt, unfortunately... By the way, welcome to spudfiles!

Also, I'd suggest a trip over to the spudfiles wiki. You'll find it useful!
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Unread postAuthor: elitesniper » Sun May 04, 2008 4:24 pm

We don't want him pressurizing it and all of a sudden the chamber flys out and nicks him in the____(insert body part here)
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Unread postAuthor: DYI » Sun May 04, 2008 8:16 pm

I don't see any real extreme problem with this (unless the PVC is non pressure rated) other than him possibly dying of a heart attack when the glue between the PVC and ABS parts separates as he's charging it :lol:
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Unread postAuthor: dongfang » Mon May 05, 2008 10:05 am

Hi,

If the PVC and the ABS parts were joined with epoxy, and they were cleaned and roughened properly before assembly, this might actually hold up some pressure (yeah - enough to make a really scary BANG when the chamber comes off after all).

If it were mine, I would get a long hose and a pressure gauge (nice to have anyway). Then, bury the gun in sand / put it in a ditch, and cover it with something soft and heavy... and get something big and heavy between the gun and you. Wear safety goggles (also nice to have). Then inflate it, and see if it bursts.. (no no it will not blow up the whole neighborhood). Report to us at what pressure it went ;)

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Unread postAuthor: taytayswims » Mon May 05, 2008 4:45 pm

i have the abs pipe screwed into the 2-1' reducer and epoxyed them to ensure a nice seal.
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Unread postAuthor: dongfang » Mon May 05, 2008 6:59 pm

Hi,

OK, so the PVC parts are joined with the ABS parts over a thread?

That's actually not so bad. If only you had joined all the PVC and all the ABS parts by the book - that is, using the right cements and primers.

I used to sometimes epoxy PVC parts - it is much easier to make final adjustments, and to glue fittings on the middle of a pipe etc. where the PVC cement sets too fast, won't wipe off and scars the surfaces of everything.
BUT recently I found out that the strength of these bonds is very weak compared to solvent welding (that's the special PVC cement). It prevents the parts from simply slipping apart, but it doesn't weld them together. I have seen several cases of one of two parts epoxied together failing. That wouldn't happen with solvent welding - either they would both hold or both fail, in my experience. The failures were because of inadequate bumpers (it were moving parts). For static parts, I still feel OK with epoxy.

There has been a religious war here some time ago, over PVC vs. ABS. Never mind that, just have fun, don't get hurt, don't push that gun very hard, and use the proper cement next time ;)

Regards
Soren
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