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6 inch coaxial

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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Unread postAuthor: SpudMonster » Sun Jun 15, 2008 7:23 am

Boss, you win the idiot competition. I'll try to list the stuff wrong with the picture you paint. But, you say, "it's safer than it looks" no, it isn't.

I: "I built this pneumatic cannon hastily"
Hastily is the operative word. Haste is bad. Haste means half-assing crap to meet deadlines. Haste means getting it done to the exclusion of all else, including safety and common sense.

II: "Originally I had planned on casting an aluminum reducer"
Why the hell do you want to make a materials transition from aluminum to PVC, when appropriate fittings exist for such an application? Even if you had some funky, exotic transition you had to make, a bit of time with a lathe and a chunk of PVC stock would give you the necessary item, exactly to your specifications.

III: "I just added more reinforcing plastic to the end caps"
If it ain't good enough, just tack some more crap on 'till it don't blow up.

IV: (this one is good) "The barrel is 2 inch white pvc, rated at 300 psi, 4 feet long, coupled to a 24 inch long cellular core, black PVC pipe. (Because that's what I had)."
Fisrt off, it's not "black PVC" dumbass. It's ABS, also known as Acrylonitrile butadiene styrene. ABS != PVC. Full stop. You're making a materials transition again here. There is no glue that will safely bond both plastics to our standards. Only threaded connections are acceptable for ABS/plastic joints. Not glued, not bolted, not riveted, and not taped. Threaded.
Next, it's cellcore for christ's sake. What posessed you to think that it was a good idea for the chamber? There is NO pressure rating at all, and it's FOAM. You've got a chamber made of foam sandwiched between two solid films of plastic. BRILLIANT!
And finally, you were too cheap to go get the right materials. Half-assed construction at work here.

V: "Originally I screwed the valve to the end-cap-pipe-coupling with 36 short wood screws. -I don't know where I got that idea- "
Oh god... Next?

VI: "The plastic reducer exploded at 70 +/-5 Psi after a few launches, taking the black PVC portion of the barrel at the coupling with it and got launched a few hundred feet into the air. "
No surprise, considering the construction.

VII: "If I wrap the entire 5 foot long chamber in a 10+ layers of pretensioned epoxy/aluminum foil, or one layer of 19 gauge bailing wire, 200-250 psi should be safe, assuming an aluminum piston."
BALING WIRE?! And Aluminum foil. ALUMINUM FOIL. MOTHERFUCKING ALUMINUM FOIL. 0.001" thick Al sheet, with epoxy.


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Stupid quote of the now:
joestue "The chamber is rated to 100 psi, it don't say that on the side, but the 2 inch stuff is rated to 300, so by analogy..."
"only the last two feet hear the end is black cellular core"
"I'm taking it to 300 psi"
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Unread postAuthor: Carlman » Sun Jun 15, 2008 7:35 am

couldnt of said it better myself SpudMonster :D

hes an idiot when it comes to spudguns and common sense so i just hope that WHEN it does go all wrong he writes to tell us about it from his hospital bed.
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Unread postAuthor: joestue » Sun Jun 15, 2008 7:43 am

the black plastic is the barrel, the safest place for it.

but you forgot about the 6 inch chamber, it's not rated.

a 'wrapped' pressure vessel is extremely common, and in fact one of the only economical methods of creating large vessels, for example, 5 foot diameter, 5K psi etc.

stress is always 2x circumferentially, that's why pipes burst length wise, a layer of 19 gauge baling wire or 10 or so layers of aluminum/epoxy will result in a pipe exploding axially, a safer proposition in my book. ie do not point a weapon at anything you do not intend to shoot....

secondly, you guys are getting wrapped up around the 2 feet of abs cellular core, and yes i did know it was cellular core, in fact i had to cook the cannon for a few days to harden the compromised plastic, after the glue attacked it.

I will certainly replace the valve with a machined aluminum one, but for the record, the most it saw was about 70PSI.

And i expect it to fail above 120psi, comparing the deflection at 70 with the deflection of the failed unit at 60, on the shot before the failure.
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Last edited by joestue on Sun Jun 15, 2008 7:53 am, edited 1 time in total.

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Unread postAuthor: Carlman » Sun Jun 15, 2008 7:53 am

joestue wrote:And i expect it to fail above 120psi, comparing the deflection at 70 with the deflection of the failed unit at 60, on the shot before the failure.


if you expect it to fail at 120 now what about over time when the plastic has been expanded and rapidly brought back to its original state after 100s of shots.

you are better off starting again mate, with pressure rated pvc, or with the way your going build it out of metal.
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Unread postAuthor: Matheusilla » Sun Jun 15, 2008 7:54 am

Why would you even want to "expect it to fail"?
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Unread postAuthor: joestue » Sun Jun 15, 2008 7:56 am

more like 10 shots, but i am going to have my friend destructively test it.

i was never intending at the time to go past 60-70psi!

100 is an absolute limit for the chamber.
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Unread postAuthor: Fnord » Sun Jun 15, 2008 8:09 am

more like 10 shots, but i am going to have my friend destructively test it.


I wouldn't even let my enemy destructively test it.

If it was only $15, just scrap it and start over. If there are any rated parts AT ALL, you can save them for future stuff. I'd save the 2" abs barrel for a combustion.
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Unread postAuthor: joestue » Sun Jun 15, 2008 8:14 am

lol fill it with water.
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Unread postAuthor: SpudMonster » Sun Jun 15, 2008 8:46 am

And I still don't get why you are too cheap to use the proper materials to build the cannon, only had 15 bucks invested initially (a small loss by any stretch) and yet casting and machining aluminum and buying epoxy/aluminum foil (still laughing here) is no problem.
Does not compute Will Robinson.
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Stupid quote of the now:
joestue "The chamber is rated to 100 psi, it don't say that on the side, but the 2 inch stuff is rated to 300, so by analogy..."
"only the last two feet hear the end is black cellular core"
"I'm taking it to 300 psi"
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Unread postAuthor: DYI » Sun Jun 15, 2008 9:38 am

This is right up there with the Mythbusters' tree cannon in terms of big-ass ghetto guns. If that was the effect you were going for, great job. Got any videos of it in action? Because without those, I'm having a hard time believing that it can even hold pressure, much less fire successfully.

I don't think it's worth mentioning how you're going to be horribly dismembered by this thing when it fails, because there's been a page of that already.
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Unread postAuthor: MaxuS the 2nd » Sun Jun 15, 2008 9:42 am

joestue wrote:a safer proposition in my book. ie do not point a weapon at anything you do not intend to shoot....


But you're ok with holding a bomb?

I don't understand your problem with just going out and getting the correct materials and following the correct safety procedures.
I don't understand it even more so because materials in your part of the world are dirt cheap.
Just remember that if you die, it will give us a bad name..
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Unread postAuthor: daxspudder » Sun Jun 15, 2008 10:29 am

love the new signature spudmonster, it says it all when it comes to this thread. This is one long drawn out hate session that I actually don't feel has been an act of flaming. joestue get rid of that gun before it gets rid of you or someone else. If you cant afford to buy the right parts, don't build a CANNON. You treat it like its some toy or something, while it is fun like a toy, there is a degree of severity associated with mistakes. If you think it would be stupid to shoot someone, then realize your risking doing much worse to yourself than being shot every time you pressurize that piece of junk. If you think building a good gun is too expensive, it wont compare to the medical bills caused from a failure that doesn't go "well"(not hitting you by some freak chance) good luck on your future guns, and take the advice of the people here, stop being argumentative, and nonchalant about your safety in all your replies. I promise people aren't wasting their time to tell you this stuff for no reason.
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Unread postAuthor: starman » Sun Jun 15, 2008 10:32 am

Do my eyes deceive me or is that actually twine string emanating from the "pilot"? .....and a mention of bailing wire.... Could it be this is the mythical twine string and bailing wire setup we've all heard tell of?.. :roll:

Also, the whole 5 lb weight on a string trigger thing throws me somewhere in the middle of a Rube Goldberg cartoon.

Joestue, seriously man... :shock:
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Unread postAuthor: Gippeto » Sun Jun 15, 2008 10:38 am

There is a very simple premise that you SHOULD be trying to implement here;

DO IT RIGHT...OR DON"T DO IT AT ALL.

Don't put your pride ahead of your safety, or that of your friends.

You have received an awful lot of replies here by people who know what they are doing.

Don't you think there might just be a reason they are telling you to scrap it and start over?
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Unread postAuthor: SpudMonster » Sun Jun 15, 2008 11:20 am

joestue wrote:but you forgot about the 6 inch chamber, it's not rated.


Are you trying to save face here? It's not working.

Prima: "look at my half-assed pile of dog crap"
Secunda: "That has some problems"
Tertia: "You're an idiot. Stop using that cannon, it's dangerous"
Quarla: "Here is a list of some of the crippling flaws of said cannon"
Quinta: "Jesus fork, you're actually going to take it to 300 PSI?!"
Prima: "But guys, look! Quarla, you're ignoring the fact that the chamber is unrated too! See? You didn't notice another flaw in my design!"
  • 0

Stupid quote of the now:
joestue "The chamber is rated to 100 psi, it don't say that on the side, but the 2 inch stuff is rated to 300, so by analogy..."
"only the last two feet hear the end is black cellular core"
"I'm taking it to 300 psi"
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