18 replies to this topic

[death by cheez]

Here it is. I still need the fitting for the schrader (sp) attachment and the attachment itself. Otherwise it is being worked on as I type. Here is what I have so far:

Materials: 5 feet of 3" PVC.

2 feet of 1 1/2" PVC, or 1 foot if you can get it that small.

2X 3" to 1 1/2" PVC converter.

3" PVC endcap.

1 1/2" ball valve.

PVC primer and cement.

1 Schrader (sp?) plug thingy to attach a air compressor/ bike pump.

Attachment the Schrader valve threads into.


Materials 1.


Materials 2.


Cut the 3" PVC into to 2 pieces of equal length, roughly 2 1/2 feet pieces.


Prime the endcap and put the cement on it. Then attach it to one of the 3" PVC sections.


This is what the converters and the ~6 inch pieces of 1 1/2" PVC will look like when you are done with priming/ cementing.


This is what they will look like with the valve attached.



Attach the valve setup.


This is what it should look like when you are done, minus the Schrader (sp?) valve thingy for the pump.

Questions, comments, flames, concerns?

EDIT: I HAVE THE LAST PART. WILL BE INSTALLED TONIGHT, TOMORROW FOR SURE.

Edited by death by cheez, 14 February 2009 - 05:53 PM.

[TantumBull]

Its good for a first homemade. I used to make shit like that when I was into making air guns that shot marbles or darts made outta sharpened coat hangers and paper cones. Not really practical for nerf, though. But definitely fun to mess around with.

[Pineapple]

Shucks. I should have told you about it earlier.

The only thing I see that might be a setback is your barrel.

Notice that it's same diameter, same length as your air storage. In order to chuck something of that size (potatoes are out, you'll probably need to shoot oranges or Nerf balls or Vortex footballs), you need a considerable amount of air disproportionate to your barrel volume (3.14 x radius [squared] x length)... I forget the optimum combination as spudfiles posted, but that's the reason why the air storage is usually a bit larger than the barrel, or the barrel is obviously smaller in diameter than the airtank.

ALSO... it appears that it's strictly glue joints holding the valve to the air tank/ barrel. That's a lot of strain to put on those glued joints. Usually we use threaded joints at the valve, or you can fabricate some reinforcement braces (channel iron or even a 1 x 4) to keep the strain of moving your cannon around, off of the glue joints.

Finally, take it slow. Use a good compressor with a pressure gauge on it (you ARE using a compressor for that right? A hand pump will take a while to fill that bad boy), and pressurize slowly. Go with 10 psi, then leak check the glue joints (using soap water), then play with the firing and air release.

You might just find yourself going back down to a 2", or even a 1 1/2" PVC barrel. Only your R&D will determine your happy zone. The fun part comes now!


-Piney-

[nerfer9]

Do you mind if you make the pics bigger so I can see them?

[badger]

Click on the pictures to enlarge them.

[veginator]

What ever you do, do not pressurize that cannon. Those bell reducers you have used to go from 3" to 1 1/2" are a special kind of fitting. They are called DMV fittings or drain waste vent. They absolutley not pressure rated and you have basicly built a bomb. Next time when make sure that your fittings and pipe are not DMV.

[Doom]

I have no serious comments on this design other than that it's good to see some experimentation and it reminds me a lot of this water balloon gun, so much that you probably should try water balloons.

Also, look into replacing the bell reducers you have there. I know a lot of people have used them (myself included), but they're not pressure rated. To stay on the safe side I wouldn't use more than 40 psig and plan on scrapping that part and replacing each bell reducer with a coupler and reducing bushing. Be sure each fitting says NSF-PW or NSF-61 on it. (Hmm... seems veginator beat me to it).

Notice that it's same diameter, same length as your air storage. In order to chuck something of that size (potatoes are out, you'll probably need to shoot oranges or Nerf balls or Vortex footballs), you need a considerable amount of air disproportionate to your barrel volume (3.14 x radius [squared] x length)... I forget the optimum combination as spudfiles posted, but that's the reason why the air storage is usually a bit larger than the barrel, or the barrel is obviously smaller in diameter than the airtank.

For pneumatic guns there is no ideal ratio as what is ideal depends on far too many factors. The only way to figure out what's ideal is through either computer simulation or much testing. What is ideal also of course depends on what you want. In general you could want energy efficiency, less air used per shot, or the most shots per tank of gas. All 3 can have different optimal configurations, though they usually overlap quite a bit.

If you just want maximum performance and don't care about efficiency (which is what the ratio talk usually is concerning), then a bigger gas chamber is better. You do reach a point of diminishing returns however, but as I said, there's no simple way to know where it is. A 2:1 ratio should be safe, however, I've found that ratios as low or lower than 0.4:1 are fine too for some configurations, so realize that 2:1 will not typically be very efficient.

Edited by Doom, 06 February 2009 - 10:22 AM.

[Vulcanman101]

I can guess where the Antioch part came from monty Pythons the holy grail where the are trying to get into that cave and Arthur says Holy man bring down the Holy Hand Grenade of Antioch funniest thing ever!!.

[Naija Blue]

It looks like it is going to be amazing! and I'm thrilled you went with that name.






-NAI-

[Pineapple]

Holy cow.

And I don't mean the cannon.

I never even thought about waste pipe and DWV fittings/ bell reducers. And a small part of my work involves using a lot of PVC pipes (irrigation system on a 6 acre retreat center).


Veg and Doom are right on this one. And I'd especially trust Doom because he's been playing with stuff like this for a long while... but both seem to have some authoritative knowledge of the subject.

I sincerely hope that no one is using stuff like ABS and foamcore pipes in their cannons. Sure it's cheaper, and like Bob said, it does SOMEWHAT hold some pressure, but the dynamics of air pressure vs. water pressure I know are not the same (and I'm just talking out of my butt and making some common-sense assumptions).

The reducing fittings I usually use are NOT the bell-shaped ones, but the slip/slip, or the threaded/slip pressure rated fittings more suited for irrigation purposes. They indeed look significantly different than those bell shaped waste fittings. I never realized myself about using pressure-rated fittings, primarily because I don't mess too much with sewer/drainage in my work.


Re-think this puppy out. We don't want to hear about you getting hurt.


-Piney-

[death by cheez]

I have a friend who made this EXACT same thing, but for tennis balls. He put it up to 80 PSI 200+ times without it even leaking so much as a spritz of air. And it still hasn't done anything bad. I am not finished with this anyways, but thanks for the tips and concerns. I will heed your advice, but I have one problem. I have no money left . I spent all of my birthday money on this beast. I will make sure to reinforce the heck out of it though. I will get on that ASAP. Besides, the MAX pressure I am willing to put this to is 50 PSI. This stuff has an 80 PSI pressure rating anyways. I don't want my mom's cousin (cop) called on me for harming some one. We are also using shells for this, I just need to find one whether it be a can or bottle that will fot snugly. More pics to come today hopefully.

Thanks for the feedback guys, since this is my first homemade. (Can you believe we thought of this thing last year around March and just put it to the test?)

[death by cheez]

Sorry for the DP.

Edited by death by cheez, 07 February 2009 - 09:15 AM.

[death by cheez]

Well I am planning on encasing the tank in metal. Though I do have plenty of old too-small jeans lying around...

[Doom]

Expecting that what you build won't fail is the bigger mistake. Planning for failure is the way to go. In fact, I built many of my pressure reservoirs with an intentional weak spot, in the hope of encouraging failures to go in a specific, well-contained direction.

Going by failure characteristics and ignoring the pressure rating is still doing it wrong. The only way to get both is with metal, which generally is the recommended material for pressurized gas. Aluminum pipe isn't expensive, and I plan to use it for this reason.

With that being said, operating at reasonable pressures (< 60 psig), using safety valves (like the pop-safety valves on McMaster-Carr), having a safety factor over the pipe's already conservative rating, using pressure rated fittings, and making sure to not abuse the pipe, PVC pipe is absolutely fine. And in general ABS is fine under the same conditions, but I'd still feel uncomfortable not using pressure rated materials.

Well I am planning on encasing the tank in metal. Though I do have plenty of old too-small jeans lying around...

I've read that short of kevlar, nothing reasonably light will contain a burst PVC pipe.

Edited by Doom, 07 February 2009 - 02:42 PM.

[death by cheez]

Thanks, that was helpful. About how expensive would it be for enough aluminum plating to cover a 2 1/2 foot long tank that has a 3" diameter? I am guessing maybe $20-$30.

[Doom]

Aluminum would be the pressure vessel material, not the cover material. Don't use a cover--as I said, anything reasonably light would not be adequate.

Perhaps I should have qualified my statement as well. Aluminum is inexpensive in small diameters. Once you get up to like 3 inch NPT, it's very expensive. I'd start looking at scrap yards at that point, but these aluminum pressurized containers I've found might be adequate too. You also could weld it, but if you use common aluminums like 6061-T6 you'd have to retreat it. Do some research and see what you can come up with.

Edited by Doom, 07 February 2009 - 03:00 PM.

[pizlo]

What ever you do, do not pressurize that cannon. Those bell reducers you have used to go from 3" to 1 1/2" are a special kind of fitting. They are called DMV fittings or drain waste vent. They absolutley not pressure rated and you have basicly built a bomb. Next time when make sure that your fittings and pipe are not DMV.

This guy is absolutely correct, those fittings are NOT pressure rated and pvc shards are SHARP. Also your valve us a HUGE air restriction because its tiny and it opens REALLY slow because its a plastic ball valve. If you liek this kind of thing head over to spudfiles.com, thats where real homeade guns like this are, not nerf.


And you guys can stfu about the metal, pressure rated PVC pipe is safe even beyond its rating so don't go beyond it, and don't let it get brittel in the cold and you will be fine.

Edited by pizlo, 07 February 2009 - 06:47 PM.

[A side of nerf]

I can vouch for Veginator's knowledge on this subject. He spent 2 years working with spud guns before he started working with nerf.

[death by cheez]

I just posted this on FU. And thanks again for all this feedback, it has been helpful and means a lot.

And I will be doing a test soon to see how many unopened pop cans it can shoot through.

Edited by death by cheez, 09 February 2009 - 03:41 PM.