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Unread postAuthor: Technician1002 » Sat Oct 30, 2010 5:58 pm

My bad.. I meant 4 inch pressure rated PVC. I edited my post. DWV PVC is bad.
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Unread postAuthor: Fnord » Sun Oct 31, 2010 12:26 pm

You should seriously consider making the chamber from steel fittings. ABS is fine for a barrel but safety is a priority when you're firing around crowds, and pvc doesn't have much of a "real world" safety factor. Bump it or stress it in a non-normal way and it breaks.
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Unread postAuthor: Technician1002 » Sun Oct 31, 2010 5:19 pm

Steel might not fit the stated $300 budget, unless they have access to a machine shop and can modify an old propane tank or other inexpensive steel tank. All steel pipe in larger sizes gets expensive quickly.

ABS is much less expensive, lightweight, has gentle failure modes, and works fine at the relatively low pressure with a good safety margin.
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Unread postAuthor: POLAND_SPUD » Sun Oct 31, 2010 5:38 pm

metal BV + metal fittings... they are pretty cheap in europe so I don't think they are that expensive in the USA
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Unread postAuthor: MrCrowley » Sun Oct 31, 2010 5:44 pm

I guess the problem with that is you wouldd have to use higher pressure in the smaller volume metal cannon to achieve the same performance as a low pressure, high volume, ABS cannon. In this case, high pressure would be bad unless you had a lot of dead volume for it to expand in before acting on the projectile.
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Unread postAuthor: Technician1002 » Sun Oct 31, 2010 6:01 pm

I have never seen relatively cheap iron pipe and fittings in sizes larger than 2 inch.

Here are some 4 and 3 inch fitting prices.
http://www.plumbingsupply.com/galvanizedfittings.html

This is a partial price list to go from a 4 inch chamber to the 2 inch valve and back to a 3 inch barrel.
4 X 3 bell reducer $77.78,
3 X 2 bell reducer $33.77.
2 inch close nipple $4.42 2 needed
Add shipping.

You could shop Ebay, but can you meet the two week deadline?

Simply welding a 2 inch nipple into an obsolete 11 lb propane tank is a cheaper way to use a steel chamber than assembling one from iron pipe and fittings.

Used steel tanks or 4 inch ABS are alternatives to 4 inch steel plumbing part prices.
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Unread postAuthor: POLAND_SPUD » Sun Oct 31, 2010 6:27 pm

who said you need 2" fittings ? 1" ones are pretty cheap. I haven't really done any tests with fragile ammo but you can build a pretty compact gun that has a large bore barrel and small chamber
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Unread postAuthor: Technician1002 » Sun Oct 31, 2010 6:38 pm

1 inch restricts the flow rate to less than 1/4 of a 2 inch. They have a distance goal to meet. A better flow allows lower pressure to be used. They want a 200 foot range, not 75.

I'm hoping we can toss the samples a few hundred feet, it will be on a football field so it would be nice to get them up into the stands.
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Unread postAuthor: POLAND_SPUD » Sun Oct 31, 2010 7:23 pm

That can be met. That would require higher pressures but it is doable.

Tech, the QDV might be a cool design but it isn't really the kind of stuff I'd advise for a complete beginner. It would make sense if you sold him or rented one of your guns - but he wants to build it himself

Damn, I wouldn't even build one myslef as I am just too lazy. if you've got parts you can assemble a BV or QEV pneumatic in 5 minutes or so - and you've got to try really really hard to #### something up.
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Unread postAuthor: Technician1002 » Sun Oct 31, 2010 7:29 pm

I didn't recommend a QDV. I posted the photo for the chamber construction materials. It is not advisable for a beginner to attempt a QDV.

It took 3 months to walk an experianced builder to build one for a competition. 2 week deadline with a beginner is not to be considered.

They don't have the time to build a QDV.. I recommended a 2 inch sprinkler valve.

Last time I checked 2 inch QEV's are pretty rare.
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Unread postAuthor: Fnord » Mon Nov 01, 2010 12:51 pm

Guys, this is a freaking sandwich launcher! He doesn't need a 4" chamber. If you want a fast 2" valve, using a huge chamber will be LOUD even at low pressures. I don't think your audience will apreciate having to wear ear protection. A 2" chamber is fine, and can always be made bigger by throwing on a coupling and another section of pipe.

So, maybe $30-40 for a large 2"?

Edit: look at the extremely wimpy gun I made in ggdt! Even it can get close to 200ft
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Unread postAuthor: Technician1002 » Mon Nov 01, 2010 1:37 pm

The chamber and valve was selected for longer acceleration with lower force. A small chamber limits the majority of the force to the first portion of projectile travel and the small valve limits the maximum velocity. This results in a launcher that works best with a very short barrel with a short acceleration spike. My Marshmallow Cannon will very happily strip the wrappers off hard candy on launch.

The large cannon at low pressure does a great job lofting a 1 Lb load of wrapped candy on a parade route. :D

Having the sandwiches delivered still wrapped and assembled is a stated goal.

Blasting the wrapper off it ruins the aerodynamics.
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Unread postAuthor: Fnord » Mon Nov 01, 2010 2:47 pm

A small valve can also limit the initial spike. When I tried simulating a 2", it did increase the performance, though at a cost of over 100 extra gees.

Really, I don't see how you're going to get this thing below 400 or so gees and still get 200ft range.

You can do it with maybe 30 psi, but at that point metal would be completely unnecessary... just use ABS, even of the dwv variety(with a pop-off).
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Unread postAuthor: Technician1002 » Mon Nov 01, 2010 4:52 pm

This is exactly the pressure range and materials I was suggesting in my first post in this thread.

With your budget and time limit, pick up a 2 inch sprinkler valve, modify it, build a 4 inch chamber 3 feet long and add a 3 inch barrel about 3.5 to 4 feet long. Use 15-30 PSI depending on distance needed.


And;

Due to the difficulty in obtaining pressure rated 4 inch PVC and fittings, 4 inch Cellular Core DWV pipe is recommended.
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Unread postAuthor: Moonbogg » Tue Nov 02, 2010 1:24 am

he isn't replying anymore. Lets stop steam rolling this guy with differing opinions and wait for some feedback on what HIS thoughts are. Just my two cents, wether they were wanted or not.
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