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Doing it Right The Second Time Around

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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Doing it Right The Second Time Around

Unread postAuthor: Braddubya » Tue Jan 06, 2009 3:00 am

So some of you may have seen my last attempt at building a cannon I dubbed Big Boy.

http://www.spudfiles.com/forums/my-new-cannon-big-boy-and-some-questions-with-pics-t16969.html

It showed incredable (atleast to me) potential but than it was brought to my attention that not all PVC is created equal and Big Boy has been retired. So now im looking to avoid any more problems on my next build. I read the Wiki on pressure rated PVC and found a place that I believe sells it. BUT before I go out and spend more money on PVC are there anymore things I should be looing out for? Any problems you guys have run into would be great to know.

I also have some specific questions.

1. Will standard PVC purple primer and clear cement work for this application?

2. Assuming I use all pressure rated parts what max psi will my cannon be capable of?

3. How is the best way to have interchangable barrels that will work from large sizes like 3" down to 1.5"?

4. As mentioned above are there any other potential problems I should be looking out for or tips on building a good cannon?

Thanks all in advance!
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 06, 2009 3:54 am

Since you're clearly intending to make this a good one and put in the effort, why not consider a piston valve? You'll be amazed at the power difference.
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Unread postAuthor: MrCrowley » Tue Jan 06, 2009 4:20 am

1. Will standard PVC purple primer and clear cement work for this application?

I believe you need prime and cement for pressure rated PVC pipe and fittings. My primer and cement says that on the container, i'm sure it's the same in America also.

2. Assuming I use all pressure rated parts what max psi will my cannon be capable of?

Larger diameter fittings have a lower pressure rating, in my Wiki article I linked to you, I posted a table that shows the ratings for the different schedules and diameters:
http://www.pvcplus.com/PVC%20schedule_40_&_80_pipe.htm

It says 4" Sch 40 is rated to 220PSI but as a general rule, not many people take their Sch 40 PVC cannons over 150 PSI with any diameter (small or large). Some take them up to 175PSI and even 200PSI, but I feel that is risking your safety. So with a cannon as large as yours, I'd stick below 150 PSI, if you make a piston valve, stick to 130PSI and below (because of the larger stresses involved on the fittings with a high powered valve like a piston).

3. How is the best way to have interchangable barrels that will work from large sizes like 3" down to 1.5"?

If you make a piston valve, i'd make one inside a 4" PVC tee. So i'd probably have a 4"-2" PVC bushing, and then a 2" female threaded adapter inside this bushing.

For your 2" barrel, just chuck on a 2" male threaded adapter, for a 1.5" barrel, get a 2"-1.5" bushing and then a 2" male threaded adapter, for a 3" barrel, get a 3"-2" bushing and then a 2" male threaded adapter.

You will need a 2" coupler for all three barrels as well (because most threaded adapters and bushings have the same outside diameter as the pipe, so you'll need a coupler so the fitting can fit in one end, and the barrel in the other end (google PVC coupler if you are unsure what it is)).

4. As mentioned above are there any other potential problems I should be looking out for or tips on building a good cannon?

None that I can think of, you're first cannon was built fairly well aside from the obvious already pointed out. Just take your time and look up cannons on this site like 'SWAT' and 'Americannon'.

:)

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Unread postAuthor: Braddubya » Tue Jan 06, 2009 3:40 pm

Jackss you read my mind

and thank you Mr Crowley that helps alot!


Anyone else have some advice?

Edit: I also have another question. Why not use a threaded end and cap for a piston valve? I mainly just see the version with a regular cap thats attached with screws. and how can this configuration hold pressure?
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Unread postAuthor: MrCrowley » Tue Jan 06, 2009 4:53 pm

Why not use a threaded end and cap for a piston valve? I mainly just see the version with a regular cap thats attached with screws. and how can this configuration hold pressure?

A threaded adaoter and end cap aren't oftenly used because the threaded adapter usually has an inside diameter smaller then the diameter of the piston. So the piston can't fit through.

Though there are a few threaded adapters you can find, which have an inside diameter large enough, for a piston to fit through. I can't remember what they are called but you can easily identify one at a hardware store if you look at the one used on my cannon here:
http://www.spudfiles.com/forums/v-a-l-p ... 14702.html

Quite a few people had, and still have, trouble finding threaded fittings with a large enough diameter for the piston to fit through, so they resorted to using an o-ring and bolting an end cap to the pipe. The o-ring helped to seal it, but more often then not it would leak. It's really only been more recently are we seeing most people finding and using these threaded adapters with threaded bushings for a servicable piston.
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