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First Build: Questions on Pressurization

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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First Build: Questions on Pressurization

Unread postAuthor: Suedo » Wed Feb 12, 2014 3:00 pm

I am building my first cannon ever, and have decided on a barrel sealing piston design using SCH40 PVC. I have attached a hand drawn diagram of my build plans. To my knowledge the build is pretty solid and the piston will be be made out of Bondo (a lightweight car filler) and will be using pieces of sheet rubber on top and bottom to act as a bumper (not sure if correct term). The only thing I am not quite sure of is the method in which I will fill the cannon. I have both a bike pump and air compressor (125psi max operating pressure) and would ideally like to use the air compressor to fill it.
So my question is it as simple as using an air compressor quick connect (correct terminology? I'm referring to the air hose input) and drilling a hole into the PVC and threading it in where the Schrader valve would normally be placed? Or would I have to use a series of copper pieces in combination with it? I would prefer not to have to spend extra money on copper pieces and have never soldered pipe before. I also have reservations on using copper and PVC together as from what I've read the two don't thread together perfectly. I'd also love to hear any suggestions and improvements for my design! Thanks.
P.S. I am basing my cannon largely on this design seen in this video



I have also made slight modifications.
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Hand-Drawn sketch
Last edited by jrrdw on Wed Feb 12, 2014 3:05 pm, edited 1 time in total.
Reason: Fixed video.
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Re: First Build: Questions on Pressurization

Unread postAuthor: noname » Wed Feb 12, 2014 4:09 pm

The copper fittings he uses are entirely optional. You will have to attach a schrader/quick connect in one of a couple ways though.
You can use a reducing bushing that threads into the PVC threads on one side and the trigger valve and fill valve on the other, or you can thread the fill and trigger valves directly into the PVC. You can use any brass/steel/copper fittings you want in the pilot area behind the piston for looks, comfortability, etc.
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Re: First Build: Questions on Pressurization

Unread postAuthor: Suedo » Wed Feb 12, 2014 11:22 pm

Thanks noname!
Is there one choice that's better than the other for having that tight air-lock seal that maximizes air flow and piston reaction time? For example an additional tee one with the trigger valve (PVC ball valve) and on the other port the quick connect hooked up to a series of couplings that would allow it to thread straight in? Or would you recommend something seen in the original hand-drawn model with the ball valve in the back and a quick connect threaded right into the side of the PVC pipe?
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Re: First Build: Questions on Pressurization

Unread postAuthor: 1nxtmonster » Thu Feb 13, 2014 8:29 am

Good luck even opening a 1.5 inch ball valve. I got one for my burst disk cannon, but to no avail. You couldn't open it without some sort of 2 foot long lever. for a piston valve, the pilot valve (the ball valve in your case) doesn't need to be as large as the main valve, a 3/4 ball valve should work fine for you.

And as for where to put the valves and stuff, just thread the schrader valve straight into the side of the pvc pipe, and put the ball valve on the back.
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Re: First Build: Questions on Pressurization

Unread postAuthor: Suedo » Thu Feb 13, 2014 5:04 pm

Thanks for the info 1nxtmonster! Based on your suggestions I have created a new schematic based on your suggestions. I am going to be building the cannon next week as I'm off from school, so you and noname have been beyond helpful for this newbie! :lol:
Last thing, how dramatically would performance improve if I switched out the ball valve for a modded sprinkler valve with a blowgun? I am aware they improve performance on simple pneumatic cannons, but would this improvement greatly increase my power for this piston operated cannon?
Thanks.
P.S. If everything goes well I'll be sure to post this cannon in the finished cannons forum.
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Hand-drawn Schematics v2
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Re: First Build: Questions on Pressurization

Unread postAuthor: 1nxtmonster » Fri Feb 14, 2014 3:42 pm

A faster pilot equals a faster piston valve, but as long as you don't use something tiny for a large piston valve (like a blowgun), it's pretty much up to how much money you want to put into it. Yes, a sprinkler valve would actuate the piston valve faster than a ball valve, and yes it's a good idea to do so, but only if you want to spend the money. A sprinkler valve would be easier and more consistent to open as an added bonus.

If bondo doesn't work out for the piston, you could give epoxy a try. Liquid epoxy (the runny two part stuff) is great for making pistons, and cast molding using clay as the mold.

I believe JSR has a more in depth tutorial here:

how-to-database/topic8919.html

But basically you just take a scrap piece of the pipe that the piston will slide in, grease it up with cooking spray, and fill with epoxy (but bondo would be about the same method)
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