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The Short: I'd love some help with my plans for an 8ft, 3in. piston, all metal pneumatic cannon.
The Long: I've been building guns out of PVC since grade school, mostly sprinkler solenoid or ball valve. One was ten feet long and worked pretty well. Now I'm out of college, and working on a farm where I've acquired welding and shop skills. Take a look at my plans for a coaxial piston valve spud gun. I have huge welders and lathes at my disposal, and all of our scrap metal is overkill for a spud gun...so I feel a personal obligation to build one!
I've been trolling Spud Files since high school and have answered a lot of questions by searching through the forum. Still, I have some more questions for all of you. Bear with me, as I haven't done any computer drawings yet. It's all by hand right now. Eight foot, 2.3" barrel (weighs about 50 pounds). 6 foot, 5" diameter air chamber (looking for around 1,000cu.in. chamber for a 2:1 or higher ratio to the barrel). 3" piston, either lathed from aluminum or composite/platic. Eventually I'd like to use this as a high pressure gun (500PSI?).
You'll see in the photos that the OD of the barrel (port) is 2 and 11/16 inches. The piston floats in a 3 inch ID stainless pipe (found in scrap and threaded it). That's a 1.3:1 piston:seat ratio. I feel like that's a relatively small ratio, so it'll border on QEV status. Would that mean that I should have a large or small pilot volume? I'll have at least a one inch pilot valve, so it should dump the pilot air pretty quickly.
I've read that the piston should open at least d/2 (half the diameter of the port opening), but fun diameter distance would be best. Is this true? I'm concerned about having too much of the piston wall being exposed to the chamber pressure, since there isn't much surface area exposed on the face of the piston when closed.
I shouldn't need O-rings on the piston, since it's barrel-sealing. It'll be milled down to fit perfectly in it's 3" stainless housing. If properly padded, should this piston be aluminum, or will I be okay using a high density plastic cylinder? What weight should I be aiming for on the piston?
Does anyone have experience with shooting canned food as ammunition? The mini tomato paste cans fit so beautifully into the barrel I found.
Thanks so much for looking into my project. I've left out so much of my math and considerations; I'm trying to keep this short. Also wanted a place to find encouragement and document my process. Wish me luck! Imager Album below...sorry I haven't rotated some of the images.
Ratio is fine. Pilot volume should always be as small as possible for optimum performance.
Beyond (0.25 x seat diameter) piston movement, you get no extra flow advantage from more opening - however since the piston will tend to impact the back end and bounce back, you want a bit more than that - but not too much, because then pilot volume is too large - so (0.5 x seat diameter) is a good rule of thumb.
HDPE tends to deform from repeated impact, I would consider Delrin as an alternative.
In terms of weight, as light as possible - a heavy piston accelerates slower and will slow down valve opening. It should however still be built strong enough to resist smacking against the endcap, a rubber bumper is obligatory.
It it's a tight fit (assuming the pipe it's riding in is a perfect cylinder) then o-rings should not be required.
Go for it, especially if you have a distant hard surface to shoot at.
Good luck with your project!
Thanks for the help! I'll get on it.
I've been using #2 cans (standard Chef Boyardee size) for years in my big 3"x10' barrel sprinkler-valve cannon. I've only ever tried 150psi once (max water pressure for the valve) and the can held together just fine. At 120psi, my usual firing pressure, it will throw a can of ravioli almost 400yds with enough energy left to explode the can and cover everything inside of ten feet in cheesy tomato sauce. It also put a can full of ice THROUGH the door skin of a 94 Dodge Shadow ES. (My own car, calm your &#%$)
I've got a design in my head for a MUCH more powerful pneumatic, but I need to find someone that can do flow modeling to design the valve for me. I'd like to be the first pneumatic cannon to "pie" a can.
The Official High-Tech Redneck
"There is no such thing as overkill." ~Solomon Short
Work has begun! I started with the plasma cutter and bench grinder to make some spacers to hold the barrel centered in the air chamber. I have three questions for you all.
1) The piston (made of delrin, thanks to your suggestions) will be milled down to fit into the piston housing. Since the air chamber is pretty large and will take a while to fill, would you suggest drilling a hole into the piston from the back, then out the side near the piston face? This would allow air to flow into the chamber quickly. I'd assume it shouldn't be too large because I wouldn't want too much back flow when the pilot volume is released. (I've sketched this out on the linked imgur photo as dotted lines on the piston)
2) On that note, I've seen it discussed here (possibly only in jest) that for large pistons, a smaller one inch piston could be an efficient pilot valve. Would you advise this for my three inch piston? I know I could buy a 1" QEV but would prefer to build something that can withstand a good 500-600 psi for future tinkering (the rest of this steel in the project is very thick and will easily handle high pressures).
3) Last, the following imgur photo shows my plans for the piston ratios. The piston can open to 1.25" (0.5 x seat diameter), the piston itself is 3" long (I just doubled the 1.25" gap and added .5" to be safe), and there is a one inch bumper of heavy rubber. Does this appear to be a decent combination of pisont length, opening gap, etc?
Plot twist! I found some flanges to use as an end cap system instead of a threaded cap. This will make for easy servicing of the piston and piston housing. Stoked! Photos will be up on the imgur album soon.
I wouldn't drill any holes because it would hurt performance. Let's assume you're filling through a schrader valve, flow restriction in the valve itself is tighter than a gap of a thousandth of an inch between 3" piston and piston housing - so you're not going to help fill rate by drilling any extra holes.
If your piston is tight, a 1/4" QEV is more than enough as a pilot valve. I have in the past actuated a 1" piston valve using just a schrader valve as a pilot - the critical elements are having a very tight piston and a limited pilot volume.
Looks very decent to me, should work a treat. Use at least 1/8" thick rubber for the piston sealing face and try and use a washer as close to the barrel inner diameter as you can.
Re: 200 Pound Farm Cannon
Thanks for your help! I was considering filling with a 1/4" compressor hose, plumbed with a quick disconnect and a ball valve. My previous guns/cannons were fitted with shrader valves. My only concern is that a shrader valve would be quite slow to fill a 8" by 30" chamber. However, if you suggest that it's critical to have a tight seal on the piston, without any holes drilled in it, then I may just need to accept the fact that it'll take a few minutes to fill.
I have lots of types/thicknesses/densities of rubber to choose from around the shop. I should be able to find good pieces for the piston face seal and piston bumper.
I'll keep cracking away at this project.
Well I've finished the work on the stainless piston housing and acetal (Delrin) piston. It's a slip fit, no wobble, no resistance either. However, it's such an exact fit that it can hold water like a cup without leaking. Air may pass through but it would be slow. Since I made two pistons (because...why not), I can stick one into each end of the pipe, and I can't push the two together. The air pressure between the two rods prevents that.
I would still like to consider drilling a small hole near the edge of the piston which would allow air to pass through to the chamber. It would be very small, and could even have some sort of rubber flap check valve in it to prevent back flow during firing.
I was actaully contemplating the piston air flow issue with cannons, I was thinking that instead of drilling the piston for airflow make it entirely tight like you have and just plumb a small check valve externally from the rear of the piston housing to the chamber. I was thinking purposely picking one with a high spring pressure such that the piston will be properly seated before the air begins flowing into the chamber. Just a thought.
Yes! The same idea came to me yesterday. Although I hadn't thought of using a high-pressure-cracking check valve. That'd be perfect for seating the piston first. I was thinking of using a small ball valve at first. Thanks!
I had to take off a couple mils on the piston. Even though acetal swells less than nylon, the tolerance is so small that it got stuck in the piston housing at 80 degrees in the seat of my truck. Should be okay now. Assembly time! I'll have a pretty imgur album up in a couple weeks if all goes well.
I machined the piston down a bit. It's still a really tight fit, but air flow past the piston should be sufficient to fill the chamber in good time. I've begun assembly, but am waiting for some stainless steel welding wire to build the flange rear end. Should have this thing wrapped up in a few weeks!
Also, would it be proper to make a new post with the link to the imgur album of the finished product, or just attach the album link in a comment here?
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