first air cannon

[dotd]

I got bored a couple days ago and decided to make an air gun out of the blue. A buddy of mine made a combustion cannon in high school, but I figured I wanted something with a bit more punch. So my cannon would be pneumatic.

I'm a tad leary of using pvc to hold compressed air, so I looked around a bit and figured I would never use an old hydraulic cylinder I had. Thus, it became the air chamber. I welded a 1.5" pipe nipple on the end and screwed a ball valve onto it. Screw a second short pipe nipple into the "out" side of the valve, and duct tape a 2" pvc barrel on. (I'll put it on proper at a later time.) Voila, air cannon.

The cylinder is 4" in diameter and about 24" long. The air volume is a tad over 300 cubic inches. Not as much as I'd like, but not bad for stuff found in the tool shed. The valve is a plain 1.5" ball valve, rated 600 psi. It's pretty easy to turn, but not nearly fast enough to fully open before the projectile is long gone.

The only valve that would be quicker would be a butterfly valve, but I don't want to lay down the cash for one ($25+ for 1.5" from mcmaster.com). I've seen the designs people have come up with for quick exhaust systems, or other things with diaphragms and pistons and such, but they all seem like they would be more troublesome in actual use than practical for having a day with the boys and a bunch of junk that just barely fits in the barrel... ;D Anything beyond, "Hit the air compressor switch! Now pull the lever!" loses the fun value.

However, I would like to see what kind of power this thing is capable of, so a better valve is needed. I'd like to keep it idiot-proof to use, and (hopefully) cheap, however, I'm a performance-oriented kinda guy, so I'm willing to make it as complex as necessary. Any ideas for a valve come to mind? Or other suggestions?

Pictures:


The cylinder was originally used on a scissor platform for the US Postal Service. It lifted something like 5000 pounds. All the modification I did to the cylinder was remove the piston and weld the pipe nipple to the end cap.


Yes, that's the ugliest weld I've ever seen, too, but it doesn't leak. My welder is a small 115v jobbie, so I had to do a couple passes. (still no excuse for the ugliness)

[Lukas]

Jee, thats a heck of a gun! I'd suggest a piston valve myself, but I don't know what sort of pressure you want, and whether you could make one out of what you have. If you go that route, you can use the ball-valve as the pilot for the piston. That will save money. I really don't recommend sprinkler valves, even as popular as they are, b/c of the airflow restriction, especialy for something that size.

[dotd]

I consider the standard piston valve too much work to use. Actual construction of it doesn't matter, that wouldn't be a problem. But if I want to go out for two hours with some buddies, I don't want to have to think about the inside of the thing the entire time. A gun should be reliable, right?

I was thinking about spending more than 20 minutes on another cannon. Take a 20 lb propane tank (never been filled), weld a 3" ID tube through the center with a ring of large holes to vent, and put a piston with O rings on each end in the tube. This seals the air holes, and you fire by yanking on a rope tied to the piston (or something like that). This only works if the sliding piston is really light, and doesn't have a good deal of friction.

Or I could just put a butterfly valve on this one and call it done. It works pretty well as-is.

[Lukas]

Hey, who says pistons are unreliable? Mine is awesome, now that I got the bugs worked out (give me a break, it was my first pneumatic and I designed it totally myself). Where'd you get the impression that they aren't?

Hmm, interesting idea. I'd thought of something like that, a valve that has balanced force on each end until you pull it far enough to open. I guess that would be classified as a chamber-sealing piston?

[dotd]

Unreliable in one person's mind may be perfectly fine in another's. I've read a few forums, and I've heard people talking about how their piston broke, or how they had to spend a full day stopping a leak, or something along those lines.

I could have made my cannon with a typical barrel sealing piston, etc. However, making it and making sure it works is more work than I want to put in, especially since I would have (and want) to do a bit more than glue a few pieces of plumbing together. PVC is out of the question for me, just a bit too dangerous considering how roughly I treat things.

The description I gave of a sliding piston is basically how I understood how a cannon called Big Red works (home.vlnx.net/~dthames/spudguns.htm, but the site appears to have been forgotten and most pages are missing), except his piston was pneumatically actuated. Again, more work than I want to put into it. Although, his cannon was extremely impressive and well built.

After playing a little more with the cannon I just made, I doubt I'd want to spend the cash on a tank, pipe, and various bits of steel. If I do anything else air cannon related, it would be putting a butterfly valve on this one, although I doubt I really need it because the ball valve with a ripcord on the handle is quite impressive as is. I feel that it being able to put a broom handle 3 feet into a bale of straw (shooting at it the "long way") is more than equivalent to shooting through a person.

[blowemupspudguy]

THAT IS THE BEST CHAMBER I HAVE EVER SEEN!!! im gonna get one...to bad it prolly weighs more than i do

[dotd]

blowemupspudguy said:

THAT IS THE BEST CHAMBER I HAVE EVER SEEN!!! im gonna get one...to bad it prolly weighs more than i do

It weighs about 25 pounds by itself. Unless you can find an old piece of junk cylinder like I had, you'd have to pay at least $100 for a new cylinder. For that much, you could probably come up with something much better. And unless you have access to a welder, I don't know of a generally easy way you could attach a valve or barrel unless you took it to a machine shop and had them cut some threads in it for you, which would also be pretty expensive.

[blowemupspudguy]

i can find a cylinder, and i do have a welder, so i could do it. it would be nice to have a better valve. i might try the chamber with an old tractor cylinder.

[sgort87]

I'd say weld a huge piston valve right to the stinkin air compressor tank then put the barrel on that. You know it will hold the pressure cuz its made for it. It's also usually rounded more than normal chambers which means a better air release speed. Just make sure you don't screw up the weld.