This project is also documented on my website at: http://www.mbeckler.org/pneumatic_cannon/
Step 1: Safety Warning!
If you only remember one thing from all my safety warnings, remember this: PVC shrapnel will not show up on an X-Ray. The doctors will have an extremely hard time picking all the plastic shards out of your body if something goes wrong. You have been warned.
organicengines adds a helpful reminder to keep your PVC out of direct sunlight, because the light will degrade the plastic and make it brittle. Be careful!
Yes, this could be a dangerous project. I'm not a lawyer, but I'd like to say that I'm absolving myself of all responsibility for your actions. If you decided to build one of these cannons, by all means have fun, but please be safe. I'm not responsible for anything except for fun. If any of the Instructables administrators or moderators feel this project is too dangerous for this website, I will gladly remove it as soon as possible.
Another safety point: This is a cannon, which is a type of gun. Guns are meant to be dangerous, they would be pointless if they weren't. If you have no experience handling guns, please be very careful with this project, and do yourself a favor and take a firearms safety course. It will be a good experience even if you never shoot a gun again. If you have experience with guns, please be careful regardless. When I went through hunter's safety, they taught us three important rules of firearms safety:
1. Treat every gun as if it is loaded, so always point it in a safe direction. This means not at you, your dog, or anybody standing around. Point it up in the air, or down at the ground.
2. Always keep your finger off the trigger (valve switch) until ready to fire. Enough said.
3. Know your target and what's beyond. Plywood, fences, and garage walls are not strong enough to stop some of the things you can shoot with this cannon. I've put clean holes through both walls of an empty 55 gallon steel drum, so know what's behind your target. Try to not shoot this cannon in residential areas, or use really low pressures and soft projectiles. Soft projectiles include rolled-up, wet socks (more on this later), water, and loose snow.
That being said, this cannon can provide hours of entertainment, as well as a bit of education, in projectile physics and the ideal gas law.
Step 2: Planning
As is customary for my projects, I've included a diagram of the project. Hopefully it helps you to understand what I've done. Here, I've included both the 1.5" and 0.5" barrels. They are interchangeable, by unscrewing them from the valve, and attaching another barrel. It is important to note that while I have placed specific couplings and bushings in the diagram, the important thing is to find what works for you. If you can find an easier/cheaper way to convert a 1.5" pipe to 1" male threads, by all means go for it. I built with what I had, or could get from the local home improvement store. Some home improvement stores won't carry all the pieces you need, so you might have to search elsewhere. I personally have had good experiences with http://www.plumbingsupply.com for finding exotic parts.
Safety Alert!!! An astute reader ( bigbob12345 ) pointed out that most bell-reducer couplers are NOT PRESSURE RATED. Please go read this page on the spudfile wiki. It is suggested that instead of using the bell reducer, that you simply use a series of regular pressure-rated bushings to connect the 3" pipe to the 1" pipe.
A note about length of pipes: The lengths of the main pressure chamber and barrels are up to you. The first cannon I ever built I used 4' pipes for both, which worked quite well, but was a bit heavy and a bit long. That cannon was broken years ago, so when I rebuilt this new cannon (that I am documenting here), I've chosen to use shorter pipes. I used a 2' pressure chamber, and cut the barrels to be about 4" longer than the end of the pressure chamber, when both are connected to the valve.
About the Valve: The brand of in-ground sprinkler system at my local hardware store was Orbit. The specific valve I used is an Orbit WaterMaster 1" Automatic Anti-Siphon Valve, with standard 1" threaded fittings, 24 Volt solenoid, and 125 psi maximum pressure. Product Webpage The anti-siphon aspect doesn't matter to me, but I happen to like the U-shape of the valve, which allows for a compact, easy to carry/hold/shoot cannon.
About the tire valve stem: I picked up a two-pack of replacement tire valve stems from the local auto parts store. Be sure to get generic valve stems, not anything specific to a particular brand of tire. You want to be able to drill a hole in the endcap, slide the valve stem through, and tighten it down airtight. The particular tire valve stems I purchased had rubber gaskets on the inside, which really help make a good seal. Another advantage to the standard tire valve stem, is that nearly every air compressor has an attachment to fill tires, so you shouldn't need any additional hardware for your compressor.
About the pipe: The pipe should be regular Schedule 40. My 3" pipe is rated for 260 PSI, the 1.5" pipe is rated for 330 PSI, and the 0.5" is rated for 600 PSI. The valve can only handle up to 125 PSI, and since I want at least a small safety margin, I'm only going to run this cannon at up to 100 PSI. This gives me a comfortable safety rating on all the PVC involved, and I have never had one of these valves fail on me, regardless of pressure.
The first image is my plan for the cannon. I hope you can understand what i'm trying to indicate. The drawings took me way too much time.
The second image is a picture of the sprinkler valve's box.
The third image is a blown-apart view of the actual fittings for the pressure chamber side of the cannon.
The fourth image is a blown-apart view of the adapters for the 1.5" barrel.
Step 3: Cutting and Sanding
I heard somewhere that you should rough-up the bonding surfaces with sandpaper to help increase bond strength. I used regular sandpaper to rough-up both sides of the bond. You'll want to get a wet paper towel to clean up the PVC dust, which probably causes lung cancer, so don't breathe it in. Then get a dry paper towel to dry off the fittings.
Step 4: Gluing the PVC
The glue really stinks, is very flammable, and will cause numerous health defects and possibly death merely by working with it. Not really, but its nasty stuff. As you can see in the first image, I'm working outside, because the glue causes headaches and cancer (not really), on my dad's planting bench. I've put down an old piece of plywood, which I covered with about 10 sheets of newspaper. You're going to spill the primer and glue, and you don't want to mess up something important.
You will probably also want to have a large grip pliers, such as the channel-lock pliers I have here. They will help you grip pipes and fittings, and it's much easier to wipe glue off the pliers than your fingers. I'll point out that when you are test fitting your fittings together, they will feel really tight, like you can't get them in all the way. When you get the primer and glue on the pieces, they will slide together like butter. Don't worry about that, but be sure not to cut your pipe too short, as you'll be able to slide it in further with the glue on it than you could while it was dry.
You might not want to glue all the fittings, as someday you might want to get them apart. In the case of breakage, you'll appreciate having a modular design, so you can replace just the broken parts. My first cannon fell off a table at a science fair, breaking the connections to the valve, which was glued on. The valve is perfectly fine, sitting in the corner of my room on the floor, but with broken off connections glued into it, it's pretty much worthless to me now.
As to the actual gluing process, here's what I do. I've received no formal training on PVC gluing, but this is what seemed to work. Don't forget to sand your connections as per the previous step. I take the purple primer brush and wipe it all around the connection area, where the bond will be made. Do this on both parts to be glued. Wait a few seconds, then take the actual glue. Spread it over the same area as the primer, and quickly set the brush back in the can. Take both pieces, and slide them together. It's pretty cool how easily they fit together when properly lubricated. Take your paper towel or rag and wipe off any excess you see, be sure to check inside, too.
As for the drying / curing time, I have always left the pieces for at least overnight, sometimes 24 hours. For this particular project, I left the pieces on the bench (they smelled bad) overnight until about 12:00 the next morning.
Step 5: Assembly
Step 6: Pressure Test
Step 7: Firing Circuit
Some people like to have a rocker switch as a safety, and then use a pushbutton switch for firing. What you do is up to you.
Step 8: Ammo, Firing, and Results
Filling your barrel with water right from the garden hose is also quite fun, good in squirt-gun fights. Of course, try to keep the barrel mostly upright.
In the first picture, you can see my brother preparing to ram a wet sock projectile down the 1.5" barrel. We used the other barrel, the 0.5" barrel, as a ram-rod, although nearly anything long and thin will work.
Video One: Here, my brother and I are shooting one of the aforementioned wet socks using the 1.5" barrel. We are standing behind our garage and shooting into the neighbors yard. The sock landed by the small shrub that you can see in the video. The cannon was pressurized to about 40 psi.
Video Two: Here, my brother and I are shooting small plastic beads with the 0.5" barrel. Since the beads were smaller than the inside diameter of the barrel, and didn't form an air-tight fit, we used a small piece of paper towel as wadding. You can see the paper towel floating down to the ground, a few feet past the barrel. The beads all landed near the same shrub the sock landed by.
Bolts: My brother discovered that a small bolt fit nicely in the 0.5" barrel. He shot it into a piece of plywood. The bolt drove through the wood about 0.25 inches. Please don't shoot bolts at people or houses.