Since man first discovered compressed air, he's been shooting stuff with it. How can I prove this? I can't. Let's just call it "male intuition". In this instructable, I am going to show you how to make your very own boomstick, or for you scientific types, a projectile velocity enhancer do-hickey. This hand held pneumatic cannon has a one inch (inner diameter) barrel that is perfect for shooting things like super bouncy balls or those cylindrical ice cubes you get in the one pound bags at the gas station.
Step 1: Safety First
Compressed air is dangerous. Period. No two ways about it. Anytime you compress a gas it really really wants to escape, and will find any opportunity to do so. This includes holes, cracks, or thin walls. In this project we will be compressing air inside a PVC tube that has the potential to break or crack and turn into a plastic grenade. ALL the PVC you use to make this cannon MUST be Schedule 40 PVC and make sure you look at the pressure test data printed on the side of the pipe. DO NOT use the core filled stuff. If you look at the cross section of a piece of PVC and you can see a yellowish material sandwiched between two thin white walls, put it back and find schedule 40 pipe. The core filled stuff will not hold the pressures we are going to use for this cannon. And remember, you are building just that, a cannon. You MUST treat this thing like it's a loaded weapon. Never point it at anything you don't intend to shoot. Only charge it when you intend to fire it, and always ensure bystanders stay at a safe distance. If you do not have some experience with high velocity projectiles, you may want to get someone who does to help you out with this one. This includes you young builders. This is definitely a project for you AND your parental unit or guardian.
Step 2: Gather Your Materials
Everything you need to make a pneumatic cannon can be found at you local big box hardware store or the auto parts store.
Here are the things you will need:
2 ft. of 2 in. PVC pipe
3.5 ft. of 1 in. PVC pipe
1 - 2 in. slip PVC cap (the type that goes over the end, not the one that goes into the pipe)
1 - 2 in. female to 2 in. female PVC connector
1 - 2 in. male slip to 1 in. female slip PVC bushing
1 - 1 in. slip to 1 in. slip to 1 in. slip PVC T connector
1 - 1 in. male slip to 1/2 in. female threaded PVC bushing
1 - PVC ball valve with 1 in. female threaded ends
2 - 1 in. male threaded to 1 in. female slip connector
1 - brass 1/2 in. male threaded to 1/4 in. female threaded bushing
1 - 200 psi 1/4 in. gauge
1 - schrader valve (the kind that is on you car tire. I got mine from an auto parts store)
Drill bit big enough to accommodate your valve stem
Saw (I used a miter saw, but you can use a hacksaw or whatever you have)
A couple of small adjustable wrenches
Sand paper (anything will do as long as it's not TOO high or low grit)
Some cloth or paper towels
PVC cement (you can get the primer and cement as a set and they both have brushes attached)
JB Weld (you should always keep this stuff handy, it works miracles)
A source of compressed air (I used my compressor, but a regular bike tire pump worked also)
All said and done this project cost me about 27 bucks and two work sessions of around an hour a piece. I already had the PVC cement and primer, the Teflon tape, the JB Weld, and all the tools.
Step 3: Valve Placement
Drill a hole in the 2 inch cap. I used a 3/4 inch paddle bit, but whatever you have that is the right size will do. The hole should be big enough to accommodate the stem of the valve you have chosen. Valves will differ depending on where you get them and what style you choose. Just make sure you get on that can not be pushed through the hole in the PVC cap when the chamber is pressurized. For stability and to prevent any air from seeping out around the valve, I JB Welded the inside if the valve and cap. Also, if someone sets the cannon down on the valve, this will prevent it from being pushed through into the chamber. JB Weld is great stuff and if you choose to go this route, once it's dry, you will never EVER get the two pieces separated short of using some TNT. You could probably just as effectively use epoxy or some other sealant, so if you have some lying around, try that.
I didn't let my JB Weld dry long enough before assembling and testing the cannon and my valve did exactly what I was trying to avoid. It pushed through. I know, bonehead move. I was able to fish it out, hold it in place, pressurize the chamber, let it re-seat, and JB weld the outside. This time I let it dry for two days and have had no problems.
Step 4: Prime, Glue, and Assemble the Pressure Chamber
Lightly sand both ends of the 2 ft 2 inch PVC pipe to give the primer and glue something to hold onto. Don't go crazy, just rough it up a little. Clean the ends with a clean rag. Any junk left on the ends could jeopardize the air seal. Repeat this for the cap (after the J Weld has dried) and the 2 inch to 2 inch connector. Apply the primer (the purple stuff) to the pipe, the cap, and the connector and let it sit for about 10 or 15 minutes until it looks dry. Then apply the cement in the same fashion, making sure in both cases to completely cover all the surfaces that will be joined. Once you have applied the cement, slide your pieces together making sure they seat completely down onto one another. You shouldn't need to use a mallet or a vise, the cement makes the pieces slide quite easily together. When you push the pieces together some smooshage will occur. Yes, smooshage. You all know what I mean. Wipe up the smooshed out cement with a paper towel or rag to keep everything looking attractive.
Step 5: Cut the 3 Ft. Barrel and Connection Pieces
Measure out three feet of 1 inch PVC pipe for the barrel and cut it using your saw of choice. Make sure you don't cut off one of your little piggies in the process. Saws are dangerous. Then measure and cut 2 3 inch pieces to be used as connectors. Remember: measure, cut, measure, cut, measure, cut. Don't measure all your pieces at the same time then cut them all at once. They won't come out to the right length. I know this is intuitive to most of you, but I'm trying to be all-inclusive.
Gather together the pieces that will make up the gauge/valve assembly (see first picture) and sand, clean, and prime them. Apply the cement as you put the pieces together remembering to cement both sides of all the joints. Make sure not to get any smooshage in the threaded parts, as this with prevent you from screwing on the valve later. Wipe all smooshage just like before. Refer to the pictures for an assembly reference. At this point you will also want to attach the other threaded piece to one end of the barrel.
Step 7: Assemble Chamber and Gauge/Valve Assembly
Sand, clean, and prime the 2 inch male side of the 2 inch to 1 inch bushing on the gauge/valve assembly and the inside of the 2 inch connector on the pressure chamber. Cement them together. You will now have two pieces that look something like the third picture. Now that all the cementing and smooshage cleaning is done, wait for everything to dry. Go find something to do to take your mind off how badly you can't wait to shoot your rapidly coming together boomstick. I was fortunate enough, at this point, to have to go to work at my job at "the biggest toy store there is" so you should go keep yourself busy and give this about 6 or 7 hours to dry, preferably over night.
Step 8: Tape Those Threads
Now that everything is nice and dry, use the Teflon tape to wrap all your threads, both brass and PVC. Don't go super thick on the tape. Just wrap each piece a couple of times. The tape will help keep everything together and provide an air tight seal. Using two wrenches, tighten the gauge and the brass bushing together (see second picture). Don't go crazy tightening them down. Snug them up and then give them one extra squeeze, that's as tight as they need to be.
Step 9: Screw Everything Together
Now it's time to put it all together. Screw the brass bushing into the valve/gauge assembly. DO NOT tighten it down to much! If you crank down on the PVC it was crack, and then you are out of business. Just snug it up taking care to face the gauge back toward the pressure chamber so that it can be read during pressurizing and firing. Decide where you want the valve handle to be so you can turn it comfortably. I'm left handed, so I put it on the left. Screw the valve into the valve/gauge assembly until it's tight and then orient the handle. Finally, screw your barrel into the output side out the valve. Again, careful not to tight or you will crack the PVC. And there you have it. Wipe the smooshage from your mouth and charge it up. I recommend charging it up to about 10 psi and make sure everything holds for about a minute. Every minute or so add 10 psi and move away. If you get up to 40 psi and everything looks and sounds good, stop and let it sit for 10 minutes or so. After waiting, check the gauge and make sure it is roughly at the same psi you left it. If you've lost anymore that a few psi, you have a leak some where. Luckily, I didn't, but if you do, discharge the air using the schrader valve and address your leak. Once you are sure the cannon is air tight, load it (you might want to use a bit of rag on your first shot to be safe), charge it to a low pressure, and see if it shoots. Have fun with it, but remember, don't be a knucklehead. This bad boy can cause serious damage to person and property.
Step 10: Final Thoughts
As you may have already read in my note in step 3, you should wait until your JB Weld (or epoxy) is ALL THE WAY dry before testing the cannon. It was a pain in the neck to go back and fix it after I punched the valve through the hole in the chamber. Also, after a couple of shots, I realized this thing has the potential to be REALLY dangerous. I spent 5 years in the Marine Corps. shooting all kind of weapons, and this thing definitely has the ability to seriously hurt, or even kill, someone. Never ever ever ever shoot anyone with this cannon. Ever. Look at the picture below and imagine if that 2X4 was your arm or leg. That was a shot at 60 psi from about 2 feet away. Don't be stupid like me and load a screw driver into your cannon. That was a dumb move, and the result was quite frightening. The next pictures are of a cannon a buddy and I made for a trajectory and momentum project we put together for my Calculus 2 class that shoots baseballs and lacrosse balls. It turned out really well (the project received an A). It has a 3 inch barrel, a 4 inch pressure chamber, and uses a 24V sprinkler valve. I rigged up a trigger with a safety switch that utilizes about 10 feet of wire as a standoff safety precaution. If you have any questions about the boomstick or the bigger cannon, let me know.