Introduction: Launch Supersonic Spuds With a PVC Air Cannon!
Have you ever felt a desire to launch a potato into the sky at a high speed? Or maybe on the colder side, be a one man army in every snowball war with a pneumatic snowball launcher? Yes? No? Maybe? Well, in this tutorial I am going to show you how to do just that, with your very own air cannon! Before we get started though, I have to add a disclaimer:
Disclaimer: I am not responsible for any injuries related to or having to do with this device. I am also not responsible for any property damage caused by this device. Use at your own risk!
~ A hacksaw, or other saw to cut the PVC pipe
~ PVC primer and cement, to fasten the PVC together
~ A drill and assorted drill bits, to make the various holes for the fill valve, pressure gauge, etc.
~ A tapping set, for making the threads in the holes we will drill
~ A Dremel, or other rotary drill/tool and a sanding drum, to cut the muzzle cutter
~ An adjustable wrench, or an assortment of regular wrenches for tightening the various fittings on the sprinkler valve
~ A bike pump or air compressor (air compressor highly recommended!), to fill the air tank with...surprise! Air!
~ A pair of Vise-Grips, helpful for tightening different fittings, but also for holding the larger tapping cutter.
~ Teflon tape, for sealing all threaded connections
~ Epoxy, I used 5 minute epoxy, we will use it to seal up parts of the sprinkler valve
For the air tank-
~ 16" length of 4" PVC pipe (schedule 40 works great for all PVC parts)
~ 4" PVC end cap
~ 4" to 4" female-to-female PVC coupling
~ 4" to 2" PVC reducer
~ 2" to 1 1/2" PVC bushing
~ 1 1/2" to 1" female threaded PVC reducer/adapter
~ 0-100 psi pressure gauge
~ tire fill valve, like you would find on a car or bike tire
For the sprinkler valve-
~ sprinkler valve (duh!). I got mine from Amazon since I couldn't find one at any nearby hardware stores
~ 1/2" male-to-male compressor fitting
~ blowgun (the kind used to blow dust off things) with a 1/2" opening
For the U-connector-
~ 2 2" 90 degree PVC street connectors/elbows
~ 1 1/2" to 1" female threaded PVC reducer/adapter
~ 2" to 1 1/2" PVC bushing
~ 2" to 2" female threaded adapter
For the barrel-
~ 4' length of 2" PVC pipe
~ 2" to 2" male threaded PVC adapter
That's it for materials, so let's get started!
Step 1: Assembling the Air Tank
(air reservoir, if you want to be really technical)
So that you may better understand the way the parts fit together, I have made an exploded view of the air tank.
We will be working from the back of the air tank to the front, starting with the fill valve assembly, so first things first, we have to drill the hole for the fill valve. The final hole size will be 7/16", but to get a clean hole we will drill in stages. Before we start drilling the holes, we need to make a small indentation so the drill bit doesn't skate across the PVC. After you make the indent, use a 1/8" drill bit to bore out a small hole. Next, I used an 11/32" bit to enlarge the hole.
Our next drilled hole will be the final size of 7/16". I had a bit of trouble (I'm so phunny) with the bit grabbing the plastic too hard. Just go slow, and don't push on the drill too much, and you should be fine. I used a countersink bit to make the valve fit perfectly into into the hole.
After putting it into the hole, screw the washer and nut (in that order) onto the threads on the valve.
Now we have to glue the end cap onto the pipe, so get out the PVC primer and cement. First, use the primer to swab the inside of the end cap and the outside of the pipe. Do the same with the cement, then push the two pieces together and rotate the end cap on the pipe a quarter turn.
Next, we need to drill and tap a hole for the pressure gauge. The overall width of the threaded end of the gauge is 1/2", but the hole we must drill has to be slightly smaller to accomodate the threads we will tap, so use a 7/16" drill bit to bore the hole.
After drilling the hole, I used a 1/4"-18 NPT tapping drill to cut the threads into the air tank. I did not have a tapping wrench big enough to fit the tapping bit into, so I used (carefully) the Vise-Grips to hold it. The proper way to tap a hole is to turn the bit clockwise, then back slightly counterclockwise. Continue this pattern until the bit has traveled all the way through the material, then unscrew the bit.
Now we need to screw the pressure gauge into our newly tapped hole. Take the teflon tape, wrap a few turns onto the threads of the gauge and start screwing it into the hole. Once it gets hard to turn, use a wrench to tighten it as much as possible; we don't want any air leakage.
Great, we're almost done!
Now get the 4" to 4" coupler, because we are going to glue it onto our pipe (with the end cap, pressure gauge and fill valve already attached). Glue it on following the same procedure as before (swab both with primer, swab both with cement, push together and rotate a quarter turn). After the coupling comes the 4" to 2" reducer. Glue it onto the coupler. Next, put on the 2" to 1 1/2" reducer, followed by the 1 1/2" to 1" threaded reducer. You know the drill...
After you complete this, congratulations! You have completed one of the three main components of the air cannon! Next up, the sprinkler valve...
Step 2: Modifying the Sprinkler Valve
Now, we could leave the sprinkler valve as-is, and trigger it electronically, but who wants to do that when we could modify it to open ten times faster? It all comes down to performance. In order to work properly, the valve needs to dump all of the air stored in the air tank into the barrel as fast as possible. This is why an ordinary valve operated by hand won't work; a human simply cannot open it fast enough, so we use a sprinkler valve normally triggered electronically. We are going to modify it to work pneumatically, however, since it will open faster that way. I won't go over how the valve works, but if you want to find out how it works you can find a diagram and explanation of one here. Let's get started...
I used an Orbit sprinkler valve for my build, which seems to be a popular choice for many air cannon enthusiasts. Unfortunately, I couldn't find one at any nearby hardware stores, so Amazon to the rescue! This is the valve I bought, and no, it is not actually that color green.
First things first: get rid of the solenoid. It's the black cylinder screwed into the top of the sprinkler valve. Then you need to remove the cover of the sprinkler valve, so unscrew all of the screws that go around the edge of the top. Once you have that off, put the screws in a safe place; we don't want to lose them. You can also move the whole bottom assembly aside, since we are only interested in the top. When I took the top off, the diaphragm assembly was stuck to the top. Carefully peel it off the top and set it onto the bottom assembly, making sure not to get anything on it.
Now that we have the top separated from the diaphragm, we need to drill a hole in the center of the top to fit in our male-to-male compressor fitting, so we need to get our 7/16" bit (I know it seems like this is the only bit we use, but the pressure gauge and the male-to-male fitting are both 1/2" compressor fittings, so it makes sense that they would use the same bit). Make a mark as well as you can in the center of the sprinkler valve, but do it from the inside since we need to make sure it doesn't get in the way of any of the parts. Use the center punch to make an indent in the plastic, and then drill out the hole with the 7/16" drill bit. Tap the hole as before, using the 1/4"-18 NPT thread tap.
In order to make the valve pneumatically operated, we need to epoxy the holes that normally connect to the solenoid, which controls the valve. Mix up the epoxy, but keep in mind that you only need a tiny bit because the holes we need to block are really small. Then cover both of the holes in the solenoid's socket, one which is right in the center, and another that looks like a slot positioned in the side of the socket. On the inside of the lid/top, there is another two holes, one of which is on the end of a black plastic piece, and one that is square-shaped (this one will require the most epoxy). These two are the most important, so you could probably get away with not gluing the holes in the socket, but just to be on the safe side I am gluing all of them.
Take some Teflon tape and wrap it around both sides of the male-to-male fitting, and screw it into the tapped hole. Be careful that you don't screw it in too far, or the sprinkler valve won't function correctly.
Screw the blowgun onto the compressor fitting, making sure to use Teflon tape. Tighten it, and you're done!
Great job, you're two thirds done! Get out your primer and cement because we need to do a bit of gluing in the next step...
Step 3: Making the U-connector
Okay, so the U-connector will connect the air tank and sprinkler valve to the barrel. The first thing we have to do is glue the two 2" 90 degree street fittings together to make the base "U" shape. After that, glue the 2" to 1 1/2" reducer onto one side of the U, followed by the 1 1/2" to 1" threaded reducer. On the other side of the U, glue on the 2" to 2" threaded coupling.
Okay, I know this step was really short, but only one more until we get to assemble the components and have a finished air cannon! Next, we have to make the most important part: the barrel...
Step 4: The Barrel
So, now we have to make the barrel. I won't go into detail on this part; you know what it's for. You what? Really? Oh, okay. The barrel is what we will put whatever we want to shoot into. Snowballs, spuds, T-shirts, you name it. Are we all caught up? Good. Now, the barrel is the simplest part of the cannon, but the most important too. Cut a 4' long piece of 2" PVC pipe and get out the Dremel (or other rotary tool), because we need to make the muzzle cutter. The muzzle cutter is the section of the barrel that cuts the excess potato or snowball as you load it into the cannon. Put a sanding bit on the Dremel, and set it to the lowest speed to avoid burning or melting the plastic. Now, start sanding around the end of the barrel. You may need to travel around the barrel multiple times to get the angle right, but don't worry too much about it; as long as it cuts the potato it doesn't have to be too sharp. I wanted to up the awesome factor, so I used the sanding bit to cut notches into the cutter, which will look almost like a serrated knife. I used a coarse sanding bit, so I went over the entire muzzle cutter with a piece of regular sandpaper afterwords. That's it for the muzzle, now we just have to attach the threaded adapter. Get out the primer and cement, and cement the adapter to the other end of the barrel. This will enable interchangeable barrels in the future (if you want), but it mostly helps with assembling the cannon.
Now that you're done with the barrel, let's move on to the final stage (before the test fire!), assembling the cannon!
Step 5: Assembling the Air Cannon
Finally! The time when we assemble everything together and see if it works (which it will)!
First, take the air tank and, after wrapping Teflon tape around it, screw the 1" male-to-male threaded coupling into the output of the air tank. I carefully used pliers to make sure it was screwed in tight, then I took the sprinkler valve and screwed it onto the coupling with the air tank attached to it (remember the Teflon!). Keep in mind however, that there are arrows on each side of the valve that tells the direction of airflow. Screw it on so that the arrows point away from the air tank. You should be able to just tighten the sprinkler valve by hand, but when you tighten it, make sure that in the end the blowgun lines up with the pressure gauge on the air tank.
Next, screw the other threaded coupling into the opposite end of the sprinkler valve (again, remember the Teflon!). Get the U-connector and begin screwing it onto the threaded coupling (Teflon!). Now, since the barrel is going to screw into the U-connector, we have to line it up carefully so that the barrel will run alongside the pressure gauge and the blowgun. Look at the pictures above for a better explanation, but you'll understand once we put the barrel on.
Finally, screw the barrel into the U-connector (remembering to use...guess what? Teflon tape! Yeah!).
Congratulations! You have finished the air cannon! Give yourself a good pat on the back; the hard part is over. Next, we have to take a test fire!
Step 6: 3...2...1...Fire!!!
Time for a test fire!
Air cannon operation instructions
1. Load the air cannon. Use a broomstick to push whatever your ammo is down into the barrel. Make sure it gets to within an inch (about) of the U-connector, but don't push it into the U-connector.
2. Pressurize the air cannon. Use an air compressor or bicycle pump to fill the air tank to about 80 psi. I took test shots starting at 20 psi and increased the pressure by 20 psi each shot, until I got to 80 psi. You may hear a slight hissing noise as you fill the air tank, but this is normal; the sprinkler valve has to equalize the pressure between its chambers. The hissing should stop after reaching about 50 psi, if it doesn't however, try to find any leaks. An easy way to do this is to get some soapy water and spray it onto the suspect area. If there is air leaking there, you will see bubbles from the soapy water. I wouldn't go above 100 psi; although PVC is commonly used for air cannons, it is not meant for very high pressures. For snowballs, use low pressures (20-40 psi); when I tested shooting a snowball at 50 psi, it came out as a cloud of snow. Also, I wrapped the air tank in duct tape to protect it and make it a little stronger in cold weather.
3. Fire! Find a safe area outside, and squeeze the blowgun's trigger to release all of the air and propel the projectile out of the barrel!