Friday, October 27, 2017

PVC Air Cannon with Sprinkler valve




I've been wanting to build a simple, electrically operated air gun for quite some time now. There are a number of cool things you can do with them (remote detonation and sentry gun, to name a couple), not even counting the ease of use that a simple electric trigger has over a rubber powered ball valve (as on my air rifle), which has to be reset after each shot.

But I already had a rifle. Sure, it doesn't have an electric valve, but still. I don't need another one.

Yet.

This moment anyway.

So logically, I built a cannon. (What else?)





Parts and Materials:

- 2" PVC pipe (two foot length) (all PVC parts are schedule 40)

- A short length of 3/4" PVC pipe

- 2" end cap

- 2" straight coupling

- A fitting that goes from the 2" straight coupling to the 3/4" threaded fitting. I couldn't find a fitting that went down to 3/4" directly, so I had to use several fittings to step down.

- 3 (or 4?) 3/4" male threaded couplings.

- 3/4" Sprinkler Valve

Here is one similar to what I'm using: https://www.amazon.com/57280-Automatic-Inline-Control-Valve/dp/B00004S1V2/ref=sr_1_12?ie=UTF8&qid=1508190041&sr=8-12&keywords=sprinkler+valve

- Switch. You could use just about any kind of switch, I went with a simple rocker switch.

Here are some just like mine: https://www.amazon.com/FBApayipa-Solder-Rocker-Switch-Toggle/dp/B01N2U8PK0/ref=sr_1_7?ie=UTF8&qid=1508347007&sr=8-7&keywords=rocker+switch

https://www.amazon.com/Gardner-Bender-GSW-18-Heavy-Duty-Replacement/dp/B00004WLKC/ref=pd_sim_60_2?_encoding=UTF8&psc=1&refRID=644JPEKGPPR324ZQV2PA

- Bike tire valve (I cut mine off of an old inner tube)

- PVC cement and primer

- Thread sealing tape

And I think that's about everything.

Sorry if my parts list is rather foggy, at the store I'm really not thinking of individual parts; I'm more just throwing the stuff that fits together in a bag and calling it good. 

Really though, you can't get any simpler than this cannon design. I'm sure you can figure it out. After all; where there's a will, there's a way. 

Better hope you know somebody named Will..........

Here is a diagram of how the sprinkler valve works, for those who want to understand it better. I explained it a bit in the video, if that helps. I always like to study and understand what I'm working with.




Step 1: Design

My design for this cannon was basically the simplest one I could come up with. Literally tank + valve + barrel. Nothing fancy here. One of my goals for this project was to make the simplest air cannon I could. Not sure why. 

The main thing that might vary on your cannon is the barrel and how you attach it. I designed this gun so I could use interchangeable barrels, which is a very cool feature in my opinion.


Step 2: Cutting Pipe to Length

With your parts dry fitted together, you can see where the PVC needs to be cut down. My design, being so simple only requires two cuts. I cut the 2" pipe down to 20" (air tank), and cut about a 1" long piece of the 3/4" pipe to connect two of the male couplings together.





Step 3: Installing Valve

Being the redneck fellow that I am, I opted for the cheap method and used a valve I cut out of an old bike inner tube.

A better idea would be to buy a metal valve at the hardware store, which can be threaded right into the PVC of the air tank.

Not that the redneck option is more likely to fail, actually I have used them on several airguns; and they have held up wonderfully in my experience (no worries there). But a metal valve would be nice.

First I drill a hole in the top of the end cap, just large enough for the end of the valve to slide through. Then I glue the two permanently together using five minute epoxy. I rigged the parts up in my vice so I wouldn't have to hold them together while the epoxy cured. 








Step 4: Coffee Break

Most important part of the whole build.


Step 5: Cleaning PVC

If the air tank wasn't going to have to hold together at 80 psi, cleaning the PVC before gluing wouldn't matter.

But it is. And if you are anything like me, you generally don't appreciate parts of your gun flying off at random times.

It gets annoying.

Yeah.

So to be extra sure that everything will hold together, it is best to clean the PVC well before slapping cement and primer on it. I used some denatured alcohol to get the surfaces as clean as possible.



Step 6: Priming and Gluing the junk up.....

The second most important part of the whole build. (Better get your priorities in order. Coffee first. Always.) 

First of all, do this in a well ventilated area or outdoors. This junk smells like nothing else, and according to the warnings on the can, is as bad for you as it smells. Don't breathe it.

Its fairly easy to use. Apply the primer, let it dry for a few minutes, and apply the cement (don't go crazy with it, but don't put too little either). Push the pieces fully onto each other, one quarter turn; and boom. Instant bond.

I'm always extra careful in this step. Remember that the parts must hold considerable pressure. If one of your joints doesn't get a good bond, you could be in serious trouble later when pressurizing this thing. Follow the directions on the can, and you should be good.















Step 7: Connecting Valve

The sprinkler valve threads right onto the male adapter coming from the air tank. Because the threads often don't make a perfect air seal, be sure to use some thread sealing tape.

My solenoid valve is also a one directional valve, with arrows telling me what side the inflow and outflow needs to be. I believe most solenoids are, so be sure to get that right. Otherwise the valve won't work.

And it would be nice if the valve worked. 




Step 8: Handle

At this point, the gun was about finished, all but the wiring.

But it was missing something. I realized it needed a handle somewhere.

No problemo. A few minutes and a scrap piece of 1x6" (ish) and it was good to go. Yeah, I attached it with duct tape (greatest invention of mankind). A better method would be to use a couple of those ratcheting pipe clamps, but I didn't have any on hand and didn't feel like waiting till I could get to the store.










Step 9: Wiring up the Solenoid

First a confession: the pics you have here are of me wiring it wrong. Somehow I managed to miswire the most simple circuit in existence.

No worries, look at the diagram and you can't mess it up.

(^^ I'm lying. You still can.)

Anyway. I rewired it off camera and got it right.

At first I thought a single 9v battery would be sufficient to power the valve. It worked to open the valve at low pressures great, so I just assumed the same would go for as much pressure as i'd put in it.

I was wrong. As soon as I got the air pressure above 40 psi the single 9v would not open the valve. I quickly hooked up an 18v drill battery, and it opened no problem. So as you can see from the pictures, I wired together two 9v's, which of course brings me up to 18v. And that has worked great.

For those wondering why I am just duct taping everything together: I am not doing that just because I am too lazy to figure out a better way. Actually, I am planning on experimenting with other ways of triggering this gun in the future (different switches, remote detonation, motion sensor, etc), so I am not wanting to set the design in stone here. You can do it more permanently.








I am calling this spud gun a cannon, as that was my main purpose for this gun, but since I made it to take interchangeable barrels, it can really be anything I want!

So far I have made four barrels for the gun; a 3", 2", 1&1/2", and a 3/4" rifle barrel.


TESTING TIME!!!


Pump up the gun to 80 psi. I'm not going to be going higher than 80 psi for this gun, as that is what the valve is rated at. The PVC is safe past 100, but as I don't feel like blowing out my valve i'm calling the limit at 80.

FIRE!!!!!

Unfortunately, the gun ended up being not quite as powerful as I'd hoped. It is not as powerful as my combustion powered spud gun, and only fired tubers about 80 yards. Not very impressive. Looking back, I should have used a larger air tank. It is plenty of air for my smaller barrels, but when I go up to the larger diameter barrels (which take more air volume to pressurize sufficiently) the shots are underpowered. Bummer. 

Watch the video if you want to see the full test. A description is about worthless.

To wrap up: if I were to build this again, I would make the air tank about twice the size it is now. I would also use a larger diameter solenoid, which would hopefully increase the amount of air released into the barrel at the instant the valve is opened. That should give me the increased power for larger barrels that I am looking for. Oh well, it's a learning experience. The cannon is still plenty of fun. 

I'm thinking about painting the thing. Any suggestions?



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