second air gun, piston valve.

[Sparrow]

My first cannon was just a ball valve water balloon launcher that wasn't very efficient.
However, I'm really happy with this thing, as you can screw on the barrel you want, and the tank you want.
I made a pretty nice piston valve with a strong spring I got from a sprinkler. I wanted to put a bumper in, but never did as I really wanted to finish it before the end of the weekend (and so glued the cap on without thinking).
However, I don't plan on putting more than 40 psi in it, and I want an opinion as to if the spring is going be enough to stop what ever damage it is that it will cause.
I also have a very cheap little air compressor I wanted to mount on the side, but it takes up to 30 sec to get the tank to 20 psi, and thats just not good enough.

ROUGH estimate as to how fast it shoots grapes are 30m/s at 10 psi.

What still needs to be done:
handle shaped and covered.
Stock finished
velcro strips replace pull ties.
Bolt made for barrel.
Tennis ball barrel made (what it was built for)
Bigger tank for tennis balls.

[Gun Freak]

Looks nice and clean, will do some damage with a golf ball barrel and bigger chamber It the pilot valve a 1/4" ball valve?

Your valve is not serviceable? Thats a big no, no. Better hope there is no problems with your piston...

[Sparrow]

Yea, its just a ball valve, I made another valve, but I found pulling the ball valve open fast was plenty good enough. i was just wondering how worried I should be about having a spring instead of a stopper. The piston is made out of pvc pipe with a foam core.

I know, (ashamed) I am not proud of the piston not being serviceable. I didn't plan it like that, but think I may make a better one later anyway. The piston has a foam front that we used on various bump proofing projects, and the only thing I can imagine breaking is the spring coming off its seat.

[Technician1002]

I recommend downloading the excellent program GGDT (Gas Gun Design Tool). It could provide a better estimate of your projectiles.

http://www.spudfiles.com/spud_wiki/index.php?title=GGDT

[saefroch]

I made a pretty nice piston valve with a strong spring I got from a sprinkler.

Your spring is assisting in returning the piston to seal, correct? You may be losing some performance, but if you can shoot a grape at any significant velocity at 10 psi, I wouldn't worry too much about the performance loss.

I like the overall design, and I'd ask for piston pictures, but I guess that's not possible.

[jackssmirkingrevenge]

I know, (ashamed) I am not proud of the piston not being serviceable.

No shame in that, I don't think I've ever made a servicable piston valve the trick is to make them in such a way that they will last the life of the launcher.

[Technician1002]

I found piston service to be essential. My Apple Cannon tends to get dried moldy apple sauce in the valve. Disassembly for cleaning and re greasing is part of the maintenance. When I forget to wash it out, dried apple juice glues the piston in place requiring a hammer and stick to remove it.

I got a candy wrapper in the o ring of the large launcher so it failed to seal.

The Mouse Musket regularly broke pistons, so they needed replacement.

Service is expected to be needed, much like having replaceable tires on a car unless you consider the life to be less than the life of the piston lube and cleanliness.

[Sparrow]

I saw the GGDT program, but I first have to install the python software, I shot the grape with the barrel horizontal from one meter, then i worked out the speed by how far it went before hitting the ground (with the time it takes to fall 1m and the distance it traveled) so I didn't take in air resistance.


If I show you the materials I used to make the piston out of, I'm sure most spud gun builders will be able to make a nice 3d model in their head, I'll post a diagram soon.

@Technician
I never though of that, I guess i'll just not stuff any fruit down the barrel. But since I can unscrew the barrel, isn't there some kind of lubricant I can spray into the valve?

Heres the valve:


I tried to keep the space behind the piston as small as possible as I wanted efficiency and not 100%power.

[mlz3000]

It looks awesome!

I'm planning on making a piston valve cannon as my second as well, with a similar design (I too like the ability to change out my barrel and chamber.)

Did you have any trouble getting the valve to seal?

Do you have any tips for me building a piston valve as my second cannon?

[Sparrow]

Thanks! I think that making your piston light but strong should be pretty good as it gets out of the way quickly and does not bash out the back of your chamber so easily. Is you make the piston, you don't need to seal the outside, just put a piece of rubber the diameter of the piston on the back of the chamber, and when the piston comes back it will seal the pilot valve exit.

I could go on, but most of the things I will tell you comes from imagination and assumption, you should ask these experts who have experience.

[Technician1002]

On making pistons seal, the most important aspect is to be sure the valve seat is perfectly flat as well as the face of the piston. Any lumps, bumps, or crooked will make a leak that will prevent it from sealing up to pressurize.

A piece of sandpaper on the end of a wooden dowel works well to spin against the valve seat to polish it flat and smooth. If you turn a piston on a lathe, or drill press, the face can be machined flat and true. With the surfaces mating well, it will be easy to get it to seat and fill with air.

I used PVC pipe caps as pistons in my Mouse Musket. Part of the installation was sanding the face of the piston on a board with a flat piece of sandpaper. The high spots sanded off first. When the high spots were sanded down to the low spots so the entire face was sanded, it was then flat. You can see the sanded face of the piston in this photo. The ring on the face is where it set against the o ring on the valve seat.

[jackssmirkingrevenge]

I tried to keep the space behind the piston as small as possible as I wanted efficiency and not 100%power.

More efficiency means more power for the same parameters (for example a more efficient valve but the same pressure, chamber size, barrel length etc.) so they go hand in hand

Here's some thoughts on piston travel and pilot volme.

Do you have any tips for me building a piston valve as my second cannon?

If you know your materials well, it should be a doddle.

This was my third pneumatic which I built back when I was 17, I knew nothing of plumbing then but knew plenty about model aircraft, and the choice of materials reflects it, but the idea is that basically you take the principle and adapt to the tools, materials and know how you have to hand.