My mini coax. piston gun plans, will it work?

[Tsukiten]

Hey guys..
I've brainstormed for ages to think of good designs for pneumatic guns, and thanks to some inspiration from here I have plenty now... Lets say you could just give me some parts and I can build some high FPS full auto portable bb gun... But the #1 problem is;
I don't have the experience
The motivation (because everything I try to build, fails)
And most of all; the only stuff you can get here is crap... (no blowguns, sprinkler valves, etc)
I have dozens of half-finished projects, but neither fully finished due my lack of parts and experience building one...
In a last attempt of building a succesful piston gun... I've come up with a very simple project to build a small, easy and effective small piston gun.
If this one fails I might give up building pneumatics

Could you guys give me some tips on building a very basic piston gun?
Here are my plans; is it okay?

In the pic below:
-Most likely the piston will be (hot) glue/candle wax, (bad idea?)
-The other olive-green stuff is epoxy
-Chamber and cap will be made of PVC, barrel from some other material
-Pilot valve is a modded lighter head (pretty good flow)
-Piston sealing face; some squishy rubber
-Bikevalve to pump it up
-Purple stuff is lube (be will vaseline or sewing-machine oil)


(click to enlarge)

It's detailed to make sure you guys can understand every part and can easily see what's wrong or not... (And I used to pixel-art )

That's it, any tips/alternative materials/warnings?
Thanks for reading and please reply...

[The Kid]

looks good i would say use the hot glue or epoxy putty for the piston (theres a how to by jsr someware). How the in the holy virgin mary mother of jesus christ do you use a lighter head for a pilot valve, why not just a ball valve cause the lighter head may look sort of ghetto.

and btw are you using o rings on this piston.

[Hawkeye]

You're better off with an end cap rather than a plug. You can limit the pilot volume that way. With the threaded plug there is an unnecessary amount of air in the area the piston can't move back into because it is narrower.

[Hubb]

Well, for starters, it looks good. Be forwarned, however, that vasoline is petroleum based and may deteriorate the PVC over time. Find a water based lubricant.

I would choose hot glue over wax, simply because it is stronger.

See if you can't cut down on the pilot volume and a bit. If the diagram is to scale, the pilot volume is massive.

I don't know about the modified lighter head but it may work. If it don't just go with a small ball valve.

But good luck with it and get it built. It looks like it has great potential.

[Tsukiten]

looks good i would say use the hot glue or epoxy putty for the piston (theres a how to by jsr someware). How the in the holy virgin mary mother of jesus christ do you use a lighter head for a pilot valve, why not just a ball valve cause the lighter head may look sort of ghetto.

and btw are you using o rings on this piston.

A ball-valve is too heavy/big and hard to open on such small gun, and a lighter with the flow-reducing stuff rigged out of it it gives very good flow by just pressing a button...
No o-rings btw...just a tight-fitting piston with some lube..

[Tsukiten]

You're better off with an end cap rather than a plug. You can limit the pilot volume that way. With the threaded plug there is an unnecessary amount of air in the area the piston can't move back into because it is narrower.

Yeah but it's hard to find the parts, PVC in my country sucks...
But I guess it's easier to find an end cap than a plug... So I might do it anyway.

[The Kid]

don't use a large ball valve with the chunky hanles use one like on my ghetto http://www.spudfiles.com/forums/ghetto- ... 11787.html cost like 5 bucks eaisy to open probely perfect for a cannon like yours but its really your creation

[jackssmirkingrevenge]

The lighter valve will definitely not have enough flow to pilot your piston, if you keep your piston travel to about 0.25 times the barrel diameter even a schrader valve will be enough as a pilot, meaning you can have one valve to both fill and fire it.

It's a better idea to have the barrel on the removable endcap, that way you can still remove the piston but it will allow you to have a minimal pilot volume.

This is my first piston cannon design, the only epoxy I used was Araldite (you can use JB weld or similar) and it was good for 80 psi at least.



The design below used a separate schrader valve for firing in order to make it more ergonomic, with the schrader struck by a spring loaded hammer released by the trigger. A simpler way of doing this would be to stick a blowgun into the back end instead.

[Tsukiten]

Hmmm...well I'll tell more about that lighter...
All the lighters have some parts in it to help make a smaller flow to have a nice candle-sized flame and lets no liquid butane out.
But when I removed all but the main valve, it empties a lighter in 3~5 seconds...
If that still isn't good enough what would be the 'minimum' pilot volume?

And @ JSR's drawings:
Balsa wood? 0.o Is that air-tight?
And wouldn't the rubber thing in a syringe move too slow?

Edit: Oh does that design need to be pumped at 2 valves?

[Hubb]

I would think by having the lighter dump that fast, it should work...but now you should be concerned about the schrader moving the piston when you remove your pump.

[jackssmirkingrevenge]

If that still isn't good enough what would be the 'minimum' pilot volume?

The simple fact is that if more air leaks around you piston than can escape through your pilot, the piston will never move. I would play safe and stick with a schrader valve at least.

Balsa wood? 0.o Is that air-tight?

Pretty much, i used to put a layer of araldite on the inside surface to seal it well.

And wouldn't the rubber thing in a syringe move too slow?

Not really, the small (5-10ml) syringes normally have low friction and move fast enough. You can always use some water-based lube to help it along.

Oh does that design need to be pumped at 2 valves?

Since the syringe seal is 100% airtight, yes. Alternatively, you can have a small equilisation hole between the chambers.