SpudFiles

Hey everyone.

A friend and I are building a 3" piston valve air cannon made from metal. We are planing to to do shooting at around 400psi or more we also might be mounting it to a car mayby if not a stand .

Edit:
Just some questions:
1.what is a good sealing face for high pressure?
2. what is a good bumper?
3.What is a good strong and light material that i can use for the piston?
4.Will i need a spring if a i fill it from the chamber and if so what type of tension should the spring be?
5.If i fill from the chamber will i need a o-ring around the piston?

here are some pics of what we have:

all three parts:


this is where the piston will be housed in:


Size comapairson:


the tee:


There are still alot of things that we still need to buy but this will properly be a long term project because of the cost of all the parts.

Cooper.

LovableAirGuns wrote: Just some questions:
1.what is a good sealing face for high pressure?
2. what is a good bumper?
3.What is a good strong and light material that i can use for the piston?
4.Will i need a spring if a i fill it from the chamber and if so what type of tension should the spring be?
5.If i fill from the chamber will i need a o-ring around the piston?

1. Rubber
2. Rubber
3. Forget about 'light', focus on building a strong piston first and foremost. A bolt and washer piston would work. Basically a long threaded bolt with metal washers like so.
4. Yes. The spring will need to be strong enough to force the piston against the barrel/seat while air passes by the piston and fills up the pilot volume behind it yet weak enough so that it wont close the piston prematurely and limit flow when you fire the cannon. Only you will be able to tell us how strong the spring needs to be once your piston is completed.
5. No.

cheers but there is one problem where am i going to find washeres big enough to fit in side the brass male to make connector?

you would probably be able to get a 3inch hole cutter or you could make one similiar to how they make the check valve here. by drilling a hole in sheet metal and filing it down using a drill press

A fastener store. I can find 3" zinc washers no problem, just search the yellow pages or Google for 'fasteners store', hardware stores like Home Depot, Bunnings or whatever probably don't have washers that big.

Take your 3" brass nipple to the store with you and tell the people there what you need.

I ever do a 1" 1/2 qev, and i've had some problems that you could avoid.

I a good idea to put the piston in the brass part, because the galva is not clean.
Prefer the brass pieces, they are so cleanest and you will not have problem to the position of internal barrel and the sealing between it and piston.

I've use inox steel piston whit o-ring in a groove on the face to to sealing whit internal barrel.

If you choose to make you own HM piston, it will be cool but it must be long, because the piece where is short, and it must avoid to put the internal barrel to close to the exaust and piston.

And if you choose to machining a piston, do not ajust it too, because when you will tighen the conection, yo will deform the brass piece qui the piston will bind. I leave 6/100mm between the two piece, to have a good slide.

Sorry for my bad english :/

Mimikool your english is pretty good but i got an idea what if i make it from pvc with epoxy inside? sound good?

they make the check valve here. by drilling a hole in sheet metal and filing it down using a drill press

They dont cut out something from a sheet of metal they bought a washer and filed it down to fit

A fastener store. I can find 3" zinc washers no problem, just search the yellow pages or Google for 'fasteners store

I will keep the fasteners store in mind but i might go for the pvc piston unless it dosent work then ill go to the fasteners store.

PVC is brittle and doesn't do well under shock. Not a good idea. An ABS piston, 1/3 filled with epoxy and 2/3 filled with foam (you want the epoxy at the front of the piston so the sealing face has something air tight and strong to secure to) would be better. You'll need to find a ABS pipe/fitting that fits perfectly inside the 3" brass nipple though, could be a problem.

I can get ABS here but it took me awhile to find a place. You could also use teflon/nylon/delrin/UHMW rod etc, material like that is good to use but could be expensive and require a little bit of machining (you probably could get it machined to size for $20 or a 12 box of beer).

I wouldtn't reassured with a piston in polymer or plastic, epoxy, unless they are full, but it's not an heavy material so it's possible.

Because i thought to a metal piston, perforated in his ass to become less heavy.

With this diameter, forces applyed on the piston are enormous for 400psi.

What the diameter of the internal barrel, and barrel?

PVC is brittle and doesn't do well under shock

but what if it was reinforce with epoxy or would it still be brittle.
mmmmmm what other ideas are there than using stuff that is hard to find?

What the diameter of the internal barrel, and barrel?

At the moment i dont know because all the parts are at my friends house and i dont see him very often :/

perforated in his ass to become less heavy

Some expressions don't translate too well

al-xg wrote:

perforated in his ass to become less heavy

Some expressions don't translate too well

hahahaha

Ah ah, yes sorry, i try to avoid to use google translate because it give me bad translations, but sometimes i must use it, and myself make mistakes to ^^ And you understood what i meant so..! :p

More the différence between the Tee and the piston diameter will be great, the less the forces applied behind the piston will be great to.

So if you make you piston in brittle material, it's better to have a large internal barrel, but it will be less efficient.

what other ideas are there than using stuff that is hard to find?

Wood can work for a piston. It would probably be your least expensive option, assuming you have a 3" hople saw and some sand paper. You could even use it in place of a three inch washer. Just cut circles from some 3/4" stock, preferably hardwood, but pine or plywood may also work.

I don't want to have to prattle about the rediculoud strength to weight ratio of some species of wood. But there is a reason people still build with it [/quote]

Wood is a very good option witch I might use for the piston until I get a lathe and machine something lighter although what should I
use as a bumper since a wooden piston will be quite heavy. Soft rubber?