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First Airgun

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Alster370 » Tue Apr 19, 2011 7:57 am

Got the rest of the fittings this morning, just waiting on the epoxy. Should hopefully be here tommorow. Had a go at casting some pistons, sadly I had no grease so I made two using hot glue as the material, and butter & ac90 for lubricant. :D

Image

Then I assembled a sort of mock up of what most of the gun should look like, excluding barrel, pilot and shrader. But hopefully you should be able to tell where they will be on the final build. Oh and this is not the actual size of gun either, I used random copper pieces to assemble it so expect the final gun to be smaller, with much less space between the tees & elbows etc.

Ahh why dont my pictures appear on the post :(

Image
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 19, 2011 8:43 am

Alster370 wrote:Ahh why dont my pictures appear on the post :(


Image

Image

You have to post the "direct url" in the img codes.
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Unread postAuthor: Gun Freak » Tue Apr 19, 2011 6:46 pm

Technician1002 wrote:Naw

That's a new one :lol:
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Unread postAuthor: Alster370 » Fri Apr 22, 2011 2:49 pm

Im having trouble getting my piston to slam back, ive made over 5 different piston sizes with no sucess. :cry:

Here is the gun so far:
Image

The piston itself:
Image

Where the piston is before pressure:
Image

Where the piston is under pressure:
Image

It seals well enough but when the pilot is opened it doesnt slam back it just sits there and then all the air is released though the pilot valve. This is when I do a test at 20psi as i dont want to epoxy the barrel in until I know that the piston works.
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Unread postAuthor: Gun Freak » Fri Apr 22, 2011 2:58 pm

You made a fatal mistake though...
If you are using a hot glue piston, it will not survive forever. It will most likely deform, in which case you need to take it out to replace. Your design doesn't permit removal of the piston though. Also, why doesn't your piston have a sealing face?

I dont want to epoxy the barrel in until I know that the piston works.

Did you epoxy all of the joints? :?
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Unread postAuthor: Alster370 » Fri Apr 22, 2011 3:55 pm

Your design doesn't permit removal of the piston though

I know. I wanted it to be mostly maintanance free, like a commercial gun would be but to do that I need a solid piston and well I dont have the right material or proper tools to do that at the moment. The gun itself will probably be in use for about 6 months before I decide to make another one so hopefully It would last. I could also do a piston of epoxy too, but that would be more expensive.


Why doesnt your piston have a sealing face?

its not finished yet. and it seals well enough for my low pressure tests. my main focus has actually been getting the piston to operate properly first, then sealing the barrel for higher pressures, second.

Did you epoxy all of the joints?

yes it works pretty well. its airtight and its 4500psi epoxy too so strength is not an issue. I would solder but I dont have any flux or a propane torch to heat up the connections with.
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Unread postAuthor: Gun Freak » Fri Apr 22, 2011 4:33 pm

Did you roughen the joints before epoxying them?
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Unread postAuthor: Alster370 » Fri Apr 22, 2011 4:39 pm

yeah I sanded them and didnt brush any of the "copper grit" off either. I put epoxy on the pipes themsleves then went round the seams of the two joints with epoxy, like you would with solder ring fittings.
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Unread postAuthor: Crna Legija » Fri Apr 22, 2011 10:20 pm

your piston is way to big you could 1/2 it, and it should meet the seat about 1/2way in the tee. also is the piston a bit bigger than the barrel?

how low is your low pressure testing?, my first homemade piston valve would not fire under 70 psi. also lube it up good
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Unread postAuthor: Alster370 » Sat Apr 23, 2011 3:46 am

your piston is way to big you could 1/2 it

Il do that today then

piston a bit bigger than the barrel

yes by about 1.5-2mm

the seat about 1/2way in the tee

The trouble is there is a little bevel 1/3 of the way in each side of the tee to stop the pipe from going all the way though the tee. I suppose I could sand it off if nessessary.

lube it up good

Im using some white grease I got from a motorcylcle shop yesterday, seems to work fairly well.

how low is your low pressure testing

around 40-50 psi, I glue the barrel in with hot glue, then pump to 40 or so psi, release the pilot to see if the piston slams back or not. Do you think I should just epoxy it up and hope it works at high pressure?

My questions:

Even if there is a sufficient friction on the piston, say 2/3psi will it still function?

can a piece of innertube be used as a suitable sealing head for the piston?

Also my piston takes a while to move forward and seal the chamber, like 2-3 pumps on the floor pump and will not move when the shock pump is used. :(
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Unread postAuthor: Gun Freak » Sat Apr 23, 2011 10:08 am

can a piece of innertube be used as a suitable sealing head for the piston?

Yes it will work fine if you attach it securely.

Also my piston takes a while to move forward and seal the chamber, like 2-3 pumps on the floor pump and will not move when the shock pump is used.

That is because your piston isn't airtight and your shock pump doesn't have enough air flow. You can fill it to the max of your floor pump then switch to your shocky and it will work then.
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Unread postAuthor: Alster370 » Sun Apr 24, 2011 3:33 am

Still working on the piston, managed to slide the pipe going to the pilot further out from the piston tee so now The piston has more space. Il keep working on it for a couple of days but the idea of buying a QEV is rather tempting. I know that most are limited to 10bar, but I assume they can operate at higher pressures? I have an SK pheumatics literally down the road so I might go and pay them a visit. :D
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Unread postAuthor: Gun Freak » Sun Apr 24, 2011 12:17 pm

managed to slide the pipe going to the pilot further out from the piston tee so now The piston has more space.

That just created more pilot volume and allowed more acceleration room for the piston... you only harmed it by doing that.

I know that most are limited to 10bar, but I assume they can operate at higher pressures?

European QEVs have been taken up to 800 psi I believe.
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Unread postAuthor: Alster370 » Sun Apr 24, 2011 2:03 pm

That just created more pilot volume and allowed more acceleration room for the piston... you only harmed it by doing
that


It was nessesary, because now the pipe is not in the the tee as much , so the piston can be the same diameter all the way across its length. When the pipe was fully in, a piston like that wouldnt have moved so it had to have the back at a smaller diameter to fit into the pipe so it could actually move. This is what I believe caused the sealing problems, as the new piston seems to be sealing much better. I hope that all made sense. :?

European QEVs have been taken up to 800 psi I believe.


Good to know that, I only intend to run at 350psi max, as I cant physically pump any higher with my shock pump. This is with the pump secured the the frame of the gun too.
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Unread postAuthor: Alster370 » Sat May 07, 2011 5:41 am

Well its been a while so heres an update.

Ive redone the gun with comp fittings again, and im currently leak proofing the gun at the moment. I managed to obtain a 1/2bsp QEV and it works at about 50psi min. I think this is because my pilot is a 15mm ball valve and that its too big??

Pictures hopefully late afternoon
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