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Help With making a coaxle pistion seal. Update, New pictures

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Help With making a coaxle pistion seal. Update, New pictures

Unread postAuthor: Jeeperforlife » Sat Dec 05, 2009 1:20 pm

My gun is built, but i can't seem to get the piston to seal. The piston has a 1/8" thick rubber washer for a sealing face glued to the 3" end cap. I am using a fridge pump to fill it. My fear is not enough volume to make the piston seal. The piston has a O-ring and a shrader valve I got to break free at only 6 PSI for a check valve. but it seems to be leaking past some how.
The specs are
4"X5' chamber
2"X5' Barrel
Pistion made with 2 3" end caps.
1" sprinkler valve for pilot.
Any ideas, sugestions or comments on the seal or any thing else on the Gun?

Marcus

Image

Image

Image

Image

Image

Edited for spelling
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Last edited by Jeeperforlife on Thu Dec 10, 2009 5:18 pm, edited 1 time in total.
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Unread postAuthor: psycix » Sat Dec 05, 2009 2:28 pm

That seems like a very good gun to me, clean build. Well done!

The only mistake is the sealing face of the piston. It should be flat and not rounded. Unless it's all precisely machined, a rounded surface will never fit exactly to the porting.
It might work, but I think this is causing your problems.
So... make the sealing face flat and you should be good. Also the barrel should be cut square.

Don't forget air may also leak past the bolt used to hold the sealing face.
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Last edited by psycix on Sat Dec 05, 2009 2:31 pm, edited 2 times in total.
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Unread postAuthor: qwerty » Sat Dec 05, 2009 2:29 pm

Could it be leaking through the bolts either side of the piston?
EDIT: you beat me to it :evil:
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Unread postAuthor: jackssmirkingrevenge » Sat Dec 05, 2009 3:04 pm

psycix wrote:That seems like a very good gun to me, clean build. Well done!


Agreed, it will be a powerful beast once you get it working well :)

How about cutting a disk of whatever material you used for your bumper and putting that between the rubber washer and the piston body?
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Unread postAuthor: jmadden91 » Sat Dec 05, 2009 6:14 pm

It could also be the low flow from the fridge compressor. Granted the o ring on your piston should allow it to work still but perhaps test a regular compressor also
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Unread postAuthor: Jeeperforlife » Sun Dec 06, 2009 2:03 am

Thanks for the props.
I will redo the sealing washer tomorrow if I get a chance. Unfortunately I do not have access to a full size compressor right now.
Let me see what else I can do and I will post back when I can.

As soon as I get it firing reliable I will build the tripod, handles, and barrel extension, then I can get some real damage done.

Marcus
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Unread postAuthor: Brian the brain » Sun Dec 06, 2009 9:28 am

You could also use some fittings and pipe
to accumulate the pressure provided by the fridgy,
before unleashing it via a ballvalve into the cannon.

This will force the piston forward and probably seal.
Kinda shocking it into position.


Good luck and nice build!
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Unread postAuthor: Jeeperforlife » Sun Dec 06, 2009 10:55 pm

Brian the brain wrote:You could also use some fittings and pipe
to accumulate the pressure provided by the fridgy


Bingo It worked!
I also replaced the sealing face with a flat one but it is leaking. I will play with it more in the morning before work to try to see what I need to finish it. I am lucky I can buy the parts, fit everything together and even glue them wile working. :D
Once I become satisfied with the firing of it I will finish the handles and tripod. It will take some time but it is getting cold so it will be a wile before I can really try it any way.
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Unread postAuthor: c11man » Sun Dec 06, 2009 11:06 pm

hey congrats

now just get that real comperssor and youl be good to go
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Unread postAuthor: Jeeperforlife » Thu Dec 10, 2009 4:32 pm

Update;

I did some changes. I added a chamber for precharging the gun, and added some handles.

The small chamber is charged to 100 PSI and opened with a ball valve. That closes the piston and makes filling very easy. I can shoot at as low as 10 PSI.

The handel was built just to mock up something to get ideas for the real handel. Problem is these are very comfortable and easy to use so I don't have a reason to change them so I am going to keep them unless something comes to mind. Another reason I wanted handles was to divert the air from the piolet chamber away from me. The tape on the blow gun will be replaced with a band clamp.

I did my first shot today with something in the barrel. I shot a 1" iron pipe cap out with a sabot, it was at 30 PSI and dented it very badly when it hit the basement wall.

Here is a picture of the side of the changes.

Image

Here is a top shot.

Image

This is my favorite shot. This is what you see in firing positition.

Image

I modified my piston to give it a flat sealing surface but it is still leaking badly. I am going to build a new completely new piston. It will be painted but I don't know what colors yet, My wife wants to paint it and I said she could do any thing she wants to it.
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Unread postAuthor: psycix » Fri Dec 11, 2009 7:41 am

Nice. The precharge chamber looks good, not messy at all.

The only "thing" is that you've taped the blowgun to the handle. Maybe you could build it inside the handle?

Painting would surely give it the finishing touch. Flat black, army-green or camo looks always good on a cannon IMO. But you could also make it more colorful. The painting should fit the cannons "attitude".
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Unread postAuthor: Jeeperforlife » Sun Dec 13, 2009 12:47 am

psycix wrote:Nice. The precharge chamber looks good, not messy at all.

The only "thing" is that you've taped the blowgun to the handle. Maybe you could build it inside the handle?

Painting would surely give it the finishing touch. Flat black, army-green or camo looks always good on a cannon IMO. But you could also make it more colorful. The painting should fit the cannons "attitude".


Thanks,
The blowgun will change but I am not sure how yet. I would like to use a push button valve in the end of the 1" pipe but I want my 6 year old son to beable to use it and it maybe to hard for him to push. As far as painting goes I don't know what she has planned or what the "attitude" will be.

I rebuilt the piston. The new one is a much better seal and I was able to shed a few ounces at the same time. Here is some pictures of the new pistion.


Image

Image

Image

I got the stuff to start the stand. I will work on it this week.
Marcus.
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Unread postAuthor: jhalek90 » Thu Dec 17, 2009 6:31 pm

That piston looks like it should work. haha 8)
Cant wait to see it finished
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Unread postAuthor: Jeeperforlife » Thu Dec 17, 2009 9:48 pm

I got some work done on the stand. Depending how much I get done I will post up some more pictures tomorrow.
Marcus
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Unread postAuthor: daccel » Fri Dec 18, 2009 2:55 am

Looking good.

Sounds like you have it sorted, but if you are still having sealing problems, what about putting chalk or something on the face that would get marked and show where it is/isn't sealing?
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