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piston troubles

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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piston troubles

Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 11:25 am

this in a new piston valve cannon i have started out of boredom and i am having trouble sealing the barrel.
the air chamber and all the fittings are 1 and 1/4in pvc
the barrel is 1/2in pvc
the original that is shown is made of a pine dowel and has a cut out rubber sealing face.
if you guys could give me a few pointers on what type of sealing faces you use and tell me what the problem is with my piston that would be great.
cheers
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IMG_0252.JPG
this is the rear part of the gun where the barrel meets piston etc
IMG_0253.JPG
view into the air chamber and barrel
IMG_0256.JPG
original piston. masking tape is to help it fit in the 1 1/4in coupling
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Unread postAuthor: clemsonguy1125 » Mon Jul 19, 2010 11:28 am

From what I can see the barrel is off center, it may just be the picture though, is the sealing face lubricated
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Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 11:37 am

clemsonguy1125 wrote:From what I can see the barrel is off center, it may just be the picture though, is the sealing face lubricated

the piston is with gear lubricant
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Unread postAuthor: clemsonguy1125 » Mon Jul 19, 2010 11:38 am

Ok, then im pretty sure your barrel is off center
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Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 11:45 am

it seems that way but its the lighting since my cameras light is to the left it illuninates the the picture unevenly
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Unread postAuthor: Technician1002 » Mon Jul 19, 2010 11:47 am

More important than the barrel being off center, what is most important is the sealing faces come together with no gaps along any of the surfaces. A wood dowell with a piece of sandpaper glued on the end and taped for a close fit can be used to polish the end of the barrel to make the end of the barrel square in relation to the piston cylinder. The second step would be to make sure the seal on the piston face is true. If the face is not flat or tilted at all, it will not make a good seal. Both of those surfaces must be true for this type of piston to work.

Time for true confessions. My Dragon cannon has the valve seat a little off center by about 1/8th inch. I discovered this while polishing the valve seat with sandpaper. When the surfaces were true, but still off center, it still worked.
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Unread postAuthor: Gun Freak » Mon Jul 19, 2010 12:04 pm

From what I can tell, you pilot volume is MASSIVE, and you are only using a blowgun pilot. That isn't going to work, because your piston doesn't have a check valve. As for the sealing, the barrel looks off center, but if you say its a trick of the camera then ok, also is the barrel cut evenly? If it is, then the only other thing I can think of is the rubber or there is a leak at the sealing face connection.
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Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 12:05 pm

thanks Technician. ill do that right now. i now this sealing face seals bc i used the same type of rubber on my spare parts gun http://www.spudfiles.com/forums/spare-p ... 20616.html
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Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 12:06 pm

Gun Freak wrote:From what I can tell, you pilot volume is MASSIVE, and you are only using a blowgun pilot. That isn't going to work, because your piston doesn't have a check valve. As for the sealing, the barrel looks off center, but if you say its a trick of the camera then ok, also is the barrel cut evenly? If it is, then the only other thing I can think of is the rubber or there is a leak at the sealing face connection.

well its not sealing in the first place so ill add a ball valve if need be later
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Unread postAuthor: Gun Freak » Mon Jul 19, 2010 12:10 pm

Well, the need will be :wink:
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Unread postAuthor: Technician1002 » Mon Jul 19, 2010 12:16 pm

When I made a piston for my Mouse Musket, one very important step is to make sure the face of the piston is true. Here is a photo of a piston I made on a drill press. The last step was taking a cut on the face of the piston to make it flat and true.
Image

A light weight piston is faster than a heavy piston.

The sharp corner of the chisel was slowly moved under the spinning piston to cut the face true.

When done the short coaxial piston valve had very good power. Here is a hole made with a 25 cent gumball.
Image
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Unread postAuthor: clemsonguy1125 » Mon Jul 19, 2010 12:58 pm

I found a good way of making pistons a few weeks ago, find a rubber stopper thats fits loosely in the housing, them take a piece of pipe the same diameter of the piston housing and lubricate it. put a piece of tape over the bottom and then the stopper. Pour epoxy over and around the stopper. When dry remove it. Ive used the a few times and have always had them work great. I can make a diagram if you need it.
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Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 1:06 pm

clemsonguy1125 wrote:I found a good way of making pistons a few weeks ago, find a rubber stopper thats fits loosely in the housing, them take a piece of pipe the same diameter of the piston housing and lubricate it. put a piece of tape over the bottom and then the stopper. Pour epoxy over and around the stopper. When dry remove it. Ive used the a few times and have always had them work great. I can make a diagram if you need it.


yes that works fantastically with hot glue also. i may end up doing that for this project but that will take a lot of epoxy
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Unread postAuthor: urgle the danish cow » Mon Jul 19, 2010 1:21 pm

ok im going to remake the piston using the idea clemsonguy brought up but i know that if i did an epoxy casting in the exact same diameter pipe that it will not fit easily into the cannon. what should i do
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Unread postAuthor: clemsonguy1125 » Mon Jul 19, 2010 1:30 pm

I used it in an exact diameter pipe, I greased it up and it was fine, You could always sand it a little. I tried it with hot glue and had issues but its been done plenty of times before. this is the how I made the stick guns piston, ill take a picture later if you want.
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