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piston issue

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 issue

Unread postAuthor: jon_89 » Mon Jul 30, 2007 9:50 pm

i just built a coaxial piston gun with a one inch chamber and a half inch barrel. my piston is currently just a one inch dowel with a neoprene washer glued to the face. when i goto fill it with air the air just comes right out the barrel. does anybody have any suggestions on how to fix this or make a different piston? thanks a bunch
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Unread postAuthor: hi » Mon Jul 30, 2007 9:55 pm

i made one like that. i just JB weled the seal face on and now it works fine.

the problem with mine was the seal face keept getting sucked in the barrel, so when i glued it to the piston it never moved again.
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Unread postAuthor: Modderxtrordanare » Mon Jul 30, 2007 9:57 pm

Does your piston fit properly inside?
Can it move back and forth easily?
Where are you filling from?
How much room does the piston has to travel?
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Unread postAuthor: jon_89 » Mon Jul 30, 2007 10:04 pm

i am filling from behind the piston. it moves back about an inch if that. it is kind of a snug fit as in moving back and forward. what do you mean does it fit properly?
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Unread postAuthor: Modderxtrordanare » Mon Jul 30, 2007 10:18 pm

jon_89 wrote:i am filling from behind the piston. it moves back about an inch if that. it is kind of a snug fit as in moving back and forward. what do you mean does it fit properly?


By "does it fit properly" I was trying to gauge the clearance your piston had from the sidewalls of the interior of your chamber.

An ideal traveling distance for your piston would be 1/4th the inside diameter of your barrel. I would definitely shorten that inch down, for efficiency's sake.

Now, for the problem of the piston not sealing. I am being led to believe that the air is equalizing around the piston faster than you can put it in, thus preventing the pressure differential needed to seal the piston.

Give your piston a wrap or so in duct tape until it is really snug, lube it up, and try filling.



Another question, is the surface of your sealing face flat and perfectly smooth?


Edit:
mopherman wrote:
jon_89 wrote: it moves back about an inch if that.

Well hell! theres your problem. it should move 1/8 -1/4 of an inch for a1/2 barrel


.125" to be precise. :D
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Last edited by Modderxtrordanare on Mon Jul 30, 2007 10:24 pm, edited 1 time in total.
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Unread postAuthor: mopherman » Mon Jul 30, 2007 10:20 pm

jon_89 wrote: it moves back about an inch if that.

Well hell! theres your problem. it should move 1/8 -1/4 of an inch for a1/2 barrel
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Unread postAuthor: chaos » Mon Jul 30, 2007 10:29 pm

mopherman wrote:
jon_89 wrote: it moves back about an inch if that.

Well hell! theres your problem. it should move 1/8 -1/4 of an inch for a1/2 barrel


yer so make your piston a bit snugglier with the housing (use electrical tape, duct tape tends to grab), put some washers or a spacer of some kind behind the piston to stop it moving so far back.
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Last edited by chaos on Mon Jul 30, 2007 10:35 pm, edited 1 time in total.
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Unread postAuthor: jon_89 » Mon Jul 30, 2007 10:30 pm

i think that may be the problem is the face is not flat because i cut it wit a hacksaw. do you know of anyway to cut it perfectly flat or should i go with a different piston design? i have tried wrapping it in tape and it did not work. i ahve also tried a spring.
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Unread postAuthor: chaos » Mon Jul 30, 2007 10:41 pm

i say get a file and make it as flat smooth and square as you possible can, if its no flat no piston will seal it.

hope ya work it out,
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Unread postAuthor: Modderxtrordanare » Mon Jul 30, 2007 10:57 pm

Chop saw. Or some sand paper and alot of patience.
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Unread postAuthor: jon_89 » Mon Jul 30, 2007 11:02 pm

ok i have been sanding for 10 minutes and have lost patience. do you guys have an idea for a good easy piston that that is gaurenteed to seal?
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Unread postAuthor: MrCrowley » Mon Jul 30, 2007 11:14 pm

What are you filling the gun up with? Hand pump? Air Compressor? If you use a hand pump try pumping alot faster.
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Unread postAuthor: jon_89 » Mon Jul 30, 2007 11:28 pm

a shock pump i just bought today.
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Unread postAuthor: noname » Tue Jul 31, 2007 12:09 pm

You might as well give it up because your gun is going to suck. There isn't going to be enough chamber volume because of how much room there is between 1" and 1/2" PVC. You'd be able to pull it off if you were using 300 psi, but not with PVC.
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Unread postAuthor: dongfang » Tue Jul 31, 2007 2:21 pm

Hi

do you guys have an idea for a good easy piston that that is gaurenteed to seal?


PVC endcaps are great, if you can find one that fits well inside your chamber.

If you follow the advice to get a larger chamber, I'd try in your place:

- Get a bushing that fits the size of the barrel. Glue a piece of barrel pipe into one end. It should just fit inside the bushing, making the wall thicker.
- (the complicated part): Find an o-ring that fits on top of the end of the bushing (the end with the pipe glued in). Its inner diam. should be a little large than that of the pipe, and its outer diam. a little smaller than that of the bushing.
- Cut a groove in the end of the pipe/bushing, for the o-ring. Requires a vertical mill or a lathe --- sorry..... The o-ring should be recessed half way or a little more.
- Glue the thing to the end of the barrel.

If it doesn't seal against your piston, and the piston really is flat and squarely cut: Glue some fine sandpaper to the head of the piston with water soluble glue. Use this as a tool to grind the o-ring into alignment (while mounted in the cannon). Remove sandpaper again.


If you don't have so fancy tooling: Carbon paper is your find when you want to find out where things touch and where they don't, and you can't see it. A halogen lamp is good when you can look.

Regards
Soren
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