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Building my first cannon...some questions

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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Unread postAuthor: jackssmirkingrevenge » Mon May 28, 2012 12:34 am

Have a look round the bins of your local tyre repair shop :) a truck inner tube is about 1/8" thick good quality rubber.

Right now I used some rubber from a mouse pad I had laying around...I'm assuming thats the cause of my leak, but could it possibly be something else?


It could be that:

- the sealing face of the piston is misaligned and not at 90º to piston travel

- the end of the barrel is misaligned (ie the edge is not parallel to the sealing face) or not properly finished (roughly cut, not properly sanded and smoothed)

I realized with the coax design, I have nothing holding the barrel in the center of the chamber properly...should I maybe put screws in to hold it down and make sure it doesn't move? I'm curious what people normally do.


Various solutions to this, some people have used screws in the past. It's not really a problem for shorter chambers though.
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Unread postAuthor: aj23 » Tue May 29, 2012 9:36 pm

Ok so now everything is solved, except for some leaking through the barrel still. I guess the piston isn't getting a proper seal. The face of the piston is a metal plate, with a piece of rubber (cheap stuff tho) bolted on top. The barrel was never cut, but maybe when it was glued it wasn't 100% straight.

Is there any way to compensate for any irregularities and have the seal be 100% with no leakage?
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Unread postAuthor: jackssmirkingrevenge » Wed May 30, 2012 12:03 am

aj23 wrote:Is there any way to compensate for any irregularities and have the seal be 100% with no leakage?


Having an extra thick piston face seal would help, but otherwise carefully finishing the end of the barrel is the best solution. It helps if you lightly chamfer the edges with sandpaper too to avoid it cutting into the rubber seal. I understand it might be glued in place and inaccessible now but keep it mind for your next build ;)
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Unread postAuthor: mattyzip77 » Wed May 30, 2012 9:22 am

Try putting another washer, a thicker washer, or a piece of felt behind your sealing face! Did u epoxy around the head of the bolt thats holding the washer in place?
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Unread postAuthor: mattyzip77 » Wed May 30, 2012 12:33 pm

At home depot, they have these reddish colored sheets of rubber they are like 8 x 8 inches. As for the 2 inch neoprene disc washers, they are with like the bolts and stuff and you have to look through those drawers that pull out. As far as centering your barrel, you need to get the right size bushing and modify it, "cut it down". There are pics around here in the forum, you just have to find them. Mouse pads SUCK! You would have better luck with a old bike inner tube!! Do you have like a local hardware store like ace or true value in your area? Or a actual plumbing supply place you could call??
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Unread postAuthor: aj23 » Wed May 30, 2012 9:53 pm

Would you mind me asking if you have a home depot link for those rubber sheets? I can't seem to find them online, and when I asked in store they told me there was pretty much no rubber I could use.

As for the washers, would you put those on top of the rubber, or below usually? My piston center and barrel wouldn't line up exactly, so I think if I put this on front it would lead to further leakage? I ended up centering with screws. It's not exactly the center, but its good enough and works for a quick fix...i needa get this done within the next couple days.

I'm not sure of any local hardware stores around here, but would definitely have a couple local plumbing stores...just not sure what to ask for when I call lol (I called a couple asking for neoprene and that didn't work out too well).
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Unread postAuthor: jackssmirkingrevenge » Wed May 30, 2012 11:53 pm

Something like this maybe?

http://www.homedepot.com/h_d1/N-5yc1v/R ... reId=10051

only 1/16" thick though so I would put a stack of at least 4. A spot of Araldite or JB weld to make sure it's sealed up from the join does wonders too.
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Unread postAuthor: aj23 » Fri Jun 01, 2012 3:04 pm

Ok so now that issue might be fixed, but another has arisen.

When the cannon was tested, the potato simply slowly went though the barrel and dropped on the floor. Water vapour could be seen coming out of the barrel, but it didn't seem like there was much pressure based on the movement of the potato. This was at 50 PSI too.

Could it be maybe the piston not moving back properly, or anyone have any idea?
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Unread postAuthor: samiam0295 » Fri Jun 01, 2012 3:23 pm

Was the piston lubricated? also are you sure that the pilot chamber is pressurizing properly? It could also be the pilot volume not venting fast enough, is the sprinkler valve modified, or are you using the electric solenoid?
Do you have a bumper behind the piston to stop it from smacking into the back of the chamber?

For anyone interested, Home Depot sells 5" rubber drain stoppers that can simply be cut to size. They are 1/8" thick white rubber.
http://www.homedepot.com/h_d1/N-25ecodZ ... reId=10051
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Unread postAuthor: mattyzip77 » Fri Jun 01, 2012 8:02 pm

Dude please tell me that you are not putting that on your cannon!!!! It may work for a few shots, if your lucky!!!!!!
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Unread postAuthor: samiam0295 » Fri Jun 01, 2012 11:23 pm

reasoning? Its thick rubber, I am going to super glue it to the face of my piston...
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Unread postAuthor: Crna Legija » Sat Jun 02, 2012 10:48 am

With any real pressure the sealing face will get ripped right off if its only super glued on, that's you every one has a lager washer and bolt holding them down.
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Unread postAuthor: mattyzip77 » Sat Jun 02, 2012 12:42 pm

Its not the best rubber in the world, you will never get a 100% seal with it.. And as Crna Legija said above you gotta bolt it. As I said before, it may work for a few shots, then its going to break.
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