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Help with my barrel sealing piston.

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 my barrel sealing piston.

Unread postAuthor: Callipygous » Sat Jul 10, 2010 1:50 am

Last weekend I made my first barrel sealing piston cannon with whatever I could find at home depot that seemed like it would work.

The piston I ended up using is a core cut from a 2x4 with a 3 1/4" hole saw, then sanded down to fit the inside of a 3" chamber. The hole saw has a pilot drill in the center of it to help you make a neat cut, so the piston has a 1/4" hole down the center. We put a bolt through that hole with some washers on each side and a piece of rubber on the sealing side.

so from back to front we have: head of the bolt, fender washer, rubber washer, wood, 3" rubber disk cut from a baking pan, rubber washer, fender washer, nylock nut.

At the end of our first real day of firing, the 3" piece of rubber ripped. I can pretty easily just cut another piece, but it seems pretty obvious that this isnt a good long term solution. Before it ripped it wasn't a perfect seal anyway. Good enough to fire, but it had a slow leak.

What I'm looking for is a good, relatively cheap method of sealing my piston to the end of the pipe. Whether that involves a small modification to my current setup, or a complete rebuild of the piston and sealing pipe, doesn't really matter to me. I designed it so the sealing pipe is removable so pretty much anything is fair game for alteration.

Things I've been considering include:
1. Gluing the rubber gasket to the wood, since the washer clearly didn't provide enough support.
2. Putting glue/epoxy down the center pilot hole, since I'm not sure, but I think that might be the source of the leak that would put that sort of pressure on the gasket.
3. Making a whole new piston by cutting a section of 3" pipe and using it as a form to line with something and then fill with epoxy to make a perfectly fitted piston, and putting the gasket in while the epoxy is still setting.

If anyone sees obvious flaws in some of these, or some reason why one of them is a better idea, or an entirely different idea that can be done with basic tools and a trip to the hardware store, I would love to hear any input.

Also, any thoughts on why the gasket had that kind of pressure on it would be appreciated. Like I said, this is my first attempt at this style of valve, so I don't fully understand the forces at work. I was sort of hoping the bulk of the force would be exerted on the wood, and the rubber would mostly just be a soft material to help fill any imperfections in the piston/sealing pipe.
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 10, 2010 2:03 am

Depending on what kind of epoxy you're using, there is the option to cast the entire piston out of it. Since it's large diameter though, a solid piston would be a bulky and expensive affair, however without some thought you could cast a light and efficient cup style piston like the example below:

Image

In general I would advise against a solid wood piston, it's bulky and puts a lot of stress on your launcher on firing, as well as having greater inertia and accelerating more slowly.

I would also look into the possibility of aquiring some 1/8"-1/4"thick neoprene sheet, some stout mouse pads fit the bill.
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Unread postAuthor: Callipygous » Sat Jul 10, 2010 2:30 am

You sir, are a godsend. Me and my brother have been trying to find an easy source of airtight rubber sheeting, and mouse pads never crossed my mind.

couple questions though...

1. whats the purpose of the bolt/washer in this design? is it hard to keep the neoprene in place while the epoxy sets? does it just provide extra support for the rubber?

2. I read your post on how to cast a solid piston, but I don't see a description of how to do it with a hollow one like you are recommending. Cast what is basically a small solid piston inside the 90% tube, and then cast a larger one between that and the full size tube? do you have a recommendation for keeping the 90% tube centered during casting? It looks like the 90% tube is in 2 different pieces, is that necessary, and if so how do i keep all of that lined up properly?

also, my outer tube is 3" pvc, and my inner tube is 2" pvc (and yes, i am now aware that that makes the gap between the two the main choke point of my gun : P). recommendation for how i get something between those two?
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 10, 2010 3:18 am

Imagine your chamber tube has a 3" internal diameter, while your barrel tube has a 2.25" outer diameter and 2" inner diameter.

This means that the internal area of the tube is approximately 3.14 inches<sup>2</sup>. The external area of the barrel is is approximately 3.98 inches<sup>2</sup>, and inner area of the chamber tube is approximately 7.07 inches<sup>2</sup>.

This means that the area of the chamber between the inner wall and the outer wall of the barrel is approximately 3.09 inches<sup>2</sup> - so hardly any restriction at all ;)

As to piston casting, this should answer all of your questions:
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Unread postAuthor: grock » Sat Jul 10, 2010 3:23 am

the bolt and the washer stop the sealing surface from being blown out the chamber, because the force going around it and out the barrel will usually rip it off after a few shots

one thing i saw once was someone cut pieces of pipe to make a square around the barrel, which was also slightly smaller then the chamber, thus making a spacer with little flow-blocking. you could also make a solid spacer with epoxy, and drill a lot of holes in it, which would make a decent spacer, but one that resticts flow less

i never really got into epoxy casting, im sure JSR will answer that shortly

EDIT: i see i was beaten to the punch, most likely because im only typing on commercials. anyway...

if you do decide to things in tape, id recommend using a razor blade to make a straight line on the first edge in the wrap, and make use a razor blade to make a straight edge on the last edge. if you make the last edge as close to above the first edge as possible, it will be even more perfectly centered
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 10, 2010 3:51 am

Further to the above advice, this is a typical setup I use to align the barrel when making an airsoft coaxial. It is important of course that the foam disk is cut accurately, if you are not sure how to do this you can wrap that section in thin strips of tape instead of the disk.
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Unread postAuthor: Stolly32123 » Sat Jul 10, 2010 5:14 am

Do you need epoxy to do this? and what is the foam disk for?
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 10, 2010 5:26 am

It's all in the how-to, have a look ;)
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Unread postAuthor: Callipygous » Sat Jul 10, 2010 8:00 pm

I'm having a hard time finding this marine epoxy you speak of. The link you gave seems to be to an expired ebay bid. Do you have a link to a store inventory, or maybe a description of what im looking for? I found some stuff that is meant for repairing fiberglass, but i dont know how to tell if its viable for this application : P
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Unread postAuthor: Gun Freak » Sat Jul 10, 2010 8:03 pm

He doesn't know, the lables of the epoxy cans in his massive epoxy storage room have all rotted off because of the humid and hot climate in his batcave :D
Just search "low viscosity marine epoxy resin" and you should find something similar to the stuff he uses.
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Unread postAuthor: jackssmirkingrevenge » Sat Jul 10, 2010 8:08 pm

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