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Resurrecting a project

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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Resurrecting a project

Unread postAuthor: Los Frijoles » Sat Nov 17, 2007 9:02 pm

I was rooting through my crap barrel today looking for something to do and I found this gun. It was a failed project that I started about a year ago and I think I can resurrect it.

Dimensions:
14-ish inch 1.5" chamber
16-ish inch .5" barrel

Here is the gun now:
Image
Whole Gun
Image
Gun Apart
Image
The Piston (or what you can see of it)

There is a threaded hole for a shrader on the back. The piston has either yellow neoprene foam or no seal at all (I can't really tell). The piston consists of a sanded down 1' endcap. I can remove the piston, but based on past experience of this, it has to be for a good reason since I have to take a metal rod, shove it through the shrader hole, and pound until it pops out. It appears that the lubricant used for the gun is vaseline.

I would really like to finish this, so here are my questions:

What can I use for a better seal than the foam? The last test I did of this that worked, the seal exploded and a hole was blown right in the middle. An idea of how to attach the seal to the piston would be nice too.

How can I improve the seal between the barrel and the piston? Also in the last test, there was considerable air leakage into the barrel.

What would be a better lubricant to use than vaseline? The piston moves, but it seems kind of sticky (e.g. not a smooth movement with bumps and jerks).
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Unread postAuthor: Hubb » Sat Nov 17, 2007 9:09 pm

1) A better seal would be rubber from an innertube or neoprene. Using the neoprene (which is a better choice in my opinion) you could simply bolt it to the piston.

2) This is a theory of mine to help seal the piston to the barrel. It might be worth your time. Another thought would be to take a rubber ball that is close in size to the barrel, only slightly larger, and cut it in half and fix it to be your seal face. This may even out the irregularities in the barrel face that cause the leakage.

3) Find a lubricant that is water based, such as KY Jelly. PVC wears down over time with petroleum.

4) Good luck with it and looking forward to it.
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Unread postAuthor: Los Frijoles » Sat Nov 17, 2007 9:17 pm

I did the rubber ball thing, but that didn't seem to work very well. It was exceptionally hard rubber.

That felt idea seems very good. I originally chose foam because of that same theory of 'make it squishier and it will get a better seal' but obviously my way of doing it didn't work too well.

On McMaster, neoprene film (I assume that to mean sheets) is $9 for a 1x3' piece. This is a bit out of my price range seeing as I don't have a job and my parents just bought a car (makes it not a good time to mooch). I read in another topic that neoprene washers were at home depot. Will these work?

Oh, I didn't mention that my piston is currently oriented cup-like towards the barrel. Should I switch this to conform with everyone else's idea of a piston or will this work?
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Unread postAuthor: Hubb » Sat Nov 17, 2007 9:34 pm

Normally, it is best to have a piston that is straight, but your case my be different. It is hard to tell without pictures. Go ahead and remove the piston, as you are going to have to eventually to resurrect this.

As far as the washer go, they will work but you might have to add another small metal washer to the front to help hold it in place.

Since you're pressed for money, might I suggest trying to use rubber from an old innertube. You can layer it to get a thicker seal.

Now, on to my felt idea. It would seem to me that it would work better with a thinner piece of rubber. The only problems I foresee with it is the securing it to the piston. If it is not secured good, it may be prone to being sucked down the barrel.

This brings up another thought. The innertube rubber is thin and may try to rip off upon firing it. What I would suggest on this thought, is to layer the innertube and glue it together with rubber cement (that IS what's it's used for, right?). Then secure it to your piston.
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Unread postAuthor: Los Frijoles » Sat Nov 17, 2007 9:49 pm

The only problem I can foresee with this is that since my barrel is .5", it makes it hard to find a screw small enough to still get a good seal and at the same time not being so small that the rubber rips right off. Also, my barrel is slightly crooked, due to a failed experiment in trying to flare out the end using a pot of boiling water :oops:. I partially fixed the problem with the straight connector seen in the pictures, but it is still a bit there.
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Unread postAuthor: Hubb » Sat Nov 17, 2007 9:55 pm

Have you tried using a washer? Try to find one that is the same diameter as the ID of your barrel. That will give your piston plenty of support.
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Unread postAuthor: mark.f » Sun Nov 18, 2007 8:26 am

Beans, for a sealing face, I would use either innertube rubber or sidewall rubber from a tubeless tire. The outside is pretty smooth rubber, (unless they've still got all those little prickly things sticking out of it, but most older tire don't), and the inside is nice and strong. It might make a really good sealing face if you press it between two weights overnight.

Also, Hubb, I'd have to disagree on the Vaseline thing being damaging to PVC. The chemical compatibility sheets I've read say PVC pipe is OK for use with refined petroleum oils up to 140 degrees Fahrenheit.
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Unread postAuthor: Los Frijoles » Sun Nov 18, 2007 7:09 pm

I just discovered why I had the piston inverted (the cup side towards the barrel). If I un-invert the piston, there will not be enough space for the piston to move around.

Also, I know that a piston travel of half the barrel diameter is a good or minimum amount (not sure which), but does this extend to this end of the spectrum with a .25" travel? Should I saw off a bit of the barrel to make the travel longer?
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