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Metal T, Barrel Sealing

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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Metal T, Barrel Sealing

Unread postAuthor: Infernal2 » Mon Oct 16, 2006 9:16 pm

I'm attemtping my first metal piston gun. Now, from my understanding of the barrel seal concept a sealing face close to the diameter of the barrel diameter is desirable due to opening times. The only issue i'm having is one of finding something to back tread into the back of my barrel bushing.

I'm tried various sizes of nipples and what not. However, instead of backthreading a pipe or epoxy -ing a pipe to go into the T, couldn't I just seal against the barrel side of the T? Will this not provide enough vacuum due to larger pilot area? I'm trying to compensate for this by using a high flow mini-piston (basically a hex bolt, washer, spring, two nuts, and a fom rubber sealer).

Any help is greatly appreciated.
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Unread postAuthor: mark.f » Mon Oct 16, 2006 9:36 pm

Ha ha!!! I just made a coaxial out of metal pipe a few hours ago.

What I did was:

Take pipe wrench and a 1 x 60" steel pipe, (as a handle).

Take desired nipple the size and thread of the tee or bushing and thread it in normally.

Tighten fully with the wrench as you normally would.

Clamp bushing or tee in crescent wrench, and clamp pipe wrench around nipple, (if it's short, I don't know how you would do this without damaging the threads). Then, tighten until the end of the pipe pokes through the back of the bushing, (you basically enlarged the threads with shear force).

Untighten the nipple, flip the bushing around, and screw in the nipple through the back with a wrench. This may take some dicking around to get the nipple started.

And there you have it.
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Unread postAuthor: Infernal2 » Mon Oct 16, 2006 10:01 pm

LOL, that's awesome.... I think that may work.... maybe.
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Unread postAuthor: Infernal2 » Tue Oct 17, 2006 7:26 pm

Well, this pain in the ass has gone through three piston incarnations. I can definetly say that sealing against the exhaust port is fricking stupid in a Metal threaded T. It might work great but the overall design sucks as far as I'm concerned.

So, after using a 1 1/4 - 3/4 bushing I decided to not go with welding due to limited workspace and I used a high grade plastic epoxy to set a 3/4 copper pipe and coupling. This has extended by exhaust almost all the way to the back of the T (fill side). I've made a piston out of 1" PVC, metal hardware (nuts, bolts, etc.) and then sandwiched pan liner and foam as a sealing face.

This slides nicely into the 1 1/4 close nipple at the back of the T and rests nicely against the reducer. I've tested it with my 3/4 ball valve and it seems to work great.

I still have to make my custom sprinkler valve for a trigger as I don't want to use a plastic one, nor by a 3/4 QEV (too damn expensive).

I also added a 3/4 ballvalve to the barrel as a safety, plus it has the added benefit if I want to use the gun in a low power application I just remove the piston.
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