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Mini Coaxial Piston Help

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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Mini Coaxial Piston Help

Unread postAuthor: ethal1222 » Fri Feb 09, 2007 4:59 pm

Hello all,

I just finished building my first piston gun. It's a mini coaxial version with a chamber made from 2" SCH 80 pipe and a 1/2" SCH 40 barrel. My piston is a 1" coupling wrapped in duct tape (crude I know) to fit the 2" SCH 80 pipe with a neoprene washer on the end to seal against the barrel. I used a blowgun as the trigger valve.

The piston slides forward and seals against the barrel perfectly every time I pressurize the gun. It doesn't leak at all. But, when I hit the trigger, all the pressure drains through the blowgun and the piston doesn't move at all.

I have seen guns very similar to this one (also with pretty crude pistons) that the builders say work very well. So please, criticize me, tell me what I'm doing wrong, what I'm overlooking, what I should do differently, or just offer some general advice. I just want to get this thing working.

Thanks.
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Unread postAuthor: LGM » Fri Feb 09, 2007 5:06 pm

You are using a blowgun to vent it? If so, you will need at least a quarter inch ball valve. A modded sprinkler valve would be really neat, professor amadeus made a coaxial with the same barrel and chamber sizes as yours and used one of those.
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Unread postAuthor: ethal1222 » Fri Feb 09, 2007 5:21 pm

Hey LGM,

Modifying the blowgun occurred to me. Do you know what exactly was done to the one you're talking about?

I have some extra 1/4" ball valves so maybe I'll try using one of them instead.

Keep the tips coming though.

Thanks.
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Unread postAuthor: LGM » Fri Feb 09, 2007 5:25 pm

Modding the blowgun still won't be enough to vent it. Just replace the blowgun with the ball valve, they should have the same size thread.
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Unread postAuthor: ethal1222 » Fri Feb 09, 2007 5:27 pm

Sorry about that last question LGM,

I misread what you wrote. Adding a sprinkler valve also occurred to me. If all else fails, I'll try that (I'm trying to keep this gun as sleek and low profile as possible).
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Unread postAuthor: Hawkeye » Fri Feb 09, 2007 6:15 pm

You shouldn't have trouble venting it with a blowgun. I really don't see why so many people have trouble with them. I have built large copper guns that are vented with a blowgun. I think the key is to try to make your piston a better fit.
Don't use the crudeness of other functioning pistons as a lowered standard for your own.
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Unread postAuthor: ethal1222 » Fri Feb 09, 2007 6:24 pm

I just replaced the blowgun with a 1/4" ball valve and the gun works like a charm now!

Here's a photo.
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