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taking a shot ...

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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taking a shot ...

Unread postAuthor: tape fist » Sun Feb 15, 2009 3:48 am

I`ve only experimented with PVC pneumatic cannons and wanted to broaden my horizons and make a copper one.I was wondering by what you see in my plans,how much psi do you think i could take this up to?The reason there are pieces that seem like there is a waste and they shouldn't be there is because i wanted to add more length to stock so it wouldn't be to close to the handle were the blow gun is.
Give me any suggestions on how i may better this design and even make it cheaper but i wound like to keep every thing thread so i don't have to deal with welding.
Thank you
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Unread postAuthor: starman » Sun Feb 15, 2009 4:05 am

Assuming you get the propane tank mod solid, the blow gun is your weak link there...practical limits of the gun in the 200-300 psi range. Others may have different opinions but that's going to be the limit with stardard air compression and even that is with a fridge compressor or hand shock pump. HPA, CO2, NI can offer much more of course...not sure I would take much further though.
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Unread postAuthor: inonickname » Sun Feb 15, 2009 6:34 am

It's around a 99% chance that the blowgun is the lowest rated part in that. Check the pressure rating on the blowgun. Some people have used them well in excess of their pressure rating, but I really wouldn't run that line..

Remember to solder/braze/thread together the pieces! Compression fittings are suicide... (brazing will reduce the strength by wrecking the annealing, consider re-annealing or soldering.)
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Unread postAuthor: Gippeto » Mon Feb 16, 2009 3:10 am

Threaded copper fittings are kind of rare here, so I'll leave that alone.

A couple of points;

The 90 deg. elbow leading off of the chamber will hurt flow. I would change it out for a couple of street 45's.

The opening in the propane bottle/chamber is going to be a major flow restriction. It might be fine on a launcher with a smaller barrel (<3/8"), but it won't fly with a 3/4" barrel.

Brazing copper is just a waste of time for our purposes. (JMO) The temperatures required for brazing will anneal the copper, lowering the safe working pressure.

And I rather doubt most spudders have the facilities to re-temper an assembled canon, if it were possible to do so.

Soldering/sweating is the way to go.

95/5 Tin/Antimony is the best solder strength wise, but 50/50 Lead/Tin will suffice for most.

If you want to work with copper, download this, and give it a read. :)

http://www.copper.org/publications/pub_ ... ndbook.pdf

More interesting reading on brazing joint/lap strength calculations;

http://www.thefabricator.com/TubePipeFa ... cfm?ID=575
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Unread postAuthor: MRR » Tue Feb 17, 2009 2:58 pm

That's pretty the same gun that i build. I always had trouble with the ,full body, hot glue piston and a blow gun. It simply didn't produced enough negative pressure to make the piston hit back. A 1/2" ball valve works fine though.

I would consider to try another piston design. A M6 bolt, washer and rubber gasket works great with a blow gun.

If you like, you can take a look at it (old version of the gun)

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