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m72 LAW Design questions

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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Unread postAuthor: jackssmirkingrevenge » Tue Jul 24, 2012 11:50 pm

If you have limited tooling and budget, you might want to consider what would be the least complex and most efficient solution:

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Use the squishy properties of a Nerf Vortex to squeeze it through a restriction. Your trigger will feed pressure behind it, which builds up until the projectile pops out like the proverbial champage cork.
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Unread postAuthor: Mr.Tallahassee » Wed Jul 25, 2012 12:02 am

Guys, Do not use pvc washers on a piston. They will snap extremely easily. If you want to make washer, use a hole saw for metal and cut them your self. Sand the edges smooth to keep from cutting the o-rings in the piston. Seriously, don't use pvc washers.
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Petitlu wrote:My balls are made of lead and weigh 22g but I can make heavier or lighter ...
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Unread postAuthor: Zorlord » Wed Jul 25, 2012 1:31 am

@JSR: I'd do that, but i need a way to safely regulate and measure the pressure.

@Tallahassee: Okay, will do. I have hole saw bits, i just cannot seem to make square cuts using a drill..
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 25, 2012 2:46 am

Considering that going "valveless" gives you zero valve opening time and maximum flow, you will not need that much pressure for good performance, 50 psi or so should do - some GGDT modelling will give a better idea.
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Unread postAuthor: iknowmy3tables » Wed Jul 25, 2012 9:20 pm

I wouldn't consider nerf footballs to be air tight projectiles, especially when they age
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Unread postAuthor: Zorlord » Wed Jul 25, 2012 10:50 pm

Well i started gluing today, and ive come to realize that PVC glue sets FAST. I was trying to push the 1 inch pipe through the one bushing so it would be exposed inside the chamber, and unfortunately i thought the glue would set slower than it did.. So now my one inch pipe is a fraction of an inch on the inside of the bushing..

Sigh... Hopefully my piston will still seal against the bushing face..
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Unread postAuthor: Mr.Tallahassee » Wed Jul 25, 2012 11:44 pm

Theoretically, it would work even if you had an entirely smooth face. Air can get into the smallest of gaps. I am pretty sure this has been discussed somewhere before. So long as you have a smooth face to seal against, it should work fine.
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Unread postAuthor: Live1nce » Thu Jul 26, 2012 12:42 am

You should post some pictures of your project as you go! :D It would be interesting to see how you do with minimal tools.
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Unread postAuthor: Zorlord » Thu Jul 26, 2012 12:48 am

Well the inside of the bushing is smooth, but its also rather sloped.... Oh well. Going to totally wing it... If it doesnt work im out 8 dollars of fittings and leftover 2 inch pipe.. Not that big of a deal.

I shall take pictures tomorrow then! (Or today rather.. My sleep cycle is super off and im wide awake.

So far I've used a mitre block and a hand saw to cut my PVC pipe to the lengths that i needed. And then i just glued everything together using pvc glue.. Next step is figuring out how the hell to make those piston faces...


Stay tuned folks...


EDIT:

Well, looks like my ability to glue sucks.. One of the bushings didnt completely glue to my chamber, so when i did a quick pressure test, all the air leaked out lol. Good news is that when held so the washers would seal the barrel, they held, or at least put up more resistance than the leak i had..

Im looking at this as a learning experience.. Pics will be up soon.

EDIT #2:

Pics!

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I now realize the coupling i cut off was a DWV coupling.. Good thing it failed now rather than later.. also, pictures of my piston are up.. it seals the barrel fairly well (leaks a bit, but not terribly bad), but when i trigger the ball valve, all the air shoots out the back.. Im assuming i need a better seal on the back of the piston?
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Unread postAuthor: Live1nce » Fri Jul 27, 2012 2:35 am

Bummer about the leaky glue joint. I once was able to fix a leaky joint on a cannon by using some 5 minute epoxy and globbing it around the leak. As long as the hole is small enough, it should be able to hold the pressure ok. Especially if your only using it for airsoft, you won't be pumping in anything over 60psi really.

The air coming out the back without the piston moving could be a few things. I have no experience with this type of cannon, and have only made one coaxial, and it was a piece of junk. lol So someone correct me if i'm wrong with my assumptions here..... It could be possible that your not releasing enough air out the ball valve quick enough. What size is the ball valve? Judging by the photo, it looks like quarter inch, which I would think should be more than sufficient enough. But like you said, a better seal might do the trick. You could try putting an O-ring around the rear piston and drilling a very small hole through the face of it so air can still pass through.

Everything looks good so far though! I think you may inspire me to build one of these projects for my airsoft loadout as well. 8)


Oh and BTW: what is that rubber piece for the barrel seal? Where did you get it?
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Unread postAuthor: Mr.Tallahassee » Fri Jul 27, 2012 2:49 am

You definitely need an o-ring seal on that piston. Use a very small drill bit to put an equalization hole in the piston so that your chamber will fill and still allow piloting. I actually used washers and did a very similar valve. Without the seal, it will not work unless your piston has a tolerance ranging in hundredths of an inch.
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Unread postAuthor: Crna Legija » Fri Jul 27, 2012 2:58 am

id just give the pistons a layer or two of tape to thicken it up a bit.
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Unread postAuthor: Mr.Tallahassee » Fri Jul 27, 2012 3:01 am

Crna Legija wrote:id just give the pistons a layer or two of tape to thicken it up a bit.
Geez, I always over think things. :lol:
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 27, 2012 4:01 am

When you can't afford the best tools or materials, you need to be a little creative. Here's what I had done using a piston from a ordinary cheap-o footpump to make a toolies piston: http://www.spudfiles.com/forums/viewtop ... tml#304901

Also, that bracelet.

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:D
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Unread postAuthor: Zeus » Fri Jul 27, 2012 4:26 am

JSR, it's 550 paracord, it's practical and manly...

Yeah, you're right.

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