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preloaded pneumatic cartridge

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 » Sat Jan 16, 2010 2:33 am

maybe you should try and make a valve which can take alot more


Or I could keep the schrader and have it recessed as in my original design.

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Unread postAuthor: chinnerz » Sat Jan 16, 2010 3:13 am

i was talking about the core
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 16, 2010 3:23 am

chinnerz wrote:I was talking about the core


Not really worried about that :)
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Unread postAuthor: mega_swordman » Sat Jan 16, 2010 4:33 am

chinnerz wrote:I don’t know how much abuse the Schrader valve can take, maybe you should try and make a valve which can take alot more


Although plausible, the problem I see would be keeping the homemade schrader down to the same size and still function as well as the original. It is possible, but it would be a tad bit difficult to construct.

Personally, I think all you need is some way to cover the outside of the schrader, that way it doesn't have the end sticking out. As for the mouth of the cartridge, shouldn't you be able a ring around either the inside or around the outside to help keep it in shape?
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Unread postAuthor: john bunsenburner » Sat Jan 16, 2010 5:30 am

These are prototypes, so it doesn't really matter how many shots they survive. Right now we need to know if it is feasable, or if we are wasting our time, if we know this works we can take a closer look at making it work even better, having long lives for the cartridges, or better valves.

As for the shot gun shell type cartridges; will you use buck shot type ammo, stick to a single pellet, or have slugs as ammo?
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 16, 2010 6:46 am

I really want there to be blowback potential so I'm making a new cartridge, 0.22" in a shorter but much wider (24mm O/D) body, giving it around 5 times the volume of my previous 0.22" cartridge. Also, the ratio between cartridge face area and calibre is 19:1, and at just 5 psi there will be a force of over 3lbs attempting to push the cart out of the breech.

The disadvantage is of course the more involved construction, which requires casting with marine epoxy and longer fabrication time, but if it works it will be worth it.
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waiting for the epoxy to cure...
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Unread postAuthor: JDP12 » Sat Jan 16, 2010 9:11 am

Nice... Can't wait to see resuts!!

However I've been wondering. Why is it you always put the piston in a smaller tube and epoxy that into the chamber? Wouldn't it be simpler just to mAke the piston the diameter of the body?
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Unread postAuthor: chinnerz » Sat Jan 16, 2010 9:37 am

i too wonder this, i think it may have something to do with reduced pilot area, and increased actual tank area, or that id will depressurise quicker
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 16, 2010 9:41 am

JDP12 wrote:Why is it you always put the piston in a smaller tube and epoxy that into the chamber? Wouldn't it be simpler just to make the piston the diameter of the body?


Cutting solid rubber pistons with a sharpened tube is the quickest and easiest way to make them, which makes the method ideal for cartridges that have to be produced in quantity. However by definition I'm limited by the size of tube that fits in my drill, so if the chamber body is any bigger than half an inch or so I don't have this option.
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Unread postAuthor: dewey-1 » Sat Jan 16, 2010 10:13 am

JSR;

The latest .22 version looks promising!
If it works out let me know and I will do CAD drawings for you.

Do you use a vacuum pump to eliminate air bubbles after mixing epoxy?
In my past experiences with potting/encapsulation of electronic circuits, the removal of air bubbles was of utmost importance.
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 16, 2010 10:30 am

dewey-1 wrote:The latest .22 version looks promising!
If it works out let me know and I will do CAD drawings for you.


More volume and more area, let's hope it works out :) I would have preferred a leaner cartridge as it makes centering the "barrel" much easier but from my extensive experimentation it's clear this isn't going to work out, the calibre/cartridge face ratio has to be considerable even at high pressures. CAD drawings would be great for when it comes to modelling a launcher.

Do you use a vacuum pump to eliminate air bubbles after mixing epoxy?
In my past experiences with potting/encapsulation of electronic circuits, the removal of air bubbles was of utmost importance.


What I usually do, especially in winter, is pour the epoxy parts into a container then let it sit in a hot water bath for a few minutes. This makes the mixture very thin, ensuring that the two parts will be thoroughly mixed and that it will flow well into the cast, and also that any bubbles generated by mixing (and there are usually a lot because I do with with my drill) will rise to the surface.
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Unread postAuthor: dewey-1 » Sat Jan 16, 2010 11:46 am

JSR;

If you can ever get a used small vacuum pump, do so. You will be surprised how many air bubbes will come out even after using heat to lower viscosity. It will defintely increase the strength of your casting!

You could use a small oil less compressor like this for a vacuum.
The vacuum chamber is hooked up to the compressor air intake.
Here are some ideas;

http://cgi.ebay.com/Vaccum-Pump-Air-Com ... 1e59ff90e4

http://cgi.ebay.com/Non-lubricated-Labo ... 27ae8c3495
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 16, 2010 12:05 pm

Interesting thought, I never considered it.

I use vacuum a lot in the lab, both for filtering and drying, so I have plenty of sources. A vacuum pump almost anyone can make is a water aspirator, all you need is running water ;)

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I also have an ultrasonic bath available that would do the same job.
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Unread postAuthor: qwerty » Sun Jan 17, 2010 5:25 am

What I usually do, especially in winter, is pour the epoxy parts into a container then let it sit in a hot water bath for a few minutes. This makes the mixture very thin, ensuring that the two parts will be thoroughly mixed and that it will flow well into the cast, and also that any bubbles generated by mixing (and there are usually a lot because I do with with my drill) will rise to the surface.


Thanks for that tip, i always found it really hard to pour epoxy when it didn't even come out the container!

Does it not set even if you're using 5 minute stuff when it's in hot water?

EDIT: damn!, i just realized i bought the same signature again :roll:
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Unread postAuthor: kozak6 » Sun Jan 17, 2010 5:53 am

Have you tried loading multiple projectiles into a cartridge?

Do you think this could increase recoil to the point of allowing cycling?
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