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auto-piston prototype

Built a BB Machine Gun? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Copperboy » Tue Jan 22, 2008 11:21 am

Wonderful thing there JSR! :P It's really shows the strengths and uses of epoxy. If the ROF changes with the tension of the spring, it would be an easy task to have some sorts of ROF switch, talk about potential!! :P
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Unread postAuthor: Brian the brain » Tue Jan 22, 2008 4:43 pm

Congrats JSR!!
ROF of the mmg I made was diapointing...about 1 every half second, and it took too much gas to work at 20 bar
It went like pshhhht-BANG!!pshhhhhhhhht-BANG.( it was powerful, don't get me wrong)

You inspired me so much with this video, I decided to convert my mmg to a smaller caliber ( 11 mm ballbearings) and use the pop-off as my main valve!

The jet of gas coming from the snapvalve was over a foot long, so I thought it would work well.
It does...
It blows the ball bearings through the same guitar just like it did with the marbles, but it now runs on the gas it used to waste only!!
New bolt works much better aswell.
This new configuration saves me 1 QEV...wich will be used on...

TUM-TUM-TUUUUUUUUMMM:

Sawed-off take 2!!!


Anyway..I gotta see that vid again!!Very cool!
Brrrrrttttttttt

Good luck on the rest of the build!
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 22, 2008 10:40 pm

I tried fitting to to the blow-forward breech and it cycled beautifully, I could make a magazine etc. but it's too much work for something that might break inside in a manner which would make it impossible to repair, so I've decided to start over.

Brian, I think we have the same idea - I think this design is too complex internally, and also, I have my doubts that the chamber is up to full pressure when the pop-off fires. What I was thinking of making was a large, heavy dudty pop-off as the main valve. The design would be something like the one proposed here and identical in concept to what skywalker suggested here

Image

Basically close to your snap-valve, but I think the provision of the rod which allows a restrictor valve to close is an important addition to the design, this will ensure that the firing chamber will empty completely, allowing the piston to seat before air is introduced again.

it now runs on the gas it used to waste only!!


Another justification to use this design, being European and all that - more efficiency!

I *really* want a shell ejecting mechanism to reload new rounds and I'm confident this valve will give enough blast to reliably cycle such an action. Watch this space (as you've all learnt to do patiently by now :roll: :D )
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Unread postAuthor: SPG » Wed Jan 23, 2008 6:52 am

Jack, if you have the airsupply closing then of course one issue I can see will be that the "firing pressure" will be less than the "filling pressure", as logically the chamber volume increases as the piston moves back but no more air can come in.

Obviously this will make a difference to the force exerted on the spring, as of course greater volume = reduced pressure which actually means reduced force. The way that's being overcome is by the stepped piston. What will be the hard bit will be tuning the piston size and the spring rate, but if you can build in even one adjustment you should be good to go.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 23, 2008 7:23 am

the "firing pressure" will be less than the "filling pressure"


only slightly, proportional to the small volume displaced by the moving piston.

You need to block off the airflow, because there is a period of movement where the outlet is closed and the piston is still moving to seat - if enough pressure builds up in that interval, the piston will "hang" open and it won't cycle.
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Unread postAuthor: daberno123 » Fri Jan 25, 2008 6:53 pm

Is there any reason to be using a ball shaped piston on the fill side? Won't it not let any air into the firing chamber?

EDIT: Never mind i thought your arrow was pointing to the space around the rod as the firing area. I get it now
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Unread postAuthor: Antonio » Tue Jan 29, 2008 2:15 pm

Hey jack, I am not sure where I c the factor'' that makes sure the rate of fire is low. In your previous design you had a small tube leading to the buffer part. I cant really c where this is in this design.
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 2:20 pm

That was just a concept drawing, in practice the spring strength would need to be adjustable
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Unread postAuthor: Antonio » Tue Jan 29, 2008 2:42 pm

jackssmirkingrevenge wrote:That was just a concept drawing, in practice the spring strength would need to be adjustable


Yeah k, sounds coola. So how are you progressing on the project? I am curious at how its doing as it looks so damn simple''. Btw if u need ideas for the lower poppet valve just ask :)
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 2:57 pm

hehe you would certainly be the man to talk to.

I've actually stopped work on this for now while I deal with some other stuff, but I haven't abandoned the idea.
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Unread postAuthor: Antonio » Tue Jan 29, 2008 3:04 pm

I could give this design a try sometime. The only limitation that I have is that the valve head area will prob be the same as my hammer/bolt'' area. But by changing the pressure of the springs it could still be achieved. As I dont work with pistons the seal wouldnt be that good as well. But yeah this design doesn leave the hammer/bolt to stick as you said. Thats the key. The thing that I am afraid of though is that this is only a part of a gun which consists of 3 parts. The trigger (blowgun) this part and the autobreech design part. So afterall its a fairly big project thats rolling here.
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 11:08 pm

That was the reasoning behind this idea, it combines the valve with the breech in one go.
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Unread postAuthor: rna_duelers » Wed Jan 30, 2008 1:15 am

Speaking of pistons,my newest epoxy creation doesn't fit.I was wondering if you could bless my next epoxy piston casting JSR?
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 30, 2008 1:25 am

My work seems more of a curse that a blessing these days :roll:

How come it didn't fit? I usually find my pistons need a light sanding in order to move smoothly. Also be careful testing movement if the chamber tube is flexible, what you might think is a blocked piston could be a result of squashing the tube while holding it.
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Unread postAuthor: rna_duelers » Wed Jan 30, 2008 1:31 am

I'm not sure why it doesn't fit,I've given it a light sand and it fit ok,but now it seems worse then before.I might just have to cast another one.I think there could be something were my piston is housed wrong,it seems to get tighter as it goes towards the barrel.If its so then I will have to tapper the piston.
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