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Semi-auto piston cannon?

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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Semi-auto piston cannon?

Unread postAuthor: PinHead » Fri Aug 10, 2007 4:31 pm

Has anyone made one? And what methods have people used to accomplish this? I did a search through the showcase but I didn't find much. I'm working on a concept at the moment that would work on a barrel-sealing valve, but I was wondering what anyone else might have already done.
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Unread postAuthor: Velocity » Fri Aug 10, 2007 4:36 pm

Yes... just use a pop-off valve as the pilot valve.
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Unread postAuthor: PinHead » Fri Aug 10, 2007 4:41 pm

I assume that you mean a safety valve, which dumps automatically at a certain pressure? So you just hook up your air source, and it would fire as soon as it reached the desired pressure?

I was thinking more of something that could be powered by a 20 oz. CO2 cylinder (probably going to end up being a paintball grenade launcher, in this case) and be able to run on a continuous air supply, that would be pressurized and ready to fire at all times. The problem that I see, as illustrated by the sticky on piston valves, is that you need a separate valve for pressurizing the chamber and for a pilot. Has anyone been able to combine that into a single 'trigger' pull?

It also seems to me that if you had a continuous air supply, opening a pilot valve without discontinuing your air supply would cause you to lose a loooooot of air. Not good for running on a compact bottle of CO2.

I'm pretty new to pistons and haven't made one yet, so forgive me if I sound really ignorant here. Just checking out some ideas.

rmich, do you have any examples of the pop-off valve type?
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Unread postAuthor: Skywalker » Fri Aug 10, 2007 5:16 pm

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Unread postAuthor: turbohacker » Fri Aug 10, 2007 5:33 pm

You do understand that a piston will reseat itself if pressure is always going into the gun, don't you?

Making a real semi automatic gun will require some special parts, mainly a bolt, to close off the hopper while firing the round.

Good luck if you plan on undertaking this project, however I would recomend building a simple piston or diaphragm gun first. This is, of course, your choice. :D
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If it walks like a dog, sratches like a dog, and barks like a dog, than its probobly a...-TURTLE, it's a TURTLE!!! Cant you see?!!!

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Unread postAuthor: PinHead » Fri Aug 10, 2007 6:03 pm

I do understand that the piston reseats itself as soon as the pilot is shut (with a constant supply of pressure). My point is that when you open the pilot, you lose the air from behind the piston (obviously necessary, it's what fires the gun, I understand that of course), but then you lose quite a bit of air from your continuous pressure supply too, which is still flowing. If you're talking about a bottle of CO2, that's not a good thing. Am I making any sense at all when I say that?

I do plan on making a simple piston gun to start. The semi auto design that I have in mind can be added directly on at a later date... assuming that it works, anyways.

Skywalker, I was asking specifically if anyone had made a semi-auto using a piston design. More specifically, a barrel-sealing or chamber-sealing tee valve piston. Other than the 2 designs by clide, the others seem to be sprinkler-based. I already considered one of those, but I thought I'd check this route out to be different.
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Unread postAuthor: turbohacker » Fri Aug 10, 2007 6:12 pm

If your piston is effecient (moves back 25% of diamiter of barrel) than the air loss due to pilot volume will be almost nothing, unless you hold down the trigger.
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If it walks like a dog, sratches like a dog, and barks like a dog, than its probobly a...-TURTLE, it's a TURTLE!!! Cant you see?!!!

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Unread postAuthor: PinHead » Mon Aug 13, 2007 10:38 pm

I think I see what you mean now. Essentially then, any piston with a constant flow of pressure would basically be considered semi, seeing as the piston would always reseat itself after every shot when you let off the trigger. That's assuming that you have a loading mechanism to go with it, of course; but I'm working on that. I have a pretty good idea in mind, so we'll see how well it works.
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Unread postAuthor: spud yeti » Wed Aug 15, 2007 4:35 am

Im building a semi-auto paintballgun. It is acuated by a piston in the end (with other custom parts of course) Its nearly finished, but I think I will save it till the next comp (I have faith in it :D )
If I were to PM you some plans, pretty please

A) dont share them to anyone,
B) wait till I post mine until you post yours (if you build it of course), and
C) give me some credit :wink:

But I am willing to send you my designs if you wish

Oh ja, and clide's "slider" semi-auto cannon is also a piston cannon
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really good quote/phrase here
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Unread postAuthor: chaos » Wed Aug 15, 2007 5:20 am

I converted my S400 to semi auto not to long ago, just continues pressure source a self loading magazine/breech and it worked pretty good, i only had one problem with it which was the pressure source (propane) which did not meet my standards on power :D lol
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Unread postAuthor: mark.f » Wed Aug 15, 2007 6:16 am

Any semi-automatic design has basically two parts to it.

The valve, and the mechanism to trigger the valve repeatedly and close off the pressure source when firing.

The loading mech., which is usually just gravity fed with something to interrupt the feed of ammo when firing, (or else you'll just get a burst of ammo). This can be as simple as a paddle or as good as a blow-forward bolt. Here's a hint: if you're using small projectiles, don't use the paddle method. I found it didn't really work good with marbles.

As far as the first component, your pilot valve could be some sort of three way valve, where in on position it supplied pressure to the gun and closed off the exhaust, and in the next position it opened the exhaust and closed off pressure to the gun. Cheapest three-way I've seen are three-way ball-valves, but if you want an easier pilot, you could make it even more complex by piloting and charging the piston pilot through a Q.E.V., (which are smaller and much easier to pilot), and piloting the Q.E.V. with some sort of toggle-switch styled three-way valve. I've got a three-way ball-valve lying around that I was going to use for propane metering, but it didn't work like I wanted it to because, between positions, it allowed propane directly from the pressure source, (not the metering pipe), into the chamber, throwing off the mix. If you wanted that I might be able to arrange for shipping to you, (I don't need it anymore).
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Unread postAuthor: PinHead » Wed Aug 22, 2007 5:55 pm

It's funny that you mention making a three way valve, because that's just what I had in mind, although not a ball valve but an inline one similar to those used on autococker-style paintball guns.

Basically, the center hole is constantly open, which leads to the pilot chamber behind the piston, and the 2 pistons slide back and forth to close off the ports to the exhaust and the air source. A trigger could be put at one end to push and a spring at the other, to make it slide back.

Please excuse the crappy quality jpeg of my diagram.
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