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Semi-auto piston under constant air flow.

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 under constant air flow.

Unread postAuthor: Leonard » Sat Aug 09, 2008 7:45 pm

Hey guys! Just a quick question for you and hopefully some inspiration if there is anything new in what i am presenting.

The air input in this design is the vertical tube at the bottom, where there is an arrow pointing into it. By leaving constant air flow through the input, can i achieve a semi-auto piston by opening the exhaust with a solenoid valve for a second and then closing it?

Image

I fear that the piston might not actuate if my pilot as less flow than my barrel. What do you guys think? This is a for piston semi-auto design, loosely based on a spool valve design (obviously the bolt part in not drawn, i'll keep it for later :wink: )
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Unread postAuthor: clide » Sat Aug 09, 2008 11:26 pm

It would probably work, but it would be easier and most likely more efficient to just hook up a 3-way valve for the exhaust.
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<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult

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Unread postAuthor: noob of noobs » Sat Aug 09, 2008 11:50 pm

@ Leonard: I think that looks like a great idea. If you can get a good seal between the exhaust vavle and the input valve with that piston, I think it'll work. The extra pressure from the air source might actually help push the piston forward if it can't get behind it.

EDIT: With the three way valve, how would you use it? You'd have to turn it one way to put gas in, and turn it another to get gas out right? So your entire trigger action would involve 2 turns, right?
I think a better investment would be to simply use 2 valves. For my design, I'm considering using a blowgun to allow propane into the gun triggered by my left hand and a blowgun used to trigger the gun in my right hand. It'd still be 2 actions, but seems a bit easier to use than turning the same valve twice.

Or you could just use the design you already have planned out and hope it works.
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Unread postAuthor: clide » Sun Aug 10, 2008 11:53 am

noob of noobs wrote:EDIT: With the three way valve, how would you use it? You'd have to turn it one way to put gas in, and turn it another to get gas out right? So your entire trigger action would involve 2 turns, right?
I think a better investment would be to simply use 2 valves. For my design, I'm considering using a blowgun to allow propane into the gun triggered by my left hand and a blowgun used to trigger the gun in my right hand. It'd still be 2 actions, but seems a bit easier to use than turning the same valve twice.

Spring load the 3-way valve so you only have to pull it in one direction, then you just release to recharge the gun. http://www.spudfiles.com/forums/semi-auto-t160.html

Or you could use an electronic 3-way with a momentary switch for even faster actuation.
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<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult

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Unread postAuthor: Leonard » Sun Aug 10, 2008 4:07 pm

Thanks for the help guys. I could surely use a three way solenoid, if i wanted to build a gun that would cost over 200 bucks to make. Im basicly working on the most efficient semi-auto design V.S. lowest cost and number of tools required.
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Unread postAuthor: clide » Sun Aug 10, 2008 4:38 pm

Ebay is a pretty good source for cheap valves. Although they sometimes take a bit of searching to find the right keywords.

One of the downsides to this design is that you need to rely on an restrictive equalization hole (or piston blow-by) for filling. With a 3-way valve you can put a check-valve in the piston to create fast fill times, however with this design increasing flow to the chamber side may cause some valve seating problems and probably a further loss of efficiency.
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Unread postAuthor: Leonard » Sun Aug 10, 2008 4:53 pm

The barrel will be 6mm, so you can guess that the chamber will be very small. I will be using high pressures (possibly up to 500psi), so volume of air will not be all that important.
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Unread postAuthor: jackssmirkingrevenge » Sun Aug 10, 2008 10:27 pm

You don't really need the complication of a spool valve, have a look at what I had done here:

Image

The chamber is refilled through a restricted opening from the firing side while the pilot is filled by air blowing by the piston, held shut by a spring in the meantime.

Three-way valves are a good idea, you might want to look at how pneumatic nailguns work for some inspiration. You can also pick up such devices quite cheaply off auction sites and with a bit of ingenuity and lots of epoxy, they can be readily converted for this purpose ;)

I would also suggest reading through this thread which includes a discussion on a similar system, incorporating a piston driven bolt reloading mechanism - though as far as I know no actual prototypes were built.
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Unread postAuthor: Leonard » Sun Aug 10, 2008 11:08 pm

I get where you're coming from. But, considering what are my goals, i'd like you to take a look at the full design, including the bolt :

Image

This can be completed with four part :
- 1/2" copper pipe
- 9/32 brass pipe (barrel)
- 10/32 brass pipe (perfect fit around barrel)
- Epoxy

I've already made a working prototype of the piston/bolt that worked with two blow gun. One to fill and one to pilot. Slow firing since i had to depress the fill blow gun and press the pilot, but it worked pretty well! Here it is :

Image

Image

So basicly im just looking into upgrading the working version i have with the pilot method i am describing in this thread. I managed to make the first prototype for under 20 dollars (excluding c02 fill system, obviously) and i had satisfing performance.

I can understand that this design would be hard to get to work properly, but i just dont wanna copy your ideas as i have done in the past. I came up with something relatively new with the prototype i just posted, i'd like to get it to work even better (full auto with a pop-up as pilot and blowgun as fill). Sharing the results with the spudfiles will be more satisfing if i build something new ^_^

Thanks for your help, trow in some feed-back on the pilot im presenting.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 11, 2008 7:56 am

I understand the need to innovate, though If I could make one suggestion, a smaller secondary piston as shown in the attached diagram would make it much easier to re-seal the piston.

Alternatively, you might want to consider making your own three-way-valve-on-a-budget using an ordinary ball valve and a drill ;)
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Unread postAuthor: al-xg » Mon Aug 11, 2008 10:02 am

Oh, just saw a similar design on the french forum, either coincidence or someone got inspired :)

http://patator.frbb.net/depannage-f10/qev-semi-auto-t3097.htm

And my drawing of a possible set up, using brass fittings (not very easy to get pressure PVC in France).
A washer and a spring would go between the pilot valve and the piston, to push it forward.

Image

With this set up the main chamber or filling port, is on T no1, so the only action needed to fire is the opening and closing of the pilot valve.
Reloading of the ammo could be achieved in many ways.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 11, 2008 10:33 am

Love the hand-drawn exploded view, class :D
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Unread postAuthor: al-xg » Mon Aug 11, 2008 10:41 am

Hand drawn into MS Paint :wink:

Oh and I just read the topic title ( I just saw the pictures the firt time :oops: ), and there are actualy working constant flow semi-auto piston canons on the french site too.
At normal spudgun pressures they don't waste too much air, and are really quite simple to design and make.
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Unread postAuthor: trollhameran » Mon Aug 11, 2008 10:47 am

My pump action prototype is just a normal piston gun, but if i leave the compressor on, it works semi auto, you just have to open and close the ball valve quickly so you don't waste to much air. I was thinking though, if you grind of the stops on the ball valve handle and bolt another handle perpendicular to that, you could turn the handle right around like a crank, which would give you two shot for each full turn of the handle. http://www.spudfiles.com/forums/pump-action-piston-gun-prototype-now-with-working-magazine-t15576.html
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Unread postAuthor: Leonard » Mon Aug 11, 2008 12:04 pm

Al-xg, on se voit bientôt sur le forum français! Merci de ta participation dans ce sujet. As-tu déjà construit le prototype illustré dans ton dessin?

Thanks everybody, i'll get to the building right away, you'll hear from me in a week or two with a (hopefully) working prototype.
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