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I too has semi-auto

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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I too has semi-auto

Unread postAuthor: jor2daje » Wed May 19, 2010 8:15 pm

Hey guys, been a while since Ive posted on here, Ive been playing with my shopsmith making pens and such, but I while I was browsing ebay for pen kits, I came across a deal I couldnt pass up, a pneumatic cylinder with 1.5" stroke for $2.00.

So I ordered it and while I was waiting for it to arrive a designed a modification to one of my near semi-auto airsoft guns that uses a 3-way dcv and a 1/4" qev. My first attempt failed miserably because I had the cylinder running parallel to my barrel and below it, this created to much torque when it tried to move the breach and ended up breaking about 6" of my barrel.

I realized the piston had to move inline with the barrel, and it just so happend the shaft of my piston was a perfect fit into my 6mm aluminum barrel so I decided to use the shaft itself as a bolt. The only problem now was how to let air flow from the qev that was perpendicular to the barrel. My solution was to bore out about a half inch of the front of the pistons shaft and drill 3 holes to allow air to flow through it and out the barrel.

This design works beautifully and was very easy to build, I just drilled a couple holes in a block of walnut and used that as the structure to hold the aluminum tubes together.

Its not much to look at, just a couple of steel and brass fittings but it definitely is effective, when testing the bolt in my garage I accidentally shot an airsoft bb into a cardboard box of bear and broke a bottle. :shock:

High speed video and more pictures to come, I got rid of the rubberbands now, they were just for preliminary testing.

[youtube]http://www.youtube.com/watch?v=f5W2v1sD4HY[/youtube]
[youtube]http://www.youtube.com/watch?v=NRXvk0hOf8E[/youtube]
[youtube]http://www.youtube.com/watch?v=y8So0LGif6c[/youtube]
[youtube]http://www.youtube.com/watch?v=w8T0V5YMrvA[/youtube]
[youtube]http://www.youtube.com/watch?v=AzSKh-fo4aU[/youtube]
[youtube]http://www.youtube.com/watch?v=TQsaoU7NxY0[/youtube]
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Attachments
CIMG0886.JPG
where I bored out the piston shaft and drilled holes.
CIMG0879.JPG
after firing.jpg
after firing.jpg (23.31 KiB) Viewed 6585 times
after loading.jpg
after loading.jpg (23.52 KiB) Viewed 6585 times
CIMG0889.JPG
the propane tank attachment so I can fire anywhere.
Last edited by jor2daje on Thu May 20, 2010 11:54 pm, edited 7 times in total.
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Unread postAuthor: PVC Arsenal 17 » Wed May 19, 2010 8:20 pm

I admire everything about this gun. Excellent work. The bored cylinder was a smart solution and it reminds me of a problem I had to work around in a very similar situation long ago. Only difference was that I sort of sharpened the rod end to a pencil point instead.

Nice job.
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Last edited by PVC Arsenal 17 on Wed May 19, 2010 8:27 pm, edited 1 time in total.

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Unread postAuthor: POLAND_SPUD » Wed May 19, 2010 8:23 pm

we has semiauto!! 8)

nice :D
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Unread postAuthor: jor2daje » Wed May 19, 2010 8:32 pm

Thanks guys

@pvcarsenal I was considering a pencil point thing too but I though it might push bbs back up into the mag.
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Unread postAuthor: mark.f » Wed May 19, 2010 8:33 pm

Looks good. Nice to see some "gas-operated" semi-automatics around...
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Unread postAuthor: Lockednloaded » Wed May 19, 2010 8:54 pm

very cool! love the concept. POLAND SPUD used a four way in conjunction with a pneumatic ram, but this is much simpler. A cannon with the same firing mech as this may just be my summer project. what kind of three way valve? what brand of cylinder? i will probably build one of marble calibre, so will it work well with a 1/2" QEV
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Unread postAuthor: POLAND_SPUD » Wed May 19, 2010 9:00 pm

MK I and II used a 4way (5way) because I didn't have a 3 way at that time

MK III uses a 3 way

(...) marble calibre, so will it work well with a 1/2" QEV
it will, but you'd get more power if you use a 3/4" QEV
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Unread postAuthor: Lockednloaded » Wed May 19, 2010 9:06 pm

depends mainly on the size of my marbles and the availability of a CHEAP 3/4" QEV
P.s. you think you could go full auto with a pop-off? can this system handle it?
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Unread postAuthor: jor2daje » Wed May 19, 2010 9:12 pm

The cylinder is a Bimba, the valve is mead mv20 3-way valve.

cylinder: http://cgi.ebay.com/Bimba-Single-Acting ... 2a061bafb0

and yes you could probably go full auto in small calibres, the spring retracts very quickly, I'll try to get a highspeed of the loading mech.
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Last edited by jor2daje on Wed May 19, 2010 9:27 pm, edited 1 time in total.
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Unread postAuthor: POLAND_SPUD » Wed May 19, 2010 9:26 pm

to achieve fully automatic fire you need either:
1. solenoid valve and time relay/555timer
2. time delay valve

it could be achieved with a pop-off but IMO it makes it a bit more complicated and you acctually lose some performance by choosing that route
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Unread postAuthor: PVC Arsenal 17 » Wed May 19, 2010 9:28 pm

Lockednloaded wrote:depends mainly on the size of my marbles and the availability of a CHEAP 3/4" QEV
P.s. you think you could go full auto with a pop-off? can this system handle it?


I'd bet it would be quite tricky.

Even with a proper trigger, it's easy to "short stroke" a gun with a cylinder. It happens when the QEV is not allowed to remain open long enough for the entire chamber volume to be dumped. Since part of your chamber volume consists of the air inside the cylinder, the cylinder may cycle only partially if at all since there is still pressure inside it.

I would imagine that with a pop-off, you would need very precise timing and perfect coordination with operating pressures to achieve the right "balance". That's not to say it can't be done of course... but the incorporation of a cylinder makes it more difficult.
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Unread postAuthor: Lockednloaded » Wed May 19, 2010 9:29 pm

well you could put a tee before the 3-way, the tee connects to a blow gun with a pop off. then you have select fire
EDIT: missed the last post ^
yea, but getting it working right would be worth the trouble.

and what are the threads in that particular cylinder?
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Unread postAuthor: jor2daje » Wed May 19, 2010 9:41 pm

the threads are just standard 1/8" same with the 3-way valve.

I added a vid showing just how quickly the cylinder opens and closes.
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Unread postAuthor: POLAND_SPUD » Wed May 19, 2010 9:43 pm

Apart from what pvc_arsenal has said... psycix's pop-off experiment has proved that only some air (NOT all) in the chamber is exhausted... this means two things:
1. decrease in performance
2. the air cylinder won't retract

you could improvise and build a circuit with a 3 way that would switch from on position to the other... but that isn't much simpler than the two options that I posted in my previous post

ohh I just realised that there is a simpler way...
I mean...in theory it is simpler but in real life it might be a bit difficult to implement it...
It really is very simple... I'll add a pic
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Attachments
mechanical.GIF
for simplicity sake this pic shows just the 3 way (but it can be a 5 way) and the air cylinder (this one has two rods - for simplicity)...

as the cylinder retracts the rod hits the stem of the valve and it switches it from one position to the other

at that moment air rushes to the air cylinder and it starts extending....

the stem and the rod are connected with a steel wire that pulls the stem back to its original position when the air cylinder is fully extended

the cycle repeats
mechanical.GIF (3.57 KiB) Viewed 6508 times
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Unread postAuthor: Lockednloaded » Thu May 20, 2010 3:00 pm

I see what you are saying, i hadn't really read the results of the pop-off experiments thoroughly.
sorry to hijack a little, but this doesn't deserve its own thread...
would a 1/8 3-way pilot a 3/4" QEV effectively? if not what is the maximum size QEV i should use?
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