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SKC2 semiauto, almost.

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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SKC2 semiauto, almost.

Unread postAuthor: spudamine » Wed Dec 23, 2009 3:56 pm

Here’s a cannon I’ve been building on and off for some time now, it’s not finished and probably never will be due to some shortfalls in the design but I thought I’d share it anyway, that’s why it’s not in the finished cannons.

[youtube]http://www.youtube.com/watch?v=QwZQyAVx0TA[/youtube]
Basic function is a ½” QEV with a slide valve for filling and piloting. The fill reservoir consists of two 88g Co2 tanks fitted into a custom manifold/butt plate.
The chamber contains a piston forming an air cylinder which operates when the chamber is filled. The piston is coupled to a moving barrel which slides over a round from the magazine and pushes it into the breech. On piloting the round is fired, the chamber empties and a spring returns the barrel forward allowing another round to be taken from the magazine.
I’ve gone for a moving barrel here over a bolt for efficiency reasons, given the projectile is a 65mm long dart a bolt would have added significant dead space, quite an efficiency killer given the small chamber.
The only large jobs left are making a stronger spring and catch for the magazine, some detents to position the round accurately enough for the barrel to fit over it, and a linkage between the trigger and slide valve. (I did experiment with mechanically linking the slide valve and air cylinder for proper semi-auto which worked but was far too fiddly).
As mentioned above I’m not sure if it’s worth finishing this gun for several reasons. As I’ve alluded to in other threads the slide valve is not very effective at high pressures and if you don’t move it very fast you lose a lot of air. The reload system is also problematic and requires quite a bit of fiddling to get it working properly, the problem is getting it to work over a range of pressures, if I was to do this again a regulator for the reload system would be a must, plus you could quite easily lose a finger with this thing.
Any questions are welcome, one day I will actually finish another cannon but it’s probably not going to be this one as I’ve just started work on something new…
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Unread postAuthor: veginator » Wed Dec 23, 2009 3:58 pm

Awesome job. :) Reminds me of the assault rifle on halo.
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 23, 2009 4:02 pm

Looks like it had a lot of potential, shame if you don't finish it. Perhaps having a separate reloading mechanism was over ambitious, and some sort of mechanism driven by the pressure of firing would have been easier to work with.
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Unread postAuthor: spudamine » Wed Dec 23, 2009 5:13 pm

Awesome job. Reminds me of the assault rifle on halo

you have a point, it wasn't intentional I guess it might of rubbed off on me after all those hours playing halo.

Perhaps having a separate reloading mechanism was over ambitious, and some sort of mechanism driven by the pressure of firing would have been easier to work with.

I hear you, but I really liked the idea of getting my reload for free without taking a hit in efficiency, especially given the long distance the machanism has to travel, I guess the most ambitous thing was trying to get the thing to fire darts.
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Unread postAuthor: POLAND_SPUD » Wed Dec 23, 2009 7:20 pm

I hear you, but I really liked the idea of getting my reload for free without taking a hit in efficiency, especially given the long distance the machanism has to travel, I guess the most ambitous thing was trying to get the thing to fire darts
lol I've got to tell you that I totally agree with you :D

I did experiment with mechanically linking the slide valve and air cylinder for proper semi-auto which worked but was far too fiddly
replace the slide valve with a small QEV and buy a 3 way valve... if I were you I would buy one from clippard (as they have a couple of valves rated to 300 psi) or at least a direct acting manually actuated valve (as they seem to handle high pressures better)

getting a 3 way valve that can handle high pressures well is really the biggest problem with this design... I've been thinking about using hydraulic valves but so far I couldn't find one cheap enough


darts as ammo have to be a real PITA... well at least as far as reliable loading is concerned... I don't think it would take too much time to convert this thing so that could use spherical projectiles

the problem is getting it to work over a range of pressures, if I was to do this again a regulator for the reload system would be a must, plus you could quite easily lose a finger with this thing.
exactly my conclusion.. I am considering building a gun that would have two separate circuits (low pressure one for loading and, possibly, piloting the main valve)

I really like the way in which ammo loads... I was considering a similar design but I thought that it might not work too well

good job.. I like it
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Unread postAuthor: spudamine » Thu Dec 24, 2009 4:37 am

getting a 3 way valve that can handle high pressures well is really the biggest problem with this design... I've been thinking about using hydraulic valves but so far I couldn't find one cheap enough

Agreed, do you think a hydraulic valve would work? I bought a hydraulic check valve last year for my pump and had to modify the seals to get it to work with air, that's why i'm using a mini ball valve on this.
darts as ammo have to be a real PITA... well at least as far as reliable loading is concerned... I don't think it would take too much time to convert this thing so that could use spherical projectiles

I could get round a lot of problems by using spherical ammo, the gun would then be a bit overbuilt and I'd end up changing a lot, I could use a regular spring for the mag though which would save a lot of hassle.

I really like the way in which ammo loads... I was considering a similar design but I thought that it might not work too well


I think it should work ok for round ammo, it's reversing the directiopn of the cylinder which was tricky to do whilst keeping it compact.
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Unread postAuthor: POLAND_SPUD » Thu Dec 24, 2009 7:38 am

Agreed, do you think a hydraulic valve would work
IDK really.. that's the problem..
if I find one that would be air actuated or manually actuated I will buy it...
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Unread postAuthor: kenbo0422 » Thu Dec 24, 2009 9:31 am

Without knowing exactly how your slide valve is set up:

What about using a hammer to open it? It would be quick. :idea:
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Unread postAuthor: twizi » Thu Dec 24, 2009 2:58 pm

i like the cp mini reg has some potentiol
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Unread postAuthor: al-xg » Thu Dec 24, 2009 7:28 pm

You could just use a smaller slide valve, or just use a small pilot volume, a blowgun and restrict the fill flow.

What was the problem with the reloading system ? If the spring is just strong enough to push the barrel forward, it should work at any pressure shouldn't it ?

I was going to use the same setup but then instead of moving the whole barrel I just had a bolt load the projectile from the back, the QEV can't be inline though and the piston diameter must be larger than the barrel diameter.
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Unread postAuthor: POLAND_SPUD » Thu Dec 24, 2009 8:33 pm

You could just use a smaller slide valve
still I think it would be better to use a smaller QEV to assist piloting and then connect a manually actuated DCV or a smaller slide valve)...
IIRC there are special high pressure slide valves used with HPA and CO2... if you look at slide valves from that perspective they might seem useful


however, I realized that switching time of the valve, not the flow is the biggest problem in most situations
the ideal pilot valve would be one that opens instantly

but a slide valve is technically a direct acting manually actuated 3 way valve...
so its opening time = how fast you open it

add to that the fact that slide valves have relatively low flow and you can see why slide valves are not the best choice

(but it doesn't mean that spool/poppet valves are ideal... quite a lot of them will leak if you use pressures higher than their rated pressure... some might work ok at 400 psi, some might even work well with 500 psi but some might start leaking at 200 psi)
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Unread postAuthor: psycix » Fri Dec 25, 2009 7:18 am

Looks good!

I like the manual loading, even though it doesn't really add anything but coolness.
The pressure gage installed in the T looks very clean, how did you do that?
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Unread postAuthor: spudamine » Mon Dec 28, 2009 7:51 am

You could just use a smaller slide valve, or just use a small pilot volume, a blowgun and restrict the fill flow

This would help somewhat but As Poland spud said I don't think i'm ever going to get a really nice semi action with one of these valves.
What was the problem with the reloading system ? If the spring is just strong enough to push the barrel forward, it should work at any pressure shouldn't it ?

It's just a little dodgy the force of the air cylinder slamming back with 500psi behind it tends to jamb things up so it does not spring forwards straight away or the momentum is lost and it won't travel all the way forwards. The clearance between the barrel and its housing is a little bit close as well, about 0.05mm, if everythink is clean and lubed it works beautifullly but as soon as some crap gets in there it starts to go wrong. I think i've made the spudgun version of the SA80 :D

I like the manual loading, even though it doesn't really add anything but coolness.
The pressure gage installed in the T looks very clean, how did you do that?


It's useful if you need to move the barrel forwards when the chamber is pressurised. As for the gauge, I just tapped a 1/8 NPT thread into the T and used a paintball mini gauge, the only tricky bit was finding a NPT tap in Europe. This particular brass T has spanner flats so is easily thick enough to take threads.
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 28, 2009 8:43 am

spudamine wrote:I think i've made the spudgun version of the SA80 :D


That raised a genuine lol :D
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Unread postAuthor: POLAND_SPUD » Mon Dec 28, 2009 10:01 am

I don't think i'm ever going to get a really nice semi action with one of these valves
If I can add something... today I've noticed that one of these valves (the one with a roller) has no seals at all...

the spool and the valve body are machined so precisely that they seal metal-to-metal... it doesn't leak at all (if you use lubricated air)

I found this info in its data sheet a couple of weeks ago but I couldn't really believe it until today ..

I was testing the semi auto MKIII at 500 psi when the valve jammed... at first I thought that it must have broken down because of high pressure... but when I opened it and removed the spool (I had to use a lot of force to do this) I noticed that:
A - it really has metal-to-metal seals
B - the valve jammed because some miniature bit of Teflon tape got inside... one blow of air and it works great

it probably can handle much more than just 500 psi.. I might have found the ideal valve

ohh btw I think that my fridge compressor broke down again.... and again it seems that the starting relay was the culprit

:?


It's just a little dodgy the force of the air cylinder slamming back with 500psi behind it tends to jamb things up

that's why I think it might be a good idea to have two separate circuit on a gun:
a HP one for the main valve (piloted filled via the main 3 way valve)
and
a low pressure circuit for controlling the air cylinder and air piloting the main 3 way valve


I know it makes things a bit more complicated... but in fact it will have a couple advantages:
- fast and consistent opening times of the main 3 way valve (as it's air piloted), which results in good performance and consistency of the QEV
- you can use higher pressures (as pressure rating of the air cylinder is no longer an issue)
- you can attach the trigger wherever you want to
- less gas used for each cycle (since you don't have to pressurise the air cylinder with air at, let say, 40 bar... 5-10 bar might be enough)



ohh btw do you happen to know the exact volume of a 88g co2 cartridge??
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