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Semi-auto valve for under $20

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Semi-auto valve for under $20

Unread postAuthor: jackssmirkingrevenge » Tue Nov 29, 2011 11:41 am

This will hopefully encourage many of you to venture beyond single shot pneumatics, for very little cash and assembly.

Here it is in action at 110 psi:



I hooked up a 12 inch 6mm barrel and did some chrony testing at 120 psi. The firing chamber volume is around 3.5 cubic inches, a little wasteful but that's what I had ;)

Using 0.25g airsoft BBs, I got an average velocity of 590 fps, more than enough to punch through a soup tin. With some adapted 1 gram 0.22" pellets I got an average of 355 feet per second, fairly decent.

In short, even at relatively low pressures with a relatively short barrel, this valve setup packs quite a punch. For say a field legal airsoft sniper, if you fit a longer barrel you could get away with much, much less pressure, a regulated main chamber would last a long time.

Conversely, increasing barrel length and/or calibre, and upping the pressure, you have enough power available to do some serious damage. I might do some testing with steel BBs and marbles down the line.

Some photos, my test setup with Combro CB-625 chrony at the end of the barrel, and a mangled soup tin.

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The items needed are illustrated below.

QEV disassembly:

Image

Note the sticker describing the three ports:

"P" denotes the pilot port, where the slide valve should be attached. Your air supply/main chamber should then be attached to the other end of the slide valve. In this case I hooked it up directly to my compressor. This port is compatible with 1/4" BSP.

"A" is where the firing chamber should be attached. For each shot, all the air that is in this chamber will be emptied. In this case I made a small chamber from a section of PVC pipe with epoxy endcaps, with a 1/4" socket screwed into one end to attach to the QEV. This port is compatible with 1/4" BSP.

"R" is where the barrel or magazine should be attached. This port is compatible with 3/8" BSP.

A note on thread sizes:

While NPT threads are common in the United States, BSP threads are widely used in many other countries.

WARNING: Never, never try to mate a BSP fitting with an NPT or NPS fitting if the pressure holding capability is at all critical.

NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).


The threads are weirdly described on eBay but in my experience so far it's safe to assume that if the threads have a diameter similar close to 1/2" or 12.7mm, they are probably 1/4" BSP.

US members should be able to find adapters for BSP parts easily.

Some 3D rendering courtesy of dewey-1:

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Image

slide valve:

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Note that moving the slide valve away from the QEV fires the valve, and also blocks off the air supply, making for a very efficient system.

Some modifications to make the valve more ergonomic are adding some sort of trigger to the collar to allow it to be pulled with a single finger as on a conventional gun, and a spring to return the collar to its "rest" postion when the trigger is released.
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qevebay.jpg
The QEV type I used, widely available for less than $10 including shipping
slidevalveebay.jpg
The slide valve type I used, widely available for less than $10 including shipping
Last edited by jackssmirkingrevenge on Sat Nov 02, 2013 10:46 am, edited 4 times in total.
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Unread postAuthor: Goats spudz » Tue Nov 29, 2011 12:29 pm

i made one of these but the power sucks :(
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Unread postAuthor: warhead052 » Tue Nov 29, 2011 12:37 pm

Jack, can you also add the full auto to this when your chamber cure's?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 29, 2011 12:43 pm

Goats spudz wrote:the power sucks :(


You left out pressure, chamber volume, barrel length and diameter, projectile size and weight, what power was and what acceptable power would have been... could you be slightly more specific ;)

can you also add the full auto to this when your chamber cure's?


Full auto needs the air supply attached to where the chamber is now, the pop-off replacing the slide valve and the QEV piston replaced with a non-cup shaped one, or poked with a hole. I'm not willing to do the latter but can quickly lathe the former, hmmm... Also it needs a spring to reset.
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Unread postAuthor: warhead052 » Tue Nov 29, 2011 12:45 pm

Ok. Well while we have this chat going, would it be possible to make a semi/full auto by adding a tee to the pilot, and putting in the slide check and the pop off? Then you wouldn't need to poke a hole or anything.
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Unread postAuthor: wyz2285 » Tue Nov 29, 2011 1:00 pm

kind boring to me :wink:
I would use a 3-way, and use the pilot air for reload :)
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 29, 2011 1:41 pm

wyz2285 wrote:I would use a 3-way, and use the pilot air for reload :)


The problem is that reloading happens at the same time as firing...

would it be possible to make a semi/full auto by adding a tee to the pilot, and putting in the slide check and the pop off?


Not really, because of how slide valves work.

Well, tried a quick full auto conversion as described earlier, replacing the rubber piston with a Delrin disk and put a light spring behind it, and put my cheap-o eBay safety valve in the pilot port... et voila ;)



The video doesn't show how damn loud it is, my ears are still ringing...

Here's the original and new piston, nothing complicated:
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Unread postAuthor: warhead052 » Tue Nov 29, 2011 1:47 pm

Is it possible to add a blow forward bolt to it, in other words, a full auto BFB?
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Unread postAuthor: wyz2285 » Tue Nov 29, 2011 1:57 pm

The problem is that reloading happens at the same time as firing...

If done correctly, it may not be a problem, at least I hope :roll:
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 29, 2011 1:57 pm

warhead052 wrote:Is it possible to add a blow forward bolt to it, in other words, a full auto BFB?


Yes, but I'm not doing requests ;P

Personally I would avoid the blow forward valve, because it uses up energy that would otherwise have been used to fire the projectile.

I'm thinking of trying out a Monsoon style gas operated action...
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Unread postAuthor: Goats spudz » Tue Nov 29, 2011 3:57 pm

jackssmirkingrevenge wrote:
Goats spudz wrote:the power sucks :(


You left out pressure, chamber volume, barrel length and diameter, projectile size and weight, what power was and what acceptable power would have been... could you be slightly more specific ;)

can you also add the full auto to this when your chamber cure's?


Full auto needs the air supply attached to where the chamber is now, the pop-off replacing the slide valve and the QEV piston replaced with a non-cup shaped one, or poked with a hole. I'm not willing to do the latter but can quickly lathe the former, hmmm... Also it needs a spring to reset.

pressure: 500psi, chamber vol:7cm by 0.4cm hydraulic pipe or sometimes 17cm by 1.5" sodastream bottle, barrel length 94cm, bore 6mm projectile size 6mm 0.16g bb, and it will go through a cat food tin but if i fill the sodastream bottle and put it behind the slide valve for portable semi-auto the power is 1 good shot then you get more movement from a dead badger
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Unread postAuthor: JDP12 » Tue Nov 29, 2011 10:20 pm

does anyone know of a valve on mcmaster that this would be equivalent to? I'm assuming its some sort of 3-way valve, but any recommendations would be appreciated.

EDIT: Nevermind, I found loads of the valves just like JSR described on ebay..


JSR, what sort of performance do you get out of this and what diameter projectile do you use? just out of curiosity?
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 30, 2011 12:22 am

Goats spudz wrote:if i fill the sodastream bottle and put it behind the slide valve for portable semi-auto the power is 1 good shot then you get more movement from a dead badger


I don't think you can expect more from such a system, the sodastream bottle is relatively small (less than 200cc) compared to the volume of fiting chamber + QEV internals + slide valve + fittings and 500 psi isn't all that much in the world of PCPs either, you would probably have been better off with a hammer valve.

Nevermind, I found loads of the valves just like JSR described on ebay.


Dude... that was the whole point of this thread! ;)

what sort of performance do you get out of this and what diameter projectile do you use?


Haven't actually tried it with a barrel yet. Porting is 9.5mm so I wouldn't use it for anything bigger than 3/8" projectiles, for larger bores you should get a larger QEV, something like this which looks like it has 1/2" BSP threads.
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Unread postAuthor: JDP12 » Wed Nov 30, 2011 12:58 am

Goats Spudz... increase your firing chamber size and pressure, that is a miniscule firing chamber when you consider thats the air actually being used to propel the projectile.

JSR- do you think you'll actually make a gun out of this? By gun I mean take it further than a prototype? Once winter break comes I'll have some more money, definitely ordering this setup, using this to create a sexy pistol... I haven't built an actual gun in a loong time, it'll be good to.
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 30, 2011 1:35 am

JDP12 wrote:do you think you'll actually make a gun out of this? By gun I mean take it further than a prototype?


I might add an ergonomic trigger and add an airsoft barrel and magazine to have a little "fun" gun at work :D
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