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Question on Semi-Auto

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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Unread postAuthor: moonmaster » Wed Aug 26, 2009 8:38 am

Thanks again everybody. I think I'll probably go with one of those slide check valves, considering I couldn't find spool valves anywhere. I've decided against the blow forward breach in favor of an air cylinder actuated breach. Any good sources for air cylinders? Mcmaster part number is preferred, since I already have a huge order pending with them.
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Unread postAuthor: POLAND_SPUD » Wed Aug 26, 2009 9:38 am

spool valves are directional control valves... spool is the part inside the valve that looks like uhmm... a spool :wink:

have a look here...

the term 'spool valves' refers to the way they are built while 'directional control valves' refers to what they do (that is they direct flow)

also remember that there are manually and solenoid actuated valves... for a semi a manually actuated valves are ok but if you want to build a full auto gun you'll need a solenoid valve + timing circuit (of course you could also build a full auto with a manually actuated valve + oscillating valve...)

if I were you I would buy a manually actuated valve first as they are more straight forward to work with and build a semi first.. then you can switch to a solenoid valve to convert the gun into a full auto gun
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Unread postAuthor: moonmaster » Wed Aug 26, 2009 11:00 am

If I understand right, I would want a 3 way 3 port valve, but all the valves matching that description are 10-32. Does mcmaster have adapters to npt?
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Unread postAuthor: PVC Arsenal 17 » Wed Aug 26, 2009 12:21 pm

The 3-way valves you'll find on McMaster are normally closed valves. They will work, but you'd be better off finding a normally open valve.

A normally closed valve will work like this:
Press down trigger, gun fills
Release trigger, gun fires

A normally open valve will work like this:
Press down trigger, gun fires
Release trigger, gun fills

A normally open valve will allow you to operate your gun like a proper semi-automatic weapon. (Pull trigger, gun fires)

If you're having trouble finding one, I highly recommend the Clippard MJVO-3 This valve was used on my TFS-Semi and btrettel's upcoming FANG 2.

I have also used the Pneumadyne A11-31-44, though it doesn't perform as well as the MJVO-3.

Both valves are operated by pressing a button. It is easy to make a "trigger" for them using the Pneumadyne Thumb Operator located under Valve Accessories on this page. It needs to be modified slightly to achieve a full stroke when used with the MJVO-3, but it makes a very good trigger. I used one on my TFS-Semi.

Image
Top right is the MJVO-3
Bottom right is the A11-31-44


Edit: Not all 3-way valves have three threaded ports. The exhaust port is usually unthreaded and simply exhausts to the atmosphere.

Edit 2: If you decide NOT to take the spool valve route and want to use a sleeve valve instead, here's a video demonstrating how one works in conjunction with a QEV.

Edit 3: What is the length of your ammo? How will your loading system work? If you give us more info, we can help you find the cylinder you need. The main factor is probably the length of the stroke. If you're using a "projectile pusher" style loading system like btrettel's FANG or my first semi-auto Nerf gun Project, the length of the stroke must be at least the length of your ammo so that the bolt can completely seal off the breech opening.
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Unread postAuthor: moonmaster » Wed Aug 26, 2009 1:17 pm

Well, I think I found a piston on Mcmaster. It has a stroke lenght of 2", which will be enough since this will fire 1.5" nerf darts.

Oh and screw the sleeve valve and QEV idea. I was going to do that until I clicked your links. These seem cheaper and more efficient.

EDIT: I will still need a QEV with one of those valves, right?
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Last edited by moonmaster on Wed Aug 26, 2009 1:21 pm, edited 1 time in total.

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Unread postAuthor: PVC Arsenal 17 » Wed Aug 26, 2009 1:19 pm

Just replied to your post on NerfHaven. This conversation is happening in two places at once! :lol:

Edit: Ah I used the exact cylinder you just found when I was testing out an inline projectile-pusher setup. Video.

Edit 2: Yes, you will need a QEV. These valves are capable of operating cylinders on their own, however then you have no way of redirecting the vented air down the barrel (unless your valve has a threaded exhaust port). Just use a QEV. It will make the cylinder cycle faster and it will dump the air down the barrel faster.
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Unread postAuthor: moonmaster » Wed Aug 26, 2009 1:35 pm

I'll still need a QEV right? If I used a 3 way 3 port valve (normally open) could I just use the exhaust to fire the gun? And are the ports npt on the MJVO-3?
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Last edited by moonmaster on Wed Aug 26, 2009 1:41 pm, edited 2 times in total.

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Unread postAuthor: PVC Arsenal 17 » Wed Aug 26, 2009 1:37 pm

Read my edit. The input and output ports on the MJVO-3 are 1/8" NPT (female). The exhaust ports are unthreaded.
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Unread postAuthor: moonmaster » Wed Aug 26, 2009 1:43 pm

You posted while I was editing, heehee! If I used a valve with a threaded exhaust, could I use that to fire the projectile without a QEV, or is that kind of valve too slow?
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Unread postAuthor: PVC Arsenal 17 » Wed Aug 26, 2009 1:45 pm

You could, but would it perform well? Probably not. Certainly not as well as one used with a QEV.
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Unread postAuthor: moonmaster » Wed Aug 26, 2009 1:53 pm

Okay, It's QEV time then. I think I finally have all the aspects of this gun figured out, with your help of course.
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Unread postAuthor: PVC Arsenal 17 » Wed Aug 26, 2009 1:58 pm

If you need help with the construction, feel free to ask. Make sure you read my latest post on NerfHaven. There are a few things to consider if you're going with this design.
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Unread postAuthor: POLAND_SPUD » Thu Aug 27, 2009 3:18 am

to tell you the truth I am not exactly familiar with mcmaster as I live in Poland so the site is of little interest for me simply because I order stuff locally


@PVC_arsenal some valves with all threaded ports are in fact both NO (normally open) and NC (normally closed) valves... simply becasue there are two positions:

1)flow from port 1 to port 2 while port 3 is blocked off
2)from port 2 to port 3 with port 1 blocked off

so depending on hte port you use it can be both NO and NC valve...

....and quite a lot of of piloted spool valves (that is valves which have smaller pilot valves inbuilt in them) can be converted from NO to NC in a minute by simple rotating the piloting thing 180 deg
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