Adventures in Airguns The process of creating a Homemade Airgun
Posted 21 May 2011 - 03:02 PM
So without further ado, I'll outline my thinking and progress thus far. (First post coming in a few minutes)
Posted 21 May 2011 - 03:27 PM
Piston valve tanks (also known as backpressure tanks) are the things powering the Hornet, Big Salvo, Sonic Bazooka, and SSPB, among others. The mechanism relies on pressure differentials. For easier explanations, I'll simply add the links below:
Secret Strike Internals
You fill the tank, equalizingthe pressure throughout, with air essentially leaking past /around the piston to the rest of the tank. The initial pumping seals off the barrel. Once you've pressurized, you open a valve to release the air behind the piston. With the front of the piston suddenly under more force, the piston flies back, sealing off the leak, and letting the rest of the air go down the barrel.
There are several different methods of piston construction. Spudding (potato launching, for the uninitiated) often involves large piston valve setups. The massive amount of air that needs to be moved, however, means that you need a nice ball valve or some other method to quickly leak out the air - not ideal for Nerfing. The cmmon nerf designs look more like the following from the Autopsy thread:
In these, there is often a rubber tip on the front of a plastic tube, which sits inside another tube ("I'm a dude, playing a dude, disguised as another dude!"). It operates on the same principle, but is meant to maximize the tank space used for launching the projectile.Once the tiny tubes get sealed off by the rubber piece, the rest of the air comes out the barrel. This is a reason why Big Salvo tanks are so awesome. In addition, you leak less air to start the process, which means you can use things like blast buttons. (Use the search function, there have already been some efforts in this area).
So that sums up what I'm trying to accomplish: a working homemade piston valve, with relatively cheap and easy construction. Whether it's possible or not, I don't know. Well, actually, I do, it should be possible. The real question is whether I can make a working design(s), or if someone else has the insight to do it. It's a totally open area for Nerf innovation, and we may end up with several different designs.
In any case, let's make this a learning experience, shall we?
Posted 21 May 2011 - 03:47 PM
Piston valves require a free-moving piston, generally speaking. A few variations require o-rings for sealing things off, but usually you have a slightly-not airtight piston, as this allows leakage. You then rely on quick air dumping to drive the piston back before too much air leaks back around the piston, and nothing happens. Alternatively, you could make the piston air-tight, but place a check valve in the piston to allow one-way air movement. This of course means you'd better have excellent seal and lubrication. Bu we're talking about the NIC, don't we already do this to maximize the power of our springers?
Furthermore, there is a relationship between the size of the piston and the size of the barrel. Let's assume an overly large piston, and a small barrel you've sealed off. You start to fire the piston, and it does so easily (you have massive surface area driving the piston back). But there's the possibility of the pressure relieving out the barrel faster than the pilot area, and so the piston seals up again without all the air coming out.
Now shrink the ration of piston diameter to barrel diameter, approaching close to a 1:1 ratio. You start the process, and air leaks out the blast button or what have you. This process is slow and will leak a bit more air in the process to get the piston moving, but once it does, the piston should fly back the rest of the way and allow the remaining air to evacuate via the barrel.
There are a few ways to mitigate the weaknesses of different designs, but you should just remember the basics. You need a fast way to bleed air (in relation to tank size; remember the spud cannons need ball valves and all that), a good piston with slight but not serious leakage, and good seals on the barrel.
(Post will be edited if needed for info correction/addition)
Posted 21 May 2011 - 03:59 PM
There's multiple possibilities, but those are the ones I'm testing right now.
On a more humorous note, attempt one was, inf fact, just having the piston sit against the front wall. So essentially, barrel area = piston face and it didn't work. But the way I set it up led to a lot of other ideas in the process, so it was a good thing.
OBOB: I realize those things, but thanks for clarifying! There is a good balance you have to find, between how much air you bled, how fast the pilot flies closed, etc. It's just seeing what works And I did list the check valve possibility, I am going to try that on one design that failed earlier.
This post has been edited by Buffdaddy: 21 May 2011 - 04:16 PM
Posted 21 May 2011 - 04:22 PM
Posted 21 May 2011 - 07:33 PM
I think that pretty much sums up the thread. I don't see the problem with a build log, but maybe you could hold off on posting until you have something built that you can log? What you're doing is cool and all, but right now it looks suspiciously like an idea thread.
Posted 21 May 2011 - 07:41 PM
But for now, does this satisfy your curiosity?
This post has been edited by Buffdaddy: 21 May 2011 - 07:42 PM
Posted 21 May 2011 - 07:56 PM
Posted 22 May 2011 - 08:50 AM
I've been going along the spud cannon line of thinking for the past few days. I have two designs for it, with roughly the same chamber volume for firing the dart. I'll show the process on the second (and working) version.
Here's the pieces of the chamber, laid out for viewing pleasure:
1 1/4" PVC bushing, coupler, 3 inches of 1 1/4" PVC, another coupler, another bushing. The e-tape wrapped thing you see?
This is a 3/4" PVC endcap, wrapped in a bit of e-tape. A craft foam bumper gets placed on the front, for sealing off the barrel.
This is the rear bushing of the chamber, with a craft foam ring to seal off the piston when it moves back. I could only find 1 1/4" to 3/4" bushings, so you need another bushing to get down to the 1/2" PVC we all know and love.
For the front, we have 1/2" CPVC hammered into 1/2" PVC. The CPVC sticks about 2 3/4" into the chamber, and gets capped with a 1/2" to 3/4" bushing. Why? To have a nice flat surface for sealing with the piston.
For the rear of my blaster, to reduce space for testing, I have a 3/4" PVC cross with a dual-action bike pump shoved in the bottom, a 3/4" ball valve on the back, and a McMaster OPV on top. Hardly war worthy, but good for testing.
This shows the larger version of my piston, made with 2" PVC parts (it failed....). I'm using 2-part epoxy to seal it off; something I can use sparingly to get a decent seal, but get apart with a pair of channel lock pliers.
This rig is hardly war-worthy, but it at least proves the concept. Pumping to 50 psi, I'm getting about 70 feet with bb stefans. There's obviously more work to do.
Posted 22 May 2011 - 05:49 PM
On the old: It would make sense to invert the sliding cap. That way, you have a smaller area of air you have to suck out to actuate the piston, and you can get more power from the same size cylinder, or the same from a smaller one.
New discovery: 3/4" CPVC endcaps slide nicely on 1.2" PVC. You'll see it tomorrow.
Posted 23 May 2011 - 12:28 AM
It relies on the builder being able to find a PVC check valve constructed such that you can attach a trigger rod:
my local OSH hardware stores have had about 50% of their stock be this style. I've also seen them at Ace.
I've been able to make it fire without modding any of the PVC parts shown, other than the plug on the far left (Unless glue counts, then the left two were "modded" but the rest were just threaded together, because I wanted to be able to return most of it if the concept failed). The plug just gets a hole through it, for the trigger rod, which attaches to the threaded end of the pin through the valve. An o-ring over the rod, and a PVC disc that compresses it a little, completed the back seal. The beige piece on the far right is a CPVC coupler, where I plugged in a pump with a CPVC end for pressurizing, then removed that (to then insert a barrel). Popped great in testing, even a little too powerful for my own preferences.
But this is not back-pressure, and I haven't worked on it at all for a year.
Just Some Bob,
You'll be glad to know that using a check valve as an air-tank blaster trigger does work!
3DBBQ from Taiwan successfully created his JSPB V6 design based on such a concept, and it even includes a semi-auto firing feature (ie. pump up the air tank, then can fire 6-7 shots individually in sequence), and that was done by simply positioning a ball valve behind (only slightly opened) as a rudimentary adjustable air regulator to control the air flow from the main tank space to the secondary tank space.
This was 3DBBQ's V6 design (imho, the dude has amazing blaster style and colour schemes):
You can see the video if it operating here:
And his check valve trigger build guides (the photos should explain everything quite clearly, no need to translate):
He posted up that design back in end-2009 (i think he might have also posted something about it on NerfHaven too, not sure though)... when i saw it back then i immediately drove out to get the parts (we seem to have similiar types of pipes, joints and valves in SG too) and built a version of it to try out... and it works! It fires in semi-auto style too, just like in his video. Anyways, here was my replicated build:
Main difference is mine uses a tire valve instead, 'cos i preferred to have the option to connect it to a manual pump or portable air compressor.
BuffDaddy, sorry for "hijacking" your thread with a different air-tank blaster trigger setup, just sharing some info to add on to Just Some Bob's post.
This post has been edited by SgNerf: 23 May 2011 - 01:10 AM
Posted 23 May 2011 - 01:57 PM
HOW DARE YOU!!!!!!
No matter, we're kinda just sharing our experiments here.
Posted 23 May 2011 - 07:15 PM
Simply put, the E-tape method isn't working very well. Unless it's EXACTLY perfect, you're either not moving the piston at all, or leaking too much air when it does. Which means we gotta start using O-rings.
I can fix my old designs later, but I did a new one, as that's what I saw when sketching out what I need in a blaster.
Here's the flash animation of how it works: (EDIT: Not my own, borrowing from the web)
Essentially, imagine a Big Salvo or Hornet tank with an expansion jutting out the side, and with a large piston instead of a tiny one.
Doing it like this would make things so much better, as you can A) NOT have a long chamber with lots of dead space behind the dart, Use the reservoir as a handle for your blaster, and C) Regulate your range by the size of your handle. Yup, you could make it shoot as you see fit.
We have our chamber, made with all the flat surfaces we need for proper sealing. We have a piston, with o-rings sealing it nicely, and a check valve to let air pass, and we have the tiny bleed hole that fills the reservoir. Using lung pressure only, I can already feel a tiny puff of air coming out the barrel as I operate it manually, so the design seems solid. We'll have more work later, and hopefully a few of the other designs I had will also work.
This post has been edited by Buffdaddy: 23 May 2011 - 07:27 PM
Posted 23 May 2011 - 07:21 PM
Posted 25 May 2011 - 04:51 PM
You're making great progress nonetheless and it'll be interesting to see how this project will benefit the hobby.