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I recently became interested in building a hybrid (at least at some point in the future) but I still have a few issues floating around in my head before I commit to trying to design one. Hopefully you guys with more experience can help me out.
My goals include using some sort of piston valve/air spring setup for a reasonably quick firing cycle. I would also like it safe enough to be hand held at maybe 2x-4x mixes (after extensive remote tests) with a max load of anywhere between 5-8x. I'm planning using a 2"x12" sch 40 steel chamber with a 0.75"x60" aluminum or copper barrel. The fueling system will most likely be manometric. I'm assuming the primary ammunition will be marbles or something similar.
The main design issues will likely be the following:
*Fittings: I have read that run-of-the-mill malleable iron fittings (although only rated to 150psi) are safe for most guns with under 10x mixes. However, I want to be sure since I'm looking for something that can be held and fired with some margin of safety.
*The piston: From what I've seen, piston hybrids have been built multiple times now with good success. Nonetheless, I think this will be by far the biggest design challenge. I'll need something that can seal very well with the barrel and air spring while being strong enough to handle the extreme pressures. I don't have the ability to weld or machine highly customized components nor do I have the budget to have this done for me. The piston will need to be highly improvised yet work well. Aside from this, using a piston also seems like it will have its advantages. The opening pressure could be adjustable (through the air spring pressure) to work optimally for each mix number. I also believe a relatively high "burst" pressure could be achieved without a high-pressure air spring as long as the valve porting isn't much wider than the valve seat. This would keep the net directional force on the piston low until near-peak firing pressure is reached at which point the barrel seal would finally break. Of course, the air spring must then do its job as a cushion and (hopefully) prevent the piston from slamming into the back of the valve. The only other issue might be a small amount of pressure left in the chamber after firing due to the piston closing before it could all be released. However, I would assume that the projectile would be long gone before this happens and causes some performance-killing vacuum effect.
*Compactness: I don’t want my cannon to become too heavy or bulky. Using a manometric fueling system should eliminate the problem of having metering pipe sprawling from my gun. I’m guessing I could simply attach a ball valve/pressure gauge/quick connect to the end of the chamber through which both the fuel and air could be delivered and measured with the single gauge. The ball valve could then be closed in order to protect the gauge and quick connect valve from damage.
*Ignition: Stun guns are banned where I live, but I have found that electric BBQ igniters work pretty well. I was able to achieve a reliable 4-gap setup with one in my advanced combustion cannon. Perhaps I could make an improvised “spark plug” by somehow using epoxy to secure one or both of the igniter wires into a small 1/4” or 1/8” NPT pipe nipple. The nipple could then be screwed through a fitting into the chamber and viola! Of course, the epoxy would need to be strong enough to handle the firing pressure (not sure about this). Has anyone tried anything similar?
I'm really only posting my thoughts. I had a long/boring day at work to think about how I could build a hybrid and this is pretty much the result. I won't have time to actually build this thing until at least next year, but it's fun to think about.
Why improvise a spark plug when honest to goodness real spark plugs can be had at the auto parts store for a couple bucks?
If you can find a tap for spark plug threads (I have no idea what they are personally) a spark plug will work great. If not, then you can make one with a schrader valve, a 1/4" x 1/8" bushing, a wire, and some epoxy.
I did the latter for my hybrid and it worked fine with a BBQ igniter at 6x, using generic Ace brand epoxy.
I know you may not be exactly sure, but what would your overall project budget for this be?
You can. You'll get them (or at least order them) at the same auto parts store.
I've done it twice in my life for two vastly different projects (Vera being one of them). Neither time was it difficult. IIRC, they're fine threads, but still standard (although possibly metric depending on what exactly you get...so check before you buy the plug).
sorry, couldn't resist...
my sparkplug tap looks like this...
tested to 9x.
Do you have any experience building piston valved pneumatic launchers? If you don't a small size marker gun even would give you and idea of what works and what doesn't. Take a look at the hybrid showcase I think dyi's 20x cannon matches your specs exactly except for the mix number.
Contrary to popular belief this isn't 4chan, it's not an image board post with words not pictures. The OP sounds well aware of JB weld and the like.
sorry, I've been doing that a lot lately... Especially considering how much I HATE 4chan...
@ Jonnyboy: Yes I have built a piston valved pneumatic cannon. It actually has good performance for a PVC launcher. I built it a year back but never got around to puting it in the showcase.
@D_Hall: I didn't know spark plugs were so cheap since I never needed to buy one myself. I guess I'll just tap it into a small endcap. It certainly beats getting messy epoxy all over the place.
@Moonbogg: I haven't really decided on a set-in-stone budget yet. I'm hoping to keep it under $300 (total guestimate).
I'm more concerned about the fittings than anything else. With an 8x mix, I'm looking at a max theoretical pressure of ~900psi. DYI's gun uses the iron fittings with the 20x mix (as most of you know), but this must really be pushing the safety factor.
Last edited by niglch on Tue Aug 18, 2009 11:17 pm, edited 3 times in total.
Don't tap into an endcap use two pieces of chamber pipe to get to a suitable length then connect them with a tee fitting which reduces down to spark plug threads. Much easier and gives ideal spark placement.
You should be ok. Remote test at a higher mix then you'll ever use for hand held operation.
Spark plugs don't use NPT.
I just realized how similar your proposed cannon is to mine. The only difference is the barrel. What porting diameter are you going for? I went for a 1/2" porting and my barrels outgrew it quickly. I will suggest that you go higher, above your barrel size for maximum performance.
You mentioned that there may be pressure left in the chamber after the valve closes. In my hybrid (essentially the same as yours) the chamber is at substantial vacuum when I open the valve to the Quick Connect to re-fuel. Enough to be considered "venting" at as little as 2x.
What will your piston slide in and how do you intend to bring the "barrel" into the tee?
If your piston will slide in a nipple, don't bother getting a seamless nipple, because when you tighten the fittings onto its end the pipe will compress the ends and jam your piston in place (speaking from experience and the resulting hours of wondering why my piston was so darn tight). So put that part of the cannon together, with teflon tape, and then take it apart. Now it is time to bore out the nipple. you could use a lathe, possibly a drill, and maybe a cylinder hone if you feel ambitious. I will leave anything else to you.
To bring the barrel into the tee on my cannon (1/2" copper barrel), I used a 1/2" copper to 3/4" NPT thread piece and drilled it out to 6/8". This won't be possible in larger sizes. I will let you figure out exactly how you intend to do that.
Another thing I would recommend is a breech that doesn't involve moving the barrel. Also, for an over-under type cannon, it is a good idea a barrel support, especially if you intend to fire heavy projectile.
@ Ramses -
I think the porting will be something like 1-1/4" for the 3/4" barrel. I have made up a *very* preliminary design for my valve (looking at it now, I can already see a bunch of improvements I could make), but it should give some idea of what I'm trying to do. As you can see, the piston slides and seals against the 1.25" pipe nipple and is stopped from moving too far by the bumper. I do not plan on bringing the barrel into the chamber at all like in many pneumatic valves. I will simply have something that seals against the bushing or coupling to which the barrel attaches.
When you say you have a substantial vacuum, do you mean that you actually have less than atmospheric pressure left in the chamber and air is actually sucked in when you open the valve? That's very interesting. Perhaps it is from the combustion products which cool after the shot.
so the part of the "spool" that seals the barrel is slightly smaller than the one at the back that seals the "pilot" off. That is a great idea. The back part of the "spool" will need to be longer or the whole piston will have a tendency to "tip" under gravity. You will also need to dramatically decrease deadspace in the pilot area.
You may also consider not using any threaded components. You can make your cannon very clean by using a section of unthreaded pipe and then buying disc shaped material and drilling holes and connecting the caps with full length B7 studs.
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