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Breech design question

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Breech design question

Unread postAuthor: viking4141 » Thu Jan 22, 2009 6:12 pm

On a breech loading spud gun, is it safe to modify the PVC screw-on endcap so that it operates as an interrupted screw breech (i.e. remove 1/2 of the thread surface area so that the breech can be inserted and turned 1/4 turn to be fastened)?
link, in case anyone doesnt understand what I mean
http://en.wikipedia.org/wiki/Interrupted_screw
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Unread postAuthor: SpudsMcGee » Thu Jan 22, 2009 6:15 pm

I don't see any problems, but I'm not the brightest spudder.
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Unread postAuthor: sputnick » Thu Jan 22, 2009 6:24 pm

On what type of spudgun? combustion is the only kind I would use, I assume it is possible, and also a decent idea,

If you are worried, try firing it remotely a few times, (maybe 5 or 6) then if it holds with no damage, then I would assume it's safe.

Also, how yould you file out the inside of the fitting?
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Unread postAuthor: viking4141 » Thu Jan 22, 2009 6:38 pm

probably just dremel it out. the only thing im worried about is just that the remaining thread won't be strong enough, but that might depend on what i use for propellant, too
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Unread postAuthor: Hubb » Thu Jan 22, 2009 6:49 pm

I'd kinda like to see that on a spudgun. I'm sure it would hold okay with a combustion. If you have any concerns about it, though, just
try firing it remotely a few times, (maybe 5 or 6) then if it holds with no damage, then I would assume it's safe.
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Unread postAuthor: viking4141 » Thu Jan 22, 2009 7:08 pm

hubb017 wrote:I'd kinda like to see that on a spudgun. I'm sure it would hold okay with a combustion.

I might post pics when this gets done, which may be quite a while, as it isnt the highest priority now (all i have now, is the barrel, chamber pipe, and the ignition system, a grill igniter), and speaking of, is it a good idea to mount the grill lighter in the breech cap?
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Unread postAuthor: starman » Thu Jan 22, 2009 7:43 pm

I've seen it done before but I can't recall where I saw it. Sanding down the gaps in the female threads would be one of your challenges as well as doing it fairly precisely. You also would probably have to limit it to 3 or 4 of the deepest threads and sand away the rest.

1x combustion only for sure. I imagine getting a reliable pneumatic seal with standard NPT PVC threads configured this way isn't going to be happening.
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Unread postAuthor: psycix » Fri Jan 23, 2009 6:12 pm

Arent those DWV endcaps known to blow out sometimes?
Removing threads wouldnt help it.
But you could always try.
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Unread postAuthor: sputnick » Fri Jan 23, 2009 6:15 pm

If they did, I would assume its from the outer piece flexing out and letting the threads fly out, not from the threads failing, so I would assume a tightened pipe clamp around the threads could remedy that...
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Unread postAuthor: viking4141 » Sat Jan 24, 2009 1:25 am

psycix wrote:Arent those DWV endcaps known to blow out sometimes?
Removing threads wouldnt help it.
But you could always try.

sputnick wrote:If they did, I would assume its from the outer piece flexing out and letting the threads fly out, not from the threads failing, so I would assume a tightened pipe clamp around the threads could remedy that...

Now, do you mean the side of the end cap splitting?, and then it is expanded wide enough for the screw in part to just fly off the back?
Can you clarify this for me a bit?
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Unread postAuthor: psycix » Tue Jan 27, 2009 8:40 am

To avoid confustion I want to ask, are we talking about a screw-on cap or a srew-in cap?
Even though I would imagine a srew-in to be stronger, I have heard stories of them flying out. I also have seen a picture, in which, like sputnick said, the adapter where the cap screws into was cracked.

A screw-on cap (which is found more uncommon) sometimes seals off at the end of the adapter, meaning that the female threads do not feel any air pressure from inside. The male threads do, and thus the male threads get flexed out and that means that the threads get even tighter.

Sputnick, you are right about the way of failing. It indeed does not have much to do with the threading itself.
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Unread postAuthor: Radiation » Tue Jan 27, 2009 2:00 pm

I have a question, In what way will this be implemented in your design? Is it for venting? Because to me It seems like a ball valve would be a lot less work and just as effective. Plus this will require a very precise modification to the threads to not cause a leak or even worse a blowout.
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Unread postAuthor: psycix » Tue Jan 27, 2009 2:21 pm

I think he wants to breechload it.
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Unread postAuthor: Radiation » Tue Jan 27, 2009 3:25 pm

Ahhh duh, title says it all. Still camlock would be a better option IMO.
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Unread postAuthor: ramses » Tue Jan 27, 2009 3:34 pm

they do make steel, pressure rated plugs. I saw a 4" one at home depot, but I forget how much. but for the cost, you could buy a union or camlocks, or anything like that. Those wouldn't work well in coax, though, you would have to use a big ball valve.
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