auto piston v3.0

[jackssmirkingrevenge]

looks like it would workthough the rod looks a tad unsopported and would need to be well rounded off to avoid damaging the flap.

[Skywalker]

Yeah, both good points. I still haven't figured out a good way to make that valve. There ought to be a way to build one into some brass fittings on the way from the blowgun; that would eliminate a few building steps.

[jackssmirkingrevenge]

Instead of that rubber flap you could simply fit a schrader valve

I think this is the design that is the simplest and with the most potential, i've decided to go down the same route

[Skywalker]

Hahah, a schrader, why didn't I think of that . . .
Well, I'm glad you have finally seen the light I'll be interested to see how it works out. If I like it, I will probably build a copy. It's simple enough I think I might be able to make it work. Which means it's very simple.

(Edit: is there any way to increase the flow through a schrader? I'm a little worried about the flow rate: it'll be fine for a small-bore, but a marble gun with that kind of flow wouldn't cycle too fast. You know, I'm fairly certain Hawkeye has some experience making check valves inside brass parts. I think that might be the best way to go.)

[jackssmirkingrevenge]

I'm glad you have finally seen the light

I always thought this design would be better, but I had to give the pop-off pilot a go first. I'm currently making what should end up looking like the below diagram, at the moment I'm casying my largest ever epoxy piston, a massive 1.3 inches in diameter massive I tell you! Quite a few "pourings" to go before it will be ready for testing. Note that the rod is not connected to the piston, allowing the latter to move back enough to open the vent to the breech.

[Skywalker]

I see, the piston makes contact with the rod as it closes. Why the complicated air control valve, though? Wouldn't a simple check-valve style be better? I imagine you have a reason for making it that way, I'm curious what that is.

massive 1.3 inches

I suppose that is massive, for epoxy. And for a bb gun too . . . What calibre barrel are you going to use, and what ammo?

[jackssmirkingrevenge]

Why the complicated air control valve

It lets me use 6mm o/d aluminium tube for the vented portion riding in 6mm i/d brass that will keep the rod supported and nice and centred, besides it's not *that* complex really

The biggest piston I ever made was just over 1 inch, it's massive by my standards because I usually work on smaller calibres and launchers in general. I'm planning on using 20mm o/d, 17mm i/d PVC conduit piping as a barrel, just about right for marbles but since I'm planning on using cartridges I can use any ammo I fancy

[iknowmy3tables]

wow I should have keeped up with this thread, well most of the designs I see are just check valves or presure relif valves (which usually just shut after a 15psi drop)

well if you just want to make a piston type valve that will inpercisely pop air out in fast burst than you can try what I'm doing and do a varient of clide's HEAR valve and be ghetto on the front and middle (smallest and largest) seals and turn them into sealing faces

[Skywalker]

I figured it had to do with stabilizing the rod. I guess it's also easier to hit a big target with it than a small target. Eh, it's complicated enough for me, though.

Ok, I wanna see that cartridge loader! Dang, if you get the whole shebang to work, it'll be mighty impressive!

[jackssmirkingrevenge]

Here's the enormous piston by adding an internal structure (basically a cruxiform syringe plunger wrapped in duct tape) to the casting it's not only light but also saves on epoxy, while still being strong enough for the job. This is how it looks like fresh popped from the mould, still needs some finishing.

[Skywalker]

Very cool. Nice to get live updates on the progress. How big is the tube that piston seals against?

@ your profile pic.

[jackssmirkingrevenge]

How big is the tube that piston seals against?

Ideally it would be the same 17mm piping I'm going to use as a barrel, but that would mean the spring has to hold back around 37 lbs of force at 110 psi, so I think it will be closer to 13mm, that will cut the force required to around 22 lbs.

[Skywalker]

But that'll cut your firepower. The air will go faster through that opening than through the barrel, and then it will slow down in the larger area in front of the piston, and then have to speed up again down the barrel. That's going to cost some energy.

Also, if you are looking at 1.3" piston then your firing pressure will be 6x higher than the closing pressure, so if it's 110 psi, you'll have 17psi left in your firing chamber after the shot. Not much, from an efficiency standpoint. I was thinking about using a sealing area about 1/4 the piston area (so half the diameter).

Surely you can come up with a 37lb spring if you can get 22 lbs! At the least, all you have to do is run the tensioner bolt in.

[jackssmirkingrevenge]

The issue is that that's quite a bit of stress, which the launcher will be constantly under, unlike normal pistons where the stress is only there when the launcher is pressurised. Besides, given the way air is transferred through the holes in the cartridge, there will still be a choke point for the flow.

[Skywalker]

hmm, I can see that it would be under stress, but why's that a problem? and anyway, you could just pull the spring out when you are going to shoot it. But I can see it being a difficulty to assemble the thing with the spring putting that much force on the setup.

Ahh, so you are still going to have the air go in thru the sides of the cartridge . . . does that mean you are going with a scaled-up version of your last blow-back breach?