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making epoxy pistons

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Unread postAuthor: inonickname » Sun Nov 15, 2009 7:44 am

I use 1/4" rubber and I've already replace it twice in my co-ax. And it's quite small.

Closing in on 10k as well I see.
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 15, 2009 7:51 am

inonickname wrote:I use 1/4" rubber and I've already replace it twice in my co-ax. And it's quite small.


Ah, but is there enough barrel area to support it?

Closing in on 10k as well I see.


In less than three years, I don't know whether to be proud or ashamed :D
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Unread postAuthor: Yamroll » Mon Apr 25, 2011 8:43 pm

So, seeing as I nearly split my thumb in half like an overcooked bratwurst today, I am just thinking about the design of the coax I'm making.

It's a 2" sch 40 outer chamber with a 1" sch 40 barrel.

It turns out, 1.5" ABS test caps (the Really thin, brightly coloured flimsy ones) are a pretty good fit for the inner diameter of the 2" sch 40.

SO, I jammed the test cap on the end of a piece of 1.5" PVC, and then filled the cap about 1/3 of the way with West Systems marine grade epoxy (some holes drilled in the PVC for the epoxy to to leak into to keep the whole thing from sliding apart).

Drilled a hole in the center, slid a bolt through with the rubber washers, nylock nutted it on.

Seems solid, like it'll produce a good seal on the barrel.

My question is this: how heavy is 2 heavy for this kind of set up?

I think my bumper design should be sufficient, but the whole piston weighs 3.0 oz, seems a bit high.

Would you guys say that sounds acceptable?
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Unread postAuthor: velocity3x » Mon Apr 25, 2011 9:03 pm

Yamroll wrote:I think my bumper design should be sufficient, but the whole piston weighs 3.0 oz, seems a bit high. Would you guys say that sounds acceptable?


I think it depends on your design. One of my guns has a 1.75" dia, 6.5oz. delrin piston with a 1.8" retract and air bumper It has an operating pressure of 1kpsi and never had a problem.
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Unread postAuthor: Yamroll » Mon Apr 25, 2011 9:08 pm

Hmm, yeah, I figure it'll travel about 1", haven't hammered that out yet. Think I've got a decent bumper plan too... Details later.

Pain meds knocking me out for now
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Piston size

Unread postAuthor: Technician1002 » Mon Apr 25, 2011 11:35 pm

For a 2 inch chamber and a 1 inch barrel, may I recommend using a smaller diameter piston? This gives a closer ratio for better performance and a lighter piston. My original Mouse Musket piston was a 1 inch pipe cap. See the Mouse Musket in my sig for breech details to use a 1.5 inch piston cylinder in a 2 inch chamber.

You definitely want a decent bumper. The hollow pipe caps as I originally used did not work well with a bumper. As a result the original pipe cap pistons became a consumable item requiring regular replacement. After moving to a solid piston, a more effective bumper could be used. Unfortunately, the Mouse Musket developed a chamber crack which forced retirement.
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 26, 2011 12:31 am

Yamroll wrote:It turns out, 1.5" ABS test caps (the Really thin, brightly coloured flimsy ones) are a pretty good fit for the inner diameter of the 2" sch 40.


Something like this?

Image

Looks like a good idea if suitably reinforced.

SO, I jammed the test cap on the end of a piece of 1.5" PVC, and then filled the cap about 1/3 of the way


How thick is that? you probably could have gotten away with less, about 1/8"-1/4" epoxy thickness would have been more than enough.

West Systems marine grade epoxy


The brand I started out with, great stuff :D

(some holes drilled in the PVC for the epoxy to to leak into to keep the whole thing from sliding apart)


Not necessary, plug them! The beauty of the test cap is that it can act as a cup piston, essentially it's a one way valve which allows air from the pilot into the chamber but not vice versa.

I think my bumper design should be sufficient, but the whole piston weighs 3.0 oz, seems a bit high.


Sounds reasonable though as said before you could probably make it lighter.

Hmm, yeah, I figure it'll travel about 1", haven't hammered that out yet.


For full flow, intheory you don't need more than 1/4" of travel.

Here's a piston design I had made using a flimsy but tight fitting deodorant cap:
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