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Piston material

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Piston material

Unread postAuthor: jitup » Fri Dec 12, 2008 6:03 pm

Hello once again,

I am working on (almost finesed with except for the valve) a 3 inch piston gun. I want to pour the piston, but I do not have any epoxy, like JSR likes to use, available in my area. I came up with a couple ideas of what I else I could use. I would appreaciate your input.

1 polyester resin (used for fiber glassing)
2 poly urathane (for wood floors) If I do this should I use oil or water based?
3 Bondo (will this crack?)

I am open to suggestions. I do not have any stock to machine a piston out of, and I do not have a lathe so I cannot make a would one.

I would prefer not to have to order any thing off the internet.

Thank you.

BTW this gun will be Bad @ss when it is finished :twisted:
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Unread postAuthor: psycix » Fri Dec 12, 2008 6:19 pm

I have the same problem, but did encounter polyurethane at the hardware store.
Did some research but I think polyurethane is too weak and too "jelly".
Could strengthen it up with another material though.
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Unread postAuthor: jitup » Fri Dec 12, 2008 7:20 pm

I thought the same thing about the poly urathane. All I hve to do for this gun is finish the piston and than glue it all together, So I am trying to find something I will get a good seal with so I do not have to use o-rings (no lathe)

any suggestions will be helpful

would polyester resin or bondo work. (I think bondo might crack what dose everyone else think.)
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Unread postAuthor: Gippeto » Fri Dec 12, 2008 7:30 pm

Another idea;

Do you have a hole saw?

You could cut plugs from wood, and either laminate them together, or mount two on a piece of all thread. (one front, one rear)

Hold them in place with nuts and washers. Seal the wood with poly urethane or fibreglass resin.

Think "spool". :)
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Unread postAuthor: jitup » Fri Dec 12, 2008 7:35 pm

so do you mean like this

) (
)--------------(
)--------------(
) (

I do not understand What I would need the resin for if I did this.
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Unread postAuthor: MrCrowley » Fri Dec 12, 2008 8:00 pm

Gippeto wrote:Another idea;

Do you have a hole saw?

You could cut plugs from wood, and either laminate them together, or mount two on a piece of all thread. (one front, one rear)

Hold them in place with nuts and washers. Seal the wood with poly urethane or fibreglass resin.

Think "spool". :)

Like this?
Image

http://www.spudfiles.com/forums/v-a-l-p ... 14702.html

I sealed it with varnish. It's several sheets of ply, but I made a new one out of a solid wood so I could add an o-ring.
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Unread postAuthor: Daltonultra » Fri Dec 12, 2008 8:15 pm

If you are doing a large piston, you might think about using sheet rubber as a face on a wood piston. McMasterCarr sells butyl rubber in tons of different sizes, shapes, and thicknesses.
When I build my big piston gun, I'll be using Delren plastic for the piston, and facing it with 1/4" hard butyl rubber. It will be more than strong enough to resist being torn out and fired down the barrel. I can also use it in layers as a piston bumper.

As for using a hole saw, finding the exact right size to cut a piston might be hard. If you have access to a drill press and GOOD clamps, you can use an adjustable circle cutter to get the exact right size. This is how I intend on cutting my Delren piston.
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Unread postAuthor: jitup » Fri Dec 12, 2008 9:01 pm

@ Daltonultra
I have a drill press, but I am unsure what a circle cutter is, can you please go into detail.

Edit: Never mind I know what it is. I think I have one.

I will upload a beter illistration of what I will probably do.

BTW this is not proportional, or centered. Just a quick 5 min MS paint sketch
Thanks Gippeto
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Attachments
piston.JPG
The hole is going to be 1/4 in so the 3" by 2' plus a little 1.5" tube chamber dose not take a week to fill
piston.JPG (12.94 KiB) Viewed 616 times
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Unread postAuthor: Gippeto » Fri Dec 12, 2008 10:54 pm

That's the idea. :)

But leave the equalization hole out. You won't need it.

Trust me. :)

Also, consider using a second rubber disk to act as a check valve on the rear disk.

Assemble in this order (front to back);

Nut, washer, rubber sealing face, wood disk, washer, nut, (length of rod), nut, washer, rubber disk, wood disk, washer, nut.

When filling, the air will push past the rubber disks (IF you fill from the pilot like most everyone does.), when piloting, the rubber disks will be pushed against the wood disks and the valve housing walls, helping to reduce total piloted volume. :)

What are you planning as a bumper?
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Unread postAuthor: CasinoVanart » Fri Dec 12, 2008 11:59 pm

Polyurethane is the bomb for pistons, trust me :wink: All you need i poly and a piece of chamber oh and some silcone or similar sealant.
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Unread postAuthor: frankrede » Sat Dec 13, 2008 12:35 am

Why not order epoxy online?
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Unread postAuthor: ALIHISGREAT » Sat Dec 13, 2008 4:05 am

1) Epoxy
2) wood
3) bolt and washers
4) Ready made HDPE rod (what i used, it happened to be exactly the right diameter, this is probably the easiest method.)
5) metal
6) PVC endcaps
7) i've seen a spray-can-cap used before
8) i'm pretty sure a mini-pritt-stick lid is a perfect fit for 22mm copper
:lol:

anyway, there are many options, but for a massive 3" piston, i would discount most of them making wood, bolt and washers and PVC endcaps, i reckon epoxy will be too heavy if you cast it solid, you'll need something light (but solid) as a core
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Unread postAuthor: psycix » Sat Dec 13, 2008 2:25 pm

but I made a new one out of a solid wood so I could add an o-ring.
*cough* mass *cough* heavy chgrmm*

I would say only have a solid back for the o-ring.
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Unread postAuthor: Gippeto » Sat Dec 13, 2008 2:48 pm

Subtle as a brick to the head. :wink:

None the less, a valid point. :)
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Unread postAuthor: jitup » Sat Dec 13, 2008 4:31 pm

To late to cast. I bought all the stuff to day to make the one above.

Iwill put a piece of rubber on the back but Gippeto, shouldn't I put the rubber closest to the barrel on the outside of the wood to act as a sealing face?

as for the bumper I will use rubber tube I have, from a talk box for guitar that I made. The thing blew up and I do not plan on remaking it. It is 1 inch house, the clear stuff.
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