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Piston Material and various other question

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 and various other question

Unread postAuthor: coryS » Sun Oct 11, 2009 10:08 pm

Alright so I have a couple more questions for you guy's. What should i make my piston for my co2 cannon out of? I have access to a lathe and many other tools. I was thinking delrin or teflon but if you guy's have any other ideas i'd be happy to listen. Also what should the dimensions be (mostly interested in length) and should it have o-rings? The piston is going to fit inside 2 inch SCH 40 PVC and needs to function from 15-100 psi. Thank you =]
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Unread postAuthor: Technician1002 » Sun Oct 11, 2009 10:57 pm

Image

I have found HDPE works very well in that pressure range. It improves the speed as it is lighter than most other materials. The UHMW version is even better.

I would recommend o rings unless you polish the inside of the pipe to a smooth finish. Otherwise the leakage between the pilot and chamber can be excessive. In these photos, there is no ring as the inside of the pipe was sanded to a smooth finish instead.

I do have a video online on using o rings in air cannons. High moving friction can cause the cannon to fail to fire. Using a floating ring is highly recommended. Do a search for my username on youtube to find it.

The piston needs to be long enough to prevent twisting and binding. This piston is a little longer than 1/2 the diameter and works well. The photo is adding the eq port from the chamber (edge) to the pilot area.
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Unread postAuthor: tghhs » Mon Oct 12, 2009 3:45 am

Well here's a pic of my new (White) Teflon piston, machined on drill press, Floating O-ring grooves cut with needle file's.

(Black piston used for this fps examination, http://www.spudfiles.com/forums/audacit ... 19423.html)

I showed dad this he thought i'd used the lathe, (not allowed to atm coz of one finger incident already :P) Anyway, i'm going to get some lessons, so hopefully some cooler ones coming up :)
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Attachments
New piston.jpg
The new one with floating O-ring
New piston.jpg (10.25 KiB) Viewed 349 times
piston and barrel.JPG
Old one, still works well, longer that it needs to be i feel as weight is on the heavy side.
piston and barrel.JPG (20.99 KiB) Viewed 349 times
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Unread postAuthor: psycix » Mon Oct 12, 2009 6:04 am

Delrin, Teflon, HDPE and UHMW make good pistons. You might want to hollow it out so it becomes even lighter.
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Unread postAuthor: jmadden91 » Mon Oct 12, 2009 8:31 am

Hubb has made a brilliant tutorial which can be found here. May not be custom machined but will definitely spark some ideas.

2 Inch Piston "How To"
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Unread postAuthor: coryS » Mon Oct 12, 2009 8:39 pm

Thank you so much for you help guy's :D Just 2 more questions and I promise i'm done. Where can i go about purchasing these materials and how much is the cost gunna run? Thank's again
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Unread postAuthor: Moonbogg » Mon Oct 12, 2009 9:07 pm

If you have a credit card you can get them from
http://www.mcmaster.com

For 2" diameter UHMW solid bar the price is $6.50 per foot. Similar sizes will vary a little in price
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Unread postAuthor: Technician1002 » Mon Oct 12, 2009 9:15 pm

Another source can be a local plastics shop. I'm a fan of avoiding shipping charges for small orders.
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Unread postAuthor: Moonbogg » Mon Oct 12, 2009 9:21 pm

Technician1002 wrote:Another source can be a local plastics shop. I'm a fan of avoiding shipping charges for small orders.


I seem to forget that not everyone has a massive McMaster-Carr wearhouse right down the street from them :D :D :D
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Unread postAuthor: SpudBlaster15 » Mon Oct 12, 2009 9:55 pm

UHMWPE would be my first choice. If you can't find it locally, Nylon works well and is more readily obtained.
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