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how far the piston must open?

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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how far the piston must open?

Unread postAuthor: far_cry » Tue Sep 16, 2008 5:46 pm

i am almost finish my co-axial rifle
the barrel is 10mm (outside) and a 8 mm bore, the chamber is 3/4" steel pipe he had 2cm diameter inside.
how much the piston must open far away from the barrel when release the air?
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Unread postAuthor: Mr.Sandman » Tue Sep 16, 2008 5:50 pm

looks like it will be fine but your piston doesnt need to move that much but how long is your piston?
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Yeah, it's that important.
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Unread postAuthor: mark.f » Tue Sep 16, 2008 5:58 pm

For a barrel with an I.D. of 8mm I would suggest 2 millimeters.
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Unread postAuthor: far_cry » Tue Sep 16, 2008 6:07 pm

Mr.Sandman wrote:looks like it will be fine but your piston doesnt need to move that much but how long is your piston?

i have to build it soon ,i think about 1 to 1.5 cm
and i think about 1 cm gap
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Unread postAuthor: jook13 » Tue Sep 16, 2008 6:23 pm

They say that a good rule of thumb is to have the piston travel be at around 1/4 of the barrel diameter... I usualy go with a tad more, just to be safe.
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Unread postAuthor: Velocity » Tue Sep 16, 2008 7:06 pm

markfh11q wrote:For a barrel with an I.D. of 8mm I would suggest 2 millimeters.


I agree. However, just in case, you might want to bump it up to 3mm, to make sure you are not choking flow.
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Unread postAuthor: far_cry » Tue Sep 16, 2008 7:19 pm

thank you fellas that's will help me a lot
just in case what if i make the gap 1 cm instead 2 mm ??
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Unread postAuthor: daberno123 » Tue Sep 16, 2008 7:39 pm

far_cry wrote:just in case what if i make the gap 1 cm instead 2 mm ??


More flow, but 1 cm is way overkill and all it will do is add to your pilot volume, therefor reducing overall performance. Do what Mark and Velocity said and aim for 2-3 mm.
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Unread postAuthor: Sticky_Tape » Tue Sep 16, 2008 7:43 pm

You will ALSO be exspanding the air in your chamber making the gun less powerful. Once the piston opens past 1/4 the barrel i.d. then you are just exspanding air into the extra area but it wouldn't make much of a difference.
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You can tell how awesome a cannon is by the pressure used.
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Unread postAuthor: far_cry » Tue Sep 16, 2008 8:11 pm

roger comrades
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 17, 2008 12:03 am

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Unread postAuthor: ALIHISGREAT » Wed Sep 17, 2008 1:32 am

personally i would aim for 5mm
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Unread postAuthor: BlackH » Wed Sep 17, 2008 4:21 am

I think 5mm too
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Unread postAuthor: far_cry » Wed Sep 17, 2008 4:00 pm

ok i will try thanks
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