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Machined Aluminium Coax - Planning to build

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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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 1:39 am

If you bore out part of the centre to make it shaped like a cup, you lose weight without losing stability in the chamber.
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Unread postAuthor: Hrawk » Sun Oct 28, 2012 1:48 am

I can do that. Would you recommend filling it with something, hot glue perhaps to keep dead space to a minimum ?

Made up the other end cap today.

Arrrrrgh ! Where did I put my 1/4" BSPT tap ? Cant find it anywhere :(
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 2:09 am

Hrawk wrote:I can do that. Would you recommend filling it with something, hot glue perhaps to keep dead space to a minimum ?


Don't need filling if you configure it like this:

Image

Just an idea of the potential weight savings from another thread:

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Made up the other end cap today.


Looking very good, this is going to be the bee's knees when it's done :)
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Unread postAuthor: Hrawk » Sun Oct 28, 2012 2:39 am

That's awesome, thankyou!
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Unread postAuthor: Brian the brain » Sun Oct 28, 2012 8:11 am

The piston is far too long, it doesn't need to be more than 50-75% the diameter of the chamber to be stable. Also, if you can make it out of acetal, it will be half the weight of an alloy pistol and more than strong enough


Considering it only needs to travel back about 1/4 of the barrel diameter a simple disc would do.
In a space confined to no more that what that disc needs to travel in, it couldn't turn.

A spring would help it to stay true.
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 12:16 pm

It's safer to have some support, even a slight skew could equal enough friction to jam it.
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Unread postAuthor: Brian the brain » Sun Oct 28, 2012 12:40 pm

If you'd use a thick rubber disc backed by a smaller aluminium one I doubt it.
The aluminium disc has to be big enough to form an actual usefull backing but it doesn't have to cover the rubber all the way to the sidewalls.
It could easilly stay a couple of mm's clear of it.

The flexible material is all that contacs the sidewalls.
Imagine this:


Never had the piston of a QEV jam..
:wink:
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Can't ask for a better compliment!!
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 12:43 pm

If it's flexible then fair enough, hell make it a diaphragm ;)
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Unread postAuthor: Brian the brain » Sun Oct 28, 2012 12:48 pm

Nope....a pistaphram...
Low weight, good flow..

:D
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: sharpshooter11000 » Sun Oct 28, 2012 4:10 pm

Looking great so far, can't wait to see this thing finished!
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Unread postAuthor: Hrawk » Mon Oct 29, 2012 1:52 am

Didn't get a whole lot done today.

My o-rings arrived, woot, I actually got something right! Thankyou Parker O-ring Handbook. http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf

Machined up the barrel and chamber. Yeah I know, I scratched the heck out of the barrel with the steady rest on the lathe. Hopefully it will sand and polish out.

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Unread postAuthor: Hrawk » Mon Oct 29, 2012 1:55 am

I plan on plunging a 20mm slot drill into the piston to make the cup shape as JSR advised.

I have the 1/8" BSPT tap to tap the schrader into the bottom of the blow gun, once again following smirking Jack's advice. Not sure if I want to just cut the hook off or cut the whole thing a bit shorter.

Might even make a new chamber to get rid of that ugly scratch . . .
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 29, 2012 8:15 am

Looking great :) are you going to put some sort of bumper between rear endcap and piston?
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Unread postAuthor: matti » Mon Oct 29, 2012 10:40 am

Btw.. how are you going to connect the barrel to the endcap ?
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Unread postAuthor: Hrawk » Mon Oct 29, 2012 4:36 pm

I have a heap of those little self adhesive silicone feet to use as a bumper. 7 of them fit quite nicely on the end cap (each one is about 8mm round).

To attach the caps I will be drilling and tapping for 3 or 4, M5x8mm button head screws.

I'd considered forgetting the o-rings and epoxying it all together but I prefer to have the ability to strip it down / rebuild it if needed.

Why didn't I thread them on ? Good question. Short answer; I suck at cutting threads.
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