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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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Machined Aluminium Coax - Planning to build

Unread postAuthor: Hrawk » Mon Oct 22, 2012 3:11 am

Hello all,

I picked up a heap of 32 & 12.7mm drawn aluminium tube the other day in readiness for my next build.

Thought I'd share my plans with you and hopefully get your feedback before I put tool to metal.

Main tube is 32mm OD, 28.8mm ID. Barrel is 12.7mm OD, 9.86mm ID. Piston and end caps will be machined from 6060 stock.

Air input compliments of a stainless 1/8" BSPT to schrader valve, tapped into the end cap. Source will be my shop compressor with a 115psi max.

Pilot will be a run of the mill blow gun.

My main objective is to make something that is neat in appearance with a reliable action.

If this works ok, I will then look at moving the blow gun forward via some right angle adapters and hose and adding a stock.

Anyhow, pics are attached and I look forward to your feedback.
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Re: Machined Aluminium Coax - Planning to build

Unread postAuthor: jackssmirkingrevenge » Mon Oct 22, 2012 4:40 am

Hrawk wrote:before I put tool to metal.


Sounds like you're getting an alternative piercing done :D

Air input compliments of a stainless 1/8" BSPT to schrader valve, tapped into the end cap. Source will be my shop compressor with a 115psi max.

Pilot will be a run of the mill blow gun.


I would hook up a connector to the open end of the blowgun and use that for filling, that way you avoid boring an extra hole and dead space.

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.

Remember, lighter pistons open faster and put less stress on the launcher.

If you shape it like a spool, you get even less weight and friction:

Image

Assuming the chamber tube extends to the muzzle end of the barrel, I would keep 50% or less of this length as a chamber, then put the endcap and use the rest of the tube as a suppressor, adding some porting to the muzzle and maybe some baffles. Your C:B ratio otherwise will be far too big, remember that for every inch of barrel-in-chamber length, there is almost 7 times more chamber volume than barrel volume. If your barrel length is 1 metre, the actual chamber volume on needs to be around 30cm long for optimum performance.
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Unread postAuthor: POLAND_SPUD » Mon Oct 22, 2012 7:37 am

why there are two holes there? you just need one (for a 3 way valve)
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Unread postAuthor: ramses » Mon Oct 22, 2012 8:08 am

Also, it looks like you may be choking flow with the barrel spacer.
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Unread postAuthor: dewey-1 » Mon Oct 22, 2012 9:35 am

Here is a better approach for the barrel support as Ramses was stating.
All are 12.7mm diameter.
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 22, 2012 9:37 am

Image

Image

;)
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Unread postAuthor: matti » Mon Oct 22, 2012 10:54 am

but.. why the double O-rings ? I have seen some used in pistons, but you only need one O-ring to seal end plug. Also take the good advices on the C:B ratio, barrel support and shortening the piston (and use plastic).
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Unread postAuthor: Hrawk » Mon Oct 22, 2012 11:45 pm

Thanks for the feedback...

Very nice barrel support / spacer you have there. I need to rework my design a bit I think.

Why the double O-ring ? Well, not being up to speed on what pressures I can expect them to hold, I figured 2 would be better than one, they are pretty cheap after all.
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Unread postAuthor: Zeus » Tue Oct 23, 2012 12:17 am

But more o-rings means more stiction.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 23, 2012 12:21 am

Zeus wrote:But more o-rings means more stiction.


Not a problem for endcaps, and having two o-rings helps to keep the part centred and less likely to skew and leak.
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Unread postAuthor: Hrawk » Sat Oct 27, 2012 6:39 pm

And so it begins.

I've shortened the chamber to 40cm, about 32 effective length when assembled.

Piston and front cap machined.
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Unread postAuthor: sagthegreat » Sat Oct 27, 2012 9:00 pm

if the piston is the block looking one then how will it seal?
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Unread postAuthor: Hrawk » Sat Oct 27, 2012 9:11 pm

I have not added the rubber to it yet.
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 12:53 am

Shineh! That piston is going to punish the launcher though. It only needs to be half as long, and if you have the option of making it out of acetal/PTFE/UHMWPE please take it, a heavy piston does no favours to performance or launcher longevity.
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Unread postAuthor: Hrawk » Sun Oct 28, 2012 1:33 am

Hmmmm, ok.

I'll shorten the piston a bit. Currently it's 21.5mm sitting inside a 28.8 ID tube.

I've been looking at purchasing a length of delrin rod which I can later make the piston from. Just using what I have on hand at the moment.
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