My first airgun, need help

[martinpeim16]

Hello everyone!

I am trying to build my first airgun with QEV and barrel seal piston. As of now I have finished pretty much the whole body, but I am having problems making my blowoff gun triggering my piston to pop. i have charged my air tank (1.25L bottle) to 30psi and pressed the trigger but air escapes without moving the piston at all. I am using a Nylon tap valve for a piston which just fits into the QEV. The QEV is a homemade QEV from a brass 1/2" tee and a 12mm pipe inside.
Something tells me its due to the small pilot area i have, as it did actually worked before when i was using a simple pilot valve.

Attached are pictures of my airgun so far.

Many many thanks, please let me know if you require any more information to assist.

[Gun Freak]

Is the piston wider than the seat? What I mean is, is there any area on the face of the piston for the chamber pressure to act on?

[martinpeim16]

yes, the seat is 12mm in diameter while the piston is 17mm. So i got 2.5mm for the chamber pressure to act on. would that be too little?

[Gun Freak]

No, that would be plenty. Is the piston almost airtight in the 12mm pipe? Sounds like its letting too much air by, so the pressure can't drop quick enough to move the piston and open the QEV.

[martinpeim16]

That's what i thought too. The piston is pretty much the same size as the QVE tube, so a tiny gap is left for air to flow. I just added some lubricant for valves and it actually worked once after trying 3 times. So its sometimes working but most of the time its not. is my piston maybe too small? I have seen ppl with longer pistons.

[Gun Freak]

You can try putting something around the skinny part of your piston so that more of it is the same diameter of the piston tube, that way it will have a better seal. Also try using more than 30 psi and it might work.

[martinpeim16]

I found out the problem. The blow gun i used only lets the air out at a slow speed, thus not been useful a a quick exhaust. I will have to try to make a pilot valve.

[jackssmirkingrevenge]

I think the problem is the fact that your piston is on a thin stem. This is preventing the pilot area from venting quickly.

Try drilling it out as indicated here:

[martinpeim16]

Thanks for the help, i drilled the holes and and it works better now. so far when i pump the bottle to 50psi i get a strong shot, i think if i get it to 100psi ill be able to have 2-3 shots. I got a new blowgun and made a fix to allow more airflow. Im also changing the airtank to a 1kg fire extinguisher.

I heard you can use a relief valve to get full-auto but i can only find them at 100+psi levels, so i would require a huge airtank to use. Are there any relief valves that operate at smaller psi? say 20psi so i get 5 rapid fire at 100psi.

Thanks.

[jackssmirkingrevenge]

I heard you can use a relief valve to get full-auto but i can only find them at 100+psi levels, so i would require a huge airtank to use. Are there any relief valves that operate at smaller psi? say 20psi so i get 5 rapid fire at 100psi.

You can buy one of these cheap adjustable QEVs from eBay, they can be set to pop at very low pressures:

http://www.ebay.com/itm/0-5-Thread-Air- ... 2326092520

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[martinpeim16]

I can't seem to find a psi range on the description for this valve on eBay, what would be the minimum and maximum range?

[martinpeim16]

OK, i got a new problem now Like you guys said, my piston is not good for my gun. I need a very fast flow to push my piston back, and when it does it doesn't throw much of a punch. How can I make a better piston? and what material works best? Pls help

[jackssmirkingrevenge]

I can't seem to find a psi range on the description for this valve on eBay, what would be the minimum and maximum range?

I was using mine at 50-100 psi

OK, i got a new problem now Sad Like you guys said, my piston is not good for my gun. I need a very fast flow to push my piston back, and when it does it doesn't throw much of a punch. How can I make a better piston? and what material works best? Pls help

Have you thought about letting epoxy into your life

http://www.spudfiles.com/forums/making- ... t8919.html

... or just spend 5 euros on a QEV from eBay...

[martinpeim16]

wow, Ill try making one out of epoxy, thanks. While i do this, i have one more question i been tying to answer. My pilot area is quite small, and to allow the the air flowing out of my pilot valve to move the piston backwards you would require a strong flow. Even with this new piston, how is the pilot valve suppose to move it backwards with air force when the flow is slow? Im only operating at around 50psi, if i was at 300psi i would not have a problem i suppose. Thanks, slowly learning a lot here

[jackssmirkingrevenge]

Seriously... just buy a QEV.

http://www.ebay.com/itm/Pneumatic-QE-04 ... 43bbd50668

It costs almost nothing and makes you life easier