SpudFiles

I'm going to be making a 2" coax with a 3/4" barrel out of some spare parts I have. Since I want this gun to be kinda used like a rifle, there will be one handle about 18" from the back and another at the back. I was wondering how I should pilot this thing since a ball valve out of the back won't be ergonomic. Any suggestions would be appreciated and all I can think of now is a sprinkler valve but I would prefer not to do that since it's so bulky. I can post a diagram of my planned layout later this evening to help clear things up. Thanks so much

***Update: diagram of potential gun layout posted lower on the page
Pictures of piston and cannon layout on page 2

it would be really cool to use a pop off valve if you have a tight piston.

1/2 inch QEV or homemade piston valve.

what were you looking to spend? if it were me id use the qev for another gun lol.

Well I want to keep it cheap since it is a spare parts gun. And I'm not familiar with qev's anyhow.

It really depends on piston tolerances and pilot volume. If you followed the d/4 rule with a cast piston (i.e. perfect fit, low friction) or an o-ringed piston, a modded blowgun is a feasible option

1'' sprinkler valve moded and use a rod it open your pilot valve.

I would suggest housing your piston in a smaller pipe than 2". If you are using a 3/4" barrel, I would use a modified bushing at the back of the chamber to pass a piece of 1" pipe into the chamber for the piston to slide in. This will both decrease pilot volume and piston bounce experienced with barrel sealing pistons significantly larger than the barrel seat diameter.

I do recommend having a piston only slightly larger than the barrel. If it is too big, the valve will start to open while there is still a relatively high pressure in the pilot. This can cause the valve to only partially open or re-close early as the chamber pressure drops faster than the pilot pressure.

A large piston requires a huge pilot valve to get them to pop open and stay open properly.

There is more info on the valve ratio on piston valves in the wiki.

So if I ended up using a piston housed in 1" pipe would a modded blowgun be feasible to use as a pilot if my piston was turned on a lathe and very tight fitting?

yes, but since your going to lathe it down you might as well put a floating O-ring on it.

mark.f wrote:I would suggest housing your piston in a smaller pipe than 2". If you are using a 3/4" barrel, I would use a modified bushing at the back of the chamber to pass a piece of 1" pipe into the chamber for the piston to slide in. This will both decrease pilot volume and piston bounce experienced with barrel sealing pistons significantly larger than the barrel seat diameter.

Ok attatched is my planned layout. I think it looks alright. It's kinda vague so any questions will be gladly answered. Still not sure about a pilot valve.

It will use ~18" of 2" pipe for the chamber, ~36" of 3/4" pipe for the barrel and a small piece of 1" pipe for the piston to ride in. This gives around a 1.8:1 chamber to barrel ratio.

The piston will be turned out of wood (unless a better option can be found) and I think I will decide to add a floating O-ring.

If you make it out of wood make sure you seal the wood with polyurethane or varnish. it will absorb all the water in the comprized air and get stuck if you don't.

Wood isn't often used as a piston material because of its pitiful impact strength and unfortunate fracture mechanics. If you slam it into something, it's apt to just snap in half along the grain.

saefroch wrote:Wood isn't often used as a piston material because of its pitiful impact strength and unfortunate fracture mechanics. If you slam it into something, it's apt to just snap in half along the grain.

I know it's not ideal but I could use a strong piece of oak, especially in a piston this small. But I'd still like to come up with something stronger. I may end up ordering some HDPE (or pvc) and o-rings from mcmaster for this project.

My main concern now is the pilot. I want to be able to remotely trigger this thing so the trigger can be placed on one of the handles of the gun. I'm not sure what kind if valve to use to make this happen for a 1" piston though.

Edit: Update- I'm going to turn the piston down from a 1/2" PVC endcap. I am still in search of o-rings that will fit though. I know 1" od are too small and 1-1/8" are too big so I need to find somewhere that has 1-1/16" or 1-1/32"