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Unread postPosted: Fri Jan 27, 2012 11:06 am
Author: CpTn_lAw
You need one thing that will be much more important than a spring behind a shorter piston.
You need to think by yourself, i'm kinda upset it seems we didn't even have the conversation a few weeks back. Everything you are asking right now was answered several times in your previous topic, let alone every other pneumatic rifle topics on this very website. May I ask a simple question? How old are you?

Stop talking "i'm gonna make that, and that, or no! better idea, this is way more cooler that this" This is getting annoying and you are only assuring us you don't know anything about what you claim to know.

Now, start reading carefully as i will answer one last time to your "already answered" questions.

Multi shot is indeed possible with a spring behind the piston, but not only. If the air source is plugged behind the piston, then simply allowing the air to push the piston against the sealing point will do the same as a spring.

Build a simple gun first before trying stuff your clearly lack the ability to do. One shot, simple piston valve will give you plenty of fun, and will be a tremendous increase in skill for you. After that, when you really understand what it's all about, try modding it, optomizing it.

It won't get much easier than a simple co-axial piston valved gun.

Unread postPosted: Fri Jan 27, 2012 2:27 pm
Author: wyz2285
Alright, so if I make a shorter piston and add a spring behind it, when the pilot valve is opened the piston will shoot backwards momentarily and then close again? Allowing for multiple shots??

Are you trying to make one that work like this?
It´s not a popular way, but it does give you multi shot(also semi if you have automatic reload)
Here is a more powerful version, but same mechanism.

Unread postPosted: Fri Jan 27, 2012 6:48 pm
Author: DerpWithAGun
Thanks wyz! I'm going to follow the guide you have on that QEV and change it a little bit depending on what parts are available. I hope a blowgun trigger will be enough to pilot this, your gun is 1/2", mine is a giant 3/4". If not I'll make my own valve.

For now I'm just going to make a single shot and on my lathe a multiple shot semi auto.

Unread postPosted: Sun Jan 29, 2012 12:48 am
Author: DerpWithAGun
Hey guys,

I been building my gun for a couple of days now. Planning and trying out different parts until eventually, the piston housing, the sealing area on the barrel and the Tee peice was all assembled. Today I worked on the piston and I have come across a problem. Simply, The piston doesn't want to seal. With a spring behind it, it pushes against the barrel tightly. Then I started to blow down the barrel and air was rushing out of the 90 degree elbow thats on the Tee. Here are some pictures of my assembly and the piston: (Btw, I did not make the piston from scratch, I found it into the section with all the tap seals in it.)

Image
This is the whole assembly.

Image
This is the piston's housing with the spring socket.

Image
This is the piston it'self.

Forgot to take a picture of the barrel but doesn't matter, it is just an aluminum pipe that is a little bit more than half the diameter of the piston.

All help appreciated.

EDIT: I just placed in another 2 strong springs, (To simulate actual air pressure pushing against the piston) and still the same result, air isn't "leaking" out, it's as if the piston isn't there in the first place.

Unread postPosted: Sun Jan 29, 2012 12:49 am
Author: evanmcorleytv
Good work so far!!

Unread postPosted: Sun Jan 29, 2012 4:19 am
Author: wyz2285
you do know that for the gun to work you need a weak spring right?

Unread postPosted: Sun Jan 29, 2012 4:26 am
Author: DerpWithAGun
I'm aware of that wyz, I put a very strong spring to simulate high pressure pushing the piston closed. I do have a very weak spring which I will use.

Does anyone know why my piston isn't sealing properly?

Unread postPosted: Sun Jan 29, 2012 8:58 am
Author: dewey-1
DerpWithAGun wrote:Hey guys,

and I have come across a problem. Then I started to blow down the barrel and air was rushing out of the 90 degree elbow thats on the Tee.


Since when does a barrel have more pressure than the chamber side?
This is an invalid test to check the seal integrity.

Almost all valves will do the same thing.

Higher pressure on the barrel (valve output) side will force the piston/diaphragm open because the chamber side (valve input) is at a low pressure.

Unread postPosted: Sun Jan 29, 2012 9:18 am
Author: DerpWithAGun
Yeah, I was kind of thinking that, so I tested it a different way. I covered up the elbow with my hand and blew down the back of the gun, (where the pilot and fill valve will be) And air still escaped past the piston and came out of the barrel.

I think I may know why now, the barrel actually consist of 3 pieces. A PVC connector with a male threaded end and a female non-threaded end which I sawed off. I had a piece of PVC pipe that fit VERY tight inside the connector which I hammered in. It was such a tight fit I came to the conclusion that I didn't need epoxy there. The third part is an aluminum length of pipe which was epoxied into the PVC pipe. The PVC pipe is so tight in the connector that it took 5 VERY hard swings with a rubber mallet to get it into place. Do you think that maybe the gun is leaking from there? I'll just epoxy it tomorrow and we'll see what happens.

Thanks for your contribution! :D

Unread postPosted: Sun Jan 29, 2012 6:10 pm
Author: warhead052
Is it sealing against a washer? If so, then air is leaking through the gap between the washer and the walls of the chamber, and you need to fix that with epoxy.

Unread postPosted: Sun Jan 29, 2012 6:37 pm
Author: DerpWithAGun
no no no warhead. I think you are talking about the second picture with the washer in the middle? That it actually the back of the gun where the springs, and there is a washer there because the spring would just go through. Anyway, that thing is 110% air tight! 4 layers of teflon tape on the connector and reducer, then, where the washer is, it has 1 O-Ring on either side which is clamped tightly together by the connector and reducer. :)

Give me a few minutes, I'll upload a picture of where my gun is sealing.

Unread postPosted: Sun Jan 29, 2012 8:13 pm
Author: Engineererrrr
I have a noob question, how do i make a piston for a 3/4 chamber to cover a 1/2 opening, fyi im doing the same project. Also couldn't you have used a reducer pipe and sand off the indents so the barrel fits all the way through the reducer into the chamber.

Unread postPosted: Sun Jan 29, 2012 10:19 pm
Author: DerpWithAGun
Engineererrrr there are many ways to make a piston for a 3/4" chamber. The picture above of my piston valve is 3/4". You can make a hotglue piston which are okay. You can also make pistons with O-rings or pistons out of a small diameter threaded rod. As for using a reducer pipe, that depends. If the reducer is made a PVC then that wouldn't be necessary as what I have already done is just as good. If the reducer is made from galvanized steel or brass then definitely not because those "indents", namely threads are very hard to take off. The only way to do it like that is with a machine that can cut off the threads.

Unread postPosted: Sun Jan 29, 2012 10:59 pm
Author: Engineererrrr
ok i mean a reducer pipe from copper pipes. You can, if im seeing this rite, convert after your "t" everting encased in a copper pipe with a filling like glue or rolled up tape, then when it comes to your chamber find a reducer that fits your chamber to your barrel, the reducer has no treads. The "indents" are small stubs inside the smaller diameter for soldering purposes. If you removed those your barrel can slide into your chamber forming the coaxial chamber-barrel area. After measuring the barrels depth into the chamber solder it air tight.

So what is a better type of piston compared to others for this purpose and how to make?

Unread postPosted: Mon Jan 30, 2012 12:21 am
Author: DerpWithAGun
Oh I understand you now. The only thing is I'm not building a coaxial styled gun.

The Piston is relatively easy to make. The best and easiest piston to make would probably be a nut, washer and threaded rod piston. Hot-glue or epoxy pistons are okay though. If you have machinery like a lathe or a drill press, there are some guides in the how-to section of this forum that show you how to make some REALLY good pistons with O-rings on them. I don't know where I can find you a guide for making the threaded rod piston sorry.