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Piston and boltaction diagram

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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Piston and boltaction diagram

Unread postAuthor: john bunsenburner » Sun May 31, 2009 6:48 am

Well now I have my lathe I also have the finalized idea for my first gun. the diagram may seem a little confusing as it has no labeling an dis hand drawn. How ever I am happy to answer all questions about it. the reason it is un labeled is because the labeling just made a huge mess out of it. Please not this diagram is to scale. One block is equal to 5mm. The red(it is found at the ends of the barrel housing which is the long 5mm thick tube that ends with a slightly wider space on the right) are areas of which i am unsure of. They are:
the area at which the barrel is pressed against an object to provide a seal
the area that stops the spring from falling out and also gives the piston something to press against.
Another big problem of mine is where to fill the gun's pilot area from, perhaps the back O-rings of the piston can be left out, i am not sure if that would work.

These areas will need a bit more brainstorming and I would be very happy if you could share ideas you have with me.

A few things you do need to know to make the diagram comprehendible:
The tubes that come of and are across from each other on the piston housing both lead to the tank, while the tube in the back goes to the QDV pilot of the gun.

Now without further talking i present you the diagram:
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My piston housing and boltaction mech.jpg
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Unread postAuthor: Gippeto » Sun May 31, 2009 10:21 am

Hello John, a rather ambitious first project. :)

I had noticed a couple of "things" to point out;

If you wish to use a "QDV" type valve, it does not seal against the barrel. The two o-rings on the piston are all you need.

There is no "pilot area", the piston is physically "set" (by means of a rod etc.), and the chamber is filled. To activate the valve, the piston is again "physically" moved until the port to the chamber is uncovered.

At that point, air pressure from the chamber will finish opening the piston.

You must limit piston travel. If you don't, the pistons travel will create a larger volume of "dead space". Dead space reduces the "effective" chamber pressure, by allowing the chamber pressure to expand (reducing pressure) before doing the "desired" work. ie. firing the projectile.

Allow only enough piston travel to have the ports open fully. Approx 25% - 50% more port area than barrel id area should be all you need. Making "wider" ports rather than "longer" ports would also be good.

I would reduce the piston diameter to 10mm. It's still too large at 10mm, but smaller pieces are trickier to machine, and larger will just increase "dead space (volume)" too much.

I have to assume that the 0-ring and spring are to retain the barrel??

If so, leave the spring out of it, turn a sleeve on your lathe to compress the o-ring. This will work better, and more predictably.
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Unread postAuthor: john bunsenburner » Sun May 31, 2009 10:29 am

Well the idea is a air piloted QDV type gun. It should thus open faster and gives the the same posibilities as a piston type gun would provide me with similar flow. The O-ring you are talking about is not an O-ring instead it is a nut, or piec eof metal on the barrel tha tpushes the barrel back onto the red "thing"(most probably simply a lip of aluminium) to provide a seal.
What my problem is with the 50% movement is that it seems to be fairly hard to make a piston housing that is 1.25cm long, though you are welcome to tell me it is not in which case i will attempt.
Any more I need to know or consider?
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Unread postAuthor: MRR » Sun May 31, 2009 10:49 am

I think that the piston won't actuate when it is in closed position. The supply to the chamber is right between the two o-rings and even if you take away the left o-ring (to pressurize the pilot), there will be no pressure in front of the piston that pushes it back. The air will simply leak through the pilot valve.

You should extend the barrel, so that the tubes to the air chamber are not covered by the piston.

edit: I hope I get it right that the piston is only piloted by a QDV...
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Unread postAuthor: Gippeto » Sun May 31, 2009 11:00 am

Not 50% movement. You need to calculate what the piston travel should be. Thusly;

A 5mm barrel has a barrel id area of 19.635mm2.

I'm recommending a total port area of "up to" 150% of that, which is 29.453mm2.

If you had two ports, they would each have 14.726mm2 area.

If we use round ports instead of rectangular (better) ports, the diameter of the port hole would be 4.33mm. (drill the holes to the closest size you have, say 4mm)

So the total piston travel should be no more than is required to fully uncover the ports. (4mm + the width of the o-ring groove + the amount of material left at the edge of the groove + half the distance between the two piston o-rings)

Please explain why you feel that boring the housing is a problem, perhaps I can help??
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Unread postAuthor: john bunsenburner » Sun May 31, 2009 11:21 am

Gippeto wrote: but smaller pieces are trickier to machine


You stated the reason your self. Small parts are tricky and fiddely to machine. And I am scared it could become problematic for an unexperienced person like me. Why is a rectangular port better than a circular one?

MRR: You have mis understood: The valve i am using is a new type really. It works like a QDV how ever is pilote by air. It is a pneumatic QDV. And i have decided to pilot it with a normal QDV simply because they are easy to make and can me made to be triggered in a realistic manner.

The piston rests on the end of the piston housing and seals the two holes leading to the barrel. When the pilot is opened the piston slams back and exposes the two holes while stoping more air from going into the pilot valve and being waster. The air from the chamber then goes into the barrel and pushes the projectile out.
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Unread postAuthor: spot » Sun May 31, 2009 12:51 pm

When the piston moves past the rectangular port, it shows more surface area. Picture: (sketch :p See attachment) As you can see, for the same amount of piston travel the rectangular ports offer better flow.
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Unread postAuthor: john bunsenburner » Sun May 31, 2009 1:13 pm

True, but that would most probably play an insignificant role, would it not? also the cutting of rectangular ports and using quick disconnects ect. just makes it too difficult to try so they will remain round. Thanks for the clearification, I love new members that come wielded with knowlage that they want to share, its always great so thanks for that.
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Unread postAuthor: Technician1002 » Mon Jun 01, 2009 12:04 pm

@ John and Gippeto. I think from the drawing that this valve is not a QDV even though it has 2 orings. I think the smaller forward seal at the barrel is the real seal, not the froward o ring. This face area of the piston under pressure is what is used to initially open the valve when the pilot is vented.

Am I on track? If so, this may help the confusion on what starts this opening when the pilot is vented. This piston if it operates that way may require small bypass ports both to the face of the piston outside the seal (not in the barrel opening) and to the pilot area as a normal eq port.
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Possible valve seat and piston EQ porting
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Unread postAuthor: john bunsenburner » Mon Jun 01, 2009 3:11 pm

Well that would not work as in your diagram a lot of air will be lost because it just flows down the pilot through the EQ port. The valve seat is most probably a metal disk of some sort, most probably just a little lip.
After chatting with insomniac i have noticed(after he pointed it out) that the following two flaws are found in my plan:
1. When the air is released in the pilot there may be a small vacuum provided but it will be unlikely to slam the piston back as fast as a co ax barrel sealer
2. The vacuum that would be caused by the piston moving back could pull the tight fitting projectile into the piston chamber which would cause a dry fire and could cause damage to the piston if I do not notice that the lead pellet got sucked in and refill.

To counter act these problems I have made a little plan which is extremely complicated, and after looking at tech's diagram have another very simple plan how ever I would like to know weather or not my concerns are valid and what ideas you have before I present mine.

EDIT: Oh nvm about me keeping the idea for my self, have a look at this and tell me what you think:
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My piston housing and boltaction mech v.2.jpg
Last edited by john bunsenburner on Mon Jun 01, 2009 4:09 pm, edited 1 time in total.
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Unread postAuthor: Hotwired » Mon Jun 01, 2009 4:05 pm

There are times when I come in for a browse and wonder if someone just robbed my train of thought.

But turns out you didn't go down the same rails I did, close though. No cookie :)


If I was intending to make this valve, what I would do is stick a tension spring behind the piston.

On filling the pressure builds up behind the piston until it is pressed against the barrel port with the spring now stretched out but unable to contract because of the greater force of the gas.

Pressure can then by whatever means you see fit leak past the piston to fill the chamber.

When you exhaust the pilot gas the only force left on the piston is the spring trying to pull it back. When the spring pulls the piston open there is also the pressure of the chamber gas from the other side assisting opening speed further.


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Unread postAuthor: mark.f » Mon Jun 01, 2009 4:57 pm

John told me to upload this.

EDIT: Oops.

The piston is two part with a check valve. However you decide to join the parts is up to you. Probably would use brazing or a strong epoxy (both can be undone).

The addition of the equalization hole and volume in front of the piston should now allow to the piston to move backwards (since there is air in front of it to push it backwards upon firing).

The 800 PSI ports will need to be really close to the edges of the O-rings.
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Unread postAuthor: john bunsenburner » Mon Jun 01, 2009 5:00 pm

Thanks for the upload mark, and at hot: great minds think(ill spare you the rest)....:D
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Unread postAuthor: Gippeto » Mon Jun 01, 2009 7:04 pm

@ mark;

Now the piston has "frontal area", that's good.

Ummm, how are you filling the chamber? :)

That would work better without the o-rings IMO (well..maybe one floating o-ring :wink: ), and then you wouldn't have to bother with a check valve or multi piece piston either.


John, is this supposed to be a "regular"piston valve then? You may be un-necessarily complicating things if this is the case.


Boring the housing is not a problem with a lathe.

I'll make an assumption that you don't have any milling cutters or reamers. You'll have to use a drill bit.

Use a starter drill to make a "dot" in the end of the stock (this "dot" will keep the more flexible small drill bit from wondering off of center), then a small drill bit (say 3mm) to make a pilot hole. Drill to full depth, backing out to clear the chips frequently. (peck drilling)

Drill the hole to depth working up to a drill bit one size smaller than final dimension. (You can go up by 3mm in size each step, more if you have the power to do so.)

Drill to depth with the correct dimension drill bit. Use copious amounts of cutting fluid, a sharp drill bit, and feed the bit in very slow.

This will (hopefully) produce a half decent finish in the hole.

Measure the diameter of the hole, and turn the piston .002" undersize.
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Unread postAuthor: mark.f » Mon Jun 01, 2009 9:14 pm

That was just a detail of the valve. And really it's not significantly different from a normal piston valve.
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