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Author Topic: PCP airgun design  (Read 494 times)

Offline Ian.b

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PCP airgun design
« on: December 27, 2018, 01:43:49 AM »
Hi, I am new here, whilst research home/custom built airguns I came across the nice builds by Wingman on here, so signed up.


I am thinking of constructing an airgun, the choice of airsupply will be either a 13 cu inch paintball cylinder/buddy bottle or to construct an air tube like in Wingman's builds.

My first question is if using a buddy bottle, I figure I will then still need a plenum before the valve to supply sufficient air quick enough due to the small oneway valve in the bottle, it is suggested that airguns need 1cc per ft/lb, so for example if I wanted a 50 ft/lb power airgun I would need a 50cc plenum, is this correct?

Many thanks.
« Last Edit: December 27, 2018, 03:16:25 AM by Ian.b »

Offline Novagun

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Re: PCP airgun design
« Reply #1 on: December 27, 2018, 07:29:58 PM »
PCPs are not my choice of airgun so I can't help but Wingman or Dvinme will know. Hope they read the forum soon so you get an answer.

Offline Pauly5

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Re: PCP airgun design
« Reply #2 on: December 27, 2018, 11:45:00 PM »
 I'm pretty sure most un regulated pcps empty straight from valve to barrel. But if you have a regulator fitted, either factory or after market, there will be a plenum that the regulator supplies air to, then the valve comes after that. I've never built one, but have fitted regulators.
If you want to know about regulators, watch Robert Lane regulators on you tube. Very interesting and well explained.
Keep us informed on what you do, we are always keen to see a build project.

 
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Offline Ian.b

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Re: PCP airgun design
« Reply #3 on: December 28, 2018, 05:23:43 AM »
Sorry if I didn't explain very clearly,  what I am wondering is if a high power airgun needs a store of air that the valve has high flow access to rather than the air for each shot having to go through the possible 'bottleneck' of the buddy bottle oneway valve.  This may not be a problem as I have never seen the size of a buddy bottle valve.

Thanks

Offline Pauly5

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Re: PCP airgun design
« Reply #4 on: December 28, 2018, 09:01:13 AM »
Hi Ian.b,
I think I get what you mean. Do you mean the shut off bottle valve that lets you remove the tank while there is still air in it? I think that by default there is a void between that valve and the firing valve, but there would be no need for a plenum, or "air store" section, as the gun would be tuned for the usable tank pressure. Only regulated guns would need that separate plenum for shot to shot storage of regulated pressure.

I had a BSA lonestar that was high power, and the only thing between the pressure of the tank and the pellet was the valve and transfer port.  So the only thing that stops the full force of the tank from dumping behind the pellet is the firing valve.


Does that help, or have I got it wrong again?  ;D ::) Of course i'm not an expert on this and a few others here will know heaps more, but this is what I know from my experience with playing around with my guns.
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Offline Ian.b

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Re: PCP airgun design
« Reply #5 on: December 28, 2018, 10:56:16 AM »
Yes, thats what I am asking as the photos of buddy bottles I have seen seem to have fairly small one way valves. 

I had just read on another forum you ideally need around 1cc per 1 Fpe, this made me wonder if it was necessary to have a volume of air sufficient for the shot in the gun right by the valve or atleast with an unrestricted path to the valve.


This project is just an idea currently, air tube or buddy bottle will depend on w hether I can get suitable tubing, I have a lead on some 32mm 6082 T6 seamless tube, it isnt ideal as it has rather thick wall of 5mm, but is the grade ok for airtubes, Ihave seen aftermarket tubes for an HW100 on ebay made of this.

Would boring the tube out effect its properties and reduce its strength, I realise 3mm wall will be weaker due to the increased diameter and thinner wall, but will machining futher negatively effect its properties because of how the tube is formed?
« Last Edit: December 28, 2018, 11:19:55 AM by Ian.b »

Offline Novagun

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Re: PCP airgun design
« Reply #6 on: December 28, 2018, 06:26:24 PM »


If you haven't already done so then the "Pistol Build Challenge'  might be useful. It is in this category. There are other pieces of information that I recall that have been discussed and I think that they are also in the modifications category. I can't recall any titles but I know the information is there somewhere. Just a matter of reading anything that looks promising.
It was Wingman I think, in fact I am sure, so look for his contributions.

Offline Pauly5

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Re: PCP airgun design
« Reply #7 on: December 28, 2018, 10:11:30 PM »
Unfortunately, this is now getting out of my league. As much as I would love to have the tools to do this I don't, and so lack  a lot of metallurgy knowledge.
Here's a link to another thread here that may be of use, otherwise I also recommend looking through wingman posts to start.

http://nzairgun.com/forum/index.php?topic=1252.0

http://nzairgun.com/forum/index.php?topic=2418.msg20033#msg20033

http://nzairgun.com/forum/index.php?topic=2403.0

Happy reading.
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Offline Mintie

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Re: PCP airgun design
« Reply #8 on: December 28, 2018, 10:47:46 PM »
Yes you will need a plenum between the regulator and the firing valve, the general rule is 1cc per fpe you are chasing but that also depends on caliber etc too. Generally the plenum is a solid piece that the reg screws into and it then screws into the other half of the valve, the valve spring is usually seated against the end of it.

In an unregulated rifle the whole tube in front of the valve is effectively the plenum, some unregulated rifles use a pot infront of the valve that sort of regulates (just by having a very small hole in it) the amount of air that the valve has available to use which can help to flatten out the shot curve.

On specific info and your engineering questions you will need to speak to Wingman.
Lots of airguns

Offline Ian.b

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Re: PCP airgun design
« Reply #9 on: December 29, 2018, 03:46:47 AM »
Thanks for the replies, so 1cc per FPE only applies to the plenum size of regulated guns, I can see that if you use an aiir tube with the firing valve at one end the valve has full access to it, but if you use a removable screw on buddy bottle how much volume does the gun need next to the valve or do the buddy bottles have large enougb openings for this not to be a  issue?


I had seen posts elsewhere about building an airgun from the book by H M Buckley, but that involved cutting quite a few threads in your lathe and I have never done this, when I saw Wingman's builds using screws to pin the valve and end caps etc. I liked the idea  and also using the trigger from a Marauder helps to avoid the complications of a reliable trigger.

Just reading through the link on the pistol builds.
« Last Edit: December 29, 2018, 03:55:28 AM by Ian.b »

Offline Mintie

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Re: PCP airgun design
« Reply #10 on: December 29, 2018, 08:06:44 AM »
Ahhh right sorry I didnt click to what you were getting at, you are asking about using a bottle without having a reg on it? That would mean it cant be removed from the rifle while being pressurized and it couldnt be swapped to another bottle under pressure in the field. The inside neck thread on most bottles is M18 so by the time you have a male thread inside that your "bottle neck" is probably going to be 8mm or less which might end up being restrictive to the flow through the valve, it would probably perform better with a plenum space to feed from.
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Offline Pauly5

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Re: PCP airgun design
« Reply #11 on: December 29, 2018, 10:01:30 AM »
There you go! I have learnt something today.
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Offline Ian.b

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Re: PCP airgun design
« Reply #12 on: December 29, 2018, 12:31:04 PM »
Thanks Mintie, that helps, the one way valves in the bottle.neck look rather small in the pictures I have seen and then I guess it also needs a pin to open the valve when it is fitted to the gun, so if I will need some air storage on the gun it may be best to use a tube rather than a bottle?

« Last Edit: December 29, 2018, 12:35:39 PM by Ian.b »

Offline Mintie

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Re: PCP airgun design
« Reply #13 on: December 29, 2018, 03:11:28 PM »
I think the tube would be a much better way to go, make sure your ID matches an easily available reg from Huma in case you choose to regulate it in the future.
Lots of airguns

Offline Pauly5

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Re: PCP airgun design
« Reply #14 on: December 29, 2018, 08:01:33 PM »
Mintie,  would it need it if it was low power?
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