Building a PCP airgun.

[NeoTech]

Well i have been looking around on the interwebz for a while and have been figuring out that i could build my own pcp airgun.. That is something that really interests me. For one i live in sweden, and all types of rifles are highly regulated here. - But there is a loop hole, Airguns.. So here is the plan. Building a Bullpup PCP airgun, similar to the edgun. Singleshot of course. I will build everything but the barrel.. i have no clue how to rifle a barrel myself sooo, i will buy that.. I found some cheap ones from bulgaria, cost about £20 so its not really a hassle.

For this i need to learn some things. And this is why i do this mainly.
1) Threading.. inner and outer threads in metric..
2) Boring, making a long precision turned hole in a block with a 4 jaw chuck..
3) Milling and flycutting internal cavities on my mini mill.

So i started out this evening.. i got some roundstock home and have scrounged up a aluminium block. I have never ground a hss tooling in my life, so i started out and looking at some tutorials about that, and ground a HSS flycutter for my mini mill.

So bare in mind, this is my first ever attempt on flycutting.. ever...


Below are a solidworks drawing i have been working on. I know there is a book about it. But i really wanna try to do it buy myself based on my own ideas.

[DaveH]

Neo,

Good job with the fly cutter, looks very good
 
DaveH

[Rob.Wilson]

Hi NeoTech

Looks like your passed the grinding HSS cutter test with flying colours 

Interesting project too 

Rob

[shipto]

Nice job on the fly cutting but I am a little confused about your picture.
If I am looking at it correctly you pull the cocking bolt back and it opens up for the pellet at the same time. I am assuming your not going to have a magazine as there does not seem to be room, so each pellet will have to be inserted by hand.
one problem I see is that the spring will make it neccesary to hold back the cocking bolt while you insert the pellet this would require three hands or holding the gun with your knees.
The other problem is that i dont see anything other than the lighter spring holding the pellet probe in place, this would mean that some of the force would be lost and could make the power erratic.
Please feel free to correct me. I look forward to following your build, I have "the modern pneumatic airgun" by H.M. Buckley and did make a gun a while back but lacked the funds to do it properly with the correct materials so it was far from ideal.

[WillieL]

So bare in mind, this is my first ever attempt on flycutting.. ever...

Well that is a whole lot better looking that my first attempt was! 

[NeoTech]

Nice job on the fly cutting but I am a little confused about your picture.
If I am looking at it correctly you pull the cocking bolt back and it opens up for the pellet at the same time. I am assuming your not going to have a magazine as there does not seem to be room, so each pellet will have to be inserted by hand.
one problem I see is that the spring will make it neccesary to hold back the cocking bolt while you insert the pellet this would require three hands or holding the gun with your knees.
The other problem is that i dont see anything other than the lighter spring holding the pellet probe in place, this would mean that some of the force would be lost and could make the power erratic.
Please feel free to correct me. I look forward to following your build, I have "the modern pneumatic airgun" by H.M. Buckley and did make a gun a while back but lacked the funds to do it properly with the correct materials so it was far from ideal.

Correct its a singleshot, handloaded version, due to the fact that i go with the bullpup design, meaning that the block will end up in the shouldrest i think i would be able to cock it and hold it open with one hand.. like onehanded mantle move on a glock..  It depends a bit on the size of your hands of course. =)

The pellet probe issue, well yes.. I thought a spring with enough force would do the trick, but i have got it pointed out several times now that it doesnt so i need to rethink that. The thing is that the dimensions of the block is so small that i cant really take off more material without comprimising the integrity of it. Maybe i could do some "pull out before pullback" and make it somewhat springloaded. I dislike those "push forward push down to lock" solutions, mostly cause they slow me down.

I have used several pcp air rifles over they years, and the best have been the dual pullbacks with seperate for loadpin and seperate for cocking. And of course the force lever ones, like the new FX 400'd.

Material, well so far im in 250gbp or so. i think i dont have to get more other than maybe more alu blocks if i screw the first one up. ;)
For stock i consider trying to make "Micarta" which is sandwiched epoxy drown cloth. So need to hit a thrift store and grab all the jeans i can  find! ;D

[Brass_Machine]

One of the things I want to do is build an air rifle. When I move of course, because of the laws here. So I will be following this build with great interest.

I am curious as to how you are going to design this? What did you do for research? Where are you getting the info? I ask, because I would like to learn as well.

Good luck!

The fly cutting looks good!

Eric

[NeoTech]

One of the things I want to do is build an air rifle. When I move of course, because of the laws here. So I will be following this build with great interest.
I am curious as to how you are going to design this? What did you do for research? Where are you getting the info? I ask, because I would like to learn as well.
Good luck!
The fly cutting looks good!
Eric

Well i'm gonna try to update this thread regular, my blog will be anyways.

Well designing.. I did what every blackbelt in google-fu would of done.. i Googled images of existing rifles, i looked for exploded view schemas on many different manufacturers rifles and i concluded what was the basic parts of a rifle and what was "fluff".. i can see reason for fluff in commercial parts.. But in my case i'm happy if it shoots. ;)

Next was looking up materials that could whitstand pressure. I went with aluminium tubes, steel tubes and different other alloys. I ended up with hydraulic tubing. its a 5mm walled steel tube
that can take around 6000 bar burst pressure, and are rated for around 2000 bar.. I guess it will not blow up in my face.. ;)
I did find different aluminium tubing, but aluminium is rather soft and i wouldnt wanna risk it. For fittings, valve and refill i will go with brass. Its tested througout the industry on higpressure vessels and couplings.. if it works for them it works for me.. 0.8-1mm pitch thread and a bunch of teflon o-rings and some dow-33 grease will probably handle the pressure well.

Another reason for brass fittings is that the air block is aluminium and steel + alu... not a good mix overtime i hear.. =)

The mechanics of a gun then.. Well i have been tinkering with devices that shoot stuff, launch stuff and blow up stuff on a regular basis for a couple of years.. I just went into solidworks and simulate the mechanics over and over until i find something that dont bind, break or blow up on me. The most basic is though, force drives hammer forward opening pin valve, force in airtube closes airvalve, letting small amounts of air out.. air valve is assisted by spring at lower pressures. Escaped air passes through a canal into barrel forcing pellet out of barrel. After that we will see. ;D

[shipto]

ok first things first. I suggest you need to have the pellet probe locked in place during firing in one way or another, I personally like the push and lock down mechanism but whatever suits you. I would get rid of the light spring behind the pellet probe so that once the trigger has got hold of the hammer it will be free and easy to load the pellet.
weight is another issue to think about, H.M. Buckley suggests using aluminium tube for the reciever and having used the very same tube you are thinking of using I suggest the same. unless you are only thinking about using it for plinking in the garden you will rue the extra weight after a while and concidering you will probably only fill to 200 to 250 bar ally will be well up to it.
I would love to talk in real time to you about this topic if you have skype or something.

[airstream]

Personally i would not use alloy tubing full stop for a air cylinder, stick with steel but 5mm wall is a bit over the top, unless the outside diameter is about 50mm. 2.5mm wall is ideal if using around a standard size  30mm od tube, its not to heavy and gives good air capacity. just make sure you hydro statically test the tube to a third above your working pressure to make sure your end plugs are safe.

[cfellows]

Seamless stainless steel, 1.25" x .080" wall is rated at 9,000 PSI burst pressure.  That's probably 3 times what you'll be pumping your reservoir up to. 

I used 1" OD, .065" wall for my air gun.  It was designed for CO2, but I've pumped it up to 2,000 PSI with compressed air.  It rated at  9,750 PSI burst pressure.

Are you going to thread the ends of your reservoir tube or use socket head cap screws in a radial pattern to hold the end plugs in place?

Here's a picture of my reservoir, admittedly small for air, but fine for CO2...



I used 4 socket head cap screws in a radial pattern to hold each of the aluminum end plugs in.  The screw head is countersunk into the aluminum plug so the shear force is across the head.  Simple O-ring on each end plug keeps it sealed.

Chuck

[Jonny]

I cant emphasise how dangerous this can be, your dealing with pressures in excess of oxy acetalene bottles and not tested!
Swat up on what you are doing, if its not you its others around.

Its not so much the rated tube, its the plugs in the ends which are the most vulnerable usually.
80 though wall i wouldnt want to put 200 bar in whether or not its passed a 300 bar test.
Mind one well known manufacturer uses 27.3mm bore with M 1.25 pitch Blows at 245 bar thats about 60 thou wall!

30mm o/d with 2.5 thick wall hydraulic tube is pushing it for 350 bar with pinned ends. Yet going 20 thou thinner and 1/16" larger dia adds to weakness, dont assume for one minute that will hold 4500psi. These are nothing less than a potential bomb and have seen for decades what you boys do over there. Cap head screws should take the load on the head not the thread.
Cant quite see but think thats done right on the 1" o/d tube, heads going through the tube and in to the end plugs, the thread just stops them dropping out when no pressure in. Unlike a certain manufacturer your way and predecessor using mush head screws on top of o/d of tube with 3/16" screws having the shear on end plugs.

To pin the cylinder on 1 1/4" stainless G316S11 X 1/8" thick wall i used M5 cap heads 17 years ago. Took it up to 6300 psi before 1/8"BSP off the shelf thread sheared. Th end plugs moved as well stressing the materials also stainless steel.

I wont remind you of the numerous failings due to incompetant tatters, even if Ginb didnt make that cylinder, he messed with it and should have slung it back if he knew what he was doing! Carreers another totally dangerous as is any CO2 pressure vessel operating at more than rated value ie air at 200bar.

[Jonny]

Neo think your missing the trigger mech.
If going underneath that will be very very deep.
Also not much point in the middle line of hole, the bottom can go there and trigger where the cylinder line is.

Or like number 21 i started and put on hold March 2003, missed the deadline and just sat round ever since.

Barrel and bolt top line. Knock open valve in middle like the predecessor to the Rapid and tube with trigger on lower line.

The ends will bellmouth after the thread on this tube at 4800 psi, done in 98.

All the parts are there in rough state inc the reg, it only needs the O rings and theoretically should work if it aint rotted away.

[Jonny]

Number 2  Sept 93 old school style, 30 o/d x 3mm thick wall threaded 1 3/4" in to the tube with 26 TPI brass thread.
Note the long breach block for long scopes and ultra short hammer to loading from memory 2", most are over 5". Short hammer stroke 2.4mm not even to this day has anyone come close except the electric solenoids.

In use up to September 2005 three times per week rain sleet or shine, then finally decided to pack up 4 days before the worlds after 12 years.

Number 7 1996 rotting away using Tungum alloy tube 30 o/d x 2mm thick wall. Same burst pressure as 30x2.5 hydraulic steel. It works but never finished because it looks like brass and it isnt. Could have plated.


No miller at this point just a Myford ML7 toy lathe.

[NeoTech]

I have been reading up on creating pressure vessels..  conclusion is that you shouldnt try to create anything beyond 200bar yourself. as stated by Johnny, the caps will become small projectiles if failing..   And i have gone way overboard on thickness of tubes. =)

About the trigger mechanis, yes they will be sitting underneth, but they will be kinda flat with a rotating sear, similar to a crossbows mechanics, with a small lever that keeps it from rotating. I have some drawings on a napkin i will post images when its cadded and simulated.  cause i'm building a bullpup design i'm not really that worried about the stacking height of the components..

And on other note, my project is a bit stalled.. cause i suck at machining and put a 12mm hole througy my block took three hours (100mm long, 12mm in dimater, on one of the edges of the block)..  and the whole setup almost killed me. ;)

[David Jupp]

One thing to be very much aware of if using Aluminium and its alloys - no matter how low the stresses are, fatigue will be an issue so the item will have a finite life (determining what that might be may not be simple).  Unlike steel which shows a 'knee' in the S-N curve, Aluminium does not have a limiting stress below which fatigue will not occur.

[Jonny]

Quite right David and why Airstream above wont use aluminium for pressure vessels.
I have returned and now on my third complaint to a well known German manufacturer using aluminium for the cylinders. Thats 6 i have come across, two brand new and one recall with visual cracks. Think they are listening this time, but has took then 5 years to do so with their typical we know better attitude.

Its not entirely the strength of the tube, its the end plugs and how they are fitted that seems to go amiss.
There are plenty of very iffy production rifles around that have 232 bar slapped in them, i wouldnt put 70bar in.

[David Jupp]

Aluminium Dive Cylinders have additional checks on the threads where the valve screws in at each routine inspection.  For steel cylinders a visual on the threads and check with go/no-go gauge is adequate, for Aluminium this is supplemented by Magnaflux examination to pick up cracking which is a known issue.

[tjensen]

Nice work.
2 questions:
1. Book about it?
2. Bulgarian barrels?
Thanks

[NeoTech]

There was a book in circulation that is hard to get hold off these days. The author published it by himself, its basicly a guide to build PCP airrifles.

And yeah, now adays bulgaria is not the prime source slovenia is.. check royal.si for cheap rifles that can be stripped for necessary hard to find items you can't make your self.
Pay attention to your local laws beforehand tho. Might be illegal where you live.

[tjensen]

Thanks