XS60C PCP Repeater project

[rsterne]

Out of curiosity I tried a stock hammer spring today.... The recess was not deep enough in the back of the hammer to get enough preload without a spacer on the spring (the spring was too short), but with a 1/4" spacer there was plenty of adjustment to get the gun over 100 FPE at 2200 psi.... I backed off the preload until the velocity was the same as what I had yesterday, and you could hear the brraaapppp of hammer bounce, and the shot count dropped to 12 shots from 15 with the Daystate pellets.... This was not unexpected, as the preload was right around zero, which seems to be the worst place to emphacize that.... So, the gun will work with a stock spring, but is much more efficient with a stiffer spring rattling around loose when uncocked.... When I get some longer, weaker springs in a couple of weeks, I will try those to see if there is any more efficiency hiding it there, as they make the gun easier to cock for the same hammer energy than a short stiff spring....

This fine tuning of a gun that is already working great is always fun, you learn sooooooooooooo much.... and it can help you cure a gun that isn't working the way you want at a later date....

Bob

[stalwart]

I realize that, for you, finding a sweet spot like this isn't voodoo, but it still has to tickle you almost as much as it does me.

A .30 that can be tuned for .22 short power and all the way to .22 LR is a thing of beauty to me. .22 mag is not far off....

[rsterne]

Yes, and when I started a few short years ago, it was a rare airgun that could achieve .22 CB Cap power.... I first did that with a modded Disco in .22 cal and was pretty proud of myself....

Bob

[stalwart]

More tools trickling in... got a reamer to finish fit the side screws in the 3K Bobcat.  The screw shown is just pushed in backwards to show fit. Next is bonding, so got a few pics before I hide the work forever... .

[stalwart]

Is there any real benefit to sleeving a liner with 2024 aluminum as opposed to 6061?

[rsterne]

The modulus of elasticity of 6061 is 10,000 ksi..... For 2024 it is 10,500 ksi.... so 5% stiffer in the same cross section / diameter.... The tensile strengths, however, are vastly different.... Carbon fibre Unidirectional laminate runs about 26,000 ksi, and steel about 29,000 ksi.... Typical carbon tubes are anywhere from 14,000-20,000 depending on the properties (thread direction and density) of the laminate.... Higher modulus carbon fibre is available, at about twice the price.... the pure fibres run 33,000 ksi (standard) to 55,000 ksi (high modulus)....

http://www.gatewaytoairguns.org/GTA/index.php?topic=83050.0

Bob

[RMM]

More tools trickling in... got a reamer to finish fit the side screws in the 3K Bobcat.  The screw shown is just pushed in backwards to show fit. Next is bonding, so got a few pics before I hide the work forever... .

Lookin good Eric, lookin good. 

[stalwart]

Glad you like it, Mike.

It should be a fun piece... I expect this won't (along with one I will be keeping) be the last I end up building.

[RMM]

Glad you like it, Mike.

It should be a fun piece... I expect this won't (along with one I will be keeping) be the last I end up building.

This is just the beginning.  I hope others see the potential that you and Bob pulled out of this $100 gun and belly up to the bar to purchase one, or many, of the various versions offered.  I know I'm having a ball with my Bobcat, experimenting and excersing new skills perpetuated by the gusto and willingness to share you and Bob project.  I'm lovin it. 

P.S.  You can keep the other one till it's finished, then please return it to ME.  LOL!!!!

[RMM]

Even though I see none of the experts fiddling with a reverse valve spring valve I whittle on.  I am aware of the threaded spring guide and jam nut coming loose plus the forces exterted on the poppet (the poppet and valve stem are threaded) but there's just something intriguing about the design I wish to, well say, whittle on.  With that said I ask for help.

The specific problem I'm running into is, things are not concentric and the Delrin spring guide nut and jam nut touch the non concentric I.D. of the OEM backer block.  With that said I made a new backer block out of 6061 and the same interference is occuring.  So heare comes my deduction.  I need a sring of smaller O.D. with the same spring rate as the 60 spring.  The 60 spring is .035 WD,  325 O.D., .826 L.  With the pics I provide and my request for a smaller O.D. spring (a new threaded guide nut and jam nut) what do ya'll think?  P.S. I'm not familiar with springs and or selecting them.

With the pics shown below and what I've written above with please disect and throw some engineering speak in layman terms at me.

Please note the jam nut was introduced after this pic.  The valve is an rsterne Maxi Valve.

[QVTom]

Quite common in custom builds.  I think the reason it is not seen as much when modding existing platforms is due to the possible infringement on hammer stroke which seems to be at a premium in most designs.

Tom

[rsterne]

Here is a Spring Calculator....  http://www.msdspring.com/SpringCalculator%5CDefault.asp

Put in the specs on your spring and get the spring rate for it.... Then put in the smaller OD you want, and reduce with the wire size until you end up with the same spring rate.... Once you find a range of specifications that appear to meet your requirements, then put those into the Century Spring search tool at....

http://www.centuryspring.com/Products/compression.php

See what common spring size from their catalogue might work for you, write down the dimensions, and take your calipers to the nearest hardward store and go looking....  Alternately, if you have a spring about the right stiffness (spring rate), put it, and your existing XS spring on a rod (to keep them lined up) and push them towards each other.... If they collapse at the same rate, they are the same stiffness.... Overall, a 20% change in rate either way probably won't matter....

Bob

[RMM]

Quite common in custom builds.  I think the reason it is not seen as much when modding existing platforms is due to the possible infringement on hammer stroke which seems to be at a premium in most designs.

Tom

Ahhhhh!  Now I see.  If I wish to increase hammer stroke the spring etal will be in the way.  Thanks!!

[RMM]

Thanks Bob.  I'm going to the site immediately after this post.  With what Tom pointed out is there enough stroke with my setup to throw some 25gr pellets?? 

P.S. I've been going back and rereading this thread (I'm up to pg 27) and setting me up a Words of Wisdom condensed tutorial, PCPs in general plus reference to the 60 mods.  After a while I was finding things running together in my forever busy head and I thought it was time to get organized.  I hope this works.  LOL!!

[rsterne]

Depends on the caliber.... Are you going to lose stroke, or can you still drive the end of the stem to flush on the backing block (and is that point in the same place)?.... Does the end of the new stem sit further back from the backing block when the valve is closed?.... The former governs how far the valve opens, and the latter governs the hammer stroke before contact, which governs the hammer energy and momentum.... Have you done anything at the other end to increase the hammer stroke, like I did in the Grizzly?.... Lots of ways to ruin a good plan, like anything else every mods means (at least) two more, most of the time....

Bob

[RMM]

Bob the air rifle is a Bobcat with a fixed .22 60 barrel (it's a good barrel).  What I did today was get prepared for a possible possible increase in stroke.  First it has to breath, I think. 

I cut enough of the threaded end off the aluminum cylinder to leave enough threads for the valve body to tighten down the o-ring + 2 threads.  Here's some dimensions,  intake port = .203", exhaust port = .190", barrel port = .168", Delrin poppet = .307,  valve throat = .438" and the stem protrusion above the baker block is .377 (backer block .630" length) so I have stem to cut to length.  I had planned work on more stroke since this 60 is my platform to play with the knowledge shared by the GTA mentors, plus I'm learning about machining to boot.  This stuff is addictive, thanks.



This distorted out of focus pic shows how I can't take pictures!!  LOL!!  What it's supposed to represent is how I beveled and polished the concave end of the slot all the way to the valve throat.  I did lightly bevel the throat mouth but it could use a little more.



This pic is the valve tester I use.  Screw the nut off the bottom of the tube, slip the bottom in first, stem sticking out, put the nut on and pump it up.  I might add after lightly lapping the poppet on a lapping block I made this valve holds air. 

[rsterne]

Your stem in the top photo sticks out further than normal, which reduces the hammer travel before impact.... It will allow more maximum lift before the hammer hits the backing block, but you are unlikely to need more than 1/8" anyway.... I only allowed 0.200" on the Grizzly, and it uses about 0.070-0.080" when properly tuned.... I would cut off half the protrusion of the valve stem, even then you are losing hammer stroke....

I don't see how you could drill the throat of the valve out to 0.438", or why you would, that just increases the sealing area and the force to drive it off the seat.... I note that you say the poppet is 0.307",  so that makes no sense anyway....

Bob

[RMM]

Yes the stem is way too long for a reason.  The first stem I made was way to short!  LOL!! 
My plans were to cut it down to between .110" and .125".  But a question arises for me with lift.  Will the lift change with various pressures? 

Terminology is the key, and I don't have the key, yet.  The area the poppet rides on I called intake port and that area hasn't been changed so the hole is .203" and upon the seat rides the .307" poppet. 

The area I called throat the threads reside on the otherside thus the .438", so please correct my terminology.  I felt I shouldn't increase that area because of the tension of the spring.  The only way I figured to increase that area without machining was to decrease the size of the poppet.  I could probable machine a wee bit more off the poppet but I thought a larger ledge for the valve spring to ride on would be prudent. 
 

[rsterne]

What you call the intake port is the throat.... The area surrounding the head of the poppet (0.438") is the ID of the valve body.... That area, minus the diameter (area) of the poppet head can restrict the flow but usually doesn't, it is more a matter of the two 90* bends it causes at the seat.... I enlarged that to 0.50" on the Grizzly, as well as using an MRod poppet which is 0.33" diameter to get as much flow as possible to the seat of the valve, but the main restriction is the throat and ports areas.... I like to see lots of area on the inlet side of the seat (into the valve, and around the poppet head) so that any calculations made on port areas on the exhaust side are accurate, and not influenced by the intake side....

I am confused by your photos and description.... Have you moved the spring to sit on the stem, or is it still inside the valve?.... If it is on the stem, you can eliminate all of the aluminum inlet side expect a couple of threads to locate the O-ring, and that doesn't need to be compressed, it can just sit in a groove.... Here is a QB79 valve I did like that....

 

The spring and mount shown in the 2nd photo mount on the tank block in the QB79, they would be missing if they were on the other end on the stem of the poppet, of course.... The threaded aluminum ring is glued in place on the brass valve body, with just clearance for the O-ring.... The pressure forces it to seal just fine....

The lift does indeed increase as the pressure decreases in an unregulated PCP.... However, when the hammer strike is properly adjusted for the pressure you are running at, the lift will almost certainly be less than 0.100".... In the Grizzly, at 100 FPE with a 0.266" throat, it is only 0.070" with pellets and 0.080" with bullets....

Bob

[RMM]

If you'll look back in this thread I've made a reverse valve spring version of your maxi valve but found out it's not the best valve to use on a 60.  I was having problems with the spring/ spring guide and jam nut touching the I.D. of the backer block tilting the valve face from the seat and causing an air leak.  If I increased the backer block I.D. large enough to eliminate the interfernce the nose of the 60 striker would beat itself into the new I.D. changing lift and eventually jamming the nose into the blocker hole.  This reverse valve spring design will work great with a flat face striker. 

The light bulb of valve design change also came on when QVTom mentioned using the reverse spring valve and working with stroke.  If I wish to effectively increase stroke, as you and stalwart have, in my Bobcat I had to use a valve with the spring on the intake side of the valve.  Now if I need to increase stroke I'm ready, but I'll wait and see if the stock stroke works with this new valve.  As mentioned I would like to drive .22 up to 25gr from this Bobcat eventually settling for a specific pellet and weight. 

Thanks for the terminology correctiion(s) plus your tutorials I'm learning a lot. I didn't dare open up the I.D. of the valve body as I thought I might get too close to the threads.  The O.D. of the poppet is the same size as the O.D. of the valve spring as I didn't want to loose too much poppet ledge (support for the spring) to support the spring under compression.   

Thanks for the lift analogy, but I have another question.  You have a conical spring in your Grizzly valve, another stroke of genius, does the spring mimic the 60 spring rate which comes to 20.43 lbs per http://www.msdspring.com/SpringCalculator%5CDefault.asp .   I would assume the valve spring rate is
a big factor in determining lift and dwell. I'm starting to put all the pieces together and feel I'm getting close to using math, maybe. 

P.S. In addition the second pic is looking at the bottom of the first pic.