Any Interest in 6mm ? - Build Thread

[lloyd-ss]

........................ I don't intentionally use the setup for destructive testing.

  Ha ha. I like that! 

Finn, thank you for that info.  Do you test actual parts, but keep the stress below the yield point, or do you make an extra part and stress it higher? Or does it just depend on the situation?
Your testing sounds good, like scuba tank testing, which I think is usually done to 5/3 of the rated working pressure. Do you use air, oil, or water to pressurize the part? Just curious because it sounds like something I might want to set up for non-destructive testing.
Thanks, Lloyd.

[oldpro]

I have run into an interesting little hiccup with the regulated gun.... The new valve I made is as identical as I could to the old one.... It functions fine, and reaches the same plateau velocity at the same pressure, minus a few fps for the 165 cc plenum instead of the 300 cc reservoir.... However, it takes MUCH less hammer strike to reach the plateau, and it appears to have very little velocity adjustment range before hitting the cliff.... I am not 100% sure about the latter, because I cannot reduce the hammer strike to the point where the velocity drops much, and then one more turn (but right at the edge of my adjustment) it quits opening....

The only thing I noticed on disassembly is that the shiny mark on the poppet that should indicate where it is sealing is near the throat (5/16") instead of near the outer edge of the poppet (3/8"), where it was on the original valve.... Both surfaces were turned at 90 deg. to the axis of the valve, and have been lapped in.... The big end of the poppet is 3/8", the small end is 1/4".... with a 1/32" bleed hole to the exhaust port and a 1/16" "jet" in the front.... I know that the area of the seal on the poppet can affect how easy it opens, do you think that is what I am seeing.... If so, would cutting a thou or two of concavity in the poppet to force the seal to the outside edge prove that and cure it?....

Lloyd, what do you think?.... Travis?....

Bob

This is why I use a 75% of bore as my bare minimum your running 66% Our commercial units are at 80%to avoid low power tune issues but still easy cocking.  280 small end 350 big end=80% thats our commercial size

[rsterne]

Thanks, guys.... I will try making a very slight concave on the bottom of the poppet to force the sealing diameter to the outside edge.... That should make it act like the previous one....

I am still trying to wrap my head around why these valve require so little hammer strike.... There must be something going on after the poppet cracks off the seat, driving the valve open.... Since I am running effectively no jet (a 1/16" hole in the front) the only thing I can see causing the valve to "blow open" is the delay that the 1/32" vent through the poppet, from the exhaust port to the inner chamber, causes in the pressure in that chamber rising to HPA pressure.... Once that chamber reaches HPA pressure, the only closing force left is the stem diameter times the pressure, which we expect.... but if the chamber pressure lags the exhaust port pressure, during that lag time there is an opening force on the poppet, right?....

Bob

[oldpro]

Thanks, guys.... I will try making a very slight concave on the bottom of the poppet to force the sealing diameter to the outside edge.... That should make it act like the previous one....

I am still trying to wrap my head around why these valve require so little hammer strike.... There must be something going on after the poppet cracks off the seat, driving the valve open.... Since I am running effectively no jet (a 1/16" hole in the front) the only thing I can see causing the valve to "blow open" is the delay that the 1/32" vent through the poppet, from the exhaust port to the inner chamber, causes in the pressure in that chamber rising to HPA pressure.... Once that chamber reaches HPA pressure, the only closing force left is the stem diameter times the pressure, which we expect.... but if the chamber pressure lags the exhaust port pressure, during that lag time there is an opening force on the poppet, right?....

Bob

Bob I cant remember if it was you or Lloyd I was sharing data about the lifting force pushing up on the oring gland on the big piston.I have tons of data to support it.

[lloyd-ss]

I can understand the 1/32"  bleed hole possibly causing a delay on the refill of the piston chamber, especially if the hole is partially clogged or undersize.  In the original prototype, I had about a .048 hole, which was too large, and the valve tried to close immediately after opening. It was very easy to open, but the refill rate was too fast and the valve did not want to stay open. I had to keep increasing the hammer strike to get any dwell from the valve. I finally threaded a thin wire thru the bleed hole and bent it a both ends to retain it, and that cut the refill rate back down and then the valve started behaving.

Travis, I remember you explaining about the lifting force, but honestly, I was never able to grasp what was happening. I am sure something is going on, I am just not picturing it yet. But I will try again.
Lloyd

[oldpro]

I can understand the 1/32"  bleed hole possibly causing a delay on the refill of the piston chamber, especially if the hole is partially clogged or undersize.  In the original prototype, I had about a .048 hole, which was too large, and the valve tried to close immediately after opening. It was very easy to open, but the refill rate was too fast and the valve did not want to stay open. I had to keep increasing the hammer strike to get any dwell from the valve. I finally threaded a thin wire thru the bleed hole and bent it a both ends to retain it, and that cut the refill rate back down and then the valve started behaving.

Travis, I remember you explaining about the lifting force, but honestly, I was never able to grasp what was happening. I am sure something is going on, I am just not picturing it yet. But I will try again.
Lloyd

If we take the data of the area of the small end piston then we can figure out the closing force applied to the large piston to the seat and there fore figure how much spring and hammer energy needed to unseat the poppet correct. Except it doesnt calculate and the energy needed is half that in my case that is with a 280/350 poppets. So remember the poppet I made with no small end and it had and oring rod going threw the fixture and out threw another oring at the fill end as it would be on a mrod to atmosphere. I couldnt hold the valve closed with my body weight when filling the tube the oring under the large poppet was lifting the piston. Ill find the pics I sent I have them on my shop lap top.

[lloyd-ss]

Travis, I think i remember about those pictures now; you texted them to me and I wasn't able to open them.  A question about that test set-up: was the poppet sealing at its extreme O.D., or was it sealing at the throat I.D., similar to what Bob is seeing?
Lloyd

[oldpro]

280 throat and 350 poppet setting on a flat base no raised lip

[rsterne]

I really have to restrict myself to one change at a time.... 

I took the valve apart today and drilled the vent in the poppet out to 0.040" (from 0.032") and undercut the seat on the poppet to move the sealing edge back out to the outside edge, like on the other valve.... Well, the gun is back to requiring the same SSG gap with the lightest bullets as before, instead of less hammer strike when the seal was further in, on the edge of the throar.... and I attribute that to moving the seat out to where it should be.... However, as I increase the bullet weight I seem to have lost velocity, and more hammer strike is required to hit the plateau.... I didn't do a lot of testing, because the valve is leaking slowly out through the barrel, which I think is the seat leaking again.... I'm not 100% sure, but I think the larger vent hole through the poppet, which in theory should not blow open the valve (as quickly) because the pressure inside the balance chamber should be rising quicker.... is doing that with the heavier bullets, but little change with the lightest one....

The more I work on this valve, the more confused I become.... 

Bob

[oldpro]

 Bob I went down that road and spent the better part of a month trying to find out how why what and where. It boils down to changing jets for different tunes and getting the correct ratio of the twin poppets. The Ratio is so important it really makes the whole tuning thing easier and has a wider spectrum. Ill suggest you up the % to at least 70% but I personally would go higher. Yes it will require more spring energy but its still a huge reduction from what a standard valve will do. JMHO

[MJP]

Bob, while it didn't seem like much you just increased the surface are of the hole some 50% from the original, for me that is a big change.

Marko

[rsterne]

Yes, Marko, that was what I intended, to bring the area of the poppet seat back up to what is was on the first valve I built, which functioned well.... The second one, when sealing at the OD of the 5/16" throat, was so easy to open I could not get a light enough hammer strike with the original spring that worked with the first valve in the other gun.... The change to seating around the outside edge of the 3/8" poppet brought it back into a range that I can adjust.... That part of the change worked.... but of the two vent holes (0.032"" and 0.040") the smaller one seemed to have less hammer strike change when you changed bullet weight.... 

Travis, I know you have changed to a much larger small piston, and then are using the jets for "tuning".... but I need to wrap my brain around WHY the various changes work, and not just accept that they do.... I know this will be a painful process, but I just can't do something without knowing why I'm doing it.... especially when some of the changes are counter-intuitive to me.... I know you are trying to help, and believe me, I greatly appreciate it.... but I think this is a process I may just have to go through, as painful as it is....

Bob

[MJP]

I was referring to change from .03 to .04 on the poppet port

Marko

[lloyd-ss]

................................ but I need to wrap my brain around WHY the various changes work, and not just accept that they do.... I know this will be a painful process, but I just can't do something without knowing why I'm doing it.... especially when some of the changes are counter-intuitive to me................................................. but I think this is a process I may just have to go through, as painful as it is....

Bob

+1
Bob,
I TOTALLY understand.   Very well said!
Lloyd

[rsterne]

Ahhh, OK, Marko.... now I understand.... Yes, I agree, that is a large change in the vent, from 0.032 to 0.040".... about 56% more area.... but it was the smallest increment I had in drills larger than 1/32".... I can now go up in smaller increments, using numbered drills getting larger from #60.... there are 3 sizes between that and 3/64" (0.047").... The 3/64" is 2.25 times more area than the 1/32" (and 38% more than the 0.040"), and I probably can't go much larger than that without hitting the bottom of the large O-ring groove in the poppet.... I think that large changes (eg. 56% and then 125%, from 0.032" to 0.040" to 0.047") may tell us what we need to know, at least I hope so....

Lloyd and I both feel that if the vent was large enough, the pressure rise in the balance chamber would rise to nearly HPA pressure very quickly, maybe before the pellet even moves, leaving the major closing force at that point in the cycle just the stem area times the pressure (like in a conventional valve).... The smaller the vent hole through the poppet, the slower the pressure rise in the balance chamber, and the greater the tendency for the poppet to "blow open".... As you know, if the balance chamber stayed at atmospheric pressure, the valve (once cracked) would blow open, and stay open, until the reservoir was empty....

As it stands, the reduction in hammer strike for a given velocity/energy in the shot is actually much greater than required.... It is difficult to reduce the hammer strike enough, and when the valve blows open (like the Cothran), it begins to operate like a switch, either on or off (open or closed), which is what causes the "cliff" instead of a plateau/knee/downslope like a conventional valve, which makes it easy to tune to a bell curve, or for efficient regulated operation.... Travis has increased the diameter of the small piston to make the valve more tunable, and then fine tunes that with his jet.... While this process obviously works, it seems to me to be using a bandage to hold a splint to a broken leg, instead of not breaking the leg in the first place.... and believe me, I am NOT trying to be insulting to his solution.... I just think that with a better understanding of WHY these valves operate in the fashion they do, we can simplify them, broaden their range, and ease their tuning.... as least that is my hope....

My Dad always used to say "the man who knows how will always be found working for the man who knows why".... words of wisdom I have never forgotten, right up there with his other favourites...."the difficult we do immediately, the impossible takes slightly longer".... or the dreaded "you mean it CAN'T be done?" (immediately followed by him doing it, of course).... 

Bob

[rsterne]

OK, today I actually made some progress instead of going backwards.... I think.... 

I pulled the valve out of the regulated gun yet again, and drilled the vent hole in the poppet out to 3/64" (0.047").... This is now 50% larger than I started with, which means the area is 2.25 times as large.... The idea is to get the balance chamber to fill from the exhaust port the instant the poppet cracks off the seat, hopefully before the pellet even starts to move down the barrel.... This should prevent (or at least reduce) the valve's tendency to "blow open" the instant it is cracked, which I am pretty sure is the reason for the "cliff" on this and the Cothran valve.... While the Cothran has virtually no velocity adjustment between the plateau and falling over the cliff, the ART/SS valve has a pretty fair velocity adjustment range where you can reduce the velocity, and improve the efficiency, before the valve stops cycling properly, and the velocity plummets over the cliff....

The first tests with this valve (compared to the first valve I made which is in the unregulated gun) had a narrow adjustment range, and required even less hammer strike than the first valve, in fact so light I could barely back the hammer strike down to find the cliff.... I took it apart and drilled the vent hole out to 0.040" and made a slight concave in the poppet seat to move the seal back out to the outside edge, and got back to where the first valve was, with a bit more hammer strike required (the larger vent), and a similar adjustment range between the plateau and cliff.... but the valve was leaking and had to come apart again.... I drilled the vent out even bigger, to 0.047", and it looks like I fixed the leak.... The result is that it took more hammer strike to get the same velocity, although still wayyyyyyyyyyy less than a conventional valve with a 5/16" throat at 2800 psi would require.... What I also got, just as I had hoped for, was a MUCH larger adjustment range between the velocity and where the valve stopped functioning.... In fact the cliff has basically disappeared....  .... I can adjust the velocity down to under 500 fps by just increasing the SSG gap, something I have NEVER seen before with my version of the ART/SS valve....

All is not perfect, however, because as you increase the hammer strike, and the velocity approaches the plateau, I go through a period of severe hammer bounce.... not a machine-gun but a real BUURRPPP.... and then as the SSG gap gets very small the hammer bounce disappears, probably because the valve is open until the bullet leaves the muzzle (gun is really loud and kicks) which is dropping the pressure in the exhaust port and not slamming the valve closed as hard.... I had been running no preload on the spring on the SSG, which isn't great for resisting hammer bounce, so I added 2 lbs. of preload, and then 4 lbs., and the period of hammer bounce got less each time, but of course I had to increase the SSG gap to get off the plateau and back to the adjustable portion of the curve.... Instead of running about 9 turns of gap (with no preload), I am running about 12 turns (with 4 lbs. preload).... However, I am delighted with the wide velocity adjustment range, so I think I'm on the right track, and the gun is still VERY easy to cock....

One of the cures for hammer bounce is a light hammer, of course, and it works well on a gun where you have lots of hammer strike available (without excess cocking force).... In a gun with an SSG, it generally reduces the gap because of the drastic reduction in hammer momentum, which allows more lift and less dwell.... The plan is to make a MUCH lighter hammer, I think I will try one with no metal core at all, just straight MDS.... May as well start there and back up if I have to.... 

Bob

[oldpro]

 What size pilot hole is in the thimble?

[rsterne]

Still a 1/16" hole in the front of the thimble.... You said going larger makes no difference, right?.... I am trying to achieve "instant" venting at both ends.... The lack of any cliff is VERY encouraging, I have never seen a balanced valve without one before.... My aim is to have a valve that tunes like a conventional one, but with less hammer strike.... but I have no need to make it feather light, as long as it is reasonable....

Bob

[oldpro]

Still a 1/16" hole in the front of the thimble.... You said going larger makes no difference, right?.... I am trying to achieve "instant" venting at both ends.... The lack of any cliff is VERY encouraging, I have never seen a balanced valve without one before.... My aim is to have a valve that tunes like a conventional one, but with less hammer strike.... but I have no need to make it feather light, as long as it is reasonable....

Bob

Do you have any smaller drill sizes? Or can you thread it and make your own jet? 4mm etc. thread

[rsterne]

The "jet" is just an 8-32 pan head screw with a hole drilled through it.... I can make any size I want up to 3/32".... I have number drills down to #60 (0.040"), below that I only have 1/32" (0.032") or 1/64" (0.016") but I'm pretty sure that tiny a drill would just break in a steel screw....

Have you ever got your velocity below 500 fps on a bullet where the plateau is over 900 without hitting a cliff (50% adjustment range) ?....

Bob