GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rkr on January 24, 2023, 08:29:29 AM
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I made a new valve block for my .45 Evanix from aluminium and I'm having huge trouble in getting the PET-P/Ertalyte poppet valve to seat. Is aluminium a poor choice of valve seat material or is it just me not doing it right? Or is it PET-P that's the problem? The old valve block which I guess was iron or low grade steel didn't have such problems with PET-P. It's 10mm straight face poppet in conical valve seat and 9mm valve tunnel.
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I don’t have a lot of experience with aluminum valve bodies but I can’t fathom why aluminum would present unique challenges for the poppet to seal off against it. A couple of things come to mind as I’m reading your description. The first one is just that a tapered seat demands precise concentricity of both the poppet (stem to poppet OD) and valve (stem bore to tapered seat). If you have the parts in your hand and pull the stem and spin the poppet lightly against the seat with your fingers, does it feel perfectly uniform throughout a full 360deg rotation?
The other thing is that while PET-P machines to a very clean surface finish, I get a tenacious wire edge at the corners. And of course for a tapered valve seat, the very corner of the poppet is the critical sealing area. If you haven’t already tried, what I would do is take a Sharpie and color both the corner of the poppet and the valve seat, and then apply a light abrasive (e.g. J-B compound) and grab the stem with a drill and spin it against the seat for a few seconds. Then clean away the abrasive and inspect both surfaces where the Sharpie was removed with the aid of magnification. If the ring on either part lacks uniformity or smoothness, you can continue with the abrasive until they do.
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Quick question…I was just looking at the dimensions, a 10mm poppet over a 9mm throat. Is that small of a sealing margin known to have worked in the past? For example, At 3000psi operating pressure, I’m calculating a compressive load of 15,500psi carried by the rim of the poppet. According to an https://www.matweb.com/search/DataSheet.aspx?MatGUID=281e3a595e624a06bf49d9c6138092e5 (http://[url=https://www.matweb.com/search/DataSheet.aspx?MatGUID=281e3a595e624a06bf49d9c6138092e5)]example datasheet for PET-P[/url], that may be exceeding the compressive strength rating of the material.
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I have used Aluminum seats on many of my custom valves & retrofits. ( 7075 or 4064 is best. 6061 T6 also works ) * Poppet stem material must be HARD !!!
Have found a Concave seat surface of @ 15* really helps center the poppet head.
The poppet head when machined needs to have a very crisp unbroken edge along the side to bottom ( Stem side ) so that the contact making the seal is a very thin margin.
As used this thin margin will quickly flatten out a tad once pressure and some cycles happen.
* Ideally using PEEK for the head, size it @ .060" larger than throat .. IMO
Manufactured CONCENTRIC in all dimensions is critical in having the assembled valve seal correctly.
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I just made a peek poppet for my AEA, it has an aluminum seat as well and I had to chuck up the poppet in a drill and burnish it into the seat to get it to seal.
I pulled it out to look at it and the poppet barely had a mark on it and the aluminum seat still had the anodizing on it.
Delrin seems to seal easily in comparison.
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I just made a peek poppet for my AEA, it has an aluminum seat as well and I had to chuck up the poppet in a drill and burnish it into the seat to get it to seal.
I pulled it out to look at it and the poppet barely had a mark on it and the aluminum seat still had the anodizing on it.
Delrin seems to seal easily in comparison.
Indeed it does ... PEEK will allow faster opening and a crisper shot cycle & if air filling gun is kept clean I won't wear out for the life cycle equivalent of many Delrin ones.
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Thank you for the feedback. This is a 2mm stem valve and I use similar valve in my .257 Evanix - 8.0mm poppet in a 7.0mm valve tunnel where it has worked great for hundreds of shots while giving big reduction in hammer force needed. The difference is that I have PEEK poppet in that valve.
Now that you mention problems in machining PET-P, I do recall my earlier valve in this gun which also had PET-P poppet developing leak in range that was caused by the poppet. The seat is as concentric as I could make in my lathe so I think the problem may be in poppet material and my ability to machine it. Perhaps I should try Delrin/acetal poppet, or would I be better off with PEEK? Delrin seals easier and with 2mm stem there's much less force slamming it back to seat so it might just work.
Come to think of it, what was that new material that's even harder than PEEK that people were using for poppets?
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Harder than PEEK
I mien dang ... peek good as it works, something even more requiring of precision to actually work sounds like a bit much honestly.
I do recall last year conversations about some semi transparent yellowish material, but don't remember material name .. or ever played with it.
Dug around ... PEI
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203824.0;attach=417976;image)
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A while back I put together this comparison of some commonly available plastics.
comp. tens. Izod water Ball coeff. wear
str. str. impact hardness abs. density indent friction factor
----- ----- -------- --------- ------ ------- ------ -------- ------
UHMW-PE 2000 3100 no break D62-D66 0 0.034 5700 0.14 0.1
PTFE ? 3900 3.5 D50 0 0.078 - 0.09 4080
ABS 5000 6500 7.0 R105 0 0.038 13500 ? ?
polycarb. 12000 9500 13.0 M70, R118 0.12 0.043 16500 ? ?
nylon 6/6 12500 11500 0.6 M85, R115 7 0.042 20000 0.25 162
w/30% GF 20000 27000 2.1 M101 0.7 0.049 30000 0.31 150
acetal 15000 9500 1.0 M88, R120 0.2 0.051 19500 0.21 340
PET-P 15000 12400 0.5 M93, R125 0.07 0.051 19500 0.20 121
PAI 19000 13000 1.0 M113, E75 0.35 0.053 ? 0.35 637
PEEK 20000 16000 1.0 M100,R126 0.1 0.047 27500 0.27 467
w/30% GF 26000 15000 1.4 M103 0.1 0.056 37500 0.25 187
PEI 22000 16500 0.5 M112 0.25 0.046 ? 0.39 605
Sources:
http://sterlingplasticsinc.com/materials/
http://www.matweb.com
[url]https://www.polytechindustrial.com/products/plastic-stock-shapes[/url]
The yellow material Scott is referring to is PEI which has a slightly higher compressive strength and hardness than PEEK. It machines very nicely even with my amateurish hand-sharpened HSS cutter. An example:
(https://i.imgur.com/5a1zzwx.jpg)
It seems to work very similarly to PEEK in terms of how easy it is to knock open. And likewise, similarly unforgiving of surface quality. Less expensive, though it's not like it takes a lot of material for a poppet.
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I did the sharpie trick (or actually polishing paste on the seat) that Nervoustrigger suggested and indeed there's a section of poppet edge that's not touching the seat properly. I guess that was the problem although I can't quite understand how i managed to do that when spinning the poppet in a lathe. Still, that PEI sound quite interesting, perhaps I'll give it a go in the future as the PEEK rod I have is only 9.5mm in diameter so I need to order something bigger anyway.
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I did the sharpie trick (or actually polishing paste on the seat) that Nervoustrigger suggested and indeed there's a section of poppet edge that's not touching the seat properly. I guess that was the problem although I can't quite understand how i managed to do that when spinning the poppet in a lathe. Still, that PEI sound quite interesting, perhaps I'll give it a go in the future as the PEEK rod I have is only 9.5mm in diameter so I need to order something bigger anyway.
I don't know if you had the same issue I had when turning peek on my lathe but I got chatter when cutting the bevel and had to use sandpaper to remove the chatter marks.
I was feeding very slowly, almost burnishing the bevel when the cutter took a nice bite out of the peek but unlike soft metals, the chatter was only 1 chip on the face, not a washboard pattern at all.
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Yup ... RAZOR sharp HHS tooling I've found best for such materials.
Minimal rake and place on work piece center line of ever so *slightly lower for best finish.
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Yup ... RAZOR sharp HHS tooling I've found best for such materials.
Minimal rake and place on work piece center line of ever so *slightly lower for best finish.
C6 Carbide with a LOT of rake did it to me. I dont use HSS, I find it creates more work for me. I have a flat C4 bit I can shape to cut the bevel with the rod chucked up.... maybe that will help?
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Great subject for the Workshop, thanks for posting, also the chart of materials.... 8)
Bob
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I made a 10.5mm delrin poppet and it sealed right away. So, poor machining from my part.
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I made a 10.5mm delrin poppet and it sealed right away. So, poor machining from my part.
Indeed higher yield materials shape shift easier and there for seal easier.
So folks understand while reading this post ....
We go to harder materials because it takes less strike energy to get poppet to come off the seat, wears far better yielding greater service life.
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I made a 10.5mm delrin poppet and it sealed right away. So, poor machining from my part.
Indeed higher yield materials shape shift easier and there for seal easier.
So folks understand while reading this post ....
We go to harder materials because it takes less strike energy to get poppet to come off the seat, wears far better yielding greater service life.
Exactly, and what I'm doing now is to keep the energy needed to get the poppet off the seat and reducing the energy needed to keep the valve open - or shall we say reducing the valve closing force. It will be interesting to see what the results are.
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I made a 10.5mm delrin poppet and it sealed right away. So, poor machining from my part.
Indeed higher yield materials shape shift easier and there for seal easier.
So folks understand while reading this post ....
We go to harder materials because it takes less strike energy to get poppet to come off the seat, wears far better yielding greater service life.
I'm using this peek poppet in an AEA VARMINT and the bolt is incredibly hard to pull. Just using this poppet, I have been able to back off the hammer spring 4 full turns and keep the same velocity, 850 fps.
The bolt is still stiff but it's now not such a chore, the trigger lightened up a smidge too. I also got 4 more shots off a fill which may not seem like a lot but I'll take it!
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I made a 10.5mm delrin poppet and it sealed right away. So, poor machining from my part.
Indeed higher yield materials shape shift easier and there for seal easier.
So folks understand while reading this post ....
We go to harder materials because it takes less strike energy to get poppet to come off the seat, wears far better yielding greater service life.
I'm using this peek poppet in an AEA VARMINT and the bolt is incredibly hard to pull. Just using this poppet, I have been able to back off the hammer spring 4 full turns and keep the same velocity, 850 fps.
The bolt is still stiff but it's now not such a chore, the trigger lightened up a smidge too. I also got 4 more shots off a fill which may not seem like a lot but I'll take it!
Yup easier to open
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I've experimented with most of the material available.
Even tried glass filled Peek and Pet P but found it abrasive to the seat.
I cut the valve seat concave and then polish with a hard felt bullet.
A little oil and light abrasive takes out any imperfections in short order.
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I've experimented with most of the material available.
Even tried glass filled Peek and Pet P but found it abrasive to the seat.
I cut the valve seat concave and then polish with a hard felt bullet.
A little oil and light abrasive takes out any imperfections in short order.
Which worked the best for your needs?
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They all work good but I find the seat geometry vs seal fit is much more important.
Made this test fixture and pump the valve to 3K psi.
Then drop an 11 gram hammer down the tube about 5".
They all break the seal at 3K.
Peek is a good choice and will provide long reliable service.
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When making a poppet if it's thread on or pressed on I don't turn it to the final diameter or cut the face. I leave it a little oversize for a couple hours because the poppet material changes shape from the pressure of the stem.
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When making a poppet if it's thread on or pressed on I don't turn it to the final diameter or cut the face. I leave it a little oversize for a couple hours because the poppet material changes shape from the pressure of the stem.
Fantastic advice! Thank you!
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Also the cheaper gear drive lathes tend to induce chatter into the plastic surface from just gear rattle . No matter how sharp and slick the cutter is . It’s a pain in the butt sometimes
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Also the cheaper gear drive lathes tend to induce chatter into the plastic surface from just gear rattle . No matter how sharp and slick the cutter is . It’s a pain in the butt sometimes
My old 9X19 is belt drive, I removed all the gears and power feeds from it and replaced the bearings, trued up the 3 jaw chuck. The cross slide could be tighter but there is no chatter on aluminum or brass so the peek suprised me.
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Also the cheaper gear drive lathes tend to induce chatter into the plastic surface from just gear rattle . No matter how sharp and slick the cutter is . It’s a pain in the butt sometimes
My old 9X19 is belt drive, I removed all the gears and power feeds from it and replaced the bearings, trued up the 3 jaw chuck. The cross slide could be tighter but there is no chatter on aluminum or brass so the peek suprised me.
Tooling rejecting to cut ... not sharp enough IMO
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Also the cheaper gear drive lathes tend to induce chatter into the plastic surface from just gear rattle . No matter how sharp and slick the cutter is . It’s a pain in the butt sometimes
My old 9X19 is belt drive, I removed all the gears and power feeds from it and replaced the bearings, trued up the 3 jaw chuck. The cross slide could be tighter but there is no chatter on aluminum or brass so the peek surprised me.
Tooling rejecting to cut ... not sharp enough IMO
We covered this above, new carbide, too much rake.
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Making peek poppets for those of us without machine tools.....it jusy takes time and patience, and the willingness to start over from scratch.
Here is a series of operations learned the hard way...
Rough cut the poppet material to size, drilling a hole as close to concentric as possible.... I press the 2mm stem in the poppet material first, using a tight interference fit, drilled with a 1/16" bit. Most of my poppets are for 5/16 or smaller throats.....That's about as big as I've stretched the 2mm stems up to 3500psi... If larger, use an appropriate stem, but it is critical to have a tight interference stem to poppet head fit.
From there, chuck the stem in my dremel, turn it wide open, and use a combination of files and sandpaper to shape and finish the poppet. For my cobra valves I cut the oring groove with a hacksaw blade carefully held against the poppet spinning at 10k rpm, or however fast my dremel.spins.
Sealing margin finish is accomplished by lightly polishing with 500 then 1000 grit sandpaper, again with the stem chucked in a dremel running WFO.
Paint the sealing margin of the poppet amd valve seat with sharpie as lay up dye, lightly burnish against the seat to find high/low spots. Repeat the sandpaper process if required. You may find your valve seat needs attention....do not be surprised if this happens.
I've made a few dozen poppets using this process. Typically no problems getting them to seal. Have several that have gone through >10k shot cycles....
IMO, high speed is what allows me to basically free hand a peek poppet that seals.with no leaks.
Of course this is far from the right way, but it is a way that works for me.
There is a time and place for peek, and a time and place for delrin. Using my crude methods, delrin is actually more difficult for me to work with, believe it or not.
Hopefully this helps someone who has been hesitant to try making their own poppets....