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PostPosted: Tue Dec 26, 2017 9:47 pm 
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I ended up having to insert a pin into the rear of the trigger on my A/R to keep the pull weight spring from bending. ( I used a lighter replacement from the P/A grab-bag ) Also had the same "lip" on the sear engagement corner contact face. This is where the dremel with polishing felts and valve grind compound really shines :mrgreen:

-D.S.

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PostPosted: Tue Dec 26, 2017 9:58 pm 
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Well, trigger mods always seem to need tweaking.... The lighter spring I used was too light, so I went back to the original one, at reduced preload.... This produced an "acceptable" trigger, but I really need a spring in between the two.... Using a longer, weaker spring with increased preload will give a more constant trigger pull over the 1st stage travel.... which is what I wanted.... but the light spring I had was just too light....

I noticed one other thing I'm not crazy about, and I have seen this before with QB triggers.... When you lighten the trigger spring, the sear has a tendency to not reset if you pull the trigger through the first stage and then release it without firing the gun.... The reason is that the load from the hammer is too great to overcome the friction between the sear and the trigger.... The term for this is "balk firing", and it is VERY common in Springers, where you usually have to re-cock the gun to reset the sear.... I can't tell you if the stock Chief trigger was doing this or not, I didn't check for it.... but the surfaces were so rough I would bet on it, even with the stiff spring.... It is something to keep in mind, if you start to take a shot and then change your mind, you can leave the sear "hanging" on the verge of firing.... The way to clear it is to recock the gun, by pulling the bolt back all the way to take the hammer load off the sear.... It is a good practice to get into, and especially necessary if your sear doesn't reset when you partially pull the trigger and then back off on the shot....

While I had the gun apart, I pulled the barrel and receiver and checked out the ports.... They are pretty small, and all the same size.... The barrel port, receiver port and valve exhaust port are all 3.5mm (0.138"), and the flat O-ring type seal between the valve and receiver is also the same ID.... In addition, the valve in my gun is slid back a bit, so the exhaust port is partially covered by the seal, restricting it even further.... That 25 fps that Ribbon had over my gun could be right there.... ::) .... The valve has slid back because the holes in the tube for the valve screws are a bit larger than the screw heads.... With 2000 psi of pressure pushing back on the valve with 1178 lbs. of force, the valve slides back until the screw heads are tight against the back of the holes, and the exhaust port, that should have lined up with the seal and receiver port, is back from where it should be.... A seal with an oversized hole would probably help, or if, as I am, you plan to enlarge the ports, you can carve more out of the front of the exhaust port than the back, to re-center the port relative to the hole in the tube and the ports in the receiver and barrel....

Anyways, with the gun back together, I tethered it to a regulator set to 1800 psi and proceeded to go through the entire range of preload adjustment, shooting five JSB 18.1 gr. Heavies at each turn of preload and recording the average.... The results are in the chart below....

Image

This is a classic curve, with a "plateau" where additional preload adds NO velocity and only wastes air.... a "knee" where the velocity starts to drop, and then a "downslope" where every time you reduce the preload the velocity drops.... Stock preload on my gun was 4 turns out, right at the beginning of the plateau.... Therefore adding more preload does nothing but make the gun louder and waste more air.... On the other hand, the knee of the curve is at 5-6 turns out, and that is a good place to experiment.... Less preload than that looses more velocity.... Now this was at 1800 psi, at the 2000 psi fill pressure, the entire curve would move up (faster) and to the left (more preload).... That 4 turns out stock setting is probably right at the top of the knee at 2000 psi.... Anyways, now that I knew where the knee was, I could try reducing the preload to see what happened to the shot strings.... Here are the results....

Image

Only shots within 4% of the maximum velocity are shown on this chart.... The red string is from yesterday, but I omitted the shots below that 96% velocity point, leaving only 8 shots within a 4% ES, the velocity dropping with every shot.... pretty dismal the way it came from the factory.... This is what happens when you tune a gun on the plateau.... I reduced the preload 2 more turns, and shot the string in black at 6 turns out.... The starting velocity was only 742 fps, it wasn't until shot #5 that the velocity climbed within 4% of the 794 fps peak (which is why the first 4 shots have been omitted from the black line).... I got 22 shots, ending at 1400 psi, so the usable string was 18 shots within a 4% ES, starting at 1900 and ending at 1400.... I increased the preload a turn, to 5 turns out, and shot another string, recorded above in blue.... This is about as good as it gets for my 2000 psi fill.... The first shot was within my desired 4% ES, the string peaked at 809 fps, and I got 16 shots with the string ending at 1500 psi.... Here is the summary of these 3 strings.... all of which were shot with JSB 18.1 gr. Heavies, from a 2000 psi fill....

4 turns out - 8 shots, average 814 fps (26.6 FPE) @ 0.98 FPE/CI
5 turns out - 16 shots, average 794 fps (25.4 FPE) @ 1.31 FPE/CI
6 turns out - 18 shots, average 779 fps (24.4 FPE) @ 1.41 FPE/CI (1900 psi usable fill)

So, there is the key to tuning your stock Beeman Chief.... Try backing out the preload adjuster a turn or so, to get a proper bell-curve and double your usable shot count, with a big increase in efficiency....

Bob

PS, Doc.... I don't know how you put a pin in the trigger, mine is so hard I can't drill it for a spring seat pin.... :shock:

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PostPosted: Tue Dec 26, 2017 11:21 pm 
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Hey Bob,

What's your opinion of this gun compared to the Benjamin Discovery and the tune you did for that rifle?

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PostPosted: Tue Dec 26, 2017 11:41 pm 
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So far, I am a bit concerned that the usable pressure range is only 500 psi to stay within a 4% ES.... The Disco was more like 800 psi.... That will limit the shot count.... The Chief has a bit more FPE that the original Disco, although I understand Crosman changed the hammer spring and gained power but lost shot count.... The shorter barrel of the Chief (21" vs 24") will likely eat up any extra reservoir volume the Chief MAY have compared to the Disco.... I won't know the length of the reservoir until I pull the valve....

Not a lot to choose from between the two designs, so far.... The Disco fill pressure can be increased by using 10-32 screws to mount the valve instead of 8-32.... I would be hesitant to do that with the Chief, because we don't know the material the tube is made from.... might be OK, might not.... :roll:

We need to have somebody test a Chief tube to destruction to know for sure....

Bob

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Airsonal;
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PostPosted: Wed Dec 27, 2017 3:07 am 
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rsterne wrote:
PS, Doc.... I don't know how you put a pin in the trigger, mine is so hard I can't drill it for a spring seat pin.... :shock:


I had to cut into the back of the trigger with a pointed dremel grindstone to get through the hardening, and it took more than 2 stones to get through it.( the pink stones cut nicer and last longer than the grey ones ) Plain old carbide steel drill bits and oil at slower speeds in the press do a good, if time consuming finish. Ended up using a tiny roll pin, and it seems to work well. I kind of like that the trigger and sear components are old school hardened, from a safety stand point....

-D.S.

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PostPosted: Thu Dec 28, 2017 5:10 pm 
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Today I used the degasser to dump the pressure (works well).... It comes with a 5mm allen key, the same size you need for the velocity adjuster, one tool does both jobs.... I then stripped the gun down completely to examine the valve and porting.... Here is a photo of all the relevant parts in stock form....

Image

The barrel port, receiver port, seal, exhaust port, and even the through hole in the gauge block are ALL the same size, 3.5 mm (0.138").... The gauge uses straight threads and seals against an O-ring in a 10mm deep blind hole in the gauge block, in similar fashion to the Disco.... There is a screwdriver slot in the front of the gauge block, at right angles to the gauge port, and the hole is the same depth as the side of the slot nearest to the gauge port.... This means that as long as you stay the other side of the slot (even touching it is OK), you will have sufficient material for the O-ring to seal against.... Therefore it should be no problem to open up the gauge port to allow vastly increased flow into the valve....

The inlet hole in the front of the valve is 5 mm (0.197"), and the valve throat is the same.... The valve stem is 3.15mm (I thought it would be 3 mm, but it is actually 1/8").... This means that the valve throat has an area of only 0.0182 sq.in.... which is the equivalent of a hole of 0.152" (3.9 mm).... So, the valve throat is bigger than the rest of the porting, but just barely.... There is quite a bit of room (over 1/8") between the front of the exhaust port and the valve seat, so enlarging the exhaust port, within reason, should be possible without breaking into the back of the valve and ruining it.... Incidently, the probe on the bolt is quite large, at 0.152" (nearly 4 mm), so the area available for flow to the pellet (.22 cal = 5.5 mm) is 0.199 sq.in..... which is the equivalent to a hole of 0.159" (4 mm)....

There is very little volume between the gauge port and valve, they are only about 3/16" apart, so about 2 cc there and another 3 cc inside the valve.... The reservoir, from the back of the fill fitting to the front of the gauge port, is 13-5/8" long x 22 mm (0.866") diameter, and there is about 1 cc additional volume inside the fill fitting.... I would calculate the reservoir volume to be (132 + 1 + 2 + 3) = 138 cc, and will be using that for any future calculations.... Note that this is almost identical to a Disco, and they run the same 2000 psi pressure, so they both have the same volume of air available.... enabling direct shot count comparisons....

The poppet is VERY interesting.... The material is VERY soft, more like a hard rubber (maybe 150D? ) than a Delrin type plastic.... you can dent it with your fingernail.... The back (seat) side is cone shaped, with the sealing surface around the outer edge, and the OD is quite large, at 10.5 mm.... In fact, the back edge is belled out slightly (presumably from contact with the seat) to 10.8 mm (0.425").... There is a clear mark on the back from contact with the seat, and the ID of the sealing perimeter is 9 mm (0.354"), so the seat is larger than that.... A good value to use for calculating the force required to crack the valve would be at least 3/8", so at 2000 psi that would require 220 lbs., and with the soft poppet material, that force is applied over a longer distance, increasing the hammer energy required significantly....The valve spring is moderately stiff, and it sits against a brass spacer on the poppet.... I'm not sure if that is to increase the preload (which it does), or to prevent the end of the spring from digging into the soft poppet material.... The front of the poppet forms a spring seat/guide.... Incidently, the ID of the valve is 13mm (0.51"), and there are two O-ring grooves directly over that section, and they are 0.676" diameter.... That leaves only (0.676-0.51)/2 = 0.083" of material supplying strength to the valve body.... so boring out the ID of the valve is not a good idea.... There is just over 1 mm of clearance around the back end of the poppet (between it's OD and the valve ID), which is a total area of 0.0314 sq.in.... While that is larger than any of the other flow passages, it doesn't leave much extra room, being the equivalent of a hole of roughly 0.200" in diameter....

All seals, on the valve, both ends of the gauge block, and on the front fill fitting (not shown) consist of two quite large O-rings (2.5 mm CS).... Now in theory two rings are no better (some say not as good) in providing a seal (because O-rings work better with a large pressure differential across them).... However, it does provide redundancy, in case one is damaged on installation.... and the rear gauge block O-rings and the valve O-rings must slide across the hole in the tube for the gauge.... which if it has any sharp edges makes a dandy cheese-grater (O-ring slicer).... You can bet I will be carefully deburring that hole before reassembly....

One final thing, the three valve locating screws are not very confidence inspiring.... They have Phillips heads, and are only 4 mm x 0.7 mm, and a rather loose fit in the holes in the tube to boot.... I think I will be looking for some better screws....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
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PostPosted: Thu Dec 28, 2017 10:37 pm 
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I modded the gauge port and the inlet side of the valve this afternoon....

Image

I milled three overlapping holes, offset, through the gauge block, and then removed the excess material with a Dremel.... the hole is about 7/32" x 1/2" and D-shaped with rounded corners.... There is still about 1/16" of material between the hole and the O-ring seat for the gauge.... The valve spring is larger than 5/16" ID, so I drilled the front end of the valve to that size.... The end of the valve, and both ends of the gauge block, are countersunk just to ease the flow through them.... I gained a 2-3 cc's of volume, so I will use 141 cc (8.6 CI) for future calculations....

I carefully examined the valve attachment as well.... There is a steel backing block between the valve and the hammer, into which thread the flathead screw holding down the front of the receiver from the top, and the screw holding on the stock from the bottom.... That block in a QB is tight up against the back of the valve, so that those screws do some of the work in holding the valve.... In the Chief, there is a gap of about 1/16" between the back of the valve and the front of that block.... If you install the block and drop the valve in place, the back of the three tapped holes for the valve screws are flush with the back of the holes in the tube.... This means that the three valve screws are "on their own", holding all the force pushing back on the valve (1180 lbs. @ 2000 psi).... Those three 4 mm x 0.7 mm pan head screws look like they could be purchased at any hardware store that carries Metric, they certainly don't look of particularly high quality.... and the threads are in shear.... If the screws are at the minimum of the tolerance diameter at the thread root, that is only 0.118".... Assuming they are all taking equal load, I ran the numbers for three typical screw grades.... and the best SHCSs you can buy....

Grade 2 screws (74ksi UTS).... Safety margin, 60% shear - 1.2:1.... I REALLY hope Beeman isn't using cheap screws !!!
Grade 5 screws (120ksi UTS).... Safety margin, 60% shear - 2.0:1
Grade 8 screws (150ksi UTS).... Safety margin, 60% shear - 2.5:1
Metric Grade 12.9 screws (170ksi).... Safety margin, 60% shear - 2.8:1

This means that the screws should really be grade Metric 12.9 screws, which is basically the best SHCSs you can get.... The screws used are 5 mm long, so the proper McMaster Carr screw would be the following.... https://www.mcmaster.com/#91290a138/=1avzg91

I wish I had access to those screws, I would at least KNOW what they are, although they don't quite have the 3:1 safety margin I would like (and 3.5:1 is preferred).... I'm betting they are better quality than the pan head Phillips screws that Beeman use.... I could be wrong, of course, maybe Beeman had some special screws made, anything is possible.... One other advantage to the McMaster Carr screws is that the head is 7 mm diameter, which will be a far better fit in the 7 mm holes in the tube.... and way better than the 6.4mm head on the existing ones.... Incidently, in case you thought about shimming the backing block up to sit against the valve and share the load, I have a word of caution about that idea.... It could be done, if done carefully, so that the valve screws were still sharing the load.... HOWEVER, it would mean that you should not even remove the stock without degassing the gun.... If you make that block part of the load bearing system, then don't forget you did that.... I prefer having the proper screws to locate the valve, and then I can remove the stock and receiver without any concern about the valve anchoring system.... just the way it is supposed to work now....

Guess what.... There isn't a hope of me finding 4 mm x 0.7 mm short SHCSs locally, and McMaster Carr won't sell to Canada.... Rather than wait weeks to get someone to source them and forward them to me.... I am going to drill out the valve and fit three 10-32 low-profile SHCS, just like the whizzy mod you do for a Disco for higher pressure use.... Now this doesn't mean you can run more pressure in the Chief, we still don't know what the tube is made of.... In addition, the tube ID on the Chief is 22 mm (0.866") instead of the 0.745" on the Disco.... That means the Chief has 35% more force on the valve at the same pressure.... 2000 psi in the Chief is like 2700 psi in a Disco.... In fact, at 2100 psi, which is where the Chief (assuming a 1006 steel tube) has a 3.5:1 safety margin the three 10-32 low-profile SHCSs, with their 140ksi UTS, will only have a 3.7:1 safety margin.... That suits me just about perfectly.... I will have to drill the holes in the tube out to 5/16" for the larger screw heads, but that will decrease the stress in the bearing area.... While I am at it, I am going to drill the hole in the top of the tube to 5/16" as well, in preparation for a larger OD Teflon transfer port....

This project is finally starting to see some direction....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
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PostPosted: Fri Dec 29, 2017 11:14 pm 
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I modded the valve today.... drilled the throat to 1/4" (originally 5 mm = 0.197")....

Image

Then I drilled and retapped the screw holes to 10-32.... and milled a recess for a 5/16" transfer port, and then milled the exhaust port out to 0.210", angled at 10 deg. towards the seat to improve the flow.... then used a small spherical burr on the Dremel to fair it into the throat....

Image

The throat area is slightly more than the exhaust port, and that in turn is slightly more than the bore if I use a 3/32" bolt probe, which is the plan.... I have yet to drill out the barrel port and make a transfer port.... The receiver will be drilled through 5/16" and I will be using a Teflon transfer port of that OD.... For the next tests I will be using a transfer port of 0.166" ID, which is 75% of the bore size.... but I can (and probably will) go larger at a later date....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Dec 31, 2017 11:25 pm 
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The next step was to fit the valve to the tube.... This involved careful setup to redrill the three valve screw holes and the hole for the port all at 90 deg. to each other and in the same plane.... All four holes were drilled to 5/16", to fit the heads of the 10-32 low-profile screws, and the new 5/16" OD transfer port.... Then the receiver had to be drilled out for the transfer port to pass through, and the barrel machined to accept the top of the transfer port.... Here is what the parts look like after this was done....

Image

There were two additional modifications made to the receiver.... Note on the side, just below the notch for the loading port (above in the photo as the receiver is upside down) a 6-32 setscrew near the back of the notch.... There is a matching one on the other side.... These setscrews tighten against the sides of the back of the loading tray section of the barrel, greatly increasing the stability of the barrel in the receiver.... Also note that the M5 flat-head screw behind the transfer port is already in place.... It is trapped between the barrel and receiver, once the barrel is in place....

With the new Teflon transfer port, the barrel/receiver assembly must be lowered over the transfer port, which fits like a peg in the two holes shown in the above photo.... It will be about 0.010" longer than the space available, so that it is in compression when you tighten that flat-head screw.... The way this is assembled means you cannot install the receiver and then slide the barrel into place from the front afterwards.... I will admit that this was an unforeseen problem, but fortunately there was an easy solution....

Image

The photo above is a top view of the barrel/receiver assembly.... Note there is a hole drilled in the barrel loading tray to allow access to the Phillips drive in the flat-head screw that holds down the receiver.... This enables you to drop the flat-head screw into the receiver, slide the barrel into place and fix it with the (now) three setscrews (the original M5 on top, and the two 6-32s on the sides at the back of the loading tray).... and then drop the assembly over the transfer port, tightening the flat-head screw (through the new hole in the barrel loading tray) as you lower it into place.... and finally using that screw to put the transfer port into compression to seal it.... I admit (sheepishly) that I was very lucky that this solution was possible.... otherwise I would have worked myself into a rather embarrassing corner.... Sometimes it is better to be lucky than smart.... ::) .... Here is a photo of the barrel, modified with the new port, and showing the access hole for the flat-head screw....

Image

There is a 5/16" diameter flat machined to accept the top of the Teflon transfer port.... It is machined down flush with the bottom of the O-ring grooves on either side of it, and breaks through into the sides of the O-ring slots.... If the O-rings were still needed, this would be a problem, but of course they are not needed now that the Teflon transfer port will seal directly against the barrel in compression.... I may still install them, they won't do any harm, of course.... but they really aren't necessary any more.... Here is another view of the new porting system....

Image

Looking through the receiver and into the barrel port (on the left), you can see the oblong port machined into the barrel, and the way it blends into the 0.21" diameter circular shape at the TP parting face.... The port was machined with a 5/32" end mill, with the back of the port vertical and flush with the back of the 0.21" TP, and the front angling forward at a 30 deg. angle.... This makes the barrel port, at the borelilne, 0.17"W x 0.25"L, which gives it the same area as the 0.21" diameter round TP.... On the right, you are looking through the main tube, into the valve, and you can actually see right into the valve chamber, showing the vastly increased area for flow.... The only remaining job was to modify the bolt to thin the probe, the results of that are shown below....

Image

Before starting changes on the bolt, I loaded a JSB 18.1 gr. Heavy pellet with the stock bolt.... The larger diameter but fatter probe pushed it just flush with the front of my angled barrel port.... However, because of the taper inside the pellet, and the smaller probe I wanted, I knew that the new probe would have to be longer.... It turned out that increasing the overall length of the bolt by just 1/16" was all that was needed.... However, the new, longer barrel port was so much longer that you could actually see the front O-ring on the bolt (there were two, tiny O-ring installed stock).... Before installing the new longer 3/32" probe (in a hole drilled in the end of the bolt), I machined off the bolt shorter, to remove the front portion of the front O-ring gland, leaving just the small diameter bottom portion of the gland intact.... The front of the bolt is now flush with the back of the transfer port, so that it will not obstruct the airflow in any way.... This leaves just the single rear O-ring in place to seal the breech.... I see no reason for two, that is a redundancy that I can live without (I hope).... I guess I will find out when I fire the gun for the first time.... If it does leak, I can always machine it to take a slightly thicker Metric O-ring, with more compression on it when in battery, to insure a seal.... The new, thinner probe seats an 18.1 gr. JSB Heavy just comfortably ahead of the new angled barrel port.... The area around it for airflow is still the smallest part of the porting system, but is the equivalent of a 0.200" diameter port.... compared to the 0.138" diameter of the original porting system.... This means I have increased the area of the ports by 110%.... so it is 2.1 times the area I started with....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jan 01, 2018 9:17 pm 
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I reassembled the Chief this afternoon, and had time to shoot a few pellets through the Chrony.... I had thought that although I increased the throat diameter, since I did not change the OD of the poppet I would not need much increase in hammer strike.... While that may be true to "unstick" the valve, it takes more dwell to get more velocity.... I started out with LESS velocity with the same settings at 2000 psi, and had to crank the preload to the max.... When I did that, I got 974 fps with the 18.1 gr. (38 FPE) and 874 fps with the 25.4 gr. pellets (43 FPE).... but with the peak velocity occurring at only 1500-1600 psi.... Pretty good power at that pressure.... 8)

I will be pulling the valve out again and changing the poppet to one made from Delrin.... I'm pretty sure that will not only reduce the hammer strike required to "unstick" the valve.... but will also increase the usable pressure range as well, giving more shots per fill....

I thought I should at least shoot one string before I changed out the poppet.... Here it is, added to the previous data....

Image

It is shown over to the right because only the shots between 1750 psi and 1300 psi were within a 4% ES.... It sure shows what big ports do.... 8) .... The 10 shots used 450 psi, so that works out to 1.38 FPE/CI.... pretty good at an average of 958 fps (36.9 FPE)....

Bob

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Airsonal;
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PostPosted: Thu Jan 04, 2018 5:50 pm 
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Had a couple of days helping around the house, taking down Christmas decorations, touching up paint in the Motel.... but got back into the shop today and made a PEEK poppet for the Chief.... I have never used one in such a low pressure gun before, and wasn't sure if it would seal OK.... because the material is so much harder.... but I faced it off square to the stem with a razor sharp tool bit in the lathe and the lapped it to the seat in the valve with Solvol Autosol like I usually do, and it sealed up immediately.... Here is the new poppet, below the stock one.... I removed the brass spring seat/spacer, so there is less spring force closing the valve as well....

Image

I made the spring seat slightly larger in diameter for a better fit inside the spring, and to keep it centered better on the smaller (3/8") OD poppet.... In addition, I tapered the poppet down to only 5/16" at the seat, still quite a bit larger than the 1/4" throat, but a lot smaller than the soft stock poppet, which had an OD at the seat of nearly 7/16".... Not only does this provide a lot better flow around the poppet near the seat, but the much smaller area should take a lot less hammer force to "unstick" at the same pressure.... and in addition the much harder PEEK will compress a lot less, so the distance the poppet travels when hit by the hammer before it breaks the seal is much less also.... The hammer energy (inch.lbs.) used up to "unstick" the valve is the force (lbs.) times the distance (in.) the poppet compresses.... The force should be reduced from about 277 lbs. at 2000 psi to about 156 lbs.... and I'm guessing the compression on the PEEK is less than half.... so the hammer energy required to "unstick" the valve should only be about 1/4 or less that of a stock poppet....

Well, on reassembling the gun, I found that all the above is NOT just theory.... With the stock poppet, even at maximum hammer spring preload I could not max. out the velocity of the gun.... With the new PEEK poppet, I can't even get down to the knee of the curve, even at MINIMUM preload.... Here is the comparison....

Image

Now we're beginning to see just how much the larger ports increased the performance.... The plateau velocity with the same 18.1 gr. JSB Heavies increased from 811 fps (26.4 FPE) to 1053 fps (44.6 FPE), both with the gun tethered to a regulator set to 1800 psi.... That is a 69% increase in FPE from a port diameter increase of 45%.... Frankly, I'm a bit shocked, that surpassed my expectations.... Part of the unexpected gain may be due to the much larger passages on the inlet side of the valve.... which means that ALL of the reservoir volume is available to feed the valve in an unrestricted manner.... instead of having to flow through that 0.138" hole in the gauge block.... which likely caused some pressure drop inside the valve during the shot.... Normally, if your FPE increases in proportion to your port diameter increase, you have done well....

I am basically to the point now that the 18.1 gr. pellets are too light, so I changed over to the 25.4 gr JSB Monsters.... The plateau velocity with those was 955 fps (51.5 FPE) at only 1800 psi.... so my power goal has been met with 200 psi to spare.... :o 8) .... I left the preload adjusted to the minimum and shot a string, starting at 2000 psi, with each pellet.... Here are the results, along with the previous data for comparison....

Image

The black line is the way the gun came stock.... the green line with the big ports and stock poppet (at max. preload).... and the red line with the big ports and PEEK poppet (at min. preload).... all three with the 18.1 gr. Heavies.... The blue line is that current setup, but with the 25.4 gr. Monsters.... Obviously I need a weaker hammer spring in order to get back some adjustment.... time to go looking.... ;D

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
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Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Thu Jan 04, 2018 7:08 pm 
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Wow!! That is some difference! Quite a power increase! Exciting stuff!

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PostPosted: Thu Jan 04, 2018 8:42 pm 
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Wow very nice work


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PostPosted: Thu Jan 04, 2018 9:46 pm 
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Well I didn't need to look far for a spring.... I set the stock spring by compressing it to coil bind (shortened it about 1/4") and then closed up the last coil on the cut end.... I ended up with a spring that was 2.38" long instead of the 2.66" I started with, with 16 coils of 0.046" wire and an OD of 10 mm (0.394").... BTW, a spring calculator says this is about a 12 lb/in spring.... The hammer weighs 80 grams and has a stroke of 0.63".... When I installed the set spring in the gun, I could still get to maximum velocity at 1800 psi, but I could adjust it down all the way to 600 fps with the 25.4 gr pellets.... Here is the data added to the previous graph....

Image

The dotted line is with the set spring.... you can see it reduced the preload by about 8 turns.... The knee of the curve, at 1800 psi, is at about 5-6 turns out.... I filled the gun to 2000 psi, and using the 25.4 gr. JSB Monsters and the preload set to 5 turns out I shot a string.... It would have been great for tethering the gun at 1800-2000 psi, because the first 6 shots were within about a 1% ES, just over 900 fps (46 FPE).... However, it was missing the first part of the bell curve, so I turned the preload out 1 more turn and shot another string.... This time I got 12 shots within a 4% ES.... I have added both strings to the previous graph to make comparisons easy....All the 25.4 gr strings are in blue, using a 2000 psi fill pressure.... The 6 turn out string ended at just over 1300 psi, so I am getting a wider usable pressure range with the PEEK poppet, as I expected.... nearly 700 psi instead of 500....

Image

The last tune above (6 turns out) was 12 shots averaging 871 fps (42.7 FPE), with an efficiency of 1.27 FPE/CI.... The last part of the string I can hear some valve bounce, and the pressure and velocity is dropping pretty quickly, so the gun could benefit from a shorter, stiffer spring with a gap, or an SSG.... I don't have an appropriate spring that will slide over the spring guide pin on the hammer, but I plan to look and see if I can make an SSG fit.... At 6 turns out, there is about 1/10" of preload, which is just about the worst condition to encourage hammer bounce.... I'm pretty sure I can increase the shot count.... but I'm already getting more total energy, at about the same efficiency, and I've gone from 16 shots at 25 FPE to 12 shots at nearly 43 FPE, and from 794 fps with an 181 gr. pellet to 871 fps with a 25.4 gr.... My best string is 68% higher FPE than the best string I had on a stock gun.... :o

Incidently, I did test the power at 2000 psi at maximum preload with the stock spring.... I got 966 fps (52.6 FPE) with the 25.4 gr. Monsters.... and 882 fps (58.0 FPE) with the 33.6 gr. Beasts....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Fri Jan 05, 2018 10:12 am 
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Nice work Bob. That thing must bark like a Great Dane.

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