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PostPosted: Sun Feb 08, 2015 8:52 pm 
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You totally lost me, Jon.... A 1 degree movement on a 20 tpi thread is moving the nut 0.00014" or 0.0035mm.... The tension on the Bellevilles will change by less than 1 lb. per degree.... One of the guys who is using barrel tension to reduce group movement and size is using ~ 800 lbs. on tension on his barrel.... I get the impression you are talking about a totally different subject.... Since it seems disconnected with my goal, I will just blunder along in blissful ignorance....

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!
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PostPosted: Mon Feb 09, 2015 8:01 am 
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Bob were both working on similar thing but to discount whether or not tensioning works or not you have to take tensioning out of the equation. Easily done just do away with the disc springs and same process of revolving the nut 10 degree increments and test shot groups. I'll tell you why after.
Fine setup is within 1 degree.


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PostPosted: Mon Feb 09, 2015 12:34 pm 
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I completely understand how moving or changing a barrel weight can affect barrel harmonics.... I have used a series of seven muzzle weights in 5 gram increments on a Crosman barrel in the past, and I can watch the groups tighten up and open up again as I move through the range of weights.... Unfortunately, if you change the velocity, or the pellet, the timing of when the pellet exits the barrel changes, and you need a different barrel weight to find the smallest group.... In addition, the POI changes as well.... I have no doubt that using smaller increments, or screwing a nut back and forth, could allow me to fine tune between the group sizes of the 5 gram weight gaps.... However, I have to ask myself why bother with all of that when changing the velocity or pellet makes it all completely useless?....

Using the Bellevilles to tension the barrel has two goals.... First to increase the resonant frequency of the barrel and reduce the amplitude of the vibrations (like tensioning a guitar string).... Second, to take advantage of the stiffening effect of the shroud, which is twice the barrel diameter.... I have already, in the initial tests, proven that the POI change as you change velocity and pellet is reduced by a factor of three to five (ie to 20-30% of what it was).... That, IMO, is no small thing, and gives me great hopes for a reduction in group size as well.... The experience of those using tensioned barrels (where a simple nut provides the tension on the barrel, no Bellevilles) suggests that there is quite a large reduction in group size between "snug" and "tight", but virtually no difference between "tight" and "very tight".... So far, nobody I am aware of is using a torque wrench to provide measureable, repeatable barrel tension to quantify those numbers, although one estimate on a 1/2" barrel is a tension of about 800 lbs. beyond which no further gain was noticed.... In addition, as temperature changes, depending on the different coefficients of thermal expansion of the barrel and shroud, the tension on the barrel could easily change by 50%.... The addition of the Bellevilles is an attempt to reduce that percentage change with temperature to 10-15% or less, depending on the materials chosen.... Using more Bellevilles could reduce that even further.... My goal is to provide a tension that remains between "tight" and "very tight" over a wide range of temperatures, hence maintaining the reduction in group size already proven possible using barrel tension....

If weight was not an issue, then simply increasing the diameter of the steel barrel is the simplest, and best, solution.... The stiffness of a barrel varies with roughly the fourth power of the diameter.... In other words, if you double the diameter, the barrel is 16 times as stiff, and for the same disturbance on firing, will have 1/16th of the deflection at the muzzle.... That is why the "rail guns" used in bench rest competitions have large diameter, solid steel barrels.... However, doubling the diameter increases the weight by (at least) a factor of four, which may not be desirable in a field gun.... The use of barrel tension is one way to increase barrel stiffness without a large weight penalty.... I am hoping that by incorporating Bellevilles into that system that I will achieve a degree of repeatability and consistency not previously done by barrel tensioning, or at least not to my knowledge.... Another method is by using a Carbon Fibre sleeve glued onto the barrel to increase the diameter and stiffness, since CF is nearly the same stiffness as steel (and high-modulas CF can be stiffer) while being one fifth of the weight.... Adding a 5/8" OD CF sleeve to a 1/2" OD barrel will more than double the stiffness, and going to 3/4" OD will double that again.... For .25 cal, a 1/2" OD steel barrel 24" long would weigh 1.0 lb.... a 3/4" OD steel barrel would weigh 2.6 lbs.... and a 1/2" steel barrel with a 3/4" OD CF sleeve would weigh 1.3 lbs.... the same stiffness for half the weight....

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: Thu Feb 19, 2015 2:46 pm 
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Location: England.
Good stuff Bob.

Most of my tests are based on 15mm dia barrels with bonded on original 20mm o/d steel sleeves. Some sleeves removed, some been cut back 50mm beyond muzzle to adapt a mechanism direct to barrel.

Not many rifles around where you can remove barrel without altering the tension, usually its a bad idea to have anything touch the breach block but in tensioning instance will alter the frequency.
Not sure many barrels would take 800Lbs (1084 Nm) pulling force before pulling out the breach, that's some force approaching end plugs in a cylinder. If you take a normal M10X1.5 bolt the max recommended force to tighten for Grade 8.8 is 58 Nm!. Thats exceeding over 18 times the max recommended force and of course barrels could be softer. http://www.npfasteners.com/pdfs/max-rec ... torque.pdf
Most rifles use one or two M3 to M6 grub screws and would probably move or pull out at around 200Lb pulling force.

Another factor is most rifles barrels end well short of cylinder by 100mm or more, people like to end flush.
Personally I would take performance over looks any day.

Something worth thinking about first rifle made 1992 I turned down a Hammerli barrel to 12.2mm o/d with half moon radius 2.6mm depth on underside to give clearance over cylinder. Long breach block barrel turned down think 9.2mm x 180mm into breach. It twanged like a tuning fork for several seconds after firing but the key point here is the groups it left regularly at 55 and 60 yards most would be happy with at 6 yards.
Barrel was bare and did same with all pellets 120ft/sec apart.

Good point about the Belleville washers (disc springs) one reason I always opted for compression springs in regs the last 23 years being more forgiving. Might need a long stack of disc springs to allow some compression and expansion plus heat. Stacking can play havoc even to the hardened experts, got some stories there.


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PostPosted: Thu Feb 19, 2015 7:32 pm 
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Big difference between the torque you tighten a bolt to, and the tension it applies.... A 7/16"-20 TPI thread in steel would normally be tightened to 400 in.lbs.... At that torque, the tension in the bolt is over 4500 lb.f.... To generate 800 lbs. of force you would only have to torque that size thread to 70 in.lb (~6 ft.lb.).... While 800 lbs. is indeed a lot of force, it's tiny compared to what threads can stand.... I have three 10-32 setscrews anchoring my barrel and they don't budge at 500 lbs. force, I'll let you know how they do at 800 when I try the 1/2" barrel and bigger Bellevilles.... The better way is to thread the barrel into the breech, of course....

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 Feb 20, 2015 11:50 am 
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Out of interest, if you were threading a barrel with a TP, how would you go about getting the thread to tighten as the TP lined up? Or, if that's not practical, how else to approach the problem - a lock nut on the outside, or maybe shims?


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PostPosted: Fri Feb 20, 2015 12:00 pm 
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Use a thimble for TP

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PostPosted: Thu May 07, 2015 11:51 pm 
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I finally had a chance to do some testing of the tensioned Crosman barrel today.... The results, unfortunately, are not conclusive.... The gun was set up shooting 18 gr. JSB Heavies at 950 fps.... I sighted the gun in at 50 yards with 1 turn (6 flats) of tension in the Bellevilles, and then reduced the tension 1 flat (1/6th turn) at a time until I got to no tension, and then removed the shroud and tried the barrel free floating.... I then set the tension back to 1 turn, and increased the tension 1 flat at a time until I got to 9 flats (1.5 turns), which put the Bellevilles nearly fully compressed, so the tension would be about 450 lb.f.... I shot 10 shots at each setting, and here are the targets.... The small squares are 0.15" and the large black squares are 0.60"....

Image

Image

Image

The groups all showed horizontal stringing, and there wasn't much wind, most of the time I couldn't feel it on my face and the leaves at the target weren't moving, or at least I didn't notice the wind, with the exception of the target at 7 flats.... If I could see any trend, it would be that the groups from 5 through 9 flats were the tightest, followed by the group with no tension.... and the group with 4 flats of tension (2/3 turn) was the worst.... Unfortunately, there was no real marked trend, other than a lot of tension seemed better than light to moderate tension.... Each time I changed the tension the POI moved on the target, with a large shift to the right occurring at 7 and 8 flats of tension.... The single hole to the left on the 8 flat target was not part of the group.... The group at 9 flats (1.5 turns) of tension had 9 shots within 1"....

It is possible that the small amount of wind spoiled the test and hid any significant results.... With that pellet at that speed, a 2 mph crosswind will move the pellet 0.6", or the width of the large black squares.... The group with the smallest vertical dispersion by a large margin was the group at 9 flats, with the most barrel tension.... I left the gun set up like that....

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 May 08, 2015 12:40 am 
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Location: Winnipeg, Canada
I should break out the old harmonic dampening device again, and see if it makes a difference on the new build.
This, of course is in an entirely different direction from what Bob is doing- it's a dampening weight.

Would I that the bbl tuning screw system from the Win 52 target/sporting series would apply to airguns. A much simple fix that the air gun tube negates...

Dunno if I'm going to get a test in this Saturday or not. Fingers crossed...

Regards,

Doc Sharptail

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PostPosted: Fri May 08, 2015 6:25 am 
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Interesting results Bob. All these groups look very respectable for the distance to me - how do you rate them compared to your other rifles? The fact that it's mostly horizontal spread in all configurations makes me wonder if perhaps there was more wind in the flight path than you realized - those vertical spreads are really very good indeed - you must have done a fine job on the power plant.

Jim


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PostPosted: Fri May 08, 2015 10:04 am 
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The gun was tethered, so the velocity spread was only about 1%.... Even so, I agree that the vertical dispersion is quite small, particularly in the case of the last group with 1.5 turns of tension (~450 lbs.) on the Bellevilles.... That group is really just a 5 shot ragged hole, a 4 shot ragged hole, and a single shot slightly further to the right.... At that distance, a 2 mph crosswind from the left could cause the split between the two groups and a 3+ mph puff the outlier.... It takes a 4 mph wind to feel it on your face and for leaves to just begin to move.... in other words if the wind is less than 3 mph, you have no real indication it is even there....

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 May 08, 2015 10:16 am 
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Location: England.
Seen similar horizontal streaks with moving weights but also vertical, large spread and same hole. Before the soft touch paint job last year I was getting a full charge 90 shots in to width of four small black blocks at 55 yards in a drift, two pellets high (three blocks) as couldn't fine tune the setting enough.

Funny enough I did plan on setting the latest incarnation up today but have to do it tomorrow, hope its not raining or windy.

I cant see a pattern developing above.
I would dismiss wind at 950ft/sec unless over 20mph.


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PostPosted: Fri May 08, 2015 2:25 pm 
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Quote:
I would dismiss wind at 950ft/sec unless over 20mph.

Seriously, Jon?.... At 950 fps with a BC of 0.036, at 50 yards the drift is 5.59".... I guess anything up to 5" of drift doesn't matter to you.... :roll:

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 May 08, 2015 3:03 pm 
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Location: Victoria, BC and Clarkston, WA
Doc Sharptail wrote:
I should break out the old harmonic dampening device again, and see if it makes a difference on the new build.
This, of course is in an entirely different direction from what Bob is doing- it's a dampening weight.

Would I that the bbl tuning screw system from the Win 52 target/sporting series would apply to airguns. A much simple fix that the air gun tube negates...

Dunno if I'm going to get a test in this Saturday or not. Fingers crossed...

Regards,

Doc Sharptail


I have done dampeners too Doc. Some I even experimented with Mercury inside a steel vessel suspended just past the forensic of a spotter weight .308 barrel in a Remington 700 Action. It helped with the whip and recoil of the medium weight barrel. My other time was in a sleeved Rem. 700 action and a 32" .308 match chambered Shilen barrel that was 1 1/8" it's whole length. I cut a .22 dovetail the whole length of the barrel to move the tungsten weights along. As I recall 3" was the optimal placement of 8 ounces of tungsten.

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PostPosted: Fri May 08, 2015 3:09 pm 
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This Jon yokel just keeps coming back to spout his nonsense eh? Seems to have a particular bug up his nose about you Bob, I suppose because he considers himself the 'master' of UK custom airguns. From the examples of his work I've seen online, well, hard to make out much through the small, dark images from days gone by, and the chipped paint finishes on some of the work, but suffice to say I'd be scared to pump some of that stuff up to pressure. And here he is in your thread saying things like torque = axial tension on a rod. And now that somehow wind magically ignores pellets if they're moving fast enough. It's head-scratching stuff to be certain.


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