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 Post subject: Regulated Disco Double
PostPosted: Sun Nov 20, 2016 3:48 pm 
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Location: Coalmont BC
Over a year ago, I suggested to Lloyd Sikes at the AirGun Lab, developer of the Disco Double, that he might consider making a regulator cartridge that could be installed inside the front upper plug that he supplies with his DD kits.... He thought it was a good idea, but life got in the way, so we didn't get around to doing the drawings until this Spring, and Lloyd made a prototype which exceeded our expectations.... He has since had some CNC'd, and has them in stock.... In addition, he has made an agreement with Eric Kuehn of Stalwart Arms to assemble and market the Disco Double kits.... I'll let Eric and Lloyd answer your questions about prices and availability (through PMs, please), the purpose of this thread is to build a Regulated Disco Double, and put the new regulator through it's paces.... Lloyd graciously supplied me with a regulated DD kit, and I purchased the rest of the parts shown in the photo below towards this project....

Image

Starting at the top, the barrel is a .22 cal TJ's with a 1/2" OD, that Sean Pero and I designed a couple of years ago.... I have had it sitting on the shelf, awaiting a suitable project, and now is the time.... It is 0.217" land, 0.213" groove, with a 16" twist.... I have two projectiles in mind, both recent developments.... One is the new 34 gr. JSB Beast pellet, and the other is the recently released NOE .217 cal 30 gr. Bob's Boattail, which I designed to work in this barrel.... My mould is in the mail as I write this, and it has a Lyman HP pin in one cavity which will drop the weight a couple of grains.... Both of these will fit and function in a PRod magazine, and I will be building my own breech to hold that and the shrouded barrel.... The lightest pellet I plan on using is the 25 gr. JSB Monster, although I know that the 18 gr. Heavies are deadly in this barrel, as are the 16 gr. Exacts and Polymag Predators....

The next item down is a Discovery main tube from Crosman.... I will be drilling out the transfer port hole to accept a larger, 5/16" OD TP, and deburring it, plus drilling a tiny vent hole 9/16" from the front on the bottom, otherwise I plan no modifications to it.... The O-rings are part of the DD kit, which is below the main tube.... Not shown are the hammer and SSG, which have yet to be made.... I also plan on using a Carbon Fibre tube as a shroud, which will place the barrel in tension to stiffen it.... The tension will be adjustable using my Belleville washer setup at the muzzle.... and of course the associated parts have yet to be machined....

The regulated Disco Double tube kit arrived assembled, just as you see it in the photo above.... It consists of the same 120 cc reservoir used in the DD kit, good for at least 3000 psi safe working pressure.... At first glance, it appears almost identical to the original unregulated DD kit, I'll get to the differences in a minute.... To the right of the DD kit are a PRod gauge block, and a much modified Disco valve which I have already written about in an earlier thread.... In addition to bore-size porting throughout (0.219" exhaust port, 0.266" throat with MRod poppet).... the valve is fitted with high-tensile 10-32 screws allowing a MSWP of over 3000 psi....

Below the DD kit are a pair of barrel bands I got from Lloyd with the DD kit.... There are a variety available, these are for use with a BNM breech fitted with a 1" shroud.... I will be using a 7/8" CF shroud on my gun, so I will have to fit them with a larger cross-section O-ring to hold snugly on the smaller shroud.... To the right of those are a PRod trigger group and typical Crosman skeleton stock.... I will eventually modify the LoP and cheek rise on that to fit me....

Now to the important part, the new AirGun Lab Regulator.... It is a cartridge design, using thick 1/2" OD Bellevilles, with the piston retained in the rear of the upper plug (inside the upper tube) with a snap-ring as shown.... The air enters the upper reservoir through the hole in the center of the piston, which is threaded 6-32 for easy removal of the piston....

Image

Looking at the front of the regulator, under the upper plastic cap, you will see an adjustment screw.... This allows moving the floating seat fore and aft, which allows you to externally adjust the setpoint pressure.... This is an exceptional feature, and part of our design criteria.... It was made possible by the position of the regulator at the front end of the upper tube, which now becomes the plenum.... a very large one at 135 cc.... The air pressure in the gun should be reduced below the current (or intended) setpoint (whichever is lower), so that the regulator piston is "open" and no pressure is on the seat before changing the adjustment, to avoid damaging the (replaceable) HP seal on the end of the piston.... IIRC, the adjustment range is about 500-2500 psi, Lloyd please correct me if I have that wrong.... :-[ .... The two protective caps, one for the adjustment screw and the other to cover the built in male Foster fill fitting, are identical, and interchangeable.... You can see them in place in the first photo....

Image

The side view, below, shows that the new upper plug is a bit longer than the previous (unregulated) version, to allow room for the regulator piston.... Note that there are two O-ring grooves, but only one O-ring.... The reference vent for the center (spring) portion of the regulator is between those two O-ring grooves, on the lower side, hidden by the bottom tube.... When the regulator is in place, the O-ring is placed in the aft groove (as shown), and the venting occurs towards the front, through the hole I will drill in the main tube, just inboard of the threads.... If you want to run the gun without the regulator, you simply remove the regulator cartridge, and move the O-ring to the front groove.... The vent hole in the regulator is now behind the O-ring, which prevents venting.... This is an elegant and simple way to allow removal of the regulator without having to replace the upper tube.... because the vent hole in the tube is inboard of the threads, but outboard of where the O-ring seals.... Again, this shows the attention to detail that occurred with the synergy that Lloyd and I have when working on projects like this.... I take great pleasure in working with him because of that....

Image

Anyway, there is the starting point for this exciting new project, and the details on the new AirGun Lab regulator for the Disco Double kits.... In addition to it being an option on the new DD kits (they all now share the new upper plug that accepts the regulator cartridge).... they are also available as a retrofit for the previous DD kits which were unregulated.... All you need is the new upper plug with regulator, shorten the hollow connecting bolt a bit (to miss the adjusting screw) and drill the vent hole in your main tube, and you're good to go.... You could even reinstall the original DD upper plug, or go back to a Crosman fill fitting, and the vent hole in the tube isn't an issue because of it's location.... This means complete interchangeability of parts, with no obsolescence, again one of our design criteria.... I'm looking forward to getting started on my new Regulated Disco Double....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
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PostPosted: Mon Nov 21, 2016 5:34 pm 
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Today I drilled out the transfer port hole in the Disco tube to 5/16", and while I had it mounted in the mill I lengthened the front of the cocking slot for the hammer by 0.20".... I will be increasing the hammer stroke, and just wanted to make sure the cocking pin won't hit the end of the slot on firing.... I then deburred all the slots and holes, paying particular attention to the gauge hole, which makes a great cheese grater the way Crosman machine it, but not very kindly to the O-rings you have to slide past it.... I pay particular attention to the front and especially the back inside edge, rounding it so that the O-rings can slide past it without getting torn.... Not doing that the first time you have a Disco apart is a big mistake.... I then carefully cleaned the inside of the main tube....

I mentioned before that the bands I got from Lloyd that hold the tubes together were for a BNM breech with 1" shroud.... The upper portion has an O-ring groove machined into it for a #022 O-ring, that will hold that diameter shroud.... I will be using only a 7/8" OD shroud, but I found that by using #212 O-rings I could used the same bands, as they have the same OD but the smaller ID I require.... However, the groove in the bands was too narrow, so I chucked them I my lathe and put a taper on the side of each groove so that they were 0.140" wide at the inside, in sort of a half-hexagon shape.... I did NOT have to cut the groove any deeper, just wider at the inner edge.... With a 70D O-ring installed, they fit the 7/8" tubing I have nicely, a snug, sliding fit with the O-ring compressed about 0.010".... This will hold the shroud well, without being rigid.... and there is no way the O-ring can pop out once the tube in installed....

I checked the fit of the valve, ground a notch in the top rear to miss the forward breech screw location, and also ground a notch in the bottom rear to allow better venting of the space between the hammer and valve through the hole behind the bottom screw.... I then disassembled and carefully cleaned the valve, lubed the stem and O-ring and put it back together.... I then lubed the valve and gauge block O-rings, slid the valve into place, installed the 10-32 screws, then installed the gauge port and a 3000 psi gauge.... As the O-rings pass the gauge hole, be VERY careful they don't get cut.... I use my fingernail, or a small flat-bladed screwdriver, to coax them past my nicely rounded edge, but you still need to be careful and work slowly.... I rolled a #016 O-ring onto the upper plug of the Disco Double kit, where it butts against the end of the main tube, lubed the O-ring that seals inside the tube, and screwed the tube onto the upper DD plug that contains the regulator.... Just before it contacts the O-ring, have a look at where the gauge is in relation to the lower DD tube, and stop threading it on when the gauge lines up with the lower tube and the O-ring is under the least compression possible.... Its only purpose is to fill the gap between the end of the upper tube and the upper plug for appearance and to keep the dirt out.... There is plenty of thread engagement without cranking it that last turn and squeezing the O-ring out of the groove....

I then temporarily installed the trigger group so that I could align the position of the lower tube with it.... I slid the narrow tube band (the one without the Picatinny rail) onto the two tubes, right back to the end of the lower tube, leaving only the flange on the aluminum plug past the band.... Once I was happy with the alignment of the tubes I snugged the setscrews on the bottom of it, turned the assembly around and viewing from the front made sure the tubes weren't twisted and sat parallel with each other.... I then tightened the setscrews in the tube band, being careful not to tighten them so much they would crush the tube, which is thin there because that is where it is threaded.... This completed the pressure assembly, and it now looks like this....

Image

At this point, it was ready for a leak test.... I slowly put in 1000 psi, and checked for leaks with soapy water on a small brush.... Finding none, I the gradually filled the reservoir, 200 psi at a time, allowing for the pressure to equalize and kept my eye on the gauge, which reads the pressure downstream of the regulator, so it will tell you the setpoint.... It stopped and held at 1600 psi, so I stopped filling at 2000 psi, waited a few minutes to be sure it hadn't crept up (it hadn't budged), and since I was still attached to my Great White I bled the line very slowly, which will often leave the check valve in the Foster open and allow you to bleed air from the bottom tube.... It worked, and once I got the tube down to 1400 psi I checked the gauge on the gun, and it still read 1600 (as it should).... I then quickly bled the air from the line to my tank, leaving 1400 psi in the lower tube and 1600 in the upper.... I wanted to increase the setpoint of the regulator, both because I expected to need a bit more pressure for this project, and because I wanted to play with the adjustment.... That meant that I had to bring the pressure in the upper tube down to less than the current setpoint, so I didn't damage the HP seat in the regulator....

The only way to do this, or at least the easiest way, was to fire the gun, so I temporarily installed the trigger group, a hammer, spring, and RVA, and making sure the transfer port was facing away from me, pulled the trigger.... I wish I had rembered to remove my hearing aids and put in my ear plugs, because it was LOUD!.... Fortunately, my hearing aids have a "clamping circuit" that prevents loud noises from getting through, so the actually act like (rather noisy) ear plugs.... Just the one shot dropped the pressure to 1400 psi, so now both tubes were below the current setpoint.... Since I was going to increase the setpoint, I didn't need to remove any more air.... If I had been going to lower it, then the pressure in both tubes would have to be lowered BELOW the new setpoint to make sure you don't damage the HP seat.... To increase the pressure you have to move the seat OUT, towards the front of the gun.... This allows the piston to move further before closing the HP seat, which compresses the Belleville springs further.... That takes more force, which means more pressure, and you end up with a higher setpoint.... To lower the pressure you need to do the opposite, you need to move the seat IN.... If you didn't bleed air from the gun, then you would be screwing the adjustable metal seat into the plastic seat on the end of the piston, and could easily damage it.... To repeat....

Adjuster OUT (CCW) increases the setpoint pressure.... Adjuster IN (CW) decreases the setpoint pressure....

If the screw is hard to turn, you probably forgot to bleed the pressure down, STOP and make sure both tubes are BELOW the lowest pressure setpoint you need.... The slot in my adjuster, viewed from the front, was at about a 45* angle, from 10:30 to 4:30 o-clock.... I decided to try 1/8 turn CCW, so I set the slot horizontal, at 9:00 to 3:00 o'clock.... The screw moved with noticeable drag from the O-rings that seal it, but was not difficult to adjust.... I then reconnected the Foster to my Great White and added 200 psi at a time, watching the pressure gauge to see how much I moved the setpoint.... It was almost exactly 200 psi, it now settled in and stayed at 1800, which is pretty close to where I think I need it.... certainly close enough for initial trials.... I continued filling the lower tube to 3000 psi.... let it sit for a while to cool, and topped it up before coming in to take the photo and write this post.... The assembly will sit under pressure while I continue to work on other parts of the gun as a leak test.... Note that if the gauge eventually shows a pressure drop, if would indicate that the leak is downstream of the regulator.... If there was a leak upstream of the regulator, the gauge (in theory) would not give you an indication of that.... However, the next time you went to fill from a tank, you could hear the check valve in the Foster click below the pressure you left the gun at....

Bob

_________________
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: Tue Nov 22, 2016 9:23 pm 
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Since I am using a PRod pistol grip trigger group and skeleton buttstock, I needed a forearm of some kind.... I ordered a pair of tube bands from Lloyd with the thoughts that I could probably come up with a way to mount a slotted piece of black 1-1/4" ABS Pipe between them to serve that purpose.... Here is the adapter I made, shown next to a tube band....

Image

It is made from a piece of 1-3/8" bar stock, drilled out to 1" and then bored out to 1.096" to the same depth as the tube band.... I then machined away two 90* segments, leaving the other two to fit around the lowest portion of the band.... This will mount the tubing concentric with the lower tube, as it is 1-3/8" ID so will just slide over the adapter.... I had to machine away the top portion to miss the upper tube (the Disco main tube), and the ABS pipe will have to be slotted to miss that tube as well.... Here are the two adapters, slid onto the tube bands, and sitting in the appropriate locations on the double tube assembly.... The front one can be positioned anywhere I want, depending on the length forestock I choose....

Image

As you can see, the adapters are on the inside of the tube bands, and the split piece of pipe will simply slide over them.... I can extend the pipe towards the rear, closer to the gauge if desired, as the setscrew portion of the rear band is below the diameter of the adapter.... I had to just file the corners slightly to prevent them dragging on the ID of the pipe.... On the front, you can see the Picatinny mount which is part of the front tube band.... I projects high enough above the adapter to clear any standard Weaver mount or ring, but I may have to notch the front of the tube a bit to clear the BiPod mount I have, it's a bit wider than the tube band on both sides....

This is going to work perfectly.... I will drill both sides of each adapter and tap the tube band for small screws, and also drill the sides of the plastic pipe and tap the adapters so that everything is rigid when assembled.... That, and cutting a wide slot in the pipe, are all that remain to complete the forestock....

Bob

_________________
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: Wed Nov 23, 2016 9:24 pm 
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I completed the Forestock today.... The stock itself is a foot of 1-1/4" ABS pipe with a slot cut in the top to clear the main tube.... It is mounted with four 6-32 pan head screws tapped into the adaptors I made yesterday, which are secured to the tube bands I got with a pair of 6-32 setscrews each.... Here is what the assembly looks like.... I slid a shroud I borrowed from my other Disco Double into place just to see what it looks like....

Image

As you can see, the Picatinny mount protrudes below the stock at the front, and it can be used for any accessory, such as a flashlight, laser, or a BiPod.... I fitted the PRod trigger group and the Skeleton stock, along with a BiPod for the photo below.... There is no breech on the gun, but it gives you an idea of what it will look like when finished....

Image

I used a holesaw to cut a circular hole in the bottom rear of the stock and then tapered the sides for appearance.... In profile view the stock hides the gauge, but it is easily readable from the bottom, as in the photo below....

Image

The forestock is in a great location, and is long enough for shooting offhand or prone, and the BiPod is in an ideal location, it can be folded forward, and remains within the overall length, even if the legs are extended.... I'm very pleased with the overall proportions and appearance, and although it doesn't yet have a breech or barrel, it's shaping up to be extremely light and well balanced....

Bob

_________________
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 Nov 24, 2016 4:47 pm 
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Today I made the hammer for the new gun, and using a bunch of parts I had laying around and/or borrowed from my other Disco Double, I assembled an SSG.... They aren't anything special....

Image

The hammer is a simple cylinder, 1.30" long (like a stock Disco hammer) and the front end is as per usual, flat in the middle with a taper on the edge for the sear to catch.... It is, however, drilled to 3/8" to allow larger hammer springs, the hole is 3/8" deeper than stock (7/8" total), and the cocking pin is moved back as far as I could.... There is a notch on the top at the front to clear the rear breech screw location, should I decide to use that.... With the notch in the valve I have the option of either location, or I can (and probably will) drill and tap the tube in between for a larger SHCS for the new breech.... The hammer weighs virtually the same as a stock Disco hammer, at 57 grams....

The SSG is the original type, where the guide runs through the gap adjuster (the large hex bolt) with an O-ring between that and the preload adjusting nut.... The spring is from a QB78, and is currently set up for about 0.4" preload (just under 7 lbs.).... As shown in the photo, the gap is zero, the end of the spring guide just touches the end of the hole in the hammer when it is sitting on the valve stem.... I have 0.72" of hammer travel total, so with a 0.020" gap (1/2 turn) I will have 0.7" of travel "under power", and the maximum force when cocked with be 19 lb.f.... I know from past experience that should be more than I need for this gun, so fully expect to be able to use a lighter spring or at least less preload when the gun is completed.... but for now this should work, and be able to reach maximum velocity (ie the plateau) in .22 cal with the 34 gr. JSBs, which is the heaviest I will be using....

Everything in or below the main tube is now done, so I guess it's just about time to start thinking about making the breech and machining the barrel to fit....

Bob

_________________
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: Sat Nov 26, 2016 2:58 pm 
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As you can see Bob , we're watching . Can't wait to see how it turns out, might get me to switch from the bottle fed kick I'm on. I'd be in at 32 shots at 40fpe in .25 cal .

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PostPosted: Sat Nov 26, 2016 3:44 pm 
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My other Disco Double will do 26 shots at 55 FPE average within a 4% ES on a 3000 psi fill (unregulated).... That was without an SSG, and I never tried restricting the barrel port to a lower velocity, so that 32 shots at 40 FPE may be within reach.... With the regulator in place, I rather doubt it.... as you will be trading shot count for a narrower ES....

Bob

_________________
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 Nov 28, 2016 7:39 pm 
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I had a chance to get started on the breech (receiver) today.... I finished the design and drawings over the weekend, so it was time to make chips.... I started with a piece of 1" x 1-1/2" 6061-T6 Aluminum 7.5" long.... The breech is only 7/8" x 1-1/4", so the first order of business was to reduce the dimensions to that by milling.... My lathe attachment only has 6.5" of travel, and the largest end mill I have is 1", so I had to make multiple passes and reposition the work, so it took most of the morning just to do that.... After lunch I centerdrilled both ends where I wanted the barrel (equidistant from the top and sides for appearance), and then mounted it between centers on my lathe, and gripped it in a 4-jaw chuck.... I turned the stub to fit inside the shroud, and then drilled a 3/8" hole just over half way through.... I then swapped the chuck to the 3-jaw, reversed the breech, held it by the shroud stub (which is centered with the hole for the barrel) and drilled from the other end until the two 3/8" holes met.... They were pretty close, as I could run the (long) drill right through the entire length.... I then swapped over to a 1/2" long drill and drilled the hole straight through that size in one operation from the back end, with the shroud stub still held centered in the chuck.... That cleaned up any slight misalignment in the 3/8" holes, and gave me a 1/2" bore straight through from end to end....

Image

I used an adjustable reamer to carefully bring the bore up to the size of the Cold Rolled Steel rod I will use for the bolt, which is a couple thou smaller than the barrel OD, so the barrel will have to be polished to fit.... However, the bore is straight enough I can slide the piece of CRS for the bolt straight through.... It's the best alignment I have ever had between the bolt and barrel, that usually causes me problems.... so I'm off to a good start....

Bob

_________________
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: Wed Nov 30, 2016 11:54 pm 
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Yesterday I got the concave in the bottom of the breech machined with a 7/8" ball end mill.... Today I spent the day in the shop and got all the holes drilled and tapped and the notches milled out for the magazine and the front barrel band, and the slot for the cocking pin.... plus the dovetail for the scope.... Here are photos of the breech as it looks at the moment....

Image

Image

I tried mounting it, and it bolts onto the tube properly, and the stub for the shroud lines up great with the barrel bands, the shroud slides right into place.... I still have to machine the slot for the bolt handle, and the recess in the bottom for the cocking pin to swing to the opposite side to the handle.... I also have to make the band that fits into the notch in the front and holds the front of the breech down tight to the main tube....

Bob

_________________
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 Dec 02, 2016 10:54 pm 
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I spent most of the afternoon and evening modifying a pair of .25 cal MRod mags to use the .22 cal wheels, so now I have .22 cal mags that are interchangeable with my .25 and .30 cal mags.... This will allow me to use any of the three calibers in the Regulated Disco Double.... It was far from a simple "drop in the .22 cal wheel", which is what I had been told.... If you want to read the saga of this conversion, you can do so here.... http://www.gatewaytoairguns.org/GTA/ind ... pic=117960.

Bob

_________________
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: Sat Dec 03, 2016 8:46 pm 
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Today I did some more work on the breech.... I machined the J-slot for the bolt handle and drilled and tapped the back for the velocity adjuster....

Image

The bolt is a probeless, retracting style, with a flat nose that chambers the round when pushed forward, then you rotate the handle downwards and withdraw it into the lower slot.... In the lower slot you can see a 1/8" steel pin, the position of which is adjustable by the 8-32 SHCS on the back of the breech.... Up from the bottom, near the back, I drilled and tapped a 6-32 hole for a setscrew pressing a small piece of Delrin against the bottom of the 8-32 screw as a brake, to keep it from self-adjusting.... There is a total of 9/32" of adjustment, which works out to 9 turns on the adjusting screw.... When fully retracted, the front of the bolt face is flush with the back of the barrel port, and fully forward it chokes it off almost completely.... This allows a very wide range of velocity adjustment, currently it is set about in the middle for the photo, so you could see the pin that limits the distance the bolt can be retracted....

Bob

_________________
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 Dec 05, 2016 6:09 pm 
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Location: Coalmont BC
Today I worked on the barrel tensioning system.... I have been using barrel tensioners for a few years now.... They are nothing new, of course, even Crosman used a tensioning system in (IIRC) its Mk.1 CO2 pistol, to stiffen and protect the thin "soda straw" barrel.... What I did differently was to utilize Belleville Washers at the muzzle to provide a variable tensioning system, that would look after a large portion of the thermal expansion as well.... Barrel tensioners are beginning to be more widely used to stiffen our rather long and slender barrels in "upscale" limited production PCPs, two notable ones are the FLEX and the Slayer (and possibly the Thomas as well? ).... The basic idea is to use a shroud, with a much larger OD than the barrel, and by tensioning the barrel at the muzzle and putting the identical compression load in the shroud, the assembly acts like it had the much larger OD of the shroud, instead of the smaller OD of the barrel.... As you increase tension, just like on a guitar string, the vibration frequency of the barrel increases, and the amplitude of the vibration decreases.... If the shroud is mounted solidly to the receiver (or part of the receiver as in my Monocoque design), then even the primary vibration of the barrel (the barrel waving up and down or side to side like a flagpole) can largely be eliminated or at least minimized.... The goal, of course, is to make the barrel stiffer and increase accuracy.... The guns I mentioned above all do this extremely well.... The tension can be considerable, Tom at American Air Arms estimated the tension on the Slayer barrel at about 800 lbs. achieved by torgueing the barrel nut....

If the barrel and the shroud are made of the same material, then as the temperature changes, they will change length by the same amount, and the tension will stay a constant.... The same thing will happen if the Thermal Expansion Coefficients are identical, or at least very similar.... However, if the barrel and shroud expand at different rates, the tension will either increase or decrease, depending on which expands more.... If the tension is applied by torqueing a nut, just a few thou can make a huge difference in the tension.... That is where I got the idea of using a short stack of Belleville washers (disc springs) at the muzzle.... They act like a very stiff spring, and so a few thou change of length (and therefore preload on the washers) won't change the tension near as much as with a rigid connection.... The stack of five 1" OD x 1/2" ID x 0.073" Bellevilles I am using in this case have a spring rate of 15,000 lb/in, which over their total travel of 0.090" means they can develop up to 1335 lbs. of force.... If the preload is 0.060", the tension will be 900 lbs. and a 0.010" change due to expansion will only change the tension by about 150 lbs.... Without the Bellevilles, a 0.010" differential change in length could drop the tension to virtually zero.... So to summarize, the intention of the Bellevilles is to allow for differential thermal expansion between the barrel and the shroud.... and to make adjustment easier and more consistent....

One of the things a shroud can be used for is to use the internal volume to reduce the residual air pressure at the muzzle by having the shroud longer than the barrel.... In some PCPs that length is filled with baffles to further attenuate the report.... However, just the shroud can do quite a bit, providing some of the muzzle blast is contained within it.... In the Slayer, the nut used to tension the barrel is perforated to allow some of the HPA to bleed back into the shroud.... Up until recently, I could not figure out a way to do that with the Bellevilles in the way.... While working on another project with John Kopaz and Lloyd Sikes I suggested the idea of using a vented barrel extension to position the Bellevilles ahead of the muzzle, and vent some of the residual muzzle pressure back into the shroud.... I have dubbed this arrangement the "Vented Barrel Tensioner", or VBT.... At its heart is a tube threaded onto the muzzle, with four 1-caliber holes at 90* to each other to allow some of the muzzle blast to enter the shroud.... The front of that tube is pulled on by the Belleville washers.... There is a collar which the tube slides through to center it and transmit the compression load from the Bellevilles into the shroud.... The front of the tube is threaded, and a nut, made to enclose the Bellevilles, allows them to be compressed to tension the barrel.... Here is what it looks like....

Image

The photo above shows the VBT assembly on the front of the shroud.... There are a pair of flats on the front to adjust the tension with a 5/8" wrench.... The assembly adds less than 3.0" of length past the original muzzle, one of the great advantages of using this system....

Image

With the shroud removed, you can see what the assembly looks like.... The Bellevilles are hidden within a chamber formed by the adjustment nut and the compression collar that butts against the front of the shroud.... In this photo, the Bellevilles are under no compression, while in the top photo, they are fully compressed.... Note the difference in how much of the flange on the collar you can see behind the chamber portion of the nut, which gives a visual indication of the amount of tension applied.... The total travel is only 0.090"....

Image

In the third photo, the adjustment nut has been removed, and you can see the chamber inside for the Bellevilles.... It threads onto the front of the perforated tube, which threads onto the front of the barrel.... I used 1/2"-20 NF threads on both ends, and in fact the threads on the front of the barrel are exactly what is required to mount a Hatsan Air Stripper, leaving my options open.... There is a slight recess in the back of the perforated tube for the O-ring on the barrel, the purpose of which is to allow the tube to be tightened onto the muzzle but allow a slight amount of movement at that joint.... This allows the tension to keep the tube perfectly aligned with the bore.... The tube is drilled through using the 29/64" tap drill to a depth of 1.8", and there are about 1" of threads inside.... This creates a chamber in front of the muzzle where the four holes are drilled to vent the muzzle blast out into the shroud.... The remainder of the tube is currently drilled to 1/4" (this gun is a .22 cal), although I may drill it out slightly larger, and it would have to have a larger hole for larger calibers.... There is enough material to handle at least a .30 cal barrel with plenty of clearance for the bullet....

Image

The last photo shows the Bellevilles washers and the collar they rest against that fits snugly inside the shroud and transmits the compression load to it.... The perforated tube was made from a piece of 5/8" CRS, with the front portion machined down to 1/2" to fit through the Bellevilles and threaded for the adjustment nut, shown to the left of it.... The shroud tube I am using is 7/8" OD Carbon Fibre, with a 3/4" ID, which only leaves a 1/16" gap around the holes.... While this is way more than the bore area, using a larger 1" OD shroud would allow more room for the muzzle blast to turn the 180* to flow back into the shroud, and would also greatly increase the available volume inside the shroud for expansion.... It would also eliminate the step down in OD from the 1" Bellevilles, bringing the shroud OD to the same size as the collar OD and the Bellevilles, making the VBT appear less bulky (relative to the larger shroud).... The VBT could also be scaled up for larger barrel diameters (to handle larger calibers), by using larger Belleville washers, of course....

Although this system doesn't use any baffles to reduce the noise signature.... and there ARE air rifles sold in Canada that use the shroud to reduce the report.... the use of ANYTHING to reduce the report may be questioned in Canada.... This system can easily be altered so that no air enters the shroud.... All that is needed is to eliminate the perforated tube and shorten the shroud accordingly.... The adjustment nut would then be threaded directly onto the muzzle of the barrel, and nothing else changes.... except the system would just be a "Barrel Tensioner" (BT) rather than a vented VBT.... The external appearance, other than the length, is identical, of course.... Rather than completely rewrite this post, or not post the information at all, I decided adding this disclaimer would suffice.... At this point in time I have no idea if there is any change to the report anyway.... :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: Wed Dec 07, 2016 7:31 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
I threaded the muzzle of my .22 cal barrel today, and it turns out it is 28", not 25" as I thought.... I will use is as is for now, until I find out what the velocity is with the 34 gr. Beasts, and maybe shorten it later (who am I kidding!).... Anyway, I assembled it for the first time to day, so that I could see it, take a photo, and weigh it....

Image

The overall length is 46.5" and it weighs 6 lbs. 10 oz.... I still have to make the bolt and the front breech band, and alter the stock, but it is easily going to make my 7 lb. goal without the scope.... so I'm a happy camper.... I think it looks pretty slick as well, and it balances right in the middle of the forestock.... 8)

Bob

_________________
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 Dec 09, 2016 5:15 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
After shovelling snow this morning, I got back into the shop and made the front breech retaining band and the bolt and handle....

Image

I notched out the ID of the band with a small mill so that it can be slid over the tube from the rear with the valve screws in place, something I always seem to forget and have to do by hand afterwards with a file.... :roll: .... The bolt nose is only roughed out oversize, it will be finished in diameter and to length after the barrel is chambered and the O-ring groove cut.... It also needs the hole for the cocking lug drilled and tapped into the bottom, but at least the main body and handle are done....

Bob

_________________
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: Wed Dec 14, 2016 11:42 am 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
Minus 27*C here this morning.... put the heat on in my shop, hopefully it will be warm enough by lunchtime to do some bullet casting.... The plan is to break in my new NOE 30 gr. mould....

Bob

_________________
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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