Author Topic: Regulated Disco Double  (Read 14650 times)

rsterne

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Re: Regulated Disco Double
« Reply #15 on: November 24, 2016, 01:34:28 PM »
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....



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

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rsterne

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Re: Regulated Disco Double
« Reply #16 on: November 28, 2016, 04:38:46 PM »
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....



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
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Monkeydad1969

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Re: Regulated Disco Double
« Reply #17 on: November 28, 2016, 06:21:53 PM »
Looks like another winner for you, Bob.

Joe
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Re: Regulated Disco Double
« Reply #18 on: November 30, 2016, 08:40:24 AM »
 Good start keep up the good work. Keep us posted.

rsterne

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Re: Regulated Disco Double
« Reply #19 on: November 30, 2016, 08:54:13 PM »
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....





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
« Last Edit: November 30, 2016, 08:59:55 PM by rsterne »
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Monkeydad1969

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Re: Regulated Disco Double
« Reply #20 on: November 30, 2016, 09:01:47 PM »
I like the way the breech secures to the band.

Joe
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Hobbyman2007

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Re: Regulated Disco Double
« Reply #21 on: December 01, 2016, 06:47:23 AM »
I sure wish the manufacturers (crosman/Benjamin) would follow your lead on the design of your breech Bob. Nothing worse than that absolutely useless 4x40 breech screw crosman uses. Nice work

Monkeydad1969

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Re: Regulated Disco Double
« Reply #22 on: December 01, 2016, 07:14:24 AM »
I sure wish the manufacturers (crosman/Benjamin) would follow your lead on the design of your breech Bob. Nothing worse than that absolutely useless 4x40 breech screw crosman uses. Nice work

Ditto.
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rsterne

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Re: Regulated Disco Double
« Reply #23 on: December 01, 2016, 08:04:04 AM »
I still use a small screw there, I drill and tap for a 4-40 between the two Crosman locations.... I use a standard SHCS, I can get away with that because my breech is thicker in that area.... It ties down the center of the breech near the transfer port just to prevent flex where the huge cutout is for the magazine.... Most of my standard Crosman breeches end up with a slot ground in the 4-48 screw head....  ::)

The front band I use has two long 4-40 screws tapped into the band, which is flat on the top.... It fits between the front of the trigger group and the gauge, there is just enough room there for a 1/4" wide band....

Bob
  • Coalmont, BC

rsterne

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Re: Regulated Disco Double
« Reply #24 on: December 02, 2016, 07:47:35 PM »
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/index.php?topic=117960.

Bob
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Monkeydad1969

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Re: Regulated Disco Double
« Reply #25 on: December 02, 2016, 08:13:46 PM »
Seems you've out done yourself, Bob.  Any pics for us to look at?

Joe
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rsterne

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Re: Regulated Disco Double
« Reply #26 on: December 02, 2016, 10:03:44 PM »
Here are pics of the finished mags, Joe....



You can see the #011 O-ring through the clear cover, it partly obscures the numbers on the wheel.... In the back view, if you look carefully you can see a ring of clear plastic around the head of the pellet, where the hardened glue made the through hole smaller.... Note how perfectly the pellet is centered in the bore now....

Bob
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rsterne

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Re: Regulated Disco Double
« Reply #27 on: December 03, 2016, 05:45:45 PM »
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....



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
« Last Edit: December 03, 2016, 05:49:20 PM by rsterne »
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Monkeydad1969

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Re: Regulated Disco Double
« Reply #28 on: December 04, 2016, 04:20:16 PM »
Thanks for those awesome pics, Bob.  Keep us in the loop.

Joe
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rsterne

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Re: Regulated Disco Double
« Reply #29 on: December 05, 2016, 02:53:12 PM »
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....



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....



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"....



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....



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....

Bob
« Last Edit: December 05, 2016, 03:17:31 PM by rsterne »
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