Regulated Disco Double

[Buldawg76]

I am sure you know this and likely will never get wet enough to happen but super glues are water soluble so just don't get it soaking wet or if it does dry it right away.
Medical super glue is not water soluble so just a FYI. Its what we used on Cadillac Avaniti to fix the many water leaks the convertible tops always had by gluing little pieces of foam in the places it leaked to build up the sealing surfaces , all convertibles leak sooner or later and in the case of the Avanti it was sooner instead of later.

Mike

[rsterne]

The ZAP-A-GAP is definitely waterproof, I have used it in model yacht construction for many years.... It is so much superior in quality to crazy-glue you wouldn't even think it was a related product, and is available in three different viscosities, ZAP (thin - wicking type, instant), the medium I used (for joints 0.005-0.010") usually applied before assembly (bonds within seconds), and a high viscosity, gap filling version, for joints up to 0.030" (30-sec. to 2 min. to set, depending on joint thickness and material).... The medium and thick can be used with the spray catalyst.... It has a strength of up to 5000 psi, and will even bond oily pieces of metal....

Bob

[Buldawg76]

Bob
I have never seen that brand of super glue so that is good to know as well. I have a friend that works at the army depot here rebuilding tanks and stryker vehicles and when the loctites and glues they use go out of date they just throw it away with some never being opened so he brings home a lot of the types I use to give to me. Their super glue they use is a medium thickness that dries very fast but I need to see if there is a catalyst to use with it to instantly set it also. It is also waterproof since the vehicles see all types of weather conditions. 

Mike

[rsterne]

Most good hobby shops that deal in R/C aircraft carry the "ZAP" brand adhesives.... The pump spray catalyst is even available in one that won't melt Styrofoam.... There are other brands available, but I found that brand far superior to any of the others.... It works on balsa, spruce, birch ply, most other woods, plastic, metal, glass, ceramic, carbon fibre, rubber, etc.etc.etc.... I often use it to repair my wife's china figurines, and most of the time she can't even find the repair (I use the thin for that).... Of course it bonds skin INSTANTLY.... 

Bob

[Buldawg76]

Bob
That's the problem there are no hobby shops in my area anymore so I probably can get it online at Tower Hobby or some of the other hobby online dealers. That's pretty much what the military glue I get is like and I have yet to find something it will not stick together. I was told to, keep it is the fridge to prolong its shelf life tremendously. I have used it to fix some of the wife coffee cup collection as well.

Yes it does work very well on skin also and I use the surgical gloves when I use it just for that reason and to keep my skin oils from contaminating the glue.

Mike

[rsterne]

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

[AmBraCol]

I haven't told you lately, but I greatly appreciate you sharing your builds with us.   

[Sfttailrdr46]

Bob and Lloyd are co-conspirators in the world of creative modifications and what is possible to do and improve AG performance 

[Buldawg76]

You would think that the manufactures of the high end gun that sell for 1500 and up could incorporate some of the features that Bob and Lloyd use in their builds. It certainly is not rocket science or any real extremely difficult machine work that a CNC could easily be programmed to complete quite easy.

If they made them with those options they would sell like hotcakes.

Mike

[Sfttailrdr46]

  For the most part they already do How many times do you go on one of the vendor web sites to order a rifle or pistol only to find out it is out of stock and will not be available for several weeks to a couple of months. Our sport/hobby is catching fire and more people are discovering the pleasure of shooting air guns every day.

Just look at the explosive growth in membership here on the GTA Forums. When I joined 3 1/2 years ago the membership was less than 10,000 now we are fast approaching double that number to close to 20,000 and I am pretty sure we will reach that number sometime early next year

[Buldawg76]

Don
That is all very true and I am hopeful that the manufactures are reading and taking head to the things we the consumer are desiring in the air guns of today and the future. I believe the coming years will be very interesting for the consumers and drain our pockets even faster than it does now.

Mike

[rsterne]

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

[Buldawg76]

Bob
I can see how the front of the barrel is machined to tension the barrel since its threaded to accept the VBT at the front. So my question is at the breech end I see what appears to be 8 holes ahead of the loading gate in the breech that I assume are for set screws tightened against flats on the barrel when its inserted into the breech, will those 8 set screws have enough grip to allow for the 800 or 900 pounds of tension to be applied to the barrel without it being pulled forward in the breech once the nut is tightened.

I am sure you will have machined flats at the set screw interfaces on the barrel so will the set screws be installed such that they are contacting the trailing edge of those flats to prevent the barrel migration forward in the breech or will the breech and barrel also be threaded together to prevent forward migration and the set screws there just to keep the barrel from rotating when the nut at the muzzle is tightened to apply the tension.

Mike

[lloyd-ss]

Bob,
I like the tensioner set-up and the way it mitigates temperature changes.  The outside tube/shroud is CF, correct?  What wall thickness? Have you noticed much accuracy change based on the amount of tensioining force, or do you put a LOT of tension on the barrel?  At the breech end, is the barrel secured with set screws, or some other method.  In general, is the full length of the barrel within the shroud unsupported?
Thanks,
Lloyd

[Machinist]

Nice engineering and nice looking.  I also wondered how thick the carbon fiber tube is.  How do you cut the carbon fiber to length?

Steve

[rsterne]

Mike, I have used a similar setup with just three 10-32 screws at 700 lbs. of tension and it is solid as a rock.... with nothing more than the cup points of the setscrews digging into a simple filed flat in the top (only) of the barrel.... Travis uses a snap ring to hold the barrel in the FLEX.... The new breech has eight 10-32 screws, and I plan on turning two shallow grooves for the setscrews, so I'm confident there will not be a problem.... I know 1200 lbs. is a lot of force, but it's only 150 lbs. per setscrew....

Lloyd, yes, the outer shroud is CF, with a 1/16" wall, so 7/8" OD x 3/4" ID.... The barrels are CrMoly TJ's with a 1/2" OD.... I haven't had much time to play with this, but my results mirror Tom's.... He found that as you increased tension, group size shrunk, and then at a certain point, no further reduction seemed to occur.... He estimated that was at around 500-600 lbs. tension on a 1/2" OD barrel, IIRC.... He uses a torque wrench, and calculating the axial load it works out to about 800 lbs.... This means that for small temperature changes, you stay within the "safe" range.... He found that with that much tension, the barrel sometimes "sings", just like a guitar string, when the gun is fired, but any kind of an intermediate support, near but not at the center, eliminates that completely.... He didn't say if the intermediate support changed the accuracy, which I would take to mean it didn't, it just eliminated the high pitched "twang".... If I notice a twang, I will make an aluminum collar with an O-ring groove on the OD to fit snug inside the shroud, and set-screw it to the barrel, about 4/7ths of the way between the front stub of the receiver and the back of the front compression collar....

Thermal expansion coefficients (in Microns per inch of length per degree) are:

1026 Steel - 6.7
4130 CrMoly - 6.2
6061 Aluminum - 13.0
2024 Aluminum - 12.7
3AL-2.5V Titanium - 5.3
Woven CF Laminate - 2.1

So, if we have a 20" length, and a 50*F temperature change, 20 x 50 = 1000, so you can use those for the length change in thou.... Subtract one from the other to get the relative length change.... Aluminum shroud, CrMoly barrel = 0.0065" tighter.... CF Shroud, CrMoly barrel = 0.0041" looser..... for a 50* rise in temperature with a 20" length.... You can imagine what that would do for a rigid, torqued setup, compared to the Bellevilles....

You can cut the CF with a saw and sand the ends.... but it does dull saw blades VERY fast....

Bob

[Hobbyman2007]

I've been using plumbers pipe cutters to cut the carbon fibre tubes . Makes nice clean cuts and no strands of the fibre get damaged like when using a saw. I've also used the same cutter to scribe barrels before cutting them . Here's the exact model I use .http://www.homedepot.com/p/104-Tubing-Cutter-32985/100021967
I'm sure with a little ingenuity I could use it to choke a newly cut barrel.

[Sfttailrdr46]

I've been using plumbers pipe cutters to cut the carbon fibre tubes . Makes nice clean cuts and no strands of the fibre get damaged like when using a saw. I've also used the same cutter to scribe barrels before cutting them . Here's the exact model I use .http://www.homedepot.com/p/104-Tubing-Cutter-32985/100021967
I'm sure with a little ingenuity I could use it to choke a newly cut barrel.

If you go to the Rigid tools web site they make a wheel that will put an impression into the metal without cutting the metal which might work to create a choke in the barrel

[rsterne]

Installing a roller instead of a cutting wheel in a heavy duty pipe-cutter has been used for years to choke barrels, with a great degree of control, in conjunction with gauge pins to measure the change in bore (land) diameter at the muzzle....

Bob

[Sfttailrdr46]

  I didn't know that I just know from HVAC work