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Thread: .257 cal Monocoque PCP

  1. #21
    Moderator rsterne's Avatar
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    Today I worked on the Thimble which contains the Barrel Port and Loading Tray.... The 3/8" diameter stub on the barrel fits into the front of it to insure alignment with the bore, and the joint is sealed with an O-ring, which also seals it into the Breechblock in front of the Transfer Port.... I haven't yet machined the O-ring groove behind the transfer port in it.... The barrel port in this part was machined with a 3/16" end mill starting at 30* to match the transfer port at the back of the port, and then angled to 45* to make the front of the port longer to match the bore area.... The port ends up just over 3/16" wide x 5/16" long at the boreline, but blends into the 17/64" round transfer port.... The finishing touches were done with a Dremel....



    The hole at the back fits over the head of an 8-32 screw in the bottom of the Breechblock, that takes the end load from the barrel screwing against it, and the air pressure inside it.... You can see that screw in the bottom of the loading tray in the photo below....



    Once again, I am extremely pleased with the way everything fits and lines up.... This relative small part took about 6 hours to make.... Next up is the Bolt, which will carry the O-ring that seals the breech inside this Thimble.... It is the retractable style I developed a while back, with no probe to restrict the airflow.... Then comes the complicated job of machining the "J" shaped bolt slot in the Breechblock and Monocoque tube.... I'm not looking forward to that....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  2. #22
    Moderator rsterne's Avatar
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    Today I made the Bolt, except to fit the handle, and cut the O-ring groove in the Loading Tray / Thimble behind the transfer port.... The Bolt still has to be cut off about where the line is drawn, which will be flush with the back of the Breechblock when fully open....



    The Bolt has a very long travel, when it is fully forward the shoulder on the Bolt is up against the Loading Tray.... This is necessary because of the long bullet, necessitating a long loading port, plus the nose of the bolt has to go past the transfer port, which had to be far enough from the front of the loading port to allow for the O-ring to seal when retracted so that the nose of the bolt clears the back of the transfer port, without the O-ring dropping into the port when fully forward.... It will retract about 3/8" from the full forward, loading position to the firing position....

    It's time to do some more thinking to make sure I don't forget anything before I continue machining the tubes.... The Monocoque tube and Breechblock both need the slot machined for the bolt handle.... and the Reservoir tube has yet to have any machining done.... although all the internal parts are now complete....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  3. #23
    Moderator rsterne's Avatar
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    Only got a few hours in the shop today.... but I got the holes in the top and bottom of the lower tube machined.... That allowed me to bolt together some of the sub-assemblies for the first time.... It actually is beginning to look like something other than a pile of parts....



    The barrel and Monocoque tube are not in place in the photo, because I wanted you to be able to see the general arrangement of how the parts fit together.... It was great to actually be able to shoulder it and see how the ergonomics work.... I'm quite pleased, although the gun is definitely long.... The LOP as shown in the photo is 14".... I still have to machine the side holes for the valve screws, the hole on the left for the gauge, and the slot on the right for the cocking handle for the hammer.... Then it remains to machine the slot for the bolt handle in the Monocoque tube and the Breechblock.... The next time I put it together it should look close to the finished product.... Long, thin, and heavy....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  4. #24
    Moderator rsterne's Avatar
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    Today I had to figure out how the drill the side valve holes at exactly 90* and aligned with the top and bottom valve holes.... Normally I use a 5C collet in a 4-sided holder, but the maximum diameter collet is 1", and the tubes are 1.25".... In addition, they are 4 feet long, and difficult to handle when clamping in my short, 2.5" wide milling attachment.... The trick is to make sure that the hole drilled through top to bottom is exactly vertical, ie square with the C/L of the chuck.... I got the idea to make a fixture that I can clamp in the chuck of the lathe with two 3/8" holes at exactly 90* to the chuck, that will straddle the diameter of the tube and allow me to put a 3/8" bar through the existing 3/8" holes in the tube.... I didn't have any metal tubing large enough, so I used a piece of 1-1/4" ID plastic pipe, mounted it in the milling attachment, and drilled a 3/8" hole straight through at 90* to the wall of the pipe, and exactly on the centerline.... It's crude, but it should work....



    The next step is to make sure that when the tube is clamped in the milling attachment that the centerline of the tube lines up with the vertical centerline of the chuck.... I put the tube in the milling vise temporarily, and using an edge finder found the top of the tube.... It is then a simple matter to raise the vise up the radius of the edge finder (0.100") plus the radius of the tube (0.625") and the tube will then be vertically centered relative to the chuck....



    Next I put the alignment tool in the chuck, lined the hole up roughly vertical, and put the tube loosely in the vice and lined the hole in the tube up with the holes in the tool....



    Then I carefully slid a 3/8" diameter bar through the vertical hole, while rotating and sliding the tube around until the bar passed through all four holes.... and then I tightened up the vice, checking as I did so to make sure the rod would still slide easily....



    At that point, I removed the tool from the lathe chuck, and checked the bar to make sure it was vertical relative to the ways of the lathe.... As you can see, it was....



    The last set was to use the edge finder again to locate the center of the existing holes in the tube, so that when I drilled the side holes, they would line up perfectly....



    This gives you an idea of what can be involved just in setting up to drill a hole accurately in a piece of tubing.... I'm pleased to say that it paid off, and the side holes ended up nearly perfectly aligned with the ones on the top and bottom of the tube.... One hole need a touch with a file on one side, and the other needed to be moved over about 0.005" with a Dremel on one side so that all four valve screws could be inserted and will take equal loads when the tube is pressurized.... When I can use the 5C collet in the square holder, I usually don't have to correct the holes at all, but this worked well, considering the multi-step setup required.... Once I had the cross hole drilled, I then used that, with a 3/8" pin in the chuck, to lay out the hole for the gauge in the left side, and the slot for the cocking handle in the right side.... Here is a photo of the top and left side....



    and here is a photo of the bottom and right side of the reservoir tube.... You can see the hammer through the cocking slot and sear slot....



    This was a time-consuming process to go through just to drill three holes and cut one slot.... but that's the way it goes when you have equipment with severe limitations like a milling attachment on a lathe.... You can usually do it, but figuring out HOW and getting set up is the hard part.... The back end of the Reservoir Tube is now complete, the only machining left to do on it is to cut it to length and drill the four holes in the front to bolt in the fill fitting....

    Bob
    Last edited by rsterne; Dec 24 2015 at 07:42 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  5. #25
    Moderator rsterne's Avatar
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    Gee, guess what!?!.... I found a beautiful Millet 6-25 X 56 SF scope under the tree this morning!.... Things were quiet around here this afternoon, so I decided to cut the tubes to length and assemble the Monocoque PCP for the first time.... Here is what it looks like....



    I still have to add the bolt, fill fittings and barrel stretcher, and it already weighs just over 13 lbs.... without the scope!.... It is 49.25" long, although it looks longer.... It does balance nicely, in the back half of the forestock.... Then I removed the buttplate, and installed the BiPod (actually a TriPod, I guess) and the scope.... I think it looks pretty awesome....



    This is pretty much a bench gun, although it could certainly be used prone for varminting, using the BiPod and with or without the buttstock.... I really like the easy adjustability of the BiPod for height and tilt, using the handwheels.... It was a pretty cool Christmas present to put it together and look at it for the first time....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  6. #26
    Moderator rsterne's Avatar
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    Today I drilled the holes in the front of the tubes for the retaining screws for the Fill Fitting and Barrel Tensioner....



    Here is a photo of the muzzle, showing the recess for the Belleville washers and nut that will tension the barrel....



    That completes the machining on the Reservoir tube.... All that remains on the Monocoque tube is the slot for the bolt handle....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  7. #27
    Moderator rsterne's Avatar
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    Yesterday and today I machined the J-slot for the bolt handle in both the Monocoque tube and the Breechblock.... I also installed a Velocity Adjuster that works by choking the barrel port by preventing the bolt handle from withdrawing fully into the lower part of the J-slot.... I consists of a 3/16" rod running in a milled slot, and a 10-32 screw to control the position of it.... In addition, there is a small set-screw which presses a plastic plug against the side of the adjusting screw to act as a brake to prevent it from moving by itself.... Here is the Breechblock, showing the J-slot, bolt handle, and adjuster.... In this photo, it is closed about 30% from wide open....



    I'm very pleased with the positioning of the bolt handle and cocking handle.... I was a bit concerned that your hand might hit the bolt handle when cocking the gun, but I can wrap my big mitt around it no problem and my knuckle doesn't (quite) touch the bolt handle during cocking.... Here is a photo of the gun in the cocked position....



    You can see that the bolt handle is fully back in the J-slot in that photo, which means that the nose of the bolt is completely clear of the barrel port.... Here is a photo of the gun unloaded, with the cocking handle forward (uncocked) and the breech open for loading.... There is lots of clearance between it and the scope....



    I also made a brake for the hammer spring preload position adjuster today, to prevent that from adjusting itself as well.... This completes the major part of the machining on this project.... I can assemble everything now, except I am still waiting for the bolts to secure the Valve and Fill Fitting, so I can't as yet pressure test it.... I measured the reservoir today, and it is a whisker over 400 cc, not including the volume inside the valve, so I should have about 25 CI of air available.... If I can manage to tune for a shot string from 3800 psi down to 3000 (55 bar), that works out to 1375 CI, which would work out to about 7 shots at 200 FPE, if I can get the gun to produce 1 FPE/CI at that power level (which is likely a stretch).... I am more likely to tune it for tethered use at 3800 psi, and to get 3 shots within a very narrow ES off tether.... What the FPE level ends up when tuned like that.... is now the big question....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  8. #28
    Moderator rsterne's Avatar
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    Finally got my valve screws, so I got to assemble the reservoir today and pressure test it.... Filled it 1K at a time, checking for leaks (none) and it's currently sitting at 3500 psi.... I'm topping off my Great White tank with the ShoeBox to complete the fill....

    I slid the hammer down the tube, with it angled about 20*, and was rewarded with a nice loud POP and the valve sealed up again 100%, so things are looking good so far....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  9. #29
    Moderator rsterne's Avatar
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    It's now sitting at 4500 psi, no leaks I can find, so I'll leave it for a week and see what happens....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

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