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Thread: .257 cal Monocoque PCP

  1. #11
    Moderator rsterne's Avatar
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    I received my new 33" Barrel today.... it slugged out right at 0.257" groove as requested, with a 7" twist.... I set to work fitting it to the Breechblock which I had previously drilled 1/2" all the way through.... I drilled the front to 33/64" and tapped it to a depth of 2" with 9/16"-18 NF threads, and then counterbored it 3/4" deep to 9/16" to fit the OD of the barrel.... The Barrel was turned down on the end to 3/8" to fit inside a steel Thimble which will carry the barrel port, next there is a 1/2" diameter portion to fit the bore of the Breechblock, and then the threads.... This was all done in a 4-jaw chuck so that it was concentric to the bore.... There is an O-ring which is compressed to seal the Barrel to the Thimble, and simultaneously seal the Barrel into the ID of the Breechblock when it is tightened against the Thimble.... Here is a photo of the Barrel machining, the roughed out Breechblock (made from 1-1/8" OD 2024-T3 round bar), and the two screwed together....





    My hand is on the mend, it felt good to get back into the shop today.... Tomorrow I hope to turn the OD of the Breechblock to fit inside the upper CrMoly Monocoque tube.... Once the rough fitting is done, there will be a lot of work required to finish and mount the Breechblock in the Monocoque tube....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  2. #12
    Moderator rsterne's Avatar
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    Yesterday I got a piece of 1.5" black Delrin rod from Whitewolf (thanks, Kim!) and started turning it into a Buttstock today.... After a few attempts at trying to hold it, I managed to mill off both sides to leave me with a piece 1" thick, which I have drilled and tapped for the 3/8" threaded rod as shown.... It will soon look much prettier.... .... The other part in the photo is the partially finished fill fitting for the front of the Air Reservoir.... It is tapped 1/8" NPT on the front for a male Foster, and the holes for the four 1/4"-28 x 1/4" SHCS retaining bolts are drilled and tapped at 90*.... It needs shortening and the O-ring groove machined yet....



    I usually use a 5C Collet in a square holder to drill holes at 90*, but the parts for this project are over 1" diameter, so they wouldn't fit.... The solution was to turn a 1" long x 1" diam. stub on the end of each part so that I could hold it in the collet as shown below.... The other parts below are the "Barrel Stretcher" which fits inside the front of the Monocoque tube, anchored by three 10-32 x 1/4" low profile SHCSs to stand the load of the Belleville washers that will put the barrel in tension.... and the mount for the Hammer Spring Adjuster which I made previously....



    The upper (long) screw in the holder for the adjuster goes through the Breechblock, below the bolt, and the spacer, securing the rear of the two tubes together.... The shorter (lower) screw just mounts it into the lower tube, and will have a short piece of Delrin rod inside the hole pressing against the side of the 9/16" bolt that carries the spring guide, to lock the adjustment in place....

    I also got the 10" long Breechblock turned to fit inside the Monocoque tube today, which prepares it for some of the machining to come.... I still have to make the Valve and Hammer.... Then comes the difficult job of machining the 4 foot long CrMoly tubes to hold everything....

    Bob
    Last edited by rsterne; Dec 10 2015 at 07:54 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  3. #13
    Moderator rsterne's Avatar
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    This morning I finished up the Buttstock.... It turned out quite well, with lots of adjustment.... The LOP can be varied from 13-15", and the drop is adjustable over a range of 1-1/4" in 1/4" increments.... The shaft in the butt pad is 1/8" off center, so 1/2 turn changes the height of the butt by 1/4" and then there are three holes on 1/2" centers.... The threaded rod is glued and pinned into the butt pad, as well as threaded, and the wingnut locks the rotation....



    This afternoon I did most of the machining on the Breechblock insert.... pretty much everything except the slot for the bolt handle.... I will have to duplicate everything you see here in the Monocoque tube, so accuracy and layout is the key here.... I'm quite pleased with the way it turned out....






    The two holes with low profile 10-32 screws in them are to take the tension load of the barrel when the Belleville washer stack at the muzzle is compressed.... There are three screws that bolt the upper and lower tubes together, a 10-32 at the back and two 8-32s forward, either side of the transfer port.... The transfer port is milled at a 30* angle towards the muzzle to ease the flow transition into the barrel.... The Picatinny scope rail bolts to the Breechblock though holes in the Monocoque tube, with the forward screw tapped into the tube itself.... The opening for loading is milled flush with the centerline of the bore on the right side, and just past the vertical centerline, so just over 1/4 of the material has been removed.... This should make the assembly much more rigid vertically than milling out the entire top half of it.... There will be a steel thimble (actually in 2 pieces) that will have the barrel port, chamber, and loading tray machined into it.... The 8-32 screw under it will take the end force trying to push it back on firing.... It is set deeply so that the head is in shear for extra strength.... I will likely mill the slot for the bolt handle into both the Monocoque tube and the Breechblock in one operation after they have been assembled.... or possibly machine the tube first, then install the Breechblock into it and mill the slot through the slot in the tube.... I'm sure it will be the most difficult part of the whole procedure....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  4. #14
    Moderator rsterne's Avatar
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    Today I machined the double convex Tube Spacer to match the breechblock, as the mounting screws and transfer port penetrate it to connect the two tubes.... The recess in the middle is a pocket to clear the barrel tensioning retainer screw....



    On the bottom at the front I had to machine a recess that will hold the Forestock tube in place tight against the top of the Reservoir.... you can see it in the photo below.... I also got the Hammer made today, including the cocking handle, as shown....



    The total weight of the hammer and handle is 182 grams, and it will have a total travel of 1.5".... The captive spring arrangement and adjuster was shown previously, and the moving part weighs 53 g. for a total of 235 g.... That means that only 77% of the total energy ends up in the hammer, the rest is absorbed by the O-ring on the guide when it comes to a halt before the hammer coasts the last bit to open the valve.... That is the price you pay for easier cocking and less hammer bounce....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  5. #15
    Moderator rsterne's Avatar
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    Today I attached the Front Cheekpiece Mount to the back of the Breechblock....



    It is mounted below the centerline of the tube so that the inside of the Cheekpiece tube sits against the top of the upper Monocoque tube.... I also machined the front of the barrel to fit the 1/2" ID Bellevilles and threaded it 1/2"-20 NF for the nut that will provide the barrel tension.... The aluminum sleeve that bolts into the front of the Monocoque tube is 9/16" ID to fit on the larger part of the barrel, while the Bellevilles ride on the 1/2" section....



    The front of the Monocoque tube is about 1/10" ahead of the muzzle to protect the crown.... If I wish, I can use a Hatsan Air Stripper instead of the nut to tighten against the Bellevilles to provide the barrel tension and also strip the air away from the back of the bullet....



    This completes all the parts that fit inside the Monocoque tube.... except to machine the bolt slot which I will do after machining the tube.... I am still undecided if that will be my next step, or if I will make the valve first.... and put off machining the two tubes until the end....

    Bob
    Last edited by rsterne; Dec 13 2015 at 05:17 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  6. #16
    Moderator rsterne's Avatar
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    I couldn't put it off any longer, so today I did most of the machining on the Monocoque Tube.... all the holes for the various bolts, plus the loading port and transfer port are done, plus the additional 10-32 tapped hole for the front of the Picatinny scope rail.... Here is the top view....



    Everything fit really well, all the bolts fit into the holes, but a couple were tight enough to require a touch with a file or Dremel bit to make them easy to assemble.... Careful deburring was the order of the day to make sure that the Breechblock didn't hang up when slid inside.... I was particularly proud of how well the loading port lined up.... Below is the bottom view, and you can see that the transfer port and mounting bolts also line up with the Spacer....



    All in all, I am very pleased.... The key to the accurate machining was using the 5/16" hole behind the loading port where the heads of the screws are that take the tension and compression loads of the barrel and tube.... Using a 5/16" pin in the chuck of the lathe through that hole, both for setup (measuring everything from that point), and for indexing 180* for the holes on the bottom, did the trick....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  7. #17
    Moderator rsterne's Avatar
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    Today I machined the top of the PRod trigger group to fit the 1-1/4" OD x 0.095" wall tube.... It normally fits a 7/8" OD x 0.065" wall tube, so one of the concerns is that the sear projects far enough into the thicker tube I am using to safely hold back the hammer.... I used the 1-1/4" diameter router bit I have to cut the larger radius, and at the point it barely touched the center of the recess I stopped and checked the seat engagement.... It was about 1/32", which although I feel was perfectly safe (after all, the hammer can't raise up by more than the clearance in the tube), I decided I wanted a bit more.... I noticed that the boss in the casting that holds the sear pivot pin was about 0.010" below where the router cut, so I mounted the trigger group back in the milling attachment and milled down until I just touched the edge of that.... The arrow in the photo below points to that shiny mark, right beside the sear.....



    With the trigger group machined to that depth, there is over 1 mm of sear engagement (I'll get an exact measurement after I cut the slot in the tube), and I'm happy with that.... One other thing that is important is that the slot in the tube to clear the sear must be slightly more than the full length of the sear.... The parts of the sear in front and behind the actual sear now project slightly above the bottom of the tube recess in the trigger group, so if the slot is too short, it would affect the engagement depth, and in fact could even prevent the sear from cocking properly.... All the other parts (the trigger and the intermediate link) are still well below the tube and do not interfere....

    The PRod trigger is very strongly built, and I see no problem with it easily withstanding way more than the 20ish lb. maximum load I will be asking it to hold....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  8. #18
    Moderator rsterne's Avatar
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    Today I made the front half of the Valve, and cut the O-ring groove in the Fill Fitting while I was set up for that....



    You will notice that I am using a conical valve spring to eliminate any restriction on the inlet side of the valve poppet.... I hope to work on the back half of the Valve tomorrow....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  9. #19
    Moderator rsterne's Avatar
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    I worked on the Valve today, and completed the machining on the body....



    While the rear half was in my square Collet holder so that I could index it 90* to drill and tap the four 1/4"-28 mounting holes, I also drilled the port and the two 8-32 screw holes beside that which hold the two tubes together at the front of the Breechblock.... In addition, I screwed the valve front into place and drilled and tapped 1/8"-NPT for the gauge which mounts on the left side.... I was extremely pleased with how well everything lined up.... Here is a photo of the assembly (without the tubes)....



    The two long 8-32 screws which hold everything together dropped right through the holes and screwed in with my fingers.... I still have to make the poppet, and install an O-ring seal for the stem in the back of the valve, but this is the last major internal part.... It won't be long I will be able to machine the lower tube to hold the valve, hammer, and fill fitting.... Unfortunately, I am still waiting for the 1/4" long 1/4"-28 SHCSs which hold the valve and fill fitting in place against the air pressure, so I won't be able to pressure test it until they arrive....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  10. #20
    Moderator rsterne's Avatar
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    I made the rest of the valve parts today.... Here is the finished poppet and stem seal....



    The poppet is made from 3/8" diameter PEEK, and I threaded the end of the 1/8" drill rod stem to 5-40 thread, drilled the PEEK with a #38 drill (tap size for 5-40) and counterbored it 0.10" to 1/8" and then just wound the stem into the PEEK, letting it cut its own threads until it stopped.... It was so solid I couldn't back it out.... I then cut the head to length and turned it for the spring seat right on the stem.... It sealed first time.... The stem seal is a #006 70D O-ring sitting in a 1/4" diameter recess in the valve body, held in place with a short piece of 7/16" bolt faced off flat on both ends and a screwdriver slot ground in the end for installation.... It has only enough drag to barely hold the weight of the valve body, so considering the forces on this valve shouldn't even be noticed.... The inside of this valve is huge....



    The throat (visible in the bottom of the rear section on the left) is 5/16" ID, so the seat margin is only 0.030".... The valve is drilled 11/16" ID, leaving a 5/32" gap all the way around the poppet head (4 times the throat area).... The front of the valve is drilled 13/32", the same as the front ID of the conical valve spring, and so has twice the area of the throat, allowing the air to enter the valve from the reservoir much easier than it can get out.... I don't think there should be any problem with this valve flowing air....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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