.257 cal Monocoque PCP

[rsterne]

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

[rsterne]

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

[rsterne]

Made up a mount for the Scope Rail to hold it for machining to cut the radius on the bottom and put in 40 MOA of angle.... This was a time consuming and nerve-wracking job.... Here is the results....



You will notice that the height of the lower portion of the rail is less at the front (left) than at the back.... The difference in the 6" long rail is just under 0.070" which works out to 40 MOA and should put me pretty close to the optical center of the scope at 200 yards.... I had to add one more mounting screw at the back....



I machined the taper into the bottom using a 1-1/4" diameter carbide router bit I got from China for less than $15 including postage.... It cuts very smoothly, and the radius fits the tube perfectly.... Setting the scope rail up so that I could machine it was tricky, though.... I had to bolt it to a backing strip of aluminum (1 x 1.5") with screws in 2 of the 3 holes and move the screws along so that I could machine the entire length.... I was JUST able to manage it without having to move the jig in the milling attachment....

Bob

[rsterne]

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

[rsterne]

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

[rsterne]

This is one of those days you almost dread, and then feel great when it's over..... provided everything works the way it should.... The spacer between the two tubes is 10" long and concave on top and bottom to fit the 1-1/4" diameter tubes, and keep them parallel and 1/4" apart.... It could have been made from a piece of 3/4" x 1" aluminum bar stock, but the only piece I had long enough was 1-1/2" wide.... so the first thing I had to do was saw it lengthwise and then mill it to 1" wide.... Since it was 10" long and I can only machine a bit over 6" of length, that required two setups, and they were critical to match up parallel and exactly 1" wide.... Fortunately, that setup of the vice on the milling attachment on my lathe was also necessary for milling both sides to a radius using the 1-1/4" diameter router bit I just bought.... but again it meant two setups per side, critical that the part be square to the machine in both axis to end up with the same thickness at the center of the web from end to end, to end up with the tubes parallel in height.... and also with the grooves centered in the width of the part, to make sure the tubes end up parallel side to side.... I'm pleased to say that it went very well.... Here is the finished blank....



It fits the tubes perfectly, in fact better than previous breeches where I have machined the bottom with a ball-end mill.... I would definitely use a carbide router bit for this job again, especially with them being a fraction of the cost of a high-speed steel ball-end mill.... It needs to be run fairly fast, with not too deep a cut.... I was running 1270 RPM and removing 0.020" per pass once the full width of the router was engaged.... and using the power cross-feed, that was smoother than going by hand.... Including shallower finishing passes, that meant for about 15-17 passes per setup, times 6 (including milling the width to 1"), so about 100 passes.... It took me 5 hours to produce the piece you see above.... There is a bit more machining to do to this part, it has to be drilled for the transfer port and the three bolts that fasten the upper and lower tubes together.... plus a recess in one end to hold the back of the Forestock in place.... However, that won't happen until the tubes are finished and this will be machined to match.... Besides, I've had enough for one day.... It's time to have a drink and relax.... It's been a very productive few days, and it's time for a break....

Bob

[rsterne]

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

[rsterne]

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

[rsterne]

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