.257 cal Monocoque PCP

[rsterne]

The bulk of the materials arrived yesterday, and I spent the day finalizing most of the design.... It doesn't look much like an airgun at the moment....



Plastic pipe is for the forestock and cheek piece.... 1 1/4" OD CrMoly tubing for the monocoque upper tube (0.065" wall) and air reservoir (0.095" wall).... The 1 1/8" 2024-T3 Aluminum round bar is for the breech block, valve, front plug, and all other internal mounts.... The rectangular aluminum barstock is for the external parts; the spacer between the tubes, the adjustable stock mounts, and the front bipod/stock mount....

Soon to be converted into a pile of chips, and usable parts, I hope....

Bob

[rsterne]

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

[rsterne]

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