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Messages - rsterne

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1
Ask Bob / Re: Tank sizes
« on: February 18, 2018, 07:29:21 PM »
To convert from the inside "water volume", usually in cc or CI.... to the amount of air it will hold in CF.... you need to know the fill pressure (in bar).... If the tank is listed in cc, convert to CI by dividing by 16.4 (cc/CI).... If you know the pressure in psi, convert to bar by dividing by 14.5 (psi/bar)....

First multiply the water volume by the fill pressure, to give the number of CI or air inside the tank at that pressure.... Then divide by 1,728 to convert CI to CF....

For example if the inside volume is 144 cc, that is (144 / 16.4) = 8.78 CI.... If the fill pressure is 3,000 psi, that is (3,000 /14.5) = 207 bar.... When filled to 207 bar, that tank will hold (8.78 x 207) = 1,817 CI of air at 1 bar.... Converted to CF, that is (1,817 / 1,728) = 1.05 CF....

The above is according to Boyle's Law for ideal gasses, and works up to about 3,000 psi.... It gets a bit more complicated when the tank is filled to more than 3,000 psi because of the VanDerWaals effect.... If we use the above calculation for a standard 1 hour SCBA tank (which is 550 CI water volume) we get the following at 4500 psi (310 bar)....

(550 x 310) = 170,500 CI.... (170,500 / 1728) = 98.7 CF (according to Boyle's Law).... However, that tank only holds 88 CF of air at 4,500 psi.... about 11% less than you calculated.... Up to 3,000 psi, you can use the basic Boyle's calculation as I detailed it.... At 4,500 psi, you need to reduce that by 11% because of the VanDerWaals effect.... At 6,000 psi the effect is much greater, about 18% compared to at 3000 psi.... At 10,000 psi a tank only holds 64% as much air as the simplified calculations tell you.... Sorry, but there is no easy formula to use once the pressure goes over 3,000 psi....

Bob

2
I held it in a 5C collet mounted in a Hex block in my milling attachment in the lathe, oriented so that the beveled edge of the weight was a 90 deg. to the axis of the lathe.... and used a 1/16" end mill (actually a tiny router bit) in the chuck.... I advanced the work towards the chuck in 0.005" increments, making two passes for a depth of 0.010" for the marks.... The crossfeed of the carriage governed the length of the marks.... The six faces of the hex block gave me half the marks, then I rotated the weight 90 deg. in the hex block and did the other six.... Sounds easy, but it took me all afternoon to do the 24 index marks....

Bob

3
Here are the completed harmonic barrel tuners.... The muzzle of the barrel is about 3/8" inside the front of the mount, about where the outside threads start....



The scale is like a clock face, with the quarters having longer index marks.... with 12 o'clock being where the setscrew is.... Each mark equals 0.003" of movement on the tuning weight.... By starting with the weight against the shoulder at the front, you can record the position using "hours" for the turns and "minutes" for the index marks, so 9-1/3 turns I would record as 9:20.... and 15-3/4 turns as 15:45 etc.etc.... I'm really pleased with the way they turned out....

Bob


4
I worked on making the Harmonic Tuners today.... I got two of the brass weights made, and one of the mounts that screws onto the muzzle.... Here is what it looks like when assembled....



The threads are 15/16"-28.... I like 28 TPI for this job because 10 deg. of rotation is 0.001" of movement (1 "hour" on a clock face is 0.003", 1 turn is 0.036").... The brass weight is made from a piece of 1.5" bar stock 1" long, the setscrew presses on a short piece of Teflon rod which acts as a brake to prevent the weight from moving by itself.... The mount is made from a piece of 1" CRS.... There is a small collar of that diameter at the front, with a 3/16" hole drilled though it to allow it to be tightened onto the muzzle with a short piece of 3/16" bar stock.... The remainder is threaded, the 2" of threads giving over 1" of adjustment for the weight.... The center portion is threaded inside 1/2"-20 NF to thread onto the barrel, and the back is counterbored 0.80" ID for 1" of length to fit over the CF sleeve.... The front is drilled out 3/4" ID, it only projects 1/2" past the muzzle to allow the HPA from the shot to escape quickly without affecting the bullet.... and it also serves to protect the crown which is a simple 90 deg. angle....

The mount weighs a couple of ounces, and the movable brass weight weighs just over 5 oz. at the present time, although I may put chamfers on the corners for appearance after initial testing.... I'm really looking forward to trying this system, I know it is tremendously successful in rimfire benchrest, where it can drastically affect the group size....

Bob

5
Dave, the problem is that boattails make the bullet act like it is longer.... That's good for BC, but requires a faster twist for stability.... That 98 gr. BBT in theory needs about an 8" twist.... I designed a 90 gr. BBT, but no interest at NOE, so it has never even reached Group Buy status.... It's just a bit longer than Doug's 91 gr. and in theory needs a 9" twist... but I suspect it may well work in the 10" in this build.... I made the loading port 0.80" long to accommodate it....



The 78 gr. BBT is in Group Buy, but not enough guys so far....  http://noebulletmolds.com/smf/index.php/topic,1628.0.html

It would be nice to see that bullet made.... should be fine in an 11" twist, so the 10" is a no-brainer, at least in theory....

Bob

6
I got to work on the barrels today.... The first order of business was to machine the breech end to 1/2" OD for 1" of length so that it will fit into the receiver.... In addition, I had to machine down the OD of the carbon fibre tube slightly to fit inside the 25/32" forward section which is drilled to a depth of 2.5".... I did that with the end of the tube running on a live center in the tailstock of the lathe to insure concentricity between the ID and OD, and it worked out great.... Both the CF sleeve and the barrel slide perfectly into the receiver and can be rotated individually, showing they are concentric with each other and the holes in the receiver are as well.... This was a bit of a nervous time, as any inaccuracy there would have been hard to correct.... At the muzzle end I turned the barrel down to 1/2" as well and threaded it 1/2"-20 NF so that it can accept either a Hatsan Air Stripper (as a backup) or my intended Harmonic Tuner, which I have yet to make.... Here is a photo of the barrel ends, breech on top, muzzle below....



and here is a photo of them with the CF sleeve in place (but not bonded yet)....



You can see where I had to skin down the outside of the CF tube to fit the 25/32" hole in the receiver, just a few thou was all that was required.... The next job will be to cut the chambers in the barrels before bonding on the CF sleeves.... I think the reamer I made for my .257 Monocoque will work, but I will have to make a new chamber reamer for the 6 mm.... I received some samples from Nick at Nielsen Specialty Ammo in the mail today (perfect timing, thanks Nick), as I would like to make sure the chamber in the .257 will accept as many different bullets as possible.... Here is a photo of the selection of .257 bullets I now have to try....



Left to Right they are.... My 98 gr. BBT HP, the 91 gr. Noble from Nick and his 85 gr. rebated boattail HP, an 88 gr. RCBS 82301, an 82 gr. NOE 260-80 FN, and a 73 gr. Lyman 257420.... I can also cast HP versions of the 82301 and 257420, so I will have lots of bullets to choose from.... I don't expect the 98 gr. BBT (or the FN version of it) to work in the 10" twist of the TJs barrel I have.... so if the chamber has to be too short to fit all the other bullets for it to chamber, so be it.... but just looking at the position of the ogives in the above photo, I think I can cut a chamber that will work with all of them.... If anything, the bullet with the Ogive starting the furthest forward is Nick's swaged HPBT.... Here are the 6mm Bullets.... the TJs barrel I have is also 10" twist....



Left to Right they are.... 58 gr. Bowman HP from Arsenal, 59 gr. NOE 245-64 HP and 69 gr. NOE 245-74 HP.... I can also cast FN versions of all three bullets.... The weights of the FN versions are 61 gr., 63 gr. and 73 gr. respectively.... The 63 gr. NOE is a shortened, FB version of the 73 gr. beside it, with the GC shank removed.... I also have a mould for a 65 gr. shortened version of the NOE 260-80, done the same way, but that will be too light for the power of this .257 cal (I hope)....

Bob

7
Yes, many airguns need a slight recess in the back bottom of the vertical bolt slot so that the bolt doesn't pop up.... The only disadvantage of the J-slot is if you switch back and forth from a gun that doesn't have one, you can forget to pull back to clear the bolt nose from the barrel port.... and the result is a very weak shot....

Here are the receivers with everything now done except to machine the 20-25 MOA slope in the scope mounts and cut the dovetails in them....



On the lower front corner you can see the semi-circular recess milled to act as a clamp for the plastic pipe forestock.... It pinches the pipe between the bottom of the receiver and the main air tube, mounting it solidly at the back in the lowest possible position so that it doesn't rattle around, and clears the barrel.... The velocity adjusting screws are installed in the back, in line with the lower bolt slot.... The 8-32 SHCS pushes on a 1.5" long steel pin that can prevent the bolt handle from retracting fully in the slot.... I have about 10 turns of adjustment, the back receiver has the adjuster fully retracted (maximum velocity) and the one in front has the adjuster set to minimum velocity.... I labelled the steel pin which is sticking out into the lower bolt slot, which holds the bolt forward so that the nose obstructs the barrel port, reducing airflow (to almost zero when fully closed as shown).... You can't see it in these photos, but there is a vertical hole coming up from the bottom, in line with the bottom of the adjusting screw.... It is tapped for a 6-32 setscrew which presses on a 1/4" long piece of thin Teflon rod which acts as a brake to prevent the velocity adjusting screw from moving by itself....

Bob

8
Alan, the main reason for the "J-Slot" is to allow the bolt to be retracted completely out of the airflow to allow full bore-area porting throughout the airflow path.... There is an added benefit, which I have not 100% decided to take advantage of in these builds.... You can install a long pin with an adjusting screw at the back of the receiver, in line with the lower slot, to adjust how far the bolt face can be retracted.... This allows complete control over the velocity from maximum all the way down to about 200-300 fps (you can literally see the pellet in the air).... The further in you screw the adjuster, the smaller the barrel port....

Bob

9
I worked on the receivers more today.... I drilled and tapped the six holes for the set-screws that will secure the barrel.... The original plan was for 10-32 screws at 120 deg. to each other.... but I was unable to get them of the proper length here in town, but I was able to get 5 mm x 0.8 mm Metric setscrews that are 10 mm long.... You can enlarge 10-32 threads to M5 by simply running the tap through them, as they are the same pitch (within less than 1%) and the Metric screws are 0.008" larger than the 10-32s.... I dislike mixing SAE and Metric fasteners on a project, but the alternative would have been to wait weeks to get the 10-32s.... I justified it by the fact that the barrels are 14 mm and the CF sleeve is 20 mm OD.... so at least everything to do with the barrels is Metric....  ::)



I had one other little glitch when setting up to drill the lower holes for the setscrews.... In order for the receiver to clear the carriage on my lathe (I was using my milling attachment to drill and tap the holes accurately) I could not drill them 30 deg. below the horizontal, I had to use 35 deg.... This is of no practical importance, but means that the angle from the top to each side screw is 125 deg. and the angle between the bottom screws is 110 deg.... instead of them all being 120 deg....

I also set up and milled the slots for the bolt handle today.... The front receiver is finished, but dinner hit the table before I got the vertical part of the rear slot done.... I'll do that tomorrow before I set up to fly-cut the recess in the front bottom of the receivers to locate the plastic pipe of the forestock....

Bob

10
I got lots more done on the receivers today.... This reduced the weight of them quite a bit.... and vastly improves the appearance....



The top and upper edges are now milled down, leaving raised areas where the dovetails for the scope mounts will be.... I also bevelled the top corners as well.... When I machine the dovetails I will be angling them down 20-25 MOA at the front to put the barrel on POA at 100 yards with the scope still optically centered....

Bob

11
PCP, C02, and Helium Powered Airguns / Re: SS valve in the .22 Mrod
« on: February 09, 2018, 08:13:13 PM »
With that plateau, your maximum usable velocity would be 948 fps @ 2 turns preload.... so anything in that 1-2 turn area should be fine.... I'm a bit surprised that your small plenum is requiring that much pressure to get those velocities.... I guess the shorter barrel is part of the reason....

Bob

12
PCP, C02, and Helium Powered Airguns / Re: SS valve in the .22 Mrod
« on: February 09, 2018, 07:13:03 PM »
I am surprised that is all you are getting with those monster ports at that pressure.... Have you tried adjusting the preload and recording a curve to see where the plateau and the knee are?.... My 2260 HPA shoots 18 gr. JSBs at 960 fps on only 1600 psi with only 0.166 ports on a modded Disco valve.... 24" barrel not 20" though....

Bob

13
PCP, C02, and Helium Powered Airguns / Re: SS valve in the .22 Mrod
« on: February 09, 2018, 06:41:02 PM »
Unless I misunderstood, he is already over 80% of bore, it's a .22 cal running 3/16" ports (85%).... I wouldn't go any bigger if that's the case....

Bob

14
Airgun Ammo / Re: .30 cal Heavy Pellet from NOE - Buy is Closed
« on: February 09, 2018, 05:56:20 PM »
The moulds for this pellet are done and in the store.... http://noebulletmolds.com/NV/index.php?cPath=374_378_532

If you participated in the Group Buy, you should get your Discount Code shortly....

Bob

15
I have three 6 mm moulds, Dave.... the 60 gr. 6mm Bowman from Arsenal (4 cavity with 2 HollowPointed by Erik).... the 74 gr. NOE.... and the shortened 64 gr. version of that.... both with Lyman HP pins in one cavity....

Today I milled out the loading port area in the two receivers.... Instead of milling it straight across, I left the left-hand side of the receiver intact from the boreline over, so that I don't lose so much rigidity.... These guns are single-shot, so no need to cut it away more than that.... I did this on my Monocoque and it works well, but of course you must load the bullet from the right side.... Drilling a 1/2" hole straight through the receiver to assure the alignment of the bolt and barrel means that when you machine the loading port you don't have a tray to guide the bullet into the bore.... so that is made from a separate piece of 1/2" diameter steel.... I drilled one for 6 mm and the other for .257 before milling them to shape.... You can see one of the inserts sitting on top of the receiver in the foreground.... along with a flat-head 4-40 screw which holds it in place.... The back receiver already has one installed....



With the loading tray in place, the right side is level with the centerline of the bore.... and the left side is vertical, tangent to the left side of the bore.... You don't need to look at what you are doing to load, just place the bullet on the right hand lip of the tray, and roll it to the left and close the bolt....

Bob

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