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Messages - rsterne

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1
The pilot is 0.232" and slides easily into the lands (which I measured at 0.234").... It then steps down to 0.220" and then a taper of 1 deg. per side starts from there.... That continues until it reaches the diameter of the grooves 0.243" (about in the middle of the flutes) and then continues back parallel at that diameter to cut the chamber.... It then steps down again to 0.220" for enough length allow you to feed the reamer deep enough to cut the chamber plus about 1/4".... The shank is simply the 3/8" drill rod I started with.... It is long enough to bottom in the Jacob's chuck in my tailstock, so that it can't slide back.... Both chucks have to be very tight to prevent the barrel, or the reamer, from slipping in them.... as the loads are very high.... If you feed too fast, you can weld the reamer into the barrel, and break it.... Don't ask how I know that....  :-[

You can actually see where the tapered leade stops and the straight chamber starts because of the lighting (a total fluke, from the flash).... The lighter blue reflection is on the taper, and where it stops is where the angle changes (only 1 deg.).... Behind that the reamer is straight.... The only part that actually does any cutting is about the last 3/16-1/4" of the taper, which cuts a taper in the rifling to form the leade in the barrel.... Ahead of that (over 3/4 of the tapered part) is smaller than the lands and doesn't touch anything.... The parallel section rubs along the chamber and polishes off any high spots, leaving it parallel, and hopefully the right diameter that a sized bullet just slides in until the nose hits the tapered leade.... It is critical to stop feeding the reamer at the right depth so that the bullet is seated against some part of the leade.... Too short a chamber and it is hard to close the bolt.... and too long a chamber and the bullet will not be against the leade, and get a "running start" before it engages the rifling.... There us usually about a 1/8" range of length of bullet that will fit any given chamber properly....

Bob

2
Here is what the finished reamer looks like....



It was hardened, polished, and them tempered for 30 min. at 465*F, raising the temperature to 480*F for the last 5 minutes because it was a little too pale a straw colour.... The result is a nice brown, the way I like my reamers....

Bob

3
Ask Bob / Re: Tank sizes
« on: February 19, 2018, 07:21:36 PM »
I made an error with the Helium, it is corrected in blue in the post above....

One other thing that the VanDerWaals effect causes that I mentioned briefly above.... For each 500 psi increase in pressure above about 2500 psi, you put less air in the tank.... This means that if you are using a regulator to tether to a high pressure tank, and shooting at, say, 2000 psi.... as the tank pressure drops by 500 psi, you will get fewer shots at high pressure than at low.... Let's say that you get 100 shots from 2500 psi down to 2000 on an SCBA tank....  If the same tank is filled to 4500 psi, from 4500 down to 4000 you will only get about 80 shots.... If you are tethered to a 6000 psi tank of the same internal volume, from 6000 psi down to 5500 you will only get about 60 shots... The higher the pressure, the fewer the shots you will get from a 500 psi drop.... This also means that you won't get as many fills from a 6000 psi welding tank as you thought, particularly when the tank is full.... the pressure will drop very fast at the beginning compared to when it is at only "half full" at 3000 psi....

Bob

4
I made the 6mm reamer this afternoon.... still not hardened and tempered but it turned out dimensionally accurate....



It is made from O1 Drill Rod.... The next step is to heat it to cherry red and quench it in oil to harden it.... Then I will polish it, hone the cutting edge again, and then temper it in the oven.... tomorrow's jobs....

Bob

5
I cut the chamber in the .257 barrel today, using the reamer I made for my Monocoque....



Here are all the bullets that will fit.... They were all chambered to a depth of 0.60" from the end of the barrel, which will put the base just ahead of the front of the barrel port.... All bullets were sized to 0.257" before testing how they chambered.... When the chamber is too short, you sure know it, as you have to push hard enough to mark up the base, and it's pretty obvious when you get the length right....



I enhanced the contrast as much as possible so that you can see the rifling marks, but they are still hard to see on all except the longest two BBTs (112 gr. FN and 98 gr. HP), where they are clear on the back of the head.... The shortest bullet is the Lyman 257420 73 gr. and it barely touches the leade.... If you push it in by hand, you can only feel resistance for about the last 1/16", and basically cannot see any marks, even with a loupe, unless you push it in another 1/16", where you feel significant resistance.... I don't expect the two longest bullets to work in the 10" twist in this barrel.... but since I can chamber them I will be able to test that and prove or disprove the validity of the Kolbe Twist Calculator numbers for them.... Many of my BBTs seem to be stable in a slower twist than it gives, so there is a chance that the 98 gr. HP may work.... providing I have enough power to push it....

The other four bullets all slid in between 0.50-0.54" and then took a bit of a push (not much, mind you) to seat them to 0.60".... If you look carefully you can see shallow marks from the tapered leade, which they are sitting firmly against when chambered.... Those 4 bullets, from L to R, are the 91 gr. Noble, the 85 gr. NSA BTHP, the 88 gr. RCBS 82301, and the 82 gr. NOE 260-80 FN.... If I find one particular bullet shoots the best and want to play with different seating depths, I can do that by changing the length of the bolt nose....

Next step is to make the bolt, cut the O-ring groove and polish the chamber behind the barrel port.... then cut the CF sleeve to final length and glue it on.... and then machine the barrel port.... I was very pleased with the way the 3-flute reamer worked, so I am going to copy that design when I make the one for the 6mm barrel.... just smaller diameters for the pilot and chamber of course....

Bob

6
Ask Bob / Re: Tank sizes
« on: February 19, 2018, 08:37:54 AM »
Absolutely correct, Bob.... ALL the 1 hour size SCBA tanks hold 88 CF of air, not 98 (or 100) as advertised by some companies.... Likewise, the 45 min. tanks are 66 CF and the 30 min. are 44 CF.... The best way, if you are unsure, is to go by the internal volume (water capacity) of the tanks to compare them....

Incidently, this is why the pressure in your 4500 psi tank drops faster from 4500 to 3000 than from 3000 to 1500 psi.... and why a 6000 psi welding tank does not hold twice the air it would at 3000 psi.... Incidently, you can't put as many CF of Helium in a tank at the same pressure as you can with air (and the loss is linear).... Here is a graph showing how air, Nitrogen and Helium compare to an ideal gas....



Note that if you filled a 1 hour SCBA tank with Helium to 4500 psi it would only contain about 84 CF of Helium.... and Nitrogen would be about 86 CF....

Bob

7
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

8
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

9
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


10
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

11
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

12
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

13
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

14
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

15
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

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