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Messages - rsterne

Pages: [1] 2 3 ... 81
1
I have a 40ish year old Atlas 12 x 36 Lathe with the Quick Change Gear Box, and of course it only does Imperial threads, from 4 to 120 TPI.... A couple of weeks ago, I found out on the Internet that by replacing two gears you can do all the common Metric threads from 0.5 mm to 6 mm pitch, and keep the Imperial threading capability from 8 to 120 TPI, all you lose is the coarse threads, which I never need.... I ordered the gears through Amazon, and for about $100 got the 44T and 52T change gears I needed to perform this conversion.... They are Boston Gears # GB44B and GB52B.... direct replacements for the Atlas change gears....



Here is the original setup.... The gears are covered in a very sticky black grease called "Keystone No. 29".... which is the most tenacious grease, made for open gears, you can imagine.... Get it on your hands, and everything you touch turns black.... The good thing about this stuff is that it stays put, and doesn't need replacing, I have never touched the grease in 40 years, all I do is the regular oiling of the pivot points and shafts....



Note the twinned 48T idlers, it is those we will replace.... Above and to the left of them is a "sliding gear" (shown in the usual "OUT" position) which is engaged with a 16T driving gear activated by the tumbler gears for forward-neutral-reverse of the lead screw.... That 16T gear is twinned with a 32T behind it (with a guard between), both rotating at the same speed, on a double-keyed hub, just like the 48T idlers, which also rotate together.... This setup does all the Imperial threads from 8 to 120 TPI.... If you move the slider to the "IN" position, to engage the 32T driving gear, the lead screw turns twice as fast, giving the ability to do threads from 4 to 7.5 TPI.... It is those coarse threads I will be giving up.... The first step is to drop the sliding gear down and move it out of the way and remove the twin 48T idlers and the shaft they rotate on....



I slid the double-keyed hub part way out of the gears so that you can see it.... It causes the two gears to rotate together on the flanged bushing below the gears, which is held in place by the adjusting bolt and nut.... The bolt slides in a slot to allow the mesh between the idler and the input gears of the QCGB to be adjusted.... The thick washer goes on the outside of the gears, and when the nut is tightened the flanged bushing is held in place, and the gears, on that double-keyed hub, rotate on it.... I smeared a thin film of the #29 grease on the sides of the new gears, lubed the double-keyed hub inside and out with Moly grease, and slid it into place, then lubed the flanged bushing with Moly grease and slid it inside the hub with the flange on the bushing against the inside of the 44T gear.... This places the 44T on the inside, acting as a spacer for the 52T gear, but they will rotate at the same speed, locked together by the double-keyed hub.... The thick washer goes on the outside, under the nut, with a dab of Moly grease on the inside surface....



I reinstalled the idler assembly by sliding the bolt head into the adjusting slot, and slid it over so that the new 52T idler gear was in mesh with the input gears of the QCGB.... It is important to adjust the mesh of the gears so that there is just a bit of "backlash" between then.... not tight, but not too loose either.... just a bit of rattle between them is the correct setting.... Once you have the correct mesh, then tighten up the nut on the shaft to hold the flanged bushing in place.... The gears, on their double-keyed hub, will rotate on that bushing.... Note that in the photos below I haven't yet gooped up the teeth of the gears with the Keystone #29, I brushed that on after adjusting the mesh and taking the photos.... I didn't want that sticky mess all over my camera....



The next step is to slide the hub for the sliding gear over towards the idlers and adjust the mesh between the slider and the 52T idler and retighten the nut to hold the slider in place.... You then swing the sliding gear adjusting assembly upwards to engage with the 16T drive gear, check for proper mesh, and tighten the hand lever to hold it in place.... With the gears adjusted in this way, you are set up for Imperial threading, exactly the same as before, and the TPI chart on the front of the QCGB is correct for all threads from 8 to 120 TPI, and the 9 feed rates.... The fact that you changed from a 48T idler to a 52T idler makes NO difference to the gear ratios.... In a spur gear train, only the size of the first and last gear matter, the others only change the direction.... NOTE that in this position, the new 44T gear is not in mesh with any others.... it is acting only as a spacer for the 52T idler gear....

To change over for Metric threading, all that is required is to move the sliding gear to the "IN" position, and readjust both gear meshes....



In the "IN" position, the sliding gear will engage with the new 44T idler, which is smaller, so you will have to loosen the nut on the shaft of the sliding gear and move it in towards the idler.... NOTE this is different from the original setup where both idlers were 48T, so you didn't have to readjust the position of the slider relative to the idler.... Adjust the mesh, tighten the nut to hold the slider in place on the lever, and swing the lever up to engage the slider with the 32T drive gear (instead of the 16T).... Adjust the mesh, and tighten the hand lever.... The lathe is now setup for Metric threading, and you can no longer use the threading chart on the QCGB.... you need the one below....



The upper part of the chart, in black, are the Metric thread pitches (in mm) now available.... There are a few lesser used Metric pitches which you can do while in the Imperial mode, and those are show in red.... They aren't perfect, but are close enough (within 0.8%) unless you are making a very long thread.... You can approximate 1.6 mm, 0.8 mm, 0.7 mm, 0.45 mm, 0.4 mm and 0.35 mm pitches by using the 16, 32, 36, 56, 64 and 72 TPI settings respectively.... Those were available before, you probably just didn't know it....

So how does this work?.... Well, it speeds up the lead screw.... Using the inner 32T drive gear doubles it to start with.... The key is the two new gears, pinned together on that double keyed hub, so that they rotate at the same speed.... When you move the slider to the "IN" position, and engage it with the new 44T idler, it turns the 52T gear at the same speed.... This speeds up the input gear for the QCGB by 52 / 44, which is 1.181818 times.... then multiply by 2, and you get 2.363636 times as fast for the lead screw as in Imperial mode.... If you select the 60 TPI position in the QCGB, you end up with 60 / 2.363636 = 25.385 TPI.... almost exactly a 1 mm pitch.... When you try and cut a 1 mm pitch thread, you are actually cutting one that is 1.0006 mm pitch.... In 10 threads, the total error is only 0.0002".... ie negligible....

I hope any of you that have an old Atlas 12" with the Quick Change Gear Box may find this "How-To" article useful.... I can't take credit for it, but it sure works well.... One other thing to be aware of is that your threading dial is now pretty much worthless, since it is intended for Imperial threading.... About the only way you can use it would be to always use the number 1 position, and ALSO make sure that your carriage is in the SAME position when you engage it.... If you have a reversing motor, you can leave the half-nuts engaged all the time, and reverse out of the thread, of course.... The other alternative is to leave the half-nuts engaged and turn the spindle by hand with a hand crank, like this....



Bob

2
Ask Bob / Re: Tuning a Regulated PCP
« on: November 23, 2017, 04:56:09 PM »
So is the 765 fps the fastest you can get at your current setpoint, regardless of hammer preload?.... If that is the case, then yes, you need to increase the setpoint slightly to get closer to 12 FPE.... The increase in FPE should, in theory, be proportional to pressure, so if you are at 125 bar now, then 130 bar should give 11.44 FPE and 135 bar about 11.88 FPE.... Yes, you will lose some shots, both because the efficiency will be slightly less (more power) and also you will have less pressure drop from full to setpoint.... Currently you have 205-125 = 80 bar of air usable, with 135 bar you would only have 205-135 = 70 bar.... At best, your shot count should drop to 80 x 70 / 80 = 70 shots.... but probably more like 65 shots at 11.9 FPE, maybe a bit less....

or, as you say, be happy with what you have now....

Bob

3
Today I drilled and tapped the 16 mm x 1.5 mm holes for the cylinders, and cut the O-ring grooves.... The most difficult part was centering each hole in my 4-jaw chuck, with it being such an awkward shape.... If I had it to do over, I would have left the block rectangular until after this operation.... Once completed, this allowed me to thread in the tanks for the first time, and they fit great, just a few thou apart.... I laid a 2240 tube on the top for this photo....



You can see how nicely the cylinders nestle up against the tube, the bottom of the cylinders is only 1-1/8" below the bottom of the tube.... Here is a photo of the back of the block, showing the threaded holes.... You can see the O-ring in place in the left hand tank hole....



The plugs for the air passages are in place.... The left hand one is tightened, the right hand one in the photo is backed off so that you can see the O-ring that seals them.... When I drilled the holes in the end of the cylinders to enlarge them, I remembered that since the male Foster is inline with that hole, and there is a check valve in the Foster that is 1/4" in diameter, I had to drill the holes in the cylinders small enough that it would not fall inside.... so I made them 7/32".... This is still way larger than the air passages in the block, which are just over 5/32", so they will offer no resistance to the flow.... The photo below shows the enlarged hole in the end of the cylinders, one of which is installed in the block....



I weighed the assembly, which is now complete except for the stub that inserts into the front of the QB tube (replacing the stock one).... The two tanks, block and plugs weigh 1 lb. 14 oz.... While that is heavier than an aluminum bottle, I don't feel it is excessive.... particularly when the purpose is for shooting from a bench....

Bob

4
General Airgun Forum Discussion / Re: Happy Thanksgiving!
« on: November 23, 2017, 09:41:52 AM »
You guys are 6 weeks overdue!.... Have a great one!....

Bob

5
General Airgun Forum Discussion / Re: Gauge Revisit
« on: November 22, 2017, 11:05:25 AM »
Very nice!.... I assume 1/8" NPT back feed?.... What is the OD?....

Bob

6
There have been many discussions about downrange ballistics, and since Helium has come on the scene, we now can play with velocities that were unheard of not that long ago.... I have done up some charts to compare what happens with a 100 gr. bullet with a BC of 0.200, compared to a 200 gr. bullet with a BC of 0.400.... The caliber is irrelevant, as long as it is constant, but because of the weights chosen, it might help if you think about a .308 cal powered by Helium for the higher velocities (or by air for the lower ones).... I used the G1 Ballistics Model, so think of these as flat-based Spitzers.... The BC is the important number, along with the muzzle velocity....

If you shoot a 100 gr. bullet at 1500 fps, that is 500 FPE, and if you double the bullet weight, about the best you might expect is about 1100 fps, which is 537 FPE.... Conversely, if you tune the gun to shoot a 200 gr. at 500 fps, you might hit around 700 fps with a 100 gr.... That is why the charts for the light bullet use a range of MVs from 700-1500 fps.... while the charts for the heavy bullet show a range of 500-1100 fps.... I am simply trying to get a valid comparison between a light bullet driven fast and a heavy bullet driven at just slightly higher FPE, just as you might achieve in reality.... Anyways, here are the charts for what is happening at 200 yards downrange in a 10 mph crosswind....



Note that even starting at 1500 fps, the bullet is subsonic at 200 yards (it has lost 450 fps).... and has lost just over half it's energy.... However, if you start it at only 1000 fps, it has only lost 150 fps, and retains over 70% of it's energy....



In the second chart, the bullet drops about twice as far starting at 1000 fps than it does at 1500 fps (80" instead of 40").... but it only has about 10" of drift instead of 15"....



By comparison, the heavy bullet (with it's superior BC) retains 79% of its energy starting at 1100 fps, and 86% starting at 900.... It has only lost about 120 fps when starting at 1100 fps, and only about 80 fps if launched at 900....



In the last chart, you will note that for a given muzzle velocity, the heavier bullet, with its superior BC, has a flatter trajectory compared to the lighter bullet in the second chart.... In fact, if you compare the heavy bullet at 950 fps to the light one at 1000 fps, they have the same drop.... and the heavy bullet has 5" of drift, compared to 10" for the light one....

My personal preference is to use whatever bullet your gun will shoot in the 900-1000 fps range.... I know that some prefer a lighter bullet moving faster, and that's fine too.... All I am trying to convey here is what happens if you take that into the Supersonic range, where the drag dominates bullet performance.... Yes, you can get a flatter trajectory, as after all, velocity is the dominant factor in determining that.... much moreso than the BC.... There are compromises to be made, however.... and hopefully the above will give you something to ponder to determine what those are....

Bob


7
An MRod or Disco style gauge.... hopefully about a 3000 psi scale....

Bob

8
This is a project I have been developing on the GTA for a while now.... Here is a rough drawing of the concept, which is to mount two empty 88 gr. CO2 cylinders under a QB78 main tube, reversed back towards the trigger group.... This will provide a nice wide base to the forearm for shooting off bags on a bench.... A pair of cylinders weigh 1 lb. 9 oz. and they each have an internal volume of 121 cc....



Well I finally got some shop time today, and I decided to work on the tank block to mount two 88gr. CO2 cylinders reversed under a QB78 main tube.... Using the previously posted drawing as a basis, I produced the following....



The upper hole is tapped 1/4" NPT for a threaded stub that will mount the block to the front of a QB78 main tube, retained using the original threaded CO2 cap.... You can see the two air passages crossing in the bottom of the hole.... They were drilled with a #21 drill (0.159"), which is the tap drill size for a 10-32 screw.... The total area of the two passages is equal to a 0.225" hole, so a similar size hole will be drilled through the 1/4" NPT stub to feed air into the QB78 tube.... Since that is larger than anything that would be used for the exhaust port, that means the entire 290+ cc's will be available to the valve....



There is an 11/32" hole drilled through in line with each 88 gr. cylinder, and that is tapped from the front 1/8" NPT for a gauge and a male Foster fill fitting.... The air passages will be plugged with low-profile 10-32 SHCSs, sealed with a #008 O-ring in the 5/16" recesses milled in the lower corners of the block.... At the moment the holes for the 88 gr. cylinders are only milled 1/2" diameter and 3/4" deep.... I will be centering each hole in a 4-jaw chuck in my lathe to machine the O-ring groove, and drill and tap the portion outboard of the O-ring to 16 mm x 1.5 mm to accept the cylinders....

The block weighs 5 oz., so the completed weight with the stub and the two cylinders should be just under 2 lbs.... The stock will have to be shortened a bit, so the total weight increase will be less than that.... I plan an operating pressure of 1800 psi maximum, in keeping with what CO2 can reach at 120*F, and because that is the MSWP of CO2 cylinders....

Bob

9
Airgun Ammo / Re: .308 cal 106 gr. Airgun Bullet at NOE
« on: November 19, 2017, 09:12:17 PM »
Spitzers are not necessarily much better for BC in the Subsonic range.... The point is very important when Supersonic where there is a shock wave surrounding the nose.... and you want the shape of the nose to be inside that shock wave.... Subsonic things work a lot differently....

Bob

10
Ask Bob / Re: Tuning a Regulated PCP
« on: November 19, 2017, 04:35:06 PM »
As you lower the pressure the plateau velocity will lower as well.... Only if you are below the knee of the curve will the velocity increase with reduced pressure.... If that happens then you can assume you are somewhere on the downslope.... ie more hammer strike would increase velocity....

Bob

11
Airgun Classifieds / Wanted QB78 Tube and Front Cap - Got It !
« on: November 19, 2017, 11:42:25 AM »
For an experimental project, I need a QB78 tube and the front cap that threads over it and retains the CO2 plug.... If you have one you wish to donate, please PM me....

Looks like I have one coming.... Thanks for looking....


Bob

12
Ask Bob / Re: Tuning a Regulated PCP
« on: November 19, 2017, 10:26:35 AM »
Without the ability to try different hammer spring preload settings, it is pretty tough to know where you are on the power curve.... If you are up on the plateau, then reducing preload will decrease air use without a loss in velocity.... If you are down on the downslope, then any reduction in preload will lose velocity....

Bob

13
Airgun Ammo / .225 cal 35 gr. RF & HP at NOE
« on: November 19, 2017, 08:54:52 AM »
New Group Buy at NOE for a 35 gr. GC bullet in .225 cal, and also a smooth sided 37 gr. version.... both with a Lyman HP pin available in one cavity.... http://noebulletmolds.com/smf/index.php/topic,2298.0.html

Bob


14
Airgun Ammo / Re: .357 cal 136 gr. BBT at NOE
« on: November 17, 2017, 11:15:46 AM »
Just a reminder that this is at last call.... http://noebulletmolds.com/smf/index.php/topic,1629.0.html

Bob

15
Airgun Ammo / Re: .308 cal 106 gr. Airgun Bullet at NOE
« on: November 14, 2017, 07:03:05 PM »
According to the Kolbe Twist Calculator, anything 26" or faster should work fine....

Bob

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