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The Garage / Converting an Atlas 12" x 36" Lathe for Metric Threading
« Last post by rsterne on Today at 02:30:38 PM »
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 the half nuts.... 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.... NOTE make sure you unplug the lathe from the wall before inserting the handle for safety....

I know it is counter-intuitive to use a power tool by hand.... but when threading up to a shoulder, or other critical work.... it sure reduces the pucker-factor.... The crank is pretty simple, it grips the inside of the spindle bore with a mechanism similar to a bicycle seat post.... You tighten the bolt with a 9/16" socket through the center of the chuck, before inserting your work....

Once again, I can't take credit for this idea.... This one came from Lloyd Sikes, and many thanks to him, I have used it a LOT....

I was just thinking (probably a bad idea...), what if you could refill the CO2 cylinders with helium? At the same pressure of course.

One other thing. I wonder if anyone has ever pressure tested any CO2 cylinders?
Well executed bob .. i like it !
Welcome to the Rabbit hole.  Beware of the Monkey!

Michael :- )
General Airgun Forum Discussion / Re: Happy Thanksgiving!
« Last post by int3man on Today at 09:13:19 AM »
Gobble Gobble, Happy Thanksgiving one and all!

Michael :- )
Ask Bob / Re: Tuning a Regulated PCP
« Last post by charmin99 on November 23, 2017, 05:37:49 PM »
Thanks Bob,

Just wanted to check my thinking was correct and yes the 765 fps is the fastest.

I will have a think about it after shooting off a few more pellets (200 so far for just for this..) and see how it behaves, scope and everything set up for current setting now and as you well know this is an iterative job and personally I find you've had enough fiddling after a while... haha.

I can well live with 60 ish shots but as I said the flatness is key for me, just that over here in the UK, wind and rain on a Sunday morning shooting you can do with that last few fps...

If I do it I will let you know how it went, but can I just say many thanks, I have learnt an awful lot, yours is probably the best and most concise, relevant info I have found to date.

Many thanks

Ask Bob / Re: Tuning a Regulated PCP
« Last post by rsterne 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....

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....

Ask Bob / Re: Tuning a Regulated PCP
« Last post by charmin99 on November 23, 2017, 03:55:26 PM »
Hello Bob,

Last question..

OK, so I have found the knee by trying various preload and dwell settings on the hammer and spring changing (costly..) and some calculation, also opened up the transfer port to optimise available setpoint pressure.

I have plotted it out and as your example it is pretty flat with a 2ft/sec variation with an amazing flat shot string of 80 then quickly falling off below at what would be the setpoint pressure.
Set point 125 bar, cylinder fill 205 bar.

The original un-regulated setup gave quite a power curve with only 38 useable shots and chronoed at 11.6 Ft/lb - 786 Ft/sec with JSB Exact 15.44

Now I have 11.0 Ft/lb 765 Ft/sec with same pellets but 80 string shots flat as a pancake...

The question, as the variation is now very good and more likely due to chrono variation I am quite happy with the result as I shoot HFT I want repeatability, however if I wanted to increase the velocity up, to say 11.6Ft/lb (785Ft/sec) and re-adjust the setpoint up, now the balance between the firing valve, striker and transfer port are set, if I just adjust the setpoint up marginally to the 11.6Ft/lb I guess I will loose a few flat shot, loose  some of the efficiency and sacrifice that for a bit higher power, is that adjustment to the setpoint now going to be a linear adjustment or close to it or would I have to start all again, which I couldnt face to be honest.

Or as you say, "enjoy what you've got.."



General Airgun Forum Discussion / Re: Happy Thanksgiving!
« Last post by Alan on November 23, 2017, 11:54:34 AM »
You Canadians are always ahead of the game! That's why you miss out! Well, anyway, you have a boxing day so you're ahead of us yet.
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