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....
Bob
