South Bend Lathe threading questions

I have some South Bend 16" Lathe threading questions. I have the book on running the lathe but still have some questions.
1) Once the halfnut is engaged, should the face on the threading dial stop turning? Mine does.
2) I was trying to cut 10 tpi but got 20. My tumblers are set correctly. I used the halfnut. I believe that for even threads I can engage the halfnut on any number. I did this also but got 20.
3) I have a micrometer stop to the left of my apron. If I set that to a position how does the autofeed or halfnuts know to disengage when it makes contact?
4) On my cross slide I replaced the bronze nut. But I am having a problem with the shim (gib?). When I go into the center it moves fine but when I retract the cross slide the shim is about 1/2 out the back and I can not move the cross slide back. I have an adjusting nut on the front but it only moves the shim in further and does not provide for a backing out of the shim. Perhaps there is a piece broken off the end of the shim where the screw sits.
Thanks Steve
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    I don't have significant experience with a South Bend, let alone a 16" one, but I'll try to answer your questions anyway.

    Yes. This is normal. The dial shows when your carriage is in the proper relationship to the leadscrew to close the half nuts. Once they are closed that relationship is locked in until you open them again, so the reading is constant.

    This suggests to me that someone has changed the gear train in the headstock to cut a thread not found on the gearbox. You'll need to compare the gears between the tumbler reversing gears and the input to the quick-change gearbox to see whether they are what is described as standard in the manual, or perhaps on a setup plate inside the gear cover. (There is no such plate in my Clausing gear cover, because the exiting banjo does not support anything other than the default geartrain. There is a separate plate for the metric gear setup, which includes a replacement banjo.)
    Aha! Looking at the old copy of _How to Run a Lathe_, I see that it is set up similar to mine (page 25 of edition 50 (1944)) has an image of the threading plate for the 16" South Bend). There are *three* things which determine the pitch.
1)    The tumbler setting. (It should be in the third slot, starting     from the left.)
2)    The position of the sliding gear sticking through a hole in     the gear cover on the left of the lathe. This makes a factor of     eight change in the thread pitch (e.g. from 10 TPI to 80 TPI).
3)    The "top lever" above the quick-change gearbox must be in the     center position. It gives 5 TPI when to the left, and 20 TPI     when to the right (as it appears you have yours.) Mine is a     paddle which is twisted to the right or left. Yours appears to     be a lever with a turned handle which has to tilt to the right     or left. You may have to rotate the chuck by hand to engage the     gears when you switch this, as with the others.
    If yours has a sliding gear (as this manual suggests), then it also probably does *not* have a banjo which can accommodate other gear setups.

    The purpose of the threading dial is to make sure that each time you close the half nuts, the tool follows exactly the same path. It can't really *change* the thread pitch, it can (if used wrong) only cause two (or more) sets of threads between each other -- all of the pitch which your gearing is set up for.

    In most lathes, it does *not* know. When I use mine for *turning*, I disengage the longitudinal feed before it touches, and use the handwheel to move the carriage until the carriage contacts the micrometer stop. Your lathe proably has a clutch, so it if is correctly adjusted, you can feed into the stop and it will *hold* the carriage from moving past that point (the clutch slipping the whole time), but I don't like depending on the clutch (and I don't actually have one).
    For *threading*, you are not going through the clutch, so threading up into contact with the micrometer stop is asking for disaster. Once it is in contact, you proably *can't* apply enough force to disengage the half-nuts, and something will break. Perhaps there is a shear pin somewhere in the leadscrew, or perhaps some teeth will break off the gear, or perhaps the leadscrew will break the end bearing and that will need to be replaced. In the case of my Clausing, what breaks is the teeth on a gear which now costs about $180.00. (Don't ask me how I know this. :-)
    *Some* machines have an automatic disengaging stop for threading, but they were much more expensive than the South Bend when new. I *think* that the Hardinge lathes have this feature.
    If you set up the micrometer stop when threading, *don't* clamp it down, so you don't wind up with a serious crash if it starts to touch the carriage while the half nuts are engaged.
    *Before* you start cutting the threads, turn a runout groove where the threads end, and as soon as your threading tool enters that groove, disengage the half nuts. The faster your spindle speed, and the coarser the thread, (and the slower your reactions), the wider this groove needs to be so you can be sure to disengage the half-nuts before you sail through the runout groove. (Of course, if you are threading a section of the workpiece which is larger in diameter than the workpiece on either end of the threads, the rest of the workpiece serves as its own runout groove so you don't have to cut one. And if you are threading up to a shoulder, you certainly need the runout groove, and you need to use as slow as spindle speed as you can get out of the machine -- at least until your reflexes are trained. (And, if you are tired, run it slower than you otherwise would.)

    This says that your dovetail is worn in the center of travel (where it spends most of its time), and therefore is loose. Any adjustment to the gibs to make that tight enough will be way too tight for the ends of travel.
    You need to re-machine the dovetails, and use a somewhat thicker gib, to get everything working right again.
    Best of luck,         DoN.
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Thanks for everyones help. I used a dial indicator on my apron to try and calculate all the combinations. First of all my lead screw is 6 tpi. And my gear box has only 2 below levers. Also I do not have a slide in gear option as my Heavy 10 does. My current stud gear is has 24 teeth. However there are 2 gears located on the external side of my gearbox. A 48T and a larger one that is in use. I can only assume that this 48T is the replacement for the Stud Gear for the lower tpi threads.
What I noticed was that combinations B-E work fine. But the A position "sometimes" works. I tried multiple times to calcuate the tpi and got both combinations of A and B or double A. What I think is happening is that my lever may be catching on B and not engaging A sometimes. Has anyone ever experienced this before?
The slot in the Gib is exactly what I need! Thanks
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This would be the dual-tumbler gearbox with both tumbler levers at the bottom. The older gearbox had one lever at the top and a single tumbler lever at the bottom.

This is correct. That other "larger" gear that is meshed with the idler gear should be 64T.

The gear at 'B' is slightly larger in diameter than the gear at 'A'. If there is enough wear on that left-side lever, it's possible that its gear is slightly meshing with the gear at 'B' instead of with the gear at 'A'. This would give you 20tpi instead of 10tpi.
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says...

Yes. The dial will be turning when the leadscrew is running and the halfnuts are open. Once you snap them closed the carriage will start to move, and the dial will stop turning.

If this is really happening then you need to investigate the geartrain further. Something may be amiss. Are you certain you are gaging the thread pitch correctly?

It won't. A SB lathe like that will not 'know' to disengage the halfnuts at all, you need to do this.

Most likely the gib is tapered, and has a screw to move it farther into the slide. The screw should be engaged in a slot in the gib so the same screw head that provides the force to move it in, will also prevent it from moving in too far - and allow you to withdraw the gib when you back the screw out. Sometimes the end of the gib that the outside of the screw bears on, snaps off.
Jim
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I didn't see anyone mention that the thread indicator needs to be engaged on the same mark for each pass to prevent doubling up the threads [10/20 tpi]. At least that's how my old Atlas works. Bugs
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