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
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.
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
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,
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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
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.
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.
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