On 2008-07-11, John Husvar wrote:

[ ... ]

Now from someone nearly totally clueless about threading:

How does one cut a left-hand thread?

I've somehow got the idea that if you cut from right to left, you get a
right-hand thread.

If you cut from left to right with the machine turning the same
direction, do you get a left-hand thread?

Yes. Ideally you want to move the compound from the position of
29.5 degrees crank towards tailstock to the same angle towards headstock
instead so you can follow the flank of the trailing edge of the cut,
only making a very shallow cut there for each pass while making a deeper
cut on the leading edge.

And you don't do the full depth in a single pass -- unless it is
a very fine and shallow thread -- say something like 80 TPI or finer.

And also -- aside from setting the gearbox (or changing the
gears, depending on the lathe involved), you also want to use the
half-nuts to engage carriage motion -- not the power feed which works
through the handwheel. (Of course, some lathes don't have the power
feed, only the half nuts, so the question becomes moot.)

The only experience I have with a lathe is turning to a diameter. My kid
knows more than I. He made nice signaling cannon during a summer work
experience program, tapered barrel and all, but has forgotten threading
procedures since it was 20 years ago.

The main thing for the first threads is to make sure that you've
got plenty of space for reaction time to stop the cut -- don't make your
first threads cut to a shoulder. :-)

Easiest is if you have a reduced diameter both before and after
the area to be threaded. If you don't have that, cut a runout groove to
the depth of the thread at the end of your thread. The needed width
will vary with the coarseness of the thread, the speed of the spindle,
and your reaction time. Before you actually cut the thread, back the
tool out far enough so it will clear the increased diameter of the
workpiece at the end of the thread, and try some practice runs -- using
the dial to determine when to engage the half nuts, and seeing how
consistent you can be as to where you disengage the half nuts. That
will give you an idea how wide the runout groove needs to be for this
thread. Coarser threads move the carriage faster and require faster
reaction times. Really fine threads give you a lot more leeway in
reaction time.

Enjoy,
DoN.

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