On 2008-07-10, Ignoramus16954 wrote:
I am trying to recall some 20 year old high school memories.

Were those high school memories here int eh US, or in Russia?
If the latter, then you were probably cutting metric threads, and the
machine may have been different enough to add to the confusion.

If I am
cutting a thread on a lathe, and the lathe is reversible, then I can
cut the thread one way, stop/reverse the lathe, return, set the cutter
deeper, and repeat, right?

I do not need to withdraw the cutter, move it back, find the proper
spot to restart, etc. Right or wrong?

Wrong -- because there is backlash in the leadscrew/half-nuts
connection, so it does not return along the same path that it took when
cutting.

And stop/reverse requires a very fast reverse if you are cutting
towards a shoulder.

Better to cut a runout groove to the depth of the thread, and
wide enough to allow for your reaction time with the half nuts, use that
to disengage the half nuts, crank the cross-slide back far enough to be
*sure* to clear the threads, and hand crank back to off the start end of
the thread, then wait for the threading dial to reach the right point
(what is the right point depends on the thread pitch being cut -- for
some pitches, any mark on the dial will do. For others, every numbered
mark. For others every *even* numbered mark, and for yet others only
the same mark as was used the first pass. (Where you are doing every
even mark -- if you start on an odd mark, it can be every odd mark
instead.) Check the manual for the lathe -- it will tell you which
selection of marks to use for a given thread. Basically, if the pitch
can be evenly divided by four you can use any mark on my lathe, but
yours may be different.

Oh yes -- also start with the cross slide set to 0 on the dial
and the cutting tool touching the workpiece, and with the compound set
to zero as well, and with an angle of 29.5 degrees for US/Metric
threads. Then for each cut -- you stop in the runout groove (using the
half nuts), leave the spindle running forwards, withdraw the cross slide
by one full turn (or two or more if necessary for a deep coarse thread),
crank back to the starting point, crank the cross-slide back in the same
number of turns, stopping on zero, and then advance the compound-slide
to set the depth of cut of the next pass, and re-engage the half-nuts
when the threading dial reaches the right point. There are stops made
for the cross-slide which allow you to simply crank back in until you
hit the stop each time which can save you a bit of time and fiddly work,
but you still feed the depth of cut using the compound -- at least for
US practice.

Enjoy,
DoN.

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