I learned this yesterday at work.

Instead of setting the compound rest on a lathe to 29º for single point
threading, I have been taught to set the compound to 90º (parallel to the
bed, z axis feed).

The tool is then advanced (incrementally of course) to the calculated root
depth of the thread (refer to the machinery's handbook or others). This will
not result in a usable thread. The crest of the thread should not be sharp
at this point. There should still be flats at the top.

Once this depth is achieved, the compound rest is advanced toward the
headstock to clean the forward flank of the thread, and then retracted to
clean the rear flank of the thread. These forward and rear cleaning cuts are
made incrementally and may require more than one pass (for each).

Using the three wire method to measure the threads, one can easily achieve a
tight-fitting thread.

This technique makes multistart threads very easy to produce. You do have to
watch out when doing the forward and rear flank cleaning cuts. Each "start"
must have the same forward cleaning cut depth and rear cleaning cut depth
(although the forward and rear depths do not have to be equal).

Another issue that must be observed is that a radius on the nose of the
threading tool will adjust the calculated depth of the root of the thread.
Usually not a big issue if your tool is essentially "sharp" as there is a
fair amount of clearance built into a female thread.

Does anyone else do it this way? The instructor who taught me is a very
knowledgeable German-educated millwright (different meaning in Germany,
apparently) and the first time I tried it I had far more success than using
the old 29º method.

Thought this might help someone.

Regards,

Robin