"Jim Wilkins" wrote in message
...
On Oct 13, 1:13 am, "Harold and Susan Vordos" wrote:
...
Actually, it's so you can calculate undercut diameters, or to determine
the
configuration of a tool for a thread that would have to be machined to a
shoulder, allowing for enough depth of tool without excess width. ...
Harold

Thanks. I hadn't really looked at the C391 that closely.

The double depth table matches the numbers for root / minor diameter
on my threading table spreadsheet which were calculated as 2 * 0.6495/
Pitch, assuming a tool tip width of 0.125/Pitch, both from Machinery's
Handbook.

I usually leave the tip 'slightly rounded' and feed the compound in
0.75/Pitch from initial contact, which should be correct for the
proper tip width but is oversize with the finer tip, then finish to
fit or wire measurement.

There's nothing wrong with using the double depth as a guide, but to use it
for sizing threads makes no sense. The tolerance for the pitch diameter is
generally smaller than the tolerance for the minor, and that has relevance
only when the tool is properly ground for the given pitch. The flat that is
required on the tool tip gives cause for greater error than might be
imagined. It's all too easy to go too deep with the formula, especially
when you must take into account the starting pass, which, again, you're at
the mercy of the major diameter. The tip radius you speak of keeps you out
of trouble, but may well be out of tolerance for the geometry of a given
thread.

Theory for threads does not translate well into operation without gauging.
People that rely on compound movement for sizing threads are commonly known
for making scrap.

Please bear in mind, I'm speaking from the perspective of one that had to
satisfy independent, critical inspection. I also worked in places where
there
was no inspection, ever, and no scrap produced. Anything turned out was
assembled and shipped. sigh

Harold