"Wes" wrote in message
...
"Ed Huntress" wrote:


"Wes" wrote in message
...
"Ed Huntress" wrote:

That's the question I'm asking. And that's what I did. But that's not
what
Bob is saying -- and what Bob is saying is the same thing said by every
source I've ever seen.

Plunge straight in, they say. Why?, I ask. It nearly doubles the chip
load,
and I can't see a good reason for it. There probably is one, but I don't
get
it.

I'll wait to have it explained.

I've floated this question a couple of times before. I never got a solid
answer, just possibilities.

I've think I remember you asking before.



Now if we were using cnc's we could get creative in how
we cut the profile. I seem to remember reading in the Sandvik book a
technique for
alternating between both flanks to reduce cutting forces.

Google is my bud.

D. Alternating flank infeed

This method alternately feeds the insert along both thread flanks, and
therefore it uses
both flanks of the insert to form the thread. The method delivers longer
tool life because
both sides of the insert nose are used. However, the method also can
result in chip flow
problems that can affect surface finish and tool life. This method is
usually only used
for very large pitches and for such thread forms as Acme and Trapeze.

http://www.cncinformation.com/cnc-wo...ing-on-a-lathe

This wouldn't work very well using engine lathes with screws with
backlash
which is about
all of them, mine for sure.

That's interesting, but, as you say, it isn't what the original
instruction
was about. We need someone who's even older than me to answer it. Most
such
people are dead. d8-)

There seems to be a problem with people winking out that have the old
skills. I'm reading
one of Tubal Cains books when I visit the reading room.

I wonder if John from amdinc has experience cutting acme? He repairs a
lot of big
machinery.

Wes

I can't think of who John is.

There are two old machining books handy, and I just looked in my 1940
edition of _American Machinist's Handbook_. Nothing there. I also looked in
Colvin and Stanley, _Turning and Boring Practice_ (1943). Nothing there,
either.

There are a few interesting bits about single-point turning of threads,
however, one of which relates to what you said above about CNC turning.
Colvin mentions a method that sets the compound perpendicular to the cross
slide, and then using the compound to cut one flank, and then run back to
cut the other. There is no detail; it sounds like it requires some careful
thought. The finishing pass(es) are taken by plunging straight in. He says
this is "said to be a common method used in English shops."

Apparently plunging straight in was the more common method in production,
even for conventional 60-degree thread forms. It's also interesting that
Acme threads derived from a group of miscellaneous flat-bottom threads known
as "******* threads." The 29-degree angle, says Colvin, probably was chosen
because Brown & Sharpe published a simple method for laying out cutting
tools with 29-degree angles.

Some time I'll dig into my other books to see what they say about cutting
Acmes. I have four or five others from that era.

--
Ed Huntress