"John Martin" wrote in message
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On Apr 23, 11:38 pm, "Ed Huntress" wrote:
"John Martin" wrote in message

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On Apr 23, 11:07 am, "Ed Huntress" wrote:





"John Martin" wrote in message

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On Apr 22, 2:04 am, "Ed Huntress" wrote:

When you cut threads by feeding straight in, you're cutting both
flanks
of
the thread at once. That produces twice the tool load for a given
depth
of
cut.

If you have a good lathe, your tools are sharp, and you're cutting
aluminum,
it may work just fine. But cutting clean internal threads depends on
getting
things pretty close to right, so the extra tool load is something to
watch
out for if you get some chatter.

--
Ed Huntress
Of more importance is that cutting on one flank loads the cutter
against the lead screw. Cutting both flanks at once, the carriage can
float instead on being driven by the leadscrew, and you can end up
with a drunken thread.
John Martin

Maybe you can answer something that's bugged me for a long time, John:
Why
do the traditional lathe books say you should advance the cut when
cutting
Acme threads by plunging straight in, rather than on the appropriate
angle
for cutting one flank at a time? I don't recall ever having seen an
explanation.

--
Ed Huntress- Hide quoted text -

- Show quoted text -
Some may say that, some don't.
Machine Tool Operation, Burghardt:
"5. The side of an Acme thread is 14 1/2 deg., therefore move the
compound rest 14 1/2 deg. to the right when cutting a screw, to the
left when boring an internal thread. Set the thread stop and feed for
successive cuts by moving the compound-rest handle."
Perhaps the audience has something to do with it. Most of the machine
tool books were written by and for guys using big, heavy lathes. The
heavier the carriage, the less likely it is to jump ahead due to soft
spots in the work or the alignment of the planets. With a lighter
lathe, and less friction and mass in the carriage, it becomes more a
factor.
John Martin

Maybe. I just never could see why they'd recommend the angled cut for 60
deg. threads, and then the straight-in cut for Acme. It seems to me that
the
Acme is going to produce a much heavier load on the tool.

I've only cut 10 tpi Acme threads (because I have only one Acme tap, and
it's 10 tpi), and it's not a big deal on my South Bend to cut those. But
bigger, coarser ones would be a real challenge.

--
Ed Huntress- Hide quoted text -

- Show quoted text -

Not at all sure who your "they" is, Ed. As I quoted, Burghardt - who
is certainly among the top few most widely read of the machine tool
writers - said to set the compound to the flank angle and feed with
it.

I don't remember which books, John. It may have been the Atlas or South Bend
lathe books. If I think of it I'll look for it and see what they say. It's
been years since I've looked.


Bruce may be on to something with regard to the measurements. For 60
degree V threads, with the compound set to 30 degrees, it's pretty
easy to figure the compound feed necessary to hit full thread depth -
it's the same as the pitch. No fussing with trig tables.

Again, though, it seems that not all the writers said the same thing.

John Martin

--
Ed Huntress