On Wed, 23 Apr 2008 23:38:00 -0400, "Ed Huntress"
wrote:


"John Martin" wrote in message
...
On Apr 23, 11:07 am, "Ed Huntress" wrote:
"John Martin" wrote in message

...
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.

I have roughed out large acme threads using the compound and finished
them with a form tool. But my guess (for whatever it is worth) is that
the reason you usually plunge cut acme threads is because they are
easier to measure that way. If you grind the tool correctly you just
jam her right in there until the calipers just slip over root diameter
and you're right.

The 60 degree threads are easy to eyeball because when they get pretty
sharp they are almost done.

Remember that a lot of the procedures were developed when things just
weren't as accurate or easy to measure. One example: Once you chucked
up a piece of stock you never, never, never, took it out of the
machine until it was finished and parted off. It was just too damned
hard to get it back in the chuck running true.


Bruce-in-Bangkok
(correct email address for reply)