In article ,
Harold & Susan Vordos wrote:

"DoN. Nichols" wrote in message
...

[ ... ]

If I had a choice for this, I would probably use a Geometric Die
Head to do it. Make the first cut as far as the turret would feed, then
(after the head released), reposition the stock through the spindle, and
close the Geometric on the existing thread, perhaps an inch or two from
the end of the cut thread. Then switch spindle on again, and feed until
end of turret travel again.

If I were dealing with an Acme thread, or even a particularly
coarse thread, I would use a Geometric Die Head with the rough/finish
switch lever, and do two passes for each section.

Actually, I've never seen Geometric chasers for Acme threads,
but I could imagine them existing.

Like you, I've never seen the inserts for Acme, but I can't help but think
that using a die head for a thread of any length of substance would be a
mistake, not that chasing it wouldn't present its own problems. If you're
seen Acme taps, you know that they're considerably longer than a typical V
type tap, so I'd assume the chasers for a die head would have to have a
corresponding increase in length.

Agreed. I have (so far) *one* Acme tap, and would by preference
single-point the thread in a single setup. But with the task stipulated --
cutting a thread longer than the travel of the carriage -- the requirement
to pick up the previous part of the thread and continue with another
section of the thread -- it strikes me that a die head would close
nicely on the previously cut part of the thread to continue.

As for the longer thread cutting requirement for Acme --
consider that Geometric chasers are available either plain, or with a
projection to allow work closer to a shoulder, and that normally only
the first few threads do the cutting, allowing many re-sharpenings.
With a larger than normal Geometric head, it should be possible to pick
up enough extra length so it would at least duplicate the extra length
of the tap -- at the cost of many fewer resharpenings.

Die heads were suspect enough when I worked at Sperry that they were not
used. We single pointed all threads, some of which were really a tough job.
One of them was a long manganese bronze shaft, 5/8" diameter, center
distance, roughly 4" long, was cut 16 pitch, with the ends turned and ground
for bearings, plus another thread or two. There were various configurations
depending on the function. These went to the control system that fired the
missile.

I have and use a die head, but I've yet to see a thread that came from a die
head that rivaled one that was single pointed, chased, although I have seen
some quite pretty threads from die heads.

One of the benefits of the die heads is that they can cut a full
thread form -- and are intended to work from stock that is a few
thousandths oversize, so you can mike over the crests to get a good idea
whether they are oversized, undersized, or right on, without the need for
the more expensive thread pitch diameter micrometer -- or juggling the
thread wires and the tables or formulae to get the right measurement.
It does make setup a bit easier at least.

I normally use the Die heads when making batches of parts with
the turret. The typical thread that I am cutting is 5/8-27 (microphone
mount thread), and I am cutting it in 360 brass. Yes, I could
single-point the thread, but in a situation like that (cutting to a
shoulder, piece after piece for an afternoon and an evening), the
Geometric die head is the better choice for me. (It is also a *lot*
quicker than single pointing it, even with the rough/finish lever and
two passes.)

The other thread is a small metric -- for concertina endbox
screws. M2.5x0.45 -- again done with the turret. A second lathe (A
Taig, FWIW) is set up with a watchmaker's collet and a form tool to
crown the heads, and then the Nichols horizontal mill with a fixture for
holding twenty of them at a time in four rows for slitting with four
slitting saws.

Neither of these parts have a long enough engagement relative to
the diameter for drift of thread pitch to be a problem, so I don't know
how good that would be with the Geometric heads.

But non-production threads I normally do single-point.

Enjoy,
DoN.
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