Let's say I have a small lathe (length) and need to make a threaded screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in practice
can this be done with enough accuracy for something like a replacement lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded rods
from MSC, etc. - this is more of a "how to use my lathe" machining question.

Thanks.
Steve.

SteveF wrote:
.. in practice can this be done with enough accuracy for something
like a replacement lead screw for a lathe?

No.

"SteveF" wrote in message
. com...
Let's say I have a small lathe (length) and need to make a threaded screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in practice
can this be done with enough accuracy for something like a replacement
lead screw for a lathe? Yes, I am aware of parts on Ebay and long
threaded rods from MSC, etc. - this is more of a "how to use my lathe"
machining question.


Steve,
It's been about a quarter century since I've done this (CNC spoiled) but
I'll give it my best shot. First off set your compound slide at 30 degrees
and square up your tool and cut the first section of thread. In all
likelihood your lathe has a threading dial on it and you engage the feed at
the same number at every pass. If so, then what you need to do after you
rechuck the part is to engage the feed on the same number, feed up close to
a cut thread and shut the spindle off leaving the feed engaged. If your
lathe doesn't have a brake be careful as the carriage will continue to feed
as long as the spindle is rotating. Now use a combination of cross slide
travel and compound slide travel to nest the tool into the previously cut
thread. Note the numbers on your compound feed dial as they will be
different than your first thread section. Depending on how fussy you want to
be you can use an eye loupe or some Dykem, and rotate the spindle slightly
by hand to make sure the tool is lined up perfectly with the previously cut
section. Once you are satisfied that the tool is lined up with the thread,
back the tool out, disengage the feed, back the carriage up, start the
spindle and begin threading as normal by picking up the same number on the
threading dial.

HTH
Dan

"SteveF" wrote in message
. com...
Let's say I have a small lathe (length) and need to make a threaded screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in practice
can this be done with enough accuracy for something like a replacement
lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded
rods
from MSC, etc. - this is more of a "how to use my lathe" machining
question.

Thanks.
Steve.

Given the proper setup, it's possible, but would be a serious challenge,
especially if you are new at machining. You'd need a follow rest, possibly
a steady rest, be able to feed the material through the headstock, and be
able to pick up threads properly. Having considerable skill and experience
would be the most valuable asset, along with soft jaws. Depending on how
you pursued the project, you might be limited to length, but you could cut
one longer than the center length of the machine.

Harold

"Harold & Susan Vordos" wrote in message
...

"SteveF" wrote in message
. com...
Let's say I have a small lathe (length) and need to make a threaded
screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in
practice
can this be done with enough accuracy for something like a replacement
lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded
rods
from MSC, etc. - this is more of a "how to use my lathe" machining
question.

Thanks.
Steve.

Given the proper setup, it's possible, but would be a serious challenge,
especially if you are new at machining. You'd need a follow rest,
possibly
a steady rest, be able to feed the material through the headstock, and be
able to pick up threads properly. Having considerable skill and experience
would be the most valuable asset, along with soft jaws. Depending on how
you pursued the project, you might be limited to length, but you could
cut
one longer than the center length of the machine.

Harold

I should have mentioned that the end product would likely be of
questionable quality.

Harold

"Grant Erwin" wrote in message
...
SteveF wrote:
.. in practice can this be done with enough accuracy for something
like a replacement lead screw for a lathe?

No.

Hey! Not true, at all!

It's something of a chore to re-index a piece of threading work, but it's
not that bad. Takes patience and time, that's all.

Consider: Mark a thin scribe line down the length of the work to show the
rotational indexing position. Position your tool in the thread very near
the chuck, and lock the half-nuts. Then unchuck, and re-index the work in
rotation by the scribe, and longitudinally by leaving a small amount of the
already-threaded portion remaining beyond the chuck jaws, and using the
(never moved) bit to find the correct longitudinal position in the chuck.

Keep lost motion in mind, and snug the tool to the "cutting" face of the
thread (by moving the work to the tool) before each re-chuck.

LLoyd

In article , Grant Erwin says...

SteveF wrote:
.. in practice can this be done with enough accuracy for something
like a replacement lead screw for a lathe?

No.

LOL. The real long answer is, the task will be so time-consuming, and
the outcome so uncertain, that you are better off doing it another
way. Borrow a larger lathe. Purchase the lead screw and fit to
your machine. etc.

Short answer though is correct. Umm, not really. No. Also
you did not specify any kind of lead accuracy tolerance on your
requirement. A lot depends on that. If the tolerance is
"it's gotta be the right number of tpi," then the answer could
be yes.

If you need ten microinch lead tolerance over the entire thing,
then definitely no.

The truth lies somewhere in between.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

On Thu, 20 Jan 2005 05:16:33 GMT, SteveF wrote:
Let's say I have a small lathe (length) and need to make a threaded screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in practice
can this be done with enough accuracy for something like a replacement lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded rods
from MSC, etc. - this is more of a "how to use my lathe" machining question.

Can you use the lathe to turn the stock, and use a die to cut your
threads, or is this for a thread you don't have a die for? Novice question
on my own part as well.

"Dave Hinz" wrote in message
...
On Thu, 20 Jan 2005 05:16:33 GMT, SteveF wrote:
Let's say I have a small lathe (length) and need to make a threaded screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in practice
can this be done with enough accuracy for something like a replacement
lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded
rods
from MSC, etc. - this is more of a "how to use my lathe" machining
question.

Can you use the lathe to turn the stock, and use a die to cut your
threads, or is this for a thread you don't have a die for? Novice
question
on my own part as well.


Well, that gives me two more questions for the group.

One - Assuming some experience on the part of the operator in cutting
threads and a good lathe, how much less accurate is using a die than cutting
with the lathe?

Two - How could one measure the thread to thread distance accurately to see
how much error was created when reindexing after moving? Looks like thread
micrometers only measure the diameter.

Steve.

In article , Dave Hinz says...

Can you use the lathe to turn the stock, and use a die to cut your
threads,

A die won't give a precision lead to the part. Because this is
a leadscrew for the lathe, that's not the best way to make
the part.

Leadscrews not only have to have the correct thread geometry,
but they are effectively length standards in and of themselves.

A die-cut leadscrew would work, but it would impart all the
lead error into whatever parts were then fabricated when
using it on the lathe.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

Two - How could one measure the thread to thread distance accurately to
see
how much error was created when reindexing after moving? Looks like
thread
micrometers only measure the diameter.


Make a rig with two nuts, possibly (probably?) each made with some way of
getting rid of play/backlash. Fix them so they don't rotate but allow some
motion along the length of the screw. Start with them both on one side of
your "join", measure the distance between (marks on) them, then move them so
one is past the join and measure again.

On Thu, 20 Jan 2005 17:05:01 GMT, SteveF wrote:

"Dave Hinz" wrote in message
...

Can you use the lathe to turn the stock, and use a die to cut your
threads, or is this for a thread you don't have a die for? Novice
question on my own part as well.

Well, that gives me two more questions for the group.
One - Assuming some experience on the part of the operator in cutting
threads and a good lathe, how much less accurate is using a die than cutting
with the lathe?

It would depend on the quality of the die, compared to the accuracy
of re-aligning the stock for the later cut(s). I'd like to think
that a good die is going to give you the quality you'd need?

Two - How could one measure the thread to thread distance accurately to see
how much error was created when reindexing after moving? Looks like thread
micrometers only measure the diameter.

I'm interested in the answers you'll get to this. What kind of
accuracy are you looking for?

On 20 Jan 2005 09:14:31 -0800, jim rozen wrote:
In article , Dave Hinz says...

Can you use the lathe to turn the stock, and use a die to cut your
threads,

A die won't give a precision lead to the part. Because this is
a leadscrew for the lathe, that's not the best way to make
the part.

Ah, I missed the part about it being a lead screw for a lathe.

Leadscrews not only have to have the correct thread geometry,
but they are effectively length standards in and of themselves.
A die-cut leadscrew would work, but it would impart all the
lead error into whatever parts were then fabricated when
using it on the lathe.

So...how does one cut a clean leadscrew when one doesn't have
a clean leadscrew to start from? Someone made the first one...

"Dave Hinz" wrote in message
...
On Thu, 20 Jan 2005 17:05:01 GMT, SteveF wrote:

"Dave Hinz" wrote in message
...

Can you use the lathe to turn the stock, and use a die to cut your
threads, or is this for a thread you don't have a die for? Novice
question on my own part as well.

Well, that gives me two more questions for the group.
One - Assuming some experience on the part of the operator in cutting
threads and a good lathe, how much less accurate is using a die than
cutting
with the lathe?

It would depend on the quality of the die, compared to the accuracy
of re-aligning the stock for the later cut(s). I'd like to think
that a good die is going to give you the quality you'd need?

Two - How could one measure the thread to thread distance accurately to
see
how much error was created when reindexing after moving? Looks like
thread
micrometers only measure the diameter.

I'm interested in the answers you'll get to this. What kind of
accuracy are you looking for?

If I can see or feel the break in the thread where the stock was moved,
there really isn't any point in measuring it. A measurement would need to
detect .01 or less.

Steve.

In article , Dave Hinz says...

So...how does one cut a clean leadscrew when one doesn't have
a clean leadscrew to start from? Someone made the first one...

Screw threads have been fabricated for a long time, using manual
methods like chasing. The real trick to to create the metrology
needed to check them, and refine them. Basically you are talking
jo blocks and maudsley.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

"Dave Hinz" wrote in message
...
On 20 Jan 2005 09:14:31 -0800, jim rozen wrote:
In article , Dave Hinz says...

Can you use the lathe to turn the stock, and use a die to cut your
threads,

A die won't give a precision lead to the part. Because this is
a leadscrew for the lathe, that's not the best way to make
the part.

Ah, I missed the part about it being a lead screw for a lathe.

Leadscrews not only have to have the correct thread geometry,
but they are effectively length standards in and of themselves.
A die-cut leadscrew would work, but it would impart all the
lead error into whatever parts were then fabricated when
using it on the lathe.

So...how does one cut a clean leadscrew when one doesn't have
a clean leadscrew to start from? Someone made the first one...

I would guess (note the word "guess") that the better way would be to turn a
couple of threads onto the end of the stock and then stick it into a
precision nut which is the work holder for a grinding machine and grind the
thread. The grinding wheel is always going to be a constant distance from
the nut so just apply constant pressure to continuously turn the stock and
work it into the nut and a screw of unlimited length could be done
accurately. Maybe I can make an attachment for my lathe!

Steve.

"Gene Kearns" wrote in message
news
On 20 Jan 2005 19:17:36 GMT, Dave Hinz wrote:


Two - How could one measure the thread to thread distance accurately to
see
how much error was created when reindexing after moving? Looks like
thread
micrometers only measure the diameter.

I'm interested in the answers you'll get to this. What kind of
accuracy are you looking for?

Assuming you don't have every precision measuring tool known to man...
use what you've got of known (or best) accuracy... in this case, the
lead screw and a dial indicator.

Since a thread is nothing more than a v-groove, it shouldn't be all
that difficult to devise a measuring tool that uses a small round
feeler ( flat sides and round bottom would be easier to use with the
indicator) riding in the cut groove. You are looking for horizontal
displacement as you ride from "old" cut to "new" cut. With patience
and a ten-thousandths indicator, you are probably going to get closer
in that short distance than the lead screw is in a foot.....

So if I clamp a nut to my steel surface plate and then mount the dial
indicator nearby so it rides in the thread groove I should be able to rotate
the screw with consistent pressure against the nut to remove backlash and
detect variations. Cool.

Steve.

You know, I don't think you can do it really accurately, but, if you
examine how a lead screw is used, you could probably do it well enough
that it does not run into the boundary area often. Do it in 3 passes
instead of two and you will probably never hit it, how often do you use
the end 3 inches of thread on your lead screw?

Lloyd E. Sponenburgh wrote:
"Grant Erwin" wrote in message
...

SteveF wrote:

.. in practice can this be done with enough accuracy for something
like a replacement lead screw for a lathe?

No.


Hey! Not true, at all!

It's something of a chore to re-index a piece of threading work, but it's
not that bad. Takes patience and time, that's all.

Consider: Mark a thin scribe line down the length of the work to show the
rotational indexing position. Position your tool in the thread very near
the chuck, and lock the half-nuts. Then unchuck, and re-index the work in
rotation by the scribe, and longitudinally by leaving a small amount of the
already-threaded portion remaining beyond the chuck jaws, and using the
(never moved) bit to find the correct longitudinal position in the chuck.

Keep lost motion in mind, and snug the tool to the "cutting" face of the
thread (by moving the work to the tool) before each re-chuck.

LLoyd

"SteveF" wrote in message
. com...

"Dave Hinz" wrote in message
...
On Thu, 20 Jan 2005 05:16:33 GMT, SteveF wrote:
Let's say I have a small lathe (length) and need to make a threaded
screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in
practice
can this be done with enough accuracy for something like a replacement
lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded
rods
from MSC, etc. - this is more of a "how to use my lathe" machining
question.

Can you use the lathe to turn the stock, and use a die to cut your
threads, or is this for a thread you don't have a die for? Novice
question
on my own part as well.


Well, that gives me two more questions for the group.

One - Assuming some experience on the part of the operator in cutting
threads and a good lathe, how much less accurate is using a die than
cutting with the lathe?

Die cutting would not be very accurate for a long thread. The die will want
to wander and the concentricity will be bad. I missed the part about cutting
a lead screw in the OP. Precision lead screws are usually ground. Although
some ball screws and lead screws today are cut out of hardened blanks on
thread whirling machines. http://tinyurl.com/5bn5d
Non precision lead screws (those not used in a machine tool) are for the
most part thread rolled. Long threads like those on all-thread rod are
through-feed rolled on a double roll type thread rolling machine.
You can easily make a long machine screw to the accuracy required by cutting
the threads in sections on a manual lathe. The problem with cutting a lead
screw on a lathe is that all of the accumulated error in the lathes drive
train, plus deflection, run out, slide wear, and thread form errors in your
tool all stack up and prevent you from making a lead screw that's as good as
the one in the lathe.


Two - How could one measure the thread to thread distance accurately to
see how much error was created when reindexing after moving? Looks like
thread micrometers only measure the diameter.


That is what the optical comparator was invented to do, check thread form
and thread lead.
http://www.qualitydigest.com/may02/html/optcomp.html
Modern manufacturers of ball screws and precision lead screws use special
electronic gaging machines that check the lead, run out, and accumulated
error.


Dan

|
| It would depend on the quality of the die, compared to the accuracy
| of re-aligning the stock for the later cut(s). I'd like to think
| that a good die is going to give you the quality you'd need?

Seems to me that if you used a die you would likely get good threads,
but if you pulled or pushed on the stock while it was passing through the
die you would gain or lose a thread or more over the length of the screw.
Again, it's a tolerance stackup, but over a long item it adds up to be a
whole bunch, especially on a lead screw.

On Fri, 21 Jan 2005 04:25:07 GMT, carl mciver wrote:

|
| It would depend on the quality of the die, compared to the accuracy
| of re-aligning the stock for the later cut(s). I'd like to think
| that a good die is going to give you the quality you'd need?

Seems to me that if you used a die you would likely get good threads,
but if you pulled or pushed on the stock while it was passing through the
die you would gain or lose a thread or more over the length of the screw.
Again, it's a tolerance stackup, but over a long item it adds up to be a
whole bunch, especially on a lead screw.

Great information. Question though, is that an SAE "whole bunch", or
a metric "whole bunch"?

In article ,
Harold & Susan Vordos wrote:

"SteveF" wrote in message
.com...
Let's say I have a small lathe (length) and need to make a threaded screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in practice
can this be done with enough accuracy for something like a replacement
lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded
rods
from MSC, etc. - this is more of a "how to use my lathe" machining
question.

[ ... ]

Given the proper setup, it's possible, but would be a serious challenge,
especially if you are new at machining. You'd need a follow rest, possibly
a steady rest, be able to feed the material through the headstock, and be
able to pick up threads properly. Having considerable skill and experience
would be the most valuable asset, along with soft jaws. Depending on how
you pursued the project, you might be limited to length, but you could cut
one longer than the center length of the machine.

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.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

"DoN. Nichols" wrote in message
...
In article ,
Harold & Susan Vordos wrote:

"SteveF" wrote in message
.com...
Let's say I have a small lathe (length) and need to make a threaded
screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in
practice
can this be done with enough accuracy for something like a replacement
lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded
rods
from MSC, etc. - this is more of a "how to use my lathe" machining
question.

[ ... ]

Given the proper setup, it's possible, but would be a serious challenge,
especially if you are new at machining. You'd need a follow rest,
possibly
a steady rest, be able to feed the material through the headstock, and
be
able to pick up threads properly. Having considerable skill and
experience
would be the most valuable asset, along with soft jaws. Depending on
how
you pursued the project, you might be limited to length, but you could
cut
one longer than the center length of the machine.

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.

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.

Harold

What I would think of doing is cut the thread from the headstock end
toward the tailstock end, using no tailstock but supporting the long
unthreaded part with a center rest and other outboard expedients. I
would start by cutting a doot of thread normally and flipping the screw
over and screwing it into a special 'nut' in the lathe jaws.

I would make a long tight fitting nut and slit one side and
semi-permanently (glue?) it to the chuck so it could not move. Then
screw in the just threaded part and clamp down and cut threads away from
the chuck. When you need to move the screw just loosen the chuck jaws
and screw in the section you just threaded.

There might be a small discontinuity where you flipped the screw, but
that could be cut off later.

--
Free men own guns, slaves don't
www.geocities.com/CapitolHill/5357/

"Dave Hinz" wrote in message
...
| On Fri, 21 Jan 2005 04:25:07 GMT, carl mciver
wrote:

| Seems to me that if you used a die you would likely get good
threads,
| but if you pulled or pushed on the stock while it was passing through
the
| die you would gain or lose a thread or more over the length of the
screw.
| Again, it's a tolerance stackup, but over a long item it adds up to be a
| whole bunch, especially on a lead screw.
|
| Great information. Question though, is that an SAE "whole bunch", or
| a metric "whole bunch"?

Neither. It's an "Oh, s**t!" whole bunch.

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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Email: | Voice (all times): (703) 938-4564
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--- Black Holes are where God is dividing by zero ---

In article ,
Nick Hull wrote:
What I would think of doing is cut the thread from the headstock end
toward the tailstock end, using no tailstock but supporting the long
unthreaded part with a center rest and other outboard expedients. I
would start by cutting a doot of thread normally and flipping the screw
over and screwing it into a special 'nut' in the lathe jaws.

I would make a long tight fitting nut and slit one side and
semi-permanently (glue?) it to the chuck so it could not move. Then
screw in the just threaded part and clamp down and cut threads away from
the chuck. When you need to move the screw just loosen the chuck jaws
and screw in the section you just threaded.

Hmm ... how about soft jaws on the chuck, bored to the right
radius, and single-point threaded? Your initial thread on the workpiece
(cut before you mount and prepare the soft jaws) would have to be long
enough so you could then spend the time to properly align the tool with
the threads (perhaps by disengaging the tumbler gears until you get
things in sync), but once that is done, your remaining problem is
starting the thread without a lead-in groove or a shoulder. If you have
a lever setup to feed the tool into engagement and back out, you could
do pretty well, as long as you don't engage too far back after several
thread passes have already been made.

There might be a small discontinuity where you flipped the screw, but
that could be cut off later.

If there is a discontinuity there, there will be one each time
you shift the workpiece. It is better to tune things to eliminate the
discontinuity from the start.

But certainly an interesting approach.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

SteveF wrote:

this is more of a "how to use my lathe" machining question.

I have absolutely no idea if this is going to work, but it is just an
idea to save on 2nd, 3rd, etc, realignments (which probably saves
nothing if you are making it for your own lathe).

1) Make first bit of threaded screw.
2) make threaded tube to match screw, tube extends through head stock,
3) make "lock nut" (or 2?)
4) mount threaded tube into jaws,
5) load lock nut onto thread part of screw
6) load threaded bit of screw into tube
7) lock threaded screw into threaded tube
8) align in jaws and tail stock
9) realign tool.
10) cut next section
11) unlock lock nut(s)
12) screw threaded rod through threaded tube and relock[1]
13) got to 10.

Theorectically you could make it as long as you like, but you need
steadies on the long bit,

[1] each turn just advances the rod the size of the pitch.