Subject: Stupid Question: Threading

From: [email-masked] (Gary huntress)
Date: Mon, 3 Jan 94 13:20:25 GMT

I've heard dozens of teachers and professors say that "There is no
such thing as a stupid question".  Well, I'm gonna test that theory
right now!

I don't really understand the basics of cutting external threads on
a lathe.  Everything that I've read here about lathe specs implies
that lower RPM is better for threading, but I'm also
(mis)remembering a rule of thumb that material rates at the cutter
should be somwhere between 100-200 feet per minute for typical
materials.  But considering a 1" rod at 100 RPM, thats about 26
feet per minute.  Aren't you hogging off too much material at too
low a speed?  Threads are cut in one pass aren't they?

Gary "a metal working wannabe" Huntress
[email-masked]
[email-masked]

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From: Gregory P. Widin <[email-masked]>
Date: Mon, 3 Jan 94 18:30:52 GMT

Stupid Question:  Threading

In article
<[email-masked]> Gary
 huntress, [email-masked] writes:

>  Aren't you hogging off too much material at
too
>low a speed?  Threads are cut in one pass
aren't they?

I routinely cut threads, when they are short or
up to a shoulder, using a HAND-crank that fits
in the lathe mandrel--it doesn't get much slower
than this.  The finish is quite good if the tool
is sharp and I use a good cutting lubricant.

Threads are emphatically _not_ cut on one pass,
at least on my lathe.  In fact, it's often
appropriate to take a second pass at no
additional infeed in order to work out spring in
the workpiece, especially if it's slender and/or
cut without tailstock support.

Greg

---

From: [email-masked] (James Kirkpatrick)
Date: 3 Jan 94 23:13:08 MST

In article <[email-masked]>, [email-masked] (Gary  huntress) writes:
|>
|>I've heard dozens of teachers and professors say that "There is no
|>such thing as a stupid question".  Well, I'm gonna test that theory
|>right now!
|>
|>I don't really understand the basics of cutting external threads on
|>a lathe.  Everything that I've read here about lathe specs implies
|>that lower RPM is better for threading, but I'm also
|>(mis)remembering a rule of thumb that material rates at the cutter
|>should be somwhere between 100-200 feet per minute for typical
|>materials.  But considering a 1" rod at 100 RPM, thats about 26
|>feet per minute.  Aren't you hogging off too much material at too
|>low a speed?  Threads are cut in one pass aren't they?
|>
|>Gary "a metal working wannabe" Huntress
|>[email-masked]
|>[email-masked]

You are partially correct.  Threading speeds do seem lower than would
be required for other sorts of operations.  However, most external
(or internal!) threading done on a lathe is actually done in several
passes.  One of the big reasons you want slow speed is so you can
disengage the carriage travel when you get to the end.  Remember that you
usually are traveling from the tailstock towards the headstock.  If you
do not disengage or shut down in some manner, the carriage will smash
into the headstock, or your cutting tool will be broken by the chuck,
or some other difficulty.  If the lathe is whirring along at 1000 RPM
you simply don't have enough time to disengage the half-nuts at the
proper moment.  (I was lucky once in that I only tore up a toothed drive
belt, not a gear, when the carriage ran into the gear housing because I
wasn't quick enough, and the setup didn't have much room for error).

Some say that about 45 RPM is just right.  My Grizzley 8" goes down to
200 RPM which is too fast to be comfortable.

The work must include some sort of cut-out area to give you time to open
the half-nuts and stop the carriage movement.  Then you unwind the cross
feed so the tool clears the work, run the carriage back towards the
tailstock, run the tool in to where it was plus a few thousandths, wait
for the proper mark on the threading dial, and re-engage the half-nuts
to take another pass.

Of course some unusual setups might be different, and there may be
lathes that let you thread in the other direction.  But the above is
generally true as far as I know.  (I can't remember if it is possible
to turn a left-hand thread on "common" lathes).

Jim

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From: [email-masked] (John Wishneusky)
Date: Tue, 4 Jan 1994 16:27:59 GMT

>[email-masked]

In my experience, (community college courses attended mostly by young
guys in the machinist trade) threads are usually cut with many passes
each of several thousandths.  You can take more at the start, less at
the end as you creep up on the final dimension.  The machinists who
taught the courses recommended a flat-topped cutting tool (zero back
and side rake) ground to the thread profile, carefully aligned to the
lathe axis.  This approach makes getting the thread profile right
because the angle ground on the tool matches the thread profile because
it's applied in a plane that passes through the thread's axis.  I think
it also decreases the cutter's efficiency, but what'd you rather have
accurate threads or fast threads?

The slow cutting speed comes about because you've got to be quick in
disengaging the half-nut and backing out the cross slide (unless you
like/want/accept a continuous groove at thread depth at the end of the
thread) then you reach the end of the thread.  Then you move the
carriage back to start another pass, reset the cross-slide to zero and
advance the compound the amount desired for the next cut and wait for
the right time to engage the half-nut as indicated by the thread
indicator.  100 rpm tests my reflexes adequately, particulary on large
threads where many passes are required. It's not bad on the first few
cuts because they're shallow and if I leave the half-nut engaged a
little too long, the tool can handle extending the cut through a
previously uncut region (unless I'm so close to a shoulder or the
chuck that I'm going to crash the tool).  On the final passes, if I
don't stop at the right time, I'm asking the tool to hog a very heavy
cut and it doesn't like that.

All the above speaks to my experience cutting steel, in brass or
aluminum heavier (perhaps full depth) cuts'd be easier.

John

---

From: [email-masked] (Dion Hollenbeck)
Date: Tue, 4 Jan 1994 21:16:58 GMT

>>>>> "Greg" == Gregory P Widin <[email-masked]> writes:

Greg> Stupid Question:  Threading
Greg> In article
Greg> <[email-masked]> Gary
Greg>  huntress, [email-masked] writes:

>> Aren't you hogging off too much material at too

Greg> Threads are emphatically _not_ cut on one pass,
Greg> at least on my lathe.  In fact, it's often
Greg> appropriate to take a second pass at no
Greg> additional infeed in order to work out spring in
Greg> the workpiece, especially if it's slender and/or
Greg> cut without tailstock support.

In gunsmithing, we typically will make 30 to 100 passes in cutting
threads depending on the thread pitch and diameter, material, etc.  

Slow speeds are used because it is easier to stop the travel when the
lathe is going slowly.  We always put the lathe in back gear to slow
it down to slowest speed possible for thread cutting.  You don't hog
off too much material at too slow a speed because you take maybe only
a .005" deep cut in aluminum or brass and .0025" in SS or other hard
metals per pass.  When you get down to the last few passes, you want
to only take off .001" or less and then make several passes with no
additional infeed travel, and when the threads finally begin to fit,
then you rub them with white chalk and cutting oil thoroughly and run
the cutter over them again without any additional infeed, thereby
polishing them.

I don't know about the precision you are attempting to achieve, but
typically we attempt to achieve zero free play and so smooth that it
feels like you have a large amount of free play.  I have made threads
which you could not rock the nut on the shaft at all and still you
could spin it with your finger and have it travel several inches up
the shaft before stopping.  This is not to brag, but merely to put in
perspective why I advocate so many shallow passes.  If you don't care
about this kind of accuracy, water down my instructions to where they
are appropriate to your situation.

-- 
Dion Hollenbeck (619)455-5590x2814     Email: [email-masked]
Staff Software Engineer                       [email-masked]
Megatek Corporation, San Diego, California    ucsd!megatek!hollen

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From: [email-masked] (Andy Dingley)
Date: Tue, 4 Jan 1994 22:37:45 +0000

In article <[email-masked]>
           [email-masked] "John Wishneusky" writes:

> (unless you
> thread) then you reach the end of the thread.

This is usually regarded as a Good Thing anyway. It makes cutting the thread
easier, as stopping the leadscrew travel becomes less critical. It also makes
it possible to screw a threaded part right down to the shoulder.

In the UK, a popular gadget for DIY toolmakers to make is a quick-retract
toolholder. A big lever pulls the tool out of the cut, so as to let you rack
the slide back. When you put the lever back, the tool returns to exactly the
same cut depth. The cross slide is now only used to increase the cut depth.
They're good gadgets for heavy cutting, as they certainly speed things up.
Do you have these things in the US ?

---

From: Gregory P. Widin <[email-masked]>
Date: Wed, 5 Jan 94 13:57:04 GMT

In article <[email-masked]>
Andy Dingley, [email-masked]
writes:

>toolholder.

The only design I know of for a retracting
toolholder is the Geo Thomas version published
in Model Engineer, and in GHT's new book from
Tee Publishing.  GHT also included a small unit
that accomplishes the same function which can
fit in the slot of a tool post (block style, not
the usual American lantern tool post).  I have
never seen either in the U.S.

Greg

Greg Widin                3M Hardgoods and
Electronics Support Division
[email-masked]                                 
    Building 260-6A-08  
(612) 736-6708                                  
  St. Paul, MN   55144

---

From: [email-masked]
Date: Wed, 5 Jan 1994 15:28:51 GMT

> threads depending on the thread pitch and diameter, material, etc.  

seems like quite a few passes what pitch and dia are you talking

> the cutter over them again without any additional infeed, thereby

When cutting thread there are a few things to keep in mind to make it easier
that I did not see mentioned or that I missed.

The cross slideshould be offset 29.5 degrees for a 60 tool instead of going
strait into the material. Reason being that it eliminated stress and chatter of
the tool bit. As you feed in on this angle most of the material is taken off at
the leading edge of the tool bit and only a fraction at the trailing edge.

            /\/\/\/\/\/\/\/\ work
                tool /\
                      \ 29 | degrees
                       \   |
              angle of  \  |
             cross slide \ |
                          \|
 
Invest in a set of thread measuring gauges I forgot the proper name but it is a
set of guage pins in sets of three ranging in sizes from about .020 to .125.
the three pins used with a mike and the chart supplied will allow you to
acurately measure threads.
                                                                    
When possible have a relief at the end of the treads.  /\/\/\/\/\___/--
This allows space for you to disengage the halfnut    |
Also if properly radiused it does not weaken the peice \/\/\/\/\/---\__

Cuts can be heavy at first but as you get deeper take shallower cuts
Some stuff deleted

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From: [email-masked] (Dave Golber)
Date: 5 Jan 1994 16:49:37 GMT

>Do you have these things in the US ?

Many years ago, I was permitted to use a Hardinge tool-room lathe
(wedge bed ... Powerful, silent and smooth as silk.  
Are there women like this lathe?)
It had this kind of gadget for threading.

Dave Golber

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From: [email-masked] (Ed Richley)
Date: 5 Jan 1994 18:00:15 GMT

All of our Hardinge HLV's here at work have a retracting
mechanism built into the compound rest. You just pull this little
lever, and out comes the tool (except on the one where the lever 
is missing...).

Also, the 29 degree setover is preferable (or so I have read) for
threading so that the non-cutting side of the tool actually
does a little clean-up work to finish the back side of the thread.
That way, the final cut is really a form cut, instead of a form
cut on the left side, and a bunch of little jaggy steps on the right side.

Ed Richley
[email-masked]

---

From: [email-masked] (John Wishneusky)
Date: Wed, 5 Jan 1994 18:44:40 GMT

>it possible to screw a threaded part right down to the shoulder.

The machinist who taught the course when I learned single-point
threading implied it was a matter of pride to be able to make a nicely
finished thread without relying on a groove at the end of the thread,
though he might have been pulling our legs a bit.
>Do you have these things in the US ?

I haven't seen one of these, it sounds convenient.  Any chance it
originated for a turret lathe without a compound on the cross-slide?

John

---

From: [email-masked] (Dion Hollenbeck)
Date: Thu, 6 Jan 1994 21:57:37 GMT

>>>>> "John" == John Wishneusky <[email-masked]> writes:

John> The machinist who taught the course when I learned single-point
John> threading implied it was a matter of pride to be able to make a nicely
John> finished thread without relying on a groove at the end of the thread,
John> though he might have been pulling our legs a bit.

Nope.  I have done it myself.  What you do is to stop the lathe
completely by turning off the motor so that the toolbit cuts almost up
to the end of the thread while leaving the feed locked *on*.  Then
take a very large crescent wrench and use it on the jaws of the chuck
to turn the lathe by hand until your tool cuts fully to the end of the
thread.  Back off the cross feed, take out of gear and go back for
another pass.  While doing this, you *must* make many shallow passes.

---

From: [email-masked] (Dion Hollenbeck)
Date: Thu, 6 Jan 1994 22:01:44 GMT

>>>>> "rtl002" == rtl002  <[email-masked]> writes:

>> threads depending on the thread pitch and diameter, material, etc.  

rtl002> seems like quite a few passes what pitch and dia are you talking

The last barrel I fitted had 1" major diameter with 14 pitch threads.
Another non-gun project had 3.25" diameter with 8 pitch threads.
Regardless of what the application, I was taught to make lots of
shallow passes.

---

From: [email-masked] (gary preckshot)
Date: 6 Jan 1994 21:58:50 GMT

In article <[email-masked]>, [email-masked] (Dave Golber)
wrote:
> >toolholder. A big lever pulls the tool out of the cut,...
[....]
> ...

Hardinge has had this kind of compound since 1915.  They also do not use a
threading guage and you do not disengage the half-nuts.  You thread toward
(or away from, if you're doing lefthand threads, Jim) the headstock and
reverse the lead screw rotation simultaneously with withdrawing the
compound.  79 years of excellence!

Gary

---

From: Doug White <[email-masked]>
Date: Fri, 7 Jan 94 09:24:22 -0500

> Regardless of what the application, I was taught to make lots of

Now that I finally have a lathe, I've been watching this thread (sorry,
I couldn't resist) closely.  I haven't cut threads since I took a 
machine shop class 20 years ago, and I'm itching to find a good project
to practice on.

Anyhow, my question relates to the 'lots of shallow passes' approach to
cutting.  I was talking to a machinist friend who said that the down
side of this is that you're putting a lot of extra wear on your tools.
He recommends taking fairly healthy cuts until you get close, and then 
slowly finishing up with finer and finer cuts.  The wear on a tool
(as long as you don't overheat it) is pretty much a function of the
number of passes you make, so this makes sense.  What I haven't
developed is a good 'feel' for how heavy a cut I can take without
straining anything.  

I work with a wide range of materials, and some is scrap of unknown
properties.  Does anyone have any suggestions for empirically determining
the maximum cut and feed for a given material?  What you can get away
with will also depend on how much cutting oil you've got pouring over
everything as well.  I'm usually not in a big rush, so I frequently
cut 'dry' with carbide tools just to minimize the mess I create.  Is
this a no-no for any particular reason?  I used to work on a variety
of machines, and each one had a different 'stiffness', so an easy cut
on one machine would produce deflection and chatter in another.  As
you can see, there are lots of variables that I haven't seen addressed
in any of the books I've got.

Any help would be greatly appreciated.  Thanks.

Doug White
MIT Lincoln Laboratory

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From: [email-masked]
Date: Fri, 7 Jan 1994 16:09:13 GMT

> Regardless of what the application, I was taught to make lots of

Dion I can cut those threads in 15 passes or less!!!!
(Dion)> Go ahead Bob Cut those threads!!!
But seriously with so many passes don't you find that your tool dulls. I've
been a machinist for 10 years and a tool and die maker for 2 years and at times
I've been required to cut threads from 10-32 to 6"-8. Materials ranging from
plexy,lexan,alum,brass,cast,D-2,cpm,titanium,stainless,and even inconel.
And I don't believe I ever exceeded 30 passes and many times had to hold
strict unreasonable military tolerences. I guess I'm just from a different
school(the one where the Boss looks over your shoulder holding a stopwatch
because he underbids the job).                                   BoB

---

From: [email-masked]
Date: Fri, 7 Jan 1994 16:45:39 GMT

> side of this is that you're putting a lot of extra wear on your tools.

This has been my experiance

> Any help would be greatly appreciated.  Thanks.
There are so many factors that have an effect on how much material you can take
off while cutting threads. I don't believe that there is an effective way
of determining the amount you take off in one pass. Experiance is the best
teacher.

If you cut dry you have a chance of overheating the tool, material galling to
the tool, and increased wear. I would suggest that you use an acid brush to
apply some cutting oil this will minimize the mess verses pouring it on. Some
tougher materials are going to require more of a coolent/lubricant, here I use
a spray bottle with a water soluble machining fluid(good on stainless). I have
cut dry at higher speeds using some ceramic inserts that seem to last fine but
are expensive.                                  Bob

---

From: [email-masked] (John Wishneusky)
Date: Fri, 7 Jan 1994 16:45:58 GMT

>another pass.  While doing this, you *must* make many shallow passes.
I meant pulling our legs about how tricky a technique and how much a
matter of pride it was.  I saw him demonstrate cutting such threads
without stopping the lathe.  His point was that we should be agile and
consistent enough to disengage the halfnut and back out the crossfeed
to produce a nicely finished thread without a full groove at the end.
I generally cut threads that way with reasonable results myself.  You
don't really have to always disengage at exactly the same point as long
as the tool's held securely enough not to shift if it ends up plowing
through a heavier cut at the end of the thread occasionally.

John

---

From: [email-masked] (gary preckshot)
Date: 7 Jan 1994 17:39:52 GMT

In article <[email-masked]>, [email-masked] wrote:

> But seriously with so many passes don't you find that your tool dulls.

Bob is certainly right, especially if threading work-hardening materials
like 300 series stainless steels.  I've never met a production machinist
that took small cuts all the way through if the machine and the tool could
hog.  My usual practice is to take as large a cut as the material
cross-section, the tool, and the machine drive can take, and reduce the cut
to a finishing cut about 2 or three finishing cuts from the end.  This way
I can tell how much spring the piece will have when I make the finish cut
and I can set the compound accordingly.  Saves time, tools, and scrap.  All
you really have to do is learn to measure accurately.  Works for threads as
well as any other shape.

---

From: [email-masked] (Stephen Shearer)
Date: Fri, 7 Jan 1994 21:27:30 GMT

>compound.  79 years of excellence!
The company Metal Lathe Accessories, Box 88, Pine Grove Mills, PA 16868
and it looks like Town Farm, Outwood Lane, Bietchingley, Redhill, Surrey,
RH1 4LR, England also carries the kits and plans for a "QUICK RETRACTING
TOOLPOST (MLA-16D)". The drawings are $6.50, complete kit $33.50 or $52.75
depending upon basic kit or long kit.

The info says: Operating the handle withdraws the tool instantly from
the work when cutting either external or internal threads.

It looks nice, I have thought of getting the plans and modifing it
for use on my EMCO C5, as it has no half-nut.  First I want to finish
the crank I am making for the spindle bore, so I can manualy make threads
up to a shoulder.

Steve