I learned this yesterday at work.

Instead of setting the compound rest on a lathe to 29º for single point
threading, I have been taught to set the compound to 90º (parallel to the
bed, z axis feed).

The tool is then advanced (incrementally of course) to the calculated root
depth of the thread (refer to the machinery's handbook or others). This will
not result in a usable thread. The crest of the thread should not be sharp
at this point. There should still be flats at the top.

Once this depth is achieved, the compound rest is advanced toward the
headstock to clean the forward flank of the thread, and then retracted to
clean the rear flank of the thread. These forward and rear cleaning cuts are
made incrementally and may require more than one pass (for each).

Using the three wire method to measure the threads, one can easily achieve a
tight-fitting thread.

This technique makes multistart threads very easy to produce. You do have to
watch out when doing the forward and rear flank cleaning cuts. Each "start"
must have the same forward cleaning cut depth and rear cleaning cut depth
(although the forward and rear depths do not have to be equal).

Another issue that must be observed is that a radius on the nose of the
threading tool will adjust the calculated depth of the root of the thread.
Usually not a big issue if your tool is essentially "sharp" as there is a
fair amount of clearance built into a female thread.

Does anyone else do it this way? The instructor who taught me is a very
knowledgeable German-educated millwright (different meaning in Germany,
apparently) and the first time I tried it I had far more success than using
the old 29º method.

Thought this might help someone.

Regards,

Robin

"Robin S." wrote in message
...
snip-----

Does anyone else do it this way? The instructor who taught me is a very
knowledgeable German-educated millwright (different meaning in Germany,
apparently) and the first time I tried it I had far more success than
using
the old 29º method.

That is nothing new, but I don't endorse it unless you are cutting multiple
entry threads on an item chucked instead of running between centers, or
you're cutting threads other than 60° form. It is especially effective on
Acme or square threads. The real problem with that method is if you're
cutting (60°) threads that will get inspected by a comparator, and I've been
there, done that, many times, the form will likely be out of tolerance so
you risk making scrap, even though rings or a snap gage may say the thread
is acceptable. The flat on thread forms has a tight enough tolerance that
the side motion necessary to clean up the threads risks cutting the flat too
wide. Threading with a sharp pointed tool is not a solution, for that is
also a scrap thread.

The second problem with that threading method is chip flow and tool tip
loading, generally leading to tip failure. If you're threading at high
speeds using carbide, the problem may be lessened, even insignificant, but
that threading method otherwise is generally a recipe for tool failure.
CNC threading at high speeds seems to work fine when so fed, something many
guys do because they don't want to pay for the feature that permits "proper"
threading. I imagine the chip flow characteristics are much improved by
running the cut at the "sweet spot", but I have nothing to document my
opinion.

My threading practice has always been to feed at 29°, but to take the final
few passes by plunging with the cross slide, but taking light cuts, no
greater than a couple thou, and as low as a half thou when going for a fine
finish. Requires a sharp tool, but your suggested method does, too. The
advantage of finishing by my method is to eliminate the chance of ruining
thread form by widening the flat.

Harold

The purpose of the 29 deg. for the compound is to allow for the heavy cut to
be on one side of the threads while just dusting the other side. In truth,
you can set the compound at any angle you desire and do the cutting as you
deisre. Some ways are just easier to do than others.
Personally, I set the compound whereever it is and work from there.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!

"Bob May" wrote in message
...
The purpose of the 29 deg. for the compound is to allow for the heavy cut
to
be on one side of the threads while just dusting the other side. In
truth,
you can set the compound at any angle you desire and do the cutting as you
deisre. Some ways are just easier to do than others.
Personally, I set the compound whereever it is and work from there.

--
Bob May

Bob,
That can get you in trouble, especially if you're chasing a thread that will
engage a long nut. When you single point thread, the whole idea of
offsetting the compound as we do, aside from chip flow, is to keep the
carriage loaded against the lead screw. That's why you set the compound
differently for internal threading as opposed to external threading, or
differently for left as opposed to right hand threads.

What can happen is when you feed the compound on the wrong flank of the
thread, the pressure of the cut can overcome the friction of the carriage
and move it away from the screw, and it often does. The net result is
what is termed a drunken thread. The pitch diameter may be correct, but
the lead won't be, and the thread binds in the nut. It's a smart policy
to always set your compound appropriately for the thread being single
pointed.

Harold

In article , Harold & Susan Vordos says...

What can happen is when you feed the compound on the wrong flank of the
thread, the pressure of the cut can overcome the friction of the carriage
and move it away from the screw, and it often does. The net result is
what is termed a drunken thread.

I've always heard that term used for threads that were
die-cut. And that it can be caused by a die cocking
on the part and still threading along.

I would think that threading with the compound set
along the axis, but with the compound screw not
loaded up against the force of the cut, would cause
a thread that simply had the wrong pitch when
it was done - by the amount of backlash in the
compound screw, basically.

Jim

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"jim rozen" wrote in message
...
In article , Harold & Susan Vordos says...

What can happen is when you feed the compound on the wrong flank of the
thread, the pressure of the cut can overcome the friction of the carriage
and move it away from the screw, and it often does. The net result is
what is termed a drunken thread.

I've always heard that term used for threads that were
die-cut. And that it can be caused by a die cocking
on the part and still threading along.

I would think that threading with the compound set
along the axis, but with the compound screw not
loaded up against the force of the cut, would cause
a thread that simply had the wrong pitch when
it was done - by the amount of backlash in the
compound screw, basically.

Jim

In this scenario, you'd realize the compound had shifted because the tool
wouldn't pick up the thread on a succeeding pass. The problem I speak of
tends to go unnoticed because the pitch wanders back and forth, usually at
the same spot in the thread, although that wouldn't necessarily be true.
The problem can come on the first pass, which then steers the carriage so
the drunken condition repeats at the same place each time and you don't
notice it happening. The amount of movement can be very minor, but that's
all it takes to screw up the thread.

I experienced just such a thread on a part for Thiokol many years ago while
in the employ of my last employer, just before starting my own shop. I was
running a Cincinnati tray top, a machine for which I have little regard,
cutting an acme thread. The carriage was very loose, and the hand wheel
was not balanced. As the handle of the hand wheel came over the top, the
weight of the handle bumped the carriage ahead slightly, until the lead
screw caught up with it. Needless to say, it did so repeatedly until the
thread was finished. Had I been running the machine for some time, I'd have
been aware of the machine's propensity to cut threads poorly, but it was my
first day on that particular machine. One of the rare returned items I
experienced while working there. Took a little searching to understand why
the thread was not good.

Harold