The nominal size of a thread is the major diameter of the internal
thread, or the nut. In other words the tap's major diameter. This is
the same for metric and imperial threads.

When cutting an external thread on the lathe, the stock must first be
turned to the proper diameter.

When watching machinists on Youtube, even the very experienced ones,
they all turn their stock to the nominal size, and they all end up
with a tight fit. Not understanding what has gone wrong, they will
often call it a "machinist's fit" and ignore the problem.

I have searched and searched, but I have not been able to find a table
or formula that tells me the proper size to turn the stock to before
cutting external threads on the lathe.

What am I missing?
--
RoRo

On Sat, 09 Sep 2023 12:28:16 +0200, Robert Roland <fake@ddress.no>
wrote:

>The nominal size of a thread is the major diameter of the internal
>thread, or the nut. In other words the tap's major diameter. This is
>the same for metric and imperial threads.
>
>When cutting an external thread on the lathe, the stock must first be
>turned to the proper diameter.
>
>When watching machinists on Youtube, even the very experienced ones,
>they all turn their stock to the nominal size, and they all end up
>with a tight fit. Not understanding what has gone wrong, they will
>often call it a "machinist's fit" and ignore the problem.
>
>I have searched and searched, but I have not been able to find a table
>or formula that tells me the proper size to turn the stock to before
>cutting external threads on the lathe.
>
>What am I missing?

See
https://www.engineersedge.com/screw_threads_chart.htm

--
Cheers,

John B.

"Robert Roland" wrote in message
news:9fhofip38igknn00qlv1g4qr8kf2d4al09@4ax.com...

The nominal size of a thread is the major diameter of the internal
thread, or the nut. In other words the tap's major diameter. This is
the same for metric and imperial threads.

When cutting an external thread on the lathe, the stock must first be
turned to the proper diameter.

When watching machinists on Youtube, even the very experienced ones,
they all turn their stock to the nominal size, and they all end up
with a tight fit. Not understanding what has gone wrong, they will
often call it a "machinist's fit" and ignore the problem.

I have searched and searched, but I have not been able to find a table
or formula that tells me the proper size to turn the stock to before
cutting external threads on the lathe.

What am I missing?

RoRo

----------------------
If the threads fit there isn't a problem.

According to Machinery's Handbook the Unified tip truncation is 0.125X the
vee thread height and the pitch diameter is 0.375X below it. Your answer
could be figured from the depths of sharp vee and Unified threads and the
pitch diameters, which are in different tables. Then add the tolerance for
the class of fit. I'd put the data in a spreadsheet and then compute and
print the 29 degree infeed for each pitch on a copy of the threading gearbox
chart.

It looks to me like the OD can be the nominal size in the maximum material
condition so that's how I cut them, and commercial nuts and tapped threads
fit. Custom parts don't have to be universally interchangeable and I think a
closer fit with greater root diameter is stronger. The OD of several of my
4-flute 1/4-20 taps averages 0.252" and the tap must be my gauge, not the
book value.

"John B." wrote in message
news:uskofipli0srh1vqn9gbog3tb19urb4441@4ax.com...

On Sat, 09 Sep 2023 12:28:16 +0200, Robert Roland <fake@ddress.no>
wrote:

>...
>I have searched and searched, but I have not been able to find a table
>or formula that tells me the proper size to turn the stock to before
>cutting external threads on the lathe.
>
>What am I missing?

See
https://www.engineersedge.com/screw_threads_chart.htm

Cheers,

John B.

------------------------

There's your answer, thanks John. The diameter can be the nominal value for
a 3A fit and only 0.001 or so less for 2A. If you want to virtue signal you
could use half way between the min and max.

"Jim Wilkins" <muratlanne@gmail.com> on Sat, 9 Sep 2023 07:47:34 -0400
typed in rec.crafts.metalworking the following:
>"Robert Roland" wrote in message
>news:9fhofip38igknn00qlv1g4qr8kf2d4al09@4ax.com...
>
>The nominal size of a thread is the major diameter of the internal
>thread, or the nut. In other words the tap's major diameter. This is
>the same for metric and imperial threads.
>
>When cutting an external thread on the lathe, the stock must first be
>turned to the proper diameter.
>
>When watching machinists on Youtube, even the very experienced ones,
>they all turn their stock to the nominal size, and they all end up
>with a tight fit. Not understanding what has gone wrong, they will
>often call it a "machinist's fit" and ignore the problem.
>
>I have searched and searched, but I have not been able to find a table
>or formula that tells me the proper size to turn the stock to before
>cutting external threads on the lathe.
>
>What am I missing?
>
>RoRo
>
>----------------------
>If the threads fit there isn't a problem.
>
>According to Machinery's Handbook the Unified tip truncation is 0.125X the
>vee thread height and the pitch diameter is 0.375X below it. Your answer
>could be figured from the depths of sharp vee and Unified threads and the
>pitch diameters, which are in different tables. Then add the tolerance for
>the class of fit. I'd put the data in a spreadsheet and then compute and
>print the 29 degree infeed for each pitch on a copy of the threading gearbox
>chart.
>
>It looks to me like the OD can be the nominal size in the maximum material
>condition so that's how I cut them, and commercial nuts and tapped threads
>fit. Custom parts don't have to be universally interchangeable and I think a
>closer fit with greater root diameter is stronger. The OD of several of my
>4-flute 1/4-20 taps averages 0.252" and the tap must be my gauge, not the
>book value.

A thing to remember is that there are also three levels (iirc) of
fit - loose (smallest bolt, largest hole), "normal" - what we're all
used too, and tight (largest bolt and smallest hole). [I'd have to
stop and go look up the technical terms, it has been a while.]
--
pyotr filipivich
"With Age comes Wisdom. Although far too often, Age travels alone."

"pyotr filipivich" wrote in message
news:uf3pfip6jagqp4tl4hhdhksn0h4hj10g68@4ax.com...

A thing to remember is that there are also three levels (iirc) of
fit - loose (smallest bolt, largest hole), "normal" - what we're all
used too, and tight (largest bolt and smallest hole). [I'd have to
stop and go look up the technical terms, it has been a while.]
pyotr filipivich

---------------------------

Threads cut on a lathe are almost certainly for a specific task, not general
sale, and can be as tight or loose as the task requires, regardless of the
established tolerance classes.

"Jim Wilkins" <muratlanne@gmail.com> on Sat, 9 Sep 2023 12:26:36 -0400
typed in rec.crafts.metalworking the following:
>"pyotr filipivich" wrote in message
>news:uf3pfip6jagqp4tl4hhdhksn0h4hj10g68@4ax.com...
>
>A thing to remember is that there are also three levels (iirc) of
>fit - loose (smallest bolt, largest hole), "normal" - what we're all
>used too, and tight (largest bolt and smallest hole). [I'd have to
>stop and go look up the technical terms, it has been a while.]
>pyotr filipivich
>
>---------------------------
>
>Threads cut on a lathe are almost certainly for a specific task, not general
>sale, and can be as tight or loose as the task requires, regardless of the
>established tolerance classes.

Ayup.

Just make sure they're correct form.

Instructor knew of a situation where the large nuts to secure an
industrial pressure cooker would not come off (ruining the batch of
whatever). Turns out the threads were at 55 degrees, and had fit
going on, but the pressure / heat / etc had made them deform and
seize. Oops.
--
pyotr filipivich
"With Age comes Wisdom. Although far too often, Age travels alone."

"pyotr filipivich" wrote in message
news:p7nsfidmsja0jt570tnhie2sju527egaff@4ax.com...

"Jim Wilkins" <muratlanne@gmail.com> on Sat, 9 Sep 2023 12:26:36 -0400
typed in rec.crafts.metalworking the following:
>"pyotr filipivich" wrote in message
>news:uf3pfip6jagqp4tl4hhdhksn0h4hj10g68@4ax.com...
>
>A thing to remember is that there are also three levels (iirc) of
>fit - loose (smallest bolt, largest hole), "normal" - what we're all
>used too, and tight (largest bolt and smallest hole). [I'd have to
>stop and go look up the technical terms, it has been a while.]
>pyotr filipivich
>
>---------------------------
>
>Threads cut on a lathe are almost certainly for a specific task, not
>general
>sale, and can be as tight or loose as the task requires, regardless of the
>established tolerance classes.

Ayup.

Just make sure they're correct form.

Instructor knew of a situation where the large nuts to secure an
industrial pressure cooker would not come off (ruining the batch of
whatever). Turns out the threads were at 55 degrees, and had fit
going on, but the pressure / heat / etc had made them deform and
seize. Oops.
pyotr filipivich

-------------------------

Whitworth, just different enough to cause trouble. I have an old horizontal
bench mill with a 1/2"-12tpi drawbar thread.

Whitworth made a survey of the threads British shops used and took the
average for the angle, instead of designing scientifically like Briggs and
Sellers in the USA.
https://en.wikipedia.org/wiki/British_Standard_Whitworth

https://www.detroitnippleworks.com/pipe-thread-standards/
https://en.wikipedia.org/wiki/United_States_Standard_thread

On Sat, 09 Sep 2023 18:15:44 +0700, John B. <slocombjb@gmail.com>
wrote:

>>What am I missing?
>
>See
>https://www.engineersedge.com/screw_threads_chart.htm

Awsome. Thanks.
--
RoRo