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?
...
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?
"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.
"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.
"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.
What am I missing?
See
https://www.engineersedge.com/screw_threads_chart.htm
| Sysop: | Keyop |
|---|---|
| Location: | Huddersfield, West Yorkshire, UK |
| Users: | 484 |
| Nodes: | 16 (2 / 14) |
| Uptime: | 51:52:07 |
| Calls: | 9,628 |
| Calls today: | 1 |
| Files: | 13,695 |
| Messages: | 6,159,569 |
| Posted today: | 1 |