Minor Diameter calculation

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John - 20 Jan 2010 14:05 GMT

Many years ago my father used an equation to find the minor diameter
of a thread. Unfortunately I have forgotten it and my father is no
longer around to ask.

I have looked at various equations, calculation programs and tabulated
information on the internet and they all seem to produce different
answers.

Does anybody know an equation which from experience will produce
reasonably accurate result when calculating the minor diameter of a
thread?

Thanks.

John H

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Charles Lamont - 20 Jan 2010 14:57 GMT

> Many years ago my father used an equation to find the minor diameter
> of a thread. Unfortunately I have forgotten it and my father is no
[quoted text clipped - 7 lines]
> reasonably accurate result when calculating the minor diameter of a
> thread?

There is no universal equation.

There is a formula for BA threads only, because they are almost unique
in being a mathematically derived series.

In general, you need to know the size and thread designation, and if not
a standard thread, the thread form and pitch. Thread form and pitch will
enable you to find the theoretical thread depth, and that gives you the
minor diameter.

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Charles Lamont

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mecc_smith - 21 Jan 2010 01:41 GMT

If the threads are very old they may be Whitworth which was also
formula driven.

> > Many years ago my father used an equation to find the minor diameter
> > of a thread. Unfortunately I have forgotten it and my father is no
[quoted text clipped - 20 lines]
> --
> Charles Lamont

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Charles Lamont - 21 Jan 2010 09:40 GMT

> If the threads are very old they may be Whitworth which was also
> formula driven.

The relationship of pitch to diameter in Whitworth threads was
determined empirically and does not follow a formula.

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Charles Lamont

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dave sanderson - 09 Feb 2010 22:58 GMT

> Many years ago my father used an equation to find the minor diameter
> of a thread. Unfortunately I have forgotten it and my father is no
[quoted text clipped - 11 lines]
>
> John H

for metric threads taking pitch from the designation gives a pretty
good
approximation, at least for tapping drills.
E.g. M6x1 -> tapping drill 5mm

Dave

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Model Forum / General / Models / February 2010

Minor Diameter calculation