On Sat, 15 Sep 2007 22:23:52 -0500, Richard J Kinch
wrote:

There's an odd thread, called the Leica thread, that is used on
photographic enlarger lenses. It is specified as 39mm in nominal diameter
but 26 tpi in pitch.

I'm wondering what other monstrous metric-English combinations are out
there?
============
I was told that this resulted from the "Marshal Plan" where many
surplus US [inch] machine tools were shipped to Europe after WW2.

While transposing gears are a possibility, you can no longer use
the half-nut, so there is a significant production penalty to
generate metric threads with an inch lead screw.

Thus the "compromise" where diameters, lengths etc. were metric
and the threads the closest possible inch equivalent, i.e. 26 tpi
close to 1.0 mm pitch [25.4 tpi]

No documentation, but sounded plausible.


Unka' George [George McDuffee]
============
Merchants have no country.
The mere spot they stand on
does not constitute so strong an attachment
as that from which they draw their gains.

Thomas Jefferson (1743-1826),
U.S. president. Letter, 17 March 1814.