[AT] OT: the inch is somewhat metric Cast iron work on a replica cannon

[Stephen Offiler]

There is a fairly complex history leading to the definition of 1 inch =
25.4mm exactly.  There were some troubles with parts interchangeability in
WW2 because different definitions were used in different countries.  But
the man who invented extreme precision, Carl Johannson - inventory of the
gage block - arrived upon this definition while working with Ford Motor Co.
in the 1920's.  It took a while for this definition to become universal.
If you find this subject of interest, I suggest:

https://youtu.be/gNRnrn5DE58

The very earliest numerical-control (NC) machines had no computer, but
rather read instructions from a punched paper tape, and these were in the
research stage in the 1940's and began to appear in practice in the
1950's.  It was not until the 1970's that computers and digital displays
began to appear on the shop floor.   Conversion from metric to inch in the
displays was not an issue.  Even if it was, the conversion could have been
accomplished regardless of the definition, because even prior to the
universal adoption of 1" = 25.4mm exactly, the errors were on the order of
25.39993 to 25.40005 which is well beyond the precision of the machines in
those days.

And finally, it is a rare engineering course anywhere in the USA that does
not present problems to students using both metric and inch systems.

SO


On Sat, May 4, 2019 at 9:42 PM James Peck <jamesgpeck at hotmail.com> wrote:

> Early in WW2, the US, UK, and Canada all had a different length inch. To
> ensure parts made in the US fit into machines made in the UK, etc., they
> needed a common inch. They solved it by defining the inch as 25.4 MM. I bet
> one heck of a lot of calipers and micrometers had to be recalibrated.
>
> This made it possible for CNC machines to switch from inch to metric on
> the displays.
>
> Some university courses would have test problems using either system or
> both. That is the real world.
>
> [Rena Glover Goss] It has been too long since I did this, Stephen.  I used
> to teach this material to engineering and engineering technology students,
> but have been retired for 18 years, and gave all my metric taps, dies,
> drills, and wrenches to one of my grandsons when I downsized.
>
> I used to get criticized by faculty colleagues because I taught my
> drafting and graphics courses only in metric units.  I told them the
> students really didn't know how to use any of the four systems of
> measurement, and that they would develop a decent sense of at least one of
> them if they didn't have to be confused by dealing with the others.  I
> don't think my colleagues necessarily "bought" my argument, but I did find
> that things were much simpler when students only had to deal with a single
> measurement device.  I think I still have metric, architectural, mechanical
> engineering, and civil engineering scales laying around--but not nearly as
> many of them as I used to.
>
> So tell me-- How long is a surveyor's chain, and what are the units in
> it?  This is not an esoteric question.  Our family is currently dealing
> with a real estate transfer that dates to the original survey for the
> Wabash-Erie canal.  The concepts of Range and Township were still
> reasonably new at that time, and the units of measure corresponded to the
> most current technology.
>
>
> [ Stephen Offiler] Larry:  yes, with metric threads, you find the tap
> drill simply by  subtracting the pitch from the major diameter.  M15 x 1.0
> gives a 14mm  tap drill.  Using this formula, you always end up with 77%
> thread  engagement for any thread, any pitch.  I'm not quite following
> your  comment about the reduction you use.
>
> [Rena Glover Goss]   THAT'S NO FAIR, SPENCER.  You were not supposed to
> figure out how   simple those relationships are in the metric system when
> compared to   any other system of threading.  I used a reduction by 1.5
>  millimeters so the thread engagement came closer to 75%, as is used   in
> the SAE system.  There are no charts for pilot drills in the metric
> system--they simply aren't needed.
> .
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