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

[Ken Knierim]

Fascinating video.
Thanks for sharing, Steve!

On Sun, May 5, 2019 at 3:57 AM Stephen Offiler <soffiler at gmail.com> wrote:

> 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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