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Subject:Re: Standards of converting to metric From:"CB Casper" <knowone -at- surfy -dot- net> To:"TECHWR-L" <techwr-l -at- lists -dot- techwr-l -dot- com> Date:Fri, 04 Feb 2005 07:12:54 -0800
>> Does anyone know of a standard regarding the conversion of imperial
measurement to metric and how this needs to be documented? Typically,
we list a description of a part, give a length in U.S. measurement and
then list the metric conversion in parentheses and smaller font behind
the measurement. PART B, 4 3/4" (121) <<
Good approach, but always, always, always use units, no exceptions,
ever. Use the same convention throughout the document, the product,
and the company. The standard must be the same everywhere.
>> When I first started this job six years ago, I was originally told
(probably by the other writers) that we only need to give the conversion
of a measurement in its first instance in the document. <<
Depending on the use of the measurements, different people use
different levels of accuracy. A buyer is interested in how many pounds
(kilograms) of material to buy, while the machinist is interested in
how to cut the part to accuracies required by engineering, to the
accuracy of a thousandths of an inch or more accurately.
If there are tolerances involved, 3.530" +.001" / -.003" then the
converted measurement of tolerance should be at least a digit
more accurate that the original measurement to assure that the
original tolerances are met.
>> One suggestion was to list the conversion formula at the beginning of
the doc and call it good. <<
AARRRGGGHHH!. Never, or could you tell by my previous discussion.
The use and some other similar backwards country are the only ones
left using the old system of measurement. This means that if the
product goes anywhere outside of the US for fabrication, they will
be using the metric system. Your company needs to control the
measurements and tolerances of the parts. If you leave the conversion
to someone else, Murphy's law WILL come back and bite you.
Most US shops now have the ability to work in either system as
the machine tools are now electronic and can go back and forth
with ease. 25 years ago, measurements were designed in to the
the machines, with measurements on the machine itself, and each
machine tool was limited to one or the other, to a great extent. These
machine tools are very expensive, so buying a new machine was a
major capital expense.
Real world example from my first job:
Military hardware designed as a cooperative French-German design.
built by a US company in a shop that had never used metric, and
most of the machine tools were US-units only.
To the metric dimensions on engineering drawings, production planning,
material lists, all units were set as Metric with US measurements in
parentheses. However our tool shop rebelled, and required that all tool
drawings be in US measurements with Metric in parentheses. For those
without close knowledge, a tool in this case is something used during
the process of building the part. A die for molding, a fixture for holding
a part for machining, etc., not the deliverable part itself.
With these two opposite systems in place, you can guess what happened.
A couple of tools were built way out of proportion to what they were
supposed to be, because the numbers got switched. What was supposed
to be 10mm became 10 inches. There was some considerable scrambling
involved to get the tools re-built to proper dimensions.
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