At 22 October 2005, 10:05 AM, Daniel wrote:
In mechanical design using inches, rarely does one see dimensioning
below 1/16 inch or 1.6 mm.
Maybe 50 years ago.
In manufacturing industries precisions of 0.2 mm (0.008") are
mundane, precisions of 0.05 mm (50 micrometers, 0.002") are used
frequently, and state-of-the-art machines, utilizing laser
interferometers, can handle precisions approaching 0.001 mm (1
micrometer, 40 microinches).
Even fine woodworking frequently uses 1/64" increments, or about
0.015" or 0.4 millimeters.
The use of the millimetre is actually a better replacement and gives
better accuracy then the fractional inches it competes with.
Millimeters are no more precise than inches. Depends entirely on how
the number accompanying the unit of measure is specified (how many
digits, tolerances, etc.). Inches can be used precisely, millimeters
can be used sloppily, and vice versa.
Products converted from fractional inches can easily within standard
fractional tolerance ranges be converted to whole millimetres
without any loss of precision. A dimension of 1/8 inch can easily
become a dimension of 3 mm.
LOL! You would be laughed out of any modern machine shop in the world
if you tried to make a claim like that!
1/8" = 0.125" = 3.175 mm. The error of 0.175 mm is about 7
thousandths of an inch, which is entirely "rejectable" for many
parts, such as ball bearings, precision shafts, connector pin
spacings, etc. Any competent machinist would be embarrassed to be
"seven thou" off a specification.
I am attaching a PDF of a mid-range horizontal machining center (a
fancy name for a mill) from Giddings & Lewis (a leading American
machine tool manufacturer). Note two things:
(a) It is specified entirely in metric, then colloquial equivalents
are provided. This is very common any more.
(b) For a mid-range tool, positioning accuracies are around 7 micrometers.
Jim Elwell
