"David R. Birch" on Wed, 12 Jun 2013 21:23:30 -0500
typed in rec.crafts.metalworking the following:
My company occasionally receives customer CAD files that have all or
most dimensions in inch fractions. What puzzles me is that when I
convert the fractions to decimal inches, the actual values turn out to
be only approximations of the fractions.

On one drawing, there was a line of holes regularly spaced and
dimensioned fractionally from one end. Based on the fractions, the
spacing seemed to be 6-11/32" hole to hole. After I converted the
fractions to decimal, the location of each hole from the end was not an
exact decimal equivalent and the hole to hole spacing was from 6.3405 to
6.3445". The distances didn't get progressively longer, just wandered
around the decimal equivalent of 6-11/32".

In another case on the same drawing, several hole diameters were given
as 1/8", yet changing it to decimal showed one line of holes to be .173"
and another on the same part .117".

Why does this happen? How do you input 1/8" and get something other than
.125"?

depends on if you have small values of .125 and large values of
1/8"

Or more exactly - precision of the actual dimension is enough to
generally fit into the 1/8" spec.
Alternatively, the drawing may "read" 1/8", but the actual
dimension is something else. (I know I can override dimensions in
AutoCAD, so that a dimension showing on screen/paper is correct, but
the measurement isn't.)

Thirdly, you may have a conversion error somewhere along the way,
from when they converted the measurement into fractional inches, to
how it was stored in the file, to how it is converted into the CAD s/w
you use.

David
--
pyotr filipivich
"With Age comes Wisdom. Although more often, Age travels alone."