David R. Birch wrote:

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".

You have to carry the decimal fraction to 5 places to keep roundoff
error from accumulating.
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"?
I think you need to learn your CAD software better. Someting is just wrong,
and I can't believe it is that broken. One possibility is this is a
graphical CAD package, and the only data actually stored in the drawing
is the LINES. The lines as shown on the screen are only approximations
of the true data, smashed to fit onto the pixels of your screen. If it
then makes dimensions off the on-screen representation, that would
explain everything. This would not be considered a CAD package by
most people, but a "DRAWING" package. Trying to use it for precision
manufacturing drawings or the production of CAM data would be a big
mistake.

Jon