On 6/13/2013 2:10 PM, Jon Elson wrote:
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.

Rounding error isn't going to turn one 6-11/32 length to to 6.3446 and
the next to 6.3417.

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.

I know my software just fine, the problem is in the drawings we're getting.

Someting is just wrong, and I can't believe it is that broken.

Exactly, and the reason I'm asking is that I also can't believe it is
that broken and I also can't imagine how I could duplicate the distortion.

How did he do that and what can I tell him to make him stop?

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

You're talking about the raster output of a graphics package vs. the
vector output of CAD software. What we are receiving is a vector AutoCAD
DWG drawing with the dimensions all in inches and fractions of inches,
but the values of those fractions are not precise decimal equivalents,
or even consistently skewed.

David