On 6/12/2013 10:23 PM, 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".

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"?

David

As others have said, I think that the displayed dimensions have been
added manually, rather than reflect how the objects were put down. The
design was probably down by a that-looks-about-right approach and then
the dimensions added manually, rather than asking the CAD app to display
what it has in its data base/file.

So, when you "... [changed] it to decimal ...", the CAD goes to it's
data & displays it in decimal. It doesn't convert the manually added
dimension. If you now went back and changed it to fractions it would
show the fractional value of its data (e.g. 3/16 for .173), _not_ the
1/8. The 1/8 is a "note" that is gone (unless UNDO can bring it back) -
I hope you have a copy of the original file.

As to the 6-11/32 dimension: what do you mean that it "seemed" to be
that? Was it called out/dimensioned that or what? Again, like the 1/8
hole, when you convert to decimal, the program goes back to the design
data, which may or may not be 6-11/32

You need to clarify this mode of communication with your customer. It
seems as though he is using the CAD file for its display value. I.e.,
he expects you to print it out & build to the drawing. In which case
you would have to convert the fractions to decimal independently of the
CAD program and add them to the drawing. Or, if you need a file for
CNC, build it from scratch, using the fractions as shown on his original
drawing.

Bob