The origins of NTS (Not To Scale) dimensions date back
to drafting on paper. On paper it was often simpler to revise a
dimension rather than redraw the correct object with dimensions.
Typically a drawing with gross dimensional change would have dimensions
with a notation of “NTS”. With the advent of CADD (Computer Aided
Drafting) it makes little since to only “fix” a dimension and have the
underlying object (lines, circles, arcs, et.) incorrect. One of the
major assumptions one makes in dealing with a drawing done in CADD is
that it is drawn correctly. The big assumption one has with CADD is “it
was created in CADD... it must be right”. I know of an engineering firm
that does all drawings on CADD, not because it is “faster” or “more
productive”, but because their clients assume the drawings are correct
and therefore do not question the accuracy of the drawings and file back
charges for mistakes...
What happens if someone is sloppy in the
revision process and only changes the dimensions by typing in
(over-riding) the new “correct” value. The results of over-ride
dimensions can be disastrous. The following scenario points out the need
to know what dimensions are real (associative) or fudged (over-ride).
Lets say that a widget (a fictitious part) drawing is composed of a
4”x5” rectangle with dimensions. The widget drawing is revised and the
rectangle is changed by the designer to be 4”x5 ½”. Instead of the
changing the rectangle the CADD draftsperson “X” merely re-dimensions
the 4”x5” rectangle to 4”x 5 ½”. The widget drawing is plotted and
everyone in the office is amazed by the speed of draftsperson “X”. But
all is not well in “CADD-land”... Trouble is brewing... The widget
drawing is now inserted as a block in and assembly. DXF files are
created for the CAM (Computer Aided Manufacturing) department, based on
the geometry of the geometry generated by the widget... OhOh!
To make the above scenario fit for your own recipe-for-disaster. Take
the above scenario and: Substitute the word Widget with what ever you
are creating, i.e. bents, doors, or bearings, et.
Substitute the word
assembly with Bridge, House, or Motor et.
Substitute CAM department
with supervisor, department head, or client, et.
Q: So how is it possible to overcome the “speedy-draftsperson-X” NTS
A: Well one would need a utility that would read the
drawing data base and check for over-ride dimensions.
CADD Group has created an NTS checking program (with the aid of custom
Programming from Robert Cheek of Maple Creek Programming) to search an
entire drawing and place the text string NTS inside every offending
over-ride dimension. A FREE version of NTS is available for downloading
from this site (click here to
download). A Licensed version ($29.95) is also available by
Frank Zander at Contract CADD Group (604) 591-1140