- [Instructor] Before we talk about
geometric dimensioning and tolerancing,
let's take a look at some limitations
of standard dimensioning using plus/minus tolerancing.
Most engineers will be familiar
with standard dimensioning practices.
Standard dimensioning tools
are intrinsic features of typical CAD programs.
A feature or geometry is drawn
and then a dimension is assigned to it
to define the geometry or location of that feature.
There are many style guides online
which describe best practices
for adding dimensions to your 2D drawing or 3D part.
While those guides might make the dimensions more readable,
they fall short of fully and completely
defining the geometry.
Standard dimensions don't include a way
to define fixed reference features
that can be used as a consistent starting point
for taking measurements.
Dimensions can also be placed
without taking into consideration
the ease or difficulty of measuring the actual dimension.
Often, standard dimensions can be interpreted
in multiple ways
and not necessarily the way the designer intended.
For example, a rectangular block
with four holes drilled through it
would have a different acceptable range of dimension layer
if the position of the hole centers was determined
relative to the sides of the part
versus being measured relative to the other hole centers.
Standard dimensioning does not explicitly define
characteristics like the flatness of surfaces,
parallel or perpendicular planar faces,
or the shapes of curved edges.
These characteristics must be inferred from the drawing.
It's commonly understood that very tight tolerances
make things more expensive
and that too loose tolerances
means parts may not fit together.
With plus/minus tolerancing,
it is possible that parts which physically fit together
can be rejected for being out of tolerance.
For example, a round hole located
using plus/minus tolerances
will have a square zone of acceptable locations
for the center of the hole,
despite the fact that the set of locations
where a round peg will still fit in the hole
is described by a circle.
Plus/minus tolerancing has no explicit controls
for the overall shape of objects
as defined by characteristics like parallelism,
perpendicularity, profile, flatness, or circularity.