- [Instructor] The American Society of Mechanical Engineers'
Geometric Dimensioning and Tolerancing Standard
defines a set of 16 fundamental rules
for properly dimensioning and tolerancing a part
based on the ASME Y14.5-2009 standard.
The complete set of rules can be found
in section 1.4 of the standard guide.
Although all 16 of these rules are important, this video
will highlight the more commonly encountered rules.
All dimensions must have a tolerance.
There are exceptions to this rule.
For example, dimensions that are identified
as reference dimensions are considered for information use
only, and therefore, tolerances don't apply.
Maximum and minimum dimensions, by definition,
are dimensional limits, so further
tolerance limits don't make sense.
The ASME standard also allows dimensions
to be called out as stock, meaning the tolerance will be
dependent on commercially available material specifications.
Dimensioning and tolerancing shall be complete,
so that there is a full understanding
of the geometry of each and every feature.
These dimensions must fully define the ideal geometry,
as well as the allowable variation of that geometry.
In general, dimensions must be explicitly called out.
Measurements or assumptions of size
cannot be taken directly from an engineering drawing.
Each necessary dimension of a part
shall be shown in the CAD file or in the drawing.
No dimension other than those necessary
to completely define a part's geometry should be used.
The use of reference dimensions should be minimized.
Dimensions must be clearly defined
so that they have one and only one possible interpretation.
Ideally, dimensions shall be chosen
and arranged to communicate the functional
or the assembly mating relationship of a part.
Drawings should sufficiently define a part
with enough detail the manufacturer
can intelligently decide what method to use
in order to create the desired geometry.
The drawing should constrain the geometry,
but should not define the process
used to create that geometry.
As long as a given process can provide
the desired tolerances, that process should be acceptable.
Exceptions to this rule include instances where
additional requirements of quality assurance, environmental
regulation, or other external factors might apply.
Dimensions should be readable with minimum confusion.
The ASME standard publication provides examples of proper
dimension locations for different types of geometry.
There are also a number of style guides available online.
Although geometric dimensioning and tolerancing rules
require each feature to have
an explicitly defined dimension,
the ASME standard states that if no angular dimension
is explicitly shown, lines which look like
they're at 90 degree angles in a drawing
are assumed to be right angles,
and lines that appear coincident on a drawing are assumed
to be collinear, and at zero degrees to each other.
Most materials expand and contract
as their temperatures change.
Rather than having to specify the room temperature and
which measurements are to be made
on every single drawing, the geometric dimensioning
and tolerancing rules specify that
all dimensions and tolerances are applicable
at 20 degrees Celsius, unless otherwise specified.
All dimensions and tolerances apply
to the part in its unconstrained free-state condition.
Once a part is removed from any manufacturing fixtures
that may constrain its shape, it's considered
to be in a free-state condition.
In certain cases, a part must be constrained
in order to take a measurement.
For example, a flexible part might need
to be mounted on a fixture that allows
repeatable measurements to be made.
All tolerances apply to the full depth, length,
and width of a part feature, unless
multiple tolerances are explicitly defined for that feature.