- [Instructor] In getting started with geometric
dimensioning and tolerancing, it is helpful to be familiar
with several frequently used terms,
including datum, features and feature of size,
feature control frame, maximum material condition,
and least material condition.
A datum is a reference geometry
that serves as an origin for measurements.
Datums can be points, axis, planes,
or some combination of these.
A datum feature is the CAD geometry
that the datum is attached to.
Typically mating surfaces in an assembly
are chosen as datum features.
This allows measurements to be made
which reflect the ability of the parts
to be properly mated together.
Typically, a datum feature is the geometry
that an inspector would place onto a feature
to serve as the reference point
for validating the geometry of a part.
Parts are designed by creating a collection
of geometric forms, such as surfaces,
edges, pins, holes, tabs, and more.
These geometric forms are commonly referred to
as part features.
If a given part feature has a set of two opposing elements,
such as a cylindrical face or a slot,
and a dimension can be drawn between those two elements,
it is considered to be a feature of size.
Features of size can be directly toleranced.
Unlike features of size, features without size
require the use of geometric constraints
to control their form.
Feature control frames allow the designer
to specify the constraints for a given feature
required for the proper fit of that feature.
For example, the feature control frame shown here in red
dictates how much the top surface of this cube
can vary from parallel with respect to the bottom surface.
More elements can be added to the feature control frame
as needed to exactly specify
the constraints on that feature.
Material condition describes how much material is left
after a feature is fabricated
at either the maximum or minimum tolerance conditions.
In the diagram shown here, with dimensions in millimeters,
the maximum material condition of the hole
would represent the amount of material left over
after we drill an 18-millimeter hole through the part.
For the peg, the maximum material condition
is when the peg is 22 millimeters in diameter.
When considering the least material condition,
in the case of the hole, it is the largest allowable hole.
With the dimensions shown here,
that is a 22-millimeter in diameter
hole through the part.
For the peg shown in the drawing,
the least material condition is when the peg
is 18 millimeters in diameter.
We can see that a peg, at its maximum material condition,
will fit in a hole of least material condition
and vice versa.
We can take advantage of these material conditions
to create tolerances that ensure parts
will properly fit together.